CONTRIBUTION À L’ÉTUDE DE LA CONSERVATION DES …bictel-fusagx.ulg.ac.be/ETD-db/collection/available/FUSAGxetd... · congélation et un post-traitement de séchage par entrainement

COMMUNAUTE FRANÇAISE DE BELGIQUE ACADEMIE UNIVERSITAIRE WALLONIE-EUROPE

UNIVERSITE DE LIÈGE GEMBLOUX AGRO-BIO TECH

CONTRIBUTION À L’ÉTUDE DE LA CONSERVATION DES GRAINES DE GRENADE

(PUNICA GRANATUM L.) PAR DÉSHYDRATATION OSMOTIQUE

Brahim BCHIR

Dissertation originale présentée en vue de l’obtention du grade de docteur en sciences agronomiques et ingénierie biologique

Promoteur : Prof. Christophe BLECKER 2011

Copyright. Aux termes de la loi belge du 30 juin 1994, sur le droit d’auteur et les droits voisins, seul l’auteur a le droit de reproduire partiellement ou complètement cet ouvrage de quelque façon et forme que ce soit d’en autoriser la reproduction partielle ou complète de quelque manière et sous quelque forme que se soit. Toute photocopie ou reproduction sous autre forme est donc faite en violation de la dite loi et des modifications ultérieures.

BRAHIM BCHIR. (2011). Contribution à l’étude de la conservation des graines de grenade par déshydratation osmotique. Gembloux, Belgique, Université de Liège-Gembloux Agro-Bio Tech. 198 p., 19 tabl., 16 fig. Résumé :

L’objectif des travaux entrepris au cours de cette thèse visait à mettre en place un procédé global de conservation des graines de grenade (Punica granatum L.). Ce procédé se base essentiellement sur une déshydratation osmotique (DO), associée à un pré-traitement de congélation et un post-traitement de séchage par entrainement. Dans ce contexte, plusieurs paramètres d'optimisation du transfert de masse ont été étudiés, tels que la nature de la solution d’immersion (saccharose, glucose, glucose/saccharose et jus de datte « sous-produit » enrichi en saccharose), la température (30, 40, et 50°C) et l’état du fruit (frais, congelé). En outre, nous avons mis en relation ces conditions avec certaines propriétés des graines : leur texture, leur structure, et leur couleur.

L’étude des paramètres de déshydratation (perte en eau (WL), gain en solides (SG), et réduction en poids (WR)) a montré qu’en utilisant des graines congelées et indépendamment de la température et de la solution utilisée, la majorité du transfert de masse s’effectue pendant les vingt premières minutes de traitement. A l’issue de cette période, la perte en eau est estimée à 46%, 41%, 39%, et 37% respectivement dans les solutions de saccharose, glucose/saccharose, de jus de datte et du glucose. La DO des graines fraîches est caractérisée par une cinétique plus lente, mais une perte finale en eau plus importante. Comme le montrent les analyses en microscopie électronique, cela s’explique par une déstructuration cellulaire survenant à la suite de la congélation des graines, ce qui vient conforter les résultats des observations microscopiques. Les mêmes techniques ont également indiqué une modification de texture/structure induite par le processus de DO. D’autre part, l’utilisation d’une solution de saccharose (55°Brix) et d’une température de 50°C favorise un meilleur transfert de masse.

La détermination des différentes fractions d’eau dans la graine par calorimétrie différentielle (DSC) a montré une augmentation d’un facteur ~2,5 fois de la fraction d’eau non congelable (eau liée) et une réduction de ~3,5 fois de la fraction d’eau congelable (eau libre) favorisant ainsi une meilleure conservation du fruit. Le suivi de la qualité intrinsèque des graines au cours de la DO a montré une perte d’une quantité non négligeable de certains composés (protéines, cendres) de la graine vers la solution, ce qui pourrait avoir une influence majeure sur la qualité organoleptique et nutritionnelle du fruit.

La DO seule ne pourrait pas maintenir une stabilité du produit au cours de la conservation. En effet, l’activité d’eau du produit fini après DO est proche de 0,90. Ainsi, dans un but plus appliqué, un traitement complémentaire de séchage par entrainement (2 m/s durant 4 heures) a été mis en place, à différentes températures (40, 50, et 60°C), afin de réduire l’activité d’eau à une valeur inférieure à 0,65. Afin d’optimiser le traitement de séchage, nous avons étudié en premier lieu l’effet de la température sur l’évolution de la matière sèche, de l’activité d’eau et du pourcentage de séchage des graines. D’autre part, plusieurs paramètres de qualité des graines de grenade (l’activité antioxydante, la teneur en composés phénoliques, les anthocyanines, la couleur, et la texture) ont été étudiés à différentes températures.

Ce travail est une contribution à l’étude des propriétés physico-chimiques des graines de grenade (Punica granatum L.) au cours des procédés de congélation, de déshydratation et de séchage. Les caractéristiques du produit fini peuvent justifier de nouvelles voies de transformation et d’exploitation des graines de grenade. Mots clés : Grenade, Déshydratation osmotique, Congélation, Séchage, Texture, Structure, Analyse calorimétrique différentielle, Perte en eau, Activité antioxydante, Composés phénoliques.

BRAHIM BCHIR. (2011). Contribution to pomegranate seeds conservation by osmotic dehydration. Gembloux, Belgium, University of Liege-Gembloux Agro-Bio Tech, 198 p., 19 tabl., 16 fig. Abstract:

The aim of this work was to create a complete conservation process of pomegranate seeds (Punica granatum L.). This process is essentially based on osmotic dehydration (OD), which was associated to freezing and air-drying process. Several parameters were studied to optimize the process such as osmotic solution (sucrose, glucose, and sucrose/glucose and date juice with sucrose added), temperature (30, 40, and 50°C) and state of the fruit (fresh and frozen). All these conditions were linked to seed proprieties (texture, structure, and colour).

The study of osmotic dehydration parameters (water loss (WL), solids gain (SG) and weight reduction (WR)) showed that most significant changes of mass transfer took place during the first 20 min of dewatering using frozen seeds, independently of temperature and sugar type. During this period, seeds water loss was estimated at 46% in sucrose, 41% in sucrose/glucose mix, 39% in date juice, and 37% in glucose. Mass transfer was slower starting from fresh fruit but led to a higher rate of WL at the end of the process. This fact can be explained by scanning electron microscopy, which showed damage of seed cell structure after freezing. This has practical consequences in terms of the modification of seeds texture. The same process also revealed a modification of seed texture and cell structure after osmotic dehydration. Using a sucrose solution and a temperature of 50°C favoured the best mass transfer. The determination of different water fractions of seed by differential scanning calorimetry (DSC) showed that the % of frozen water decreased 3.5 times contrary the % of unfreezable water that increased 2.5 times. This favours a better seeds conservation. During osmotic dehydration, there was a non negligible leaching of natural solutes from seeds into the solution, which might have an important impact on the sensorial and nutritional value of seeds.

Using only osmotic dehydration could not maintain the stability of seeds during conservation. In fact, after the osmotic process, water activity of seeds was found to be higher than 0.9, allowing to the development of microorganisms and some undesirable reactions. As a consequence, a drying of the pomegranate seeds (during four hours) was investigated at three different temperatures (40, 50, and 60 °C) with air flow rate of 2 ms-1. Prior to the drying process, seeds were osmodehydrated in a sucrose solution (55°Brix) during 20 min at 50°C. The drying kinetics and the effects of OD and air-drying temperature on antioxidant capacity, total phenolic, colour, and texture were determined.

This work is a contribution to the study of physico-chemical properties of pomegranate seeds (Punica granatum L.) during freezing, osmotic dehydration and drying. After the global process, the pomegranate seed characteristics lead to new industrial developments. Keywords: Pomegranate; Osmotic dehydration; Freezing, Drying; Texture; Structure; Differential scanning calorimetry analysis; Water loss; Antioxidant activity; Phenolic compound.

Dédicaces

Je dédie ce travail :

Aux deux êtres les plus chers, mon père Mohamed Naceur BCHIR et ma mère

Radhia HIZEM, pour leur amour, leur soutien et leurs sacrifices, en témoignage

de ma grande estime et mon amour pour eux.

A mes frères Aymen, Habib et Rached pour la motivation et les encouragements

incessants qu’ils m’ont fournies en élaborant ce travail.

A mon cousin Samir pour la confiance qu’il m’a toujours accordée.

Remerciements

Au terme de ce travail qui a été réalisé dans le cadre d’une collaboration entre le

laboratoire de Biophysique et Ingénierie des Formulations de Gembloux Agro-Bio Tech

(Université de Liège, Belgique) et le laboratoire d’Analyse Alimentaire de l’ENIS (Université

de Sfax, Tunisie) je voudrais remercier les personnes qui, de près ou de loin, ont participé à

son aboutissement.

Tout d’abord, je remercie Monsieur Claude DEROANNE, ancien responsable de l’Unité

de Technologie des Industries Agroalimentaires, pour m’avoir accueilli au sein de son Unité

et pour la gentillesse qu’il m’a témoignée.

Mes plus chaleureux remerciements s’adressent à mon promoteur de doctorat, Monsieur

Christophe BLECKER (Chef du service de Technologie des Industries Agroalimentaires)

qui était un grand soutien moral pour moi et m’a toujours encouragé pendant les moments

difficiles. De plus, les conseils qu’il m’a prodigués ont toujours été clairs et précis, me

facilitant l’accomplissement de ce projet.

Je remercie Monsieur Hamadi ATTIA, Responsable de l’Unité Analyses Alimentaires de

l’Ecole Nationale d’ingénieurs de Sfax, de m’avoir offert l’occasion d’y travailler et pour

l’intérêt qu’il a accordé à mes travaux tout au long de leur réalisation.

Je remercie également Monsieur Souhail BESBES (Maître de conférences à Université de

Sfax, Tunisie), qui nous a donné l’idée de travailler sur ce projet et m’a suivi tout au long de

ce travail et dispensé ses conseils avisés.

Mesieurs Benoît HAUT et Frank DELVIGNE, ont accepté de juger ce travail et d’en

être des rapporteurs. Je leur dois une nette amélioration de la qualité de ce document. Qu’ils

trouvent ici toute ma reconnaissance.

Monsieur Michel PAQUOT, Monsieur Georges LOGNAY, Monsieur François BERA,

Madame Sabine DANTHINE ont été membres de mon comité de thèse. Ils m’ont assuré un

accompagnement scientifique de qualité.

Je tiens également à remercier Madame Lynn Doran pour son aide au laboratoire et sa

profonde gentillesse.

Nicole Rucquoy, Claire Ndayisenga, Thomas Bertrang, Fabian Rouffiange, Vanessa

Ardito, Marjorie Servais, Guy Delimme, Stéphane Guillaume, Sandrino Filocco, Alain

Someville, Dominique Cortese, Maguy Pétré, Isabelle Van de Vreken etc. ont tous

contribué au bon fonctionnement de ce travail car la bonne ambiance qui régnait dans le labo

grâce à eux est un élément indispensable pour mener à terme de longues manipulations.

Je remercie mes collègues Mazen Ibrahim, Romdhane Karoui, Jean-Michel Giet,

Gaoussou Karamoko, Prudent Anihouvi, Caroline Vanderghem, Gilles Olive, Christine

Anceau, Emilie Arnould, Paul Callewaert et Olivier Roiseux. Faire la recherche à leur côté

a été un réel plaisir.

Tous mes remerciements vont également aux membres du personnel du laboratoire

d’Analyse Alimentaire de l’ENIS (Sfax, Tunisie) qui m’ont chaleureusement accueilli et

m’ont permis de réaliser ce travail dans une ambiance excellente.

LEXIQUE

aw water activity ;

ANOVA analyse de la variance ;

AA antioxidant activity (%) ;

Abs absorbance;

°Brix degré brix (%) ;

C* chroma ;

C° degré celcius ;

Cp chaleur spécifique (J·kg-1·K-1) ;

Deff coefficient de diffusion (m2.s-1) ;

DII déshydratation-imprégnation par immersion ;

DM dry matter ;

DO déshydratation osmotique ;

DPPH 2,2,-diphenyl-2-picryl-hydrazyl ;

DR drying rate (g water/g dry matter min-1);

DSC differential scanning calorimetry ;

FM fresh matter;

FS fresh seed;

GAE gallic acid equivalents;

h° angle de teinte ;

Hfus enthalpie de fusion (j.g-1);

HMF hydroxy-méthyl-furfural;

HPLC high performance liquid chromatography;

K1, K2 Peleg’s parameters;

L* luminosité;

min minute;

PPO polyphenol oxidase;

R2 coefficient de corrélation (%);

RA: relative activity (%);

RMN résonance magnétique nucléaire;

SG solids gain (g/100 g fresh seeds);

SEM scanning electron microscopy;

TEAC trolox-equivalent antioxidant capacity ;

TAA total antioxidant activity (%);

t time;

Tf melting point (°C);

Tg’ glass transition temperature (°C);

UFW unfreezable water (g.g-1 dry matter);

WL water loss (g/100 g fresh seeds);

WR weight reduction (g/100 g fresh seeds);

% pourcent.

http://www.clinchem.org/cgi/content/extract/50/5/952

TABLE DES MATIÈRES

Introduction générale ...................................................................................... 1

Chapitre 1 : Synthèse bibliographique............................................................. 7 Publication I : Synthèse des connaissances sur la déshydratation osmotique ...................... 7

Résumé ................................................................................................................................... 8

1. Introduction ...................................................................................................................... 11

2. Généralités sur la déshydratation osmotique.................................................................... 13

2.1. Cinétique de la déshydratation osmotique .................................................................. 13

2.2. Principaux facteurs influençant les performances de la DO ....................................... 17

2.2.1. Propriétés des tissus biologiques............................................................................ 18

2.2.2. Concentration et composition de la solution osmotique ........................................ 18

2.2.3. Température de la solution osmotique ................................................................... 20

2.2.4. Durée du traitement ................................................................................................ 21

2.2.5. Mode de mise en contact des phases, effet de l’agitation et du rapport

solide/solution ........................................................................................................... 21

2.2.6. Mise en œuvre de la déshydratation osmotique ..................................................... 22

2.3. Application de la déhydratation osmotique................................................................. 24

2.3.1. Pré-traitement thermique........................................................................................ 24

2.3.1.1. Blanchiment ...................................................................................................... 24

2.3.1.2. Congélation ....................................................................................................... 25

2.3.2. Méthodes combinées de la DO............................................................................... 25

2.3.2.1. Imprégnation sous vide ..................................................................................... 25

2.3.2.2. Haute préssion hydrostatique ............................................................................ 26

2.3.2.3. Ultrasons............................................................................................................ 27

2.3.2.4. Irradiation .......................................................................................................... 27

2.3.2.5. Chlorure de sodium ........................................................................................... 28

2.3.2.6. Centrifugation.................................................................................................... 29

2.3.2.7. Traitement par champ éléctrique pulsé ............................................................ 29

2.3.3. Stabilisation des produits déshydratés osmotiquement par des traitements

physiques................................................................................................................... 29

2.3.3.1. Séchage.............................................................................................................. 30

2.3.3.2. Congélation ....................................................................................................... 31

2.4. Equipements pour la DO ............................................................................................. 31

2.5. Qualité des produits végétaux traités par DO.............................................................. 32

2.5.1. Saveur..................................................................................................................... 33

2.5.2. Couleur ................................................................................................................... 33

2.5.3. Texture ................................................................................................................... 34

2.5.4. Réhydratation ......................................................................................................... 34

3. Valorisation des fruits conservés en solutions sucrées..................................................... 35

4. Conclusion........................................................................................................................ 35

5. Références bibliographiques ............................................................................................ 37

Chapitre 2 : Cinétique de transfert de masse durant la déshydratation

osmotique des graines de grenade ........................................................ 44

Résumé ................................................................................................................................ 45

1. Objectif et stratégie expérimentale........................................................................... 45

2. Principaux résultats .................................................................................................. 47

Publication II : Osmotic dehydration of pomegranate seeds: mass transfer kinetics and

differential scanning calorimetry characterization.................................................... 48

1. Introduction ............................................................................................................. 50

2. Material and methods .............................................................................................. 51

3. Results and discussion............................................................................................. 56

4. Conclusions ............................................................................................................. 71

5. References ............................................................................................................... 73

Chapitre 3 : Effet de la congélation sur la cinétique de transfert de masse

durant la déshydratation osmotique des graines de grenade .............. 77

Résumé ................................................................................................................................ 78

1. Objectif et stratégie expérimentale........................................................................ 78

2. Principaux résultats ............................................................................................... 80

Publication III : Osmotic dehydration of pomegranate seeds (Punica granatum L.): Effect

of freezing pre-treatment .......................................................................................... 81

1. Introduction ........................................................................................................... 83

2. Material and methods ............................................................................................ 84

3. Results and discussion........................................................................................... 90

4. Conclusions ......................................................................................................... 100

5. References ........................................................................................................... 102

Chapitre 4 : Utilisation du jus de datte comme milieu d’immersion pour la

déshydratation osmotique des graines de grenade ............................ 107

Résumé .............................................................................................................................. 108

1. Objectif et stratégie expérimentale........................................................................ 108

2. Principaux résultats ............................................................................................... 110

Publication VI : Osmotic dehydration kinetics of pomegranate seeds using date juice as an

immersion solution base ........................................................................................ 111

1. Introduction ............................................................................................................ 113

2. Material and methods ............................................................................................. 114

3. Results and discussion............................................................................................ 120

4. Conclusions ............................................................................................................ 131

5. References .............................................................................................................. 133

Chapitre 5 : Effet des conditions de séchage sur les propriétés

physicochimiques des graines de grenade déshydratées osmotiquement

.............................................................................................................. 139

Résumé .............................................................................................................................. 140

1. Objectif et stratégie expérimentale.............................................................................. 140

2. Principaux résultats ..................................................................................................... 142

Publication V : Effect of air-drying conditions on physico-chemical properties of

osmotically pre-treated pomegranate seeds ............................................................ 143

1. Introduction ............................................................................................................... 145

2. Material and methods ................................................................................................ 147

3. Results and discussion............................................................................................... 154

4. Conclusions ............................................................................................................... 169

5. References ................................................................................................................. 170

Chapitre 6 : Discussion générale, conclusions et perspectives ................... 178

6.1. Discussion générale................................................................................................ 179

6.2. Conclusion générale et perspectives ...................................................................... 187

6.3. Références bibliographiques .................................................................................. 190

Listes des figures.......................................................................................................... 194

Listes des tableaux ...................................................................................................... 196

Productions scientifiques .......................................................................................... 198

Introduction générale

Introduction générale ___________________________________________________________________________

Introduction générale

Ce sujet entre dans le cadre d’une collaboration entre le laboratoire de Biophysique et

Ingénierie des Formulations de Gembloux Agro-Bio Tech (Université de Liège, Belgique) et

le laboratoire d’Analyse Alimentaire de l’ENIS (Université de Sfax, Tunisie), qui s’articule

autour de la valorisation d’agrofournitures, dont principalement les produits et sous-produits

du palmier dattier Phoenix dactylifera L. Ainsi, plusieurs projets antérieurs ont été menés

dans une perceptive de caractérisation et de valorisation de la pulpe ou du noyau de datte, de

la sève du palmier dattier, du citron et des graines de pin d’alep et de cumin (Masmoudi et al.,

2007 ; Ben-Thabet et al., 2007 ; Cheikh-Rouhou et al., 2008 ; Besbes et al., 2009).

Le présent travail de thèse est consacré à l’étude d’un autre fruit important en Tunisie: la

grenade.

Selon les données de l’organisation des Nations Unies pour l’alimentation et l’agriculture

(FAO), la production mondiale de grenade était de 1,5 millions de tonnes en 2009, l’Iran étant

le premier producteur mondial avec une production annuelle de 700 000 tonnes, suivi de

l'Inde, l’Etats-Unis, la Turquie, l'Espagne, Israel, la Tunisie, la Grèce, Chypre et l'Egypte.

La filière grenade a connu un essor remarquable en Tunisie. En effet, la moyenne de

production par an (la saison de récolte dure de 3 à 4 mois) des grenades, qui n’était que de

51 000 tonnes au cours de la période 1992-1997, est passée à 62 000 et 70 000 tonnes

respectivement entre 1997-2001 et 2001-2008, soit une augmentation de 22% et 38%,

respectivement. En 2009, la production a atteint 75 000 tonnes. La variété El-Gabsi de qualité

organoleptique très appréciable, et donc de haute valeur marchande, représente environ 35%

de ce tonnage (Emna, 2010).

1


La grenade est le fruit d’un arbuste appelé grenadier, de nom latin Punica granatum L.

appartenant à la famille des punicacées. Il s’agit d’une baie, de taille et de poids variable

(diamètre entre 7 et 12 cm ; poids compris entre 50 et 800 g selon les variétés), à écorce dure

et coriace, de couleur brune à rouge ou jaune-beige, qui renferme dans des « loges »

délimitées par des cloisons épaisses appelées membranes intercarpellaires, de nombreuses

graines (de 200 jusqu’à 800 graines par fruit). Ces graines sont composées par un pépin, peu

agréable mais comestible, enrobé d'une pulpe translucide juteuse de couleur rose ou rouge, à

saveur aigrelette, plus ou moins sucrée et acide selon les variétés. Les graines constituent la

partie comestible de la grenade (Espiard, 2002).

Les graines de grenade sont riches en éléments nobles qui leur confèrent des

caractéristiques organoleptiques et nutritionnelles très intéressantes. Elles constituent une

excellente source d’hydrates de carbone (glucose, fructose), de minéraux (le magnésium, le

potassium, le calcium), de vitamines (vitamine C, B3...), d’acides organiques, et de

polyphénols. Ces derniers (flavonoïdes, tanins etc.) confèrent aux graines de grenade

d’importantes propriétés anti-oxydantes (Hernandez et al., 1999; Jaiswal et al., 2010). Edas,

(2009) a montré que le jus de grenade présente une activité antioxydante (18 – 20 Trolox-

Equivalent Antioxidant Capacity ‘TEAC’) trois fois supérieur à celle du vin rouge et du thé

vert (6 – 8 TEAC).

Les caractéristiques chimiques des graines de grenade ont suscité l’intérêt des

scientifiques et des industriels. En effet, nommée « ingrédient de l’année » en 2004 par la

société d’études de marché Mintel, la grenade fait l’objet d’un nombre croissant d’études, et

son incorporation dans des formulations alimentaires et cosmétiques est devenue courante.

Depuis 2005, plus de 475 produits à base de grenade ont vu le jour en Amérique. Cette

croissance s'explique principalement par l'attrait des propriétés bénéfiques de la grenade sur la

santé humaine, en relation avec sa composition (Storey, 2007). En effet, la majorité des études

2

http://fr.wikipedia.org/wiki/Pulpe




publiées sur la grenade sont focalisées sur ses caractéristiques nutritionnelles et leurs impacts

sur la santé humaine, en relation avec sa composition (Adsule et Patill, 1995; Garca et al.,

2004 ; Aslam et al., 2006). D’un point de vue pratique, la plupart de ces travaux de

recherches se sont vus concrétisés dans des projets industriels. Plusieurs marques de produits

cosmétiques (Archipelago, Ushuaïa, Tocophea etc.) incorporent les extraits de grenade dans

leur gamme des produits, pour ses propriétés antioxydantes, anti-inflammatoires et

antimicrobiennes. D’autre part, plusieurs médicaments à base de grenade sont utilisés pour

lutter contre les états inflammatoires, ainsi que l’athérosclérose et les maladies

cardiovasculaires (Curtay et al., 2008). Dans le domaine alimentaire, divers produits à base de

graines de grenade ont été présentés récemment sur le marché mondial : le jus, le vin, la

crème-glacée, etc. (Storey, 2007). Malgré cette diversification de produits, des solutions

doivent être apportées, notamment au niveau de la conservation des graines de grenade, en

particulier dans un pays comme la Tunisie.

La consommation actuelle des grenades en Tunisie est cantonnée à l’état frais durant la

saison de récolte, en raison d’un manque de valorisation industrielle et essentiellement dû à

des problèmes de conservation. En effet, Le potentiel de stockage des grenades est limité par

l’apparition d’un brunissement de l’écorce, et des pourritures provoquées par des

champignons, affectant ainsi la qualité organoleptique intrinsèque du fruit. Ces grenades non

consommées en l’état sont écartées au niveau des stations de conditionnement et de

transformation.

Afin d’échelonner la période de consommation de la grenade en Tunisie, de mieux

exploiter les excellentes propriétés nutritionnelles, biologiques et thérapeutiques de ce fruit, et

de developper un nouveau mode de transformation. La déshydratation osmotique (DO) est

une technique qui permet de répondre à toutes ces attentes, selon diverses études menées sur

plusieurs fruits (abricot, ananas, banane, etc.) et légumes (carotte, haricot, oignon, etc.)

3


(Uddin et al., 2004; Shi et al., 1998, Jena et Das, 2004; Rastogi et Raghavarao, 2004;

Dermesonlouoglou et al., 2008 ; Garcia-Segovia et al., 2010 ; Kowalska et al., 2008). En effet,

cette technique de conservation, économe en énergie, est susceptible de prolonger la période

de disponibilité des produits alimentaires, et leur confère des propriétés sensorielles nouvelles

et appréciées. Elle permet ainsi aux acteurs de la filière agroalimentaire d’écouler leur

production à de meilleurs prix et aux consommateurs d’en disposer tout au long de l’année.

Cette technique est un outil facile à mettre en place, surtout dans les pays en voie de

développement, en raison de son faible coût.

Cependant, la DO est un procédé relativement lent. Il est donc impératif d’utiliser des

procédés complémentaires afin d’augmenter la perméabilité des membranes cellulaires pour

faciliter la libération de l’eau. Un pré-traitement thermique (blanchiment, congélation) est très

souvent utilisé (Kowalska et al., 2008). Le procédé de congélation reste un excellent pré-

traitement pour la déshydratation osmotique, permettant d’améliorer significativement le

transfert de masse (Torreggiani et Bertolo, 2001). Cependant, les changements structuraux

dans la paroi cellulaire peuvent mener à une diminution de la fermeté du produit après

congélation (Torreggiani et Bertolo, 2001).

Les produits issus du procédé de déshydratation osmotique sont classés parmi les produits

à taux d’humidité élevé (Garcia-Martinez et al., 2002). Aussi le produit n’est pas encore

microbiologiquement stabilisé, et l’activité de l’eau peut y être élevée (supérieur à 0,9). Pour

éviter l’altération du produit au cours de l’entreposage, plusieurs post-traitements ont été

proposés tels que le séchage, la friture, etc. (Fernandes et al., 2006).

Le séchage est le procédé le plus communément utilisé par les scientifiques et les

industriels (Ade-Omowaye et al., 2003). L’utilisation du séchage dans les industries

agroalimentaires a de multiples objectifs : accroître la durée de conservation des produits;

stabiliser les produits agricoles et transformer les produits par des réactions biochimiques ou

4


biologiques. Cependant, cette technique est coûteuse en énergie. En effet, le séchage des

produits végétaux nécessite environ 5000 kJ/kg d’eau évaporée (Mujumdar, 2006). Afin de

réduire le coût énergétique global de l’élimination de l’eau, plusieurs auteurs proposent la

combinaison entre la déshydratation osmotique et le séchage (Wang et Sastry, 2000; Ade-

Omowaye et al., 2003 ; Fernandes et al., 2006). En effet, la pré-déshydratation diminue la

teneur en eau initiale dans le produit induisant une réduction du temps de séchage et du besoin

énergétique pour le séchage complémentaire (Fernandes et al., 2006).

L’étude du procédé de conservation des graines de grenade par déshydratation osmotique

semble avoir échappé, tant à l'investigation scientifique qu'à l'exploitation l'industrielle. C'est

cette approche originale que nous nous sommes proposés d'étudier.

Dans ce contexte, le présent travail de thèse s’est attaché à délivrer les bases scientifiques

et techniques pour l’étude des possibilités de conservation des graines de grenade (variété

Tunisienne El-Gabsi) par déshydratation osmotique.

Pour y parvenir, nous avons tout d’abord optimisé la déshydratation osmotique des graines

de grenade en analysant les effets de l’influence de la température (30, 40, 50°C) et de la

composition des solutions osmotiques (saccharose, glucose, saccharose/glucose) sur la

cinétique de transfert de masse, ainsi que sur les caractéristiques physico-chimiques des

graines de grenade. Afin d’explorer l’effet du pré-traitement de congélation sur le procédé de

DO, nous avons comparé, dans un deuxième temps, la cinétique de transfert de masse, la

structure des cellules et la texture des graines fraiches et congelées avant et après DO. Le

troisième volet a porté sur la valorisation d’une deuxième agrofourniture tunisienne, le jus de

datte, dont l’utilisation permet une réduction du coût économique global du procédé. En

Tunisie, le saccharose est un produit importé alors que le jus de datte est issu des dattes

déclassées (à bas prix) pour des raisons de texture et de forme. Ainsi ce volet avait comme

objectif la détermination de l’influence du milieu d’immersion (solution à base de jus de datte

5


6

en substitution du saccharose) sur la cinétique de DO, et la caractérisation des changements

structuraux et texturaux des graines de grenade avant et après DO. Pour une meilleure

stabilisation des graines de grenade au cours de la conservation, nous avons abordé l’effet des

conditions du post-traitement de séchage par entrainement sur les caractéristiques physico-

chimiques et rhéologiques des graines déshydratées osmotiquement.

Afin de mieux élucider l’influence du pré-traitement de congélation, du traitement de DO

et du post-traitement de séchage sur la qualité des graines de grenade, nous avons mis en

œuvre des techniques fines tels que la calorimétrie différentielle à balayage («differential

scanning calorimetry », DSC), la microscopie électronique à balayage, et la texturomètrie.

Avant d’aborder l’ensemble des résultats et leurs discussions, un passage en revue de la

littérature présente les principales connaissances sur le procédé de déshydratation osmotique

des fruits et légumes. (*)

___________________________________________________________________________

* Les références bibliographiques sont reportées en fin du document.

Chapitre 1 : Synthèse bibliographique ___________________________________________________________________________

Chapitre 1:

Synthèse bibliographique :

Ce travail a fait l’objet de la publication suivante :

Bchir, B., Besbes, S., Giet, J., Attia, H., & Blecker, C. (2010). Synthèse des

connaissances sur la déshydratation osmotique. Biotechnologie Agronomie

Société et Environnement, (in press).

7


Titre : Synthèse des connaissances sur la déshydratation osmotique

Résumé :

Parmi les procédés de conservation des produits végétaux, la déshydratation

osmotique présente un intérêt économique et nutritionnel certain. Cette technique, économe

en énergie, est susceptible de prolonger la période de disponibilité des produits alimentaires,

et leur confère des propriétés sensorielles nouvelles et appréciées. Elle permet ainsi aux

acteurs de la filière agroalimentaire d’écouler leurs productions à de meilleurs prix et aux

consommateurs d’en disposer tout au long de l’année. Cette technique est un outil facile à

mettre en place, surtout dans les pays en voie de développement, en raison de son faible coût.

Le présent article a pour objectif de présenter une synthèse de la littérature concernant la

technique de déshydratation osmotique, afin d’en rappeler les bases théoriques et pratiques,

mais aussi d’en préciser les nouvelles tendances et voies de recherches récentes.

8


Synthèse des connaissances sur la déshydratation osmotique

*Brahim Bchir1, Souhail Besbes2, Jean-Michel Giet1, Hamadi Attia2, Christophe

Blecker1

1 : Unité de Technologie des industries agro-alimentaires, Université de Liège Agro-Bio

Tech, Passage des Déportés, 2-B-5030 Gembloux, Belgique.

2 : Unité Analyses Alimentaires, Ecole Nationale D’ingénieurs de Sfax, Route de Soukra,

3038 Sfax, Tunisia

* Corresponding author: Tel: +32/ 8162273, Fax: +32/81614222

E-mail address: [email protected]

9


Résumé :

Parmi les procédés de conservation des produits végétaux, la déshydratation

osmotique présente un intérêt économique et nutritionnel certain. Cette technique, économe

en énergie, est susceptible de prolonger la période de disponibilité des produits alimentaires,

et leur confère des propriétés sensorielles nouvelles et appréciées. Elle permet ainsi aux

acteurs de la filière agroalimentaire d’écouler leurs productions à de meilleurs prix et aux

consommateurs d’en disposer tout au long de l’année. Cette technique est un outil facile à

mettre en place, surtout dans les pays en voie de développement, en raison de son faible coût.

Le présent article a pour objectif de présenter une synthèse de la littérature concernant la

technique de déshydratation osmotique, afin d’en rappeler les bases théoriques et pratiques,

mais aussi d’en préciser les nouvelles tendances et voies de recherches récentes.

Summary:

Among the preservation processes of vegetal products, osmotic dehydration presents

an economic and a nutritional interest. This technique consumes a low quantity of energy,

prolongs the period of availability of foodstuffs, and gives new and appreciated sensory

properties to products. Therefore, the producers can sell their productions with better prices

and the consumers are able to consume fruits and vegetables throughout the year. This

technique is very easy to set up, especially in the developing countries due to its low cost. The

aim of this article is to present a synthesis of the literature concerning the osmotic dehydration

technique, and also to specify the new tendencies and directions of recent research.

Mot-clés : Déshydratation osmotique, conservation, nouvelles tendances

Keywords: Osmotic dehydration, preservation, new tendencies

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1. INTRODUCTION

Les industriels du secteur agro-alimentaire sont aujourd’hui confrontés à deux

problèmes majeurs : d’un part l’attente croissante des consommateurs pour des produits de

hautes qualités nutritionnelles et organoleptiques et, d’autre part, l’augmentation des coûts

énergétiques. En réponse à ces défis, les techniques de stabilisation et de conservation des

aliments, tels que le séchage ou les techniques du froid, connaissent des améliorations

constantes, et sont de mieux en mieux intégrées aux filières industrielles.

Une catégorie de méthodes de séchage fait intervenir une ou plusieurs étapes de mise

en contact des denrées avec une solution aqueuse concentrée en sels (ex. saumurage des

légumes, viandes, poissons ou fromages), en acide (ex. marinage des produits carnés) ou en

sucres (confisage et semi-confisage des fruits). Ce traitement vise à réduire, à moindre coût, le

risque d’altération de la qualité nutritionnelle et organoleptique du produit traité (Ade-

Omowaye et al., 2003). Le confisage est une des techniques traditionnelles dont les

développements récents ont donné naissance aux procédés dits de « déshydratation

osmotique » (DO) ou de « déshydratation-imprégnation par immersion » (DII) (Albagnac et

al., 2002).

La déshydratation osmotique présente un certain nombre d’atouts par rapport aux

techniques traditionnelles de séchage. En particulier, l’aliment est traité à plus basse

température (entre 5 et 85°C), et à l’abri de l’oxygène (puisqu’il est immergé), ce qui est

particulièrement favorable pour les produits sensibles aux réactions de dégradation oxydatives

et thermiques (Lerica et al., 1985; Garcia-Segovia et al., 2010).

De plus, la DO permet de reduire la charge microbienne, et ainsi de prolonger la

période de conservation des produits (Castello et al., 2009).

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La déshydratation osmotique est attribuée au phénomène d’osmose, qui se manifeste à

travers les membranes cellulaires « semi-perméables » (perméables à l’eau, mais moins aux

solutés) des tissus. Raoult-Wack, (1994) a montré que le moteur de ce transfert est une

différence de concentration entre la solution et le matériau à traiter. Il se traduit par deux

écoulements simultanés à contre-courant : une diffusion de l’eau des cellules du produit (la

solution la moins concentrée) vers la solution hypertonique où l’aliment est plongé

(déshydratation) et une entrée de soluté de la solution vers l’aliment (imprégnation).

L’aliment peut ainsi perdre jusqu’à 50% de la teneur initiale en eau en moins de 3 heures

(Lerica et al., 1985). La sortie d’eau s’accompagne généralement d’une perte de solutés

propres au produit alimentaire. Ce transfert, quantitativement négligeable par rapport aux

deux premiers, soulève des critiques quant à son impact sur les qualités organoleptiques et

nutritionnelles du produit transformé (Albagnac et al., 2002).

Afin d’améliorer l’efficacité du processus de déshydratation osmotique, divers

traitements peuvent être appliqués pour faciliter la diffusion de l’eau: ultrasons, irradiation,

champ électrique pulsé, etc. Pour une conservation de très longue durée, le produit obtenu

après déshydratation osmotique peut encore subir un traitement complémentaire, tel qu’un

séchage à l’air ou une congélation (Garcia-Segovia et al., 2010).

Employée industriellement depuis les années 60, la DO fait récemment l’objet d’un

certain regain d’intérêt. Au cours de la dernière décennie, la progression constante du nombre

d’articles scientifiques publiés annuellement sur le sujet en témoigne. Le présent travail se

propose de rappeler les bases de la déshydratation osmotique, d’exposer les différents facteurs

influençant ce processus et de présenter les dernières avancées scientifiques en termes

d’amélioration des performances de la technique.

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2. GENERALITES SUR LA DESHYDRATATION OSMOTIQUE

2.1. Cinétique de la déshydratation osmotique

Les cinétiques de transfert de matière dans les produits végétaux (Tableau 1a-1b)

peuvent se décomposer en deux phases : une première phase, responsable de l’essentiel des

transferts d’eau et de solutés, suivie d’une seconde phase, pendant laquelle la perte en eau

ralentit fortement tandis que les débits d’entrée en solutés continuent d'augmenter

régulièrement (Kowalska et al., 2008). Il semble probable que les membranes cellulaires

soient victimes d'une perte de leur caractère semi-perméables, permettant progressivement

aux solutés de pénétrer dans la cellule (Raoult-Wack, 1994). La durée de la première phase est

très variable suivant le produit traité, d’une demi-heure à deux heures dans les conditions les

plus courantes (morceaux de petites tailles, de l’ordre du cm3). Ces transferts se déroulent à

travers les parois et membranes cellulaires du produit. À l’intérieur de ces derniers, les

espaces intercellulaires servent de lieux d’accumulation ou de passage pour les substances

échangées (Raoult-Wack, 1994; Lenart, 1996; Kowalska et al., 2008).

Deux approches sont employées afin d’étudier la cinétique de la déshydratation

osmotique :

L’approche classique qui se base sur la détermination de deux paramètres. En effet, des

travaux antérieurs ont prouvé que deux paramètres peuvent quantitativement représenter le

processus osmotique. Ces paramètres sont la perte d’eau (« Water loss », WL), indiquant l’eau

qui sort du matériel cellulaire vers la solution, et le gain en solides (« Solids Gain », SG). Ces

paramètres sont habituellement déterminés par la mesure des solides totaux, ou par analyse

chimique (Krokida et al., 2000 ; Riva et al., 2005; Garcia-Segovia et al., 2010).

13


Tableau 1 : Application de la déshydratation osmotique sur des fruits (1a) et légumes (1b).

Fruits Conclusions majeures Auteurs

Abricot L’utilisation d’une température de 50°C, une concentration de saccharose de 60°Brix et un rapport solution/produit de 10:1 offrent une meilleure déshydratation.

Khoyi et Hesari, 2007

Ananas La perte en eau maximale (0.64-0.77 kg d’eau/Kg de tranche) a été observée à une température comprise entre 30°C et 60°C.

Jena et Das, 2004

Banane L’utilisation de deux bains successifs aboutit à une réduction de poids de 60% et un gain de soluté de 2%.

Jiokap Nono et al., 2001

Cerise Le temps, le rapport solution/produit, et le sucre utilisé ont un effet significatif sur le transfert de masse.

Sunjka et Raghavan, 2004

Châtaigne La meilleure déshydratation a été obtenue à haute concentration en soluté et basse température.

Chenlo et al., 2007

Fraise Les cellules ne sont pas affectées par la nature du soluté. Le coefficient de diffusion de l’eau est très affecté par le Pré-traitement de blanchiment.

Ferrando et Spiess, 2001

Grenade L’augmentation de la température favorise la perte en eau et le gain en solutés de la graine. L’analyse par DSC montre une réduction de la mobilité de l’eau au cours du procédé. D’autre part, Tg’ dépend du type de sucre et de la teneur en eau dans la graine.

Bchir et al., 2009

Mangue La congélation a un effet positif sur la perte en eau, et un faible effet sur le gain en solutés. L’utilisation d’une solution de saccharose de 45°Brix à 30°C offre une meilleure perte en eau et un gain en soluté des mangues congelées.

Floury et al., 2008

Melon L’utilisation d’une solution de maltose (40-60°Brix) entraine une augmentation de la perte en eau et une diminution du gain en soluté, contrairement au saccharose.

Ferrari et Hubinger, 2008

Kiwi Toute augmentation de température et de la concentration en sucre se traduit principalement par une augmentation des vitesses de transfert d’eau, les transferts de soluté n’étant que peu affectés. L’utilisation de solutés de déshydratation différents (saccharose, saccharose/sucre inverti, sirop de glucose) n’induit que de faible variation au niveau des transferts de matière.

Vial et al., 1990

Papaye L’augmentation de la température (de 30 à 70°C), de la concentration en saccharose dans la solution de déshydratation (de 45 à 72°Brix), la présence de calcium (0.05 M), l’absence de blanchiment préalable et/ou le remplacement du saccharose par un sirop de glucose de faible dextrose équivalent favorisent la perte en eau et freinent la pénétration du sucre.

Heng et al., 1990

Pastèque L'augmentation de la température et de la concentration de la solution osmotique provoque une augmentation du transfert de masse.

Falade et al., 2007

Pomme L’utilisation de sirops vieillis n’a pas d’influence significative sur les cinétiques de transfert de masses.

Jiokap Nono et al., 2001

(1a)

14


Légumes Conclusions majeures Auteurs Carotte L’augmentation de la masse moléculaire du soluté entraine une

augmentation du coefficient de diffusion de l’eau. Le traitement par DO avant séchage améliore efficacité du séchage par entrainement.

Uddin et al., 2004

Citrouille La DO entraine une modification de la structure cellulaire (45°Brix, 25°C, 20 :1, 9h).

Mayor et al., 2008

Concombre Le traitement par DO avant congélation améliore la fermeté du fruit et prolonge sa période de conservation par rapport à la DO seule.

Dermesonlouoglou et al., 2008

Goyave La DO sous vide à une température de 40 et 50°C offre le meilleur coefficient de diffusion d’eau.

Panades et al., 2008

Haricot La perte d'eau maximale a été obtenue quand des tranches de fruit de 10 millimètres ont été immergées dans une solution de concentration en sucrose de 60° Brix, maintenue à 60 °C pour 2 h, alors que l'imprégnation maximum était obtenue quand les tranches de 5 mm ont été immergées dans une solution de 50° Brix, maintenue à 60 °C durant 6 h.

Abud-Archila et al., 2008

Oignon Le traitement avec une solution de 40% de saccharose entraine une augmentation de la vitesse de destruction cellulaire au début du procédé. Le maltose et le tréhalose ont un effet protecteur sur la membrane plasmique.

Ferrando et Spiess, 2001

Pommes de terre

Les conditions optimales pour la DO de la pomme de terre sont : température : 22°C ; concentration en saccharose : 54% ; concentration en sel : 14% et le temps de traitement de 329 min.

Eren et Kaymak-Ertekin, 2007

Tomate et tomate cerise

Le traitement physique par perforation du fruit a permit une perte d’eau plus élevée que par traitement chimique.

Shi et al., 1998

(1b)

Dans la littérature, des travaux de modélisation ont recours au coefficient de diffusion

(Deff) issu de l’équation de Fick (Eq. 1) (Crank, 1975) :

xCSDeffm δδφ ..−= (Eq. 1)

Où Φm est le flux de matière traversant la surface S pendant l’unité de temps t (Kg/s) ;

S est la section normale à la direction du flux (m²); C est la concentration (Kg/m3); Deff est le

coefficient de diffusion (m²/s) ; x est la distance sur un axe parallèle à la direction du flux (m).

L’approche fine qui se base sur l’étude des paramètres déterminés à partir de la

calorimétrie différentielle à balayage et la résonance magnétique nucléaire (RMN). Ces

techniques permettent d’approcher les liaisons de l’eau dans le produit.

15


Calorimétrie différentielle à balayage

La calorimétrie différentielle à balayage (« differential scanning calorimetry », DSC) est

une méthode thermo-analytique permettant la mesure de propriétés physiques d’un

échantillon soumis à un programme de température. Le principe de la DSC est de comparer le

flux de chaleur nécessaire pour maintenir la température d’un échantillon (capsule contenant

l’échantillon) égale à celle d’une référence (capsule vide), chauffés ou refroidis à une vitesse

contrôlée. Ce flux de chaleur à une température donnée est directement proportionnel à la

chaleur spécifique (Cp) du matériau à cette température (Eq. 2).

T)f(t,dtdT

pCdtdQ

+= (Eq. 2)

Où Q est le flux de chaleur absorbé ou libéré par l’échantillon (mW g-1), Cp est la

chaleur spécifique de l’échantillon (J g-1), T est la température (°C), t le temps et f(t,T) est

une fonction dépendante du temps et de la température.

Des transformations thermodynamiques de 1er ordre, comme la cristallisation ou la fusion,

vont se traduire respectivement par un pic exothermique de cristallisation ou un pic

endothermique de fusion. La position de ces pics indique la température de transformation

(T°fusion ou T°cristallisation); leur aire est proportionnelle à l'enthalpie du processus. Une

transformation de 2ème ordre sera caractérisée par une marche, trahissant un saut de la chaleur

spécifique (Cp), dont le point d’inflexion correspond à la température de transition vitreuse

(Tg). Cette transition correspond durant le chauffage, au changement réversible de la phase

amorphe d’un polymère d’une forme vitreuse (relativement durs, à faible mobilité de l’eau) en

une forme visqueuse ou caoutchoutique (mou, à plus forte mobilité de l’eau). La

transformation est inverse au refroidissement. La détermination de la température de

transition vitreuse, la température de fusion et de cristallisation et de l’enthalpie de fusion et

de cristallisation au cours du temps permet de quantifier la teneur en eau libre et liée dans le

16


produit ainsi que d’étudier la cinétique de déshydratation du produit (Cornillon, 2000;

Ohkuma et al., 2008).

Résonance magnétique nucléaire (RMN)

La méthode par RMN repose sur l’analyse de l’interaction entre l’eau et la matrice

macromoléculaire. Les molécules d’eau peuvent interagir de manière labile ou permanente,

définissant ainsi des compartiments distincts dits d’eau faiblement liée (ou vacuolaire) et

d’eau d’hydratation (ou de surface). Au cours des processus d’échange, les protons de l’eau

interagissent avec ceux de la matrice soit par échange chimique direct, soit par une interaction

magnétique (dipôle-dipôle). Pendant la détection RMN (base résolution impulsionnelle) de

l’aimantation des noyaux, certains protons peuvent changer d’état (de compartiment) et cette

modification affecte le signal RMN enregistré. Le retour à l’équilibre de l’aimantation est

caractérisé par deux temps de relaxation des protons de l’eau. La relaxation T1 ou relaxation

spin-réseau caractérise le retour à l’équilibre des populations spins. La relaxation T2 ou spin-

spin représente l’amortissement de l’aimantation dans le plan transversal. La détection et

l’analyse de cette perturbation (transfert d’aimantation) sont à la base de la quantification des

proportions d’eau libre et d’eau d’hydratation, et de l’analyse de la cinétique des échanges

entre ces deux systèmes (Riggs et al., 2001). Actuellement, d’autres méthodes comme

l’imagerie par résonance magnétique (IRM) sont mises à profit afin d’étudier la structure du

produit et de quantifier les échanges internes (Derossi et al., 2008).

2.2. Principaux facteurs influençant les performances de la DO

Les cinétiques de transfert d’eau et de solutés dépendent de trois facteurs (Rastogi et

Raghavarao, 2004; Dermesonlouoglou et al., 2008 ; Garcia-Segovia et al., 2010):

Les propriétés intrinsèques des tissus traités: la structure poreuse, la taille, la forme, la

superficie du produit;

17


Les conditions opératoires de traitement: temps, température de traitement, pression,

agitation de la solution, composition de la solution;

Le mode de mise en contact des phases entre aliment solide et solution liquide.

2.2.1. Propriétés des tissus biologiques

Tout ce qui est préjudiciable à l’intégrité des tissus, telle qu’une maturation trop

avancée, la mise en œuvre de pré-traitements thermiques, chimiques ou enzymatiques peut

entraver la perte en eau tout en favorisant le gain en soluté. La grande variabilité observée

dans le comportement des végétaux au cours d’un traitement de DO est généralement

attribuée aux différentes propriétés tissulaires. Ces dernières incluent la compacité des tissus,

l’importance relative des espaces intra- et extra-cellulaires, la porosité et la teneur initiale en

matières sèches (Lenart, 1996). En effet, la porosité de l’aliment affecte sa texture et influence

sa fermeté. Les changements de porosité causés par le processus osmotique favorisent l’action

des forces d’entraînement non-diffusionelles, tels que des gradients de pression (Nieto et al.,

2004). La majorité des produits végétaux (Tableau 1a-1b) sont découpés en cube ou en sphère

avant le traitement de déshydratation osmotique, ce qui facilite le transfert de matière grâce à

un contact direct entre les cellules et la solution (Kowalska et al., 2008).

Lors d’une DO, quelques structures cellulaires peuvent devenir endommagées, tandis

que les autres restent pratiquement inchangées. Les traitements osmotiques impliquent ainsi

un stress cellulaire par suite de la réduction de l’eau disponible dans les cellules, ce qui

modifie leur physiologie (Rastogi et Raghavarao, 2004).

2.2.2. Concentration et composition de la solution osmotique

La différence de concentration en soluté entre le produit à traiter et la solution est le

moteur du transfert de masse en DO. La perte en eau est plus importante lorsque cet écart est

initialement élevé (Raoult-Wack, 1994). En effet, Vial et al. (1990) ont montré que toute

augmentation de la concentration en sucre se traduit principalement par une augmentation des

18


vitesses de transfert d’eau, les transferts de soluté n’étant que peu affectés. Pour la

déshydratation des fruits, des solutions de sucre concentrées de 50 à 70°Brix ont été

employées (Lerica et al., 1985; Corrêa et al., 2010). Néanmoins, il existe une concentration

seuil (entre 50 et 65°Brix) au-delà de laquelle l’imprégnation décroît (Raoult-Wack, 1994;

Lenart, 1996).

La composition des solutions (type, masse moléculaire du soluté) mises en œuvre en

DO est un facteur clé du procédé (Corrêa et al., 2010). Les solutions sont préparées à partir de

solutés cristallins solubles ou de solvants miscibles à l’eau, utilisés seuls ou en mélange. Les

constituants doivent être dépourvus de toute toxicité, présenter une solubilité suffisante et

être, idéalement, bon marchés (Rastogi et Raghavarao, 2004). Le choix du soluté est le

résultat d’un compromis entre les exigences technologiques et la qualité du produit final,

c'est-à-dire ses caractéristiques physicochimiques (pH, structure…), ses propriétés

nutritionnelles et organoleptiques (texture, couleur…), ses propriétés fonctionnelles

spécifiques (pouvoir aromatique, sucrant, colorant, état de surface collant ou brillant - dans

l'exemple du glucose) et son pouvoir dépresseur de l’activité en eau. Les solutions à base de

sucres (saccharose), sont les plus couramment utilisées dans la DO des fruits (Lenart, 1996).

Toutefois, il est essentiel de prendre en compte le coût de ces solutés, qui peut se révéler

prohibitif (Giraldo et al., 2003).

Utiliser différents solutés en mélange permet de tirer parti de l’effet respectif de

chacun (masse molaire, propriétés de diffusion...), mais aussi de développer des interactions

spécifiques (soluté/soluté et soluté/aliment) pour mieux maîtriser les niveaux de

déshydratation et d’imprégnation (Giraldo et al., 2003). En pratique, l’utilisation de sucres de

masse molaire élevée (hydrolysats d’amidon de faible indice d'équivalent dextrose ), en

mélange avec le saccharose, conduit à des niveaux de déshydratation plus élevés et des

niveaux d’imprégnation plus faibles que ceux obtenus avec une solution de saccharose. Au

19


contraire, l’utilisation de solutés de masse molaire plus faible que le saccharose, tels que les

sucres invertis (fructose, glucose), permet d’obtenir des niveaux d’imprégnation plus élevés

(Saurel et al., 1995 ; Torreggiani et Bertolo, 2001 ; Corrêa et al., 2010). Pour intégrer au

mieux ces deux impératifs, et optimiser la déshydratation tout en limitant l’imprégnation, il

faut utiliser une solution mixte mettant en œuvre deux solutés de masse molaire bien

distinctes (Saurel et al., 1995). L’intérêt de solutions ternaires, ou plus complexes, associant le

saccharose avec d’autres sucres de masse molaire différente, ou associant sucres et chlorure

de sodium, a été mis en évidence expérimentalement (Lenart, 1996). L’addition de NaCl à une

solution osmotique semble augmenter la force d’entraînement lors de la déshydratation. Ce

phénomène est attribué à la capacité du NaCl d'abaisser l’activité de l’eau (aw) (Kowalska et

al., 2008). LeMaguer et Sharma, (1997) ont montré que l’utilisation d’une solution osmotique

contenant 44% de saccharose et 7% de NaCl permet d’optimiser les conditions de

déshydratation osmotique des carottes.

2.2.3. Température de la solution osmotique

Le rôle de la température en DO a été étudié sur un large éventail de températures (5 -

85°C), le domaine de travail devant être adapté pour chaque famille de produit (Lerica et al.,

1985, Floury et al., 2008). Une température opératoire comprise entre 20 et 40°C est souvent

considérée comme optimale sur le plan qualitatif (Lerica et al., 1985). A ces températures, la

semi-perméabilité des membranes cellulaires de différents végétaux est à peine affectée.

L’extraction de l’eau est alors possible seulement par des processus osmotiques. Les transferts

d’eau sont favorisés par des températures élevées (Floury et al., 2008). Aussi, le sucrage et

confisage des fruits sont habituellement réalisés à 60°C. Cependant, une température trop

élevée n’est pas souhaitable car la température est l’un des facteurs responsables de la rupture

des tissus végétaux et des membranes. Par exemple, les membranes plasmatiques

commencent à subir des dommages irréversibles et une perte de sélectivité à 55°C (Thebud et

20


Santarius, 1982). Ceci provoque une modification de la structure et de la texture du matériau,

mais aussi le développement de réactions de brunissement enzymatique et de dégradation de

la couleur. Pour chaque fruit et légume existe en outre une température seuil, au-delà de

laquelle la qualité du produit est affectée et les transferts de soluté prennent le pas sur le

transfert d’eau (Floury et al., 2008).

2.2.4. Durée du traitement

La durée du traitement est un facteur important à considérer, quels que soient les

produits traités. Généralement, la perte d’eau, la réduction de masse et le gain en solides

augmentent avec le temps de traitement (Rastogi et Raghavarao, 2004 ; Kowalska et al.,

2008).

Marchal et al. (2005) ont rapporté un changement de sélectivité au cours de la

déshydratation, c'est-à-dire que le rapport de la perte en eau sur le gain en solide (WL/SG)

décroît au cours du temps. Ce phénomène déjà mentionné, est attribué à la mort des cellules

qui accompagne l’augmentation de la concentration en sucre dans le tissus (Mavroudis et al.,

2004). Ceci conduit à la perte de fonctionnalité de la membrane cellulaire et peut affecter la

qualité du produit. Lenart, (1996) a montré que la durée de déshydratation de morceaux de

pomme ne doit pas dépasser une durée de 15 min, à une température comprise entre 70 et

90°C.

2.2.5. Mode de mise en contact des phases, effet de l’agitation et du rapport

solide/solution

Dans le cas de tranches de fruits, les coefficients de transfert d’eau libre et de

saccharose de la solution de déshydratation augmentent, non seulement avec la concentration

en saccharose, mais aussi avec l’agitation, (Vial et al., 1990). Marouzé et al. (2001) ont

montré que le transfert de masse nécessite une agitation entre la solution et le produit, qui,

pouvant être discontinue, permet un gain d’énergie. En effet Mavroudis et al. (2004) ont

21


mesuré les effets de l’agitation sur le transfert de masse en terme de nombre de Reynolds, et

ont montré qu’une forte agitation augmente la perte en eau. Au cours du temps, la perte en

eau s’avère moindre lorsque la DO est réalisée en écoulement laminaire plutôt que turbulent.

Le gain en solide n’est que peu affecté par le niveau d’agitation, ce qui s'explique par

l’existence d’une couche limite diluée autour de l’aliment (Giroux et Marouzé, 1994).

Les transferts de matière dépendent fortement de la manière dont sont mises en contact

les phases solide (fragile et « légère »; l'aliment) et liquide (« lourde » et visqueuse, la

solution hypertonique). Une viscosité élevée du liquide augmente la résistance externe (à

l’interface solide/liquide) aux transferts de matière et nécessite la mise en œuvre d’un système

de brassage adapté, compatible avec la fragilité des produits.

Des études ont montré qu’un rapport pondéral, solution de déshydratation/tranches de

fruit, trop grand (facteur de dilution trop marqué), rend difficile la détermination des

différentes substances diffusées et le suivi efficace du phénomène osmotique. Par contre, un

rapport petit ralentit le taux de diffusion (Adamrounou et al., 1994).

Pour une meilleure efficacité de la DO, les systèmes de mise en contact des phases

doivent permettre de réduire la dispersion des temps de séjour, forcer l’immersion des

produits, réduire les effets de couche limite et préserver la forme et la fragilité des produits.

2.2.6. Mise en œuvre de la déshydratation osmotique

Chaque fruit ou légume présentant ses propres conditions optimales de déshydratation

osmotique, il est difficile de mettre en avant une méthodologie universelle.

Typiquement, tant pour les fruits que pour les légumes, le régime turbulent est préféré,

avec des temps de séjour compris entre 5 et 480 min. L'optimum de température se trouve

habituellement entre 25°C et 80°C, et le rapport produit/solution évolue généralement entre

1/2 et 1/20.

22


Le soluté le plus fréquemment rencontré est le saccharose, employé à des

concentrations comprises entre 38°Brix et 65°Brix pour les fruits, entre 35°Brix et 60°Brix

pour les légumes. L'usage du sel comme soluté est souvent réservé à la déshydratation des

légumes.

Il apparait ainsi nécessaire, industriellement, d'optimiser les conditions de traitement

spécifiquement pour chaque produit. Les tableaux 2a et 2b fournissent à ce sujet quelques

exemples représentatifs.

Tableau 2: Les conditions optimales de déshydration osmotique des fruits (2a) et légumes (2b)

Fruits

Paramètres Ananas Châtaigne Grenade Kiwi Melon cantaloup

Mangue

Concentration en soluté :

Saccharose

62°Brix 60°Brix 50°Brix 60°Brix 38°Brix 65°Brix

Température 30°C 25°C 50°C 40°C 41°C 35°C

Temps 360 min 480 min 20 min 150 min 132 min 360 min

Rapport (produit/solution) 1:6 - 1:4 - 1:20 -

Références Singh et al., 2008a

Chenlo et al., 2007

Bchir et al., 2009

Cao et al., 2006

Corzo et Gomaz,

2003

Madamba et Lopez,

2002

(2a)

Légumes

Paramètres Carotte Chou-fleur Pois patate

Poivre vert

Radis Tomate

Saccharose 52°Brix - 60°Brix - - 35°Brix

Sel - 12°Brix - 5.5°Brix 25°Brix 5°Brix

Concentration en solutés

Sorbitol - - - 6% - -

Température 49°C 80°C 60°C 30°C 36°C 60°C

Temps 150 min 5 min 120 min 240 min 95 min 120min

Rapport (produit/solution) 1:10 2:4 1:8 1:3 1:15 1:4

Références Singh et al.,

2008b

Vijayanand et al., 1995

Abud-Archila et al., 2008

Ozdemir et al., 2008

Petchi et Manivasagan,

2009

Souza et al., 2007

(2b)

23


2.3. Application de la déshydratation osmotique

La DO est un procédé relativement lent. Il est donc important de trouver des méthodes

qui augmentent le transfert de masse sans affecter la qualité du produit. Ainsi, un traitement

permettant d’augmenter la perméabilité des membranes cellulaires et de faciliter la libération

de l’eau pendant la DO est obligatoire. Parmi les pré-traitements utilisés, on peut citer les

méthodes thermiques de blanchiment et de congélation (Kowalska et al., 2008). D'autres

procédés consistent à remplacer un traitement unique (ici, la DO) par la combinaison de

plusieurs techniques de conservation modérées, respectueuses du produit, et pouvant accélérer

les transferts. Ces techniques peuvent être l'application du vide, les hautes pressions

hydrostatiques, l’ultrason, l’irradiation, la centrifugation, le champ électrique pulsé etc.

2.3.1. Pré-traitement thermique

2.3.1.1. Blanchiment

Le blanchiment est un traitement thermique, réalisé par immersion du produit dans un

bain d’eau chaude, par passage dans une atmosphère de vapeur ou par chauffage ohmique. Sa

durée est de quelques minutes, dans une gamme de 85°C à 100°C. Il permet de détruire les

enzymes susceptibles d’altérer les légumes ou les fruits avant leur traitement ultérieur (dans

notre cas c’est la déshydratation). Ce procédé prévient ainsi un certain nombre d’altérations

organoleptiques, telles que des modifications de flaveurs et de couleurs (dégradation de la

chlorophylle, brunissement des pommes, etc.). Il limite également certaines pertes

nutritionnelles comme la destruction des vitamines, et permet l’élimination de l’air et des gaz

occlus dans les tissus végétaux facilitant la réhydratation (Dermesonlouoglou et al., 2008).

Vis-à-vis de la DO, le blanchiment facilite le transfert des matières dissoutes, comme

il l’a été rapporté pour les tranches de pomme et de tomate précédemment blanchies à la

vapeur (Dermesonlouoglou et al., 2008 ; Kowalska et al., 2008). En revanche, le blanchiment

24


est opéré à des températures trop élevées pour les constituants cellulaires, causant un risque

accru d'altération qualitative et de pertes par dissolution.

2.3.1.2. Congélation

L’eau est le principal composant de la majorité des aliments congelés du commerce.

Une part notable de cette eau est liée à divers degrés: dans des complexes colloïdaux

macromoléculaires, dans des structures gélifiées ou fibreuses à l’intérieur des cellules, et dans

les hydrates. Lors de la congélation, la nucléation de la glace et la croissance des cristaux

apportent de nombreuses modifications au produit (Talens et al., 2003). Les composantes

cellulaires solubles peuvent atteindre la saturation et précipiter, détruisant ainsi la turgescence

des tissus; des modifications de pH peuvent affecter les complexes colloïdaux ; des

changements très marqués de pression osmotique peuvent rompre les membranes semi-

perméables, ce qui facilite le transfert de masse au cours de la déshydratation osmotique

(Floury et al., 2008 ; Kowalska et al., 2008 ; Dermesonlouoglou et al., 2008 ; Bchir et al., In

press). La vitesse de congélation et la température finale de conservation sont des points

critiques pour le maintien des propriétés sensorielles, fonctionnelles ou biologiques après la

congélation. Une congélation très lente peut conduire à un exsudat excessif à la

décongélation, alors qu’une congélation très rapide permet de préserver la texture de certains

produits (Talens et al., 2003).

2.3.2. Méthodes combinées à la DO

2.3.2.1. Imprégnation sous vide

Le procédé de DO pour les produits végétaux est généralement mis en œuvre à

pression atmosphérique (Raoult-Wack, 1994). Toutefois, l’application d’une dépression

stationnaire augmente la vitesse de déshydratation (Corrêa et al., 2010). La présence de gaz

occlus dans les espaces intercellulaires de la structure poreuse du produit traité apparaît

comme la cause principale de la modification des cinétiques de transferts de matières (Fito,

25


1994; Wu et al., 2009). Pendant la DO sous vide, le gaz est expulsé du tissu tandis que

l’écoulement capillaire augmente. L’augmentation de la vitesse du transfert d’eau est

principalement attribuée à l’action combinée du vide et de l’écoulement capillaire, qui dépend

lui-même du volume de gaz occlus dans le tissu. Par conséquent, l’accélération des transferts

de matière en DO sous vide est d’autant plus marquée que la porosité du produit est plus

importante (Corrêa et al., 2010). Cependant, sous vide pulsé, c’est l’imprégnation qui est

favorisée. En effet, lorsque le produit revient à pression atmosphérique, la solution concentrée

pénètre massivement dans les pores du produit alimentaire, ce qui a pour conséquence

ultérieur d’augmenter la surface de contact entre le produit et la solution et d’accélérer ainsi

les transferts de matières (Fito, 1994). Corrêa et al. (2010) ont obtenu par cette technique une

perte d’eau plus élevés que dans le cas d'une DO ordinaire, en utilisant le vide seulement

pendant 15 min. La technologie du sous vide partiel cyclique trouve tout son potentiel lorsque

l’on souhaite formuler le produit à l’aide d’additifs. Ainsi la texture des fruits peut-elle être

améliorée par immersion sous vide dans une solution contenant de la pectine-méthylestérase

ou différentes solutions salines, par exemple à base de chlorure de calcium ou de nitrate de

calcium (Javeri et al., 1991).

La DO sous vide permettrait d’obtenir des produits de qualité organoleptique et

physicochimique supérieure à celle des produits traités par DO à pression atmosphérique. De

plus, cette alternative réduit les coûts énergétiques globaux (Fito, 1994).

2.3.2.2. Haute pression hydrostatique

Les traitements à haute pression permettent une stabilisation microbiologique

significative des aliments, tout en préservant les qualités organoleptiques et nutritionnelles de

manière plus importante que les traitements thermiques. Certaines études (Taiwo et al., 2002;

Rastogi et Raghavarao, 2004) ont mis en évidence le fait que le pré-traitement à haute

pression crée un compactage de la structure cellulaire accompagné d'une libération de

26


composants cellulaires. Ce phénomène a pour conséquence la formation d’un gel par liaison

d'ions divalents avec la pectine estérifiée qui limite le coefficient de diffusion des solides. A

haute pression (jusqu’à 600 Mpa) les membranes cellulaires sont réversiblement

perméabilisées. Ce phénomène est imputable aux transitions de phase des bicouches

lipidiques de la membrane cellulaire. Cet effet est recherché pour l’élaboration rapide des

fruits sucrés tout en préservant l’aspect, les qualités organoleptiques et nutritionnelles du

produit frais.

Cependant, la technologie haute pression reste coûteuse, du fait des contraintes de

fabrication des enceintes et par leur capacité limitée.

2.3.2.3. Ultrasons

L’application d’ultrasons (onde à fréquence supérieur à 20 000 Hz) a déjà fait ses

preuves dans l’augmentation du taux de transfert de masse pour la déshydratation osmotique

de tissus poreux comme des cubes de pomme. Simal et al. (2001) ont obtenu par cette

technique une perte d’eau de 27% et un gain de solide de 23% plus élevés que dans le cas

d'une DO ordinaire. L’application d’ultrasons produit un phénomène de cavitation, qui

consiste en la formation de bulles de gaz dans le liquide, qui engendrent, en éclatant, des

fluctuations de pression (Fernandes et al., 2009). Cet effet facilite la diffusion pendant le

processus osmotique et accélère le dégazage du produit, tout en préservant la saveur, la

couleur, et les composants nutritifs les plus sensibles à la chaleur (Simal et al., 2001). Cette

technique permet aussi l’inactivation des enzymes et des bactéries en cassant leurs membrane

cellulaires (Jambrak et al., 2010).

2.3.2.4. Irradiation

La structure intérieure du tissu des produits agricoles peut être lysée par γ-irradiation.

Il en résulte une plus grande perméabilité des cellules, d’où un transfert de masse amélioré

pendant un séchage à l’air (Wang et Sastry, 2000). Rastogi et Raghavarao, 2004, ont rapporté

27


que la combinaison de la γ-irradiation avec la DO peut résoudre le problème de la diminution

des transferts de masse pendant un séchage convectif.

2.3.2.5. Chlorure de sodium

Comme il l'a déjà été mentionné, le type d’agent osmotique utilisé, et par conséquent

sa masse moléculaire, affecte fortement la cinétique d’extraction d’eau et de gain en soluté.

D’après Simal et al. (2001), le chlorure de sodium est un excellent agent osmotique, en raison

de sa faible masse moléculaire qui se reflète par sa grande mobilité pendant le transfert de

masse.

Dans le cas des solutions sucrées-salées (Tableau 3), des effets fortement antagonistes

sur le gain en solutés ont été identifiés. L’imprégnation en sel est en particulier limitée par la

présence de sucre. Cet effet « barrière » du sucre sur la pénétration du sel a été mis en

évidence sur des produits végétaux (Lenart, 1996). Il serait dû à la formation, dans l’aliment,

d’une couche périphérique fortement concentrée en sucre. En même temps, la présence de

sucre diminuerait fortement le coefficient de diffusion du NaCl (Lenart, 1996). La présence de

sel empêche par ailleurs dans certains cas la formation d’une croûte superficielle (croûtage du

produit) causée par le sucre.

Tableau 3 : Différents travaux utilisant une solution ternaire pour le traitement des fruits par déshydratation osmotique.

Solutés Concentration

totale (g/100g solution)

Saccharose/NaCl (p/p)

Produit/Soluté (p/p)

Références

Saccharose 50 35/15 1/20 Hawkes et Filk, 1978

50 40/10 1/20 Hawkes et Filk, 1978

+ 50 47.5/2.5 1/20 Biswal et Le Maguer, 1989

50 45/5 1/20 Biswal et Le Maguer, 1989

NaCl 60 59/1 1/5 Lerica et al., 1985

61 59/2 1/5 Lerica et al., 1985

28


2.3.2.6. Centrifugation

Cette technique peut être utilisée pour augmenter la perte en eau tout en retardant le

gain en solide. Azuara et al., (1998) ont appliqué une force centrifuge de 64 g pendant la

déshydratation osmotique à 30°C de disques de pomme et de pomme terre. Ils ont observé que

ces conditions augmentent le transfert de masse (perte en eau) de 15% tout en retardant

considérablement (80%) le gain en solide.

2.3.2.7. Traitement par champ électrique pulsé

Le traitement par champ électrique pulsé, pour une intensité de champs électrique

comprise entre 0.5 et 15 kV/cm, entraîne une augmentation de la perméabilité des membranes

végétales. Son action instantanée, sa courte durée d’application (moins d’une seconde), et la

possibilité de traiter des aliments solides à basse température, rendent le champ électrique

pulsé plus promoteur qu’un traitement thermique dans une perspective de diffusion ou

d’extraction d’eau (séchage) ou de métabolites (Ade-Omowaye et al., 2003). Un autre

avantage du champ électrique pulsé réside dans le fait qu’il n’augmente pratiquement pas la

température du produit. Ade-Omowaye et al. (2003) ont montré que la cinétique de DO à

20°C du paprika traité à 2.5 kV/cm est comparable à celle d’une DO réalisée à 55°C sur le

même produit sans application de champ électrique.

2.3.3. Stabilisation des produits déshydratés osmotiquement par des traitements

physiques

Les produits issus du procédé de déshydratation osmotique sont classés parmi les

produits à humidité intermédiaire (PAI), à taux d’humidité élevé (Garcia-Martinez et al.,

2002). Aussi le produit n’est pas encore microbiologiquement stabilisé, et l’activité de l’eau

peut y être élevée. Plusieurs traitements ont été proposés pour parfaire le processus: séchage,

congélation, pasteurisation, friture etc. Les plus communs sont le séchage par air (Ade-

Omowaye et al., 2003) et la congélation (Agnelli et al., 2005).

29


2.3.3.1. Séchage

Le séchage provoque un abaissement de l’activité d’eau du produit, c'est-à-dire que

l’eau reste peu disponible pour les micro-organismes et pour les réactions chimiques. On

considère généralement qu’un produit est stable lorsque son activité de l’eau est inférieure ou

égale à 0,65 (Thebud et Santarius, 1982).

L’utilisation du séchage dans les industries agroalimentaires a de multiples objectifs :

accroître la durée de conservation des produits; stabiliser les produits agricoles (maïs, luzerne,

riz, lait, ...) pour amortir le caractère saisonnier de certaines activités; et transformer les

produits par des réactions biochimiques ou biologiques (produits de salaison, touraillage de

malt, etc.). Cependant, cette technique est couteuse en énergie: le séchage des produits

végétaux nécessite environ 5000 kJ/kg d’eau évaporée (Mujumdar, 2006).

La combinaison de la déshydratation osmotique avec le séchage permet d’améliorer la

qualité des produits (Fernandes et al., 2006) et de réduire le coût énergétique global de

l’élimination de l’eau. En effet, la pré-déshydratation diminue le temps de séchage et le

besoin énergétique du séchage complémentaire (Fernandes et al., 2006). En effet, la DO

n'exige qu'entre 100 et 2400 kJ/kg d’eau enlevée, selon les applications (Mujumdar, 2006).

Après une période de mise en régime, la cinétique de séchage par l'air est caractérisée par une

période à vitesse constante, correspondant à l’évaporation de l’eau de surface qui est

constamment renouvelée par transport interne et qui se traduit par une variation linéaire de la

teneur en eau en fonction du temps. Cette première période est suivie d’une ou plusieurs

étapes à vitesses décroissantes, où les forces capillaires n’acheminent plus suffisamment

d’eau en surface pour compenser l’évaporation (Ade-Omowaye et al., 2003). Pour les produits

alimentaires et biologiques, le séchage est limité par la résistance des parois cellulaires, par la

migration des solutés qui obstruent les pores et par le croûtage de la surface (Wang et Sastry,

2000; Ade-Omowaye et al., 2003).

30


2.3.3.2. Congélation

La congélation des aliments est un excellent moyen de maintenir pendant longtemps,

presque inchangées, leurs valeurs nutritionnelles. Cette préservation de la qualité s’explique

tant par l’abaissement de la température qui ralentit les réactions biochimiques et inhibe les

activités microbiennes que par la réduction de l’activité de l’eau du substrat (Floury et al.,

2008).

La possibilité de prétraiter les produits par une déshydratation partielle (voire par

imprégnation) avant congélation semble prometteuse (Talens et al., 2003 ; Wu et al., 2009).

Cette technique, dite de déshydro-congélation, permet la réduction de la quantité d’eau dans

le produit afin de diminuer la quantité de cristaux formés, le temps de congélation et de

décongélation. Il en résulte une meilleure conservation des propriétés du fruit. La

déshydratation osmotique constitue de ce point de vue un pré-traitement efficace (Talens et

al., 2003 ; Dermesonlouoglou et al., 2008).

Au point de vue énergétique, on notera que, sans déshydratation préalable, la

congélation des fruits nécessite entre 250 et 340 kJ/kg d’eau congelée, et entre 150 et 320

kJ/kg d’eau congelée pour les légumes (Mujumdar, 2006).

2.4. Équipements pour la DO

Diverses configuration de mise en contact des phases ont été proposées et étudiés

(Figure 1). En pratique, l’opération peut être réalisée en contacteur continu ou discontinu,

avec ou sans agitation. Certains auteurs proposent simplement une immersion forcée des

morceaux dans le liquide (Marouzé et al., 2001). Cette solution est d’ailleurs utilisée dans la

plupart des entreprises de semi-confisage de fruits. D’autres systèmes plus élaborés ont été

proposés en vue d’une mise en œuvre du procédé en continu (contact permanent entre le

solide et le liquide) (Raoult-Wack, 1994; Giroux et Marouzé, 1994).

31


Immersion sans agitation

Immersion avec agitation

Immersion avec agitation intermittente

* Mélange hydraulique * Mélange hydraulique et mécanique * Déplacement aliment/solution * Arrosage monocouche * Arrosage multicouche

Film de solution entourant l’aliment

* Immersion avec agitation continue mécanique * Lit infiltré fixe en lot * Lit infiltré en lot * Lit infiltré mobile avec lent co-courant.

Mise en contact des phases

Figure 1. Les différents systèmes de mise en contact des phases (solution osmotique et l’aliment)

2.5. Qualité des produits végétaux traités par DO

La déshydratation osmotique permet le maintien des qualités nutritionnelles, voire

l’amélioration des qualités organoleptiques de produits souvent fragiles, ainsi qu’une

meilleure résistance à des traitements ultérieurs (séchage, stockage…) (Albagnac et al., 2002).

En effet, en tenant compte de la possibilité de transferts de masse dans les deux sens (gain de

solutés et perte de solutés), la déshydratation osmotique permet la formulation de nouveaux

produits (Albagnac et al., 2002). Selon Raoult-Wack, (1994), la déshydratation osmotique

permet de modifier les propriétés fonctionnelles des produits en les imprégnant des solutés

souhaités. La DO augmente le rapport sucre/acide, améliore la texture et préserve la couleur

pendant la déshydratation et le stockage (Raoult-Wack, 1994). Toutefois, il convient de noter

que les apports en soluté, et notamment en sucres, ne vont pas toujours dans le sens d'une

amélioration des propriétés nutritionnelles.

En évitant le contact avec l’oxygène de l’air, la DO limite les réactions d’oxydation,

mais aussi les pertes de composés volatils par entraînement. Elle est efficace même à

32


température modérée (souvent inférieure à 50°C), ménageant ainsi les composés

thermosensibles tels que les arômes, pigments et vitamines (Vial et al., 1990).

L’effet de la DO sur les différents attributs de la qualité est détaillé ci-après.

2.5.1. Saveur

Au cours de la DO, l’introduction de soluté modifie inévitablement le rapport

acides/sucre, ce qui adoucit la saveur du produit final. Cependant, en séchage ou en DO,

l'élimination d'eau ne doit pas se faire au détriment de la saveur (i.e. des arômes). En règle

générale, tout facteur tendant à augmenter la viscosité de la solution osmotique diminue la

diffusivité relative des arômes. Ainsi, il vaut mieux en DO diminuer la température et

augmenter la concentration du produit en matières sèches pour conserver les arômes

(Torreggiani et Bertolo, 2001).

2.5.2. Couleur

La couleur est un attribut très important des aliments, car elle influence l’acceptabilité

par le consommateur (40% du critère d’acceptabilité) (Falade et al., 2007). Des couleurs

anormales, suggérant la détérioration de la qualité ou du caractère comestible, sont des causes

de rejet par le consommateur. Beaucoup de réactions peuvent affecter la couleur pendant le

traitement thermique des fruits et de leurs dérivés. Les plus communes sont la dégradation des

pigments (chlorophylle, β-carotène,…) et les caroténoïdes et la chlorophylle, et les réactions

de brunissement telles que la réaction de Maillard des hexoses, et l’oxydation de l’acide

ascorbique. Au cours de la DO, les solutés introduits réduisent les modifications de la couleur

du produit, notamment en limitant la dégradation des pigments chlorophylliens et

caroténoïdes. L’activité enzymatique de polyphenol-oxydase responsable du brunissement

enzymatique est alors inhibée. La sensibilité des produits au brunissement non enzymatique

est également limitée (Torreggiani et Bertolo, 2001).

33


2.5.3. Texture

Le départ d’eau, ainsi que son remplacement par d’autres molécules, implique des

contraintes mécaniques qui modifient la conformation du matériau. Ainsi, le produit se

rétracte sous l’action des fortes densités de flux (Castello et al., 2009; Garcia-Segovia et al.,

2010). D’autre part, au cours du processus de déshydratation, les polysaccharides (pectine,

hémicelluloses, cellulose) qui constituent la membrane cellulaire sont partiellement

solubilisés, modifiant ainsi la fermeté du produit (Nunes et al., 2008). Ces modifications sont

quantifiables par l’analyse de la texture du produit; ces mesures sont basées sur la résistance à

la pénétration par une sonde (Torreggiani et Bertolo, 2001).

2.5.4. Réhydratation

Les produits déshydratés sont souvent conçus pour être réhydratés ultérieurement. La

capacité et la vitesse de réhydratation, qui est souvent longue (plusieurs heures), sont décrites

comme les principaux critères de la qualité des produits finis. La réhydratation des fruits et

légumes a été bien étudiée (Taiwo et al., 2002; Rastogi et Raghavarao, 2004). Une étude de la

cinétique de réhydratation peut être effectuée pour mesurer l’ampleur nette des dommages

subis par le produit pendant les étapes de transformation antérieures (Rastogi et Raghavarao,

2004).

La réhydratation est influencée par plusieurs facteurs, groupés en tant que facteurs

intrinsèques (composition chimique du produit, traitement de pré-séchage, formulation de

produit, techniques et conditions de séchage) et extrinsèques (composition du milieu

d’immersion, température, conditions hydrodynamiques). Certains de ces facteurs induisent

des changements de structure et de composition du tissu végétal, ce qui influence les

propriétés de reconstitution lors de la réhydratation (Taiwo et al., 2002). Par exemple, plus la

concentration en sucre est grande ou plus la période de la DO est longue avant séchage,

meilleure est la réhydratation, le sucre empêchant probablement le rétrécissement du tissus

34


végétal lors du séchage à l’air (Neumann, 1972). Dans l’étude sur le céleri sec, Neumann

(1972) a rapporté que la réhydratation à des températures élevées diminue le temps exigé pour

atteindre la capacité maximum de sorption d’eau et le contenu d’humidité finale.

3. VALORISATION DES FRUITS CONSERVÉS EN SOLUTION SUCRÉES

Les fruits ont été depuis longtemps conservés par le sucre. La fabrication de produits

déshydratés osmotiquement remonte à la haute antiquité. Aujourd’hui, la conservation des

fruits par les sucres est devenue une industrie à part entière, et est appliqué à une vaste gamme

de produits, comme les fruits confits (Espiard, 2002), les compotes et purées (Espiard, 2002),

la confiture (Espiard, 2002 ; Albagnac et al., 2002), les marmelades et pâtes de fruits (Espiard,

2002 ; Albagnac et al., 2002). On trouve sur le marché un nombre de produits apparentés

ayant tous subis des transformations visant non seulement à obtenir des fruits stables à

température ambiante (grâce à une réduction de leur activité d’eau), mais à aussi leur conférer

une texture et une saveur spécifique (Albagnac et al., 2002).

4. CONCLUSION

A travers cette étude bibliographique, nous espérons apporter des bases scientifiques et

techniques pour l’étude de la conservation des produits végétaux par déshydratation

osmotique. La maîtrise de cette technique revêt une importance primordiale pour les

industries alimentaires car elle permet un gain d’énergie et, moyennant la prise en compte de

l’effet des apports en solutés par imprégnation, une préservation de la qualité nutritionnelle

des produits traités. Ainsi, l’utilisation de la DO permet un meilleur contrôle et une maîtrise

de la qualité des produits finis. Il en découle un élargissement de la gamme des fruits traités,

une diversification des caractéristiques des produits obtenus, et le développement de produits

nouveaux. En effet, cette technique permet de définir des voies de valorisation de plusieurs

fruits et légumes. Prenons l'exemple des graines de grenade, fruit jusqu’ici consommé presque

exclusivement frais, pendant la période de récolte. De récents travaux (Bchir et al., 2009;

35


Bchir et al., In press) ont évalué l’aptitude des graines de grenade en DO, en fonction de la

température et du choix des solutés. Ces travaux ouvrent la voie à de nouveaux modes de

consommation, comme les graines confites, qui pourraient être introduites dans d’autres

produits transformés. De la sorte, il devient possible d’exploiter bien mieux les excellentes

propriétés nutritionnelles, biologiques et thérapeutiques de ce fruit.

Soulignons enfin que l’introduction de la DO dans le processus de transformation des

fruits permet en général une réduction du nombre d’étapes et/ou de durée totale du traitement,

et de bénéficier de l’effet protecteur des solutés incorporés. La littérature souligne aussi que,

d’une manière générale, la déshydratation osmotique doit être complétée par un traitement

thermique, chimique, mécanique ou physique afin de parfaire la stabilisation du produit fini.

36


5. REFERENCES BIBLIOGRAPHIQUES

Abud-Archila M., Vazquez-Mandujano D., Ruiz-Cabrera M., Grajales-Lagunes A., Moscosa-

Santillan M., Ventura-Canseco L., Gutierrez-Miceli F. & Dendooven L. 2008.

Optimization of osmotic dehydration of yam bean (Pachyrhizus erosus) using an

orthogonal experimental design. Journal of Food Engineering, 84, 413-419.

Adamrounou L.T., Conway J. & Castaigne F. 1994. Influence de la déshydratation partielle

par osmose sur la composition de tranches de pomme. Sciences des Aliments, 14, 75-

85.

Ade-Omowaye B.I.O., Rastogi N.K., Angersbach A. & Knorr D. 2003. Combined effects of

pulsed electric fierld pre-treatment and partial osmotic dehydration on air drying

behaviour of red bell pepper. Journal of Food Engineering, 60, 89-98.

Agnelli M.E., Marani C.M. & Mascheroni R.H. 2005. Modelling of heat and mass transfer

during (osmo) dehydrofreezing of fruits. Journal of Food Engineering, 69, 415-424.

Albagnac P.G., Varoquaux J. & Montigaud coord C.I. 2002. Technologies de transformation

des fruits /Paris.-Londres.-New York : Ed. Tec & Doc,cop.-XXII-498 pages.

Azuara E., Beristain C.I. & Gutiérrez G.F. 1998. A method for continuous kinetic evaluation

of osmotic dehydration. Lebensm-Wiss.U. –Technol, 31, 317-321.

Bchir b., Besbes S., Attia H., & Blecker C. 2009. Osmotic dehydration of pomegranate seeds:

Mass transfer kinetics and DSC characterisation. International Journal of Food

Science and Technology, 44, 2208-2217.

Bchir b., Besbes S., Attia H., & Blecker C. Osmotic dehydration of pomegranate seeds

(Punica granatum L.): Effect of freezing pre-treatment. Journal of Food Process

Engineering. In press (DOI: 10.1111/j.1745-4530.2010.00591.x).

Biswal R.N. & Le Maguer M. 1989. Mass transfer in plant material in contact with aqueous

solution of ethanol and sodium chloride: equilibrium data. Journal of Food Process

Engineering, 11,159-176.

Cao H., Zhang M., Mujumdar A., Wh D. & Sun J. 2006. Optimization of osmotic dehydration

of kiwifruit. Drying Technology, 24, 89-94.

37


Castello M., Igual M., Fito P. & Chiralt A. 2009. Influence of osmotic dehydration on texture,

respiration and microbial stability of apple slices (Var. Granny Smith). Journal of

Food Engineering, 91, 1-9.

Chenlo F., Moreira R., Fernandez-Herrero C. & Vazquez G. 2007. Osmotic dehydration of

chestnut with sucrose: Mass transfer processes and global kinetics modelling. Journal

of Food Engineering, 78, 765-774.

Cornillon, P. 2000. Characterization of osmotic dehydrated Apple by NMR and DSC.

Lebensmittel-Wissenschaft und-Technologie-Food Science and Technology, 33, 261-

267.

Corrêa J., Pereira L., Vieira G. & Hubinger M. 2010. Mass transfer kinetics of pulsed vacuum

osmotic dehydration of guavas. Journal of Food Process Engineering, 96, 498-504.

Corzo O. & Gomez E. 2003. Optimization of osmotic dehydration of cantaloupe using desired

function methodology. Journal of Food Engineering, 64, 213-219.

Crank, J. 1975. The mathematics of diffusion (2nd ed). Oxford: Clarendon Press.

Dermesonlouoglou E.K., Pourgouri S. & Taoukis P.S. 2008. Kinetic study of the effect of the

osmotic dehydration pre-treatment to the shelf life of frozen cucumber. Innovative

Food Science and Emerging Technologies, 9, 542-549.

Derossi A., De Pilli T., Severini C. & McCarthy M. J. 2008. Mass transfer during osmotic

dehydration of apples. Journal of Food Engineering, 86, 519-528.

Eren I. & Kaymak-Ertekin F. 2007. Optimization of osmotic dehydration of potato using

response surface methodology. Journal of Food Engineering, 79, 344-352.

Espiard E. 2002. Introduction à la transformation industrielle des fruits. TEC&DOC-

Lavoisier. pp. 181 - 182.

Falade K., Igbeka J. & Ayanwuyi F. 2007. Kinetics of mass transfer and colour changes

during osmotic dehydration of watermelon. Journal of food engineering, 80, 979-985.

Fernandes F., Gallao M. I. & Rodrigues S. 2009. Effect of osmosis and ultrasound on

pineapple cell tissues structure during dehydration. Journal of Food Engineering, 90,

186-190.

38


Fernandes F., Rodrigues S., Gaspareto O. C. P. & Oliveira E.L. 2006. Optimization of

osmotic dehydration of papaya followed by air-drying. Food Research International,

39, 492-498.

Ferrando M. & Spiess W. 2001. Cellular of plant tissue during the osmotic treatment with

sucrose, maltose and trehalose solutions. Journal of Food Engineering, 49, 115-127.

Ferrari C. & Hubinger M. 2008. Evaluation of the mechanical properties and diffusion

coefficients of osmodehydrated melon cubes. International Journal of Food Science

and Technology, 43, 2065-2074.

Fito, P. 1994. Modelling of vacuum osmotic dehydration of food. Journal of Food

Engineering, 22, 313-328.

Floury J., Le bail A. & Pham, Q.T. 2008. A three-dimensional numerical simulation of the

osmotic dehydration of mango and effect of freezing on the mass transfer rates.

Journal of Food Engineering, 85, 1-11.

Garcia-Martinez E., Martinezmonzo J., Camacho M.M. & Martineznavarrete, N. 2002.

Characterisation of reused osmotic solution as ingredient in new product formulation.

Food Research International, 35, 307-313.

Garcia-Segovia P., Mognetti C., André-Bello A. & Martinez-Monzo J. 2010. Osmotic

dehydration of Aloe vera (Aloea barbadensis Miller). Journal of Food Engineering,

97, 154-160.

Giraldo G., Talens P., Fito P. & Chiralt A. 2003. Influence of sucrose solution concentration

on kinetics and yield during osmotic of mango. Journal of Food Engineering, 58, 33-

43.

Giroux, F. & Marouzé, C. 1994. Etude de dispositifs permettant l’agitation des produits dans

les procédés de déshydratation imprégnation par immersion. In : GFGP. Agitation et

mélange en biotechnologies alimentaire et industrielle. Editions TEC & Doc Lavoisier,

Paris, pp. 29-34.

Hawkes J. & Flink J.M. (1978). Osmotic concentration of fruits slices prior to freeze

dehydration. J Food Process Preser, 2, 265-284.

Heng K. Guilbert S. & Cuq J.L. 1990. Osmotic dehydration of papaya: influence of process

ariables on the product quality. Sci Alim, 10, 831-848.

39


Islam M.N. & Flink J.M. 1982. Dehydration of potato. II Osmotic concentration and its

effects on air drying behaviour. Journal of Food technology, 17, 387-403.

Jambrak A., Herceg Z., Subaric D., Babic J., Brnic M., Brncic S., Bosiljkov T., Cvek D.,

Tripalo B. & Gelo J. 2010. Ultrasound effect on physical properties of corn starch,

Carbohydrate Polymers, 79, 91-100.

Javeri H., Toledo R. & Wicker L. 1991. Vacuum infusion of citrus pectinmethylesterase and

calcium effects on firmness of peaches. Journal of Food Science, 56, 739-742.

Jena S. & Das H. 2004. Modelling for moisture variation during osmo-concentration in apple

and pineapple. Journal of Food Engineering, 66, 425-432.

Jiokap Nono, Y., Nuadjea G.B., Raoult Wack A.L. & Giroux, F. 2001. Comportment de

certains fruits tropicaux traités par déshydratation imprégnation par immersion dans

une solution de saccharose. Fruits, 56, 75-83.

Khoyi M. & Hesari J. 2007. Osmotic dehydration kinetics of apricot using sucrose solution.


Kowalska H., Lenart A. & Leszczyk D. 2008. The effect of blanching and freezing on

osmotic dehydration of pumpkin. Journal of food engineering, 86, 30-38.

Krokida M.K., Karathanos V.T. & Maroulis Z.B. 2000. Effect of osmotic dehydration on

colour and sorption characteristics of apple and banana. Drying Technology, 18, 937-

950.

LeMaguer H. & Sharma S. 1997. Design and selection of processing conditions of pilot scale

contactor for continuous osmotic dehydration of carrots. Journal of Food Process


Lenart A. 1996. Osmo-convective drying of fruits and vegetables: technology and application.

Drying Technology, 14, 391-413.

Lerica C.R., Pinnavaia T.G., Dalla Rosa M. & Bartolucci L. 1985. Osmotic dehydration of

fruit: influence of osmotic agents on drying behaviour and product quality. Journal of


Madamba P. & Lopez R. 2002. Optimization of the osmotic dehydration of mango

(Mangifera indica L.) Slices. Drying Technology, 20, 1227- 1242.

40


Marchal L., Allali H. & Vorobiev E. 2005. Blanchiment de fraise par chauffage ohmique:

incidence sur la cinétique de déshydratation imprégnation par immersion, Récents

Progrès en Génies des procédés, Numéro 92- ISBN 2-910239-66-7, Ed. Lavoisier,

Paris, France.

Marouzé C., Giroux F., Collignan A. & Rivier M. 2001. Equipment design for osmotic

treatments. Journal of Food Engineering, 49, 207-221.

Mavroudis N.E., Dejmek P. & Sjoholm I. 2004. Osmotic treatment induced cell death and

osmotic processing kinetics of apples with characterised raw material properties.


Mayor L., Pissarra J. & Sereno A. 2008. Microstructural changes during osmotic dehydration

of parenchymatic pumpkin tissue. Journal of Food Engineering, 85, 326-339.

Mujumdar, A. S. 2006. Handbook Of Industrial Drying, 3rd Edition. Taylor and Francis

Group, LLC, 688-700

Neumann H.J. 1972. Dehydrated Celery: effects of pre-drying treatment and rehydration

procedures on reconstitution. Journal of Food Science, 37, 437-441.

Nieto A.B., Salvatori D.M., Castro M.A. & Alzamora S.M. 2004. Structural changes in apple

tissue during glucose and sucrose osmotic dehydration: Shrinkage, porosity, density

and microscopic features. Journal of Food Engineering, 61, 269-278.

Nunes C., Santos C., Pinto G. Lopes-da-silva J.A., Saraiva J.A. & Coimbra M.A. 2008. Effect

of candying on microstructure and texture of plums (Prunus domestica L.). LWT- Food


Ohkuma C., Kawai K., Viriyarattanasak C., Mahawanich T., Tantratian S., Takai R. & Suzuki

T. 2008. Glass transition properties of frozen and freeze-dried surimi products: effect

of sugar and moisture on the glass transition temperature. Food Hydrocolloids, 22,

255-262.

Ozdemir M., Ozen B., Dock L. & Floros J. 2008. Optimization of osmotic dehydration of

diced green peppers by response surface methodology. Food Science and Technology,

41, 2044-2050.

Panades G., Castro D., Chiralt A., Fito P., Nunez M. & Jimenez R. 2008. Mass transfer

mechanisms occurring in osmotic dehydration of guava. Journal of Food Engineering,

87, 386-390.

41


Petchi M. & Manivasagan R. 2009. Optimization of Osmotic Dehydration of Radish in Salt

Solution Using Response Surface Methodology. International Journal of Food

Engineering, DOI: 10.2202/1556-3758.1584.

Raoult-Wack A.L., 1994. Recent advances in the osmotic dehydration of foods. Food Science

and Technology, 5, 255-260.

Rastogi N.K. & Raghavarao K.S.M.S. 2004. Mass transfer during osmotic dehydration of

pineapple: considering Fickian diffusion in cubical configuration. Lebensmittel-

Wissenschaft und-Technologie, 37, 43-47.

Riggs P.D., Kinchesh P., Braden M. & Patel P.M. 2001. Nuclear magnetic imaging of an

omotic water uptake and delivery process. Biomaterials, 22, 419-427.

Riva M., Campolongo S., Leva A.A., Maestrelli A. & Torreggiani D. 2005. Structure property

relationships in osmo-air-dehydrated apricot cubes. Food Research International, 38,

533-542.

Saurel R., Raoult-Wack A.L. & Rios Guilbert S. 1995. Approches technologiques nouvelles

de la déshydratation- imprégnation par immersion (DII). Industries Alimentaires et

agricoles, 2, 7-13.

Shi J., Le Maguer M., Wang S. & Liptay A. 1998. Application of osmotic treatment in tomato

processing effect of skin treatments on mass transfer in osmotic dehydration of

tomatoes. Food Research International, 30, 669-674.

Simal S., Sanchez E.S., Bon J., Femenia A. & Rossello C. 2001. Water and salt diffusion

during cheese ripening: effect of the external and internal resistances to mass transfer,


Singh B., Panesar P. & Nanda V. 2008b. Optimization of osmotic dehydration process of

carrot cubes in sucrose solution. Journal of Food Process Engineering, 31, 1-20.

Singh C., Sharma H. & Sarkar B. 2008a. Optimization of process conditions during

dehydration of fresh pineapple. Journal of Food Science and Technology-Mysore, 45,

312-316.

Souza J., Medeiros M., Magalhaes M., Rodrigues S. & Fernandes F. 2007. Optimization of

osmótica dehydration of tomatoes in a ternery system followed by air-drying. Journal

of Food Engineering, 83, 501-509.

42


43

Sunjka P. & Raghavan G. 2004. Assessment of preatreatment methods and osmotic

dehydration for cranberries. Canadian Biosystems Engineering, 46, 335-340.

Taiwo K.A., Angersbach A. & Knorr D. 2002. Influence of high intensity electric field pulses

and osmotic dehydration on the rehydration characteristics of apple slices at different

temperatures. Journal of Food Engineering, 52,185-192.

Talens P., Escriche I., Martinez-Navarrete N. & Chiralt A. 2003. Influence of osmotic

dehydration and freezing on the volatile profile of kiwi fruit. Food research

international, 36, 635-642.

Thebud R. & Santarius K.A. 1982. Effects of high-temperature stress on various bio-

membranes of leaf cells in situ and in vitro. Plant Physiology, 70, 200-205.

Torreggiani D. & Bertolo G. 2001. Osmotic pre-treatments in fruit processing: chemical,

physical and structural effects. Journal of Food Engineering, 49, 247-253.

Tortoe C., Orchard J., Beezer A. & O’Neil M. 2007. Potential of calorimetry to study osmotic

dehydration of food materials. Journal of Food Engineering, 78, 933-940.

Uddin M., Ainsworth P. & Ibanoglu S. 2004. Evaluation of mass exchange during osmotic

dehydration of carrots using response surface methodology, Journal of Food


Vial C., Guilbert S. & Cuq J. 1990. Osmotic dehydration of kiwi-fruits: influence of process

variables on the colour and ascorbic acid content. Sci Alim., 11, 63-84.

Vijayanand P., Chand N. & Epieson W. 1995. Optimization of osmotic dehydration of

cauliflower. Journal of Food Processing and Preservation, 19, 229-242.

Wang W.C. & Sastry S.K. 2000. Effects of thermal and electrothermal pretreatments on hot

air drying rate of vegetable tissue. Journal of Food Process Engineering, 23, 299-319.

Wu L., Orikasa T., Tokuyasu K., Shina T. & Tagawa A. 2009. Applicability of vacuum-

dehydrofreezing technique for the long-term preservation of fresh-cut eggplant: effects

of process conditions on the quality attributes of the samples. Journal of Food Process


Chapitre 2: Déshydratation osmotique des graines de grenade ___________________________________________________________________________

Chapitre 2:

Cinétique de transfert de masse durant la déshydratation osmotique des graines de grenade


Bchir, B., Besbes, S., Attia, H., & Blecker, C. (2009). Osmotic dehydration of

pomegranate seeds: mass transfer kinetics and differential scanning calorimetry

characterization. International Journal of Food Science and Technology, 44,

2208–2217.

44


Résumé *

Titre : Cinétique de transfert de masse durant la déshydratation osmotique des graines de

grenade.

Objectif et stratégie expérimentale :

L’objectif de ce premier volet visait à optimiser le procédé de déshydratation

osmotique (DO) des graines de grenade. La DO a été menée durant 120 min en utilisant

différentes températures (30, 40, et 50°C) et solutions sucrées (saccharose, glucose, et

saccharose/glucose 50:50 w/w) présentant un extrait sec soluble de 55°Brix. L’étude de la

cinétique de transfert de masse a été basée essentiellement sur la détermination de la perte en

eau, du gain en solides et de la réduction en poids au cours du temps. D’autre paramètres tels

que la température de transition vitreuse, la température de fusion, et l’enthalpie de fusion ont

été déterminés par calorimétrie différentielle afin d’étudier l’évolution des différentes

fractions d’eau (libre, liée) dans la graine au cours du procédé.

Les différentes étapes du procédé sont reprises de façon synoptique dans la figure 1’.

_________________________________________________________________________ * Ce résumé permet de présenter de façon synthétique, en français, l’axe de recherche de l’article qui a été publié en anglais.

45


Graines congelées :

(-50°C)

Conditions

Traitement : déshydratation osmotique

Solution (55°Brix): - Saccharose, - Glucose - Saccharose/glucose

Température (°C) : - 30 - 40 - 50

Temps (min): - 0 - 80 - 20 - 100 - 40 - 120 - 60

Rapport : graine/solution: - 1/4

Paramètres

Paramètres physico-chimiques : - pH, aw - MS, Deff, conductivité - °Brix, couleur (L*, a*, b*)

Paramètres de transfert de masse : - Perte en eau - Gain en solides - Réduction en poids

Paramètres thermiques : - Température de transition vitreuse - Température de fusion - Enthalpie de fusion - Teneur en eau congelable et non congelable

Figure 1’ : Différentes étapes du procédé de déshydratation osmotique.

46

Chapitre 2: Déshydratation osmotique des graines de grenade ___________________________________________________________________________ Principaux résultats :

Les transferts de masse les plus significatifs sont intervenus pendant les 20 premières

minutes du traitement. A l'issue de cette période, les pertes mesurées en eau des graines

étaient de 46%, 37%, et 41%, en utilisant les solutions respectives de saccharose, glucose et

saccharose/glucose (50:50 p/p).

L'augmentation de la température s'accompagne d'une augmentation du transfert de

masse, attribué à un effet sur le coefficient de diffusion de l’eau et du soluté.

La calorimétrie différentielle à balayage (differential scanning calorimetry, DSC) a

fourni des informations complémentaires sur les changements de mobilité de l'eau et du soluté

dans la graine au cours de la déshydratation. Le rapport eau libre / eau liée a été divisé par dix

(de 5 à 0,5 à la fin du procédé), ce qui peut contribuer à une meilleure conservation du fruit.

L'analyse DSC révèle également que la température de transition vitreuse dépend du type de

sucre utilisé pour la déshydratation des graines de grenade.

47


Osmotic dehydration of pomegranate seeds: Mass transfer

kinetics and DSC characterization

Brahim Bchira, Souhail Besbesb, Hamadi Attiab, Christophe Bleckera, *

a Department of Food Technology, Gembloux Agricultural University, Passage des Déportés,

2, B- 5030 Gembloux, Belgium

b Laboratory of Food Analyses, Sfax National of School Engineers, Route de Soukra, 3038

Sfax, Tunisia

*Corresponding authors Tel: +32(0)81/62.23.08

*Fax: +32(0)81/60.17.67

*E-mail address: [email protected]

48

Chapitre 2: Déshydratation osmotique des graines de grenade ___________________________________________________________________________ Abstract

Osmotic dehydration of pomegranate seeds was carried out at different temperatures

(30, 40, 50°C) in a 55°Brix solution of sucrose, glucose, and mixture sucrose & glucose

(50:50 wt/wt). The most significant changes of water loss and solids gain took place during

the first 20 min of dewatering. During this period, seeds water loss was estimated to 46% in

sucrose, 37% in glucose and 41% in mix glucose/sucrose solution. The increase of

temperature favoured the increase of water loss, weight reduction, solids gain and effective

diffusivity. Differential scanning calorimetry data provided complementary information on

the mobility changes of water and solute in osmodehydrated pomegranate seeds. The ratio

between % frozen water and % unfreezable water decreased from 5 to 0.5 during the process.

That involving the presence of very tightly bound water to the sample, which is very difficult

to eliminate with this process. It also appeared that glass transition temperature depends on

the types of sugar.

Keywords: Osmotic dehydration; water loss; solids gain; Differential scanning

calorimetry; pomegranate.

49

Chapitre 2: Déshydratation osmotique des graines de grenade ___________________________________________________________________________ 1. Introduction

Pomegranate (Punica granatum L.) is one of the most important fruits in Tunisia. Its

total production in 2008 reached more than 70,000 tons. Pomegranate is composed by a non

edible part formed by 30% of skin (external part) and 13% of internal lamel and an edible part

formed by seeds (50-70%). Pomegranate seeds are composed by 15% pips (woody part), this

part determines the hardness, and 85% pulp (the juicy part) depending on cultivar (Al-

Maiman & Ahmad, 2002).

The edible part of the fruit contains considerable amounts of sugars, vitamins, organic

acids, phenolic compounds and minerals (Espiard, 2002). In Tunisia the research of addition

value to pomegranate seeds is very limited and presents a traditional feature such as jam

preparation or direct consummation of fruit during the crop season (between September and

December). However, other perspectives of transformation and exploitation of the

pomegranate seeds should be undertaken to give value addition to this typical fruit. As

reported in the literature, seeds could be used for preparation of grenadine (Adsule & patill,

1995), fresh juice (Espiard, 2002), jelly (Maestre et al., 2000), jam (Espiard, 2002), wine

(Altan & Maskan, 2004), spice (Adsule & Patill, 1995), paste, flavoring and coloring drinks

and mainly in some new cosmetics application (Espiard, 2002). Moreover, recently, more

than 475 new products containing pomegranate (food and drinks) were born on the American

market. These included, chewings-gum called pomegranate Power, sausage of chicken to the

pomegranate, ices, breads, and biscuit with pomegranate... (Storey, 2007).

The demand for healthy, natural and tasty processed fruits increased continuously, not

only for finished products, but also for ingredients that can be included in some food

formulation such as ice-cream, cereals, dairy, confectionery and bakery products. In fact, over

the last few decades, a lot of research studies about processing of fruits and vegetables

(Vivanco et al., 2004), meat and fish (Ivan et al., 2007) were developed using osmotic

50

Chapitre 2: Déshydratation osmotique des graines de grenade ___________________________________________________________________________ dehydration (OD). This process consists in the immersion of the product in a concentrated

solution (sugar, salt, sorbitol, glycerol), generating a partially dehydrated and impregnated

product (Torreggiani & Bertolo, 2001). Osmotic dehydration has a lot of benefit, like the use

of a low energy and cost compared to other dehydration methods. In addition, it involves

effective inhibition of polyphenoxidase, prevention of loss of volatile compounds, even under

vacuum and reduction of heat damage to color and flavor during dehydration (Krokida et al.,

2001). Raoult-Wack et al. (1991) noticed that OD did not strongly deteriorate the texture of

fruits. This effect was explained by the protective function of sugars in the fruit tissue.

Nowadays, the industry uses this technique for some previously cutted fruit like apple,

banana, mango, apricot, between others. This process has not been used for the conservation

of whole pomegranate seeds, neither by scientifics nor by industrials.

The aim of this work was firstly to investigate the kinetics of osmotic dehydration and

to determine the influence of osmotic conditions, such as temperature and osmotic solutions

on mass transfer during osmotic dehydration of whole pomegranate seeds. And secondly to

characterize the internal changes in osmotically dehydrated pomegranate seeds in sucrose,

glucose and mixture sucrose & glucose using differential scanning calorimetry (DSC). Also to

quantify the different states of water in the seeds, and to study the influence of osmotic

dehydration process on different parameters like the glass transition temperature.

2. Material and Methods

2.1. Preparation of pomegranate seeds

Fresh pomegranate fruits (Punica granatum L.) of El Gabsi variety were obtained from

a local research centre in Gabes, Tunisia. Pomegranates fruits were collected at full ripeness

stage, having the same size. The fruits (20 kg) were washed in cold tap water and then frozen

at -50°C. Pomegranates were thawed, during 1hour at room temperature, and seeds were

51

Chapitre 2: Déshydratation osmotique des graines de grenade ___________________________________________________________________________ recuperated in bottles just at the moment of osmodehydrated process. During the thawing of

the seeds, 24ml of juice per 100g of fresh matter were percolated.

2.2. Osmodehydration process

Sucrose, glucose and their mixture (50:50 wt/wt) were dissolved in water in order to

obtain 55°Brix solutions. About 10 g of seeds was soaked in the sugar solution and were

placed in bottles (Schott) of 100 ml. The volume ratio between the seeds and the sugar

solution was kept at one part of seeds and four parts of solution (1:4). Osmotic dehydration

process was conducted during 20 to 120 min in a shaking water bath (GFL instrument D

3006, Germany; oscillation rate 160 rpm) at different temperatures (30, 40, & 50°C).

2.3. Mass transfer kinetics

Seeds were removed from the immersion solution at selected time intervals (0, 20, 40,

60, 80, 100, and 120 min) and were quickly rinsed (with distilled water) and the excess of

solution at the surface was removed with absorbent paper. Water activity and soluble solids

were then measured as described below. The material was weighed before and after

osmodehydration to calculate the percentage of weight reduction (WR). The moisture content

was determined to calculate water loss (WL) and solids gain (SG), based on the following

equations (Mavroudis et al., 1998):

WR (%) = 100.)(

i

fi

WWW −

(1)

(3) WL (%) = SG + WR

SG (%) = 100.)(

i

sisf

WWW −

(2)

Where Wi is the initial weight of the sample (g), Wf the final weight of the sample (g),

Wsi the initial total solids content (g) and Wsf the final total solids content (g). Each value is

the mean of three determinations.

52

Chapitre 2: Déshydratation osmotique des graines de grenade ___________________________________________________________________________ 2.4. Mathematical modelling

Peleg’s equation parameters were obtained using eqn (4) (Peleg, 1988). This two-

parameter model was redefined by Palou et al. (1994) in terms of soluble solids and moisture

content and describes sorption curves that approach equilibrium asymptotically.

tkktMCtMC

210)(

+±= (4)

Where )(tMC is the amount of water or solids at the instant t (g/g dry matter (DM)),

is the initial amount of water or solids (g/g DM), k1 and k2 are Peleg’s parameters and t

is the time (s).

0MC

The value of the amount of water loss or solids gain at the equilibrium was then

calculated using eqn 5 (Park et al., 2002).

20

210

1)(limk

MCtkk

tMCMCteq ±=

+±=

∞→ (5)

Pomegranate seeds do not have a spherical shape, Alvarez et al. (1995) pointed out that

diffusion problem for any geometry can be reduced to the analytical solution corresponding to

a sphere, by modifying the Fourier number , using shape factor. 2/0 RtDFffe=

In order to determine the water and solutes effective diffusion coefficient the following

assumptions considerations were taken into account: homogeneous body, the external

resistance to mass transfer is negligible compared with internal resistance, the initial moisture

content was uniform throughout the sample, and the diffusion coefficient is constant (Crank,

1975).

The solution for Fick’s equation law for diffusion out a sphere is given by equation 6,

with using the following boundary conditions of internal resistance (Crank, 1975; Alvarez et

al., 1995):

53

Chapitre 2: Déshydratation osmotique des graines de grenade ___________________________________________________________________________ Uniforme initial amount : t = 0, 0<r<R, MC(t)=MC0; Symmetry of concentration: t >0, r =

0, 0)(=

∂∂

rtMC ; Equilibrium content at surface : t >0, r = R, MC(t)=MCeq

[ ]∑∞

=

−=−

−=

10

2

0

exp)(

nnn

eq

eqAouS FB

MCMCMCtMC

W μ (6)

Where: Bn= 6/μn2; μn= nП; F0= Deff, AorS t/R2; n = 1, 2, 3,…

Where Deff, AorS is the effective diffusivity of water loss or solids gain (m2 s-1); n is the

number of series terms, R is the equivalent radius of sphere (m), r is the distance in the radius

direction (m), and t is the time (s). WA and WS are the dimensionless amount of water loss and

solids gain, respectively; MCeq is the equilibrium amount of water loss or solids gain (g/g

DM) calculated using eqn 5.

As stated earlier, in this work pomegranate seeds were assumed to be ellipsoids, having

three characteristic diameters (2rM1~2rM2≤2RM ). According to Alvarez et al. (1995) the shape

factor (Ψ) eqn 7 is defined as Ss/Sp, and Ss is the surface area of a sphere of volume equal to

that of seeds with surface area Sp, which is assumed to be an ellipsoid. The intrinsic

diffusivity Deff is given by Ψ2 D’eff. It can be concluded that the diffusion coefficient

calculated from eqn (6) is D’eff and that it must be corrected by the factor Ψ2 when the product

shape can be assumed as an ellipsoid.

21

2

2

2

)/(1sin)/(1

22

4

MM

MM

MM

e

p

s

RrRr

Rrr

RSS

−⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−+

==−ππ

πψ (7)

2.5. Physico-chemical analysis of seeds

All analytical determinations were performed in triplicate. Values were expressed as the

mean± standard deviation.

The dry matter was calculated according to AOAC (1995). Approximately, 5 g of seeds

were oven dried at 103°C ± 2°C, until constant weight.

54


Total nitrogen was determined by the Kjeldahl method. Protein was calculated using the

general factor (6.25) (AOAC, 1995).

To determine total lipid content, about 5g of seeds were mixed with chloridric acid. Fat

was then extracted with a soxtherm automatic S 306 AK solvent extractor equipped with six

Soxhlet posts (Gerhardt soxtherm, Switzerland) and command unit (Gerhardt Variostat,

Switzerland) using petroleum ether 40-60°C in each Soxhlet post. The result was expressed as

the percentage of lipids in the dry matter.

To determine ash content, about 5 g of seeds were incinerated in a muffle furnace (type

Gelman, Germany) at about 550°C for 8h. The total ash content was expressed in dry weight

percentage (AOAC, 1995).

Carbohydrate content was estimated by difference of mean values, 100-(Sum of

percentages of moisture, ash, proteins and lipids) (AOAC, 1995).

aw was measured using an aqualab (Switzerland) instrument at 20 °C.

The soluble solids of seeds were determined according to AOAC (1995) methods. It

was measured by an ATGO digital refractometer (DBX-55, Switzerland) at 20°C and

expressed in °Brix.

pH measurements were performed using a Hanna instrument 8418 pH meter

(Switzerland) at 20°C.

The CieLab coordinates (L*, a*, b*) were directly read with a spectrophotocolorimetre

Mini Scan XE (Germany) with a lamp (D 65). In this coordinate system, the L* value is a

measure of lightness, ranging from 0 (black) to +100 (white), the a* value ranges from -100

(greenness) to +100 (redness) and the b* value ranges from -100 (blueness) to +100

(yellowness).

Conductivity was measured using a conductimeter (LF 597-5; Germany) instrument at

20°C.

55


Absorbance was measured using a spectrophotometer (Shimadzu UV 240, Cambridge,

USA) and the wave length used (λ ) was between 200 and 700 nm.

Differential scanning calorimetry (DSC) was performed on the pulp previously

separated from pip. A 2920 TA Instruments (New Castle, Delaware, USA) with a

Refrigerated Cooling Assessory and modulated capability was used. The cell was purged with

70 ml min−1 of dry nitrogen and calibrated for baseline on an empty oven and for temperature

using two temperature and enthalpy standards (indium, Tonset: 156.6 °C, ΔH: 28.7 J g−1;

eicosane, Tonset: 36.8 °C, ΔH: 247.4 J g−1). Specific heat capacity (Cp) was calibrated using a

sapphire. The empty sample and reference pans were of equal mass to within ±0.10 mg. DSC

curves were recorded during heating from –50 to 40°C at a scan rate of 5°C/min. All these

DSC experiments were made using hermetic aluminium pans. The analysed sample mass was

about 3.50 ±0.25mg.

2.6. Statistical Analysis

Statistical analyses were carried out using a statistical software program (SPSS for windows

version 11.0). The data was subjected to analysis of variance using the general linear model

option (Duncan test) to determine significant differences between samples (P<0.05).

3. Results and discussion

Chemical composition of pomegranate seeds before osmotic dehydration is shown in

table 1. Pomegranate seeds are rich in carbohydrate (~85%) followed by protein (~8%), lipid

(~5%) and Ash (~3%). This composition is quite similar to pomegranate seeds cultivated in

Egypt (El-Nemr et al., 1990).

56


Table 1. Chemical characteristic of pomegranate seeds

Seeds Dry matter (DM %) 16.00 ± 0.05 Protein g/100g DM 7.79 ± 0.86 Lipid g/100g DM 4.55 ± 0.40 Ash g/100g DM 2.87 ± 0.19 Carbohydrate g/100g DM 84.93 ± 0.25 pH 4.17 ± 0.20 Aw 0.989 ± 0.002 °Brix 15.50 ± 0.09

As in previous works, three different parameters were followed during osmotic

dehydration: water loss (WL), solids gain (SG) and weight reduction (WR). They were

responsible for total mass change, shrinkage and changes in the fruit liquid phase

concentration that defines the water activity, the quality and the stability of the final product

(Kowalska & lenart, 2001 and Falade et al., 2007).

3.1. Osmodehydration using sucrose solution

3.1.1. Mass transfer kinetics

The effect of dehydration time on WL, WR, and SG was studied in pomegranate seeds

at different temperatures (30, 40 and 50°C). The most significant changes took place during

the first 20 min of dewatering as shown in Fig. 1a. During this time, WL in seeds was 37, 39,

and 46 % respectively for 30, 40 and 50°C. After this period of dehydration, the percentage of

water loss varied slightly and ranged on average close to 35, 38, and 43 % for 30, 40 and

50°C respectively. The same trend was also observed for WR (Fig. 1b). Under the same

conditions, SG was also increased significantly during the first 20 min (Fig. 1c) reaching 5.4,

6.9, and 7.2 % respectively for 30, 40, and 50°C, and tend to be stable at the end of the

process. A similar curve has been reported in the osmotic dehydration of watermelon (Falade

et al., 2007). Statistical analysis showed that WL, SG and WR varied significantly with time.

However, the significant difference founded at course of time was not higher. This fact could

be because the majority of the transfer was done during the first 20 min of the process. As

57

Chapitre 2: Déshydratation osmotique des graines de grenade ___________________________________________________________________________ consequence we suggest to stop the process after 20 min as it implies no addition of thermal

energy to the system.

Table 2. Evolution of osmotic dehydration parameters in sucrose solution at different temperatures 30, 40, and 50°C

30°C

0 min 20 min 40 min 60 min 80 min 100 min 120 min

°Brix of solution 55.00±0.00a 51.10±0.63b 49.80±0.10b 49.10±0.70b 49.40±0.10b 49.40±0.20b 49.30±0.20b

°Brix of seeds 15.50±0.09a 39.55±0.63b 42.85±0.07c 44.55±0.07d 45.10±0.28d 46.25±0.07e 46.60±0.14e

pH of solution 8.27±0.03a 4.89±0.09b 4.68±0.08c 4.54±0.06cd 4.52±0.04cd 4.48±0.05d 4.46±0.08d

Conductivity of solution

(μs/cm)

0.90±0.01a 31.75±0.63b 36.05±1.62c 39.80±0.84d 40.80±0.70de 42.20±1.27de 43.20±1.27e

Dry matter of seeds (%) 16.00±0.05a 39.30±1.30b 41.20±1.40b 45.20±2.60c 45.20±0.10c 46.80±0.70c 48.20±0.10c

40°C

°Brix of solution 55.00±0.00a 50.00±0.84b 50.40±0.84bc 49.40±0.28cd 49.20±0.14d 49.15±0.12d 49.05±0.14d

°Brix of seeds 15.50±0.09a 40.85±0.07b 43.35±0.21c 45.40±1.31e 45.60±1.13e 45.85±0.35e 46.85±0.63e

pH of solution 8.27±0.03a 4.70±0.04b 4.50±0.02bc 4.46±0.09c 4.42±0.17c 4.43±0.12c 4.40±0.04c


(μs/cm)

0.90±0.01a 33.00±1.83b 37.35±1.76bc 40.60±2.97cd 43.95±3.74d 42.15±2.89cd 44.00±1.27d

Dry matter of seeds (%) 16.00±0.05a 40.90±2.97b 44.20±0.18bc 45.90±1.02c 47.30±0.15c 47.80±0.20c 49.00±0.06c

50°C

°Brix of solution 55.00±0.00a 49.70±0.07b 49.30±0.01bc 49.10±0.21c 48.90±0.28c 49.00±0.21c 49.00±0.21c

°Brix of seeds 15.50±0.09a 41.60±0.14b 45.30±0.14c 46.40±0.14d 46.90±0.14d 48.70±0.28e 49.10±0.07e

pH of solution 8.27±0.03a 4.60±0.27b 4.50±0.20b 4.50±0.01b 4.40±0.02b 4.30±0.10b 4.30±0.07b


(μs/cm)

0.90±0.01a 35.50±0.35b 39.00±1.06c 40.10±0.07de 40.50±0.21e 41.00±0.49e 41.20±0.28e

Dry matter of seeds (%) 16.00±0.05a 42.70±0.08b 46.80±0.65c 47.80±0.83cd 48.30±0.41cde 48.60±0.73de 49.50±0.76e

All values given are means of three determinations. Means in line with different letters are significantly different (P<0.05)

58


The rapid loss of water in the beginning at various temperatures (30, 40, and 50°C) is

due to the large osmotic driving force between the dilute sap of seeds and the surrounding

hypertonic medium. Then, slower water transfer is mainly influenced by the reduction of the

difference in concentration between the seeds and osmotic solution which could involve a

slower driving force. Indeed, the reverse trend of °Brix observed in seeds and osmotic

solution confirms these facts (Table 2). The trend observed in SG for the different

temperatures studied (Fig. 1c) could be explained by migration of sucrose to the seeds through

their cell membranes due to the important gradient of sugar between the seeds and the osmotic

solution.

From the results shown in Fig. 1, it can be concluded that the increase of temperature

from 30 up to 50 °C lead to an increase of water loss, weight reduction, and solids gain. The

increases of temperature at 40°C involved the same evolution of WL as using 30°C. However

a significant difference was found in comparing to 30°C from time superior to 60 min. At

50°C significant difference was already observed after 20 min. A similar evolution for WR

and SG as function of time and temperature was found. Nevertheless, the increase of

temperature to 40 and 50°C showed a significant difference of SG compared to 30°C from 20

min. A Higher value of different parameters was always observed using 50°C. As

consequence we suggest using 50°C to have the best dehydration. Ferrari & Hubinger (2008)

showed that the increase of temperature leads to irreversible damage and a loss of selectivity

of cell membrane involving a higher osmotic pressure at the product/solution interface.

Moreover, higher temperature raised diffusion coefficients indiucing higher mass transfer

rates (table 3). This behavior has also been reported in the osmotic dehydration of apricots

(Khoyi & Hesari, 2007) and melon (Ferrari & Hubinger, 2008).

59


Table 3. Water and solids effective diffusivities calculated by Fick’s model

Water loss Solids gain Sugar T (°C) Deffw (m2s-1) R2 (%) Deffs (m2s-1) R2(%)

50°C 9.44 × 10-12 99.92 4.81 × 10-12 99.72 40°C 8.09 × 10-12 99.89 4.72 × 10-12 99.88

Sucrose 30°C 7.43 × 10-12 99.66 4.21 × 10-12 99.80

50°C 9.10 × 10-12 99.93 5.44 × 10-12 99.92 40°C 7.88 × 10-12 99.77 4.54 × 10-12 99.89

Sucrose/glucose 30°C 6.98 × 10-12 99.77 4.48 × 10-12 99.87

50°C 8.74 × 10-12 99.83 9.54 × 10-12 99.88 40°C 6.69 × 10-12 99.68 6.52 × 10-12 99.89 Glucose

30°C 5.41 × 10-12 99.65 4.94 × 10-12 99.50

In this study, the increase of temperature had a more significant effect on WL than on

SG (Figs 1a and 1c). Indeed, after 120 min when the temperature increased from 30 to 40°C,

the gain of WL and SG was 4.4% and 0.5%, while when the temperature rised from 40°C to

50°C, the gain was 5.7% and 1.1% respectively. In addition effective diffusivity of water was

also higher than that of the solids (Table 3). This effect is generally attributed to the influence

of natural tissue membranes and to the diffusive properties of water and solutes as a function

of their respective molar mass (Falade et al., 2007). In a study on osmotic dehydration of

model foods (aqueous agar-agar gels), Raoult-Wack et al. (1989) suggested that this apparent

contradiction could be due to a reciprocal influence between the water and solute transfers

where sugar penetration by diffusion and sugar release within the water out flow are

combined.

60


0

5

10

15

20

25

30

35

40

45

50

0 20 40 60 80 100 120

Time (min)

% W

L

30°C 40°C 50°C

0

5

10

15

20

25

30

35

40

45

0 20 40 60 80 100 120

Time (min)

%W

R

30°C 40°C 50°C

0

2

4

6

8

10

12

0 20 40 60 80 100 120Time (min)

%S

G

30°C 40°C 50°C

a

b

c

Figure 1: Variation of water loss (WL) (a) weight reduction (WR) (b) and solids gain (SG) (c) with time and temperature (30, 40, 50°C) using sucrose solution during osmotic

dehydration.

61

Chapitre 2: Déshydratation osmotique des graines de grenade ___________________________________________________________________________ 3.1.2. Evaluation of the Peleg and Fick mathematical models

The Peleg’s equation parameters (K1 and K2) were determined for water loss and solids

gain (eqn 5), as shown in table 4. This model showed a good fit to the experimental data, with

correlation coefficients (R2) close to 0.99. The parameters K1 and K2 did not exhibit a clear

trend with the increase in temperature. Ferrari & Huninger (2008), Khoyi & Hesari (2007)

and Park et al. (2002) verified the same fact in similar studies with melon, apricots and pears,

respectively, using sucrose as the osmotic agent.

Table 4. Values of Peleg’s equation parameters for water loss and solids gain

Water loss Solids gain Sugar T (°C) k1 k2 R2 k1 k2 R2

30 37.8782 0.2362 0.9993 15.8206 0.0300 0.9988 40 36.1447 0.2316 0.9999 13.4587 0.0302 0.9996

Sucrose 50 28.0531 0.2317 0.9999 12.0808 0.0286 0.9992

30 62.8484 0.2328 0.9999 12.6530 0.0308 0.9967 40 44.9577 0.2315 0.9995 8.9232 0.0360 0.9997

Glucose 50 25.5221 0.2346 0.9999 4.8449 0.0299 0.9997

30 44.4009 0.2301 0.9999 13.6050 0.0289 0.9995 40 34.7331 0.2317 0.9998 13.6054 0.0288 0.9995

Sucrose/glucose 50 29.7106 0.2311 0.9998 10.5526 0.0286 0.9998

Table 3 shows the effect diffusivity values for water and solids calculated using Fick’s

model (eqn 6), which also presented a good fit to experimental data, showing an average

correlation coefficients (R2) close to 0.99. The experimental values for effective diffusivity

were to an order of magnitude between 4×10-12 to 9×10-12 for solids gain and 5×10-12 to 9×10-

12 m2s-1 for water loss. Park et al. (2002) working with pear cubes found that Deff ranged from

0.35×10-9 to 1.92×10-9 m2s-1 for water loss and from 0.20×10-9 to 3.60×10-9 m2s-1 for solids

gain at different temperature (40-60°C). Lazarides et al. (1997) found values ranging from

1.42×10-10 to 4.69×10-10 for moisture diffusivity and from 0.73×10-10 to 2.41×10-10 m2s-1

solute diffusivity of apple slices at different temperature (20-50°C) and sucrose solution

concentrations (45-65%). So, a comparison of the diffusivities values to those reported in the

62

Chapitre 2: Déshydratation osmotique des graines de grenade ___________________________________________________________________________ literature showed that our results were lower. This comparison should however take into

account the experimental conditions, the different estimation methods employed, the variation

in food composition and its physical structure. Indeed, given their small size, pomegranate

seeds can be kept intact and do not need to be cut. On the contrary, pear, apple or kiwis, due

to their large sizes, have to be cut in small volumes. Thus, cutting these fruits creates more

externals lesion leading to a higher contact of cells with the osmotic solution in a shorter time

inducing a higher diffusion.

3.1.3. Physico-chemical Characteristics of the osmodehydrated fruit preparation

The changes that occurred in pomegranate seeds and in the osmotic solution, a function

of time and temperature, are shown in tables 2 and 5. As it was expected all parameters

(°Brix, pH, conductivity…) evolved in the same trend like mass transfer parameters (WL, SG,

WR). In fact, statistical analysis shows a significant difference (P<0.05) at the beginning of

the process (20 min). The increase of temperature showed that 50 °C gave the lowest °Brix,

and pH of the solution, and the highest °Brix in the seeds. At the beginning of the process (20

min) the °Brix in the solution decreased as the °Brix of seeds increased, after that °Brix

tended towards an equilibrium (Table 2). This was a consequence of osmosis, inducing a

balance of concentration between the seeds and the sucrose solution. The diffusion of some

solutes from pomegranate seeds to the aqueous solution, could explain the decrease of pH and

the increase of the conductivity in the osmotic solution. The measure of colour parameters L*,

a*, b* showed a slight reduction of L* and a slight increase of a* and b* (Table 5). This

variation could be explained by a migration of pigment from pulp to solution. Indeed

absorbance reached a peak at 510 nm. According to the literature, this peak corresponded to

the pink pigmentation (flavonoids) in pomegranate seeds (Sestili et al., 2007). Table 6 shows

that water activity was reduced from 0.989 to 0.903, confirming water loss during the process.

Moreover, using 50°C involved the lowest water activity (0.903) compared to the other

63

Chapitre 2: Déshydratation osmotique des graines de grenade ___________________________________________________________________________ temperatures (30°C: 0.925 and 40°C: 0.915). Therefore, water activity is very temperature and

time dependent.

Table 5. CieLab coordinates of sucrose solution at different temperatures 30, 40, and 50°C 30°C CieLab

coordinate 0 min 20 min 40 min 60 min 80 min 100 min 120 min

L* 65.76±0.01a 63.04±1.04ab 61.41±2.05b 60.18±2.04b 61.33±1.47b 61.00±2.12b 60.13±0.62b

a* -0.60±0.02a 0.54±0.07b 1.35±0.42c 3.44±0.29e 2.29±0.38d 3.44±0.16e 3.15±0.49e

b* 3.60±0.01a 6.78±0.15c 5.92±0.58bc 6.25±0.68bc 5.95±0.35bc 5.44±0.08b 6.79±0.36c

40°C

L* 65.76±0.01a 59.64±1.73bc 59.58±0.49bc 60.96±1.05b 59.33±0.58bc 57.47±0.03c 58.76±0.65c

a* -0.60±0.02a 3.82±0.34c 4.62±0.70c 2.31±0.02b 4.58±0.73c 5.81±0.27d 6.56±0.15d

b* 3.60±0.01a 4.38±0.36ab 4.85±0.63ab 5.70±0.57bc 6.27±0.891bc 7.01±0.08de 8.18±1.55e

50°C

L* 65.76±0.01a 61.40±0.96b 60.90±0.40b 60.50±0.47bc 59.80±0.75bc 59.60±1.32bc 58.80±0.15c

a* -0.60±0.02a 2.40±0.92b 2.90±0.134bc 3.30±0.46bcd 4.10±0.30cd 3.70±1.14bcd 4.60±0.24d

b* 3.60±0.01a 6.50±0.26b 7.80±0.07bc 8.10±0.17bc 9.60±1.98c 8.90±0.04c 9.30±0.09c All values given are means of three determinations. Means in line with different letters are significantly different (P<0.05)

3.1.4. Thermal properties of seeds as measured by DSC

Osmotic dehydration of pomegranate seeds was investigated by DSC to determine the

kinetic of seeds dehydration and the state of water during the process at 50°C. DSC

thermograms (Fig. 2) presented 2 main thermal events between -50°C and 30°C. The first was

a change in heat capacity (∆Cp) and the second was an endothermic peak. ∆Cp can be related

to a freeze-concentration glass transition temperature (Tg’) due to the presence of sucrose,

protein, fibre (pectin, lignin, hemicellulose and cellulose), and water in the sample. As

reported in such products, carbohydrates and proteins can be described as amorphous food

polymers constituent by not arranged chains (Roos, 1995). The endothermic transition could

be attributed to the melting of crystallized water (free and freezing bound water). In fact,

during the cooling only free water and freezing bound water were crystallized to give ice. And

during the heating, frozen water undergoes a fusion of ice and unfreezable water does not

64

Chapitre 2: Déshydratation osmotique des graines de grenade ___________________________________________________________________________ undergo any change. The endothermic transition was used to calculate the amount of

unfreezable water (UFW) estimated as follows:

DMHHW

UFW watersamplewater )/( ΔΔ−= (8)

Where ∆Hsample is the heat of melting expressed in j/g, ∆Hwater is the normalized heat of

melting of pure water (351.2+/- 1.2 j/g), Wwater is the total water content of sample expressed

in g of water and DM is the dry matter content expressed in g DM (Goni et al., 2007).

Time: 20 min AW: 0.954




Time: 100 min AW: 0.905

Time: 120 min AW: 0.903

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

Hea

t Flo

w (W

/g)

-60 -40 -20 0 20 40

Temperature (°C)Exo Up Universal V3.0G TA Instruments

Figure 2: DSC thermogram obtained for pomegranate seeds soaked in sucrose solution at 50°C.

65


A considerable increase in Tg’ was observed up to 20 min and after this period Tg’

varied much slower and reached -34.87 °C at the end of the process (Table 6). Moreover

melting point (Tf), unfrozen water (UFW) and enthalpy of fusion (∆Hfus), strongly decreased

during the first 20 min and after this time, Tf and ∆Hfus showed a slight decrease but UFW

tended to be constant for the rest of the process. A similar evolution was observed by

Cornillon, (2000) using apple soaked in 63 g sucrose/100 g solution for osmotic dehydration.

The increase of the glass transition (Tg’), the lower enthalpy of fusion (∆Hfus) and water

activity (aW) were consistent with the fact that less and less water was present in the seeds as

the dehydration process occurred (Cornillon, 2000). Sá et al. (1999) found that Tg’ for

polysaccharides water systems reach to a maximum with decreasing water content, inducing

the decreased mobility of the polymer chains. Indeed, it is well known that water has a

negative Tg’ (-173°C) as opposed to the different constituents of seeds that have a positive

Tg’ (sucrose: 62°C; pectin: 160°C; Hemicellulose: 150 - 220°C; cellulose 220 - 250°C;

protein: 77 - 112°C) (Roos, 1995). Thus, the presence of water permits to reduce the Tg’ of

sample. It is well know establish that water drastically decreased the Tg’ of food, due to its

plasticizing effect on amorphous polymers (Roos, 1995). A comparison with published

literature data on water-sucrose solutions and Tg’ seeds (at the end of the process) showed

close values, indicating that sucrose was predominant in seeds. In fact Tg’ of a sucrose

solution (49°Brix) and Tg’ of seeds (also 49°Brix), at the end of the process, were

respectively -32.21 and -34.87°C. It is well known establish previously that diffusion of

sucrose in the seeds induced an increase of Tg’ (Sá et al., 1999). Moreover pomegranate seeds

present a decrease of freezing temperature, which can be explained by the depressing effect of

solute, such as sugars, diffusing in the seeds. Indeed, several works reported that the increase

of solutes in fruits was closely related to the decrease of the freezing temperature (Ohkuma et

66

Chapitre 2: Déshydratation osmotique des graines de grenade ___________________________________________________________________________ al., 2008). The increase of the relative amount of sucrose and the reduce amount of water in

the cell corresponded to the shift of Tg’.

On analyzing the DSC data, we observed that the amount of unfreezable water (water

that did not freeze) declined in the beginning of the process from 3.98 to 1.09 g.g-1 DM and

tended to stabilize in the course of time reaching ~ 0.90 g.g-1 DM. This fraction is strongly

associated with the polymer matrix and showed neither exothermic nor endothermic peak on

DSC curves. The decrease of UFW in seeds could be due to the higher percentage of water

loss at the beginning of the process. In fact, dry matter increased from 16% to 43% min and

water activity decreased from 0.989 to 0.954 at 50 °C after 20 min. The stability of UFW was

the result of water loss and solids gain, and implicate that water become more bound in seeds.

Moreover, the analyzing of the % of frozen water (calculated by dividing the enthalpy of

fusion of sample by the enthalpy of fusion of pure water) and the % of UFW (calculated by

subtracting the % of total water per the % of frozen water) showed different evolution in

course of time. In fact, the % of frozen water decreased 3.5 times contrary the % of UFW that

increased 2.5 times. This was a consequence of water loss and sugar gain during the process.

Many authors found that the increase in the unfreezable water weight fraction in fruit can

mainly be attributed to a significant accumulation of osmolites such as soluble sugars (Goni et

al., 2007 and Ohkuma et al., 2008). Moreover, the most significant changes took place during

the first 20 min of the process, that confirming previous results. During this time, the % of

frozen water decreased from 70% to 28% whereas the % of UFW increased from 14% to

29%. After this period, the % of frozen water and UFW slightly varied and ranged on average

close to 20 and 34% respectively. The same trend was also observed for water activity

indicating that less and less free water was available in seeds. The final product presented a

higher % of UFW than % of frozen water; this is an advantage for a better conservation of

seeds. Nevertheless, water activity of osmodehydrated seeds was higher (superior to 0.9)

67

Chapitre 2: Déshydratation osmotique des graines de grenade ___________________________________________________________________________ involving certain undesirable reactions, such as non-enzymatic browning, fat oxidation,

vitamin degradation, enzymatic reactions, and protein denaturation. As a consequence, other

treatments (pasteurization, freezing, drying) will be necessary to ensure a good conservation

of the seeds.

Table 6. Differential scanning calorimetry results for pomegranate seeds over soaking time in sucrose, glucose and mixture sucrose & glucose solution at 50°C

Time (min) Tg’ (°C)

midpoint

Tf (°C)

onset

∆Hfus

(J/g)

unfreezable water

(g.g-1 DM)

AW

Sucrose solution

0 -41.88 0.22 233 3.98 0.989a

20 -34.19 -4.90 93.86 1.09 0.954b

40 -33.64 -5.65 81.17 0.91 0.946c

60 -33.61 -5.92 79.02 0.88 0.942c

80 -34.43 -8.84 58.74 0.92 0.923d

100 -35.35 -9.48 47.52 0.92 0.905e

120 -34.87 -8.56 61.33 0.85 0.903e

Sucrose and glucose solution

0 -41.88 0.22 233 3.98 0.989a

20 -35.39 -7.43 78.88 1.08 0.958b

40 -35.62 -6.49 83.20 0.96 0.945c

60 -35.42 -7.78 70.04 0.94 0.931d

80 -35.50 -7.36 76.19 0.88 0.919e

100 -34.74 -7.39 75.47 0.84 0.910ef

120 -35.03 -6.92 82.96 0.80 0.906f

Glucose solution

0 -41.88 0.22 233 3.98 0.989a

20 -40.65 -8.32 83.99 1.25 0.956b

40 -40.03 -10.04 82.87 0.93 0.947b

60 -40.65 -10.92 72.74 0.95 0.930c

80 -40.08 -11.25 70.43 0.89 0.925c

100 -40.87 -11.42 67.22 0.90 0.911d

120 -40.60 -11.88 65.20 0.93 0.910d

Means in column with different letters are significantly different (P<0.05)

68

Chapitre 2: Déshydratation osmotique des graines de grenade ___________________________________________________________________________ 3.2. Osmodehydration using glucose and mixture sucrose & glucose solution

Using glucose and mixture sucrose & glucose solution time and temperature had a

similar effect on different parameters (WL, SG, WR, Deff, °Brix, DM, pH …) as with the

sucrose solution. With regard to the kind of solute employed, comparing the values of WL,

SG and Deff at the same temperature, osmotic dehydration with glucose leads a decrease of

water loss and a slightly increased solids gain (Figs 3a and 3b). Moreover the difference on

°Brix between seeds and solution was very weak using glucose solution. In addition glucose

solution induces a lower Deff of water and a higher Deff of solids contrary to sucrose solution

(Table 3). Fig 3a shows that the sucrose solution allowed a better water loss followed by the

sucrose/glucose mix and the glucose solution. These dissimilarities were attributed to the

specific surface of seeds and the differences between molecular weight of glucose and

sucrose. In fact, mass transport can be described by the Fick’s second Law, depending on the

coefficient (Deff) which is influenced by the radius of solute (Saurel et al., 1994). Briois et al.

(1998) showed that the speed of diffusion of sugar molecules in fruit membrane cells was

negatively correlated with their molecular size. Indeed with a low molar mass (glucose)

solute, the effect of dilution was responsible for the faster reduction of the difference in

average concentration between liquid phase and solid phase, inducing decrease of the water

loss in the course of time. As consequence glucose allowed easily diffusion (Masmoudi et al.,

2007).

69


0

5

10

15

20

25

30

35

40

45

50

0 20 40 60 80 100 120

Time (min)

% W

L

Sucrose Glucose Sucrose/Glucose

a

0

2

4

6

8

10

12

14

0 20 40 60 80 100 120Time (min)

% S

G

Sucrose Glucose Sucrose/glucose

b

Figure 3: Comparison of water loss (WL) (a) and solids gain (SG) (b) using different osmotic

solutions (sucrose, glucose and mixture sucrose & glucose) at 50°C.

70


Tf, ∆Hfus, aW and UFW evolve in the same way as with the sucrose solution except the

Tg’ for glucose seeds that slightly increase as a function of time (Table 6). As it was shown in

sucrose solution in this case, Tg’ of water-glucose and seeds (at the end of the process)

showed close values, indicating that glucose was predominant in seeds. In fact Tg’ of a

glucose solution (55°Brix) and Tg’ seeds, at the end of the process, were respectively -40.6

and -44.9°C. Roos, (1995) and Liu et al. (2007) showed that Tg’ was strongly dependent on

the molecular weight, it decreased with decreasing molecular weight. Table 6 shows that the

addition of small molecules decreased the value of Tg’. In fact, the use of a glucose solution

showed the lowest Tg’ and Tf. Thus glucose solution induces reducing of water loss compared

to the others osmotic solution. Roos (1995) found that higher sucrose content in the mix

induced a higher Tg’. Table 6 shows that the amount of UFW at the end of the process was

very closed. In fact, UFW varied between 0.80 and 0.93 involving the presence of very tightly

bound water to the sample, which is very difficult to eliminate with this process even after

120 min.

4. Conclusion

Osmotic dehydration process could be used for the conservation of pomegranate seeds.

The rate of different parameters was directly related to temperature, time, and solute. In fact

process showed that operating 20 min at 50°C offered the best result. As a consequence, it

could be better to stop the process after 20 min as it implies no addition of thermal energy to

the system. Osmotic dehydration reduced water activity from 0.989 to an average of 0.900. At

this aW value a complementary treatment such as drying, freezing and pasteurization should

be necessary to ensure its good conservation.

The nature of sugar used for the dehydration solution involves modifications in the

evolution of mass transfers and effective diffusivity during the process. In fact, sucrose

(higher molecular weight) induced the best effective diffusivity of water involving the best

71

Chapitre 2: Déshydratation osmotique des graines de grenade ___________________________________________________________________________ dehydration of seeds, contrary to glucose (lower molecular weight) that induced the best

effective diffusivity of solids and than impregnation. Therefore we suggest using sucrose

solution to have the best dehydration. On the other hand, the study of physico-chemical

characteristics of osmodehydrated pomegranate seeds showed the lost of solutes from the

seeds to the osmotic solution during the osmotic dehydration process. So these solutions could

be used as natural additives (flavour and color) in the industry.

DSC data provide complementary information on the mobility changes of water and

solute in osmotically dehydrated pomegranate seeds. Indeed, it was possible to determine a

strong decrease in water mobility involving an increase of glass transition as more solute and

water migrated respectively into and out of seeds. It also appeared that Tg’ depends on the

types of sugar. In fact Tg’ of seeds using a sucrose solution was higher than Tg’ using glucose

or glucose/sucrose mix solution. After osmotic dehydration, the product presented a higher %

of UFW than % of frozen water, this is an advantage for a better conservation of seeds.

The finished product has an attractive colour and presents a good texture in mouth, a

pleasant sugar taste and a good aroma. It would be interesting for the continuation of this

work to assess the sensory properties (texture, flavour, etc.) and to substitute the standard

solutions by new solutions (date juice) that bring new organoleptic properties to the finished

product.

72

Chapitre 2: Déshydratation osmotique des graines de grenade ___________________________________________________________________________ References:

Adsule, N.R., & Patil, N.B. (1995). Pomegranate. In: Salunkhe, D.K., Kadam, S.S., ed.

Handbook of Fruit Science and Technologies: Production, Composition, Storage and

Processing. New York, USA: 455-463.

Al-Maiman, S.A., & Ahmad, D. (2002). Changes in physical and chemical properties during

pomegranate (Punica granatum L.) fruit maturation. Food Chemistry, 76, 437-441.

Altan, A., & Maskan, M. (2004). Rheological Behavior of pomegranate (Punica granatum L.)

juice and concentrate. Journal of Texture Studies, 36, 68-77.

Alvarez, A .C., Aguerre, R., Gomez, R., Vidales, S., Alzamora, S.M., & Gerschenson, L.N.

(1995). Air Dehydration of Strawberries: effects of Blanching and Osmotic

Pretreatments on the Kinetics of Moisture Transport. Journal of Food Engineering,

167-178.

AOAC (1995) Official methods of analysis. 15th edn. Association of Official Analytical

Chemists.

Briois, E., Dusautois, C., & Heno, P. (1998). Applications alimentaires des sirops de glucose

à haute teneur en maltose. Industries Alimentaires et Agricoles, 7/8, 79-81.

Cornillon, P. (2000). Characterization of osmotic dehydrated Apple by NMR and DSC.

Lebensmittel-Wissenschaft und-Technologie-Food Science and Technology, 33, 261-

267.

Crank, J. (1975). The mathematics of diffusion (2nd ed). Oxford: Clarendon Press.

El-Nemr, E., Ismail, A,. & Ragab, M. (1990). Chemical composition of juice seeds of

pomegranate fruit. Journal of Food Science, 7, 601–606.

Espiard, E. (2002). Introduction à la transformation industrielle des fruits. TEC&DOC-

Lavoisier (pp 181-182).

Falade, K., Igbeka, J., & Ayanwuyi, F. (2007). Kinetics of mass transfer and colour changes

during osmotic dehydration of watermelon. Journal of food engineering, 80, 979-985.

73

Chapitre 2: Déshydratation osmotique des graines de grenade ___________________________________________________________________________ Ferreari, C.C., & Hubinger, D.M. (2008). Evaluation of the mechanical properties and

diffusion coefficients of osmodehydrated melon cubes. International journal of Food


Goni, O., Munoz, M., Cabello, J., Escribano, M., & Merodio, C. (2007). Changes in water

status of cherimoya fruit during ripening. Postharvest Biology and Technology, 45,

147-150.

Ivan, L., Nicholas, J., Gary, & D. (2007). A simple centrifugal dehydration force method to

characterize water compartments in fresh and post-mortem fish muscle. Cell Biology

International, 31, 516-520.

Khoyi, M., & Hesari, J. (2007). Osmotic dehydration kinetics of apricot using sucrose

solution. Jouranl of food engeneering, 78, 1355-1360.

Kowalska H., & Lenart A. (2001). Mass exchange during osmotic pretreatement of

vegetables. Journal of food engineering, 49, 137-140.

Krokida, M. K., Oreopoulou, V., Maroulis, Z.B., & Marinos-Kouris, D. (2001). Effect of

osmotic dehydration pretement on quality of French fries. Journal of food


Lazarides, H.N., Gekas, V., & Mavroudis, N. (1997). Apparent mass diffusivities in fruit and

vegetable tissues undergoing osmotic processing. Journal of food Engineering, 31,

315-324.

Liu, Y., Bhandari, B., & Zhou, W. (2007). Study of glass transition and enthalpy relaxation of

mixtures f amorphous sucrose and amorphous tapioca starch syrup solid by differential

scanning calorimetry. Journal of food Engineering, 81, 599-610.

Maestre, J., Melgarejo, P., Tomas, A. & Garcia-Viguer, C. (2000). New food products drived

from pomegranate. Cahiers Options méditerranéennes. 243 - 245.

Masmoudi, M., Besbes, S., Blecker, C., & Attia, H. (2007). Preparation and characterization

of osmotidehydrated Fruits from Lemon and Date By-products. Journal of Food

Science and Technology international, 13, 405-412.

74

Chapitre 2: Déshydratation osmotique des graines de grenade ___________________________________________________________________________ Mavroudis, N. E., Gekas, V., & Sjohlm, I. (1998). Osmotic dehydration of apples, shrinkage

phenomena and the significance of initial structure on mass tranfer rates. Journal of

food Engineering, 38, 101-123.

Ohkuma, C., Kawai, K., Viriyarattanasak, C., Mahawanich, T., Tantratian, S., Takai, R., &

Suzuki, T. (2008). Glass transition properties of frozen and freeze-dried surimi

products: effect of sugar and moisture on the glass transition temperature. Food

Hydrocolloids, 22, 255-262.

Palou, E., Lopes-Malo, A., Argaiz, A. & Welti, J. (1994). The use of Peleg’s equation to

model osmotic concentration of papaya. Drying Technology, 12, 965-978.

Park, K.J., Bin, A., Bord, F.P.R. & Park, T.H.K.B. (2002). Osmotic dehydration Kinetics of

pear D’anjou (Pyrus communis L.). Journal of Food Engineering, 52, 293-298.

Peleg, M. (1988). An empirical model for the description of moisture sorption curves. Journal

of Food Science, 53, 1216-1219.

Raoult-Wack, A. L., Guilbert, S., Le Maguer, M., & Rios, G. (1991). Simultaneous water and

solute transport in shrinking media-part 1: application to dewatering and impregnation

soaking process analysis (osmotic dehydration). Drying Technnology, 9, 589-612.

Raoult-Wack, A. L., Lafont, F., Rios, G., & Guilbert, S. (1989). Osmotic dehydration: study

of mass transfer in terms of engineering properties. In Drying 89, ed. A. S. Mujumdar

and M. Roques, Hemisphere Publishing Corporation (pp 487 – 495). New York.

Roos, Y. (1995). Characterisation of Food Polymers Using State Diagrams. Journal of food


Sá, M.M., Figueiredo, A.M., & Sereno, A.M. (1999). Glass transitions and state diagrams for

fresh and processed apple. Thermochimica Act, 329, 31-38.

Saurel, R., Raoult-Wack, A.L., Rios, G., & Guilbert, S. (1994). Approches technologiques

nouvelles de la déshydrataion-impregnation par immersion. Cahier Scientifique,

Volume N°112.

75


76

Sestili, P., Martinelli, C., Ricci, D., Fraternale, D., Bucchi, A., Giamperi, L., Curcio, R.,

Piccoli, G., & Stocchi, V. (2007). cytoprotection effect of preparations from various

parts of Punica granatum L. fruits in oxidatively injured mammalian cells in

comparison with their antioxidant capacity in cell free systems. Pharmacological

research, 56, 18-26.

Storey, T. (2007). La grenade le fruit médicament, Magazine NEXUS, Santé, 51, 46-54.

Torreggiani, D., & Bertolo, G. (2001). Osmotic pre-treatments in fruit processing: chemical,

physical and structural effects. Jouranl of food engineering, 49, 247-253.

Vivanco, P. D, Hubinger, M. D., & Sobral, P. J. A. (2004). Mass transfer in osmotic

dehydration of Atlantic Bonito (Sarda) fillets under vacuum and atmospheric pressure.

Internationnal Drying Symposium, 14, 2105-2112.

Chapitre 3: Effet de la congélation sur la cinétique de transfert de masse ___________________________________________________________________________

Chapitre 3:

Effet de la congélation sur la cinétique de transfert de masse durant la déshydratation osmotique des graines de

grenade


Bchir, B., Besbes, S., Attia, H., & Blecker, C. (2010). Osmotic dehydration of

pomegranate seeds (Punica granatum L.): Effect of freezing pre-treatment.

Journal of Food Process Engineering, DOI: 10.1111/j.1745-4530.2010.00591.x

77


Résumé *

Titre : Effet de la congélation sur la cinétique de transfert de masse durant la déshydratation

osmotique des graines de grenade


Ayant démontré précédemment que l’utilisation d’une solution de saccharose à

55°Brix et d’une température de 50°C, favorise la perte en eau au cours du procédé de DO,

cette partie visait, tout en appliquant ces conditions, à déterminer l’impact du pré-traitement

de congélation sur la cinétique de DO et la qualité organoleptique des graines de grenade,

durant sept heures de traitement. Pour ce faire, nous avons comparé l’évolution des

paramètres physico-chimiques et de transfert de masse des graines fraiches et congelées au

cours de la DO. Nous avons aussi mis en œuvre une technique fine de microscopie

électronique à balayage afin d’observer les modifications structurales des cellules, et

développer une procédure particulière d’analyse de texture des graines de grenade.



78


Graines de grenade congelées

Graines de grenade fraîches

Conditions


Solution (55°Brix): - Saccharose

Temps (min): - 0 - 60 - 240 - 10 - 80 - 300 - 20 - 120 - 360 - 40 - 180 - 420

Température (°C): - 50


Paramètres

Paramètres physico-chimiques : - pH, aw, - MS, Deff, conductivité - °Brix, couleur (L*, a*, b*) - Texture (hardness et toughness) - Structure cellulaire (microscopie électronique à balayage)


Figure 1’ : Différentes étapes du procédé de déshydratation osmotique des graines fraîches et

congelées.

79


Principaux résultats :

Le pré-traitement de congélation des graines a entraîné une réduction du temps de

déshydratation attribuée à une augmentation des coefficients de diffusion de l’eau et de soluté.

Dans le cas des graines congelées, les principales pertes en eau (46% de réduction) et gain en

solide (7% d'accroissement) surviennent pendant les 20 premières minutes du traitement. La

DO des graines fraîches est caractérisée par une cinétique plus lente, mais une modification

finale plus importante (62% de perte en eau). Par conséquent, la congélation est un pré-

traitement intéressant pour favoriser la DO.

La microscopie électronique à balayage ainsi que l'analyse de texture ont montré une

altération physique de la structure des graines après congélation, ayant une répercussion

directe sur la fermeté du fruit. Les mêmes techniques ont également indiqué une modification

de texture/structure induite par le processus de déshydratation osmotique.

80


Osmotic dehydration of pomegranate seeds (Punica

granatum L.): Effect of freezing pre-treatment

*Brahim Bchira, Souhail Besbesb, Hamadi Attiab, Christophe Bleckera,

a Gembloux Agro-Bio Tech, University of liège, Passage des Déportés, 2, B- 5030 Gembloux,

Belgium

b Unité Analyses Alimentaires, Ecole Nationale d’Ingénieurs de Sfax, Route de Soukra, 3038

Sfax, Tunisia


*Fax: +32(0)81/60.17.67


81

Chapitre 3: Effet de la congélation sur la cinétique de transfert de masse ___________________________________________________________________________ Abstract

Osmotic dehydration of pomegranate seeds was compared using fresh and frozen seeds.

The process was carried out at 50°C in a 55°Brix solution of sucrose. Freezing pomegranate

seeds before osmotic dehydration involved an increase of effective diffusivity and a reduction

of dehydration time. The most significant changes of water loss (46 g/100g of fresh seeds

(FS)) and solids gain (7 g/100g of FS) took place during the first 20 min for frozen seeds.

After this period, seeds water loss and solids gain ranged on average close to 43 and 8 g/100g

of FS, respectively. Osmotic dehydration was slower starting from fresh fruits but led to a

higher rate of water loss (62 g/100g of FS) at the end of the process. Both scanning electron

microscopy and texture analysis showed a destruction of cell structure and seed texture during

the pretreatment (freezing). The same techniques also revealed a texture/structure

modification induced by the osmotic dehydration process.

Practical applications

In Tunisia the research of addition value to pomegranate seeds is very limited and

presents a traditional feature such as jam preparation or direct consummation of fruit during

the crop season (between September and December). However, other perspectives of

transformation and exploitation of the pomegranate seeds should be undertaken to give an

added value to this typical fruit. Osmotic dehydration can be an alternative process to increase

the conservation time of fruit and to give a new osmodehydrated fruit that can be included in

some food formulation such as ice-cream, cereals, dairy, confectionery and bakery products.

Keywords: Osmotic dehydration; water loss; solids gain; effective diffusivity; scanning

electron microscopy; texture analysis; pomegranate.

82

Chapitre 3: Effet de la congélation sur la cinétique de transfert de masse ___________________________________________________________________________ 1. Introduction

Pomegranate (Punica granatum L.) is one of the most important fruits in Tunisia. Its

total production in 2008 reached more than 70,000 tons. Pomegranate is composed by a non

edible part formed by 30% of skin (external part) and 13% of internal lamel and an edible part

formed by seeds (50-70%). Pomegranate seeds are composed by 15% pips (woody part), this

part determines the hardness, and 85% pulp (the juicy part) depending on cultivar (Al-

Maiman and Ahmad, 2002).The edible part of the fruit contains considerable amounts of

sugars, vitamins, organic acids, phenolic compounds and minerals (Espiard, 2002).

Pomegranate seeds need preservation methods to increase their shelf-life due to insect

attacks and microorganism growths. Among these methods we notes; drying, freezing,

pasteurization, osmotic dehydration etc. (Raoult-Wack et al., 1991). Osmotic dehydration has

received a considerable attention due to its low energy and cost compared to other

dehydration methods. Osmotic dehydration can be an alternative process to increase the

conservation time of fruits (Kowalska et al., 2008). Therefore, this method allows to consume

pomegranate seeds during the off season (other than September -December). Other benefits of

osmotic dehydration include effective inhibition of polyphenoxidase, prevention of loss of

volatile compounds, even under vacuum and reduction of heat damage to color and flavor

during dehydration (Krokida et al., 2001). The other major application is to reduce the water

activity of many food materials so that microbial growth will be inhibited (Bolin et al., 1983).

Osmotic dehydration gives two major simultaneous counter-current mass transfer fluxes,

namely water flow from the product to the surrounding solution and solute infusion into the

product (Escriche et al., 2000; Lewicki et al., 2005). There is a third flow of natural solutes

such as sugars, organic acids, minerals and salts leaching from the food into the solution

(Waliszewski et al., 1997), which is quantitatively negligible, but may have an important

effect on the organoleptic and nutritional value of the product.

83


Mass transfer in osmodehydrated tissues was described by many scientifics (Lazarides

et al., 1995 and Jiokap Nono et al., 2001). Nowadays, the industry uses osmotic dehydration

for some previously cut fruit like apple, banana, mango, and apricot, amongst others.

However, this process has not been used for the conservation of whole pomegranate seeds by

industrials. Moreover, to our Knowledge, we are the first to have previously studied the

osmotic dehydration of pomegranate seeds (Bchir et al., 2009).

The cellular membrane exerts high resistances to transfer and slows down the rate of

osmotic dehydration (Erle and Shubert, 2001). Therefore pre-treatments such as freezing,

high-pressure, high intensity electric field pulse have been reported to enhance mass transfers

(Tedjo et al., 2002; using mangos fruit).

The aim of this research was to investigate the kinetics of osmotic dehydration and to

determine the influence of freezing, on mass transfer during osmotic dehydration and to

characterize textural and structural change in osmotically dehydrated pomegranate seeds.

2. Material and Methods

2.1. Preparation of pomegranate seeds

Fresh pomegranate fruits (Punica granatum L.) of El Gabsi variety were obtained from

a local research centre in Gabes, Tunisia. Pomegranates fruit were collected at the same

ripening stage, having the same size. The fruit (20 kg) were washed in cold tap water and then

frozen at -50°C. Some pomegranates (20 kg) were conserved at 4°C until analysis. Seeds were

recuperated immediately prior to the osmodehydration process.

2.2. Osmodehydration process

Sucrose was dissolved in water in order to obtain 55°Brix solutions. About 10 g of

seeds was soaked in the sugar solution and were placed in bottles (Schott) of 100 ml. The

volume ratio between the fruit and the sugar solution was kept at one part of fruit and four

parts of solution (1:4). Osmotic dehydration process was conducted during 10 to 420 min in a

84

Chapitre 3: Effet de la congélation sur la cinétique de transfert de masse ___________________________________________________________________________ shaking water bath (GFL instrument D 3006, Germany; oscillation rate 160 rpm) at 50°C

(Bchir et al., 2009).



40, 60, 80, 120, 180, 240, 300, 360 and 420 min for frozen and fresh) and were quickly rinsed

(with distilled water) and the excess of solution at the surface was removed with absorbent

paper. Soluble solids were then measured as described below. The material was weighed

before and after osmodehydration to calculate the percentage of weight reduction (WR). The

moisture content was determined to calculate water loss (WL) and solids gain (SG), based on

the following equations (Mavroudis et al., 1998):

WR g/100g of fresh seeds = 100.)(

i

fi

WWW −

(1)

WL g/100g of fresh seeds = SG + WR (3)

SG g/100g of fresh seeds = 100.)(

i

sisf

WWW −

(2)



the mean of three determinations.

2.4. Mathematical modelling

Diffusion coefficients were calculated using Fick’s second law equation applied to a

sphere, by modifying the Fourier number , using shape factor, due to an

ellipsoids shape of pomegranate seeds.

2/0 RtDFffe=

The following assumptions were taken into account in order to determine the effective

diffusion coefficient: homogeneous body, the external resistance to mass transfer was

negligible compared with internal resistance, the initial moisture content was uniform

throughout the sample, and the diffusion coefficient was constant (Crank, 1975).

85


The solution for Fick’s equation law for diffusion out of a sphere is given by equation 4,

using the following boundary conditions of internal resistance (Luikov, 1968; Crank, 1975):

Uniforme initial amount: t = 0, 0<r<R, MC(t)=MC0

Symmetry of concentration: t >0, r = 0, 0)(=

∂∂

rtMC

Equilibrium content at surface: t >0, r = R, MC(t)=MCeq

[ ]∑∞

=

−=−

−=

10

2

0

exp)(

nnn

eq

eqAouS FB

MCMCMCtMC

W μ (4)

Where: Bn= 6/μn2; μn= nП; F0= Deff, AorS t/R2; n = 1, 2, 3,…

Where Deff, AorS is the effective diffusivity of water loss or solids gain (m2 s-1); n is the

number of series terms, R is the equivalent radius of sphere (m), r is the distance in the radius

direction (m), and t is the time (s). WA and WS are the dimensionless amount of water loss and

solids gain, respectively; MCeq is the equilibrium amount of moisture or solids content (g/g

DM) calculated using Peleg’s equation.

Peleg’s equation parameters were obtained using eqn (5) (Peleg, 1988). This two-

parameter model was redefined by Palou et al. (1994) in terms of soluble solids and moisture

content and describes sorption curves that approach equilibrium asymptotically.

tkktMCtMC

210)(

+±= (5)

Where )(tMC is the amount of moisture or solids at the instant t (g/g dry matter

(DM)), is the initial amount of moisture or solids (g/g DM), k1 and k2 are Peleg’s

parameters and t is the time (s).

0MC

The value of the amount moisture or solids content at the equilibrium was then

calculated using eqn 6 (Park et al., 2002).

20

210

1)(limk

MCtkk

tMCMCteq ±=

+±=

∞→ (6)

86


As stated earlier, in this work pomegranate seeds were assumed to be ellipsoids, having

three characteristic diameters (2rM1~2rM2≤2RM ). According to Alvarez et al. (1995) the

diffusion coefficient (D’eff) must be corrected by the factor Ψ2 when the product shape can be

assumed as an ellipsoid. The shape factor (Ψ) eqn 7 is defined as Ss/Sp, and Ss is the surface

area of a sphere of volume equal to that of fruit with surface area Sp, which is assumed to be

an ellipsoid (Alvarez et al., 1995). The intrinsic diffusivity Deff is given by Ψ2 D’eff (Alvarez et

al., 1995).

21

2

2

2

)/(1sin)/(1

22

4

MM

MM

MM

e

p

s

RrRr

Rrr

RSS

−⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−+

==−ππ

πψ (7)

2.5. Physico-chemical analysis



Dry matter and moisture contents

The dry matter was calculated using oven drying, according to AOAC method 934.01

(1990). Approximately, 5 g of seeds were oven dried at 103°C ± 2°C, until constant weight.

Moisture content was estimated by difference of mean values, 100% - % of dry matter

(Chenlo et al., 2007).

Protein content


general factor (6.25) (AOAC, 1990, method number: 920.152).

Lipids content

To determine total lipid content, about 5g of seeds were mixed with chloridric acid. Fat

was then extracted with a soxtherm automatic S 306 AK solvent extractor equipped with six

Soxhlet posts (Gerhardt soxtherm, Switzerland) and command unit (Gerhardt Variostat,

87

Chapitre 3: Effet de la congélation sur la cinétique de transfert de masse ___________________________________________________________________________ Switzerland) using petroleum ether 40-60°C in each Soxhlet post. The result was expressed as

the percentage of lipids in the dry matter.

Ash content

To determine ash content, about 5g of seeds were incinerated in a muffle furnace

(Gelman, Germany) at about 550°C for 8h. The total ash content was expressed in dry weight

percentage (AOAC, 1990, method number: 940.26).

Carbohydrate content


percentages of moisture, ash, proteins and lipids) (Barminas et al., 1999).

Total soluble solids and water activity

The soluble solids of seeds and osmotic solution were determined by measuring the

°Brix at 20°C using an ATGO digital refractometer (DBX-55, Switzerland). Water activity

(aw) was measured using an aqualab (Switzerland) instrument at 20 °C.

pH and conductivity


(Switzerland) at 20°C. Conductivity was measured using a conductimeter LF 597-5

(Germany) instrument at 20°C.

Color





(yellowness).

88

Chapitre 3: Effet de la congélation sur la cinétique de transfert de masse ___________________________________________________________________________ Microscopic observations

Microscopic observations were carried out according to Attia et al. (1993). Samples

were deposed on supports and underwent successively three operations. First, the dehydration

was carried out in an increasing ethanol gradient, going from 10 to 100% (volume/volume).

Secondly, the drying was performed at critical point CO2 (75 bars, 40 to 42 °C) using an Bal-

Tec apparatus (Bal-Tec CPD030). Finally, the metallization with gold was carried out using

an Bal-Tec apparatus (Bal-Tec MET 020). The observations were performed with a scanning

electron microscope (SEM) Philips XL 30 (Philips, France).

Texture analysis

Texture analysis were carried out using a texture profile analyzer (TA.XT2; Stable

Micro Systems, UK), with 75 mm compression probe. The operating conditions of the

instrument were as follows: 1.5 mm/s pre-test speed, 0.5 mm/s test speed, 10.0 mm/s post-test

speed, 0.10 N trigger force and 85% sample deformation. The hardness and toughness of

seeds were the means of 20 single seed measurements. Hardness (N) of seed was taken as the

force in compression which corresponded to the breakage of samples, while the toughness (N

mm) is the energy required to crush the sample completely (Kingsly et al., 2006; Al-Said et

al., 2009).


Statistical analyses were carried out using a statistical software program (SPSS for windows

version 11.0). The data was subjected to analysis of variance using the general linear model

option (Duncan test) to determine significant differences between samples (P<0.05).

89

Chapitre 3: Effet de la congélation sur la cinétique de transfert de masse ___________________________________________________________________________ 3. Results and discussion

Chemical composition of pomegranate seeds before osmotic dehydration is shown in

table 1. Seeds are rich in carbohydrate (~85% DM) followed by protein (~8% DM), lipid

(~5% DM) and ash (~3% DM). This composition is quite similar to pomegranate seeds

cultivated in Egypt (El-Nemr et al., 1990). Seeds also had a low pH (~4.17), this could be

attributed to their high content in organic acids such as citric and malic acids, which are

important for sensory properties and preservation (Poyrazoglu et al., 2002).

Table 1. Chemical characteristic of pomegranate seeds

Seeds Dry matter (DM %) 16.00 ± 0.05 Protein g/100g DM 7.79 ± 0.86 Lipid g/100g DM 4.55 ± 0.40 Ash g/100g DM 2.87 ± 0.19 Carbohydrate g/100g DM 84.93 ± 0.25 pH 4.17 ± 0.20 aw 0.989 ± 0.002 °Brix 15.50 ± 0.09


The effect of dehydration time on water loss (WL), weight reduction (WR), and solids

gain (SG) was studied starting from both frozen and fresh pomegranate seeds at 50°C, using

sucrose solution. The water loss increased in time during the osmotic process using fresh

seeds, but mostly before 120 min, reaching 47 g/100g of fresh seeds (FS). After this period,

only a slight increase was observed during the rest of the process reaching 62 g/100g of FS

after 420 min (Fig. 1). The WR and SG for fresh seeds followed also the same evolution

reaching 55 and 7 g/100g of FS at the end of the process, respectively. Several works reported

similar dewatering and impregnation kinetics for osmotic dehydration of many fresh fruits

(Khoyi and Hesari, 2007 and Falade et al., 2007).

90


0

10

20

30

40

50

60

70

0 50 100 150 200 250 300 350 400 450

Time (min)

SG

and

WL

(g/1

00g

of fr

esh

seed

s)

Figure 1. Comparison of WL and SG using fresh (× WL, ● SG) and frozen (ΔWL, ○SG) seeds during osmotic dehydration process.

Using frozen seeds, the most significant changes took place during the first 20 min of

dewatering as shown in Fig. 1. During this time, WL in seeds was 46 g/100g of FS, and after

that it varied slightly and ranged on average close to 43 g/100g of FS. The same trend was

also observed for WR. Under the same conditions, SG of frozen seeds was also increased

significantly during the first 20 min reaching 7 g/100g of FS, and tended to be stable at the

end of the process. Similar trends for osmotic process of frozen pumpkin, apple and carrot

were observed by Kowalska and lenart, (2001) and Kowalska et al. (2008).

The increase of WL, as shown in Fig. 1, at the beginning of the process is due to the

large osmotic driving force between the dilute sap of seeds and the surrounding hypertonic

medium. Then after this period, the slower water transfer is mainly influenced by the

reduction of the difference in concentration between the seeds and osmotic solution which

could involve a slower driving force. Indeed, the reverse trend of °Brix observed in seeds and

91

Chapitre 3: Effet de la congélation sur la cinétique de transfert de masse ___________________________________________________________________________ osmotic solution confirms these facts (Table 2). During the first 20 min, water loss was the

results of both osmotic process and defrosting of seeds. The trend observed in SG (Fig.1)

could be explained by migration of sucrose to the seeds through the cell wall and

accumulating between the cell wall and the cellular membrane, due to the important gradient

of sugar between the seeds and the osmotic solutions.

Using fresh seeds the dewatering was slow at the beginning of the process compared to

frozen seeds. In fact, during 60 min the percentage of WL in frozen seeds was 11 g/100g of

FS higher than observed in fresh seeds. However, a higher rate of WL (62 g/100g of FS) was

obtained using fresh seeds after 420 min of the process. These results are in accordance with

previous findings (Kowalska et al., 2008) comparing osmotic dehydration of fresh and frozen

pumpkin and carrot. Moreover, Fig. 1 shows that SG in fresh seeds was lower than those of

frozen seeds. In fact after 60 min of the process, SG of frozen seeds was 6 g/100g of FS

higher than fresh seeds. Even after 420 min, the percentage of SG of fresh seeds was lower.

So pretreatment before osmotic dehydration proved to increase solids gain in comparison with

samples without pretreatment. Our results were similar to those observed by Kowalska et al.

(2008) showing that the use of freezing before OD increased the percentage of solids gain.

Differences in water loss and solids gain between fresh and frozen seeds can be

explained by the damaged cell structure of frozen fruit due to freezing. In fact, Delgado and

Rubiolo (2005) showed that during freezing a part of the aqueous fraction freezes out and

forms ice crystals that damage the integrity of the cellular compartments. Thus, the cellular

membranes loose their osmotic status and their semi-permeability, favouring a large osmotic

driving force between the dilute sap of the seeds and the surrounding osmotic solution

(Kovacs and Meresz, 2004; Torreggiani and Bertolo, 2001). This fact was behind the higher

WL and SG for frozen seeds, at the begging of the process. The difference in water lost from

92

Chapitre 3: Effet de la congélation sur la cinétique de transfert de masse ___________________________________________________________________________ 180 min was due to the fast reduction of the difference in concentration between seed and

osmotic solution using frozen seeds (Table 2).

Different studies on various fresh fruit dewatering (papaye apple, pumpkin, carrot, and

kiwi) mentioned that the most significant changes were observed between 30 and 100 min

using sucrose solutions (60 and 62 °Brix) at different temperatures (30 and 50°C) (Saurel et

al., 1994; Kowalska et al., 2008). All these authors reported that WL ranged between 39 and

50 g/100g of FS. Results obtained in this study showed that WL in frozen seeds was in

accordance with the literature, however fresh seeds were slightly higher. This comparison

should however take into account the experimental conditions and the differences in states of

the various fruits. Indeed, given their small size, pomegranate seeds can be kept intact and do

not need to be cut. On the contrary, in literature most fruits (apple, kiwis…) used for the

osmotic process have to be cut into small volumes, due to their large sizes. Thus, cutting these

fruit creates more external lesions leading to a higher contact of cells with the osmotic

solution in a shorter time. The longer time needed to dewater pomegranate fresh seeds could

be explained by their external membrane ability to protect cells.

The value of water loss was higher than solids gain and depended on the advancement

of the dewatering process. In addition effective diffusivity of water was also higher than that

of the solids (Table 3). According to Vial et al. (1991) the difference between WL an SG was

essentially due to the diffusional differences between water and sugar as related to their

different molar mass as well as to the presence of “semi-permeable” vegetal membranes.

Raoult-Wack et al. (1989) described an antagonistic effect of water and solute transfer,

signaling that this is probably due to the combination of sugar penetration by diffusion and

sugar transportation by the water out-flow, as a function of the water flow rate.

93


Table 2. Evolution of osmotic dehydration parameters in sucrose solution using frozen and fresh seeds

Frozen seeds

0min 10min 20min 40min 60min 80min 120min 180min 240min 300min 360 min 420min

°Brix of

solution

55.00

±0.00e

52.30

±0.14d

49.70

±0.07c

49.30

±0.01bc

49.10

±0.21ab

48.90

±0.28ab

49.00

±0.21ab

48.60

±0.21a

48.70

±0.35a

48.60

±0.21a

48.60

±0.14a

49.00

±0.28ab

°Brix of seeds 15.50

±0.09a

29.30

±0.14b

41.60

±0.14c

45.30

±0.14d

46.40

±0.14e

46.90

±0.14e

49.10

±0.07f

49.20

±0.70f

49.30

±0.35f

49.20

±0.07f

49.30

±0.14f

49.00

±0.28f

pH of solution 8.27

±0.03c

4.71

±0.02b

4.60

±0.27ab

4.50

±0.20a

4.50

±0.01ab

4.40

±0.02a

4.30

±0.07a

4.40

±0.01a

4.30

±0.07a

4.40

±0.02a

4.40

±0.01a

4.39

±0.14a

Conductivity

of solution

(μs/cm)

0.90

±0.01a

26.57

±0.04b

35.50

±0.35c

39.00

±1.06d

40.10

±0.07de

40.50

±0.21def

41.20

±0.28defg

42.90

±0.84fg

42.10

±2.19efg

42.90

±1.13fg

43.60

±1.13g

48.2

±2.55h

Dry matter of

seeds (%)

16.00

±0.05a

34.88

±1.05b

42.70

±0.08c

46.80

±0.65d

47.80

±0.83de

48.30

±0.41ef

49.50

±0.76f

49.29

±0.03f

49.30

±0.17f

49.50

±0.16f

49.60

±0.17f

49.60

±0.01f

L* 65.76

±0.01f

62.21

±0.76 e

61.40

±0.96cde

60.90

±0.40cde

60.50

±0.47bcde

59.80

±0.75bcd

58.80

±0.15b

59.90

±0.58bcd

59.90

±0.05bc

56.20

±0.60a

55.40

±1.63a

55.20

±1.98a

a*

-0.60

±0.02a

2.09

±0.02 b

2.40

±0.92b

2.90

±0.13bc

3.30

±0.46 bcd

4.10

±0.30 bcde

4.60

±0.24 bcde

5.30

±0.14bc

5.30

±0.77 cde

5.90

±0.34de

6.30

±0.17e

6.38

±0.79e

CieLab-

coordina

-te of

solution

b* 3.60

±0.01a

5.36

±0.15b

6.50

±0.26bc

7.80

±0.07cd

8.10

±0.17de

9.60

±1.98e

9.30

±0.09de

9.30

±0.10de

9.30

±0.22ed

9.40

±0.43ed

9.40

±0.02ed

9.42

±0.92ed

Fresh seeds

°Brix of

solution

55.00

±0.00g

54.90

±0.25g

54.9

±0.14g

54.75

±0.07g

54.55

±0.21g

53.95

±0.07f

53.30

±0.14e

52.75

±0.35d

52.05

±0.07c

52.40

±0.28cd

51.15

±0.07b

48.45

±0.31a

°Brix of seeds 15.50

±0.09a

17.55

±0.07ab

19.45

±0.07ab

20.30

0.01b

25.45

±0.35b

26.95

±0.07c

30.80

±0.07cd

34.60

±0.63d

40.90

±0.42e

45.80

±0.28f

48.4

±0.56f

48.40

±0.57f

pH of solution 8.27

±0.03e

5.82

±0.03d

5.62

±0.02cd

5.15

±0.64bc

5.39

±0.02cd

5.21

±0.01c

4.70

±0.24ab

4.60

±0.11a

4.60

±0.09a

4.60

±0.08a

4.40

±0.02a

4.38

±0.27a

Conductivity

of solution

(μs/cm)

0.90

±0.01a

3.13

±0.04b

3.61

±0.23bc

4.10

±0.17bc

4.83

±0.07cd

6.00

±0.31de

7.10

±0.96e

9.10

±0.13f

10.70

±0.60g

13.50

±1.45h

15.20

±0.46i

15.50

±0.99i

Dry matter of

seeds (%)

16.00

±0.05a

18.25

±0.35ab

20.79

±0.10b

23.53

±1.01c

30.94

±1.44d

33.06

±0.86d

49.62

±0.24e

50.52

±0.47e

51.73

±1.41ef

53.33

±0.55f

53.94

±0.56f

53.93

±0.45f

L* 65.76

±0.01e

64.68

±0.16cde

64.5

±0.12de

64.00

±0.96cde

64.10

±0.69cd

64.00

±0.51cd

64.20

±0.27c

63.6

±0.12cd

63.60

±0.12bc

63.80

±0.83bc

62.50

±0.18ab

61.33

±1.43a

a*

-0.60

±0.02a

-0.59

±0.14a

-0.59

±0.01a

-0.53

±0.02a

0.16

±0.02b

0.35

±0.02bc

0.80

±0.14cd

0.50

±0.22bc

1.00

±0.33d

1.50

±0.01e

1.80

±0.44e

1.81

±0.24e

CieLab-

coordina

-te of

solution b* 3.60

±0.01a

3.54

±0.01a

3.48

±0.99ab

4.68

±0.01b

5.41

±0.13b

5.22

±0.52b

5.50

±0.19b

5.81

±0.44b

6.03

±0.43b

6.38

±0.63b

6.08

±0.80b

6.12

±0.47b

Means in line with different letters are significantly different (P<0.05)

94


Table 3. Water and solids effective diffusivities calculated by Fick’s model and values of

Peleg’s equation parameters

Water loss Solids gain Deffw (m2s-1) R2(%) K1 K2 R2 (%) Deffs (m2s-1) R2(%) K1 K2 R2(%)Frozen 9.44×10-12 99.92 28.05 0.23 99.99 4.81×10-12 99.72 12.08 0.03 99.92 Fresh 0.58×10-12 98.96 670.00 0.21 99.66 0.20×10-12 98.12 318.63 0.02 99.51

3.2. Evaluation of the Peleg and Fick mathematical models

The Peleg’s equation parameters (K1 and K2) were determined for water loss and solids

gain (Table 3). This model showed a good fit to the experimental data, with correlation

coefficients (R2) close to 0.99. Effective diffusivity values for water and solids were

calculated using Fick’s model, which also presented a good fit to experimental data, showing

an average correlation coefficients (R2) close to 0.99. Freezing pretreatment involved a strong

increase of effective diffusivity of water and solids compared to untreated samples (Table 3).

3.3. Physico-chemical characteristics of the osmodehydrated fruit preparation

The effect of time on physico-chemical characteristics of the osmodehydrated seeds

(frozen and fresh) and osmotic solution during osmotic process are shown in table 2. Using

frozen seeds, the most significant change occurred during the first 20 min of dewatering. In

fact, statistical analysis shows a significant difference (P<0.05) between 0, 10 and 20 min for

all parameters. The fresh seeds, on the contrary, showed a progressive evolution. At the

beginning of the process the °Brix of the solution decreased as the °Brix of seeds increased,

after that °Brix tended towards an equilibrium. This was a consequence of osmosis, inducing

a balance of concentration between the seeds and the sucrose solution. The decrease of the

osmotic solution pH and the increase of conductivity could be attributed to the diffusion of

some organic acids from pomegranate seeds to the aqueous solution. Moreover, the measure

of color parameters L*, a*, b* showed a slight variation of these parameters. This variation

95

Chapitre 3: Effet de la congélation sur la cinétique de transfert de masse ___________________________________________________________________________ could be explained by a migration of pigment from pulp to solution and the non-enzymatic

browning (Masmoudi et al., 2007).

Compared to fresh seeds, the speed of the osmodehydration process using frozen

seeds was higher. Indeed, using frozen seeds the concentration equilibrium (°Brix) was

reached earlier between the two compartments (seeds and osmotic solution) compared to fresh

seeds. In fact, after 60 min °Brix and pH (solution) for frozen and fresh seeds reached 46 and

25°Brix; 4 and 5, respectively. Moreover, conductivity of frozen seeds solution was 6 time

higher than conductivity of fresh seeds solution. These differences could be explained by the

increase of the exchange through the seeds membranes due to the irreversible damage and a

loss of selectivity of cells induced by freezing.

These results showed the utility of freezing for a better transfer of solutes and water

respectively into and out of the fruits.

3.4. Microstructure and texture analysis

To better understand the effect of freezing and osmodehydration treatment at cell level

and on texture characteristics, scanning electron microscopy (SEM) and texture analysis were

used. Fig. 2a shows a control sample of pomegranate seed tissue, which did not receive any

other treatment only the preparation for SEM. The bright regions in the micrograph are

mainly the cytoplasmic membrane and the cell walls; the darker regions are holes where cell

contents were before. Frozen cells have a different appearance than fresh cells (Fig. 2b). In

fact, tissues of fresh seeds showed isodiametric cells with a regular shape and well-organized.

On the contrary, frozen seed cells appeared torn and irregular in shape, due to the loss of

turgor, with the presence of many empty regions (regions which were not occupied by cells).

This difference is due to the freezing treatment. Indeed, empty regions indicated that ice

nucleation and crystal growth damaged the cell wall. Numerous studies described that during

freezing a gradual breakdown in the organization of the protoplasmic structure and, in most

96

Chapitre 3: Effet de la congélation sur la cinétique de transfert de masse ___________________________________________________________________________ cases, the rupture of the plasmalemma with subsequent loss of turgor pressure in cells. In

addition, some degradation and separation of cell walls were also noted (Jewell, 1979;

Torreggiani and Bertolo, 2001). Decompartmentalization caused by ice crystals prevents the

return of water to the intracellular medium during thawing, causing loss of turgidity. This has

practical consequences in terms of the loss of the ability to act as a semi-permeable membrane

or diffusion barrier, and also the modification of fruit texture (Kovacs and Meresz, 2004).

a

b

c

d

Figure 2. Scanning electron microscopy photographs of fresh (a), frozen (b) and osmodehydrated fruits prepared with fresh (c) and frozen (d) seeds.

97


Texture analysis showed three zones characteristic of seed compression (Fig. 3). The

first (a) correspond to the pulp resistance (~13 N). The second (b) correspond to the fracture

of pip (~30 N), and the third zone (c) correspond the crushing of seed characterised by the

increased in force through a short distance. In fact, using the same analytical procedure, Al-

Said et al. (2009) showed that pulp and pip hardness varied between 9-14 N and 24-45 N

respectively, for pomegranate seeds cultivated in Oman. The peak force attained during the

test is referred to as hardness and area under the curve as toughness. Statistical analysis shows

a difference (P<0.05) between fresh and frozen seeds (Table 4). Frozen-thawed seeds have the

lower values of both hardness (47±2 N) and toughness (54±3 N mm) then fresh seeds (53±4

N and 74±3 N mm respectively). This can be explained by cells membrane deterioration

during freezing inducing the loss of binding capacity among cell walls, which is in accordance

with the result observed by SEM. As consequence frozen seeds lose their firmness and reduce

their thickness. Hence the probe penetrates more the seed and touches the pip at a smaller

distance compared to fresh seed. In fact, the distance before pip crushing using fresh seeds

(5.45±0.73mm) was higher than frozen seeds (4.34±0.51mm).

Distance (mm)

Forc

e (N

)

c

b a

Figure 3. Characteristic force-distance curve for texture analysis using fresh seeds.

98


Figure 2c and 2d show that osmotic dehydration process changed the tissue structure

compared to an untreated sample. In fact cells appeared irregular in shape, slightly distorted

but well-organized. This fact was probably due to the solubilisation of polysaccharides

(cellulose, hemicellulose and pectin) that composes the cell walls, the water loss and the pre-

concentration of sucrose on the surface of the tissue (Raoult-Wack et al., 1991; Torreggiani

and Bertolo, 2001; Delgado and Rubiolo, 2005). Indeed, pectin is the major constituents of

the middle lamella and thus contributing to the cell adhesion and firmness (Nunes et al.,

2008). Moreover water loss induces the plasmolysis of cells and solids gain gives consistency

to the tissues. Nunes et al. (2008) showed that diffusion of sucrose into the fruit tissue, during

osmotic dehydration process, and its interaction with the cell wall and middle lamella pectic

polysaccharides might result in the formation of a jam-like structure that gives consistency to

the tissues. Likewise, Delgado and Rubiolo (2005) showed that osmotically dehydrated

strawberry tissue with the sucrose concentration used did not greatly affect the tissue

structure.

Textural properties of seeds are closely linked to cellular structure (Torreggiani and

Bertolo, 2001; Sajeev et al., 2004). Indeed, the osmotic dehydration process induced an

increase of textural parameters (toughness and hardness) compared to the untreated sample

(Table 4). Statistical analysis shows a difference (P<0.05) between hardness and toughness of

seeds before and after osmotic process. In fact, hardness and toughness increased to 17 N and

13 N mm using frozen seeds and 36 N-15 N mm for fresh seeds. This could be a consequence

of the exchange (water loss and solids gain) between seeds and osmotic solution. Sajeev et al.

(2004) showed that the increase in textural parameters can be attributed to the dehydration of

cormels (Taro: Colocasia esculenta L.). As a consequence of this exchange, the products will

more or less lose weight and will shrink eventually. Indeed, the peaks of pip crushing using

frozen and fresh seeds were observed at 3.7±0.34 mm and 3.2 ± 0.34 mm, respectively. Thus

99

Chapitre 3: Effet de la congélation sur la cinétique de transfert de masse ___________________________________________________________________________ seeds reduce their thickness between an average close to 0.64 and 2.25 mm, after osmotic

dehydration process, using frozen and fresh seeds respectively.

Table 4. Textural properties of pomegranate seeds

Without treatment Osmotic dehydration treatment

Frozen seeds Fresh seeds Frozen seeds Fresh seeds

Toughness (N mm) 54.55±3.96a 74.22±3.45b 67.21±5.55b 89.59±5.85c

Hardness (N) 46.73±2.47a 53.41±4.23a 63.46±3.04b 90.10±5.01c Means in line with different letters are significantly different (P<0.05)

Fresh osmodehydrated seeds had higher texture parameters compared to frozen

osmodehydrated seeds. Table 4 shows a statistical difference (P<0.05) between hardness and

toughness of osmodehydrated fresh and frozen seeds. These results are in accordance with

previous findings (Van Buggenhout et al., 2007) using fresh and frozen carrots. In fact,

hardness and toughtness of osmodehydrated fresh seed was 26 N and 22 N mm higher than

those observed in osmodehydrated frozen seed, respectively. In addition, thickness of

osmodehydrated fresh seeds was 0.6 mm lower then osmodehydrated frozen seeds. These

differences were due to higher water loss using fresh seeds. In fact, Kingsly et al., (2006)

found that hardness and toughness of pomegranate seeds decreased when the moisture content

increased.

4. Conclusion

Freezing treatment prolongs the conservation of pomegranate seeds, however it involves

a destruction of cell structure and seed texture. As a consequence frozen seeds can not be

consumed directly. However, osmotic dehydration process could add value to frozen

pomegranate seeds. Indeed, freezing before osmotic dehydration provided 1.4 and 3.5-times

more water loss and solids gain respectively, than an untreated sample at the beginning of the

process. So as a consequence, the process could be stopped after 20 min, implying a

100

Chapitre 3: Effet de la congélation sur la cinétique de transfert de masse ___________________________________________________________________________ substantial gain of time and thermal energy. On the contrary for fresh seeds, it is better to

continue the process up to 420 min.

After the osmotic dehydration process, microstructure and texture analysis revealed a

modification of seed texture and cell structure. This could essentially be due to water loss and

solids gain. On the basis of seed texture, osmodehydrated frozen fruit may be considered

more suitable for incorporating in many foods products while osmodehydrated fresh fruit, due

to their hard texture, could be air-dried to give dried fruit.

The finished product has an attractive colour and presents a good texture in mouth, a

pleasant sugar taste and a good aroma. Osmotic dehydration reduced water activity from

0.989 to an average of 0.900. At this aw value a complementary treatment such as drying,

freezing and pasteurisation should be necessary to ensure its good conservation.

101

Chapitre 3: Effet de la congélation sur la cinétique de transfert de masse ___________________________________________________________________________ References:

Al-Maiman, S.A., and Ahmad, D. 2002. Changes in physical and chemical properties during


Al-Said, F.A., Opara, L.U., and Al-Yahyai, R.A. 2009. Physico-chemical and textural quality

attributes of pomegranate cultivars Brown in the Sultanate of Oman. Journal of Food


Alvarez, A .C., Aguerre, R., Gomez, R., Vidales, S., Alzamora, S.M., and Gerschenson, L.N.

(1995). Air Dehydration of Strawberries: effects of Blanching and Osmotic

Pretreatments on the Kinetics of Moisture Transport. Journal of Food Engineering, 25,

167-178.

AOAC. 1990. Official methods of analysis of the Association of Official Analytical Chemists.

Edited by K. Helrich. Published by the Association of Official Analytical Chemists,

Inc., 15th edition, Arlington, Virginia; USA. 1298 pp.

Attia, H., Bennasar, M., Lagaude, A., Hugodo, B., Rouviere, J., and Tarodo De La Fuente, B.

1993. Ultrafiltration with microfiltration membrane of acid skimmed and fat-enriched

milk coagula: hydrodynamic, microscope and rheological approaches. Journal of

Dairy Research, 60, 161-174.

Barminas, J.T., James, M.K., Abubakar, U.M. 1999. Chemical composition of seeds and oil of

Xylopia aethiopica grown in Nigeria. Plant Foods for Human Nutrition, 53, 193-198.

Bchir, b., Besbes, S., Attia, H., & Blecker, C. 2009. Osmotic dehydration of pomegranate

seeds: Mass transfer kinetics and DSC characterization. International Journal of Food

Science and Technology, 44, 2208–2217.

Bolin, H.R., Huxsoll, C.C., and Jacko, R. 1983. Effect of osmotic agents and concentrations

on fruit quality. Journal of food Science, 48, 202-205.

Chenlo, F., Moreira, R., Fernandez-Herrero, C., and Vazquez, G. 2007. Osmotic dehydration

of chestnut with sucrose: Mass transfer processes and global kinetics modelling.


102

Chapitre 3: Effet de la congélation sur la cinétique de transfert de masse ___________________________________________________________________________ Crank, J. (1975). The mathematics of diffusion (2nd ed). Oxford: Clarendon Press.

Delgado, A.E., and Rubiolo, A.C. 2005. Microstructural changes in strawberry alter freezing

and thawing processes. Lebensmittel-Wissenschaft und-Technologie-Food Science and

Technology, 38, 135-142.

El-Nemr, E., Ismail, A., and Ragab, M. 1990. Chemical composition of juice seeds of

pomegranate fruit. Journal of Food Science, 7, 601–606.

Erle, U., and Shubert, A. 2001. Combined osmotic and microwave vacuum dehydration of

apples and stawberries. Journal of Food Engineering, 49, 193-199.

Escriche, I., Garcia-Pinchi, R., and Fito, P. 2000. Osmotic dehydration of kiwifruit (Actinidia

chinensis): fluxes and mass transfer kinetics. Journal of Food Process Engineering, 23,

191-205.

Espiard, E. 2002. Introduction à la transformation industrielle des fruits. TECandDOC-

Lavoisier (pp 181-182).

Falade, K., Igbeka, J., and Ayanwuyi, F. 2007. Kinetics of mass transfer and colour changes

during osmotic dehydration of watermelon. Journal of Food Engineering, 80, 979-

985.

Jewell, G.G. 1979. Fruits and vegetable. In J.G. Vaughan (Ed.), Food Micriscopy (pp. 1-34).

London: Academic Press Inc.

Jiokap Nono, Y., Giroux, F., Cuq, B., and Raoult-Wack, A. 2001. Etude de paramètres de

contrôle et de commande du procède de deshydratation-impregnation par immersion,

sur système probatoire automatise: application au traitement des pommes "Golden".


Khoyi, M., and Hesari, J. 2007. Osmotic dehydration kinetics of apricot using sucrose

solution. Journal of Food Engineering, 78, 1355-1360.

Kingsly, A.R.P., Singh, D.B., Manikantan, M.R. and Manikantan, Jain, R.K. 2006. Moisture

dependent physical properties of dried pomegranate seeds (Anardana). Journal of


103

Chapitre 3: Effet de la congélation sur la cinétique de transfert de masse ___________________________________________________________________________ Kovacs, E., and Meresz, P. 2004. The effect of harvesting time on the biochemical and

ultrastructural changes in Idared apple. Acta Alimentaria, 33, 285-296.

Kowalska, H., Lenart, A., and Leszczyk, D. 2008. The effect of blanching and freezing on

osmotic dehydration of pumpkin. Journal of Food Engineering, 86, 30-38.

Kowalska, H., and Lenart, A. 2001. Mass exchange during osmotic pretreatment of

vegetables. Journal of Food Engineering, 49, 137-140.

Krokida, M. K., Oreopoulou, V., Maroulis, Z.B., and Marinos-Kouris, D. 2001. Effect of

osmotic dehydration pretement on quality of French fries. Journal of Food


Lazarides, H.N., Katsanidis, E., and Nickolaidis, A. 1995. Mass transfer during osmotic pre-

concentration aiming at minimal solid uptake. Journal of Food Engineering, 25, 151-

166.

Lewicki, P.P., and Porzecka-Pawlak, R. 2005. Effect of osmotic dewatering on apple tissue

structure. Journal of Food Engineering, 66, 43-50.

Luikov, A.V. 1968. Analytical Heat Diffusion Theory. Academic Press, Inc., New York, NY.

685 pp.

Masmoudi, M., Besbes, S., Blecker, C., and Attia, H. 2007. Preparation and characterization


Science and Technology International, 13, 405-412.

Mavroudis, N. E., Gekas, V., and Sjohlm, I. 1998. Osmotic dehydration of apples, shrinkage



Nunes, C., Santos, C., Pinto, G. Lopes-da-silva, J.A., Saraiva, J.A., and Coimbra, M.A. 2008.

Effect of candying on microstructure and texture of plums (Prunus domestica L.).

LWT- Food Science and Technology, 41, 1776-1783.

Palou, E., Lopes-Malo, A., Argaiz, A. and Welti, J. (1994). The use of Peleg’s equation to

model osmotic concentration of papaya. Drying Technology, 12, 965-978.

104

Chapitre 3: Effet de la congélation sur la cinétique de transfert de masse ___________________________________________________________________________ Park, K.J., Bin, A., Bord, F.P.R. and Park, T.H.K.B. (2002). Osmotic dehydration Kinetics of

pear D’anjou (Pyrus communis L.). Journal of Food Engineering, 52, 293-298.

Peleg, M. (1988). An empirical model for the description of moisture sorption curves. Journal

of Food Science, 53, 1216-1219.

Poyrazoglu, E., Gokmen, V., and Artik, N. 2002. Organic Acids Phenolic Compounds in

pomegranates (Punica granatum L.) Grow in Turkey. Journal of Food Composition

and Analysis. 15, 567 - 575.

Raoult-Wack, A. L., Guilbert, S., Le Maguer, M., and Rios, G. 1991. Simultaneous water and



Raoult-Wack, A. L., Lafont, F., Rios, G., and Guilbert, S. 1989. Osmotic dehydration: study

of mass transfer in terms of engineering properties. In Drying 89, ed. A. S. Mujumdar

and M. Roques, Hemisphere Publishing Corporation (pp 487 – 495). New York.

Sajeev, M.S., Manikantan, M.R., Kingsly. A.R.P., Moorthy, S.N., and Sreekumar, J. 2004.

Texture Analysis of Taro (Colocasia esculenta L. Schott) Cormel during Storage and

Cooking. Journal of Food Science, Vol. 69, Nr. 7.

Saurel, R., Raoult-Wack, A.L., Rios, G., and Guilbert, S. 1994. Mass transfer phenomena

during osmotic dehydration of apple II. Frozen plant tissue. International Journal of

Food Science and Technology, 29, 543-550.

Tedjo, W., Taiwo, K.A., Eshtiaghi, M.N., and Knorr, D. 2002. Comparaison of pretreatment

methods on water and solids diffusion kinetics of osmotically dehydrated mangos.


Torreggiani, D., and BERTOLO, G. 2001. Osmotic pre-treatments in fruit processing:

chemical, physical and structural effects. Journal of Food Engineering, 49, 247-253.

Van Buggenhout, S. Grauwet, T. Van Loey, A., and Hendrickx, M. 2007. Effect of high-

pressure induced ice I/ice III-transition on the texture and microstructure of fresh and

pretreated carrots and strawberries. Food Research International, 40, 1276-1285.

105


106

Vial, C., Guilbert, S., and Cuq, J. 1991. Osmotic dehydration of kiwi fruits: influence of

process variables on the colour and ascorbic acid content. Science des aliments, 11,

63-84.

Waliszewski, K. N., Texon, N.I., Salgado, M.A., and Garcia, M.A. 1997. Mass transfer in

banana chips during osmotic dehydration. Drying Technology, 15, 2597-2607.

Chapitre 4 : Utilisation du jus de datte comme milieu d’immersion ___________________________________________________________________________

Chapitre 4:

Utilisation du jus de datte comme milieu d’immersion pour la déshydratation osmotique des graines de grenade


Bchir, B, Besbes, S., Karoui, R., Paquot, M., Attia, H., & Blecker, C. (2010).

Osmotic dehydration kinetics of pomegranate seeds using date juice as an

immersion solution base. Food and Bioprocess Technology, DOI:

10.1007/s11947-010-0442-1.

107


Résumé *

Titre : Utilisation du jus de datte comme milieu d’immersion pour la déshydratation

osmotique des graines de grenade


Tout en transposant les conditions mises au point et les méthodologies d’analyse des

chapitres précédents, cette partie visait à valoriser une autre agrofourniture tunisienne : le jus

de datte. Ainsi, nous avons substitué la solution de saccharose utilisée comme milieu

d’immersion, par du jus de datte possédant un extrait sec soluble de départ de ~ 20°Brix et

ajusté à 55°Brix par du saccharose. L’étude de la cinétique de transfert de masse a été basée

sur la détermination de la perte en eau, du gain en solides et de la réduction en poids au cours

du temps. Les produits élaborés ont été caractérisés du point de vue physico-chimique (pH,

aw, MS, Deff, °Brix, couleur). D’autres techniques plus fines ont été développées, la

microscopie électronique à balayage et la texturométrie, permettant d’élucider les

modifications structurales des cellules et texturale de la graine avant et après DO.



108



(-50°C)

Conditions


Solution (55°Brix): Jus de datte: - Deglet-nour - Allig - Kintichi


Temps (min): - 0 - 20 - 80 - 5 - 40 - 100 - 10 - 60 - 120



Paramètres

Paramètres physico-chimiques : - pH, aw, - MS, Deff - °Brix, couleur (L*, a*, b*) - Texture (hardness et toughness) - Structure cellulaire (microscopie électronique à balayage)

Figure 1’ : Procédé de déshydratation des graines de grenade dans du jus de datte.

109



Du fait de la richesse en sucre du jus de datte (~ 53 g de saccharose /100 g MS et ~ 35

g de sucre réducteur /100 g MS). Son utilisation a permis d’apporter ~ 35% du sucre pour la

préparation de la solution d’immersion à 55°Brix, et ainsi de réduire la quantité de saccharose

ajoutée.

L’étude de la cinétique de déshydratation osmotique a montré que les changements les

plus cruciaux du transfert de masse sont intervenus pendant les 20 premières minutes du

procédé, indépendamment des variétés de datte utilisées. Ces conditions ont permis une perte

d'eau de ~ 39 %, et un gain en solides de ~ 6 %. Cette période est suivie d'une phase de

stabilisation, plus lente, amenant la perte en eau à ~ 40 % et le gain en solides à ~ 9 %.

Cependant ces valeurs sont légèrement plus faibles que celles retrouvées lors de l’utilisation

d’une solution de saccharose à 55°Brix.

Une lixiviation des solutés de la graine vers la solution a été observée durant le

procédé. Cette perte n'est quantitativement pas négligeable, et pourrait avoir un impact

important sur les caractéristiques sensorielles et valeurs nutritives des graines.

La microscopie électronique a indiqué que le processus de déshydratation osmotique

induit des modifications structurales des cellules, ce qui est confirmé par les analyses

texturales. Les modifications texturales (hardness et toughness) sont plus faibles que celles

enregistrées lors de l’utilisation d’une solution d’immersion à base de saccharose. Ainsi, la

teneur en saccharose est apparue comme le principal facteur influençant la texture de la

graine, mais aussi la viscosité du jus de datte.

110


Osmotic dehydration kinetics of pomegranate seeds using

date juice as an immersion solution base

*Brahim Bchira, Souhail Besbesb, Romdhane Karouia, Michel Paquotc, Hamadi Attiab,

Christophe Bleckera

a Gembloux Agro-Bio Tech, University of Liege, Passage des Déportés, 2, B- 5030

Gembloux, Belgium

b Laboratory of Food Analyses, National Engineering School of Sfax, Route de Soukra, 3038

Sfax, Tunisia

c Department of Industrial Biological Chemistry, Gembloux Agro-Bio Tech, University of

Liege, Passage des Déportés, 2, 5030 Gembloux, Belgium


*Fax: +32(0)81/60.17.67


111


Abstract:

Pomegranate seeds were osmodehydrated using date juice added with sucrose (final

°Brix: 55) as immersion solution. The kinetics of osmotic dehydration showed that the most

significant changes of mass transfer took place during the first 20 min of the process,

regardless of date juice varieties. During this time, seeds water loss and solids gain were

estimated to ~ 39 % and ~ 6 %, respectively. After 20 min of the process, the percentage of

water loss and solids gain varied slightly and ranged on average close to ~ 40 % and ~ 9 %,

respectively. During osmotic dehydration, there was a leaching of natural solutes from seeds

into the solution, which is quantitatively not negligible, and might have an important impact

on the sensorial and nutritional value of seeds and date juices. Both scanning electron

microscopy and texture (compression) analysis revealed that osmotic dehydration process

induced modifications of seed texture and cells structure. Sucrose was found to be the

essential element which influences the texture of seed and the viscosity of date juice.

Additionally, natural sugar present in date juice permits to substitute 35% of the total quantity

of sucrose added to the osmotic solution.

Keywords: Pomegranate; Date juice; Osmotic dehydration; Texture analysis;

Scanning electron microscopy.

112


1. Introduction

Pomegranate (Punica granatum L.) and date (Phoenix dactylifera L.) have always

played an important part in the economic, the social lives and food habits in Tunisia. In 2008,

the production reached 70,000 and 126,000 tonnes respectively for pomegranate and date.

Pomegranate seeds contain considerable amounts of sugars (e.g., glucose, fructose),

vitamins (e.g., vitamin c, vitamin B3) , organic acids (e.g., oxalic acid, citric acid), phenolic

compounds (e.g., anthocyanin, flavonoid) and minerals (e.g., Magnesium, potassium)

(Espiard 2002; Yunfeng et al. 2006). In Tunisia, research giving added value to pomegranate

seeds is very limited. Traditional use such as jam preparation and/or direct consumption of

fruit during the crop season, is limited to a period between September and December of each

year.

Tunisia is currently classified the 10th world producer and the first exporter of dates.

During 2007-2009, Tunisian production of date has reached an average of 120.000 tonnes per

year with dominance of Deglet Nour variety constituting about 60 % of the total production

due to its very good sensory quality and a high commercial value. In Tunisia, Deglet Nour,

Allig and Kentichi are the most consumed varieties (Besbes et al. 2009). This production

progress is unfortunately accompanied by a substantial increase of loss (about 30% of the

total production) during picking, storage, commercialization and conditioning processes

(Besbes et al. 2005a). The lost dates, commonly named “date by-products”, are not consumed

by humans due to their inadequate texture (too soft or too hard), or visual properties (Besbes

et al. 2009). Up to now, “date by-products” are discarded or utilized in limited cases for

animal feed. Owing to their high content in sugar, dates were also used for the preparation of

some food products (e.g., date juice, syrup, date preserves or date jellies). Studies concerning

the use of these by-products to develop new products are scarce and are concerned mostly

with metabolites or biomass production (Besbes et al. 2009; Besbes et al. 2005b).

113


Bchir et al. (2009) showed the possibility of pomegranate seeds to be dehydrated

osmotically using different osmotic solutions (55°Brix) (Sucrose, glucose and mix

sucrose/glucose solutions). In the research studies of Besbes et al. (2009) a high level of

carbohydrate (total sugar ~ 86 g/100 g dry matter; total dietary fiber ~ 9 g/100 g dry matter) in

date was described. Due to the high sugar content, date is used like source of sugar more than

like fruit (direct consumption) (Barreveld 1993). Indeed, dates were used to confer to milk

and tea a sweetened taste appreciated by the consumers (Munier 1973).

Nowadays, the industry uses osmotic dehydration for some previously cut fruits like

apple, banana, mango, apricot and strawberry among others, to obtain fruits with a good

aroma, and nutritional content. In addition, osmotic dehydration is a pre-drying step for

textural reasons, allowing to reduce convective air drying due to infused sugar binding

available water and thus partially reducing water activity. The infused solids provide a

superior texture to many dried fruits. Regarding tart or astringent fruits, such as cranberry and

tart cherry, the osmotic pre-treatment provides a favorable acid:sugar ratio resulting in an

overall better flavour.

This paper reports on value addition to pomegranate seeds and date juice from the most

produced and consumed Tunisian varieties (Date: Delglet Nour, Allig and Kentichi;

Pomegranate: El Gabsi) through their use in the formulation of dehydrated fruits using

osmotic dehydration process. The kinetics of osmotic dehydration, physico-chemical

characteristics, textural and structural change of osmodehydrated fruits was investigated.

2. Material and methods

2.1. Samples

2.1.1. Pomegranate seeds

Fresh pomegranate fruits (Punica granatum L.) of El Gabsi variety were obtained

from a research centre in Gabes (Tunisia). Pomegranate fruits were collected at full ripening

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stage, having the same size. Pomegranate is composed of a non edible part formed by 30% of

skin (external part), 13% of internal lamel and an edible part formed by seeds (50–70 %).

Pomegranate seeds are composed of 15 % pips (woody part), that determines the hardness,

and 85 % pulp (the juicy part) (Al-Maiman & Ahmad, 2002). Seeds have an ellipsoids shape,

13±1 mm length, 7±1 mm breadth, 628±2 kg/m3 bulk density, and 2±0.2 mm of pip thickness.

An average weight of an individual seed was 0.504±0.04 g.

Twenty kilograms (20 kg) of pomegranate were washed in cold tap water and then

frozen at -50 °C. A digital thermometer BT20 (Bresso, Italy) was placed in the pomegranate

core to measure the temperature changes during one hour of thawing at room temperature.

Temperature of pomegranate core reached -7.5 °C, after one hour of thawing. Seeds were

recuperated immediately prior to the osmotic dehydration process.

2.1.2. Date by-products

Date by-products (Phoenix dactylifera L.) of ‘‘Deglet Nour’’, ‘‘Kintichi’’and ‘‘Allig’’

varieties were obtained from the National Institute of Arid Zone (Degach, Tunisia). One batch

of 20 kg (of each variety) of date fruits having the same origin and ripening stage was used.

Dates were directly pitted, washed in running tap water at 25 °C, and left for 24 h at an

ambient temperature of 25 °C. Then, dates pulps were milled to obtain date paste, which was

stored at -20 °C until analyses (Masmoudi et al. 2007).

2.1.3. Date Juice Preparation

The date juice was prepared by adding water to date paste at a ratio of 3:1 (v/w) as

described by Masmoudi et al. (2007) and Yousif et al. (1990). The date paste water mixture

was boiled gently with continuous stirring for 5 min. The extract was filtered through fine

mesh cheesecloth. The obtained date juice presented total soluble solids about 21 °Brix. It was

cooled at room temperature and then stored at -20 °C until analyses.

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2.2. Osmodehydrated fruit preparation

2.2.1. Osmotic solution preparation

Sucrose was dissolved in date juice (total soluble solid varied between 19.03 and

24.70 °Brix) in order to obtain 55 °Brix solution at 25°C. It was then kept at 4 °C until

analyses and use. 55 °Brix was selected as an average of the values used in the literature. In

fact, osmotic solution concentration varied between 50 and 70 °Brix, depending on the fruit to

be dried (Saurel et al. 1994; Corrêa et al., 2010).

2.2.2. Osmodehydration process

About 10 g of seeds was soaked in the osmotic solution and were placed in bottles

(Schott) of 100 ml. The volume ratio between the fruit and the sugar solution was kept at one

part of fruit and four parts of solution (1:4). Osmotic dehydration process was conducted

during 5 to 120 min in a shaking water bath (GFL instrument D 3006, Germany; oscillation

rate 160 rpm) at 50°C (Bchir et al. 2009; Bchir et al. 2010).



40, 60, 80, 100 and 120 min) and were quickly rinsed (with distilled water) and the excess of

solution at the surface was removed with absorbent paper. Soluble solids were then measured

as described below. The material was weighed before and after osmodehydration to calculate

the percentage of weight reduction (WR). The moisture content was determined to calculate

water loss (WL) and solids gain (SG), based on the following equations (Mavroudis et al.

1998):

WR g/100g of fresh seeds = 100.)(

iWfWiW −

(1)

SG g/100g of fresh seeds = 100.)(

iWsiWsfW −

(2)

WL g/100g of fresh seeds = SG + WR

(3)

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the mean of two determinations.

2.4. Physico-chemical analyses

All analytical determinations were performed in duplicate. Values were expressed as the


Dry matter and moisture contents

The dry matter (DM) was calculated according to AOAC (AOAC 1990) method 934.01

(1990). Approximately, 5 g of samples were oven dried at 103 °C ± 2 °C, until constant

weight. Moisture content was estimated by difference of mean values, 100 % - % of DM

(Chenlo et al. 2007).

Protein content


general factor (6.25) (AOAC 1990, method number: 920.152).

Lipids content

To determine total lipid content, about 5 g of samples were mixed with chloridric acid.

Fat was then extracted with a soxtherm automatic S 306 AK solvent extractor equipped with

six soxhlet posts (Gerhardt soxtherm, Switzerland) and command unit (Gerhardt Variostat,

Switzerland) using petroleum ether 40-60 °C in each Soxhlet post. The result was expressed

as the percentage of lipids in the DM (Bchir et al. 2009).

Ash content

To determine ash content, about 5 g of samples were incinerated in a muffle furnace

(Gelman, Germany) at about 550 °C for 8h. The total ash content was expressed in dry weight

percentage (AOAC 1990, method number: 940.26).

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Carbohydrate content


percentages of moisture, ash, proteins and lipids) (Barminas et al. 1999).

Determination of individual sugars

Sugars were extracted with 10 ml of ethanol solution (80%) according to Bouabidi et al.

(1996). The extracts were centrifuged (2000 tr/min, 30 min) and filtered (0.45 μm). Sucrose,

glucose, and fructose were analyzed with a High Performance Anion Exchange

Chromatography coupled with Pulse Amperometric Detection (HPAEC-PAD) on a Dionex

DX500 chromatographic system. The flow rate was 1 ml/min, the pressure and temperature

were 1000 psi and 80 °C, respectively. External standards of fructose, glucose, and sucrose

were used for quantification.

Total soluble solids and water activity

The soluble solids of pomegranate seeds juice, date juice and osmotic solution were

determined by measuring the °Brix at 25 °C using an ATAGO digital refractometer (DBX-55,

Switzerland). Water activity (aw) of seeds and date juice were measured using an aqualab

(Switzerland) instrument at 20 °C.

pH, conductivity and acidity


(Switzerland) at 25 °C. Conductivity was measured using a conductimeter LF 597-5

(Germany) instrument at 25 °C. Acidity was determined by titration with Na OH (0.1N) to pH

= 8.1 ± 0.2 as described by Grigelmo and Martin (Grigelmo and Martin 1999).

Color




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(yellowness). The hue angle (h°ab) and chroma or intensity (C*) were calculated according to

the following equations:

(5) h° = arctan (b*/a*) (4) C*= (a*2+b*2)1/2

Viscosity

Viscosity of osmotic solution was measured at 25 °C with a CV 120 Bohlin rheometer

(Bohlin instruments, UK) using a cone plate geometry (radius=40 mm, angle=4 °). The gap

between the cone and plate geometry was set to 150 μm. Shear rates from 0 to 70 s−1 were

utilized for flow behaviour determination.

Microscopic observations

Microscopic observations were carried out according to Attia et al. (1993). Samples

were deposed on supports and underwent successively the following three operations. First,

the dehydration was carried out in an increasing ethanol gradient, going from 10 to 100 %

(volume/volume). Secondly, the drying was performed at critical point CO2 (75 bars, 40 to 42

°C) using a Bal-Tec apparatus (Bal-Tec CPD030). Finally, the metallization with gold was

carried out using a Bal-Tec apparatus (Bal-Tec MET 020).

The observations were performed with a scanning electron microscope (SEM) Philips

XL 30 (Philips, France).

Texture analysis


Micro Systems, UK), with 75 mm compression probe. The operating conditions of the

instrument were as follows: 1.5 mm/s pre-test speed, 0.5 mm/s test speed, 10.0 mm/s post-test

speed, 0.10 N trigger force and 85 % sample deformation. The hardness and toughness of

seeds were the means of 20 single seed measurements. Hardness (N) of seed was taken as the

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force in compression which corresponded to the breakage of samples, while the toughness (N

mm) is the energy required to crush the sample completely (Kingsly et al. 2006; Al-Said et al.,

2009).


Statistical analyses were carried out using a statistical software program (SPSS for

windows version 11.0). The data was subjected to analysis of variance (ANOVA) using the

general linear model option (Duncan test) to determine significant differences between

samples (P<0.05).

3. Results and discussion

3.1. Physico-chemical properties of seeds and date juices

The composition of pomegranate seeds and date juices from studied cultivars was

investigated. Results showed predominance of carbohydrate in seeds (84.93±0.25 g/100 g

DM) and a significant amount of protein (7.79±0.86 g/100 DM). Pomegranate seeds also

contained a relatively low level of ash (2.87±0.19 g/100 g DM) and lipid (4.55±0.40 g/100 g

DM). Carbohydrate level showed a low content of sucrose (1.09±0.11 g/100 g DM) and a

high level of glucose and fructose (i.e., respectively, 27.95±1.86 and 32.90±2.47 g/100 g

DM), and is in accordance with Fadavi et al. (2005) who found a similar content of glucose

and fructose from pomegranate seeds cultivated in Iran. Seed composition is similar to that

found in literature. Indeed, carbohydrate, protein, lipid and ash values varied respectively

between, 76.0 % - 86.0 %; 4.4 % - 8.7 %; 1.3 % - 9.1 %; 2.6 % - 3.5 % of (DM) (Espiard

2002; Fadavi et al. 2005). Seed had a low pH (4.17±0.20) this could be attributed to their high

content in organic acids such as citric and malic acids, which are important for sensory

properties and preservation (Poyrazoglu et al., 2002).

Date juices exhibited a high content of carbohydrate (~96.14 - 97.60 g /100 g DM) and

a total soluble solid varied between 19.03 and 24.70 °Brix. Lipid, ash protein and acidity

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contents were relatively low (~0.14 - 0.17; ~1.23 - 2.34; ~0.76 - 1.37 g/100 g DM and ~8.3

meq/100g respectively). A slight difference between varieties was observed. The composition

is higher than that found with Yousif & Al-Ghamdi (1999) and Al-Farsi (2003) for Sefri,

Sullag and Habash date varieties. Using the same method of juice extraction and the same

date variety, Masmoudi et al. (2007) showed a relatively high sucrose content (48.43 g/100 g

DM) flowed by fructose and glucose (37.19 g/100 g DM) in juices of Deglet Nour date

variety. The high content of natural sugar could allow an added value to date juice (e.g., used

as immersion solution). In addition, starting with date juice permitted to reduce by 35 % the

amount of sugar added to the osmotic solution.

3.2. Kinetics of osmotic dehydration

3.2.1. Mass transfer kinetics

The osmotic process was studied in terms of water loss (WL), solids gain (SG) and

weight reduction (WR). The values of these parameters were calculated according to Eqs. (1),

(2) and (3). An increase on WL, SG and WR of seeds were observed with the increase of

immersion time for all process conditions (using different date juices), at the beginning of the

osmotic dehydration process. Fig. 1 shows that the most significant changes took place during

the first 20 min of the process. During this time, WL in seeds was estimated to ~ 39 %. After

this period of dehydration, the percentage of water loss varied slightly and ranged on average

close to ~ 40 %. The same trend was observed for WR. Using the same conditions, SG was

increased significantly during the first 20 min reaching ~ 6 %. However, a slight increase of

SG was observed during the rest of process tending to be stable at the end. Statistical analyses

showed a significant difference during the first 20 min, regardless the considered parameters.

Different studies on various osmodehydrated fresh fruit (e.g., papaye apple, pumpkin, carrot,

guavas and kiwi) mentioned that the most significant changes were observed between 30 and

100 min using sucrose solutions (50 and 62 °Brix) at different temperatures (30 and 50 °C)

121


(Saurel et al. 1994; Kowalska and Lenart 2001; Kowalska et al. 2008; Corrêa et al. 2010). All

these authors reported that WL ranged between 39 and 50 g/100 g of fresh matter. Results

obtained in this study showed that WL in seeds was in accordance with the literature. Bchir et

al. (2009) showed a slightly higher result (43 % of WL and 9 % of SG after 20 min of the

process) but a similar dewatering and impregnation kinetics for the osmotic dehydration of

pomegranate seeds in sucrose solution at 50°C; this difference could be explained by the

difference in the solids soluble composition of different osmotic solutions.

0

5

10

15

20

25

30

35

40

45

50

5 10 20 40 60 80 100 120

Time (min)

SG, W

L a

nd W

R (g

/100

g fr

esh

seed

s)

Figure 1. Water loss (WL: ♦), weight reduction (WR: ○) and solids gain (SG: ×) from

the osmodehydrated seeds using Deglet Nour date juice as an immersion solution base.

An initial rapid increase on water loss at various osmotic solutions could be due to a

large osmotic driving force between the dilute sap of seeds (15.50 °Brix) and the surrounding

hypertonic medium (55.00 °Brix), and also due to the thawing of seeds during the process.

122


Indeed, Raoult-Wack et al. (1991), Shi et al. (2009) and Ruiz-Lopez et al. (2010) showed that

the rate of mass transfer during osmotic dehydration is influenced essentially by the difference

of concentration between samples and the surrounding osmotic solution. In addition, mass

transfer could be influenced by the loss of cell membrane selectivity inducing by the

pretreatment (e.g., Freezing). In fact, Delgado and Rubiolo (2005) and Aguilera et al. (2003)

showed that during slower freezing, a part of the aqueous fraction freezes out and forms ice

crystals that damage the integrity of the cellular compartments. Thus, the cellular membranes

lose their osmotic status and their semi-permeability, favoring a large osmotic driving force

between the dilute sap of the seeds and the surrounding osmotic solution. Slower water

transfer after this period is mainly influenced by the reduction of the difference in

concentration between the seeds and osmotic solution and hence a reduced driving forces

(Allali et al., 2008 and Uribe et al., 2010). Indeed, °Brix observed in seeds and osmotic

solutions tend to be equal at the end process (Table 1). Moreover rapid loss of water and

uptake of solids near the surface in the beginning caused structural changes leading to

compaction of these surface layers and increased mass transfer resistance for water and solids

(Delgado and Rubiolo 2005, Fathi et al., 2009). The trend observed in SG could be explained

by migration of sucrose to the seeds through the cell wall and accumulation between the cell

wall and the cellular membrane, due to the important gradient of sugar between the seeds and

the osmotic solutions. In fact, osmotic solution has a higher content of sucrose than

pomegranate seeds.

3.2.2. Evolution of osmodehydrated fruit parameters

Table 1 shows the change that occurred in seeds and osmotic solutions parameters as

function of time during the process. No difference was observed, regardless the osmotic

solutions used. As it was expected all parameters (°Brix, pH, conductivity,...) evolved in the

same trend like mass transfer parameters. Soluble solids content in pomegranate seeds

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increased as the °Brix solution decreased at the beginning of the process (20 min); after that

they tended towards equilibrium (49.10 °Brix). This was a consequence of osmosis, inducing

a balance of concentration between the seeds and the osmotic solution (Raoult-Wack et al.

1991). An explanation could arise from the migration of the osmotic solution (high sucrose

content) sugars to the pomegranate seeds (low sucrose content) through the cells membrane.

Thus, seeds were progressively concentrated in sugars and lost a part of its water content. The

difference between the total soluble solids in the pomegranate seeds and the osmotic solutions

at the end of the treatments was relatively small. This could support solute impregnation and

limit water loss. In fact, Saurel et al. (1994) reported that low concentrations in solutes

facilitate their impregnation in the fruits, whereas high concentrations allow much more

dehydration and lower water activity.

The increase of conductivity and the decrease of the osmotic solutions pH could be

attributed to diffusion of some solutes and organic acids from pomegranate seeds to the

aqueous solution. Many authors found similar kinetics during the osmotic dehydration of

various fruits (Delgado and Rubiolo 2005; Masmoudi et al. 2007; Fabiano et al., 2008;

Kowalska et al., 2008; Azuara et al., 2009). Moreover, the measure of color parameters L*,

a*, b* showed a slight variation of these parameters. In fact, hue angle (h°) and chroma (C*)

decreased from 1.4 - 27.9 to 1.2 - 23.0 respectively. This variation could be explained by a

migration of pigment from pulp to solution and the enzymatic or non-enzymatic browning

(Monsalve-Gonzalez et al. 1994; Nisha et al. 2010).

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Table 1. Evolution of osmotic dehydration parameters of pomegranate seeds and

osmotic solution (using Deglet Nour date juice)

0 min 5 min 10 min 20 min 40 min 60 min 80 min 100 min 120 min

°Brix of solution 55.00 ±0.00

53.90 ±0.50

52.40 ±0.40

50.30 ±0.35

49.50 ±0.01

49.30 ±0.07

49.20 ±0.01

49.10 ±0.07

49.10 ±0.01

°Brix of seeds

15.50 ±0.09

22.65 ±0.60

30.90 ±0.67

42.90 ±0.28

45.00 ±1.06

47.70 ±0.13

48.80 ±0.49

49.00 ±0.28

49.10 ±0.42

pH of solution 5.55 ±0.24

5.50 ±0.34

5.46 ±0.11

5.40 ±0.05

5.30 ±0.07

5.10 ±0.12

5.10 ±0.07

5.10 ±0.08

5.00 ±0.04

Conductivity of solution (μs/cm)

82.24 ±2.31

95.13 ±1.13

125.00 ±0.51

131.90 ±0.01

134.40 ±2.54

142.5 ±2.19

147.4 ±0.84

147.8 ±0.35

149.10 ±1.55

Dry matter of seeds (%)

16.00 ±0.05

20.80 ±1.05

26.90 ±0.16

35.42 ±1.43

41.45 ±0.64

43.66 ±0.06

45.34 ±0.09

46.43 ±0.18

46.95 ±0.58

aw of seeds 0.9890 ±0.002

0.9760 ±0.001

0.9570 ±0.002

0.9470 ±0.003

0.9445 ±0.001

0.9420 ±0.001

0.9400 ±0.001

0.9385 ±0.002

0.9355 ±0.002

L* 52.99 ±0.96

52.00 ±0.36

51.56 ±0.17

51.10 ±0.08

50.80 ±2.15

50.40 ±2.28

48.00 ±1.12

47.20 ±0.04

47.10 ±0.70

a* 4.30 ±0.27

4.35 ±0.72

4.41 ±0.15

4.50 ±0.18

5.10 ±0.67

5.40 ±0.48

6.80 ±0.64

7.70 ±0.38

7.80 ±0.46

b* 27.58 ±0.50

27.00 ±0.40

26.08 ±0.23

25.70 ±0.19

25.20 ±0.53

24.50 ±1.44

23.30 ±1.39

22.00 ±1.18

21.60 ±1.81

h° 1.41 ±0.25

1.40 ±0.05

1.38 ±0.04

1.35 ±0.10

1.32 ±0.07

1.30 ±0.03

1.29 ±0.10

1.23 ±0.20

1.22 ±0.30

CieLab coordinate of solution

C* 27.91 ±1.50

27.52 ±0.50

26.81 ±1.30

26.10 ±1.10

25.85 ±0.70

25.55 ±0.05

25.18 ±0.70

23.30 ±1.04

23.01 ±0.50

3.3. Osmodehydrated fruits characteristics

3.3.1. Physico-chemical characteristics of the osmodehydrated fruit preparations

The changes that occurred in pomegranate seeds after osmotic dehydration process are

shown in table 2. No significant difference was observed, regardless the osmotic solutions

used. Carbohydrate and total soluble solids in osmodehydrated seeds increased by ~11% DM

and ~34 °Brix, respectively. This increasing is due to the diffusion of sucrose from osmotic

solution (high sucrose content) to the seeds (low sucrose content). In fact the determination of

individual sugars in seeds, showed an increase of sucrose content (59.24±1.03 g/100 g DM).

Contrary to fructose and glucose level, that decreased (10.31±0.51 and 10.75±0.05 g/100 g

DM, respectively). Moreover protein and ash content decreased strongly by ~7.5 % and 1.9 %

DM, respectively; contrary to lipid level that varied slightly (from 4.5 to ~3.8 % DM)

compared to untreated seeds. In this sense, protein, mineral and organic acid diffusion by

125


osmosis contributed to a better flavour of the osmotic solutions. Albagnac et al. (2002)

showed that the solute diffusion flow from fruit to osmotic solution have an important effect

on the sensorial and nutritional value of the product.

Table 2. Physico-chemical properties of pomegranate seeds before and after osmotic dehydration process

Deglet Nour Kintichi Allig

Pomegranate seeds

Juices Components Before OD After OD Dry matter (%) 16.00±0.05 46.95±0.58a 47.69±1.36a 47.80±0.38a

Protein g/100g DM 7.79±0.86 0.35±0.08a 0.30±0.07a 0.27±0.01a

Lipid g/100g DM 4.55±0.40 3.57±0.11a 4.11±0.16a 3.74±0.31a

Ash g/100g DM 2.87±0.19 0.90±0.02a 0.88±0.04a 0.86±0.05a

Carbohydrate g/100g DM

84.93±0.25 96.85±0.70a 96.74±0.45a 96.76±0.24a

aw of seeds 0.989±0.002 0.935±0.002b 0.919±0.001ab 0.912±0.005a

°Brix of seeds 15.50±0.09 49.10±0.42a 49.85±0.28a 48.90±0.28a

Means in line with different letters are significantly different (P<0.05)

Water activity (aw) of the osmodehydrated fruit remained at high levels (~0.913). This

could be explained by the final total soluble solids (~49 °Brix) insufficient for an important

decrease of aw values. After osmotic process seeds can be kept in a refrigerator for a short

time and in a freezer for a long period. Rastogi et al. (2002) showed that osmotic dehydration

did not generally produce stable products. As consequence it must be used as pre-processing

stage before a complementary processing such as pasteurization, freezing, drying, etc.

(Fernandez et al. 2006; Mujumdar & Law 2010 ; Devic et al. 2010).

3.3.2. Microstructure and texture analysis

To better understand the effect of osmodehydration treatment at cell level and on

texture characteristics, scanning electron microscopy (SEM) and texture analysis were used.

Microstructural changes during osmotic dehydration are important in order to understand the

changes which occurred in the kinetics parameters and compositional changes of seeds

126


(Garcia et al. 2010). Images of transversal cross sections of treated and untreated seeds are

presented in Fig. 2. Fig. 2a shows the control sample of frozen pomegranate seed tissue,

which did not receive any treatment other the preparation for SEM. The bright regions in the

micrograph are mainly the cytoplasmic membrane and the cell walls; the darker ones are

holes where ice and cell contents were before SEM treatment. Cells appeared torn and

irregular in shape, with the presence of many empty regions (regions which were not occupied

by cells) (Fig. 2a). Empty regions indicate that ice nucleation and crystal growth damaged the

cell wall during the pre-treatment (freezing). This had influence in terms of semi-permeable

membrane loss, diffusion barrier, and/or modification of seeds texture (Aguilera et al. 2003;

Maity et al. 2009).

Fig. 2b shows that osmotic dehydration process change the tissue structure compared

to untreated sample. In fact cells appeared shrinking, distorted, and their contour appeared

irregular and wrinkling. This fact was probably due to the solubilisation of polysaccharides

(cellulose, hemicellulose and pectin) that composes the cells walls, the water loss and the pre-

concentration of sucrose on the surface of the tissue during the process (Raoult-Wack et al.

1991; Torreggiani & Bertolo 2001; Delgado and Rubiolo 2005; Garcia et al. 2010). Indeed,

pectin is the major constituents of the middle lamella and thus contributing to the cell

adhesion and firmness (Nunes et al. 2008). Moreover water loss induces the plasmolysis of

cells and solids gain gives consistency to the tissues. In fact, Nunes et al. (2008) showed that

diffusion of sucrose into the fruit tissue, during osmotic dehydration process, and its

interaction with the cell wall and middle lamella pectic polysaccharides might result in the

formation of a jam-like structure that gives consistency to the tissues. There are several

experimental findings in the literature that are consistent with our claims regarding the

occurrence of cells structure modification during osmotic processing (Delgado and Rubiolo

2005; Nunes et al. 2008).

127


c

a

b

Forc

e (N

)

c

b

a

Distance (mm)

Figure 2. Scanning electron microscopy photographs of frozen (a) and osmodehydrated seeds (b) and a characteristic force-distance curve for texture analysis using frozen (•) and osmodehydrated (‒) seeds (c)

128


Textural properties of seeds (composed by pulp and pip) are closely linked to cellular

structure (Torreggiani & Bertolo 2001; Sajeev et al. 2004). Fig. 2c shows a characteristic

force-distance curve for texture analysis using frozen and osmodehydrated seeds. It shows

three zones characteristic of seed compression. The first (a) is attributed to the pulp resistance

(~13 N). The second (b) correspond to the fracture of pip (~30 N), while the third (c) zone is

ascribed to the crushing of seed, characterised by the increase in force through a short

distance. In fact, Al-Said et al. (2009) showed that pulp and pip hardness varied between 9-14

N and 24-45 N respectively, for pomegranate seeds cultivated in Oman. The peak force

attained during the test is referred to as hardness and area under the curve as toughness.

Osmotic dehydration process induced an increase on toughness (61.6±4.7 Nmm) and hardness

(62.8±3.5 N) compared to untreated sample (toughness: 54.5±3.9 Nmm and hardness:

46.7±2.4 N). ANOVA shows a significant difference (P<0.05) between treated and untreated

sample. This could be a consequence of the exchange (water loss and solids gain) between

seeds and osmotic solution. Many authors showed that the hardness and toughness increased

when the moisture content decreased since water act as plasticizer (Sajeev et al. 2004; Kingsly

et al. 2006; Al-Said et al. 2009). No significant differences (P<0.05) in textural properties

(Toughness and hardness) of osmodehydrated fruit using date juice added with sucrose or

only sucrose osmotic solution (55°Brix) were observed. Therefore sucrose was an important

factor influencing fruit texture. In fact, during osmotic process sucrose (major component

present in osmotic solution) passes through the cell wall and accumulates between the cell

wall and the cellular membrane, where it forms a hypertonic solution leading to water out flux

through the cellular membrane (Mascheroni & Spiazzi 1997). As consequence of this

exchange, the products will more or less lose weight and will shrink eventually. Hence the

probe penetrates more in the seed and touches the pip at a shorter distance compared to

untreated seed. Indeed, the peak of pip crushed was observed at a short distance (3.39±0.23

129


mm) compared to untreated seeds (4.34±0.51 mm). Thus seeds reduce their thickness between

an average close to ~0.95 mm, after osmotic dehydration process. Delgado and Rubiolo

(2005) and Falade et al. (2007) reported similar results with carrot and banana immersed in

sucrose solution.

3.3.3. Apparent viscosity of the osmotic solutions

An increased in the date juice viscosity (about 83 %) was observed, following the

addition of sucrose. This was due to the high molecular weight of sucrose (Masmoudi et al.

2007). During the osmotic process, the viscosity was reduced by half. This fact was due to the

migration of water from seeds to the osmotic solution. The apparent viscosity of all osmotic

solutions at the end of the process decreased when the shear rate increased until 10s-1,

corresponding to a non-Newtonian rheological behaviour (shear-thinning). After this rate,

viscosity trend to be constant until the end of the test (Fig. 3). Until a shear rate of 10 s-1, the

flow behaviour could be due to the presence of different molecules (carbohydrates, proteins,

sucrose) in osmotic solutions (Aylin & Medeni 2005). After 10 s-1 the flow behaviour was

essentially due to the presence of sucrose. Indeed Fig. 3 shows that sucrose solution (49.4

°Brix) had the same flow behaviour and similar viscosity then osmotic solutions (~49.4 °Brix)

after 10 s-1. At our point of view, the difference between osmotic solutions viscosity was due

to the variation of carbohydrate and proteins level in date juices.

130


0

0,01

0,02

0,03

0,04

0,05

0,06

0 0,4 0,8 2 4 10 30 50 70

Shear rate (1/s)

App

aren

t vis

cosi

ty (P

a/s)

Deglet Nour

Allig

Kintichi

Sucrose

Figure 3. Flow behaviour of the osmotic solutions after 120 min of the process

4. Conclusion

Date juice added with sucrose could be considered as a potential good immersion

solution used to the osmodehydration of pomegranate seeds. In fact, the use of date juice

presented two important advantages. It: (i) involves an increasing of an economic gain by

reducing the amount of sucrose added to the osmotic solution, and (ii) allows a high

nutritional value product with high natural sugar content. As consequence, date juice could be

used for the conservation of many fruits using osmotic dehydration process.

The rate of different osmotic dehydration parameters was directly related to time.

There was a leaching of natural solutes from seeds into the solution during the process, which

is quantitatively not negligible, and might have an important impact on the sensory properties

and nutritional value of seeds and date juices. During osmotic treatment not only the

composition of the tissue is changed but also the structure since modification of cells structure

was observed by SEM, which was confirmed by texture analysis.

131


Osmotic dehydration found to be a good alternative to valorize pomegranate seeds and

date, suggesting the use of these products as ingredients in food formulations.

Acknowledgment

This research was supported by Gembloux Agro-Bio Tech, University of Liege (Belgium) and

National Engineering School of Sfax (Tunisia).

132


References

Aguilera, J. M., Chiralt, A., & Fito, P. (2003). Food and product structure. Trends in Food


Albagnac, P.G., Varoquaux, J., & Montigaud, C.I. (2002). Technologies de transformation

des fruits /Paris.-Londres.-New York, Ed. Tec & Doc, cop.-XXII, pp. 498.

Al-Farsi, M. (2003). Clarification of date juice. International Journal of Food Science and


Allali, H., Marchal, L., & Vorobiev, E. (2008). Blanching of Strawberries by Ohmic Heating:

Effects on the Kinetics of Mass Transfer during Osmotic Dehydration. Food

Bioprocess Technology, DOI: 10.1007/s11947-008-0115-5, in press

Al-Maiman, S.A. & Ahmad, D. (2002). Changes in physical and chemical properties during


Al-Said, F.A., Opara, L.U., & Al-Yahyai, R.A. (2009). Physico-chemical and textural quality

attributes of pomegranate cultivars Brown in the Sultanate of Oman. Journal of Food


AOAC. (1990). Official Methods of Analysis of the Association of Official Analytical

Chemists, 15th Ed., (K. Helrich, ed.), Association of Official Analytical Chemists, Inc.,

Arlington, Virginia, VA.

Attia, H., Bennasar, M., Lagaude, A., Hugodo, B., Rouviere, J., & Tarodo, B. (1993).

Ultrafiltration with microfiltration membrane of acid skimmed and fat-enriched milk

coagula: hydrodynamic, microscope and rheological approaches. Journal of Dairy

Research, 60, 161-174.

Aylin, A., & Medeni, M. (2005). Rheological Behavior of pomegranate (Punica granatum L.)

juice and concentrate. Journal of Texture Studies, 36, 68 - 77.

Azuara, E., Flores, E., & Beristain, C. (2009). Water Diffusion and Concentration Profiles

During Osmodehydration and Storage of Apple Tissue. Food and Bioprocess


133


Barminas, J.T., James, M.K., & Abubakar, U.M. (1999). Chemical composition of seeds and

oil of Xylopia aethiopica grown in Nigeria. Plant Foods for Human Nutrition, 53, 193-

198.

Barreveld, W.H. (1993). Date palm products, FAO agricultural services bulletin No. 101.

Bchir, B., Besbes, S., Attia, H., & Blecker C. (2009). Osmotic dehydration of pomegranate



Bchir, B., Besbes, S., Attia, H., & Blecker, C. (2010). Osmotic dehydration of pomegranate

seeds (Punica granatum L.): Effect of freezing pre-treatment. Journal of Food Process

Engineering. DOI: 10.1111/j.1745-4530.2010.00591.x, in press

Besbes, S., Hentati, B., Blecker, C., Deroanne, C., Lognay, G., Drira, N.E. & Attia H.

(2005a). Voies de valorisation des sous produits de dattes: Valorisation du noyau.

Hygiène Microbiologie Alimentaire, 49, 1-9.

Besbes, S., Cheikh Rouhou, S., Blecker, C., Deroanne, C., Lognay, G., Drira, N. E., & Attia

H. (2005b). Voies de valorisation des sous produits de dattes: Valorisation de la pulpe.

Microbiologie Hygiène Alimentaire, 18, 3-11.

Besbes, S., Drira, L., Blecker, C., Deroanne, C., & Attia, H. (2009). Adding value to hard date

(Phoenix dactylifera L.): compositional, Functional and sensory characteristics of date

jam. Journal of Food Chemistry, 112, 406-411.

Bouabidi, H., Reyens, M., Roussi, M. (1996). Critères de caractérisation des fruits de

quelques cultivars de palmier dattiers (Phoenix dactylifera L.) du sud tunisien. Annales

de L’INRAT, 69, 73-86.

Chenlo, F., Moreira, R., Herrero, F., & Vazquer, G. (2007). Osmotic dehydration of chestnut

with sucrose: Mass transfer processes and global Kinetics modelling. Journal of Food


Corrêa, J., Pereira, L., Vieira, G., & Hubinger, M. (2010). Mass transfer kinetics of pulsed

vacuum osmotic dehydration of guavas. Journal of Food Engineering, 96, 498-504.

134

http://www.sciencedirect.com/science?_ob=MiamiImageURL&_imagekey=B6V24-48XKN8B-D-1&_cdi=5692&_user=532081&_check=y&_orig=search&_coverDate=12%2F31%2F1991&view=c&wchp=dGLbVzb-zSkWz&md5=fdbcaddad77b8455014fad3033c68e53&ie=/sdarticle.pdf�


Delgado, A.E., & Rubiolo, A.C. (2005). Microstructural changes in strawberry after freezing

and thawing processes. Lebensm. Wiss. u. Technol, 38, 135-142.

Devic, E., Guyot, S., Daudin, J., & Bonazzi, C. (2010). Kinetics of Polyphenol Losses During

Soaking and Drying of Cider Apples. Food and Bioprocess Technology,

DOI: 10.1007/s11947-010-0361-1, in press.

Espiard, E. (2002). Introduction à la transformation industrielle des fruits. TEC and DOC-

Lavoisier, Paris; France, pp. 181-182.

Fabiano, A.N., Oliveira, F., & Rodrigues, S. (2008). Use of Ultrasoud for Dehydration of

Papayas. Food and Bioprocess Technology, 1, 339-345.

Fadavi, A., Barzegar, M., Azizi, H., & Bayat, M. (2005). Physicochemical Composition of

Ten Pomegranate Cultivars (Punica granatum L.) Grown in Iran. Journal of Food

Science Technology, 11, 113 - 119.

Falade, K., Igbeka, J., & Ayanwuyi, F. (2007). Kinetics of mass transfer and colour changes

during osmotic dehydration of watermelon. Journal of Food Engineering, 80, 979-985.

Fathi, M., Mohebbi, B., & Razavi, S. (2009). Application of Image Analysis and Artificial

Neural Network to Predict Mass Transfer Kinetics and Color Changes of Osmotically

Dehydrated Kiwifruit. Food and Bioprocess Technology, DOI: 10.1007/s11947-009-

0222-y, in press.

Fernandes, F.A.N., Rodrigues, S., Gaspareto, O.C.P., & Oliveira, E.L. (2006). Optimisation of

osmotic dehydration of bananas followed by air-drying. Journal of Food Engineering,

77, 188-193.

Garcia, P., Mognetti, C., André-Bello, A., & Martinez-Monzo, J. (2010). Osmotic dehydration

of Aloe vera (Aloea barbadensis Miller). Journal of Food Engineering, 97, 154-160.

Grigelmo-Miguel, N., & Martin-Belloso, O. (1999). Characterization of dietary fiber from

orange juice extraction. Food Research International, 31,355-361.

135


Kingsly, A.R.P., Singh, D.B., Manikantan, M.R., & Manikantan, R.K. (2006). Moisture

dependent physical properties of dried pomegranate seeds (Anardana). Journal of Food


Kowalska, H., & Lenart, A. (2001). Mass exchange during osmotic pre-treatment of

vegetables. Journal of Food Engineering, 49, 137-140.

Kowalska, H., Lenart, A., & Leszczyk, D. (2008). The effect of blanching and freezing on


Mascheroni, R., & Spiazzi, E. (1997). Mass transfer model for osmotic dehydration of fruits

and vegetables-I. Development of the simulation model. Journal of Food Engineering,

34, 387-410.



Science and Technology international, 13, 405-412.

Mavroudis, N. E., Gekas, V., & Sjohlm, I. (1998). Osmotic dehydration of apples, shrinkage



Maity, T., Raju, P., & Bawa, A. (2009). Effect of Freezing on Textural Kinetics in Snacks

During Frying. Food and Bioprocess Technology, DOI: 10.1007/s11947-009-0236-5,

in press.

Monsalve-Gonzalez, A., Barbosa-Canovas, G.V., Cavalieri, R.P., McEvily, A.J. & Iyengar R.

(1994). Inhibition of enzymatic browning in apple products by 4-hexylresorcinol. Food


Mujumdar, A., & Law, C. (2010). Drying Technology: Trends and Applications in

Postharvest Processing. Food and Bioprocess Technology, DOI: 10.1007/s11947-010-

0353-1, in press.

Munier P. (1973). Le palmier dattier. Paris : Maison neuve et Larose, pp. 141-221.

136


Nisha, P., Singhal, R., & Pandit, A. (2010). Kinetic Modelling of Colour Degradation in

Tomato Puree (Lycopersicon esculentum L.). Food and Bioprocess Technology,

DOI: 10.1007/s11947-009-0300-1, in press.

Nunes, C., Santos, C., Pinto, G., Lopes-da-silva, J.A., Saraiva, J.A., & Coimbra, M.A. (2008).

Effect of candying on microstructure and texture of plums (Prunus domestica L.).

LWT- Food Science and Technology, 41, 1776-1783.

Poyrazoglu, E., Gokmen, V., & Artik, N. (2002). Organic Acids Phenolic Compounds in

pomegranates (Punica granatum L.) Grow in Turkey. Journal of Food Composition

and Analysis, 15, 567- 575.

Raoult-Wack, A. L., Guilbert, S., Le Maguer, M., & Rios, G. (1991). Simultaneous water and



Rastogi, N.K., Raghavarao, K.S.M.S., Niranjan, K., & Knorr, D. (2002). Recent

developments in osmotic dehydration methods to enhance mass transfer. Trends in


Ruiz-Lopez, I., Castillo-Zamudio, R., Salgado-Cervantes, M.A., Rodriguez-Jimenes, G.C., &

Garcia-Alvarado, M.A. (2010). Mass Transfer Modelling During Osmotic Dehydration

of Hexahedral Pineapple Silices in Limited Volume Solutions. Food and Bioprocess


Sajeev, M.S., Manikantan, M.R., Kingsly, A.R.P., Moorthy, S.N., & Sreekumar, J. (2004).

Texture Analysis of Taro (Colocasia esculenta L. Schott) Cormels during Storage and

Cooking, Journal of Food Science, 69, 315-321.

Saurel, R., Raoult-Wack, A.L., Rios, G., & Guilbert, S. (1994). Mass transfer phenomena

during osmotic dehydration of apple II. Frozen plant tissue. International Journal of


Shi, J., Pan, Z., McHugh, T., & Hirschberg, E. (2009). Effect of Infusion Method and

Parameters on Solids Gain in Blueberries. Food and Bioprocess Technology, 3, 271-

278.

137


138



Uribe, E., Miranda, M., Vega-Galvez, A., Quispe, I., Claveria, R., & Di Scala, K. (2010).

Mass Transfer Modelling During Osmotic Dehydration of Jumbo Squid (Dosidicus

gigas): Influence of Temperature on Diffusion Coefficients and Kinetic Parameters.

Food and Bioprocess Technology, DOI: 10.1007/s11947-010-0336-2, in press.

Yousif, A.K. & Al-Ghamdi, A S. (1999). Suitability of some date cultivars for jelly making.

Journal of Food Science and Technology, 36, 515-518.

Yousif, A.K., Abou Ali, M. & Abou-Idreese, A. (1990). Processing evaluation and storability

of date jelly. Journal of Food Science and Technology, 27, 264-267.

Yunfeng, L., Changjiang, G., Jijun, Y., Jingyu, W., Jing, X., & Shuang, C. (2006). Evaluation

of antioxidant properties of pomegranate peel extract incomparison with pomegranate

pulp extract. Journal of Food Chemistry, 96, 254–260.

Chapitre 5 : Effet des conditions de séchage ___________________________________________________________________________

Chapitre 5:

Effet des conditions de séchage sur les propriétés physico-chimiques des graines de grenade déshydratées

osmotiquement


Bchir, B., Besbes, S., Karoui, R., Attia, H., Paquot, M., & Blecker, C. (2010).

Effect of air-drying conditions on physico-chemical properties of osmotically

pre-treated pomegranate seeds, Food and Bioprocess Technology, DOI:

10.1007/s11947-010-0469-3.

139


Résumé *

Titre : Effet des conditions de séchage sur les propriétés physico-chimiques des graines de

grenade déshydratées osmotiquement


Nous avons précédemment étudié la déshydratation osmotique des graines de grenade

et l’analyse de l’effet de paramètres tels que la température (30, 40, 50°C), la solution

osmotique (saccharose, glucose, saccharose/glucose, jus de datte) et l’état du fruit (congelé et

frais) sur la cinétique de déshydratation ainsi que sur les caractéristiques physico-chimiques

des graines. D’après les résultats obtenus, on peut conclure que pour une meilleure

déshydratation des graines de grenade, les conditions suivantes sont les plus intéressantes: un

traitement de 20 min, une température de 50°C et une solution de saccharose (55°Brix). Les

graines issues de la DO présentent une activité d’eau proche de 0,9 ce qui permet la

croissance des micro-organismes et d’autres réactions indésirables (oxydation des lipides,

dégradation des vitamines…) au cours de l’entreposage. Ainsi dans un but plus appliqué, un

traitement supplémentaire de séchage par entrainement (2 m/s durant 4 heures) a été mis en

place afin de réduire l’activité d’eau à une valeur inférieure à 0,65. Dans ce cadre, nous nous

sommes intéressé à l’étude de l’effet des conditions du post-traitement de séchage sur les

caractéristiques physico-chimiques des graines de grenade déshydratées osmotiquement.

Avant le procédé de séchage, les graines de grenade ont subi une DO pendant 20

minutes à 50°C en utilisant une solution de saccharose (55°Brix). Ensuite un traitement de

séchage par entrainement (2 m/s durant 4 heures) a été mis en place à différentes températures

(40, 50, et 60°C). En premier lieu nous avons étudié l’effet de la température de séchage du

traitement sur l’évolution de la matière sèche (MS), l’activité d’eau (aw) et le pourcentage de

séchage (DR). Afin de déterminer l’évolution des différentes fractions d’eau dans la graine en

fonction de la température, une méthode plus fine a été adoptée qui consiste à analyser les

propriétés thermiques de la pulpe par calorimétrie différentielle. Le séchage par air chaud est

de moins en moins performant au regard des exigences croissantes en matière de qualité des

produits finis. Ainsi, nous avons étudié l’effet de la température (40, 50, 60°C) de séchage sur

plusieurs paramètres de qualité des graines de grenade tels que : activité antioxydante,

composés phénoliques, anthocyanines, couleur et texture.

Les différentes étapes du procédé sont reprises de façon synoptique dans figure 1’. _________________________________________________________________________

* Ce résumé permet de présenter de façon synthétique, en français, l’axe de recherche de l’article qui a été publié en anglais.

140



(-50°C)

Conditions



Solution (55°Brix): - Saccharose

Temps (min): - 20


Conditions

Post- traitement : séchage par entrainement

Température (°C): - 40 - 50 - 60

Temps (min): - 0 - 120 - 30 - 180 - 60 - 240

Vitesse de l’air (m/s): - 2

Paramètres

* MS, aw, DR, Deff , Tg’, ∆H * Activité antioxydante, composés phénoliques, anthocyanines * Couleur (L*, C*, h°), texture (hardness, toughness) * Activité de la polyphénol oxydase (PPO), teneur en hydroxy-méthyl-furfural (HMF).

Figure 1’ : Différentes étapes des procédés de déshydratation osmotique et de séchage des

graines de grenade.

141



Le procédé de séchage mis en œuvre permet d’atteindre l’activité de l’eau cible de

0,65. Le temps nécessaire pour atteindre cette valeur diminue avec la température (240, 120,

et 60 min pour respestivement des températures de 40, 50, et 60°C), en relation avec

l’accroissement du coefficient de diffusion de l’eau et du taux de séchage.

D’autre part, la déshydratation osmotique et le séchage exercent une influence

significative sur la qualité nutritionelle des graines. En effet, le procédé de DO entraîne une

réduction de l'activité de scavenging du radical diphenylpicryl-hydrazyl (DPPH). Cette

réduction est suivie par une diminution des teneurs en composés phénoliques, en

anthocyanines. Ce phénomène est accentué par des températures de séchage de plus en plus

élevées. Cependant, ces valeurs restent comparables (voires supérieures) à celles observées

chez d’autres fruits n’ayant subi aucun traitement.

Les paramètres chromatiques (L*, C* et h°), ainsi que l’indice de brunissement, sont

affectés par le procédé de séchage qui conduit à une variation de la couleur des graines de

grenade. La faible activité PPO et la teneur réduite en HMF indiquent que les réactions

enzymatiques et non-enzymatiques n'ont pas une influence majeure sur le brunissement de la

graine. Cela confirme les faibles valeurs de l’indice de brunissement. Outre la couleur, la

combinaison entre la DO et le séchage a influencé la forme et la texture, puisque les graines

ont perdu jusqu'à 55% de leurs épaisseurs après séchage à 60°C.

142


Effect of air-drying conditions on physico-chemical properties of

osmotically pre-treated pomegranate seeds

*Brahim Bchira, Souhail Besbesb, Romdhane Karouia, Hamadi Attiab, Michel Paquotc,

Christophe Bleckera

a Gembloux Agro-Bio Tech, University of Liege, Passage des Déportés, 2, B- 5030

Gembloux, Belgium

b Laboratory of Food Analyses, National Engineering School of Sfax, Route de Soukra, 3038

Sfax, Tunisia

c Department of Industrial Biological Chemistry, Gembloux Agro-Bio Tech, University of

Liege, Passage des Déportés, 2, 5030 Gembloux, Belgium


*Fax: +32(0)81/60.17.67


143


Abstract

The drying of pomegranate seeds was investigated at 40, 50, and 60 °C with air

velocity of 2 m/s. Prior to drying, seeds were osmodehydrated in 55 °Brix sucrose solution for

20 min at 50 °C. The drying kinetics and the effects of osmotic dehydration (OD) and air-

drying temperature on antioxidant capacity, total phenolics, colour, and texture were

determined. Analysis of variance (ANOVA) revealed that OD and air-drying temperature

have a significant influence on the quality of seeds. Both anthocyanin and total phenolic

contents decreased when air-drying temperature increased. The radical diphenylpicril-

hydrazyl activity showed the lowest antioxidant activity at 60 °C. Both chromatic parameters

(L*, C* and h°) and browning index were affected by drying temperatures, which contributed

to the discolouring of seeds. The final product has 22, 20, 16 % of moisture; 0.630, 0.478,

0.414 of aw; 151, 141, 134 mg gallic acid equivalent/100g Fresh Matter (FM) of total

phenolics; 40, 24, 20 mg/100g FM of anthocyanins; and 46, 39, 31 % of antioxidant activity,

for drying temperatures of 40, 50 and 60 °C respectively. In view of these results, the

temperature of 40 °C is recommended as it has the lowest impact on the quality parameters of

the seeds. Differential scanning calorimetry data provided complementary information on the

mobility changes of water during drying. Glass transition temperature (Tg’) depends on

moisture content and as consequence on drying conditions. In fact, Tg’ of seeds dried at 60 °C

(Tg’= -21 °C) was higher than those dried at 50 °C (Tg’= -28 °C) or 40 °C (Tg’= -31 °C) and

osmodehydrated seeds (Tg’= -34 °C). During OD and drying process, the texture of seeds

changed. The thickness of seeds shrank by 55 % at 60 °C.

Keywords: pomegranate; osmotic dehydration; drying; antioxidant activity;

differential scanning calorimetry; texture.

144


Introduction

Pomegranate (Punica granatum L.) presents a virtual explosion of interest as a

medicinal and nutritional product. Recently, more than 475 new products containing

pomegranate (food and drinks) were created in United State, including a chewing gum named

“pomegranate power”, a chicken sauce containing pomegranate, etc. (Storey 2007).

The edible part of the fruit (seeds) contains a considerable amount of sugars, vitamins,

polysaccharides, minerals and polyphenols (Espiard 2002). Due to their polyphenol

compounds (e.g., anthocyanins), condensed tannins (e.g., proanthocyanidins) and

hydrolysable tannins (e.g., ellagitannins and gallotannins)) pomegranate seeds possess anti-

oxidative properties (Hernandez et al. 1999; Jaiswal et al. 2010). In fact, these compounds are

able to reduce the formation of free radicals compounds that cause oxidation reactions

associated with biological complications such as aging, cardiovascular disease, and

carcinogenesis (Rosenblat and Hayek 2006).

Despite all these advantages, the consumption of pomegranate seeds is limited to the

crop season due to problems of preservation (Defilippi et al. 2006). Indeed, the major cause

limiting the potential of pomegranates is the development of decay, which is often caused by

the presence of fungal inoculum in the blossom end of the fruit. During long term storage,

rinds scalds symptoms appear as a superficial browning (Defilippi et al. 2006).

Preservation methods can be used to increase the shelf-life of fruits, among them there

are drying, pasteurization, osmotic dehydration etc. (Raoult-Wack et al. 1991). Freezing is

also a preservation method; however, this treatment involves modifications of the texture and

cell structures (Bchir et al. 2010a; Bchir In press). As consequence, frozen fruit cannot be

consumed directly after thawing. Nevertheless, freezing could be an excellent pre-treatment

for osmotic dehydration of fruit and vegetable, improving significantly mass transfer during

osmotic process. Our previous investigations showed that freezing before osmotic dehydration

145


provided 1.4 and 3.5 times more water loss and solids gain, respectively, than an untreated

pomegranate seeds (Bchir et al. 2010a). Osmotic process has received considerable attention

as a pre-drying treatment to reduce energy consumption and improve food quality (El-Aouar

et al. 2003; Ruiz-Lopez et al. 2010). According to Pokharkar et al. (1997) and Uribe et al.

(2010) the main advantages of the osmotic dehydration process are: retention of natural colour

without addition of sulphites and high retention of volatile compounds during subsequent

drying.

After osmotic process, water activity of sample was found to be higher than 0.900,

allowing development of microorganisms (e.g. bacteria, fungi), and some undesirable

reactions such as; enzymatic and non-enzymatic browning reactions, fat oxidation, vitamin

degradation, and protein denaturation during storage (Bchir et al. 2009; Bchir et al. 2010b).

As a consequence, a complementary treatment such as drying may enable better conservation

of pomegranate seeds.

Drying is the most commonly used method for food dehydration since it is the most

rapid process; it inhibits enzymatic degradation, limits microbial growth and produces a

uniform dried product (Harbourne et al. 2009; Uribe et al. 2009). In this context, various

fruits and vegetables such as onions (Singh and Sodhi 2000), red pepper (Doymaz 2007),

garlic cloves (Sharma et al. 2003), ear corn (Friant et al. 2004), apricots (Doymaz 2007), and

mulberry (Doymaz 2007) have been dried, despite several negative reactions such as

shrinkage, loss of colour, texture and nutritional-functional properties (Arabhosseini et al.

2009; Miranda et al. 2009).

The aim of the present study was to: (i) investigate the kinetics and influence of air-

drying temperature on mass transfer; and (ii) determine the impact of drying temperatures on

antioxidant activity, phenolic, anthocyanin content, colour development and texture of

pomegranate seeds.

146

https://springerlink3.metapress.com/content/?Author=Elsa+Uribe

https://springerlink3.metapress.com/content/?Author=Akbar+Arabhosseini


Materials and methods

Materials

Fresh pomegranate fruits (Punica granatum L.) of El Gabsi variety were grown and

harvested in Gabes during autumn (2009). Tunisia pomegranate fruits were collected at full

ripeness stage (weight: ~ 500 g; skin colour: red; juice colour: pink; juice pH: ~ 4.4; °Brix: 15

g/100g; skin thickness: ~ 4 mm). Pomegranate is composed of a non edible part composed of

30% skin (external part), 13 % internal lamel, and an edible part formed of 50 – 70 % seeds.

The seeds are composed of about 15 % pips (woody part), which determines hardness, and 85

% pulp which contains juice (Espiard 2002). The investigated seeds presented the following

characteristics; shape: ellipsoids; length: 13±1 mm; breadth: 7±1 mm; pip thickness: 2±0.2

mm; average weight of an individual seed: 0.504±0.04 g; bulk density: 628±2 kg/m3.

Twenty kilograms of pomegranate were frozen at -50 °C for one month. Before

osmotic dehydration process, pomegranates were thawed at room temperature for one hour. A

digital thermometer BT20 (Bresso, Italy) was placed in the pomegranate core to measure the

temperature elevation during one hour of thawing at room temperature. Temperature of

pomegranate core reached -7.5 °C, after thawing. Seeds were immediately separated manually

prior to the osmotic dehydration process.

Osmotic dehydration treatment

About 100 g of frozen seeds were osmodehydrated in sucrose solution (55°Brix) for

20 min at 50 °C using a shaking water bath (GFL instrument D 3006, Germany; oscillation

rate 160 rpm). The time and temperature combination was selected on the basis of our

previous findings which showed that osmotic dehydration of pomegranate seeds for 20 min

using sucrose solution at 50 °C gives higher mass transfer rate (Bchir et al. 2009; Bchir et al.

2010a). Sucrose solution was heated at 50 °C before adding the seeds to the bottles (Schott,

Saint- Gallen, Switzerland) of 500 mL. The volume ratio between the seeds and the sugar

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solution was kept at one part of seeds and four parts of solution (1:4) (Bchir et al. 2009). After

osmotic dehydration process, seeds were removed from the solution, quickly rinsed (with

distilled water, 20min) and the excess of solution at the surface was removed with absorbent

paper.

Air-drying experiment

Approximately 200 g of osmodehydrated seeds were uniformly spread on perforated

stainless trays and dried at three temperatures 40, 50 and 60 °C for 240 min. These

temperatures have been selected according to those mostly used for fruit and vegetable in the

literature (Kingsly and Singh 2007; Erbay and Icier 2009).

Dried seeds were taken out from dryer at different time intervals (30, 60, 120, 180 and

240 min). Drying experiments were carried out with a laboratory scale drier by operating it at

a air velocity of 2.0 ± 0.1 m/s. The drying cabinet (Memmert tcp 800, Schutzart, Germany)

was equipped with an electrical heater, a fan, and temperature indicators.



Physico-chemical analysis

Dry matter, moisture contents and water activity

The dry matter (DM) was calculated according to AOAC method 934.01 (1990). For

the different time intervals, approximately 5 g of seeds were oven dried at 103 °C ± 2 °C until

constant weight. Moisture content was estimated by difference of mean values, 100 % - % of

DM (Chenlo et al. 2007). Water activity (aw) was measured using an aqualab (Switzerland)

instrument at 20 °C.

Surface colour measurement

The CIELAB coordinates (L*, a*, b*) were directly read with a

spectrophotocolorimeter Mini Scan XE (Germany) with a lamp (D 65). In this coordinate

148


system, L* value is a measure of lightness, ranging from 0 (black) to +100 (white), a* value

ranges from -100 (greenness) to +100 (redness) and b* value ranges from -100 (blueness) to

+100 (yellowness). The total colour difference (∆E*) was determined by using the following

equation (Romano et al. 2008):

Where L*, a*, b* and L0*, a0

*, b0* are the current and the initial CIELAB coordinates,

respectively.

The Hue angle (h*ab) and chroma or intensity (C*) were calculated according to the following

equations:

(2) (3) C* = (a*2+b*2)1/2 h* = arctan (b*/a*)

(1) ( ) ( ) ( )2*0

*2*0

*2*0

** bbaaLLE −+−+−=Δ

For Hue colour index, 0° or 360° represents red, and 90°, 180° and 270° represent

yellow, green and blue, respectively.

Browning index

The methodology applied for determination of browning index was that proposed by

Vega-Galvez et al. (2009). Pomegranate seeds were placed in distilled water at 40 °C for 6 h,

using a solid to liquid ratio of 1:50. Then, water was first clarified by centrifugation

(Beckman coulter J-E, Indianapolis, USA) at 3200×g for 10 min. The supernatant was diluted

with an equal volume of ethanol at 95% and centrifuged again at 3200×g for 10 min. The

browning index (absorbance at 420 nm) of the clear extracts was determined in quartz

cuvettes using a spectrophotometer (Shimadzu UV 240, Cambridge, USA).

149


Polyphenol oxidase extraction and activity measurement

A portion of pulp (10 g) was dipped in a McIlvaine buffer solution (1:1) at pH = 6.5.

The buffer contained NaCl 1 M and 5% polyvinylpolypyrrolidone. The homogenate was

centrifuged (8,000 rpm, 30 min) at 4 °C. The solid residue was discarded and the supernatant

was filtered through a Whatman # 1 paper. The resulting liquid constituted the crude enzyme

extract.

Polyphenol oxidase activity was determined by placing 3 ml of 0.05 M cathechol and

75 μl enzyme extract in a 1 cm path cuvette. Assays were carried out at 410 nm using a

shimadzu UV 240 spectrophotometer (Cambridge, USA) (Robert et al., 2002). A change in

absorbance at 410 nm per minute and millilitre of enzymatic extract correspond to one unit of

PPO activity. The initial rate of the reaction was computed from the linear portion of the

plotted curve. Results were expressed as relative activity (RA, %) calculated by Eq. (4)

0

100AARA = (4)

Where A and A0 are the current and the initial PPO activity, respectively (Robert et al., 2002).

Hydroxylmethylfurfural analysis

The analysis of hydroxymethylfurfural (HMF) was carried out by High Pressure

Liquid Chromatography (HPLC). Approximately, 1 g of pulp was placed in 25 ml flask; 2 ml

each of Carrez I and II reagents were added with stirring for 30 min and the volume made up

with Milli-Q water. After standing for 30 min, the supernatant was filtered through a filter of

0.45 μm and then injected in to the chromatograph (Rada-Mendoza et al. 2002).

HPLC determination was carried out, following the method of Vinas et al. (1992),

using a ZorBax 300SB-C18 column (4.6×150 mm; waters) at 30 °C. Mobil phase was a

mixture of methanol/water (10/90, v/v) with isocratic elution with 1mL min-1 flow rate and 20

μl injection volume. The UV detector PDA was set at 280 nm. Quantification was carried out

by the external standard method, using a commercial standard of HMF (Sigma, New Jersey,

150


USA). A standard curve was obtained by using HMF standard dissolved in distilled water at

various concentrations (ranging from 0 to 104 μg/ml).

Antioxidant activity

Antioxidant activity was determined using pomegranate seeds extract solution.

Approximately 5 g of pomegranate seeds were crushed and mixed with 15 ml methanol -

water solution (4:1, v/v) at room temperature (20 °C) for 5 h under stirring. The extracts were

then filtered and centrifuged (Beckman coulter J-E, Indianapolis, USA) at 4,000g for 10 min

and the supernatant was concentrated under reduced pressure at 40 °C for 1 h using a rotary

evaporator (Buchi B-461 water Batch, Switzerland) to obtain the crude extract. The crude

extract was kept in dark glass bottles inside the freezer until use (Biglari et al. 2008).

Antioxidant activity of pomegranate seeds was determined using the 2,2,-diphenyl-2-

picryl-hydrazyl (DPPH) method (Vega-Galvez et al. 2009). Two (2) ml of DPPH radical

(0.15 mM in ethanol) was added to a test tube with 1 ml of the crude extract. The reaction

mixture was vortex-mixed for 30 s and left to stand at room temperature in the dark for 20

min. The absorbance was measured at 517 nm using a spectrophotometer (Shimadzu UV 240,

Cambridge, USA). The spectrophotometer was equilibrated with 80% (v/v) ethanol. Control

sample was prepared without adding extract. Total antioxidant activity (TAA) was expressed

as the percentage inhibition of the DPPH radical and was determined by the following

equation:

1001[%] ×⎟⎟⎠

⎞⎜⎜⎝

⎛−=

control

sample

AbsAbs

TAA (5)

Where TAA is the total antioxidant activity and Abs is the absorbance.

Phenolic content

Total phenolic were determined using Folin-Ciocalteau reagents. Crude extract (40 μl)

or gallic acid standard were mixed with 1.8 ml Folin-Ciocalteu reagent (predilued 10-fold

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with distilled water) and allowed to stand at room temperature for 5 min, and then 1.2 ml of

sodium bicarbonate (7.5%) was added to the mixture. After standing for 60 min in darkness at

room temperature, absorbance was measured at 765 nm.

A standard curve was obtained by using gallic acid standard solution at various

concentrations (ranging from 0 to 2 mg/100g). The absorbance of the reaction samples was

compared to that of the gallic acid standard and the results were expressed as mg gallic acid

equivalents (GAE)/100 g sample (Biglari et al. 2008).

Anthocyanin content

Anthocyanin content was determined using the pH-differential method described by

Kirca et al. (2007). Aliquot (1 g) of crush pulp was mixed with 80 ml of distilled water. The

mixture was sonicated (15 min) and centrifuges (1500×g for 10 min) and the upper phase was

used for assay. Two samples of 1 ml were taken from the upper phase and each one was

placed in 25 ml flask. The first flask was diluted with buffer solution pH 1 (1.49 g KCl/100

ml and 0.2N HCl) and the second one with buffer solution pH 4.5 (1.64 g sodium acetate /100

ml). After standing for 30 min at room temperature, absorbance was measured at 510 and 700

nm, using spectrophotometer (Shimadzu UV 240, Cambridge, USA). Pigment content was

calculated, based on cyanidin-3-glucoside using the following equation (Kirca et al. 2007):

(6)

100××××GVDM

eLAbs

wAnthocyanin (cyanidin-3-glucoside equivalents, mg/100 g) =

Where Abs (absorbance) = (Abs510nm-Abs700nm) pH1 - (Abs510nm-Abs700nm) pH4,5;

Mw (molecular weight) = 449.2 g/mol, for cyanidin-3-glucoside ; D = dilution factor ; L = path

length in cm; e = 26 900 molar extinction coefficient of cyanidin-3-glucoside [L × mol-1 ×

cm-1]; V = final volume [ml]; G = sample weight [mg].

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Texture analysis


Micro Systems, UK), with 75 mm compression probe as described by Bchir et al. (2010a).

The operating conditions of the instrument were as follows: 1.5 mm/s pre-test speed, 0.5

mm/s test speed, 10.0 mm/s post-test speed, 0.10 N trigger force and 85% sample

deformation. The hardness and toughness of seeds were the means of 20 single seed

measurements. Hardness [N] of seed was taken as the force in compression which

corresponded to the breakage of samples, while the toughness [N mm] is the energy required

to crush the sample completely.

Differential scanning calorimetry

Differential scanning calorimetry (DSC) was performed on the pulp previously

separated from pip. A 2920 TA Instruments (New Castle, Delaware, USA) with a

Refrigerated Cooling Assessory and modulated capability was used. The cell was purged with

70 ml min−1 of dry nitrogen and calibrated for baseline using an empty oven and for

temperature and enthalpy using two standards (indium, Tonset: 156.6 °C, ΔH: 28.7 J g−1;

eicosane, Tonset: 36.8 °C, ΔH: 247.4 J g−1). Specific heat capacity (Cp) was calibrated using a

sapphire. The empty sample and reference pans were of equal mass to within ±0.10 mg. DSC

curves were recorded during heating from -50 to 40 °C at a scan rate of 5 °C/min. All these

DSC experiments were made using hermetic aluminum pans. The analyzed sample mass was

about 3.50 ±0.25 mg.

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Drying rate and effective diffusion coefficients

The drying rate (DR) was calculated using the equation (Shi et al. 2008);

tMM

DR t−= 0 (7)

Where DR is overall drying rate [g water/g dry matter min-1]; M0 is moisture content of seeds

at time 0 [g water/g dry matter]; and Mt is moisture content of seeds at time t [g water/g dry

solid].

Diffusion coefficients (Deff) were calculated using Fick’s second law equation applied

to a sphere, by modifying the Fourier number using shape factor, due to an

ellipsoids shape of pomegranate seeds as has been reported in our previous investigation

(Bchir et al. 2009).

2/0 RtDFffe=

Statistical Analyses

Statistical analyses were carried out using a statistical software program (SPSS for

windows version 11.0). The data sets were subjected to analysis of variance using the general

linear model option (Duncan test) in order to investigate differences between samples

(P<0.05).

Results and discussion

Chemical composition of pomegranate seeds and osmodehydrated seeds (Table 1)

showed a predominance of carbohydrate in pomegranate seeds (84.93±0.25 g/100 g DM) and

a high amount of protein (7.79±0.86 g/100 DM), in agreement with previous findings of

Espiard (2002) and Fadavi et al. (2005). Pomegranate seeds were found to contain low levels

of ash (2.87±0.19 g/100 g DM) and lipid (4.55±0.40g/100 g DM). The DM and water activity

were about 16 % and 0.989, respectively.

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After osmotic dehydration process, carbohydrate and total soluble solids in

osmodehydrated seeds increased by 10 % and 62 %, respectively. This increasing is due to the

diffusion of sucrose from osmotic solution (high sucrose content) to the seeds. On the

contrary protein and ash contents, decreased from 7.8 and 2.9 to 0.5 and 1.0 g/100 g DM,

respectively. The amount of lipid was found to vary slightly (i.e. 4.5 to 4.0% DM).

DM of osmodehydrated seeds increased by 27 % and water activity slightly decreased

to 0.954 ±0.002. As consequence complementary treatments such as drying would be required

to reduce water activity to 0.650; to control bacteria, fungi and yeast development (Fabiano et

al. 2008; Pinho et al. 2009; Miranda et al. 2009).

Table 1: Chemical characteristic of pomegranate seeds

Untreated seeds Osmodehydrated seeds

Dry matter [%] 16.00 ± 0.05 42.75±0.33 Protein [g/100g DM] 7.79 ± 0.86 0.51±0.02 Lipid [g/100g DM] 4.55 ± 0.40 4.03±0.81 Ash [g/100g DM] 2.87 ± 0.19 1.04±0.04 Carbohydrate [g/100g DM] 84.93 ± 0.25 94.41±0.97 °Brix 15.50 ± 0.09 41.50±0.50 aw 0.989 ± 0.002 0.954±0.002 DM: Dry matter

Drying kinetics

The effect of drying time on dry matter (DM) water activity (aw), and drying rate (DR)

was studied in osmodehydrated seeds at different temperatures (40, 50 and 60 °C). From Fig.

1, DM increased from 42 % to 78, 80 and 84 % after 240 min of the process time, for drying

temperatures of 40, 50 and 60 °C respectively. Moisture content decreased by 26 % and 64 %

after osmotic dehydration and drying compared to untreated seeds, respectively. Water

activity decreased from 0.954 to 0.700, 0.565, and 0.430 in 180 min for drying temperatures

of 40, 50 and 60 °C respectively. After 180 min a slight decrease was observed (40 °C: 0.630;

50°C: 0.478; 60°C: 0.414). Under the same condition, DR decreased (from 2.21 × 10-2 ;

155


2.00× 10-2 ; 1.20 × 10-2 to 0.50 × 10-2 ; 0.35 × 10-2 ; 0.30 × 10-2 g water/g dry matter min-1,

for drying temperatures of 60, 50 and 40 °C, respectively), during the first 180 min, tending to

be stable at the end of the process. Statistical analysis (ANOVA) did not show a significant

difference (P>0.05) between 180 and 240 min for all the investigated parameters. Similar

findings have been previously reported in many works (Kingsly and Singh 2007; Falade and

Onyeoziri 2010; Fathi et al. 2010).

The drying kinetics of seeds could be subdivided in two phases. The first period (until

180 min) corresponds to an easy diffusion of water from the inside to the surface of seeds and

the evaporation of free water on the seeds surface during drying; the second one (from 180 to

240 min) corresponds to a difficult diffusion of water. This could be due to the modifications

in seed surface during the drying. In fact, many authors showed that after a few hours of

drying, the product becomes denser, tougher and often leathery in nature with a case hardened

surface which does not facilitate moisture diffusion (Doymaz 2007; Carlos and De-Michelis

2009). This behaviour could be favoured by the pre-treatment (osmotic dehydration). Indeed,

Mandala et al. (2005) showed that sugar surface impregnation during osmosis favours sugar

crystallization, in some parts of the outer layers of apple tissue, forming a barrier to the

movement of water during drying.

From the results showed in Fig. 1, it can be concluded that increasing temperature of

drying from 40 to 60 °C resulted in quicker removal of water and shorter drying times to

reach aw of 0.650. In fact, using a temperature less than 60 °C resulted in a higher water

activity and a lower drying rate. The increase of temperature at 50 °C induced the same

evolution of aw and DM as with 40 °C. At 60 °C significant difference was observed after 60

min and 30 min for aw and DM, respectively. Moreover, using a drying temperature of 60 °C

caused a reduction in the drying time by four times, in order to reach a water activity (aw) of

0.650 as compared to that at 40 °C. These findings are in agreement with previous studies

156


reported for various dried fruits and vegetables (Miranda et al. 2009; Gokhale and Lele 2010).

Park et al. (2002) and Shi et al. (2008) found that the increase of air-drying temperature (from

40 to 80 °C) induced an increase of heat energy which speeded up the movement of water

molecules and resulted in higher moisture diffusivity.

The calculated values of effective diffusivity (Deff) at different temperatures are presented

in table 2. It can be seen that the values of Deff greatly increased with the increasing air-drying

temperature. Effective diffusivity values for dried pomegranate seeds are similar to those

estimated by different authors for other vegetables (Madamba et al. 1996; Ahrné et al. 2003;

Doymaz, 2007). Table 2 showed that effective diffusivity values and experimental data of

Peleg’s equation parameters (K1 and K2) presented a good fit, showing an average correlation

coefficients (R2) close to 0.99.

Table 2: Effective diffusivities calculated by Fick’s model and values of Peleg’s equation

parameters (K1 and K2).

Drying temperature

Deff [m2 s-1] R2 [%] K1 K2 R2 [%]

40 °C 2.85 × 10-10 97.57 5.94 × 103 2.00 99.54 50 °C 3.74 × 10-10 99.67 9.43 × 103 1.86 99.31 60 °C 4.49 × 10-10 98.92 17.82 × 103 1.48 99.78

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(a)

0

0,2

0,4

0,6

0,8

1

1,2

0 50 100 150 200 250

Time [min]

Wat

er a

ctiv

ity

60 °C 50 °C 40 °C

1.2

0.8

1

0.6

0.4

0.2

(b)

0

10

20

30

40

50

60

70

80

90

0 50 100 150 200 250Time [min]

Dry

mat

ter

[%]

60 °C 50 °C 40 °C

158

Figure 1: Variation of water activity (a) and dry matter (b) of pomegranate seeds as a

function with time and temperature (40, 50, 60 °C).


The investigation of the effect of air-drying temperature on the mobility changes of

water in dried seeds by differential scanning calorimetry (DSC) confirmed the previous

results regarding aw and Deff. From the results obtained, it was possible to determine a

significant decrease in water mobility after OD and drying process. Indeed, DSC results

showed that after 20 min of OD, the % of frozen water (free water) decreased from 70 % to

28 % (determined by dividing the enthalpy of fusion of sample by the enthalpy of fusion of

pure water). After 240 min of drying, free water in seeds was eliminated. In fact, Fig.2

showed a disappearance of the endothermic peak after 240 min of drying at different air-

drying temperatures, compared to osmodehydrated seeds. This is due to the loss of total free

water fraction in seeds. In fact, endothermic peak could be attributed to the melting point of

crystallized water. During the cooling only free water was crystallized to give ice while

during heating, frozen water undergoes a fusion of ice.

On other hand, DSC thermograms (Fig. 2) showed a considerable increase in glass

transition temperature (Tg’) as the air-drying temperature increased. In fact, Tg’ of seeds

dried at 60 °C (Tg’= -21 °C) was higher than those dried at 50 °C (Tg’= -28 °C) or 40 °C

(Tg’= -31 °C) and to the osmodehydrated seeds (Tg’= -34 °C). The increasing of Tg’ could be

induced by a progressive loss of non-freezing water (tightly bound water) of seeds during the

drying process. Sá et al. (1999) found that Tg’ for polysaccharides water systems reach to a

maximum with decreasing water content, inducing the decreased mobility of the polymer

chains. Glass transition temperature was determined from the change in heat capacity (∆Cp).

∆Cp can be related to the glass transition temperature (Tg’) due to the presence of sucrose,

protein, fibre (pectin, lignin, hemicellulose and cellulose), and water in the sample. As

reported in such products, carbohydrates and proteins can be described as amorphous food

polymers constituted by not arranged chains (Roos 1995).

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T°: 40°CT°: 50°CT°: 60°C

DO

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

Hea

t Flo

w (W

/g)

-60 -40 -20 0 20 40

Temperature (°C)Exo Up Universal V3.9A TA Instruments

Figure 2: DSC thermograms obtained for osmodehydrated (OD) and dried pomegranate seeds at different temperatures (40, 50, 60 °C).

Hot air-drying temperature is very important for the dehydration, but it is limited by

the heat sensitivity of seeds and the expected quality of the final product (Erbay and Icier

2009; Jaya and Das 2009; Mujumdar and Law 2010). Therefore, the physicochemical

properties of seeds at different air-drying temperature were analysed.

Physico-chemical properties

Antioxidant activity

Antioxidant compounds are considered as an indicator of the quality of food

processing due to its low stability during thermal process (Bilgari et al. 2008; Saxena et al.

2010). Antioxidant activity (AA) was determined in terms of stable free radical DPPH. (2,2,-

diphenyl-2-picryl-hydrazyl) according to the method described by Vega-Galvez et al. (2009).

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https://springerlink3.metapress.com/content/?Author=Alok+Saxena


Antioxidant activity of pomegranate seeds (84 %), cultivated in Tunisia, was found to be

slightly higher compared to other pomegranate seeds (62-72 %) cultivated in India (Kulkarni

and Aradhya 2005). After osmotic dehydration process, seeds showed a rapid decrease of

antioxidant activity (i.e. until 58 %). This value remained interesting compared to other fruits

and vegetables (Bilgari et al. 2008; Miranda et al. 2009; Kuljarachanan et al. 2009).

Antioxidant activity continues to be reduced during drying, regardless the considered

drying temperature. In fact, AA reached 46, 39, and 31 %, after 240 min for drying

temperatures of 40, 50, and 60 °C respectively (Table 3). In spite of this decrease, AA%

remained higher than those observed in date and close to tea and coffee antioxidant activity

value (Bilgari et al. 2008). As shown the lowest antioxidant activity was recorded using a

higher air-drying temperature (60 °C). ANOVA analysis showed a significant difference

(P<0.05) of AA% as a function of air-drying temperatures. Similar results have been reported

by Miranda et al. (2009) and Kuljarachanan et al. (2009) during the increase of air-drying

temperature from 50 to 90 °C of Aloe Vera and lime. This reduction of AA% could be

explained due to loss of different components (i.e. phenolics acids, flavonoid, and ascorbic

acid), which are responsible for the antioxidant activity of pomegranate seeds, during heating

(Nicoli et al. 1999; Kulkarni and Aradhya 2005). Vega-Galvez et al. (2009) concluded that

although natural antioxidants are lost during heating, the overall antioxidant properties of

foods could be maintained or enhanced by the formation of new antioxidant compounds with

enhanced antioxidant properties. In fact, increase in AA% following thermal treatment has

been reported in many vegetables (Choi et al. 2006; Kang et al. 2006). As consequence, in

this study the destruction rate of antioxidants during heating was higher than the formation

rate of these compounds.

161


Table 3: Value of total phenolic anthocyanin and antioxidant activity of untreated, osmotic dehydrated and dried seeds

Total phenolic

[mg/100g] Total anthocyanin

[mg/100g] Antioxidant activity [%]

Untreated seeds 326.68e±1.40 82.30d±1.42 84.23e±0.31 Osmotic dehydrated seeds

184.39d±1.15 68.43c±0.30 57.88d±1.07 40 °C 151.76c±1.93 40.11b±1.53 46.23c±0.56 50 °C 141.14b±1.23 24.03a±0.14 39.04b±0.80

Dried seeds

60 °C 134.58a±1.14 20.10a±0.28 31.17a±1.16 Means in column with different letters are significantly different (P<0.05)

Total phenolic content

Pomegranate seeds’ phenolic content 326.7±1.4 mg gallic acid equivalent/100g fresh

matter (FM) (Table 3). This value is in agreement with previous finding in pomegranate

seeds, which varied between 230 and 510 mg gallic acid equivalent/100g FM (Kulkarni and

Aradhya 2005). During osmotic dehydration treatment, a decrease of 40 % (184 mg/100g)

compared to the initial phenolic content was observed (Table 3). This value was lower to that

found in fruits (apple and cherry: 500 mg/100g, strawberry: 330 mg/100g) and higher

compared to vegetable (25 – 100 mg/100g) (Yang et al. 2006).

Moreover, pomegranate seeds showed a regress in total phenolic during the drying

from 184 mg/100 g FM (osmotic dehydrated seeds) to 151, 141 and 134 mg gallic acid

equivalent/100g FM for drying temperatures of 40, 50 and 60 °C, respectively (Table 3) in

agreement with other findings Nicoli et al. (1999); Erbay and Icier (2009). In spite of this

reduction during drying, values remained higher compared to those observed in vegetables

(Yang et al. 2006).

The reduction of total phenolic compounds after osmotic process was due to the

migration of phenolic compounds from pulp to osmotic solution induced by a large osmotic

driving force. This fact was due to the higher difference in concentration between dilute seeds

sap (15 °Brix) and the surrounding hypertonic medium (55 °Brix) (Raoult-Wack et al. 1991).

162


This behaviour has been reported in the osmotic dehydration of pomegranate seeds (Bchir et

al. 2009; Bchir et al. 2010a). During drying, total phenolic compounds significantly decreased

indicating the negative effect of higher temperature on total phenolics compounds. This could

be ascribed to thermal degradation of naturally occurring anti-oxidative compounds present in

pomegranate seeds as flavonoids (anthocyanins), phenolic acid (Madrigal-Carballo et al.

2009; Devic et al. 2010). This result corroborates the findings of Jukunen-Tiitto (1985) and

Harbourne et al. (2009) who reported that an increase in temperature from 40 to 70 °C caused

a decrease of the flavonoid content in willow leaves and meadowsweet. Moreover, enzymatic

and non-enzymatic reaction could be a responsible for the decrease of phenolic compounds in

seeds supported by the increase of the temperature (Jeantet et al. 2006). In fact, phenolic

compounds are the substrate for polyphenol oxidase enzyme. Also, Maillard reaction (non-

enzymatic reaction) use phenolic compounds having, carbonyl functions, like a substrate

(Jeantet et al. 2006).

Total anthocyanin pigments content

Similar to antioxidant activity and total phenolic, anthocyanin pigment decreased from

82 to 68 mg/100g during the first 20 min of osmotic dehydration process (Table 3). This fact

could be due to the migration of anthocyanin pigment from pulp to the osmotic solution

induced by the driving osmotic force. Antioxidant activity was lower to those observed in

strawberry (450 – 700 mg/100g) and in range compared to blueberry (25 – 495 mg/100g),

mulberry (67 – 107 mg/100g) (Cisse et al. 2009).

During heating (from 40 to 60 °C), a decrease in the anthocyanin pigment

concentration was also observed for pomegranate seeds (Table 3). In spite of this decrease of

antioxidant activity, values remained closer to those observed in pulm (30 mg/100g), grapes

(30 -750 mg/100g) and blueberry (25 – 495 mg/100g) (Yang et al. 2006). The highest

concentration of anthocyanin (40 mg/100g FM) was recorded by using the lower temperature

163


(40 °C). In fact, after drying anthocyanin content was reduced by 41, 64 and 70% for drying

temperatures of 40, 50 and 60 °C respectively. Similar trends were observed for pomegranate

seeds anthocyanins (Jaiswal et al. 2010) and black carrot anthocyanins (Kirca et al. 2007)

during heating. The degradation of anthocyanins could be due to enzymatic (polyphenol

oxidase) reaction. In fact, Raynal et al. (1989) reported that polyphenol oxidase playing

important role in oxidative degradation of anthocyanins during the drying of plums.

Moreover, Cemeroglu et al. (1994) founded that the degradation rate of anthocyanins in sour

cherry increased with increasing heating temperature (e.g. 60, 80 °C). In fact, the increase in

the temperature enhanced the modification rate of the anthocyanin chemical structure

favouring its degradation (Jackman and Smith 1996).

The decrease of anthocyanin content contributes to the decline of the colourful

appearance of seeds (Jaiswal et al. 2010).

Relation between antioxidant activity, total phenolic and total anthocyanin pigments

Phenolic compounds, including anthocyanins, display strong antioxidant activity;

contributing significantly to the antioxidant capacity of fruits (Nicoli et al. 1999; Jeantet et al.

2006; Fathi et al. 2009). In fact, the decrease of phenolic compound by 17 % involved a

decrease of the antioxidant activity by 20 % at 40 °C. The percentage of loss in antioxidant

activity remained slightly higher than that observed with total phenolic at different air-drying

temperature. Contrary to others studies, these results showed that the production of new

antioxidant compounds during drying was very weak (Shi et al. 2008; Vega-Galvez et al.

2009).

Colour

The effect of osmotic dehydration and air-drying temperature on seeds colour was

illustrated in table 4. Five chromatics coordinates was used to characterise the changes of

seeds colour during these processes. Seeds colour was found to be dependent on air-drying

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temperature and osmotic process. After osmotic dehydration hue angle (h°) and lightness (L*)

values increased, while an opposite trend was observed for chroma (C*) values. Furthermore,

a* and b* colour parameters showed a slight decrease during osmotic process. These

variations indicated that seeds become less dark during OD. This could be explained due to

the migration of pigment from pulp to the osmotic solution inducing by osmotic driving force.

During drying, hue angle and lightness values decreased with the increase of air-drying

temperature from 84° and 29 to 69° and 23, respectively. This changes indicated the reduction

of colour from a more green (when Hue > 90°) to an orange-red (when Hue <90°) and seeds

turning darker (decreasing of L*). Chroma values, increased with the increase of air-drying

temperature showing that seeds colour became more saturated. Moreover, a* and b* colour

parameters showed a slight increase during drying (Table 4). This modification in seeds

colour is mainly due to the effect of temperature on heat-sensitive compounds such as

carbohydrates, proteins and vitamins, which cause colour degradation during drying process.

According to Mandala et al. (2005) the decrease of “L*” values and the increase of “a*”

values correspond to the increase of fruit browning. To better understand the effect of air-

drying on seeds colour, browning index and total colour difference were determined. It can be

observed that an increase of temperature led to a significant formation of brown products.

Indeed, the maximum browning index was occurred at 60 °C (0.075) as compared to 50 °C

(0.064) and 40 °C (0.051). Similar observations were reported by Miranda et al. (2009) and

Vega-Galvez et al. (2009) using Aleo vera and red pepper, respectively. The total colour

difference (∆E*) value increased slightly with the increase of air-drying temperature (40 °C:

3.0± 0.5; 50 °C: 5.1± 0.2 and 60 °C: 9.8± 0.8). This indicated that seeds became brownish

(Romano et al. 2008). However, ∆E* was lower to those observed in many dried fruits

(Pereira et al. 2007; Chong et al. 2008). In addition, browning index was very low indicating

that air-drying temperature does not have a great influence on the browning of seeds. This

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could be due to the osmotic dehydration pre-treatment. Indeed, Ponting (1973) and Krokida et

al. (2001) showed that dehydration of foodstuffs (e.g. potato) by immersion in osmotic

solutions before convective air-drying, improves the quality of the final product since, it

prevents oxidative browning.

The formation of brown compounds in seeds may be related to both enzymatic and

essentially non-enzymatic (Maillard reaction) reactions (Miranda et al. 2009).

Table 4: Effect of air-drying temperature on chromatics coordinates and on textural properties of seeds.

Dried seeds

Untreated

seeds Osmotic

dehydrated seeds 40 °C 50 °C 60 °C

h° 63.50±2.30 84.60±3.40 77.44±3.10 73.33±3.00 69.47±1.00 C* 15.29±0.10 11.71±0.50 14.30±0.06 15.54±0.48 19.08±0.17 L* 26.31±1.10 28.91±0.17 27.79±1.10 25.90±0.33 22.95±0.01 a* 12.44±0.14 7.80±0.77 10.20±0.41 11.50±0.70 14.65±0.25

Chromatics coordinates

b* 8.90±0.03 8.60±0.02 9.91±0.11 10.40±0.06 12.21±0.01

Hardness [N] 46.73±2.47 63.46±3.04 101.54±4.06 118.61±3.47 123.63±4.91

Toughness [N mm]

54.55±3.96 67.21±5.55 87.01±4.52 92.33±3.24 103.38±4.12

Pip crush [mm] 4.51±0.34 3.70±0.34 2.10±0.17 1.80±0.12 1.65±0.11

Enzymatic browning

Browning colour could be induced by polyphenol oxidase (PPO) present in

pomegranate seeds. PPO was extracted from the pulp and the relative activity was measured

as a function of air drying temperature. Results showed a relative activity of 27 % for PPO in

osmotically dehydration seeds.

The increase in air-drying temperature involved a decline in PPO relative activity. The

relative activity decreased by 4 and 5 % at air drying temperatures of 50 and 60 °C

respectively as compared to that at 40 °C. Our results were in agreement with previous

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findings showing that PPO is a heat-labile compound (Mandala et al. 2005; El-Aouar et al.

2003; Jaiswal et al. 2010).

The presence of PPO in seeds could be responsible for phenolic compounds

(flavonoids, tannins, lignins, phenolic acids) degradation involving colour modification

(Jaiswal et al. 2010). In fact, Saxena et al. (2010) showed that tissue browning is mainly due

to the oxidation of phenolic compounds into quinine compounds under aerobic conditions by

PPO, then the quinine compounds undergoes polymerization forming brown polymeric

pigments, leading to browning. However, Lenart (1996) found that the presence of sugar on

the surface of the osmodehydrated sample, is a barrier for the contact with oxygen thus

reducing the oxidative reactions and the resultant discolouring of the fruit.

The inactivation of PPO by drying prevents the browning reaction in seeds. However

in precedent paragraph, we found that the browning colours increased slightly as function of

air-drying temperature. Therefore, there is another reaction that induced browning reaction.

Many authors found that during drying non-enzymatic browning (Maillard reaction,

caramelisation) was responsible for browning of fruits (Maskan 2001; Lewicki 2006; Miranda

et al. 2009).

Non-enzymatic browning

Maillard reaction, also called sugar-amino browning reaction, which is a form of non-

enzymatic browning, is a chemical reaction between an amino acid and reducing sugar under

heating conditions (Rada-Mendoza et al. 2002). The reactive carbonyl group of the sugar

interacts with the nucleophilic amino group of the amino acid to create hundreds of different

compounds. 5-Hydrorymethylfurfural (HMF) is one of the major intermediate products in the

Maillard reaction (Rada-Mendoza et al. 2002).

It was observed that increasing the air-drying temperature leads to enhanced HMF

content (40 °C: 0.017 mg/100 g FM; 50 °C: 0.019 mg/100 g FM and 60 °C: 0.024 mg/100 g

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FM) compared to osmodehydrated seeds (0.011 mg/100 g FM). However, HMF values of

different seeds were very low showing that air-drying temperatures do not have a great

influence on the formation of HMF. This could be due to the low content of protein, in

osmodehydrated seeds. In fact, protein is a necessary substrate for the Maillard reaction

(Rada-Mendoza et al. 2002). That is confirming the lower browning index and the decrease of

AA% as function of temperature. In fact, low HMF content and the decrease of AA% induced

through the enhancing of the temperature, show that the rate of destruction of antioxidant

compounds was higher than the rate of formation of these compounds. Indeed many authors

found that Maillard reaction let the formation of many antioxidant compounds (e.g.

melanoidins) (Shi et al. 2008; Vega-Galvez et al. 2009).

Texture analysis

Texture analysis of osmodehydrated and dried pomegranate seeds were studied over

time periods of up to 20 min and 4h, respectively (Table 4). Two textural parameters

(Hardness and Toughness) were used to characterize seeds texture modification. Based on the

results, hardness and toughness were affected by osmotic process and air-drying temperature.

In fact, after OD, seeds hardness and toughness increased by 17 % and 13 %, respectively

compared to untreated seeds. During drying, hardness increased by 38, 55 and 60 N, while

toughness also increased by 20, 25 and 36 N mm at drying temperatures of 40, 50 and 60 °C

respectively. This behaviour could be explained by the structural collapse of seeds induced by

the increased water loss during osmotic and drying process (Mandala et al. 2005). As a

consequence of this exchange, the products will more or less lose weight and will shrink

eventually (Aversa et al. 2009). Indeed, the peaks of pip crushing decreased after osmotic

process and drying (Table 4). Thus, compared to untreated seeds, those that were only

osmodehydrated reduced thickness by 18 % and those that were also dried at 40, 50 and 60 °C

lost 43, 51 and 55 % thickness, respectively. Similar results have been reported by Mandala et

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al. (2005), and Bchir et al. (2010a) using the textural changes during the drying of apple and

chempedack and osmotic dehydration of pomegranate seeds, respectively.

Conclusion

Osmotic dehydration (OD) and drying process could be used for the conservation of

pomegranate seeds. Indeed, OD followed by drying allowed to reduce water activity until a

value less than 0.650 after 60, 120 and 240 min at drying temperatures of 60, 50, and 40 °C,

respectively. To reduce energy consumption and improve food quality, it would be interesting

that drying stopped after these times. From the obtained results, it is recommended to use 40

°C since the low influence on the quality parameters of seeds was observed.

The determination of PPO activity and HMF content after drying showed that

enzymatic and non-enzymatic reactions (Maillard reaction) have not a market effect on

browning index, showing the benefit effect of pre-treatment (osmotic dehydration) on colour

stability.

During drying not only the composition of the tissue is changed but also the textures

since seeds reduce their thickness to maximum 55 % using 60 °C. Differential scanning

calorimetry data showed a relation between Tg’ and texture parameters. In fact, water loss of

seeds induced an increase of hardness and toughness and also an increase of Tg’.

These processes permit a microbiological stability but also a degradation of the

nutritional quality of the fruit that remained slightly lower compared to other fruits and

vegetables. It should be interesting to use seeds as ingredients in food formulations.

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References

Ahrné, L., Prothon, F., & Funebo, T. (2003). Comparison of drying kinetics and texture

effects of two calcium pre-treatment before microwave-assisted dehydration of apple

and potato. International Journal of Food Science and technology, 38, 411-420.

AOAC. (1990). Official Methods of Analysis of the Association of Official Analytical 436

Chemists, 15th Ed., (K. Helrich, ed.), Association of Official Analytical Chemists, Inc.,

437 Arlington, Virginia, VA.

Arabhosseini, A., Padhye, S., Huisman, W., Boxtel, A., & Müller, J. (2009). Effect of Drying

on the Color of Tarragon (Artemisia dracunculus L.) Leaves. Food and Bioprocess

Technology, DOI: 10.1007/s11947-009-0305-9, in press.

Aversa, M., Curcio, S., Calabrò, V., & Iorio, G. (2009). Experimental Evaluation of Quality

Parameters During Drying of Carrot Samples. Food and Bioprocess Technology,

DOI: 10.1007/s11947-009-0280-1, in press.

Bchir, B., Besbes, S., Attia, H., & Blecker C. (2009). Osmotic dehydration of pomegranate



Bchir B., Besbes S., Attia H., & Blecker C. (2010a). Osmotic dehydration of pomegranate

seeds (Punica granatum L.): Effect of freezing pre-treatment. Journal of Food Process

Engineering, DOI: 10.1111/j.1745-4530.2010.00591.x, in press.

Bchir B., Besbes S., Karoui R., Paquot M., Attia H., & Blecker C. (2010b). Osmotic

Dehydration Kinetics of Pomegranate Seeds Using Date Juice as an Immersion

Solution Base, Food and Bioprocess Technology, DOI: 10.1007/s11947-010-0442-1,

in press.

Bchir, B., Besbes, S., Giet, J., Attia, H., & Blecker C. Synthèse des connaissances sur la

déshydratation osmotique. Biotechnologie Agronomie Société et Environnement, in

press.

Biglari, F., Abbas. F., & Easa, A. (2008). Antioxidant activity and phenolic content of various

date palm (Phoenix dactelifera) fruits from Iran. Food Chemistry, 107, 1636-1641.

170

https://springerlink3.metapress.com/content/?Author=Akbar+Arabhosseini

https://springerlink3.metapress.com/content/?Author=Sudhakar+Padhye

https://springerlink3.metapress.com/content/?Author=Willem+Huisman

https://springerlink3.metapress.com/content/?Author=Anton+van+Boxtel

https://springerlink3.metapress.com/content/?Author=Joachim+M%c3%bcller

https://springerlink3.metapress.com/content/?Author=Maria+Aversa

https://springerlink3.metapress.com/content/?Author=Stefano+Curcio

https://springerlink3.metapress.com/content/?Author=Vincenza+Calabr%c3%b2

https://springerlink3.metapress.com/content/?Author=Gabriele+Iorio


Carlos, A., & De-Michelis, A. (2009). Comparison of Drying Kinetics for Small Fruits with

and without Particle Shrinkage Considerations. Food and Bioprocess Technology,

DOI: 10.1007/s11947-009-0218-7, in press.

Cemeroglu, B., Velioglu, S., & Isik, S. (1994). Degradation kinetics of anthocyanins in sour

cherry juice and concentrate. Journal of Food Science, 59, 1216-1217.

Chenlo, F., Moreira, R., Herrero, F., & Vazquer, G. (2007). Osmotic dehydration of chestnut

with sucrose: Mass transfer processes and global Kinetics modelling. Journal of Food


Chong, C., Law, C., Cloke, M., Hii, C., & Abdullah, L. (2008). Drying kinetics and product

quality of dried Chempedak. Journal of Food Engineering, 88, 522-527.

Choi, Y., Lee, S., Chun, J., Lee, H., & Lee, J. (2006). Influence of heat treatment on the

antioxidant activities and polyphenolic compounds of Shiitake mushroom, Food

Chemistry, 99, 381-387.

Cisse, M., Dornier, M., Sakho, M., Ndiaye, A., Reynes, M., & Sock, O. (2009). Le bissap

(Hibiscus sabdariffa L.): composition et principales utilisations. Fruits, 179-193.

Defilippi, G., Whitaker, B., Hess-Pierce, B., & Kader, A. (2006). Development and control of

scald on wonderful pomegranates during long-term storage. Postharvest Biology and


Devic, E., Guyot, S., Daudin, J., & Bonazzi, C. (2010). Kinetics of Polyphenol Losses During

Soaking and Drying of Cider Apples. Food and Bioprocess Technology,

DOI: 10.1007/s11947-010-0361-1, in press.

Doymaz, I. (2007). Air-drying characteristics of tomatoes. Journal of Food Engineering, 78,

1291-1297.

El-Aouar, A., Azoubel, P., & Murr, F. (2003). Drying kinetics of fresh and osmotically pre-

treated papaya (Carica papaya L.). Journal of Food Engineering, 59, 85-91.

Erbay, Z., & Icier, F. (2009). Optimization of hot air drying of olive leaves using response

surface methodology. Journal of Food Engineering, 91, 533-541.

171

https://springerlink3.metapress.com/content/?Author=Emilie+Devic

https://springerlink3.metapress.com/content/?Author=Sylvain+Guyot

https://springerlink3.metapress.com/content/?Author=Jean-Dominique+Daudin

https://springerlink3.metapress.com/content/?Author=Catherine+Bonazzi


Espiard, E. (2002). Introduction à la transformation industrielle des fruits. TEC and DOC-

Lavoisier, pp. 181–182.

Fabiano, A.N., Oliveira, F., & Rodrigues, S. (2008). Use of Ultrasoud for Dehydration of

Papayas. Food and Bioprocess Technology, 1, 339-345.

Fadavi, A., Barzegar, M., Azizi, H., & Bayat, M. (2005). Physicochemical Composition of

Ten Pomegranate Cultivars (Punica granatum L.) Grow in Iran. Journal of Food

Science Technology, 11, 113 - 119.

Falade, O., & Onyeoziri, N. (2010). Effects of Cultivar and Drying Method on Color, Pasting

and Sensory Attributes of Instant Yam (Dioscorea rotundata) Flours. Food and

Bioprocess Technology, DOI: 10.1007/s11947-010-0383-8, in press.

Fathi, M., Mohebbi, B., & Razavi, S. (2009). Application of Image Analysis and Artificial

Neural Network to Predict Mass Transfer Kinetics and Color Changes of Osmotically

Dehydrated Kiwifruit. Food and Bioprocess Technology, DOI: 10.1007/s11947-009-

0222-y, in press.

Fathi, M., Mohebbat, M., & Razavi, S. (2010). Effect of Osmotic Dehydration and Air Drying

on Physicochemical Properties of Dried Kiwifruit and Modeling of Dehydration

Process Using Neutral Network and Genetic Algorithm. Food and Bioprocess

Technology, DOI: 10.1007/s11947-010-0452-z, in press.

Friant, N., Marks, B., & Barkker-Arkema, F. (2004). Drying rate of corn. Transaction of the

ASAE, 47, 1605-1610.

Gokhale, S., & Lele, S. (2010). Optimization of Convective Dehydration of Beta vulgaris for

Color Retention. Food and Bioprocess Technology,DOI: 10.1007/s11947-010-0359-8,

in press.

Harbourne, N., Marete, E., & Jacquier. J., & O’Riordan, D. (2009). Effect of drying methods

on the phenolic constituents of meadowsweet (Filipendula ulmaria) and willow (Salix

alba). LWT-Food Science and Technology, 42, 1468-1473.

172


Hernandez, F., Melgarejo, P., Tomas-Barberan, F.A., & Artes, F. (1999). Evolution of juice

anthocyanins Turing ripening of new selected pomegranate (Punica granatum) clones.

European Food Research and Technology, 210, 39-42.

Jackman, R.L., & Smith, J. (1996). Anthocyanins and betalains. Natural food colorants,

Blackie Acadenamic and professional, 244-280.

Jaiswal, V., DerMarderosian, A., & Porter, J. (2010). Anthocyanins and polyphenol oxidase

from dried arils of pomegranate (Punica granatum L.). Food Chemistry, 118, 11-16.

Jaya, S., & Das, H. (2009). Glass Transition and Sticky Point Temperatures and

Stability/Mobility Diagram of Fruit Powders. Food and Bioprocess Technology, 2, 89-

95.

Jeantet, R., Croguennec, T., Schuch, P., & Brulé, G. (2006). Science des aliments, TEC and

DOC-Lavoisier, pp. 121–161.

Jukunen-Tiitto, R. (1985). Phenolic constituents in the leaves of northern willows: methods

for the analysis of certain phenolics. Journal of Agricultural and Food Chemistry, 33,

213-217.

Kang, K., Kim, H., Pyo, J., & Yokozawa, T. (2006). Increase in free radical scavenging

activity of ginseng by heat-processing. Biological and Pharmaceutical Bulletin, 29, 750-

754.

Kingsly, A., & Singh, D. (2007). Drying kinetics of pomegranate arils. Journal of Food

Engineering, 79,741-744.

Kirca, A., Ozkan, M., & Cemerglu, B. (2007). Effects of temperature, solid content and pH on

the stability of black carrot anthocyanins. Food Chemistry, 101, 212-218.

Krokida, M.K., Oreopoulou, V., Maroulis, Z.B., & Marinoskouris, D. (2001). Effect of

osmotic dehydration pre-treatment on quality of French fries. J. Food Engin., 49, 339–

345.

Kuljarachanan, T., Devahastin, S., & Chiewchan, N. (2009). Avolution of antioxidant

compound in lime residues during drying. Food Chemistry, 113, 944-949.

173


Kulkarni, P., & Aradhya, S. (2005). Chemical changes and antioxidant activity in

pomegranate arils during fruit development. Food Chemistry, 93, 319-324.

Lenart, A. (1996). Osmo-convective drying of fruits and vegetables: Technology and

application. Drying Technology, 14, 391-413.

Lewicki, P. (2006). Design of hot air drying for better foods. Food Science and Technology,

17, 153-163.

Madamba, P., Driscoll, R., & Buckle, K. (1996). The thin-layer drying characteristics of

garlic slices. Journal of Food Engineering, 29, 75-97.

Madrigal-Carballo, S., Rodriguez, G., Krueger, C., & Dreher, M. (2009). Reed, J.

Pomegranate (Punica granatum) supplements: Authenticity, antioxidant and polyphenol

composition. Journal of Functional Food, In press, DOI: 10.1016/j.jff.2009.02.005, in

press.

Mandala, I., Anagnostaras, E., & Oikonomou, C. (2005). Influence of osmotic dehydration

conditions on apple air-drying kinetics and their quality characteristics. Journal of Food


Maskan, M. (2001). Kinetics of colour change of kiwifruits during hot air and microwave

drying. Journal of Food Engineering, 48, 169-175.

Miranda, M., Maureira, H., Rodriguez, K., & Vega-Galvez, A. (2009). Influence of

temperature on the drying kinetics, physicochemical properties, and antioxidant capacity

of Aloe Vera (Aloe Barbadensis Miller) gel. Journal of Food Engineering, 91, 297-304.

Mujumdar, A., & Law, C. (2010). Drying Technology: Trends and Applications in

Postharvest Processing. Food and Bioprocess Technology, DOI: 10.1007/s11947-010-

0353-1, in press.

Nicoli, M., Anese, M., & Parpinel, M. (1999). Influence of processing on the antioxidant

properties of fruit and vegetables. Trends in Food Science and Technology, 10, 94-100.

Park, J., Bin, A., & Brod, F. (2002). Drying of pear d’Anjou with and without osmotic

dehydration. Journal of Food Engineering, 56, 97-103.

174


Pereira, N., Antonia, M., & Ahrne, L. (2007). Effect of microwave power air velocity and

temperature on final drying of osmotically dehydrated bananas. Journal of Food


Pinho, R., Guiné, F., Henrriques, F., & Barroca, M. (2009). Mass Transfer Coefficients for the

Drying of Pumpkin (Cucurbita moschata) and Dried Product Quality. Food and

Bioprocess Technology, DOI: 10.1007/s11947-009-0275-y, in press.

Pokharkar, S. M., Prasad, S., & Das, H. (1997). A model for osmotic concentration of banana

slices. Journal of Food Science and Technology, 34, 230- 232.

Ponting, J.D. (1973). Osmotic dehydration of fruits. Resents modifications and applications.

Process Biochemistry, 8, 18-20.

Rada-Mendoza, M., Olano, A., & Villamiel, M. (2002). Determination of

hydroxymethylfurfural in commercial jams and in fruit-based infant foods. Food

Chemistry, 79, 513-516.

Raoult-Wack, A.L., Guilbert, S., Le Maguer, M., & Rios, G. (1991). Simultaneous water and


soaking process analysis (osmotic dehydration). Drying Technnol., 589–612.

Raynal, J., Moutounet, M., & Souquet, J.M. (1989). Intervention of phenolic compound in

plum technology. Changes during drying. J. Agric. Food Chem. 37, 1046-1050.

Robert, C., Soliva, F., Martinez, P., Sebastian, M., & Martin, O. (2002). Kinetics of

polyphenol oxidase activity inhibition of avocado preserved by combined methods.


Romano, R., Baranyai, L., Gottschalk, K., & Zude, M. (2008). An approach for monitoring

the moisture content changes of drying banana slices with laser light backscattering

imaging. Food and Bioprocess Technology, 4, 410-414.

Roos, Y. (1995). Characterisation of food polymers using state diagrams. Journal of Food

Engineering, 24, 339–360.

175

https://springerlink3.metapress.com/content/?Author=Raquel+Pinho+F.+Guin%c3%a9

https://springerlink3.metapress.com/content/?Author=Francisca+Henrriques

https://springerlink3.metapress.com/content/?Author=Maria+Jo%c3%a3o+Barroca


Rosenblat, M., & Hayek, T. (2006). Aviram, M. Anti-oxidative effects of pomegranate juice

(PJ) consumption by diabetic patients on serum and macrophages. Atherosclerosis, 187,

363-371.

Ruiz-Lopez, I., Castillo-Zamudio, R., Salgado-Cervantes, M.A., Rodriguez-Jimenes, G.C., &

Garcia-Alvarado, M.A. (2010). Mass Transfer Modeling During Osmotic Dehydration

of Hexahedral Pineapple Silices in Limited Volume Solutions. Food and Bioprocess


Sá, M.M., Figueiredo, A.M., & Sereno, A.M. (1999). Glass transitions and state diagrams for

fresh and processed apple. Thermochimica Act, 329, 31–38.

Saxena, A., Maity, T., Raju, P., & Bawa, A. (2010). Degradation Kinetics of Colour and Total

Carotenoids in Jackfruit (Artocarpus heterophyllus) Bulb Slices During Hot Air

Drying. Food and Bioprocess Technology, DOI: 10.1007/s11947-010-0409-2, in

press.

Sharma, G., Prasad, S., & Chahar, V. (2003). Moisture transport in garlic cloves undergoing

microwave-convective drying. Food and Bioproducts Processing, 87, 11-16.

Shi, J., Pan, Z., McHugh, T., Wood, D., & Hirschberg, E., Olson, D. (2008). Drying and

quality characteristics of fresh and sugar-infused blueberries dried with infrared

radiation heating. LWT-Food Science and Technology, 41, 1962-1972.

Singh, H., & Sodhi, N. (2000). Dehydration kinetics of onions. Journal of Food Science and


Storey, T. (2007). La grenade, le fruit medicament, Magazine NEXUS. Santé, 51, 46-54.

Uribe, E., Miranda, M., Vega-Galvez, A., Quispe, I., Claveria, R., & Di Scala, K. (2010).

Mass Transfer Modelling During Osmotic Dehydration of Jumbo Squid (Dosidicus

gigas): Influence of Temperature on Diffusion Coefficients and Kinetic Parameters.

Food and Bioprocess Technology, DOI: 10.1007/s11947-010-0336-2, in press.

Uribe, E., Vega-Gálvez, A., Scala, K., Oyanadel, R., Torrico, J., & Miranda, M. (2009).

Characteristics of Convective Drying of Pepino Fruit (Solanum muricatumAit.):

176


https://springerlink3.metapress.com/content/?Author=Tanushree+Maity

https://springerlink3.metapress.com/content/?Author=P.+S.+Raju

https://springerlink3.metapress.com/content/?Author=A.+S.+Bawa

https://springerlink3.metapress.com/content/?Author=Elsa+Uribe

https://springerlink3.metapress.com/content/?Author=Antonio+Vega-G%c3%a1lvez

https://springerlink3.metapress.com/content/?Author=Karina+Di+Scala

https://springerlink3.metapress.com/content/?Author=Romina+Oyanadel

https://springerlink3.metapress.com/content/?Author=Jorge+Saavedra+Torrico

https://springerlink3.metapress.com/content/?Author=Margarita+Miranda


177

Application of Weibull Distribution. Food and Bioprocess Technology,

DOI: 10.1007/s11947-009-0230-y, in press.

Vega-Galvez, A., Di Scala, K., Rodriguez, K., Lemus-Mondaca, R., Miranda, M., Lopez, J.,

& Perez-Won, M. (2009). Effect of air-drying temperature on physico-chemical

properties, antioxidant capacity, colour and total phenolic content of red pepper

(Capsicum annuum, L. var. Hungarian). Food Chemistry, 117, 647-653.

Vinas, P., Campillo, N., Hernandez-Cordoba, M., & Candela, M.E. (1992). Simulation liquid

chromatographic análisis of 5-(hydroxymethyl)-2-furaldehyde and metil anthranilate in

honey. Food Chemistry, 44, 67-72.

Yang, R.Y., Tsou, S. C. S., Lee, T. C., Chang, L. C., Kuo, G., & Lai, P. Y. (2006). Moringa, a

novel plant rich in antioxidants, bioavailable iron, and nutrients. In: C. T. Ho (ed)

Challenges in Chemistry and Biology of Herbs.American Chemical Society,

Washington, D.C. pp. 224-239.

Chapitre 6:

Discussion générale, conclusions et perspectives

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Chapitre 6 : Discussion générale, conclusions et perspectives ___________________________________________________________________________

6.1. Discussion générale

Les fruits et les légumes, de par leur caractère saisonnier, sont disponibles en abondance

pendant de courtes périodes de l’année. La conservation des végétaux au cours des récoltes

peut prolonger leur période de disponibilité. Elle permet ainsi aux producteurs d’écouler leurs

productions à de meilleurs prix et aux utilisateurs de pouvoir les consommer sur de longues

périodes de l’année à des prix raisonnables. Parmi les méthodes de conservation, la

déshydratation osmotique vise à réduire, à moindre coût, le risque d’altération de la qualité

nutritionnelle et organoleptique du produit traité (Ade-Omowaye et al., 2003). Cependant, si

cette technique est appliquée sur de multiples fruits et légumes (pomme, abricot, banane,

carotte, etc.), elle n’a jamais été étudiée auparavant sur les graines de grenade.

Dans ce contexte, le présent travail de thèse s’est attaché à délivrer les bases

scientifiques et techniques pour l’étude des possibilités de conservation des graines de

grenades (variété Tunisienne El-Gabsi) par déshydratation osmotique.

Pendant la DO, les transferts de masse dépendent, d’une part des propriétés intrinsèques

des tissus traités, et d’autre part des conditions opératoires de traitement (pression,

température, composition de la solution d’immersion, durée de traitement). Dans un but

d’optimisation du procédé, nous avons envisagé dans un premier temps d’évaluer l’influence

de la température (30, 40, 50°C), de la composition des solutions osmotiques (saccharose,

glucose, saccharose/glucose) et du temps (0, 20, 60, 120, 180, 240 min) sur la cinétique de

transfert de masse et sur les caractéristiques physico-chimiques de la préparation du fruit

déshydraté osmotiquement.

Deux approches ont été employées afin d’étudier cette cinétique. L’approche

« classique » qui se base sur la détermination de trois paramètres, la perte d’eau (« Water loss

», WL), le gain en solides (« Solids Gain », SG) et la réduction en poids (« weight

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réduction », WR). Ces paramètres sont déterminés par la mesure des solides totaux (Krokida

et al., 2000 ; Riva et al., 2005; Garcia-Segovia et al., 2010). Nous avons aussi eu recours à une

modélisation de la DO au moyen des solutions analytiques de l’équation de Fick, permettant

ainsi de déterminer les coefficients de diffusion de l’eau et du soluté. La deuxième approche,

plus fine, se base sur l’étude des paramètres thermiques (Tg’, ∆H, Tf, UFW) déterminés à

partir de la calorimétrie différentielle à balayage. Cette technique permet de suivre l’évolution

des différentes fractions d’eau (libre et liée) dans le produit au cours du traitement.

Sur base des résultats obtenus, indépendamment de la solution ou de la température, la

cinétique de transfert de matière pouvait se décomposer en deux phases. Une première phase

rapide d'une durée systématiquement voisine de 20 min, durant laquelle l’essentiel des

transferts d’eau et de solutés s’opèrent. Elle était suivie d'une seconde phase marquée par la

forte diminution de l’intensité des échanges. La perte en eau, le gain en solides et la perte de

poids ont subi des variations tout au long du procédé, dont l’amplitude, passées les 20

premières minutes de la phase initiale de transfert, est suffisamment faible pour préconiser

l’arrêt du procédé.

L'augmentation de la température de traitement a eu pour effet de favoriser les transferts

de matière, en augmentant les coefficients de diffusion de l’eau et de solutés. La

détermination des coefficients de diffusion a également montré que la nature du soluté a un

effet significatif sur la cinétique de transfert de masse. En effet, à une même température, le

coefficient de diffusion de l'eau dans une solution de saccharose est supérieur à celui du

glucose, de masse molaire plus important. A l'inverse, le coefficient de diffusion des solutés

est deux fois plus élevé dans le glucose que dans le saccharose. Ainsi on préconise d’utiliser

le saccharose pour une meilleure déshydratation des graines de grenade.

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L’analyse des propriétés thermiques de la pulpe des graines de grenade déshydratées

osmotiquement à 50°C, en utilisant différentes solutions, a confirmé les résultats précédents.

La détermination des fractions d’eau non congelable et congelable a montré une évolution

différente au cours du temps. En effet, le produit final présente une teneur en eau non-

congelable supérieure de 7% à celle de l’eau congelable. Cela favorise une meilleure

stabilisation du produit au cours de l’entreposage. D’autre part, ces résultats montrent qu’il

existe une fraction d’eau fortement liée au produit et qu’il est impossible de l’enlever avec ce

procédé.

On peut conclure à ce stade que pour une meilleure déshydratation des graines de

grenade, les conditions suivantes sont les plus intéressantes: un traitement de 20 min, une

température de 50°C et une solution de saccharose (55°Brix).

Ayant adopté ces conditions opératoires, l’étude a ensuite envisagé une comparaison

entre les graines fraiches et congelées afin de déterminer l’impact du pré-traitement de

congélation sur la cinétique de transfert de masse et sur la qualité organoleptique des graines

de grenade. Pour ce faire, nous avons étudié l’évolution des paramètres physico-chimiques

(MS, °Brix, couleur, conductivité, pH, aw, Deff, etc.) et de transfert de masse (WL, SG, WR).

Afin de mieux élucider l’influence du pré-traitement de congélation sur la qualité

organoleptique des graines de grenade, nous avons mis en œuvre les techniques de

microscopie électronique à balayage et de texturométrie.

Les résultats ont montré que l’utilisation des graines fraîches entraine une

déshydratation plus lente au début du procédé, comparé aux graines congelées. Cependant le

pourcentage de perte en eau le plus élevé a été obtenu à la fin du traitement en utilisant des

graines fraîches. En contrepartie, le pourcentage de gain en solides des graines congelées était

plus important que celui des graines fraîches, même après 360 minutes.

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Cela s’explique, comme le montrent les analyses en microscopie électronique, par une

destructuration cellulaire survenant à la suite de la congélation des graines. En effet, après

congélation les cellules apparaissaient déchirées, irrégulières (en forme) et avec une présence

de zones vides. Ces régions sont probablement engendrées par la croissance des cristaux de

glace au cours de la congélation entrainant ainsi une destruction cellulaire. Cette

décompartimentation provoquée par les cristaux de glace empêche le retour de l'eau au milieu

intracellulaire pendant la décongélation, causant ainsi la perte de sa turgescence. Ceci a des

conséquences pratiques en termes de perte de capacité de la paroi cellulaire d'agir en tant que

barrière semi-perméable ou de diffusion, mais aussi sur la texture du fruit (Kovacs et Meresz

2004).

Il convenait ainsi d’évaluer l’impact de la congélation sur les propriétés sensorielles, en

particulier la texture. Pour cela une méthode d’analyse de la texture a été adaptée en fonction

de la spécificité de la matière première. L’analyse texturale des graines congelées a montré

une diminution des paramètres texturaux (hardness et toughness) traduisant ainsi une perte de

la fermeté. Comparée à la graine fraiche, la graine congelée a perdu 9% de son épaisseur. Cela

confirme les observations précédentes par microscopie électronique.

Dans le contexte de la valorisation d’une deuxième agrofourniture tunisienne, nous

avons substitué une partie du saccharose, utilisé pour la préparation de la solution

d’immersion, par du jus de datte, qui contient naturellement ~19 % de matière sèche

composée essentiellement par du sucre (~ 53 g/100 g MS de saccharose et ~ 35 g/100 g MS

de sucre réducteur). Le jus de datte utilisé permet de valoriser les écarts de triages de datte qui

représentent 30% de la production, ce qui constitue un tonnage énorme avoisinant 30.000

tonnes/an pour la Tunisie et avoisinant 2.000.000 tonnes/an dans le monde (Besbes et al.,

2009). Ces quantités importantes de dattes ne sont pas consommées par l’Homme pour

plusieurs raisons : faible qualité gustative, texture trop dure, contamination par des

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champignons ou des insectes ou tout simplement parce qu’elles sont dévalorisées face à des

dattes plus attractives.

L’étude de la cinétique de DO des graines de grenade dans des solutions à base de jus de

datte montre le même profil que celui observé lors de l’utilisation des solutions de glucose ou

de saccharose à 55°Brix. Cependant, il présente des valeurs de perte en eau et de gain en

solides légèrement plus faibles. Cela pourrait être dû à la présence de plusieurs types de

sucres dans le jus de datte, en comparaison d’une solution qui contient seulement de

saccharose à 55°Brix. Ici encore, les changements les plus cruciaux sont intervenus pendant

les 20 premières minutes du procédé, entraînant une perte en eau à ~ 39%. Après cette période

de déshydratation, le pourcentage de perte d'eau a encore légèrement augmenté, atteignant une

moyenne de ~ 40%. La même tendance a été observée pour la réduction en poids, le gain en

solides.

Après DO, les modifications des caractéristiques physico-chimiques des graines ont été

étudiées. Les résultats ont montré une modification de la qualité intrinsèque des graines de

grenade caractérisée par un gain de sucre et une perte significative de protéines et d’éléments

minéraux. Concernant la solution d’immersion, une amélioration de la qualité sensorielle a été

observée, suite a un gain de solutés issu des graines. Ainsi, la solution osmotique pourrait être

incorporée dans des formulations alimentaires, contribuant ainsi à une meilleure valorisation

du produit fini.

Notre étude a également porté sur l’observation microscopique et l’analyse de texture

des graines de grenade avant et après DO. Les résultats ont montré, après DO, des cellules

déformées, déchirées, avec des formes plus irrégulières. Cela est probablement dû à la

plasmolyse des cellules, causée par la perte en eau par osmose, mais aussi à la solubilisation

des polysaccharides (cellulose, hémicellulose et pectine) qui composent les membranes des

cellules. Ces modifications ont un effet direct sur la texture de la graine. En effet, les résultats

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de l’analyse texturale ont montré une modification de la fermeté des graines, induite par une

réduction de leurs tailles, ce qui vient conforter les résultats des observations microscopiques.

Les modifications texturales (hardness et toughness) sont plus faibles que ceux enregistrées

lors de l’utilisation d’une solution d’immersion à base de saccharose. Ainsi, la teneur en

saccharose est apparue comme le principal facteur influençant la texture de la graine, mais

aussi la viscosité du jus de datte.

La DO seule ne pourrait pas maintenir une stabilité du produit au cours de la

conservation. En effet, l’activité d’eau du produit fini après DO est proche de 0,9. Ainsi dans

un but plus appliqué, un traitement supplémentaire de séchage par entrainement (2 m/s durant

4 heures) a été mis en place afin de réduire l’activité d’eau à une valeur inférieure à 0,65.

L’objectif étant d’améliorer la conservation des graines de grenade.

Avant le procédé de séchage, les graines de grenade ont été soumises à une

déshydratation osmotique en utilisant les conditions précédentes (20 minutes de traitement,

une température de 50°C et une solution de saccharose à 55°Brix). Afin d’optimiser le

traitement de séchage, nous avons étudié au premier lieu l’effet de la température (40, 50,

60°C) et du temps du traitement (0, 30, 60, 120, 180, 240 min) sur l’évolution de la matière

sèche (MS), l’activité d’eau (aw) et le pourcentage de séchage (DR) des graines. Une méthode

plus fine a été adoptée, qui consiste à analyser les propriétés thermiques de la pulpe des

graines de grenade par calorimétrie différentielle à différentes températures, afin de

déterminer l’évolution des différentes fractions d’eau dans la graine après 240 min de

traitement.

Les résultats obtenus ont montré que, quelle que soit la température de séchage utilisée,

la cinétique de séchage des graines apparaît comme la juxtaposition de deux périodes

distinctes. La première période, durant les premières 180 minutes, correspond à une diffusion

facile de l’eau, de l'intérieur de la graine vers sa surface et ainsi une évaporation de l'eau libre

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pendant le séchage. Cette évaporation est due à une différence de pression partielle d’eau

entre l’air et la surface du produit (Jeantet et al., 2006). La deuxième période, de 180 à 240

minutes, correspond à une diffusion de l'eau rendue difficile à la suite de la modification

structurale de la graine pendant le séchage (Torreggiani et Bertolo, 2001).

La variation de la température de séchage a eu un effet significatif sur le coefficient de

diffusion, l’activité d’eau, la matière sèche, le pourcentage de séchage, et sur les paramètres

thermiques relevés en DSC. L’analyse des propriétés thermiques de la pulpe des graines de

grenade à différentes températures a montré une élimination totale de l’eau libre à partir de

40°C après 240 min de traitement. En considérant le profil thermique obtenu, on peut

considérer qu’il s’agit d'un produit assez stable.

La valeur cible de l’activité d’eau (0,65) a été atteinte après 60, 120, et 240 min

respectivement à 60, 50, et 40°C. Ainsi, on préconise d’arrêter le procédé après ces temps

respectifs afin de minimiser le besoin énergétique du procédé.

Le séchage est de moins en moins performant au regard des exigences croissantes en

matière de qualité des produits finis. Ainsi nous avons jugé utile d’étudier l’effet de la

température (40, 50, 60°C) de séchage sur plusieurs paramètres de qualité des graines de

grenade tels que l’activité antioxydante, la teneur en composés phénoliques, les

anthocyanines, la couleur, et la texture.

Le séchage par entrainement, ainsi que la déshydratation osmotique, ont exercé une

influence significative sur la qualité des graines. En effet, le procédé de DO a entraîné une

réduction de l'activité de scavenging du radical diphénylpicryl-hydrazyl (DPPH). Cette

réduction est suivie par une diminution des teneurs en composés phénoliques et en

anthocyanines. Ce phénomène est accentué par des températures de séchage de plus en plus

élevées. Cependant, ces valeurs restent comparables (voires supérieures) celles observées

chez d’autres fruits (date, raisin etc.) (Yang et al., 2006 ; Bilgari et al., 2008).

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Les composés phénoliques, y compris les anthocyanines, contribuent significativement à

l’activité antioxydante du fruit. En effet, une réduction mesurée de la teneur en composés

phénoliques de 17% a été acompagnée par une réduction de 20% de l’activité antioxydante à

40°C. Le pourcentage de perte de l’activité antioxydante diminue en fonction de

l’augmentation de la température et reste toujours supérieur à celui de la perte des composées

phénoliques, ce qui montre que la production de composés antioxydants au cours du séchage

est très faible.

L’étude de la qualité organoleptique a montré que les paramètres chromatiques

(luminosité : L*, saturation : C* et angle de teinte : h°) ainsi que l’indice de brunissement ont

été affectés par le procédé de séchage, qui a contribué à une modification de la couleur des

graines de grenade. Afin de pouvoir mieux cerner l’origine de cette décoloration, il était

impératif de déterminer l’activité polyphénol oxydase (PPO) et la teneur en

hydroxyméthylfurfural (HMF). Dans cette optique, les résultats ont montré une faible activité

PPO et des teneurs réduites en HMF dans les graines de grenade après séchage. Par

conséquent, les réactions enzymatiques et non-enzymatiques (réaction de Maillard) n’ont pas

une influence significative sur le brunissement de la graine. Cela confirme les faibles valeurs

de l’indice de brunissement. Outre la couleur, la combinaison entre la DO et le séchage a

influencé la forme et la texture, puisque les graines ont perdu jusqu'à 55% de leur épaisseur

après séchage à 60°C.

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6.2. Conclusion générale et perspectives :

Ce travail a permis de mettre en place, pour la première fois, un procédé global de

conservation des graines de grenade (Punica granatum L.). Ce procédé se compose de trois

étapes : un pré-traitement de congélation suivi par une déshydratation osmotique et terminé

par un post-traitement de séchage par entrainement. Plusieurs paramétres d’optimisation du

procédé ont été étudiés, nous permettant de retenir les conditions suivantes : une

déshydratation osmotique pendant 20 min en utilisant des graines congelées, une température

de 50°C et une solution de saccharose avec un extrait sec soluble de 55°Brix. Concernant le

séchage par entrainement, nous préconisons d’utiliser une température de 40°C, afin de

reduire la dégradation de la qualité nutritionelle des graines après 240 min du traitement.

A la fin de ce procédé, l’activité d’eau de la graine est inférieure à 0,65. Un tel niveau

d’activité d’eau assure une bonne stabilité au cours de l’entreposage. L’étude de la qualité

nutritionnelle des graines après DO et séchage révèle des pertes significatives en activité

antioxydante, composés phénoliques, anthocyanines qui laissent néanmoins au produit des

valeurs nutritionnelles intéressantes, et proches de celles d’autres fruits n’ayant subi aucun

traitement.

Cette thèse a également contribué à une valorisation d’une deuxième agrofourniture

tunisienne : les écarts de triage des dattes. A partir de ces dattes, nous avons préparé du jus

contenant naturellement du sucre, qui représente 35% de la quantité totale de sucre des

solutions d’immersion à 55°Brix. Cette substitution permet une amélioration de la qualité

organoleptique des graines, au travers d’un gain de solutés naturels au cours de la DO, mais

aussi une réduction du coût économique du procédé.

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Cette recherche doctorale a conduit à plusieurs développements méthodologiques

adaptés aux caractéristiques des graines de grenade, en ce qui concerne en particulier

l’analyse de la texture et de la structure des graines.

Notons enfin qu’au dela des conclusions qui viennent d’être tirées, ce travail montre des

caractéristiques du produit fini qui semblent pouvoir justifier de nouvelles voies de

transformation et d’exploitation des graines de grenade. Pour consolider ce concept on se

propose dans un futur travail :

D’optimiser et de comparer l’effet des différents méthodes de séchage telles que :

le séchage solaire, la micro-onde sur les propriétés physico-chimique (vitamines, minéraux,

etc.) et antioxydante (composés phénoliques, etc.) des graines de grenade, afin de réduire la

dégradation de leurs qualité organoleptique et nutritionnelle.

D’étudier l’incorporation des graines de grenade séchées dans une matrice

alimentaire tel que le yoghourt ou le pain, et la caractérisation organoleptique (texture,

couleur, etc.) du produit fini. Cela permettra la consommation de ce fruit sous plusieurs

formes tout au long de l’année.

D’étudier la stabilité microbiologique (coliformes, flore totale et fongique) et

physico-chimique (teneur en anthocyanines et en polyphénols etc.) au cours du stockage des

graines séchées et incorporées dans des formulations alimentaires, ce qui permet de suivre le

vieillissement du produit au cours du temps.

D’envisager une étude sensorielle, autre que la texture en particulier, pour

déterminer le niveau de la satisfaction du consommateur envers les graines séchées et

incorporées dans des formulations alimentaires.

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Pour que ce travail de laboratoire concernant la conservation des graines de grenade

soit clos et que l’on puisse passer à son extrapolation à l’échelle industrielle, quelques

éléments restent à réaliser tels que :

- L’exploitation des résultats obtenus à l’échelle laboratoire pour passer à l’échelle

pilote puis par extrapolation à l’échelle industrielle ;

- L’étude des équipements industriels de DO et de séchage ;

- La réalisation d’une étude du marché et du coût économique global du procédé.

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6.3. Références bibliographiques

Ade-Omowaye B., Rastogi N., Angersbach A. & Knorr D. (2003). Combined effects of

pulsed electric field pre-treatment and partial osmotic dehydration on air drying

behaviour of red bell pepper. Journal of Food Engineering, 60, 89-98.

Adsule, N.R., & Patill, N.B. (1995). Pomegranate. In: Salunkhe, D.K., Kadam, S.S., ed.

Handbook of Fruit Science and Technologies: Production, Composition, Storage and

Processing. New York, USA, pp.455-463.

Aslam, M.N., Lanky, E. P., & Varani, J. (2006). Pomegranate fractions promote proliferation

and procollagen synthesis and inhibit matrix metalloproteinase-1 production in human

skin cells. Journal of Ethno-Pharmacology, 103, 311-318.

Ben-Thabet, I., Attia, H., Besbes, S., Deroanne, C., Francis, F., Drira, N., & Blecker, C.

(2007). Physicochemical and Functional Properties of Typical Tunisian Drink: Date

Palm Sap (Phoenix dactylifera L.). Food Biophysics, 2, 76-82.

Besbes, S., Drira, L., Blecker, C., Deroanne, C., & Attia, H. (2009). Adding value to hard date

(Phoenix dactylifera L.): compositional, functional and sensory characteristics of date

jam. Journal of Food Chemistry, 112, 406-411.

Biglari, F., Abbas. F., & Easa, A. (2008). Antioxidant activity and phenolic content of various

date palm (Phoenix dactelifera) fruits from Iran. Food Chemistry, 107, 1636-1641.

Cheikh-Rouhou, S., Besbes, S., & Blecker, C. (2008). Black cumin (Nigella sativa L.) and

allepo pine (Pinus halepensis Mill.) seeds oils: stability during thermal oxidation at

60°C and 100°C. Microbiol. Hyg. Alim. 19, 12-20.

Curtay, J.P., Jacob, L., Jung, R.R., & Kaplan, M. (2008). Jus de grenade fermenté, la grenade,

“aliment-plus” un nouvel outil puissamment cardiovasculaire et anti-cancer dans

l’arsenal de la nutrithérapie, Ed. Macro pietteur, Paris, pp. 2-73.

Dermesonlouoglou, E.K., Pourgouri, S., & Taoukis, P.S. (2008). Kinetic study of the effect of

the osmotic dehydration pre-treatment to the shelf life of frozen cucumber. Innovative

190

http://www.springerlink.com/content/119976/?p=b397cc6760c14364921c2660fb185063&pi=0


Food Science and Emerging Technologies, 9, 542-549.

Edas, S. (2009). Polyphénols et jus de grenade. Pharmacognosie, 7, 1-5.

Emna, A. (2010). Impulser l’investissement agricole privé. Magazine presse économique

Tunisie, n°3, pp. 15-16.

Espiard, E. (2002). Introduction à la transformation industrielle des fruits. TEC&DOC-

Lavoisier, Paris, France, pp. 181-182.

FAOSTAT. (2009). Bases de données statistiques de la FAO, Food and Agriculture

Organization of the United Nations, Rome.

Fernandes, F., Rodrigues, S., Gaspareto, O., & Oliveira, E. (2006). Optimization of osmotic

dehydration of papaya followed by air-drying. Food Research International, 39, 492-

498.

Garca, M., Suana, D., Dulce, A., & Acinda, N. (2004). The effect of two methods of

pomegranate (Punica granatum L) juice extraction on quality during storing at 4°C.

Journal of Biomedicine and Biotechnology, 5, 332 - 337.

Garcia-Martinez, E., Martinezmonzo, J., Camacho, M., & Martineznavarrete, N. (2002).

Characterisation of reused osmotic solution as ingredient in new product

formulation. Food Research International, 35, 307-313.

Garcia-Segovia, P., Mognetti, C., André-Bello, A. & Martinez-Monzo, J. (2010). Osmotic

dehydration of Aloe vera (Aloea barbadensis Miller). Journal of Food Engineering,

97, 154-160.

Hernandez, F., Melgarejo, P., Tomas-Barberan, F.A., & Artes, F. (1999). Evolution of juice

anthocyanins Turing ripening of new selected pomegranate (Punica granatum L.)

clones. European Food Research and Technology, 210, 39-42.

Jaiswal, V., DerMarderosian, A., & Porter, J. (2010). Anthocyanins and polyphenol oxidase

from dried arils of pomegranate (Punica granatum L.). Food Chemistry, 118, 11-16.

Jeantet, R., Croguennec, T., Schuch, P., & Brulé, G. (2006). Science des aliments, TEC and

DOC-Lavoisier, Paris, pp. 121–161.

191


Jena, S., & Das, H. (2004). Modelling for moisture variation during osmo-concentration in

apple and pineapple. Journal of Food Engineering, 66, 425-432.

Kovacs, E., & Meresz, P. (2004). The effect of harvesting time on the biochemical and

ultrastructural changes in Idared apple. Acta Alimentaria, 33, 285-296.

Kowalska, H., Lenart, A., & Leszczyk, D. (2008). The effect of blanching and freezing on


Krokida, M.K., Karathanos, V.T., & Maroulis, Z.B. (2000). Effect of osmotic dehydration on

colour and sorption characteristics of apple and banana. Drying Technology, 18, 937-

950.


of osmodehydrated fruits from lemon and date by-products. Journal of Food Science

and Technology international, 13, 405-412.

Mujumdar, A. S. (2006). Handbook Of Industrial Drying, 3rd Edition. Taylor and Francis

Group, LLC, 688-700.

Rastogi, N.K., & Raghavarao, K.S. (2004). Mass transfer during osmotic dehydration of

pineapple: considering Fickian diffusion in cubical configuration. Lebensmittel-

Wissenschaft und-Technologie, 37, 43-47.

Riva, M., Campolongo, S., Leva, A.A., Maestrelli, A., & Torreggiani, D. (2005). Structure

property relationships in osmo-air-dehydrated apricot cubes. Food Research

International, 38, 533-542.

Shi, J., Pan, Z., McHugh, T., Wood, D., Hirschberg, E., & Olson, D. (2008). Drying and

quality characteristics of fresh and sugar-infused blueberries dried with infrared

radiation heating. LWT-Food Science and Technology, 41, 1962-1972.

Storey, T. (2007). La grenade, le fruit médicament, Magazine NEXUS. Santé, France, 51, 46-

54.



192


193

Uddin, M., Ainsworth, P., & Ibanoglu, S. (2004). Evaluation of mass exchange during

osmotic dehydration of carrots using response surface methodology, Journal of Food


Wang, W.C., & Sastry, S.K. (2000). Effects of thermal and electrothermal pre-treatment on

hot air drying rate of vegetable tissue. Journal of Food Process Engineering, 23, 299-

319.

Yang, R.Y., Tsou, S. C. S., Lee, T. C., Chang, L. C., Kuo, G., Lai, P. Y. (2006). Moringa a

novel plant rich in antioxidants, bioavailable iron, and nutrients. In: C. T. Ho (ed)

Challenges in Chemistry and Biology of Herbs.American Chemical Society,

Washington, USA, D.C. pp. 224-239.

Liste des figures

194

Liste des figures ___________________________________________________________________________

Chapitre 1 : .............................................................................................................................. 7

Figure 1. Les différents systèmes de mise en contact des phases (solution osmotique et

l’aliment) pour une déshydratation osmotique (Marouzé et al., 2001)..................... 32 Chapitre 2 : ............................................................................................................................ 44

Figure 1’. Différentes étapes du procédé de déshydratation osmotique.............................. 46 Figure 1. Variation of water loss (WL) (a) weight reduction (WR) (b) and solids gain (SG)

(c) with time and temperature (30, 40, 50°C) using sucrose solution during osmotic dehydration ............................................................................................................... 61

Figure 2. DSC thermogram obtained for pomegranate seeds soaked in sucrose solution at 50°C .......................................................................................................................... 65

Figure 3. Comparison of water loss (WL) (a) and solids gain (SG) (b) using different osmotic solutions (sucrose, glucose and mixture sucrose & glucose) at 50°C......... 70

Chapitre 3 : ............................................................................................................................ 77

Figure 1’. Différentes étapes du procédé de déshydratation osmotique des fraiches et

congelées................................................................................................................... 79 Figure 1. Comparison of WL and SG using fresh (× WL, ● SG) and frozen (ΔWL, ○SG)

seeds during osmotic dehydration process................................................................ 91 Figure 2. Scanning electron microscopy photographs of fresh (a), frozen (b) and

osmodehydrated fruits prepared with fresh (c) and frozen (d) seeds........................ 97 Figure 3. Characteristic force-distance curve for texture analysis using fresh seeds.......... 98

Chapitre 4 : .......................................................................................................................... 107 Figure 1’. Procédé de déshydratation osmotique des graines de grenade dans du jus de

datte......................................................................................................................... 109 Figure 1. D Water loss (WL: ♦), weight reduction (WR: ◌) and solids gain (SG: ×) from

the osmodehydrated seeds using Deglet Nour as an immersion solution base....... 122 Figure 2. D Scanning electron microscopy photographs of frozen (a) and osmodehydrated

seeds (b) and a characteristic force-distance curve for texture analysis using frozen (•) and osmodehydrated (‒) seeds (c)...................................................................... 128

Figure 3. Flow behaviour of the osmotic solutions after 120 min of the process ............. 131 Chapitre 5 : .......................................................................................................................... 139

Figure 1’. Différentes étapes des procédés de déshydratation osmotique et de séchage des

graines de grenade................................................................................................... 141 Figure 1. Variation of water activity (a) and dry matter (b) of pomegranate seeds as a

function with time and temperature (40, 50, 60 °C) ............................................... 158 Figure 2. DSC thermograms obtained for osmodehydrated (OD) and dried pomegranate

seeds at different temperature (40, 50, 60 °C) ........................................................ 160

195

Liste des tableaux

196

Liste des tableaux ___________________________________________________________________________

Chapitre 1 : .............................................................................................................................. 7 Tableau 1. Application de la déshydratation osmotique sur des fruits (1a) et légumes (1b)

................................................................................................................................... 14 Tableau 2. Les conditions optimales de déshydration osmotique des fruits (2a) et légumes

(2b). ........................................................................................................................... 23 Tableau 3. Différents travaux utilisant une solution ternaire pour le traitement des fruits par

déshydratation osmotique ......................................................................................... 28 Chapitre 2 : ............................................................................................................................ 44

Tableau 1. Chemical characteristic of pomegranate seeds .................................................. 57 Tableau 2. Evolution of osmotic dehydration parameters in sucrose solution at different

temperatures 30, 40, and 50 °C................................................................................. 58 Tableau 3. Water and solids effective diffusivities calculated by Fick’s model ................. 60 Tableau 4. Values of Peleg’s equation parameters for water loss and solids gain .............. 62 Tableau 5. CieLab coordinates of sucrose solution at different temperatures 30, 40, and 50

°C .............................................................................................................................. 64 Tableau 6. Differential scanning calorimetry results for pomegranate seeds over soaking

time in sucrose, glucose and mixture sucrose and glucose solution at 50 °C........... 68 Chapitre 3 : ............................................................................................................................ 77

Tableau 1. Chemical characteristic of pomegranate seeds .................................................. 90 Tableau 2. Evolution of osmotic dehydration parameters in sucrose solution using frozen

and fresh seeds .......................................................................................................... 94 Tableau 3. Water and solids effective diffusivities calculated by Fick’s model and values of

Peleg’s equation parameters...................................................................................... 95 Tableau 4. Textural properties of pomegranate seeds ....................................................... 100

Chapitre 4 : .......................................................................................................................... 107

Tableau 1. Evolution of osmotic dehydration parameters of pomegranate seeds and osmotic

solution (using Deglet Nour date juice) .................................................................. 125 Tableau 2. Physico-chemical properties of pomegranate seeds before and after osmotic

dehydration process ................................................................................................ 126 Chapitre 5 : .......................................................................................................................... 139

Tableau 1. Chemical characteristic of pomegranate seeds ................................................ 155 Tableau 2. Effective diffusivities calculated by Fick’s model and values of Peleg’s

equation parameters (K1 and K2) ............................................................................ 157 Tableau 3. Value of total phenolic, anthocyanin and antioxidant activity of untreated,

osmotic dehydrated and dried seeds........................................................................ 162 Tableau 4. Effect of air-drying temperature on chromatics coordinates and on textural

properties of seeds................................................................................................... 166

197

Productions scientifiques ___________________________________________________________________________

PRODUCTIONS SCIENTIFIQUES

Publications et communications scientifiques réalisées dans le cadre du Doctorat

1. Publications

Bchir B., Besbes S., Giet, J., Attia H., & Blecker C. 2010. Synthèse des connaissances sur la

déshydratation osmotique. Biotechnologie Agronomie Société et Environnement (in

press).

Bchir B., Besbes S., Attia H., & Blecker C. 2009. Osmotic dehydration of pomegranate seeds:

Mass transfer kinetics and DSC characterisation. International Journal of Food


Bchir B., Besbes S., Attia H., & Blecker C. 2010. Osmotic dehydration of pomegranate seeds

(Punica granatum L.): Effect of freezing pre-treatment. Journal of Food Process

Engineering, DOI: 10.1111/j.1745-4530.2010.00591.x.

Bchir B., Besbes S., Karoui R., Paquot M., Attia H., & Blecker C. 2010. Osmotic dehydration

kinetics of pomegranate seeds using date juice as an immersion solution base, Food

and Bioprocess Technology, DOI: 10.1007/s11947-010-0442-1.

Bchir B., Besbes S., Karoui R., Attia H., Paquot M., & Blecker C. 2010. Effect of air-drying

conditions on physico-chemical properties of osmotically pre-treated pomegranate

seeds, Food and Bioprocess Technology, DOI: 10.1007/s11947-010-0469-3.

2. Communications

Bchir B., Roiseux O., Attia H., Deroanne C., & Blecker C. Contribution to the valorisation of

pomegranate (Punica granatum L.). 11ème édition du BioFrorum organisé par

l’université de liège, Belgique, 11 octobre 2007, (Poster).

Bchir B., Besbes S., Karoui R., Paquot M., Attia H., & Blecker C. Utilisation du jus de datte

comme milieu d’immersion pour la déshydratation osmotique des graines de grenade

(Punica granatum L.). Journée scientifique de la Société Royale de Chimie sur le

thème « Chimie verte » organisé par l’Université de Liège Gembloux Agro-bio tech,

Belgique, 14 octobre 2010, (Poster).

198

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