Food Chemistry 9 (1982) 115-129
THE CHEMISTRY OF FLAVOUR AND TEXTURE GENERATION IN CHEESE
Laboratoire de Recherches sur les Aromes, lnstitut National de la Recherche Agronomique, 21034 Dijon Cedex, France
J. C. GRIPON & L. VASSAL
Laboratoire de Biochimie et Technologie Laitieres, lnstitut National de la Recherche Agronomique, 78350 Jouy-en-Josas, France
(Received: 11 December, 1981)
Cheese texture and flavour are obtained through a series of chemical changes which occur in the curd during the early stages of ripening. The lowering of pH and Eh, a result of lactic bacteria metabolism, greatly influences texture through water and mineral contents, but has also further repercussions on some chemical changes. Lipid hydrolysis leads to free fatty acids which serve as a substrate for further reactions. Proteolysis influences texture, but mainly flavour, as it results in the formation of peptides and amino acids which, for flavour, leads to aroma compounds through enzymatic and, perhaps, purely chemical reactions.
Milk has unique nutritious properties but, as it is highly perishable, unless properly heat-treated and refrigerated, it has a very short shelf-life. For centuries cheese making has been the only means of preserving the most valuable constituents of milk, and among the great variety of cheese types some can be considered as products with real long-term storage possibilities.
Starting from a liquid which, although it is not flavourless, has normally a very bland aroma, the cheese maker can, using different technologies, create a series of new products varying even within the same type of cheese over a large range of texture and flavour properties. Differences can also be encountered between cheeses
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116 J. ADDA, J. C. GR1PON, L. VASSAL
from the same batch; this is particularly true for soft types, and it is not unusual to find detectable sensorial differences, even between the two sides of the same cheese. These texture and flavour properties are not obtained until after a ripening period, the length of which varies with the type of cheese, and cannot be maintained at their best for an indefinite period of time. This means that what we have to observe is not constant with time. As a consequence of the heterogeneous nature of the product, and of the complexity of its constitution, the chemical basis of cheese flavour and aroma has not yet been elucidated, despite a large number of publications, most of which have placed emphasis on the volatile aroma, permitting, in some cases, one to obtain an insight into the broad mechanism, but, regrettably, still leaving many questions unanswered. The texture of cheese, even though it is recognised as important for consumer preferences, has not, on the whole, been very extensively studied. This is why it is difficult to discuss the chemical basis of texture and flavour in cheese, the more so as no new results have been published since the subject was excellently reviewed by others (Behnke, 1980; Law, 1981).
Protection of the valuable milk constituents against spoilage is achieved by raising the dry matter content through clotting of milk protein, with subsequent elimination of whey, lowering the pH by lactic acid starters, and adding a certain amount of salt to the curd (salting or brining).
The fresh curd thus obtained is still rather bland in flavour and has a texture which differs considerably from the well-ripened products. These properties will be obtained after a series of enzymatic or non-enzymatic reactions. Water-soluble substances, fat and protein will follow an evolution which can be more or less controlled by varying the parameters such as moisture content, pH and Eh of the curd.
INFLUENCE OF pH AND PHYSICO-CHEMICAL CONDITIONS
The basic reaction in cheese making is the production of lactic acid by starters. Carbohydrates are fermented via the well known hexose diphosphate pathway to pyruvic acid. Lactic acid is then formed from pyruvic acid, which acts here as a hydrogen acceptor, so that the reduced NAD can be reoxidised for a further oxidation of glucose.
The lactic acid production makes the pH drop to a certain value which determines the future formation of the cheese. In Camembert cheese, for example, the pH drops to about 4.6, and sometimes lower, as a consequence of the large amount of lactose which remains in the curd still rich in water at the end of draining. The acidity of the curd leads to an important, almost total, solubilisation of the phosphates and calcium, linked to the protein micelles, and temporarily lowers the activity of the lipolytic and proteolytic enzymes (Mocquot, 1971). The calcium level is an important factor, as this element acts as a cement in the cheese body. The difference
FLAVOUR AND TEXTURE GENERATION IN CHEESE 1 17
in cohesion between the body of an Emmental (0'9 to 1.0 ~oCa) and that of a soft cheese (0.2 to 0.3 ~o Ca) is obvious. This, together with the water content, limits the size of each type of cheese and, consequently, the possible ripening time. Mineral equilibrium also plays a r61e in the texture modification during ripening. Thus, as Camembert ripens, there is an important Ca-transport which, initially, is more or less uniformly present in the curd, towards the outside of the cheese (Metche & Fanni, 1978). The core, with a low Ca content, keeps acidic and firm with a low proteolysis. Lactic acid later serves as a substrate for surface flora, allowing the pH to rise to a level where enzymes become more active, leading to a highly flavoured product. Moreover, the increase of pH itself may also contribute to the softening of cheeses such as Camembert (Noomen, 1977).
The technique of washing the curd~ which is sometimes used in soft cheese technology, results in less acidity and more minerals with, as a consequence, a different body. The more neutral flavour is a consequence of the lower activity of the Penicil l iurn and metabolism is limited by the low level of available lactic acid. This may serve as an example of the late influence of lactic acid fermentation on the cheese's development.
Another consequence of the development of a lactic acid flora is the lowering of the Eh (Galestoot & Kooy, 1960) to a potential of about - 130 mV or lower. As we shall see later, the existence of a negative potential will help to explain why some conversions occur and others are made impossible. At least, in Cheddar, reducing conditions achieved artificially have been shown to be essential for the production of key aroma compounds (Manning, 1979).
As well as metabolising lactose, starters can also use citrate as a substrate. This results in the production of pyruvic acid from oxaloacetic acid with acetic acid and CO 2 as by-products. This pyruvate, as it is not needed for reoxidising the reduced NAD, is used to produce diacetyl, according to a mechanism which is pH- and oxygen-dependent (Collins, 1972; Dwivedi, 1973).
Pyruvate is first decarboxylated to acetaldehyde TPP complex, which reacts with a molecule of acetyl CoA to form, directly, diacetyl. In many instances, diacetyl is reduced enzymatically to acetoin, which is subsequently reduced to 2,3-butylene glycol.
LIPIDS AND THEIR DEGRADATION
Fat plays a very important r61e in the development of a good texture, and it is well known that a higher fat content leads to a less firm and elastic body. During recent years there has been an increased interest in cheeses (Cheddar and Swiss) with lower fat content (20 to 30 ~o only) and the consumer has been able to notice the excessive firmness and lack of smoothness of such cheeses. These differences can be explained by the presence of more protein matrix in the cheese (Emmons et al., 1980). From a
118 J. ADDA, J. C. GRIPON, L. VASSAL
strictly practical point of view, it is necessary to raise the water content of the non-fat matter in order to obtain a texture more identical to that of normal cheese.
The reduction of the size of fat globules does not produce a distinct difference in the texture of Cheddar and the slight decrease in firmness and elasticity noticed in cheeses made from homogenised milk could result from a small increase in the water content (Emmons et al., 1980).
Fat composition can also have an influence on the texture. A relationship has indeed been observed in Emmental between firmness and iodine value (IV) (Steffen, 1975). A higher IV (i.e. a more unsaturated fat) resulted in a softer body. Along the same lines, it appears that Gruy6re cheese, made from the milk of grazing cows, or cows fed on green fodder supplemented with coprah oil, had a more open texture than cheeses made with the milk from cows not receiving any supplement (Mocquot, 1979).
During the ripening period, the amount of free fatty acids differs according to the type of cheese. In Camembert (Kuzdzal-Savoie & Kuzdzal, 1966) it can be up to 10 ~o of total fatty acids. Although the influence of lipolysis on texture has not been really investigated, it is generally considered that it has no great influence on the rheological properties of the cheese.
The r61e of fat is also important for the perception and formation of flavour. It is commonly observed that cheese made from skimmed milk does not develop a full aroma (Ohren & Tuckey, 1969). If the fat content is increased above a certain limit, the flavour is not improved, and there may even be more frequent off-flavours. Substituting vegetable or even mineral oil for milk fat seems to favour a certain aroma-development--at least in Cheddar (Foda et al., 1974). This seems to prove that one important action of fat is to dissolve and hold the flavour components. The fact that milk fat has to be used in order to obtain real cheese flavour emphasises the influence of the composition of milk fat on flavour genesis, whilst experiments reincorporating milk fat into skim milk, with or without the use of an emulsifying agent, seem to suggest that the fat-water interface has an important influence on flavour development, although it is not yet fully understood.
As in every type of food with a high fat content, lipolysis and oxidation are likely to occur. Lipolysis is known to be an enzymatic reaction. Milk lipases have been shown to be more active than starter lipases in Cheddar (Reiter & Sharpe, 1971). They seem to hydrolyse the fat selectively and to be able to attack triglycerides, whilst lactic streptococci lipases seem to be active mainly on mono- and diglycerides (Stadhouders & Veringa, 1973). The free fatty acids pattern of cheese shows, on the whole, a certain specificity towards the liberation of long chain fatty acids (Kuzdzal & Kuzdzal-Savoie, 1966; Umemoto & Sato, 1975). Free fatty acids will partition between water and lipid phase and be present as soaps. The liberated fatty acids are involved in several types of reaction which vary in importance according to the type of cheese considered.
FLAVOUR AND TEXTURE GENERATION IN CHEESE l 19
In cheeses where mould growth occurs, production of methylketones is very important. This production follows a two-step scheme: the fatty acids are first oxidised to fl-ketoacids, which are then decarboxylated to the corresponding methylketones with one carbon atom less (Hawke, 1966). Both resting spores (Lawrence, 1966) and mycelium (Lawrence & Hawke, 1968) seem equally efficient in the conversion. Besides this main mechanism, it seems that certain short chain carbonyl compounds could also result to a limited extent from the metabolic activity of the mould on the fl-ketoacids (Dartey & Kinsella, 1971). The latter are normally present in small quantities in milk fat, i.e. in the ketoglycerides which represent about l ~o of milk fat. This second mechanism, which is based on the constitutive fl-ketoacids, is the main pathway in cheeses where mould growth is not involved in the ripening.
The amount of ketones produced during the curing does not depend directly on the amount of available fatty acids precursor (Anderson & Day, 1966) as 2- heptanone always predominates in Blue cheese (ewe or cow~espite considerable variations between samples) while 2-nonanone is the more abundant ketone in the soft type. Many factors affect the rate of formation of individual ketones: temperature (Dolezalek & Hoza, 1969), pH (Jolly & Kosikowski, 1975; Lawrence & Hawke, 1968), physiological stage of the mould (Fan et al., 1976) and the ratio of concentration of fatty acid to dry weight of spores (Fan et al., 1976). It appears that free fatty acids do not accumulate in the mixture during methylketone formation, which means that the lipolysis rate does not normally exceed the oxidation rate of the liberated fatty acid, thus avoiding the toxic effect of the fatty acids, which is more noticeable on the mycellium than on the resting spore (Fan et aL,1976). This toxic effect removes the efficiency of the technique of initiating lipolysis by homogeni- sation of milk fat compared with the use of microbial lipase for enhancing the rate of flavour development.
Data on the concentration of individual methylketones during blue cheese ripening show large fluctuations, which suggest interconversion mechanisms. Indeed, methylketones are further metabolised by P. roqueforti into the cor- responding secondary alcohol, the reaction being reversible under aerobic con- ditions. The rate of ketone disappearance again depends on the influence of the physiological stage of the mould and the concentration of ketones (Fan et al., 1976). The presence of nonenone in Blue cheese and Camembert, together with that of undecenone and tridecenone in Camembert made from milk heavily contaminated with Pseudomonas (Dumont et al., 1977), raises the question whether these ketones are formed from monounsaturated fatty acids normally present in milk fat or whether another mechanism is to be postulated. The existence of a pathway from monounsaturated fatty acids to unsaturated ketones would mean a preferential lipase activity, as the medium chain length monounsaturated fatty acids are present in milk fat in much smaller quantities than the corresponding saturated acids.
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Another possible reaction, in which polyunsaturated and, perhaps, monounsatu- rated, fatty acids can be involved, is oxidation. The amount of oxidation in cheese is, however, rather limited, as milk fat would normally be very susceptible to oxidation in the conditions (pH, and copper content when copper vats are still in use) which prevail in cheese. The existence of a low redox potential, together with the presence of natural antioxidants, could prevent the initiation of oxidation mechanisms, or create conditions in which the primary oxidation products are further reduced. The second hypothesis would explain the existence of 1-alkanols (inclu...