From the foregoing considerations it will be evident that the rheological properties of any material are consequent upon its structure. The three major constituents of cheese, casein, fat, and water, each contribute to the structure and therefore to the rheological properties in their own specific way. At normal room temperatures the casein is solid, the fat is an intimate mixture of solid and liquid fractions, giving it what may be described as plastic properties, while the water is liquid. The casein forms an open meshlike structure (46-48). Within this mesh is entrapped the fat, which had its origin as the fat globules of the milk. The water is more precisely the aqueous phase, since dissolved in it are the soluble constituents of the milk serum together with any salt which has been added during the cheese-making. Some of the water is bound to the protein and therefore largely immobilized; the remainder is free and fills the interstices between the casein matrix and the fat. This structure is common to all types of cheese. The differences among various types of cheese are brought about by the influence of different manufacturing regimes on that structure. In addition to the characteristic differences arising from different procedures, adventitious variations may also occur within the same type of cheese. These reflect differences in the original milk or in the conditions during the manufacturing or maturing processes.
It is the casein within the cheese which is responsible for its solid nature. The primary structure is a three-dimensional cage whose sides consist of chains of casein molecules (49,50). This provides a structure of considerable inherent rigidity. The chains are not linear (51), but have an irregular, somewhat gnarled structure. This may be deformed elastically under the action of external forces and this elasticity modifies the rigidity of the cage.
During the clotting process these chains have been formed by the joining together of individual casein particles in the serum. This serum surrounds the fat globules so that each cage may be expected to encase at least one globule or cluster of globules (52). The distribution of sizes of these cells will be controlled by the distribution and size of the fat globules. For instance, when the milk is first homogenized, the cheese made from it will have a more uniform distribution of cells than cheese made from fresh milk (53). The complete cheese curd at this stage consists of an aggregate of these cells of casein plus fat and the whole is pervaded by the aqueous phase (52).
If a force is applied to such a structure, the deformation will be primarily controlled by the rigidity of the cage, modified by any elasticity in its structural members. In the absence of the fat and water this would behave simply as any other open-type structure and its deformation would be characterized by a single modulus of rigidity or elastic ity. However, the deformation of any of the cells is limited by the fat within it. At very low temperatures the fat would be solid and this would only add to the rigidity. At the normal temperatures at which cheese is matured and used the fat has both solid and liquid constituents and has its own peculiar rheological properties. Any deformation of the casein matrix would also require the fat to deform. At the same time, the water between the fat and the casein acts as a lubricant. As a result, the rigidity of the fat is added to that of the casein in a complex manner and it is this which gives rise to the peculiar viscoelastic properties of the cheese.
Even this is not the whole picture. The final cheese is not merely a continuous aggregate of cells as just described. During manufacture the curd is cut into small pieces at least once to allow any excess serum to drain away. As the serum drains away, the casein matrix shrinks on to the fat globules, making a more compact whole. The granules so formed may then be further distorted, as in the cheddaring process, or may be allowed to take up a random distribution, as in a Cheshire cheese. The final cheese mass is an aggregation of these granules which forms a secondary structure having its own set of rheological properties. Even this may be further modified by subsequent processes such as milling, which gives rise to a tertiary structure, and by pressing, which distorts the whole (54).
This rudimentary account of the factors contributing to the rheological properties applies to any cheese. During the course of manufacture, and subsequently during ripening, the basic structure may be modified by mechanical or thermal treatment, or the casein itself may be acted upon by bacteria and any residual enzymes. These agencies may change the organization of the structure or they may cause contiguous fat globules to coalesce. Finally, water may be lost by evaporation from the surface.
Before discussing the effect of the structure of the cheese on its rheological properties it is appropriate to consider differences which may occur within a single cheese or between cheeses from the same batch which might be expected to be alike. A cheese which matures in contact with the air will develop a pronounced rind and may show considerable variation throughout its body. The surface layers lose moisture more rapidly than the inner portions. This results in a difference in composition, which may be reflected in the rheological properties. It may also affect the progress of the maturation process itself. Another source of variation in some cheeses, particularly some of the larger varieties with long maturation periods, is that they are turned at intervals: The top and bottom layers are subjected to alternate low and high compressive forces due to their own weight, while the middle will have a more uniform treatment. This again may be reflected in the nature of the maturation. An otherwise uniform cheese will show a distribution of firmness as in Figure 11. A practical consequence of this is that measurements made on or near the surface of a large cheese will not be characteristic of the main body of that cheese.
Differences between cheeses may be expected to be greater and the magnitude of the variations will depend to some extent on the method of measurement. Penetration
methods, where the instrument only acts locally on a limited quantity of the sample, give greater differences than compression testing in which a larger volume of the sample is involved. Variations of the order of 10% of the mean value of a parameter have been found on different samples from the same batch in a compression test (37); this probably indicates the limit of reproducibility which one may reasonably expect when making measurements on cheese. Differences between batches may be considerably larger. As an extreme example, one experimenter measuring eight different lots of cream cheese obtained readings which ranged from 54 to 251 units (55). Before leaving the topic of variation, one other source which must always be borne in mind is that between laboratories nominally making similar measurements. Because this is not a problem unique to either rheology or cheese, it will be disregarded in what follows.
Although it is the interaction of the properties of the principal constituents which gives cheese its viscoelastic-ity, it is profitable to consider some of the features associated with each of the constituents. First, consider the casein, which, it has been pointed out, gives the cheese its solid appearance. Because the casein forms chains within the spaces around the fat globules, there must necessarily be a minimum amount of casein below which any continuous structure cannot exist. This will depend on the number, size, and size distribution of the fat globules and on the size and size distribution of the casein micelles themselves. Once the quantity of casein required for this minimum structure has been exceeded, any additional casein will serve to strengthen the branches and the junctions. It is to be expected that, irrespective of the type of cheese, there will be a general relationship between the amount of casein present in the cheese and its firmness. Figure 12 shows this relation for some ten different types of hard cheese (29). Naturally, since the data refer to cheeses of very different provenance, there is considerable scatter, but it is possible to draw a regression line through the points and this indicates that about 25% of the weight of the cheese must be casein in order to provide a rigid framework and that above this limit more protein only strengthens it.
The requirement that there be sufficient casein to build a structure around the fat has been clearly shown in mea-
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