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10 20 30 40 Initial stress (kPa)

Figure 15. Amount of relaxation after previous stress.

one makes the assumption that the cheese behaves as a Voigt body, a modulus of rigidity can be calculated from the inphase component of the signal received and a viscosity from the out-of-phase component. For a true Voigt body both of these should be independent of the frequency. In fact, neither was constant with either Cheddar or Gouda cheese, even when the strain was as low as 0.04. The inference is that even at this strain some failure in the rigid structure had taken place. Either internal cracks or slip planes had developed within the cheese (42). However, everyday experience suggests that cheese should have some rigid structure. The stress on the lower layers of a large cheese, such as a Parmigiano or a Cheddar, is of the order of 3 to 4 kPa; yet such a cheese retains its shape more or less indefinitely.

Summarizing the previous paragraphs, it appears that the rheological role of the casein is to provide a continuous elastic framework within the individual granules. It is likely that where the casein chains lie in the granule surface and are contiguous with chains in neighboring granules they may be held together by physicochemical bonds which develop during maturation and give rise to some rigidity in the aggregation (54). However, these bonds will be sparser than those within the original network, since they form only where chance contacts occur. This results in a weaker secondary structure. These bonds may be broken when a positive strain is applied, giving a Maxwell body type of response, but are strong enough to preserve rigidity under the cheese's own weight. Some further support is given to this view by the fact that individual granules have a much higher modulus than the whole cheese (60).

The role of the other major constituents is more clearly documented than is that of the casein. The fat derived from the original milk is very roughly one third of the total mass. Its rheological properties are very sensitive to temperature changes and, as is to be expected, this sensitivity is imparted to the whole cheese. At 5°C (around normal refrigeration temperatures) many of the glycerides in the milk fat are solid. The proportion of solid fat decreases with rise of temperature, particularly sharply in the 12-15°C region, which is close to the ripening temperature for many varieties. Above this region the proportion of solid decreases further until at around 35°C almost all is liquid. This is near the temperature that a small portion will rapidly attain if it is placed in the mouth and chewed. The ratio of the solid to liquid fat components is the principal factor influencing the rheological properties of the fat (61). Although factors such as breed and species of animal, pasture, and herd management have some effect on this ratio, temperature is by far the most important. Fat also supercools readily, so that the thermal history is almost as important as the absolute value of the temperature itself. During maturation and storage cheese is usually kept at a lower temperature than that at which it was made. Originally most of the glycerides will have been liquid and at the lower temperature these will slowly solidify. Most of the change will take place in the first day or two, but solidification will continue progressively, maybe for several weeks before final equilibrium is reached.

Measurements made on Cheddar, Cheshire (39), Emmentaler (62), Gouda (63), and Russian (64) cheeses at a range of temperatures, using different instruments, all show the same general trend (Fig. 16). It may be mentioned that different instrumental methods give rise to somewhat different results. In general, penetrometers appear to indicate a rather steeper dependence of firmness on temperature than compression tests. Nevertheless, bearing in mind the difference in origin of the cheeses and the limitations in accuracy of the individual measurements, already discussed, this curve shows clearly the influence of the (test) temperature on the firmness of the cheeses.

Differences in the glyceride composition of the original milk fat are also reflected in the final cheese. Using the iodine number as an indication of the ratio of solid to liquid fat at any one temperature, the firmness of Swiss cheese was shown to vary seasonally with the solidity of the fat (65) (Fig. 17). The lower the iodine number, the greater the degree of saturation of the glycerides and the higher the a> c

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