L

41 42 43

Water content (%)

Figure 22. Relation between rate of casein breakdown and moisture content.

pressed, may sometimes appear as crystals embedded in the fat-protein mass. Then it is not usually present in large enough quantities to make any sensible contribution to the rheological properties of the whole. Almost all of the salt is present only in solution in the water. There its effect on the properties of the whole cheese is minimal. Any serious contribution of the salt to the rheological properties of the whole cheese is by indirect action (67). A high concentration of salt increases the osmotic pressure, diverting a significant quantity of water from the structural bonds of the casein network (68,69). This may be seen in the effect on the ripening of some samples of Mozzarella cheese (70). When the salt content was low (0.27%) the modulus decreased from 120 to 45 N/m2 in five weeks, but when salt was present in excess of 1% no change was observed within the same period.

In cheese where the salt is added by immersion in brine there is a further consideration. The inward diffusion of the brine results in a concentration gradient being set up within the cheese. This has a twofold effect. First, the presence of the salt simply excludes some of the water; this in turn reinforces the moisture gradient simultaneously arising from the evaporation of water from the surface layers. Probably more important is the fact that the diffusion of the salt into the cheese is a slow process (71). In the early stages of ripening the salt concentration in the inner regions is unlikely to be sufficient to limit the protein degradation described above. The presence of more water and an enhanced proteolytic activity affect the rheological properties in the same sense, giving rise to a weaker structure and therefore a less firm cheese. It is therefore probable that the variation in firmness within a single cheese, which, as described earlier, is closely related to the moisture distribution, is accentuated by bringing.

Some cheeses also contain a significant quantity of gas. In the Swiss-type cheeses this is concentrated in the eyes; the remainder of the cheese has a close waxy texture and is virtually free of any entrapped gas. In hard and crumbly cheeses the gas may simply be air which has infiltrated into cracks which have developed in the mass. The signif icance of either eyes or cracks is that measurements made on the whole cheese or on its surface are not a fair representation of the properties of the main body of the cheese. Hidden cracks or eyes will give rise to irregularities when measurements are made by any form of penetrometer and probably account for a substantial amount of the observed scatter. In compression measurements they will give rise to premature breakdown. They also account for the fact that difficulties are encountered when any attempt is made to assess the rheological properties of cheese by studying the propagation of ultrasound through it. The velocity of propagation is more influenced by the scattering at the discontinuities than by the properties of the cheese mass.

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