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Figure 12. Relation between firmness of cheeses and total protein content.

surements on two other varieties of cheese. In some Mes-hanger cheese (56) it was found that, unless the casein occupied more than about 37% of the volume of the cheese not occupied by the fat, it had virtually no rigidity and behaved like a soft paste. In a similar way, it has been shown quite dramatically (Fig. 13) that in Mozzarella cheese (57) the protein must exceed about 42% of the nonaqueous part of the cheese for any rigidity to exist. Again, this corresponds to a minimum casein content of about 25% for a rigid structure.

Although it has been claimed that the casein matrix gives rigidity to the cheese, there is still a theoretical question which has not been answered. Is the matrix continuous throughout the whole cheese, so that it may be treated as a solid body, or do the dislocations in the structure which have been introduced by cutting allow it to flow, albeit im perceptibly, however small the stress? The evidence is inconclusive. In theory, an examination of the stress-strain curves at very low strains should decide the issue. If there is a continuous structure, there may be elastic deformation but no flow until a finite stress is reached which is sufficient to rupture some of that structure. Force-compression curves should indicate whether the cheese is of the Maxwell body or the Voigt body type. Commercially available instruments are generally not precise enough in this region, for they are not designed for this purpose. More precise measurements on cheese analogs have already been mentioned as showing that these began to break down at very small strains (42). Measurements of the recovery after compression of a few real cheeses (58) showed that even the smallest strains applied broke down some structure in a Mozzarella cheese, but a Cheddar cheese and a Muenster were able to recover completely (Fig. 14).

Cheddar cheese has also been studied by means of a relaxation experiment (44). Samples were compressed under different stresses, and hence at different rates, to the same ultimate deformation and the subsequent relaxation of the stress was observed. Using Peleg's treatment (43) it was found that the value of Hk2 in equation 20 was dependent on the stress applied. The greater the stress, the more the structure broke down, although only the same strain was reached; the relation was curvilinear, particularly at the lowest stresses (Fig. 15). Extrapolating this curve back to zero stress should indicate whether any breakdown occurred. A zero value of l/k2 would indicate a solid structure. Unfortunately, any extrapolation of this curve would be speculative. It is possible that it would lead to a positive intercept on the vertical axis. If this were so, it would lead to the conclusion that there was no continuous structure throughout that cheese, but to establish this with confidence would require measurements at much lower stresses.

Another series of experiments which could throw light on this question has been carried out (59). A small sinusoidal vibration was applied to one surface of the cheese sample and the stress transmitted through it observed. If

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