Variation With

The development of firmness during the ripening and subsequent storage of cheese has always been of particular interest to both the practical cheese-maker and the rheol-ogist. During this period all three principal constituents undergo change. Moisture evaporates from the surface of the cheese if this is exposed to the air; additional moisture migrates from the interior to replace that lost and some of this also evaporates. Eventually a moisture gradient is set up, but even the center will be drier than it was originally. The protein in the matrix undergoes progressive change as the available water is reduced and the residual enzymes and bacteria continue to act on it (46,47,75). Furthermore, on the purely physical plane, as the water content decreases, the matrix may shrink or collapse under pressure so that voids do not form and the protein becomes denser. Some of the glycerides in the fat slowly crystallize, resulting in a more solid mass of fat.

These changes which take place in the structural components of the cheese are reflected in force-compression curves. Figure 23 shows curves for two cheeses, Cheddar (47) and Gouda (63), when green and when mature. During aging the strain sustainable by the cheese before breaking down at the yield point (point B in Fig. 7) decreased more or less exponentially with time in both cases. Figure 24 shows this for one batch of Cheddar cheese. At the same time the stress at the yield point diminished. The strength of the matrix had clearly been reduced through aging. In

Strain

Figure 23. Force-compression curves for young and mature cheeses:--, Cheddar (mature); —, Cheddar (young);—, Gouda

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