Physical Chemistry of Foods

It is often (implicitly) assumed that the modulus of a material is a

measure of its strength. Figure 17.4 shows that this can be very wrong. Even

within one structural class, i.e., polymer gels, the relation can be quite poor,

as illustrated in Figure 17.4b.

Structure Breakdown. When a linearly elastic material is

deformed and then allowed to relax, the stress–strain curve is fully

reversible, as illustrated in Figure 17.5, frame (a). For a larger

deformation, the curve is generally not linear, and perceptiblehysteresis

tends to occur, as depicted in frame (b). Nevertheless, the deformation is

fully reversible. This means that deformation/relaxation has left the

structure unaltered; repeating the test on the same specimen leads to an

identical result. The hysteresis is due to energy dissipation, caused by flow of

solvent through the gel network if it concerns a gel, as mentioned in Section

5.1.3.

Most soft solids undergo anirreversiblechange in structure upon large

deformation; examples are in Figure 17.5, frames (c–e). In frame (c), relating

to a starch gel, for instance, the hysteresis is considerable. Part of the

FIGURE17.4 Variation in textural properties. (a) Stress (s) versus strain (eH)
curves and fracture points ( 3 ). (b) Relation between elastic shear modulusGand
fracture stress (sfr) for some types of gels of various concentrations of the network-
forming material. (From approximate results by H. Kimura et al. J. Food Sci. 38
(1973) 668.)