Figure 17.30a shows results on the modulus of wafers, made at various
gas contents. Equation (17.21) is precisely obeyed. Wafers have a very low
water content, and the material is in a glassy state, which explains the very
high values of the modulus (upper curve). Upon storing the wafers in air of a
high relative humidity, the wafers become soft: the matrix material takes up
water and obtains a rubbery state (cf. Figure 16.3). It is seen that the
modulus then is markedly decreased. Upon large deformation of the
original wafers, fracture occurred at a stress of about 400 kPa. Deformation
of the ‘‘soft’’ wafers led to buckling at a stress of about 15 kPa.
Figures 17.30b and c show results on a model wheat bread. (This was
made of wheat starch, xanthan gum, water, and yeast, but normal white
bread made of wheat flour gives much the same results.) In frame (b), fresh
FIGURE17.30 Mechanical properties of baked cellular products. (a) The Young
(compression) modulusEuas a function of product densityjof wafers kept in air of
various water activities (indicated near the curves). (After results by G. E.
Attenburrow et al. J. Cereal Sci. 9 (1989) 61.) (b) Compression and decompression
(indicated by arrows) of the crumb of a model wheat bread (r/rm& 0 :35), giving the
stresssversus the Cauchy straineC. Complete densification would presumably be
reached ateC& 0 :7. The solid line shows the first compression and decompression,
the broken line the second compression. The bread was 4 hours old. (c) Same
experiment, same bread, but now 7 days old. (After results by C. J. A. M. Keetels et
al. J. Cereal Sci. 24 (1996) 15.)