bread is deformed. The matrix material is rubbery, and the stress–strain
curve is comparable to that in Figure 17.29d, although a clear buckling
point cannot be identified (presumably because of the unevenness of the
cellular structure). Decompression shows some, though not much, hyster-
esis, and a second compression curve (after the first decompression) is quite
similar to the first one. Frame (c) relates to the same bread after keeping it at
room temperature for 7 days, when it was considered very stale. Staling is
due to retrogradation, i.e., forming of starch microcrystallites, which makes
the matrix much stiffer (Section 6.6.3). The compression curve is now typical
for yielding (sy&9 kPa), although the stress overshoot is so large that
considerable local fracture must have occurred as well. The hysteresis
between compression and decompression is very large, and the second
compression curve confirms that the first compression has caused
considerable irreversible structure breakdown.
Extrusion cookingis used to make several foods of a cellular structure,
such as crispbreads, various snacks, and some breakfast cereals. These are
primarily starch based products. In the extruder the ingredients are highly
compressed and made to flow—which results in a continuous mass—and
heated to a high temperature. This implies that the water present is strongly
superheated; upon leaving the extruder through a die, whereby the pressure
is suddenly released, steam forms and the material strongly expands. A
cellular structure is generally formed. The matrix often has such a low water
content that it becomes brittle upon cooling, which then means that the
product is not asoftsolid.
The relative density is generally between 0.6 and 0.25. Equation
(17.23) tends to be approximately obeyed. Although micrographs suggest
that the cells are closed, the relation between modulus and density is like
that of a sponge, the scaling exponent being about equal to 2. Deviations
from theory seem to be largely due to unevenness of the structure: the cells
often show a wide size distribution, and the density may vary, especially
between outside and inside of the specimen.
Plant Tissues. Most tissues obviously are cellular soft solids. Plant
cells have cell walls, many of which are fairly rigid. However, three quite
different factors contribute to the stiffness of plant tissues:
The rigidity of thecell wall
The strength of themiddle lamella, which mainly consists of pectins
and acts as a ‘‘glue’’ between the cells
Theturgor in the cell, i.e., the osmotic pressure inside the cell being
higher than outside