Physical Chemistry of Foods

amylose and the gel resembles a polymer gel of the general type depicted in
Figure 17.10b.
When the starch concentration is markedly higher, full swelling of
starch granules cannot occur, and the system becomes closely packed; for
potato starch this occurs above 5% starch, for most other starches at
distinctly higher concentrations (up to 15%). Upon cooling, a gel is formed,
consisting of closely packed granules, with a thin layer of amylose gel as a
kind of glue between the granules. For most types of starch, including wheat
starch, the swollen granules are more or less isometric, but in potato starch
they lock into each other, almost as in a jigsaw puzzle: see Figure 17.26b. In
either case, the modulus of the gel is given by the modulus of the swollen,
gellike granules, which is higher for a higher starch concentration (i.e., less
swelling) and for longer keeping at low temperature (i.e., stronger
retrogradation). For wheat starch, the fracture stress is primarily given by
the fracture stress of the amylose ‘‘glue,’’ since the swollen granules can slip
past each other without being greatly deformed. For potato starch, it is
primarily the stiffness of the grains that determines the fracture stress, since
it needs considerable deformation (or even fracture) of the grains before
they can slip past each other.
To give some idea of the magnitude of the parameters, we will consider
30 %starch gels, aged about 2 weeks. We then have roughly

Starch type Wheat Potato

Young’s modulus 300 900 kPa

Fracture stress 100 400 kPa

Fracture strain 0.3 0.5

For a younger gel, the modulus and the fracture stress are much smaller, the
fracture strain being larger. Especially for a very young potato starch gel,
elastic deformability is high,efrbeing of the order of 3.

Doughs. A dough is made by kneading a mixture of grain flour and
water (and possibly a litle salt). The water content is, e.g., 45%. The dough
contains partly swollen starch grains, making up, e.g., 70%of the volume.
The structure is thus like that of a concentrated starch gel, but there are
differences as well. The continuous ‘‘phase’’ is a more or less homogeneous
viscoelastic liquid, consisting of water, protein, other nonstarch solids, and
part of the amylose. The whole dough then is aviscoelasticsystem, and it