the presence of other components also play a role. It is often assumed that
lipids may have considerable effect. As mentioned, some lipids (especially
those having an acyl chain of 12 to 16 carbon atoms) form inclusion
complexes with amylose. This does somewhat affect the properties of fairly
concentrated starch gels, but it has never been shown to a have a significant
effect on retrogradation as such.
Quite in general, great care should be taken in directly relating
retrogradation, for instance as measured by an increase of melting enthalpy,
to physical changes occurring. Mechanical properties, whether evaluated
sensorily or by rheological measurement, mostly depend, in a very involved
manner, on the number and the properties of such structural elements as
microcrystallites. Moreover, modified starch is often used; for instance,
chemical cross-linking causes a decrease in gelation temperature and in the
extent of retrogradation.
6.7 RECAPITULATION
Polymers are macromolecules that consist of long chains, for the most part
linear, of identical or similar units called monomers. They derive special
properties from their molecular structure, of which the possibility of
assuming very many conformations is the most striking one.
Conformation. In solution, the average conformation tends to be
much expanded, causing the molecules to occupy relatively large volumes.
For simple polymers, theory is available to predict properties. The most
important variables are molecular mass or size, the stiffness of the chain,
and the solvent quality, for instance as expressed in the solvent exclusion
parameter b. The greater these parameters, the more expanded is the
polymer chain in solution. This implies that the relative viscosity of a
polymer solution (relative to that of the solvent) can be very high and
increases with increasing molar mass, for the same mass concentration. The
viscosity is an apparent one, since it strongly decreases with increasing strain
rate.
Mostnatural polymers are greatly heterogeneous, having specific
structures, and polymer theory can only be used in a broad, semiquanti-
tative sense. For most polysaccharides and for some proteins (gelatin,
casein) the theory nevertheless is useful. For most polysaccharides, the
solvent quality of water is rather poor, but many of them nevertheless have a
quite expanded conformation, because they are relatively stiff molecules
(starch being an exception). For globular proteins other theory is needed.
Proteins and several of the polysaccharides are polyelectrolytes.