Note The reader may be cautioned that the term gelatinization
temperature is used rather indiscriminately. The method used to
obtain it may have been loss of birefringence, loss of x-ray
diffraction pattern, differential scanning calorimetry (where the
result depends on the scanning rate), or even increase of apparent
viscosity in a particular instrument (e.g., an ‘‘amylograph’’). These
methods may give quite different results, depending in a different
way on water content, especially the last one. Moreover, cultivars of
one species may vary in gelatinization temperature.6.6.3 Retrogradation
When keeping a gelatinized starch solution or paste at ambient temperature,
physical changes are observed, which are generally lumped under the name
retrogradation. These changes can be of various types: a solution becoming
turbid, precipitation of part of the starch, gel formation, or a once formed
gel becoming stiffer and more brittle. It greatly depends on conditions,
especially starch/water ratio and temperature, what change will occur. These
changes have the same origin: ordering of molecules or parts of molecules
occurs, and such a change can be quantified by calorimetry. Upon heating,
energy is consumed in disordering the structure over a certain fairly narrow
temperature range, and thisDHmay be considered as a melting enthalpy.
DHincreases with time; see e.g. Figure 6.26.
Dilute Systems. We will first consider what will happen in an
amylosesolution. Actually, amylose is very poorly soluble in water at room
temperature (although it is well soluble in some salt solutions, notable KCl;
cf. Table 6.1). It readily forms helices in water, of which at least part are
double helices. These helices tend to align, forming parallel stacks that may
be consideredmicrocrystallites. X-ray diffraction shows the chain packing to
be similar to that of B type crystallites in native starch. As much as 70%of
the amylose may become crystalline. It depends on amylose concentration
what the consequences will be. An amylose solution at 65 8 C has a chain
overlap concentrationc*(see Section 6.4.2) of about 1.5%. If a more dilute
solution is cooled,precipitationof amylose will occur.
In moreconcentratedamylose solutions, agelwill be formed. The gel
stiffens during keeping, although the correlation between the increase inDH
and the increase in stiffness is far from perfect: Figure 6.26. Much the same
happens in a gelatinizedstarchsolution. Here, amylose and amylopectin
have phase separated, and the swollen granules contain virtually no
amylose. This implies that the amount of water available for amylose is