Physical Chemistry of Foods

solvent mixtures:

1

TmðjÞ



1

Tm^0

¼

R

DHm

q½ð 1 jÞwð 1 jÞ^2 Šð 6 : 14 Þ

whereTm^0 is the melting temperature of the pure polymer,DHmthe melting
enthalpy per mol of monomer,qthe volume ratio of monomer to solvent
molecule, andwthe solvent–segment interaction parameter (which equals
ð 1 bÞ=2; see Section 6.2.1). For water contents below 30%, which is about
the range for which Eq. (6.14) would be valid, a relation as depicted in
Figure 6.24b can be reasonably predicted, ifwis assumed to be 0.5 or even
somewhat largerðb4 0 Þ. (Incidentally, this implies, again, that starch, like
many polysaccharides, is not very hydrophilic.) Exact agreement with
Eq. (6.14) is not to be expected, since the crystalline ordering in starch is
irreversible (see, however, Section 6.6.3). The extrapolated value for melting
of pure starch would be about 250 8 C; it cannot be experimentally
determined.
There is, however, an important complication. Data as presented in
Figure 6.24a have been obtained by slow heating of starch granules in an
excess of water. During this process, the granules can take up water, which
they tend to do with increasing temperature. During melting, the water
content will be close to the equilibrium content at that temperature, which is
the plateau value obtained at water contents above 50 or 60%(Figure
6.24b). If starch–water mixtures of lower water content (about 35–60%) are
heated, two melting temperatures may be observed, the plateau value and a
higher one. Some results obtained by differential scanning calorimetry are
shown in Figure 6.25. The explanation is presumably that part of the
crystallites have been in a position to take up sufficient water to melt at a
low temperature, whereby little water is left to be taken up by the remaining
crystallites. It should be realized that the diffusion rate of water in a
concentrated starch system can be very small (see Figure 5.16a). It is also
seen in Figure 6.25 that the melting occurs over a wider temperature range
at low water content, indicating that the crystallites are less perfect. Heating
in combination with water uptake may thus alter the crystallites before they
melt.
In thegelatinization process, four stages can be distinguished. They
occur when an aqueous starch suspension is heated while stirring.

1. Below the melting point, thegranules swellsomewhat by taking up
   water, increasing in volume by 30–40%. Some of the amylose
   leaches from the granules.


2. Meltingis discussed above. During melting, more amylose leaches.
   Melting does not necessarily imply that all supermolecular