10.6.3.1 Gelatinization BehaviorThe

10.6.3.1 Gelatinization Behavior
The gelatinization temperature of modified starches is affected, either because
that is the aim of the modification or because it is an inevitable consequence.
DSC has been used to study the influence of chemical modification on the
gelatinization parameters, and for most modifications a decrease in To
and Tm,
as well as in ∆H, has been found. These findings have been observed for
hydroxypropyl distarch phosphates (maize, waxy maize, and tapioca) [295],
for hydroxypropyl potato starch [296], and for a range of modifications of
wheat starch (including hydroxyethyl and hydroxypropyl, acetate, distarch
phosphate, aluminum octenyl succinate, and acetylated starches) [297]. In
some cases (i.e., oxidation and crosslinking) an increase in Tmwas observed
[201,297]. For an acetylated high-amylose starch, ∆Hwas found to increase
compared with the native starch [201]. The changes in DSC parameters
observed will increase with increasing MS, as has been observed for hydroxyalkyl starches [296–298].
When the DSC thermograms of a modified starch and its native counterpart
are compared, the profiles look very much the same at high water contents
[201,298]; however, when the measurements are performed at limited water
levels (volume fraction of water = 0.55), some interesting results have been
obtained [296]. At this intermediate water content, a biphasic gelatinization
endotherm was observed for the native starch. The peak at the low temperature
side of the double endotherm was found to decrease in size with increasing
MS for hydroxypropylated potato starch. This was interpreted as being due to
a change in the conformation of the starch chains in the amorphous regions
and thus a change in the influence of the amorphous regions on the melting
of the crystallites.
10.6.3.2 Rheological Behavior
One reason for the use of modified starch can be that a more viscous or more
thus notable, and several examples are available. Peak viscosity as well as
setback values were increased for a hydroxypropyl distarch phosphate waxy
maize compared with its unmodified counterpart, and acetylated distarch phosphate smooth pea and acetylated smooth pea both gave improved viscosity
curves compared with the unmodified starch, whereas distarch phosphate
smooth pea starch gave somewhat lower viscosities [205]. For hydroxypropyl
potato starch, it was found that the pasting temperature and the peak temperature both decreased with increasing MS, whereas peak viscosity increased.
The setback values were rather similar [294]. Crosslinking increased the viscosity of potato starch and waxy maize starch, but the concentration at which
a measurable viscosity was obtained was also increased [155]. Fundamental
rheological measurements have shown that a crosslinked waxy maize starch
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stable paste is required (see Table 10.5). Changes in rheological properties are