is observed after gelatinization [3

is observed after gelatinization [33]. The temperature range during which the
crystallinity is lost and the rate at which it is lost depend on the water content
and on the type of starch [7,99]. The temperature range increases with decreasing water content, and at a water content below 50% the temperature for
complete loss of crystallinity approaches 100°C. The loss of crystallinity seems
to occur in two steps. At first, the loss occurs at a very low rate, but then at a
temperature typical of the starch the rate increases dramatically [7]. In smallangle x-ray scattering (SAXS) a d-spacing between 260 and 296 Å is observed
[53]. This spacing seems to be related to the birefringence and disappears on
heating. The Bragg peak that occurs at about 10 nm, thought to be due to the
alternating crystalline and amorphous layers, disappears during gelatinization
[51,52,54].
10.3.2.3 Endothermic Transitions
Starch gelatinization is an endothermic process, with enthalpy values in the
range of 10 to 20 J/g. Waxy wheat starch has higher transition temperatures
and enthalpies than nonwaxy starch [99a], but when the enthalpy values were
compared on an amylopectin basis they were identical. In a study of ten
different starches (including A-, B-, and C-starches), a relation between transition enthalpy and the amylopectin unit-chain distribution was found; that is,
the enthalpy increased when the amylopectin unit-chain length increased [99b].
Another trend was a negative correlation between transition temperatures and
amylose content. DSC has become perhaps the most important tool for studying starch gelatinization [88,100,101]. A typical DSC thermogram is given in
atures, in excess water [81]. In more concentrated systems, the loss of birefringence has been shown to coincide with the high-temperature part of the
double endotherm [97,102].
The relation between the endothermic processes and the loss of x-ray
diffraction intensity is also illustrated in Figure 10.3. It is evident that the DSC
thermogram shows other thermal events besides the melting of crystallites.
The origin of double endotherms at certain water contents has been much
discussed since they were first reported [88,100]. A first approach was to treat
the melting according to the Flory–Huggins approach — that is, as the equilibrium melting of the polymer crystals (starch crystallites) in the presence of
a plasticizer (water). Although this approach gave much insight into the process
it is not strictly correct because equilibrium melting is not obtained during the
DSC scan. The occurrence of double endotherms has also been explained as
being due to a transition between different polymorphic forms of starch [5];
however, x-ray diffraction studies do not support such transitions [7,98].
The suggestion that the melting of crystallites is preceded by a glass transition seems to be a more fruitful approach [49]. The location of Tgis very
© 2006 by Taylor & Francis Group, LLC
interval as the DSC endotherm (see Table 10.3), or at slightly lower temperFigure 10.3. Loss of birefringence occurs in about the same temperature