dependent on the water content [103

dependent on the water content [103]; therefore, the hydration of starch, or
perhaps rather the distribution of water inside the starch granule, is of great
importance. When starch is placed in excess water at room temperature, the
water content inside the granule at equilibrium will be 0.54 g water per g starch
[80], which is much lower than the final value of the water-holding capacity,
or swelling volume, of the gelatinized starch. It has therefore been suggested
that, during the initial part of the gelatinization, endotherm hydration of amorphous parts occurs, causing the Tg
to be passed. In excess water, this process,
as well as the melting of crystallites, occurs very quickly, whereas at lower
water contents both processes occur much more slowly; therefore, they separate
into two distinct peaks in the DSC thermogram. At even lower water contents,
a situation will finally be reached when no more water is distributed to the
amorphous starch and the melting has to occur at the initial water content. The
plasticizing effect of water is then low, meaning that the melting temperature
of the crystallites is very high. This is exactly what is observed: At water contents
below 30% (w/w) only one or no endotherm at all is detected at temperatures
below 100°C [88,97,100]. According to the study by Perry and Donald [93a],
it is more correct to describe gelatinization as depending on the availability of
a solvent. They suggested that gelatization begins when the molecular mobility
in the amorphous regions of the starch granule reaches a critical level. This
mobility depends on the total plasticization, which in turn will depend on both
the presence of plasticizing solvent (e.g., water) and thermal energy.
The occurrence of glass transition in starch systems has been deduced
from a shift in the baseline during heating of an aqueous starch suspension
[103–105]. Such a shift in the baseline can be due to a glass transition [5,49],
but it may also be due to differences in heat capacity between the solid state
(starch granule) and the liquid state (gelatinized starch granule) [104].
FIGURE 10.3The DSC endotherm of a wheat starch suspension (58.6% water),
together with the change in x-ray diffraction intensity during heating. (Data from
Svensson et al. [7].)
100
X-ray intensity (%)
80
60
40
20
0
30 40 50
Temperature (°C)
60 70 80 90 100
Endothermic heat flow
© 2006 by Taylor & Francis Group, LLC
408 Carbohydrates in Food
The occurrence of a glass transition during the early stages of gelatinization was suggested by results of DSC experiments carried out by heating starch
systems to different temperatures during the DSC scans and then cooling and
reheating [49], as well as by the finding that the crystallinity disappears
stepwise [7]. In some starches, it has been possible to identify the Tg
just
preceding the gelatinization endotherm [5,106].
10.3.2.4 Morphological Changes
The morphological changes occurring when starch is heated in excess water
have been studied using the scanning electron microscope (SEM) [19,63]. In
some starches (e.g., potato and maize), the granules seem to swell to a similar
degree in all directions, resulting in swollen granules that are approximately
similar in shape to the original ones, just larger. The size of starch granules
during heating has been determined for some maize starches [107]. For waxy
maize, the diameter increased from 15.6 to 39.6 µm at maximum swelling; for
normal maize, the corresponding values were 14.9 to 33.3 µm; and for crosslinked waxy maize, the corresponding values were 14.5 to 31.5 µm. In another
investigation, the diameter of gelatinized starch granules was found to be 46 ±
15 µm for maize, 34 ± 24 µm for wheat, and 85 ± 25 µm for pea [108]. The
large standard deviation indicates a broad size distribution of gelatinized granules. For wheat, barley, and rye starches, the swelling is restricted in one
dimension, resulting in complicated folding of the granules [63]. The same
morphological changes occur, albeit shifted to higher temperatures, when the
water content is decreased [109]. For these cereal starches, it should then be
possible to determine the degree of heat treatment in a process by studying the
shape of the granules.
10.3.2.5 Swelling
Disordering of the crystalline domains in the starch granules is thus the first
step in gelatinization. Methods such as measuring the loss of birefringence,
DSC, or x-ray diffraction are able to measure this process in one way or another.
The values for gelatinization temperature ranges measured with these methods
industry), this first step is necessary but is not enough. For the development of
useful functional properties, such as water-holding capacity or rheological
properties, the events coming after the melting of the crystalline structure are
the important ones. It has already been noted that the starch granule absorbs
water. This absorption leads to the morphological changes described and to a
considerable swelling of starch granules. The swelling is often measured as an increase in gel volume, and some typical results are given in Figure 10.4
The swelling begins at a temperature corresponding to To
in DSC measurements, but it continues to much higher temperatures than the Tc[29]. The
© 2006 by Taylor & Francis Group, LLC
usually coincide (see Table 10.3). For the application of starch (e.g., in the food