starch is physically and chemically inert, and it shows little digestion in the
human gut. To change it into a functional product, it is generally heated in
an excess of water. This causes what is calledgelatinization, which involves a
number of changes, including water uptake.
The most conspicuous change may bemeltingof the crystallites. When
a suspension of starch granules in water is heated, their birefringence
disappears. Examples are in Figure 6.24a; in each separate granule, the
temperature range over which birefringence disappears is significantly more
narrow. The melting is also observed with x-ray diffraction and calorimetry.
The melting enthalpy ranges from 12 to 22 J per g dry starch, i.e., 40–50 J
per g crystalline material. This is much less than the melting enthalpy of
crystalline sugars, which is generally of the order of 400 J?g^1. This
indicates, again, that the degree of ordering in starch crystallites is far less
perfect than in a sugar crystal.
The melting temperature depends on water content, as depicted in
Figure 6.24b. This phenomenon is comparable to the melting point
depression commonly observed for impure solid materials (e.g., imperfect
crystals). Flory has derived an equation for the melting temperatureTmas a
function of the volume fraction of polymerjin concentrated polymer–
FIGURE6.24 Gelatinization of starch from wheat (- - -), potato (——), and maize
(corn)ð Þ. (a) Percentage of starch granules showing birefringence as a function of
temperatureTduring heating up. (b) Melting temperatureTmas a function of water
content (%w/w). (c) Water uptake in g per g dry starch during heating in an excess of
water as a function of temperature. (Approximate results from various sources.)