THESIS

units are known to inhibit retrogradation (Shi and Seib, 1992). The retrogradation of
waxy rice starch appears to be directly proportional to the mole fraction of unit chains
of amylopectin with DP 14-24 and inversely proportional to mole fraction DP 6 - 9.
Once crystalline regions are formed, their reversal is dependent on molecular type:
amylose, 100 o C to 120 o C; amylopectin, as low as 50 o C for samples having a low
degree of order. When formed in gels, crystalline regions act as physical cross links in
a gel network. In reviewing crystallinity, the X-ray polymorphic forms have been
described in terms of structure, their function in granules, changes that occur with
annealing, loss due to melting (gelatinization), and recrystallization upon
retrogradation. Through macro-crystalline domains, crystallinity also contributes to
granule ultra structure. These structures are often revealed by etching granule
examination. Most studies require the use of transmission or scanning electron
microscopy but large growth rings can be seen with an optical microscope. Observed
structures show periodic density fluctuations that vary in size and shape, depending
on the extent to which amorphous regions or imperfect crystallites have been
disrupted (Maningat and Juliano, 1980).
2) Starch concentration
Starch concentration influences the extent of retrogradation. The
maximum extent of retrogradation is obtained in a starch concentration about 60%
(w/w). Retrogradation process is very sensitive to temperature (Zeleznak and
Hoseney, 1986). The aging of wheat starch gels at 4, 21, and 30 o C was studied with
DSC (Lengton and LeGrays, 1981). Crystallinity increased with time and
crystallization occurred at the highest rate and to the greatest extent in storage at 4 o C.
Retrogradation is very sensitive to the water content in starch gels. Lengton and
LeGrays (1981) observed that crystallization during aging occurred only in gels with
starch content between 10 and 80%, and maximum crystallization, measured with
DSC, occurred in gels with 50–55% starch. Other DSC studies have confirmed that
maximum crystallinity occurs in gels with 50–60 % starch (w/w) (Zeleznak and
Hoseney, 1986).
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