THESIS

A high GT value is uncommon, particularly in high amylose rice. A low
ambient temperature during ripening may increase amylose content and independently
reduce GT (Nikuni et al. 1969). The GT affects the degree of cooking of rice because
of the cooking gradient from the surface to the core of the grain. Because GT
correlates directly with cooking time, a low GT favors fuel conservation, provided
eating quality is not adversely affected. GT also affects the molecular properties of
amylopectin (Donald et al. 1997).
2.5.1 Rice starch changes during gelatinization
Rice starch begins to gelatinize between 85 o C and 95 o C, the exact
temperature dependent is the specific varieties (Bhattacharya, 1979). For example,
different starches exhibit different granular densities, which affect the granules that
can absorb water. Since loss of birefringence occurs at the time of initial rapid
gelatinization (swelling of the granule), loss of birefringence is a good indicator of the
initial gelatinization temperature of a given starch. The largest granules, which are
usually less compact, begin to swell first. Once optimum gelatinization of the grains
has occurred, unnecessary agitation may fragment the swollen starch grains and cause
thinning of the paste. Further, it occurs in those parts of the grain where the water
content is sufficiently high (water to starch ratio ≥ 0.75) (Hoseney, 1990). Starch
swell are transformed progressively to an essentially amorphous form with loss of
organized structure. Ultimately granule structure is completely lost and a thin paste (<
~ 4%) or gel (> ~ 4%) is formed. Evidence of this loss of order can be seen by
irreversible granule swelling, loss of birefringence, and loss of crystallinity.
2.5.2 Loss of birefringence
In polarized light ungelatinized starch granules show birefringence,
resulting in the “maltese cross” pattern (Fitt and Snyder, 1984). When To is reached
the birefringence begins to disappear. One of the most common methods for
determining the gelatinization temperature range is to follow the loss of birefringence
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