THESIS

2.5 Rice starch gelatinization
Gelatinization describes the irreversible collapse (disruption) of
molecular order within a starch granule when heated in excess water. When an
aqueous suspension of starch suspension of starch is heated above the gelatinization
onset temperature (To), the granules absorb large amounts of water and swell
considerably impart a substantial increase to the viscosity of the suspension (Thomas
and Atwell, 1999). Further heating of starch granules in excess water results is
granule swelling and additional leaching of soluble component (primary amylose).
Swelling of granules attains a maximum value at elevated temperatures and is
subsequently followed by granular disruption and exudation of the granule contents
into the suspension matrix (Whistler et al. 1984) as shown in Figure 7. Final
gelatinization temperature (GT) of starch granules refers to the water temperature at
which at least 90 percent of the starch granules have gelatinized or lost birefringence
(Maltese cross) or swollen irreversibly in hot water. GT is classified for rice starch
granules as low (55 to 69.5°C), intermediate (70 to 74°C) and high (74.5 to 80°C). GT
is indexed in the breeding programmed by the alkali spreading value based on the
degree of dispersion of six grains of milled rice in 10 ml of 1.7 percent potassium
hydroxide after 23 hours soaking at 30°C (Swinkels, 1985).
Figure 7 Idealized diagram of the swelling and gelatinization of a starch granule
Source: Swinkels, (1985)
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