THESIS

1.2.4 Retrogradation gel images
Retrogradation gel images of 0 - 48 hr debranched high amylose
rice starch preheated at 75, 95, and 121 o C for 30 min are shown in Figure 16. Briefly,
all debranched starch solutions were heated in boiling bath for 30 min to stop any
further enzymatic activity and to unwind any double helices, melt the crystallites and
retrograde starch which might have formed during the initial heating and incubation at
55 o C during 4 to 48 hr debranching. Visible precipitates formed immediately after
removing and cooling the debranched solution from the boiling. As the cooling
progressed (at 4 o C for 16 hr), increases in turbidity and precipitate formation were
noticed. The 16 to 48 hr debranched of rice starch preheated at 75 o C, 95 o C and 121 o C
had higher turbidity gel than 0 to 4 hr debranched samples that moderate clarity and
harder gelation than 8 hr debranched samples.
The turbidity and clarity differences between the studied
starches may affected by the degree of retrogradation, granule swelling and the
association between amylose and amylopectin (Swinkels, 1985). Very slowly
retrogradation gel was visible observed for the control sample (non enzyme) cooling
for first hour in an ice bath at 4 o C. In contrast, the 16 to 48-hr debranched of rice
starch gel retrograded much more rapidly during the first hour of cooling in the same
condition.
The major factors influencing rice starch retrogradation would
be starch content, ratio of amylose to amylopectin, and the branch chain length of
amylopectin. Although both amylose and amylopectin components are involved in the
process of starch retrogradation, the linear fraction, amylose, favors recrystallization
in a rapid mode (Slade and Levine, 1988a).
When starch is gelatinized under high moisture content and
allowed to cool, alignment of order amylose molecules and debranched amylopectin
with each other leads to the formation of a rigid gel. A rearrangement between
starches occurs and a three-dimensional network is rapidly constituted. The higher the
95