Gelatinization property is one of the most important rheological indicators
of cooking quality and processing characteristics of rice starch. Juliano
et al. (1964) found a significant correlation between disintegration
of rice endosperm starch granule in alkali (KOH) solution and gelatinization
temperature of milled rice. Rice with low gelatinization temperature disintegrates
almost completely in 1.7% KOH solution, whereas rice with intermediate
gelatinization temperature shows partial disintegration. Rice with high
gelatinization temperature remains largely unaffected in 1.7% KOH solution.
Amylopectin side chains play a distinct role in the disintegration of
rice endosperm starch granules in alkali solution and their gelatinization
in urea solution (Nishi et al. 2001, Umemoto et al. 2002).
Starch granules containing amylopectin enriched in shorter chains (A+B1)
are more easily disintegrated in alkali solution than starch granules
having amylopectin enriched in longer chains. Varietal differences in
amylopectin fine structure were reported by Nakamura et al. (2002).
This report deals with the variations in alkali digestibility of Bangladesh
rice cultivars and the relationship between alkali digestibility and amylopectin
Alkali digestibility of 575 Bangladesh rice cultivars as well as two standard
cultivars, Kinmaze and IR36, were determined on a single grain basis according
to the method of Little et al. (1958). The amylopectin structure
of 15 rice cultivars were determined by HPAEC-PAD as described by Nishi
et al. (2001).
All of the alkali digestion types, i.e. high, intermediate and low, were
observed in the rice cultivars tested (Fig. 1). Low alkali digestibility
consisted of the alkali spreading scores 1 and 2, as determined by the
degree of spreading of rice kernels in 1.7% KOH solution after 24 hours
of immersion at room temperature. Intermediate alkali digestibility comprised
alkali spreading scores 3, 4 and 5; high alkali digestibility consisted
of alkali spreading scores 6 and 7. More than 90% of the cultivars studied
possessed alkali spreading scores of 1 and 2 (Fig. 2), suggesting that
most of the Bangladesh rice cultivars tested are resistant to alkali.
Clear differences were observed in the proportions of amylopectin side-chain
between Kinmaze and IR36, which exhibited high and low alkali digestibility,
respectively (Table 1). Kinmaze amylopectin had a higher proportion of
fraction fa (A chain) and a lower proportion of fraction fb1
(B1 chains) than IR36. The differences in proportions of fb2
(B2 chains) and fb3 (B3 and longer chains)
were less obvious. These results are consistent with those of Umemoto
et al. (2002).
Amylopectin side-chain fractions varied significantly among Bangladesh
rice cultivars tested (Table 1). However, little difference in amylopectin
structures was observed between low alkali digestion type and IR36 (Table
1). Compared to IR36, intermediate alkali digestion type had a higher
proportion of fa and fb1 and a lower proportion of fb2
and fb3 fractions, except BGD341 (Table 1). BGD341 showed a
pattern similar to Kinmaze, i.e. more fa fraction and less fb1
fraction than the other intermediate type cultivars. Amylopectin from
majority of the high alkali digestible cultivars, except BGD236, contained
a significantly higher proportion of fa and slightly lower proportion
of fb1, fb2 and fb3 fractions compared
to that of IR36 (Table 1). The proportion of fa fraction of cultivars
having the score 7 was comparable to that of Kinmaze. Umemoto et al.
(2002) reported that starch synthase IIa (SSIIa) gene is responsible for
the elongation of amylopectin short chains (A+B1) which directly
influence alkali disintegration of starch granules. Our results, however,
suggest that genes other than SSIIa also play additional roles in determining
the physico-chemical behavior of starch granules.
The amylopectin showing low alkali digestibility had the lowest proportion
of fa fraction, while that exhibiting high alkali digestibility tended
to show the highest proportion of fa. Amylopectin having intermediate
digestibility contained an intermediate proportion of fa fraction. In
contrast, amylopectin with high alkali digestibility possessed a lower
proportion of fb1 fraction compared to those with intermediate and low
alkali digestibilities. These findings indicate that gelatinization behavior
of starch granules in rice is predominantly determined by amylopectin
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Little, R. R., G. B. Hilder and E. H. Dawson, 1958. Differential effect
of dilute alkali on 25 varieties of milled white rice. Cereal Chem. 35:
Nakamura, Y., A. Sakurai, Y. Inaba, K. Kimura, N. Iwasawa and T. Nagamine,
2002. The fine structure of amylopectin in endosperm from Asian cultivated
rice can be largely classified into two classes. Starch/Starke 54:
Nishi, A., Y. Nakamura, N. Tanaka and H. Satoh, 2001. Biochemical and
genetic analysis of the effects of amyloseextender mutation in
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Umemoto, T., M. Yano, H. Satoh, A. Shomura and Y. Nakamura, 2002. Mapping
of a gene responsible for the difference in amylopectin structure between
japonica-type and indica-type rice varieties. Theor. Appl.
Genet. 104: 1-8.