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 fine structure.

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 fractions

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 structure.

References

Juliano, B. O., G. M. Bautista, J. C. Lugay and A. C. Reyes, 1964. Studies on the physicochemical properties of rice. J. Agric. Food Chem. 12: 131-138.

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: 111-126.

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: 117-131.

Nishi, A., Y. Nakamura, N. Tanaka and H. Satoh, 2001. Biochemical and genetic analysis of the effects of amyloseextender mutation in rice endosperm. Plant Physiol. 127: 459-472.

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.