Long glucan chains reduce in vitro starch digestibility of freshly cooked and retrograded milled rice
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Understanding the structural factors related to the starch digestibility of cooked milled rice grains is important in mitigating the impact of diet-related diseases. In this study, changes in starch structure of rice during in vitro digestion and retrogradation is reported for 8 varieties of indica milled rice with apparent amylose ranges from 3 to 34. Results showed that retrogradation is effective in decreasing starch hydrolysis rate (k), while addition of exogenous lipid had no significant effect. Moreover, reduction in digestibility after retrogradation was more effective in high amylose than low amylose or waxy rice. Structurally, strong negative correlations were found between k and starch fractions rich in long glucan chains such as long-chain amylose (LCAM) or intermediate-chain amylose (ICAM). Conversely, strong positive correlation was seen between k and short-chain amylopectin (SCAP). Decreased hydrolysis of LCAM and low levels of SCAP was observed in retrograded rice. This shows that LCAM becomes less susceptible to digestion which consequently results to increased hydrolysis of SCAP as the preferred substrate in rice.
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