Fine structure of starch biomacromolecules and digestibility: The regulative role of amylose and amylopectin in the digestive hydrolysis of starch in rice

Abstract

The digestibility of starch in staple foods has rarely been examined at the bio-macromolecular level. This study addresses this by investigating the fine structures of amylose and amylopectin to understand their roles in starch digestibility in cooked white rice. Using the static INFOGEST protocol and oral processing by human volunteers, we assessed the starch digestion characteristics of 13 rice varieties, with amylose and amylopectin chain length distribution being analyzed using size-exclusion chromatography and high-performance anion exchange chromatography, respectively. Kinetic modelling revealed that chewed white rice follows a typical parallel digestion pattern, with rapidly ($S_F)$ and slowly digestible starch ($S_S$) being digested simultaneously at distinctly different rates. Amylose content (AC) and amylose weight were significantly and positively correlated with the digestion rate and extent of $S_S$, whereas the digestion rate and extent of $S_F$ were closely linked to amylopectin, particularly its short and intermediate chains (degree of polymerization 13–36). Compared to low-amylose rice (AC < 25 %), high-amylose rice exhibited significantly higher $S_S$ but with a lower digestion rate, attributed to its higher AC with shorter chains and fewer short to intermediate Ap branches. These findings provide insights into starch structure-digestibility relationships, aiding the development of rice varieties with slower digestion rates.