Using starch structure to choose rices with an optimal combination of palatability and digestibility

Abstract

Steamed white rices that are popular with consumers are usually quick to digest, which is however disadvantageous to metabolic health. However, varieties which are slow to digest are generally regarded as less palatable. Starch molecular structure is a major determinant of both properties. Starch structure includes distinct attributes, such as the chain-length distribution of debranched chains and the size distribution of (undebranched) whole molecules. While some structural attributes control both properties, it is found that there are some structural features which are significant determinants for one property and not the other. Here, a novel means of using this fact to overcome these competing demands of slow digestibility and acceptable palatability is explored. The chain-length distributions (CLDs) and in vitro digestibility of 98 varieties, and palatabilities of 9 market varieties, were measured using size-exclusion chromatography (SEC) for the CLDs, and human panelists for palatability. The CLDs were parameterized with biosynthesis-based models, one each for amylopectin and for amylose. Four structural features of the CLDs (βAp,3, hAm,2, hAm,1 and hAp,5), involving the amounts of short and long chains in various CLD regions, showed significant correlations, with correlation coefficients having opposite signs for both digestibility (0.597**, −0.451**,-0.350**, and −0.413, respectively) and palatability (0.769*, −0.396, −0.491 and −0.398, respectively), and which thus were the key factors in the conflicting demands of slower digestibility versus acceptable palatability. It was also found that different structural parameters were dominant for digestibility and palatability; for instance, some structural features of hAp,1 and βAp,5 were found to optimize palatability and digestibility independently (hAp,1 - digestibility, −0.547**; βAp,5 – palatability, 0.339). Hence optimizing these two separate sets of parameters provides a means of finding an overall set of structural parameters which can be used to give the best combination of lower digestibility and acceptable palatability. Because the genes controlling these CLDs have been established, this can help rice breeders to develop varieties which give the best compromise for simultaneously optimizing both properties.