| This work examined the structural features and physicochemical properties of rice starch in relation to kernel chalkiness, kernel breakage on milling, rough rice drying conditions, and storage-functionality changes. Three long-grain rice cultivars with similar endosperm gross composition but different in functional properties, differed in starch molecular size and amylopectin chain length distributions. Starch fine structure analyses by high-performance size-exclusion chromatography and high-performance anion-exchange chromatography with pulsed amperometric detection were effective in discriminating rice functional properties that can hardly be differentiated by the conventional quality test procedures. Fine structure data correlated with kernel translucency, X-ray diffraction patterns, thermal properties, and pasting characteristics. Starches from the chalky kernels of six rice cultivars generally contained less amylose (more amylopectin) and more short branch-chain amylopectin (less long branch-chain amylopectin) than the starches from translucent kernels. Thus, starch synthesis in chalky kernels may slightly favor glucan chain branching over chain elongation. Rough rice drying conditions caused no apparent changes on starch fine structures. Flour pasting properties were affected by drying treatments but those of starch were not, indicating that the kernel components removed by the alkali-steeping extraction of starch were important to the observed drying-related changes in rice functionality. Significant changes in starch fine structure were observed mainly on the rough rice stored at 38°C for 9 months (accelerated aging). The decreased amylose-amylopectin ratio, shortened amylopectin average chain length, and the shift in chain length distribution to shorter branch chains were implicative of molecular-level starch degradation. The flour and starch samples showed incongruent trends in pasting and thermal properties, thus hinting on the role of not only starch but also its interaction with non-starch kernel components in rice aging. |
