Unraveling the shell-core heterogeneity in cooked rice: Impact of milling degree on molecular structure and texture formation

Abstract

During rice cooking, leached starch and other soluble components form a surface film, resulting in the development of a characteristic shell–core structure. However, the compositional, structural, and physicochemical differences between shell and core flours, as well as their distinct contributions to rice texture, remain unclear. Therefore, in this study, the shell and core flours from cooked rice grains subjected to varying degrees of milling (DOMs, 0–60 %) were isolated and characterized. Results suggested that shell starches exhibited shorter chain lengths and smaller granule sizes, favoring the formation of lamellar structures with lower retrogradation tendency. In contrast, core starches contained longer chains and larger granules, which promoted compact aggregate formation and higher retrogradation propensity. As DOM increased, the shell flour showed elevated levels of amylose and short-chain amylopectin, enhancing rice stickiness. Meanwhile, the core flour exhibited a lower retrogradation tendency, accompanied by greater porosity and reduced structural integrity of core region, ultimately reducing rice hardness. These findings provide new insights into the spatial heterogeneity of starch structure and functionality in cooked rice, offering a theoretical basis for the quality improvement of rice-based products.