Effect of Combining Germination and Heat–Moisture Treatment of Unpolished Red Rice on the Physicochemical Properties and Digestibility of Its Starch

This study investigated changes in the physicochemical characteristics of starches obtained from unpolished red rice (URR) under germination and combined germination and heat-moisture treatment (HMT). Starches isolated from native, germinated, and germinated combined with HMT exhibited high purity, an A-type crystalline structure, and irregular polyhedral granules measuring 3-8 <i>μ</i>m in size. Germination did not influence the pasting temperature, trough and final viscosities, or setback of red rice starch. However, it increased the maximum viscosity, breakdown, and swelling power while reducing solubility. Germination elevated the concentration of rapidly digestible starch (RDS) and decreased the amount of resistant starch (RS). Notably, the increments in RDS and reductions in slowly digestible starch (SDS) and RS were positively correlated with longer germination durations (6-24 h). When HMT was applied after germination, increases were observed in pasting temperature, solubility, and RS content, while decreases occurred in maximum viscosity, breakdown, swelling power, and RDS content. Consequently, starches extracted from red rice germinated for 6 h and subsequently modified by HMT exhibited desirable characteristics, including the lowest RDS content (54.09%) and the highest RS content (30.39%) among all tested samples. These starches could be utilized in the production of low-carb products.