Hydrocolloid-Assisted Hydrothermal Modification of Intact Rice Kernels for Reduced Digestibility and Cooked-Rice...

Cooked rice is a high-glycemic-index staple, and reducing its starch digestibility is therefore important. This study evaluated hydrocolloid-assisted hydrothermal treatment of intact kernels for lowering cooked-rice starch digestibility while preserving texture and appearance. The kernels were mildly heated near the starch onset gelatinization temperature with κ-carrageenan, locust bean gum, xanthan gum, or guar gum (GG), each at 0.3%, then dried under humidity-controlled conditions (50°C, 80% relative humidity). GG was further examined at 0.1%-0.5%. Hydrothermal treatment alone lowered the estimated glycemic index (eGI) from 91.3 to 86.4 without significantly changing hardness, although adhesiveness increased. Adding a hydrocolloid during mild heating altered digestibility and texture: xanthan gum lowered eGI but deformed the kernels and raised hardness and adhesiveness, whereas GG gave the lowest eGI, raised adhesiveness but not hardness, and better-preserved kernel integrity. Increasing the hydrocolloid concentration reduced digestibility: at 0.3%, GG lowered eGI to 80.8. Raising it to 0.5%, however, lowered eGI only slightly further (to 79.2) while markedly increasing adhesiveness and causing kernel agglomeration. Thus, 0.3% GG most effectively reduced digestibility while preserving cooked-rice quality. Differential scanning calorimetry indicated that the reduced digestibility did not arise primarily from internal starch reorganization: the thermal profile of the treated kernels became more native-like even as digestibility fell. It was therefore more likely associated with a GG-rich surface layer that restricted kernel swelling. Overall, hydrothermal treatment with 0.3% GG is a simple, commercially viable strategy for lowering the starch digestibility of cooked rice while retaining texture and appearance comparable to native rice.