Molecular structure and physicochemical properties of starch isolated from hydrothermally treated brown rice flour

Abstract

The impact of single and dual hydrothermal treatments on the molecular structure, crystalline structure, in vitro digestibility, and physicochemical properties of starches isolated from hydrothermally treated brown rice flour was investigated. Annealed rice starch (ANN) exhibited no significant change in molecular structure, whereas heat-moisture treated rice starch (HMT) showed decreased apparent amylose content, molecular weight of amylopectin, average chain length of amylopectin, and long amylopectin branch chains (DP ≥ 37). The relative crystallinity, intensity ratio of 1045 cm−1–1015 cm−1, and gelatinization enthalpy remained unchanged in ANN, but significantly decreased in HMT. In dually modified starches, the HMT-ANN had higher gelatinization temperatures, gelatinization enthalpy, relative crystallinity and ratio of 1045 cm−1–1015 cm−1 than the ANN-HMT. Hydrothermal treatment substantially increased starch digestibility in the granular state, showing increased rapidly digestible starch (RDS) content and decreased resistant starch (RS) content. However, in the gelatinized starches, ANN and HMT showed decreased RDS level and increased slowly digestible starch (SDS) and RS levels. The changes in amounts of RDS, SDS, and RS in ANN, HMT and dually modified starches could be attributed to the alteration of crystalline and molecular structures.