The impact of rice protein on in vitro rice starch digestibility

Rice protein is a major non-starch component that may affect rice starch digestibility. Removal of protein from rice grain causes increased rice starch digestibility, however, the impact of individual proteins on rice starch digestibility is yet to be determined. Total rice protein content was calculated from total nitrogen and individual proteins, namely globulin, glutelin and prolamin were analysed by an optimised high-performance liquid chromatography (HPLC) method, and correlated with in vitro rice starch digestibility. Unexpectedly, total protein and protein composition estimated by HPLC were not negatively correlated with in vitro starch digestibility. The extraction of glutelin and prolamin was incomplete which may be due to the starch-protein interaction. The level of residual rice protein showed a negative correlation with in vitro rice starch digestibility. Addition of rice protein, namely 2.0% albumin, 2.5% globulin, 5.0% glutelin and 2.0% prolamin separately to rice flour decreased the in vitro rice starch digestibility by 8.2%, 6.3%, 10.7% and 6.5% respectively. Addition of 5%, 10% and 15% acetic acid extracted rice proteins to flour decreased in vitro rice starch digestibility in a concentration dependent manner. This study suggests rice starch digestibility can be manipulated by changing the protein content and composition through breeding new varieties or adding certain proteins before cooking.