Addition of rice protein inhibits rice starch digestibility by enlarging the hydrogel pore size and promoting the formation of resistant starch with a DP around 150

Abstract

The two main components in rice grains are starch and protein, while their mutual interactions in determining the overall rice starch digestibility are unknown. To this end, the impact of rice protein addition on the digestibility of six rice starches with distinct molecular structures was investigated. Notably, the Low GI rice variety exhibited the fastest starch digestibility without protein addition. However, when protein was added, the maximum digested amounts of starch and protein were significantly reduced, especially with the Low GI rice, which exhibited the slowest starch digestibility. Mechanistically, the addition of rice protein enlarged the pore size and thickness of the starch hydrogel, particularly in Low GI rice starch with an amylose content of approximately 39 %. This effect may result from partial inclusion interactions between protein and amylose, as indicated by XRD, which create additional space, as visualized through SEM, facilitating the formation of resistant starch in the hydrogel network. Furthermore, the resistant starch had an approximate chain length of DP 150 and a hydrodynamic radius of 5 nm. Collectively, these results suggest that adding rice protein can reduce rice starch digestibility, while the effects depend on both the starch molecular size and chain-length distributions.