Rice protein digestion by sake Koji enzymes: Comparison between steamed rice grains and isolated protein bodies from rice endosperm

The digestion of proteins in steamed rice grains by sake koji enzymes under simulated sake mash conditions was analyzed by comparing the hydrolysis of steamed rice grains and heat-treated protein bodies (PBs) isolated from seven rice samples including four endosperm-storage protein mutants. The disappearance of peptides in the digest of isolated PBs was faster than that of steamed rice grains; however, more insoluble proteins formed in the case of isolated PBs. Not all of the hydrolyzed PB proteins were immediately solubilized in the digestion tests. High-molecular-weight peptides were more abundant in the solubilized digest of steamed rice grains than in that of isolated PBs. Variance in Ile, Ser, Glu, and Gly levels in the digest of steamed rice grains was relatively high among the seven samples, but was not found to be high in digests of isolated PBs. These results indicate that factors that may be derived from the steamed rice grains profoundly affect the digestion of proteins in steamed rice grains by sake koji enzymes.