Rice Genotype Related to Starch Biosynthesis: Effects on Rice Starch Structure and Enzymatic Digestibility during Shochu and Sake Production.

The enzymatic digestibility of rice grains is a crucial property affecting the efficiency of the starch-to-alcohol conversion during shochu and sake brewing. However, the effects of the genetic background of rice on its enzymatic digestibility have not been clarified. To bridge this gap, we probed the relationships among starch biosynthesis genes, starch structure, and the enzymatic digestibility of steamed rice during brewing. In shochu and sake koji digestion tests, the digestibility of the Alk/Wxa genotype was lower than that of the alk/Wxb genotype. The four examined genotypes (Alk/Wxa , alk/Wxa , alk/Wxb , and alk/wx) differed in terms of the digestibility of steamed rice under commercial-enzyme, shochu koji, and sake koji conditions, with digestibility following the order of alk/wx > alk/Wxb > alk/Wxa > Alk/Wxa . The retrogradation rate of steamed rice under sake conditions followed the order of alk/wx < alk/Wxb < alk/Wxa < Alk/Wxa . Digestibility showed positive and negative correlations with the amylopectin short-chain ratio and amylose content, respectively. Differential scanning calorimetry analysis indicated that crystalline amylopectin in the higher-crystallinity and higher-gelatinization-temperature Alk variety was not completely gelatinized by steaming, which resulted in reduced digestibility. Starch biosynthesis genotype (Alk/Wx) influenced the amylopectin short-chain ratio and amylose content, which resulted in a diversity of the enzymatic digestibility of steamed rice under shochu and sake production conditions. This study provides fundamental insights for selecting or breeding rice cultivars for shochu and sake brewing.