The NRIB report also explains the mechanism linking higher temperatures during the earing period to the hardness of the rice.
The starch contained in rice grains comes in two kinds: amylose and amylopectin. Amylose molecules are made up of long, branching chains whereas amylopectin has short, dense side chains. Cooler temperatures during earing cause amylopectin to develop shorter side chains, and warmer weather means longer side chains. The graph on p.2 of the NRIB PDF shows an inverse relationship between average temperature one month after earing and the digestibility of steamed rice.
Rice steaming at Dewazakura’s second site in Yamagata City, Yamagata Prefecture.
The NRIB report goes on to briefly state that rice with short amylose side chains is broken down more easily by (kōji) enzymes in the main ferment, while rice with longer side chains is harder to break down. And this then affects the solubility of the rice and therefore its suitability for sake brewing.
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