Structural Differences in Amylopectin Affect Waxy Rice Processing

Abstract

We studied the varietal differences in the rice cake hardening rate, which is one of the important characters in waxy rice processing. Waxy upland rice cv. Kanto-mochil72 had fewer short chains (dp 7 to 10) of amylopectin and more intermediate-length chains (dp 12 to 21) than cv. Tsukuba-hatamochi and cv. Mangetsu-mochi, which harden much more slowly when made into rice cakes, mochi, than does Kanto-mochil72. Based on zymogram analysis of the soluble fraction from the developing endosperm, the activity of starch synthase 2 (SS2), a candidate enzyme responsible for the difference in the amylopectin chain length was detected with Kanto-mochil72. SS2 activity was absent, however, in Tsukuba-hatamochi and Mangetsu-mochi. Using the progeny derived from a cross between Kanto-mochil72 and Mangetsu-mochi, we showed that differences in amylopectin chain length corresponded to the gelatinisation temperatures of the rice flours and rice cake hardening rates. The progeny of those rich in short amylopectin chains showed lower gelatinisation temperature and slower rice cake hardening rate than the progeny of those having amylopectin with fewer short chains. These results indicate that the differences in chain length distribution of amylopectin, polymorphisms in starch synthase activity, differences in rice flour gelatinisation temperature, and differences in rice cake hardening rate are not merely correlated with each other but are intimately linked since they are related to amylopectin properties.