AMYLOPECTIN STALING OF COOKED MILLED RICES AND PROPERTIES OF AMYLOPECTIN AND AMYLOSE

Starches of waxy rices that showed varietal differences in hardness te sting of cooked rice after amylopectin staling and high-amylose conten t (AC) rices differing in gel consistency (GC) and starch gelatinizati on temperature (GT) were studied to determine the factors related to v arietal differences in amylopectin staling of cooked rice. Intermediat e- and high-GT starches showed greater amylopectin staling of gelatini zed rice by hardness testing values or differential scanning calorimet ry (DSC) endotherm than did low-GT starches in both waxy and nonwaxy r ices. Isoamylase-debranched amylopectins of waxy rices differed in the ratio of weight-average degree of polymerization (DPw) fractions, but these fraction ratios were not simply related to differences in amylo pectin staling of cooked rice. Among high-AC rices, amylopectin from l ow-CT starch was confirmed to have higher iodine affinity (2.3-2.5%) t han amylopectin from intermediate-CT starches (1.7-1.8%), regardless o f GC. Within high-AC starch of the same GT type, soft-CC rice correspo nded with more A + B-1 DPw 16-18 and less B-3 DPw 150-200 fractions of debranched amylopectin and low DPw of amylose. Amylopectin of amylose extender mutant of IR36 was confirmed to have a longer chain length t han ordinary rice amylopectin: the debranched amylopectin has more B-2 DPw 47-51 fraction, less A + B-1 DPw fraction, but no B-4 fraction wi th DPw >200. Only high-AC amylopectin had debranched fraction with DPw >120.