Cereal Chem. 81(4):515-520The molecular structure and pasting properties of starches from eight buckwheat cultivars were examined. Rapid viscograms showed that buckwheat starches had similar pasting properties among cultivars. The actual amylose content was 16-18%, which was lower than the apparent amylose content (26-27%), due to the high iodine affinity (IA) of amylopectin (2.21-2.48 g/100 g). Amylopectins resembled each other in average chain-length (23-24) and chain-length distributions. The long-chains fraction (LC) was abundant (12-13% by weight) in all the amylopectins, which was consistent with high IA values. The amyloses were also similar among the cultivars in numberaverage DP 1,020-1,380 with 3.1-4.3 chains per molecule. The molar-based distribution indicated that all the amyloses comprised two molecular species differing in molecular size, although the weight-based distribution showed a single species. A comparison of molecular structures of buckwheat starches to cereal starches indicated buckwheat amylopectins had a larger amount of LC, and their distributions of amylose and short chains of amylopectin on molar basis were similar to those of wheat and barley starches.

Long chains (LC) were fractionated by 1-butanol precipitation from isoamylase-debranched amylopectins of rice, maize, wheat, buckwheat, and sweet potato. The structure of the precipitate (LCppt) was characterized by high-performance size-exclusion chromatography (HPSEC) with pre-column labeling with 2 aminopyridine. HPSEC showed that LCppt corresponded to the LC fraction of unit-chain distribution of amylopectin. LCppt had a number-average degree of polymerization of 330 490 and an average-number of chains of 1.2 1.4, indicating that the structure of LCppt is very similar irrespective of its botanical sources and is distinct from amylose. HPSEC revealed that size distribution of LCppt was also similar among the specimens examined. The distribution was distinct from and much narrower than that of amylose. Still, LCppt consisted of a significant amount (by weight) of chains as long as amylose. LCppt contained 5.1 9.4 mol% of branched molecules, which had 5 chains per molecule (except for 10 of indica rice IR36), indicating incomplete debranching of the side chains on LC by isoamylase. The structure of the isoamylase-resistant branches is unknown. In this respect, LCppt and amylose appear to have a common branched structure, which is unusual in starch α-glucans.