Studies on sweet potatoes—III. Distribution of unit chains of branched and unbranched molecules of starch

Freshly prepared sweet potato (SP) starch was separated quantitatively into amylose and amylopectin by a concanavalin A precipitation method. Amylose was further fractionated into two subfractions containing essentially linear (Am) and moderately branched (Ax) molecules, by hot-butanol extraction. The molecular weight determined by GPC of Am was 3 × 106 Da, the values for Ap and Ax were 1.5 × 107 and 5 × 106 Da, respectively. The β-amylolysis limit values were 57, 70 and 92%, respectively, for Ap, Ax and Am. The native, as well as the derived β-limit dextrins of Ap and Ax, were debranched with isoamylase and their unit chain lengths, measured by GPC and SE-HPLC methods, showed Ap (C̄L, 21.0) to have a trimodal distribution of unit chains with D̄P values of 52, 37 and 20, whereas Ax had tetramodal unit chains with D̄P values of 40, 30, 21 and 9. Am was more or less a linear molecule. In vitro SP starch was more digestible than legume and cereal starches.