Structural studies of linear and branched fractions of chickpea and finger millet starches

Structural studies using beta-amylase and pullulanase were carried out to explain at a molecular level the differences in the digestibility of legume (chickpea, Bengal gram, BG) and cereal (finger millet, ragi, Rg) starches. In vitro, BG starch was less digestible (∼45%) with glucoamylase than the Rg starch (>70%). Of the various fractionation methods employed, concanavalin A precipitation gave pure amylopectin (Ap). The crude amylose on subfractionation with hot 1-butanol yielded pure amylose (Am) and an intermediate fraction (Ax). The content of “true” Am and Ax was higher in BG (33 and 6.5%) than in Rg starch (22 and 6%, respectively). The molecular weight of BG starch fractions was much higher than those of Rg starch fractions. Beta-amylolysis and debranching studies indicated BG-Am to contain sparsely distributed sidechain branches, whereas Rg-Am was more linear. The BG-Ap was comprised of very long B (dp 70), long B (dp 56), long A/short B (dp 25), and short A (dp 17) chains; whereas Rg-Ap had long B (dp 48), short B (dp 25), and short A (dp 17) chains. The structure of Ax lay between those of Ap and Am.