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 chickpea and finger millet starches. In vitro, chickpea starch was less digestible (approx.45%) with glucoamylase than the finger millet 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 chickpea (33 and 6.5%) than in finger millet starch (22 and 6%, respectively). The molecular weight of chickpea starch fractions was much higher than those of finger millet starch fractions. Beta-amylolysis and debranching studies indicated that chickpea-Am contained sparsely distributed side-chain branches, whereas finger millet-Am was more linear. The chickpea-Ap was comprised of very long B (degree of polymerization (dp) 70), long B (dp 56), long A/short B (dp 25) and short A (dp 17) chains; whereas finger millet-Ap had long B (dp 48), short B (dp 25) and short A (dp 17) chains. The structure of Ax was between those of Ap and Am.