Digestibility Characteristics of Isolated Starch from Sorghum and Corn Grain

Abstract

Purified starch was isolated from nine varieties of grain sorghum [Sorghum bicolor (L.) Moench] differing widely in seed and endosperm characteristics and from four varieties of corn. Six-hour in vitro dry matter disappearance (IVDMD) values were similar (P>.05) for most purified, isolated varietal starches in yr 1 and 2. In yr 3, however, IVDMD of one waxy starch was higher (P<.05) than that of starch isolated from a bird-resistant sorghum. Likewise, overall mean 6-h IVDMD values were nearly identical for all sorghum and cornstarches. In vitro gas production (IVGP), a procedure utilizing amyloglucosidase and commercial baker's yeast to assess susceptibility of the isolated starch to enzymatic degradation, was higher (P<.05) for sorghum starch than for cornstarch in both yr 1 and 2. Among sorghum types, isolated waxy starch (high in amylopectin) generally gave higher IVGP values than nonwaxy starches. Some differences in IVGP between nonwaxy starches were apparent, suggesting that other factors, such as granule size or chain length, may affect starch degradation. As with IVGP, enzymatic digestion via incubation with an a-amylase enzyme generally favored the waxy starches, although differences among isolated sorghum starches appeared small. Differences in a-amylase digestion between sorghum and cornstarches were small. These studies suggest that digestion by rumen microbes is equally rapid for cornstarch and sorghum starch. Lower feeding value of grain sorghum cannot be ascribed to major differences in rumen digestibility of the isolated starch. Differences in amylase susceptibility, and, therefore, postruminal digestion, may exist. Results suggest that factors other than starch type, e.g., protein or tannins, must play significant roles in limiting availability of starch granules and the subsequent digestibility of starch in sorghum grain by ruminants.