Preparation of water-soluble and water-swellable starch acetates using microwave heating

Abstract

Starch acetates of degree of substitution 0.1–1.5 were prepared by heating corn starch, acetic acid and acetic anhydride in sealed, stirred, teflon vessels in a microwave reactor. Reaction efficiencies were typically >90% at reaction temperatures of 150–160 °C for 4–7 min. Starch acetates prepared by reaction at 160 °C to DS values of 0.4–0.9 were >90% soluble in room temperature water. Acetylation rates and water solubilities decreased in the order waxy>normal>high amylose starch. Significant reduction in molecular weights of the starches occurred during microwave reactions. Acetylation occurred preferentially at the C6 position (70%) based on 13C NMR results. Acetylation at lower temperatures (120 °C) gave starches of DS 0.2–0.3 which were water insoluble but highly swellable. These starches retained their native crystallinity, implying that acetate substitution was a block pattern.