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

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.