Effects of acetic acid/acetic anhydride ratios on the properties of corn starch acetates

Corn starch was pre-treated with acetic acid and then acetylated by acetic anhydride under microwave irradiation. The effects of molar ratios of these two reagents on the acetylation of starch were investigated. Starch acetate with a high degree of substitution (DS, 2.93) was obtained at a molar ratio (acetic acid/acetic anhydride) of 1:1. However, the DS should tend to decrease with a change of this ratio. The FT-IR analysis indicated characteristic absorption peaks, with increasing DS materialised by an increase of the carbonyl CO group and a decrease of the hydroxyl O–H group, at about 1750 cm−1 and 3450 cm−1, respectively. The X-ray diffraction patterns of acetylated starch showed an amorphous structure. Degree of crystallinity, surface morphology, water solubility and water absorption index of corn starch were also affected by the changes in reagent ratios. The glass transition (Tg) and melting (Tm) temperatures of acetylated starches also decreased after acetylation.