Distribution of acetyl groups alters the properties of acetylated starch

To explain why the viscosity of starches acetylated with acetic anhydride (AA), despite its higher reactivity, was lower than that of starches acetylated with vinyl acetate (VA), waxy (WRS), normal-amylose (NRS), and high-amylose (HRS) rice starches were selected for acetylation, and their structures were investigated. FT-IR analysis confirmed that acetyl groups replaced the hydrogen atoms. The acetylation primarily targeted the C-6 and C-3 positions, with the C-6:C-3:C-2 substitution ratio being influenced by amylose content and the type of acyl donor. For both acetylation reagents, the substitution sites of amylopectin were mainly located at C-6. With acetic anhydride treatment, C-6 substitution decreased to 55.6% in WRS-AA and 20.5% in NRS-AA, and 12.7% in HRS-AA, indicating that acetyl groups were predominantly located at C-3 on amylose. In contrast, vinyl acetate resulted in acetyl groups on amylose primarily localized at both C-3 and C-2, and led to a more uniform distribution of acetyl groups. Peak, final, and setback viscosities increased after acetylation, with vinyl acetate exerting a more pronounced effect than acetic anhydride, which indicated that a more uniform distribution of acetyl groups facilitated enhanced starch swelling and retrogradation. This research provided a strong foundation for explaining the mechanism of starch acetylation.