DisplayTitle

High degree of substitution (DS) starch acetate is a useful thermoplastic material interested by materials industry; however, conventional preparation of such involves heavy uses of organic media which lead to high cost and purification difficulties. In this study, waxy, normal, and high-amylose maize starches were acetylated to various DS in aqueous medium catalyzed by concentrated NaOH and the substitution pattern of the acetyl group on glucose monomer was characterized for the first time. The DS values were in the order of high-amylose maize starch (DS 1.72) > waxy maize starch (DS 1.60) > normal maize starch (DS 1.03) when same amount of acetylating reagent was added. The acetylation efficiency was improved by a stepwise procedure, repeatedly filtering and resuspending the starch between each addition of acetic anhydride. Starch acetates were further fully esterified to DS 3.0 using acetic anhydride-d6 or propionic anhydride in pyridine. Complete substitution remarkably improved resolution for 1H NMR spectrum and allowed acetyl group at different hydroxyl positions to be quantitatively calculated. Because propionyl -CH2- signal overlapped the acetyl C6 proton signal at 2.1 ppm, we confirmed that deuterated acetyl group was a preferred secondary substituent over propionyl group. For starch acetates prepared in aqueous medium, C2 and C3 were preferred reaction sites at DS lower than 0.14. At DS 1, the distribution of acetyl groups was ca. 44%, 32%, and 24% at C2, C3, and C6 positions, respectively, regardless of the ratio of amylose to amylopetin in native starch. Future works involving applications and structure-function relations of starch acetates and other derivatives can be studied based upon these results.