Synthesis and Characterization of Starch Acetates with High Substitution

Acetylation of high‐amylose (70%) maize starch to high degree of substitution (DS) was studied by reacting starch with acetic anhydride using 50% aqueous NaOH as the catalyst. DS increased with increasing reaction times and increasing ratios of acetic anhydride to starch. Reaction efficiency (RE) increased with longer reaction times and decreased with increases in the ratios of acetic anhydride to starch for extended reaction times. Increasing the amount of NaOH increased both DS and RE. A series of starch acetates with DS values of 0.57–2.23 were prepared and their crystalline structures, chemical structures, thermal stability, and morphological properties were investigated. After acetylation, and as DS increased from 0.57 to 2.23, the crystalline structures of starch steadily disappeared. The carbonyl group's peak at 1,740 cm^‐1 appeared in the FTIR spectra. The intensity of this peak increased with a decrease in the peak intensity of the hydroxyl groups at 3,000‐3,600 cm^‐1, indicating that the hydroxyl groups on starch were replaced by the acetyl groups. Thermal stability of starch acetates increased. The smooth surface of the starch granules became rough with acetylation. Further acetylation led to the loss of the starch granules and the formation of beehive‐ and fibrous‐like structures.     

Starch Acetate‐Maleate Mixed Ester Synthesis and Characterization

A new starch acetate‐maleate mixed ester with different degrees of substitution (DS) for each ester group was prepared. High‐amylose (70%) corn starch was first reacted at 123°C with ≈4.5 equivalents of acetic anhydride for different times, and then the intermediate products were further reacted at 80°C with 0.27–1.1 equivalents of maleic anhydride for a fixed time. ¹H NMR and alkaline saponification were used to determine DS values of acetyl and maleate groups. Increasing the reaction time from 60 to 120 min for starch and acetic anhydride increased DS of the acetyl group, while DS of the maleate group increased with increasing amounts of maleic anhydride. There was a good agreement between the two methods for DS determination. The reaction efficiency of acetylation increased from ≈30 to 50% with reaction time, and maleation decreased from ≈30 to 20% with increasing amounts of maleic anhydride. The mixed esters were characterized by ¹H NMR, ¹³C NMR, FTIR, DSC, and TGA. Characteristic peaks in ¹H NMR, ¹³C NMR, and FTIR spectra indicated the presence of acetyl and maleate groups in starch molecules. The exothermic peak in the DSC curve and the increase in thermal stability from the TGA curve were attributed to thermal cross‐linking of the double bond of the maleate group.