Rapid preparation of starch esters by high temperature/pressure reaction

This study was undertaken to determine if starch esters of moderate to high degree of substitution (DS) could be prepared more rapidly, efficiently and with fewer byproducts. The method selected was to heat mixtures of dry corn starch, glacial acetic acid and anhydrides under pressure in small (60 μl) stainless steel sealed pans. Starch acetates of DS 0.5–2.5 were prepared at temperatures of 160–180 °C in 2–10 min. Reaction efficiencies were essentially 100%. Reaction rates increased with increasing acetic acid concentration and decreased with increasing acetic anhydride concentration. The acetic acid remaining in the samples could be completely removed by vacuum stripping at 120–190 °C. Starch succinates of DS 1.0–1.5 were prepared at temperatures and times similar to those for starch acetates above. Longer reaction times (20–60 min) were required for the preparation of starch octenylsuccinates and dodecenylsuccinates having moderate DS values (∼0.5). Significant acetylation due to esterification of starch with acetic acid also occurred in these systems for long heating times (>20 min). This results in release of water and probably accounts for some of the apparent degradation (reduction in viscosity, browning) seen at long heating times.