Optimum conditions for the precipitation of chitosan oligomers with DP 5-7 in concentrated hydrochloric acid at low temperature

Abstract

Chitosan was partially hydrolysed with 35% hydrochloric acid for 2 h at 80 degrees C and the hydrolysate stored at - 20 degrees C after dilution with water to precipitate higher molecular weight (MW) chitosan oligomers. When the hydrolysate was not diluted with water, no precipitate was formed but 7.3% chitosan oligomers were precipitated at a dilution ratio of 1.0 (ml water/ml hydrolysate). The time for precipitation was not significantly changed after storing the hydrolysate at - 20 degrees C for 1 day. In addition, the precipitation yield was not significantly influenced by the concentration of HCl used for the hydrolysis except at less than 5.0 (ml HCl/g chitosan). However, the yield of precipitated oligomers changed with partial hydrolysis time. For 0.5 and 2 h hydrolysis, 10.1 and 7.3% of the oligomers were precipitated, respectively, but only 3.1% of the oligomers were obtained after a 4 h reaction. When methanol was added to the hydrolysate, the precipitation yield increased up to 70% but the amounts of lower MW chitosan oligomers in the precipitated oligomers also increased with the increase of higher MW. The precipitated oligomers were mainly composed of pentamers and hexamers. (C) 1999 Elsevier Science Ltd. All rights reserved.