The reaction of chitosan was carried out in a flask witha plug. Dried sample (Ig) was dissolved in 25 mL of2.8% acetic acid; then, 25 mL of ethanol was added,and a little pyridine was dropped into the mixingsolution until the solution became clear. Acetic anhy-dride was charged. After it was stirred at ambienttemperature for a predetermined time, the reactionmixture was precipitated with ethanol, and the pre-cipitate was washed with acetone to remove excessreactant. The N-acetylated chitosan was dried at 50°C.