Ultrasound-assisted deacetylation of beta-chitin: influence of processing parameters

摘要

Ultrasound-assisted deacetylation (USAD) produces chitosan by treating beta-chitin suspended in 40% aqueous sodium hydroxide with high-intensity ultrasound irradiation. Chitosans resulting from the application of non-isothermal and isothermal USAD processes were characterized using NMR spectroscopy, X-ray diffraction, scanning electron microscopy, viscosity measurements and size-exclusion chromatography. The characteristics of the USAD-produced chitosan strongly depend on the processing temperature and ultrasound parameters, namely the irradiation amplitude and duration, with viscosity-average molecular weight in the range 100000-750000 g mol~(-1) and average degree of acetylation in the range 6.0-25.0%. Moreover, appropriate adjustment of the USAD parameters allows the production of chitosan with high molecular weight (M_w ≈ 10~6 g mol~(-1)) and low degree of acetylation (ca 7.0%) after a single-step short-time processing (30 min) carried out at low to moderate temperature (50-80 °C). The occurrence of cavitation, a phenomenon whose intensity depends on the irradiation amplitude and duration as well as on the processing temperature, is responsible for reducing the average size of the polysaccharide particles, favoring the accessibility of sodium hydroxide to the reactive sites and supporting a more efficient deacetylation of beta-chitin as compared to the thermochemical process.