Effects of Ball Milling, Ultrosonic or Hydrochloric Acid Treatment on the Microstructure and Enzymatic Deacetylation Efficiency of Chitin

Chitin is the second most abundant natural polysaccharide on Earth, which can be deacetylated to produce chitosan. However, due to the high crystallinity and poor solubility of chitin, its efficiency of enzymatic deacetylation is very low. To investigate the effects of different treatments on the microstructure and enzymatic deacetylation efficiency of chitin, chitin was modified by ball milling, ultrasonic or hydrochloric acid treatment in this study. First, the optimum conditions of ball milling and ultrasonic treatments were determined. Then, the microstructure of modified chitins was characterized by Fourier transform infrared (FTIR) spectroscopy, elemental analysis, X-ray diffraction, thermogravimetric-differential scanning calorimetry (TG-DSC) and scanning electron microscopy (SEM). Finally, the deacetylation efficiency of chitin deacetylase from Rhodococcus sp.11-3 for chitin was measured. The results showed that the optimum conditions of ball milling and ultrasonic modification were 1800 r/min and 45 min, and 400 W and 45 min, respectively. Among the three treatments, ball milling had the best modification effect, resulting in an 88.14% reduction in the viscosity-average molecular mass of chitin. The intermolecular hydrogen bond network of chitin was destroyed and some glycosidic bonds were broken; the degree of deacetylation increased; the crystallinity decreased from 96.30% to 73.04%; thermal stability was destroyed; and the treated chitin showed a stacking structure and was more loose and porous. In addition, chitin deacetylase had higher deacetylation efficiency for ball-milled chitin, and the yield of acetic acid was 3.4 times higher than that from raw chitin. In conclusion, ball milling treatment can improve the enzymatic deacetylation efficiency of chitin by changing its physicochemical properties and microstructure.