Optimization of the fermentation conditions of Rhizopus japonicus M193 for the production of chitin deacetylase and chitosan

•Bioconversion of chitin to chitosan by R. japonicus fermentation was studied.•Fermentation conditions for CDA production were optimized statistically.•Chitosan from bioconversion had superior quality than that of chemically extracted. To improve the production of chitin deacetylase (CDA) for the bioconversion of chitin to chitosan with desirable functionality, the effect of the nutritional requirement on the CDA production from Rhizopus japonicus M193 fermentation was investigated under submerged conditions. Nutritional elements including glucose (g/L), inoculum level (%), and MgSO4·7H2O (g/L), as well as culture time (d) were identified as the most critical factors for the CDA production based on the results from Plackett–Burman design (PBD). Taguchi design with orthogonal array was further employed to optimize R. japonicus M193 fermentation conditions based on the results from PBD, in which 2.5% chitin, 5g/L glucose, 5% inoculum level, 0.6g/L MgSO4·7H2O, and 5d culture time were identified as the optimal fermentation conditions. Under this condition, the maximum CDA production, DDA and MM of produced chitosan were 547.38±12.06U/L, 78.85±1.68%, and 125.63±3.74kDa, respectively. Obtained chitosan displayed similar physicochemical and structural properties to those of commercial chitosan extracted using chemical method based on the results from Fourier transform infrared spectrometer (FT-IR), thermogravimetric analysis (TGA)–differential scanning calorimetry (DSC), and nuclear magnetic resonance (NMR) assays, while the use of chemical reagents was significantly reduced.