Xylitol production from rice straw hemicellulosic hydrolysate by Candida tropicalis GS18 immobilized on bacterial cellulose-sodium alginate matrix

Xylitol is gaining great attention in current pharmaceutical markets. Commercial production by chemical hydrogenation of xylose is however associated with several inevitable drawbacks. Biotechnological route of xylitol production is a superior alternative, if performed at a lower process costs. The goal of present study was to evaluate the efficiency of bacterial cellulose-sodium alginate (BC-AG) composite as an immobilization matrix for Candida tropicalis GS18 in the production of xylitol through a repeated batch fermentation of rice straw hemicellulosic hydrolysate. Different concentrations of bacterial cellulose, sodium alginate, and calcium chloride were screened to select the stable composite for immobilization. Comparative studies for xylitol production using adsorbed yeast cells on BC-AG composite and suspended cells in different production media like synthetic, non-detoxified, and detoxified hydrolysate were carried out. BC-AG composite prepared by using 3.0% alginate and bacterial cellulose in 1:1 ratio through cross-linking with 3.0% CaCl₂ solution was found to be most suitable for cell immobilization. The maximum xylitol yield and productivity obtained from immobilized cells on BC-AG composite in synthetic and detoxified media were 0.67 g/g; 0.70 g/L/h and 0.55 g/g; 0.52 g/L/h, respectively. The composite was successfully used for up to five repeated batch fermentations in synthetic and detoxified hydrolysate media. However, the composite was unstable in the non-detoxified medium. Hence, BC-AG composite was found to be an effective immobilization matrix for bioconversion of xylose into xylitol.