MICROBIAL-PRODUCTION OF XYLITOL FROM D-XYLOSE USING CANDIDA-TROPICALIS

Candida tropicalis DSM 7524 was used to produce xylitol from D-xylose. The fermentation conditions were optimized during continuous cultivation. The strain employed showed no great dependence upon temperature in a range between 30-degrees-C and 37-degrees-C. It achieved is best yield of xylitol from D-xylose at a pH value of 2.5. Such low pH values allow non sterile cultivation, which is a major economic factor. With an oxygen uptake rate of 0.8-1 ml oxygen per litre culture medium, the C. tropicalis produce xylitol at a yield of between 77% and 80% of the theoretical value. Higher yeast extract concentrations prevent the conversion of D-xylose into xylitol. D-xylose acts as a growth inhibitor in higher concentrations. The maximum xylitol yield was reached at a D-xylose concentration of around 100 g/l. In a non sterile batch culture with substrate shift 220 g/l xylitol were produced from 300 g/l D-xylose at a xylitol productivity rate of 0.37 g/(lh). In order to increase the specific yield, C. tropicalis was immobilised on porous glass and cultivated in a fluidized bed reactor. In a continuous non sterile cultivation with immobilised cells 155 g/l D-xylose produced 90-95% g/l xylitol with a productivity of 135 g/(lh).