Xylitol production from a mutant strain of Candida tropicalis

To characterize the kinetics of growth, sugar uptake and xylitol production in batch and fed-batch cultures for a xylitol assimilation-deficient strain of Candida tropicalis isolated via chemical mutagenesis.

Chemical mutagenesis using nitrosoguanidine led to the isolation of the xylitol-assimilation deficient strain C. tropicalis SS2. Shake-flask fermentations with this mutant showed a sixfold higher xylitol yield than the parent strain in medium containing 25 g l⁻¹ glucose and 25 g l⁻¹ xylose. With 20 g l⁻¹ glycerol, replacing glucose for cell growth, and various concentrations of xylose, the studies indicated that the mutant strain resulted in xylitol yields from xylose close to theoretical. Under fully aerobic conditions, fed-batch fermentation with repeated addition of glycerol and xylose resulted in 3.3 g l⁻¹ h⁻¹ xylitol volumetric productivity with the final concentration of 220 g l⁻¹ and overall yield of 0.93 g g⁻¹ xylitol.

The xylitol assimilation-deficient mutant isolated in this study showed the potential for high xylitol yield and volumetric productivity under aerobic conditions. In the evaluation of glycerol as an alternative low-cost nonfermentable carbon source, high biomass and xylitol yields under aerobic conditions were achieved; however, the increase in initial xylose concentrations resulted in a reduction in biomass yield based on glycerol consumption. This may be a consequence of the role of an active transport system in the yeast requiring increasing energy for xylose uptake and possible xylitol secretion, with little or no energy available from xylose metabolism.

The study confirms the advantage of using a xylitol assimilation-deficient yeast under aerobic conditions for xylitol production with glycerol as a primary carbon source. It illustrates the potential of using the xylose stream in a biomass-based bio-refinery for the production of xylitol with further cost reductions resulting from using glycerol for yeast growth and energy production.