Analysis and optimization of a two-substrate fermentation for xylitol production using Candida tropicalis

Xylitol, a functional sweetener, was produced from xylose using Candida tropicalis ATCC 13803. A two-substrate fermentation was designed in order to increase xylitol yield and volumetric productivity, Glucose was used initially for cell growth followed by conversion of xylose to xylitol without cell growth and by-product formation after complete depletion of glucose. High glucose concentrations increased volumetric productivity by reducing conversion time due to high cell mass, but also led to production of ethanol, which, in turn, inhibited cell growth and xylitol production. Computer simulation was undertaken to optimize an initial glucose concentration using kinetic equations describing rates of cell growth and xylose bioconversion as a function of ethanol concentration, Kinetic constants involved in the equations were estimated from the experimental results. Glucose at 32 g L-1 was estimated to be an optimum initial glucose concentration with a final xylose concentration of 86 g L-1 and a volumetric productivity of 5.15 g-xylitol L-1 h(-1). The two-substrate fermentation was performed under optimum conditions to verify the computer simulation results. The experimental results were in good agreement with the predicted values of simulation with a xylitol yield of 0.81 g-xylitol g-xylose(-1) and a volumetric productivity of 5.06 g-xylitol L-1 h(-1).