A stoichiometric analysis of biological xylitol production

In the biological production of xylitol from xylose it has been demonstrated that a low-level supply of oxygen to the culture (similar to 0.1 volume of air per volume of culture medium per minute) yields an increase of extracellular accumulated xylitol up to 0.65 g(xylitol) g(xylose)(-1) in our experiments. In spite of the abundant experimental evidence regarding xylitol production, the advances in the mathematical description of the process are relatively scarce. In this work, a stoichiometric model considering the main biochemical reactions (metabolic fluxes) involved in microaerobic xylitol production is proposed. The main metabolic reactions in xylitol production by Candida parapsilosis were incorporated in order to establish a stoichiometric reaction rate analysis of the network. The reaction rates in the metabolic network were calculated both for determined and underdetermined systems. A comparison between predicted and experimentally measured or reported yields has shown that the proposed stoichiometric model correctly depicts the xylitol yield within a 12.8% average error for several xylose-assimilating yeasts. The expected xylitol concentrations for a production process were also estimated after a sensitivity analysis of the metabolic network. An analysis of the estimated metabolic fluxes provided insight into some physiological events involved in the production of xylitol by yeasts. (C) 2010 Published by Elsevier B.V.