Influence of oxygen availability on cell growth and xylitol production by Candida guilliermondii.

Oxygen availability is the most important environmental parameter in the production of xylitol by yeasts, directly affecting yields and volumetric productivity. This work evaluated the cell behavior in fermentations carried out with different dissolved oxygen concentrations (0.5-30.0% of saturation), as well as a limited oxygen restriction (0% of saturation), at several oxygen volumetric transfer coefficients (12 < or = kLa < or = 70 h(-1)). These experiments allowed us to establish the specific oxygen uptake rate limits to ensure high yields and volumetric productivity. When oxygen availability was limited, the specific oxygen uptake rate values were between 12 and 26 mg of O2/of g cell x h, resulting in a yield of 0.71 g of xylitol/xylose consumed, and 0.85 g/[L x h] for the volumetric productivity. According to the results, the effective control of the specific oxygen uptake rate makes it possible to establish complete control over this fermentative process, for both cell growth and xylitol production.

Modeling of the production of xylitol from d-xylose in a batch and continuous membrane bioreactor

This work presents a mathematical model that describes of the behavior of the main variable involved in the continuous bioconversion of xylose to xylitol by Candida guilliermondii in a stirred-tank bioreactor with total cell recycle, a promising process to obtain this polyol because it increases productivity in function of the high cellular density. The fermentation system was modeled considering cellular mass, xylose, xylitol and dissolved oxygen concentrations, as well as xylose reductase enzymatic activit as dependent variables. The model is based on the assumption of the intra/extracellular xylitol transport and it is able to represent inhibition by xylose on the main intracellular reaction (conversion of xylose to xylitol by xylose reductase) as well as the influence of the oxygen limitation in all steps of the bioconversion. The results of the mathamatical model simulation were compared to experimental data. The model represents quite well the yeast performance in the production of xylitol.