CONVERSION OF GLUCOSE XYLOSE MIXTURES BY PICHIA-STIPITIS UNDER OXYGEN-LIMITED CONDITIONS

Oxygen has a large effect on the growth, substrate utilization, and product formation of yeasts. A good understanding of the influence of oxygen on the conversion of mixed substrates by Pichia stipitis is necessary for a good control of the process. A model was developed to describe the effect of the oxygen supply rate on the conversion of a mixture of glucose and xylose by P. stipitis. This model was validated by means of batch and continuous culture experiments under oxygen-limited conditions. The diauxic conversion of a glucose/xylose mixture by Pichia stipitis can be described by a competitive inhibition of one sugar on the uptake of the other sugar. The influence of xylose on the glucose conversion is negligible. Glucose, however, has a strong inhibitory effect on the xylose conversion. The xylose conversion is inhibited completely at glucose concentrations of 2.3 g l-1 and higher. The model can predict batch conversions of a glucose/xylose mixture. For extremely oxygen-limited conditions, the specific xylose uptake rate equals the anaerobic xylose uptake rate. The continuous fermentations are well described by the model for the oxygen-limited parts of these fermentations. Maximum biomass concentrations of 3.5-4 g l-1 were reached because of other limitations than oxygen. The simulations show that the ethanol yield is maximal at an oxygen uptake rate close to zero. For this situation, the volumetric ethanol production will be low as a result of the low biomass concentration. Therefore, the optimal conditions for ethanol production from hemicellulosic wastes with P. stipitis will be a compromise between the ethanol yield and the volumetric production rate.