A flocculating strain of Pichia stipitis for the conversion of glucose/xylose mixtures

In the production of ethanol from glucose/xylose mixtures by Pichia stipitis, the sugar uptake rate is the rate limiting step in the process. Therefore, a high biomass concentration is needed for a complete conversion of a mixed substrate in a single reactor. A flocculating strain of P. stipitis can offer the advantage of a high biomass concentration in the reactor that is almost independent of the dilution rate. The sugar concentrations in an oxygen-limited continuous culture were varied and CaCl 2 was added to promote flocculation. Flocculation occurred under these conditions, and a flocculating type of P. stipitis was isolated. The flocculating strain of P. stipitis, which formed pseudomycelium, is deposited at the CBS as P. stipitis CBS 7505. To study the possibilities of P. stipitis CBS 7505, continuous culture experiments were carried out in an airlift-loop reactor. A biomass concentration of 14.1 g l −1 was possible at a dilution rate of 0.05 h -1, leading to a simultaneous conversion of glucose and xylose. Computer simulations showed the conversion of glucose is probably oxygen limited. The results of flocculation are comparable with those of coimmobilization of P. stipitis and Saccharomyces cerevisiae. The specific and volumetric ethanol production rates of the two processes are almost identical. However, a higher ethanol yield was found in the flocculation experiments, but more xylose is converted in the case of coimmobilization. The conversion of waste streams that contain hexose as well as pentose sugars with flocculating P. stipitis is possible. Due to the low yield of ethanol, the extra profit made in comparison to that obtained from the conversion of the same waste stream with S. cerevisiae is minimal.