Cofermentation of glucose and xylose with immobilized Pichia stipitis in combination with Saccharomyces cerevisiae

In glucose- and xylose-containing waste streams, xylose is only converted if the glucose concentration is sufficiently low not to inhibit xylose conversion. In order to obtain a low bulk glucose concentration, thereby promoting the conversion of xylose, the possibilities of a continuous culture with immobilized P. stipitis and suspended Saccharomyces cerevisiae were studied. The results of these experiments have been compared with the experimental results obtained from only immobilized P. stipitis and from coimmobilized P. stipitis and S. cerevisiae. The various processes for the simultaneous conversion of a mixture of glucose and xylose were modeled and compared with experimental results for a better understanding of the phenomena occurring. In a process with immobilized P. stipitis and suspended S. cerevisiae , conversion of xylose was possible. The xylose conversion was lower than with coimmobilization, although the bulk glucose concentration was lower and the P. stipitis concentration in the reactor was four times higher. Model simulations showed that the glucose profiles in the beads were quite different for the various processes. The differences in the xylose conversion rates could be explained by the different glucose profiles in the beads. The higher xylose conversion with coimmobilization is a result of the very low glucose concentration in the beads with this process as compared with immobilized P. stipitis and suspended S. cerevisiae. The model could predict the steady-state bulk glucose and xylose concentrations for the various processes. From the simulations, it follows that coimmobilization of P. stipitis and S. cerevisiae is the most promising process for the complete conversion of a mixture of glucose and xylose. For a complete conversion, a high initial P. stipitis concentration must be used because P. stipitis does not grow under the conditions of coimmobilization.