Reactors in series for the complete conversion of glucose/xylose mixtures by Pichia stipitis and Saccharomyces cerevisiae

Substrates that contain hexose as well as pentose sugars can form an interesting substrate for the production of ethanol. If Pichia stipitis is used for the conversion of such substrates, the pentose sugars are only converted when the hexose concentration is very low. This is a consequence of the sequential use of substrate. A system with reactors in series might be a good process for the sequential use of hexose and pentose sugars. The concentration of the preferred substrate will be low in the last reactor, thus allowing a more optimal pentose conversion. Two different systems with reactors in series were tested. The first system had three reactors with P. stipitis as the sole yeast. The second system consisted of two reactors: Saccharomyces cerevisiae was inoculated in the first reactor to convert hexose rapidly, and the second reactor contained P. stipitis to convert the pentose sugars. In the three-reactor system with P. stipitis, an almost complete conversion of a glucose/xylose mixture was possible. Aeration of the second reactor accelerated the sugar conversion. The volumetric ethanol production rate is, however, relatively low: 0.51 g l −1 h −1. With two reactors and two yeasts, only 20% of the xylose is converted. This is caused by the competition for oxygen between the two yeasts in the second reactor. The conversion of a mixture of glucose and xylose in a system containing reactors in series is possible. Low dilution rates have to be applied to obtain sufficiently low glucose concentration for the conversion of xylose; consequently, the volumetric production rates are low. The system with a combination of S. cerevisiae and P. stipitis can only be competitive when the yeasts are separated.