EFFECTS OF CULTURE CONDITIONS ON THE CO-FERMENTATION OF A GLUCOSE AND XYLOSE MIXTURE TO ETHANOL BY A MUTANT OF SACCHAROMYCES-DIASTATICUS ASSOCIATED WITH PICHIA-STIPITIS

Substrates that contain hexose as well as pentose sugars can form an interesting substrate for the production of ethanol. Pichia stipitis and a respiratory-deficient mutant of Saccharomyces diastaticus were used to convert such a substrate into ethanol under continuous culture conditions. With a sugar mixture (glucose 70%/xylose 30%) at 50 g/l, the xylose was entirely consumed when the dilution rate (D) did not exceed 0.006 h-1 whereas the glucose was entirely consumed whatever the D. The study of influence of initial substrate concentration (S0) was performed at D=0.015 h-1. Under these conditions the substrate was entirely consumed when its initial concentration did not exceed 20 g/l. With S0=80 g/l the residual xylose concentration reached 20.5 g/l. At low D or at low S0, P. stipitis was the dominant species in the fermentor. Increasing the D or S0 resulted in the wash-out of P. stipitis mainly because of its low ethanol tolerance.