Ethanol fermentation of mixed-sugars using a two-phase, fed-batch process:method to minimize D-glucose repression of Candida shehatae D-xylose fermentations

Candida shehatae cells pre-grown on D-xylose simultaneously consumed mixtur es of D-xylose and D-glucose, under both non-growing (anoxic) and actively growing conditions (aerobic), to produce ethanol, The rate of D-glucose con sumption was independent of the D-xylose concentration for cells induced on D-xylose. However, the D-xylose consumption rate was approximately three t imes lower than the D-glucose consumption rate at a 50% D-glucose: 50% D-xy lose mixture. Repression was not observed (substrate utilization rates were approximately equal) when the percentage of D-glucose and D-xylose was cha nged to 22% and 78%, respectively. In fermentations with actively growing c ells (50% glucose and D-xylose), ethanol yields from o-xylose increased, th e % D-xylose utilized increased, and the xylitol yield was significantly re duced in the presence of D-glucose, compared to anoxic fermentations (Y-ETO H,Y-xylose = 0.2-0.40 g g(-1), 75-100%, and Y-xylltol = 0-0.2 g g(-1) compa red to Y-ETOH,Y-xylose = 0.15 g g(-1), 56%, Y-xylitol = 0.51 g g(-1), respe ctively). To increase ethanol levels and reduce process time, fed-batch fer mentations were performed in a single stage reactor employing two phases: ( 1) rapid aerobic growth on D-xylose (mu = 0.32 h(-1)) to high cell densitie s; (2) D-glucose addition and anaerobic conditions to produce ethanol (Y-ET OH/xylose = 0.23 g g(-1)). The process generated high cell densities, 2 x 1 0(9) cells ml(-1), and produced 45-50 g L-1 ethanol within 50 h from a mixt ure of D-glucose and D-xylose (compared to 30 g L-1 in 80 h in the best bat ch process). The two-phase process minimized loss of cell viability, increa sed D-xylose utilization, reduced process time, and increased final ethanol levers compared to the batch process.