Ethanol fermentation of mixed-sugars using a two-phase, fed-batch process: method to minimize D-glucose repression of Candida shehatae D-xylose fermentations
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cells pre-grown on D-xylose simultaneously consumed mixtures of D-xylose and D-glucose, under both non-growing (anoxic) and actively growing conditions (aerobic), to produce ethanol. The rate of D-glucose consumption was independent of the D-xylose concentration for cells induced on D-xylose. However, the D-xylose consumption rate was approximately three times lower than the D-glucose consumption rate at a 50% D-glucose: 50% D-xylose mixture. Repression was not observed (substrate utilization rates were approximately equal) when the percentage of D-glucose and D-xylose was changed to 22% and 78%, respectively. In fermentations with actively growing cells (50% glucose and D-xylose), ethanol yields from D-xylose increased, the % D-xylose utilized increased, and the xylitol yield was significantly reduced in the presence of D-glucose, compared to anoxic fermentations (YETOH,xylose = 0.2–0.40 g g−1, 75–100%, and Yxylitol = 0–0.2 g g−1 compared to YETOH,xylose = 0.15 g g−1, 56%, Yxylitol = 0.51 g g−1, respectively). To increase ethanol levels and reduce process time, fed-batch fermentations were performed in a single stage reactor employing two phases: (1) rapid aerobic growth on D-xylose (μ = 0.32 h−1) to high cell densities; (2) D-glucose addition and anaerobic conditions to produce ethanol (YETOH,xylose = 0.23 g g−1). The process generated high cell densities, 2 × 109 cells ml−1, and produced 45–50 g L−1 ethanol within 50 h from a mixture of D-glucose and D-xylose (compared to 30 g L−1 in 80 h in the best batch process). The two-phase process minimized loss of cell viability, increased D-xylose utilization, reduced process time, and increased final ethanol levels compared to the batch process.
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