Glucose repression of xylitol production in Candida tropicalis mixed-sugar fermentations

Glucose repressed xylose utilization inCandida tropicalis pre-grown on xylose until glucose reached approximately 0–5 g l^–1. In fermentations consisting of xylose (93 g l^–1) and glucose (47 g l^–1), xylitol was produced with a yield of 0.65 g g^–1 and a specific rate of 0.09 g g^–1 h^–1, and high concentrations of ethanol were also produced (25 g l^–1). If the initial glucose was decreased to 8 g l^–1, the xylitol yield (0.79 g g^–1) and specific rate (0.24 g g^–1 h^–1) increased with little ethanol formation (<5 g l^–1). To minimize glucose repression, batch fermentations were performed using an aerobic, glucose growth phase followed by xylitol production. Xylitol was produced under O₂ limited and anaerobic conditions, but the specific production rate was higher under O₂ limited conditions (0.1–0.4 vs. 0.03 g g^–1 h^–1). On-line analysis of the respiratory quotient defined the time of xylose reductase induction.