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

Abstract

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.