Increase of xylitol productivity by cell-recycle fermentation of Candida tropicalis using submerged membrane bioreactor

Candida tropicalis, an osmophilic strain isolated from honeycomb, produced xylitol at a maximal volumetric productivity of 3.5 g l−1 h−1 from an initial xylose concentration of 200 g l−1. Even at a very high xylose concentration, e.g., 350 g l−1, this strain produced xylitol at a moderate rate of 2.07 g l−1 h−1. In a fed-batch fermentation of xylose and glucose, 260 g l−1 xylose was added, and the xylitol production was 234 g l−1 for 48 h, corresponding to a rate of 4.88 g l−1 h−1. To increase xylitol productivity, cells were recycled in a submerged membrane bioreactor with suction pressure and air sparging. For each recycle round in cell-recycle fermentation, the average concentration of xylitol produced, fermentation time, volumetric productivity, and product yield were 180 g l−1, 19.5 h, 8.5 g l−1 h−1, and 85%, respectively. When cell-recycle fermentation was started with the cell mass concentrated twofold after batch fermentation and performed for 10 recycle rounds, we achieved a very high productivity of 12 g l−1 h−1. The productivity and total amount of xylitol in cell-recycle fermentation were 3.4- and 11.0-fold higher than those in batch fermentation, respectively.