Biotransformation of mixed sugars to bioethanol and xylitol by Candida tropicalis using sequential fermentation: an innovative strategy towards biorefinery development

Abstract

Xylose conversion to xylitol instead of bioethanol is more profitable as it is a high-value chemical with several applications in food and pharma industries. The current study explores the ability of C. tropicalis for bioethanol and xylitol synthesis under anaerobic and aerobic conditions, respectively. Firstly, the effect of agitation (100, 200, 300 and 400 rpm) on glucose fermentation was evaluated. The maximum bioethanol yield and productivity of 0.49 ± 0.008 g/g and 3.62 ± 0.007 g/L/h were achieved at 200 rpm in 8 h. Subsequently, sequential fermentation was performed for bioethanol and xylitol synthesis from the glucose-xylose mixture. Bioethanol in the medium decreased xylose assimilation due to its inhibitory effect on C. tropicalis, resulting in poor xylitol yield. Bioethanol separation via distillation and xylose fed-batch fermentation with glycerol supplementation (2 g/L) was adopted to improve xylitol production. The maximum productivity of bioethanol and xylitol from glucose-xylose (59.04 ± 0.283 g/L, 50.27 ± 0.095 g/L) mixture was 3.52 ± 0.019 g/L/h and 2.19 ± 0.003 g/L/h respectively. These results were analogous to those obtained in the separate glucose and glycerol-supplemented xylose fed-batch fermentation experiments. The suggested strategy in the present work shows potential for developing biorefineries based on lignocellulose materials for the co-production of biofuels and biochemicals.