Optimization of pH and acetic acid concentration for bioconversion of hemicellulose from corncobs to xylitol by Candida tropicalis

Hemicellulose hydrolysate from corncobs, separated by diluted sulfuric acid and sequently detoxed by boiling, overliming and solvent extraction, was used for xylitol production by Candida tropicalis W103. The effect of glucose and acetate in hydrolysate on xylitol production was investigated. It was found that glucose in hydrolysate promoted growth of Candida tropicalis while acetate at high concentration was inhibitory. The acetate inhibition can be alleviated by adjusting pH to 6 prior to fermentation and a substrate feeding strategy. Under these optimum conditions, a maximal xylitol concentration of 68.4 g l−1 was obtained after 72 h of fermentation, giving a yield of 0.7 g g−1 xylose and a productivity of 0.95 g l−1 h−1.