8-26: Enhanced xylitol and ethanol yields by fermentation inhibitors in steam pretreated hydrolysates

A systematic study of the effects of low concentrations of fermentation inhibitors on the fermentation of xylose to xylitol and hexoses to ethanol by yeast Candida guilliermondii in the steam pretreated hydrolysates from sugarcane bagasse, hybrid poplar, switchgrass, yard waste and giant reed was conducted. The influence of different inhibitor concentrations on the growth and fermentation abilities of C. guilliermondii cultivated in lignocellulosic hydrolysates was measured during liquid batch cultures. In the presence of up to 8g/L of acetic acid along with furfural, 5-HMF, and phenolics in sugarcane bagasse hydrolysate, the ethanol and xylitol yields were boosted up to 20% compared to the control (100%). For all the steam pretreatment severities tested for hybrid poplar the ethanol yield of theoretical was enhanced as much as 47% compared to the control (100%). However, xylitol yield was negatively affected by the higher concentrations of inhibitors in hybrid poplar hydrolysates collected after six different steam pretreatment conditions. It was shown that acetic acid, furfural, 5-HMF, and phenolics at certain concentrations can be called rather enhancers than fermentation inhibitors. Customizing the steam pretreatment conditions for production of fermentation inhibitors at unique concentration can be used as a novel approach of improving fermentation of xylitol and ethanol by Candida guilliermondii. This approach will allow us in the future to find the optimum concentration of inhibitors to increase ethanol and xylitol yields during fermentation of hexoses and pentoses. Provided that the inhibitors in lignocellulosic hydrolysates are identified, the fermentation process can be improved in several ways.