Enhanced Xylitol and Ethanol Yields by Fermentation Inhibitors in Steam-Pretreated Lignocellulosic Biomass

Abstract

A systematic study was conducted on the effects of specific concentrations of fermentation inhibitors acetic acid, 5-hydroxymethylfurfural (5-HMF), furfurals, and phenolics on the fermentation of xylose to xylitol and hexoses to ethanol by Candida guilliermondii in water-soluble fractions (hydrolysates) from steam-pretreated sugarcane bagasse, hybrid poplar, switchgrass, mixed wood, and giant reed. 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 to8 g/Lof acetic acid along with furfural, 5-HMF, and phenolics (found in hybrid poplar hydrolysate and sugarcane bagasse), the ethanol and xylitol yields increased up to 140% and 88%, respectively, compared to a control containing no inhibitors(100% and 68, respectively). The highest ethanol but lowest xylitol yields compared to controls were measured in hybrid poplar hydrolysate,which was then selected forthesystematic study for production of specific concentrationsof the fermentation inhibitors to enhance ethanol and xylitol yields. For all six steam pretreatment severities tested for hybrid poplar, the presence of inhibitors caused the ethanol yield to be enhanced by as much as 22% compared to the control. However, the xylitol yield was negatively affected by the higher concentrations of inhibitors in the six hybrid poplar hydrolysates. Overall this study showed that fermentation inhibitors are not necessarily harmful compounds. At specific concentrations, acetic acid, furfural, 5-HMF, and phenolics might be called fermentation enhancers rather than inhibitors.