Improving cellulosic ethanol fermentation efficiency by converting endogenous water-soluble carbohydrates into citric acid before pretreatment.

Water-soluble carbohydrates in raw lignocellulosic biomass are converted into hydroxymethylfurfural (HMF) in the acid-based pretreatment, thus increasing the detoxification intensity and reducing the fermentation efficiency of cellulosic ethanol. Therefore, reducing water-soluble carbohydrates in raw corn stover is crucially important to reduce the inhibitors' generation and improve the ethanol fermentation efficiency. In this study, aerobic solid-state fermentation of corn stover by inoculating Aspergillus niger spores converted 83% of the endogenous water-soluble carbohydrates into citric acid, leading to the decrease of 41% of HMF generation and 8% of sulfuric acid usage during the dry acid pretreatment. The reduced inhibitor generation improved the ethanol fermentability by 11% more ethanol than that of the corn stover without water-soluble carbohydrates' removal. This suggests that the removal of the water-soluble carbohydrates before pretreatment significantly reduced the inhibitors' generation in pretreatment and improved the fermentation efficiency of cellulosic ethanol.