The generation of fermentation inhibitors during dilute acid hydrolysis of softwood

Abstract

The influence of the severity of dilute sulfuric acid hydrolysis of spruce (softwood) on sugar yield and on the fermentability of the hydrolysate by Saccharomyces cerevisiae (Baker’s yeast) was investigated. Fermentability was assessed as the ethanol yield on fermentable sugars (mannose and glucose) and the mean volumetric productivity (4 h). The hydrolysis conditions, residence time, temperature, and sulfuric acid concentration were treated as a single parameter, combined severity (CS). When the CS of the hydrolysis conditions increased, the yield of fermentable sugars increased to a maximum between CS 2.0–2.7 for mannose, and 3.0–3.4 for glucose above which it decreased. The decrease in the yield of monosaccharides coincided with the maximum concentrations of furfural and 5-hydroxymethylfurfural (5-HMF). With the further increase in CS, the concentrations of furfural and 5-HMF decreased while the formation of formic acid and levulinic acid increased. The yield of ethanol decreased at approximately CS 3; however, the volumetric productivity decreased at lower CS. The effect of acetic acid, formic acid, levulinic acid, furfural, and 5-HMF on fermentability was assayed in model fermentations. Ethanol yield and volumetric productivity decreased with increasing concentrations of acetic acid, formic acid, and levulinic acid. Furfural and 5-HMF decreased the volumetric productivity but did not influence the final yield of ethanol. The decrease in volumetric productivity was more pronounced when 5-HMF was added to the fermentation, and this compound was depleted at a lower rate than furfural. The inhibition observed in hydrolysates produced in higher CS could not be fully explained by the effect of the by-products furfural, 5-HMF, acetic acid, formic acid, and levulinic acid.