Phenols and lignin: Key players in reducing enzymatic hydrolysis yields of steam-pretreated biomass in presence of laccase

Phenols are known as inhibitors for cellulases and fermentative microorganisms in bioethanol production processes. The addition of laccases removes the phenolic compounds and subsequently reduces the lag phase of the fermentative microorganism. However, the application of laccases diminishes glucose release during the enzymatic hydrolysis. In this study a model cellulosic substrate (Sigmacell) together with lignin extract, whole steam-pretreated wheat straw (slurry) and its water insoluble solid fraction (WIS) were subjected to enzymatic hydrolysis to evaluate the effects of laccase treatment in presence of lignin and phenols. The presence of laccase in enzymatic hydrolysis of Sigmacell with lignin extract reduced glucose yield by 37% compared with assays without laccase. Furthermore, this reduction was even more marked in presence of phenols (55% reduction). Interestingly, when hydrolyzing WIS, the addition of phenols coupled with laccase treatment did not show a reduction when compared with only laccase addition. This fact suggests the key role of lignin in the hydrolysis inhibition since in WIS the ratio cellulase per gram of lignin was much lower than in Sigmacell experiments. Finally, the lower cellobiose and xylose recoveries point out that phenolic oligomers formed by laccase oxidation play important roles in the inhibition of endoglucanases, cellobiohydrolases and xylanases. To conclude, the proportion of lignin and the composition of phenols are key players in the inhibition of cellulases when the enzymatic hydrolysis is combined with laccases detoxification.