Heads and tails of laccases in bioethanol production
Conference contribution, 2015
In a lignocellulosic biorefinery, the sugar platform could lead to bioethanol production through biochemical routes. The bioethanol production process is, however, hindered by the recalcitrant structure of lignocellulose and a pretreatment is needed to increase biomass digestibility. Current pretreatment technologies generate inhibitory compounds that hamper the sugar conversion to ethanol by the fermenting microorganism.
High ethanol titers are necessary to make the process economically-viable. This could be reached by using high substrate loadings, which implies high inhibitor concentration in the broth. Laccases are powerful biocatalysts to overcome the effect of inhibitory compounds. Laccases are multicopper oxidases that catalyze the oxidation of substituted phenols, anilines and aromatic thiols to their corresponding radicals. This capacity allows laccases to act specifically on phenolic compounds present in pretreated materials.
In our studies, the potential of laccases as detoxification agents has been demonstrated by removing 70 to 100% of total phenols. Laccases trigger the fermentation of slurries non-fermentable without laccase treatment by increasing dramatically the ethanol yield (from 0.1 g/g to 0.36 g/g). The implementation of the laccase detoxification step boosts ethanol production at substrate loadings as high as 25% (w/w) reaching 58.6 g/L ethanol concentration for a cost-effective industrial ethanol production.
Despite the great phenolic reduction, sugar recovery is reduced after laccase addition. Our results suggest that laccase-derived products exert a negative effect on enzymatic hydrolysis. An increase in Klason lignin together with changes observed in the ATR–FTIR spectra supported a grafting process that would limit the accessibility of cellulolytic enzymes to cellulose.