Delignification and Saccharification Enhancement of Sugarcane Byproducts by a Laccase-Based Pretreatment

Sugarcane bagasse and straw, two major agro-industrial byproducts generated by the sugarcane industry, contain significant amounts of carbohydrates that can be hydrolyzed and then converted into ethanol or other valuable compounds. However, access to them is limited by the presence of lignin, a recalcitrant polymer that protects cell-wall polysaccharides from enzymatic hydrolysis. This work demonstrates the ability of an enzymatic pretreatment, based on the lactase from Pycnoporus cinnabarinus, and 1-hydroxybenzotriazole as mediator, to remove and/or modify lignin in sugarcane bagasse and straw residues, improving their subsequent saccharification. Up to 27% and 31% decreases of relative lignin content in ground sugarcane bagasse and straw, respectively, were achieved by the lactase-mediator pretreatment followed by alkaline peroxide extraction. Moreover, the lignin removal directly correlated with improvements in enzymatic saccharification, increasing glucose releases by around 39% and 46% for bagasse and straw, respectively, compared with those of the corresponding controls. Lignin depolymerization and degradation were made evident in the 2D-NMR spectra by a significant reduction in the number of aliphatic side chains involved in the main beta-O-4' and beta-5' interunit linkages, together with a remarkable removal of p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S) lignin units as well as the associated p-coumarates and ferulates, with respect to polysaccharides.