17-48: Limitation of the enzymatic hydrolysis of cellulose by the presence of lignin

Lignocellulosic cell wall is recalcitrant to enzymatic hydrolysis, mainly because of the lignin. The objective of this work was to evaluate the limiting effect of lignin from different sources in the enzymatic hydrolysis of cellulose. Two samples of lignin from kraft process (softwood and hardwood) and a sugarcane lignin (grass) were used. As polysaccharide source, eucalyptus pulp from paper industry containing cellulose (76.2%) and hemicellulose (14.8%) was used. The hydrolysis experiments were performed with 2 and 10 FPU / g of substrate, and supplementing with β-glucosidase (4 and 20 UI / g of substrate). The effect of lignin in the enzymatic hydrolysis was assayed by adding simultaneously the enzyme extracts and the lignin samples. With softwood and hardwood lignin samples, we observed that with lower enzyme loading (2 FPU and 4 UI / g of substrate), the cellulose conversion was 30% lower than the control (without lignin). No effect was observed with sugarcane bagasse lignin. However, none of the lignin samples had negative effect in the cellulose hydrolysis with higher enzyme loading, showing that it was sufficient to overcome the limiting effect of the tested lignin samples. To evaluate the effect of soluble compounds from lignin, assays were conducted with acetate buffer previously incubated with lignin samples. Soluble fractions of lignin samples inhibited the enzymes, despite of the enzyme loading. These results show that the aromatic soluble compounds have high inhibitory effect on cellulases and the comprehension of the inhibition mechanisms can be the key to enhance the polysaccharide hydrolysis.