Reduction of surface area of lignin improves enzymatic hydrolysis of cellulose from hydrothermally pretreated wheat straw

Abstract

Enzymatic hydrolysis of cellulose after pretreatment of wheat straw (WS) was investigated for the first time in relation to lignin surface area (SA). Lignin SA in solid residues from WS autohydrolysis (AH) and successive NH₃ (aq) extraction was determined using cationic dye adsorption. AH at increasing severity decreased up to 45% and 53% of WS lignin SA and specific surface area (SSA), respectively. Cellulose-to-glucose conversion from AH solid fractions from 24 h reaction with 15 FPU g⁻¹ cellulase activity increased linearly from 31% to 91% with decreasing lignin SA. When AH solid fractions were extracted with NH₃ (aq), both lignin SA and SSA increased in the corresponding solid residues, SSA up to 92%. As a consequence, cellulose-to-glucose conversion decreased in spite of the lower proportion of lignin in the solid residues after the NH₃ (aq) extraction. Up to 85% sugar yield was obtained from the single-stage AH process but when combined with NH₃ (aq) extraction the two-stage process yielded at most 71% of the original straw sugars. These results show that, independent of the lignin content, reduction of surface area of lignin improves the enzymatic hydrolysis process.