Targeted hemicellulose removal and lignin modification greatly enhance cellulose accessibility/hydrolysis and overall sugar recovery from steam treated wood using low enzyme loadings

Due to its relative simplicity and effectiveness, variations of acid-catalyzed steam pretreatment is the predominant pretreatment method used in commercial cellulosic ethanol plants. Acid-catalysed steam pretreatment has been shown to improve cellulose accessibility, by targeted solubilisation and recovery of the hemicellulose component. However, this results in lignin enrichment of the substrate that restricts cellulose accessibility. The detrimental influence of lignin is further exacerbated by condensation which increases the hydrophobicity of the lignin and the likelihood of non-productive cellulase-lignin interactions. Lignin condensation also limits the downstream extractability of the lignin component and thus, its potential utility. Rather than just solubilising the hemicellulose component, modified steam pretreatment options provide an opportunity to address “the lignin challenge”. The presented work will describe several examples of lignin modification during steam pretreatment to improve cellulose accessibility and lignin extractability. In particular, the inclusion of lignosulfonates (3-6%) during the pretreatment of poplar was found to improve hemicellulose recovery by 10% and enzymatic hydrolysis of the cellulose by 17%. The addition of lignosulfonates or sodium sulfite resulted in changes to the lignin as the acid group content increased significantly. Combining sulfite with sodium carbonate resulted in the recovery of 85% of the original carbohydrate in the water insoluble fraction. This cellulose and hemicellulose rich component could be completely hydrolyzed within 48 h using a protein loading of 10 mg/g substrate. It was apparent that lignin modification also resulted in improved substrate swelling, as determined by the Simons’ staining technique.