Extent of Enzyme Inhibition by Phenolics Derived from Pretreated Biomass Is Significantly Influenced by the Size and Carbonyl Group Content of the Phenolics

In the current biorefinery concept, hydrothermal-based pretreatments such as steam pretreatment are often used to first open up the plant cell wall structure and increase the accessibility of the cellulose component to cellulase enzymes. However, this pretreatment process also generates water-soluble phenolics that are strongly inhibitory to the cellulase enzymes, therefore limiting the efficiency of the enzymatic hydrolysis of the pretreated biomass. Although some earlier work assessed the influence of phenolics on cellulose hydrolysis, the characteristics of the phenolics that most influenced inhibition have not yet been elucidated. As biomass-derived phenolics are heterogeneous monomer/oligomeric/polymeric aromatic compounds that have diverse functional groups and varied molecular size, the current work investigated the influence of the major structural properties of the pretreatment-derived phenolics, such as their molecular weight and the nature of their hydroxyl and carbonyl groups, on enzyme inhibition. It was apparent that phenolics derived from steam-pretreated softwood were more inhibitory than the phenolics derived from steam-pretreated hardwoods with phenolics with a smaller molecular size and greater carbonyl content being more inhibitory to enzyme-catalyzed cellulose hydrolysis.