Different pretreatment approaches catalyze biomass hydrolysis and other reactions to var­ious extents. Therefore, the composition of the liquid and solid process streams resulting from different pretreatment approaches can be widely different. For example, dilute sul­furic acid pretreatment processes can solubilize nearly all the hemicellulose but very little lignin or cellulose. Other pretreatment approaches, primarily the alkaline processes, are more effective at solubilizing lignin, but leave extensive amounts of the hemicellulose frac­tion as an insoluble component of the pretreated solids. These factors greatly impact the relative composition of the pretreated solids and the requirements for effective enzymatic saccharification in subsequent processing steps.

While there are considerable economic drivers to reduce the overall severity of the pre­treatment operation (lower reactor materials cost, lower temperature and/or residence time, lower losses of resulting sugars to degradation, lower requirements to adequately “condition” pretreatment hydrolyzates for subsequent fermentation processes), less aggressive pretreat­ment conditions will generally result in less sugar release (primarily from hemicellulose) in the pretreatment step. This will shift more of the hydrolytic sugar production requirement from the pretreatment step to the enzymatic hydrolysis step and will have an impact on the amount and type of enzymes required to achieve high sugar yields from both cellulose and hemicellulose in less severely pretreated biomass.

Recently, greater attention is being given to the understanding and development of hemi — cellulase and other “accessory” enzyme systems needed to effectively debranch and depoly — merize residual insoluble hemicellulose and soluble hemicellulose oligomers resulting from less severely pretreated biomass processes (2). Systematic studies are needed to determine whether augmentation or partial substitution of cellulase activity with various hemicellulase and accessory enzyme activities can improve the cost-effectiveness of biomass conversion processes by increasing sugar yields.