2-46: Will steam pretreatment of wood residues always require a post treatment step to ensure maximum sugar recovery?

Wood represents an abundant source of sugars with the cellulose and hemicellulose components being among the largest sources of carbohydrate in the world. However, a pretreatment step is always required to both recover the hemicellulose in a useable form and to enhance the effectiveness of the enzymatic hydrolysis of the cellulose. Steam pretreatment has been shown to be effective on a range of lignocellulosic feedstock’s including woody and non-woody residues. There are several mechanistic reasons behind the difference in the recalcitrance of steam pretreated woody substrates compared to the non-wood feedstocks. For example, the residual lignin remaining after the steam treatment of wood restricts the enzymatic hydrolysis both by limiting the cellulose accessibility and binding irreversibly to the cellulase enzymes. Thus, for these and other reasons some form of post treatment will likely always be a prerequisite when wood is the substrate if significant hydrolysis yields at low enzyme loading are required. The types of post treatment that we have assessed have their origins in the forest products/pulp and paper sector.   A typical pulping regime is a multi-step process consisting of various wood handling, pulping and bleaching stages. Thus, it is highly likely that the pretreatment and fractionation of wood-to-ethanol will also require a multi-step process if we are to use low enzyme loadings. The pre-and-post treatment strategies that have been evaluated and used to enhance the overall sugar yield with minimum use of enzymes will be discussed with reference to the experience gained by the pulp and paper sector.