The amount and types of enzymes required for the saccharification of cel­lulose and hemicellulose are strongly dependent on the biomass being hy­drolyzed and the type and severity of pretreatment. Ultimately the selection of biomass feedstock will be based on local availability and economy of sup­ply. In the early stages of commercial development, feedstocks with the great­est potential for demonstrating economic viability of an integrated process are likely to be developed first. These likely will include processing residues that are already available at processing plants such as corn fiber, soybean hulls, sugarcane bagasse, wood waste, and paper mill waste. The selection of both desizing and pretreatment processes may also be strongly influ­enced by local economics, especially with regard to co-location with existing wood, coal, or municipal solid waste-burning power plants, where inexpen­sive power and steam are available. With a diversity of potential substrates, different thermochemical pretreatments will be utilized to balance accessibil­ity to enzymatic attack with destruction of valuable sugars. Variations in the severity of the pretreatment (pretreatment severity is defined as the combined effect of temperature, acidity, and duration of treatment) must also be varied to maximize both sugar and fermentation compatibility. For example, an acid pretreatment, run at high temperature, high pressure and for long periods of time is considered more severe than a neutral pH water pretreatment run under the same temperature and pressure conditions. A low severity pretreat­ment will solubilize less of the hemicellulose fraction, increasing the amount of hemicellulase enzymes required, but may also reduce the production of by-products toxic to the fermentation, increasing the ethanol yield from the fermentation.

3.1