Thermal chemical pretreatments are currently necessary to enable cellulase access through the hemicellulose sheath of the plant cell wall microfibrils, thus exposing the crystalline cellulose core. This pretreatment must be just severe enough to create this access, but not so severe as to divert sugars to non-fermentable or toxic compounds (6-8). Today, the de­polymerization of arabinoxylans (hemicellulose in hard woods and grasses) in cell walls is accomplished with good results by a variety of hot acid, hot water, and alkaline treatments. Final conversion of liberated soluble oligosaccharides is often accomplished using hemi — cellulases. Soft woods contain hemicellulose composed primarily of galactoglucomannan, which liberates galactose, glucose, and mannose upon depolymerization. These sugars are all fermentable by natural yeasts.

We recognize that the capital cost of the pretreatment unit operations is a critical factor for enabling the future biorefinery. High pretreatment capital cost is primarily due to the materials of construction required by conditions of high severity. In this context, severity is based on the pretreatment acidity, temperature, and time at temperature. New combinations of biological preconditioning (before thermal chemical pretreatment) and better thermal chemical pretreatments prior to enzymatic conversion have promise for overcoming this barrier.

The reactions of plant cell wall chemical constituents and ultrastructure to pretreatments must also be understood at a more detailed level. For example, basic research is required to understand the relationships between feedstock plant structure and composition. Simply, we need to develop better chemical and enzymatic treatments. Solving the yield challenge requires the integration of the complexities of plant structure, chemical pretreatment, and enzyme action. This integrated approach is a new and critical research paradigm.