The pretreatment conditions affect the solubilization, recovery, and composition of the solubilized hemicelluloses. Depending on the raw material and pretreatment conditions, high molecular weight fractions, oligosaccharides, monomers, or sugar degradation prod­ucts are formed. Longer pretreatment times (at around 190°C) and additives, such as SO2, lead generally to better recovery of monomers. Less severe pretreatment conditions lead to solubilization of xylan and/or formation of oligosaccharides which can be hydrolyzed into monomers by enzymes (101). The total enzymatic hydrolysis of substituted oligomers needs the synergistic action of endoenzymes and accessory enzymes. The raw material, pretreat­ment conditions, and thus the structure of the solubilized hemicellulose oligomers should be known for the identification of enzymes needed.

The solubilized fraction from steam-pretreated birch wood contained about 10% acetyl groups, which were liberated with a culture filtrate from T. reesei (77). Synergy between xylanases, p-xylosidase, and acetyl esterase of T. reesei was shown to be essential for the production of xylose from steamed birch xylan. Hydrolysis of the high molecular weight fraction of steamed birch wood hemicellulose by xylanase alone produced only about 10% of the amount of xylose produced by the whole set of enzymes (see Table 10.2). The work of characterizing oligomers and solubilized polymers from other substrates and pretreatment techniques is ongoing, with the details being scarce and widely varied. In essence, the best enzymes for a specific hydrolysate must be worked out empirically on a case-by-case basis. Knowledge of the structures involved, either in the native feedstock or hydrolysate, will provide significant clues; however, the lack of details regarding enzyme substrate specificities will still necessitate extensive screening and synergy studies.

Acknowledgment

This work was supported by the US DOE Office of the Biomass Program.