Until recently, pretreatments have usually been designed for extensive removal of hemi — cellulose in order to improve the enzymatic hydrolysis of cellulose. Thus, the impact of the enzymatic hydrolysis of fiber-bound hemicellulose on cellulose hydrolysis has not been considered important, as evaluated by the number of publications. The amount of hemi — cellulose in the solid substrate resulting from pretreatment varies from about 1% to up to 25%, depending upon the pretreatment. Acidic and high temperature pretreatments tend to hydrolyze and remove more hemicellulose, while alkaline and low temperature processes generally leave higher residual hemicellulose in the solids and greater amounts of oligomers in the hydrolyzates. Ammonium fiber expansion/explosion (AFEX) is essentially a dry pro­cess and results in virtually no change in the solids composition, as there is no liquid phase to partition components.

The removal of xylans during the pretreatment has been shown to correlate with the hydrolyzability of the raw material (87). There was a clear correlation with the residual hemicellulose content of pretreated spruce and the degree of hydrolysis. It is however, difficult to conclude whether this correlation is affected also by other factors, such as further chemical modifications caused by the severity of the pretreatment. In another study, various raw materials with different levels of residual hemicellulose contents did not seem to follow this hypothesis. Neither residual xylan nor glucomannan seemed to correlate with the degree of cellulose hydrolysis when additional p-glucosidase was supplemented. On the other hand, xylanase activity in the preparation was shown to increase the hydrolysis conversion and rate (88).

Hemicellulolytic activity in commercial cellulase preparations has been expected to pro­vide the necessary hydrolysis of the residual hemicellulose in the solid matrix. The activities of accessory enzymes in the commercial preparations vary, and in most reports, are not even measured. Their role in the solubilization of matrix bound hemicelluloses can only be speculated. Even less is known about the effect of these unquantified activities on the soluble oligomer fractions resulting from pretreatment. Although extensive studies on the activities required for enzymatic hydrolysis of these compounds have not been carried out, interest and research into this area are rapidly expanding. From the known structures and known enzyme activities, one can begin to evaluate enzyme mixes for their efficacy on these complex substrates.