Influence of enzyme loading and physical parameters on the enzymatic hydrolysis of steam-pretreated softwood

Softwood is an interesting raw material for the production of fuel ethanol as a result of its high content of hexoses, and it has attracted attention especially in the Northern hemisphere. However, the enzymatic hydrolysis of softwood is not sufficiently efficient for the complete conversion of cell ulose to glucose. Since an improvement in the glucose yield is of great imp ortance for the overall economy of the process, the influence of various pa rameters on the cellulose conversion of steam-pretreated spruce has been in vestigated. The addition of beta -glucosidase up to 50 IU g(-1) cellulose t o the enzymatic hydrolysis process resulted in increased cellulose conversi on at a cellulase loading up to 48 FPU g(-1) cellulose. Despite very high e nzyme loading (120 FPU g(-1) cellulose) only about 50% of the cellulose in steam-pretreated spruce was converted to glucose when all of the material f ollowing pretreatment was used in the hydrolysis step. The influence of tem perature, residence time, and pH were investigated for washed pretreated sp ruce at a dry matter (DM) content of 5% and a cellulase activity of 18.5 FP U g(-1) cellulose. The optimal temperature was found to be dependent on bot h residence time and pH, and the maximum degree of cellulose conversion, 69 .2%, was obtained at 38 degreesC and pH 4.9 for a residence time of 144 h. However, when the substrate concentration was changed from 5% to 2% DM, the cellulose conversion increased to 79.7%. An increase from 5% to 10% DM res ulted, however, in a similar degree of cellulose conversion, despite a sign ificant increase in the glucose concentration from 23 g L-1 to 45 g L-1. Th e deactivation of beta -glucosidase increased with increasing residence tim e and was more pronounced with vigorous agitation.