The tested thermostable fungal enzymes, classified as cellobiohydrolases (CBHs) or endoglucanases (EGs) based on the activity determinations, were chosen by preliminary screening and characterisation. Several thermostable CBHs from various thermophilic organisms were purified and characterised (Voutilainen et al, manuscript in preparation). The gene of the most poten­tial CBH isolated from Thermoascus aurantiacus was fused with the T. reesei CBHI cellulose binding domain (CBM). In addition, an EG from Acremo — nium thermophilum, a в-glucosidase and a xylanase from T. aurantiacus were used to compose the thermostable mixtures. Fermenter supernatants pro­duced in pilot scale were used to obtain the thermostable cellulase mixtures. The optimal ratio of EG to CBH amount (measured as protein of the enzyme mixtures) was determined on the basis of FPU activity of the preparations. The highest FPU activity was obtained by an EG to CBH protein ratio of 3 : 10, which corresponded well to the respective ratio of the native T. ree — sei enzymes. This ratio also gave the highest sugar yields in the hydrolysis of the steam pretreated corn stover substrate (results not shown) and was used as the standard basis for various mixtures. Three different mixtures were used in this work, differing with respect to the xylanase activity (Table 3). The xylanase-free preparation (TM 1) was first used for the spruce substrate

and the xylanase-containing preparations (TM 2 and TM 3) for the corn stover substrate. As it has frequently been observed that xylanases enhance the hydrolysis of lignocellulosic substrates containing even low amounts of re­sidual xylan [9], preparations with xylanase activity were later used for both substrates.

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