The hydrolysate released from the pretreatment is typically treated with en­zymes in order to break down cellulose and hemicellulose into hexoses and pentoses that are then further fermented to ethanol. Enzyme costs are, how­ever, generally high, so that the search for new enzymes with high efficiency that can be produced at low costs is the key to overcome the bottleneck of this process step [18] (see also Viikari, this volume). Another possible way to reduce treatment costs is to implement recycle loops in order to feed back­washed enzymes into the vessel of enzymatic hydrolysis [19,20]. Enzymes could further be produced at the plant using a stream of the pretreated ma­terial in an on-site enzyme production.

The hydrolysis step is optimized by performing the enzyme treatment to­gether with yeast fermentation of glucose (simultaneous saccharification and fermentation, SSF). The temperature optimum of the enzymes is, however, of­ten higher than the optimum for yeast. This can depress the advantages of SSF compared to separation of the two processes.

4.3