In SHF, enzymatic hydrolysis for conversion of pretreated cellulose to glucose is the first step. Produced glucose is then converted to ethanol in the second step. Enzymatic hydrolysis can be performed in the opti­mum conditions of the cellulase. The optimum temperature for hydrol­ysis by cellulase is usually between 45°C and 50°C, depending on the microorganism that produces the cellulase. The major disadvantage of SHF is that the released sugars severely inhibit cellulase activity. The activity of cellulose is reduced by 60% at a cellobiose concentration as

low as 6 g/L. Although glucose also decreases the cellulase activity, the inhibitory effect of glucose is lower than that of cellobiose. On the other hand, glucose is a strong inhibitor for ^-glucosidase. At a level of 3 g/L of glucose, the activity of ^-glucosidase reduces by 75% [27, 80]. Another possible problem in SHF is contamination. Hydrolysis is a lengthy process (one or possibly several days), and a dilute solution of sugar always has a risk of contamination, even at rather high temperatures such as 45-50oC.