**5.1 Saccharification of lignocellulose**

After lignocellulose being pretreated, the polysaccharide-enriched material is hydrolyzed to single sugars (hexoses and pentoses) with enzymes. The commercialized enzyme to hydrolyze cellulose and hemicellulose is in fact a mixture of some different kinds of enzymes, commonly called cellulase, extracted from microorganism. These enzymes cleave glycosidic linkages in carbohydrates, typically via inverting or retaining mechanisms, the latter of which proceeds via a two-step mechanism that includes formation of a glycosyl-enzyme intermediate [36].

In contrast with hemicellulose, which can be hydrolyzed more easily, cellulose is somewhat stable with crystalline structure, resisting depolymerization. To enzymatically hydrolyze cellulose effectively, the following three cellulose enzymes are popularly together employed [37]:


The cellulase enzyme activity can be measured by a standard method described in Technical Report NREL/TP-510-42628 (National Laboratory of the US Department of Energy Office of Energy Efficiency and Renewable Energy). Enzymatic hydrolysis gives an advantage to the growth of microorganism in fermentation although there is a little difference in optimal temperature for the two processes.
