**5. Enzymatic hydrolysis of polysaccharides**

After pretreatment to partially remove lignin and loose up polysaccharide structures, polysaccharides need to be hydrolyzed into sugar molecules which will be converted into ethanol by fermentation [38]. The hydrolysis can be accomplished chemically via acid-catalyzed cleavage of glycosidic bonds or by enzymes produced by microbes. Enzymatic method is more popular due to less impact on the environment and higher selectivity in the hydrolysis. Glucose and xylose are the main products in hydrolysates from the enzymatic breakdown of polysaccharides.

Enzymes produced by the filamentous fungi such as *Aspergillus nidulans*, *Aspergillus niger*, *Penicillium* spp. and *Trichoderma reesei* are dominant in commercial biorefinery [38]. Among different types of cellulases, endoglucanases attack the internal glycosidic bonds in the amorphous cellulose regions, causing fragmentation of the cellulose structure, and exoglucanase works of the termini of β-glucan molecules to release glucose molecules one at a time, while β-glucosidase attacks catalyzes the hydrolysis of the glycosidic bonds to terminal non-reducing residues in beta-d-glucosides and oligosaccharides to release one or two glucose units at a time [60]. The costs of cellulases are high, spurring the development of methods to recycle hydrolysis enzymes [61]. Inclusion of hemicellulose's, such as endoxylanases, xylosidases, exoxylanases and other accessory enzymes, such as esterase's and arabinosidase's, in the hydrolysis step improves the efficiency of enzymatic hydrolysis of lignocellulosic biomass and helps reduce enzyme loading and costs [62].

Various strains of yeasts and bacteria are being investigated with the goal of developing a consolidated process of hydrolysis and co-fermentation of glucose and xylose, without the need for adding exogenous cellulases [63].
