The most used enzyme to hydrolyze cellulose fibers thus enhancing its posterior fibrillation has been cellulase.Despite being mainly produced by fungi, some bacteria and actinomycetes have also been reported to present cellulase activity [151]. Generally, cellulases can be divided into three classes: (1) endoglucanases (endocellulases), which hydrolyze amorphous regions of cellulose; (2) cellobiohydrolases (exoglucanases), which progressivelycleave the ends of cellulose crystalline or amorphousregions, producing disaccharides (cellobiose) and tetrasaccharides,and (3) glucosidases (cellobiases), which hydrolyzethe di- and tetrasaccharides into glucose [152]. Atleast two cellobiohydrolases are found in natural cellulases:cellobiohydrolase I and cellobiohydrolase II, whichattack reducing and nonreducing ends of the cellulosechain, respectively [153]. In the enzymatic hydrolysis, cellulasescan be irreversibly bound to lignin, so its removalis strictly necessary for an efficient hydrolysis [154].