Enzymatic hydrolysis has an upper edge over acid hydrolysis to produce sugars for alcohol fermentations. Enzymes are naturally occurring plant proteins that cause certain chemical reactions to occur. There are two technological developments: enzymatic and direct microbial conversion methods. The chemical pretreatment of the cellulosic biomass is necessary before enzymatic hydrolysis. The first appli­cation of enzymatic hydrolysis was used in separate hydrolysis and fermentation steps. Enzymatic hydrolysis is accomplished by cellulolytic enzymes. Different kinds of “cellulases”, i. e., endoglucanases, exoglucanases, glucosidases, and cellobiohydrolases are commonly used [75, 107] to cleave cellulose and hemi — cellulose. The endoglucanases randomly attack cellulose chains to produce poly­saccharides of shorter length, whereas exoglucanases attach to the non-reducing ends of these shorter chains and remove cellobiose moieties, glucosidases hydrolyze cellobiose, and other oligosaccharides to glucose [142]. In order to enhance the susceptibility of cellulose for enzymatic hydrolysis, the pretreatment of cellulosic material is, therefore, an essential prerequisite. Physical and chemical pretreatments like ball milling, irradiation, alkali treatment, acid treatment, hydrogen peroxide treatment are highly recommended to enhance saccharification of cellulosic material after their enzymatic hydrolysis [6, 167].

So far, cellulose has been hydrolyzed with enzyme cellulase only at pilot plant scale. The process is divided into many steps and includes two basic inputs, namely, nutrients for the fungus and cellulosic material to be hydrolyzed. The nutrients supply include nitrogen and other supplements required for the growth of celluloytic microorganisms and is given in the form of sterilized nutrient medium. Cellulosic materials are pretreated. The celluloytic microorganism is grown and subsequently the enzyme is produced. The microorganism (such as fungus) is propagated as a submerged culture in a fermentation unit equipped for mixing and aerating the growth medium.

In the cellulose hydrolysis or saccharification step, the enzyme produced in the previous step comes into contact with the pretreated cellulosic materials. The enzyme solution hydrolyzes the solid cellulose to the glucose units. The product stream is continuously withdrawn from the unit. Finally, the glucose solution is separated from unhydrolyzed cellulose by filtration. The glucose solution can be used for fermentation to ethanol.

The rate and extent of enzymatic hydrolysis is affected by the pretreatment method, substrate concentration and accessibility, enzyme activity, and reaction conditions such as pH, temperature and mixing [121, 181]. Different strategies for enzymatic hydrolysis and ethanolic fermentation have been developed to address specific process engineering issues (Table 9.14).