Inherent properties of SB/SL cell wall make them resistant to cellulase mediated act for releasing monomeric sugars from hemicelluloses or cellulose. Pretreatment of SB/SL increase the accessible surface area and crystallinity of holocellulose which ameliorates the enzymatic action, in turn yielding optimum sugars recovery [14]. Table 16.4 summarizes the enzymatic hydrolysis profile of SB/SL after various kinds of pretreatment methods employed.

The hydrolysis rate of SB/SL directly depends upon the efficiency of pretreatment method used. Removal of lignin from the substrate determines the enzyme accessi­bility to the carbohydrate fraction of cell wall. A direct correlation exists between the removal of lignin and hemicelluloses on cellulose saccharification [11, 17]. Dilute H2SO4 pretreated SB followed by NAOH pretreatment (1 % NaOH, 60 min, 120 °C) produced 35g/L sugars (86.2 % hydrolytic efficiency) after 96 h of enzymatic hy­drolysis (10 FPU/g; 15 beta-glucosidase IU/g) [70]. Scanning electron microscopic (SEM) analysis also reveals the effect of dilute-acid pretreatment on native SB, alka­line pretreatment on cellulignin followed by enzymatic hydrolysis of cellulose. The un-homogeneity in structure and the disruption of first hemicelluloses followed by lignin and cellulose is clearly evident in SEM analysis (Fig. 16.3).

In the other study, oxalic acid pretreated SB (160 °C, 16 min, 3.5 % w/v Oxalic acid) produced total reducing sugars 56.3 g/g bagasse (92.30 % hydrolytic efficiency) after 120h of enzymatic hydrolysis (20 FPU/g; 25 beta glycosidase (IU/g)) [71]. Among alkaline pretreatments, hydrated ammonia based pretreatment methods have found great interest recently [8]. Hydrated ammonia precisely act on lignin removal from the SB leaving hemicelluloses and cellulose together but in fragile form which yields appreciable sugar recovery upon enzymatic hydrolysis [8]. In association, our laboratory reveals the maximum sugars recovery (28 g/L) after 96 h of enzymatic hydrolysis of ammonia pretreated SB (20 % ammonia, 24 h, 70 °C). The optimum enzyme loadings (15 FPU/g and 17.5 beta glucosidase IU/g) were used (Chandel et al. Unpublished work).

Other important factors such as substrate concentration, cellulase loading, and end-product inhibition also plays an important role for the hydrolytic efficiency of SB/SL [9, 50]. In order to enhance the surface area of holocellulosic fraction in the cell wall, surfactants like Tween-20 have been used [44, 23]. To overcome the problems of end-product inhibition and process complexities, integrated process configurations such as SSF and consolidated bioprocessing (CBP) have been found successful [14, 50].