SB/SL consists of crystalline cellulose nanoflbrils embedded in an amorphous matrix of cross-linked lignin and hemicelluloses that impairs enzyme and microbial acces­sibility [18]. Table 16.2 summarizes the detail cell wall composition of SB and SL.

In general, SB contains more holocellulose (hemicellulose + cellulose) (67.8 %) than SL (61.7 %). In contrast, lignin and ash content is less in SB (23.6 %, 1 %) than SL (36.1 %, 7.8 %) which limits the biochemical-based conversion applications of latter. Apart from carbohydrates and lignin, the cell wall ingredients such as silica, ash, and extractives along with natural moisture resides in both kind of biomass [8]. Factors such as high structural carbohydrates and less lignin in SB/SL, large availabil­ity, almost no food/feed value make it better feedstock for bio-based products than the contemporary agro residues (wheat straw, rice straw, corn stover, etc.) [2]. Holocellu­lose content in SB/SL (67.8 % and 61.7 %) is of high importance for their bioconver­sion into various products by microbial fermentation. This content (% dry weight) is fairly comparable with the other lignocellulosic materials such as wheat straw (56.1), corn stover (64.1), switch grass (61.8), and Spruce wood (71.9) [9, 10, 13].

In order to degrade the holocellulosic fraction of plant cell wall, the current pre­treatment methodologies are unattractive due to economic concerns. This is basically because of the special arrangement of cross-linked lignin with holocellulose network; biomass has evolved a superb mechanism to protect itself from microbial invasion. This mechanism of natural resistance in plant cell wall is called “biomass recalci­trance” [18]. Accessibility to the carbohydrate fraction of cell wall is a multi-scale phenomenon encompassing several orders of magnitude due to both macroscopic (compositional heterogeneity, mass transfer limitations) and microscopic barriers (holocellulose crystallinity, lignin-holocellulose linkage) [20].

Cell wall is a complex and highly arranged structure and thus resists the acces­sibility of cellulase enzymes. Pretreatment allows its breakdown and increase the amenability of enzymes for the sugars monomers recovery. Vascular bundles are consequently arranged in native SB/SL. Pretreatment disrupts the compactness of cell wall. Scanning electron and atomic force microscopic view of cell wall after dilute acid mediated pretreatment, clearly shows the disorganization of cell wall components which is pivotal for the improved cellulase action on carbohydrate poly­mer in order to yield simple sugars. From raw SB/SL (highly complex structure) to glucose or other sugars (monomers) production is mutli-sclae phenomenon spanning several orders of magnitude (10-9 meters) [18, 20].

16.2 Pretreatment of SB/SL

Pretreatment methods can be divided into four different categories: Physical, physico-chemical, chemical, and biological processes. Each method has its own specificity toward the plant cell wall fractions. Table 16.3 summarizes the effect of different pretreatment technologies applied to the sugarcane feedstock for the recovery of sugars.