Hemicellulose is a heterogenous branched structure unlike cellulose and is located in secondary cell walls that consists of pentoses ф-D-xylose, a-L-arabinose), hexoses ф-D-mannose, a-D-galactose, p-D-glucose), and some uronic acids (a-D-glucuronic, a-D-4-O-methyl-galacturonic, and a-D-galacturonic acids) (Kootstra et al. 2009). Low-molecular-weight, amorphous, and branched structure with short lateral chains makes hemicelluloses easier to decompose (Saha 2003). Removal of hemicellulose increases the digestibility of cellulose. Hemicellulose is thermochemically sensitive and further degrades into furfural and hydroxymethyl furfurals and formic acids which have been reported as fermentation inhibitors (Palmqvist and Hahn-Hagerdal 2000; Hendriks and Zeeman 2009). Therefore, severity parameters must be carefully optimized to avoid the formation of these hemicellulose degradation products which will decrease the sugar recovery.
Lignin is the third most abundant polymer found in nature. In plants, it is present in cell wall and convenes an impermeable resistance to microbial attack and oxidative stress. Lignin is an amorphous aromatic heteropolymer that consists of phenylpropane units (p-coumaryl, coniferyl, and sinapyl alcohol) held together by many different linkages (Hendriks and Zeeman 2009). Lignin is closely interlinked with cellulose and hemicellulose thus considered as major barrier in enzymatic hydrolysis. Several studies have shown removal of lignin increases the enzymatic digestibility (Chang and Holtzapple 2000; Ohgren et al. 2007; Varnai et al. 2010; Wang et al. 2013). Other detrimental effects caused by lignin include (1) nonspecific adsorption of enzymes to lignin, (2) interference with nonproductive binding of enzymes to lignin-carbohydrate complexes, and (3) toxicity of lignin-derived compounds to microorganisms (Agbor et al. 2011). Parts of the lignin are found to decompose during pretreatment and upon cooling coalesce together with altered properties (Brownell and Saddler 1987; Lynd et al. 2002). Delignification using chemicals such as alcohols and solvent causes swelling of the biomass, lignin disruption, and increased surface area leading to increased access of enzymes to cellulose fibers. The amount of lignin varies with the biomass composition, for example, woody biomass has more lignin compared to agricultural residues. Pretreatment can alter the lignin structure but might not lead to substantial delignification.
The structural modification via pretreatment is required for effective enzymatic hydrolysis. Understanding the structural limiting factors such as cellulose crystallinity index, specific surface area, degree of polymerization, and lignin and acetyl content is important for the selection of the best pretreatment for a specific biomass material.
Success of enzymatic hydrolysis and other downstream processes depends on the pretreatment efficiency. Pretreatment often starts with size reduction of the biomass followed by a thermochemical treatment to disrupt the recalcitrance of ligno- cellulosic biomass. This leads to increase in substrate porosity, lignin redistribution, and breaking of hydrogen bonds between polysaccharides and thus maximizes the exposure of cellulolytic enzymes to substrate to reach effective hydrolysis. Pretreatment can be divided into four major classes: physical, chemical, physicochemical, and biological. The type of pretreatment required depends on outcome and physical properties and chemical composition of biomass. Overall, desirable attributes of pretreatment are:
- • Chemicals cost during pretreatment and subsequent neutralization and conditioning prior to fermentation should be kept low.
- • Waste generation during pretreatment should be minimum and nonhazardous.
- • Size reduction should be minimized and as it is energy intensive and expensive.
- • Pretreatment should avoid degradation of hemicellulose sugars and should preserve them for fermentation.
- • Noncorrosive chemicals should be used to prevent pretreatment equipment.
- • Pretreatment must result into high product yields in subsequent enzymatic hydrolysis and fermentation processes with less conditioning costs and minimal loss of sugars during conditioning.
- • Pretreatment should decrease the load of enzymes needed to achieve more than 90 % conversion within a few days, preferably 3 days.
Fig. 2.2 Cellulase catalytic mechanism
- • Recovery of lignin and other constituents for conversion to valuable coproducts should be facilitated by pretreatment.
- • Overall, pretreatment should simplify the downstream processes by lowering the energy and cost input.