Woody materials including bark, wood, and mixture of other residues from the forest contain cellulose, hemi — celluloses, lignin and small amount of other biomass

contents (Figure 1.4). Cellulose is the major component in plant biomass and it is made of anhydroglucopyra — nose or glucose residues, which can be converted to glucose and act as major source of hexoses in woody feedstock (Alvira et al., 2010). Due to the hydrogen bonds in it, cellulose is a highly crystalline structure and it is difficult to hydrolyze. Unlike cellulose, hemicel — luloses are heteropolymers composed of both five — carbon sugars and six-carbon sugars, including glucose, mannose, arabinose, xylose and others (Bochek, 2003). Due to its amorphous structure, hemicellulose is easily breakable by dilute acid or base. Lignin is the third ma­jor part in wood and comprises the glue that guards woody biomass from pathogens. Lignin mainly consists of phenolic units and with available technology we cannot use lignin as a source of bioethanol. Pretreatment
of these lignocelluloses separates the sugars and lignin and disrupts the crystalline portion of the biopolymers (Hu et al., 2008). Different pretreatment methods have been explored for achieving the optimistic situations with different biomass.

In general, pretreatment methods can be divided into biological pretreatment, physical pretreatment, and chemical pretreatment according to the pretreatment procedure. Some pretreatments combine two or more of broadly explored methodologies. Table 1.1 recaps some of the broadly explored pretreatment methods as per the classification (Sun and Cheng, 2002).

Biological Pretreatment

Most pretreatment technologies require selected and expensive amenities or equipment that have high power requirements, depending on the process. In particular, physical and chemical processes require rich energy for biomass conversion, whereas, biological treatment via microorganisms is a safe and environmentally friendly method and is increasingly being promoted as a process that does not require high energy, even for lignin removal from a lignocellulosic biomass (Okano et al., 2005; Potumarthi et al., 2013; Ravichandra et al., 2013).

Phanerochaete chrysosporium is one among the best microbial models to study the lignin degradation by white rot fungi. Fungi breaks down lignin anaerobi­cally through a family of extracellular enzymes collec­tively termed as lignases (Howard et al., 2003). Two families of lignolytic enzymes are generally consid­ered to play vital role in the enzymatic degradation: peroxidases (lignin peroxidase) and phenol oxidase

(Malherbe and Cloete, 2003). Other enzymes are not fully explored including glucose oxidase, methanol ox­idase, glyoxal oxidase, and oxidoreductase (Eriksson, 2000). Another best example was Trichoderma reesei, which is a mesophilic cellulolytic fungus isolated in the mid-1950s. By the mid-1970s, an impressive collec­tion of more than 14,000 cellulolytic fungi were isolated against cellulose and other insoluble fibers (Coyne et al., 2013). Trichoderma reesei, although a good producer of hemi and cellulolytic enzymes, is unable to degrade lignin (Mekala et al., 2008; Gupta et al., 2013).

Actinomycetes are also best tested for their task in lignin biodegradation. Lignolytic enzymes like peroxi­dases, ligninase and manganese peroxidase were discovered in P. chrysosporium (Saritha et al., 2012). Based on this, P. chrysosporium was taken up for biolog­ical delignification of wood and paddy straw in ethanol production. But, the extent of delignification was inade­quate to expose a significant portion of cellulose for enzymatic hydrolysis. Thus, from the reports available, it is evident that white rot fungi and actinomycete can be used jointly to remove lignin from lignocellulosic substrates, and further studies are required to shorten the incubation time and to optimize the delignification process.