The pretreatment methods cause physical and/or chemical changes in the lignocellulosic biomass; thus, pretreatment technologies are usually classified into physical, chemical, physicochemical, and biological. For the purposes of classification, steam and water are excluded from being considered chemical agents for pretreatment since extraneous chemicals are not added to the biomass (Mosier et al., 2005). This chapter focuses on chemical, physical and physicochemical pretreatments but a brief description of biological treatments are include in order to contrast and compare them.

Biological Pretreatments

Biological pretreatments employ microorganisms mainly brown, white and soft-rot fungi, which degrade lignin, hemicellulose and cellulose in small proportion (Alvira et al., 2010). Recently, this approach has received renewed attention as biological pretreatments have several advantages over conventional physical/chemi — cal pretreatment methods, such as they are considered as environmentally friendly, low capital cost, low energy, no chemicals requirement, and mild environ­mental conditions (Saritha and Lata, 2011). However, the main drawbacks to develop biological methods are the low hydrolysis rate obtained in most biological materials and the relatively long time of the pretreat­ment compared to physical/chemical methods. Conse­quently, more space and longer processes are required, which increase the operating costs (Alvira et al., 2010; Saritha and Lata, 2011).

The white-rot fungi are able to decompose all wood fractions, including lignin because they produce various enzymes involved in lignin degradation such as lignin peroxidase, laccase, manganese peroxidase, versatile peroxidase, and H2O2-forming enzymes such as glyoxal oxidase and aryl alcohol oxidase. White-rot fungi also produce cellulases, xylanases and other hemicellulases that are required in the hydrolysis. Almost all

white-rot fungi produce manganese peroxidase and lac — case, but only some of them produce lignin peroxidase (Isroi et al., 2011).

Several white-rot fungi such as Phanerochaete chryso- sporium, Ceriporia lacerata, Cyathus stercolerus, Ceriporiop — sis subvermispora, Pycnoporus cinnarbarinus, Pleurotus ostreaus, Dichomitus squalens, Coriolus versicolor, Tricho — derma reesei, Aspergillus terreus, Aspergillus awamori, Bjer — kandera adusta, Phlebia tremellosus, Fusarium proliferatum, and Pleurotus florida have been examined on different lignocellulosic biomass (Alvira et al., 2010; Cui et al., 2012; Isroi et al., 2011; Kuhar et al., 2008; Pinto et al., 2012; Saritha and Lata, 2011; Wan and Li, 2011). Recently, some bacterial laccases have also been characterized from Azospirillum lipoferum and Bacillus subtilis (Saritha and Lata, 2011). However, they face three major chal­lenges associated with lignin structure: (1) the lignin polymer is large; therefore, ligninolytic systems must be extracellular, (2) lignin structure comprises interunit carbon—carbon and ether bonds; therefore, the degrada­tion mechanism must be oxidative rather than hydrolyt­ic, and (3) lignin polymer is stereoirregular, therefore the ligninolytic agents must be much less specific than degradative enzymes (Isroi et al., 2011).

As mentioned before, one of the most main draw­backs of biological pretreatments is the time of the pre­treatment; reported time of treatment is between 7 and 60 days (Giles et al., 2011; Mahalaxmi et al., 2010; Wan and Li, 2011). After 18 days of pretreatment, C. subver­mispora effectively delignified corn stover, switchgrass, and hardwood with glucose yields during enzymatic hydrolysis that reached 56.50%, 37.15%, and 24.21%, respectively (Wan and Li, 2011). Also, glucose yield after 21 days of pretreatment with Poria subvermispora and Irpex lacteus reached 69% and 66% of cellulose available in the wheat straw, respectively, with an ethanol yield of 62% in both cases (Salvachua et al., 2011).

Biological pretreatment has also been used before py­rolysis of biomass to produce fuel. The biological pre­treatment of corn stover can optimize the thermal decomposition, decrease the reaction temperature and reduce the gas contamination (SOx), making the biomass pyrolysis more efficient and environmentally friendly. Biological pretreatment can decrease the activa­tion energy and reacting temperature of the hemicellu — lose and cellulose pyrolysis (up to 36 °C), shorten the temperature range of the active pyrolysis (up to 14 °C), and increase the thermal decomposition rate (Isroi et al., 2011; Yang et al., 2011).

A cost-competitive biological pretreatment of lignocel — lulose requires continuous studying and testing more mi­croorganisms for their ability to delignify the plant material quickly and efficiently (Saritha and Lata, 2011). Also, integrated methods, such as, cotreatment with organic solvents, diluted acids, supercritical CO2 and ionic liquids (ILs); mutation breeding and crossbreeding of fungal mycelia to obtain engineering strains; and inte­gration of fungal pretreatment with simultaneous saccharification and fermentation to produce biofuels and value-added products should be studied (Tian et al., 2012).