Pretreatments vary from hot-water extraction and steam pretreatments (often with an oxidant or other chemical) to weak and strong acid and alkali pretreat­ments (Sun and Cheng, 2002). Physical pretreatments include mechanical communition, milling and ultra­sound methods (Agbor et al., 2011; Balat, 2011), as well as irradiation.

Chemical pretreatment methods include ammonia fi­ber explosion (AFEX), organosolv treatment and the addition of either acid or alkali (Isroi et al., 2011; Ong, 2004; Dashtban et al., 2009). The use of acid as a catalyst, normally H2SO4, targets the hemicellulose to dissolve with lignin and cellulose remaining as solids, whereas the addition of alkali, normally NaOH, mainly targets lignin, leaving mainly cellulose as a solid with hemicel — luloses (Dashtban et al., 2009; Ong, 2004). Physicochem­ical pretreatments combine a mild chemical treatment with high pressure and temperature and include methods ranging from uncatalyzed solvolysis (hydro­thermolysis) to steam explosion with chemical additives such as carbon dioxide or sulfur dioxide, AFEX and "popping" techniques (Mosier et al., 2005; Pan et al., 2005; Wi et al., 2011). Recent developments include pretreatments based on alkali soaking (NaOH) coupled with extrusion (Karunanithy and Muthukumarappan, 2011). Steam explosion consists of steaming the lignocel — lulose at high pressure followed by either a rapid or slow reduction in pressure to dissolve the hemicellu — loses into solution and allow the cellulose and lignin to remain as solids (Ong, 2004; Dashtban et al., 2009). SO2 or CO2 can be used as catalysts, although SO2 can be highly toxic to downstream fermentation microor­ganisms (Ong, 2004).

Although physical and chemical pretreatments can effectively reduce the recalcitrance of lignocellulosic compounds within a shorter time frame, they result in many environmental and cost concerns for industries. They require high-energy input alongside high — pressure reactors and can produce toxic compounds and wastewaters (Isroi et al., 2011).

Biological pretreatment methods include the use of microorganisms in order to delignify the lignocellulose material (Ravichandra et al. 2013; Dashtban et al.,

2009) . The enzymes produced by the microorganisms selectively disrupt the fibril and lignin structures of the plant cell wall and provide the advantages of lower energy demands, minimal waste production and reduced effects on the environment (Isroi et al., 2011; Dashtban et al., 2009). Microbial delignification is a gentle and effective approach to remove up to 31.59% lignin from biomass such as corn stover (Wan and Li,

2010) but results in a low rate of downstream hydrolysis. Pretreatment times required for direct microbiological methods are lengthy, being typically from 18 to 35 d. Nonetheless, enzymatic delignification is an alternative option and different "-omics" technologies are likely to yield new enzymatic delignification systems from

different white rot and brown rot fungi (Martinez et al.,

2009) .

The method chosen for pretreatment is dependent upon the lignocellulosic material and the hydrolysis to be carried out afterward. If the hydrolysis step is accom­panied by microbial enzymes, which are optimized at a lower pH (4—6), an acidic pretreatment is preferred as the first step in the bioconversion process (Dashtban et al., 2009).