Perhaps the most natural pretreatment of biomass is a purely biological method. Nature commonly employs lignin-degrading microorganisms such as white, brown or soft-rot fungi (Lee, 1997; McMillan et al., 1999). A study that investigated the effect of high-yield concen­trated recombinant MnP (rMnP), produced from the yeast Pichia pastoris on the biobleaching of kraft pulps found that rMnP applied at 30 U/g pulp for 24 h fol­lowed by alkali extraction removed a significant quan­tity of lignin from both hardwood and softwood unbleached kraft pulps (Xu et al., 2010). The rMnP — treated pulp was more susceptible to subsequent peroxide bleaching compared to the control pulp. More than 60% of the kappa number was reduced by sequential rMnP treatments and alkaline extractions. When using white-rot fungi, such as Ceriporiopsis subyer — mispora, to treat sugar maple chips, the amount of extracted hemicellulose can be increased (Barber,

2007) . The biotreatment alters the physical and chemical structures of the LB and removes a portion of the non­carbohydrate mass.

Because biological pretreatment is safe, environmen­tally friendly and energy saving it is gaining more attention (Okano et al., 2005). The downside is that bio­logical pretreatment is too slow for some industrial ap­plications and some material is lost to the microorganism as it is a consumer of hemicellulose, cel­lulose and lignin (Bohlmann, 2006). The microorganisms are also susceptible to poisoning by lignin derivatives (Hamelinck et al., 2003). Biological pretreatment by itself may not be the best solution but it could provide value when employed in conjunction with other pretreatment options.

Acid Hydrolysis

Water is a weak acid by itself; however, adding a salt to water will enhance the activity of the acid. Aqueous acids, especially those with a salt, autoseparate into hydrogen cations and hydroxyl anions, where one side of the cleaved sugar polymer receives the hydrogen cation and the other receives the hydroxyl group. One can apply acid hydrolysis either as a pretreatment or as a main hydrolysis step. A variety of acids act well at ambient temperatures to pretreat LB and prepare the material for anaerobic digestion. LB is eventually hydro­lyzed into monosaccharides, furfural, HMF and other volatile products. The lignin, however, condenses and precipitates out as a result of the pretreatment (Esteghla — lian et al., 1997; Liu and Wyman, 2003; Shevchenko et al.,

1999) .

Concentrated acids are quite powerful, act at mild temperatures and result in rapid reactions. However, H2SO4, H3PO4 and HCl are highly toxic, corrosive, and hazardous. Reactors for acid hydrolysis need to resist corrosion. Furthermore, recovering the concentrated acid from the hydrolysis effluent is important to reduce the negative environmental consequences and to reduce costs.

Hydrolysis using a DA is an effective pretreatment for LB (Hinman et al., 1992). It produces high sugar yields from some hardwoods, like poplar and aspen. In one study, poplar wood was pretreated with a 2% sulfuric acid at 190 °C for 1.1 min and this was followed by an enzymatic hydrolysis (Wyman et al., 2009). In this particular study, the xylose yield was 18.5% and the glucose yield was 64.3% where the raw material contained 25.8% xylose and 74.2% glucose (Wyman et al., 2009). In another study of aspen wood, the wood was pretreated with a 1.1% sulfuric acid at 170 °C for 30 min and followed by enzymatic hydrolysis (Tian et al., 2011). The xylose and mannose yield was 13 wt% (18 wt% theoretical contents) and the glucose yield was 85% following treatment (Tian et al., 2011). See Table 27.5 for a comparison of concentrated and DA pretreatments.

There are essentially two classes of DA pretreatment processes: high-temperature continuous-flow and low

temperature batch processes. High-temperature systems operate at temperatures over 160 °C and are appropriate for solutions with a low concentration of solids, between 5% and 10%. Low-temperature systems operate under 160 °C and are appropriate for solutions with a high concentration of solids, between 10% and 40%.

Even though a simple acid pretreatment significantly improves the rate of a hydrolysis process, it costs higher than other physicochemical pretreatment processes. One such process is steam explosion and was discussed previously in this chapter. Another consideration for an acid hydrolysis pretreatment is that one must neutralize the hydrolysate prior to subsequent enzy­matic hydrolysis or fermentation (Sun and Cheng, 2002).