Utilizing liquid water by itself, as the only pretreat­ment reagent, is an option of interest as it is environmen­tally friendly and inexpensive compared to other
pretreatment methods (Amidon et al., 2008; Liu, 2010; Mosier et al., 2005). High pressure is applied to keep the water in a liquid state while it is at elevated temper­atures (Hendriks and Zeeman, 2009). This enables the water to penetrate the cell structure of the biomass and thus hydrate the cellulose and remove the hemicellu — loses. Another feature of water is that it has a high dielectric constant. This facilitates ionic substances to disassociate and allows for the dissolution of hemicellu — loses and a portion of the lignin.

When the water temperature exceeds 150 °C, the hemicellulose begins to solubilize. The degree to which this occurs is determined by thermal, acid and alkali stability of the hemicellulose, which is dependent on the composition of the hemicellulose backbone and the branching groups. Temperature of the water can selec­tively solubilize hemicelluloses. A 75% maximum xylan solubilization in the hot water extract of sugar maple was obtained at 175 °C after 2 h, whereas only 30% of the initial xylan was removed from a 2 h treatment at 152 °C (Mittal et al., 2009). When the water temperature exceeds 180 °C an exothermal reaction begins. It is most likely related to the solubilization of the hemicelluloses (Brasch and Free, 1965).

Another result of the thermal process is that the pH of the extract decreases to 3—4 (Gregg and Saddler, 1996a). Portions of the hemicelluloses are hydrolyzed, which form acids such as acetic acid. These are released from acetylated polysaccharides in the wood. These acids lower the pH and catalyze the additional hydrolysis of hemicellulose (Liu and Wyman, 2003; Liu, 2008; Tunc and van Heiningen, 2008; Zhu et al., 2005).

Depending on the intensity of the hot water extraction, sugars may dehydrate. When hexose sugar dehydrates HMF, also known as HFM or 5-hydroxymethyl-

2- furaldehyde, is formed. When pentose sugar dehy­drates, furfural is formed. In addition to solubilizing hemicellulose, hot water treatment can lead to solubiliza­tion of portions of lignin (Ramos, 2003). Regardless, the produced compounds are usually phenolic heterocyclic compounds such as vanillin, vanillin alcohol, furfural and HMF. This is especially true in strong acidic
conditions. Additionally, these compounds tend to inhibit or toxify bacteria, yeast, methanogens and archae. This is a significant disadvantage in using hot water to extract cellulose and hemicellulose (Brownell et al., 1986).

Hot water extracts can be converted to desired products as well, i. e. via separation and fermentation (Liu et al., 2009; Shupe and Liu, 2009). Fermentation is also strongly inhibited when a hydrolysate is produced from a treatment containing 3% or more of solids or the treatment temperature exceeded 220 °C for 2 min. These conditions likely yield furfural or soluble lignin com­pounds. At temperatures in excess of 250 °C pyrolysis begins to take place (Laser et al., 2002). Therefore, one should avoid these high temperatures. Another undesir­able effect of thermal pretreatment is that it may increase the crystallinity index (CrI) of cellulose (Weimer et al., 1995). It is important to remove the soluble lignin com­pounds quickly. Since lignin is highly reactive, the disengaged lignin will recondense and precipitate onto the biomass (Liu and Wyman, 2003). This seems to be more prevalent in cases where severe pretreatment con­ditions are used. In these cases, more condensation and precipitation of lignin compounds takes place and sometimes, soluble hemicellulosic compounds such as furfural and HMF are also produced (Mittal et al., 2009) and polymerized (condensed) and deposited onto the extracted biomass.

Despite the undesirable effects above, when compared to other pretreatment methods, liquid hot water wood extraction still has a major advantage. Since a large vol­ume of water is used the solubilized hemicelluloses and lignin compounds appear in lower concentrations. As a result, the risk of undesirable degradation products is reduced. The substances in the extract can be separated and converted to desired products.

Figure 27.6 illustrates the three methods of liquid hot water reactors. They are differentiated by their configu­rations. One is cocurrent, another is countercurrent and the third is a flow-through reactor.

Briefly, in cocurrent pretreatment, the biomass and water are heated and held at the desired conditions for a specific residence time prior to allowing it to cool. In the countercurrent design, water and lignocel — lulosic material flow in opposite directions through the reactor. The flow-through reactor is designed such that hot water is passed over a stationary bed of LB and carries the hydrolysate and dissolved lignocellulosic components out of the reactor (Hendriks and Zeeman, 2009).