The lignocellulosic biomass represents the most abundant source of fermentable sugar in nature, not only for fuel ethanol production, but also for producing a wide range of fermentation products, such as additives for the food industry, indus­trial chemicals, components of balanced animal feed, and pharmaceuticals. But to utilize this renewable resource in the production of most of these products, the lignocellulosic biomass must be pretreated, i. e., the lignocellulosic materi­als should be suitably processed in such a way that their constituent sugars and polysaccharides are susceptible to the action of hydrolytic enzymes as well as of fermenting microorganisms.

In the previous chapter, the complexity of the structure of lignocellulosic bio­mass was recognized considering that the lignin and hemicellulose form a sort of seal covering the polysaccharide with the highest potential to release glucose, the cellulose. In addition, it should be emphasized that most of cellulose in biomass has a crystalline structure derived from the longitudinal alignment of its linear chains. In the crystalline cellulose, the polysaccharide-polysaccharide interactions are favored and the polysaccharide-water interactions are reduced so this biopolymer is insoluble in water. A minor fraction of cellulose has an amorphous structure (Figure 4.2). The hemicellulose chains establish hydrogen bonds with the cellulose microfibers forming a matrix reinforced with lignin. The lignin presence makes it so the lignocellulosic complex cannot be directly hydrolyzed with enzymes. In this way, factors such as the crystallinity degree of cellulose, available sur­face area (porosity of the material), protection of cellulose by the lignin, pod-type cover offered by the hemicellulose to the cellulose, and heterogeneous character of the biomass particles contribute to the recalcitrance of the lignocellulosic materi­als to the hydrolysis. In addition, the relationship between the biomass structure

and its composition adds a factor implying even more variability exhibited by these materials regarding their digestibility (Mosier et al., 2005b). Therefore, the pretreatment step of the lignocellulosic complex has the following goals:

• Breakdown of the cellulose-hemicellulose matrix

• Reduction of the crystallinity degree of cellulose and increase of the fraction of amorphous cellulose

• Hydrolysis of hemicellulose

• Release and partial degradation of lignin

• Increase of the biomass porosity

In addition, the pretreatment should contribute to the formation of sugars (hexo — ses and pentoses) through the hemicellulose hydrolysis or to the ability to form glucose during the subsequent enzymatic hydrolysis of cellulose. The pretreatment should also avoid the formation of by-products inhibiting the subsequent biopro­cesses. As a complement, the pretreatment avoids the need of reducing the biomass particle size, a very energy-consuming process. The efficiency of this process is evidenced by the fact that, when the biomass is not pretreated, glucose yields dur­ing the following cellulose hydrolysis step are less than 20% of theoretical yields, whereas the yields after the pretreatment often exceed 90% of the theoretical yields (Lynd, 1996). In this way, the pretreatment is a crucial step during the overall process for fuel ethanol production from lignocellulosic materials. However, the pretreatment is one of the most expensive steps: the unit costs of pretreatment can reach 30 cents per gallon of produced ethanol (about US$0.08/L EtOH), according to Mosier et al. (2005b). Nevertheless, the improvement of pretreatment has a great potential to reduce its costs and increase the efficiency of the overall process.

Different methods have been developed for pretreatment of lignocellulosic bio­mass, which can have physical, chemical, physical-chemical, or biological nature (Sanchez and Cardona, 2008; Sun and Cheng, 2002). The evaluation of each one of these methods is related to whether it meets all the goals mentioned above, in addition to other features involving technoeconomic criteria, such as the cost of the agent or catalyst employed, the possibility of recycling the agents or catalysts involved, the degree of technological maturity of each method, the possibility of generating lignin as a co-product, and the ability of each method to be applied to the maximum possible amount of lignocellulosic materials.