The following step is the pretreatment of the fractionated material. The main goal of pretreatment is to overcome this lignocellulosic recalcitrance, to separate the cellulose from the matrix polymers, and to make it more accessible for enzymatic hydrolysis. Reports have shown that pretreatment can improve sugar yields to greater than 90% theoretical yield for biomass such as wood, grasses, and corn [8,9]. Pretreatment technologies for lignocellulosic biomass include thermal, (thermo)chemical, physical and biological methods or various combinations thereof [5, 9].

In general, pretreatment processes produce a solid pretreated biomass residue that is more amenable to enzymatic hydrolysis by cellulases and related enzymes than native biomass. Many pretreatment approaches, such as dilute acid and steam/pressurized hot water based methods, seek to achieve this by hydrolyzing a significant amount of the hemicellulose fraction of biomass and recovering the resulting soluble monomeric and/or oligomeric sugars. Other pretreatment processes, such as alkaline-based methods, are generally more effective at solubilizing a greater fraction of lignin while leaving behind much of the hemicellulose in an insoluble, polymeric form [10]. Most pretreatment approaches do not hydrolyze significant amounts of the cellulose fraction of biomass but enable more efficient enzymatic hydrolysis of the cellulose by removal of the surrounding hemicellulose and/or lignin along with modification of the cellulose microfibril structure [11]. Biological pretreatment uses microorganisms and their enzymes selectively for delignification of lignocellulosic residues and has the advantages of a low energy demand, minimal waste production and a lack of environmental effects [7, 12, 13]. It has been suggested that there will probably not be a general pretreatment procedure and that different raw materials will require different pretreatments [10]. Table 2.1 gives an overview of the different pretreatment technologies.

The choice of the optimum pretreatment process depends very much on the objective of the biomass pretreatment, its economic assessment and environmental impact. Tech­nological factors, such as energy balance, solvent recycling and corrosion, as well as environmental factors, such as wastewater treatment, should all be considered carefully when selecting a method [5]. Diverse advantages have been reported for most of the pre­treatment methods, which make them interesting for industrial applications. Only a small number of pretreatment methods has been reported as being potentially cost effective thus far. These include steam explosion, liquid hot water, dilute acid pretreatments, lime, and ammonia pretreatments [11, 16, 18,19]. The complete depolymerization of these renew­able feedstock in a cost-effective manner with minimal formation of degradation products represents a significant challenge for microbiologists and chemical engineers. Obstacles in the existing pretreatment processes include insufficient separation of cellulose and lignin (which reduces the effectiveness of subsequent enzymatic cellulose hydrolysis), formation of by-products that inhibit microbial growth and fermentation (e. g. acetic acid from hemi — cellulose, furans from sugars and phenolic compounds from the lignin fraction [20]), high use of chemicals and/or energy, and considerable waste production. Research is focused on converting biomass into its constituents in a market competitive and environmentally sustainable way [21].