The biochemical production of different biofuels occurs through the hydrolysis of polysaccharides contained in lignocellulosic biomass into fermentable sugars and organic acids. Hydrolytic catalysis of biomass is achieved using either acid — catalysed hydrolysis or enzyme-catalysed hydrolysis. While acid hydrolysis is applicable to certain instances of biofuel production, enzymatic hydrolysis is a more economically and technologically advantageous option given the specificity of enzymes, the mild conditions required for the process and the high product yields gained.

The complex structure of lignocellulose calls for a pretreatment step in order to render the substrate more amenable to hydrolysis. Knowledge of the composition and structure of the raw materials is imperative to determine the most suitable pretreatment, to choose which enzymes to use as catalysts and to select the microorganisms most suited for optimal biofuel production. In fact, identifying the most suitable pretreatments and conditions for the fractionation of different lignocellulosic materials into its main structural components is one of the most important goals of research and development (http://www. eng. auburn. edu/cafi/ index. htm). Furthermore, the pretreatment step is currently considered the second largest contributor to the cost of second generation biofuels after feedstock acquisi­tion (Stephen et al. 2010). The effectiveness of the pretreatment will determine the yield in each successive step and therefore also the final product yield.

The heterogeneity of lignocellulose feedstocks has driven the investigation of numerous pretreatments but these can be broadly categorized into physical, chemical and biochemical pretreatments or can be a combination of these different classes of pretreatment (Alvira et al. 2010; Agbor et al. 2011). Among the different pretreatment options, organosolv, dilute acid prehydrolysis, acid-catalysed steam explosion and the novel SPORL technique (Sulphite Pretreatment to Overcome Recalcitrance of Lignocellulose) are the most promising technologies to facilitate the commercialization of biorefining of woody materials (Wang et al. 2009).

However, other pretreatments could be of interest for forestry residues that are more easily digestible. Conventional as well as some novel pretreatments and their mode of action are briefly described next with reference to specific studies on Pinus, Eucalyptus and Acacia.