One of the most promising emerging biorefinery plat­forms is the biochemical path that focuses on fermenta­tion of sugars extracted from lignocellulosic feedstocks (Carvalheiro et al., 2008). This technology involves three basic steps: (1) conversion of biomass to sugar or other fermentation-feedstock, (2) bioconversion of these biomass intermediates using biocatalysts, and (3) pro­cess products to yield added value chemicals, fuel — grade ethanol and other fuels, heat and/or electricity (Carvalheiro et al., 2008).

The first step involves a pretreatment process the goal of which is to alter or remove structural and composi­tional impediments to hydrolysis in order to improve the rate of enzyme hydrolysis and increase yields of fermentable sugars from cellulose or hemicellulose (Mosier et al., 2005). The effectiveness of enzymatic hydrolysis of pretreated lignocellulosic biomass can be significantly enhanced if lignin and its derivatives are removed or effectively modified before adding enzymes because lignin and its derivatives interfere with the path for cellulases action and they are also toxic to microor­ganisms, slowing down enzymatic hydrolysis (Qing et al., 2010; Yang and Wyman, 2008).

The ideal pretreatment process produces a disrupted, hydrated substrate that is easily hydrolyzed but avoids the formation of sugar degradation products and fermentation inhibitors (Agbor et al., 2011). Further­more, there is an overall consensus on the several tech­nical, operational and economical characteristics that a successful pretreatment should accomplish, including

(1) maximize the production of highly digestible solids that enhances sugar yields during enzyme hydrolysis;

(2) avoid the degradation of sugars including those derived from hemicellulose; (3) not require the addition of toxic compounds or minimize their use; (4) fermenta­tion compatibility, minimizing the formation of inhibi­tors for the enzymes or microorganisms in the subsequent steps; (5) effectiveness at low moisture con­tent; (6) broad applicability for multiple crops, sites ages and harvesting times; (7) not required size reduction of biomass; (8) maximize the production of other valuable by-products, e. g. lignin; (9) to be cost-effective by oper­ating in reactors of moderate size, minimizing the heat and power requirements, chemicals and capital equip­ment; and (10) be scalable to industrial size (Alvira et al., 2010; Brodeur et al., 2011; Jorgensen et al., 2007; Yang and Wyman, 2008).

This chapter reviews the advances in the most studied pretreatments and those recently proposed in scheme of the biochemical biorefinery, including kinetics, mecha­nistic and economical models proposed for describing some of these pretreatment processes.