The first step in lignocellulose conversion to bioethanol is size reduction and pre­treatment. Pretreatment of biomass is technically challenging and is a large part of the process cost and therefore will need to be optimized prior to commercialization.

The purpose of the pretreatment step is increasing the surface area and porosity, lignin removal, depolymerization of hemicellulose, hemicelluloses removal, and disruption of lignocelluloses structure. Thus the cellulose component is accessible to hydrolyzing agents and reduces the crystallinity of cellulose to further facilitate hydrolysis (Balat et al. 2008).

A successful pretreatment must meet the following requirements: (1) improve formation of sugars or the ability to subsequently form sugars by hydrolysis, (2) avoid degradation or loss of carbohydrate, (3) avoid formation of by-products inhibitory to subsequent hydrolysis and fermentation processes, and (4) minimize energy input

and be cost-effective. The pretreatment stage promotes the physical disruption of the lignocellulosic matrix in order to facilitate acid — or enzyme-catalyzed hydroly­sis. Pretreatments can have significant implications on the configuration and effi­ciency of the rest of the process and, ultimately, also the economics (Hamelinck et al. 2005) . The goal of pretreatment of lignocellulosic biomass to biofuel is depicted in Fig. 20.2.

The most pretreatments are done through physical or chemical means. In order to achieve higher efficiencies, both physical and chemical pretreatments are required. Physical pretreatment is often aimed to reduce size of the biomass. Chemical pretreatment is aimed at removing chemical barriers so that the enzyme can access cellulose for microbial destruction. Each type of feedstock requires a particular combination of pretreatment methods to optimize the yield of that feed­stock, minimize degradation of the substrate, and maximize the sugar yield.