ABSTRACT
Efficient generation of a fermentable hydrolysate is a primary prerequisite in the utilization of cellulosic biomass for biorefinery processes, including liquid transportation fuels production, particularly bioethanol. Unfortunately, enzyme catalyzed degradation of cellulose and C5 heteroxylans in cellulosic feedstocks is challenging, and the cost of the enzymatic step is still a major obstacle in the development of economically viable cellulose-based ethanol. The main issues include the natural resistance of the substrates, the complex interlacing of the polymeric substrate constituents, the crystalline nature of cellulose, and the fact that each biomass substrate requires action of multiple enzyme activities to accomplish the degradation to monosaccharides. In order to provide for the enzymatic processes to be both economically competitive
and environmentally feasible, a “minimization” of the enzymatic treatment is crucial. In addition to minimization of enzyme production costs, the “minimization” concept involves employment of the minimal number of different enzymes, in the right proportion, and at minimal dosage for achieving maximal yields on the substrates (Banerjee et al., 2010; Gao et al., 2010; Meyer et al., 2009). The enzymatic reactions, moreover, have to be accomplished at the shortest possible time with minimal energy input. A number of options are available for reducing enzyme production costs, but the minimization of the enzymatic treatment requires a detailed understanding of the substrate, the enzyme actions, kinetics, and rate-limiting factors. This chapter will outline the efforts that have been made in improving enzyme production, and will discuss the complexity of enzymatic hydrolysis of cellulosic feedstocks, and highlight the current strategies employed in the development of efficient minimal enzyme treatments for enzymatic hydrolysis of complex biomass substrates. Particular emphasis will be put on the unique complexity of enzymatic cellulose hydrolysis reactions, but the progress in the understanding of enzymatic hemicelluloses degradation will also be summarized.
