Alessandra Verardi1, Isabella De Bari2*, Emanuele Ricca1 and Vincenza Calabro1

1Department of Engineering Modeling, University of Calabria, Rende (CS) 2ENEA Italian National Agency for New Technologies, Energy and the Sustainable Economical Development, Rotondella (MT)

Italy

1. Introduction

Bioethanol can be produced from several different biomass feedstocks: sucrose rich feedstocks (e. g. sugar-cane), starchy materials (e. g. corn grain), and lignocellulosic biomass. This last category, including biomass such as corn stover and wheat straw, woody residues from forest thinning and paper, is promising especially in those countries with limited lands availability. In fact, residues are often widely available and do not compete with food production in terms of land destination. The process converting the biomass biopolymers to fermentable sugars is called hydrolysis. There are two major categories of methods employed. The first and older method uses acids as catalysts, while the second uses enzymes called cellulases. Feedstock pretreatment has been recognized as a necessary upstream process to remove lignin and enhance the porosity of the lignocellulosic materials prior to the enzymatic process (Zhu & Pan, 2010; Kumar et al., 2009).

Cellulases are proteins that have been conventionally divided into three major groups: endoglucanase, which attacks low cristallinity regions in the cellulose fibers by endoaction, creating free chain-ends; exoglucanases or cellobiohydrolases which hydrolyze the 1, 4- glycocidyl linkages to form cellobiose; and p-glucosidase which converts cello — oligosaccharides and disaccharide cellobiose into glucose residues. In addition to the three major groups of cellulose enzymes, there are also a number of other enzymes that attack hemicelluloses, such as glucoronide, acetylesterase, xylanase, p-xylosidase,

galactomannase and glucomannase. These enzymes work together synergistically to attack cellulose and hemicellulose. Cellulases are produced by various bacteria and fungi that can have cellulolytic mechanisms significantly different.

The use of enzymes in the hydrolysis of cellulose is more effective than the use of inorganic catalysts, because enzymes are highly specific and can work at mild process conditions. In spite of these advantages, the use of enzymes in industrial processes is still limited by

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several factors: most enzymes are relatively unstable at high temperatures, the costs of enzyme isolation and purification are high and it is quite difficult to recover them from the reaction mixtures. Currently, extensive research is being carried out on cellulases with improved thermostability. These enzymes have high specific activity and increased flexibility. For these reasons they could work at low dosages and the higher working temperatures could speed up the hydrolysis reaction time. As consequence, the overall process costs could be reduced. Thermostable enzymes could play an important role in assisting the liquefaction of concentrated biomass suspensions necessary to achieve ethanol concentrations in the range 4-5 wt%.

The immobilization of enzymes has also been proposed to remove some limitations in the enzymatic process (Hong et al., 2008). The main advantage is an easier recovery and reuse of the catalysts for more reaction loops. Also, enzyme immobilization frequently results in improved thermostability or resistance to shear inactivation and so, in general, it can help to extend the enzymes lifetime.

This chapter contains an overview of the lignocellulosic hydrolysis process. Several process issues will be deepened: cellulase enzyme systems and hydrolysis mechanisms of cellulose; commercial mixtures; currents limits in the cellulose hydrolysis; innovative bioprocesses and improved biocatalysts.