Latest Frontiers in the Biotechnologies for Ethanol Production from Lignocellulosic Biomass

Abstract

The development of the industrial and technological society together with the economic and environmental implications, such as global warming and decreasing oil reserves, have been driving worldwide interest in searching for renewable energies to replace fossil fuels. With respect to fossil fuels, biomass-based fuels have the advantage of decreasing greenhouse gas (GHG) emissions. In this context, ethanol produced from biomass, the so called “bioethanol”, has become a major energy carrier for a sustainable transportation sector. Bioethanol is an oxygenate fuel with an high octane number (Moon et al., 2009) and it can be used as biofuel either in its pure state (E100) or blended with petrol in various proportions, such as E85, E95, E10 containing 85%, 95% and 10% of ethanol respectively. Among these, E10 not requires any change in engine (Balat, 2009a). In addition, bioethanol has low toxicity and reduces urban air pollution because the carbon dioxide released during its combustion is virtually reused by plants during the chlorophyll photosynthesis. Currently, United States and Brazil are the largest bioethanol producers in the world from corn and sugarcane respectively. However, in some countries with low availability of agricultural lands, the production of biofuels from dedicated crops could lead to direct conflict with food productions. Lignocellulosic materials and, among them, agro-forest residues, could, offer a great potential as biomass source for bioethanol production. In fact, they are virtually abundant and low cost (Perlack et al., 2005). Lignocellulosics materials can be classified in four groups: forest residues (chips and sawdust from lumber mills, dead trees, and tree branches), municipal solid wastes (household garbage and paper products), waste paper and energy crops (Balat, 2010). Lignocellulosic feedstocks are composed primarily of carbohydrate polymers (cellulose and hemicellulose) and phenolic polymers (lignin). Cellulose (C6H10O5)x is a linear polysaccharide polymer of glucose made of cellobiose units that are packed by hydrogen bonds. The structure of this polymer is rigid and compact, so that in order to obtain glucose, the biomass needs pre-treatment that breaks its structure to facilitate the action of the enzymes. The individual cellulose chains are packed and organized into crystalline microfibrils. Within these microfibrils, cellulose is found in two forms, namely amorphous and crystalline. The crystalline form of cellulose is very difficult to degrade. Hemycellulose such as xylan (C5H8O4)m is a short polymer of pentoses and hexoses