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 sugars. The dominants sugars in hemicelluloses are mannose (six-carbon sugar) in softwoods and xylose (five carbon sugar) in hardwoods and agriculture residues (Persson et al., 2006). Hemicellulose contains also, galactose, glucose and arabinose. This polymer is amorphous and easier to hydrolyse than cellulose. Lignin [(C9H10O3)(OCH3)0.9-1.7]n is a phenyl propane polymer that contains many functional groups such as hydroxyl, methoxyl and carbonyl. Unlike cellulose and hemicellulose, lignin cannot be utilized in the fermentation process. In fact, it has high resistance to chemical and enzymatic degradation. Low concentration of various other compounds, such as extractive and ash are also present. Ash consists of minerals such as silicon, aluminum, calcium, magnesium, potassium, and sodium. Extractives include resins, fats and fatty acids, phenolics, phytosterols, salts, minerals and other compounds. The proportions of these constituents vary between different species. Hardwood has a content of cellulose and hemicelluloses around 80% of total feedstock dry matter while softwood contains around 70% of total dry matter (Balat, 2010). On the other side, lignin is more in softwood than hardwood (Balat, 2009b).Table 1 shows the composition of several lignocellulosic materials and their potential ethanol output obtainable from 1 Kg dry biomass of each type. Cellulose generally accounts for 30-60% of the biomass dry weight while the hemicellulose content varies from 10% to 40%, and the lignin content from 10% to 25% except for olive husks in which the lignin content is higher (48.4%, Table 1). Actually, the world’s largest ethanol producers are Brazil and USA, which together account for more than 65% of global ethanol production. In Europe (EU), the high oil prices and the ratification of the Kyoto Protocol in 2005 have provided additional incentives to promote the use of alternative fuels. Today, EU is the third producer of bioethanol in the world with a production that in 2009 amounted to 3.7 billion liters (www.plateforme-biocarburants.ch).