Ethanol (i. e. ethyl alcohol, bioethanol) is a liquid oxygenated biofuel employed either as a fuel or as an additive. When it is used as the latter, due to its high oxygen content, a less amount of additive is required. The increased percentage of oxygen allows a better oxidation of the gasoline hydrocarbons with the consequent reduction in the emissions of CO and aromatic compounds (Sanchez and Cardona, 2008).

Bioethanol is the most widely used biofuel for transportation applications, specially in the Western hemisphere, where it surpasses biodiesel in importance. One major problem related to bioethanol production is the availability of the raw materials that varies considerably from season to season and depends on geographic location (UNCTAD, 2006). The major global producer of bioethanol is Brazil, which produces 50% of the world fuel ethanol, using sugar cane juice (Brazil is responsible for 25% of all sugar cane production worldwide) and molasses (Murphy, 2004). In the USA, 95% of the fuel ethanol produced comes from corn, while more temperate countries like Canada uses other less efficient starchy crops like wheat, corn and barley (Murphy, 2004). Table 4.3 depicts ethanol yield from the most common crops.

Biological feedstocks that contain appreciable amounts of sugar — or materials that can be converted into sugar, such as starch or cellulose — can be fermented to produce bioethanol (Kim and Dale, 2004). Bioethanol feedstocks can be classified

into three types: (1) sucrose-containing feedstocks (e. g. sugar beet, sweet sorghum and sugar cane); (2) starchy materials (e. g. wheat, corn and barley); (3) lignocellulosic biomass (e. g. wood, straw and grasses).

The price of the raw materials can highly affect the production costs of bioethanol because feedstocks typically account for more than one-third of the production costs, and maximising bioethanol yield is imperative (Murphy, 2004). In this sense, sugar cane and sugar beet present an alcohol yield of around 3 and 1 hl/t respectively, while cereals such as wheat and corn present higher alcohol yields (3.6 and 4 hl/t, respectively). However, processing costs to produce ethanol from sugar cane and sugar beet (where sugars are easily accessible since disaccharide can be broken down by the yeast cells) are lower compared to cereals and most of all compared to starchy materials and lignocellulosic biomass (Cardona and Sanchez, 2007; Prasad et al., 2007). Starchy, lignocellulosic, urban and industrial wastes need costly pretreatment to convert into fermentable substrates.