After corn, wheat (Triticum spp.) is the most employed grain for fuel ethanol pro­duction, especially in Europe and North America, due to its high starch content (Table 3.8). Other than the sugar beet, wheat is the main feedstock for ethanol production in France (Poitrat, 1999). In fact, France was the fifth world producer of wheat in 2007 (33.21 million tons) after China (109.86 million tons), India (74.89 million tons), United States (53.60 million tons), and Russian Federation (49.38 million tons; FAO, 2007b). The wheat yield in France is 6.25 ton/ha, while in Ireland, it reaches up to 8.1 ton/ha (FAO, 2007a).

Sorghum (Sorghum bicolor) is another grain proposed for ethanol produc­tion. The United States is the world’s leading producer of sorghum with a volume of 12.82 million tons in 2007 (FAO, 2007a, 2007b). Sorghum is employed in many countries, such as Colombia, as a component of animal feed, though corn is replacing it. One of the features of sorghum as feedstock for ethanol produc­tion is the presence of significant amounts of tannins. These tannins provoke the decrease in the ethanol production rate during the fermentation process, although they do not affect either the ethanol yield or the enzymatic hydrolysis of starch contained in sorghum (Mullins and NeSmith, 1987).

One of the most promising crops for fuel ethanol production is sweet sorghum, which produces grains with high starch content, stalks with high sucrose content, and leaves and bagasse with high lignocellulosic content (Sanchez and Cardona, 2008a). In addition, this crop can be cultivated in both temperate and tropical coun­tries, it requires one third of the water needed for the sugar cane harvest and half of the water needed by corn, and it is tolerant to drought, flooding, and saline alkalin­ity (du Preez et al., 1985; Winner Network, 2002). Grassi (1999) reports that from some varieties of sweet sorghum the following productivities can be obtained: 5 ton/ha grains, 8 ton/ha sugar, and 17 ton dry matter/ha lignocellulosics.

3.2.2.1 Cassava

The cassava (Manihot esculenta) is a perennial bush achieving 2 m by height and is native to South America. The main feature of this plant is its edible roots, thus the plant is uprooted after one year of growth in order to obtain its better conditions for its consumption. The cassava root is cylindrical and oblong and can reach up to 100 cm in length and 10 cm of diameter. Its pulp is firm and presents high starch content. Cassava tubers are consumed in a cooked form and represent a crucial component in the food of more than 500 million people in America, Asia, and Africa. The cassava is not a very exigent crop, but it should be grown no higher than 1,500 m above sea level. For cassava cropping, the soils should be porous because the root requires sufficient oxygen levels to grow; it also requires good drainage. The cropping temperature should be in the range 25 to 30°C (Agronet, 2007). For this reason, cassava is one of the most important tropi­cal crops in the world. Once planted, cassava roots can be harvested after seven months and stay in the soil for three years (Alarcon and Dufour, 1998).

The world’s largest cassava producer is Nigeria, followed by Brazil, Indonesia, and Thailand (FAO, 2007b). As can be observed in Table 3.9, major cassava producers are located in Africa, Southeast Asia, and South America. The most

elevated cassava yields among the main producers are from Thailand (20.28 ton/ ha) and Brazil (13.63 ton/ha; FAO, 2007a). In general, more than 90% of cas­sava production is directed to human food, while the balance is used for produc­ing starches and snacks. The substitution of corn with cassava flour has been proposed for animal feed production taking into account its high energy content (Espinal et al., 2005a). The importance of cassava cropping from the viewpoint of its agro-industrial applications lies in its high potential for energy production in the form of starch (see Table 3.8). Espinal et al. (2005a) report that the use of improved seeds and fertilizers, and a suitable weed control allows production of 20 to 30 ton/ha of fresh roots and 10 to 12 ton/ha of dried cassava in zones where other starch-producing crops like corn, sorghum, or rice do not reach yields above 4 or 5 ton/ha.

There exist two main methods for industrial production of native cassava starch: the traditional method employed in India and some Latin American coun­tries, and the modern method of the type used by the company Alfa Laval for large-scale production. In the traditional process, fresh roots are washed and debarked before crushing in a rotary rasper. Starch is separated from the crushed pulp before passing through a series of reciprocating nylon screens of decreasing mesh size (50250 mesh). The resultant starch milk is settled over a period of four to eight hours using a shallow settling table or a series of inclined channels laid out in a zigzag pattern. Settled starch is sun-dried on large cement drying floors for approximately eight hours. During this period, the moisture content reduces from 45 to 50% down to 10 to 12%. To achieve efficient drying, sunny conditions are required with ambient temperatures of more than 30°C and relative humidity of 20 to 30%. Dried starch is ground to a fine powder and packaged for sale (FAO and IFAD, 2004). In the modern Alfa Laval-type process, roots are washed and debarked, sliced and then crushed in a rotary rasper. Starch pulp is passed through two conical rotary extractors to separate starch granules from fibrous materials, and then fed via a protective safety screen and hydro cyclone unit to a continuous centrifuge for washing and concentration. The concentrated starch milk is passed through a rotary vacuum filter to reduce water content to 40 to 45% and then flash dried. The flash drying reduces moisture content to 10 to 12% in a few seconds, so starch granules do not heat up and suffer thermal degradation.