Cassava represents an important alternative source of starch not only for etha­nol production, but also for production of glucose syrups. In fact, cassava is the tuber that has gained the most interest due to its availability in tropical coun­tries, being one of the top 10 more important tropical crops. Ethanol production from cassava can be accomplished using either the whole cassava tuber or the starch extracted from it (Sanchez and Cardona, 2008). Starch extraction can be carried out through a high-yield, large-volume industrialized process as the Alfa Laval extraction method (FAO and IFAD, 2004), or by a traditional process for small- and midscale plants (see Chapter 3, Section 3.2.2.3). This process can be considered as the equivalent of the wet-milling process for ethanol produc­tion from corn. The production of cassava with high starch content (85 to 90% dry matter) and less protein and minerals content is relatively simple. Cassava starch has a lower gelatinization temperature and offers a higher solubility for amylases in comparison to corn starch. The hydrolysis of cassava flour has been proposed for glucose production in an enzymatic hollow-fiber reactor with 97.3%
conversion (Lopez-Ulibarri and Hall, 1997), considering that cassava flour pro­duction is simpler and more economic than cassava starch production. However, it is considered that cassava ethanol would have better economic indicators if the whole tuber were used as feedstock, especially when small producers are involved (Sanchez and Cardona, 2008). Fuel ethanol production from whole cas­sava is equivalent to ethanol production from corn by dry-milling technology. For this, cassava should be transported as soon as possible from cropping areas because of its rapid deterioration due to its high moisture content (about 70%). Hence, this feedstock should be processed within three to four days after its har­vest. One of the solutions to this problem consists in the use of sun-dried cassava chips (Sriroth et al., 2007). The farmers send the cassava roots to small chipping factories where they are peeled and chopped into small pieces. The chips are sun-dried for two to three days. The final moisture content is about 14% and the starch content reaches 65%.

The first step of the process in the distillery is the grinding of the dried cas­sava chips or fresh roots (if a permanent supply is ensured; Sanchez and Cardona, 2008). Milled cassava is mixed with water and undergoes cooking followed by the liquefaction process (Nguyen et al., 2008). Liquefied slurry is saccharified to obtain the glucose, which will be assimilated by the yeast during the next fer­mentation step. The process can be intensified through the SSF as in the case of corn (Figure 11.10). If fresh roots are employed, a fibrous material is obtained in the stillage after distillation. This material can be used as an animal feed similar to the DDGS produced in the corn-based process. The wastewater can be treated by anaerobic digestion to produce biogas, which can be used to produce steam and power for the process. Nevertheless, the amount of steam generated is not enough to cover the needs of the process. Hence, natural gas or other fossil fuel is required (Dai et al., 2006).

( Milling

FIGURE 11.10 Simplified diagram for fuelethanol production from cassava.