1.2.1 Cassava chip

Similar to bioethanol production of corn grains, there are two processes for preparing cassava chips which are "Dry Milling" and "Wet Milling". In Dry Milling process (Figure 2), chips are transferred to the hopper and a metal and stone detector. The chips are then milled and sieved to obtain fine powders. Coarse powders are remilled. The fine powder having all components including fibers is slurried with water and proceeds to cooking and enzyme hydrolysis. The heat to cook slurry for liquefaction process is usually from direct steam instead of a jet cooker due to the difficulties of handling particles and contaminants in slurry. Owing to contamination of sand, conveyor system and grinding system require special treatment. Furthermore, after passing through syrup making process, an extra separation unit or hydrocyclone is required to remove sand and other impurities. The dry milling process is suitable for batch fermentation. Most of existing factories in China and some factories being installed in Thailand apply this dry milling process as it uses less equipment and investment (Sriroth & Piyachomkwan, 2010b).

As corn grains are composed of many valuable components including protein, lipid and starch, wet milling process has been developed as a separation technique in order to fractionate starch and other high-valued products including corn gluten meal with high protein content, corn gluten feed and corn germ for oil extraction. The grains are initially

cleaned and soaked in steeping water containing some chemicals such as sulfur dioxide, the most typically used one, and lactic acid to soften the grains. The soften kernels are milled to be suitable for degermination process and separated germ is used for oil extraction. Degermed ground kernels are then passed through fine mills so that the fiber can be readily separated. The protein is further fractionated from the defibered starch slurry by centrifugal separators. After fractionation of each component, starch slurry is further processed to cooking and enzyme hydrolysis for ethanol production. In wet milling process of cassava chips (Figure 3), the starch slurry is prepared from dried chips by modifying typical cassava starch production process. Unlike wet milling of corn grains with water, the chips are milled to fine powder before slurried with water. The process is sometimes named as "Starch milk" process of which the starch is then extracted from chips by a series of extractors. After depulping, the starch slurry is then concentrated by a separator and subjected to a jet cooker for liquefaction. Currently, only a few plants are using this process, because this process requires a high investment. Factories have modified the process by reducing the extractor and stipping tank unit. Wet milling generates high starch losses in the solid waste. However, the process is more controllable and can be practically applied to high solid loading and continuous fermentation process (Sriroth & Piyachomkwan, 2010b).

In contrast to wet milling process, dry-milling process does not fractionate each component, yielding a by-product of mixed components. Although more valuable products are co­produced by wet milling process, this process is capital and energy intensive and results in a lower yield of ethanol as compared to dry-milling process; one ton of corn yields 373 and 388 liters of ethanol when processed by wet — and dry — milling, respectively (FO Licht, 2006).

both germ and fiber prior to fermentation (Singh and Eckhoff, 1997; Wahjudi et al., 2000; Huang et al., 2008). This combined process improves cost reduction as compared to wet­milling process while increases value addition to dry-milling process. Although, cassava chips are corn analog and can be processed either by wet-milling or dry-milling process, the chips do not contain other valuable components. Dry-milling process is therefore generally applied for bioethanol production.