As described previously, the ethanol production from cassava feedstock involves 5 main steps (Figure 6a) which are

— Feedstock preparation: the main purpose of this step is to make cassava feedstock be physically suitable for downstream processing, i. e. cooking, starch hydrolysis, fermentation and distillation & dehydration. Details are different regarding to types of feedstock and milling process. In general, the preparation includes impurity removal (washing and peel removal of fresh roots, metal detector, sand and soil removal of slurry by hydrocyclone), size reduction by milling or rasping and fiber separation.

— Cooking: The starch is cooked to rupture the granular structure and hence improve its susceptibility to enzyme hydrolysis. Cooking is achieved at a temperature greater than a gelatinization temperature. During cooking, the high viscosity of the slurry is developed due to starch gelatinization and swelling of some particles. Cooking is, therefore, commonly performed in a presence of liquefying enzymes, i. e. a-amylase to liquefy cooked slurry.

— Starch hydrolysis: Starch is enzymatically hydrolyzed to glucose by a-amylase and subsequently by glucoamylase. The liquefaction by a-amylase is usually conducted at high temperatures at which the starch become gelatinized. After liquefaction, the liquefied slurry is cooled down to an optimum temperature for glucoamylase hydrolysis which is about 50-55°C, depending on enzyme types.

— Yeast fermentation: Glucose is then fermented by yeast. By the end of fermentation, the obtained beer contains approximately 10%v/v ethanol, depending on solid loading during fermentation.

— Distillation and dehydration: The beer is subjected to distillation to concentrate the ethanol to 95% and then dehydration to remove water, yielding anhydrous ethanol (99.5%).

Nowadays, the production process of bioethanol from starch feedstock is developed to significantly reduce processing time and energy consumption by conducting saccharification and fermentation in a same step; this process is called "Simultaneous Saccharification Fermentation", or SSF process (Figure 6b). In this SSF process, the liquefied slurry is cooled down to 32°, afterward glucoamylase and yeast are added together. While glucoamylase produces glucose, yeast can use glucose to produce ethanol immediately. No glucose is accumulated throughout the fermentation period (Figure 7) (Rojanaridpiched et al, 2003).

The material balance of ethanol process with a production capacity of 150,000 liters/day, a recommended size of ethanol plants for optimum production costs, feasible feedstock management and effective waste water treatment, from dried cassava chips by Simultaneous Saccharification and Fermentation (SSF) process is estimated from production data collected during pilot trials (at 2,500 L working volume) and factory survey (Figure 8) (Sriroth et al., 2006). The conversion ratio of feedstock (kg) to ethanol (liter, L) is about 2.5:1 for dried chips or 6:1 for fresh roots, this conversion factors are starch-quantity dependent. Based on the pilot production data, the estimated production cost, excluding the feedstock cost, of ethanol from cassava chips by SSF process is about 0.259 USD/L (Rojanaridpiched et al., 2003; Sriroth et al., 2006) which is close to a value reported by FO Licht to be 0.24 USD/L (FO Licht, 2006). The estimated production cost of cassava chips are detailed in Table 7 (Sriroth et al., 2010a).

In SSF process, the starch in cassava material has to be initially cooked and liquefied prior to SSF process. Recently, a granular starch hydrolyzing enzyme has been developed to produce fermentable sugars from native or uncooked corn starch and is then applied to cassava chips (Piyachomkwan et al., 2007; Sriroth et al., 2008). This enzyme can attack directly to uncooked starch granules (Figure 9), allowing liquefaction, saccharification and, in the presence of yeast, fermentation to occur simultaneously in one step at the ambient temperature without cooking; this process is Simultaneous Liquefaction, Saccharification Fermentation or SLSF (Figure 6c). Figure 10 demonstrates the ethanol production from cassava chips using conventional, SSF and SLSF process. It is interesting to note that by SLSF process, the total soluble solid and glucose content do not change over fermentation as starch is used in a native, granular insoluble form. The fermentation efficiency of SLSF process is reported to be comparable with cooked process (Table 8). SLSF process is energy­saving, easy to operate and can be applied economically to produce sustainable energy, at a small scale, for community.

ra«49va Chin ‘Moisture 15 %

v>uip starch content 65o/o (wet basis)

Fuel Ethanol

118.35 T/D or

150,0 L/D

Fig. 8. Mass balance of ethanol production from cassava chip by SSF (Simultaneous Saccharification and Fermentation) process; T/D = Tons/Day, TS = Total Solid,

L/D =Liter/day (Fermentation efficiency 90%, Distillation efficiency 98.5%) Source: Sriroth et al., 2006

O hr 12 hr 24 hr 48hr

ethanol production from cassava chips by Simultaneous Saccharification and Fermentation (SSF; the slurry of 25% dry solid was liquefied by 0.1% Termamyl 120L (Novozymes) at 95- 100°C, 2 hr followed by simultaneous saccharification and fermentation with 0.1% AMG (Novozymes) and Saccharomyces cirivisiae yeast at 32°C for 48 hrs) and Simultaneous Liquefaction, Saccharification and Fermentation (SLSF; the slurry of 25% dry solid was liquefied, saccharified and fermented with 0.25% granular starch hydrolyzing enzyme (Stargen™, Danisco-Genencor, USA) and Saccharomyces cirivisiae yeast at 32°C, 60 hr.

Source: Rojanaridpiched et al., 2003 ; Sriroth et al., 2007