1.2 Bioethanol production

Instead of chemical synthesis, the bioprocess, i. e. fermentation of simple sugars by microorganism is nowadays used extensively to produce ethanol from renewable sugar- containing biomass. Important ones are sugar crops, starch crops, and lignocellulosic materials derived from agricultural residues. The two former ones are recognized as the first generation feedstock for bioethanol production while the last one is the second generation feedstock. When ethanol is produced by yeast fermentation of sugar feedstock such as sugar cane, molasses, sugar beet and sweet sorghum, yeast can directly consume simple sugars and convert them to ethanol. However, starch and cellulose feedstock are a polymer of glucose and cannot directly be utilized by yeast. They have to be converted or depolymerized to glucose prior to yeast fermentation. Depolymerization or hydrolysis of starch is much simpler and more cost effective than that of cellulosic materials and can be achieved by acid or enzyme or a combination of both.

Starch is a polysaccharide comprising solely of glucose monomers which are linked together by glycosidic bonds. It is composed of two types of glucan namely amylose, a linear glucose polymer having only a-1,4 glycosidic linkage and amylopectin, a branched glucose polymer containing mainly a-1,4 glycosidic linkage in a linear part and a few a-1,6 at a branch structure. Most starches contain approximately 20-30% amylose and the rest are amylopectin. Some starches contain no amylose such as waxy corn starch, waxy rice starch, amylose-free potato, amylose-free cassava and some have very high amylose contents upto 50-70% as in high amylose maize starches. These two polymers organize themselves into semi-crystalline structure and form into minute granules, which are water insoluble. Starch granules are less susceptible to enzyme hydrolysis. Upon cooking in excess water, the granular structure of starch is disrupted, making glucose polymers become solubilized and more susceptible to enzyme attacks. At the same time, the starch slurry becomes more viscous. This process is known as gelatinization and the temperature at which starch properties are changed is named as gelatinization temperatures. Different starches have different gelatinization temperatures, implying different ease of cooking. Cassava starch has a lower cooking temperature, relatively to cereal starches; the pasting temperatures for cassava, corn, wheat and rice are 60-65, 75-80, 80-85 and 73-75°C (Swinkels, 1998; Thirathumthavorn & Charoenrein, 2005).

The starch hydrolysis by enzymes is a two-stage process involving liquefaction and saccharification. Liquefaction is a step that starch is degraded by an endo-acting enzyme namely a-amylase, which hydrolyzes only a-1,4 and causes dramatically drop in viscosity of cooked starch. Typically, liquefying enzymes can have an activity at a high temperature (> 85°C) so that the enzyme can help reduce paste viscosity of starch during cooking. The dextrins, i. e. products obtained after liquefaction, is further hydrolzyed ultimately to glucose by glucoamylase enzyme which can hydrolyze both a-1,4 and a-1,6 glycosidic linkage. Glucose is then subsequentially converted to ethanol by yeast. By the end of fermentation, the obtained beer with approximately 10%v/v ethanol, depending on solid loading during fermentation, is subjected to distillation and dehydration to remove water and other impurities, yielding anhydrous ethanol (Figure 1).