In this chapter, starch as carbon source will be primarily discussed in the application for single-step or direct bioconversion. Starch is a polysaccharide and the most abundant class of organic material found in nature. Sources of starch that are normally used in the production of ethanol are derived from seeds or cereals such as corn, wheat, sorghum, barley, soy and oat. Other sources of starch can be from tuber or roots such as potato, yam or cassava. By using starch as substrate for bioethanol production has distinct advantages in terms of its economical pretreatment and transportation compared to other types of biomass. For example cassava or tapioca tuber that has received an enormous attention in the production of biofuel in particular bioethanol in East Asia region such as China, Thailand, Malaysia and Indonesia (Dai et al., 2005; Hu et al., 2004; Nguyen et al., 2006). Cassava is a perennial woody shrub, ranks second to sugarcane and is better than both maize and sorghum as an efficient carbohydrate producer under optimal growing conditions. It is also the most efficient producer under suboptimal conditions of uncertain rainfall, infertile soil and limited input encountered in the tropic (Fregene and Puonti-Kaerlas, 2002).

Before undergo conventional or traditional fermentation, starch regardless of its sources required to be hydrolyzed. Two types of hydrolysis usually applied are mineral acid hydrolysis and enzymatic hydrolysis. The mineral acid or acid-base involved in the hydrolysis can be of diluted or concentrated form. The dilute acid process at 1-5% concentration is conducted under high temperature and pressure and has fast reaction time in the range of seconds or minutes. The concentrated acid process on the other hand uses relatively mild temperatures and the reaction times are typically much longer as compared to those in the dilute acid hydrolysis. The biggest advantage of dilute acid processes is their fast reaction rate, which facilitates continuous processing for hydrolysis of both starch and cellulosic materials. Their prime disadvantage is the low sugar yield and this has opened up a new challenge to increase glucose yields higher than 70% (especially in cellulosic material) in an economically viable industrial process while maintaining high hydrolysis rate and minimizing glucose decomposition (Xiang et al., 2004; McConnell, 2008). The concentrated acid hydrolysis offers high sugar recovery efficiency, up to 90% of both hemicelluloses and cellulose sugars. Its drawback such as highly corrosive and volatility can be compensated by low temperatures and pressures employed allowed the use of relatively low cost materials such as fiberglass tanks and piping. Without acid recovery, large quantities of lime must be used to neutralize the acid in the sugar solution. This neutralization forms large quantities of calcium sulfate, which requires disposal and creates additional expense. In addition to that, this type of hydrolysis has resulted in the production of unnatural compounds that have adverse effect on yeast fermentation (Tamalampudi et al., 2009).

Enzymatic hydrolysis of starch required at least two types of enzymes. This is due to that the starch or amylum comprises of two major components, namely amylose, a mainly linear polysaccharide consisting of a-1,4-linked n-glucopyranose units and the highly branched amylopectin fraction that consists of a-1,4 and a-1,6-linked n-glucopyranose units (Knox et al., 2004). Depending on type of plants, starch typically contains 20 to 25% amylose (van der Maarel et al., 2002) and 75 to 80% amylopectin (Knox et al., 2004). These two type linkage, a-1,4 and a-1,6-linked required an efficient starch hydrolysis agent or enzyme that can fraction a-1,4 and promote a-1,6 debranching activity. Since starch contains amylose and amylopectin, single or mono-culture cells are usually added during fermentation stage where starch has already been hydrolyzed to reducing sugar by hydrolysis agent such as acid-base or microbial enzymes in pretreatment and saccharification steps. The microbial enzyme of a — amylase cleaves a-1,4 bonds in amylose and amylopectin which leads to a reduction in viscosity of gelatinized starch in the liquefaction process. The process is the hydration of starch by heating the starch in aqueous suspension to give a-amylase an access to hydrolyze the starch. Dextrin and small amount of glucose and maltose are the end products. Exoamylases such as glucoamylase is then added during saccharafication which hydrolyses 1,4 and 1,6-alpha linkages in liquefied starch (van der Maarel et al., 2002). Enzyme has an advantage over acid — based hydrolysis. Amylolytic enzymes hydrolysis work at milder condition with the temperature lower 110°C (Cardona et al., 2010). However, enzyme is expensive especially cellulosic enzyme where it was reported the most expensive route accounted for approximately 22%-40% of total total cost (Wooley et al., 1999; Yang and Wyman, 200; Rakshit, 2006). Furthermore, fermentation of high concentration of starch to obtain high yield of ethanol is unfeasible due to reducing sugar inhibition on enzyme. This was shown in the work of Kolusheva and Marinova (2007) where the high reducing sugar produced from hydrolysis of high concentration not only inhibited the enzyme activity but also the fermenting yeast.