Ethanol fermentation is a biological process in which organic material is converted by microorganisms to simpler compounds, such as sugars. These fermentable com­pounds are then fermented by microorganisms to produce ethanol and CO2.

Several reports and reviews have been published on production of ethanol fer­mentation by microorganisms, and several bacteria, yeasts, and fungi. Among those microbes that are capable of yielding ethanol as the major product, Saccharomyces cerevisiae and Zymomonas mobilis can accumulate high concentration of ethanol in fermentation media. Historically, the most commonly used microbe has been yeast. Among the yeasts, Saccharomyces cerevisiae, which can produce ethanol to give concentration as high as 18 % of the fermentation broth, is the preferred one for most ethanol fermentation (Lin and Tanaka 2006). This yeast can grow both on simple sugars, such as glucose, and on the disaccharide sucrose. Some other yeast species which produce ethanol are summarized in Table 20.2.

As mentioned before, that the accumulation of hydrolysis products in the medium will inhibit hydrolysis process, hydrolysis and fermentation process combination in one-step reaction could be one way to overcome the problem. Known as simultane­ous saccharification and fermentation (SSF), the process allows the glucose pro­duced from hydrolysis to be fermented immediately. This allows the concentration of the glucose in the SSF medium to remain low, so that the hydrolysis process continues without significant inhibition (Takagi 1976).

SSF possess some other advantages, such as shorter the length of time required for the lignocellulosic biomass to ethanol conversion process, fewer enzymes needed compared to that for regular enzymatic hydrolysis. Furthermore, the chances of contamination are reduced because the process occurs within the same reaction vessel. However, there are fundamental problems with SSF: hydrolysis and fermen­tation both require specific temperature ranges for optimal operation. S. cerevisiae shows the best activity at temperatures around 32 °C with a pH of between 4 and 5 (Wasungu 1982). Any extreme of temperature during fermentation, either high or low, produces lower ethanol concentrations. Yeast does not grow well in temperatures lower than 20 °C or higher than 40 °C. The hydrolysis process, however, performs best at temperatures of about 50-55 °C (Palmqvist 2000). If the temperature is lower than the optimum one, the enzymes will not digest material optimally.

The presence of the ethanol in the fermentation medium during SSF has the pos­sibility of inhibiting the fermentation reaction. As the concentration in ethanol increases, the ethanol attacks the various microorganisms in the system. Both the enzymes and the yeast undergo plasma membrane degradation as the ethanol con­centration increases. Eventually, the ethanol concentration will become high enough to cause cell death in both the enzymes and the yeast (D’amore 1991). Therefore, thermostable strains capable of producing substantial amounts of ethyl alcohol at optimum temperature saccharification and suitably resistant to ethanol are needed to

Scheme 20.1 Conversion of glucose to ethanol and co-products explore (Szczodrak and Targonski 1988) . Such thermostable yeast strains make possible to conduct the fermentation at 42 °C with increased ethanol production (Sree et al 1999). One example of thermostable yeast strain is Kluyvero mycesmarx — ianus CHY1612 which is possible to shift temperature for SSF (Kang et al. 2012). Furthermore, the negative effects which excessive concentrations of ethanol have on yeast activity and cellulase within the SSF system are eliminated with a vacuum cycling reactor where the concentration of ethanol was kept at a relatively low level by its removal from the flash chamber (Roychoudhury et al. 1992).

However, more efforts have to be made in the development of microorganisms for industrial ethanol production. In addition, it is important to know the rate- limiting step. In SSF, the ethanol production rate is controlled by the cellulase hydrolysis rate and not the glucose fermentation, and hence, steps to increase the rate of hydrolysis will lower the cost of ethanol production via SSF.