Following hydrolysis, converting the resultant sugars to products is the next step. Fermentation is a bio­logical option and is the focus of this section. Both chemical/catalytic and biochemical conversions are common.

At this point, the pretreatment and hydrolysis activ­ities were designed and executed all with the intent of optimizing and preparing for fermentation, the capstone process of the bioconversion (Gamage et al., 2010). Fermentation is referred to as anaerobic digestion. Fermentation is the chemical breakdown of a substance by bacteria, yeasts, or other microorganisms to produce ethanol or other alcohols, lactic acid, lactose, and hydrogen (Chandel et al., 2007; Wheals et al., 1999).

One of the most significant factors in fermentation is the choice of organism or modification to an organism to acquire a desired product. Some organisms only metab­olize hexoses while others may metabolize both hexoses and pentoses. Saccharomyces cerevisiae is an old and very popular strain of yeast used throughout the food and fuel industries. When added to a batch of material, it will metabolize the glucose component, almost exclu­sively, into ethanol and carbon dioxide. It will generally follow the Embden-Myerhof pathway under anaerobic conditions when the temperature is controlled around 30 °C (Limayem and Ricke, 2012). S. cerevisiae grows optimally at this temperature and it also resists high os­motic pressure and it is tolerant of pH as low as 4.0 and it is tolerant of many inhibitory products (Hahn-Hagerdal et al., 2007). S. cerevisiae remains popular because of its high ethanol yield from hexose sugars; it generates 12.0—17.0% w/v, which is 90% of the theoretical maximum (Bayrock and Ingledew, 2001; Claassen et al., 1999).

Despite all its great characteristics, S. cerevisiae cannot metabolize both hexoses and pentoses and thus it is not a great organism for converting LB. In

LB, there is a significant portion of the hydrolysate containing hemicelluloses, pentose sugars such as D-xylose, which may potentially enhance yields (Martin et al., 2002). Identifying and employing an optimal organism is a great opportunity in fermenta­tion. The optimal organism ought to be high yielding, able to metabolize both hexose and pentose sugars, tolerant to high ethanol concentration and tolerant to chemical inhibitors left over from pretreatment and hydrolysis. There are numerous naturally occurring organisms that possess a subset of these characteristics, but none are ideal. To develop a more advantageous organism one might have to genetically modify an organism to achieve one’s goals. Table 27.7 lists several naturally occurring organisms and their features and liabilities (Limayem and Ricke, 2012).

Reducing operating costs and product inhibition is another important goal. There are strategies that combine hydrolysis and fermentation together. Simul­taneous saccharification and fermentation (SSF) is one strategy that has just that in mind. The needed en — zyme(s) and the corresponding organisms are added together so that enzymatic saccharification of cellulose and subsequent fermentation of the resultant sugars takes place at the same time in the same reactor (Dowe and McMillan, 2008). However, SSF requires an overall compromise between saccharification and fermentation, usually resulting in a less optimum oper­ation. Another strategy is to employ an organism that is capable of making its own enzymes for hydrolysis and of fermenting the resultant sugars. Consolidated bioprocessing lowers the cost of bioconversion by reducing enzymatic saccharification and fermentation into a single step and eliminates the need for cellulase enzymes (Ladisch et al., 2010; Lynd et al., 2005).

Despite the number of prokaryotic and eukaryotic microorganisms that convert sugars to ethanol, most remain limited in terms of cofermentation, ethanol yields, and tolerance to chemical inhibitors, high temperatures and ethanol.