The yeast Saccharomyces cerevisiae is specialized for fermentation, with approx­imately 45% of cellular proteins devoted to glycolysis and ethanol fermentation

[18]. Glucose and maltose are fermented to ethanol by S. cerevisiae via the same fermentation pathway (Figure 4.3) [19] used to make beverage alcohol. In glyc­olysis, glucose is converted through a series of reactions to pyruvate, and energy is extracted in the form of four ATP molecules. Then, pyruvate is converted to ethanol in a two-step reaction; pyruvate is decarboxylated to form the more reactive acetaldehyde, which is reduced to ethanol. The second part of the fer­mentation pathway reoxidizes NADH to NAD+ and thus serves to recover the reducing equivalents that were consumed in the conversion of glucose to pyruvate.

For each glucose fermented, two ethanol and two CO2 molecules are produced (Table 4.1, Figure 4.3). The theoretical mass yield is only 0.51 g of ethanol per g of fermented glucose. The actual yield is closer to 90-95% of 0.51 g because some glucose is converted to cell mass and side-products such as glycerol, citric acid cycle intermediates, and higher alcohols. Contaminating microorganisms can also lower the yield by converting glucose to other fermentation products such

as acetic, lactic, and succinic acids. Because ethanol is used as a fuel, it is also appropriate to consider ethanol yield on an energy basis. The thermodynamic yield can be calculated by comparing the heats of combustion for the products and reactants (Table 4.1). By this measure, converting glucose to ethanol has an amazing theoretical yield of 98-99%, which means that the yeast actually gains little energy benefit from fermenting glucose to ethanol. In other words, ethanol fermentation is an excellent process for generating fuel, because most of the energy from glucose is retained in the fermentation product.

Yeast are ideally suited for use in the fuel ethanol industry. Fermentations run 360 days a year, in tanks containing thousands of gallons of beer, with no pH adjustments and only approximate temperature control (reactors are cooled with well water). As a consequence of the absence of pH control and the pro­duction of CO2, the pH drops steadily during the fermentation and ends up below 4.0. Furthermore, the yeast withstand extreme environmental stresses including high osmolality (beginning solids of 25-30% or higher) and high ethanol con­centrations (final concentrations of 12-18% vol.), as well as organic acids pro­duced by contaminating bacteria. The constant contamination of the fermentation is a consequence of the need to run the process in an “open system”— non — aseptically — because the fermentation volumes are quite large and the selling profit margin for ethanol is very low. Fortunately, most bacterial contaminants do not grow below pH 4, and the ability of yeast to do so provides a natural method of suppressing the growth of these contaminants.

Environmental stresses are additive and often synergistic in nature, which means that a combination of many minor stresses, from the perspective of the yeast, equals a single large stress. For example, yeast have reduced tolerance to ethanol at higher temperatures and reduced tolerance to organic acids at lower pH. Despite all of these challenges, S. cerevisiae produces ethanol at rates in excess of 3 g l-1 h-1 and at yields close to 95% of the theoretical maximum. Efforts in the yeast research field are directed at developing strains that produce less glycerol, grow at slightly elevated temperatures (38°C), and withstand even higher ethanol concentrations.