The wort is cooled to 20-25°C and then inoculated with yeast (Saccharomyces cerevisiae). Fermentation, in wooden vessels of diameter 4 m and height of 5 m, and a capacity of 7500 to 45,000 liters, requires 2 to 3 days and is principally characterized by the growth of the yeast and the fermentation (through the Embden-Meyerhof-Parnas, or glycolytic, metabolic pathway) of the wort sugars to ethanol (Fig. 2).

The pH of the fermenting medium may decrease from 5.0-5.5 to 4.2-4.5 due to the production of organic acids, especially acetic, succinic, and pyruvic acids. Many other compounds, such as higher alcohols, carboxylic acids, esters, aldehydes, and ketones, are formed at this time: these compounds are sufficiently volatile to carry over into the distillate and therefore to make a major contribution to the final flavor of the whiskey. Glycerol, also formed during fermentation, is nonvolatile (b.p. 290°C) and therefore is nondistillable organic material that might otherwise have contributed to whiskey flavor. The yeast, in addition, has the ability to convert sulfur-containing amino acids into volatile sulfur-containing flavor compounds such as dimethyl disulfide (CH3-S-S-CH3).

Figure 2 Fermentation of maltose to ethanol and carbon dioxide.

Ethanol CH3CH2OH

Figure 2 Fermentation of maltose to ethanol and carbon dioxide.

In whiskey, an important component in terms of contribution to flavor is a small optimal amount of the fusel oil fraction, which consists of n-amyl alcohol, isoamyl alcohol, butanol, isobutanol, and propanol. The principal components of the fusel oil fraction are the two amyl alcohols that originate during fermentation as a result of the deamination of leucine and isoleucine. These amino acids in turn arise from the hydrolysis of grain proteins and of autolyzed yeast protein.

The product of this fermentation is a liquid (the wash) that contains about 10% ethanol. The carbon dioxide produced during fermentation is recovered and used, for instance, in the manufacture of soft drinks.

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