Yeasts and Alcoholic Fermentation

Yeasts are a group of 160 species of singlecelled fungi, relatives of molds. Not all are useful: some cause the spoilage of fruits and vegetables, some cause human disease (for example, the yeast infection of Candida albicans). Most of the yeasts used in making bread and alcoholic drinks are members of the genus Saccharomyces, whose name means “sugar fungus.” We cultivate them for the same reason that we use particular bacteria to sour milk: they make foods resistant to infection by other microbes, and produce substances that are mainly pleasant to us. Essential to the yeasts’ production of alcohol is their ability to survive on very little oxygen, which most living cells use to burn fuel molecules for energy, leaving behind only carbon dioxide and water. In the absence of oxygen, the fuel can be broken down only partly. The overall equation for the production of energy from glucose without oxygen goes like this: