Yeast

Yeast, any of a number of microscopic, one-celled fungi important for their ability to ferment carbohydrates in various substances (see Fermentation). Yeasts in general are widespread in nature, occurring in the soil and on plants. Most cultivated yeasts belong to the genus Saccharomyces; those known as brewer's yeasts are strains of S. cerevisiae.

Yeasts have been used since prehistoric times in the making of breads and wines, but their cultivation and use in large quantities was put on a scientific basis by the work of the French microbiologist Louis Pasteur in the 19th century. Today they are used industrially in a wide range of fermentation processes; medicinally, as a source of B-complex vitamins and thiamine (see Vitamin) and as a stage in the production of various antibiotics and steroid hormones; and as feed and foodstuffs.

Pure yeast cultures are grown in a medium of sugars, nitrogen sources, minerals, and water. The final product may take the form of dried yeast cells, or the yeast may be pressed into cakes with some starchy material. When a batch of yeast for baking, medicinal, or food purposes is completed, the medium in which the yeast was grown is discarded. In the making of wines, beers, spirits, and industrial alcohol, however, the fermented medium is the desired product, and the yeast itself is discarded or used to make animal feeds. See Bread; Brewing; Wine.