any of certain economically important single-celled fungi (kingdom Fungi), most of which are in the phylum Ascomycota, only a few being Basidiomycota. Yeasts are found worldwide in soils and on plant surfaces and are especially abundant in sugary mediums such as flower nectar and fruits. There are hundreds of varieties of ascomycete yeasts; the types commonly used in the production of bread, beer, and wine are selected strains of Saccharomyces cerevisiae. The small cakes and packets of yeast used in food- and beverage-processing contain billions of individual yeast cells, each about 0.003 inch (0.075 mm) in diameter.

Yeasts reproduce asexually by budding: a small bump protrudes from a parent cell, enlarges, matures, and detaches. A few yeasts reproduce by fission, the parent cell dividing into two equal cells. Some yeasts are mild to dangerous pathogens of humans and other animals (e.g., Candida albicans, Histoplasma, Blastomyces). Torula is a genus of wild yeasts that are imperfect, never forming sexual spores.

In food manufacture, yeast is used to cause fermentation and leavening. The fungi feed on sugars, producing alcohol (ethanol) and carbon dioxide; in beer and wine manufacture the former is the desired product, in baking, the latter. In sparkling wines and beer some of the carbon dioxide is retained in the finished beverage. The alcohol produced in bread making is driven off when the dough is baked. The fermentation of wine is initiated by naturally occurring yeasts present in the vineyards. One yeast cell can ferment approximately its own weight of glucose per hour.

Yeast is 50 percent protein and is a rich source of vitamins B₁, B₂, niacin, and folic acid. Brewer's yeast is sometimes eaten as a vitamin supplement.

In commercial production, selected strains of yeast are fed a solution of molasses, mineral salts, and ammonia. When growth ceases, the yeast is separated from the nutrient solution, washed, and packaged. Yeast for baking is sold in compressed cakes containing starch or in a dry granular form mixed with cornmeal.