yeast, any of about 1,500species of single-celledfungi, most of which are in the phylumAscomycota, only a few beingBasidiomycota. Yeasts are foundworldwide insoils and on plant surfaces and are especially abundant in sugary mediums such as flowernectar andfruits. There are hundreds of economically important varieties of ascomycete yeasts; the types commonly used in the production ofbread,beer, andwine are selected strains ofSaccharomyces cerevisiae. Some yeasts are mild to dangerous pathogens of humans and other animals, especiallyCandida albicans,Histoplasma, andBlastomyces.

Physical description

As fungi, yeasts areeukaryotic organisms. They typically are about 0.075 mm (0.003 inch) in diameter and have many forms, from spherical to egg-shaped to filamentous. Most yeasts reproduce asexually bybudding: a small bump protrudes from a parentcell, enlarges, matures, and detaches. A few yeasts reproduce byfission, the parent cell dividing into two equal cells.Torula is agenus of wild yeasts that are imperfect, never forming sexualspores.

Uses

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In food manufacture, yeast is used to causefermentation andleavening. The fungi feed onsugars, producing alcohol (ethanol) andcarbon dioxide; inbeer andwine manufacture the former is the desired product, inbaking it is 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 thedough is baked. The fermentation of wine andsourdough breads is often initiated by naturally occurring yeasts present in air. One yeast cell can ferment approximately its own weight ofglucose per hour.

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distilled spirit: Yeast and yeast culture

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

Commercial yeast is 50 percentprotein and is a rich source of vitaminsB₁,B₂,niacin, andfolic acid.Brewer’s yeast andnutritional yeast, which is deactivated (nonliving), can be eaten as a vitamin supplement.