yeast

American  

[yeest] / yist /

noun

1. any of various small, single-celled fungi of the phylum Ascomycota that reproduce by fission or budding, the daughter cells often remaining attached, and that are capable of fermenting carbohydrates into alcohol and carbon dioxide.

2. any of several yeasts of the genus Saccharomyces, used in brewing alcoholic beverages, as a leaven in baking breads, and in pharmacology as a source of vitamins and proteins.

3. spume; foam.

4. ferment; agitation.

5. something that causes ferment or agitation.

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verb (used without object)

1. to ferment.

2. to be covered with froth.

yeast British  

/ jiːst /

noun

1. any of various single-celled ascomycetous fungi of the genus Saccharomyces and related genera, which reproduce by budding and are able to ferment sugars: a rich source of vitamins of the B complex

2. any yeastlike fungus, esp of the genus Candida, which can cause thrush in areas infected with it

3. a commercial preparation containing yeast cells and inert material such as meal, used in raising dough for bread or for fermenting beer, whisky, etc See also brewer's yeast

4. a preparation containing yeast cells, used to treat diseases caused by vitamin B deficiency

5. froth or foam, esp on beer

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verb

1. (intr) to froth or foam

yeast Scientific  

/ yēst /

1. Any of various one-celled fungi that reproduce by budding and can cause the fermentation of carbohydrates, producing carbon dioxide and ethanol. There are some 600 known species of yeast, though they do not form a natural phylogenic group. Most yeasts are ascomycetes, but there are also yeast species among the basidiomycetes and zygomycetes. The budding processes in yeasts show a wide range of variations. In many yeasts, for example, the buds break away as diploid cells. Other yeasts reproduce asexually only after meiosis, and their haploid buds act as gametes that can combine to form a diploid cell, which functions as an ascus and undergoes meiosis to produce haploid spores. Still other yeasts form buds in both haploid and diploid phases. The ascomycete yeast Saccharomyces cerevisiae is used in baking to produce the carbon dioxide that leavens dough and batter. It has been the subject of extensive research in cell biology, and its genome was the first to be sequenced among eukaryotes. A variety of yeasts of the genus Saccharomyces are used in making beer and wine to provide alcohol content and flavor. Certain other yeasts, such as Candida albicans, are pathogenic in humans.

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Other Word Forms

• yeastless adjective
• yeastlike adjective

Etymology

Origin of yeast

before 1000; Middle English ye ( e ) st (noun), Old English gist, gyst; cognate with Dutch gist, German Gischt yeast, foam, Old Norse jastr yeast, Greek zestós boiled, Sanskrit yásati (it) boils

Explanation

Bakers use yeast to turn flat, sticky dough into tall, fluffy bread. The yeast interacts with sugar and fills the bread with pockets of air as it bakes. When you buy yeast for baking cinnamon rolls or rye bread at home, it comes in a small packet and looks like little brownish grains. The first step in using yeast is often mixing it with warm water and a little sugar. When yeast is active, or alive, the mixture quickly becomes bubbly and foamy. Yeast is also used in brewing beer and wine. The word yeast comes from a Germanic root — the German word Gischt, or "froth," is closely related.

Example Sentences

In response to these challenges, the yeast activated their protective systems.

From Science Daily • Apr. 12, 2026

To fill this gap, researchers engineered the yeast Yarrowia lipolytica to produce a precise mix of six essential sterols.

From Science Daily • Mar. 27, 2026

For that sugar to then ferment and produce alcohol, the person giving the sample would also need to have a yeast infection, she wrote.

From Los Angeles Times • Mar. 19, 2026

They discovered that when microscopic organisms such as yeast and mold fed on the kelp, the byproduct more closely resembled what they were seeking.

From The Wall Street Journal • Mar. 18, 2026

What Nim liked best about cooking bread was watching the dough she’d mixed from dry flour, yeast, and water puff into warm, fresh bread.

From "Nim’s Island" by Wendy Orr

Related Words

• fungus