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Yeast are a group of unicellular fungi commonly used to leaven bread and ferment alcoholic beverages. They belong to the division Ascomycota. Yeasts, such as Candida albicans[?] can cause infection in humans.

In the absence of oxygen, yeast produce their energy by converting sugars into carbon dioxide and ethanol (alcohol). In brewing, the ethanol is used, while in baking the carbon dioxide raises the bread and the ethanol evaporates.

An example with glucose as the substrate is

C6H12O6 (glucose) →2C2H5OH + 2CO2

They reproduce asexually through budding[?]. A new bud grows out of the parent yeast when the condition is right, then after the bud reaches an adult size, it separates from the parent yeast.

Yeasts for leavening bread may be produced commercially or caught from the environment. The use of potatoes, water from potato boiling, eggs, or sugar in a bread dough accelerates the growth of yeasts. Salt and fats such as butter slow yeast growth down.

Beer brewers classify yeasts as top-fermenting and bottom-fermenting. This distinction was introduced by the Dane Emil Christian Hansen.

Top-fermenting yeasts (so-called because they float to the top of the beer) can produce higher alcohol concentrations and prefer higher temperatures. An example is Saccharomyces cerevisiae, known to brewers as ale yeast. They produce fruitier, sweeter, real ale type beers.

Bottom-fermenting yeasts ferment more sugars leaving a crisper taste and work well at low temperatures. Examples include Saccharomyces uvarum[?] and Saccharomyces carlsbergensis[?]. They are used in producing lager-type beers.

Brewers of wheat beers often use varieties of Torulaspora delbrueckii.

Winemakers use a variety of different yeasts depending on the type of wine and the condition of the grapes. Too high a sugar or alcohol concentration slows the growth of yeast, so for very ripe grapes with lots of sugar he or she would use a yeast tolerant of those conditions. If the yeast dies before all the fermentable sugar has been converted to alcohol, the result is a "stuck" fermentation. Some yeast is chosen because it tends to develop certain aromas, such as the distinctive "banana" smells of Beaujolais from Georges DeBeouf[?]. Wild yeast are naturally present on the skins of grapes, so grape juice will spontaneously ferment unless the wild yeast are arrested by cold temperature or sulphates[?]. Depending on the strain of indigenous yeast, the result may be unpalatable or possibly more complex than if a single cultured strain were used. In general, natural yeasts are riskier than cultured, and tend to be used by tradition-oriented, Old World-style winemakers.

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Saccharomyces cerevisiae is also known as budding or baker's yeast. It is used as a model organism by biologists studying genetics and molecular biology (in particular the cell cycle) because it is easy to culture but as a eukaryote, it shares the complex internal cell structure of plants and animals.

Another important experimental model is Schizosaccharomyces pombe[?] or fission yeast.

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