The most familiar yeast in food production, Saccharomyces cerevisiae, has been used in brewing and baking for thousands of years.
S. cerevisiae feeds on the sugars present in the bread dough and produces the gas carbon dioxide. This forms bubbles within the dough, causing it to expand and the bread to rise.
Several different yeasts are used in brewing beer, where they ferment the sugars present in malted barley to produce alcohol. One of the most common is S. cerevisiae. The same strain of S. cerevisiae which can also be used in breadmaking is used to make ale-type beers. It is known as a top-fermenting yeast because it creates a foam on the top of the brew. Bottom-fermenting yeasts, such as S. pastorianus, are more commonly used to make lagers. They ferment more of the sugars in the mixture than top-fermenting yeasts, which gives a cleaner taste.
The alcohol in wine is formed by the fermentation of the sugars in grape juice, with carbon dioxide as a by-product. Yeast is naturally present on grapeskins, and this alone can be sufficient for the fermentation of sugars to alcohol to occur. A pure yeast culture, most often S. cerevisiae, is usually added to ensure the fermentation is reliable. Other yeast cultures like Pichia, Torulaspora and Kluyveromyces are naturally present or added to create special flavours in the wine. Sparkling wine, including champagne, is made by adding further yeast to the wine when it is bottled. The carbon dioxide formed in this second fermentation is trapped as bubbles.
Yeasts are also used to produce kefir products, semi-soft ripened cheeses and fermented soy drinks.