Yeast, any of a group of microscopic, one-celled fungi important for their ability to ferment carbohydrates.
Budding yeasts are true fungi of the phylum Ascomycetes, class Hemiascomycetes. The true yeasts are separated as one main order Saccharomycetales, which includes at least ten families. Most cultivated yeasts belong to the genus Saccharomyces.
Yeasts are heterotrophic, lack chlorophyll, and are characterized by a wide dispersion of natural habitats. Common on plant leaves and flowers, yeasts are also found on the skin surfaces and in the intestinal tracts of warm-blooded animals, where they may live symbiotically or as parasites. In women, who are pregnant or taking antibiotics, an infection of the vagina and vulva caused by a yeastlike fungus Candida albicans, is common. Yeasts are also found in soil and saltwater, where they contribute to the decomposition of plant and algal matter.
Yeasts multiply as single cells that divide by budding or direct division (fission), or they may grow as simple irregular filaments (mycelium). In sexual reproduction most yeasts form asci, which contain up to eight haploid ascospores. These ascospores may fuse with adjoining nuclei and multiply through vegetative division or, as with certain yeasts, fuse with other ascospores.
The most well-known and commercially significant yeasts are the related species and strains of Saccharomyces cerevisiae. These organisms have long been utilized to ferment the sugars of rice, wheat, barley, and corn to produce alcoholic beverages and in the baking industry to expand, or raise, dough. Saccharomyces cerevisiae is commonly used as baker's yeast and for some types of fermentation. The yeast's function in baking is to ferment sugars present in the flour or added to the dough. This fermentation gives off carbon dioxide and ethanol. The carbon dioxide is trapped within tiny bubbles and results in the dough expanding, or rising.
In beer brewing, Saccharomyces carlsbergensis, named after the Carlsberg Brewery in Copenhagen, where it was first isolated in pure culture by Dr. Emile Christian Hansen around 1888, is used in the production of several types of beers including lagers. S. carlsbergensis is used for bottom fermentation. S. cerevisiae used for the production of ales and conducts top fermentation, in which the yeast rise to the surface of the brewing vessel. In modern brewing many of the original top fermentation strains have been modified to be bottom fermenters. Currently the S. carlsbergensis designation is not used, the S. cerevisiae classification is used instead.