What are yeast?

Introduction to Fungi Credit: Kandis Elliot, Mo Fayyaz, University of Wisconsin, Madison

Yeast are single-celled eukaryotic microorganisms that are classified, along with molds and mushrooms as members of the kingdom Fungi. Yeasts are phylogenetically diverse, and consequently are classified in two separate phyla, the Ascomycota and the Basidiomycota. Budding yeast (also called “true yeasts”), such as the well-known species, Saccharomyces cerevisiae are classified in the order Saccharomycetales, which is under Ascomycota. Biological classification is based on characteristics of the cell, ascospore and colony. Physiological characteristics are also used to identify species. One of the most well known characteristics of yeast is the ability to ferment sugars for the production of ethanol.

Saccharomyces cerevisiae are one of the most well-known and commercially significant species of yeast. These organisms have long been utilized to ferment the sugars of rice, wheat, barley, and corn to produce alcoholic beverages, and to expand and raise dough to make bread. Most yeast reproduce asexually by an asymmetric division process called budding rather than by binary fission. A small bud emerges from the surface of the parent cell and enlarges until it is almost the size of the parent.

Yeast are unicellular fungi. The precise classification is a field that uses the characteristics of the cell, ascospore and colony. Physiological characteristics are also used to identify species. One of the more well known characteristics is the ability to ferment sugars for the production of ethanol. Budding yeast are true fungi of the phylum Ascomycota, class Saccharomycetes (also called Hemiascomycetes). The true yeast is separated into one main order Saccharomycetales.

Yeast are characterized by a wide dispersion of natural habitats. Common on plant leaves and flowers, soil and salt water. Yeast are also found on the skin surfaces and in the intestinal tracts of warm-blooded animals, where they may live symbiotically or as parasites. The common "yeast infection" Candidiasis is typically caused by the yeast-like fungus Candida albicans. In addition to being the causative agent in vaginal yeast infections Candida is also a cause of diaper rash and thrush of the mouth and throat.

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Fission (S. pombe) and budding (S. cerevisiae) yeasts Credit: Susan L. Forsberg, Nat Rev Gen 2:659

Yeast multiply as single cells that divide by budding (eg. Saccharomyces) or direct division (fission, eg. Schizosaccharomyces), or they may grow as simple irregular filaments (mycelium). In sexual reproduction most yeast 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 yeast, fuse with other ascospores.

EM image of S. cerevisiae Credit: UC Berkeley

The awsome power of yeast genetics is partially due to the ability to quickly map a phenotype producing gene to a region of the S. cerevisiae genome. For the past two decades S. cerevisiae has been the model system for much of molecular genetic research because the basic cellular mechanics of replication, recombination, cell division and metabolism are generally conserved between yeast and larger eukaryotes, including mammals.

The most well-known and commercially significant yeast 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. Yeast is often taken as a vitamin supplement because it is 50 percent protein and is a rich source of B vitamins such as niacin, folic acid, riboflavin, and biotin.

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Dr. Emil Christian Hansen, 1842-1909

In brewing, Saccharomyces carlsbergensis, named after the Carlsberg Brewery in Copenhagen, where it was first isolated in pure culture by Dr. Emil Christian Hansen (1842-1909) in 1883, 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.

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. Sourdough bread, is not produced with baker's yeast, rather a combination of wild yeast (often Candida milleri) and an acid-generating bacteria (Lactobacillus sanfranciscensis sp. nov). It has been reported that the ratio of wild yeast to bacteria in San Francisco sourdough cultures is about 1:100. The C. milleri strengthens the gluten and the L. sanfrancisco ferments the maltose. For more information about sourdough see rec.food.sourdough FAQ.

Wild yeast and acid-generating bacteria are used in making sourdough bread

The fermentation of wine is initiated by naturally occurring yeast present in the vineyards. Many wineries still use natural strains, however many use modern methods of strain maintenance and isolation. The bubbles in sparkling wines is trapped carbon dioxide, the result of yeast fermenting sugars in the grape juice. One yeast cell can ferment approximately its own weight of glucose per hour. Under optimal conditions S. cerevisiae can produce up to 18 percent, by volume, ethanol with 15 to 16 percent being the norm. The sulfur dioxide present in commercially produced wine is actually added just after the grapes are crushed to kill the naturally present bacteria, mold, and yeast.

C. albicans Credit: E. Gueho/Science Photo Library

The yeast-like fungus, Candida albicans, is commonly found in the mouth, vagina, and intestinal tract. Candida is a normal inhabitant of humans and normally causes no ill effects. However, among infants and individuals with other illness a variety of conditions can occur. Candidiasis of the mucous membranes of the mouth is known as thrush. Candidiasis of the vagina is called vaginitis. Candida also causes severe disease in persons with AIDS and chemotherapy patients.

Suggested Reading:

Campbell, I., and Duffus, J.H., eds., Yeast (1988)

Pfaff, Herman Jan, et al., The Life of Yeasts (1978)

The Early Days of Yeast Genetics. (1993) edited by Michael N. Hall and Patrick Linder. Cold Spring Harbor Laboratory Press, Cold Spring Harbor, New York.

Mortimer, R.K., Contopoulou, C.R. and J.S. King (1992) Genetic and physical maps of Saccharomyces cerevisiae, Edition 11. Yeast 8:817-902.

The Molecular and Cellular Biology of the Yeast Saccharomyces cerevisiae: Gene Expression. (1992) edited by Elizabeth W. Jones, John R. Pringle, and James R. Broach. Cold Spring Harbor Laboratory Press, Cold Spring Harbor, New York.

The Molecular and Cellular Biology of the Yeast Saccharomyces cerevisiae: Genome Dynamics, Protein Synthesis, and Energetics. (1991) edited by James R. Broach, John R. Pringle, and Elizabeth W. Jones. Cold Spring Harbor Laboratory Press, Cold Spring Harbor, New York.