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 yeasts are true fungi of the phylum Ascomycetes, class Saccharomycetes (also called Hemiascomycetes). The true yeasts are separated into one main order Saccharomycetales.
Yeasts are characterized by a wide dispersion of natural habitats. Common on plant leaves and flowers, soil and salt water. 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. The common "yeast infection" is typically Candidiasis is 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.
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. 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.
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 sanfrancisco 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.
The fermentation of wine is initiated by naturally occurring yeasts 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, molds, and yeasts.