yeast infection: see candidiasis.

yeast, name applied specifically to a certain group of microscopic fungi and to commercial products consisting of masses of dried yeast cells or of yeast mixed with a starchy material and pressed into yeast cakes. Although a number of fungi are sometimes called yeasts, the true yeasts are unicellular, consist of oval or round cells, and reproduce chiefly by budding. Under certain conditions some yeast cells secrete a thickened wall, and the cytoplasm of the single cell within divides to form four or eight cells, or spores, known as ascospores, which emerge when the wall ruptures. In a few species two cells fuse before undergoing spore formation. There are about 500 species in all.

Yeasts, especially those of the genus Saccharomyces, have long been of commercial importance because they are the chief agents in alcoholic fermentation. Because of this they are essential to the making of beer, wine, and other alcoholic beverages and industrial alcohol. Wild yeasts, those found in nature and probably carried by insects from the soil to fruits, are frequently active in the fermentation process. In breadmaking the yeasts act upon the carbohydrates in the dough, forming carbon dioxide and ethyl alcohol, which are driven off in the baking process; the escaping carbon dioxide causes the bread to rise. Since early times yeast has been used in treating various ailments; brewer's yeast has a high content of thiamine and other vitamins of the B-complex group. Yeasts are classified in the kingdom Fungi, phyla (divisions) Ascomycota and Basidiomycota.

brewer's yeast: see yeast.

Any of certain economically important and usually single-celled fungi (see fungus), most of which are classified as ascomycetes. Found worldwide in soils and on plant surfaces, yeasts are especially abundant in sugary mediums such as flower nectar and fruits. The types commonly used in the production of bread, beer, and wine are selected strains of Saccharomyces cerevisiae; the small cakes and packets used contain billions of individual yeast cells, each of which can ferment approximately its own weight of glucose per hour. Dried yeast is 50percnt protein and is rich in B vitamins; brewer's yeast is sometimes taken as a vitamin supplement. Some yeasts are mild to dangerous pathogens of humans and other animals. Candida albicans, for example, irritates oral and vaginal linings, and Histoplasma and Blastomyces cause persistent lung infections.

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