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 50% 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.