Although the word yeast often refers to Saccharomyces cerevisiae, the yeast typically employed in the production of beer, bread, and wine, yeasts are actually a broad category of unicellular fungi which reproduce by budding or fission. They may be generalists or highly specialized, and have been found to occupy an enormous range of habitats: Soil, ripe fruit, the human body, and even the upper levels of the atmosphere. Yeasts naturally exist in mixed communities with other yeasts as well as non-yeast fungi and bacteria, although they may be manipulated by humans as monocultures. Although the most famous example is Saccharomyces there are many other genera of yeast which can produce alcohol from sugars. Brettanomyces is the most famous example of non-Saccharomyces yeast which is used in beer production. In wine production, a much wider variety of yeasts have been employed (often unintentionally), and many have been found to contribute positive characteristics including esters, phenols, and glycerol.
Yeast may belong to one of two phyla: Basidomycota or Ascomycota. Saccharomyces, Brettanomyces, and most of the wild yeasts relevant to brewers and wine makers belong to the phylum Ascomycota, with a few exception. When the specific name of a yeast is given (e.g. Saccharomyces cerevisiae) the first word is the name of the Genus to which the yeast belongs, and the seconds word is the yeast's specific epithet. The genus name is often abbreviated after it has been made clear (e.g. S. cerevisiae). The rules governing yeast taxonomy and nomenclature are laid down by the most boring group of people in the world, who operate under the alias "International Botanical Congress". Some commercial yeast strains are marketed under names which appear to be species names, but are not actually recognized (e.g. B. lambicus marketed by White Labs; actually a strain of B. bruxellensis).
Teleomorphs and Anamorphs
Some yeasts have sexual and asexual forms, which are morphologically and metabolically different but genetically the same. Each form has a different name. Often the genus name changes and the specific epithet does not (e.g. Brettanomyces bruxellensis and Dekkera bruxellensis). Anamorphs are the asexual form, which reproduce clonally. Teleomorphs are the sexual form. Saccharomyces yeasts have no Teleomorphic genus, although they do sporulate to reproduce sexually. Brettanomyces yeasts are always belong to the genus Dekkera in the Teleomorphic form, while Candida teleomorphs belong to many different genera.
When defined species hybridize the resultant yeast should technically be called by the name of both species with a "x" between. A cross between Saccharomyces cerevisiae and Saccharomyces eubayanus would be written "Saccharomyces cerevisiae x Saccharomyces eubayanus". In reality hybrids may be more complex than a single cross. Lager yeast is believed to have S. eubayanus and S. cerevisiae in its parentage, and it is often known by the name S. pastorianus, although it may not constitute a true species.
Use in beer production
Killer Yeast Phenomenon
See the Packaging Page.