Yeast

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 bacteria and other fungi, 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.