Yeast

Yeast, any of single-celled plants classified with the fungi. Some cause diseases of the skin and mucous membranes, while others, notably the strains of Saccharomyces cerevisiae (baker's yeast), are used in baking, brewing, and winemaking. Yeasts employ either or both of 2 metabolic processes: Fermentation involves the anaerobic decomposition of hexose sugars to yield alcohol (ethanol) and carbon dioxide; “respiration” involves the exothermic decomposition of various sugars in the presence of oxygen to give carbon dioxide and water. Yeasts are also grown as a source of food rich in B-complex vitamins.