is any one of a number of yeast strains belonging to the species Saccharomyces cerevisiae, used to brew pale, brown, and dark ales, wheat beers, Belgian ales, and many other types of beers. Aside from lager beers and beers brewed by spontaneous fermentations, essentially all beers are brewed with ale yeast as the fermenting organism. Ale yeasts are genetically distinct from lager yeasts in that the latter are hybrid organisms formed by mating between two closely related Saccharomyces species, whereas ale yeasts appear to be purely S. cerevisiae. In contrast to lager yeasts, which tend to gather toward the bottom of the fermentation vessel, ale yeasts are characterized as “top fermenters” because many (but not all) form a thick layer of foam at the top of the wort during fermentation. See lager yeast. However, a better distinguishing characteristic between these two types of yeast is the temperature at which they best ferment: ale yeasts carry out fermentation at moderate temperatures of 18°C to 24°C (65°F to 75°F), whereas lager yeasts are able to ferment at much colder temperatures, 5°C to 14°C (40°F to 58°F). Compared with lager yeasts, ale yeasts also tend to produce more of the esters that lead to fruity and/or complex flavors and aromas; the higher ale fermentation temperatures can also accentuate this tendency. Ale yeasts comprise a diverse group of strains with many different attributes, as can be seen by even a casual perusal of the catalog lists of many of the commercial beer yeast producers. At least 180–200 different ale yeast strains are commercially available, each with different advertised fermentation characteristics and/or flavor/aroma profiles; however, it is not known whether many of the strains are similar or even identical to each other.

See also ale, yeast.