Over 1,500 species of yeast have been identified. These are predominantly single-celled fungal microorganisms able to grow in both the presence and absence of oxygen. Of these, there are basically two major strains used in brewing: Saccharomyces cerevisiae (ale) and Saccharomyces pastorianus (lager). Ale yeast operates at around room temperature (10–25°C, 50–70°F), ferments quickly, and produces the “fruitiness” characteristic of most ales. Lager yeast works at colder temperatures (7– 15°C, 45–59°F), ferments slowly, and utilizes more wort sugars, leaving a cleaner, crisp taste. Ale and lager yeast are the most commonly used worldwide, but the increase in craft brewing has led to a rise in the use of other yeast strains such as Brettanomyces spp., which are traditionally used in Lambic beer production.
Ale yeast strains are best used at temperatures ranging from 10 to 25 degrees C (50–70°F), though some strains will not actively ferment below 12 degrees C (54°F) (Kunze, 1996). Ale yeasts are generally regarded as top-fermenting yeasts since they rise to the surface during fermentation, creating a very thick, rich yeast head. That is why the term “top-fermenting” is associated with ale yeasts.
Lager yeast strains are best used at temperatures ranging from 7 to 15 degrees C (45–59°F). At these temperatures, lager yeasts grow less rapidly than ale yeasts, and with less surface foam they tend to settle out to the bottom of the fermenter as fermentation nears completion. This is why they are often referred to as “bottom” yeasts.
Although most breweries use pure yeast cultures for fermentation, spontaneous or mixed fermentation is still used for some specialty beers. These kinds of fermentations usually involve a mix of different yeast species (and bacteria as well) that appear sequentially overtime, giving the beer an added complexity. For example, Brettanomyces yeast species are commonly present during the later stages of Belgian ale styles—Lambic, Gueuze, and Flanders red ales..
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