Yeast
The Microscopic drivers of fermentation
While malt and hops tend to be in the beer ingredient spotlight, yeast are often beer’s quiet, unsung hero. Yeast drive the transformation of brewer’s wort into carbon dioxide, ethanol, and a range of esters, phenols, and other compounds that, to a large extent, define a beer’s character.
Boiled Down Version
- Yeast are single-cell fungi that convert fermentable sugars in brewer’s wort during fermentation into ethanol and carbon dioxide but also produce smaller amounts of esters (fruity), phenols (spices, medicinal), and other compounds that impact the flavor of beer.
- Beer is broadly divided into two categories based on the yeast species used in fermentation: ales and lagers.
- Ales are typically fermented with Saccharomyces cerevisiae and lagers with Saccharomyces pastorianus
- Ale yeast (S. cerevisiae) prefers fermenting at warmer temperatures (18 – 24°C), while lager yeast (S. pastorianus) prefers fermenting at cooler temperatures (5 – 14°C)
- Beers fermented with ale strains demonstrate more yeast character in the form of esters and other metabolic byproducts that affect flavor. Lagers strains tend to impart a more neutral fermentation character that creates a beer-focused on malt and hops.
- Other fermentation organisms are also used to produce some beer styles, including Brettanomyces yeasts, lactic acid bacteria, Acetobacter, and others that are beyond the scope of this article.
The Role of Yeast in Brewing
Yeast are single-cell eukaryotes that are part of the fungi kingdom. While yeast have been used for thousands of years to make beer, its role in brewing is a relatively recent discovery. In the mid-1800s, Louis Pasteur showed for the first time that alcoholic fermentation was caused by yeast, kickstarting the fields of microbiology and biochemistry (Barnett, 2003).
Yeast convert the fermentable sugars in the wort that the brewer produces into ethanol, carbon dioxide, and a wide range of other compounds in smaller amounts that impact the flavor of beer, like esters, phenols, aldehydes, and more through fermentation.
Ale vs Lager
Beer is broadly classified into two large groups, ales and lagers, defined by the yeast species used to ferment the beer. While there are many different yeast genera, the two most commonly used yeast species in beer brewing are both in the Saccharomyces genus. Ales are fermented by Saccharomyces cerevisiae which typically prefers higher fermentation temperatures, typically in the range of 18 – 24°C (65 – 75°F) (Oliver, 2012, pp. 27-33). Lagers are fermented by Saccharomyces pastorianus, which prefer cooler fermentation temperatures than ales, typically around 5 – 14°C (40 to 58°F).
Ale strains are sometimes called top-fermenting as they tend to flocculate together at the top of a fermenting beer, while lager strains are called bottom-fermenting as they tend to flocculate to the bottom. Generally, ale yeast strains impart more flavor impact on the finished beer than lager strains. Ale strains can produce higher levels of esters (fruity) and sometimes phenolic (spices, medicinal) flavors compared to lager strains that tend to be more flavor neutral, giving focus of the beer flavor to the hops and malt profile.
While Saccharomyces yeasts are typically the fermentation organisms used for making beer, a wide variety of other microorganisms are used for some specialty styles. For example, Brettanomyces yeasts, Acetobacter, lactic acid-producing bacteria, and other organisms can be found in styles like lambics, Flanders ales, and other beer styles, but that is beyond the scope of this article.
To learn more about specific strains, both Omega Yeast and White Labs offer brewing yeast for professionals and homebrewers. Their websites have great information about their yeast strains and brewing yeast in general.
References
Barnett, J. A. (2003). Beginnings of microbiology and biochemistry: the contribution of yeast research. Microbiology. https://doi.org/10.1099/mic.0.26089-0
Oliver, G. (2012). The Oxford Companion to Beer. Oxford University Press.