Beer Class: Yeast, the unsung hero of the brewery (Part 1)
The discovery of yeast, an often underestimated ingredient of the brewing process.
For many beer lovers, the star players in their favorite brews are hops and malt. Hops contribute bitterness and aroma, malt provides the sugars and body. But there's another, often understated, ingredient that is at the heart of the fundamental transformation that creates the tasty beverage we know and love: yeast. This microscopic marvel, a single-celled fungus, is the unsung hero, the silent magician behind every sip.
Fermentation is an essential part of brewing and an old adage says ‘the brewer makes wort, but the yeast makes beer’. Without it, you would simply have a sugary malt tea, essentially. This little overachiever consumes the fermentable sugars in the wort and, through a metabolic process, converts them into ethanol (alcohol) and carbon dioxide.
The brewery’s hardest worker
Impressive as that is, yeast's contribution extends far beyond just producing alcohol. If the yeast is able to consume all the sugars, it will result in a drier, lighter-bodied beer. This is called ‘attenuation’. Yeast with lower attenuation will leave more residual sugars, leading to a fuller and sweeter beer with more so-called mouthfeel, like some British ale styles.
Yeast also plays a role in a beer’s clarity as it has a tendency to clump together, known as flocculation. Some yeast strains are highly flocculent, resulting in brighter beers, while others remain in suspension, contributing to the hazy character of styles like New England IPAs.
Yeast as a flavor enhancer
But did you know yeast is a major contributor to the final flavor profile of beer? While busily fermenting, it finds the time to produce a wide range of byproducts, including aromatic compounds like esters and phenols. Esters are often fruity, with notes of apple, pear, or banana, and are a key characteristic of many ale styles. Phenols can contribute flavors like clove, spice, or even smoke. Sometimes phenols are desirable, like the signature spicy or clove character of German Weizens, but often they are not.
The specific flavors produced are dependent on the yeast strain used, the fermentation temperature, and the wort's composition. Warmer fermentation temperatures generally lead to higher ester production. Different yeast strains produce different flavor profiles, which is why a Belgian ale tastes different from a German lager, even if they're made with similar ingredients.
Brewer's yeast or baker's yeast?
A common question is: if bread dough is made with yeast, why doesn't it contain alcohol? The answer lies in the baking process. While the yeast in the dough does produce a small amount of alcohol during fermentation (the rising phase), the high temperatures of the oven during baking cause the alcohol to evaporate completely. The yeast itself is also killed by the heat. What remains is an airy loaf of bread, thanks to the carbon dioxide bubbles produced by the yeast.
Bakers once used brewing yeast, which was often sold from breweries. However, over time, specific strains of yeast were isolated and cultivated enable a faster baking process. Other breads that don’t use baker’s yeast, like sourdough, take much longer to make.
The discovery of yeast, a real game changer
No article on yeast is complete without the intriguing story of its discovery. Louis Pasteur is credited with recognizing the role of yeast in making beer, but it was actually Dutchman Antonie van Leeuwenhoek who first observed it under his microscope in 1680.
By that time we had known for millennia that there was something in the beer that made it ferment, which is why the techniques of collecting and repitching yeast were already quite advanced. It was attributed to something mysterious and known as ‘Godisgoode’. That is why it wasn’t specifically mentioned along with water, hops and malt in the German Reinheitsgebot (Purity Law).
Capturing wild yeast
In traditional brewing wooden beer sticks, also known as a yeast log or yeast ring, were used to cultivate and capture wild yeast from the air to inoculate wort for fermentation and reuse over generations.
This is still used in traditional Scandinavian brewing to preserve their Kveik yeast. In many Belgian lambic breweries, such as Cantillon, yeast is revered in similar fashion. Though your first impulse when visiting may be to pick up a broom and start cleaning, the wood where yeasts have made their home is sacred and it is left untouched.
What role did Louis Pasteur play?
As with many scientific discoveries, Pasteur did not figure out the role of yeast completely on his own. Theodore Swann proved in the 1830s that fermentation was a biological process caused by these living yeast cells, not a spontaneous chemical reaction as was widely believed at the time. A combination of Pasteur’s rock star status and his scientific prowess led to the world accepting the fact that yeast are living organisms that can produce alcohol in the absence of oxygen, but were also the cause for beer spoilage.
Though yeast was now better understood and possibly a bit domesticated, it was still made up of multiple strains and included bacteria as there was not yet a way to properly control it. Emil Hansen of the famous Carlsberg brewery discovered in 1883 how to isolate the yeast cells and make the strains pure, thereby curing the much dreaded ‘beer sickness’ that was ailing many breweries and also Carlsberg at the time. Since then many yeast strains have been developed and today over 500 are commercially available for brewing.
Author: Tina Rogers
Picture credits (from top to bottom):
- Banner picture: Coolship Spontaneous Fermenting at Allagash Brewing, USA CC BY 2.0
- Visitors watching beer fermenting in a large brewhouse, wood-engraving c. 1847, CC BY 4.0
- Production process of bread, picture by Petr Brož, CC BY 3.0
- Antonie van Leeuwenhoek (1632-1723), painting by Jan Verkolje.
- Yeast ring from 1877 or earlier, Nordic Museum, Sweden, CC BY 4.0
- Louis Pasteur (1822-1895), painting by Albert Edelfelt.
- I. Yeast species in their development. II. Disease enzymes in beer. Page 80 from Ernst Keil's journal Die Gartenlaube (The Garden Arbor), 1882.
Updated: 12-12-2025
