Beer Class: Yeast, the unsung hero of the brewery (Part 2)
The role of yeast in the brewing process
Ales versus Lagers
The world of brewing yeast is incredibly diverse. To make life simpler, they are broadly categorized into two main types: ale and lager yeast strains. Ale yeast strains are top-fermenting yeasts known as ‘Saccharomyces cerevisiae’, meaning they typically rise to the surface of the wort during fermentation. They generally prefer warmer fermentation temperatures (18-24°C) and produce a wider range of esters and phenols, contributing to the characteristic fruity and complex profiles of ales like IPAs, stouts and Belgian tripels.
Lager yeast strains are bottom-fermenting yeasts known as ‘Saccharomyces pastorianus’, preferring cooler temperatures (7-15°C) and typically settling at the bottom of the fermenter. They produce a cleaner, crisper fermentation profile with fewer esters and phenols, allowing the malt and hop characteristics to shine through in lagers such as your favorite pilsner.
Ale, a historic beer
For the most part, historical beers were all ales, and the yeast used was a complex and wild mix of microorganisms. Before the 16th century, the concept of a "lager" as we know it didn't exist. All beers were essentially what we now call ales, fermented with Saccharomyces cerevisiae or other top-fermenting yeast strains.
These yeasts were not a single, pure strain. They were a mix of whatever microorganisms were present in the air, on the grains, and on the surfaces of the brewing equipment. The fermentation was therefore spontaneous, relying on the microflora of the local environment. This is why historical beers would have had a unique "terroir"—a taste profile reflective of their specific geographical location.
The origin of lager yeast
The discovery of the lager yeast strain is a fascinating story. For centuries, brewers in Bavaria and other parts of Central Europe fermented their beer in cold cellars and caves, a process they called "lagering" (German for "to store"). Unbeknownst to them, the cold environment favored the unique Saccharomyces pastorianus strain, which was likely a hybrid of Saccharomyces cerevisiae and another wild yeast (Saccharomyces Eubayanus).
The isolation and cultivation of this specific strain in the 19th century revolutionized brewing, leading to the widespread production of the clean, crisp beers we know today. Beyond these two broad categories, there are countless specific strains, each with its own unique characteristics and flavor contributions. Brewers meticulously select yeast strains based on the desired beer style and flavor profile.
Yeast’s playground: the fermentation vessel
Before modern conical fermenters, many breweries used traditional methods like open fermentation. This involved fermenting beer in large, shallow, open vessels. While this practice exposed the beer to airborne wild yeasts and bacteria, it was a common method in many parts of the world. This practice is still used by Belgian Lambic brewers in the region of Brussels.
A notable English example of this is the Yorkshire Square system, a two-story fermentation vessel used in some English breweries. Sam Smith’s Organic Pale Ale is still made that way. The wort is pumped into the upper, shallow vessel, where the yeast begins to ferment. As the yeast grows and rises, it's skimmed off or pushed down through a central "swan neck" tube to the lower, deeper vessel. This method allows for a very specific yeast management process and contributes to a distinctive flavor profile.
Trumer Brewery in Obertrum, Austria, uses open fermentation tanks as part of its traditional brewing process for its signature beers, which they call ‘slow brewing’. While less common today due to sanitation concerns, it remains a testament to the ingenuity of early brewers.
The secret world of yeast
For a brewery, its yeast strain is a unique and often fiercely guarded asset. The specific strain of Saccharomyces cerevisiae used by a brewery like Guinness, for example, is what gives their stout its distinctive character. Brewers practice yeast propagation, a process of cultivating a new, healthy batch of yeast from a small starter culture to ensure a consistent and reliable fermentation. I heard (fact or fiction?) that Duvel uses a different yeast to bottle condition their beers than the yeast they brew with, to prevent homebrewers from propagating their yeast. Similarly, the ale yeast used by Orval seems to be a well-kept secret making it challenging to emulate their recipe.
The proprietary nature of these yeasts is so significant that many breweries store their cultures in secure yeast banks—essentially biological vaults—often in multiple locations to safeguard against contamination or loss. Well-known examples are the yeast bank of Weihenstephan near Munich and the University of Leuven in Belgium, who are guarding the soul of many famous and beloved beers. This ensures that their unique flavor profile can be maintained for generations, protecting their brand identity, legacy and consistency of your favorite beer!
Author: Tina Rogers
Picture credits (from top to bottom):
- Banner picture: Fermentation tank at De Kromme Haring Brewery in Utrecht (Netherlands), picture by Tasty Tales
- Russian Imperial Stout at 100 Watt Brewery in Eindhoven (Netherlands), picture by Tasty Tales
- Figure 2 of yeast cells image from Industrial Alcohol by John Geddes McIntosh (Public Domain)
- Conical fermenters at Walhalla Brewery in Amsterdam (Netherlands), picture by Tasty Tales
- Trumer Brewery, Obertrum (Austria), picture by Tina Rogers
- Orval Trappist beer, enamal advert, Musée de la Bière, Stenay (France) CC0 1.0
Updated: 07-01-2026
