Myth vs. Science: A Practical View on Lager Brewing

Sam Tierney, head brewer at The Propagator—Firestone Walker’s innovation brewhouse in Venice, California—separates conventional wisdom from cool science when it comes to brewing great lager. Proper technique is the key.

Sam Tierney Oct 1, 2021 - 7 min read

Myth vs. Science: A Practical View on Lager Brewing Primary Image

Photo: Courtesy Firestone Walker

As lager brewing has become more widespread in craft brewing over the past few years, brewers have looked to the past to decipher the mysteries of lager and the traditional methods used to produce it. From inherited wisdom about lagering a week for every degree Plato to seemingly arbitrary fermentation-temperature programs that take weeks to complete, it can be tough to separate myth from science.

Traditional lager brewing consists of interrelated techniques, each dependent on the others to create a system effective at producing quality beer within particular historical constraints. Using modern brewing science and engineering, we can assess these techniques on their individual merits and determine which are still needed to produce the highest-quality beer—and which can be modified or discarded.

Many brewers still think that traditional lagering times of two to three months, even for lighter beers such as helles and pilsner, produce better lagers. Is it true? To figure that out, we need to explore what lagering actually is and what it accomplishes.

Lagering—from the German lagern, or “to store”—is generally understood as the cold-conditioning period after primary fermentation. Traditionally, brewers lagered beer in lined wooden barrels in ice-cooled cellars and later in horizontal steel tanks in refrigerated cellars. Only more recently did some brewers move to jacketed unitanks for primary fermentation as well as lagering. The traditional goals of lagering are to carbonate, clarify, and mature the flavor of the beer. Let’s break each of these goals down and see how traditional and modern methods affect each.



In traditional two-vessel fermentation systems, lager fermentation starts in an open-top tank. When primary fermentation nears completion, the brewer transfers the beer to a horizontal tank in an ambiently cooled cellar, then seals the tank to build pressure. As the beer finishes fermentation in the secondary tank, the temperature slowly drops. The slow cooling rate is key here: If you were to crash-cool the beer in a day, as you can with a jacketed tank, you would shock the yeast and stop the fermentation before it could finish carbonating the beer. You would also leave elevated levels of diacetyl in the beer. Lager yeast continues to be active to around 40°F (4°C), completing the carbonation and maturation. This allows for fully carbonated beer in a tank that can only hold up to about one bar of pressure (14.5 PSI), but it also adds about a week of time between primary fermentation and the final cold-lagering phase.

That way of naturally carbonating a beer via spunding—bunging the tank before the end of fermentation—is key for German brewers constrained by the Reinheitsgebot. However, modern American brewers are free to add carbonation with pure CO2 later in the process if they wish. Brewers will argue over which methods provide the best mouthfeel, but excellent beers can be made via spunding, as well as by adding head pressure while lagering or with pinpoint injection systems post-lagering. For brewers who do wish to spund their lagers, modern tanks rated up to 25 PSI allow for full carbonation even while finishing fermentation at diacetyl-rest temperatures of up to 60°F (16°C). Employing an elevated-temperature diacetyl rest allows for a shorter primary fermentation and a faster transition to your eventual lagering temperature.


Clarification is also one of the main goals of traditional lagering, and modern advances allow various approaches not available to brewers of the past. Before advances in clarification methods, horizontal tanks were seen as crucial to efficient lagering—though it would still take weeks for a beer to clear on its own. Today, centrifuges, filters, and even fining agents such as Biofine Clear make rapid clarification possible. Horizontal tanks only really make sense if you like the way they fit in your cellar or if you wish to achieve bright beer without more modern clarification methods.


Flavor maturation is where all the moving parts start to come together. As noted above, for carbonation, we no longer need to slowly chill our beer while spunding; that allows elevated diacetyl-rest temperatures and faster maturation times. Sulfur slowly dissipates during extended lagering, but overseeing a healthy primary fermentation allows for appropriate sulfur levels before heading into lagering. Some fusel alcohols and acids slowly convert into esters during lagering, smoothing out some of the harshness associated with high-gravity fermentations. This aspect of flavor maturation is key to the development of stronger styles such as doppelbock—but for lighter styles such as pilsner, the effect is negligible, and it may even be detrimental to the fresh character desired in the style.

While lighter lagers may taste more “mature” after two months of lager time, they can lose the freshness and vibrancy that we prefer at Firestone Walker.

The Firestone Walker Approach

So, what does this all mean to the modern craft brewer? We find that using a 34/70 derivative yeast at a primary fermentation temperature around 50°F (10°C), with a diacetyl rest in the high-50s Fahrenheit (14–15°C) allows for lager fermentations that are fully complete in about 10 days and ready for lagering.

A key to optimizing this performance, and producing beers with appropriate sulfur levels, is using healthy yeast at precise pitching rates and wort oxygenation levels. We bung our tanks to 1 bar (14.5 PSI) at the end of fermentation; combined with steady head pressure during lagering, this produces nearly carbonated beer. We then top it up with a small amount of CO2 in our clarification system, if needed. After a 30–32°F (-1–0°C) lagering period of one to three weeks—the length depends on the beer—we then centrifuge and filter our lagers with diatomaceous earth. This makes for brilliantly clear beers with good haze stability.

While longer lagering times may still make sense for higher-ABV styles, modern advances in science and technology have almost eliminated the need for long tank times in lighter styles such as pils and helles. When it’s time to choose how long to lager, keep in mind your flavor goals and how each technique affects your overall timeline.

And remember, let your palate be your guide—if your beer tastes ready, it is ready!