How to Avoid Krausen Blowout (Fermentation Overflow)

Any seasoned beer brewer has had their fair share of Krausen blowouts. Sometimes these blowouts result in vigorous beer geysers that reach the ceiling! Other times they are gentle, frothing overflows, which cover the floor in sticky, almost-beer. What causes this surprising, bubbly phenomenon?

A nifty timelapse of Krausen blowout with a homebrew setup

What Is Krausen?

Krausen is a foamy byproduct of beer fermentation. After a brewer boils the malt, hops, and barley into the water, they have a liquid known as wort. Wort has all the sugars necessary for the yeast to convert into alcohol. Once the wort is ready, a brewer strains it into a fermentation vessel and adds the all-important yeast.

During this fermentation process, the yeast eats the sugars and turns them into alcohol, carbon dioxide, and that funky stuff on top of the wort, known as Krausen. The Krausen builds as the yeast does its work. If the Krausen builds too fast, it can bubble into the airlock and block carbon dioxide from escaping. The newly produced carbon dioxide then has nowhere to escape. The pressure builds and builds until…boom! A genuine Krausen blowout.

What Makes a Beer More Likely to Overflow?

High gravity beers are at a high risk of Krausen blowout. These beers result in higher alcohol content. The yeast needs to be much more active to produce this additional alcohol. Since Krausen is a byproduct of the fermentation of sugars into alcohol by yeast, a higher alcohol beer will produce more Krausen, thus increasing the probability of a fermentation overflow.

Another cause of blowouts can be brewing an out-of-season beer. Some beers are easier to ferment in the summer and others are better in the winter. For example, lagers prefer colder fermenting conditions of 45-55°F. Ales, on the other hand, do best around 68°F. This means brewing a lager in the summer would result in a more vigorous fermentation, potentially leading to a blowout. Be sure you have the right temperature conditions for the beer you want to brew before brewing it. An ounce of prevention is worth a pound of cure!

Krausen forms at the top of a five-gallon brewing jug. Notice how the vessel is inside a plastic container to make clean-up easier in the case of a Krausen blowout! Photo courtesy of Jswmesq

How Can We Brewers Avoid These Nuisance Blowouts?

There are a few different ways to decrease the risk of fermentation overflow. Controlling the temperature, using a blowoff tube, and not overfilling the vessel are three easy ways to dramatically reduce the risk of a Krausen blowout.

Temperature Control Can Reduce Risk of a Krausen Blowout

An effective way to ensure a more reliable fermentation is to control the temperature of the wort. A typical 5-gallon fermentation can increase the temperature of the wort by 10-15°F. Yeast works faster at higher temperatures. This means that Krausen builds up quicker the warmer the wort is. If the brewer doesn’t take precautions to reduce the wort temperature during fermentation, they could have a blowout.

Fortunately, there are many ways to cool off warm wort. A wet cloth around the brewing vessel can suck off some heat. Some people place the vessel in a bucket of water and add ice periodically. The most accurate way to control the temperature is to brew the beer in a space with air-conditioning and a sort of thermostat, such as the Coolbot.

homebrew, fermentation
A homebrew set up with plenty of Krausen developing. The tube coming out of the carboy on the right is a blowoff tube, whereas the carboy on the left has a traditional airlock. Notice how the blowoff tube empties into a pitcher. Photo by Makyo

Use a Blowoff Tube to Prevent Fermentation Overflow

Traditional airlocks are a culprit for fermentation blowout. If the Krausen develops too quickly, it can bubble up into the airlock and prevent it from letting air out. The pressure inside the carboy will then increase until it blows the airlock off the top.

A cheap alternative to the traditional airlock is a blowoff tube. This tube attaches to the top of a carboy and empties into another vessel. This way, any extra Krausen flows through the tube and into a separate vessel, which is filled with a sanitary solution. This process substantially decreases the likelihood of a blowout because it is much more difficult for pressure to build up.

Don’t Overfill Your Vessel

Most glass carboys are 6 or 6.5 gallons. It’s risky to fill these carboys with more than 5 gallons of beer. You must leave some headspace for the Krausen to develop. In most cases, 1 or 1.5 gallons of space should be enough to prevent the Krausen from creeping into the airlock. However, sometimes brewers accidentally make more beer than they anticipated, and they get greedy by trying to fill their carboys just a little bit more. This is risky behavior! Sooner or later, you will experience a krausen blowout!

fermentation, krausen
An industrial brewing set up with lots of Krausen floating on top. Looks tasty! Photo by Petr Kratochvil

Enjoy Your Brewing!

Remember, for most people beer brewing is a fun hobby. Don’t be discouraged if your beer ends up on the ceiling, or if you end up spending a few hours cleaning up a big mess. This is all part of the fun, and it gives you stories to share with fellow brewers. Now go make some tasty brews!

Different types of tasty beer!

Article written by Evan Levy on August 11, 2020

[Featured image courtesy of Jinx!]

Comments 4

    1. Temperature? Temperature is the best way to control active fermentation and make the yeast happy. I don’t see how that would do anything other than increase the possibility of a blowoff.

      1. Hi Dave,

        Thanks for writing in and checking out the CoolBot as a brewery cold room solution. I believe the gentleman may be referring to lowering the fermentation temperature to rein in or prevent an overactive fermentation, since generally letting the temperature increase out of control would increase the possibility of a blowout, as you well said.

        I hope that helps, thanks again for including the CoolBot team in your project, and please feel free to reach out again for any tips or brainstorming to set up your CoolBot beer cooler or fermentation chamber, we’re happy to assist.


        Guth B
        Customer Support

  1. Pressure fermentation diminishes Krausen. If I use the active fermentation to clear a keg on sanitiser Krausen nearly hits the top, under 10psi it’s 2cm max.

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