Homebrew Beer Coming Out of the Airlock (How To Deal With a Blowout!)

The first two days of fermentation is an exciting time for homebrewers because there is so much activity to watch but extremely active fermentations can cause beer, foam, and yeast to come out of the airlock, risking a blockage and blowout.

Homebrew beer can come out of airlocks during active fermentation because the explosive growth of yeast and excess carbon dioxide inside the wort can quickly expand to overfill the available headspace inside the fermenter. Using a blow-off tube will allow foam, beer, and yeast to exit the fermenter safely and avoid clogging an airlock.

It’s super common for new homebrewers to underestimate how fast headspace inside a fermenter can fill up and, unfortunately, it usually results in a messy blowout. Let’s go into this topic in more detail and learn how to prevent it!

Why your airlock is overflowing

With all of the activity surrounding brew day, it is easy to forget that fermentation is one of the most active stages of the homebrewing process and it can be quite surprising just how powerful those yeast can react to the sugar inside your wort.

During active fermentation, yeast are devouring the yeast inside the wort and converting it to alcohol and CO2 (among other things). With all of the food readily available, they also reproduce like crazy. This reproduction, coupled with the excess gasses being emitted, can fill up headspace inside a fermenter quickly and cause your beer to escape the only way it can – out of the airlock.

Unfortunately, while an airlock is meant to allow excess gas to escape without issue, they are not designed to handle solid material passing through them.

Airlocks that get yeast or other solid material inside them can quickly become blocked which causes pressure to build up inside the fermenter as the gas inside has nowhere else to go. If the pressure becomes great enough, it can actually pop the airlock and/or the bung out of your fermenter and cause a small explosion of foam, yeast, and beer.

This issue is most likely to happen during the first 48 hours of fermentation at the highest levels of activity. Once things calm down, it is unlikely that you will have any issues with an airlock overflowing with beer.

Homebrew is foaming out of the airlock

Depending on the intensity of your fermentation and the amount of headspace in your fermenter, you might just have a little foam coming out of your airlock and not necessarily beer or yeast.

But, is that safe?

Foam coming out of your airlock is a sign that there is not enough room inside the fermenter to contain the fermentation and steps should be taken to prevent a blowout even if it looks as though your airlock is working fine. As foam does contain some solid material, it is only a matter of time before your airlock becomes clogged.

So, whether you have beer, yeast, or just foam coming out of your airlock you will need to take action.

Let’s see what to do.

How to deal with an airlock blowout

Fortunately, dealing with a blowout is quite easy.

If you have beer, foam, or yeast coming out of your airlock you should replace the airlock with a blow-off tube. If you don’t have access to a blow-off tube, simply remove the airlock completely but leave the bung inside the fermenter opening. Immediately after the fermentation process has slowed down you should clean, sanitize, and replace the airlock inside the bung.

A blow-off tube can take many forms but typically it’s just a piece of 1/2 inch vinyl tubing inserted into a special bung or fermenter cap that lets gas and physical material exit the fermenter more easily when there is a build-up.

Yes, using a blow-off tube technically means that there is nothing preventing bacteria from entering the fermenter but active fermentation means that there will be a lot of positive pressure coming out of the fermenter and it will be very unlikely that anything will make its way INTO the fermenter at this time.

Don’t wait to do this, by the way. Once the airlock gets clogged you will risk an explosion of beer from the fermenter in the very near future.

As soon as possible, just reinstall the airlock to keep any harmful bacteria or wild yeast out of your fermenter and let the process finish naturally.

To prevent this from happening in the first place, I would actually recommend picking up a fermenter cap, airlock, and blow-off tube combo that allows you to easily use a blow-off tube early on and then transition to an airlock later.

Here is your shopping list:

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By the way, I have another resource on this topic if you want to learn more about the best airlocks for homebrewing beer.

Changing an airlock during fermentation

Are you worried about swapping out airlocks during fermentation?

While airlocks are designed to keep harmful bacteria out of your homebrew, a quick swap with a clean and sanitized airlock is very unlikely to cause any issues. To be sure, spray the area around the airlock with sanitizer and be sure that you have sanitized the bung (including inside the hole) and all parts of the airlock itself.

Overall, allowing your fermenter to explode will be much more dangerous for your batch than simply swapping out an airlock because if the explosion happens when you aren’t home you will leave the rest of the homebrew exposed for a long period of time.

At least with an airlock change, you’ll be able to control all of the variables!

Can you brew beer without an airlock?

While using a blow-off tube and airlock combination is the preferred method, you might hear about some homebrewers that prefer to use just a blow-off tube or maybe even no airlock at all.

Fermenting beer without an airlock will always expose your beer to harmful outside bacteria and wild yeast that could cause an infection. In general, there should always be an air gap between the outside and inside of your fermenter to ensure the highest quality beer.

Keep in mind, however, that there are some styles of beer that actually want to become infected such as farmhouse ales or lambics. In these situations, however, the brewer will usually introduce the specific bacteria or yeast they want to use and still control the fermentation with a proper airlock setup.

In all cases, I would recommend the setup I mentioned before with the fermentation cap, the blow-off tube, and the 3-piece airlock as it will give you the most flexibility and ease of use!

By the way, I have another resource on this subject if you want to read more about fermenting without and airlock.