Birallee Beer & Brewing: How to Pressurize and Purge Kegs Using CO2 from Fermentation: A Step-by-Step Guide

Once you dive into the world of pressure fermentation, it opens the doors to loads of options and possibilities for achieving things when brewing and fermenting. In this post we're going to look at how you can pressurize and purge your kegs with carbon dioxide (CO2) gas created during fermentation, saving you from wasting precious CO2 from your gas bottle.

Introduction

Fermentation involves yeast consuming the sugars made available in wort, converting them to and producing CO2 as a byproduct of this process. Typically, this CO2) is expelled via the airlock of your fermenter. However, if you are using a pressure capable fermenter, you can capture and harness this CO2 (there is lots of it) for other purposes.

Why Pressurise & Purge Your Kegs with CO2?

Oxygen is the greatest nemesis of finished beer, as even the smallest amount coming into contact with it can cause the staling effects of oxidation. Typical symptoms of oxidation include discoloration of the beer, giving it a brown and murky appearance, and a sherry like or wet-cardboard flavour developing. Hop forward styles like IPA's are especially prone to oxidation, but it can impact any beer style.

Pre-pressurizing and purging your serving keg using the CO2 created during fermentation, you can;

Remove all oxygen from within the keg

Have the keg pressurised and ready to perform a pressure transfer from your pressure fermenter

What You Need

A pressure capable fermenter (eg. Kegland's FermZilla).

Actively fermenting wort to generate the CO2 we'll be harnessing.

Gas line and disconnects to connect the gas output/post on your fermenter to the gas post on your keg

Beer line and disconnect(s) from your keg to your other keg (if transferring to another keg), or otherwise, another vessel for the sanitising solution to be displaced into

A ball lock keg with sanitising solution inside

A secondary vessel (keg, bucket, soda bottle with carbonation cap attached)

A spunding valve

Steps to Pressurise & Purge Keg using CO2 from Fermentation

Prepare the Fermenter: Before beginning, you need to have actively fermenting wort in your pressure fermenter, and some pressure already built up within it by connecting a spunding valve. Around 10-15psi is a good starting point.
Sanitize the Keg: Add the sanitising solution to your keg at the recommended dilution level and fit the lid to seal it. Some people fill the keg completely, we just use a couple of litres and shake it in the fermenter to cover and sanitize all parts of it.
Connect the Gas Line: Remove the spunding valve from the fermenter, then connect the gas out post on your fermenter (1) to the gas/in post of your keg (2) as depicted in the image below. You should hear gas filling up inside the keg as the pressure is equalized between the two vessels.
Purge the Sanitizing Solution: Connect the liquid/out post on your keg (3) to the other vessel you will purge the sanitizing solution into (4).
In our example photo above, we're using a soft drink/soda bottle with a carbonation cap attached so we can use a liquid disconnect . Alternatively, you can opt to have the end of the hose sitting loose in a bucket, or you can use a disconnect and attach it to another keg.
Release the Pressure: The sanitizing solution will now be forced out of the keg via the line that has been connected. If you're using a pressurized vessel as we are (soda bottle with carbonation cap), you will need to periodically release the pressure to keep the transfer flowing. If you've just got an open ended beer line then the sanitizing solution should flow straight out.
Bonus: This process sanitizes the liquid/out post and dip tube on your keg, as well as the disconnects and line/hose it is being transferred through.
Rebuild Pressure: If you had a decent amount of pressure before beginning, and depending on how much sanitizing solution you're using, you may be able to expel all the solution out of the keg in one go. If you run out of pressure, simply leave the liquid/out post (3) disconnected and let the pressure build up again in the fermenter and keg and repeat steps 4 and 5 above.

Once your keg has been purged of sanitizing solution you should ensure it is completely purged of oxygen. For this next phase, we'll essentially replace the connection at number 3 with a spunding valve

Connect Gas Line: Connect the gas/out post on your fermenter (1) to the gas/in post of your keg (2).
Attach Spunding Valve: Connect your spunding valve to the liquid/out post of your keg (3)
Set Spunding Valve Pressure: Set your spunding valve pressure so it maintains a constant pressure within the keg and pressure fermenter.
Maintain Pressure: You can leave this connected for the entire fermentation period if necessary. The pressure within the keg and fermenter will continue to build until it reaches the pressure set on the spunding valve. Once this pressure is reached it will constantly vent excess pressure as required to maintain it's set pressure. Since carbon dioxide (CO2) is more dense than oxygen, the oxygen will always be forced to the top of the keg and out through the spunding valve first.

Once completed, your keg is ready to receive a pressure transfer from your fermenter which is the best way to get your beer into your keg without introducing any oxygen. Check out our guide on how to perform a pressure transfer below.

Conclusion

This is our standard process that we use for every batch of beer that we make. What we particularly love about it is we're using free carbon dioxide gas created by the fermenting yeast. By following these steps, you can effectively pressurize and purge your kegs, ensuring your beer remains fresh and oxidation-free.