Tuesday 30 November 2021

How to pressurise & purge keg using CO2 from fermentation

Once you dive into the world of pressure fermentation, it opens the doors to loads of options and possibilities for ways of achieving things when brewing and fermenting.

As we all know, the process of fermentation works by yeast consuming the sugars made available in wort, which then converts the sugar to alcohol. The byproduct of this process is carbon dioxide (CO2). If you are using a pressure capable fermenter, you can capture and harness this CO2 (there is lots of it) for other purposes. 

Such purposes may include pre-pressuring one or multiple kegs so it is purged of oxygen, and is full of CO2 and ready for your pressure transfer of beer after fermentation has completed.

Another common technique is to have your keg filled (or partly filled) with sanitising solution - and then use the CO2 from your fermentation to purge, or push this sanitiser out of the keg and into another keg, or another vessel such as a bucket. Here's how it's done.

What you'll need

  • A pressure capable fermenter (eg. Kegland's Fermzilla) with fermentation happening or about to happen
  • 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
  • Your keg, with sanitising solution in it
  • A spunding valve

How to do it

  1. Put the sanitising solution in your keg and seal it shut. We typically use 1 or 2 litres of solution - you can use as much or as little as you like. Some people fill the keg completely, we just use a couple of litres and shake it in the fermenter to cover and sanitise all parts of it.

  2. Connect the gas outlet on your fermenter (1) to the gas/in post of your keg (2)

  3. Connect the liquid/out post on your keg (3) to the other vessel you will purge the sanitising 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

  4. As the gas from the fermentation is transferred via the line into the keg, the sanitising solution will be forced out (slowly). If using a pressurised bottle as we are, you will need to periodically release the pressure in the bottle to keep the transfer flowing.

  5. As an added bonus, this process sanitises the liquid/out post and dip tube on your keg, as well as the disconnects and line/hose it is being transferred through.

  6. Another option is to leave the liquid/out post (3) disconnected and let the pressure build in the keg. You can then attach the disconnect and have all the sanitiser pushed out at once, and more quickly.

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

  1. Connect the gas/out post on your fermenter (1) to the gas/in post of your keg (2).

  2. Connect your spunding valve to the liquid/out post of your keg (3)

  3. Set your spunding valve pressure

  4. You can leave this connected for the entire fermentation period if necessary. The pressure within the keg 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.

The best thing about this entire process is it's essentially using 'free' carbon dioxide that is created by your fermentation, so reduces the amount of CO2 you would otherwise use from your bottle.

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