Repurposing and Conserving CO2 During Kegging

With the current recommendations and restrictions on leaving the house, I’m trying to conserve basic brewing necessities as much as possible. This includes propane and CO2–neither of which can be ordered online (and really don’t count as necessities in the same way that groceries do, so it’s hard to justify many extra trips out to get more).

Keg purging is one brewing task that’s non-essential but nice, in terms of long-term beer quality. By keg purging, I mean replacing the ambient atmosphere in a keg (the stuff we breathe) with CO2 from a tank, to greatly reduce oxygen concentrations and postpone noticeable oxidation in the beer. My usual procedure prior to kegging is to fill a keg with StarSan, and push it out using CO2 from my CO2 tank. This doesn’t use a ton of CO2, but it still does use some up that could go to other purposes.

The easiest CO2-conserving scenario is to go without a keg purge, which is my normal procedure anyhow for many “non-delicate” beers (e.g., porters and stouts). However, I’ve noted that lighter lagers, pilsners, and blonde ales do show noticeable oxidation effects within a month or two without a keg purge. In the “good old days” of sharing growlers and homebrew happy hours and such, I could finish a keg in 4 weeks or so. Now, I expect many kegs will stay on service longer, and so I want to extend the quality as much as I can. Keg purging is nice, if possible!

One option I’ve considered is to use sugar-based carbonation (wort-driven krausening or corn sugar), which should both eat up any latent oxygen and carbonate simultaneously. I’ll likely try that for some future beers (especially more robust, darker styles), but I worry about oxidation risk from adding the sugar and also leaving the beer at slightly higher temperatures to allow the yeast to carbonate more quickly. It’s not an ideal option for light lagers.

So, how might I purge kegs to avoid oxidation and simultaneously conserve precious CO2 from my tanks? Reuse CO2 from empty kegs!

When a keg in the keezer is drained of beer, it’s full of CO2 at serving pressure. Normally, I just bleed this off before cleaning the keg. Why not repurpose the gas?

So, I hooked up a jumper between the gas ports of the empty keg and a StarSan-filled keg (the latter being the one I’ll fill with fresh, uncarbonated beer). Before doing this, I let the empty keg warm up, to give a bit more gas volume (yay, physics). I put a picnic tap on the StarSan-filled keg, hooked up the gas, and let the empty keg push out the StarSan.

It worked like a charm! The transfer took around 10 minutes, but the whole keg got drained, with a bit of residual CO2 left over. No CO2 went to waste, and I ended up with a purged keg ready to fill with pilsner!

The keg setup for CO2 purge. The keg at lower right is empty and just moved out of the keezer. The keg at upper left is empty and filled with StarSan. The red line is pushing CO2, and the clearish-white line is moving the StarSan into a sanitation tub (for use sanitizing equipment).
The colored lines and arrows here show the path of the gas and liquid. The yellow arrows indicate the flow of CO2, and the red arrows indicate flow of the StarSan.

5 thoughts on “Repurposing and Conserving CO2 During Kegging

  1. Fantastic idea Andy! I’d be willing to try it if I understood how you then fill the purged keg with fresh beer from the carboy. If so, don’t you have to open the keg to gravity feed from the carboy? Or are you fermenting in a keg? If so, won’t you push a lot of trub into the serving keg?


  2. Great questions! I should probably do a second post showing how I transfer into the keg. I feed the beer into the keg via the “beer out” port, with the tubing carrying the beer hooked up to a beverage out disconnect…fermenter–>tubing–>disconnect–>beverage out port–>keg…so, no need to open the keg back up. I crack the pressure release valve, so that excess CO2 can escape (get pushed out) as the keg fills up.


  3. Hmm…so the beer is drained from your fermenter (a carboy, or a conical?) by gravity. Do you then need a ‘beverage out disconnect’ at the upper end if the tube is just a siphon tube from a carboy or is attached to a port on the conical?


  4. Yep, the beer is drained by gravity. You do not need a beverage out disconnect on the upper end, because you just pull the beer out of a siphon tube or conical port (I have both setups, depending on what I’m fermenting in).


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