This is a combination keg/carbonation/beer storage question.

I have only fridge/freezer room for either a carboy or my (currently 1) keg.

I have about half of the corny keg left of a light cream ale, and had to boot it out of my fridge to put in an IPA for a couple/3 week cycle. It was properly carbonated (seemed to work great at 11ish PSI) before I pulled it out and am keeping in a 75ish degree room.

For fun (and science!) I drew a glass off of it yesterday and it was all foam and then flat [tasting] beer under it.

So, my questions, in "teach a man to fish" style (and assuming there are no leaks in my system, which I believe to be true):

1) what is the phenomenon that changes how the carbonation level of the beer appears when it is at room temperature? In my mind, I think of a carbonated beer as having bubbles suspended in the liquid once the container is properly pressurized. i.e. a headspace of CO2 that is of sufficient pressure to keep the CO2 suspended in the beer. When dispensing, the pressure equalizes again to maintain that pressure. It's hard for me to visualize how the beer could come out "flat" looking if the pressure level of the system hasn't changed. Note, I turned off the CO2 when I took it into room temperature, as it seemed like the right thing to do(?).

Now, at room temp CO2 expands I believe (?), but why exactly did that result in what I observed? I think if I understand that, I'll understand much more about the process.

2) Given #1, and knowing that the beer style matters as far as how it ages, what would you target as a general rule/deadline to get it cold again?

3) Given #1 and #2, should I purge the keg while it is warm and sitting until it's cold again? If so, why?

I know that's a mouthful (or 3), but I feel like I'll understand much more and be able to fish (and drink) if I get these concepts nailed down :)

Thanks! (And as always, if you have articles or chapters of books you'd point to, I'd love those as well/alternately).

(This is brew #4, so surely I'll start to run out of questions soon...)

2 Answers 2

  1. CO2 is less readily absorbed by warm liquids. Therefore, CO2 in solution comes out of solution when you warm the beer.

  2. Whenever it works for you. The warmer the beer is stored though, the sooner you should try to cool it back down. Warmer storage promotes faster aging. For me, the point where I want to start cooling it down again starts at about 80F.

  3. No, don't purge unless you want to go through the carbonation process again when you cool it down. As long as the keg remains sealed, the CO2 in the headspace will go back into solution when you cool it down.

  • Wow. You made my question seem very overly complicated :) Doing some quick research based on your answers, but I see no reason not to flag it answered shortly. Thanks!!
    – goodytx
    Apr 26, 2012 at 18:07
  • Actually, a quick additional question on #1: is the CO2 released out of the liquid on the dispense? I would assume it can't come out of the liquid inside the pressurized keg, given that it has a fixed pressure (an no leaks)? Maybe I don't fully get the pressure level measure.
    – goodytx
    Apr 26, 2012 at 18:12
  • 2
    The CO2 is migrating from being dissolved in the beer into the headspace in the keg. The reason you get more foam in your glass is because the additional CO2 in the headspace is increasing the pressure of your pour.
    – Denny Conn
    Apr 26, 2012 at 19:24

There are several factors at play here.

You're right that the CO2 will expand with increasing temperature, but since it's in the keg, a closed vessel, it can't expand, so pressure increases instead. Increased pressure increases solubility of CO2, but with increased temperature solubility of CO2 decreases, and unfortunately that change is more than the solubility increase by the increased pressure, so CO2 comes out of solution, reducing the carbonation.

Also, the increase in pressure is then what is causing all the foam, causing even more CO2 to come out of solution when dispensed, making the beer taste flat. So , you're getting hit by a double whammy here.

If you want to hook up to the CO2 tank and chill the keg again, you can just go ahead and do that. Once the beer has chilled it will need a couple of days to reach equilibrium again.

If you want to continue serving at room temperature, you have a couple of choices:

  • either, purge the keg, not entirely, but enough to reduce the pressure down to one that matches your dispensing line. This has the advantage of not needing a new beer line, but at the cost of slightly less carbonation.
  • or, get a longer beer line, one that is balanced to the pressure now in the keg. This will avoid the need to purge, keeping as much carbonation in the beer as possible at that temperature and pressure, and reduce foaming since the line is balanced to the pressure.

To find out the current pressure in the keg, you can connect your regulator to the keg. Initially, have the regulator turned down low, and increase slowly - the point the CO2 starts to flow is then the pressure in the keg. From the co2 pressure, you can calculate the length of beer line required.


  • Thanks mdma! That tells me a lot as well. I'm starting to think I should just try and chill the primary manually and keep the keg in the fridge; my room only gets to about 75 ambient (hurrah for central air in Texas) so I could probably easily keep it where it needs to be until primary is done and then this wouldn't have been a question :) Either way, I'm way more prepped now. Side note, a friend just dropped off "Homebrewing for Dummies" which seems quite decent. And not at all insulting :)
    – goodytx
    Apr 26, 2012 at 22:18
  • great. a downvote and no comment. this kind of behavior helps no-one.
    – mdma
    Apr 28, 2012 at 9:01
  • I don't see a down vote... Did I do something accidentally? Or not me? If I can fix it let me know!
    – goodytx
    Apr 28, 2012 at 12:30
  • @goodytx, not you dude, but someone else, someone who obviously considers themselves wise enough to judge this answer, but not wise enough to share that reasoning with the rest of us.
    – mdma
    Apr 28, 2012 at 12:51
  • @goodytx If you click in the number itself between the upvote and the down vote it will show you how the total votes break out. In this case three ups and one down.
    – brewchez
    Apr 28, 2012 at 22:11

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.