Ask Mr. Wizard

What is the proper way to force carbonate a keg of homebrew?



Dear Mr. Wizard,

I used my keg system for the first time recently and force carbonated my brew by chilling the beer, applying approximately 30 pounds of carbon dioxide pressure and shaking the keg. By doing this, I over-carbonated my American pale ale. Is there a way to remove the extra bubbles? Also what is the proper step-by-step procedure and temperature for force carbonating so I can avoid this the next time?

Ryan Pearce
Kenner, Louisiana

The Wiz Responds:

Removing excess carbon dioxide from this batch of beer, or de-carbonating it, at home is about as crude and unpredictable as the method used to get your poor beer in this state to begin with. Every time I read the procedure you used for carbonation I cringe and I applaud you for writing into us describing a problem with the old crank-n-shake method!

What you must do is to bleed off the head pressure in your keg, re-seat the bleed valve and allow the beer and keg headspace to re-equilibrate. This means that excess gas in the beer will flow out of the beer into the keg headspace and the keg pressure will increase to some pressure. This unknown pressure depends on how much carbon dioxide the beer contains and how full the keg is. If the keg has an infinite headspace, for example when a glass of beer is left on the
countertop for a long time period, the beer goes flat and the atmospheric pressure of your house remains unchanged. When the keg has a finite headspace, the gas will cause the pressure to increase, unless the keg is essentially empty and you will again get flat beer.

Bottom line is that the emergency de-carbonation plan only works if you have already consumed most of the beer in your keg. The method does require patience and some old fashioned sensory evaluation . . . that means you get to enjoy a couple of beers while coaxing your beer back to an acceptable carbonation level. It will take about 8 hours for the beer to re-equilibrate after you dump the head pressure in the keg. You probably can get this done in 24 hours if you are really committed. Dump the pressure the first time at midnight and go to bed. This means your first scheduled sensory trial is at 8 a.m. If you have a regular job the day for your appointment with the ill keg should be Saturday! If your prognosis indicates too much fizz, repeat the procedure and have another sample at 4 p.m. Eventually, you will either find that the carbonation is acceptable or that you have repeated this procedure in excess and now have under-carbonated beer.

Under-carbonated beer is of course what you had before applying the crank-n-shake method. I use the term “crank” to describe the random application of gas pressure resulting from cranking the gas regulator to its maximum setting of 30 psi and unleashing this pressure on the poor keg of beer. If beer could become ill from too much dissolved gas like scuba divers do when staying down too deep for too long, these beers would certainly suffer from a high rate of the bends.

Divers use dive tables, these days programmed into fancy little dive computers, to avoid the bends. Dive tables relate depth and time to dive duration and this is all based on gas solubility in blood. Depth is another term for pressure and every 33 feet of liquid adds another atmosphere of pressure. The key to safe diving is to avoid staying too deep for too long. Same is true with carbonating your homebrew . . . and yes, I did just happen to return from a dive trip, thanks for asking!

Brewers also have handy little gas tables and these tables show how much gas is dissolved in beer when the beer is in equilibrium with a given gas pressure at a given temperature. For example, beer at 40 ºF (4 ºC) with an equilibrium headspace pressure of 12 psig (the ‘g’ indicates gauge pressure where 0 psig equals atmospheric pressure) contains about 2.5 volumes of carbon dioxide or about 5 grams per liter. This is a normal carbonation level for most draft beers, including your American pale ale.

The easiest way to properly carbonate your beer is to exercise a little patience and to equip yourself with the proper tools. In the case of carbonation, a gas table is a pretty important tool. With table in hand, you can select your desired carbonation level at the temperature your beer is being stored. Ideally you should carbonate your beer at the same temperature you will use for serving. Once you know the desired level of carbonation and the beer temperature use the gas table to determine the required gas pressure. This pressure is what the regulator on your tank will be adjusted to. As divers say “plan the dive and dive the plan.” This same principle applies to carbonating homebrew and requires patience . . . and yes, I did just get back from a diving trip, thanks for asking!

Once you have your gas plan, attach your keg to the carbon dioxide tank adjusted to the pressure dictated by your gas table and wait. A batch of homebrew is small and the headspace pressure will equilibrate with the beer in about 3 days. The only thing you can do to speed this method up is to periodically shake the keg. Some people want to bubble the gas through the dip tube in the keg, but this really does not speed things up much because the gas bubbles are too large and zip through the beer before much gas diffuses into solution. It also causes foaming. Take my advice and just hang tight!

You can periodically shake the keg to speed things up, but whatever you do, avoid the temptation of cranking the regulator higher than what your gas table states. Just remember to “plan the dive and dive the plan” and while patiently waiting for your beer to carbonate daydream about sandy beaches, blue skies, cold beer, def jams and . . . yes, I did just get back from a diving trip.