Ask Mr. Wizard

Methods To Speed a Slow Forced Carbonation


Bob Allen — Dayton, Ohio asks,

I have a red ale in a corny keg at 40 °F (4 °C) under 18 lbs. (124 kPa) CO2 pressure. After a week, I pulled a sample and The beer foamed a lot, but the head settled down relatively quickly. The problem is the ale was flat as a pancake; no carbonation in it at all! I like my beers a little on the fizzy side so I’m duly disappointed. Any suggestions to help me get this ale carbonated?

reddish hued clear amber beer with  a tan head in a shaker pint glass
Photo by Charles A. Parker/Images Plus

The two most likely causes of your observation is that your beer is not carbonated to your liking or that the aggressive pour knocked enough carbon dioxide out of your beer that the perception of carbonation dropped. My bet is on a combination of the two, with the first explanation accounting for the lion’s share of the problem.

If I am reading this correctly, you are relying on the transfer of carbon dioxide into your beer through the headspace surface area. There is nothing wrong with this method, but it is slow. Your combination of temperature and pressure will eventually result in 3.0 volumes of carbon dioxide. Although this is higher than the approximate 2.5 volumes for most styles, it’s not a bad strategy because the transfer of gas from the keg headspace to the beer slows down as solubility approaches the maximum allowed by the temperature and pressure combination.

Because carbon dioxide quickly moves into flat liquids, your beer probably has a bit more dissolved carbon dioxide than it may seem. Certainly not suggesting your flat-tasting beer is OK, but the foaming you see upon pouring is likely associated with some gas flowing out of the beer as it falls into the glass. And if you are pouring with 18 pounds (124 kPa) of head pressure, the flow velocity may be a wee fast, knocking even more gas out of the beer than a quiet pour, or even whipping some air into the beer as it tumbles into your glass. Indeed, one of the arguments in favor of pouring beer into a glass for consumption, versus drinking from a bottle or can, is knocking some of the gas out of the beer and reducing the bloating feel that often accompanies rapid consumption of beer.

My view on carbonation technique has changed over the last few years and I have warmed up to an accelerated method I previously derided as the “crank and shake” approach. This method has been a longtime favorite because it’s lightning fast and does a fine job of getting the beer carbonation level close enough for routine enjoyment.

My rendition of this technique begins by chilling my keg down to serving temperature under about 15 lbs. (103 kPa) of CO2 pressure; the purpose of the pressure is simply to keep the lid sealed so that no air is sucked into the keg as the beer and gas headspace cool. After cooling the beer, I move my keg to a work table, lay it on its side, and connect my CO2 to the keg’s gas inlet. It’s best to locate the CO2 cylinder above the keg to prevent beer from flowing into the gas line and it’s also best to sanitize the gas line before carbonation.

My view on carbonation technique has changed over the last few years and I have warmed up to an accelerated method I previously derided as the “crank and shake” approach.

This is where the method becomes approximate and prone to issues if performed with too much vigor. I usually start off by cranking my gas regulator up to 25 psig (172 kPa) before starting gas flow into my beer. With the keg on its side, gas will bubble into the beer and pressurize the thin headspace that is now located along the length of the keg. Gently rocking the keg back and forth helps increase the rate of gas solubility into the beer. And as gas flows into the beer, the sound of bubbling gas can be heard. After about a minute, I reduce the pressure to about 20 psig (138 kPa) and continue slowly rocking the keg for a couple of minutes before reducing the pressure to 15 psig (103 kPa). Slow, rhythmic rocking continues until the sound of gas flowing through the regulator can barely be heard.

The last adjustment brings the gas pressure to 13 psig (90 kPa), or whatever your final set-point pressure happens to be, and keg rocking continues until the sound of gas flow all but stops. The total time for this process is less than 10 minutes and within about 15 minutes of the last rocking of the keg, a trouble-free and carbonated pint of beer can be poured. Is this method elegant and controlled? Heck no! Does it get the job done? Absolutely.

If this is not your idea of a proper process, another method to consider using is keg-conditioning using an adjustable pressure relief valve, aka a spunding valve. The awesome thing about keg conditioning with a spunding valve is that your priming sugar dose does not dictate carbonation level, provided that the priming dose is sufficient. Any excess pressure from excess priming sugar is simply vented from the spunding valve. I am not suggesting that it’s good practice to simply prime with reckless abandon but adding a bit more than required is the plan. I’ve covered this in previous columns and on BYO+ videos and will skip the process details.

Variations on the spunding theme include pressurized fermentation, transferring from primary to a secondary with residual extract, and capping off a unitank fermentation with residual extract. But when push comes to shove and a keg is not carbonated when the party is about to start, the crank and shake method does work quite well.

Response by Ashton Lewis.