Ask Mr. Wizard

Overly Foaming Draft Beer


Marc Kovach — Dover, Pennsylvania asks,

When I keg my beer, I force carbonate for two weeks at 14–15 psi. At this time, I have found carbonation is right where I like it and I get just the right amount of head in my glass. This condition lasts until I get about half way though the keg (approximately 11⁄2 months later) when I start getting half beer and half foam. This tends to be worse with hoppier beers. The 14–15 psi is what I calculated to result in 12 psi after pressure loss through my lines. My dispensing lines are about 5–6 feet long. Any ideas what may be causing this?


Your problem is caused by overcarbonation that is slowly occurring over time. Let’s dig into what you are observing over time beginning with your method of carbonation. Based on your question, I am assuming that you are not using the shake and bake method of carbonation where beer is shaken at some pressure to increase the rate of gas absorption. Your method is the set it and forget it method where gas slowly diffuses into beer from the headspace above the beer. I like this method and have no critique of the method. Your problem is your chosen pressure.

Another assumption I am making is that your target carbonation level coincides with the 12 psi at the tap you mention. This is a normal pressure for beers carbonated to 2.57 volumes of carbon dioxide and stored at 38 °F (3 °C) (see the Zahm & Nagel CO2 solubility in beer chart for reference). Carbonating through the headspace is a relatively slow process, where the rate of CO2 flow into beer slows as the dissolved concentration approaches equilibrium with the headspace pressure at whatever temperature the system is set to. In other words, your beer quickly picks up CO2 early in the process, pours well and tastes like you want it to, then it slowly picks up more gas than you want until equilibrium is reached at ~14.5 psi; if your beer is in a 38 °F (3 °C) keezer, the carbonation level is 2.8 volumes of CO2. For readers who prefer g/L of carbon dioxide and bar for pressure, this condition translates to beer at 1 bar
pressure and 5.5 g of CO2 per liter, and for the sake of easy reading I am opting to not repeat this conversion going forward.

If I have misinterpreted your question, hang with me because this general explanation doesn’t have to be correct for the remedy to work . . . but I think I am following what you are doing.

Now let’s look at your draft system. Seems that you have matched your 5–6 feet (1.5–1.8 m) of draft line to match up with 12 psi of pressure; you should be balancing the system to match the equilibrium pressure and temperature of the beer. If you do indeed want 2.8 volumes of carbon dioxide in your beer (great for some styles of beer like weizen or many Belgian-style beers), 14.5 psi at 38 °F (3 °C) is your number. However, your draft line is too short to balance this condition and explains why your beer slowly becomes foamy when poured after a few weeks. In short, your draft system is out of balance. The solution for this particular problem is to increase your draft length to balance the 14.5 psi condition. Based on my explanation above, I don’t believe this is the solution to your problem.

What you need to do is carbonate beer to a typical level of 2.57 volumes (5 g/L) by decreasing your CO2 setpoint to 12 psi (0.8 bar). Assuming you are using 3⁄16-in. ID beer line, your current draft line length fits this system. I am personally a fan of flow-control beer faucets that allow for a bit of fine-tuning when pouring. These faucets provide the best control when the draft line is a bit shorter than calculated because the shorter-than-calculated lines leave a bit of excess pressure to scrub using the flow control faucet. You may find that carbonating to 2.57 volumes takes longer when pressure is reduced to 12 psi, so you may want to do a little beer shaking or rolling the Corny keg to start your carbonation cycle.

Finally, that observation about more challenges with hoppier beers makes sense because these beers often contain very small haze particles from the reaction between hop polyphenols and malt proteins; these little dudes act as nucleation sites for gas breakout during dispense. I hope this helps you solve your dispense issues so that your brews pour properly!

Response by Ashton Lewis.