Q: It’s been seven years since I started dispensing my homebrew with a Cornelius keg system, and I’ve never been happier until now. I just came across a tank of “the mix” (nitrogen/CO2) and an old Murphy’s faucet designed for dispensing with the mix. I was quite excited until I hooked it all up. We got way too much foam, and it did not dissipate as it does with the fine products designed for this dispensing system. The regulator was set at 30 psi as it should have been. The beer was at 45 °F (7 °C). It had been pouring perfectly on straight CO2 minutes before. Is there something different that must be done in the conditioning? Is it possible that the terminal gravity was too high? Must I use unmalted barley in my grain bill? Pub owners have told me that other beers don’t do well on this system unless they’re designed for it. Why?
A: You say the beer was pouring perfectly on CO2 “minutes before” your foam problems with the gas mix. You do not say whether you had tried to dispense the beer through the Murphy’s tap under CO2. This is a critical piece of information. I have to assume that you were dispensing the beer with a conventional tap using 12–15 psi of CO2. If that is true, then here’s what I would do to track down the problem.
First, check your gas mix. It should be three parts nitrogen to one part CO2 (75%/25%). Then check your tap. I’ve never worked with a Murphy’s tap. If it’s the same as a Guinness tap, it has five little pinholes in a plastic restrictor disk. The beer is pushed through those pinholes to force the CO2 in the beer to “break out.” The restrictor plate is the secret to the formation of that nice thick creamy layer of foam in the glass. If this is the way your tap works, then 30 psi may not be enough. Guinness is dispensed under as much as 45 psi. At 30 psi, you may get foaming in the keg and in the beer line because not enough CO2 counterpressure is present to maintain the carbonation level in the keg.
So how can 30 psi not be enough? Well, this is mixed gas. The total is 30, but only 7.5 psi of that is CO2. With gas mixtures, each gas behaves as if the others were not there. In other words, if you’ve got a keg of beer at 45 °F (7 °C) at 2.2 volumes of carbonation — which translates into a bit over 11 psi of CO2 pressure — you need over 11 psi of CO2 counterpressure in the headspace to maintain an equilibrium (that’s over 44 psi using the 75%/25% mix). If you have only 7.5 psi of CO2, the gas in the beer will come out of solution.
Your informants are basically right about beers needing to be designed for the mixed gas dispense. Proper dispense design is not a matter of recipe formulation — beers brewed with barley flakes are (other things being equal) more prone to foaming than all-malt beers. But carbonation is a critical parameter. When you dispense beer through that tap, most of the CO2 dissolved in the beer breaks out into a head of foam, which can take quite a while to settle down. The beer underneath the foam is rather flat. So you don’t need a lot of carbonation in the keg.
I suggest carbonating the keg to 1.8 volumes. With a draft system and pressure gauge, it is easy to adjust the carbonation in the keg downward by bleeding off the head pressure in small steps. After each bleed, wait for the pressure to stabilize — it will bounce back up — before taking off a little more. You’re finished when it comes to rest at the correct head pressure, as determined by your carbonation chart.
So I suspect that part of the reason your foam isn’t settling down is that the beer in your keg was too highly carbonated for this dispense method. Note also that 7.5 psi of CO2 head pressure (30 psi of mixed gas) is enough to maintain 1.8 volumes of carbonation at 45 °F (7 °C). With correct head pressure and carbonation levels, most any ale can be dispensed under mixed gas.
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