A draught beer system is a combination of temperature, pressure, distance, height, resistance, and pressure. Foamy beer is caused by an unbalance of one or more of those parameters. And foamy beer is a big waste of money for any bar operator.
To prevent foaming, many bar operators rely on what is commonly called G-mix, a mixture of 75% nitrogen and 25% carbon dioxide gasses. It is called that because it is used to serve draught Guinness Stout.
That blend, however, is inappropriate and injurious to beers that haven't been nitrogentated at the brewery. Translation: serving a beer with the G-mix that wasn't originally gassed with the G-mix at the brewery will result in flat, non-carbonated beer at the tap.
Why? In physics, Henry's Law states that
At a constant temperature, the amount of a given gas dissolved in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid.Or as Garrett Oliver, brewmaster, The Brooklyn Brewery, puts it:
Here is the slightly confusing part (unless you remember your high school physics, which most of us don’t). Only CO2 can hold CO2 in your beer and keep it from going flat.What to do?
In my brewpubs, I have installed beer pumps. These are impellers that sit in-line. They simply impel the beer forward. They're powered by an air compressor; but the air does not into contact with the beer. This can be an expensive option as each beer line will need its own impeller.
I wrote about another solution in an earlier post: No laughing gas in stout. (That was a reference to nitrous oxide, commonly known as laughing gas, which is not used in beer dispense! It's nitrogen that is meant.) Use a gas blend, but reverse the blend: 75% or so CO2 and 25% Nitrogen. Pressure is thus increased, pushing the beer further, but without necessarily over-carbonating the beer.
Here's another useful primer, reprinted with permission from Garrett Oliver, brewmaster for The Brooklyn Brewery: The Truth About G-Mix - Serve better beer and save money!