Mixed-Gas Dispense is the use of a blend of gases (usually nitrogen [N2] and carbon dioxide [CO2]) to serve draught beer. There are two reasons mixed gases are used to dispense beer: either because the beer comes from the brewery with a mixture of N2 and CO2 already dissolved in the beer (as is the case with Guinness and other nitrogenated beers) or because extra pressure is required to push beer from the cellar through a very long draught line to the tap.
The physical principle at play in both cases is called Dalton’s law, which requires that different gasses in a keg of beer must behave independently of each other. In the case of a nitrogenated beer such as draught Guinness, mixed-gas dispensing is necessary because dispensing a keg of nitrogenated beer with pure CO2 would cause the N2 dissolved in the nitrogenated beer to diffuse out of the beer and into the headspace of the keg as if there were no pressure in the headspace at all. The N2 in the beer would essentially “ignore” the CO2 in the headspace and diffuse out of the beer until equilibrium was reached between the amount of N2 dissolved in the beer and the amount of pressure from N2 in the headspace. As the N2 left the beer, the CO2 would likewise diffuse into solution through the same principle. It seems counterintuitive to many people, but the fact is that although the pressure of any gas can be used to physically push a liquid from the keg to the tap, CO2 pressure is unable to hold nitrogen in solution and vice versa.
The same effect can be noted when dispensing normally carbonated beer with a 75/25 blend of N2 and CO2 widely known as “beer gas” or “G-mix” (where the “G” stands for “Guinness”). In this case, normally carbonated beer will lose CO2 into the headspace and eventually go somewhat flat, except under extreme pressures. If the beer is dispensed to the bar with 20 lb of total pressure, the 75/25 ratio of the gas means that the gas exerts 15 lb of N2 pressure, but only 5 lb of CO2 pressure. The portion of this total pressure for which one gas is responsible is referred to as that gas’s partial pressure.
Most normally carbonated beer requires 12–15 lb of CO2 pressure to maintain proper carbonation, but achieving this with a mixed gas containing only 25% CO2 would require 48–60 lb of total pressure, which is too high for most systems to withstand. At more normal dispensing pressures, normally carbonated beer dispensed with G-mix eventually falls to the same reduced level of carbonation as draught Guinness—the beer the gas was actually meant for in the first place. A failure to understand this basic principle causes many bars and restaurants to end up serving undercarbonated draught beer because they have a mistaken impression that they can serve all of their beers with the same gas blend.
However, a commercial gas blender can be used to achieve customized N2 to CO2 ratios, which is useful for pushing beer through long-draw draught systems at a higher total pressure while maintaining appropriate carbonation. To do this, the total pressure in the keg is increased by increasing the partial pressure of N2 while maintaining a constant partial pressure of CO2. Thus, an increasing ratio of N2 to CO2 will yield higher dispensing pressures without bringing higher CO2 pressure to bear, thereby avoiding overcarbonation of the keg of beer. Gas blending tends to save establishments money because preblended gas is more expensive than buying the gas separately. Also, the ability to maintain proper carbonation results in happier customers, better beer sales, and less waste.