The Science of Mixed Beer Gas
Beer gas is a vital component of the perfect pint. Pouring a pint is a simple action with a large amount of skill and engineering behind it. The work of the brewer, the work of the yeast, the work of the barman and the work of the Beer gas.
Beer gas has come to be the known term for the gas used in cellars and pubs, that maintains the fizz and physically pushes any beverage from the keg to the tap. With beer gas, you are looking for different characteristics for different drinks. A pilsner tastes different from a stout and a large factor in that taste is the gas. The magical qualities in Guinness where the bubbles don't act the way you would expect is due to the gas. Ultimately for the landlords, the biggest factors are convenience and price.
The gasses used to make up beer gas are Carbon Dioxide (CO2) and Nitrogen (N2). Each are used separately and in combination to provide the range of flavours and functions needed for the keg to tap pouring.
CO2 is used alone for very small run properties where the distance from keg to tap is very short potentially not even in a cellar environment and even then only with ciders and larger style drinks.
N2 is used alone for dispensing wine and for nitro coffee or cold brew coffee. With wine, it is used because it is none reactive and can provide the pressure to push the drink through the line. Nitro-coffee used the N2 to push through the lines and to add the bubbles to the coffee which makes it sweeter and creamier without sugar or milk being added.
For the majority of applications, a mixture of the two gasses is used. The two most common beer gas mixtures are 30/70 and 60/40. Each number represents a percentage of the mixture always denoting CO2 fist so a mixture of 30/70 (used for stouts and Guinness) is 30% CO2 and 70% N2, meaning the 60/40 is 60% CO2 and 40% N2.
The reasons for the different mixtures are drink texture and drink flavour. In lagers, the aim is a fizzy bitter drink. For this reason, the mixture is stronger in CO2, CO2 reacts with the water in the drink forming carbonic acid which gives a bitter taste to water (think Soda or sparkling water) CO2 is also a larger molecule and therefore produces larger bubbles that make a foamy head and fizzy taste.
If CO2 does all the good stuff why is it mixed with N2? Over carbonisation, in systems that have longer to go from the keg to the tap, the CO2 can separate from the lager before reaching the tap, therefore, creating too big a head (see image below) on the drink and ultimately wasting lager, which we can all agree is a crime. Using N2 at about 40% allows the benefits of CO2 without the drawback, as N2 keeps the CO2 in the liquid all the way into the glass.
The other common mixture in the 30/70, this is for your heavier drinks and the taste is already imparted there and a smoother texture is desired. For those reasons, N2 is the prominent gas used. Like in Nitro-coffee the N2 adds the smooth “creamy” texture due to the size of the molecule and therefore the size of the bubbles.
Other mixtures are available but less widely used as they are required for beverages that are a hybrid of the two common beer styles.