Carbon Dioxide, Nitrogen and Oxygen in beverages at the point of dispense

Dissolved oxygen (DO)

Units of measure: mg/L (ppm) or µg/L (ppb) where 1 ppm = 1000 ppb

Over the years the target levels for DO have continued to fall - typically now the target for DO in bright tank would be less than 50 ppb, and in many breweries this target would be below 10 ppb

Once the beer is pasteurised it is pointless measuring DO as any oxygen that had been in the beer will have reacted at the high temperature and the damage will have been done  

Carbon Dioxide (CO2)

Units of measure: g/l (grams per litre) or V/V (volumes of gas per volume of liquid), where 1 vol ≈ 2 g/l

 
Traditional ale brewing in open fermentation vessels would produce 2 g/l dissolved CO2. In the UK it is common to consume beer with this level of CO2. Bars & pubs often use “breathers” which just keep the barrel topped up with atmospheric pressure CO2, preventing air coming in to damage the beer but at the same time not over-carbonating the beer

Lager style beers in kegs have higher CO2 levels and a major challenge for brewers is to maintain the correct level of CO2 in the beer when in the pub/bar. CO2 gas is used to dispense the beer and the pressure used must take in to account the temperature of the keg - bar owners may keep the cellar very cold for a number of reasons and this can easily cause the beer to rapidly over-carbonate. For example it is not uncommon to find cellars kept at  7°C (45°F) and 30 psig being used to drive the beer out. This will lead to the beer reaching 4 vols dissolved CO2!

Mixed gas (CO2/N2) can be used as the dispense gas because N2 is very insoluble so high pressures can be used on the keg without the beer becoming over-carbonated

Nitrogen (N2)

Units of measure: ppm (mg/L) or ml/L (millilitres/litre) where 1 ml/L = 1.25 mg/L

Widely used in the Ireland and the UK, less so elsewhere, N2 is very insoluble so by itself contributes very little to the size of the head when the beer is dispensed into a glass.

For example, imagine a ½ litre glass of beer with highly nitrogenated  beer (50 ppm):

50 ppm = 50/28 x 22.4 ml/l = 40 ml/L  so the ½ litre glass contains 20 ml of N2. If the glass diameter is 5 cm the surface area on top of the beer is ~ 20 sq/cm so even if ALL the nitrogen came out of solution to form the head it would only be 1 mm high.

However if N2 is used with CO2 it stabilises the head generated by dissolved CO2. Because N2 is very insoluble it comes out of solution very easily and with small bubbles - small bubbles are more stable than large ones.

Also since the surrounding air in the pub already contains 78% N2 the bubbles are less likely to disappear into the atmosphere, hence these beer heads are thick, creamy and stable

In the brewery it is not an easy job to simply add N2 to carbonated beer, it requires experience and good technique to maintain the correct balance of gases through the process

   

Dissolved gases in beverages