An extended version of the article from Beer Buzz April – June 2019 issue.
In our last article we looked at the some of the containers that beer comes in. This time we are taking a look at something which divides many beer lovers – the role of carbon dioxide (CO2) in beer.
Let’s start by getting one thing out in the open – all beer should have carbon-dioxide dissolved in it. Whether it is a pint of Holt’s cask conditioned mild or Fosters made in Heineken’s factory in Moss Side, there is CO2 in your beer. There are many things that make the two drinks different – that one would be considered ‘flat’ and the other ‘fizzy’ is just one of them.
Carbonation, also referred to in brewing as ‘condition’, is measured in ‘volumes of CO2’. A volume is the space that the carbon-dioxide would take up at a standard atmospheric pressure at a temperature of 0° C. In other words, if a gallon of beer contained 2 volumes of CO2, the CO2 by itself would occupy the same space as two gallons.
A well-kept cask ale at cellar temperature should contain approximately 1.1 volumes of carbon-dioxide. A typical mass-market lager would be expected to contain 2.4 to 2.6 volumes of CO2 where a carbonated soft drink will typically have between 3 and 3.5 volumes. The reaction of the palate to the dissolved carbon-dioxide in your beer is part of the flavour profile – be it the gentle tingle on the tongue of good cask ale or the more pronounced ‘bite’ of a keg beer.
‘Nitro-keg’ beers (including the ubiquitous Guinness) are another category again – these tend to have low volumes of dissolved carbon-dioxide – in the 1.2 – 1.5 volumes region but also have nitrogen injected into them. The nitrogen has smaller bubbles which gives the characteristic ‘creamy’ appearance and mouthfeel.
There are several ways carbon-dioxide gets dissolved in beer, but the most common is that it is generated naturally during the fermentation process. As yeast sets to work converting sugars in the wort to alcohol, the main biproduct is CO2.
Many brewers will ferment in sealed tanks so that the naturally generated gas carbonates the beer while others ‘force carbonate’ either by applying CO2 at high pressure and low temperature or by forcing CO2 through the beer using a device known as a carbonation stone (for those of a certain age – think of a SodaStream in action).
During cask conditioning, the carbon-dioxide generated during secondary fermentation in the cask is trapped in the sealed cask and absorbed into the beer. Contrary to what some believe, fermentation in a cask does not stop under pressure. There is a relationship between pressure and fermentation rate but at the pressures found in beer production, the effect is negligible and does not slow down secondary fermentation.
Although commonly referred to as a process which takes place in the pub cellar, beer packaged with live yeast and sufficient fermentable sugars and held at suitable temperature will undergo secondary fermentation whether its in the brewery, in a warehouse or in the pub cellar.
The levels of carbonation in a sealed cask can be significantly in excess of the expected final 1.1 volumes but the brewer must take care not to allow too much fermentation as the closures on a cask – the shive where a cask will be ‘vented’ and the keystone where the tap will be placed – will only hold back a relatively low pressure.
Cask beer which has been allowed to become too warm will frequently ‘blow’ one of its closures, increased temperature resulting in an increased rate of secondary fermentation and increased generation of carbon-dioxide.
The amount of carbon-dioxide which remains dissolved in a beer is determined by two factors – temperature and pressure. Physics determines the amount of CO2 which will remain soluble in a liquid at a given temperature – the lower the temperature, the more CO2 will remain dissolved.
In a cask ale the CO2 produced during secondary fermentation stays dissolved in the beer until cask is ‘vented’ in the pub cellar – at which point any excess CO2 will then slowly escape to the atmosphere until it reaches the level which is soluble at cellar temperature. At 13⁰C this is 1.1 volumes and a vented cask beer needs time for the carbonation level to settle. Typically the time taken to vent off excess CO2 also allows other unwanted flavours in the beer to dissipate and desired flavours to develop.
In a kegged beer higher carbonation levels are maintained by applying pressure to the liquid to keep the CO2 in solution (the skills of applying the right pressures in the cellar are for another day).
In many mass market lagers, carbonation is used to stimulate the tongue and mask the generally low flavour profile. However, for many modern brewers producing ‘craft’ beers for keg dispense, the intended carbonation is very much part of the design of a beer.
‘Craft’ brewers will design their keg beers to have anything from 1.2 to 3.0 volumes of CO2. A carbonation of 2.4 – 2.6 volumes would be typical but lower levels are often used for stouts and porters while highly hopped IPAs may use higher volumes to push out hop aromas from and prevent them tasting cloying. Too high a carbonation for a given style and ‘carbonic bite’ can become overpowering and masks flavour.
It’s important to remember that all carbon-dioxide is the same gas – there is not ‘good’ and ‘bad’ CO2. Whether it is generated during fermentation in a tank at the brewery, comes from a cylinder or is generated by secondary fermentation, it is all made up of one molecule of carbon and two of oxygen and once it is in your beer, you have no way of telling how it got there.
Whether you enjoy your beer gently or highly carbonated is a matter of taste.
Cask ale lovers enjoy the creaminess that comes with low carbonation while other drinkers find cask ales ‘flat’ and ‘dull’ and seek the lift from carbonation. Different beer styles suit different levels of carbonation. There is no right and wrong.
Just enjoy the beer.