Winegrowers Supplies - Carbonation of Beer
Commercial beers generally contain a fairly consistent amount of carbonation. Among brewers the extent of carbonation is usually specified in 'volumes of CO2' (bars). Brewers would say that the average beer contains about 2.5 volumes of CO2, i.e. the CO2 dissolved in one pint of beer would fill two and a half pint glasses.
Those gas volumes are based on what's present in the beer before it is poured into the glass. The process of pouring, foam formation, etc., reduces the amount of gas that is actually present in the beer when you drink it.
Even though yeast produces a lot of CO2 during fermentation, most of it is released from the beer. As a result, fermented beer nearly always requires an additional carbonation before it is packaged. To do this, brewers have two choices:-
On the one hand, they can put the beer in a closed pressure tank, connect a cylinder of CO2, and pump some in. Using pressure gauges and perhaps some simple testing equipment, they can determine exactly when the beer contains the desired level of carbonation. Most breweries use this technique because it is easy and fast.
The second technique relies on natural methods. Brewers add a measured amount of
unfermented beer (called wort) along with some yeast. The yeast ferments the
wort to produce additional CO2. As long as the beer is stored in a
pressure-tight container (keg, bottle or serving tank) during this phase, the
newly produced CO2 will dissolve in the beer.
When actively fermenting beer is added instead of separate doses of wort and yeast, it's called 'kreusening', a German term.
When natural carbonation is carried out in a bottle the resulting beer is said to be 'bottle conditioned'. Craft brewers like this approach because it helps to protect the beer from oxidation. On the bottom of each bottle you will see a thin white or tan layer of sediment. That's the lees from the yeast that produced the carbonation.
Cask ales contain less carbonation than draft beers. Here, the
beer/yeast/wort mixture is sealed in a cask then kept at cellar
temperatures while the carbonation develops. Before serving, the cask must be
set in one place and allowed to settle. This drops all the yeast to the bottom
of the cask so that the resulting beer is clear and bright.
A properly produced cask ale can help to illustrate the impact of carbonation on beer flavour. Look for a pub that offers the same beer in cask and draft forms. Order a pint of each and compare them side by side. With the carbonic bite stripped away, cask ale usually displays a very different balance of hop and malt flavors. The resulting beer is quite different from the carbonated pint - and that's a key reason for the rising popularity of cask ale.
So that's how CO2 gets into the beer, but what's the purpose of those cylinders of CO2 found connected to kegs at pubs and restaurants:-
Keg beer is usually stored in a chiller that is some distance from the tap where it is served. Getting the beer to the tap takes some sort of energy. In the old days the barman provided this energy by walking to the keg and carrying the beer to the bar. Now, gas pressure does the work, the 'draft' system applies CO2 to the top of the keg. A tube to the tap runs from the bottom of the keg, so that when the tap is opened, gas pressure drives beer out of the keg, through the line and into your glass.
The skill in this system is setting the proper gas pressure. Too much and the beer becomes over carbonated and foams excessively. Too little pressure has the same effect. The correct pressure is different for every set-up since it depends on many variables, from the diameter of the hose and length of the run to the temperature of the beer and the height of the tap. Too often, well-intentioned barmen lower gas pressure to prevent foaming of one beer, only to discover that all of the other beers start to foam as a result.
These days, another gas, nitrogen, is used by certain brewers. Guinness took the lead in this area, developing a nitrogen draft system nearly 40 years ago. Now they put nitrogen in the canned 'Guinness Pub Draft' as well. This is done with a plastic nitrogen-soaked wafer called a 'widget'. This lurks at the bottom of the can so it can add a shot of gas into the beer at the very instant the beer is opened.
Both the draft and can systems infuse the beer with a nitrogen/carbon dioxide mixture that leads to a firm creamy head while leaving a relatively light carbonation in the beer itself. This system has become popular in Britain because it mimics some properties of cask ale.