Carbonation. While levels differ widely, carbonation is one of the defining features of beer. The effects of carbonation strongly influence a beer’s mouthfeel, flavor, aroma, and appearance. Beer without carbonation is said to be “flat,” and this is an apt description, as the beer is likely to be dull and lifeless. The two main products of the fermentation of wort sugars are ethanol and carbon dioxide gas. The carbon dioxide is readily soluble in the beer. According to Henry’s law, “at a constant temperature, the amount of a given gas dissolved in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid.” See henry’s law. So in a closed vessel such as a bottle, cask, or brewery tank the amount of carbon dioxide dissolved in the beer will vary according to the temperature of the beer and the pressure of the carbon dioxide in the headspace.

Carbonation is measured in two ways. One of these methods compares the volume of dissolved gas with the volume of liquid (vol/vol), while the other measures the weight of gas in solution compared to the volume of liquid (g/l). The device used for measuring the concentration of carbon dioxide in solution uses the principles of Henry’s law. A sample of the beer is placed in a specially designed closed container with a measured amount of headspace. The sample is agitated, either by simple shaking, or by passing a small electric current through it, to obtain an equilibrium between the gas in solution and its partial pressure in the headspace. By measuring the temperature of the sample, and the partial pressure in the headspace, the carbon dioxide in solution can be calculated. This is traditionally done using a chart that compares partial pressure and temperature but modern machines have calculators built in.

After primary fermentation an unfinished beer in an open vessel will contain around 1.0–1.2 vol/vol (2–2.4 g/l) of dissolved carbon dioxide, while a beer matured in a closed cylindroconical vessel, with a top pressure, may contain up to 2.5 vol/vol (5.0 g/l) of dissolved carbon dioxide. Traditionally, carbonation to the level required for serving was achieved in the brewery by transferring the beer into a closed vessel with some residual fermentable sugar, and allowing the fermentation to finish. In bottles, the practice of bottle conditioning involves the addition of priming sugar to finished beer, sometimes with fresh yeast. Once the beer is bottled, the yeast consumes the priming sugar, gives off carbon dioxide inside the bottle, and naturally carbonates the beer. Modern production methods allow for carbon dioxide to be added directly to beer using a porous stone or sintered steel rod. See bottle conditioning, carbonation stone, and henry’s law.

See also carbon dioxide.