Carbon Dioxide is a gas produced by yeast during fermentation and creates the “fizz” or “condition” characteristic of beer. In the anaerobic fermentation process, yeast converts the sugars in the wort to, primarily, alcohol and CO₂. Surplus CO₂ is often collected and used later for boosting the level of CO₂ in the finished beer (see carbonation). In cask-conditioned beer the CO₂ is generated principally in primary fermentation but a small amount is produced in a secondary fermentation to give the beer a gentle “tingle” (“condition”). The same principle applies in the production of bottle-conditioned beers. Priming sugars—small additions of sucrose or glucose—are often added during filling of the cask or bottle to stimulate the secondary CO₂ generation by the residual yeast. See real ale. Keg, bottled, and canned beers often have additional CO₂ added, usually post-filtration. Beers with this post-fermentation CO₂ are sometimes known as “brewery-conditioned” beers.

CO₂ will dissolve quite readily in beer, with solubility increasing with decreasing temperature. The content of CO₂ in a beer is often expressed in terms of volumes of gas at standard temperature and pressure per volume of beer or in grams of CO₂ per liter of beer. As an approximation, one volume of CO₂ is equivalent to two grams of CO₂ per liter. Cask-conditioned beers have CO₂ levels of around 1.2 vols, whereas keg beers range typically from 2–2.6 vols and bottled and canned beers slightly higher. Knowledge of the precise level of CO₂ in a keg beer is important in that gas pressure in the dispense line from keg to tap needs to be adjusted to the level of the beer in the container to prevent excessive foaming (“fobbing”) of the beer as it is delivered into the glass. See fobbing.

The high solubility of CO₂ at low temperatures is exploited in beers that are served very cold (34–37°F, or 1–3°C). As the beer is taken into the mouth, the rough surface of the warm tongue forms a focus (“nucleus”) for the CO₂ to come out of solution. This creates a sharp tingle on the tongue, which can be perceived as refreshing but can sometimes be almost painful; it is known by brewers as the “CO₂ bite.” This “bite” is caused by carbonic acid created when CO₂ is dissolved into an aqueous liquid. The lower the beer temperature, the higher the dissolved CO₂ level and the greater the bite. This sensation combines with hop bitterness to form the backbone of a beer’s palate sensation and a balance to the sweetness of malt. Without carbon dioxide, most types of beer would be considered unpalatable.

The evolution of streams of gas (CO₂) bubbles from the base of a glass of beer is viewed as being aesthetically pleasing and can aid in foam retention. This rise of gas bubbles can be enhanced by scratching or etching the base of the glass to create nuclei for gas evolution. In the UK branded glasses are often etched with distinctive brand logos to enhance the visual appeal of the beer in the glass.

The foam on a glass of beer is also largely a result of CO₂ evolution out of the liquid, and beer foam is largely comprised of the gas. See foam.