Nitrogenated Beer occurs when nitrogen gas (N₂) is dissolved in the beer, usually in addition to carbon dioxide gas (CO₂). The ratio of the two gasses is typically around 70% N₂ to 30% CO₂. Although many beers are now served in nitrogenated form, nitrogenation is most closely associated with Irish stouts.

One way that nitrogenated beer can be distinguished from its normally carbonated counterpart is its creamy, persistent head of foam. Also, nitrogenated beer is notably less carbonated than most other beers and as a result has a less acidic flavor.

When a nitrogenated beer is first poured into a glass, it has a distinctive, homogenous, foamy texture. This texture is a result of the relative insolubility of nitrogen in beer (N₂ is approximately 100 times less soluble in beer than CO₂) combined with special dispensing technology. In the case of nitrogenated beer served on draught, the special dispensing technology is a restrictor plate immediately prior to the faucet. This restrictor plate is perforated by a number of small holes, which in conjunction with the sudden pressure drop from one side of the plate to the other encourage extremely small nitrogen bubbles to form. These small bubbles rise to the top of the glass much more slowly than CO₂ bubbles and sometimes even flow downward (the downward-flowing bubbles are the ones closest to the side of the glass. They eventually work their way to the middle of the glass and rise to the top). In the case of canned or bottled nitrogenated beer, the role of the restrictor plate is served by a device called a widget, which like the restrictor plate jets nitrogenated beer through several small holes. See widget.

After its initial foamy stage, a nitrogenated pour will gradually stratify into an attractive glass of beer topped with a dense, creamy head (a process that can take several minutes). Customers often find the pulsating waves that form in the glass during the “settling” of the beer somewhat mesmerizing, and the image of the settling pint has been used very effectively in advertising. The almost imperceptibly small bubbles of N₂ are much more persistent than larger CO₂ bubbles, and the head that they form often lasts longer than the pint of beer itself. Nitrogen bubbles are more stable than CO₂ bubbles, partially resulting from the fact that the surrounding air is mostly nitrogen as well. Because of the creamy head and the lower CO₂ content, nitrogenated beers tend to have a creamy, smooth mouthfeel, even when the beer below is technically light or even thin.

Nitrogenated beer was first mass marketed by Guinness during its 200th anniversary celebration in 1959. Guinness originally called its invention “easy serve” and despite 3 years of research and development at first only released it in England because of concerns that the technology was not yet perfected. It was another 5 years until Guinness finally considered its new nitrogenated beer ready for release in Ireland in 1964.

For Guinness, the advent of nitrogenated beer coincided with the popularization of the use of metal kegs instead of wooden casks in the 1950s. Traditionally, draught beer was delivered to pubs in wooden casks still containing live yeast and unfermented beer. After a conditioning period, the publican would serve the beer by means of gravity or a hand pump, with air displacing the beer as it was served. See cask conditioning. When Guinness decided to switch to kegged beer, which was far easier to handle and more stable in the marketplace, they looked to duplicate the soft texture of the original. Nitrogenation approximates the product of this traditional serving technique much more closely than dispensing beer under pure CO₂ pressure, while not requiring the publican to devote as much attention to his inventory. Although beer served traditionally through casks is still quite popular in England and beer served under CO₂ pressure has become the worldwide standard, nitrogenated beer remains an important factor thanks to brands such as Guinness and Boddington’s, who have developed, refined, and marketed nitrogenation. Today, even craft brewers have begun to experiment with nitrogenation, although the technique requires very high nitrogen pressures in cellar tanks. Many brewers feel that nitrogenated dispense shreds notable character from beer, particularly degrading the crisp qualities of hop bitterness. Others argue that when brewers make beers intended from the outset to be served in nitrogenated form, it is possible to offset any negative effects while taking advantage of the formation of the creamy, long- lasting foam.