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Beer head (also head or Kräusen) is the frothy foam on top of beer which is produced by bubbles of gas, typically carbon dioxide, rising to the surface. A head is produced during fermentation, and consists of wort protein, yeast and hop residue. The foam is attributed to yeast eating sugars and building up CO2 in the process. The fermentation may take place in the brewery or it may be a secondary fermentation taking place in the cask or bottle. If the beer is no longer fermenting, due to filtering or pasteurising, then a head cannot naturally occur, so a head is produced by force carbonating the beer by inserting carbon dioxide and sometimes a mixture of carbon dioxide and nitrogen gas.
The density and longevity of the head will be determined by the type of malt and adjunct from which the beer was fermented. Different mash schedules and cereal sources influence head retention. In general, wheat tends to produce larger and longer-lasting heads than barley.
Consumers tend to place a lot of importance on beer heads: too much of a head is undesirable because it detracts from the mass of the drink (similar to carbonated soda drinks), but on the other hand, a beer drink is viewed as incomplete unless it has a head, and the specific form of head expected for the type of beer. Some view beer heads as important because they give off the aroma of the beer. Another opinion is that it is important for the aesthetic look of the beer. The commercial significance of the head has led to extensive academic study, and sophisticated analysis techniques.
Formation through carbon dioxide 
The carbon dioxide may be produced naturally through the activity of brewers yeast, or artificially by dissolving carbon dioxide under pressure into the liquid. The beer head is created by the carbon dioxide produced as a byproduct of the metabolism of brewer's yeast acting upon starches and sugars found in the wort.
Chemical composition 
While the actual foam activity of beer depends on the presence of carbon dioxide, it is the surface-active materials like amphipathic polypeptides from malt that determine size, shape and length of the foam.
Beer foam consists of polypeptides of five different classifications, divided by their relative hydrophobicity. As the hydrophobicity of the polypeptide groups increases, so does the stability of the foam.
Carbonation occurs when carbon dioxide is dissolved in water or an aqueous solution. This process is generally represented by the following reaction, where water and gaseous carbon dioxide react to form a dilute solution of carbonic acid.
Importance of the glass 
Properties of the glass can also influence a beer head, with a roughened surface at the base of glass known as a widget, providing for nucleation of carbon dioxide deep in the beverage rather than at the surface, resulting in a slower release of gas to the atmosphere. While glass is completely nonporous, its surface can retain oil from the skin, aerosolized oil from nearby cooking, and traces of fat from food. When these oils come in contact with beer there is a significant reduction in the amount of head (foam) that is found on the beer, and the bubbles will tend to stick to the side of the glass rather than rising to the surface as normal.
For proper foam formation, it is also important to dry the glass thoroughly after washing. Any water in the glass can prevent excitement of the gas by covering designs set in the bottom of the glass, thus making the beer flat.
Conversely, some styles such as Belgian witbier benefit from being poured into a wet glass to control the often profusive head.
Formation of 'Nitrogen head' 
The creamy head on beers such as Guinness is created by a widget in cans or bottles using nitrogen, or by the process of drawing keg beer from a keg using nitrogen or mixed gas (carbon dioxide and nitrogen). The use of nitrogen, which was pioneered by Guinness, creates a firm head with small bubbles while reducing the excessively acidic taste often produced by using carbon dioxide alone.
- German, J. B.; McCarthy, M. J. (1989). "Stability of aqueous foams: Analysis using magnetic resonance imaging". Journal of Agricultural and Food Chemistry 37 (5): 1321. doi:10.1021/jf00089a025.
- "Glass To Last". Hospitalitymagazine.com.au. 2010-02-18. Retrieved 2010-02-24.