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Beer head (also known as the head) is produced by bubbles of carbon dioxide rising to the surface.
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.
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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[1].
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[2], 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 in order 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.
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