Beer head

The foam on top of beer is called a head. It is caused by bubbles of carbon dioxide rising to the surface. The carbon dioxide may be produced naturally by the activity of the brewers yeast, or artificially by dissolving carbon dioxide under pressure into the liquid. The density and longevity of the head will be determined by the type of starch from which the beer was fermented. Wheat starch tends to produce larger and longer lasting heads than barley starch.

Causes

The foam on top of beer is caused by the acidity created by the carbon dioxide made by the activity of brewer's yeast. 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.

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.

This process yields the "fizz" and the head to beer.

Rinsing the glass

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. Also it is important to make sure the glass is well dried after washing. If there is water in the glass it 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.

Forced carbonation

The carbonation in filtered beers is created by injecting carbon dioxide artificially into the beer.

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.

References

External links

* [http://www.uni-regensburg.de/Fakultaeten/nat_Fak_IV/Organische_Chemie/Didaktik/Keusch/D-beer_foam-e.htm The Tragedy of a Nice Beer Foam Head - Peter Keusch]
* [http://oz.craftbrewer.org/Library/Methods/Other/KunzeFoam.shtml Beer Foam - Wolfgang Kunze]
* [http://www.madsci.org/posts/archives/may99/926363970.Ot.r.html There is plenty of physics involved in opening a bottle of beer - Richard Kingsley]
* [http://www.expasy.org/spotlight/back_issues/sptlt048.shtml One beer please - Vivienne Baillie Gerritsen]