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
starch tends to produce larger and longer lasting heads than barley
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.
The carbonation in filtered beers
is created by injecting carbon dioxide artificially into the beer.
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.