Beer head
Frothy foam on beer produced by carbon dioxide bubbles.
That frothy foam sitting on top of your beer is called the head (or collar). It forms when gas bubbles, mostly carbon dioxide, rise to the surface. The key ingredients for this foam are wort protein, yeast, and hop residue. The carbon dioxide itself is produced during fermentation as yeasts break down sugar-rich molecules to produce ethanol and carbon dioxide. Carbonation can happen either before or after bottling. If the beer is still fermenting in the bottle, it carbonates naturally, and the head appears when you open and pour it. If the beer has been pasteurized or filtered, it needs to be force-carbonated with pressurized gas. How dense and long-lasting the head is depends on the malt and other ingredients used. Different mash schedules and grain sources affect how well the foam holds. Generally, wheat creates bigger, more persistent heads than barley. A related phenomenon is "lacing" or "lace"—the white, foamy residue left on the glass as the head fades or as you drink. Just like the head itself, lacing is influenced by the beer's composition (proteins, hops, yeast residue, filtration). Beer enthusiasts can learn a lot from lacing, though strictly speaking, the quality of the beer isn't obvious from the head or the lace alone.
Drinkers care a lot about the head. Too much is undesirable because it cuts into the actual drink (similar to soda). On the other hand, a poured beer feels incomplete without the right kind of head for that style. Some connoisseurs value the head because it carries the beer's aroma. Others see it as important for the beer's appearance. This commercial significance has led to academic studies. At least one study suggests the head helps transport the beer after pouring by damping sloshing, turning vertical movement into horizontal movement.
Carbon dioxide can come naturally from brewer's yeast or be artificially dissolved under pressure. The head is created by this CO₂, which is a byproduct of yeast metabolizing starches and sugars in the wort.
While the foam's activity depends on carbon dioxide, it's surface-active materials—like amphipathic polypeptides from malt—that determine the foam's size, shape, and longevity. Beer foam contains five classes of polypeptides, divided by their hydrophobicity.
- Carbon dioxide source
- Produced naturally by brewer's yeast or artificially by dissolving carbon dioxide under pressure
- Foam stability factor
- Hydrophobicity of polypeptide groups; increasing hydrophobicity increases foam stability
- Nitrogen effect
- 20 ppm addition of nitrogen gas can have a large effect on foam stability
- Decay model
- Beer froth decays exponentially
- Lacing relation
- Amount of lacing is closely controlled by the specific composition of the beer
Lore & Background
The beer head is created by carbon dioxide produced as a byproduct of brewer's yeast metabolism acting upon starches and sugars in the wort. While the actual foam activity depends on the presence of carbon dioxide, it is the surface-active materials like amphipathic polypeptides from malt that determine the size, shape, and length of the foam. Beer foam consists of polypeptides of five different classifications, divided by their relative hydrophobicity. Closely related to the beer head is 'lacing' or 'lace', a white foamy residue left on the inside of the glass as the head recedes or as the beer is drunk. Beer connoisseurs can tell much by the lacing, though strictly speaking beer quality is not readily apparent by the head or the lacing. 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. The use of nitrogen creates a firm head with small bubbles while reducing the excessively acidic taste often produced by using carbon dioxide alone.
Reader's Guide
Consumers tend to place a lot of importance on beer heads: too much head is undesirable because it detracts from the mass of the drink, yet a poured beer is viewed as incomplete unless it has the specific form of head expected for the type of beer. Some connoisseurs view the head as important because it helps provide the aroma of the beer, while another opinion is that it is important for the aesthetic look. The commercial significance of the head has led to academic studies. At least one study suggests that the head assists in the transport of beer after pouring by damping oscillation (sloshing) and converting vertical movement into horizontal movement. Glassware is often designed to accentuate or accommodate the head; a roughened surface at the base of a glass known as a widget provides nucleation of carbon dioxide deep in the beverage, resulting in a slower release of gas. Glass surfaces can retain oil from skin, aerosolized oil from cooking, and traces of fat from food, which significantly reduce the amount of head. For proper foam formation, it is important to dry the glass thoroughly after washing, though some styles like Belgian witbier benefit from being poured into a wet glass to control the often abundant head. In the United Kingdom, the head is included in the measure of the beer when serving, though the Campaign for Real Ale believes it should not be counted and that customers should be entitled to ask for a full, liquid pint.
Did You Know?
- The beer head is produced by bubbles of gas, predominantly carbon dioxide, rising to the surface.
- Wheat tends to produce larger and longer-lasting heads than barley.
- The use of nitrogen in beers like Guinness creates a firm head with small bubbles and reduces the excessively acidic taste.
- Beer froth has been scientifically demonstrated to decay exponentially, a finding awarded the 2002 Ig Nobel Prize in Physics.
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