Foam, or head, atop a glass of beer is widely considered to be among its most alluring physical traits. It is the main attribute that visually separates beer from other sparkling drinks, and it is also quite important for the mouthfeel of many beers. As one drinks a beer, some of the foam is also consumed, giving the beer a thicker, smoother texture. Brewers, of course, know this, and therefore foam formation and stability are an essential part of overall beer quality. Beer foam has been studied extensively and brewers work hard to make sure their beer has good “head retention.”
Beer is, among other things, a supersaturated solution of carbon dioxide (CO2) and will not foam unless encouraged by agitation or by the presence of nucleation sites such as particles in beer or scratches on glasses. The more CO2, the more foam will be produced.
Foam is an inherently unstable phenomenon because of the huge increase in surface area within an aqueous system that is counter to the force of surface tension. That beer foam is stable, unlike that in richly carbonated beverages such as champagne and sodas, is due to the presence of surface-active agents in the beer.
The key physical process leading to the collapse of beer foam is disproportionation. In this phenomenon, gas passes from small bubbles to adjacent larger bubbles, leading to a drastic reduction in the number of bubbles and an increase in the size of remaining bubbles such that they become unattractive. The lower the temperature, the more stable is the foam. More important, it is advantageous to have a uniform distribution of bubble sizes, preferably small bubbles because then liquid beer drains more slowly from the foam, which helps stabilize the head. This focuses attention onto the sites and mechanism of foam formation; devices in dispense taps and deliberate scratching of the bottom of drinking glasses should be such as to enable the production of uniformly small bubbles.
The main foam-stabilizing agents in beer are hydrophobic polypeptides derived from grain. These molecules cross-link with the bitter iso- α-acids derived from hops to render the foam more rigid and not only more stable but also able to adhere to the sides of the glass as the beer is consumed (cling or lacing).
Other materials that can support foaming include metals. Zinc is especially efficacious and has an advantage over metals such as iron that were once used in that it does not promote oxidation. Melanoidins have some foam-stabilizing capability, meaning that darker beers frequently have superior foaming properties. Some brewers add foam-stabilizers such as propylene glycol alginate to the beer, and some brewers introduce nitrogen gas into the beer, which gives much more stable foams (though at risk to the aroma of those beers expected to have hoppy notes). This is the reason for the creaminess and retention of the heads on most draft Irish stouts such as Guinness.
The most frequent cause of poor foaming of beer is the presence of foam-negative materials, most notably lipids introduced in the process or as part of food consumed alongside the beer, and detergents that have not been properly washed from the glass or dispense lines.
Alcohol is also foam-negative, so stronger beers tend to have less foam stability pro rata. Also damaging to foam are proteolytic enzymes, either those such as papain added as haze-preventatives, or those secreted from stressed and old yeast.
Deeper heads of foam will naturally tend to last longer. Head retention can be measured by a variety of methods. These include procedures that assess the rate at which liquid beer drains from the foam (e.g., the methods of Ross & Clark and Rudin) and those that assess the extent to which the foam itself subsides (e.g., the NIBEM method).
Though most consumers want and expect to see at least a few centimeters of foam on top of a beer, there are wide cultural differences. In Britain, for example, consumers in the south, when served cask-conditioned ales, widely prefer the liquid level of a pint to nearly reach the rim of the glass. A cap of foam should float above, but no more than that. In northern England, however, the consumer expects a frothier foam, which is often achieved by a device called a “sparkler,” attached to the spout of hand-pump assembly. The sparkler causes the beer to spray violently into the glass, which foams the beer even though cask-conditioned beers contain only light carbonation. It is customary to use over-sized glasses, in which the pint measure is marked by a line near the top of the glass, but which leave plenty of room for the foam to feature. The quality of beer foam remains a topic of debate among beer drinkers, though hopefully over pints.