Different beer in glasses isolated on black

© 2023, Randy Mosher / Craft Beer & Brewing Magazine

Of all alcoholic beverages, beer alone can boast of a dense and lingering mousse topping its liquid depths. Beer’s lively texture and fluffy foam have been admired since ancient times. Understanding all the chemical miracles that come together to make it happen makes it even more amazing.

Sure, many drinks have bubbles. In champagne, these can be enchanting, lazily making their way up to the surface, but then simply self destructing. Wonderful as it can be, wine lack the unique mix of components needed for bubbles to form sustainable foam.

Carbonation strongly contributes to aroma, as bubbly beverages contain a rich mix of aroma compounds, giving off twice as much aroma as still ones, and this is true for beers as well.2 It’s also why most wine needs an incurved class with a lot of headspace to capture its volatiles. Beer isn’t so fussy, and is happily aromatic in a variety of shapes—even the plastic ‘airline’ cups used in competitions. 

The Chemistry of Foam

Beer’s long-lasting foam is dependent upon a very particular mix of components. The first and most important is a particular type of proteins known as lipid transport proteins (LTP), present in barley and malt. Since much of the biological matrix of life is water-based, it’s challenging for living things to shuttle crucial fatty acids and other water-insoluble compounds through their aqueous environment. With a barrel-shaped cavity enhanced with molecular components attractive to fats, LTPs make this possible by encouraging fatty molecules to snuggle inside and safely enjoy the ride to their destination.

An LTP’s cavity is a perfect fit for the waxy hop bittering compound isohumulone, beer foam’s second essential component. Its presence explains why the foam always tastes much more bitter than the beer itself. 

Isohumulone interacts with a third type of component called glycoproteins, which are proteins cross-linked with a carbohydrate. One of the more crucial is the glycosylated form of a barley protein ominously called “protein Z.” Since glycoproteins are produced during malt kilning, amber and dark beers often have better, more persistent foam than pale ones. The isohumulone and glycoproteins are attracted to one another and form elastic bonds that help stabilize the foam. 

Beer also has a unique body, quite different from wine or any other beverage I can think of. Its slightly viscous texture results from a number of things, but especially from a network of interconnected proteins, phenols and carbohydrates that also help support the persistence of beer’s foam. Called a colloidal state after the Greek word for “glue,” this body builder in beer has the same structure as gelatin, in which participating molecules form a network robust enough to entrap water. As in a Goldilocks situation, the protein fragments here must be of intermediate length: too small and they can’t form a network; too large and they clump together and fall out as inappropriate haze.1 In addition, hops contribute polyphenols to beer, which can also enhance its body as they do in wine. At modest levels (200 mg/L), they can also increase bitterness intensity and longevity, and also provide increased palate fullness.2

Pouring, Serving and Foam

The method of serving can make a big difference to the way a beer’s head presents itself. In the U.S., we have long been indifferent to foam—just a time-consuming nuisance to impatient drinkers. But in lager-centric regions of Europe, drinkers have learned to be suspicious of a Pils that arrives too soon after ordering, as everyone knows that it takes three full, agonizing minutes to achieve the proper result. This type of head generally requires specific ‘side-pour’ taps and also depend on well-brewed beer with plenty of available foam components for their fluffiness.

Undercarbonated beer obviously presents a challenge, and often results in a lifeless experience. The exception to this is Real Ale, which turns gentle carbonation into an asset. English fans debate the value—and quantity—of foam on their draught beer with hammer and tongs. There is a historic preference for foam in the North, where taps may be fitted with restricting “sparkler” devices. Forcing the lightly carbonated beer through this sparkler creates a dense, crema-like foam.

‘Nitro’ ales and stouts were developed to emulate real ale. In beer, nitrogen behaves quite differently than carbon dioxide. Unlike CO2, it barely dissolves, so when the pressure in a nitrogenated can is released, the gas rushes out to form the lovely cascade of foam so appealing to Irish stout drinkers. Despite its insolubility, nitrogen is actually foam-positive—another reason this trick works. 

With packaged beer, I was taught by an earlier generation of Wisconsin drinkers to pour it “straight down the middle” of the glass. This will result in half a glass of foam, so you need to wait, let it settle and repeat until you have a nice, dense foam on top. This also releases some carbonation, for a creamier and less bloat-inducing pint. Also, rather than popping, many of the bubbles gradually bleed off some of their gas, producing abundant tiny bubbles with robust skins, making for a creamy, long-lasting foam.

In Brazil “chope” (meaning ‘draught,’) is a common form of beer service. Very small (~10 oz) tapered pilsner glasses are filled with lager, often chilled a bit below freezing, then topped off separately with foam resembling whipped cream. With the exceedingly light-bodied industrial beers involved, this trick depends on two things: a ‘creamer’ faucet capable of dispensing pure foam, and foam-boosting additives called alginates, derived from marine algae, as otherwise these beers would not be up to the task. The creamer faucets are fun to experiment with; some pour normally when pulled forward, but dispensing rich foam when pushed backwards. Others simply dispense foam for topping.

Fats are the enemy of foam, as many of us who have brewed with fatty ingredients have experienced. It’s not exactly clear which ingredients, addition points and methods will cause the most trouble, but the suspects are nuts, chocolate, coffee and dairy. It does seem that fats such as butterfat, with a high melting point, seem less problematic. 

In bar service, grease or lipstick on imperfectly clean glasses is often the culprit, but since detergents have a fat-friendly side to them, they can also reduce beer’s surface tension and collapse foam, so a fat free but improperly rinsed glass is also a problem

Bubbles will not form on a molecularly smooth surface; they require rough nucleation sites. You can often see streams of bubbles emanating from tiny scratches in a glass or—horrors!—from residual crud stuck on its walls. Although it’s not always done, it’s good practice to wipe dry the inside of each washed glass, which will help remove this detritus.

Sometimes, especially for sparkling wines, glasses have nucleation sites in the bottom, traditionally made by scratching with a diamond stylus or nowadays laser-engraving. Boston Beer’s Sam Adams lager glass has a small lasered circle; within a few minutes, you can see a perfect donut of foam telegraphed up to the surface. 

Brewing for Foam

This important aspect of beer is affected by many factors in both the recipe and the brewing process, which is why good brewers take care to manage it. Certain ingredients and specific processes are considered either foam-positive or -negative. Malts are foam-positive, while adjuncts such as rice or corn are not. Hops are foam-positive. Tetra Hop, a processed hop product with the skunkiness precursor removed, is highly foam-positive, and is used for that effect. Certain metals may be highly foam-positive, but are otherwise not good for beer. Some, like the cobalt salt that was used briefly in the 1960s as a foam enhancer, are stunningly potent, but also injurious to health. 

The grain bill and how the mash is conducted also can make a difference. Classic historical mashes often included a protein rest—a necessary step with lightly modified malt containing a good fraction of high molecular-weight proteins. With fully modified modern malts, this rest can actually degrade the mid-length proteins necessary for body and helpful for head formation.

High heat intensity can also damage a beer’s protein structure. Advanced modern brewing systems are designed to limit heat input to wort, sometimes skipping a proper boil, getting pasteurization, protein coagulation and stripping of DMS and other unwanted volatiles by gentler means such as sub-boiling temperatures and thin-film evaporation.

One key point about many of beer’s head-forming components is that they may only be used once. After that, they’re useless. It’s important that carbonation in the tank be done gradually, so foam does not form. Traditional methods employ a technique called gespunding, in which the tank is capped with a pressure-relief valve towards the end of fermentation, allowing natural carbonation to build gradually and avoiding in-tank foaming.

Once again something we all take for granted in beer turns out to be another vast and fascinating rabbit-hole of wonder. I, for one, will never look a beer’s foamy finial the same way, and I hope you don’t either.

Further Reading

1a: Gribkova et al., 2022; Biomolecules. 2022 Jan; 12(1): 24.
The Influence of Biomolecule Composition on Colloidal Beer Structure
doi: 10.3390/biom12010024

2: Dietz et al., 2020; Journal of the Institute of brewing Volume126, Issue4 2020 Pages 320-342
The multisensory perception of hop essential oil: a review