Science of making good beer: It’s all about the foam

A bartender pours a draft beer in a bar in Brest, western France – Copyright AFP/File FRED TANNEAU

Scientists have discovered the basis behind one of the important elements in brewing: the long-sought formula behind stable beer foam. The research explains why different beers rely on different physical mechanisms to keep bubbles intact and why some foams last far longer than others.

For many people who enjoy a glass of beer, the experience feels incomplete without a thick, creamy layer of foam resting on top. Yet this foam often disappears quickly, collapsing before the first sip. Some varieties, however, manage to hold their foam for a surprisingly long time. The reason for this has not been determined – until now.

Researchers at ETH Zurich have uncovered the scientific explanation for these differences. The work, based on materials science, took seven years of detailed investigation.

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The idea for the study began with a straightforward question posed to a Belgian brewer: “How do you control brewing?” The brewer’s answer was brief but revealing: “By watching the foam.”

The scientists now understand the forces and structures responsible for long-lasting beer foam, offering new insight into what keeps a beer’s head intact.

Tripel, Dubbel, and Singel: Which Foams Hold Up Best?

In their analysis of Belgian ales, the scientists found a clear hierarchy. “Tripel” beers produced the most stable foam, followed by “Dubbel” beers, while “Singel” beers had the least durable head due to milder fermentation and lower alcohol content.

The researchers also evaluated two lagers from large Swiss breweries. Although these lagers can achieve foam stability similar to Belgian ales, the physics behind them vary significantly. One lager performed noticeably worse.

For many years, scientists believed that beer foam mainly stayed intact because of protein-rich layers that formed around each bubble. These proteins, which come from barley malt, can influence how easily the bubble surface flows (its surface viscosity) and its surface tension.

However, the new experiments show that foam stability is more complex than previously thought, and highly dependent on beer style.

How Proteins and Surface Forces Shape Foam Stability

In lager beers, foam stability is controlled by surface viscoelasticity. This property depends heavily on both the amount of protein in the beer and how these proteins denature. Higher protein levels result in a stiffer film surrounding the bubbles, which helps the foam last longer.

“Tripel” beers, by contrast, rely very little on surface viscoelasticity. Instead, they maintain foam through Marangoni stresses — forces created when variations in surface tension generate movement across a liquid’s surface.

The Marangoni effect (also called the Gibbs–Marangoni effect) is the mass transfer along an interface between two phases due to a gradient of the surface tension. This phenomenon was first identified in the so-called “tears of wine” effect.

Stability matters

The researchers found that foam stability depends on the structure and behaviour of the protein-rich shells that surround each bubble. In Belgian “Singel” beers, these shells behave as though many small, spherical particles are tightly packed across the bubble surface. This resembles a two-dimensional suspension (a mixture of a liquid and very fine solids), which helps maintain foam.

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“Dubbel” beers show a different pattern. The proteins create a mesh-like membrane that strengthens the bubbles even more. “Tripel” beers again stand apart, with bubble dynamics resembling those of simple surfactants, the molecules commonly used to stabilize foams in everyday products.

The precise reasons for these differences are still to be fully determined. One protein, LTP1 (lipid transfer protein 1), appears to play a major role.

It was also found that foam stability is not influenced in a straightforward or linear way. As an example, adding more surfactants to increase viscosity may actually destabilise the foam because it interferes with Marangoni effects.

The research appears in the journal Physics of Fluids, in a paper titled “The hidden subtlety of beer foam stability: A blueprint for advanced foam formulations.”