Cold evenings bring back amber beers, spicier seasonal beers and soft lagers to be drunk slowly. Yet, in front of every pint, there is always that moment in which the gaze runs to the foam: there are those who hope it remains high, soft, compact; who accepts a thinner head; those who instead find themselves with the bubbles already vanished, without understanding why. A tiny detail, of course, but which for many enthusiasts is part of the drinking experience itself.
A group of researchers from ETH Zurich, coordinated by Jan Vermant, professor of Soft Materials, thought to answer this question once and for all. After seven years of study they have reconstructed, piece by piece, the physics that governs the behavior of bubbles. Their research, published in Physics of Fluidsreveals for the first time what mechanisms stabilize beer foam and why certain styles seem almost “immortal”, while others give up in seconds.
The investigation was born from a seemingly simple question that Vermant asked a Belgian brewer: “How do you control the quality of beer?”. The answer, surprising in its immediacy, became the basis of the project: “We understand everything from foam”.
Tripel, Dubbel, Singel and lager
By analyzing several Belgian beers, the team discovered that the Tripel they are the true champions of long-lasting foam. The Dubbel follow closely, while the Singellighter and with less fermentation, show a much more fragile hold.
The comparison then continued on two lagers produced by large Swiss breweries. In some cases, the stability of the white hat approaches that of the Belgians, even if the scientific reason is completely different. One of the beers analyzed, however, performed very badly: evanescent foam, unstable surface, enormous margin for improvement. Vermant explained it with an almost ironic calm: “There is still a lot to do. If they want, we can help them.”
For years it was believed that the stability of the foam depended mainly on the barley malt proteins and their ability to create a viscous film around the bubbles. The new research, however, debunks this idea as the only explanation. The forces at play are more complex and, above all, change depending on the style.
In lagerstability depends on surface viscoelasticity. More proteins (even partially denatured) means a more rigid film: protected bubbles, long-lasting foam.
In Tripelthe opposite happens. Viscoelasticity is minimal. Instead, the bubbles are supported by the Marangoni flowsi.e. currents generated by differences in surface tension. It’s a surprisingly easy phenomenon to observe: just sprinkle tea leaves on water and then add a drop of soap. The leaves run away, move, dance at the edges. The same happens on the surfaces of the bubbles of these beers: currents that strengthen, shape and prolong the life of the foam cap.
Physics, however, does not end here. In one Singelproteins behave like tiny particles that accumulate on the surface of bubbles, creating a sort of two-dimensional suspension and providing stability. In Dubbelproteins are intertwined like a thin network. In Tripelhowever, the behavior resembles that of simple surfactants: molecules that move quickly, capable of generating flows and making the system alive.
Behind these differences, researchers have identified a common protagonist: protein LTP1lipid transfer protein 1. Changes structure, changes function, changes behavior depending on the type of beer. And apparently it is she, more than any other, who determines the quality of the foam.
A toast even outside the glass
The brewing industry is not the only one to benefit from this discovery. The physics of bubbles also affects sectors where there is no toast at all. In electric car engines, for example, lubricants can form foams that become dangerous. Vermant and his team are already working with large companies like Shell to figure out how to destroy them in a controlled way.
The same research also aims to create more sustainable surfactants, free of fluorine or silicone, capable of managing surfaces without heavy environmental impacts. Furthermore, as part of a European project, ETH is developing foam-based systems for carrying useful bacteria and, in collaboration with the expert Peter Fischer, is studying how to stabilize milk foam with more natural methods.
And so, from a Belgian pint observed with scientific eyes, we end up talking about electric vehicles, sustainability, biotechnology and nutrition. Beer foam is no longer just a soft promise on a glass: it becomes a window into living, elegant and surprisingly useful physics.
