On a warm evening, a freshly poured beer can look perfect, topped with a thick cap of foam. Yet that foam, known as ‘head’ among aficionados, often fades almost instantly. Some beers, however, keep their head intact far longer than your everyday lager.

Scientists at ETH Zurich set out to discover why. After seven years of experiments and brewery partnerships, their new study shows that foam stability depends on a precise mix of bubble physics and protein chemistry.

Foam, Fermentation, and the Physics of Bubbles

The researchers studied six beers, ranging from Swiss lagers to Belgian ales. They found that the more times a beer is fermented, the longer its foam endures. Single-fermented lagers, like many mass-market beers, produced the most fleeting heads. Double-fermented beers lasted longer. And triple-fermented Belgian Trappist ales—traditional beers made by monks for centuries—topped the list with foam that could linger far longer than the rest.

“The stability of the foam does not depend on individual factors in a linear manner,” said Jan Vermant, professor of soft materials at ETH Zurich. “You can’t just change ‘something’ and get it ‘right.’ The key is to work on one mechanism at a time—and not on several at once. Beer obviously does this well by nature.”

Beer foam is a delicate balance of proteins, bubbles, and invisible forces. Proteins from barley malt gather at the surface of bubbles, forming films that can either hold strong or rupture. But the team discovered that the story changes depending on how the beer is fermented.

In lagers, proteins remain relatively intact, creating a viscous film around bubbles. This surface viscosity stabilizes the foam, at least for a short time. In Belgian ales, fermentation alters the proteins in stages. After two fermentations, proteins form net-like structures that bind bubbles together. After three fermentations, the proteins break into fragments that act like natural surfactants. These fragments trigger surface currents known as Marangoni stresses—swirling flows that constantly redistribute material along bubble walls, keeping them from collapsing.

Emmanouil Chatzigiannakis, a co-author of the study, compared the process to a simple kitchen experiment: sprinkle tea leaves on water, then add a drop of soap. The soap creates a surface tension gradient that sends the leaves swirling outward. In beer, similar currents stabilize foam by circulating endlessly across bubble surfaces.

Proteins at Work

One protein in particular—lipid transfer protein 1, or LTP1—proved decisive. In lagers, LTP1 acts like tiny spheres packing together at the surface, while in double-fermented beers, the protein partially unfolds, knitting a stronger mesh. However, in triple-fermented beers, LTP1 fragments become amphiphilic—part water-loving, part water-repelling—just like soap molecules.

“These protein fragments function like surfactants, which stabilise foams in many everyday applications such as detergents,” Vermant explained.

That helps explain why Belgian beers can hold their heads high, while lagers slump. But foam is not equally prized everywhere. “Foam isn’t that important everywhere beer is served—it’s basically a cultural thing,” Vermant said. In Belgium, however, it is part of the drinking ritual, a visible sign of quality and taste.

Ok, But What For?

The study may sound like a playful exploration of beer, but its implications reach far beyond the pub. Foams are everywhere: in whipped cream and cappuccinos, in shaving cream and fire retardants, even in the lubricants of electric vehicles. Understanding how to stabilize (or destabilize) foams could help engineers prevent dangerous frothing in car engines, design greener detergents, or even develop protein-based systems for biotechnology.

The researchers are already working with Shell to study how to break down unwanted foams in lubricants. They are also exploring how to create sustainable surfactants free of fluorine or silicon. And, closer to the café than the garage, they are experimenting with milk foam for cappuccinos, applying lessons learned from beer to morning coffee.

For beer lovers, the study offers comfort: science has confirmed what Belgian monks may have known all along. The art of fermentation creates not just stronger flavors but stronger foams, too.

The findings appeared in the journal Physics of Fluids.