On tap: What makes beer foams so stable?
Research into stability of foams finds a valuable test subject in a tall glass of beer
image:
Wave interference (interferometry) images of beer bubbles, superimposed onto a glass of foamy beer.
view moreCredit: AIP/Chatzigiannakis et al.
WASHINGTON, August 26, 2025 – Beer is one of the world’s most popular drinks, and one of the clearest signs of a good brew is a big head of foam at the top of a poured glass. Even brewers will use the quality of foam as an indicator of a beer having completed the fermentation process. However, despite its importance, what makes a large, stable foam is not entirely understood.
In Physics of Fluids, from AIP Publishing, researchers from ETH Zurich and Eindhoven University of Technology investigated the stability of beer foams, examining multiple types of beer at different stages of the fermentation process.
Like any other foam, beer foam is made of many small bubbles of air, separated from each other by thin films of liquid. These thin films must remain stable, or the bubbles will pop, and the foam will collapse. What holds these thin films together may be conglomerates of proteins, surface viscosity, or the presence of surfactants, which are molecules that can reduce surface tension and are found in soaps and detergents.
While the researchers have spent years studying the formation of foams, they realized beer could serve as a perfect testing ground.
“The idea was to directly study what happens in the thin film that separates two neighboring bubbles,” said author Emmanouil Chatzigiannakis. “And the first thing that comes to mind when thinking of bubbles and foams is beer.”
Turning to a collection of scientific imaging and rheometry techniques, the team was able to determine how these thin films could hold together to make a stable foam.
“We can directly visualize what's happening when two bubbles come into close proximity,” said Chatzigiannakis. “We can directly see the bubble’s protein aggregates, their interface, and their structure.”
They found that for single fermentation beers, foams are held together primarily through the surface viscosity of the beer. However, for double-fermented beers, the proteins in the beer come together to form a two-dimensional structure, giving the thin films an elastic quality that keeps them intact longer.
With the multiple different ways beer foams hold together, the researchers believe that beer provides an excellent platform to study the stability of foams in general, with applications in everything from oil separation to firefighting chemicals and treating varicose veins.
“This is an inspiration for other types of materials design, where we can start thinking about the most material-efficient ways [of creating stable foams],” said author Jan Vermant. “If we can't use classical surfactants, can we mimic the 2D networks that double-fermented beers have?”
The authors also hope that their work will make its way to the brewers that inspired them, and that in the future the team will identify ways to increase or decrease the amount of foam so everyone can pour a perfect glass of beer every time.
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The article “The hidden subtlety of beer foam stability: A blueprint for advanced foam formulations” is authored by Emmanouil Chatzigiannakis, Alexandra Alicke, Lea Le Bars, Lucas Bidoire, and Jan Vermant. It will appear in Physics of Fluids on Aug. 26, 2025 (DOI: 10.1063/5.0274943). After that date, it can be accessed at https://doi.org/10.1063/5.0274943.
ABOUT THE JOURNAL
Physics of Fluids is devoted to the publication of original theoretical, computational, and experimental contributions to the dynamics of gases, liquids, and complex fluids. See https://pubs.aip.org/aip/pof.
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Journal
Physics of Fluids
Article Title
The hidden subtlety of beer foam stability: A blueprint for advanced foam formulations
Article Publication Date
26-Aug-2025
Why the foam on Belgian beers lasts so long
Summertime is beer time – even if the consumption of alcoholic beers is declining in Switzerland. And for beer lovers, there is nothing better than a head of foam topping the golden, sparkling barley juice. But with many beers, the dream is quickly shattered, and the foam collapses before you can take your first sip. There are also types of beer, however, where the head lasts a long time.
ETH researchers led by Jan Vermant, Professor of Soft Materials, have now discovered just why this is the case. Their study has just been published in the journal Physics of Fluids. The Belgian researcher and his team put seven years of work into these issues. It all started out with a simple question put to a Belgian brewer: “How do you control fermentation?” — “By watching the foam,” was the succinct reply.
Today, ETH scientists understand the mechanisms at work behind perfect beer foam. And perhaps future beer drinkers will be able to admire the head of foam in their glasses a little longer before quenching their thirst.
Lager beers have the most fleeting foam
In this study, the materials scientists showed that Belgian beers that have been triple fermented have the most stable foam, followed by double fermented beers. The head is least stable in single fermented lager beers.
Triple-fermented beers include Trappist beers, a speciality of the eponymous monastic order. A beer from a large Swiss brewery was also among the lager beers the ETH researchers examined. “There is still room for improvement – we are happy to help,” says Vermant with a smile.
To date, researchers assumed that the stability of beer foam depended primarily on protein-rich layers on the surface of the bubbles (see ETH News): proteins come from barley malt and influence surface viscosity, i.e. the stickiness of the surface, and the surface tension.
Surface stress instead of viscosity
The new experiments, however, show that the decisive mechanism is more complex and depends significantly on the type of beer. In single-fermentation lager beers, surface viscosity is the decisive factor. This is influenced by the proteins present in the beer: the more proteins the beer contains, the more viscous the film around the bubbles becomes and the more stable the foam will be.
The situation is different with multi-fermentation Trappist beers, where surface viscosity is actually minimal. Stability is achieved through so-called Marangoni stresses - forces that arise from differences in surface tension.
This effect can be readily observed by placing crushed tea leaves on the surface of water. Initially, the fragments spread out evenly. If a drop of soap is added, the tea leaves are suddenly pulled to the edge, causing currents to circulate on the surface. If these currents persist for a long time, they stabilise the bubbles in the beer foam.
A protein is decisive for foam quality
However, the protein LTP1 (lipid transfer protein 1) plays a decisive role in stabilising beer foam. ETH researchers were able to confirm this by analysing the protein content of the beers they studied.
In single fermentation beers, such as lager beers, the so-called LPT1 proteins are present in their original form. They act like small, spherical particles that arrange themselves densely on the surface of the bubbles. This corresponds to a two-dimensional suspension, i.e. a mixture of a liquid and finely distributed solids, which in turn stabilizes these bubbles.
During the second fermentation, the proteins are slightly denatured by the yeast cells, meaning that their natural structure is slightly altered. They then form a net-like structure, a kind of membrane, making the bubbles even more stable.
During the third fermentation, the already altered LPT1 proteins are denatured to such an extent that fragments with a water-repellent and a “water-loving” end are formed. These fragments reduce the interfacial and surface tensions and stabilize the bubbles to the maximum possible extent. “These protein fragments function like surfactants, which stabilize foams in many everyday applications such as detergents,” explains Vermant.
Collaboration with a major brewery
As he emphasizes: “The stability of the foam does not depend on individual factors linearly. You can't just change 'something' and get it 'right'.” For example, increasing the viscosity with additional surfactants can actually make the foam more unstable because it slows down the Marangoni effects too strongly. “The key is to work on one mechanism at a time – and not on several at once. Beer obviously does this well by nature!” says Vermant.
In conducting this study, the ETH professor collaborated with one of the world's largest breweries that was working on the foam stability of their beers and wanted to understand what actually stabilizes beer foam. “We now know the mechanism exactly and are able to help the brewery improve the foam of their beers,” says Vermant.
For Belgian beer consumers, the head is important because of the taste and as “part of the experience,” as the materials researcher relates. “But foam isn't that important everywhere beer is served - it's basically a cultural thing.”
Potential applications also in technology and the environment
The findings from beer foam research are also significant over and beyond the art of brewing. In electric vehicles, for example, lubricants can foam– presenting a dangerous problem. Vermants' team is now working with Shell, among others, to investigate how such foams can be destroyed in a targeted manner.
Another goal is to develop sustainable surfactants that are free of fluorine or silicon. “Our study is an essential step in this direction,” as Vermant underlines.
In an ongoing EU project, the researchers are also working on foams as carriers for bacterial systems. And in collaboration with food researcher Peter Fischer from ETH Zurich, they are working on stabilizing milk foam by way of proteins. “So there are many areas where the knowledge we have gained from beer is proving useful,” as Vermant concludes.
Journal
Physics of Fluids
Article Title
The Hidden Subtlety of Beer Foam Stability: A Blueprint for Advanced Foam Formulations
Article Publication Date
26-Aug-2025
