Why does some beer foam last longer than others?

Researchers have found the holy grail of brewing: the formula for stable beer foam.

But it’s not just breweries that will benefit from these findings.

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 Zurich researchers led by Jan Vermant, professor of soft materials, have now discovered just why this is the case.

Their study appears in the journal Physics of Fluids.

The Belgian researcher and his team put seven years of work into the subject. It all started out with a simple question put to a Belgian brewer: “How do you control brewing?”

“By watching the foam,” was the succinct reply.

Today, the 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.

Triple beats double and single

In this study, the materials scientists showed that when it comes to the Belgian ales studied, “Tripel” beers have the most stable foam, followed by “Dubbel” beers. The head is least stable in beers without intense fermentation and with the lowest alcohol content (“Singel”).

Two lager beers from large Swiss breweries were also among the beers the researchers examined. Beer foam stability can be comparable to that of the Belgian ales. Yet, the underlying physics at work are different. Interestingly, one of them did not score that well.

“There is still room for improvement—we are happy to help,” says Vermant.

To date, researchers have assumed that the stability of beer foam depended primarily on protein-rich layers on the surface of the bubbles: proteins come from barley malt and influence surface viscosity, i.e. the fluidity of the surface, and surface tension.

The new experiments, however, show that the decisive mechanism at work is more complex and depends significantly on the type of beer.

In lager beers, surface viscoelasticity is the decisive factor. This is influenced by the proteins present in the beer, as well as their denaturation: the more proteins the beer contains, the more rigid the film around the bubbles becomes and the more stable the foam will be.

The situation is different with “Tripel”-style beers, where surface viscoelasticity 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 stabilize the bubbles in the beer foam.

The physics of beer foam

Different beers, different brewing conditions, and thus different foam physics. The answer lies in the structure and dynamics of the protein-rich shells of the bubbles. In the Belgian “Singel,” the protein-rich shells behave as if small, spherical particles 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.

In the “Dubbel” beer, proteins form a net-like structure—a kind of membrane—making the bubbles even more stable. In the case of “Tripel,” the physics become even richer; the dynamics of the bubbles’ surface resemble those of simple surfactants, molecules that stabilize foams in many everyday applications.

The exact reasons for this different behavior are still unknown. However, it seems that the protein LTP1 (lipid transfer protein 1) plays a decisive role in stabilizing beer foam. The researchers were able to confirm this by analyzing the structure and content of the protein in the Belgian beers they studied.

Brewery collab

As Vermant emphasizes: “The stability of the foam does not depend on individual factors in a linear manner. You can’t just change one thing 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 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 precise physical mechanism and are able to help the brewery improve the foam on their beers,” says Vermant.

For Belgian beer consumers, the head is important because of the taste and as “part of the experience,” the materials researcher adds.

“But foam isn’t always important wherever beer is served—it’s a cultural thing.”

Beyond beer

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. Vermant’s team is now working with Shell, among other companies, 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 important step in this direction,” Vermant underlines.

In an ongoing EU project, the researchers are also working on foams as carriers for bacterial systems. In collaboration with food researcher Peter Fischer from ETH Zurich, they are also 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,” Vermant concludes.

Source: ETH Zurich

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