Why the foam on Belgian beers lasts so long
Researchers at ETH Zurich find the holy grail of brewing: the recipe for stable beer foam
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Summertime is beer time - even if the consumption of alcoholic beers is on the decline in Switzerland. And for beer lovers, nothing beats a head of foam sitting on top of the golden, sparkling barley juice.
But with many beers, this dream is quickly shattered and the foam collapses before you can take your first sip. However, there are also types of beer where the head lasts a long time.
Researchers at ETH Zurich led by Jan Vermant, Professor of Soft Materials, have now discovered why this is the case. The Belgian and his colleagues have been working on this for seven years. It all began with a simple question to a Belgian brewer: "How do you control fermentation?" - "By observing the foam," was the answer.
Today, the ETH scientists know the mechanisms behind the perfect beer foam. And perhaps in future, beer drinkers will be able to admire the head in their glass a little longer before quenching their thirst.
Lager beers have the most perishable foam
In this study, the material scientists show that Belgian beers that have been triple-fermented have the most stable head, closely followed by double-fermented beers. In contrast, the head is least stable in single-fermented lager beers.
Triple-fermented beers include Trappist beers, a specialty of the monastic order of the same name. The lager beers examined by the ETH researchers also include a beer from a large Swiss brewery. "There's still room for improvement - we're happy to help," says Vermant with a smile.
Until now, the researchers assumed that the stability of the beer foam depended primarily on protein-rich layers on the surface of the bubbles (see ETH News): Proteins originate from barley malt and influence the surface viscosity, i.e. its stickiness, as well as the surface tension.
Surface tension instead of viscosity
However, the new experiments show that the decisive mechanism is more complex and strongly depends on the type of beer.
Surface viscosity is the decisive factor for single-fermented lager beers. It is influenced by the proteins present in the beer: the more proteins there are in the beer, the more viscous the film around the bubbles becomes and the more stable the foam.
The situation is different with multi-fermented Trappist beers: Here, the surface viscosity is minimal. The stability is created by so-called Marangoni tensions. These are forces caused by differences in surface tension.
This effect can be observed by placing crushed tea leaves on a water surface. The fragments initially spread evenly. If a drop of soap is added, the tea leaves are suddenly pulled to the edge. This creates currents that circulate on the surface. If such currents persist for a long time, they stabilize the bubbles in the beer foam.
A protein determines foam quality
However, the protein LTP1 (lipid transfer protein 1) plays a decisive role in the stabilization of the beer foam. The ETH researchers were able to confirm this by analyzing the protein content of the beers studied.
The so-called LPT1 proteins are present in their original form in simply fermented beers, such as lager beers. They act like small, spherical particles that are densely packed on the surface of the bubbles. This corresponds to a two-dimensional suspension, i.e. a mixture of a liquid and finely dispersed solids, which in turn stabilizes these bubbles.
During the second fermentation, the proteins are slightly denatured by the yeast cells, i.e. their natural structure is slightly altered. They then form a net-like structure, a kind of membrane, which makes the bubbles even more stable.
During the third fermentation, the already altered LPT1 proteins are denatured to such an extent that there are fragments with one water-repellent and one "water-loving" end. These fragments reduce the interfacial and surface tensions and stabilize the vesicles to the maximum. "These protein fragments function like surfactants, which stabilize foams in many everyday applications such as detergents," explains Vermant.
Collaboration with a large brewery
He emphasizes: "The stability of the foam does not depend linearly on individual factors. You can't just change 'something' and get it 'right'." If the viscosity is increased with additional surfactants, this can even make the foam less stable because it slows down the Marangoni effects too much. "The key is to work specifically on one mechanism - not on several at the same time. Beer obviously does this well by nature!" says Vermant.
In this study, the ETH professor worked with one of the world's largest breweries. They are working on the foam stability of their beers. They therefore want to understand what really stabilizes the beer foam. "We now know the exact mechanism and can help the brewery to improve the foam of its beers," says Vermant.
For Belgian beer consumers, foam is important because of the taste and as "part of the experience", as the materials researcher says. "But foam is not so important everywhere where beer is drunk. It's something cultural."
Applications also possible in technology and the environment
The findings from beer foam research also have significance outside of the art of brewing. In electric vehicles, for example, lubricants can foam - a dangerous problem. Together with Shell, among others, Vermant's team is now investigating how such foams can be destroyed in a targeted manner.
Another goal is to develop sustainable surfactants that do not contain fluorine or silicon. "Our study is an important step in this direction," says Vermant.
In an 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 the stabilization of milk foam using proteins. "So there are many areas where the knowledge we have acquired with beer is useful," says Vermant.
Note: This article has been translated using a computer system without human intervention. LUMITOS offers these automatic translations to present a wider range of current news. Since this article has been translated with automatic translation, it is possible that it contains errors in vocabulary, syntax or grammar. The original article in German can be found here.
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