Why 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 Zurich 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, 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.

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 stabilises 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 stabilise the bubbles to the maximum possible extent. "These protein fragments function like surfactants, which stabilise foams in many everyday applications such as detergents," explains Vermant.

Collaboration with a major brewery

As he emphasises: "The stability of the foam does not depend individual factors in a linear manner. 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 stabilises 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."

Applications 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 important 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 stabilising 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.