The key to a perfect beer foam lies in the fermentation process, according to physicists from the federal Swiss Institute of Technology in Zurich (ETH). After seven years of research, the team has identified the factors that make foam stable and long-lasting. The findings, published in the journal *Physics of Fluids* in collaboration with the Eindhoven University of Technology, could have applications in diverse fields, from firefighting foam production to varicose vein therapy.
Beer serves as an ideal model for studying the physics of foam more broadly. The head topping the popular beverage consists of myriad small air bubbles separated by a thin liquid film that must remain stable to prevent the bubbles from bursting and the foam from collapsing.
The researchers analyzed various beer types, discovering that Belgian triple-fermentation brews have the most stable foam, followed by double-fermentation beers. The most ephemeral foam was found on single-fermentation lagers.
It was previously thought that foam stability depended primarily on protein-rich layers forming on the bubbles’ surface—proteins derived from barley malt that influence viscosity and surface tension. However, the new experiments reveal a more complex mechanism. In single-fermentation lagers, surface viscosity is the decisive factor, influenced by protein content: the more protein, the more viscous the film around the bubbles and the more stable the foam.
The situation differs with multi-fermentation Trappist beers, where surface viscosity is minimal. Here, stability relies on the so-called ‘Marangoni effect,’ a physical phenomenon occurring when a liquid is subject to non-uniform surface tension. Another critical factor for foam persistence is the protein LTP1, which becomes increasingly denatured through multiple fermentations, acting like a surfactant that enhances bubble stability.
“Foam stability does not depend linearly on individual factors. You can’t just change one thing and get the right result. The key is to work on one mechanism at a time, not multiple mechanisms simultaneously,” concluded the study coordinator, Jan Vermant.
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