Physics of beer foam: researchers uncover the secret of stable crowns

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For many beer lovers, a magnificent, stable head is the sign of a perfectly brewed and tapped beer. However, the physical and chemical mechanisms that hold the foam together are astonishingly complex and were previously only partially understood. A team from ETH Zurich and Eindhoven University of Technology now claim to have deciphered the "holy grail of the art of brewing", which they say took them seven years to do.

The study, published in the journal "Physics of Fluids", shows that there is no single mechanism, but that the type of beer and its brewing process are decisive. The team led by Emmanouil Chatzigiannakis and Jan Vermant got to the bottom of the matter by investigating various commercial beers – from Swiss lagers to Belgian Trappist beers – using high-precision methods. Using surface rheometry (determination of flow behavior), tensiometry (measurement of surface tension) and a special "dynamic thin-film balance", they were able to directly observe and measure the processes in the wafer-thin liquid films that separate the individual gas bubbles in the foam.

Two paths to a stable crown

The key finding of the study is that there are two fundamentally different stabilization mechanisms that dominate depending on the type of beer. For bottom-fermented beers such as lager, the surface viscosity is the decisive factor. Proteins accumulate at the interface between the beer and the CO₂ bubble and form a cohesive, rather rigid network. This tough layer immobilizes the surface of the bubbles and slows down the flow of liquid from the walls of the foam bubbles (known as drainage). As a result, the foam remains stable for longer because the bubbles do not dry out and burst as quickly.

The situation is entirely different with top-fermented, multi-fermented Belgian ales such as Tripel or Dubbel. Here, the surface viscosity was surprisingly low. Instead, the so-called Marangoni tensions are at work here. The proteins create mobile "islands" on the surface rather than a rigid network. When a liquid film between two bubbles becomes thinner due to drainage, concentration and thus stress gradients are created at this point on the surface. These gradients generate a flow that actively draws fluid back into the thinning area. This "self-healing effect" can even lead to visible, recirculating currents in the film, making the foam extremely robust.

Fermentation as the key

The researchers were able to link this effect directly to the brewing process. They examined three beers from the same Belgian brewery, which differed mainly in the number and duration of fermentations (Singel, Dubbel and Tripel). The result was clear: the more fermentation steps a beer undergoes, the more pronounced the Marangoni effect and the more stable the foam.

A proteomic analysis confirmed the chemical basis for this: the concentration and functionality of the lipid transfer protein 1 (LTP1), which is important for foam, increases with fermentation intensity. In beers such as dark Dubbel, the protein serpin Z4 also plays a role, the properties of which are influenced by Maillard reactions – a non-enzymatic browning reaction – during kilning, the drying of the malt.

A blueprint for better foams

The findings are not only of great importance for brewers, who can now develop more targeted strategies to improve foam quality – either by promoting surface viscosity or by optimizing the conditions for the Marangoni effect. This depends on the type of beer.

The authors also see their work as a "blueprint for advanced foam formulations" outside the brewing industry. According to the researchers, understanding the interplay between viscosity and Marangoni stresses could be useful in the development of stable foams in areas such as food technology, cosmetics, firefighting or even medical applications such as the treatment of varicose veins.

(mack)