Why do some beers retain their foam for a long time, while others see it disappear in an instant? Researchers have looked into this mystery, revealing mechanisms that go beyond simple gustatory pleasure.
Scientists from ETH Zurich, led by Jan Vermant, have spent seven years of their careers studying the stability of beer foam. Their work, published in the journal
Physics of Fluids, shows that Belgian triple-fermented beers, such as Trappist beers, have the most stable foam. In contrast, lager-type beers, with a single fermentation, see their foam collapse quickly. This difference is explained by distinct physicochemical processes related to proteins and surface tensions.
In single-fermentation beers, surface viscosity plays a key role. Proteins, derived from barley, form a viscous layer around the bubbles, stabilizing them. The richer the beer is in proteins, the longer the foam lasts. However, this simple approach is not sufficient for multiple-fermentation beers, where other forces come into play.
The LTP1 protein is a central player. In lagers, it remains in its original form, acting as small particles that densify at the surface of the bubbles. During additional fermentations, it denatures, first forming a membranous network, then fragments with surfactant properties, similar to those of detergents, optimizing stability. This evolution explains why triple-fermented beers have such a resistant foam.
For Trappist beers, stability comes from Marangoni stresses, forces resulting from differences in surface tension.
Marangoni stresses are forces that appear when the surface tension of a liquid varies across its surface. This variation can be caused by differences in temperature, solute concentration, or by the addition of substances such as surfactants.
In the case of beer, these stresses create movements on the surface of the bubbles, which help maintain their integrity. For example, if an area has lower surface tension, the liquid is pulled toward areas where the tension is higher, generating stabilizing currents.
This phenomenon is observable in everyday life, such as when a drop of alcohol is added to water, causing swirls. In materials science, there are situations where it is crucial for designing stable foams in various products, from food to cosmetics.
Understanding these mechanisms allows engineers to manipulate fluid properties for applications ranging from the pharmaceutical industry to environmental protection, where foam control is essential.