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 specialty 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.

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.

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 stabilize the bubbles in the beer foam.