A warm day with a cold newly tapped beer. For many Swedes, summer is the time of year when beer tastes the best and maybe the white foam is an important part of the experience. The foam can also be observed to determine if it has been a successful fermentation process.
Just like other types of foam, the one that lies on top of the beer consists of small air-filled bubbles that are separated by a thin layer of liquid. If the layer is not stable, the bubbles will burst. How long the bubbles remain intact depends on several things, including the amount of carbon dioxide in the bubble, the protein composition in the liquid, the surface tension and viscosity - how viscous the liquid is. Sometimes, different substances are added to make the foam more stable.
Perfect model
In a new study, researchers at, among others, Eindhoven university of technology in the Netherlands, have investigated what happens in the thin layer of liquid that separates two foam bubbles. They wanted to use beer primarily because they perceived the drink as a perfect model for how foam forms and disappears.
Six types of beer were studied - two types of bottom-fermented Swiss lager, a single-fermented Belgian ale, a double-fermented Belgian ale, and two types of triple-fermented Belgian trappist beer (brewed in monasteries by trappist monks).
According to the study, which was published in the journal Physics of Fluids, it is different physical phenomena that keep the foam in different types of beer. The triple-fermented beers had, along with one of the lager types, the most stable foam, followed by the double-fermented one. The single-fermented beer and the other lager type had the least stable foam. But how long it lasts is determined by different things in the different types of beer.
Protein affects
In lager beer, the viscosity at the surface was the decisive factor, which in turn was affected by the proteins in the beer - the more protein, the more durable the foam. With trappist beer, it was instead a more complex interaction between surface tension and viscosity that determined.
"How stable the foam is is not determined by one thing. It is not possible to just change one factor and get it in a certain way …", says Jan Vermant, one of the study authors in a press release.
The researchers point out that the study can provide knowledge that extends beyond the world of beer.
Correction: In an earlier version, there was an error regarding trappist beer.
