The frothy cap on beer is more than just a party trick. For brewers, it is a symbol of craftsmanship, and for scientists, it is a mystery of chemistry and physics. Yet anyone who has watched a pint lose its head knows that its stability is elusive. What makes some beer foam drain within seconds while others last for minutes?
Beer foam is composed of many tiny air bubbles. For decades, it was believed that special proteins form an elastic network around these bubbles. The network prevents the bubbles from merging, thus holding up the beer head. This reasonable explanation, however, does not tell the whole story.
Emmanouil Chatzigiannakis, assistant professor at Eindhoven University of Technology, discovered a hidden stabilization mechanism during a seven-year side project. His team found that the protein network does exist in some beers, such as Swiss lagers. However, the foam of other types of beer, like the Belgian tripel and dubbel, relies on “Marangoni stresses,” which are caused by small differences in surface tension across the bubble, to remain stable. “When it comes to physics, it’s the stabilization mechanism. When it comes to brewing, they are completely different types of beers,” Chatzigiannakis said.
To reach this conclusion, Chatzigiannakis and his team applied pressure to nanometer-thick films of beer—essentially the tiny amounts of beer between foam bubbles—and measured their stability as they were gradually drained. The “thin-film balance” methodology provided plenty of insights. “We use interferometry and directly visualize […] the thickness differences in nanometer resolution,” Chatzigiannakis said.
Surface rheometry provided a second way to measure how easily the proteins flow before breaking apart, known as the viscoelasticity. A rigid protein network appeared in lager foams, but not in tripels. It turned out that Marangoni stress was the mechanism that held up the frothy head in tripels.
“The main surprise was that we didn’t expect Marangoni stresses,” Chatzigiannakis said. “I was thinking that I was doing the experiments incorrectly.”
Beyond physics, Chatzigiannakis and his team conducted a protein-type analysis to investigate the biochemistry of foam, and detected lipid transfer protein 1 (LTP1) and Serpin Z4 protein. Both active promoters of foam, they varied in abundance between different beers. Tripel beers contained a higher amount of LTP1, while dubbel beers, which have shorter fermentation periods and lower alcohol content, have more Serpin Z4. , while Serpin Z4 protein appears more in dubbel beers, which have lower alcohol content. The differences in size and structure of these proteins explain why the two types of beers stabilize their foam in distinct ways.
Buying packs of beer at 8:00 am for experiments was a memorable experience for Chatzigiannakis. “Once, the cashier actually commented on it,” Chatzigiannakis said. “I was buying a lot of beer in the morning, and he said, ‘It’s quite early to drink that much beer!’”
These results could not only help brewers create more controlled, stable foams, but also extend to other industries. For example, understanding how polymer networks are stabilized and the behavior of facial foams are important applications of this research.
Next time you pour a cold beer—only if you’re over twenty-one years old!—before gulping it down, take a moment to admire the delicate froth on top—where physics, chemistry, and brewing tradition merge together in every bubble.