For beer lovers, there’s nothing quite as satisfying as a frothy head of foam on a freshly poured glass.
But while some beers lose their foam almost instantly, Belgian beers are famous for keeping a thick, creamy head long after the first sip.
Now, researchers at ETH Zurich, led by Professor Jan Vermant, have figured out why.
Their work, published in Physics of Fluids, reveals that the secret lies in a complex mix of proteins and surface forces that change with the way beer is fermented.
The project began with a simple question years ago, when a Belgian brewer was asked how he controls fermentation.
His reply was short and to the point: “By watching the foam.” That answer sparked years of research into the science of bubbles in beer.
The findings not only explain the long-lasting foam of Belgian brews but could also be useful in industries far removed from brewing.
Not all beers are created equal when it comes to foam. The researchers compared lager beers, which are fermented only once, with Belgian beers that are fermented two or even three times.
They found that lager beers, including a sample from a large Swiss brewery, had the least stable foam. Double-fermented beers did better, but the champions were triple-fermented beers such as the famous Trappist ales brewed in monasteries.
Their foam was by far the most durable.
Until now, scientists believed that foam stability depended mainly on proteins from barley malt, which form a sticky layer on the surface of bubbles. The stickier the surface, the longer the foam should last. That is true for simple lagers.
But Vermant’s team discovered that in multi-fermented Belgian beers, something else takes over. These beers rely on Marangoni stresses—surface flows created by tiny differences in surface tension.
A simple way to see this effect is to sprinkle tea leaves on water and then add a drop of soap. The soap changes the surface tension, creating currents that push the leaves to the edge. In beer, similar flows keep the bubbles alive much longer.
A key player in the process is a protein called LTP1, short for lipid transfer protein 1. In lager beers, LTP1 proteins stay in their natural form, sticking to bubble surfaces like little particles. This helps stabilize the foam but only to a limited degree.
In double fermentation, yeast cells alter these proteins, making them form a net-like film around bubbles, which adds strength. By the third fermentation, the proteins are broken down even further into fragments with both water-loving and water-repelling ends.
These act like natural surfactants, the same kind of molecules used in soaps and detergents, and they maximize the stability of the foam.
The research also showed that balance is important. Simply increasing viscosity or adding surfactants doesn’t automatically improve foam. In fact, it can make it worse by interfering with the surface flows that keep bubbles intact. Beer, it seems, has found the perfect recipe through centuries of tradition.
The study has applications beyond brewing. The same knowledge can help industries prevent unwanted foam in lubricants used in electric cars, or design eco-friendly surfactants without harmful chemicals. Researchers are even exploring foams as carriers for bacteria in environmental systems or for making milk foams more stable.
For Belgian beer drinkers, though, the impact is already clear. A lasting head of foam is more than decoration—it enhances the taste and aroma and forms an essential part of the drinking experience. Thanks to science, we now understand why Belgian beers do it better, and how that knowledge could bubble over into many other fields.
