For many beer lovers, a pint isn't complete without a rich, creamy head of foam. Yet, that satisfying foam often vanishes quickly. Now, after seven years of intensive research, scientists at ETH Zurich have unlocked the secrets behind long-lasting beer foam.
The Quest for the Perfect Head
Led by Professor Jan Vermant, the team's work, published in Physics of Fluids, began with a simple question posed to a Belgian brewer: "How do you control brewing?" The brewer's insightful answer – "By watching the foam" – sparked a deep dive into the complex physics at play.
The researchers have now identified the forces and structures responsible for durable beer foam, providing insights into what makes a beer's head last.
Belgian Ales: A Hierarchy of Foam
Analyzing Belgian ales, the scientists discovered a clear hierarchy in foam stability. Tripel beers boasted the most stable foam, followed by Dubbel beers. Singel beers, with their milder fermentation and lower alcohol content, had the least durable heads.
The team also assessed two lagers from Swiss breweries. While these lagers can achieve similar foam stability to Belgian ales, the underlying physics differ significantly. One lager's performance was unexpectedly poor, prompting Vermant to remark, "There is still room for improvement – we are happy to help."
Challenging Old Theories
For years, it was believed that beer foam was primarily stabilized by protein-rich layers around each bubble, derived from barley malt. These proteins were thought to influence surface viscosity and surface tension.
However, the new research reveals that foam stability is far more complex and highly dependent on the beer style.
Proteins and Surface Forces: The Key to Foam
In lager beers, surface viscoelasticity is the dominant factor. This property is determined by the amount of protein and how these proteins denature. Higher protein levels result in a stiffer film surrounding the bubbles, leading to longer-lasting foam.
Tripel beers, conversely, rely less on surface viscoelasticity. Instead, they maintain foam through Marangoni stresses – forces generated by variations in surface tension across a liquid's surface.
A simple demonstration involves placing crushed tea leaves on water. A drop of soap causes the leaves to be pulled outward, creating swirling currents. These currents help stabilize the bubbles, similar to what happens in Tripel foam.
Inside the Bubble Shells
The researchers found that foam stability depends on the structure and behavior of the protein-rich shells around each bubble. In Belgian Singel beers, these shells resemble a two-dimensional suspension of tightly packed particles, contributing to foam maintenance.
Dubbel beers exhibit a different pattern. Their proteins form a mesh-like membrane that further strengthens the bubbles. Tripel beers, again, stand apart, with bubble dynamics akin to those of simple surfactants.
The reasons for these differences are not fully understood, but lipid transfer protein 1 (LTP1) appears to play a crucial role. The ETH researchers confirmed this by examining the structure and concentration of LTP1 in the Belgian samples.
Brewery Collaboration and Practical Applications
Vermant emphasizes that foam stability is not a linear process. "The stability of the foam does not depend on individual factors in a linear manner. You can't just change one thing and get it right." He cautions that adding more surfactants to increase viscosity can actually destabilize the foam by interfering with Marangoni effects. "The key is to work on one mechanism at a time – and not on several at once. Beer obviously does this well by nature!"
The ETH team is collaborating with a major brewery to enhance foam stability. "We now know the precise physical mechanism and are able to help the brewery improve the foam on their beers," Vermant says.
He also notes that foam is culturally significant, especially in Belgium, where it enhances both taste and the overall drinking experience.
Beyond Beer: Foam Science in Other Industries
This research has wider implications. In electric vehicles, lubricant foaming can pose serious risks. Vermant's group is collaborating with Shell to understand how to efficiently break down these foams.
Another objective is to develop environmentally friendly surfactants that do not rely on fluorine or silicon. "Our study is an important step in this direction," Vermant says.
The team is also investigating foams as carriers for bacterial systems as part of an EU project, and working with food researcher Peter Fischer from ETH Zurich to study how proteins can stabilize milk foam. "So there are many areas where the knowledge we have gained from beer is proving useful," Vermant concludes.
