Beer foam is often viewed as the hallmark of quality. That iconic head forms as gaseous carbon dioxide bubbles rise to the surface. For more than 50 years, researchers have explored the formation of beer foam, largely as a function of the beer’s chemical composition and primarily using analytical and experimental methods. Now Antonio Delgado (University of Erlangen-Nuremberg in Germany), Wenjing Lyu (German Engineering Research and Development Center in Busan, South Korea), and colleagues have paired experimental observations with a numerical simulation to tackle foam-formation dynamics.
The team employed a new tapping system that fills a specially designed cup from the bottom. In the hygienic, no-contact filling process, the cup is placed on a nozzle that pushes beer up through temporary openings. The researchers then monitored the filling process from the side and above with a pair of cameras. To better understand the observations, they replicated the process in a numerical simulation that solves the Navier–Stokes equation for a mixture of two fluids—the liquid beer and the CO2 gas.
Delgado and his colleagues found that nearly all the beer foam forms within the first second, when the flow into the cup is especially turbulent. Once the cup has a layer of liquid that covers the bottom, the flow becomes less turbulent, and the rate of foam formation rapidly slows. The researchers closed the tap after about four seconds and continued to monitor the cup as the foam collapsed, as shown in the image.
The team compared the foam formation for beer at three different temperatures and dispensed by taps with three different pressures. The consistent trend was that warmer temperatures and higher pressures create more foam because warmer beer degases more quickly, and faster dispensing creates more foam in the early filling process. At 15 °C and 1.5 bar—the highest values studied—the time-averaged foam volume was a whopping 82.9% of the total drink volume.
But creating foam is only half the equation; for drinkers to enjoy the foam, it needs to stick around. Although the foam lasted the longest for the highest temperature and pressure studied, the relationship wasn’t as straightforward as it was for foam formation. For pump pressures of 1 bar and 0.5 bar, the foam stability was higher at the intermediate temperature of 10 °C than at 15 °C.
The reason is the distribution of bubble sizes. At lower temperatures, the bubbles have a fairly uniform size, but at higher temperatures, the sizes vary. The pressure difference between neighboring large and small bubbles causes the large bubbles to siphon off the small bubbles’ gas until the small bubbles wither away. And thus the foam dissipates faster.
For the current study, the researchers used a single home-brewed wheat beer, but previous work from other groups has shown that foam stability depends on factors that vary with beer type, such as the amount of protein. Future work could investigate the dynamics for various beers. (W. Lyu et al., Phys. Fluids 35, 023318, 2023.)
