There’s something undeniably satisfying about opening a swing-top bottle. That crisp pop, the rush of cool air, and the promise of a refreshing drink inside—it’s an experience that homebrewers and craft beer lovers alike have come to appreciate. But have you ever wondered what’s really going on when that cap flips open?
Max Koch did. And not just in passing. Koch, a researcher at the University of Göttingen in Germany, combined his professional expertise in physics with his hobby of homebrewing to explore what happens when you open one of these iconic bottles. What started as a fun experiment with a high-speed camera quickly became a deep dive into the unseen world of sound waves, shockwaves, and the physics of bubbling beer.
And the results? They were cool. Literally.
From Kitchen Curiosity to Cutting-Edge Research
It all began in Koch’s home brewery—picture bubbling fermenters, gleaming bottles, and the hum of science quietly in the background. He decided to film the moment he popped open one of his freshly brewed beers using a high-speed camera. The footage, as you might guess, was fascinating. But what he didn’t expect was that it would become the basis for a formal scientific study.
Enter Robert Mettin, head of the Ultrasound and Cavitation group at Göttingen’s Third Institute of Physics. Mettin took one look at the footage and suggested submitting it to Physics of Fluids, which happened to be preparing a special issue called “kitchen flows.” That nudge was all it took for Koch and his team to up their game.
They brought out the high-fidelity microphones, fired up computational fluid dynamics simulations, and decided to explore, in painstaking detail, what makes that signature “pop” when a swing-top bottle opens.
The Surprising Sound of Swing-Tops: More “Ah” Than “Bang”
The first major discovery? That beloved sound when you crack open a swing-top isn’t a sharp shockwave like a balloon popping. Instead, it’s a much softer, but still quick, “ah” sound.
By slowing the footage down and listening carefully, Koch’s team found that this “ah” is actually caused by a standing wave—basically, a vibration that’s trapped and oscillates up and down within the neck of the bottle. And it’s not just air vibrating. It’s the rapid condensation that forms in the bottleneck that plays a starring role here.
Once the seal breaks, the pressurized carbon dioxide (CO₂) and air mixture inside the bottle expands violently. This sudden expansion cools the gas to an astonishing -50 degrees Celsius. That’s colder than most freezers, and it happens in the blink of an eye. The extreme cooling reduces the speed of sound in the gas, which explains why the sound we hear has a much lower frequency than you’d expect.
“If you blow across the top of a full bottle like a whistle, you’ll hear a much higher tone,” Koch explained. “But the pop’s frequency is way lower because of the sudden cooling and expansion inside the bottle.”
And while the pop doesn’t seem that loud, it’s another story inside the bottleneck. There, the sound can reach decibel levels comparable to standing next to an airplane turbine—at just one meter away! Fortunately, it only lasts for a fraction of a second.
A Symphony of Science: Liquids, Bubbles, and Motion
After the initial pop, the science show isn’t over. Koch’s high-speed cameras captured what happens next—and it’s a chaotic dance of physics.
As soon as the bottle opens, the dissolved CO₂ in the beer rapidly starts to form bubbles. These bubbles change the density and pressure inside the liquid, triggering the beer to rise slightly inside the bottle’s neck. Add in the fact that most people tilt or move the bottle while opening it, and you get a sloshing motion that creates waves in the bottleneck.
But the team noticed something else, too.
The swing-top lid doesn’t just fly open and stop. As it flips back, the metal clasp hits the glass bottle with a sharp edge. That sudden impact transfers energy back into the liquid—essentially adding an extra kick to the bubble formation. This process, called gushing, is why some bottles foam over right after being opened. It turns out that even the small clink of the lid hitting the bottle can lead to a bubbly eruption.
Cracking the Code (Almost)
One of the team’s challenges was figuring out why the sound had the low frequency it did. They built a simplified physical model that accounted for the rapid cooling, the gas dynamics, and the behavior of standing waves. But not everything added up.
Their computer simulations predicted an initial sharp spike in the sound before the short “ah” resonance—the pop we hear. Oddly enough, that sharp spike never appeared in their real-life experiments.
What was missing? Koch and his team still aren’t entirely sure. “That was one of the puzzles we couldn’t quite crack,” Koch admitted. “Our simulations showed a clear initial peak, but in practice, it just wasn’t there.”
The Real Challenge? Taste Testing the Data
Of course, no experiment involving homebrewed beer would be complete without a little quality control. Koch joked that another big challenge was “drinking the homebrewed beverages and still maintaining clarity during the experiment.”
We can only imagine the scientific discipline involved in balancing physics with pint tasting.
Why This Matters (Beyond Beer Geeks)
At first glance, you might think this is all just fun and games for homebrewers. But there’s more to it than that. Understanding the physics behind common phenomena like opening a bottle helps scientists grasp the fundamental behaviors of fluids, gases, and sound waves in dynamic systems.
Cavitation (the formation of vapor cavities in a liquid), rapid decompression, and standing wave phenomena are crucial in areas ranging from industrial engineering to medicine. For example, understanding how bubbles form and collapse can help improve ultrasonic cleaning devices, ship propeller designs, and even medical ultrasound technology.
Plus, it’s a great reminder that science is everywhere—even in your kitchen, your backyard brew lab, or your favorite bottle of craft beer.
Cheers to Science!
Max Koch’s experiment serves as a brilliant example of what happens when curiosity, passion, and expertise come together. What began as a simple homebrew hobby turned into a scientific investigation published in a respected physics journal.
And next time you pop the top on a swing-top bottle, remember—you’re not just opening a beer. You’re setting off a chain reaction of pressure changes, sound waves, and thermal physics, all happening in the blink of an eye.
Here’s to the science behind the pop!
Reference: On the popping sound and liquid sloshing when opening a beer, Physics of Fluids (2025). DOI: 10.1063/5.0248739