Volcano Science Reveals Secret to the Perfect Espresso

A new espresso equation shows why grinding too fine or too coarse can ruin your coffee.

A good cup of espresso is harder to craft than you think. A small change in grind size can turn the same beans into a balanced shot, a sour trickle, or a bitter mess.

While previous efforts that sought to find the recipe for the perfect shot of espresso focused on chemistry, a new study surprisingly suggests that the secret might lie in treating the coffee ground more like geology.

Inside the portafilter, a puck of ground coffee becomes a tiny landscape of ridges, tunnels, dead ends and hidden channels. Hot water, pushed through under high pressure, must find a path. If it races through one side, the shot turns thin and sour. If it crawls too slowly, it extracts too much and turns bitter.

The new study argues that the same mathematics used to understand fluids moving through sandstone, volcanic rocks and porous industrial materials can also predict how water moves through ground coffee.

This doesn’t mean your morning espresso needs a chalkboard where you have to solve differential equations. But it does suggest that one variable matters more than many home brewers realize: grind size. So, take note.

The Secret Life of the Coffee Puck

Any espresso begins with the main raw material: ground coffee beans packed into a puck and struck with hot water at up to about 9 to 10 bars of pressure. From the outside, the puck looks like a solid blob, but it is, in fact, a network of channels and ridges.

Fabian Wadsworth, an earth scientist at Ludwig-Maximilians-Universität München, usually studies volcanic eruptions. Coffee, he told Science News, became a way to teach students about percolation. “Coffee felt like a natural way that students might get engaged with those problems,” he said.

That connection is less whimsical than it sounds. In volcanoes, gas bubbles squeeze through molten rock. In aquifers, water threads through sediment. In espresso, hot water threads through coffee. In all three cases, flow depends on the size and shape of pores.

×

Get smarter every day...

Stay ahead with ZME Science and subscribe.

Daily Newsletter
The science you need to know, every weekday.

Weekly Newsletter
A week in science, all in one place. Sends every Sunday.
No spam, ever. Unsubscribe anytime. Review our Privacy Policy.

Thank you! One more thing...

Please check your inbox and confirm your subscription.

The study’s authors put this idea into a mathematical model. They focused on two features: the connected pore space between coffee grains, and the total surface area where water touches coffee. In their study the team writes: “We find excellent agreement with our percolation theory,” suggesting that grind size, packing and surface area can predict coffee permeability.

Permeability is the keyword here. It describes how easily a fluid moves through a material. A coffee puck with high permeability lets water pass quickly. A puck with low permeability resists flow. This physics has an overly inflated effect on taste.

X-raying Coffee Like Rock

To test the model, they used two coffees: Tumba from Rwanda and Guayacán from Colombia. They ground each at 11 different settings, from very fine to coarse, producing 22 samples. Then they loaded the grounds into narrow plastic tubes and scanned them with X-ray computed micro-tomography, a technique that builds a 3D map from many X-ray slices.

The researchers could see the particles, the pores, and the channels that water might use. They then ran computer simulations through these 3D maps to calculate how fluid would move.

Coffee particles are not little spheres. They are jagged, rough, fractured bits of roasted plant material that come in various tiny shapes. This study found that roughness and shape strongly affect how much surface area water encounters.

“If you were to double the grain size, you increase the permeability by a factor of four,” Wadsworth told Science News.

In practical terms, coarse grounds open wider channels, so water moves faster. Fine grounds close those channels, meaning more water lingers.

Neither extreme guarantees good coffee.

Why too fine can ruin the shot

A common instinct among espresso beginners is to grind finer. Fine grounds expose more surface area, so they should extract more flavor. But if the coffee is too fine, the puck resists water. Pressure builds and flow becomes uneven. Water may carve narrow channels through the puck, overextracting some regions while barely touching others. The result can taste bitter, harsh or burnt.

If the coffee is too coarse, water passes through too quickly. It does not spend enough time dissolving flavorful compounds. The result can taste weak or sour.

“You can make an excellent cup of coffee with beans bought from the supermarket — the number one thing is getting the grain size right,” Wadsworth told The Telegraph.

So what’s the right grain size? For a roughly 30-second espresso, the team suggests grains in the range of about 145 to 275 micrometres. A human hair is roughly 180 micrometres thick, so the sweet spot sits around this size.

That does not make bean quality irrelevant. Roast, origin, freshness and processing still shape flavor. But the new work suggests that a good bean badly ground can fail, while an ordinary bean ground well can perform better than expected.

The Barista’s Equation

At finer settings, the researchers found a population of very small particles, often called fines. These fines can clog channels, migrate during brewing, and collect near the outlet screen. That can change permeability while the shot is still running.

The study also points out that coffee particles may swell when they absorb water. The paper notes that some estimates suggest coffee grain volume can increase by up to 30 percent, though the effect remains debated. If swelling changes particle size during brewing, then that may also have to be taken into account.

“The model will make the most sense to expert baristas with machines equipped to measure pressure and flow rates,” Wadsworth told Science News. “They might have a different language for talking about it, but I think they would be able to understand what we did and appreciate it.”

Modern espresso machines increasingly measure pressure, flow and output mass. It may be that at some point, these high-tech espresso machines might come built-in with a physical model that could adjust recipes automatically, rather than relying on users to play around with settings.

Coffee, Volcanoes and the Physics of Everyday Things

The most interesting thing about this study is perhaps its geology angle. A morning espresso is surprisingly treated like a miniature Earth science experiment.

The same concepts that help scientists understand gas escape from magma, water movement through rocks, or industrial filtration can illuminate a drink many people make before they are fully awake.

“Harnessing insights from volcanology for coffee research is genuinely exciting and shows how methods developed in one field can open new perspectives in another,” Samo Smrke, a coffee science expert at the Zurich University of Applied Sciences who was not involved in the work, told Science News.

Modern food science increasingly treats cooking and brewing as problems in physics, chemistry and materials science. Bread dough, chocolate, beer foam and coffee pucks all behave as complex materials. Their textures and flavors depend on forces too small to see but large enough to taste.

RelatedPosts

Sugar with that? Sweetening coffee or tea really changes your drink
Dr. Oz “miracle green coffee pills” pulled, marketers fined $9 million
Hamburg becomes the first city to ban coffee pods
We knew it! Coffee and cocoa cocktail boosts your attention and mood

For espresso lovers, the takeaway is simple to grasp.

Level the puck. Tamp evenly. Use water around 90 to 96°C, but above all, respect the grind.

The findings were reported in the journal Royal Society Open Science.