For a perfect espresso shot, the primary ingredient is not coffee. It's physics

Who would have thought that espressos have so much in common with X-rays and volcanoes?

In Short

• New equation predicts water flow through ground coffee
• Even tamping and contact time key for perfect espresso
• X-ray imaging confirms equation's accuracy on coffee grounds

What do volcanic eruptions and your morning coffee have in common? Well, it's more than you might think.

Researchers have developed a new equation, borrowed from the physics of how gases move through magma and water seeps through rock. This new equation can predict how water flows through ground coffee.

The findings, published in Royal Society Open Science, offer a simple answer to a deceptively simple question.

What makes a great espresso?

PHYSICS AND ESPRESSO

Espresso begins with ground coffee beans that are tamped or compressed into a compact body called a puck. Hot, pressurised water then pushes through this puck, extracting flavours, bitterness, and caffeine.

The key variable that determines whether the result is sublime or undrinkable is permeability, which is how easily water moves through the packed coffee grounds.

According to Fabian Wadsworth, an earth scientist at Ludwig-Maximilians-Universitat Munchen in Germany and lead researcher on the study, two things matter most.

First, the grounded coffee must be distributed evenly, free of clumps, and tamped with uniform pressure. It's done so that water doesn't rush through one area of the puck while bypassing another.

Second, the time water spends in contact with the coffee grounds must be carefully controlled. If the time is too long, the brewed result turns bitter. If it's too short, the shot will lack flavour and caffeine.

Wadsworth, who normally studies volcanic eruptions, said he turned to coffee as a teaching tool.

"Coffee felt like a natural way that students might get engaged with those problems," Wadsworth told Science News.

X-RAYS AND ESPRESSOS

To test the equation, the team examined two coffee roasts; Tumba from Rwanda and Guayacan from Colombia.

Both the coffees were grounded at 11 different settings each, producing 22 samples in total. Using software that converts multiple X-ray cross-sections into three-dimensional images, they mapped how fluid moved through each sample.

The results confirmed that the equation describes water flow through coffee grounds just as accurately as established equations describe gas moving through volcanic rock or water through sandstone.

Then they found another unexpected thing. Doubling the grain size of the coffee grounds increased permeability, making it easier for water to flow through by a factor of four, which has direct consequences for taste.

A BETTER BREW

Wadsworth noted that the model will be most useful to expert baristas using machines equipped to measure pressure and flow rates.

Coffee science expert Samo Smrke of the Zurich University of Applied Sciences, who was not involved in the study, called the cross-disciplinary approach "genuinely exciting," adding that it shows how methods developed in one field can open new perspectives in another.

The equation won't make a barista out of a physicist overnight, but it does mean the pursuit of a perfect espresso now has the backing of the undeniable principles of science behind it.

- Ends