How physicists tested espresso machine pressure from 1 to 12 bar

In most espresso machines, one control appears to trump the rest: pressure. Increase it and, in theory, you ought to end up with more coffee in the cup.

That assumption has guided espresso-machine design since the 1940s. A single figure became the standard across the trade: 9 bar, about nine times the pressure of the air around us.

A team of physicists in Poland decided to test that idea across pressures from mild to extreme. Close to the upper end, the machine’s behaviour stopped matching the simple expectation.

A question from baristas

The work started with a gripe from working baristas. At the Warsaw Coffee Conference, they told a visiting physics student their biggest headache was channelling.

Channelling happens when hot water bores fast paths through the coffee grounds, so part of the espresso turns bitter while the rest comes out thin and under-extracted.

Maciej Lisicki, a physicist at the University of Warsaw, took the problem at face value.

Together with colleagues, he set out to quantify how brewing pressure shapes what ends up in the cup, treating the everyday espresso routine as something to study scientifically.

The shift in mindset came naturally. “As physicists, we turn coffee into research on a daily basis. Now we made it the subject, and not the fuel,” said Lisicki.

Coffee shots at different pressures

Espresso has a reputation for being temperamental. The very same beans can yield a sweet, balanced shot one moment and a sour one the next-an irritation that previous experiments have documented carefully.

To investigate, the researchers modified a standard café machine by adding a scale and a pressure gauge, recording both at 10 times a second. They then pulled shot after shot across a range from about 1 bar up to 12 bar.

At the lower end of the pressure range, the coffee puck behaved like a neat porous medium, similar to sand or tightly packed soil.

If you push with twice the pressure, you get roughly twice the flow. In physics, that proportional relationship is known as Darcy’s law.

More pressure, less coffee trickle

That orderly pattern did not hold for long. When pressure rose beyond roughly 5 bars, the straight-line trend began to curve.

From that point, increasing pressure no longer guaranteed higher flow, and forcing it higher eventually reduced the trickle reaching the cup.

Cafés had carried hints of this for years. Many baristas had suspected that excessive pressure can choke a shot, but until now nobody had mapped the effect across a clear, controlled span of pressures.

The collapsing coffee bed

The underlying reason is compression. Under strong pressure, the wet coffee bed does not continue to act like a firm filter: its tiny voids narrow, and the compacted disc becomes harder for water to pass through.

Substances that contain liquid in pores while deforming under load are described as poroelastic-the kind of behaviour seen in saturated soil and in living tissue.

After 30 to 40 seconds, once the soluble coffee has dissolved, what remains appears to behave in that way, based on how closely the flow data aligned with the model.

X-ray scans of the pucks taken before and after brewing showed the internal structure tightening, consistent with compaction under pressure. The squeezed bed and the flattened flow curve pointed to the same mechanism.

The first 40 seconds

If you follow a single shot from the start, it plays out in three phases. Over the first 5 to 10 seconds, water penetrates the dry bed, driving out trapped air and causing the grounds to swell-an early stage other studies have captured on film.

Next comes a brisk middle period: flow increases and water extracts flavour from the now-wetted grounds.

Beyond the usual shot duration, the flow trends towards a steady level, and it was those later, stable values that revealed the squeezing effect.

Dissolution shapes the timeline as much as pressure does. By measuring how much coffee dissolved second by second, the team produced a simple model that links the opening moments of the shot to the settled flow rate at the end.

Mysteries in the cup

For the first time, researchers have measured how the coffee bed compresses under brewing pressure across a full range and incorporated it into a practical model.

The group is continuing the work. Their next step is to swap coffee grounds for transparent glass beads.

That change should allow them to observe the hidden brewing dynamics directly and keep teasing apart the physics of espresso.

“Physics is about answering mysteries that are unsolved, and it turned out that coffee has these mysteries in it just as well as galaxies do,” said Lisicki.