The Pressure Plateau: What Physics Reveals About Espresso Flow

Eighteen and a half grams of coffee, ground fine, tamped at exactly 20 kilograms of force. The barista increases pressure from 6 bar to 9 bar, expecting the shot to run faster. It doesn't.

This counterintuitive behaviour has frustrated baristas for years. The assumption seems logical: more pressure should push water through the puck more quickly. But a new study published in Physics of Fluids on June 23 provides the first systematic evidence that espresso physics doesn't work that way.

In Sofia's specialty coffee scene, where baristas at cafes like those partnering with Blue Bag Specialty Coffee dial in single-origin beans daily, this research offers something valuable: an explanation for inconsistencies that no amount of careful technique seemed to eliminate.

When Coffee Stops Behaving Like Coffee

The research team, with affiliations in Poland and Germany, built their experiment around a 2-group Sanremo Zoe Competition espresso machine modified with pressure sensors and Arduino-based data collection. They used 18.50 grams of single-origin Brazilian coffee from the Igarapé region, roasted by CoffeeLab Warsaw, ground on a Fiorenzato F64 EVO grinder.

At low pressures, coffee behaves like any porous material. Push harder, water flows faster. But somewhere between 6 and 9 bar, the relationship breaks. Flow rate plateaus instead of climbing.

The researchers describe this as poroelastic compaction. Under brewing pressure, the coffee puck deforms. The spaces between particles narrow. Water has fewer pathways, and pushing harder simply compresses those pathways further rather than forcing more liquid through.

This poroelastic compaction has been alluded to in the coffee community, but with no systematic evidence. I think we characterized this effect for the first time.

Maciej Lisicki, University of Warsaw

The Channeling Problem Gets Clearer

The study originated from a practical question. Baristas at a coffee trade event asked the researchers how to reduce channeling, the phenomenon where water finds easier paths through the puck, extracting some areas more than others.

The physics now makes sense of what baristas observe. When pressure increases beyond the plateau point, water doesn't distribute more evenly. It finds the path of least resistance more aggressively. The puck compresses unevenly, cracks form, and channeling worsens.

X-ray microtomography images of pucks before and after brewing showed swelling, cracks, and delaminations. The physical structure of the coffee bed changes during extraction in ways that pressure alone cannot control.

The researchers also measured total dissolved solids (TDS) across 5-second fractions of espresso. The earliest liquid contained approximately 25% TDS. By 60 seconds, concentration approached zero. Most extraction happens in the first moments, and flow dynamics during that window determine the cup.

What Actually Controls Consistency

These 120-second brews weren't meant to be drinkable. They were designed to observe how the puck behaves after most solubles have washed out.

One finding stands out for daily practice: stopping and restarting the brew cycle increased flow without causing additional dissolution. Running the machine multiple times through the same puck, as when trying to make a longer drink, leads to extraction inconsistency.

The research doesn't prescribe a new recipe. It doesn't argue that 9 bar is always correct. What it demonstrates is that pressure is one variable among many, and not the master control baristas sometimes assume.

Puck preparation matters. Grind distribution matters. Dose and particle size matter. The study validates what careful baristas already practice: consistency comes from controlling the variables before the pump engages.

The Craft Behind the Science

This research joins a growing body of espresso physics. A 2023 Physics of Fluids study explained how finer grinds can sometimes produce weaker espresso through uneven extraction. A 2020 study challenged conventional recipes by arguing for lower doses, coarser grinds, and faster shots.

For baristas working with small-batch, single-origin beans, where each roast behaves slightly differently, understanding why pressure plateaus changes the diagnostic process. When a shot runs slow, the instinct to increase pressure may be exactly wrong.

The study was funded in part by the University of Warsaw IDUB program.