Full Breakdown
Pressure Limits in Espresso Extraction: Polish Physicists Reveal Why Too Much Bar Can Choke a Shot
6/28/2026, 11:14:00 AM
Core Findings: Pressure, Flow, and Coffee Quality
Physicists at the University of Warsaw measured espresso extraction across pressures from ?1 bar to ?12 bar. Up to about 5 bar, water flow increased proportionally with pressure, following Darcy’s law. Above 5 bar the relationship deviated: additional pressure reduced flow, eventually slowing the shot. The effect is attributed to the coffee puck’s poroelastic compression, which narrows internal channels and impedes water passage.
Background: The 9-Bar Standard and Barista Concerns
Since the 1940s, espresso machines have been calibrated to a nominal 9 bar pressure, based on the assumption that higher pressure yields more extraction. At the Warsaw Coffee Conference, baristas reported “channeling” – uneven water paths that produce bitter and weak portions in a single shot – as a persistent problem. No quantitative study had previously examined the full pressure range.
Researchers and Methodology
The study was led by physicist Maciej Lisicki of the University of Warsaw, who responded to the baristas’ complaint. The team equipped a commercial café machine with a high-frequency (10 Hz) pressure gauge and a precision scale, recording water mass and pressure for each brew. They also captured X-ray images of coffee pucks before and after extraction to assess structural changes.
Data & Statistics: Linear Flow Up to 5 Bar, Collapse Beyond
- Pressure range tested: ?1 – 12 bar.
- Linear flow regime: 1 – 5 bar, where flow ? k × pressure (k ? constant).
- Non-linear regime: > 5 bar, where flow plateaued then declined.
- X-ray analysis showed a measurable reduction in pore space after high-pressure brewing, consistent with poroelastic compression.
- The first 40 seconds of extraction featured rapid water uptake, a middle “fast” phase, and a late steady-flow phase where the squeeze effect became evident.
Why It Matters: Implications for Espresso Machines and Barista Practice
The findings suggest that the industry-standard 9 bar may be suboptimal for many coffee blends, potentially causing over-compression, reduced flow, and uneven flavor extraction. Machine manufacturers could consider adjustable pressure profiles that stay below the compression threshold for specific grind sizes and bean varieties. Baristas may benefit from calibrating pressure based on empirical flow data rather than default settings.
Official Statements & Responses
Lisicki emphasized the shift from treating coffee as fuel to treating it as a research subject, noting that the study “provides a working model linking early extraction moments to the settled flow at the end.” The research was published in *Physics of Fluids*, underscoring its peer-reviewed status.
Criticism & Opposition
Barista communities have long suspected that excessive pressure “chokes” a shot, but prior to this work they lacked quantitative evidence. No formal industry criticism has been recorded in the sources, though the study challenges the entrenched 9-bar norm.
Conflicting Reports & Gaps
The article does not present alternative measurements of pressure effects from other laboratories, nor does it address how different grind sizes or bean origins might shift the 5-bar threshold. Future work is needed to generalize the model across diverse brewing conditions.
Verbatim Quotes
- “As physicists, we turn coffee into research on a daily basis. Now we made it the subject, and not the fuel,” — Maciej Lisicki, Physicist, University of Warsaw
- “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,” — Maciej Lisicki, Physicist, University of Warsaw
What’s Next: Future Experiments and Modeling
Lisicki’s team plans to replace coffee grounds with transparent glass beads to isolate mechanical compression from chemical extraction. Additional modeling will aim to predict optimal pressure curves for varied coffee formulations, potentially informing next-generation espresso machine design.
