A new study from researchers in Poland and Germany provides scientific evidence that higher pressure does not always result in a faster espresso extraction. Published in the journal Physics of Fluids, the research explores how a coffee puck behaves under typical brewing conditions, confirming a phenomenon long observed by professional baristas.
The team conducted experiments using a modified commercial espresso machine, brewing shots at pressures ranging from 1 to 12 bar. According to the findings, at low pressures, the flow rate increased proportionally with pressure, as would be expected with a simple porous material. However, within the 6- to 9-bar range commonly used in cafes, the flow rate plateaued, ceasing to increase even as pressure climbed higher.
Researchers explain this behavior with a concept called "poroelastic compaction," where the saturated coffee bed compresses under high pressure, constricting water pathways. This effect helps explain shot-to-shot inconsistencies, even when variables like dose, grind, and tamping are kept constant. The study also used X-ray imaging to show physical changes like swelling and cracking in the puck after brewing, offering a visual model of extraction dynamics.