Above roughly 5 bar, pushing harder on an espresso puck stops increasing flow
A University of Warsaw soft-matter group (Waszkiewicz, Lisicki and colleagues) instrumented a café-grade espresso machine and measured flow through pucks across a range of pump pressures. Up to about 5 bar the bed behaved like an ordinary porous medium (flow proportional to pressure), but above that the flow rate flattened and then fell as pressure rose: once the soluble fraction has dissolved (roughly 30-40 s in), the remaining bed is a poroelastic solid that compacts under load, and its permeability drops faster than the extra pressure can compensate. A minimal model reproduces the non-linear pressure-flow curve and the time evolution of flow during a shot. Caveat: one light-to-medium roast Brazilian coffee, one machine setup; the paper models flow and soluble concentration, not sensory outcome.
Why it matters: If your shots choke or run slow, cranking pressure above 6 bar is unlikely to help and may make the bed compact further; grind, dose and lower pressure profiles are the levers, which supports the trend toward 6-bar and declining-pressure espresso.
Published Jun 23, 2026 · collected Sep 5, 2026