Researchers from Poland and Germany, among them Maciej Lisicki of the University of Warsaw, have taken apart one of the most durable assumptions in espresso: that turning up the pressure speeds up the shot. Their paper appeared on 23 June in Physics of Fluids, with funding from the University of Warsaw's IDUB programme.
The team worked with a modified Sanremo Zoe machine, 18.5 grams of a Brazilian single origin and pressures ranging from 1 to 12 bar, running 60 long extractions across 11 pressure settings. Flow does rise with pressure at first — but across the 6-9 bar band most cafés actually use, the curve flattens out.
The reason lies in the puck. At low pressure it behaves like an ordinary porous bed; at café pressures it turns poroelastic, compressing, swelling and cracking, changes the team captured with X-ray imaging. The extra pressure goes into deforming the bed rather than moving water through it — which is why puck preparation, grind size and even distribution matter more than another bar on the gauge.
Source: Daily Coffee News