When a fluid flows through a porous bed of grains–like sand, soil, or coffee grounds–the flow rate typically increases linearly with pressure. But that’s not the case for espresso, which is typically brewed with 6-9 bars of pressure. Instead, espresso’s flow rate plateaus at around 5 bars, stays constant as pressure increases, and then drops off as pressure climbs higher still.
The reason, one group found, is that the mechanical load starts collapsing the grain bed. As the pores collapse, it becomes harder for water to penetrate the bed, despite the aid of increasing pressure. That’s why there’s no point to pushing espresso-making to pressures above 9 bars: the grain bed is too compressed for the flow rate to increase, even with the added pressure. (Image credit: K. Luhaers; research credit: R. Waszkiewicz et al. (pre-print); via Physics World)
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