Under Pressure - understanding the physics behind espresso brewing.

Espresso brewing requires the flow of hot water under high pressure (ca. 6-9 bar) through a pressed layer of ground coffee. During filtration, the coffee bed is extracted and dissolved. Usually, uniform filtration and high material extraction are observed, but sometimes the flow becomes unstable and channelling occurs, during which the flow is concentrated in finger-like structures, negatively affecting the quality of the brew. The appearance of channelling significantly influences the rate at which coffee is dissolved, as can be shown by observing the TDS (total dissolved solids) percentage of the liquid coming out of the espresso machine.

In controlled laboratory conditions, we measured TDS and the mass of the liquid coming out of the machine as a function of time to compare brews without a channel to brews with a large (0.5 cm wide) artificially induced channel. We established that whilst the model of extraction differs significantly between the two regimes, the rate at which water flows through the coffee bed eventually stabilises at the same value. Therefore, some hydrodynamic properties of the coffee bed do not change even after a large hole is introduced into the system.

Furthermore, our preliminary results show that the flow rate through the coffee puck is not pressure-dependent, contrary to Darcy’s law, which indicates the need for a different constitutive model to be determined. The results of our measurements will serve as a basis for numerical and theoretical modelling of porous bed extraction.