Press a button, a few seconds of whirring, and your morning espresso is ready. But does more pressure always mean a faster flow? Polish physicists have found that, instead of simply speeding up the brew, higher pressure can compress the coffee grounds and act like a tighter cork.
For most of us, the espresso machine's work ends when the coffee starts flowing into the cup. If the drink is watery, bitter or simply different from the last one, we tend to blame the beans or the grind.
But the real explanation may lie in what happens inside the machine, where water is forced through a tightly packed layer of ground coffee.
The issue caught the attention of physicists after a student at the Warsaw Coffee Conference asked baristas what they most wanted to learn about espresso brewing.
They pointed to channelling — a process in which water fails to flow evenly through the coffee and instead finds easier paths through the grounds.
Some areas of the coffee can then be barely exposed to water, while others are washed excessively, making the final drink difficult to predict and reproduce.
Researchers from the University of Warsaw and the Institute of Physics of the Polish Academy of Sciences turned an espresso machine into a laboratory instrument, fitting it with pressure sensors and an electronic scale.
They prepared 60 shots of espresso, using 18.5 grams of the same coffee for each one. The researchers changed only the pressure, testing 11 levels ranging from about 1 to 12 bar.
At low pressure, increasing the force pushed the water through the coffee faster, as expected.
But at around 5 bar, that relationship broke down.
Increasing the pressure further produced little additional increase in flow, while at the highest pressures the flow could actually slow down.
The coffee flowed fastest at around 5 bar in the experiment, even though espresso machines typically operate at between 6 and 9 bar.
The reason was the coffee itself.
After tamping, the grounds do not form a rigid filter. They create a soft, porous layer that behaves more like a wet sponge.
As pressure increases, it forces water through the coffee but also compresses the grounds, narrowing the spaces between them. The additional pressure therefore begins to create its own resistance.
The coffee also absorbs water and swells, changing the network of channels through which the liquid moves.
X-ray tomography showed that after brewing, the layer of ground coffee can develop cracks and areas where it becomes detached from the bottom of the filter.
The researchers also tested what happened when the pump was stopped and restarted. The interruptions increased the flow but did not increase the amount of dissolved coffee extracted.
The findings suggest that repeatedly stopping and restarting the pump may instead encourage the formation of unwanted channels through the coffee.
The team then examined the concentration of coffee compounds reaching the cup throughout the brewing process.
The first drops appeared after five to 10 seconds and were highly concentrated, with about a quarter of their mass consisting of dissolved coffee ingredients.
After about 20 seconds, the concentration fell rapidly, approaching zero after around a minute.
The highest concentration of dissolved substances arriving per second occurred at approximately the 30-second mark. After that, the machine increasingly delivered liquid containing fewer dissolved coffee compounds.
“As physicists, we turn coffee into research every day. This time, we turned it into a topic, not a fuel”, Maciej Lisicki from the University of Warsaw jokes in a press release from the American Institute of Physics.
The researchers developed a model showing how pressure, the elasticity of the coffee grounds and the spaces between them interact during brewing.
The study was conducted by scientists from the University of Warsaw and the Institute of Physics of the Polish Academy of Sciences, in collaboration with the University of Potsdam and the Max Planck Institute for Dynamics and Self-Organization in Göttingen, Germany.
The findings were published in Physics of Fluids (https://doi.org/10.1063/5.0319611).
The results suggest that pressure in an espresso machine is not simply a speed-control setting. Increasing it changes the physical structure of the coffee through which the water must pass.
For baristas and espresso-machine manufacturers, the findings could point towards more sophisticated control of pressure during brewing rather than simply using a more powerful pump to produce faster extraction. (PAP)
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