The flavor of an espresso is determined by how evenly the coffee grounds are packed before brewing.
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Mathematics now explains why grinding coffee beans too finely weakens the espresso.
Inside the espresso machine, boiling water moves under high pressure through a bed of ground coffee. One of the factors used to evaluate the resulting espresso is called extraction yield, which is a measure of how much coffee dissolves in boiling water during extraction.
In 2020, William Lee and colleagues at the University of Huddersfield, UK, found that grinding coffee into very small granules reduced extraction yield. Now they’ve investigated what’s behind the impact.
To do this, the researchers built a mathematical model of what happens when a coffee layer has two regions of different porosity. This happens when one area is packed more densely than the other. In the simulation, water passed through the porous areas more easily, so more coffee grains migrated and dissolved there. This increased the porosity of this area, allowing even more water to flow through it. This means that the coffee was extracted unevenly in his two parts of the coffee bed.
When running simulations on different ground coffees, the researchers confirmed the same trends as previous experiments. For very finely ground coffee, its non-uniformity led to a significant drop in extraction yield. According to Lee, their first idea was that the fine granules create clogs that water cannot pass through. However, their model showed that even slightly uneven filling means that water does not flow at the same rate through all parts of the coffee bed. This affects the flavor as under-extracted coffee tastes watery.
“In simulations, some parts of the coffee bed are completely unable to release the delicious chemicals we enjoy. It will be like this,” says Jamie Foster of the University of Portsmouth, UK. According to Christopher Hendon of the University of Oregon, espresso really is “the most problematic brewing method of all.” Because the combination of small granules and very warm water under great pressure means that the flow is prone to instability due to blockages and clogged pipes. generation of foam.
The researchers suggest that redesigning the part of the espresso machine that packs the granules could make coffee extraction more uniform. But Ann Smith, who worked on the project, says there’s still work to do before the team’s models fully reflect the behavior of existing machines. One example is taking X-rays of his coffee bed to determine details such as how many areas of different porosity can be present.
“I think we have a lot more work to do before we can confidently say, ‘This is exactly how espresso is brewed,’” Lee says.
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