Espresso coffee is made by first grinding roasted coffee beans into beans. Hot water is then forced through a bed of coffee beans under high pressure, and the soluble content of the coffee beans is dissolved in the water (extraction) to produce espresso.
In 2020, researchers found that more finely ground coffee beans make a weaker espresso. This counterintuitive experimental result makes sense if, for some reason, there are regions within the coffee bed where less or even no coffee is extracted. This uneven extraction becomes more pronounced when the coffee is ground finer.
In Fluid Physics, from AIP Publishing, researchers from the University of Huddersfield explored the role of uneven extraction in coffee using a simple mathematical model. They divided the coffee into two regions to examine whether uneven flow produces a weaker espresso.
One of the regions of the model system harbored more compacted coffee than the other, causing an initial disparity in resistance to flow because water flows faster through more compacted beans. Coffee extraction further decreased resistance to flow, as coffee beans lose 20-25% of their mass during the process.
“Our model shows that flow and withdrawal amplified the initial disparity in flow between the two regions due to a positive feedback loop, with more flow leading to more withdrawal, which in turn reduces resistance and leads to more withdrawal.” more flow,” said the co-author. William Lee. “This effect seems to be always on, and it’s not until one of the regions has all of its soluble coffee extracted that we see the experimentally observed decrease in extraction with decreasing grind size.”
The researchers were surprised to find that the model always predicts uneven flow in different parts of the coffee bed.
“This is important because the flavor of the coffee depends on the level of extraction,” Lee said. “Too little extraction and the coffee flavor is what experts call ‘underdeveloped,’ or as I describe it: smoky water. Too much extraction and the coffee tastes very bitter. These results suggest that even if it appears that the overall extraction is in the right level, it could be due to an underdeveloped and bitter coffee blend.”
Understanding the origin of uneven extraction and avoiding or preventing it could allow for better brews and substantial financial savings by using coffee more efficiently.
“Our next step is to make the model more realistic to see if we can get more detailed information about this confusing phenomenon,” Lee said. “Once this is achieved, we can start to think about whether it’s possible to make changes to the way espresso is made to reduce the amount of uneven extraction.”
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