Researchers at the University of Huddersfield discovered that uneven extraction in espresso preparation leads to weaker coffee when using very finely ground beans. Understanding and preventing this phenomenon could improve the taste of coffee and allow for financial savings through more efficient use of coffee.
Espresso coffee is prepared by first grinding roasted coffee beans. The hot water then forces its way through a bed of coffee at high pressure and the soluble contents dissolve into the water (extraction) to produce the espresso.
In 2020, researchers discovered that finer ground coffee beans produce 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 Physics of Fluids , from AIP Publishing, researchers at the University of Huddersfield explored the role of uneven coffee extraction using a simple mathematical model. They divided the coffee into two regions to examine whether the uneven flow actually makes the espresso weaker.
In this figure, Q is the flow rate, epsilon is the porosity (which increases as the coffee is extracted), and c is the concentration of dissolved coffee (a measure of the strength of the espresso). Credit: WT Lee, A. Smith and A. Arshad
One region of the model system hosted more compacted coffee than the other, which caused an initial disparity in flow resistance because water flows more quickly through more compacted beans. Coffee extraction further reduced flow resistance, as coffee beans lose around 20% to 25% of their mass during the process.
“Our model shows that flow and extraction widened the initial disparity in flow between the two regions due to a positive feedback loop, in which more flow leads to more extraction, which in turn reduces resistance and leads to more flow. ”, said the co-author.
Guilherme Lee. “This effect seems to be always active, and only when one of the regions has all its soluble coffee extracted do we see the experimentally observed decrease in extraction with the decrease in 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,” said Lee. “Too little extraction and the flavor of the coffee is what experts call 'underdeveloped,' or as I describe it: smoky water. Too much extraction and the coffee becomes very bitter. These results suggest that even if it appears that the overall extraction is at the right level, this may be due to a mixture of bitter and underdeveloped coffee.”
Understanding the origins of uneven extraction and avoiding or preventing it could allow for better flavors and substantial financial savings by using coffee more efficiently.
“Our next step is to make the model more realistic to see if we can gain more detailed insights into this confusing phenomenon,” said Lee. “Once this is achieved, we can start to think about whether it is possible to make changes to the way espresso is prepared. to reduce the amount of irregular extraction.”
Tell me in the comments how you usually prepare your espresso and what you think of this discovery.
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