Espresso coffee is prepared by first grinding roasted coffee beans into beans. The hot water then forces its way through a layer of coffee beans under high pressure, dissolving the soluble content of the coffee beans into the water (extraction) to produce espresso.
In 2020, researchers found that more ground coffee beans brew weaker espresso. This counterintuitive experimental finding makes sense if, for some reason, there are areas within the coffee bed where little or even no coffee is extracted. This uneven extraction becomes more noticeable when the coffee is ground finer.
in fluid physics, from AIP Publishing, University of Huddersfield researchers explored the role of asymmetric extraction of coffee using a simple mathematical model. They split the coffee into two regions to examine whether the uneven flow actually leads to weak espresso.
One region in the model system hosted more tightly packed coffee than the other, causing an initial variance in flow resistance because water flowed more quickly through the tightly packed beans. Coffee extraction reduces flow resistance even further, with coffee beans losing about 20% to 25% of their mass during the process.
“Our model shows that flow and extraction widened the initial variation in flow between the two regions due to a positive feedback loop, where more flow leads to more extraction, which in turn reduces resistance and leads to more flow,” said co-author William Lee. “This effect appears to be always active, and we will not see the experimentally observed diminishing extraction with a decrease in grind size until all of the soluble coffee in one area has been extracted.”
The researchers were surprised to find that the model always predicted uneven flow through different parts of the coffee bed.
“This is important because the taste of the coffee depends on the level of extraction,” he told me. “Too little extraction and the taste of the coffee is what experts call ‘retarded’, or as I put it: smokey water. The extraction is too much and the coffee tastes too bitter. These findings suggest that even if the overall extraction appears to be at the right level, it may be due to a combination of Bitter and unsophisticated coffee.
Understanding the origin of uneven extraction and avoiding or preventing it can enable better coffee brewing and significant 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 insights into this disconcerting phenomenon,” he told me. “Once this is achieved, we can begin to consider whether changes can be made to the way espresso is brewed to reduce the amount of uneven extraction.”