Researchers at the American Institute of Physics (AIP) have delved into the role of uneven coffee extraction using a simple mathematical model. They found that by understanding the origin of uneven extraction in brewing espresso, the overall taste of the coffee beverage can be improved.

Financial savings are also possible by using coffee more efficiently and sustainably.

Espresso coffee is brewed by first grinding roasted coffee beans into grains. Hot water then forces its way through a bed of coffee grains at high pressure, and the soluble content of the coffee grains dissolves into the water (extraction) to produce espresso.

Brewing a “weaker” espresso
In 2020, researchers found that more finely ground coffee beans brew a weaker espresso. This counterintuitive experimental result makes sense if, for some reason, regions exist within the coffee bed wher less or even no coffee is extracted. This uneven extraction becomes more pronounced when coffee is ground more finely.

In Physics of Fluids, the University of Huddersfield researchers split the coffee into two regions to examine whether uneven flow does indeed make weaker espresso.

One of the regions in the mathematical model system hosted more tightly packed coffee than the other, which caused an initial disparity in flow resistance because water flows more quickly through more tightly packed grains. The extraction of coffee decreased the flow resistance further, as coffee grains lose about 20-25% 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,” says co-author William Lee.

“This effect appears always to be active, and it isn’t 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.”

Surprise observations 
The researchers were surprised that the model always predicts uneven flow across different parts of the coffee bed.

“This finding is important because the taste of the coffee depends on the level of extraction,” underscores Lee. 

“Too little extraction and the taste of the coffee 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 the overall extraction is at the right level, it might be due to a mixture of underdeveloped and bitter coffee.”

Seeking a better brew
Understanding the origin of uneven extraction and avoiding or preventing it could enable 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 obtain more detailed insights into this confusing phenomenon,” continues Lee. 

“once this is achieved, we can start to think about whether it is possible to change how espresso coffee is brewed to reduce the amount of uneven extraction.”