A recent experiment showed that, contrary to theoretical predictions, beyond a cutoff point, grinding coffee more finely results in lower extraction. One potential explanation for this is that fine grinding promotes non-uniform extraction in the coffee bed. In new research, scientists at the University of Huddersfield investigated the possibility that this could occur due the interaction between dissolution and flow promoting uneven extraction.
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.
Espresso coffee is a beverage brewed from the roasted, ground cherries (beans) of the coffee robusta or arabica plant.
In brewing an espresso, hot (92-95 degrees Celsius ) water is forced at high pressure (9-10 atmospheres) through a bed of 15-22 grams of finely ground coffee resulting in a beverage with a mass of 30-60 grams.
Although coffee is a complex mix of nearly 2,000 chemicals, most mathematical models of coffee brewing treat coffee as a single substance using mass as a measure of the amount.
The quality of coffee can be measured by two properties: strength and extraction yield. Strength is the mass concentration of dissolved coffee solids in the beverage. Extraction yield is the mass fraction of the coffee grains that have dissolved.
Coffee grains are only partially soluble so there is a maximum value of the extraction yield which cannot be exceeded
A rough measure of coffee quality is given by the coffee quality control chart which plots strength against extraction yield.
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 where less or even no coffee is extracted. This uneven extraction becomes more pronounced when coffee is ground more finely.
In the new study, Professor William Lee and his colleagues from the University of Huddersfield explored the role of uneven coffee extraction using a simple mathematical model.
They split the coffee into two regions to examine whether uneven flow does in fact make weaker espresso.
One of the regions in the 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 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,” Professor Lee said.
“This effect appears to always 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.”
The researchers were surprised to find the model always predicts uneven flow across different parts of the coffee bed.
“This is important because the taste of the coffee depends on the level of extraction,” Professor Lee said.
“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 it looks like the overall extraction is at the right level, it might be due to a mixture of underdeveloped and bitter coffee.”
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,” Professor Lee said.
“Once this is achieved, we can start to think about whether it is possible to make changes to the way espresso coffee is brewed to reduce the amount of uneven extraction.”
The results appear today in the journal Physics of Fluids.
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W.T. Lee et al. 2023. Uneven extraction in coffee brewing featured. Physics of Fluids 35 (5): 054110; doi: 10.1063/5.0138998
