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 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…
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