Coffee lovers can be particular about their beans, agonising over where they are grown and how they are roasted.
But a study suggests that the perfect cup may depend less on the origin of the coffee than on how it is prepared once it reaches the kitchen. When ground properly, it seems cheap supermarket beans can rival far pricier options.
The research, led by Fabian Wadsworth, a volcanologist at Ludwig-Maximilians University in Munich, Germany, examined the physics of espresso making. The conclusion was straightforward: if you control the flow of water through the coffee, you control the taste. The key variable, it appeared, was how big the coffee grounds were.
Espresso is made by forcing hot water at high pressure through a tightly packed bed, or puck, of finely ground coffee. As the water passes through, it dissolves flavour compounds, including acids, sugars, oils and bitter elements. The balance of these determines whether a shot tastes rich and rounded, or harsh and unpleasant.
To understand this process, Wadsworth and his colleagues ground coffee at a range of settings, from very fine to coarse. The researchers then used X-ray scans to build detailed 3D images of the coffee puck and ran computer simulations to track how water moved through it. This allowed them to estimate how easily water could pass through the packed grounds.
The central finding is intuitive. If water flows too slowly, it extracts too much, producing a strong but often bitter shot. If it flows too quickly, it extracts too little, leading to a thin, sour taste. The best espresso lies somewhere in between.
As the study, published in the journal Royal Society Open Science, puts it, the rate of flow "dictates the extraction" of the compounds.
Grind size turned out to be a key factor. Fine grounds would pack together tightly, leaving only narrow channels for water and slowing the flow. Coarser grounds would create larger gaps, allowing water to pass through more quickly. Very small changes in grind size could therefore have large effects on flavour.
This helps explain why expensive coffee does not guarantee a good result. Poorly prepared, it can taste flat or unpleasant. Conversely, a basic supermarket blend can taste delicious if the grind is right and the flow is well controlled. The same physics applies to both.
Baristas already work with these principles. When an espresso tastes sour, they grind finer to slow the flow and increase extraction. When it tastes bitter, they grind coarser to speed things up. The research provides a clearer scientific footing for this trial-and-error approach.
None of this makes beans irrelevant. Origin, roast and freshness still matter. But the findings suggest that preparation plays a larger role than many drinkers might realise.