More than 2 billion cups of coffee are consumed every day around the world.

But why does the taste and aroma of espresso vary slightly each time, even when using the same beans and the same machine? A research team led by Professor Maciej Lisicki at the University of Warsaw in Poland announced on June 24 in the American Institute of Physics (AIP) journal *Physics of Fluids* that they have analyzed the relationship between pressure and water flow during the espresso extraction process. They revealed that the deformation of the coffee bed under high pressure, which alters water flow, is the physical mechanism behind "channeling" and extraction inconsistency.

"We found that coffee hides unsolved problems as interesting as those in galaxies," said Professor Lisicki, adding that this research could provide a scientific foundation for creating more stable and consistent-tasting espresso.

Espresso is extracted by forcing hot water through a bed of finely ground coffee at a pressure of approximately 9 bars.

Baristas meticulously adjust the grind size, tamping, extraction pressure, and extraction time of the roasted coffee, yet the taste does not always remain consistent.

In particular, "channeling"—a phenomenon where water does not permeate the entire coffee bed evenly but instead flows concentratedly through paths of least resistance—is considered a major nuisance that hinders uniform extraction, making the espresso taste overly sour or bitter and disrupting the balance of flavors.

The research team connected pressure sensors and electronic scales to a commercial espresso machine used in cafes and extracted espresso dozens of times while varying the pressure in the range of 1 to 12 bars.

The analysis showed that at low pressures, the flow rate increased in proportion to the pressure, following Darcy's law, where higher pressure leads to a proportional increase in the flow through the coffee bed. However, the situation changed once the pressure exceeded 5 bars.

Even as the pressure continued to increase, the rate of flow increase gradually diminished, and at around 9 bars—the typical pressure for espresso extraction—the flow rate reached a near-saturation point.

The research team explained that this occurs because, as extraction progresses, the coffee bed, which is a porous medium, behaves like a poroelastic material with many microscopic pores.

In a poroelastic material, as pressure increases, the coffee bed itself becomes more compressed, reducing the pores and narrowing the paths through which water can pass.

Furthermore, when the researchers analyzed the components of the extract by dividing it into 5-second intervals, they found that the concentration of Total Dissolved Solids (TDS), which determines the coffee's taste, reached about 25% in the initial extract but decreased rapidly thereafter, dropping to nearly zero after about 60 seconds.

The research team explained that about 10–12% of the initial dry coffee bed mass dissolves during the extraction process, and this dissolution process also significantly impacts changes in water flow.

They added that espresso extraction is not merely a process of water passing through coffee grounds, but a complex process in which the dissolution of coffee components and the compression and expansion of the coffee bed occur simultaneously. "This study provides important clues to understanding why coffee taste is not consistently reproduced even under the same conditions," they stated.