The physics of brewing the perfect espresso – Ars Technica

“Channeling” during brewing process can lead to non-uniform filtration and lower extraction yield.
Many variables can affect the quality of a steaming cup of espresso, including so-called “channeling” during the brewing process, in which the water doesn’t seep uniformly through the grounds but branches off in various preferential paths instead. This significantly reduces the extraction yield and thus the quality of the final brew. Scientists from the University of Warsaw have gleaned insights into the underlying physics of channeling that will help coffee lovers achieve more consistent results when brewing espresso. They presented their preliminary findings at the American Physical Society’s Global Physics Summit in Anaheim, California, this morning.As previously reported, there’s an official industry standard for brewing espresso, courtesy of the Specialty Coffee Association, which sets out strict guidelines for its final volume (25–35 mL, or roughly 1 ounce) and preparation. The water must be heated to 92° to 95°C (197° to 203°F) and forced (at a specific pressure) through a bed of 7 to 9 grams (about a quarter of an ounce) of finely ground coffee for 20 to 30 seconds. But most coffee shops don’t follow this closely, as the brewing machines allow baristas to configure water pressure, temperature, and other key variables to their liking. The result of all those variations in technique is a great deal of variability in quality and taste.Naturally, scientists find this fascinating. For instance, in 2020, Christopher Hendon’s lab at the University of Oregon helped devise a mathematical model for brewing the perfect cup of espresso over and over while minimizing waste. Hendon is a computational materials chemist, and his lab holds regular coffee hours for the Eugene campus community. The researchers focused on an easily measurable property known as the extraction yield (EY): the fraction of coffee that dissolves into the final beverage. That, in turn, depends on controlling water flow and pressure as the liquid percolates through the coffee grounds.Hendon et al. based their model on how lithium ions propagate through a battery’s electrodes, akin to how caffeine molecules dissolve from coffee grounds. They concluded that the most reproducible thing you can do is use fewer coffee beans and opt for a coarser grind with a bit less water; brew time was largely irrelevant.Three years later, Hendon’s team showed how adding a single squirt of water to coffee beans before grinding can significantly reduce the static electric charge on the resulting grounds. This in turn reduces clumping during brewing, yielding less waste and the strong, consistent flow needed to produce a tasty cup of espresso. Good baristas already employ the water trick; it’s known as the Ross droplet technique. But this was the first time scientists have rigorously tested that well-known hack and measured the actual charge on different types of coffee.This latest work focuses less on the chemical changes that occur during the brewing process and more on the mechanical and physical processes. “To a physicist, brewing coffee is a reactive flow through a complex porous medium that undergoes dynamic reconfiguration—a fascinatingly complex phenomenon,” Maciej Lisicki, a physicist at the University of Warsaw in Poland, told Ars. “This exploration is in part motivated by many myths or assumptions that function in the coffee community. Since many people have a religious attitude towards their favorite drink, there are beliefs about the details of the process (and the do’s and don’ts), which we would love to clarify and learn for sure.”Lisicki leads a team of coffee aficionados, which meshes nicely with the lab’s long interest in culinary fluid dynamics and their love of the annual Warsaw Coffee Festival. When a barista friend asked them about the phenomenon of channeling, the team decided to investigate. “Channeling is something we actually want to mitigate,” said Lisicki. “It is something the barista cannot control, so by minimizing the chance that a channel appears, you maximize the repeatability.”Channeling is similar to the phenomenon of dissolving rocks, another example of a porous medium that colleagues have studied. “We thought it would be a simple and easy project where we could apply their methods and quickly obtain valuable results, but it turns out that the process of coffee brewing is much more complex,” Franciszek Myck, who presented results at the APS meeting, told Ars. “It turns out we had to learn quite a lot on the way to be able to decompose the process of brewing into small steps and understand what is going on, both in terms of the physics and the mechanics of the machine, to be able to gain full control of the process, from the moment a roasted grain enters the lab to the magical liquid filling the mouth with all its complex aromas.”The team initially tried to use a simple home coffee machine for their experiments but eventually partnered with Coffeelab, a major roaster in Poland, and CoffeeMachineSale, the largest global distributor of roasting gear. This brought industrial-grade equipment and much professional coffee expertise to the project: state-of-the-art grinders, for instance, and a cafe-grade espresso machine, tricked out with a pressure sensor, flow meter, and a set of scales. The entire setup was connected to laboratory laptops via a microchip and controlled with custom software that allowed the scientists to precisely monitor pressure, mass, and water flowing through the coffee.The scientists measured the total dissolved solids to determine the rate at which coffee is dissolved, comparing brews without a channel to those with artificially induced channels. They found that, indeed, channeling adversely affected extraction yields. However, channeling does not have an impact on the rate at which water flows through the espresso puck.”That is mostly due to the structural rearrangement of coffee grounds under pressure,” Lisicki said. “When the dry coffee puck is hit with water under high pressure—as high as 10 times the atmospheric pressure, so roughly the pressure 100 meters below the sea surface—it compacts and swells up. So even though water can find a preferential path, there is still significant resistance limiting the flow.”The team is now factoring their results into numerical and theoretical models of porous bed extraction. They are also compiling an atlas of the different kinds of espresso pucks based on micro-CT imaging of the coffee.”What we have found can help the coffee industry brew with more knowledge,” said Myck. “Many people follow procedures based on unconfirmed intuitions or claims which prove to have confirmation. What’s more, we have really interesting data regarding pressure-induced flow in coffee, the results of which have been a surprise to us as well. Our approach may let us finally understand the magic that happens inside your coffee machine.”Ars Technica has been separating the signal from
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Source: https://arstechnica.com/science/2025/03/the-physics-of-brewing-the-perfect-espresso/