Scientific inspiration can strike at any time. For Christopher Hendon, a computational materials chemist at the University of Oregon, inspiration came at a local coffee shop where his lab holds regular coffee hours for the Eugene campus community—a fitting place, since Hendon’s research specialties include exploring the scientific principles behind really the good coffee. Among the regulars were two volcanologists, Joseph Dufek and Joshua Mendez Harper, who noted striking similarities between the science of coffee and plumes of volcanic ash, magma, and water. Thus was born an unusual collaboration.
“It’s kind of like the beginning of a joke — a volcanologist and a coffee expert walk into a bar and then walk out with a paper,” said Mendes Harper, a volcanologist at Portland State University. “But I think there’s a lot more opportunity for this kind of collaboration, and there’s a lot more to know about how coffee breaks down, how it flows as particles, and how it interacts with water.” These studies can help solve parallel problems in geophysics—whether it’s landslides, volcanic eruptions, or how water seeps through soil.
The result is a new paper published in the journal Matter demonstrating how adding a single jet of water to coffee beans before grinding can significantly reduce the static electrical charge of the resulting grounds. This, in turn, reduces the formation of lumps during brewing, achieving less waste and the strong, consistent flow needed to produce a delicious cup of espresso. Good baristas already use the water trick; this is known as the Ross drop technique, according to Hendon. But this is the first time scientists have rigorously tested this well-known hack and measured the actual charge of different types of coffee.
As previously reported, there is actually an official industry standard for making espresso, courtesy of the Specialty Coffee Association, that sets strict guidelines for its final volume (25-35 ml, or roughly one ounce) and preparation. The water should be heated to 92° to 95°C (197° to 203°F) and passed (at a certain pressure) through a bed of 7 to 9 grams (about a quarter ounce) of finely ground coffee for 20 to 30 seconds. But most coffee shops don’t follow this closely, usually using more coffee, while brew machines allow baristas to configure water pressure, temperature and other key variables to their liking. The result of all these variations in technique is a wide variety in quality and taste.
In 2020, Hendon’s lab helped create a mathematical model for making the perfect cup of espresso, time and time again, while minimizing waste. Espresso flavors come from approximately 2,000 different compounds that are extracted from the coffee grounds during brewing.
So Hendon and his colleagues focused on building a mathematical model for a more easily measurable property known as extraction yield (EY): the fraction of coffee that dissolves in the final brew. This in turn depends on controlling the water flow and pressure as the liquid percolates through the coffee grounds. Hendon et al. based their model on how lithium ions diffuse across the battery’s electrodes, which they liken to how caffeine molecules dissolve from coffee grounds.
A bunch of simulations and several thousand experimental shots of espresso later, the authors concluded that the most reproducible thing to do is to use fewer coffee beans and choose a coarser grind with slightly less water; cooking time was largely irrelevant. Conventional wisdom holds that a fine grind is best because more surface area of the resulting compacted coffee bed is exposed to the hot water, thus increasing the extraction yield. But the group’s experiments revealed that if the coffee is ground too finely, it can clog the coffee bed, thereby reducing the extraction yield. This is also a big factor in the variability of taste.
This latest research focuses on why microscopic lumps form in the first place, especially at very fine grinding levels. The culprit is static electricity arising from the crushing and friction between grains during grinding. Hendon thought that reducing that static would be a good way to get rid of those lumps. The technical term is triboelectricity, which arises from the accumulation of opposite electrical charges on the surfaces of two different materials due to contact with each other. (Not to be confused with triboluminescence, the emission of cold light when a material undergoes physical deformation—the reason Wint-O-Green Life Savers emit blue sparks when crushed, visible in the dark.)
A similar build-up of charge also occurs during volcanic eruptions. “During the eruption, the magma breaks up into very small particles that then come out of the volcano in this big cloud, and during this whole process, those particles rub against each other and charge up to the point of producing lightning,” Mendes said. Harper. “In a simplified way, it’s similar to grinding coffee, where you take these beans and reduce them to a fine powder.” Because the particle-scale physics found in volcanic plumes is quite difficult to study in nature, the collaboration with Hendon’s study of triboelectric effects in coffee provided a useful platform on a smaller scale.