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.
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