Mathematically Speaking, You’re Probably Grinding Your Espresso Too Finely

If you’re still struggling to brew a reliably good cup of espresso, maybe it’s time to change the settings on your grinder. According to a new mathematical model, most people might be pulverizing their beans a tad too much, sacrificing flavor, consistency and cost efficiency in the process.

With coarser-than-average grinds, “we reckon you can save about 25 percent coffee mass per shot,” explains Jamie Foster, a mathematician at the University of Portsmouth and author on a paper published this week in Matter, in an interview with Nicola Davis at the Guardian.

And if you’re in the business of brewing, savings like that can really add up. When Foster’s colleagues trialed their approach in a coffee shop in Oregon, it saved $0.13 per drink, totalling $3,620 for the year, reports Jason Arunn Murugesu for New Scientist. The process arguably reduces waste as well, and could boost sustainability in the coffee industry, according to a statement.

Prior to the team’s intervention, the baristas in the Oregon coffee shop were making a common mistake, the team’s results show. Most caffeine enthusiasts dump more than the recommended amount of beans into their grinder, then dial the texture down to “fine,” says study author Christopher Hendon, a computational chemist at the University of Oregon, in a statement. What comes out the other end is a bit like espresso Russian roulette: While some shots taste fine, others are too bitter or too acidic—and results vary drastically from cup to cup, Hendon explains.

The science behind this caffeination conundrum boils down to some trippy physics. To home in on the secret to coffee consistency, the researchers digitized espresso-making into a mathematical model, simulating how coffee chemicals flow out of grounds and into hot water that’s being forced through the mixture.

At face value, the typical fine grind dogma makes sense: Smaller particles have more collective surface area, giving the water more coffee to flow over and absorb flavor from. In theory, that should maximize what’s called extraction yield, or the amount of coffee that dissolves into the final beverage, imbuing it with that strong espresso taste. But the team’s model shows this strategy can easily backfire. Grinds too fine can settle and pack together in the basket of the espresso machine, clogging an otherwise even mesh and stymieing water’s journey through. As a result, some cups end up bitter, while others end up sour; a few taste strong, a few taste weak.

Instead, the team recommends grinding the beans a touch more coarsely—though not so much that you end up sacrificing that precious surface area. (Brew time doesn’t seem to make much of a difference.) This caffeinating combo, they say, hits on a sweet spot: grinds still small enough to confer lots of coffee taste, but just large enough to keep the flow of water uniform, Foster tells Davis. A coarser grind also requires fewer beans, and slightly less water that moves through the grinds faster, cutting down on time and cost.

“This is an opportunity to save a lot of money without sacrificing quality,” Hendon says in a statement.

The process still leaves plenty of room for creativity, though. Fiddling with other factors, like temperature, pressure and bean variety, can still tailor a shot to any coffee connoisseur’s taste. “A good espresso beverage can be made in a multitude of ways,” Hendon says. Following the team’s coffee credo, though, he adds, might help ensure that once you find your ideal cup, you’ll be “able to make it 100 times in a row.”

Katherine J. Wu | | READ MORE

Katherine J. Wu is a Boston-based science journalist and Story Collider senior producer whose work has appeared in National Geographic, Undark magazine, Popular Science and more. She holds a Ph.D. in Microbiology and Immunobiology from Harvard University, and was Smithsonian magazine's 2018 AAAS Mass Media Fellow.