How to brew a better espresso, according to science

New research challenges conventional wisdom that brewing a strong shot of espresso requires very finely ground coffee beans.

Well-ground beans typically are thought to be best for making strong shots because smaller grounds dissolve more readily, while water flows through such grounds more slowly — allowing more time for the water to soak up coffee.

In brewing espresso and running computer simulations of the same process, researchers found that using finer grounds generally did allow water to absorb a higher percentage of the dried coffee — but only to a point. When filtered through beans ground to the finest setting on a standard machine, the hot water extracted a lower percentage of coffee than water filtered through slightly coarser grounds, researchers report online January 22 in Matter.

Both the experiments and simulations showed that, with the finest grounds, very small particles wedge in the gaps between other particles, says Jamie Foster, a mathematician at the University of Portsmouth in England. That means water flows unevenly through the grounds, oversampling some parts while missing others, which wastes coffee and produces inconsistent flavors.

After tasting fine-ground espresso, Christopher Hendon, a chemist at the University of Oregon in Eugene, remembers detecting both the bitter notes of overextracted coffee and the sour hints of underextracted grounds. “That’s a really strange flavor profile,” he says.

Coarser grounds, however, eliminated the clogging issue and created more consistent taste. What’s more, “you’ve extracted [coffee] more efficiently,” Foster explains, so “you can afford to use less coffee.” The researchers partnered with a café in Eugene to test this strategy. According to data on sales for a year beginning in September 2018, brewing espresso shots with 15 grams of coarse grounds rather than 20 grams of fine grounds saved more than $3,600.