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
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