Ending the daily grind: Science of perfect espresso uncovered

Coffee aficionados insist that the perfect espresso requires finely ground beans, tamped down tightly, to let steaming water pass through and create the fullest flavour.

But according to scientists baristas are getting it wrong, and could save time, money and improve taste by using courser grounds, which allow the water to flow more freely.

The problem, according to mathematicians at the University of Portsmouth and the University of Oregon, lies in the fact that grounds are often packed so tightly that they form clumps which water cannot pass through.

The teams performed mathematical modeling and brewed hundreds of espresso shots to discover the key variables required to make consistently tasty coffee.

They concluded that using 15 grams of coffee rather than 20, at a coarser grind, and letting the water run for just seven to 14 seconds, instead of the usual 25, gives the best flavour, and prevents any bitterness from occurring.

They calculated that the new method, reduces the amount of coffee used in espresso by 25 percent, and would save coffee shops 10p per drink.

Dr Jamie Foster, of the University of Portsmouth said: “The conventional wisdom is that if you want a stronger cup of coffee, you should grind your coffee finer. This makes sense because the finer the grounds mean that more surface area of coffee bean is exposed to water, which should mean a stronger coffee.

“When beans were ground finely, the particles were so small that in some regions of the bed they clogged up the space where the water should be flowing.

“These clogged sections of the bed are wasted because the water cannot flow through them and access that tasty coffee that you want in your cup. If we grind a bit coarser, we can access the whole bed and have a more efficient extraction.”

Dr Foster said that for people brewing at home the perfect coarseness would be difficult to judge with the naked eye, so people should use a quarter less coffee than usual, then carry out some trial and error taste test to find out what gave the best flavour.

Researchers trialled the new method in a small US coffee shop over a period of one year and they reported saving thousands of dollars.

Dr Christopher Hendon, a chemist at the University of Oregon, said: “If you use 15 grams instead of 20 grams of coffee and grind your beans coarser, you end up with a shot that runs really fast but tastes great.

“Instead of taking 25 seconds, it could run in 7 to 14 seconds. But you end up extracting more positive flavors from the beans, so the strength of the cup is not dramatically reduced. Bitter, off-tasting flavors never have a chance to make their way into the cup.

“For the local shop owner, this is an opportunity to save a lot of money without sacrificing quality.

“For the roaster, this is an opportunity to reflect on the approach to roasting and how people are brewing their coffee. For the producer, this should encourage them to continue to produce high-quality coffee that can earn them the most money, knowing that more people will have access to it.”

Previous studies have looked at drip filter coffee. This is the first time mathematicians have used theoretical modelling to study the science of the perfect espresso - a more complicated process due to the additional pressure

The study is published in the journal Matter.