Forget expensive beans and pricey filters – if you want a stronger cup of pour-over coffee, just add water slowly, steadily and from a height, researchers say.
While there are myriad ways to make coffee – from moka pots to cafetieres and barista-style machines – pour-over coffee is an everyday staple for many. Now scientists say they have discovered how to make a stronger cup using the same quantity of ground coffee.
Dr Arnold Mathijssen, a co-author of the study from the University of Pennsylvania, said pouring the hot water slowly from a goose-neck kettle increases the contact time between the water and the coffee grounds, while pouring from a greater height increases mixing, both of which result in more coffee being extracted.
But he added: “If you pour too slowly, or if you go too high, then the jet tends to break up into these smaller droplets, and that’s what you want to avoid as well.”
Writing in the journal Physics of Fluids, Mathijssen and colleagues report how they carried out a number of experiments involving transparent silica gel particles – representing ground coffee – that were illuminated with a laser sheet, which were recorded with a high-speed camera.
While slow pouring increases contact time, the team found that at low heights the velocity of the water was too low to dig into and disturb the bed of particles. Water poured from greater heights resulted in greater agitation of the particles, but the team found that water must flow in an unbroken jet to dig into the bed of particles and displace those at the bottom of the funnel – a process that results in particles building up on the sides and then falling in, creating an avalanche-like effect that increases mixing.
“The increased height compensates for the slow pouring. You only get the avalanche if there is enough energy available,” Mathijssen said.
When the team applied their findings to coffee itself, they found pouring from a greater height resulted in stronger brews, provided an unbroken flow of water was used.
The team add that a good starting point for those at home is to reduce the amount of ground coffee by 10%, say from 20 to 18 grams per cup, then taste the coffee produced by pouring the hot water at different heights – keeping to a limit of about 30cm for safety.
Researchers have previously revealed that to make the perfect espresso every time, it is best to use coffee that is ground slightly coarser than normal.
Both that study and the new research found that the proposed adjustments meant less coffee was needed to produce a drink of given strength. Experts say this is important given that climate breakdown is already causing problems for cultivation of the Coffea arabica plant.
Prof Jamie Foster, of Portsmouth University, who was not involved with the new research but carried out the study on espresso-making, said it seemed “totally plausible” that the proposed method would lead to a stronger cup of pour-over coffee and a more optimal use of the coffee grounds, and he saw no reason why similar logic could not be applied to cafetiere-style brewing.
“Of course, there is a cheat open to those who prefer cafetiere coffee,” he said. “That is, put in a spoon and give the grounds a stir, but perhaps a cleverly chosen pour could save dirtying cutlery.”
