Maths says you need coarser coffee grounds to make a perfect espresso

A more efficient way of making an espresso could reduce the amount of coffee wasted per cup, potentially saving billions of dollars every year.

Coffee baristas must decide how finely to grind their beans and then set their machines in a way to exploit the flavours of the coffee compounds. Jamie Foster at the University of Portsmouth, UK, and his colleagues decided to investigate the best way to maximise yield from the smallest possible amount of coffee.

They created a mathematical model of the “basket” of an espresso machine, the cylindrical container that contains the ground coffee. “We had to capture how the water is flowing through this bed of packed grounds, and how various coffee chemicals move around and dissolve,” says Foster.

The mathematicians’ initial calculations were based on a single coffee particle in this cylindrical contraption. They then scaled this calculation up for an entire basket’s worth of coffee in order to get an averaged description at the level of the whole basket.

Calculating for every coffee particle without this method would have required an impossible amount of computing power, Foster says.

Baristas normally say that the finer the coffee grinds, the more efficient the resulting brew due to an increased surface area of the coffee beans. But Foster and his colleagues found that their mathematical model showed that using slightly coarser ground coffee resulted in better mixing.

This was because coarser coffee ensured no part of the basket became clogged, and this ultimately resulted in a higher extraction of coffee compounds for the same amount of beans used.

A small coffee shop in Oregon owned by friends of one of the research team tried this method between September 2018 and September 2019. The shop found it saved $0.13 per drink, resulting in $3620 saved that year, and is still using it today. “If everyone did this, it would save the industry billions of dollars,” says Foster.

A spokesperson for the British Coffee Association says the find could have a real-world impact. “A mathematical model that increases efficiency of espresso extraction from ground coffee beans is hugely insightful for coffee businesses and coffee shop retailers,” they told New Scientist.