Unevenly packed coffee to blame for weak espresso, say mathematicians

Grinding coffee beans too finely makes espressos weaker and now mathematics can explain why.

Inside of an espresso machine, boiling water moves through a bed of ground coffee under high pressure. One factor used to rate the resulting espresso is called extraction yield, which is a measure of how much of the coffee dissolves in boiling water during the brewing.

In 2020, William Lee at the University of Huddersfield in the UK and his colleagues found that extraction yield decreases when coffee is ground into very small granules. Now, they have looked into what is behind the effect.

To do this, the researchers constructed a mathematical model for what would happen if the bed of coffee has two regions with different porosity, which occurs if one area is more tightly packed than the other. In simulations, water flowed through the more porous region more easily, so more coffee granules moved around and dissolved there. This, in turn, increased the porosity of this region and led to even more water flowing through it. This meant that coffee was unevenly extracted across the two parts of the coffee bed.

When they ran their simulations for different grinds of coffee, the researchers saw the same trend as in past experiments – for very finely ground coffee, the unevenness led to a noticeable decrease in extraction yield. Lee says that their first idea was that fine granules make clogs that the water can’t go through. But their model showed that even slightly uneven packing meant that water didn’t flow through all parts of the coffee bed at the same rate. This, in turn, affects flavour because insufficiently extracted coffee tastes more watery.

“In the simulation, some parts of the coffee bed are not able to fully release the tasty chemicals that we enjoy. If we could control this unevenness we’d be empowering baristas to make tastier drinks in novel ways,” says Jamie Foster at the University of Portsmouth in the UK. Christopher Hendon at the University of Oregon says that espresso is really “the most problematic brew method of all” because the combination of tiny granules and very warm water under lots of pressure means that it is easier for flows to become unstable due to clogging or the creation of bubbles.

The team suggest that coffee extraction could possibly be made more even by redesigning the part of the espresso machine that granules get packed into. However, Ann Smith, who worked on the project, says there is still work to do before the team’s model fully reflects how existing machines work. One example would be taking X-rays of the coffee bed to determine details like how many regions of different porosity there can be.

“I think a lot more work is needed before we can confidently say ‘this is exactly how you should brew your espresso’,” says Lee.