This is how you should be grinding your coffee, according to science

In a surprising and somewhat explosive cocktail, US chemists have teamed up with volcanologists to identify the best way to grind your coffee beans.

If you’ve ever ground your own coffee beans, you’d know that coffee particles tend to clump together and stick to the grinder. Science tells us this happens because as the beans fracture and rub together the process generates static electricity.

But the problem which affects at-home brewers, and industrial coffee production, has been solved in a new study published in the journal Matter.

The study found that a small amount of water added to coffee beans immediately before grinding means less coffee is wasted and there is less mess to clean up. When making espresso, they also found that grinding with water resulted in a longer extraction time and a stronger brew.

Josh Méndez Harper explains why some baristas spray water on their coffee beans before grinding. Credit: University of Oregon

“Moisture, whether it’s residual moisture inside the roasted coffee or external moisture added during grinding, is what dictates the amount of charge that is formed during grinding,” says senior author Christopher Hendon, a computational materials chemist at the University of Oregon in the US.

“Water not only reduces static electricity and therefore reduces mess as you’re grinding, but it can also make a major impact on the intensity of the beverage and, potentially, the ability to access higher concentrations of favourable flavours.

“Pushing the concentration up by 10%–15% for the same dry coffee mass has huge implications for saving money and improving quality.”

To investigate the phenomenon Hendon teamed up with volcanologists who study similar electrification processes during volcanic eruptions.

“During eruption, magma breaks up into lots of little particles that then come out of the volcano in this big plume, and during that whole process, those particles are rubbing against each other and charging up to the point of producing lightning,” says first author and volcanologist Joshua Méndez Harper, of Portland State University.

The researchers measured the amount of static electricity produced when grinding different commercially and in-house roasted coffee beans.

They found that there was no association between static electricity and the coffee’s country of origin or processing method.

But they did find that less electricity was produced when coffee had a higher internal moisture content, and when it was ground at a coarser setting. Darker (drier) roasts produced more charge than lighter roasts, and also produced much finer particles when ground at the same setting.

A small pile of moist coffee beans on a table top
Beans wet with 5 µL of water per g of coffee. Credit: University of Oregon

They also compared espresso made with identical coffee beans ground with and without a splash of water. The water was added using a process known in the coffee industry at the “Ross Droplet Technique”.

They found this resulted in espresso shots that were more similar from shot to shot, with a longer extraction time and a stronger brew.

“The central material benefit of adding water during grinding is that you can pack the bed more densely because there’s less clumping,” says Hendon.

“Espresso is the worst offender of this, but you would also see the benefit in brew formats where you pour water over the coffee or in small percolation systems like a stovetop Bialetti. Where you’re not going to see a benefit during brewing is for methods like the French press, where you submerge the coffee in water.”

So, how much water do you need to add? According to Méndez Harper: “just a spritz works”.

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