You’re grinding your coffee wrong

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

If you’ve ever ground your own coffee beans, you’ll know that coffee particles tend to stick together and stick to the grinder. Science tells us this happens because as the grains break and rub against each other, the process generates static electricity.

But the problem, which affects 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 the coffee beans just before grinding meant less coffee was wasted and there was 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 Mendes Harper explains why some baristas splash water on their coffee beans before they grind. Credit: University of Oregon

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

“Water not only reduces static electricity and therefore reduces mess while you’re grinding, but it can also have a big impact on the intensity of the brew and, potentially, the ability to access higher concentrations of beneficial flavors.”

“Increasing the concentration by 10%-15% for the same mass of dry coffee 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 the eruption, the magma breaks up into many small particles that then come out of the volcano in this big cloud, and during this whole process, those particles rub against each other and charge up to the point of producing lightning,” says first author and volcanologist Joshua Mendez Harper of Portland State University.

The researchers measured the amount of static electricity produced when grinding different commercial and home-roasted coffee beans.

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

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

A small pile of wet coffee beans on the table top
Beans soaked with 5 µL of water per gram of coffee. Credit: University of Oregon

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

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

“The central material benefit of adding water during milling is that you can pack the bed more tightly because there are fewer lumps,” Hendon says.

“Espresso is the worst offender of this, but you’ll also see the benefit in brew formats where you pour water over the coffee or in small percolation systems like a Bialetti stove. Where you won’t see a benefit during brewing is with methods like the French press, where you submerge the coffee in water.”

So how much water should you add? According to Mendes Harper, “just one spray works.”

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