Why adding water when you grind coffee beans makes for a better brew

Adding a drop of water to your coffee beans before grinding them can reduce mess and lead to a more flavourful brew.

Coffee aficionados often add a little water to their beans before grinding, either from a wet spoon or as a spritz of moisture, because it stops coffee grounds sticking together and keeps them from flying away from the main pile. Scientists think this is because the water reduces static electricity, but the exact mechanism and the perfect amount of water to add were unclear.

To investigate, Christopher Hendon at the University of Oregon and his colleagues sprayed various amounts of water on a range of coffee beans that had been roasted for different lengths of time, with varying levels of internal moisture, and then put them through a grinder. They then measured the amount of static electricity, how large the ground coffee particles were and the flavour of the coffee brewed with an espresso machine.

They found that the most important factor in determining the electrostatic charge, and so the clumpiness of the coffee, was the total moisture in the beans before grinding. Beans roasted to a darker colour have less moisture, and this makes them more susceptible to clumping.

“It’s not about the origin or the processing method. It’s not about the quality of the coffee or the price that one might pay for those particular beans,” says Hendon. “It really boils down to the colour of coffee and the internal moisture.”

Adding around 20 microlitres of water per gram of coffee – or around half a millilitre for a typical brew – can stop the grounds clumping together and improve the consistency and flavour of an espresso, says Hendon. “If you squirt a little water on it, you’re gonna see a difference in brewing,” he says.

As well as helping people make better coffee at home, the addition of water before grinding could save hundreds of millions of dollars for commercial coffee companies, says Hendon. This is because the resulting ground coffee is less clumpy and so more water can percolate through it, reducing the amount of coffee beans you need to make a brew of a certain strength.

“This paper could make a small but important change to the process of brewing espresso coffee,” says William Lee at the University of Huddersfield, UK. “The most intriguing aspect of the study is the investigation of clumping of grains due to electrostatic charging. This could affect the taste of coffee, as water will not be able to penetrate these clumps.”