Grinding coffee beans with a drop of water yields less watery espresso

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 C.H. Hendon    28-03-2024     Leggi in PDF

Whether you use a conical burr, flat burr, or a whirly blade grinder, the process of fracturing coffee beans results in the accumulation of static charge through triboelectrification. The charge density is thought to depend on physical properties such as the material's thermal conductivity, the Seebeck coefficient, and Fermi level. In the case of coffee, the beans are comprised on countless molecules suspended in an amorphous array of degraded carbon-rich material, and the charging process is largely governed by the composition of the insoluble coffee matrix. Finding ways to control the matrix composition is therefore one chemical handle to affect the amount of charge that accumulates during grinding.

In our recent study published in Matter, we showed that the composition of the coffee matrix is affected by roast color, with darker coffees generating more static than their lighter counterparts. But the solution to static reduction is not simply to roast lighter, because consumers tend to have different flavor preferences. Instead, we show that it is not necessarily the color of the coffee that dictates the magnitude of charging, but rather the residual water content contained within the beans. The moisture content can be affected by altering roasting parameters, as well as through the direct addition of small amounts of water to the whole beans themselves. The latter results in a marked reduction in static with just a single drop per 20 g or coffee.

The static reduction doesn't only affect the cleanliness of the grinding process, but also results in the reduction in aggregate formation. Through microscopy, these aggregates are shown to be clusters of small particles adhered to large particles and, when packed into an espresso basket, result in inhomogeneities within. These inhomogeneities lead to variable extractions. Since water tends to percolate around the aggregates, the result is lower concentration of dissolved coffee material. Keeping all other variables constant, but by adding a drop of water during grinding, espresso is shown to increase in concentration as much as 16%, resulting in a more intense and less wasteful drink.

Try it for yourself — add a single drop of water to your next grind!

Christopher H. Hendon – Associate Professor of Chemistry at the University of Oregon, he leads a group with two interests: one using computational chemistry to study material defects, and the other studying the physics and chemistry of coffee production.