Static electricity during coffee grinding: Why does a few drops of water make espresso flow slower?

There is a familiar phenomenon with coffee grinder users: just after grinding, coffee powder flies out, sticks to the grinder's walls, clings to the spout, or forms small clusters. Baristas usually handle it by tapping the grinder gently, waiting for the powder to settle, or using a brush to clean it.

But the powder particles sticking together are not just a hygiene issue. They can change the way coffee particles are distributed in the espresso filter, affecting water absorption and flow rate.

The main reason lies in the phenomenon of static electricity during grinding, a topic that has been studied by materials and chemistry research groups using a charge detector, particle size analysis, and espresso brewing experiments.

Where does static electricity in the grinder come from?

When coffee beans pass through the burr grinder, they are not only crushed into small pieces but also collide and rub against the burr surface, the grinder's interior, and other coffee beans.

There are two mechanisms to distinguish:

• Triboelectrification: static electricity resulting from the interaction and friction between material surfaces.
• Fractoelectrification: static electricity related to the process of material fracture and the formation of new surfaces.

In a study published in Matter in 2024, the authors showed that most of the static electricity formed during grinding is related to the events of particle breakage, rather than just the friction between coffee powder and the grinder's surface.

This explains why some grinding conditions produce more static electricity than others. Grinding finer usually creates more breakage and contact events. In the study's dataset, darker-roasted coffee tends to produce finer particles and stronger negative static charges than lighter-roasted coffee.

Moisture inside the bean is more important than roast color

Do not just look at the roast color to predict the amount of static electricity. Research surveyed around 30 commercial coffee samples and found that the remaining moisture inside the roasted bean is a better indicator than the Agtron color scale in explaining the magnitude and sign of static electricity.

In the studied sample set, the transition from positive to negative static charge occurs when the moisture content of the roasted bean drops below around 2% by weight. This is an experimental result from the research group's sample set, not a physical threshold applicable to all coffee types and grinders.

This also shows that two batches of coffee with the same roast color may not produce the same static state. Storage history, pre-roast moisture, roast curve, and post-roast dryness can all affect the behavior of the beans during grinding.

When coffee powder clumps, water meets a different powder layer

During grinding, charged particles can attract each other and form clumps. These clumps alter the way coffee powder fills the espresso filter volume.

The issue is not just about having more or fewer fine particles. When fine particles stick to larger particles or adhere to the grinder's interior, the actual distribution of particles in the filter may differ from the initial distribution created at the burr region.

With espresso, where the powder layer is compressed into a block with significant hydraulic resistance, even small changes in the powder structure can affect water absorption and flow behavior throughout the extraction process.

Ross Droplet Technique: a small amount of water can make a difference

The technique of adding a small amount of water to the coffee bean before grinding is called Ross Droplet Technique (RDT). This practice has been used by some baristas before experimental research explained the mechanism.

In the study published in Matter, adding water to the bean before grinding reduced static electricity and the phenomenon of small particles clumping with large particles. The authors noted that adding around 20 microliters of water per gram of coffee can bring the static charge close to zero in some experimental conditions.

A subsequent study published on iScience in 2024 compared various methods of reducing static electricity, including waiting after grinding, adding water, and using an ionizing device. The results showed that adding water to the green beans before grinding both reduced the charge and affected the distribution of the beans and the amount of powder left in the machine. In this study, a water level of approximately 20–30 microlit/g helped reduce the charge by at least 50–60% in the samples surveyed.

How does the espresso experiment change?

The interesting point of the study published on Matter is that the authors did not stop at measuring the charge. They also brewed espresso with fixed parameters to observe the effect of RDT-treated coffee powder.

With the same brewing parameters, the samples ground after adding water had an extraction time nearly 50% longer. The TDS of the espresso increased from 8.2% to 8.9%.

This difference is equivalent to a 0.7% TDS point or approximately 8.5% if calculated as a relative increase. This is a change in the concentration of dissolved substances in the drink, not evidence that the flavor is necessarily better.

The research group explained that when the charge is reduced, large and fine particle clusters are broken or rearranged. Many fine particles that tend to stick in the grinder are released, resulting in a smaller average particle size in the filter basket and increased flow resistance.

Therefore, water passes through the powder more slowly. However, the study did not conduct a sensory test to demonstrate a specific flavor profile. A higher TDS only indicates that the drink is more concentrated under the test conditions.

How to try RDT at a coffee bar?

You can start with a small test, rather than applying it immediately to all types of coffee and all grinders:

1. Weigh the exact amount of green coffee beans needed for brewing.
2. Use a clean spray bottle to add a very small amount of water to the beans, not spraying directly into the grinder.
3. Mix gently to distribute the water evenly over the surface of the beans.
4. Grind immediately after moistening.
5. Keep the same dose, grind size, compression force, temperature, and brewing ratio as the control sample without RDT.
6. Record the shot time, beverage volume, TDS, and amount of coffee left in the machine.

A level of 20 microliters per gram is equivalent to 0.02 milliliters per gram. With a dose of 18 grams, the converted amount of water is approximately 0.36 milliliters. This is a conversion from the usage level in the study, not a mandatory quantity for all espresso machines or grinders.

Do not increase the water amount arbitrarily. In the iScience study, excessive water can promote the phenomenon of agglomeration due to conduction. Residual humidity in the grinding chamber also needs to be observed to prevent clogging, burr wear, or inadequate cleaning.

Is RDT suitable for all brewing methods?

It cannot be concluded as such. The experiments presented here focus mainly on the grinding and brewing of espresso on a specific burr grinder configuration. They have not demonstrated that RDT provides the same level of improvement for Vietnamese phin, V60, AeroPress, or French press.

It is also necessary to distinguish between two objectives: reducing coffee buildup in the machine and improving sensory quality. A method may help reduce static electricity, reduce coffee loss, or make the shot flow slower, but it may not necessarily create a better-tasting cup of coffee.

For coffee shops, a reasonable check is to run two parallel groups on the same type of beans, the same grinder, and the same recipe. If RDT makes the shot longer, adjust the grind size or extraction parameters according to the beverage target, rather than keeping the old time at all costs.

Static electricity is not just about coffee powder flying out and making a mess. It is a physical variable that appears right in the grinding chamber and can affect the structure of the coffee layer and the water flow resistance when brewing espresso.

For more information on grinders, roasting, and extraction, please refer to the SAVINA Coffee Library.

References

• Méndez Harper, J. et al. (2024). Moisture-controlled triboelectrification during coffee grinding. Matter, 7(1), 266–283. https://doi.org/10.1016/j.matt.2023.11.005.
• Harper, J. M., Bumbaugh, R. E. & Hendon, C. H. (2024). Strategies to mitigate electrostatic charging during coffee grinding. iScience, 27(9), 110639. https://doi.org/10.1016/j.isci.2024.110639