Water spray on coffee? The Ross Droplet Technique (RDT)

Static charge of the grinds is to blame when ground coffee does not slide into the portafilter or dosing cup, but instead sticks to the grinder body or the chute. This occurs primarily with single-dosing grinders, but grinders with bean hoppers also sometimes have static issues. However, static charge not only leads to a messy work environment but can also negatively affect espresso extraction. Yet a simple method can help: a spray of water on the beans before grinding.

This technique is called Ross Droplet Technique (RDT) and was introduced by David Ross in the Home-Barista forum in 2005. Since then, it has been refined, and even scientific studies on the topic have been presented. In this article, you will learn how to apply RDT, what studies underpin it, and how static charge arises.

How RDT works

1. After weighing the coffee in the dosing container, 2-3 drops of water are added.
2. A spray bottle is best for distributing the water evenly.
3. The container is gently shaken to distribute the water over the beans.
4. Grinding can then begin.

Studies show that 0.1 - 0.2 g of water per 18 g of coffee is sufficient to significantly reduce static charge. This means less coffee gets stuck in the grinder, and the grinds are distributed more evenly.

Why does static charge occur?

Static charge during coffee grinding arises from two main mechanisms:

• Triboelectrification: Friction between coffee particles and grinding tools generates electrical charge.
• Fractoelectrification: When coffee beans break, charge shifts occur at the fracture points.

This charging causes fine coffee particles to adhere to larger particles, forming agglomerations (clumps). The different charges (positive and negative) cause the particles to attract each other and form electrically neutral aggregates. This results in an uneven distribution of coffee bed density – areas with high density and areas with a lot of empty space are created, which complicates even extraction.

Influence of coffee variety, roast level, and humidity

• Bean moisture content: Beans with less than 2% moisture tend to be negatively charged, while moister beans are more likely to be positively charged.
• Roast level: Darker roasts produce stronger negative charges than lighter roasts.
• Particle size: Smaller particles (<100 μm) tend to be negatively charged, while medium-sized particles (100-300 μm) are more positively charged.
• Ambient humidity: Higher humidity (>60%) reduces static charge.

Scientific findings on RDT

• The reduction of static charge leads to less coffee residue in the grinder.
• Fine coffee particles remain better distributed, which makes extraction more homogeneous.
• However, the scientific evidence for improved extraction is inconclusive. Some experiments show increased extraction, while others could not find significant differences.

The most comprehensive study by Méndez Harper et al. (2024) showed that moisture is a central factor for static charge. Beans with less than 2% moisture content tend to have a negative charge, while moister beans are more likely to be positively charged. This explains why different roast levels are charged to different extents.

Is RDT safe for the grinder?

A frequently discussed point is whether water in the grinder can cause long-term damage. However, studies and practical reports suggest that the small amount of water quickly evaporates and leaves no moisture residue. In addition, many roasters industrially add up to 5% water per kilogram of coffee – significantly more than the 1-1.5% with RDT (0.1-0.2 grams per 18 grams of coffee).

Conclusion

RDT is a simple and effective method to reduce static charge during grinding. It ensures a cleaner work surface, more even grind distribution, and can positively affect espresso extraction. While scientific studies do not yet provide conclusive evidence for improved extraction, the practical benefits for many home users and professionals are clearly recognizable: Less static charge means more control over the grinding process and a more pleasant coffee preparation. And when it brings more joy, you'll taste it in the cup afterward.

Sources and further reading:

• "Moisture-controlled triboelectrification during coffee grinding" by Joshua Méndez Harper, Connor S. McDonald, Elias J. Rheingold, Lena C. Wehn, Robin E. Bumbaugh, Elana J. Cope, Leif E. Lindberg, Justin Pham, Yong-Hyun Kim, Josef Dufek, and Christopher H. Hendon

• "A study of espresso puck resistance and how puck preparation affects it" Jonathan Gagné, Coffee ad Astra

• "Assessing the impact of RDT on espresso extraction" by Jeremy and Joe from Socratic Coffee

• "BETTER WAY TO GRIND COFFEE: Deep Dive on New Hendon Paper" by Lance Hedrick