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

Static charge in the ground coffee is to blame when ground coffee doesn't flow into the portafilter or dosing cup, but instead sticks to the grinder body or chute. This happens especially with single-dose grinders, but grinders with bean hoppers sometimes have static problems too. However, static charge doesn't just create a messy work environment, it can also negatively impact espresso extraction. But there's a simple method that can help: a spray of water on the beans before grinding.

This technique is called the Ross Droplet Technique (RDT) and was introduced in 2005 by David Ross in the Home-Barista Forum. Since then it has been refined and scientific studies on the topic have been published. In this article you'll learn how to apply RDT, what studies are behind it, and how static charge forms.

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 shaken gently to distribute the water over the beans.
4. After that, grinding can begin.

Studies show that 0.1 - 0.2 g of water per 18 g of coffee is enough to significantly reduce static charge. This means less coffee sticks to the grinder and the ground coffee is distributed more evenly.

Why does static charge occur?

Static charge during coffee grinding is created by 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 sites.

This charge causes fine coffee particles to attach to larger particles and form agglomerations (clumps). The different charges (positive and negative) cause the particles to attract each other and form electrically neutral aggregates. This results in uneven density distribution in the coffee bed – creating areas of high density and areas with lots of empty space, which makes even extraction difficult.

Influence of coffee variety, roast level, and humidity

• Moisture content of the beans: Beans with less than 2% moisture tend to have negative charge, while more humid beans are more positively charged.
• Roast level: Darker roasts create stronger negative charges than lighter roasts.
• Particle size: Smaller particles (<100 μm) tend to have negative charge, while medium-sized particles (100-300 μm) are more positively charged.
• Environmental humidity: Higher humidity (>60%) reduces static charge.

Scientific findings on RDT

• The reduction of static charge leads to less coffee buildup in the grinder.
• Fine coffee particles remain better distributed, making extraction more homogeneous.
• The scientific evidence for extraction improvement is however inconclusive. Some experiments show increased extraction, while others found no significant differences.

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

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 evaporates quickly and leaves no moisture residue. Additionally, many roasteries 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 creates a cleaner work surface, more even ground coffee distribution, and can have a positive effect on espresso extraction. While scientific studies haven't yet provided definitive proof of improved extraction, the practical benefit for many home users and professionals is clear: Less static charge means more control over the grinding process and more enjoyable coffee preparation. And if it's more enjoyable, you'll taste it in the cup afterward too.

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