Clive Coffee's Analysis of James Hoffmann's RDT Experiment

We were first turned on to RDT (Ross Droplet Technique) several years ago vis-à-vis several espresso forums. On the one hand, we decided to try it out of sheer intellectual curiosity and, on the other, admittedly out of angst—frustrated that my grinder, which costs several hundred dollars, would produce grounds with so much static, resulting in a confounding mess. Is this normal? Is this something I can change, and if so, would my workflow and the flavor of my coffee benefit? We were astonished by how well it worked. Here’s an easy way to use RDT:  Take a small spray bottle, give your beans a few spritzes, shake or stir, then grind— and voila, static electricity is eliminated. While we can see the results and that something happened, we also want to understand why and what exactly happened. 

Several months ago, two behemoths of the coffee industry, James Hoffmann and Chris Hendon, sat down and discussed the implications of a scientific paper (co-authored by Hendon) that investigates the process of RDT, how RDT helps reduce static electricity and the impact it can have on brewing & extraction. 

As a result, Hoffmann produced his own RDT test and a Google Survey to crowdsource results for anyone interested in experimenting on their own. Hoffmann personally approached our resident coffee savant, Charles Kelso, to participate in this noble experiment, which he humbly and happily agreed to. Before we jump into our test results, let’s briefly get into the meat of Hendon’s research. 

A Quick Note on the Coffee Study

At its core, grinding breaks coffee beans into smaller particles. Thus, the energy produced from this process emits heat, which, in turn, generates static electricity. It’s a two-pronged process: 1. Triboelectrification and 2. Fractoelectrification—both generating interfacial heating from the grinding process, giving the coffee particles positive and negative charges. 

Ultimately, their data supports what we already understood about the benefits of RDT, namely that it eliminates static electricity. Their research demonstrated that the grounds acquire a net-negative charge when the beans have a moisture content below 2%. Additionally, they collected extensive data on the interplay between roast, moisture, and electrification, so please feel free to peruse the entire article for additional insights. Introducing moisture significantly reduces surface charging on the grounds within the grinder, preventing charge buildup and minimizing static. For those nervous about applying water to their beans, have no fear; you will not break your grinder. 

RDT: More Than a Static Elimination Technique 

Moreover, what’s even more fascinating are the elements of RDT that we didn’t know we were benefiting from. Here is the short version. They compared espresso shots made with coffee grounds using RDT with and without added water, targeting a specific shot output weight. They found that shots made with RDT-ground coffee flowed slower and had higher extraction levels, which was unexpected. The experiments used four times more water than usual, impacting the brewing process. Hendon explains that this extra water prevents the formation of what Hoffmann calls "electroclumps," ensuring a more even distribution of ultra-fine particles, leading to slower flow rates and higher extraction. This intriguing result suggests that using more water with RDT could improve consistency and extraction, prompting further investigation into Hoffmann's experiment.

Hoffmann’s RDT Experiment 

To recap the paper’s results, shots pulled with RDT flowed markedly slower. They displayed a higher extraction rate than shots pulled without, which piqued Hoffmann’s interest—mainly because this finding is the antithesis of our established understanding of RDT. The clincher is that the shots pulled in the study used four times the water we usually would use for static reduction. Hoffmann concluded that if we can modify the flow rate and increase extraction by adding more water to the beans, we can potentially alter extraction in several interesting ways to chase better and more interesting flavors. Hoffmann’s experiment explores these implications and asks if adding more water would give us more consistency and higher extraction with shorter shots. The potential is limitless. 

Hoffmann’s experiment is exceedingly simple to replicate at home: 

• Select your preferred recipe and set a specific ratio (1:2,1:1.5,1:3, etc).
• Grind a dry dose and pull the shot as usual, stopping when it hits your target weight. Record the time it takes. 
• Next, use RDT for the second dose, applying 20µl of water per gram or about 3-4 sprays for an 18-20g dose. Pull the shot, stop when it reaches the target weight, and record the time.

Clive’s RDT experiment 

In the spirit of the scientific method, we tried to replicate Hoffman’s experiment as best as possible and add to the data set. We performed this test three times with the same coffee (Kickstep Blend by The Great North) and four different espresso grinders that may produce varying results. The Great North is an espresso blend with a medium to dark profile. It's flavor profile of cocoa, caramel, and dark cherry make this a coffee that's easy to dial in and has a large range of what an acceptable tasting shot is. 

The grinders we chose: 

• Eureka Oro Mignon Single Dose—to compare our results with Hoffmann’s findings. The Oro uses flat stainless steel 65mm burrs.
• Fellow OPUS— a budget-friendly conical burr grinder (40mm stainless steel) with the slowest grinding RPM and burr set. 
• LUCCA Atom 75—chosen for its larger burrs (75mm cryo-treated diamond inside burrs) and slightly higher retention, representing grinders not designed explicitly for single dosing.
• Eureka Mignon Zero—though it might seem redundant alongside the Oro Single Dose, stay tuned for our experiment’s results. The Zero uses 55mm flat stainless steel burrs.

Clive’s Experiment: a Review of the Results 

Let’s jump straight into the results. First, the biggest surprise of the experiment is that we’d assumed that our results would more or less line up with Hoffmanns'—they didn’t. The LUCCA Atom 75 delivered precisely what we anticipated. Our dry shots consistently took just over 30 seconds, while the first RDT shot significantly slowed down, clocking in at 44 seconds. Over several shots, we observed an average shot time increase of 30%. The Fellow Opus, on the other hand, met our expectations by showing no significant change; there was less than a 1% difference in shot times with and without RDT. So far, so good.

Then came the unexpected result from the Oro Mignon. Without RDT, our shots consistently lasted around 27 seconds, but the first RDT shot dropped sharply to 22 seconds. On average, this resulted in a 21% reduction in shot time. Charles repeated the test several times with the Oro to ensure accuracy and consistently obtained the same results. This unexpected outcome prompted Charles to test the Mignon Zero, a similar grinder, to verify these findings. The Zero produced nearly identical results, averaging a 19% reduction in shot time using RDT.

Thoughts, Surprises, & Speculation

The best aspect of this experiment is the knowledge we gained. The outcomes we didn’t anticipate widened our reference frame and encouraged us to run even more tests. Charles had three primary takeaways: 

• We need more clarity on the cause and effect of RDT:  While the results varied between grinders they were impressively consistent with each grinder tested. Even though we cannot pinpoint or isolate the reason for the why (there are several whys), we can still benefit from using RDT, which is ultimately a net positive for anyone making espresso. If you’re already a hardcore RDT head or are interested in using this method, give your beans a few more spritzes of water. The paper notes that “shots with the same degree of extraction but different time parameters should have markedly different flavors.” Using this technique to unlock higher extractions quickly could unlock previously hidden flavors in your coffee. 

• Isolating specific variables can give us more clarity on RDT: Charles speculates, rather astutely, that parsing out several variables in the grinding process can shed some light on RDT—burr chamber material, grinder heat production, grinder heat absorption, burr RPM, feed rate, grind retention, and particle size distribution. 
• What Hoffmann’s survey results will reveal: We’re very excited to see how everyone’s responses impact the overall data set and individual data points, like which grinders respond more to RDT.

Did you find this topic interesting? Are you going to evangelize the benefits or RDT for all of those to hear? If you're not a true believer, or if you have insights on RDT, sound off in the comments. This is a living experiment and the more folks who actively participate, the better. Don't forget to read the full study on RDT—"Moisture-controlled triboelectrification during coffee grinding," and if you're feeling up to it, please consider contributing to James Hoffmann's survey: The RDT Experiment. As always, thanks for stopping by and nerding out on coffee with us. Cheers.