In my recent review of the Fellow Opus grinder I mentioned the retention issues as its main flaw. In reality, retention is one of the major problems of the vast majority of budget grinders, and even more expensive grinders also suffer from it to a greater or lesser extent.
One of the simplest and most effective solutions to cope with this problem is the RDT technique, which stands for “Ross Droplet Technique,” named after the user who first proposed this technique on the legendary home-barista forum.
This technique involves lightly spraying the coffee beans with a little water just before grinding them. This reduces static charge, so during grinding, static and retention are reduced, and the formation of clumps, typical when grinding coffee very finely and often the main cause of channeling, is also minimized.
In the video, I show you the huge difference between applying the RDT or not. In the first test, without using this technique, the retention in the grinder is enormous, forcing me to “blow” with the hopper lid and give several taps to force out all the ground coffee that got stuck in the burrs.
However, in the second test, I apply the RDT technique, spraying the beans three times with water using a spray bottle, which I tell you more about in my review of some Normcore accessories. In this second test, retention is minimal, and I only need to give a couple of taps to release all the coffee I previously weighed on the scale.
Besides reducing static and retention… Does it also improve extraction?
One of the questions that has been debated about this technique is whether, in addition to reducing static, retention, and clump formation, it also improves extraction.
The best study conducted on this is this one from the scientific publication Journal Matter. It explains that spraying a little water on the coffee beans before grinding them is not only beneficial in mitigating the effects of static and retention in grinders, but it also increases extraction by 10%.
This translates into an espresso that is 10% more concentrated. Basically, what we are achieving with the RDT technique is increasing the solubility of the ground coffee particles, which is especially interesting in medium or light roast beans, which are less soluble than dark roasts.
To be completely honest, I have not noticed a substantial change in the duration of the extraction, but better cleaning and maintenance of the grinder is always a good thing: with less retention, you ensure that the coffee you are using is the one you just ground, and not part of what was trapped in previous grindings.
Things to consider
Something very important to consider: grinders do not get along well with water, so there is a risk of corroding and rusting the burrs in low-quality grinders. The truth is that any reasonably decent grinder, with stainless steel burrs, will not suffer any damage from coming into contact with just a couple of drops sprayed on the coffee beans.
This brings me to another point that also needs to be considered, which is that the amount of water to spray depends greatly on the specific grinder. In the case of my tests with the Fellow Opus, I have perfectly validated that the best results are obtained by spraying three times with the spray, but in your case, it may be sufficient with one, two, or more than three.
Another thing to consider is that this technique is mainly useful for espresso, and to a lesser extent for filter coffee. In French press or cold brew preparations, it is a useless technique, as with coarse grinds, static and retention are much lower.
It should also be mentioned that the benefit is much greater with light or medium roasts, which have a higher static charge than dark roasts, something I could also verify with the Fellow Opus: grinding a medium roast coffee, I experienced a lot of retention, while with a decaffeinated dark roast coffee bean, there was hardly any.
In short, I think it’s worth exploring the RDT technique if your grinder is giving you headaches with static and retention. The benefits are several, and the drawbacks are few, considering that the effects on the quality of your espresso will come more as a result of keeping the grinder burrs clean, rather than from possible effects on improving extraction.
