Recent Studies on Coffee: Part 1 - The Secret to Perfect Espresso

There have been several new studies just published that we found interesting, so we wanted to share them, and our thoughts on them.

First up is “Researchers claim to have discovered the secret to perfect espresso” - a clickbait headline from The Guardian. Here’s the actual study: https://www.cell.com/matter/fulltext/S2590-2385(23)00568-4. Before we dig in to this “secret to perfect espresso,” let’s take a look at what we already knew, and what they did actually discover.

Anyone who has ever ground any coffee knows that static electricity is generated during grinding. The grounds stick to the exit chutes of grinders, they stick to grounds bins, they stick to each other. This static electricity often results in coffee grinding being a mess. A simple way to significantly cut down on that static electricity is to perform the “Ross Droplet Technique, or RDT,” which is nothing more than spritzing the beans with a little water mist before grinding them. We’ve known about this for years. Some grinders even have ionizers to reduce the static electricity. This has always been done simply to avoid the mess.

Here are the new discoveries. First, they found that a big part of the static electricity generated during coffee grinding happens due to the pieces of the beans rubbing against each other (triboelectrification) rather than the friction between the burrs and the beans themselves (fractoelectrification). This is significant because if you can reduce the number of fines that a grinder is producing, you will get less triboelectrification.

Second, they found that sometimes the grounds end up with a negative charge and sometimes a positive charge. Whether they end up positive or negative depends mostly on roast level and internal moisture level. So if an ion beam for de-clumping that is built into a grinder is going to work, it has to be of the correct charge. We’ve known for quite a long time that clumped grounds in an espresso puck are not good for extraction evenness and shot flavor.

Third, and this is the really interesting and important piece, they found that performing the RDT actually significantly slows down espresso shots and results in higher extraction yields, if you use enough water. “Enough water” to make this happen is more like four spritzes of water from a little spray bottle rather than the one spritz that has tended to be the common advice. Why does this happen? The water prevents the grounds from getting so charged up in the first place (and/or dissipates the charge if they do get charged up), which prevents them (particularly the fines) from clumping together and acting as boulders. For years, baristas have used another technique, knows as the Weiss Distribution Technique, or WDT, which is essentially just stirring the grounds with a toothpick or similar to break up the clumps before tamping. Fascinatingly, WDT does not appear to do a good enough job breaking up the clumps. The “four spritzes of water RDT” method prevents the clumps so well that shot times were 50% longer than non-RDT shots, and TDS (total dissolved solids) were 0.9% higher, which corresponds to a 2.25% increase in extraction yield at the 18g dose, 45g yield recipe used in the experiment. The thought is that this happens because the empty space between grounds has been reduced as a result of not having as much clumping, so there is more resistance to the flow of water, and the flow through the puck is more uniform.

Now, the researches did not actually taste the espresso to determine if the 50% longer shot time, 2.25% higher extraction shots tasted better than the non-RDTed shots. However, with espresso, higher TDS/extraction is much more consistently linked to better flavor than it is in filter, as it is much harder to get high enough extractions with espresso, especially with lighter roasts (a 20% extraction light roast espresso might be shockingly sour but a 21.5% extraction light roast espresso could taste balanced, bright, fruity, and sweet, so this 2.25% increase in extraction from RDT is massive).

For example, a solid light roast but not like 90+ point coffee might taste better and better on pourover as you increase extraction up to let's say 21-22% but then above (23%+) that you might start tasting some slightly off-putting flavors that are NOT full-on defects, nor astringency from channeling, nor roast flavor. This same coffee on espresso will likely just taste better and better as you increase extraction because you are only going from maybe 19-20% extraction to 21-22% at the very most (this is what the RDT shots reached, 18g dose, 45g yield, 8.93% TDS = 22.3% extraction...so it never reaches the point where the coffee starts tasting worse. If you grind finer to try to boost extraction more then you just get channeling and extraction goes down).

Even if you are using a darker roast for your espresso, which might taste balanced at 18% extraction, which is pretty easy to hit even without doing the four spritzes of water RDT, it is extremely likely that an RDT shot ground at a slightly coarser setting to achieve the 18% extraction will taste better than a non-RDT shot ground at a slightly finer setting to achieve the 18% extraction, because the RDT shot will have had more uniform flow (less channeling) and therefore present a softer flavor with simultaneously less sourness and less bitterness/harshness/astringency.

One final note: these results were all obtained with a Mahlkonig EK43 grinder. There is some evidence that this huge increase in shot time and extraction yield when performing the four spritzes RDT does not happen with some other grinders. However, we have not yet seen any evidence that it ever makes shots flow faster, extract less, or taste worse, so we do not see any reason not to at least give it a try. If it doesn’t do anything for you, then just stop doing it. But you may find it helps you a lot!

So, if you are brewing espresso at home, we highly, highly recommend at least trying the four spritzes RDT! In a commercial setting, it is sadly still not really feasible from a workflow and efficiency perspective. Though it does make us think about Proud Mary’s EK hoppers being sealed in freezers - perhaps espresso hoppers being sealed in very humid environments would achieve the desired results…