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Some of James Hoffmann’s late videos on the topic of the AeroPress (Amazon Affiliates link) have stirred a lot of discussion around this particular brewer. I highly recommend watching all three of his videos, because the tests and discussions he presented are of a very high quality. It is extremely rare for me to land on such a deep discussion about any coffee brewing method without having major reservations about some of the claims being made; here, everything James claimed and concluded fits with my current understanding about the physics of coffee brewing.
Here are links to the videos in question by James Hoffmann:
While I have enjoyed the AeroPress a lot in the past especially when travelling, I have always had one big complaint about it: getting a thick coffee bed to minimize uneven flow when pressing out the water limits the brew ratios that can be used. For example, using a 18 grams dose limits the ratio to about 1:14, given that only 260 mL of water can fit through the remaining space of the AeroPress chamber (you can fit a bit more water if you let some drip out while you pour).
Because of this, most AeroPress brews I had enjoyed in the past had flavor profiles that I found typical of under extracted coffee, which in the case of light-roasted coffee emphasizes bright acidity but often lacks sweetness. Jame’s videos made me think about the AeroPress brewer again at a particular moment where I happened to be discussing a recent paper about very long immersion brews by the UC Davis team with coffee scientist Samo Smrke. In the paper, scientists brewed several immersions with hour-long steep times, and showed that the average extraction yields calculated in a way that is a bit analogous to the immersion equation depended only very weakly on the brew ratio. This appears surprising because we often brew coffee with much shorter brew times, where the solubles retained inside the coffee grounds have a different profile from those that leaked from the coffee particles into the slurry.
This caused me to reevaluate my issue with the AeroPress and made me want to try brewing for much longer steep times. The thought is the following: if we can get the coffee particles and slurry much closer to equilibrium, the chemical profile inside the coffee particles will become much more similar to that of the slurry. In other words, the flavor profile of the brew will become similar to what one would get with a percolation that approaches full extraction, except for some wasted concentrated coffee that will remain entrapped inside the coffee particles. In the case of a percolation, the continued addition of clean water would allow us to leave the coffee particles filled with cleaner water, i.e., with less good stuff left behind. The big advantage of AeroPress, however, is that it is much easier to agitate the slurry and get a very even contact between the water and coffee particles.
There is also another point that James made in his videos which I had never heard before, about there being a double-humped preference in terms of brew temperatures. He mentioned that most baristas seem to enjoy light-roasted beans with brew temperatures of about 80°C and then above 90°C, with a valley of less-preferred temperatures in between. In the past I had only brewed a few times with boiling water in the AeroPress, and I did not like the results and never went above 90°C again. I thought that the very good thermal insulation of the AeroPress was probably the reason why I was experiencing this ceiling in preferable brew temperatures.
All these thoughts pushed me to try AeroPress brews with 99°C water and 10 minutes-long brew times, something I had never considered before. The only reason I did not go for 100°C exactly is that this would destabilize the stream of my Fellow gooseneck kettle and make the pouring more messy. I was immediately astounded at the extreme sweetness this gave me in the cup, and I therefore decided to experiment more and land on a repeatable recipe that would give me the most out of a coffee. James Hoffmann seemed to experience only a slight improvement between two and four-minute brews, but this could related roast level, grind size or just preference. While I agree with James that 10-minutes brews are not desirable in a cafe environment, I am absolutely willing to pay that price at home for the kind of quality increase I have experienced.
When experimenting with these brews, I noticed something that I had encountered before with the siphon brewer. When pushing out concentrated water through the coffee bed, it is still possible to draw astringent flavors that make the brew flat and boring, even though the slurry should already be close to being saturated with coffee chemicals after 10 minutes. This is very surprising if you think of whatever chemical compounds cause astringency as extracting normally by diffusion, just more slowly. If that were the case, it wouldn’t matter that coffee particles along a channel encounter a lot of fast-flowing fluid, because the fluid is already at equilibrium with the chemical compounds in the coffee particles.
If you have been following my blog for a long time, you may recall a past discussion where I hypothesized about what may cause astringency in coffee. If we assume that astringency comes from similar compounds as it does in raspberries, wine and beer — large molecules called polyphenols — then there is another possible mechanism that could fit this observation. The polyphenols in question have typical sizes of 50−70 Å (McRae et al., 2014), much larger than other molecules that cause coffee acidity for example, and I wonder if they may not behave like something more akin to coffee fines, these small fragments of coffee particles with sizes below about 50 microns. Basically, polyphenols may not extract efficiently by the process of diffusion, and they may instead stick to the sides of coffee particles by electrostatic forces, much like coffee fines. Studies in geophysics have demonstrated that fines in oils can detach from larger particles and flow along with a fluid if the fluid is fast enough to pull the fine away. The smaller a fine is, the faster the fluid must be before it can tear the fine apart from tge larger particle on which it is stuck. If we were to extend this same principle to the even smaller polyphenols, one could imagine that only very fast localized flow — channels — may be enough to efficiently carry the polyphenols into a coffee cup, regardless of whether the slurry is already saturated with other chemical compounds.
Now, please keep in mind this is only a hypothesis, and testing it probably falls squarely outside of what I can test without a proper laboratory and a much more thorough knowledge of chemistry. Regardless, it is important to keep in mind that a brew can become astringent even if a saturated slurry is pushed unevenly through a coffee bed.
In the context of the AeroPress, I found that the main difficulty in avoiding astringency is to get a flat bed of coffee before pushing on the plunger, and avoid having to press too hard on the plunger. James also mentioned that brews where he had to push harder on the plunger tasted much worse, and this matches my experience. Depending on your grinder, the optimum grind size that avoids this will be the main limiting factor in achieving average extraction yields as high as you would like with limited brew times. As Barista Hustle demonstrated in a past experiment, coffee particles larger than 500 microns would take a lot more than 10 minutes to fully extract with a one-stage immersion brew. How much solubles you will be able to extract without getting uneven flow and astringency will therefore depend on how narrow your grinder’s particle size distribution is — and, in particular, on how many fines it produces, because those have a disproportionate effect on the hydraulic resistance of a coffee bed.
One trick that allowed me to get the most consistent brews was to absolutely avoid stirring in circular motions. Doing so will cause the coffee particles to deposit into a dome-like shape, and this bed shape will cause most of the flow to happen on the edges of the coffee bed when the plunger is pushed in. My brews were flat and astringent in all cases where my AeroPress bed had a dome-shape bed when I pushed in the plunger. Instead, I ended up using a back-and-forth stirring motion, because this is quite efficient at getting the whole coffee bed wetted quickly, without introducing a rotation motion that could favor a dome-shaped coffee bed.
I also found that I obtained best results when I swirled the dripper quite vigorously—as James also recommends—but a bit later after I had stirred. I think this is true because it leaves more time for the coffee particles to deposit at the bottom in a potentially irregular bed shape, and then the swirl can rectify this and make the coffee bed a bit flatter.
During my tests, I also found that the Fellow Prismo attachment (Amazon Affiliates link) made it a bit easier to avoid astringency. I suspect that this is mostly true because of the clever metal filter design, which includes a silicon ring around it that prevents any possible bypass of water near the edges of the coffee bed. Be sure to place your AeroPress filter on top of the metal filter, otherwise it will immediately clog because of the small surface area of the exit valve. One other thing the Prismo allows you to do is pour half or so of the water first, then stir in the coffee and fill up the remaining water. This forces the coffee fines to remain suspended by buoyancy, and really helps preventing any filter clogging, as previously demonstrated by Barista Hustle. Doing this allowed me to grind way finer without needing to press any harder and without experiencing astringency.
Here is the step-by-step recipe I ended up using. While using the Prismo is facultative, I recommend it if you can get one:
- Choose a sturdy mug where the AeroPress can fit with a good level.
- Place a dry filter on and screw the lid on tightly.
- Place your dry coffee dose in the dripper and shake left-to-right to make it flat. I like to use a dose of about 18 grams. If you have the Prismo, use it and pour half of the water before the dry coffee dose.
- Start a timer, and pour 100°C water until the AeroPress is filled — this is about 260 grams of water if you are using the Prismo, or a bit more if you aren’t, because some water will drip.
- Using a spoon or the plastic stick that comes with the AeroPress, stir in a back and forth motion from the complete bottom of the dripper all the way to the top. Avoid circular stirs !
- Place the plunger a few millimeters deep onto the AeroPress chamber. This will cause a bit of water to escape even with the Prismo; don’t sweat it.
- Remove the AeroPress from your scale, and give it a swirl to level the coffee bed.
- At the 5 minutes mark, give the AeroPress another thorough swirl.
- At the 9 minutes mark (or later), start pressing on the plunger gently. It usually takes me a bit more than 1 minute to press it all the way down.
I captured these steps in the video below:
This recipe allowed me to reach average extraction yields of about 23.5% on Facsimile’s Gatomboya Kenyan, using the percolation equation. This means that this 23.5% yield is made entirely of solubles that made their way into the cup, and therefore were not wasted. Because the steep time was extremely long and the profile of chemicals retained inside the coffee particles had time to come closer to equilibrium with the brew, I have a suspicion that the taste profile will be much closer to a percolation brew extracted at an average extraction yield of about 27%, which is the number one would get by including the retained water in this calculation (by using the immersion equation). This would not be true of shorter AeroPress brews. What really impressed me with these brews was that they seemed sweeter compared to other methods I have used in the past (even including the Stagg X dripper !).
I have started to do some tests with the Tricolate, and in fact my Tricolate brews of this same coffee tasted surprisingly similar at much higher average extraction yields (about 26%). Now, this shows how evenly the AeroPress can extract flavors given enough time, but it’s important to remember that retention of solubles in the spent coffee particles makes it more wasteful — it is actually comparable to throwing out about 3% of the coffee dose. Still, I think these types of AeroPress brews are quite valuable for their repeatability, and also their ease of use during travel.
One aspect that differentiates AeroPress brews from other gravity-driven pour over brews is that they include more undissolved solids in the cup. Even with thicker Aesir paper filters (Amazon Affiliates link), more undissolved solids will make it to the cup simply because pressure was used. A typical pour over brew uses a pressure of only about 0.008 bar—the weight of a 5 cm-tall column of water, whereas the AeroPress uses pressures of about 0.5 to 1 bar. I have not found this to make the taste worse in any way, and I did not find that it reduced my perception of origin character.
In order to get this AeroPress recipe right, you will still have to figure out what grind size is right for your particular grinder. Once you recognize the feeling of astringency which quickly removes all complexity and perception of sweetness in a brew, it should become easier to conclude that you have either ground too fine or done a poor job of achieving a flat bed and a light push of the plunger. Note that darker roasts may also taste bitter or roasty if you use boiling water, and in these situations it will be much desirable in my experience to reduce your kettle temperature. In fact, I suspect this is why the inventor of the AeroPress Alan Adler recommends using 80 to 85°C water— I bet he was not drinking very light roasts.
I also tried to open the AeroPress chamber at 5 minutes to give it a second stir, because I believe large coffee particles can take a few minutes to become completely filled with water and that may seem like a key moment to further help reaching equilibrium in the slurry. However, this made it much harder to avoid astringency, because the coffee bed never seemed to come back flat and even. Because of this, I abandoned this and instead opted for a vigorous swirl at the 5 minutes mark.
I do not think it is an accident that cupping methodologies used to assess coffee quality by professionals resembles this technique (even down to breaking the crust after a few minutes). I now suspect that the very long steep times during coffee cuppings are not only important to let the slurry cool as often quoted (but don’t get me wrong, that’s important !). I think it also allows the brew to come closer to equilibrium, which is probably why many have noted that cuppings seem to taste better than most pour overs brews. They probably indeed reach flavor profiles that are close to very high average extraction yields, and the lack of a percolation step means that defects associated with uneven flow are simply absent.
- Try steeping your AeroPress brews for 10 minutes or more. The flavor profile will get much closer to pour overs brewed at high average extraction yields.
- Try using 100°C water if you are drinking light roasts, even though the AeroPress has more insulation than most pour over drippers.
- Push the plunger as gently as you can.
- Try to obtain an even bed before pushing the plunger in. To my surprise, it is still quite possible to get astringent brews if you push the water unevenly through the coffee bed, even when the slurry is saturated.
- Use the Prismo if you have one, it helps to prevent side bypass. It can also really mitigate filter clogging if you pour some water first and then the dry coffee on top of it.
- While this method can reach flavor profiles typical of very high average extraction yields like a well-prepared pour over or a cupping, it is a lot more wasteful because the slurry will retain some very concentrated water. I typically experience about 3% waste, which is similar to losing 0.5 grams out of a 18 grams dose.