The Mechanism Behind Astringency in Coffee

[Originally published on July 18 on my Patreon]

Header photo credit: @coffeeandlucas,

At this year’s Specialty Coffee Expo in Boston, I finally met Samo Smrke, a coffee scientist from whom I have learned a lot over the years, and who produces what are in my opinion some of the best scientific papers on the topic of coffee chemistry (e.g., on the topics of coffee degassing, coffee storage, and how roasting style affects the degassing). I told Samo about some of the weird observations I have made recently about astringency, which were on my mind because I could not figure out a model to explain them.

My first puzzling observation was that a long steep prepared in a Trinity One dripper could taste perfectly fine if I sampled it from the top after ten minutes, but astringent after it had gently flowed out from the bottom after opening the flow valve. This did not fit in my previous picture of what causes astringency, because I have always thought that an uneven flow of water through a coffee bed would cause astringency because it would over-extract the coffee particles along the channel where a lot of water had flowed. But here, the 10-minutes brew was very close to being saturated with coffee compounds, so I was expecting the dripping phase to be way less sensitive on the evenness of flow, but it was not ! What did I expect though ? This is coffee after all, and no easy shortcut ever works.

The second observation that was puzzling me happened during an experiment that Thibaut Paggen and I ran at the Canadian Roasting Society (CRS) one week-end. Thibaut has been busy developing a brand-new dripper called the SOL, which aims to achieve a perfectly even flow of water through the coffee bed, without bypass, and in a very modular way that will allow all kinds of different puck preparation tools to be applied to filter coffee. After playing with the dripper a bit, I was starting to be confident that the flow of water was getting very even through the coffee bed in his latest prototypes, and we met at CRS to compare different recipes that we had enjoyed at home with the prototype drippers. I tried something a bit classical, similar to what I would brew on Stagg X, with very nice results and an average extraction yield around 23% (the extraction yields quoted here are approximate to about ± 1%, as they rely on my spotty memory).

Our experimental corner at the Canadian Roasting Society. Walk-in customers were really wondering what the hell is going on here.

Thibaut was trying something a lot more radical, where he ground almost as fine as espresso, and still managed to achieve a nice drip rate above approximately 1 gram per second, which generated one of the brews I have tasted with the most intense and juicy flavors, at a staggering 26% average extraction yield. However, the brew was also unmistakably astringent. To my surprise, the astringency in that particular brew did not bother me as much as usual, and I still enjoyed the brew thanks to its incredible juiciness, but I would have definitely preferred it without the astringency. We also had the luck of being in the presence of Scott Rao, and he blessed us with his honest take on the two brews: he also found the second brew more astringent, and did not enjoy it as much as I did. This might not sound surprising to many of you, as grinding too fine has been known to generate astringency quite reliably. The part that surprised me, however, is that we could not get rid of the astringency even with a thick, high-quality paper filter, a thick bed of coffee that remained perfectly flat throughout, no bypass, and an apparently even flow of water in the absence of clogging. I thought this was pretty discouraging, as grinding very fine without astringency has been kind of a holy grail for me.

The highly modular SOL dripper will definitely be the geekiest dripper available.

When I told this to Samo, he did not appear to be surprised at all, and he told me about colleagues that had recently likely discovered what chemical is responsible for astringency in coffee. Regular readers might remember one of my older blog postswhere I discussed the chemicals responsible for astringency in wine and raspberries; however, those specific chemicals were not found in large numbers in any coffee brews when scientists looked for them (see a nice discussion on that topic by Barista Hustle). While I don’t necessarily care about the name of the chemical that is responsible for astringency in coffee (I will keep it private until the scientific paper in question is published), I very much care about how it behaves, and why it may produce such odd experiences as the two I described above. And that was exactly what Samo was getting at: this compound, as others do, has a solubility curve that depends very strongly on the water temperature, in a way that is not at all linear.

Example solubility curves for different chemical compounds. If you were to draw a vertical line at a given temperature, this would inform you about the relative fractions of each chemicals that will be dissolved. If you move around the vertical line (i.e., change your slurry temperature), you can see that the relative fractions of different chemicals will vary a lot. Note that this figure does not list chemical compounds that are specific to coffee. Figure credit:

If the solubility curves of different chemical compounds all had the same dependency on water temperature, changing the temperature while we brew would only have an impact on how fast we draw out coffee compounds during the brew (and thus on the average extraction yield). However, given that the solubility curves of coffee chemicals can have such different shapes, changing the temperature affects the relative quantities of chemicals, and therefore the flavor of the brew. The case of astringent compounds is particularly interesting, because they require a very high temperature to be dissolved efficiently in water. This is a really good thing for us, because if they were to dissolve in water, they would pass right through the coffee bed, as all the other tasty compounds do, and it would be almost impossible to avoid astringency.

This discussion really caused a light bulb to turn on in my mind, because this mechanism had the potential to explain all of my experiences with astringency. These astringent chemicals are mostly undissolved in the slurry, which means they can be filtered out if we achieve a gentle and even flow of water through a coffee bed that is thick enough. Readers of my book may be familiar with this power of filtration that a coffee bed has: it is actually much better at filtration than a paper filter itself, provided that it is thick enough and water flows evenly through it. Therefore, avoiding astringency is not directly a matter of flow evenness, it is actually a filtration problem ! In real-life scenarios, an uneven flow of water will just come with regions in the coffee bed that are not filtered well, whether this happens in a channel where the water can faster, or bypass water that flows around the edge of a coffee bed in a V60. This immediately explained my experience with the Trinity One; even though the slurry was saturated with coffee compounds, an uneven flow during drawdown would let more undissolved astringent compounds through, and into the cup.

This mechanism can also explain why grinding finer causes unavoidable astringency. If most of the astringent compounds are just “floating around” without getting dissolved efficiently, those that are trapped inside of an intact coffee particle won’t be able to come out into the slurry, which guarantees that they won’t end up in the beverage. The finer we grind, the more broken coffee cells we expose to the slurry, because the surfaces of large coffee particles, and the totality of very fine coffee particles will be made up of such broken cells. This also means that the finer we grind, the more astringent compounds we liberate in the slurry; the amount of such undissolved compounds we liberate is a direct proportion of the total surface area of coffee particles.

A schematic of undissolved astringent compounds floating off from broken cells at the surface of a large coffee particle, or from a smaller particle made of only a single broken cell.

This immediately explains why we hit a wall of astringency when we grind, regardless of how evenly the water flows. As I mentioned before, the coffee bed is a more potent filter when water flows evenly through it, but it is by no means a perfect filter. Even if the coffee bed only allowed a small fraction of undissolved compounds to go through, that fraction might become noticeable in taste as we grind finer.

It is always lovely when a simple mechanism immediately explain a lot of puzzling observations, but the part that I love even more is that it can allow us to make predictions. The first one I would make based on these observations is the following: we may be able to push down to finer grind sizes if we achieve better filtration, either with a taller bed of coffee, a thoroughly tamped bed of coffee, or with a paper filter that is thick and has micron-sized pores (or even smaller). However, be careful not to require pressure during drawdown, because brewing with a higher pressure almost always correlates with a faster microscopic flow of water between the coffee particles, and this makes water more efficient at dragging any undissolved compounds to the cup (including fines, and probably astringent molecules too). I believe this is why espresso is always quite a bit astringent, although in this particular case it can fit in much better with the texture and flavor profile. This also explains why James Hoffman also found that pressing an Aeropress harder generated a more astringent cup (something I have also experienced, along with a more cloudy cup).

Here is another prediction that is more fun and a bit wild: I think that reducing the slurry temperature drastically may make it easier to avoid astringency, while still pushing average extraction yields to very high numbers. Imagine a scenario where only 1% of all the astringent compounds dissolve in water at 90°C, which may be a typical slurry temperature (I made up this 1% for the sake of this thought experiment). If you grind fine enough that this 1% becomes perceptible in the cup, one solution to reduce astringency is just to lower the slurry temperature, and make that fraction that can dissolve even smaller.

When I realized this, I told Thibaut, my Patreon followers and Scott Rao about it, and I asked them if they were willing to help me testing this out. My first experiment was to try brewing with the Ithaki, using Thibault’s method that involves a grind size almost as fine as espresso, distributed with the Weiss Distribution Technique and tamped, but using a brew temperature of 70°C. I experienced the same thing as my followers who tried it: this allowed me to achieve surprisingly high extraction yields (27%), with no astringency I could perceive. However, the cup profile was much less interesting, in the sense that the acidity was a lot less vibrant, and the taste profile veered more towards earthy, chocolaty, nutty and vegetal flavors. Scott’s answer to my text message was priceless: “Oh yes, I noticed this years ago but the cup profiles were always boring”. I guess another approach to discovering things in coffee is having tried everything possible, 20 years ago D: My Patreon followers tried several profiles on the Decent espresso machine with a drastically falling temperature profile and shared similar thoughts; the average extraction yields were surprisingly high, but the taste profile was not as vibrant.

While this odd experiment did not immediately need to a new and revolutionary filter coffee recipe, I think it is a nice demonstration that we are now on the right track in our interpretation of what causes astringency. All we need to figure out now is a way to achieve much better filtration. I believe this is also why Omri’s Del Creatives matrix filter allowed me to obtain such tasty coffee with a profile similar to Filter 2.0. After having used it many times, however, I was left with a problem where my pressure would always creep up, and the brews became gradually more astringent, cloudy and rancid, which I believe is due to the matrix filter slowly clogging up with coffee fines and oils. Using an ultrasonic bath did help, but it did not completely reset the filter. Samo had an interesting take on this: he said “yes you need an autoclave”. That seems a bit overkill for a home brewer set-up though, but I am definitely tempted (you could imagine having many metal filters, and cleaning them up in a big batch).

The Del Creatives filter is a thick metal mesh that sits under the coffee bed, and achieves much better filtration than paper filters due to its thickness.

When Samo explained this to me in Boston, I asked him about tea, because this was something that had always troubled me: why is green tea so bad when it is brewed above approximately 70°C ? Well, the answer was simple ! There are other types of chemical compounds in green tea that taste quite harsh (bitter and astringent) which can dissolve at much lower temperatures. It is therefore much better to avoid dissolving them ! In the case of black tea, the processing that is involved might just be de-naturing the compounds in question, and removes the need for the low brew temperatures. This reminds me of decaffeinated coffee a lot; I have done many experiments with decaf that turned out not tasting astringent, and when I tried them with caffeinated coffee the results were much worse. This has led me to suspect that most decaffeinated beans are almost immune to astringency, probably because the processing in question might affect the chemical compounds responsible for astringency in coffee.

Coffee may therefore be a bit like green tea, except that the threshold temperature where the brew becomes automatically astringent and undrinkable may just be around something like 95°C instead of 70°C. In fact, I suspect this is the case based on some of my experiments with the Hario siphon. This brewer has the particularity of being able to keep a very high slurry temperature if needed, and I have never brewed anything that tasted good if I maintain a slurry temperature above 95°C consistently.

After reading this article, Samo Smrke had an interesting comment to make. He told me that he was not convinced that the undissolved astringent compounds in the cup participate to the actual sensation of astringency on the tongue, although more research was clearly needed about it. At first I thought that undissolved astringent compounds had to matter for the whole mechanism to match my observations described in the post above, but after thinking about it a bit more, I realized that perhaps not. In fact, the total quantity of astringent compounds in the cup of coffee will still be strongly affected by how well they were filtered by the coffee bed, and this may be enough to affect the fraction that actually dissolve in the cup in a way that changes our perception of astringency.

Scott Rao also made an interesting observation, and noted that with Filter 2.0 and other methods, a long contact time seems to matter for astringency, especially when the grind size is relatively fine. I suspect that this may fit within the mechanisms described above, because even if a small 1% of astringent compounds dissolve in water, exactly which molecule is dissolved or not will change from moment to moment. This means that an astringent compound within a coffee particle’s intact cell may dissolve, get out of the particle, and precipitate out again. This would likely be a very inefficient process at dragging out astringent compounds from a coffee particle (and oddly reminds me about Stephen Hawking’s mechanism to explain black hole dissipation), but it may help explain why long contact times, especially with clean water (otherwise some astringent compounds will also go back inside the coffee particles), may draw out more astringency.


  • The molecules responsible for astringency in coffee are very hard to dissolve in water.
  • Therefore, astringent compounds float around freely in the slurry, and avoiding astringency in the cup is a matter of filtration.
  • One good way of filtering out the astringent compounds is with a thick, flat and undisturbed coffee bed, through which water flows relatively slowly and evenly.
  • Grinding too fine will liberate a lot more astringent compounds by breaking more coffee cells, which will result in an astringent cup regardless of how even the water flows through the coffee bed. Even the thickest bed of coffee won’t be able to filter out all of the astringent compounds if you liberate too many of them.
  • Extremely low slurry temperatures (think 70°C) can reduce the astringent compounds’ solubility even more, allowing to grind much finer and reach very high average extraction yields without astringency, but this also removes a lot of the interesting flavor compounds and makes the brews much less lively.
  • I think we could unlock very interesting types of coffee brews if we were able to brew at regular temperatures, use a very fine grind, and rely on a secondary means to filter out everything that is undissolved from the brew.

More Even Espresso Extractions

Originally posted on Patreon on Oct 17, 2021

I’m excited to share that I have just figured out a detail about my puck preparation that seems to completely eliminate a problem I had with almost every single shot of espresso I had ever pulled so far. After you pull an espresso shot, you can knock out your puck in a plate and wait for a minute or two to see if the color of the puck is relatively even across its bottom surface. In my experience, decaf and medium to dark roasts do not show much of anything useful when doing this, perhaps because of the darker color, or because they tend to channel less with their more porous coffee particles (I don’t knows for sure that more porous particles reduce channeling, but it seems plausible). Inspecting the bottom of a spent puck is more easily done if you use a paper filter below your puck and if your grinder allows you to grind with a finer average particle size, because both of these aspects will help the spent puck to hold together instead of falling apart when you knock it out.

A typical case of dark spots under my spent espresso pucks
An extreme bad case of dark spots under my spent espresso pucks when grinding too fine.

Until recently, most of my spent espresso pucks looked like the least bad case in the first photo above, or as bad as the second photo if my grind size was way too fine. The dark spots in these photos likely correspond to regions where there are still more coffee solubles that did not make their way into the cup, probably because of uneven flow that caused a “dead zones” around which water tended to flow. If you place such a spent puck in clean water and manage to not destroy it in the process, you will see how the darker spots disappear within a minute or two. If you cut across the depth of the puck, you will also notice how the dark spots tend to be located within a shallow region at the bottom of the puck, i.e. within a few millimeters if it’s not a bad case, or up to almost half of the puck in the very worst cases I have observed (like in the photo below).

This type of spent puck diagnostic has been used for quite a while on the Decent diaspora forum by some careful observers like Stéphane Ribes. The diaspora users are actually the ones who first interpreted the dark spots as under extracted regions, which I believe is correct. When I realized that inspecting the spent pucks in this way was actually useful, I started doing so after almost every shot I made, and I realized how pervasive this effect was across all of my shots, and how they were heavily affected by the use of a paper filter below or above the puck, the WDT Weiss Distribution Technique (see this other post for more details), or by placing a BPLUS or a Flair 58 puck screen above the puck.

I know I have not yet made a detailed post about the effects of puck screens, but it has already been widely adopted by users on the Decent diaspora forum. It is a pretty neat way to keep both your espresso machine’s shower screen and your tamper extremely clean, and to be honest, just these upsides are enough for me to want to use it, as long as it doesn’t negatively impact extraction. I do not yet have hard data to back this up, but I suspect that the puck screens are in fact usually good for extraction; I think they help to distribute the flow of water more evenly than most espresso machine shower screens, or at least compared with the stock one on the Decent DE1 machine as well as the IMS Cl 200 IM which I now use. I am a bit worried that it might encourage a bit more water to flow around the edges of the puck screen especially when it is still dry at the preinfusion stage of a shot, but I do not know for sure that this would even be a problem as long as it doesn’t last into the main part of the shot. The quality of my extractions seem to have improved after I started using it, but I plan to demonstrate this more quantitatively with a future experiment. Note that Stéphane Ribes has posted many experiments on the (private) Decent forums where he compared several different puck preparation tools including puck screens and came to similar conclusions.

Another improvement I have observed recently was the use of a more paper filter with more hydraulic resistance below the espresso puck (thicker, or with less, smaller pores). As you may recall from my last blog post, I now suspect that adding a source of hydraulic resistance under the puck that is non-negligible compared with the espresso puck itself is desirable to reduce the impact of channels because it slows down the flow of water where it would tend to become otherwise too fast and as a consequence less even across the surface of the coffee bed. Lately, I have been using 55 mm pre-cut Whatman 5 filters. Please note that Whatman 5 filters (and most high-resistance paper filters) have not been designed for food-grade use, so I would be really hesitant to recommend their use for coffee. You can find the list of trace heavy metals that they could contain here; to my knowledge, the most problematic ones would be Mercury, Arsenic, and Antinomy, but after comparing the upper limits of how much they can contain with local health regulations for tap water and tuna consumption (tuna has a lot of Mercury), I decided that I was comfortable with their use for me personally, but I do not take responsibility for any impact they could have on your health because my assessment of the risk that they pose could be wrong.

Note that stacking V60 filters or similarly low-resistance filters will not give you a high amount of resistance, because the column of filters will become too tall way before they build any significant amount of resistance and water will just flow around it. You might also notice that I have moved from using the 58mm Robot Cafelat filters to now using the smaller 55 mm Whatman 5 filters. I realized that it was much more important that the paper filter does not creep up against the wall of the portafilter basket, because this creates a path of least resistance for water. I do not consider this only a hypothesis; if you compare such larger filters with 55 mm ones, you will notice how it reduces the total hydraulic resistance of the system, makes the shot taste more astringent, and reduces the average extraction yield, all consistent with the idea that it creates a channel for water. What seems to matter most is for the paper filter to cover all the portafilter holes, and ideally that it extends almost up to the portafilter walls.

I know a lot of you might ask; if adding extra resistance below the coffee puck is good, have you tried a good old pressurized basket? And yes, I have indeed ordered one and tried it. The result was a very low average extraction yield shot with a really nasty taste. I am not sure why this is, honestly; I suspect it might be because it was a cheaply manufactured basket with uneven holes that leave way too much space without holes near the basket walls. If you know a high-end 58 mm pressurized basket manufacturer, please let me know and I will be happy to give it a try.

Even if the use of deep WDT, a paper filter below the espresso puck and a puck screen all seem to have helped reduce the number and the size of dark spots under my spent pucks (as well as increase my average extraction yields and improve the overall taste of my shots), I had never managed to completely eliminate them until now. It is common to hear that good ideas come to us while sleeping, well this one falls squarely in that category as I woke up excited to try something I had not even considered for a long time. I am not too surprised I dreamed about this (or at least some part of my brain was thinking about it), because this issue has been on my mind a lot lately. What I realized is that I seemed to always obtain more dark spots around the left half of my spent pucks (or the right half before you flip it over when knocking it).

If you recall the details of my puck preparation steps with the EG-1 grinder and SSP Ultra Low Fines burrs, you might remember that I had determined that grinding directly into the portafilter was desirable to minimize clumping and obtain higher-quality extractions. I had come to this conclusion by comparing the results I was obtaining with the Weber Workshops Blind Shaker, which caused a lot of clumping for me when I shook the grounds. Note that this clumping issue might only be a problem with the SSP burrs that force you to grind much finer than other burrs for espresso, most probably because they indeed produce less fines.

The blind shaker that comes with the EG-1 grinder

However, even back then, I was still annoyed by the fact that flat burrs tend to send the coffee preferentially on one side of the portafilter (to the right side with the design of EG-1), an effect probably worsened by the use of high rpm. This is what caused me to try using deep WDT in the first place, but I had never connected these two observations and realized that this might be one cause for the dark spots under my spent pucks. Back when I started making espresso, I did not know of a great way to fix this problem, but now thanks to another neatly designed grinder by Weber Workshops (the Key, which is not out yet), I realized that I could actually WDT directly into the Blind Shaker like the Key does with the Magic Tumbler, and just not shake it !

Hence, my puck preparation now consists of the following steps:

  • Grinding in the Weber Shaker.
  • Placing a paper filter at the bottom of my basket and pre-wetting it.
  • Doing WDT in circles (at full depth) directly in the Weber Shaker for a few seconds, until the grounds are fluffy.
  • Placing the Weber Shaker on top of my basket and funnel, then lifting the central part while gently spinning the central part: this drops the coffee grounds gently across the portafilter surface with a slight donut shape.
  • Doing deep WDT in the portafilter (making sure I don’t displace the paper filter).
  • Gently placing the Flair 58 puck screen on top.
  • Tamping.

Note that I don’t need to tap because I usually dose 20 grams in the Decent 24 grams basket. These steps above are exactly what I tried today, and I was really impressed with the results. Not only my average extraction yield went up by a whopping 1%, this shot had by *far* the least dark spots I have ever seen:

While it is still not 100% perfect (there are two subtle dark spots at the upper left and one at the lower left), the difference is quite dramatic, even with the same coffee and profile. Now, I know these are only two data points, but I want to share them with you because the difference is so large it is most likely significant:

Shot 1 (ground directly in portafilter)

  • Dose: 20.3 g 
  • Shot weight: 47.7 g
  • TDS Concentration (unfiltered): 10.62%
  • Average extraction Yield (unfiltered): 25.0%

Shot 2 (ground in Weber Shaker)

  • Dose: 20.1 g
  • Shot weight: 47.7 g
  • TDS Concentration (unfiltered): 10.94%
  • Average extraction Yield (unfiltered): 26.0%

I used the same grind size (6.5 at 1500 rpm; my burrs start touching at 8.0 and fully lock at 4.0, so I am at about 125 microns from full burr lock) with 55 mm Whatman 5 filters below the puck and a Flair 58 puck screen above the puck in both cases. I pulled both of these shots with Jungle’s very nice Qabballe natural Ethiopian, and I used a slight variation of Stéphane Ribes’ Easy Blooming shot (for DE1 aficionados, I have made the exit trigger of the blooming part happen at a very low 0.3 bar pressure to obtain a longer bloom, and I chose a “flow range of action limiter” of 2.5 mL/s; here is the profile configuration file). I have come to really enjoy this profile, because it provides both mouthfeel and origin character with a good quality of extraction. What more can I ask for ?

Here is a comparison of the two shots using Miha Rekar’s awesome Decent Shots Visualizer. Here is also a screen grab of just the improved shot:


  •  I recently realized that I had dark spots under my spent espresso pucks that indicate under-extraction regions and therefore uneven flow, and I was struggling to fix them.
  • Adding a paper filter with more resistance under my espresso puck seems to have helped reducing them, as expected from my recent realization that it should reduce the impact of channels.
  • Adding a Flair 58 puck screen above my espresso puck also seems to help in most scenarios.
  • The last trick that finally got me to almost zero dark spots was to WDT directly in the Weber Shaker, drop the coffee in the portafilter, then do all the usual (deep WDT, etc.) instead of grinding directly in my portafilter with the EG-1 and SSP ULF burrs.
  • This last part (grinding in the Shaker) seems to have increased my average extraction yield by about 1% (n=2 data points).

Note added on Jul 16, 2022. After playing more with the Key and Niche grinders, I can say with confidence that dark spots under the puck are much more common, and harder to avoid, with the EG-1 and SSP Ultra low fines burrs. I strongly suspect that this is due to the lack of fines generated by the grinder, which in turn requires a much finer grind for espresso. I suspect that EG-1 with the stock burrs does not generate dark spots either, but I have yet to test it.

Please note that I have no financial ties to any of the companies mentioned above. I am talking about their products because I personally use them and I like them; I am not paid to talk about them and none of the companies above are aware that I will be talking about them. I have receive free items  from Weber Workshops (a burr set and glass cellars) but I have paid every other product I own and I have not received these items as part of any agreement. I do not accept any advertisement agreements from any companies because I want to minimize possible biases. Also note that I have decided to stop using Amazon Affiliates links for now because it came to my attention that some readers perceived that it could bias my recommendations (however I did not go into old posts to remove them). I never posted Amazon links just with the intention of adding an affiliate link, however I can understand that this can present the appearance of a bias that I prefer to eliminate.

The Physics of Coffee Class

I’m excited to announce I will give a 3-hours live class on the Physics of Coffee during the 2022 Specialty Coffee Expo in Boston !

I will discuss my most up-to-date understanding of the physics in play in both espresso and filter coffee, and what this means for the best methods to brew coffee repeatably and achieve your desired goals. This class will include some brand new data and conclusions, new questions and predictions, and we will do a live demonstration of a new dripper prototype that I think will be both geeky home baristas’ and WBC coffee competitors’ dreams come true.

If you want to understand what happens under the hood of coffee brewing in an approachable yet deep way, I think you will love it !

I will also answer live questions during this course, and none other than Scott Rao will be serving you some very fine coffee for you to enjoy during the class !

The class will happen on Saturday, April 9 in Somerville, MA. The number of spots is limited, so be sure to grab your tickets here before the event.

Restraining Flow to Mitigate Channels

In the past weeks, I have been experimenting with the AeroPress combined with the Prismo attachment, and I tried one small hack that produced a surprising result. I inserted a pasta strainer mesh like the one I described in my Stagg [X] recipe to increase the total open surface area under the AeroPress filter. As shown on the photo below, placing a paper filter directly on top of the Prismo’s metal filter will only allow water to flow through the tiny and sparse holes of the metal filter, and therefore more pressure will be required to achieve the same drip rate.

When a paper filter adheres to the metal mesh of the Prismo, water will only flow through a small fraction of the total filter area where the holes are located.

The reason why I tried to increase the total free surface below the paper filter is simply to reduce the amount of pressure required to push the AeroPress plunger. As James Hoffmann described in his recent AeroPress video, more pressure usually leads to a coffee that tastes more astringent, which reduces the perceived complexity and sweetness of the beverage. It is possible that this could be explained by an increase of channels at the microscopic level when more pressure is applied — to my knowledge this has never been demonstrated very clearly, but so far this the best hypothesis to explain the results, and there are plausible physical mechanisms that could cause channels to occur when more pressure is applied (i.e., a faster localized flow of water that causes larger drag forces on the coffee particles).

In fact, I have often assumed that it was desirable to have as much free surface as possible below a paper filter, even with gravity driven brews, in order to get the fastest drip rate for a given pressure. As I discussed in a previous blog post, physics-based simulations showed that faster drip rates are expected to correspond to a more even flow of water through a percolation medium, everything else being equal. Applying the results of these simulations, however, requires us to assume that the coffee bed acts as an immovable percolation medium, which probably breaks down under high pressures.

When I tried brewing with the pasta strainer mesh placed between the Prismo metal filter and the AeroPress paper filter, I was surprised to find that the brews were noticeably more astringent. When this happened, I stored this observation in the “confusing things” drawer in my brain, and left it there. But more recently, something else happened that caused me to think about this again.

A small company called Del Creatives generously sent me a prototype of one of their inventions. It is basically a modified espresso basket with a thick metal filter that is inserted below the coffee bed in order to provide both more resistance and a truly immovable percolation medium. This allows us to brew coarsely ground and untamped coffee and produce a beverage that resembles filter coffee, with a few important differences.

The Del Creatives basket with a 2 micron matrix filter and other parts meant to prevent the bypass of water around the metal filter.

Some parts of the prototype seem like temporary hacks, but I was immediately impressed by the device: the physics of it make a lot of sense to me, and the design seems to prevent any possible bypass of water around the metal mesh. They sent me 4 different filters with average pore sizes of 2, 5, 15 and 60 microns, and recommended that I use about 7 grams of untamped coffee with it.

The Del Creatives basket came with 4 matrix filters with average pore sizes of 2, 5, 15 and 60 microns. These thick filters are made of metal and provide a non-moving filtration medium below the coffee bed.

Experimenting with this device has been quite eye-opening to me. I think this does more than transform the Decent DE1 espresso machine into a tiny and fast batch brewer; in fact, it also adds something valuable because it de-couples the extraction medium (the coffee) from the filtration medium (the thick metal filter). Navigating brew parameters with this device felt like navigating a new kind of beverage, because the brew defects that I encountered were completely different. Instead of encountering astringency when I ground too fine, I encountered rancidity when I brewed too hot (the DE1 can achieve ridiculously high slurry temperatures compared with pour over), and a really strong flavor and odor that reminds me of burnt rubber when the shots stayed at high pressures (i.e. not counting preinfusion and blooming) for more than about 30 seconds. This latter defect happened at very high extraction yields (in the 25-27% range) and only seemed to depend on how much time was spent at high pressure. This characteristic burnt rubber odor reminded me a lot of the what my kitchen smelled like and what my shots tasted like when I pulled blooming shots with 1:10 ratios to understand how the puck resistance evolves a few months back (some of my findings were discussed in this post).

This reminded me of a discussion I had with Zurich University coffee scientist Samo Smrke where he told me that very-long espresso shots might be able to extract some chemical compounds of the coffee bean (arabinogalactans and galactomannans) that are usually insoluble in water. These large chemical compounds can become soluble if they are broken down into smaller parts, which instant coffee manufacturers have often done by extracting with acids or using pressurized water at temperatures of 130—160°C, above the boiling point at atmospheric pressure.

I think what is happening with this device from Del Creatives has something to do with my earlier observations that liberating more surface area under the AeroPress filter caused more astringent beverages. The 2 micron filter that I used actually acts as a source of resistance under the coffee bed, as well as a filtration medium. I suspect that this limits the maximum flow rate that can take place in any pores between the coffee particles, which will reduce the rate at which channels may form, and perhaps more importantly, will prevent the local flow of water to become too fast where channels form. I think this is why all my brews with the Del Creatives filter had quite high extractions yields (about 24-27%) and none of them had much detectable astringency.

When I got a hold of what caused these brew defects specific to this device, I finally landed on some recipes that tasted really great, even at surprisingly high extraction yields. This is actually the first method by which I have really enjoyed brews in the 25.5-26.5% range of average extraction yields. The beverages that it generates can easily reach 3-4% TDS concentrations (which I now love without dilution), with intense sweetness, acidity and very discernable origin characteristics. I think this is really impressive for a brew method that takes barely a minute and requires no intervention from the barista, and this tastes so nice to me that it may become my daily driver (although I’d love it if it was able to brew larger doses).

Here’s my current recipe with the Del Creatives basket, I expect this to keep evolving:

  • I use 7—8 grams of untamped coffee with a grind size slightly coarser than espresso. As a reference point, I pull espresso at about 5.5 on my EG-1 with SSP Ultra Low Fines burrs, and allongés at 6.5. I can hear my burrs start touching when the grinder is running at 8.0 or finer, and my burrs fully lock at around 3.0. My V60 brews would be located around 14.5 (which would read 4.5 after a full turn of the EG-1 dial).
  • I grind directly in the Del Creatives basket and I use a WDT tool to even out the coffee bed.
  • I place a Flair 58 Puck Screen on top of the basket to make the water distribution as even as possible because I have noticed my DE1 shower screen is not always very even.
  • I use a profile that I built on the DE1 with the following steps: (1) a fast preinfusion with 16—20 grams of water at 95°C; (2) a 23-seconds phase with a slow refilling of 90°C water at 0.5 grams per second to compensate for some water dripping; (3) a 30-seconds blooming phase without flow; and (4) a smooth transition to a 1.0 mL/s flow of 90°C water until about a beverage weight of about 73 grams has been reached. I initially use hot water because the cooler coffee bed will immediately lower the temperature of the incoming water, and then I stick with 90°C water because I have found hotter temperatures to produce a rancid taste even with light roasts.
  • I place a Stagg [X] Dripper on top of my coffee glass with an AeroPress filter inside of it to filter out a bit of the coffee oils, because the Del Creatives basket is not conveniently sized to add a paper filter to it.

You can find an example DE1 profile file here, or use the screenshots below to build it yourself:

One example of a typical brew that I really enjoyed with this recipe was with 8.0 grams of Heart’s Honduran Extreberto Caceres (a washed Pacas and Catimor grown at 1750 m.a.s.l. in Santa Barbara), with a beverage weight of 71.0 grams, a concentration of 2.9% TDS and an average extraction yield of 25.7% (not syringe filtered). This is a ratio of about 1:9 if you speak espresso language (beverage weight over dose) or about 1:11.5 if you speak pour over language (total water weight over dose).

This is what this the graphs looked like for this particular beverage:

The Del Creatives beverage that I pulled with Heart’s Extreberto Caceres as described above.

You can also explore this graph’s data here with Miha Rekar’s Decent shot visualizer.

I use the Stagg [X] with an AeroPress paper filter to filter out some of the oil produced by the Del Creatives shots.

As some of my supporters noted when we were discussing this on my Patreon Discord channel, you may expect that these Del Creatives shots are somewhat similar to Scott Rao’s famous Allongés, or the turbo shots described elsewhere (e.g. this paper and this video by Lance Hedrick) that also use coarser grounds but shorter ratios.

However, these Del Creatives shots do not necessarily rely on a fast drip rate, even if the coffee is ground coarser, and they taste distinctly different. I actually prefer them to turbo shots, and I usually prefer them to Allongés. Qualitatively, I found that turbo shots almost always have some level of astringency that annoy me a bit. I really enjoy well-dialed in Allongés, but I find it a bit annoying to have to dial them in separately from espresso shots, and they actually taste like a completely different beverage compared with the Del Creatives shots. The Allongés tend to have a bright, crisp acidity, and the Del creatives shots are thick, viscous with intense sweetness. The downside with Del creatives shots is that the burnt rubber taste can appear easily if the ratio is stretched just a bit too much.

Maybe more importantly for me, this device has added another layer to my understanding of the physics in filter coffee, which I didn’t expect would happen so early after having released my book on this topic. Namely, I now think that adding a source of resistance below a coffee bed can be beneficial to mitigate or even eliminate the impact of channels. This is particularly powerful when a pump is available, because it allows to de-couple the extracting medium from the filtration medium. I would certainly like to see more experimentation and data on this topic, but so far all my observations line up with this potential explanation.

These new observations also change what I would do if I were to design an “ideal” dripper; I would actually want it to have a valve to control the drip rate, but this source of resistance would have to be distributed uniformly across the bottom of the coffee bed, much like the small but evenly distributed pores of the Prismo metal filter. This might be a more difficult engineering challenge. Maybe two punctured circles that can rotate with respect to each other in order to gradually open or close the holes could achieve this.

I am also quite excited to see this basket from Del Creatives being fully released. It is currently a bit hard to clean thoroughly between shots (which is usually needed in its current form), and I would love to see it made to accommodate larger doses. I also found that the thick metal filters needed to be flushed very thoroughly before brewing with them (the water that came through was initially gray and smelled metallic), and it remains to be seen whether they will eventually clog after many shots (I have now done about 30 shots using the 2-micron filter without problems).


  • I recently realized that purposely limiting the maximum drip rate by adding a source of resistance below a coffee bed can be desirable to limit the amount and the impact of channels.
  • The source of resistance must not make the flow of water uneven through the coffee bed, so it can be made either of many small holes, or a single valve placed under a metal filter with even holes.
  • A new prototype basket manufactured by Del Creatives matches this new understanding and also provides a thick metal filter that acts as an immovable porous medium below the coffee bed, which de-couples the extraction medium (coffee) from the filtration medium (the metal filter).
  • Combining the Del Creatives basket with the DE1 espresso machine allowed me to brew some of my favorite coffee beverages so far, with very high concentrations (3–4% TDS) and high average extraction yields (25.5—26.5%). They taste juicy, vibrant and preserve origin characteristics.
  • Pushing the ratio too far with these brews will bring a very specific awful taste that reminds me of burnt rubber, and does not appear to cause much astringency.

Added Note: Since I published this post on Patreon back on September 10, I have come to realize that the Del Creatives matrix filters eventually started clogging for me and required increased levels of pressure after about 50 shots. After talking with Omri at Del Creatives, I realized that cheap ultrasonic baths can be used to declog them (I used this exact model – not an Amazon Affiliates link). This device is also great to clean up the Flair 58 puck screen that can accumulate some oils over time.

Reaching Fuller Flavor Profiles with the AeroPress

Reminder: as an Amazon Associate I earn small commissions from qualifying purchases made through the Amazon links below, which are identified individually. I have no association or commercial agreements with any products mentioned below.

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:

Part 1 – The AeroPress

Part 2 – Understanding the AeroPress

Part 3 – The Ultimate AeroPress Technique

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.

Ellagitannin, a polyphenol found in raspberries
(source: Wikimedia Commons)
Malic acid (top) and caffeine (bottom)
(sources: Wikimedia Commonsand PubChem)

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.

The Fellow Prismo attachment has two parts: a lower cap with a pressure valve, and an upper metal ring with a silicon gasket around it which prevents all side water bypass.

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:

  1. Choose a sturdy mug where the AeroPress can fit with a good level.
  2. Place a dry filter on and screw the lid on tightly.
  3. 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.
  4. 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.
  5. 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 !
  6. 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.
  7. Remove the AeroPress from your scale, and give it a swirl to level the coffee bed.
  8. At the 5 minutes mark, give the AeroPress another thorough swirl.
  9. 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.

On Gravimetric Measurements of Total Dissolved Solids

Header photo by Juan Silveira.

A while ago, I read an interesting idea from Robert Mckeon Aloe on Matt Perger’s Telegram channel about measuring the total dissolved solids of a live espresso shot with the Decent DE1 machine, by comparing the output gravimetric flow measurements with a bluetooth-connected Acaia scale, to the input flow above the puck at the shower screen as predicted by the DE1. I think this is a really good idea in principle, but when I read this I immediately expressed worries that I didn’t think it could be done accurately yet because of the systematic inaccuracies with which the DE1 currently estimates flow rate.

The idea is simple; the flow rate at the shower screen tells you what volume of water is incoming per unit time, and the Acaia scale measures how much total mass is falling into the cup per unit time. The key here is that this total mass is not only made up of water, but it also includes dissolved chemicals (what we usually refer to as total dissolved solids, or TDS), oils, dissolved CO2 and undissolved solids in suspension. Using the well-known mass density of water, one can calculate how much of that total weight is made up of water, subtract that and obtain the weight per unit time of everything else. This is not exactly a live measurement of TDS, because of all the other stuff in there, but it might very well be a good tracker of TDS if the dissolved solids make up most of the non-water mass.

Basically, Ray Heasman at Decent uses a purely empirical and very complicated model that takes the voltage of the DE1’s vibratory pumps as an input, and predicts how this relates to the flow of water at the shower screen. This is quite crazy, because the answer depends on so many factors, and changing a single tube in the machine can throw off these predictions. Even worse, the answer is different when the pressure changes, and the properties of the user’s electrical grid can also affect this. Somehow, Ray managed to pull this off by gathering enough data and building a predictive model that works reasonably well within typical espresso settings. I think that this is quite a feat, and it shows how much real geekery is going on under the hood of this fantastic machine.

Because the flow model depends on so many parameters, it is not rare to see the flow rate being off by 10-20% on the DE1. Therefore, I thought we could not reconstruct something useful and repeatable in terms of a live TDS curve during a shot that is based on the difference in output weight minus input flow of water. Worse, the measurements provided by the Acaia scale are very noisy, especially when estimating the change of weight per unit time. After a shot is done, it is possible to smooth the gravimetric data to make a useful comparison, but doing so live in an accurate way would be extremely hard (although probably not impossible). Readers could be tempted to also worry about the Acaia readings lagging behind (as I was), but because water is incompressible at 9 bar, the should be no lag in terms of the flow measured at the bottom of the espresso basket and that at the top of it, and the only sources of lag that remain are (1) the freefall of the fluid (about 0.136 seconds for a 91 mm fall in my case) and (2) any lag in the Bluetooth data transfer to the DE1, which I suspect is also not too significant.

Things would get much worse if one would attempt predicting the average extraction yield by summing up the TDS curve at every moment in time, because the inaccuracies would pile up, resulting in a very large measurement error. After having this discussion with Robert, I kind of forgot about the idea, maybe a bit too fast.

More recently, when I published my experiment on comparing blooming shots pulled with the Niche or the EG-1, I noted that the gravimetric flow was tapering down near the end of the shots when the DE1 was trying to keep the flow constant (blooming shots are flow-controlled and aim at a 2.2 mL/s flow rate at the shower screen). Somehow having already forgotten about the TDS discussion, I immediately blamed the big change in pressure that probably threw off Ray’s model, and I didn’t think of testing whether the TDS curves I built from the viscosity calculations could explain the difference.

DE1 graphs for the Niche and EG-1 shots I pulled in my last experiment

Yesterday, I was reading about a feature on the DE1 users forum where a new firmware upgrade allows you to correct for any constant offset between the predicted and measured flow. Decent user Ed Hyun built a small Python program to help calculate this offset by using all of your past shots, instead of running a calibration profile and waste some coffee (I am still constantly amazed by the Decent users community), and on that thread, a user named Joe Duncan resurfaced that idea of the declining gravimetric flow measurements being due to the change in TDS rather than errors in the DE1’s model. When reading this, my mind clicked that this might actually also explain the tapering gravimetric flow in my previous experiment !

The live TDS curve predictions I calculated from how the puck resistance evolved during DE1 shots, by assuming that the changes in puck resistance are only caused by a change in fluid viscosity due to a change in TDS

So I decided to go ahead and pull the data from this last experiment, and try correcting the Acaia gravimetric flow rate by multiplying it with a factor (1-TDS/100), where TDS are the live TDS graphs as a function of shot time that I derived from my calculations of the fluid viscosity. This allows to estimate the gravimetric flow of water only, without the dissolved chemicals in it. I also used tabulated values for water density versus temperature to apply the appropriate correction to the DE1 volumetric flow rates.

Indeed, this correction seems to account for most of the slope in gravimetric flow rate ! This is consistent with my hypothesis that changes in puck resistance are driven by changes in TDS and viscosity, and it also lends credence to the possibility of estimating live TDS with Robert’s idea, although we would always need to take such curves with a grain of salt because of the other solids in the beverage (oils, undissolved solids) and the current limitations of the DE1’s flow estimates.

Now, there is still a non-negligible constant shift between the gravimetric and volumetric flow rates, and that could be due to specificities of my own DE1’s tubings or my local electrical grid. Thankfully, I can easily calculate this shift and correct for it, by looking at the median ratio of flow in the last 10 seconds of each shot. Quite interestingly, this factor was slightly different for the Niche and the EG-1; The DE-1 over-predicted the Niche flow rates by 8 ± 2%, and the EG-1 flow rates by 12 ± 1%. This difference between the two grinders is on the edge of being statistically significant, and could be explained either by a systematic error in the exact TDS values I predicted from my puck resistance – EG-1 flow rates would be affected more by an error because of its higher TDS on average – or to a different amount of oils and undissolved solids between the EG-1 and the Niche.

Applying these constant shifts to the flow rate curves shown above yields the following:

This last set of figures also shows something interesting; the correction seemed to have removed all of the Niche’s slope, but not exactly all of the EG-1’s slope. This makes me favorable to the hypothesis that my viscosity-based TDS curves might be a bit systematically off in the case of EG-1, and the true TDS might actually be slightly higher a bit early in the shots, whereas this may not happen with the Niche.

All of this means that the Decent might actually be able to keep a constant flow rate even when the pressure changes significantly. The decrease in gravimetric drip rate can likely be attributed to changes in TDS and other beverage content, meaning that I was probably worrying way too much about tweaking my profile’s flow rates to achieve a constant gravimetric drip rate.


The fact that the Acaia scale-based flow rates in my previous experiment tapered off near the end of the shots was likely related to the TDS of the shots decreasing, in a way consistent with the TDS curves I had predicted.

A Comparison between Standard and Low-Fines Espresso Shots

Now that I have outlined the method by which I pull espresso shots with the EG-1 grinder and SSP ultra-low-fines burrs (or ULF burrs for short), I thought it would be interesting to compare them with a more classical espresso grinder like the Niche Zero. I do not yet have great data to show you exactly how the particle size distributions of these two grinders differ, because doing so will require measuring the very-fine coffee particles down to a dozen microns in size, something extremely hard to do with imaging methods like my grind size application. Sifting might sound like a good method for this, but it is also impractical because, without a jet of pressurized air, fine particles tend to stick to larger ones, making it really hard to measure their contribution using sieve sets. Rather, I have plans to get some laser diffraction data for both grinders; it requires expensive machines, but it is a much better solution to detect the differences in how many fine particles different grinders generate.

We will see in this post how the differences in the shot characteristics between the two grinders strongly hint at a very different quantity of fines. You may recall from one of my earlier posts that a quantity often called D10 drives the hydraulic resistance of a coffee puck (i.e., how much it resists flow at a given pressure). Simply put, D10 is the size of the particle that you would encounter at 10% of the total dose weight if you sorted every particle by increasing weight. If we used D50 instead, and stopped at 50% of the total dose weight, we would get something called a “median” particle size (ordered by particle weight); it may be surprising that D10 and not something closer to D50 drives the puck resistance, but that is a consequence of fines having a disproportionate role in affecting the resistance.

With this in mind, you can understand how using a grinder that generates less fines will require grinding much finer overall to overcome the otherwise reduced proportion of fines, and obtain a similar puck resistance. I find it easiest to see this difference visually when inspecting the spent puck of a an SSP ULF shot (top panel below) to that of a Niche shot (bottom panel below), dialed in for similar puck resistances. In these photos, we can clearly see how the particles in the Niche spent puck are much coarser on average. I think it’s easier to see this effect in a spent puck because the fines get washed away to the bottom of the puck and in the shot of espresso, so we can more clearly see the coarser particles that remain. The SSP ULF shot at the top looks more like a uniform brownie, except where I damaged it when removing the filter paper.

For today’s comparison, I decided to pull as many shots as I could in a row with the set amount of time I had, alternating between the EG-1 + SSP ULF and the Niche. I always alternate between methods with I do such comparisons, to account for anything that may evolve during the experiment, like the overall temperature of the set-up or my puck preparation changing because I am getting tired. I used a washed Colombian coffee named Asotbilbao. It is a mix of Caturra, Castillo and Colombia varieties grown at 1600-2000 m.a.s.l. in Planadas, Tolima, roasted by Andy Kyres about two weeks before I ran the experiment. A few days after it was roasted, I vacuum-sealed and froze the coffee, and I let it thaw completely before I started the experiment.

I used a similar setup than my latest few experiments; namely, the deep WDT method described in this post with Levercraft’s WDT distribution tool, one cafelat robot filter below the puck and one above the puck (as described here) with the creped sides against the coffee puck. When grinding with the Niche, I used the distribution technique shown here and inspired by Scott Rao. When grinding with the EG-1 + SSP ULF burrs, I used the methodology that I detailed in my last post about low-fines espresso shots. I used the Force Tamper, and the DE1+ Decent espresso machine with the IMS shower head and the Decent 18g basket.

I chose to use the blooming espresso profile developed by Scott Rao for the DE1, with a 1:4 ratio. I chose this particular combination in order to focus on how the puck resistance would evolve after a thorough, 30-seconds preinfusion that ensures no part of the change in puck resistance during the shot may be caused by the puck still becoming wet. After experimenting with my adaptive profiles, I have concluded that the puck not starting fully wet at the end of preinfusion was likely a significant part of what drove the quick decrease in puck resistance at the start of my shots, regardless of the grinder. Using a blooming profile therefore allows me to remove this variable of puck dryness entirely, and to see how the puck resistance evolves in time due to other factors. The choice of a 1:4 ratio may sound surprising: it will reduce any contrast between the average extraction yields of the two grinders, because the additional water will bring all coffee particles closer to being completely extracted, but it will also allow me to better see where the puck resistance stabilizes after a longer shot time. This is a compromise I was willing to make for this experiment.

Before I started the experiment, I let the DE1 warm up for about 15 minutes with the portafilter in, and I then dialled in the Asotbilbao with both grinders. I landed on grind size 10.0 on the factory-zeroed Niche, and 4.7 at 1500 rpm on the EG-1. I let both grinders at these respective grind settings for the whole experiment without touching them again. I ran one test shot with each grinder during dial-in, and I then landed on a dialled-in EG-1 shot for my third overall shot. The fourth shot on the Niche was still not dialled in, and the fifth overall shot ended up serving as my first dialled-in Niche shot. I mention this because it may be relevant for temperature stability; I often find that it takes two or three shots before the machine and portafilter all come to their peak temperature stability. In the further discussions of this experiment, I only numbered and considered the shots that were dialed in. All in all, I managed to pull six EG-1 shots alternated with five Niche shots after the dial-in step.

After every shot, I stirred the coffee with a clean spoon, and I sampled a few grams with a clean, numbered pipette. I emptied the pipette and then collected a sample again, to minimize any possible contamination, and I placed the pipette on a flat surface for an hour or so. I decided to collect all samples first during the experiment, and to let them cool in their respective pipettes while I was running the rest of the shots and cleaning up afterwards. I then started measuring them with the VST refractometer while not paying attention to the randomly assigned pipette numbers, which made the measurements blind with respect to what grinder they corresponded to. This also allowed the samples to reach room temperature before they touched the refractometer prism, a crucial point when measuring total dissolved solids or TDS, as I talked about here.

I then put the full sample on the refractometer, immediately took one “raw” measurement, sampled a drop or two back in a VST syringe, emptied the syringe and then sampled the rest of the coffee directly from the refractometer into the syringe, again to minimize any possible contamination. I then pulled the syringe piston all the way back, inserted a brand new VST syringe filter, and slowly pushed the piston until I had obtained few filtered drops on the refractometer prism. Before every raw or filtered TDS measurement, I cleaned the refractometer prism with alcohol, and I zeroed the refractometer with room-temperature distilled water before every pair of raw and filtered measurements.

I realized something important about measuring espresso TDS when carrying this experiment. I knew that TDS readings usually keep creeping up when measuring raw samples, even when they were allowed to reach room temperature, and I had always attributed this to undissolved coffee fines depositing on the surface of the refractometer prism. I was surprised to notice that this also happened with the VST syringe-filtered samples, albeit to a much lesser extent. In fact, I came to the realization that this is especially a problem when using a sample larger than 2-3 drops. I previously allowed myself to use larger samples because I had let them cool to room temperature before they ever touched the refractometer prism, but a larger sample has a lot more fines that can deposit on the refractometer prism. During the whole experiment, I always measured the TDS immediately after placing the sample on the prism and those are the measurements that I ended up adopting, however it is only after the sixth overall measurement that I started to use only three drops with the VST-filtered sample, which completely eliminated any change in TDS with subsequent measurements. As a consequence, I have adopted a measurement error of ± 0.03% TDS for the filtered samples before I started using smaller 3-drop samples, which is about the amount by which they went up in the first 15 to 20 seconds. For the remaining filtered measurements, I adopted a measurement error of ± 0.01% TDS.

I made my log files publicly available here; I also placed my 11 DE1 shot files in a zipped package here. Now, let’s discuss the results.

First, I want to talk about the DE1 graphs. You may recall that blooming shots are flow-controlled profiles, which means that the DE1 tries to maintain a constant 2 mL/s flow of water at the shower head, and adjusts the pressure accordingly. Usually, this means that the pressure initially peaks ideally somewhere in the range of 4—8 bars, and then slowly decreases as the puck resistance goes down. This is what this looked like with the Niche:

The puck resistance curves (either calculated from the shower head flow in blue or from the Acaia scale measurements in brown) go a bit crazy during the bloom; that’s perfectly normal and I don’t consider that the resistance curves are very useful in that phase. You can see that the pressure curves rose to about 5 bar and gradually declined. One shot showed way less puck resistance, and thus less pressure, probably because of an inconsistency in my puck preparation. The EG-1 + SSP ULF shots looked like this:

We can observe similar pattern here, where each shot peaks near 5 bar, but this time there is one outlier where the puck resistance was higher instead of lower. If you look carefully, however, you will notice that the rate at which the pressure decreases is quite different with the EG-1 shots. Here’s another look at the same pressure curves, but normalizing the shots to each grinder’s respective median peak:

Here, we can see something really interesting: All of the EG-1 shots saw their pressure  curves decline much faster, and the Niche shots had a pressure curve that declined more gradually. Because I used a 30-seconds bloom in these shots, the loss in puck resistance is likely not related to the puck gradually becoming wetter. I am also very skeptical of the usual claims that the puck is “losing integrity”, falling apart or developing many channels, because similar experiments where I pulled shots to 1:11 showed no evidence of further degradation in puck resistance. In fact, the two spent puck photos that I showed further above were from these other tests with a 1:11 ratio. Maybe the puck would actually fall apart with doses much below 18 grams and/or bad puck preparation, but with all the shots I am testing here, the spent pucks seemed to remain in very good condition, and I think that massive channeling is not a good explanation.

This leaves me with few viable hypotheses to explain the decrease in puck resistance. I think it is most likely that a loss in the slurry’s viscosity as it becomes less concentrated makes the fluid more able to flow through the puck as it becomes clearer. This would also naturally explain why the SSP ULF shots decline much faster than the Niche shots; having had to grind much finer, all particles of coffee are able to give out a lot more solubles faster, and their extraction also slows down faster because they each contain less stuff. Larger coffee particles start extracting slower, but they contain a lot of solubles hidden deep below their surface; these compounds take time to slowly diffuse toward the surface of the particles, and they keep leaking out for a longer time.

If you read one of my recent posts about puck preparation, I actually talked about this, and showed the following figure from a science paper by Sobolik et al. (2002) that describes how the viscosity of coffee changes with temperature and concentration:

At high water temperatures, a change of concentration in coffee solubles from 0% TDS (clean water) up to 10—15% TDS is sufficient to increase the fluid’s viscosity by 50% according to their data. The exact numbers may depend slightly on the type of coffee, but I will be surprised if it changes by orders of magnitude. If we just use a simple interpolation of Sobolik et al.’s data, it therefore seems like a change of viscosity from something above 10% TDS to something close to 0% TDS could account for changes in pressure by about a factor of two, in the right ballpark for what we are observing in the above shots. This depends on a few assumptions, however; coffee oils which I mostly ignored might play an important role here, and I used the rule-of-thumb relation where the change in pressure goes with the square of the puck resistance as observed by John Buckman and other DE1 users – I believe this is due to the porosity reducing when pressure increases, but the exact relation my deviate slightly from this. If you go back to the DE1 graphs, you will also notice that the actual flow rates measured by the Acaia scales gradually go down a bit as the pressure curve declines. This is likely due to small inaccuracies in the way in which the DE1 estimates the flow at the shower head (by just reading the pump voltages, which is genuinely crazy, so I don’t blame them). A small part of the pressure decline might certainly be explained by this miscalculation in flow, but we are talking about a ~20% decline near the end of shots, that cannot account for all of the full pressure decline.

Now, you might ask “why do we care what the underlying explanation is?” I think this matters, because it may be pointing us in the direction of something I very rarely hear, except recently when I saw another great experiment by Stéphane Ribes on the DE1 users forum: higher-uniformity burrs in general (and those that generate less fines) will give out their solubles a lot faster, and reach high extraction yields at shorter ratios compared to burrs with wider particle size distributions. When you think about it, this may actually be suggesting that using shorter ratios, not longer ratios, may be a sensible approach to these types of burrs, if we can appreciate much higher-TDS beverages.

In my experience so far, there is something that prevents any kind of espresso shots or allongés to taste good when they run for more than about 30 seconds (excluding preinfusion and bloom). This may be related to the increased amount of oils that are able to get into the beverage after this period of time; oil is much more viscous than water and takes some time to come out from the particles entirely. This observation could also be related to other, heavier chemical compounds more slowly leaking out into the beverage. Regardless, both of these interpretations would suggest that the finer-ground particles obtained with the EG-1 + SSP ULF burrs will also allow the less desirable stuff to come out faster than they do from the Niche particles, on average. If I were to stop one of these EG-1 shots at 1:2, I may be getting all the same good-tasting stuff as a 1:3 Niche shot, with less of the not-so-desirable stuff. Conversely, if I stop the EG-1 shots at 1:3, I may be getting a much higher average extraction yield than the Niche, but I might also be getting a lot more of the oils or less-desirable compounds, and maybe it would taste like something more comparable to a 1:5 shot on the Niche.

This may sound like I am saying that we should be targeting the same average extraction yields regardless of the grinder, but I don’t think this is true. In fact, even shots that I run at about 1:2.3 on the EG-1 + SSP ULF burrs seem to have higher average extraction yields compared with 1:3 shots on the Niche. However, I digress, so let’s get back to the results of this experiment.

Against my expectations, something really interesting happened with the temperatures of my shots during this experiment. First, let’s look at the average shot temperature versus shot number. We can see that, despite having preheated the machine for a while and having run test shots before the actual experiment, my first shot ran with a cooler 0.5°C average temperature, and the second shot was also slightly a bit cooler than average.

In the figure above, the two black crosses mark the shots that had outlier pressure curves in the DE1 graphs, and the horizontal dashed lines show the averages for each grinder. The shaded regions show the typical spread of each grinder (their standard deviations). Another thing jumps out of this graph; the EG-1 may be producing just slightly cooler shots on average. When I saw this, I wasn’t sure it’s not just a statistical happenstance, and I decided to zoom in on the DE1 temperature graphs, shown below.

 In this figure, the black lines represent the “goal” temperature of the DE1. The blue lines represent all of the EG-1 shots, and the red lines represent all of the Niche shots. I used dotted curves to represent the first shots with both the EG-1 and Niche. If you are not used to the DE1 you may wonder why the actual curves are so different from the temperature goals. This is perfectly normal, and it has to do with the fact that the bloom phase in the 10—40 seconds range has no flow, so there is no new water added to the slurry to actually reach the lower temperature goal. No, there isn’t a tiny fridge inside the DE1 group head in order to cool it without adding more water. When the shots start flowing again at 40 seconds, cooler temperature is added to reach the last 92°C goal, and the slurry temperature gradually cools down to meet the goal.

The part I find the most interesting about these curves is that the EG-1 shots did not seem to struggle to reach the initial 97.5°C temperature goal significantly more than the Niche shots. Rather, something happened during the blooming phase where the EG-1 slurries lost their temperature faster! There are two possible explanations for this: either the Niche heats up the grounds a whole lot more than the EG-1 does, or this is yet another consequence of the finer coffee particles produced by the EG-1 at espresso dial-in. The smaller particles provide a lot more surface area for heat to be exchanged from the slurry to the coffee particles, which means that the inside of the coffee particles will heat up faster with the EG-1 shots, and therefore drag the slurry temperature down faster. I plan to verify this with a simple experiment: all I need is to wait 10 minutes after grinding with neither the DE1 or portafilter preheated, to make sure that both the EG-1 and Niche pucks start at room temperature. If the particle size distribution is the underlying explanation, this effect will still be there; if the difference comes from heat transfer during grinding, the effect will have disappeared completely.

The dynamics of how chemicals extract from coffee particles is affected by both particle size and the slurry temperature, so I am not sure that these data immediately warrant bumping up the start temperature of low-fines shots, but it would be really interesting to try it and see what happens to the taste compared with regular blooming shots on the Niche.

Another cool thing I noticed during this experiment is that all of the low-fines shots had way less crema than the Niche shots, and when any crema was visible, it disappeared a lot more quickly. This observation does not seem to correlate much with pressure, because even the Niche shot with a very low pressure had significantly more crema than the low-fines shot with the most pressure. Even though the pressure of the low-fines shot decreased faster, it still had a higher pressure during the full shot!

Even the lowest-pressure Niche shot produced a thick and stable crema.
The crema on the EG-1 + SSP ULF shots was either entirely absent or thin and short-lived.

This indicates that the presence of coffee fines in the espresso is probably an important ingredient in maintaining the stability of crema. Like everyone, I think crema looks beautiful, but this doesn’t necessarily mean that it tastes good, and in fact it has often been reported to taste quite harsh and bitter by itself.

Now, let’s see how the average extraction yields compared between the two set-ups. First, let’s look at the those calculated from the unfiltered, raw samples (i.e., the “Instagram” extraction yields):

We can already see a very clear trend, where EG-1 shots extracted much higher despite the high 1:4 ratio used in this experiment. The measurements based on the VST-filtered samples show a similar trend:

 Yet again, the low-fines shots had a significantly higher average extraction yield. The average “raw” extraction yields were respectively 26.8% ± 0.2% for the EG-1 and 24.8% ± 0.1% for the Niche (a difference of 2.1% ± 0.3%), and those in the VST-filtered samples were 24.1% ± 0.3% for the EG-1 and 22.7% ± 0.2% for the Niche (a difference of 1.3% ± 0.3%). This difference should only get larger when using gradually shorter ratios.

This fits in perfectly with the picture that EG-1 + SSP ULF shots are generating much less fines, and therefore requiring me to grind with smaller average particle sizes. It also fits with my current interpretation of the different pressure declines, and the different temperature declines during the bloom phase. I was, however, quite surprised that the difference got less pronounced after having VST-filtered the samples. I expected the VST filtration to be removing less stuff because the low-fines shots should contain less fines in the cup. My current working hypothesis to explain this is that there is also a difference in oil content. Most likely, the finer particles of the low-fines shots allow oils to come out easier with a fixed ratio. Because oils tend to have a much higher refractive index than water or coffee, this may have thrown off the unfiltered TDS readings more so than undissolved fines did.

I also noticed that the Niche shots seemed to drip more coffee into the glass during the same 30-seconds bloom (there is one shot where I forgot to note down the measurement):

I think this can still be explained from the coarser particles of the Niche, on average. The particles initially absorb water by the effect of capillary action, and the time it will take for this to bring water to the core of a particle goes up with the second power of the particle size. This means that a coarser particle will take a lot more time to absorb all the water that it can absorb. Conversely, I think the finer particles of the EG-1 pucks were able to absorb water more quickly and leave less of it available for dripping out.

One aspect where the Niche always shines is its low grind retention. In fact, its average retention (0.18 ± 0.04 g) was a bit better than the EG-1 when the latter is used with the SSP ultra-low-fines at 1500 rpm (0.23 ± 0.02 g):

 From my experience with the EG-1 stock burrs, I think they would have the same or maybe slightly less retention than the Niche does, especially if they are used at lower rpm, but the extremely fine-grinding requirements of the SSP ULF burrs require making this sacrifice in terms of grind retention. It is likely that suiting the EG-1 with a more powerful motor and operating the SSP burrs at this ridiculously fine grind setting with a low rpm would allow us to reduce the grind retention. This would also make the grinder even more expensive, however.

Quite interestingly, I noticed that the WDT step also caused 0.06 ± 0.03 g retention on average, and the tamping step with the Force Tamper also caused an additional 0.10 ± 0.01 g retention on average. This is a slight annoyance I have with the Force Tamper, especially given that it’s not very practical to clean it up entirely between shots.

When I thought I was almost done writing this post, I decided to go a bit deeper in testing how plausible it was that the decline in viscosity may explain all of the decline in puck resistance and pressure curves. To do this, I used DE1’s live temperature, flow and pressure data from each shot to estimate the puck resistance, and interpolate the Sobolik et al. relations of viscosity versus TDS (extrapolated in log space to higher temperatures), to obtain a live graphed estimate of the  TDS that comes out of the DE1, if viscosity were the sole responsible for changes in puck resistance:

By multiplying these TDS curves with flow, and calculating the cumulated total of extracted solids, I was also able to build extraction yield curves:

 Because the change in puck resistance only tells me about a relative change in viscosity, I had to make a guess at what value the TDS curves ended at, near the end of each shot. I decided to choose whatever value would make the end of the extraction yield curve meet with the actual VST-filtered measurement for each shot.

Now, the point of this was not to make a precise estimate of what I think the TDS really was, but rather to see if the numbers on these curves looked realistic… and they indeed do! The shapes of the extraction yield curves even look like what one would expect for the washing out of solubles leaking out of coffee particles, and like what Stéphane actually measured with different shot ratios in his figure further above. I did try to measure the TDS at the end of one of my shots for a coffee that extracts similarly as the Asotbilbao, and I obtained a value of about 2.0% TDS, in the ballpark of what these curves predict.

This experiment makes me want to try blooming shots with the EG-1 + SSP ULF at slightly lower ratios, and see if I like them. It also makes me want to try a more drastically decreasing shot temperature after the bloom, and see what happens. Reaching much cooler temperatures near the end of a shot could outset the loss in viscosity due to the change in TDS; it would also reduce how aggressive the extraction is near the end of a shot, where I suspect we don’t want to extract too fast anyway. How weird would it be to pull a flat-pressure, flat-flow shot?

All the realizations that came with this experiment also help me put things in perspective, and indicates that the much finer average particle size is probably the source of so many of the differences between the two styles of espresso shots. This would also back up the claim that we often hear in the espresso community, about low-fines (or high-uniformity) shots being less well suited for darker roasts, much like finer-ground coffee and higher brew temperatures are more suited to lighter roasts with filter coffee. I bet that using unusually low shot temperatures may produce good low-fines shots even with darker roasts, probably with less crema.

I also included a few more graphs below, but I don’t find them as interesting. They show that the EG-1 shots had lower puck resistances near the end, but that’s equivalent to saying that their pressure curves declined faster. They also show that the peak resistances were relatively well aligned, but this is not surprising because this is the criterion I used (indirectly with the pressure) to dial in the two grinders.

Pulling Low-Fines Espresso Shots

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Now that I have gotten comfortable with pulling good-tasting shots with the Weber Workshops EG-1 grinder paired with SSP’s ultra-low-fines burrs, I think it is time for me to share my experience. The SSP ultra-low-fines burrs (I’ll use “ULF” for short) were not initially designed for espresso, because their explicitly stated goal is to minimize the amount of very fine coffee power produced when grinding coffee.

If you are not familiar with grinding mechanics, we usually refer to the distribution of coffee particle sizes that come out of a grinder by the term particle size distributions. They are a visual representation of how many coffee particles you get for each size of particle, and for most grinders they look something like this:

Basically, you have a peak at larger particle sizes that depends on how much spacing you have between the grinder burrs, which is usually how we adjust our desired grind size. As you change your grind size, this peak will move left or right, towards smaller or larger particle sizes. This is why I refer to this peak at the right as the “target peak”, because it consists in coffee particles of the desired size, and which we have control over. The width of the target peak can change from one grinder to the next, and it also depends significantly on burr alignment and perhaps even more importantly, on the geometry of the burr teeth, and on the burr materials and coating, both things that I do not think get their fair share of appreciation. The rotation speed of  a grinder can also affect the width of the target peak, but in my experience the exact effect it has seems to be very different for different types of burrs.

The other peak at left consists of particles of a much smaller size. Those are what I usually call coffee fines, and I’ll therefore refer to this peak as the “fines peak”; they are a consequence of crushing the coffee beans rather than cutting them cleanly, and their size depends on the microscopic structure of coffee beans. I believe it is not a coincidence that, regardless of the grinder, burr, and grind size, this peak is always located near the actual size of coffee cells (about 40 microns). I think this is a consequence of fractal-like patterns of fractures that arise when crushing beans that stop propagating to smaller scales when they get to a similar size as the coffee cells, and they have no significant structure smaller the cells to propagate into. But in short, the most important point here is that the location of the fines peak does not move left and right as you change your grind size. Rather, the amount of fines will change, which means that the height of the fines peak will change if we represent it in a figure like the one above. When we grind finer, we apply more cuts to the beans, and force them through a smaller gap, and both effects tend to increase the production of fines.

The cellular structure of roasted coffee seen under an electron microscope. The cells are about 40 microns in diameter. Credit: Rebeckah Burke, University of Rochester.

If you are familiar with my Grind Size Application, you may be wondering why you have not observed the fines peak of your grinder; this is simply because it is extremely difficult to measure the fines peak with any imaging technique. It would necessitate using a microscope to image the fines, and you will face several other hurdles like fines sticking to each other and to larger coffee particles. Laser diffraction is one of the only methods that can easily measure the fines peak, but it necessitates expensive equipment.

Fines play more than one role when brewing coffee. They very quickly give out all of their soluble content when they come in contact with water because they consist of mostly partially broken coffee cells, but this is perhaps not the most important point because the fines usually only make up a very small fraction of the coffee mass, even when they vastly outnumber the other particles in terms of number. One much more noticeable effect of fines is how they affect the dynamics of percolation, i.e. how water flows through a bed of coffee. They play a dominant role in controlling the rate at which water flows through the coffee bed, as is already well known in geophysics (e.g. see page 107 of this manual) and has recently started to be appreciated in the context of coffee (e.g., see the recent paper Eiermann et al. 2020). But they are even more tricky, because fines can often make their way between larger coffee particles and migrate through a bed of coffee with the flow of water. This is yet another already well-appreciated phenomenon in the context of geophysics, but I have not seen it discussed as much in the coffee industry. I spend a great deal of pages talking about this in my upcoming book The Physics of Filter Coffee.

Minimizing the production of coffee fines is a relatively widely desired goal when grinding for filter coffee, because fine coffee powder tends to clog paper filters more easily, and any uneven distribution of fines across a coffee bed can result in an uneven flow of water, and thus in more uneven extraction of the coffee flavors. I have used my ULF burrs for years with filter coffee and have really enjoyed them over any other combination of grinders or burrs that I got to try.

However, in the context of espresso preparation, seeking to answer whether fines are desired or not can lead you into heated debates over which is better than the other. There is one reason the answer is not that simple: using pressure makes it much less likely that fines will cause significant clogging, unless you are in the presence of an ungodly amount of fines. Some decaffeinated coffees, for example, can generate quite spectacular amounts of fines that even the worst Ethiopian beans won’t come close to; I only know this because I have now started to recognize the effect of fines on my DE1 Decent Espresso Machine’s graphs, but this is a topic for another post entirely (it’s on the top of my list of upcoming posts).

Now, fines don’t really clog espresso shots, but they still migrate through the coffee bed. In fact, they migrate way more easily than in the context of filter coffee, because the finer grinds and applied pressure cause the water to flow at much faster speeds microscopically (even when the resulting drip rate is similar). The faster flow of water is able to drag the fines more easily with it, and it is able to dislodge even the smallest of fines that may otherwise hide efficiently between the cracks and irregularities of larger coffee particles (don’t take my word for it; take Yulong Yang’s words, who wrote his Ph.D. thesis on that matter). Even if you place a paper filter below your coffee puck, fines will easily migrate through that filter with the typical amounts of pressure that an espresso machine applies. Typical drip coffee uses a pressure drop equivalent to about 0.008 bar, so really using any kind of pumps, or even an Aeropress plunger, places you in a totally different context.

If fines migrate and they don’t get trapped anywhere on the way, it means that they land into your beverage. This is not too surprising, as they play an important role in the texture and mouthfeel of typical espresso beverages. For more discussion of this topic, you can refer to the book “Espresso Coffee: The Science of Quality” (Amazon affiliate link) by Illy & Vianni. I have come to think that fines are also important for the stability of espresso crema, if that’s something you’re into. If you ask me, crema is really beautiful, but I tend to agree with James Hoffman that it doesn’t taste any good.

So, what could be the point of removing fines when preparing espresso ?

I think there are two valid reasons why one may want to try that. First, removing the mouthfeel and texture could be a valid goal, because it will increase beverage clarity and make it easier to tell apart the differences between different coffee origins, much like a well-prepared drip coffee can achieve. Now, please don’t think I am saying this is the only acceptable goal for espresso; after having tried both quite extensively, I must say that I really enjoy espresso with a thick mouthfeel like I can achieve with the stock burrs of my EG-1 grinder, or with the Niche grinder. However, I generally find these kinds of shots harder to dial in, and I eventually tend to get bored of it because I find that different coffees end up tasting a bit more similar.

A second reason why removing fines may be desirable is that it will allow you to grind way finer, and extract a lot more of the coffee solubles in the same amount of time, and with the same amount of water. Remember that fines play a very important role in determining how fast water flows through your coffee bed; you don’t need to remove such a large fraction of the fines to start seeing a significant change in your drip rate, and you may need to grind quite finer to compensate. In fact, I suspect that this is the only reason why every coffee beans require to be dialled in differently when pulling espresso shots. In my upcoming book, I presented data from my imaging software to show that coffee beans that come from vastly different origins, varieties and processing methods do not result in significant differences of target peak widths, but I think they can result in a much more variable amount of fines.

Grinding much finer can come with its own set of hurdles, however. In my example, the EG-1 grinder uses a PID-controlled low-torque motor to adjust its grind speed live and keep it as constant as possible while grinding. The rotation rate can be varied anywhere from 500 to 1800 rotations per minute (or rpm). However, the SSP ULF burrs were not designed to be used for espresso with the EG-1, and they produce so little fines that one must grind so much finer with them that the EG-1 has to be used at higher rotation rates when doing so. Otherwise, maintaining a slow rotation rate while grinding through harder beans can overload the capacity of the motor—it will stop before any damage occurs, but this means you will have to widen the burr spacing and grind the rest of your dose at higher rpm, or in other words you just wasted that dose of coffee. After having done extensive testing even with the hardest ultra light-roasted washed Kenyan beans (or I should say rocks), using 1300 rpm has never caused a motor overload in several months of use. Just to be safe, I use 1500 rpm and I feed the beans in slowly, because I really hate wasting a dose of coffee. To give an approximate idea, I feed 18 grams of beans in approximately 40 seconds while grinding.

Many in the coffee industry have indicated a preference for using slower rpm when grinding, and it has its share of advantages because it reduces popcorning (the coffee beans jumping back and forth before entering through the burrs), especially at very fine grind sizes, and it tends to reduce the amount of coffee clumping and caking, which can result in uneven flow if it is not properly rectified with further steps like the Weiss Distribution Technique. This may be a partial consequence of the reduced amount of friction that results in less static electricity, but if you grind fine enough, other phenomena like the Van der Waals attraction between coffee particles can become important too, and even without static electricity you can observe some caking and clumping that just happens as soon as high-velocity particles of coffee hit each other or the sides of your grinder’s exit chute. This leads me to yet another reason why lower rotation rates may be desirable; less caking against the grind chamber and the grinder’s exit chute means less retention and staling of coffee grounds within the grinder.

I have often seen claims that higher rotation rates also impact the flavors of coffee because of additional generation of heat, however I am still quite skeptical of this claim in many contexts. As Christian Klatt, a former service product manager of Mahlkönig pointed out in one of his talks, higher rotation rates will generate more heating of the burrs over an extended period of use, but extensive testings at Mahlkönig’s labs demonstrated that the coffee itself actually gets heated less at higher rotation rates because it travels faster between the burrs and has less time to get heated up in the process. I am therefore skeptical that lower rotation rates have a positive impact on flavor, and I cannot say with any certainty that I perceive a difference in taste between high and low rotation rates. However, I can very much understand the benefits of having less clumping, less retention and less popcorning.

In short, the hurdles that come with using the ULF burrs for espresso are all related to having to grind very fine and at faster rotation speeds, and can be summarized by:

  • the grinds clumping more easily and requiring the use of the WDT technique;
  • having to feed the beans slowly into the grinder;
  • more popcorning of the coffee beans which means a slower grinding (this is only true when grinding extremely fine with a low-torque motor);
  • more potential for making a mess on your counter, especially when grinding this fine.

However, I don’t think these hurdles should stop you from trying. After some time, I have managed to deal with all of these issues, and I have come to prefer the ULF burrs over all other options I have tried so far, especially when paired with Scott Rao’s blooming shots and the paper filter sandwich method on the DE1 machine. In the past few months, I have logged the extraction yields of my daily shots and they all fell in the range 25—29% when measured without VST syringe filters (i.e., those extraction yields are artificially too high by about 1-2% because of undissolved solids and fines in the cup). Filtering my samples with the VST syringe filters have usually yielded average extraction yields in the range 23—25%, but I have done it much less often because these single-use filters are wasteful and expensive.

In fact, I have already completed an experiment that will be the subject of my next blog post, where I compare the EG-1 and ULF burrs with the Niche grinder using twelve 1:4-ratio blooming shots, and I found a consistently that the EG-1+ULF extracted higher by 1.4 ± 0.3% in terms of average extraction yield. If you are wondering, I never drink blooming shots with ratios above 1:3, but I used 1:4 in this experiment to see where the hydraulic resistance of my espresso puck behaved past 1:3.

Taste-wise, I have found the ULF blooming shots to bring out origin characteristics more clearly, with a lot more sweetness, juiciness and fruity flavors. However, pair them with darker roasts and they will bring out the worst in them, mainly harshness and bitterness. In fact, I find these types of shots to approach filter coffee a bit more, although in a much more concentrated way. Going back to filter coffee these days has had me very disappointed in comparison; we’ll see if this is just a novelty effect, but it has already lasted for several months now. Using the stock EG-1 burrs or the Niche grinder has gotten me some great-tasting shots too, but they were a bit less intense and not as clean and distinctive.

I know many will ask me the details of how I use the EG-1 with ULF burrs to pull shots, and several have already asked that question. It is not easy at first, because it is less forgetful of bad puck preparation and requires you to grind so fine that I was initially uncomfortable with it. Hence, I will detail exactly how I do it here. There are two pieces of equipment that I think are really crucial to do a good job: a portafilter funnel (I use Decent Espresso’s tall funnel) and a good WDT tool (I use Levercraft’s tool). I do not use Weber Workshop’s blind shaker that comes with the grinder. While it did a nice job of breaking clumps and distributing for filter coffee and with more classical espresso preparation, I have not succeeded in using it with the ULF burrs. They require me to grind so fine that any kind of agitation of the grounds cause caking for me. If any of you managed to make the shaker work with ULF shots, please let me know.

After having tried several different methods and read technical reports on the handling of very finely ground materials, I have come to the conclusion that the least amount of manipulation of the coffee grounds tends to be best when dealing with such finely ground powder. The only exception to this is WDT and tamping, but I’ll come back to those. But otherwise, I highly recommend dosing directly into the portafilter. Do not use any kind of dosing cup, and do not touch the grounds with your fingers. In the context of grinding this fine, I would recommend against using the Ross Droplet Technique (RDT) method where one sprays the coffee bean with water before grinding, because humidity may increase clumping even if it eliminates static. After some preliminary tests, clumping indeed seemed slightly worse when I used RDT.

I have figured out a way to use the rails on the EG-1 such that I get almost no single grind of coffee to fly out on my counter when grinding ULF shots, and in a way that also minimizes retention. The EG-1’s design already does an amazing job at minimizing retention, but we are really working in a rough regime with ULF shots that require such fine grinding at high rpm. First, remove all of the forks on the grinder, and then place only the rail ring until there is a gap of a few millimeters between the ring and the knocking lever. The ring should be placed upside down, such that it can be lifted up, not down, when you press on it.

I place the upper rail ring upside down such that it can be pushed up, not down, with the fingers.

If you want to minimize the amount of coffee flying out, I also recommend placing something like aluminum tape that you cut out in the shape of the exit chute on top of the rail ring, as shown here:

Just make sure you place something at the bottom side of the tape such that coffee grounds don’t stick on it. I like aluminum tape because it doesn’t stick too hard on the fork either, so it’s easier to remove it without gumming up the ring.

Next, place the rail fork, also in the direction where it can be lifted up, not down, with your finger. Next, place your portafilter and funnel on the rail fork and lift the fork up until the upper side of your funnel gently touches the rail ring but without it lifting the ring. This will allow you to use the ring as a secondary knocker, by lifting it up and letting it knock against the funnel, like this:

This is really convenient, because it will allow you to dislodge any coffee particles from the rail ring itself and it will really reduce retention. In addition to this, I recommend using Doug Weber’s wiper hack to help reduce retention inside of the exit chute. You can even go further and cut-out a second wiper that sticks out toward the bottom of the exit chute, to also prevent any coffee grounds from sticking to that surface too:

I recommend you make yourself one by placing a transparent piece of plastic against the bottom of the exit chute and tracing its shape with a marker. This is what this looks like for me:

With the two wipers, my grinder chute looks extremely clean after grinding, despite the high rotation rate and very fine grind size.

In order to be able to grind fine enough for ULF shots, you have to be sure that your burrs are very well aligned. Fortunately, the EG-1’s design makes this easy, but you have to pay attention to parallel alignment if you have one of the earliest batches of the SSP ULF burrs, where the magnetic pin holes were not always placed precisely enough. In my case, this made it hard to place the lower burr completely flat against the bottom part of the burr carrier with just my hands. I also have the stock burrs, which do not have that problem at all; they are extremely easy to place in or take out with my fingers, without having to apply any force. Here’s how easy this should be in a video:

If you notice that you have to press a bit hard to get either of your SSP burrs in, you may have one from the earlier batches, and I recommend you use a wooden mallet to gently knock the burr left and right (on its teeth) until it lays perfectly flat against the bottom carrier. I like to hold the carrier against a bright background and try to see any light coming through the gap between the burr and its carrier. Repeat the gentle knocks until you can see no light passing there. I also recommend contacting Weber Workshops if you face this issue, because I have recently learned that they can send you narrower magnetic pins that completely fix this issue.

If you have the latest batches of SSP ULF burrs or the narrower magnetic pins, then all you need to care about is that your burrs and carrier are very clean before you place the burrs in, and then use Doug Weber’s method to place the burrs in:

When I use this method, I usually start near the zero dial on my grinder and the point where I can’t move the dial without causing the lower burr to co-rotate is usually very close to that zero mark. After having tightened the four screws on the burr carrier, the point where I can hear the burrs touching when I turn on the grinder is usually near the mark 8 below zero (i.e., 2 full dials below zero).

Now, you may be tempted to think that this grind size where the burrs touch is  the finest grind setting of the EG-1. And it is not at all ! One design features of the EG-1 that took me a while to figure out is that the plate against which the upper burr sits is free to move up and down (but remains parallel to the bottom burr) during grinding. I am unsure of why this is, but I suspect it has something to do with improving either the alignment or the particle size distribution of the grinder. It could also just be there to allow you to go from a very coarse to a very fine grind size without having to grind the particles that remain between the burrs. Regardless, this means that even if your burrs touch, the upper burr will be lifted slightly when grinding, and the gap between your burrs will be larger than zero. In fact, I can go at least as fine as the 4.5 mark below zero, which corresponds to 5.5 dial numbers below zero, or in other words 3.5 numbers below the points where I first hear the burrs touching.

With that set-up I usually need a grind size in the range 4.8 to 5.2 when grinding 18 to 20 grams of coffee for a blooming shot with the paper filter sandwich method. Hence, I go between 2.8 and 3.2 finer than where I start hearing the burrs touch. With more classical shot profiles, I typically use grind sizes of 5.1 to 5.5 depending on the beans.

The rest of my workflow is already pretty well detailed in one of my previous posts, so I won’t repeat it here. I will, however, share a video of my routine with an ULF blooming shot, which you can view below:

You will notice that I turn the portafilter around a little while I’m grinding. I do this to try and get the coffee grounds to fall more evenly in the portafilter; note that you can’t turn the portafilter too much without it falling down from the forks. You will also notice that I let the grinder run for a bit longer for you to hear how the sound changes when there is no more coffee particles and oil between the burrs; this is when you can most clearly hear the outside edges of the burrs rubbing against each other. You will also see how I started with a vigorous surface WDT to break up the clumps that formed from coffee caking up against the chute (I would skip this step with a lower-rpm grinder), and then I I the usual, shorter, deep WDT where I start from deep down and up to the surface while stirring.

There are two additional tricks I’d like to share. First, I enjoy placing a shallow ramekin below the grinder chute, this way it will catch any stray grounds when you knock without a portafilter on, and it’s easier to throw them out.

Another trick that I really enjoy is to use a pipette brush to do a deep clean-up between the burrs. Basically, after unplugging the grinder, just open up the magnet-locked parts of the grind chamber, clean things up with a small brush, and then go 5 full rotations coarser on the dial (this means 50 full dial marks coarser). This is enough for me to fit the pipette brush between the burrs and clean up any kind of coffee particles or oil residue on the burrs. You would be amazed how much of a difference in taste this will make after just a week or two of regular use. This is still one of the top reasons why I wouldn’t swap grinders for anything else.

How a Paper Filter Below an Espresso Puck Affects Hydraulic Resistance

In one of my latest posts, I investigated the effect of puck preparation, and in particular the addition of a dry paper filter above the espresso puck, affects the hydraulic resistance of the system during an espresso shot. While I have not yet tested its effect on average extraction yield, I did not see an obvious effect of the top paper filter on shot repeatability, although it increased the hydraulic resistance by about 6% on average. This is a small effect, and is about the same as my shot-to-shot variations of 5% caused by my imperfect puck preparation if I exclude the significant outlier shots that happen 15–25% of the time.

One of the next logical steps was to test the effect of adding a paper below the puck, which was also popularized by Scott Rao a little while ago as a method to increase the average extraction yield of espresso.

[Edit: I initially said that Scott introduced the idea of using a bottom paper filter, but thanks to Robert McKeon Aloe for pointing out that others had been doing this a long time ago on Home-Barista. Mark J. Burness also pointed out that Sang Ho Park may have been the first person to use the technique. As far as I know, this idea had remained quite obscure until Scott talked about it on Instagram.]

While the paper filter on the top might help dispense the water more evenly across the puck and potentially prevent some structural damages from the impacting water, I believe that the role bottom paper filter is quite different. One of biggest revelations I had while working on my book The Physics of Filter Coffee was related to this: lifting a paper filter that sits directly on the bottom of a dripper is often a really good thing, because it liberates all of the filter’s pores for coffee to flow through more evenly, and then the fluid can flow through the exit holes very fast.

Water flowing only through the exit holes of a dripper

There is one subtlety here that had prevented me from fully appreciating this fact: the hydraulic resistance of a paper filter blocked everywhere except for a dripper’s exit holes is often way higher than the same system where you just lift the paper filter slightly. This is true because water can flow very fast at the center of an unobstructed dripper hole, whereas it will flow at the same velocity everywhere through the exit hole if a paper filter sits directly on it. This is why liberating the full surface of the paper filter, by lifting it, is what dominates the end result: once the paper filter is lifted, the exit holes of any dripper on the market really don’t offer much resistance at all.

Because of this consideration, my hypothesis for why the bottom paper filter appears to produce higher average extraction yields (as observed by Scott Rao, Stéphane Ribes and Socratic) is that it simply allows water to flow through more paths across the coffee puck. This means that there are probably less regions of the coffee puck that remain under extracted, and on top of that, the lower overall hydraulic resistance that this results in should allow one to grind slightly finer and gain a bit of accessible surface of coffee particles to extract solubles more quickly.

This is what I set out to test with a small experiment. I pulled 10 shots with a new batch of the washed Mas Morenos Honduras coffee roasted by my friend Andy Kyres (owner of Color/Full Coffee Corp), the same green coffee I used in my last experiment. The coffee was roasted on 2020 December 12, and I opened the sealed two-pounds bag on the day of the experiment, on 2020 December 22. I decided to pull 10 shots, alternating between the use of a paper filter at the bottom only versus no filter at all. In this experiment, I also did not measure average extraction yields to maximize the number of shots I pulled in the short amount of time I had.

Yet again, the reason why I alternated between the two methods is to minimize the effect of the espresso machine or grinder getting gradually warmer, or my puck preparation slowly changing. I used the Niche Zero on grind size 13.0 (at factory zero-point) for this experiment, with the DE1 Decent Espresso machine’s “Best Pressure Profile”, much like last time. I also used the same ground distribution method, and opted for the “deep WDT” puck preparation because it allowed me to achieve more repeatable results in my last experiments. I used Levercraft’s WDT tool in its default configuration, the Force tamper at its default pressure setting, Cafelat Robot 58mm paper filter, and I recommend reading my last blog post if you would like to get more details about any of these considerations; it also includes videos of my puck preparation routine. Yet again, I pulled 3 shots before starting the experiment to ensure everything was warm enough.

The DE1 “best pressure profile” I used for this experiment

It is interesting to note that I needed to grind 1.0 dial finer on the Niche compared to the last batch of the same green coffee. This is probably related to either differences of aging, or slightly different roast profiles. Because I consider Andy very good at replicating roasts, I would favor the hypothesis of either the fact that the coffee had been more freshly roasted, or that the green aged more which could have changed the bean moisture and how it shatters, i.e. how many fines it generates, when ground.

I pre-wetted the bottom paper filter by flushing the DE1 into the dry filter, and then carefully pressed on its edges with my finger to get them to stick properly, taking care not to displace the filter. I placed the creped side of the filters up and toward the coffee puck, because I want to maximize the surface of contact between the coffee particles and paper filter to get as much of an even flow as I can. I placed a video of this here.

One of the first things that became immediately obvious during this experiment is how the use of a bottom paper filter completely fixed the issue I was discussing in my last experiment where my spent pucks had a slight hollow near the center. All five brews without a paper filter still clearly showed this central hollow at the center of the spent puck, while none of those with the bottom paper filter did.

My current puck preparation yields a central hollow in the spent puck, in all cases where no paper filter is placed below the coffee puck. In my last experiment, I also determined that using only a paper filter at the top of the puck did not fix this issue at all.
All shots that I pulled with a paper filter at the bottom of the coffee puck yielded a perfectly flat spent puck, everything else unchanged.

There is something about this initial observation that I found really surprising. The fact that the top filter did not fix the hollows, but the bottom paper filter did, leaves me with only two hypotheses to explain it, and both surprise me. The first hypothesis is that there is really almost no flow of water far from the center of the puck unless you use a paper filter at the bottom. The second one is that some coffee particles are able to pass through the portafilter holes near the center of the puck, even with the Decent Espresso baskets which have even basket hole sizes compared to other manufacturers (except VST baskets which are also very even).

A recent experiment carried by Stéphane Ribes on the Decent Diaspora forum makes me think the first hypothesis is more likely. Stéphane had the ingenious idea of cutting out spent espresso pucks and measuring how much solubles were left on the edges versus center with a subsequent immersion in clean water. His experiment clearly demonstrated that the outer edges of espresso pucks are under extracted when no paper filter is placed under the puck.

The findings of Stéphane Ribes’ radial extraction experiment that are relevant here. Stéphane found that using a paper filter at the bottom of the espresso puck really helped to avoid under extracting the edges of the coffee puck.

All of these observations point in the same direction, as Stéphane already noted way before me: current espresso baskets do not seem optimal at all for even extractions, because the basket holes do not extend close enough toward the edges of the basket. I suspect there are engineering reasons for that; such baskets may be too fragile to sustain high pressures for very long, and may break more easily. If this is the case, then using disposable paper filters may still be the best solution for more-even, home espresso, even though this is definitely not a great option for heavy use in a cafe.

Now, let’s shift our focus to what I actually intended to measure during this experiment: how the hydraulic resistance of my espresso shots were affected by the use of a paper filter at the bottom of the puck. Below, I shot DE1 graphs of the 5 shots without paper filters, followed by the 5 shots in which I used a paper filter at the bottom.

DE1 graphs without paper filters (top) and with a paper filter at the bottom of the puck (bottom). Using a paper filter at the bottom of the puck significantly reduced the hydraulic resistance, increasing flow.

Once again, these graphs contain a lot of information, which I explained in great detail in my last post. One important point I want to mention again is how I calculated the puck resistance; the DE1 usually displays them as the pressure drop (green curve) divided by the square of the DE1-estimated flow rate of water at the shower head (blue curve). This is actually an estimate of the square of the puck resistance, from which the changes in bed depth and porosity versus pressure (due to puck compression) are removed. It is useful to remove these effects because they are both reversible, and this allows you to only see how other variables like grind size, fines migration, and channels, may affect your puck resistance. Note that, as I also detailed in my last post, I believe that the initial rise in puck resistance is still due to un-corrected effects in puck compression, and the subsequent fall in most profiles that don’t have a blooming phase (as is the case here) are due to the puck gradually becoming fully saturated with water.

In the graph above and in all of the remainder of this post, I am showing the square root of the pressure curve divided by the flow, to obtain the puck resistance not squared. This makes it easier to talk about puck resistance and relate it to bed depth and other variables as per Darcy’s law. As a reminder, the orange resistance curve is calculated similarly, but using the output weight of espresso measured by the Acaia Lunar scale which I connected on the DE1 using Bluetooth. Once again, I slightly modified all resistance curves by less than 1% to account for small variations in my exact doses (all shots here have doses between 17.8 and 18.0 grams).

It is quite clear in the figures above how adding a paper filter at the bottom of the coffee puck decreased the hydraulic conductivity, making the water flow faster and the shots faster to reach similar beverage weights. In the figure below, I show only the (shower head flow-based) resistance curves compared with each other:

DE1 resistance curves calculated with and without the use of a paper filter at the bottom of the espresso puck. Using a paper filter at the bottom significantly reduced the hydraulic resistance.

This reduced hydraulic resistance really fits well with the results of Stéphane’s experiment discussed above, and the observation that the use of a bottom paper filter completely removes the problem of hollows at the center of spent pucks. To quantify this a bit further, here’s a comparison of the peak values of these resistance curves, as well as the hydraulic resistances near the end of the shots (when the beverage weights reach 40.0 grams).

As shown above, the bottom paper filter reduced the hydraulic resistance by a significant factor 1.9 ± 0.2, i.e. almost reduced it by half. The standard deviations as well as median absolute deviations of both samples were also reduced when using a paper filter at the bottom, but I believe the more relevant quantity is the fractional variation in resistance, not the absolute variation. If this is what we look at, both samples have a standard deviation of about 18% versus 19% of the average hydraulic resistance, which I believe is not significant here.

Now, let’s look at similar graphs but for the stabilized hydraulic resistance, using either the DE1-estimated values based on flow rate at the shower head, and those using the output drip rates as measured by the Acaia Lunar scale.

In my last experiment, I explained how I think that the peak value of the resistance curves is particularly sensitive to preinfusion because it happens when the coffee puck has not yet been entirely saturated with water. As a consequence, I think that looking at the hydraulic resistance near the end of a shot is a better indicator of what is going on. The hydraulic resistance values calculated from the Acaia Lunar scale are also probably more accurate, because the DE1 flow rates at the shower head are estimated based on a complicated physical model of the machine that depends on many factors such as the properties of the electrical grid the machine is used with.

Therefore, I think the most informative graph is the one showing the stable scale hydraulic resistance (the last one above). This graph shows that using a paper filter at the bottom of the puck decreased the stable hydraulic resistance by a factor 1.43 ± 0.04, and possibly reduced the shot-to-shot variation slightly: I’m getting variations of 4 ± 1 % with the paper filter and 5 ± 1 % without it. This is similar to my previous experiment, and probably not a significant difference between the two samples. I do not think the dramatically reduced median absolute deviation (blue bars) is particularly informative in the “no paper” case because of the small sample with three tightly grouped data points.

In other words, using a paper filter at the bottom of the coffee puck did not affect the variability in peak resistance much, but it significantly reduced the hydraulic resistance by about 43 ± 4%. I find this quantification really interesting, because we can compare it to the surface coverage of holes in the Decent baskets. The hole pattern of the Decent baskets have an outer diameter of about 50 mm, 8 mm smaller than the full 58 mm diameter of the basket. Therefore, if the decreased hydraulic resistance is only caused by the hole pattern not reaching the edges of the basket, we would expect a change in resistance of only (50/58)2 = 35%. The value that I found, 43 ± 4%, is a bit larger than this, and might suggest that even within the central pattern of basket holes, flow might not be perfectly even because of the spacings between the holes. Adding a paper filter below the puck may therefore make the flow about 6% more even even within the central region covered by basket holes, although this number is quite imprecise.

As a result of this experiment, I will definitely be using a paper filter at the bottom of my puck more often. It is a bit more trouble, but I now believe it is really worth it. I plan to eventually measure the effect on average extraction yield myself, and I would love it if anyone could try assessing its effect with blind tasting. 

I also noted during this experiment that all shots with a paper filter at the bottom showed their first droplets of espresso in a ring shape at the bottom of the portafilter. Although this is not conclusive evidence, it may suggest that the overall flow was still not perfectly even, and that the addition of a paper filter may have over-compensated and allowed for a bit more flow than we want near the edges of the basket. If this is true, then we may benefit from using slightly smaller paper filters, perhaps something in the range 55–57 mm. I think that comparing the average extraction yields and resistance curves of shots taken with paper filters of different diameters may turn out to be very interesting, and we might find that there is an optimum filter diameter that is slightly smaller than the full basket size.

Another explanation for the outer ring of espresso appearing first under the basket could simply be related to the fact that all of the espresso near the edges has nowhere else to escape, and therefore pools at the outer basket holes, giving us a false impression that more fluid is flowing there. If this is the case, the 58mm paper filters will probably be optimal for an even extraction. If you go back to Stéphane’s slide above, this interpretation seems likely because the edges of the puck were very slightly under extracted even when he used a paper filter below the puck.

You can find the log of my shots here, as well as the DE1 shot files and the profile I used here.

Disclaimer: I receive no financial benefits from any of the companies mentioned above, and I have no business ties to them. Decent Espresso generously offered me a 25% discount on their DE1 machine, and Weber Workshops offered me a set of SSP Ultra-low-fines burrs and their glass cellars, without obligations or expectations. All my impressions of the gear that I use are my own and never financially motivated. The owner of color/full is a personal friend.

I would like to thank Johanna “Mimoja” Amélie Schander for having coded the required Bluetooth communication codes on the DE1 and making it possible to pair the DE1 with Acaia scales.

The Four Rules of Optimal Coffee Percolation

After having brewed almost exclusively pour overs with the V60 and the Fellow Stagg [X] in the past few years, I have come to adopt a style of recipes that best suited these drippers in a way that it took me a while to fully understand. Writing my upcoming book The Physics of Filter Coffee forced me to think more deeply about the limitations of these drippers, and made me aware of their design flaws, if we can still call them flaws in a context where almost every other dripper is more flawed. More recently, I began playing with Decent Espresso Machine’s DE1, with the Tricolate dripper, and with the base of the Aeropress as a gravity dripper. While I will discuss these in other posts, these different devices opened my mind to some aspects of percolation.

After this shake-up of how my thoughts about percolation are organized, I decided to write down some of what I now believe are the most important things to consider when designing a dripper, a brew recipe, or espresso preparation methods. Here, I’ll call them the “rules of optimal percolation”; it does not mean you necessarily have to follow them, but rather I think they are some things we should always be mindful of.

Some of them may be useful in the context of immersion, but I wrote them specifically with percolation brews in mind. It is generally a lot easier to achieve good and even extraction with immersion brews, but immersion is not as potent as percolation to achieve high extractions, and in some cases percolation also makes it possible to achieve a very good beverage filtration. This is why I think it is worth putting up with all of percolation’s difficulties in the first place.

First, let me just state the four rules I have settled upon, and I will then explain them in detail.

  1. Avoid Bypass
  2. Avoid Clogging
  3. Achieve an Even Flow of Water Through the Coffee
  4. Adjust your Brew Ratio to your Grind Size

In addition to the four rules above, it is good to remember that using drippers made of insulating materials is desirable when preparing coffee with any percolation method. It is also best to avoid drippers made of materials that can store a large amount of heat, with the exception of espresso machine group heads, because those can be kept at a controlled, high temperature. In general, you just want to be able to control the temperature of your slurry during an extraction. This is famously quite hard to do with most pour over drippers, which is why I tend to prefer the Fellow Stagg [X] or the plastic V60 over most other drippers.

In the same spirit as this caveat about temperature stability, it will always be frustrating and wasteful to brew coffee unless you can repeat your best brews in a repeatable way. It is therefore always preferable to choose repeatable methods, measure your dose of coffee and water, and use a grinder, kettle or dripper that helps you replicate your results precisely. The four rules that I am about to discuss only focus on how to achieve a better evenness of extraction during the percolation phase, and ignore these considerations of repeatability.

1. Avoid Bypass

What I call bypass is any water that manages to make its way around the coffee bed, or most of the coffee bed, and will therefore not participate to extraction. While this effect doesn’t immediately sound alarming—I’ve said in the past it is just like diluting your brew with more water—I now think that it drastically reduces our flexibility when brewing coffee. Worse, bypassing water often touches the outer edges of the coffee bed, and may drag unpleasant flavors by over extracting these parts from the significant flow of clean water.

One of the major problems with bypass is that it will depend on your brew parameters, such as the filter you are using, how tall a column of water you have above your coffee bed, the depth of your coffee bed, and perhaps above all, your grind size. Imagine you are brewing coffee in a V60 dripper: the absolute amount of water that flows around the coffee bed at any moment does not depend on your grind size, if you compare apples to apples (i.e., with the same filter and the same water column heigh). It might be, for example, around 1 gram per second at the moment where you have a 5 cm column of water above the coffee bed (I made up this number). However, how much water passes through your coffee bed depends significantly on your grind size.

If you are grinding quite coarse, maybe you have 5 grams per second that are passing through the coffee bed, and therefore bypass only makes up for 17% of your total drip rate at that moment. However, if you ground fine enough that only 0.5 grams per second are actually passing through the coffee bed, bypass makes up more than 66% of your total drip rate at that moment. This is a recipe to get a weak and astringent brew. I now think this is one of the major hurdles that prevent us from grinding finer and still obtaining a good-tasting beverage with drippers such as the V60.

I have often heard baristas claiming that pouring at the center of the coffee bed avoids bypass—this is simply false. Wherever you pour, if water is able to pool on top of your coffee bed at all, it can find its way to the edges and still bypass. Neither are aggressive center pours a good solution: they will cause a crater at the center of the coffee bed, which may reduce bypass, but it will also produce a very uneven extraction by leaving some of the higher-up coffee particles under extracted.

Another way to mitigate bypass is to divide water pours into many steps, such that the column of water never gets too tall above the coffee bed. While this will definitely reduce bypass, it will significantly reduce the temperature of your slurry, in a way that is hard to control. While lower slurry temperatures may be preferable with darker roasts, I have never enjoyed them with the lighter roasts that I am used to drinking. Using many small pours will also make your brew much longer, because the water traveling through the coffee bed will not be pushed by as much weight on top of it. A longer brew time is not necessarily a bad thing in itself, however, as I will discuss further down.

Another trick can be used to reduce the impact of bypass: agitation. By causing enough agitation of the coffee bed, a barista can force even the deeper parts of the coffee bed to encounter fresh water, and increase the efficiency of extraction before much of the water can actually get around the coffee bed. While this solution certainly works, it is not without drawbacks. Too much agitation can allow coffee fines to get trapped in a paper filter, and cause clogging. We will come back to clogging in the next rule—but basically, this is the main reason why we never agitate 100% of the coffee bed for a full brew, because this would be a sure way to clog it.

Even Fellow’s Stagg [X] dripper, which I have come to prefer over the V60 in part because it suffers from less bypass with the appropriate modifications, is not completely free from bypass. I have only realized this after brewing coffee with drippers that actually do not bypass.

2. Avoid Clogging

Whenever too many coffee fines get trapped in the pores of a paper filter, the drip rate of a coffee brew can go down drastically. This is not only a problem because the brew becomes much longer: the bigger issue is that whatever water is still able to pass through will do so along smaller, and unchanging paths through the filter. This means that large regions of the coffee bed will potentially stop receiving fresh water, and will remain under extracted, while other regions will receive the bulk of the flow and contribute astringency or other unpleasant flavors caused by over extraction.

It is often not easy at all to avoid clogging; using a high-quality grinder that generates less coffee fines is a viable solution, but even with those you will encounter some coffee beans (e.g., decaffeinated or Ethiopian coffees) that still generate enough fines to potentially clog most paper filters unless you are careful about it.

Having your water pass through a large surface area of paper filter is one great way of reducing clogging, as well as using thicker paper filters. This is true because both of these tricks will increase the total volume of paper filter where fines can get trapped before clogging occurs. This is often referred to as the loading capacity of a filter. Using creped filters is also a good trick, because the rippled surface of the filter will increase the surface area of contact between the coffee particles and the coffee filter at the very small scale.

One big design flaw that I often encounter in drippers is that the concentrated water can only pass through a small region of an otherwise large paper filter. For example, the thin Kalita filters and the shallow ridges at the bottom of the Kalita dripper often cause the filter to sag down, and sit on top of the three small holes of the dripper. Before this happens, water is free to flow through the full bottom of the filter, providing a large-enough surface of filter to avoid clogging, but as soon as the filter sits on the holes, water begins flowing only through the paper directly above the three holes. This drastic reduction in the filter surface is responsible for the Kalita’s infamous clogging issues. The Chemex, Stagg [X] and Stagg [XF], among others, suffer from this problem. The design of the V60 makes it extremely robust against clogging because it has a gigantic, cone-shaped surface area of filter that is well lifted from the dripper walls by ridges. However, doing so makes the V60 extremely vulnerable to another major problem: bypass.

3. Achieve an Even Flow of Water Through the Coffee

Rules 1 and 2 above are only useful because they are in a sense required to achieve an even flow of water through the coffee bed. However, there are other ways in which water can flow unevenly through a coffee bed: having a non-level bed of coffee, a non-level dripper, a very uneven particle size distribution, bad puck preparation in the context of espresso, can all be further causes for an uneven flow of water through the coffee bed. It is therefore important to always be mindful of these potential issues.

Problems of uneven flow are often grouped within the term channeling. While technically, channeling may only refer to a hollow in the coffee bed (either microscopic or large) that allows for a large local flow of water, this problem really is of the same nature than bypass, or any uneven flow. In my experience, it is relatively easy to avoid channeling in the true technical sense with gravity-driven brews, other problems like bypass and clogging are not easy at all to deal with. In the context of espresso, channels in the true technical sense are not as easy to avoid, because mistakes in puck preparation combined with the higher pressure will favor channels.

There are some general ways of improving the uniformity of flow inside a coffee bed that are unrelated to clogging and bypass. For example, blooming the coffee bed properly to start percolation after it is entirely wet, and using a grinder that produces a more even particle size distribution are two ways to do this. A more uniform particle size distribution will not only it will make it directly easier to extract evenly in the first place, but it will also lead to a more even flow of water. Another important consideration is the drip rate of your brewer; for a fixed dripper cross-sectional area, the much slower drip rates will tend to give rise to a less even flow of water inside the coffee bed. I discussed this in a bit more details in this past blog post, and we’ll come back to this idea below.

4. Adjust your Brew Ratio to your Grind Size

When grinding coffee finer, you are exposing more cells of the coffee beans to the surface of the coffee particles, and they can therefore extract more easily. This means that the finer you grind, the less solvent and the less time you will actually need to extract everything before you start to draw out the unpleasant components that extract slower and taste worse. In other words, I believe that optimal brew ratios will be smaller (less water) when grinding finer. This also means that beverages prepared with finer grinds will necessarily be much more concentrated, and generally have somewhat higher average extraction yields.

If I had read this particular rule a year ago, I would have though “what the hell is this guy thinking?”. I would have thought that because, in practice, preparing a V60 with very finely ground coffee would taste quite bad, even if one uses much less water. But remember: grinding finer with a V60 actually causes a different problem: it increases bypass, and significant bypass will result in a weak and astringent brew. I now believe that this is why it took me so long to uncover this idea of how adjusting brew ratios to the grind size is important. Every dripper I had always used had design flaws that simply made it impossible to achieve good beverages with some combinations of ratio and grind size, because of either bypass or clogging!

It is only at the beginning of 2020 that this idea of adjusting brew ratio to grind size hit me, which prompted me to write about it in a past blog post, but lacking the proper drippers to explore it further, it remained just that, an abstract idea. However, this came back to me when I started alternating between espresso and Rao allongés on the DE1, and wondering how such different grind sizes could taste good when using different ratios, but similar brew times and pressures. When I thought more about this, I suggested that there may exist a family of good-tasting recipes, where the flow rate of the DE1 would be adjusted as a function of the grind size.

A visual representation of the family of coffee beverages that may belong to the “optimal percolation” zone, as described in this post.

However, when I wrote about this and designed DE1 “adaptive” shot profiles to explore this family of brew recipes, I did not immediately grasp the importance of the changing brew ratios. It is only after having brewed a lot with these adaptive profiles that I came to realize the brew ratio was a lot more important than keeping a fixed brew time along all possible brew recipes. Coarser-ground coffee tasted better with 1:4 to 1:6 ratios, whereas finer-ground coffee tasted better with 1:2 to 1:3 ratios, and there didn’t seem to be any grind size that doesn’t taste good, as long as the proper ratio is used, and the puck preparation was good enough to achieve an even flow of water through the puck.

Brewing with the Tricolate made me even more confident about this. To be sure I was not experiencing any bypass, I placed some food-grade silicon around the bottom part of the dripper, and I started experimenting with brewing finer-ground coffee with a shorter brew ratio. And indeed, the results were very encouraging: I was obtaining very good-tasting coffee, free of astringency, at both higher concentrations and extraction yields that I was ever able to achieve with my more classical pour over brews at a 1:17 ratio.

The bottom of the Tricolate dripper which I patched with food-grade silicon glue to make sure I would obtain strictly zero bypass of water.

It is important to stress again that you must free yourself from the other problems like bypass and clogging, before you can actually fully explore the family of good-tasting coffee beverages with different grind sizes. So far, the only drippers I encountered that allowed me to do this are: the DE1 espresso machine, the Büchner funnel, the Tricolate, and the bottom of an Aeropress used as a gravity dripper (although it is hard to avoid clogging with it). In other words: don’t expect this rule to be very useful if you are using drippers like the V60 or Stagg [X] that do not completely avoid bypass. The first two rules therefore act as some kind of a barrier that you need to cross before you can really use the concept of adapting ratio to grind finer than typical pour over grind size.

You will also have to open your mind about brew time; we are used to think that very long pour over brews (above 4 or 5 minutes) are always bad, but this is only true because they normally indicate you ground fine enough to cause significant bypass, or that your filter has clogged. In other words, they are once again different underlying problems that are not directly related to brew time. If you brew with a dripper that is completely free of bypass with a filter that does not clog, you can obtain very good tasting brews that are much longer. I personally had a few really good brews that took as long as 10 minutes recently, with the Tricolate and the bottom part of an Aeropress used as a gravity dripper.

If you try to brew something as fine as an espresso with only the force of gravity, you may obtain brew times so long that they will become truly problematic; maybe the flow of water will get so slow that flow will be uneven inside the coffee bed, or the slurry will become too cool to achieve good extraction. Scott Rao has experimented a bit more than me with the extreme end of finer grind sizes with the Tricolate, and he seems to be finding a limit, much finer than what we are used to for pour overs, but still a limit, where the brews start tasting flatter. The use of pressure with a Büchner funnel or espresso machine is probably needed at some point, and with the increased uses of pressure, there is also probably a point where tamping becomes important to avoid uneven flow. However, using pressure will almost always lead to a loss of beverage clarity, because the added pressure, and the resulting fast flow of water in the pores of the coffee bed at the microscopic level, will allow coffee fines to migrate even through relatively thick paper filters. This is not necessarily a bad thing; I was really surprised recently at how much I like Rao allongés that are not clear brews at all.