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.

A Study of Espresso Puck Resistance and How Puck Preparation Affects it

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Before I begin to characterize how different grinders or espresso preparation methods affect extraction and taste, I decided to test how repeatable my shots of espresso are. The DE1 Decent Espresso machine is pretty nice for this purpose because it shows you graphs of your live pressure, the flow of water coming out of the shower head, and how the weight of your espresso beverage changes over time if you connect a Bluetooth scale to it.

To carry this experiment, I bought a large bag of washed Mas Morenos Honduras coffee roasted by my friend Andy Kyres who owns Color/Full Coffee Corp, dialled it in on the Niche Zero grinder, and followed Scott Rao’s puck preparation technique that he posted on his Instagram TV channel. I started on the Niche first because I can compare it to other Decent users more easily, and I plan to do a similar experiment on my EG-1 grinder very soon.

My current grinding corner, where I am preparing a battery of tests to compare the 68mm conical Mazzer burrs in the Niche Zero to the 80mm SSP ultra-low-fines flat burrs in the EG-1.

For this experiment, I dosed 18.0 grams of coffee in the Niche and ground directly in the portafilter, like Scott shows in his Instagram video. I used grind size 14.0 on the Niche for the whole experiment. I made sure to send grounds as much as possible at the edges of the portafilter at first, and then filled up the rest in a circular motion. To do this, I moved the portafilter around in a “nutation motion” with my left hand while the Niche was grinding. Here’s a video of my attempt at this:

I used the tall Decent portafilter funnel to help get all of the grounds in my portafilter, and Levercraft’s WDT tool in its default configuration. Levercraft’s is my favorite WDT tool so far; I find that the large number of needles (8) and their wide angle makes it much easier to distribute the grounds evenly and obtain a level puck.

Levercraft’s WDT tool

I decided to test a few different puck preparation techniques while I was at it. First, I tried using exactly the Weiss Distribution Technique (WDT) method that Scott showed on his IGTV channel, where you only de-clump and stir the top third of the depth of the coffee puck before tamping. You then tap gently and tamp once, trying to get a coffee puck that is as level as possible. I’ll call this method “surface WDT” here. You can see a demonstration in this video:

I also tried another method, “deep WDT”, that is more popular among DE1 users on the Decent Diaspora forum, where the WDT tool is inserted at full depth and used to stir the coffee grounds more vigorously in circular patterns, slowly going up and then finishing on the surface, like I do in this video:

This is followed again by a small tap of the portafilter on the tamping mat, and by tamping. I used The Force Tamper (Amazon Affiliate link) to reduce variations due to non-level tamping. I found that holding the base of the tamper with my left hand and then tamping with the right hand allows me to obtain a more level coffee puck, like I show in this video:

I left the tamper at its default level of pressure. Scott Rao recently popularized the use of paper filters at the top of the puck to help distributing flow evenly across the top of the puck, and I decided to also include that method in today’s experiment. With the first two methods that I described above, I did not use any paper filter. So I added two more kinds of methods where I did either surface WDT or deep WDT, and also added a dry Cafelat Robot 58mm paper filter on the top of the coffee dose. I always added the filter such that the creped side was facing the coffee puck, because this is where I want to maximize the surface of contact. I just placed the dry paper filter on top of the tamped coffee bed like I show in this video:

In order to minimize any systematic effects of my getting tired, slowly improving my technique, or the grinder or DE1 getting warmer, I alternated between methods. I started by pulling 6 shots where I alternated between surface and deep WDT without paper filters, and then I started alternating between all four methods. I used the “Best Pressure Profile” that peaks at 8 bar after preinfusion, and then slowly goes down to lower pressures over time. My goal was to assess how repeatable would be the puck resistance and its decline with time, so I like to keep a fixed pressure for this experiment because I know changing the pressure from shot to shot could affect the physics of what size of coffee fines can migrate down the puck, and at what rate they do so. I also pulled three shots before starting the experiment, to dial in the coffee and ensure that the DE1’s temperature was stable enough.

Note that I did not refract any of the shots in this experiment: doing this properly takes time, and I decided to focus this experiment on repeatability of puck resistance only, as this allowed me to pull and log 24 shots in a bit less than two hours, probably twice as much as if I had refracted each of them.

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

Before we discuss the results of this experiment, it is worth talking a bit about the DE1 graphs for one of the no-paper, “surface WDT” shots described above.

DE1 graph for a representative espresso shot with the “best pressure profile”

I know the DE1 graphs contain a lot of packed information, especially if you are not familiar with the machine. The green line shows the pressure in units of bar and how it changes over time during the shot; you can see a slow rise of pressure during preinfusion (in the first 10 seconds), which causes the machine to exit the preinfusion phase when a pressure of 4 bar is reached. The blue curve shows the rate at which water flows out of the shower head, as calculated by the DE1, based on measuring the voltage of the pumps and a complicated physical model of the whole machine. As you can see above, I used a 3.5 mL/s preinfusion during the preinfusion. The early 4 mL/s start is related to the firmware set-up of my machine, and I did not bother changing it because I don’t think it matters much for this test.

When the pressure reaches 4 bar, the machine switches from a “flow-controlled” mode to a “pressure-controlled” mode where the pressure is immediately cranked up to 8.4 bar, and then it slowly decreases to about 5 bar at 35 seconds. The red curve shows the temperature near the shower head, in units of degrees Celsius divided by ten. The brown curve shows the gradually increasing weight of espresso as measured by the Acaia Lunar scale that I had connected to the DE1. You can see that the brown curves becomes similar to the blue curve at around 17 seconds: this is when the puck becomes mostly saturated with water, where the flow of water inside the puck becomes the same as the flow of water that exits the puck. Because the DE1 uses a complicated physical model of the machine to estimate the blue curves (the flow from the shower head), it is not atypical to see errors of about 10–20%. It is notoriously difficult to actually measure the flow of water through pumps with a better accuracy in the context of espresso making. This explains why the blue and brown curves are not perfectly on top of each other even late in the shots. The brown curve also includes dissolved solids and should in principle be about 10% higher than the blue curve for a short while, but this effect is dominated by the systematics of the flow calculation at the moment. Even if we had a perfect blue curve, measuring a flow rate very rapidly with a 0.1 gram-precision scale for the brown curve is hard, because the measurement error gets inflated if you try to measure the flow in a very short amount of time. I went around some of this problem by using custom smoothing algorithms I wrote (not those of the DE1), based on running-box local second-order polynomial fitting, a running “median” box followed by a running average-based smoothing box (you can safely ignore all of this sentence if it means nothing to you).

The yellow and orange curves show estimates of the puck’s resistance based on the pressure curve (green) and either of the two flow curves (blue or brown). For reasons that I explained in my last post, the resistance of the coffee puck to the flow of water can be approximated with the flow of liquid through the puck divided by the square root of the pressure drop. In the DE1’s app, these resistance curves actually show the square of the flow (in mL/s) divided by the pressure drop (in bar), and thus estimates of the square of the puck resistance, but through this post I am showing the resistance for simplicity. It makes sense that the yellow and orange curves only meet when the puck is saturated; before that moment, the whole presuppositions of Darcy’s law and the simple characterization of the puck with a hydraulic resistance don’t make much sense because it has not reached the “percolation threshold”, i.e., the point where espresso starts coming out from under the puck.

[Edit Feb. 7, 2021: in the paragraph above I had originally misquoted the puck resistance as flow divided by the square root of pressure, whereas it’s actually the square root of pressure divided by the flow. Thanks to Sam Roesch for pointing this out.]

I believe that the point where the pressure starts to rise during preinfusion corresponds to the moment where the empty headspace above the puck is completely filled with water, and now the machine has to apply pressure to get more water in the system. Water is basically an incompressible fluid in the context of espresso (you need 220 bar to compress water by 1%), but the machine somehow manages to get more water in there before espresso starts dripping out (in all cases, the first drops of espresso appear at 11-12 seconds). This means that something must be compressing, and if it is not water, it has to be the espresso puck. This effect was actually demonstrated quite neatly with transparent portafilters; the coffee puck is compressed by the espresso pump much like a spring, and it decompresses if no more pressure is applied on it. This is why the “puck resistance” as seen from above the puck (the yellow curve) starts to rise at the end of preinfusion. But then, what happens next is interesting: the resistance of the puck gradually decreases, and then stabilizes.

A demonstration of espresso puck compaction with a transparent portafilter

I used to think this was related to the slurry getting less viscous and therefore more easily flowing through the coffee pores as it gets less concentrated. A paper by Sobolik et al. (2002) showed that coffee beverages with concentrations in the range 0—10% show changes of viscosity of about 50% at 80°C (Fig. 3), which would be expected to cause a decrease in puck resistance by 50% as the coffee solubles get depleted. This is not enough to explain the full changes in resistance that we observe (about a factor 2 to 3.5 here), so while it may explain a small fraction of the decrease, it is definitely not the full explanation.

Figure 3 of Sobolik et al. (2002), showing the dynamic viscosity of water (black circles) and concentrated coffee at 10% TDS (grey circles), 20% TDS (white circles) and 30-50% TDS (triangles), as a function of temperature. The symbols show empirical measurements by Weisser (1972) and the lines are best-fitting power laws.

Another potential culprit for this decrease in bed resistance could be related to the migration of fines, a topic I discussed extensively in my upcoming book The Physics of Filter Coffee. However, the change in bed resistance is so strong and abrupt that it made me worry a third cause might be the explanation: the espresso puck might still be partially dry when we are at the end of the preinfusion. I now think that this is the most likely explanation, because adding just a 10 seconds pause after the preinfusion completely removes the peak in bed resistance, and there is no reason for the slurry to have gotten less viscous, or the fines to have moved anywhere, during a 10 seconds pause without flow. This is a topic for a future post, but I believe it will be really important to add a short pause after preinfusion to allow the puck to fully saturate and improve espresso. Scott Rao has done something similar in the past with his “blooming espresso”, but I suspect we may get some of their benefits with a pause as short as 5—10 seconds after preinfusion. Furthermore, I think that the 30 seconds blooming phase of the blooming espresso allows the puck to decompress, and I am not sure that it can be easily compressed again once the puck has been saturated—not that this is necessarily a problem anyway.

DE1 graph for a shot with a modified version of the “best pressure profile”, where a 10-seconds pause was added immediately after preinfusion. The pause drastically reduced the peak in puck resistance that would usually happen in the first 10 seconds of the shot. This indicates that the true culprit for the peak in bed resistance in normal shots might be an incomplete wetting of the puck.
Comparing only the puck resistances (as calculated from the shower head) of two shots with the same puck preparation and grind size shows how the shot with a pause (red) completely skipped the peak in bed resistance that occurred with the no-pause, normal shot (blue). The puck resistance almost caught up to the normal curve immediately after bloom. The smaller puck resistance even after 30 seconds could be caused either by variations in my puck prep, or by the fact that the coffee puck did not compress as much in the latter case. We might also be seeing a slight decrease of 20% in the red curve during the shot that could be explained by a decrease in the viscosity of the slurry.

I know this was a wordy explanation of the DE1 graphs, but I think it will really help us better interpret the results of this experiment, as well as future ones. Now, here’s what I obtained when comparing all of the “surface WDT” or “deep WDT” shots without paper filter:

Combined DE1 graphs for the “surface WDT” shots (top panel), and “deep WDT” shots (bottom panel), both without paper filters.

The top panel groups all of the shots where I used the “surface WDT” method on top of each other, and the bottom panel groups all those where I used the “deep WDT” method. You can see that the exact moment where the preinfusion ends varies a bit from shot to shot; this is related to how fast the pressure ramps up to 4 bar during preinfusion, and therefore probably related to slight differences in the puck’s resistance. I adjusted all resistance curves by small deviations in dose to make them most comparable to each other, but this was a small adjustment of less than 1% because all of my doses were between 17.8 and 18.0 grams. One thing is immediately obvious from the graphs above: the peak of the puck resistance (yellow curve) varies quite wildly from one shot to the next, with either method. The “deep WDT” method got me peaks in resistance that are closer together, and therefore a bit less variable, but there are still a few outliers.

Puck resistances calculated at the shower head for the shots with surface WDT (blue curves) and deep WDT (red curves). The deep WDT method seems to reduce variations in peak resistance slightly.

I made similar figures for the cases with a dry paper filter on top of the puck:

Combined DE1 graphs for the “surface WDT” shots (top panel), and “deep WDT” shots (bottom panel), both with a dry paper filter on top of the puck.

The graphs above show a very similar picture: the “deep WDT” method still seems to get me slightly more consistent peak resistances from one shot to the next, but the paper filters do not seem to clearly help repeatability.

We can compare the exact value of the peak puck resistance as measured from the top of the puck (yellow curve):

In this graph, I show each shot as a filled grey circle, grouped by puck preparation method. The red circle and vertical bars represent the average and standard deviation for each method, and the blue circle and vertical bars represent the median and median absolute deviations, which are similar concepts to the red circles, except that they are less affected by outliers. There are only a few clear things that seem to be happening here. First, “deep WDT” helps reduce the spread in peak resistance, and perhaps increases the average peak resistance a bit, although this effect is quite diluted by variations in my puck preparation technique. Second, the paper filters don’t clearly improve the situation, but they also seem to increase the overall puck resistance by about 7%. This is not too surprising, because the filter acts as an additional percolation layer, with its own hydraulic resistance that will contribute to increase the global resistance just a bit. In the last two groups to the right, I divided the resistances of the “paper filter” cases by 7% and combined them with the “no-filter” cases, to show all of the “surface WDT” and “deep WDT” shots together, whether I used a paper filter or not. Yet again, we see the effect of the drastically reduced median absolute deviation (blue vertical bar), and the slight decrease in standard deviation (red vertical bar). This tells us that the majority of shots are much better grouped together in terms of peak resistance, but that there are still a few shots that were very different.

Viewing the DE1 profiles for all “surface” vs “deep” WDT methods, regardless of paper filters, is also visually compelling:

Combined DE1 graphs for the “surface WDT” shots (top panel), and “deep WDT” shots (bottom panel), combining all shots regardless of whether a paper filter was used or not.

One thing I find particularly interesting about these data is that all of the outliers with the “deep WDT” method—about 15% to 25% of my shots—seem to be chokers (high resistance), not gushers (low resistance). Intuitively, I expect that mistakes in puck preparation will tend to leave low-density regions in the puck, or worse, small channels, which would both favor the occurrence of gushers. But something else seems to be happening here, that causes these outliers. I have two hypotheses for this: (1) my distribution of coffee grounds in the portafilter is generally really bad, and there were only a few cases where I had a very good puck preparation, i.e. the high-resistance cases above were the only great shots; or (2) another random but relatively rare process causes the puck resistance to go up—possibly some larger coffee particles clogging one or more holes in the espresso basket, or some coffee oil that I did not clean up perfectly from the last shot reduced the effective sizes of some of these holes. If the latter explains my observations, then adding a paper filter at the bottom of the puck should help reduce this effect, because blocking any portafilter hole would not affect the hydraulic resistance of the full system as much with a paper filter above it. Water could still go around the clogged hole quite easily by passing through the paper filer.

However, I have a slightly frightening suspicion that the true explanation is my terrible repeatability at distributing the coffee grounds in the portafilter. The reason is that I noted in my logs that some of these shots had a particularly beautiful even appearance of drips at the bottom of the portafilter holes, and a very clean and even flow of water, and they tend to correspond to the shots that had the highest peak resistances in the graph above. The more frightening aspect of this is that all 24 shots from the experiment looked ok visually; we are talking about minor visual differences here. If this is the explanation for the varied puck resistances, then it would mean puck preparation is absolutely unforgiving in terms of how much the peak resistance varies from one shot to the next. While deep WDT seems to improve the general picture by eliminating most of the worst gushers, it did not seem to improve the rate at which I obtained these higher-resistance shots. This would mean that doing deep WDT helps us to avoid the worst shots, but also that no style of WDT really fixes underlying problems in ground distribution during grinding. This also calls for future experiments to test more methods of ground distribution.

Another fact that hints at an imperfect distribution is the following: all of the shots that I pulled for this experiment, including those with a paper filter at the top, yielded spent pucks that were slightly more hollow near the center:

All the spent pucks from today’s experiment had a slight hollow near the center, indicating that water flowed unevenly through the puck.

The fact that this happened even with a paper filter at the top suggests that this was not caused by the shower head damaging the top of the puck. Rather, it might indicate that the way I distribute grounds onto the portafilter with the circular motion leaves a lower density near the center of the puck. I did a quick test of this by grinding one more shot as a mound into the portafilter and then using the deep WDT method, and the spent puck was indeed much less hollow at the center. Another way to completely fix this problem is to add a paper filter below the puck.

Now, the wide variations in peak resistance that I observed above are not particularly great news for the adaptive profile that I created for the DE1 in my last post. These adaptive profiles try to go around imperfect dial-in in grind size by adopting whatever flow occurs at peak pressure, which means that they rely on the peak resistance to decide what is the best flow for the rest of the shot. In other words, all of the shots in this experiment, which used the exact same grind size, would have yielded flow rates that varied between 1 mL/s and 2 mL/s, and generated quite different styles of beverages. A straight flow-controlled shot with a pre-determined flow rate would be even worse, however; these variations in bed resistance would be reflected in variations twice as large in pressure, and would have caused some of these shots to choke or stay at extremely low pressures, both of which seem to yield quite bad-tasting shots, instead of just different styles of shots.

More typical profiles like the “Best Pressure Profile” seem more forgiving in that regard, and this is because even though they start at wildly different flow rates, they seem to converge to more similar flow rates near the end of the shots, as the puck resistances converge to values which do not vary as much from shot to shot. This convergence effect means that the average flow during these pressure-profile shots do not vary as much, and the shots probably taste more similar as a consequence. I have indeed tasted most of these 24 shots above, and while some were definitely better than others, they mostly did not taste wildly different.

One way we can better characterize this convergence of puck resistance is by looking at the stabilized value near the end of the shot. To do this consistently across all shots, I chose the moment where any shot reached a beverage weight of exactly 40.0 grams, and compiled the puck resistance at that moment. You can see that the results, below, show much less variations, and also less dependence on the puck preparation technique:

The figure above lists the end-of-shot resistances as calculated from the yellow curves (based on the flow rate at the shower head). I made a similar figure by using instead the probably more-reliable puck resistances calculated from the Acaia Lunar scale:

Here, we seem to observe a slight improvement in repeatability when using both a top paper filter and the deep WDT method.

I believe that these data are all a strong indication that the standard DE1 preinfusion is insufficient to reach saturation of the puck before we actually start extraction. I think this is the cause for less overall repeatable shots, and probably for much less even extractions as well. This potentially has even more impact on my adaptive profiles because they rely on a moment where the puck is not yet saturated to determine what the optimum flow rate should be for a given shot.

I think that repeating this experiment with a short pause after preinfusion, or even a blooming profile that then mimics the pressure curve of the “best pressure profile”, would yield much more consistent shots.

So, what are my conclusions this unusually long post, even by my standards?

  • My shots show variations of about 5% in terms of puck resistance near the end of the shot. This is comparable to the kind of resistance change one would expect from adding or removing 1 gram from a 18.0 grams dose. I do not know yet what this corresponds to in terms of grind size, but it indicates that I have to be careful in concluding that any small changes in puck resistance was caused by something else than random variations in my puck preparation.
  • When I use the surface WDT method, my shots show significant variations in peak resistance, by about 40%. This means that using surface WDT will cause wide variations in flow rate when using the adaptive profiles. As a consequence, I will keep using the “deep WDT” method for now.
  • Using a paper filter on top of the puck does not seem to significantly improve the repeatability of my shots, nor does it improve the hollows that often form at the center of my spent pucks. It is still possible that the top paper filter improves the average extraction yield and/or the flavour, so I have not yet made up my mind about whether I should keep using the top paper filter.
  • This experiment taught me that I need to keep working on how I distribute the coffee in my portafilter while grinding. As a consequence, I will keep experimenting with other methods in the near future.
  • This experiment tells me that current preinfusion steps in most of the DE1 profiles are insufficient to fully saturate the puck before extraction. I will therefore experiment a bit to determine how I can improve preinfusion and I will revamp the adaptive profiles very soon.

For those who are interested, I made my log publicly available here for the 24 shots used in this experiment. I also zipped all of the DE1 data files from my 24 shots with the “best pressure profile” here. I also generated 7 more figures to diagnose my data and make sure that no further systematics affected my peak resistance curves; for instance to ensure that variations in preinfusion temperature preinfusion duration or the shot order did not cause issues. I made those figure available here for the more technically minded readers. I also posted them at the end of this post below.

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.

An Espresso Profile that Adapts to your Grind Size

OK, this will seem sudden to almost everyone, and even more so to readers on my regular blog, but I decided to break out the chronology of my next couple posts to talk to you about something that excites me a lot. And from that break in chronology comes an announcement that I would normally have made in an earlier post: I am now the happy owner of a DE1 Decent espresso machine, thanks to my Patreon followers who amazingly already backed me up to this level of coffee geekery equipment. I never expected this to happen this fast. I received the machine just a bit more than a month ago, and it introduced significant chaos into my coffee habits, thoughts and plans. Positive chaos, however. For those who do not know, this machine might be better described as a computer filled with sensors that drives hot water through your coffee. It a weird, and delightful concept. If even an espresso machine was suited to me and my almost unhealthy level of coffee analysis, this is the one.

In just a month of having used the DE1, I think my understanding of percolation has solidified a lot. I am still creeping out of novice territory in terms of the mechanics of actually preparing espresso and tasting it, but this thing is almost live having X-ray vision into the portafilter if you think hard about the graphs it shows you and have the right tools to interpret them (and the obsession to dream about these graphs). One of the frustrations I encountered, however, relates to preparing espresso in general: the amount of coffee that needs to be wasted while adjusting grind size, dose, and perfecting puck preparation techniques can be significant before we obtain a great-tasting, “dialled-in” shot.

This is particularly true when using the DE1 machine in flow mode, i.e., where the user decides on the desired flow rate (or, more generally, a flow profile) instead of a pressure profile. In principle, a flow mode is great because it can prevent runaway effects where the flow becomes too fast near the end of a shot, by instead automatically adapting the pressure profile of the machine to keep a flow of water that is constant in time. It can also reduce the impacts of channels where too much water takes the same path of the coffee puck, by reducing the pump pressure when this happens.

A DE1 shot in flow mode. The green curve is pressure, and the thick blue curve is flow rate (both with the same axes, in units of bar or mL/s). The dotted blue curve is the cumulated volume of water dispensed by the machine, and the harder-to-spot dashed blue line is the flow rate targeted by the machine at each moment. This shot peaked at 8 bars, which is in the range typical of a nice espresso shot.
A DE1 shot in flow mode that was ground too fine. The curves use the same definitions as above (the yellow curve is an estimate of the puck’s hydraulic resistance). The pressure reached very high values here, causing a high level of puck compression that prevents a good, even percolation. This shot tasted harsh, muted, and bitter.

In practice, however, it is harder to dial in a shot when using a flow profile. This is true because the pressure can react very strongly to small changes in coffee bed resistance (i.e., grind size, puck prep, and other factors) when the machine requires a fixed flow rate. Just to clear out any misconception, the flow rate I am talking about here is actually the volume of water per second that the machine is sending into the coffee puck. At equilibrium (after the coffee puck saturates completely), this will be the same as the drip rate out of the coffee puck because water cannot be compressed at 9 bar (very far from it). Bear in mind this is not exactly the same as the microscopic velocity of water around coffee particles.

It might seem surprising that pressure reacts so strongly to changes in bed resistance if you are familiar with Darcy’s law, which dictates that the drip rate of a fluid depends on the pressure drop across a percolation medium, in a linear way, which means that if you double the pressure, the drip rate should also double. However, Darcy’s law make numerous approximations in order to come to this conclusion, and one of them completely fails in the context of espresso making: Darcy’s law assumes that the medium of percolation is fixed, and immovable. This is wrong in many ways when we pull an espresso shot. As you increase the pressure, the puck compresses and becomes shallower (this, taken alone, would increase the drip rate), and some coffee powder gets detached and moves around through the coffee bed (the drip rate would increase when fines are liberated, then decrease if they get captured further down in the coffee bed or a paper filter). But even more importantly in this situation, the coffee particles getting compacted closer together means that the sizes of pores between the coffee particles become smaller. In technical terms, the porosity of the coffee bed becomes smaller, and this reduces the drip rate.

Looking at the classical form of Darcy’s law can be insightful for this:

In the equation above, Q is the drip rate, L is the thickness of the coffee bed, A is the cross-sectional are of the coffee puck, μ is the viscosity of the slurry, k is the intrinsic permeability of the coffee bed (how easily it lets fluids through, i.e. the reverse of resistance), a property that depends on particle sizes, shapes, roughness, and pore sizes, and Δp is the pressure differential across the puck that is applied by the espresso machine. The considerations that I mentioned earlier mean that both L and k are in reality functions of the pressure drop, and k is even a function of time while fines are still moving around. The exact ways in which L and k depend on pressure are relatively complex and not too important here, but in practice, the Decent users community has noticed that the combined effect of all these phenomena is a drip rate that depends roughly on the square root of the pressure drop:

This seems to hold as long as the pressure does not go above 10 bar, where the drip rate becomes extremely small even at high pressures. John Buckman at Decent calls this a “secondary puck compression”, which may be caused by the cellulose of coffee particles deforming and blocking any remaining pores much more efficiently. We try to avoid going above 10 bar because of this reason; it makes it very hard to manage the flow rate efficiently when it happens, and the pressure tends to peak very high and the espresso shots stall completely. If you play a bit more with Darcy’s law and split out the coffee bed’s permeability k into its grind size and porosity components, you would end up with the pressure drop Δp going with the squared power of the parts of the puck’s resistance that have to do with grind size.

I know it is popular to suggest using the Darcy-Weisbach equation whenever the assumptions of Darcy’s law fail, but Darcy-Weisbach is a slightly more general equation that accounts for friction in turbulent fluids, and rule-of-thumb calculations as well as computer simulations by Ellero & Navarini (2019; for those well versed in hydrodynamics, the microscopic Reynolds number associated with water flowing inside the coffee puck is in the range 2—12, far from the thousands required for turbulent flow) indicate that the flow of water inside a typical espresso coffee puck never comes any close to the velocities required for turbulence. Hence, Darcy-Weisbach is of no help in espresso making, perhaps unless a gigantic channel happens that would leave a visible, large hole in the spent puck.

Pardon me for this relatively technical detour—all this was mostly useful to point out that the pressure curve of the DE1 set in “flow mode” will react to the square of any change in puck resistance to keep the drip rate fixed. In other words, a coffee bed twice as more resistant will require 4 times as much pressure. As you can imagine, this volatility in the pressure curve makes it hard to obtain “standard” espresso shots at about 9 bar, and especially in avoiding a pressure that goes to 10 bar and up.

As a consequence of this, several DE1 users usually adopt a more easily approachable pressure profile when they pull espresso shots. One very popular pressure-based profile is to ramp up to about 8—9 bar right after preinfusion, and then let the pressure fall down gently as the resistance of the coffee puck gradually reduces. This reduction in puck resistance is pretty much universal, although not all coffee pucks change at the same speed. I’ll come back to this in a future post, but I believe this is mostly caused by fines getting detached and reaching the cup of coffee, and probably also due in part to the viscosity of the slurry going down as solubles get depleted and also carried in the coffee cup. Thus, slowly reducing the pressure will maintain a more constant drip rate through the espresso shot, and it seems that this is generally preferred in the coffee community. I do not know why a constant flow rate may lead to better taste, and I am definitely interested to hear if some of you have hypotheses to explain it.

If you still follow me, you will notice that neither of these two types of profiles are “ideal”—flow profiles can cause volatility in the pressure curve unless they are perfectly dialed in, and pressure profiles can lead to changes in flow rate if the rate of decrease in pressure does not match the rate of decrease in puck resistance.

A DE1 shot in pressure mode that went relatively well. The exact shape of the pressure decline did not match how the puck resistance went down exactly, and as a consequence the flow rate got a bit faster near the end of the shot, which is not ideal for taste according to Scott Rao. My (very) limited experience seems to match with this.

With all of this in mind, and after reading John Buckman’s reflections on the three (or for) “mothers” of good espresso recipes, and starting to use Rao’s Allongé profile on the DE1, I had a realization: The Rao Allongé is intended for the pressure to peak in the same range as an espresso shot (8—9 bar), with the same goal of a constant (but faster) flow rate after preinfusion that leads to a slow decrease in pressure, and a similar typical brew time around 30 seconds. The only differences are: a coarser grind, a longer ratio (more water per grams of dry coffee), and a faster flow rate. At first, this seemed a bit arbitrary to me, but then I realized that the Allongés seemed to taste noticeably better when I managed to get them to peak near 9 bar — they are built as flow profiles on the DE1, so it requires a bit of fine-tuning of the grind size too.

A DE1 shot of a Rao Allongé, recently shared by John Buckman on Twitter. Notice how similar this looks compared with an espresso shot in flow mode, except for the faster flow rate. The pressure still beaks near 9 bar, and decreases as the puck resistance also decreases. The extraction even lasts for a similar time, despite the much coarser grind size!

I can’t say I’m sure about this, but this seems to indicate something that might be fundamental about preparing coffee beverages with percolation and the use of pressure. Maybe we just want to get a flow rate that does not decrease during the extraction phase, and get the fastest flow rate that won’t enter this problematic regime where the pores of the coffee bed start closing up beyond 10 bar of pressure. I could see how a flow rate increasing during the extraction would be bad: this would cause a more potent extraction toward the end of the brew, where most of the solubles were already removed anyway, and this might be efficient at extracting the larger molecules that do not taste as good, or maybe even some cellulose that make up the walls of the coffee cells in the most extreme case. Maybe at some point in the future we may realize that decreasing drip rates near the end of the extraction are also good (or perhaps even better), but currently no one seems to be exploring this possibility, for an obvious reason: a coffee bed’s hydraulic resistance usually goes down during the extraction, so unless special care is taken, the drip rate will tend to go up, not down as this happens.

If that intuition turns out to be right, it begs another question: why would only typical espressos at 1:2 (1 part dry coffee to 2 parts water) or 1:3 ratios, and then the Rao Allongés at 1:5 to 1:6 ratios taste good? I don’t see an obvious reason to believe that there is nothing else good-tasting in between, or even on either sides of these two possibilities. Taste is obviously subjective, and the perception of taste is well known to depend a lot on concentrations, but what if those are just two recipes along a continuum of good-tasting recipes that peak near 9 bar and maintain a constant drip rate throughout the extraction?

This question led me to think about another way in which we could program the DE1 to achieve good extractions. If what really matters is peaking the pressure near 9 bar and then maintain a constant drip rate of whatever the drip rate happens to be at the moment where the pressure hits 9 bar, why not ask the machine to do just that? Apart from slight differences in preinfusion, this same profile would yield a “flow profile” espresso shot with a fine grind, and a Rao Allongé profile with an appropriately coarser grind. But then… it would also potentially yield good-tasting beverages anywhere in between, assuming that the range in beverage concentrations is also pleasing to the barista.

A visual representation of 1:2 or 1:3 ratio espresso shots and the Rao Allongé, along with a possible family of good-tasting recipes along the blue line. I am hoping that dialing in with the new profiles proposed here will move us along the good-tasting recipes (purple arrows) rather than outside of them (yellow arrows) and toward the less good-tasting regions.

If this holds to be true, it would be even better than it sounds. This would not just open up new interesting beverages, but it would also mean that getting the grind size wrong would yield a good-tasting beverage anyway. Imagine you use such a profile and get the grind size grossly wrong and grind way too coarse, and end up with a good-tasting Rao Allongé. That may not be what you were trying to achieve, but if it tastes good that would already be a start! And now imagine you just get the grind size wrong by 5-10 microns in burr spacing. Instead of getting a shot that peaks at the wrong pressure, or where the drip rate craps out completely and generates a sour or bitter brew, what you would get is a slightly different beverage type, still along the “family” of good-tasting recipes, without all the extraction defects that follow an uneven or otherwise problematic percolation.

As you can probably tell, I’m excited about this possibility. I would love to stop worrying about pulling a shot with a bag of coffee I only have 100 grams of. And I would also love to waste zero grams of good-tasting coffee regardless of the bag size.

In practice, there is still one hurdle in getting the Decent to behave like this (i.e., peak at 9 bar then keep whatever drip rate at that moment constant). There is currently no way to specify a desired drip rate on what is currently being measuring during a shot. When I asked John Buckman about this possibility, he mentioned that they plan to design a whole programming language surrounding the construction of DE1 profiles, and that this would be a great application for it. I was happy to hear this, and then in the following days I kept realizing even more points about how cool such a profile might be. This made me so excited that I decided to try and find a way to bend the current Decent software into doing it anyway, even if it wouldn’t be perfect.

After a bit of fiddling, I realized that there is actually one way to achieve such a profile with the current DE1 software. This method is limited by the finite amount of steps (20) that the DE1 will accept to execute during a single shot in advanced profile mode. Any well-behaved profile should indeed fit within 20 steps, but this one does not belong to this category, because it is trying to work around the current capabilities of the machine. Here’s the idea:

  1. Execute a normal preinfusion. For current DE1 “flow profile” shots, this is a flow profile step at 3.5 mL/s that triggers an exit to the next step whenever the pressuredrop reaches 4 bar, indicating that the puck is saturated with water or close to it.
  2. Rise the pressure and hold it at 8.6 bar for 4 seconds, as one would usually do at the start of a pressure profile on the DE1.
  3. Now, the weird part: Attempt to start a long flow profile step at a flow too fast for typical espresso, in this case 3.5 mL/s, with a trigger that exists the step if the flow ever reaches a value slower than 3.4 mL/s.
  4. Follow with a very similar step that attempts to establish slightly slower, 3.3 mL/s flow profile step that triggers out if the flow reaches 3.2 mL/s.
  5. Keep adding steps like this until you reach step #19, with a value of flow rate that would typically be too slow for espresso — in this case, 1.0 mL/s, and that triggers a skip whenever the flow rate reaches 0.9 mL/s.
  6. Add a final flow profile step that is even slower, in this case 0.5 mL/s, without any trigger for skipping that last step.
Standard preinfusion and pressure rise steps in the new “adaptive” profiles I’m proposing
Subsequent steps that scan for the current flow rate in the“adaptive” profiles

It may seem like every one of these steps starting from the third one are self-contradictory, but in practice this serves a purpose. Imagine you grind a bit too fine, and instead of reaching your goal of, say, a 2.2 mL/s flow rate during the extraction phase, you actually reach 1.8 mL/s. What will happen ? Well, the machine will peak at 9 bar because you asked it to, the flow rate will reach 1.8 mL/s, and then the machine will attempt to run step number 3, and it will ask the machine to up the flow rate to 3.5 mL/s while immediately starting to check that your flow rate is at least 3.4 mL/s. This happens so fast that the pumps won’t even have any time to react and the software will have triggered out of steps 4, 5, 6 and so on until the exit condition is not triggered anymore: this will happen at whatever step has an exit trigger slightly below your current flow rate (1.8 mL/s). In practice, this will happen at whatever step is asking the machine to reach a flow rate pretty close to 1.8 mL/s, and there you have it: you effectively asked the machine to keep whatever flow rate was currently going on.

I was honestly skeptical this would work because it is so twisted, but to my delight it worked exactly as I wanted! Now, the limit of 20 steps required a careful selection of what flow rates we want to test during steps 3 to 19. I asked some long-time Decent users (thanks Scott Rao and Stéphane Ribes) and looked a bit on the Decent user forum for what ranges of flow rates people are usually pulling their shots at. This depends a bit on roast date, and on the desired ratio (as you might expect from the discussion above), but it mostly seems to happen between 2.0 and 2.7 mL/s. For this reason, I decided to scan this range with steps of 0.1 mL/s, and also scan a wider range (0.5 to 3.5 mL/s) with more spacing between the individual steps. With the more flexible coding capabilities of the future Decent software, the machine will be able to keep exactly the flow rate you had whatever its value is, but right now this will cause the machine to adopt something close, unless you are so far from dial in that your flow rate is outside of these bounds a the moment where the pressure profile peaks.

If you think the profile seems complicated, you are right that the means to get there were complicated, but in reality using the profile should not be more complicated than any other ones, and hopefully it will be simpler because it might taste good in a wider range of grind sizes. Just view it like this: the preinfusion and the initial pressure peak are fixed, and the shape of the flow profile is also fixed. All that changes is how fast the flow rate is, starting at the pressure peak. You are dialing in the fixed flow rate with your grind size. In principle, you might be able to just pull shots and stop them at about 26—30 seconds, and then if your flow rate was a bit faster than your usual goal (typically 2.2 mL/s for light roasts), grind a bit finer next time, and vice versa. Hopefully, you’ll be able to enjoy that brew even if it had a flow rate that is slightly wrong. Now, what happens if you grind WAY too coarse ? You’ll get a sudden jump to the fastest flow rate (3.5 mL/s) after the pressure peaks, accompanied by a sudden drop in pressure. In the reverse, if you went way too fine, the pressure will suddenly jump after the ~9 bar peak and flow rate may struggle to reach even the minimum 0.5 mL/s allowed by this profile. Maybe you will even reach the forsaken range above 10 bar where the shot goes to hell. But hey, what other profiles can fix shots that far from dial-in right now ?

I have only tried this on a few shots so far, and the results seem to be promising. I have only been making and tasting espresso for a bit more than a month now, so I don’t put too much trust on my espresso palate. I also wouldn’t trust my puck prep except that I received a significant amount of help from Scott Rao to the point where I think it should now be acceptable. In short, I’d now like to share this profile to see what others think about it. If there’s something wrong about the many assumptions above, then the profile will just die out.

But otherwise, maybe it will help us waste less coffee and explore more recipes, which would be lovely. I also think that, still assuming I’m not suffering from confirmation bias when tasting my shots, there might be ways in which we can improve it in the future.

The first shot I pulled of an Ethiopian coffee bean ever with the Decent was using a prototype of the adaptive profile described here. I knew Ethiopians need coarser grinds, but I decided not to move my grind size and the shot was indeed slower than usual (notice the ~1.5 mL/s flow rate). But also notice how the pressure peaked nicely at 8.6 bar and the flow rate remained perfectly constant. The shot tasted really nice for such a bad dial in, at least according to my limited experience with espresso.

Maybe a linear decrease in flow rate over the shot will taste better? Maybe current preinfusion can also be improved, as pointed out by Stéphane Ribes in one of his amazing reports available on the users-only Decent forum? Maybe we can have a version that automatically stop based on a set shot time, capturing the fact that coarser grinds seem to do better with larger ratios, weaker concentrations, and similar shot times? We’ll see!

While I was at it, I also built two more profiles that I like, both built by Scott Rao on the DE1. The blooming shot is very similar except for a long pause between the preinfusion and the extraction phase (allowing one to reach shockingly high average extraction yields), and the Rao Allongé. Yes, yes, the recipe that “might unite standard shots and Rao Allongés” has a version for either of those two, but that’s only a temporary limitation because it would be impractical to scan all the flow rates typical of espresso shots, as well as those typical of Rao Allongés (~ 4 mL/s) with the limit of 20 steps currently required by the DE1.

I’ll let you know more about what I think when I’ve tried the profiles more extensively, and I’ll also talk about how others have liked it.

You can find the three profiles I’m proposing here:

Adaptive Shot—Intended for 1:2 to 1:3 espresso

Adaptive Blooming Shot—A modified version of Scott Rao’s Blooming Shot

Adaptive Rao Allongé—Intended for 1:5 to 1:6 Rao Allongés

Please note they will probably change in the future! Also note that the profiles above are all intended to be manually stopped after about 26-40 seconds. Do not let them run in their entirety, otherwise you will get extremely long shots that will probably be weak and over-extracted.

Disclaimer: I receive no financial benefits from Decent Espresso machines, and I have no business ties to them. Decent Espresso generously offered me a 25% discount on the machine. All my impressions of the machine are therefore my own and never financially motivated (as goes with all my past posts, and all products so far).

Brewing with the Fellow Stagg [X] Dripper

Photo by Noé Aubin-Cadot.

Reminder: as an Amazon Associate I earn small commissions from qualifying purchases made through the Amazon links below. I have no association to Fellow and don’t receive any benefits from posting this.

If you are a regular reader, you might have seen me writing a bit about the Fellow Stagg drippers. Six months ago, Fellow offered me their Stagg EKG 0.6L kettle for me to include it in my analysis of kettle streams, and they included an unexpected bonus in the package: their Stagg [XF] dripper. I had been brewing only V60s for more than a year at that point, and I did not really seek to try other dripper geometries. However, the Stagg immediately intrigued me with its design. For starters, it is a flat-bottom dripper, and comes with prepleated filters similar to Kalita filters, but produced by Fellow.

A photo of a prepleated filter in the Stagg [X] brewer.

As you probably know, I like to swirl the dripper between every kettle pour, and the base design of the Stagg drippers doesn’t make this easy. Fortunately, the rubber ring at the bottom of the dripper can be taken off easily, but even without it, swirling is a bit awkward because the dripper can capsize. I found that the base of my Olivewood Hario V60 worked perfectly if I kept the rubber ring on and placed the dripper on the Olivewood base:

I know the Olivewood is quite expensive especially if you are just going to use the base, but I don’t know about a cheaper option currently. I’m sure some of you could fix this with 3D printers!

Despite these minor hurdles, I was immediately pleased when I first brewed with the Stagg. My brews seemed noticeably sweeter than those I made with the V60, using the same coffee and grind size. One thing I did not like about the Stagg [XF] is that I needed to use a lot more water to pre-rinse the filters because they are a lot taller. They are so tall that not pre-rinsing them carefully or with enough water caused them to leave a slight papery taste in the cup. This is the only time I have ever noticed this with bleached filters, but they also weigh a lot more than typical pourover filters. Quite surprisingly, they did not impart a bad taste at all, but I prefer my coffee untainted. The Stagg [XF] dripper is also quite tall and narrow, and this makes it harder to control how and where your kettle stream lands on the slurry. I mentioned that to Fellow, and they immediately sent me their Stagg [X]!

Turns out not only I preferred the Stagg [X] over the [XF], I preferred it so much over the V60 that I only alternated between the two drippers for a dozen brews, and then I switched to using exclusively the Stagg [X] ever since. Six months and more than 400 brews later, I’m finally ready to write about this.

The reason why I prefer the Stagg [X] brews over the V60 is simple: I prefer the taste of the coffee it brews. As I mentioned, I find it sweeter and less sour. I think this is caused by two things: a higher slurry temperature due to the better thermal insulation, and a more even flow through the full coffee bed that leads to a more even extraction. I tested the slurry temperature with my K-type Thermoworks temperature logger, by placing 100 mL of hot water in different drippers and observing how the temperature evolves when the flow holes are blocked:

As the data show, the Stagg [X] slurry was indeed slightly warmer and more stable than the V60 slurry. I tried comparing the Stagg [X] to the V60 with the exact same parameters, namely a 25.0 grams dose, a 1:17 ratio, 4 pours with the exact same timing and flow rate, the same Hario tabless V60 filters and ColorFull Coffee Corp‘s delicious Sun Blood decaf. I used a 210°F (99°C) kettle temperature with the Plastic V60, and a 202°F (94°C) kettle temperature with the Stagg [X], as previous experiments showed that this was the way to obtain an average slurry temperature of 191°F (88°C) during the brew, in both cases. I measured the concentration of the brews using a VST refractometer and found 1.34% TDS for both brews, corresponding to an average extraction yield of exactly 19.9% in both cases (keep in mind that this is not low for a decaffeinated coffee). The V60 brew had quite a longer brew time, however, clocking in at 5:40 compared with 5:01 for the Stagg [X]. This longer brew time is not too unexpected, as the V shape of the V60 makes the same dose of coffee taller. The resulting brews tasted quite different: the V60 brew had almost no sweetness and had a bit of an ashy aftertaste probably due to some regions of the coffee bed getting over extracted, and the Stagg [X] brew was jammy, sweet and fruity. This comparison is interesting to me, because it means there is something more to just a better thermal insulation with the Stagg [X], despite the two brews having the same average extraction yield. I suspect that figuring out what coffee dose gets me the same shorter brew time with the V60 would reveal it to have a slightly lower average extraction yield.

This was good timing, because I was thinking a lot about dripper geometries to write the percolation chapter of my upcoming coffee book The Physics of Filter Coffee, and I was gradually discovering that water does not flow at all like I expected or wished for when brewing with the V60. By simultaneously measuring my pour rate and the drip rate of the V60 with two scales connected to my friend Francisco’s web app, I was able to deduce the height of the water in the dripper at every moment of the brew, as well as the hydraulic resistance of the whole dripper, filter and coffee bed system. Hydraulic resistance is a quantity that represents how much the combined effects of the dripper, filter and coffee bed are preventing the water from flowing down very fast. Without going too deep into the weeds (this is what the book will be for!), these data really convinced me without a doubt that some significant amount of water was bypassing the coffee bed, i.e. flowing around it rather than through the coffee.

This effect is usually called bypass, and has been discussed a lot in the coffee community. I was previously skeptical about it and I thought that very little water bypassed around the V60 bed, because I had never seen hard data to support the idea. I don’t blame the coffee community for that, because it is not easy to measure this effect accurately. Some recommend to always pour at the center of the slurry to prevent bypass, but this is like deciding to fight against the ocean by punching it. Regardless of where you pour water, it will find the path of least resistance and flow mostly there. Pouring at the center of the slurry and hoping to avoid bypass is thus a lot like pouring at the center of a dining plate to prevent water from spilling out. As soon as the water level gets high enough, it will flow around the plate as much as it likes. The same is true with the V60, and most other drippers. If the level of water becomes high enough that it touches some free paper filter that does not stick closely against a dripper wall, some of this water will go through the filter, regardless of where you pour.

Bypass is not necessarily a terrible thing when water flows completely around the coffee bed: it will just make your beverage more diluted. You would need to pour more water to reach the same average extraction yield compared with a brew where water does not bypass, and the resulting cup would thus be less concentrated. The part that I find desirable at all is where water can also flow through only part of the coffee bed, and then escape midway. When this happens, less water will reach the bottom of the coffee bed, and the bottom will remain less extracted than the surface layers of the coffee bed. This is where dripper geometry becomes important: a conical dripper will allow water to escape from the middle of the coffee bed, whereas a cylindrical dripper won’t allow this as easily.

You might think that a solution against this problem would be to agitate the coffee bed very deeply in a conical dripper, to make sure that the bottom is well extracted. You wouldn’t be wrong at all, but this brings out other complications: such deep agitation will allow very fine coffee powder to be dislodged more easily from the larger coffee particles, and this migration of the fines can completely clog the paper filter. Because clogging tends to leave very few spots of lower resistance in the filter, a large fraction of the water will be forced to flow there, and this uneven flow will usually cause small regions of the coffee bed to become over-extracted, and will result in harsher and more astringent brews. I suspect this might also affect brew clarity, for the same reason, although I have not verified this with measurements yet.

Using smaller pours is yet another strategy to minimize bypass. This will definitely minimize the amount of water that completely bypasses around the coffee bed, but it won’t prevent water from bypassing midway through the coffee bed. This strategy will also lower the slurry temperature, and lead to lower average extraction yields and different flavor profiles that I tend to like less.

Another solution could be to remove the ridges in a conical brewer, much like the upper half of the Kono Meimon dripper. However, this creates another issue: water will only flow through a small opening near the bottom of the dripper, which means that any coffee fines liberated with the flow will be trapped in a smaller surface area of paper filter. This will make the dripper more susceptible to clogging, as I quickly discovered when my friend Dan Eils sent me a 3D-printed ridgeless V60. If you are skeptical of this, I suggest to try an eye-opening experiment: take a glass V60 dripper that does not have a base at the bottom, and place a paper filter at the bottom of the opening outside the dripper. Use an elastic to hold it in place, then try brewing coffee with this. You will experience a surprisingly slow flow, if not an immediate clogging of the filter. This is a nice demonstration of how the total surface area of paper filter really matters for clogging. Remember that only the filter surface that is not pressed against a solid surface counts, because water cannot go through the filter efficiently elsewhere.

Placing a filter outside the bottom of a V60 will cause it to clog instantly.

In other words, the V60 sacrifices some evenness of extraction (the bottom of the coffee bed is harder to extract well), in exchange for a more easily maintained flow of water and less clogging. This is true because it has a very large free filter area compared to the volume of the coffee bed. Flat-bottom drippers face the challenge of having less free filter surface for each gram of coffee.

If you think about it, you will realize that this is also why both the Kalita and the Stagg have ribs at the bottom. They are there to lift the filter, and prevent it from collapsing into the dripper holes. If the filter does collapse, water can only flow through a minuscule surface area of free paper filter, and it will immediately clog. You might have experienced this with the Kalita: if you then just gently lift the filter, water will start flowing again because you have increased its free surface dramatically.

The Stagg dripper is definitely an improvement over the Kalita in this respect, because it has more holes and wider, more evenly distributed ridges. However, I don’t think they are deep enough: I have experienced clogging several times when I first started brewing with the Stagg. Ray Murakawa suggested placing a few whole coffee beans at the bottom of the dripper to avoid this. This solution works, but I didn’t really love it because it can easily lead to an uneven coffee bed and makes it harder to get an even flow. Scott Rao had a nice idea that turned out to work much better for me: cutting out a tea strainer mesh, and placing it at the bottom of the dripper.

Although I don’t think this is the ideal solution, it seems to be one of the best options that can be easily done at home. It completely fixed the issue of clogging for me when I used it with the prepleated Stagg filters. However, prepleated filters are also problematic in my view because they still allow for some bypass, and even probably allow for some water to escape from the middle parts of the coffee bed, although this will definitely not be as much of an issue than it is with a conical dripper. This is probably what pushed others like Scott Rao and Matt Leberman to prefer brews made with a V60 filter that is forced into the Stagg dripper after pre-wetting it.

This may look messy, but it is quite clever. It forces the filter to adhere on the walls of the dripper, thus minimizing bypass and giving you more surface for pouring and more volume for water. It’s also not the perfect solution because it will be hard to always place the filter in the exact same way, and thus may lead to different brew times, and different amounts of bypass. It also becomes important to use a tea strainer mesh, because the elimination of bypass means that the dripper will become even more susceptible to clogging. I tried to bring further improvements to the filter mesh, by poking it with a drill bit and causing it to have a “spiky” shape:

The idea behind this is to minimize the amount of contact between the mesh and the filter, and therefore increase the total free surface area of the filter. I found that it helped with flow, especially when the filter is pressed against the dripper walls. However, I did not get as many spikes as I would like, and they are not of the same exact height. This makes it a bit harder to obtain a flat coffee bed and an even flow.

If you prefer to use the Stagg filters and don’t mind the extra minute, you can place one in the dripper as usual, pre-wet it and then press each filter fold against the dripper with a finger until they all adhere to the dripper wall:

I won’t go into filter properties too much here (this will be discussed in great length in the book), but the quality of pores in the Stagg filters is not as great as those of the Hario tabless V60 filters. The V60 filters are also a lot thicker, even though the Stagg filters feel thicker because they are just more rigid. The better pores and increased thickness make the V60 filters more interesting to me, but I’m a bit bothered by the worse repeatability that is caused by the messy filter placement.

Here is the recipe that I currently follow when I brew with the Stagg [X] dripper.  I don’t think it’s the only good way to use it, but this has worked well for me over the past few months:

  • Use between 22 and 26 grams of coffee (I personally like to use 25 grams).
  • Use a coffee-to-water ratio that suits your grinder and personal preference of extraction yield versus beverage strength combination. I usually brew with 17:1.
  • I recommend making a nest shape in the dry coffee bed, and start pouring in the hole during the bloom step. This may seem counterintuitive, but I think this helps getting water through the bottom layers of the coffee bed a bit faster even with flat-bottom drippers.
  • Pour about 2.5 to 3.5 grams of water per gram of dry coffee dose for the bloom step. You want to have enough water that you can immediately give the dripper a good swirl to make the surface of the coffee bed even before all water has dripped out.
  • I use the same grind size as I would for V60, and the same pouring patterns (anything that covers the full surface, for example spirals). I also use the same pour height (just below the point where it splatters), and the same pour rate of 5-6 g/s (as described here, this depends on your kettle). Several people pointed out to me that the breakup length of a kettle is reduced when pouring hotter water. I think this is indeed true, but it doesn’t change my recommendation of pouring just below the height where it splatters.
  • I still use very hot water most of the time (210°F, 99°C). The only reason I don’t use a running boil is to avoid getting an unstable kettle stream, or even spurting. I lower my kettle temperature to 205°F (96°C) when I don’t know a roaster well, because it minimizes roastiness (burnt and bitter taste) when a coffee is roasted darker. I have gotten good results with kettle temperatures as low as 190°F (88°C) when coffee was roasted even darker, but I tend to prefer roasts that are well matched to hotter water.
  • I tend to use about 4 pours when I brew with the Stagg (not counting the bloom), rather than only two as I used to brew with the V60. The better heat insulation makes this possible, and you will be forced to do this if you use the Stagg [X] with non-folded prepleated filters because of the smaller dripper volume. I really like the brews that this produces, but it makes it a lot harder to keep track of pour sizes and the timing of pours, especially given that different coffees tend to flow at different rates. I currently start a new pour whenever the water level goes lower than about one inch above the coffee bed, and I am not particularly great at keeping track of my pour sizes, although I try to keep them around 90 grams each.
  • I swirl the dripper very gently just after every kettle pour for about half a second (I swirl the bloom more vigorously, and for 2-3 seconds).
  • I use prepleated Stagg filters as intended the first time I brew a bag of coffee, with the flat tea strainer mesh. If I don’t get very long brew times (about 3:45 or more for me, but this depends on your grinder), I will tend either press the filter folds against the dripper walls with the spiked tea strainer mesh, or use a V60 filter with the flat tea strainer mesh. Combining the V60 filter with the spiked mesh won’t help as much, because the V60 filter will collapse between the spikes anyway when it is forced inside the dripper.
  • Immediately take out the spent coffee and filter and rinse the dripper and mesh. Avoid to let the coffee steep in there because that will stain the dripper with stale coffee oils much faster.

You can find a video of an example brew I made here. This coffee was a really nice washed Ethiopian Yirgacheffe roasted by my friend Andy Kires (the roaster behind Colorfull). I used a 25.3 grams coffee dose and 430 grams of water for a 1:17 ratio, and I obtained a brew with a 1.53% TDS concentration, for an average extraction yield of 22.1%.

You might have guessed it from the brew guide above: although I prefer the taste of my Stagg [X] brews over the V60, and they tend to reach higher average extraction yields in part due to the higher achievable slurry temperature, I do not obtain the same level of repeatability as I did for the V60. As I showed in a previous post, my 2-pour V60 brews were typically repeatable within 0.2% extraction yield, but my Stagg [X] brews have a worse stability with a larger spread of about 0.3—0.4% in average extraction yield. I’m pretty sure this is mainly due to two things: it is harder to replicate the mesh and filter placement exactly, and it is harder to keep precise track of more kettle pours. This means the V60 was a great tool for experimenting, and it allowed me to generate many figures based on several hundred repeatable brews for my book. But now, I really enjoy this improved taste even at the cost of a little repeatability!

Note: Since I released this blog post on Patreon a few months ago, I have switched to using a coarser mesh at the bottom of the Stagg [X], which I made from a pasta strainer. I also made a version where I imprinted spikes by wrapping the mesh around a nail with pliers. I found that it works best with the V60 filter to ensure that you still get enough flow by minimizing the contact between even the mesh and the paper filter.

Note that a coarse but flat mesh works very well with the Stagg [X] filters too. I find this combination useful with coffees that otherwise choke the V60 filter or otherwise take a lot more than 5 minutes to brew.

I have also received several questions about how I place the V60 filters in the Stagg [X]; I now simply place the dry filter on top of the dripped (with the point of the V shape standing in at the bottom of the dripper) and then run some faucet water in the middle. The faucet pressure collapses the filter, and I help it fall down with my hand until the filter sticks to the walls well. I then press any folds to the walls to, like this:

The Effects of Varieties, Origin and Processing

Over the past year, I have logged the details of just over a thousand pour over brews that I prepared and drank. I had started doing this with several goals in mind, and one of those was to understand whether different coffee varieties behave differently when brewing pour overs. Over this period of time, I have learned how to improve my brew method to obtain more repeatable results and better quality extractions, and I also changed coffee brewing gear a couple times. I also realized several months back that the coffee concentrations (TDS) I was measuring with my reflectometer were initially not as precise as I thought, and I wrote a blog post about how to measure them better when I figured this out.

I decided to have a look at the brews where I used exactly the same coffee to water ratio (1:17), the same dripper (plastic V60), grind size, burr and grinder (the Weber Workshops EG-1 with SSP ultra low fines burrs), brew water (the Rao/Perger composition as described here) as well as method (which I describe here), with doses between 20 and 22 grams. I allowed for a small range in doses because in a conical dripper, this range of doses corresponds to a very small range of bed depth (4.65 cm to 4.80 cm, or a change of only 3%). The detailed list of all coffee gear I use can also be found here. These criteria narrowed the list down to 416 brews prepared in an identical way, but with a range of 192 distinct coffee bags.

One thing that jumps immediately when looking at the data is the wide range of average extraction yields that result from brewing different coffee beans. Even at the stages where my brews got consistent to within 0.2% in extraction yield between brews of the same beans, changing beans caused my extraction yields to vary widely across ~20% to ~23% average extraction yields. This figure shows the average and range in average extraction yields that I obtained when I was brewing coffee from a bag that mentioned each coffee variety:

I found it really interesting that the Kenyan variety SL34 comes much higher than SL28 in the figure above. We only rarely get to taste SL34 alone as it is almost always mixed with SL28, but that trend tells me that SL34 may contribute a lot more solubles because bags with both SL28 and SL34 seem to extract a lot more stuff than those with only SL28 !

As you can also see, varieties typically grown in Kenya (SL34, SL28, Ruiru 11) and Ethiopia (74110, 74112, Kurume and Wush Wush) produced the highest extraction yields, whereas those often grown in South America (Bourbon, Catuai, Pacas, Castillo) tended to yield lower ones. A particularly interesting case is that of Gesha, a variety originating from Ethiopia (as they all do in reality) that is now also grown in South America, often with a different spelling “Geisha”, and likely with a slightly different genetic makeup (Please see Getu Bekele’s excellent book “A Reference Guide to Ethiopian Coffee Varieties” for more on this). Again, the general trend where African coffees generate more solubles is still observed: Ethiopian Gesha extracts about 0.4% higher than South American Geisha on average.

These differences across origin countries can be viewed with the following graph:

Now, the explanation for these trends is unclear: it could be explained by differences in the genetics of the coffee, in how these genes get expressed across different environments, or it could also be caused by different agricultural or processing practices. I doubt that processing alone could explain the differences, as grouping the values along natural or washed coffees shows a weaker trend where washed coffees seem to extraction about 0.5% higher on average:

The possibilities that I just mentioned all assume that the different extraction yields are actually real, in the sense that African coffees bean tend to contain more soluble materials. However, there could be a different explanation; the different chemical profiles that correspond to different varietals might also be throwing off the calibrations of the VST refractometer, which uses the refraction index of the coffee and translates this into a concentration with their proprietary calibrations. I don’t know how sensitive these calibrations are on the exact chemical profile of the coffee, but I suspect there may be some variations in the sense that obtaining extremely accurate extraction yields would require using a different calibration for Ethiopian, Kenyan or Colombian coffees for example.

Probably the most interesting trend that appeared in the data is this one:

Each red circle in the figure above is a distinct bag of coffee. The symbol size is larger for points in denser regions, to give more focus on the general trend and away from lone outliers. The blue ellipses are contours of a 2D normal distribution that fits the data (at 1, 2, and 3 times the characteristic width), and the orientation of these ellipses are caused by the correlation between altitude and average extraction yield – the Pearson correlation coefficient is 25.6%, which means that the angle of the ellipses is significantly off from horizontal, as you can see.

It looks like coffees grown at higher altitudes produce higher average extraction yields ! This would favor the hypothesis that the differences are real and due at least in part to growing conditions, because there would not be any reason for this correlation between extraction yield and altitude to appear if it was purely caused by the VST refractometer calibration getting slightly thrown off by different chemical compositions. Specifically, some research by Vaast et al. (2006) and Worku et al. (2018) showed that higher-grown coffees tend to concentrate more nutrients into their seeds in harsher growing environments such as high-altitude farms. This may be an evolutionary strategy for the coffee tree to maximize the survival rate of its seeds that will face more difficulty in growing into new coffee trees.

Another effect that I find interesting is how varieties and origin countries might affect the total brew time. The reason to expect variations in brew time has to do with how brittle coffee beans are, because their brittleness will affect how much coffee fines are produced during grinding. Basically, more brittle coffees will generate more fines, and will result in longer brew times. This will likely be strongly affected by roasting style too, but the initial moisture, density and hardness of the green coffee or the bean size may all have an effect, which would introduce variations across varieties and origins.

One interesting thing about the figures above is that they confirm some well known facts about Ethiopian coffees: they seem to generate a lot more fines, and clog a lot more easily as a consequence. I often heard that Kenyans also behave this way, but my data does not seem to suggest they do, or at least much less than Ethiopians !

Differences in processing did not seem to affect brew time significantly:

But differences in growing altitude also show a strong correlation (a Pearson correlation coefficient of 36.3%), perhaps indicating that harsher environments may also affect the physical structure of the beans, potentially making the beans harder, drier and/or smaller, which will lead to more fines once they are ground:

Another possible explanation of these trends in brew time could be that it only has to do with roasting, but that these correlations were introduced because we tend to like different coffee beans roasted at different development levels on average. While this almost certainly happens to some extent (e.g., more expensive Ethiopians and Kenyans getting roasted more lightly to preserve more origin characteristics), I doubt it is the only explanation, and I suspect that either genes or growing environment also have something to do with it.

Note that I do not share my complete brew log publicly, but it is available through my Patreon page, specifically here. Keep in mind I do not share a live version even on Patreon, because it requires some data management before I share it. The current version has 636 brews and covers all of 2019, and I plan to update it with the next 400 brews in the coming month or so. 

The Physics of Kettle Streams

Reminder: as an Amazon Associate I earn small commissions from qualifying purchases made through the Amazon links below. I have no association to Hanna instruments and don’t receive any benefits from them.

For a while now, I have wanted to dig deeper into the physics of kettle streams and how they affect agitation when we brew coffee. It is currently hard to find any information about the effects of pour height and pour rate on coffee brewing, and yet in my experience it can definitely have an impact on the resulting cup of coffee. It probably comes at no surprise if I tell you that pouring way too fast can dig a hollow in the coffee bed and lead to clogging a filter, or to an unevenly extracted and astringent brew. What may surprise some readers a bit more is that measuring my pour rate and pour height to keep them stable is one of the things that really helped me achieve more repeatable brews. I discussed this a bit more in previous posts about what affects brew time and the repeatability of manual pour overs.

Apart from affecting the height of water at any given moment and therefore indirectly affecting brew time, the rate at which you pour water onto your coffee slurry (i.e., the mix of water and coffee grounds) also has an important effect on how much you agitate it, and whether you end up with a hollow in the coffee bed. But this is also linked to how high you pour from, and what kettle you are using ! More specifically, the width of the gooseneck, how smooth it is at its tip, how vertical the stream is, and how fast you move the kettle around will all change the properties of the water stream and how it interacts with the slurry.

When a stream of water leaves the gooseneck, it will usually have a relatively smooth and cylindrical shape. Any tiny imperfections at the tip of the gooseneck will still be imprinted on the shape of the stream, which initially do not show much, but as the stream falls down further, its imperfections will gradually grow in size. This is due to a property of fluids called the surface tension. Molecules of water are slightly attracted to each other because they are polar, which means they have a slightly negative electric charge near the oxygen atom, and slightly positive charges near the hydrogen atom.

Polarity of a water molecule.
Network of water molecules arranged to align the negative and positive charges forming hydrogen bonds, responsible for the cohesive nature of water.

This means that water is slightly attracted to itself, a property called cohesion. It can also be similarly attracted to other materials: we call this adhesion. Water experiences very little adhesion with ambient air, and as a consequence the surface of water can seem to behave like a stretched elastic membrane, for example by forcing the water to shape itself into droplets. This effect is what we call surface tension, and it is thus simply a consequence of water having more cohesion than it has adhesion with air.

An illustration of surface tension. Water molecules near the surface of water are attracted down by forces of cohesion (black arrows), whereas they have very little adhesion pulling them upwards toward the air. This causes water to behave as if its surface was a stretched elastic membrane. Source: Wikimedia commons.

Now back to our slightly imperfect stream of water; because of water’s surface tension, the regions where the stream has a bit more curvature will experience a slightly decreased inward force, and vice versa. This is what will slowly drive the imperfections to grow until they are noticeable by eye, and further cause the stream to break into droplets. This phenomenon is called a Plateau-Rayleigh instability, and can be observed easily by pouring water with a gooseneck kettle from high up above a sink.

An illustration of the Plateau-Rayleigh effect, where an initially smooth stream of water (top) gradually becomes more sinuous and eventually breaks into droplets. Source: NIST.

The length at which the perturbations become large enough to break the stream into droplets is typically called the breakup length of a stream of water. This length is affected both by the smoothness of the gooseneck and the pour rate. As you can imagine, a rougher gooseneck will have larger initial perturbations in the stream shape, and therefore a shorter breakup length. The effect of pour rate on breakup length is however much less straightforward; in fact, for each gooseneck type there is an “optimum” pour rate where the breakup length is very long, and pouring either slower or faster will reduce it.

Illustration of flow regimes and breakup length for gradually increasing pour rates (left to right), based on a simulation by Bertola & Brenn (2019).
Illustration of breakup length as a function of pour rate, with the associated regimes. Typical pour over kettles cannot pour fast enough to reach far into the wind induced regime, or to the atomization regime.

I decided to check this relation with the Brewista Artisan 0.6L kettle I used for the past year or so, with and without the flow restrictor I described in this earlier blog post. For a while now I have suspected that the Fellow kettles actually do better than Brewista in terms of their gooseneck smoothness, and Fellow generously offered to send me their Stagg EKG 0.6L model for me to do a direct comparison. In each case, I poured water from above a tall mason jar placed on an Acaia Pearl S in flow rate mode, starting low and then raising up the kettle while keeping the same flow rate. When I started hearing a splattering sound, this indicated that the breakup length coincided with the surface of the water in the mason jar, so I stopped and measured both the absolute height of the tip of the gooseneck, as well as the height of the water. Subtracting these two values allowed me to estimate the breakup length associated with a given pour rate. By repeating this with several different pour rates, I was able to verify the predictions based on the physics papers above, as well as measure a net improvement in breakup length for the Stagg EKG over the Brewista, as I suspected !

Measured breakup length as a function of pour rate for the Stagg EKG 0.6L kettle and the Brewista Artisan 0.6L kettle, with and without a flow restrictor. Each kettle had a pour rate that maximizes breakup length at about 8 g/s, and the Stagg EKG has a longer breakup length at most pour rates due to its gooseneck being smoother. Adding a flow restrictor to the Brewista reduced its optimum pour rate, but also reduced its longest breakup length.

Understanding breakup length is important because the smoothness of the water stream when it lands on the slurry will completely change how much, and how deeply the slurry gets agitated ! Engineers have already figured out these details for the construction of plunging jet reactors, which aim to maximize the rate of chemical reactions between a gas and fluid with a stream of fluid falling in a still pool and entraining some gas bubbles with it (for example, see Yin et al. 2018).

Illustration of a plunging jet reactor, where gas bubbles are entrained in a pool of liquid with a stream of falling fluid, to maximize the surface of contact between the gas and liquid and optimize their rate of chemical reactions.

Basically, the rougher a stream is where it lands on the slurry, the more air bubbles will be transported in the slurry (see Baylar et al. 2016). The air bubbles will cause friction with the flow of water below the surface, as the bubbles themselves are slowed down by buoyancy forces pushing them upwards. At the depth where water has slowed down enough, only buoyancy remains as an acting force on the air bubbles and they will rise back up to the surface. This rapid breaking of the underwater current can cause the flow to become turbulent, which is a good thing for coffee extraction because turbulence is great at distributing fresh water between all particles of coffee, and make sure that no one particle remains under-extracted.

If the stream of water is extremely irregular where it lands however, a lot of air bubbles will be dragged into the slurry and the breaking will happen so fast that agitation will not reach very deep down, potentially leaving the coffee bed undisturbed. At its most extreme, pouring from higher than the breakup length will cause only tiny droplets of water to fall on the slurry, and no agitation at all will be able to reach the coffee bed and make extraction more even.

Illustration of turbulent flow in the context of Rayleigh-Taylor perturbations, as simulated by Chiel van Heerwaarden

If we take the opposite approach and pour water at a fast rate by holding the kettle just above the surface of the slurry, the stream will be very smooth while it enters the slurry and almost no air will be dragged into it. This means that the underwater current will not slow down as much before it reaches the coffee bed, and you will instead slowly dig a hollow in the coffee bed using a laminar flow (i.e., a “not turbulent” flow). This is not very efficient at separating all coffee particles to extract them evenly, and the hollow will potentially later cause a channel and local over-extraction unless you swirl the dripper to make the coffee bed flat again.

Now that we saw how breakup length affects agitation, it should be clearer that in order to obtain a turbulent agitation of the coffee bed, we need the stream of water to be a little rough as it enters the slurry, but not too much. To little will result in non-turbulent flows, and too much will only agitate the surface of the slurry. In practice, I found best results by pouring from the height that will place your breakup length just below the surface of the slurry. You can achieve this by pouring just a bit lower than the point where you can hear a splattering sound as the stream enters the slurry. In other words, you do not want to hear that splattering sound, but you want to be pouring just below the point where you would hear it.

Illustration of different types of agitation imparted to the coffee slurry by different shapes of water streams.

Depending on how fast you are pouring and what kettle you are using, this pour height that almost causes splattering sounds will be different, because the breaking length of your stream will be different. These effects were explored by Qu et al. (2011) and Baylar et al. (2016) in the context of plunging jet reactors, and are illustrated below:

Depth of agitation and bubble entrainment as a function of pour rate. Choosing a pour rate that falls just below the slurry will maximize agitation; higher pours will cause only shallow agitation, and lower pours will cause laminar currents in the slurry. Based on experimental data from Qu et al. (2011).
Depth of agitation and bubble entrainment as a function of nozzle roughness (i.e., gooseneck roughness in the context of coffee brewing) and the impact velocity of water. Based on experimental data from Baylar et al. (2016).

The longer your breakup length is, the more velocity your stream of water will have as it enters the slurry, because it had more time to be accelerated by gravity. This means that selecting a longer breakup length will cause stronger and deeper agitation, and will agitate your coffee bed more. Unfortunately, I do not think there is a “best answer” when it comes to what should be your breakup length. If you have a breakup length that is very long, and end up pouring from very high up (without splatter !) as a result, you might impart so much agitation to the coffee bed that you could end up clogging your filter because very fine coffee particles will more easily fall at the bottom.

In my experience, I like using a breakup length of about 18-19 cm, which I achieve by pouring at 5 g/s with my Stagg EKG 0.6L kettle, and pour from a height just below that which causes splattering sounds. This works very well with my current set-up (detailed here on my blog), but you may want to select shorter break-up lengths if your grinder or coffee generates a lot of fines. I sometimes reduce my pour rate to 3-4 g/s with some Ethiopian coffees that clog easily, and crank it up to 6-7 g/s with some naturals or South Americans that have insanely fast drawdown times. I recommend experimenting with your own kettle and determine the pour rate that suits you best.

There is another detail we did not discuss that will affect the breakup length of your kettle. How vertical your falling stream of water is important – generally, I found that the most vertical streams are easier to control, preserve a good breakup length, and cause less hollows in the coffee bed. To make your stream as vertical as possible, I recommend moving your kettle relatively slowly to leave the stream undisturbed, and keep it at a relatively stable height during your brew. One nice advantage of having a high quality kettle is that they are designed to minimize the teapot effect. This is another phenomenon caused by the surface tension of water, causing it to want to stick to the outer wall of the kettle, and make the stream of water less vertical especially at slow pour rates.

Illustration of the teapot effect, causing the stream of water to bend towards the kettle especially at slow pour rates. A hydrophobic coating completely eliminates the adhesion of water to the gooseneck surface and therefore eliminates the teapot effect completely. Source: phys.org.

Now I know we discussed a lot of different things in this blog post, and it can be a little hard to keep all these variables in mind when you try to apply these concepts to your brewing method. To help you a little, I created some 2-dimensional maps of how much agitation you will cause in the slurry, and whether that agitation is turbulent or laminar. Your exact map will depend on the kettle you are using, but this should give you a good idea of how to explore your options. As I mentioned, I personally found the best results by maximizing turbulence by pouring just below the splattering point, and finding the flow rate that causes the right amount of agitation to avoid clogging and maximize evenness of extraction.

Illustration of an agitation map based on different pour heights and pour rates. I normally aim for the “sweet spot” where agitation is turbulent and significant, i.e. when pour height is just below the breakup height that would cause a splattering sound. I select the highest pour rate that does not result in filter clogging, typically 5 g/s.
Illustration of my pour strategy placed on top of the previous agitation map. I normally start at 5 g/s, and if a given coffee draws down very fast because it has a small amount of fines, I go to faster pour rates and pour from slightly higher to maintain a breakup length just below the surface of the slurry. If on the other hand a coffee generates an unusual amount of fines, I will pour slightly slower, and adjust the pour height lower so to avoid the splattering sound.

I would like to thank Fellow who generously sent me a Stagg EKG 0.6L gooseneck kettle for free so that I could compare its performance with the Brewista artisan. This gift came without any obligation on my part regarding the content of this or further blog posts.

Why Can’t we Grind Coffee Finer for Pour Over ?

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When I started reading about the craft of pour over coffee, I learned that it’s important not to grind too fine, because past a certain point the coffee tastes harsh and astringent (i.e., a drying sensation of the tongue). This was very much in line with my experience, and on top of that, grinding too fine would result in a completely clogged V60 filter that would make it impossible to get all the water through without absurdly long brew times. For these reasons, I didn’t question the concept and I was happy to grind as coarse as everyone else.

The Barista Hustle cupping experiment, showing how average extraction yield evolves with time with fine (green), medium (orange) and coarse (blue) coffee grounds.

The first thing that made be re-think this was an experiment performed by Barista Hustle, where they measured the extraction yield versus time for different cuppings made with narrow particle size distributions of different sizes, which they obtained by sifting ground coffee. If you have not already watched this video, you should definitely take a look; it provides quite compelling evidence that we are never really extracting the core of particles larger than about 100 microns within reasonable brew times and water-to-coffee ratios. To convince myself that their experiment was consistent with the idea of under-extracted particle cores, I created a simple model and fitted it to their data and found that water only seemed to extract down to a characteristic depth of about 40 microns, i.e., this is the depth where extraction is about 60% slower than surface extraction. This means a very large fraction of the coffee mass is almost never extracted at typical filter grind size !

Just the idea that we are wasting that much coffee is already not great, but I think the problem goes even further; as I argued in a previous post, the cells that are hidden deep under the surface of coffee particles still extract but slower; this will contribute flavors to your cup of coffee that have a very different profile compared to the cells near the particle surface. In other words, we are not doing a great job of extracting all the coffee cells evenly when we brew pour over. Does it mean even extractions will necessarily taste better ? No, but I think we should try it and see what happens.

Following this, Matt Perger was shouting from the rooftops about the virtues of grinding finer, and there was even talk of using a chemistry Büchner funnel for filter coffee. I thought he potentially had a point, and I became very interested in the idea of getting past the astringency limit. I started experimenting with Turkish ground coffee brewed on the siphon with limited success; it only worked well with a couple coffee beans. I also tried the Büchner funnel; my friends Matt Leberman and Mitch Hale seemed to like it, but I never managed to brew something that I did not find very astringent. Dan Eils also sent me a nice 3D printed V-shaped vacuum brewer, which he called the Vac60, but I was still unable to brew coffee free of astringency if I used it with very finely ground coffee.

Dan Eils’ Vac60 brewer.

I thought maybe agitation was the problem, so I tried using the Melodrip with finely ground coffee and a couple different brew techniques, ranging from my usual two pours to Ray Murakawa’s full two stirred blooms, multi-pour method. Again, I always found the results too astringent, and worse than what I was obtaining with a coarser grind size. These poor results eventually pushed me away from experimenting with very finely ground coffee.

The Melodrip, photo by Elika Liftee.

As I started to suspect I was not gonna be able to obtain great brews with the methods above, I decided to instead improve my grinder’s particle size distribution as much as I could. After some research, this is what led me to purchase a Weber Workshop EG-1 with the ultra low fines SSP burrs. Using my own imaging software, I verified that it indeed produced the narrowest particle size distribution I had seen amongst all the grinders I tested, ranging from several hand grinders to my previous Baratza Forté and even to industry standard Titus-aligned Mahlkönig EK43 grinders. I expected that dialing in the grinder would allow me to push my extraction yields very high, and to grind extremely fine.

The SSP Ultra low fines burrs.
Filter coffee grind size with the SSP ultra low fines burrs. Fines are nowhere to be seen !

To my surprise, this is not exactly what happened; I gained about 1% in average extraction yield compared to the Forté, but my brews suddenly tasted much cleaner, and the tasting notes were a lot more obvious, much like what you get from cupping. I attributed this to the quasi absence of fines produced by the SSP burrs that I installed on it (I still think this is the explanation). I was barely able to choke my V60 brews, even if I ground my coffee insanely fine, yet there was still a definitive limit in grind size below which my coffee tasted very astringent. Even more confusing, I did not seem to be observing a decrease in average extraction yield when I ground fine enough for my brews to taste bad, as I kept hearing should happen. I attributed this to my poor skills at brewing consistently, or measuring my coffee concentration accurately; I still think it was wiser not to trust myself too much back then, especially given that I struggled to brew two V60s in a row with brew times within 20 seconds or concentrations within 0.05% total dissolved solids (TDS) of each other.

The coffee station I assembled to brew consistent V60s.

So I spent the next several months significantly upping my consistency game. I insulated my kettle better with a Brewcoat (see my previous post about this); I found an obscure flow restrictor only available in Asia; I started logging every detail of my brews with two scales and a brew stand to record complete brew curves and see my kettle pour rates in real time; I significantly tightened my refractometer measurement procedure; I improved my measurements of brew water composition (see this post); and I even installed a measuring tape on my wall to always pour from the same height and impart the same amount of agitation to the V60 slurry. I also started weighing my dose with a precision of 0.01 grams for good measure; the effect of a 0.1 grams dose accuracy on calculated extraction yields is about the same as that of a 0.01% TDS measurement accuracy.

Consistent V60 brews across days.
My kettle pours for the consistent brews.

I don’t think measuring my coffee dose to 0.01 grams had a big impact on my repeatability, but it elucidated something that had been bothering me: sometimes my dose suddenly weighed 0.1 or even 0.2 grams less if I was very slow at preparing it. I now know that this is caused by water evaporating from the wet V60 filter, which means you will be mis-calculating your doses slightly if you take too much time to weigh your dose and compensate for that loss. I now take about 5 to 10 seconds to weigh my dose within 0.01 grams, which is fast enough to avoid any issue with evaporating water.

About 8 months ago, I passed the point where I was getting brews consistent to within about 0.4% in average extraction yield, which corresponds to differences of about 0.02% in TDS concentrations with my typical brew parameters. But only very lately, I finally passed the point where I am able to replicate brews to within 0.2% in average extraction yield across five days. Now, the fun is really starting because I can finally start to investigate how many variables influence extraction yield in a much more accurate way.

Just as I was getting these results, a new science paper about espresso rocked the media, which essentially argued that most third wave espressos are brewed with a very uneven flow, and that we might want to consider grinding espresso coarser for a more even extraction. That’s nice and all, but I don’t really drink espresso. I was still excited when I saw this figure:

How extraction yield changes with grind size for espresso brews. Figure from Cameron et al. (2019).

This shows exactly the “common knowledge” curve I was talking about earlier; if you grind too fine, average extraction yield goes down because your extraction becomes more uneven. As I had just achieved the kind of precision that allowed me to do this, I decided to build a similar curve for filter coffee. I brewed seven V60s in a row using the same coffee and grind settings ranging from 3.0 to 9.0 on the EG-1 and tried to replicate my brew technique as well as I could. Here’s what my kettle pours looked like across the six brews;

My kettle pours compared during the grind size experiment.

Not too bad, except for one brew (grind size 6.0) where my first kettle pour was a bit ahead. More interestingly, here are the TDS normalized by beverage weight tat I obtained. They are simply the average extraction yields divided by 14.4, as I normalized my results to a 20.0 grams dose and a 288 grams beverage weight:

How my beverage concentration (normalized to the same beverage weight) changed with grind size on the EG-1.

This result actually confirmed the impression I had even a couple weeks after obtaining the EG-1; I am not brewing near the maximum average extraction yield… not at all ! A blind tasting revealed the best brew in this case was at grind size 8.0, and all those high-extraction brews at grind sizes below 6 did not taste great at all !

I do not know whether this would happen with different grinders, or if this is a weird feature about a very uniform particle size distribution. Regardless, I think it might be further indication that I’m not using my grinder to its full potential. One hypothesis that immediately jumped to my mind was that the deviation from a linear trend might indicate uneven flow. The mathematical model of Cameron et al. (2019) based on the diffusion of chemical compounds in a two-sized bed of coffee particles predicted a linear trend for an espresso brewed with even flow.

I could very well be wrong about this, but I don’t see an obvious reason for a similar model to produce a very different behavior at coarser grind size. The difference between filter and espresso brews might be that the regime over which flow becomes uneven happens on a very short range of grind sizes for espresso, whereas for filter coffee it is happening over a gigantic range. Because of this, my figure may be showing an increase of extraction yield that is due to the significant increase in available coffee particles surface as I was grinding finer. Thus, I thought the deviation from a linear trend may tell us about flow evenness:

An estimation of how uneven my extraction was if I assume extraction yield should go up linearly as I grind finer.

But then I sat on this for a while, and thought more about it. Another hypothesis came to my mind, this one much simpler. What if uneven flow is not the main culprit for astringency when I grind just a bit too fine (say, at 5.0) ? What if I am just locked into this grind size restriction because of my a priori hypothesis that I should brew with a 1:17, or even 1:16 coffee-to-water ratio ?

When I brew tea (I sometimes do !), I can make almost anything astringent if I wait and steep it for too long. I think this might also be true of coffee, except for some unicorn exquisitely roasted high-quality green coffee (i.e., those coffees that tasted great with my Turkish siphon recipe). Only, percolating coffee is much more complicated because we don’t directly control steep time; we are under the tyrannic reign of water flow, unless we cut the brew with water still in the V60 – which would feel so wrong.

Locking my ratio to 1:17 means that I am using a fixed amount of solvent to extract flavors from the coffee particles. It is a relatively high ratio, which means that I am extracting aggressively fast. When I grind at my usual setting, I might be over-extracting just a small fraction of the coffee cells near the outermost surface of coffee particles, contributing a small amount of chemical compounds that cause astringency, but not enough for me to really taste that. But as I grind finer, this fraction of exposed coffee cells goes up, while my flow of water gradually goes down, causing an even more aggressive total extraction which leads to astringency.

In other words, the fine grind size is not necessarily the unique source of the problem; it might be the combination of long brew time and finer grind size that leads to a worse cup of coffee. So here’s an idea; what happens if we use less water, and then bypass the resulting cup of coffee with fresh water to obtain the preferred concentration ? This is very similar to what Scott Rao suggested in his books when dealing with very large doses in batch brewers; we end up not having enough flow, so we have to limit brew time or grind absurdly coarse. Hence the suggestion to use a smaller ratio and then bypass the result with brew water to reach the desired beverage concentration. We might want to try that even for single-cup pour over coffee, or at least when we are using a grinder that produces very little fines. If you grind so fine that your dry coffee grounds start to clump, or your filter clogs, we should not expect an improved taste regardless of the brew ratio ! It is also possible that fines will migrate into your cup, muddying the flavors if there are too many of them.

It is not guaranteed that the finer grind size will make up for the smaller amount of water; maybe our average extraction yields will go down or the brews will not taste as good, but I have started trying this out on a few coffees and my (very) preliminary results seem positive. I will definitely start investigating this more, and see how fine I can get without hitting astringency, and I’ll report back with some results ! Maybe I’ll finally get a Büchner brew I like, only with a ratio as small as 1:10 ?

I’d like to thank Matt Perger, Scott Rao and Chris Hendon for useful discussions.

Measuring Brew Water Properties

Photo header credit: Kathy Gagné

Reminder: as an Amazon Associate I earn small commissions from qualifying purchases made through the Amazon links below. I have no association to Hanna instruments, API or Red Fish and don’t receive any benefits from them.

Most of the questions I receive about coffee brewing are related to the craft of brew water, especially when it comes to verifying that its properties are consistent with one’s goal. If you are not familiar with how brew water affects your coffee, I highly recommend reading this previous post I wrote on the subject before you read what follows. I will assume that the readers are familiar with the concepts of alkaline buffer and total hardness, both of which seem to have the most impact on taste. If you have the chance to grab one of the elusive copies of the Water for Coffee book, I also highly recommend it if you would like to learn more about water.

Both my blog and the Barista Hustle website contain various useful tools (e.g., here and here) when it comes to selecting your brew water recipe and crafting it, but here I want to focus on how we can precisely measure the total hardness and alkaline buffer of any water. This can be useful to verify your water recipe came out good, or to check whether your tap water is good for coffee. If you have tap water soft enough to add minerals until you reach a desired composition, this is yet another situation where you will want to know the properties of your original water precisely.

A conductivity meter used to estimate the total dissolved solids in water.

In the past, a lot of focus has been placed on measuring the approximate total dissolved solids in water using electrical conductivity meters like this one. These devices only measure the electrical conductivity (EC) of water as their name suggests, and then try to guess the total hardness of water assuming a specific composition. In general they are not very accurate, but changes in EC measurements can be useful to quickly notice a change in water composition. Most of them claim to apply an automated correction for temperature, required because the temperature of water also affects its conductivity. Unfortunately, I have come to stop trusting any of these claims, and I suggest making sure you always measure your water EC at a fixed temperature (25°C, i.e. 77°F, is typical).

As EC meters are not very accurate, it is natural to wonder if there are better methods available to diagnose your water, and there are ! It is much more useful to directly measure the alkaline buffer (KH*) and total hardness (GH) of your water. If you don’t want to measure both, I recommend going with KH because a bad buffer can really destroy a good cup of coffee. After looking for a while, I found two sets of tools to measure KH and GH, both based on the concept of titration. We will go deep in the rabbit hole of what titration means further down, but for now let’s just focus on the titration tools I found.

*KH is not always equal to the alkaline buffer, but for most water compositions they are the same (put simply, as long as GH is larger than the alkaline buffer we can call the latter KH).

Colorimeters and Photometers

A Hanna Instruments HI-775 colorimeter for total alkalinity.

Those are definitely the nicest options if you are able to find them and are willing to spend a couple hundred dollars for your measurements. You only have to add some reactant to a sample of your water, place it in a small colorimeter or photometer device, and you will get the composition of your water as a number on a screen. It is almost as easy as using a refractometer to measure coffee concentration. With typical water recipes, they can reach precisions of about 5-6 ppm as CaCO3 in both GH and KH. This is a very good precision, and should be better than what we can actually taste in the resulting coffee. It’s also very hard to make a human mistake during the measurements, which is not the case with the other tools described below. There are some down sides however; they are usually hard to find and you will need to order reactant chemicals every once in a while. Those that I could find are manufactured by Hanna Instruments. The most relevant ones are:

  • HI-775 colorimeter for KH measurements*.
  • HI96735 photometer for general hardness (expensive !).

*Make sure to use the freshwater model, not the saline water model.

If you get tempted by the calcium and magnesium hardness colorimeters like I did (HI-719 and HI-720), don’t get them because their range is only useful for extremely soft water. I personally use the HI-775 for quick KH measurements. It provides measurements in units of mg/L HCO3-, which you need to divide by 1.219 to obtain a measurement in our typical units ppm as CaCO3.

A KH colorimeter device (left), next to a sample of water after adding reactant (middle) and a cuvette of clean water for calibration (right).

Aquarium Titration Kits

The other cheaper and more widely available option is to go to your local aquarium store and buy titration kits for GH and KH, which are sometimes sold as a single package. The big issue with these kits is that the smallest increments they can measure by default in both GH and KH is about 18 ppm as CaCO3. Maybe this is fine for aquariums, but it’s a bit sloppy for coffee brewing because we generally like to use soft water (for example, a lot of popular recipes have KH at ~40 ppm as CaCO3). They are a bit more difficult to use and prone to human error compared to colorimeters, but they can definitely do a much better job than EC meters.

These kits come with a user manual explaining how to perform the measurement; you usually need to put 5 mL of water in a small cuvette, add a single drop of reagent, shake it, then look at the color. Depending on the brand and type of titration kit you are using, different colors will indicate that you are done with your measurement. For the popular API brand, the GH kits go from orange to dark green and the KH kits go from blue to green and then yellow. Once you reach the color turning point, you can look up a table in the user manual to convert the number of reactant drops to a GH or KH measurement – typically, every drop counts for 1 german degree, or 17.85 ppm as CaCO3.

What is Titration ?

Titration is a method widely used in chemistry to measure the concentration of a specific compound in a solution that we’ll call the test solution. The idea is to use another solution, called a reactant, which has a known concentration in another chemical agent that can react with the chemical we want to measure in the test solution. That reactant also contains a color indicator that will change color quickly if none of the compound to be measured is left in the test solution.

Then, the idea is to slowly add a precisely measured volume of reactant in a known volume of test solution, until the compound we want to measure has all reacted away, at which point the color indicator will suddenly cause a change of color. You can then do some maths to calculate how much of the original chemical you reacted away before none was left, because you know exactly how much of the reactant you added.

A titration curve (red line) with its turning point (vertical dashed blue line) and the zone where the color indicator shifts (green to yellow shaded region).

What I call a “trigger” in the figure above can be the pH of the solution, or other technical properties I won’t get into. The important point is that the color indicator is sensitive to that triggering property. In the example image above, the chemical that serves as a color indicator is sensitive early in the transition, and this indicates that the best measurement will be obtained by using the reactant volume corresponding to when the color transition is completed. Some other reactants and indicators would however have the color transition happen right in the middle of the turning point, which would make the color transition so fast that there would be almost no ambiguity, and therefore the titration would be more precise.

In a laboratory environment, chemists will use very precise tools like burettes in a very clean and controlled environment, allowing them to reach very accurate measurements with precisions as good as about 1% of the measurement. We should not expect to be able to do this with aquarium tests, which are way less precise than volumetric tools. If you’d like to learn more of the technical details behind titration, I highly recommend this website.

What I found about the API GH titration tests is that they use a reactant called an EDTA tetrasodium salt, a large and complicated molecule that can capture calcium and magnesium ions, and triethanolamine, a buffering component that keeps the pH very high to make the transition point very steep. I wasn’t able to find out what color indicator they used however, which makes it unclear where exactly the color transition takes place with respect to the turning point. The KH titration test information is even more sparse; my feeling is that API only publishes as much information as they have to for safety measures, and they keep the rest as intellectual property. The best we can do in that situation is follow their instructions, which don’t clearly specify whether the correct measurement happens at the start or the end of the color transition.

Stretched Titration with Aquarium Kits

One of the most straightforward ways to obtain a better precision with an aquarium titration kit is to use a larger sample of water, and put more drops of reactant in it. You will obviously need a clear container larger than what is provided by API kits for this. Don’t use any dining glassware, as the reactants are not safe for consumption ! If you double your water sample size, then each drop of reactant will only represent a measurement of half a German degree (i.e., ~9 ppm as CaCO3). This is already much better, but it can quickly require a lot of reactant especially with harder water. I built a Google spreadsheet to help you determine your measurement and precision with this method. If you are pretty sure that you added the exact number of drops required for the color transition, leave the “Precision for number of drops” to one, but if you are not entirely sure which one of the last two or three drops caused it, you can use errors of two or three drops accordingly; the spreadsheet will include this in your final KH or GH measurement error.

I purposely set a maximum achievable precision of 5% of the measurement and 5 ppm as CaCO3 in the spreadsheet (this is true of the other methods below as well) – I don’t think it is likely we will get better measurements in a non-laboratory environment and with the specific chemicals that the API aquarium kits use for titration. You would hit a couple systematic problems if you tried to obtain a better accuracy. For example, the titration curves of the API kit have a smooth part near the transition point which will make the point where the color changes less obvious in those situations.

If I did not put these limits on precision, you may use a large sample and think you have measured KH with a 0.1% precision, when in reality you may be 5% off. Another example of a systematic error is the fact that magnesium reacts slightly differently than calcium with the API test kits, and because they do not know your water’s Mg/Ca ratio, they use the properties of calcium only to translate your reactant volume into a GH measurement. Other problems could include using a cuvette that is not perfectly clean or dry, not having shaken the reactant properly, having left water sample to evaporate a bit, or even having lost a couple milligrams of your sample when you shook it. As you can imagine, those are all things you should try to avoid !

Here are step-by-step instructions for a stretched titration:

  1. Thoroughly clean and dry the glass cuvette.
  2. Wear clean and dry protective gloves.
  3. Measure a precise volume of brew water (API’s default is 5 mL).
  4. Shake the reactant thoroughly and open it.
  5. Add a single drop of reactant and stir/shake well (depending on your cuvette).
  6. Look at the solution color in front of a white wall in a well-lit environment.
  7. Keep going until you see a color shift happen, and make sure you don’t lose the count of reactant drops.
  8. Stop counting when the full solution color has noticeably shifted even after you stir/shake the cuvette for a dozen seconds.

Back Titration with Aquarium Kits

There is one neat trick called back titration that we can use to reach a better precision without needing all the additional reactant. The idea is to put a fixed, small number of reactant drops in the cuvette, and then add your brew water one drop at a time until the reverse color shift happens. Because we reversed things, now a KH kit would go from yellow to blue when you reach the transition point. Translating your experiment into a measurement is a bit more complicated in this scenario, so I also included a back titration calculator in the Google spreadsheet to help you doing it. The more drops of reactant you started with, the better precision you will obtain, up to a limit. However, you will also need to add a lot more drops of brew water especially when measuring soft water, and it can become easy to miscount them or place a drop outside or on the sides of the cuvette if you are not focused enough.

One thing you will need to do before you can use this method is calibrate your particular pipette with a milligram-precision scale like this one so that the spreadsheet knows the average weight of one of its water drops. There is a space just for that in the Google spreadsheet; simply fill it up with the weight of 25 drops obtained with your pipette in milligrams, and the spreadsheet will do the rest. Otherwise, you could use the exact same pipette as I used (sold by Ronyes Lifescience) and leave the default values in the spreadsheet. Make sure you put the rubber cap on the size with the larger hole.

Here are step-by-step instructions for a back titration:

  1. Thoroughly clean and dry the glass cuvette.
  2. Wear clean and dry protective gloves.
  3. Shake the reactant thoroughly and open it.
  4. Put a single drop of reactant in the cuvette.
  5. Add a single drop of water and stir/shake well (depending on your cuvette).
  6. Look at the solution color in front of a white wall in a well-lit environment.
  7. Keep going until you see a color shift start to happen, and make sure you don’t lose the count of water drops.
  8. Stop counting when you first see signs that the color of the full solution has started shifting noticeably. It will keep getting more obvious if you add more water, but use the moment where you first noticed it clearly as your number of drops.
  9. If you went past the color shift or want a better precision, add another drop of reactant and then keep adding water again. You will just need to update the number of reactant drops in the calculator, and don’t forget to always count the total drops of water that you added since the very start.

Scale-Assisted Back Titration with Aquarium Kits

As it can be hard and annoying to count many drops of water, I thought of a better way to do this, but it requires using a milligram-precision scale. The idea is to use weight instead of volumetric measurements, and translate those back into GH and KH using known properties of the reactant and water. Having only access to API-brand kits, I made the required measurements for their specific types of reactants only.

To perform a scale-assisted back titration, first place your titration cuvette on the scale and note the total weight. Then add the desired number of reactant drops (remember that more drops will get you more precision), and note down the updated weight. Now start adding drops of your brew water one at a time. You can pick up the cuvette, shake it, and place it in front of a well-lit white wall to make things easier. I recommend noting the total weight at every step (without taring) in case your scale turns off automatically when you take too long. Put the cuvette back on the scale when you reached the color turning point, and note down the water weight that you needed. From these measurements you can deduce the total weights of water and reactant, and enter them in the spreadsheet to obtain your KH or GH measurement, with its measurement errors.

I highly recommend using this method at a room temperature in the range 18-25°C, because otherwise the mass density of the reactant drops and water could change too much, which would falsify the mass to volume relations that I hard coded into the spreadsheet. This method is a lot more robust against human error compared to a regular back titration. The step-by-step instructions are the same as a normal back titration; you just need to keep track of total water and reactant weights as you go.

Testing the Different Methods

I decided to measure my Rao/Perger brew water with each method to verify that the expected accuracies were reached. Here are visualizations of each measurement with its error bars obtained with the web tool, and compared to the expected value for the water recipe I used, shown as a vertical blue dashed line (40.5 ppm as CaCO3).

Measurements of total alkalinity made with the different approaches explained here (red circles with measurement errors), compared to the expected KH of the brew water recipe I crafted (vertical blue dashed line).

When to Use Which Method

In order to illustrate when each method is preferable, you need to have an idea of whether your water is very hard or soft, and what type of precision you want to achieve. To help you decide, I built a figure illustrating the fractional error you would get for KH measurements with the different methods mentioned here, depending on your water’s total alkalinity:

Fractional measurement errors  in total alkalinity for different methods discussed here.

The case for GH is very similar so I didn’t show a separate figure for it. As you can see, stretching the API titration kit to a 20 mL sample instead of 5 mL will easily get you a very good precision, but it can be very costly in terms of reagent, especially with harder water. To illustrate this, let’s look at how much reagent a stretched normal titration requires compared with back titration:

Number of reagent drops required to reach a fractional error of 10% (black)  or 20% (red) with the stretched titrations (full lines) versus back titrations (dashed lines). The back titrations require a lot less reactant.

Here we can really see the strength of back titration: it requires way less reactant to reach similar precisions. This is especially true in the case of hard water, where a lot of reactant can be otherwise needed. If you like to see the absolute error for different methods rather than the fractional errors, then I’ve made an alternate figure for you:

Red Sea Titration Test

After I wrote this post, a friend of mine Victor Malherbe told me about another nice option: the Red Sea Fish titration tests. They have titration kits for total alkalinity, calcium and magnesium hardness with the following precisions; they seem especially interesting for their KH precision:

  • KH: 2.5 ppm as CaCO3 (i.e., 0.5 meq/L),
  • Calcium hardness: 12.5 ppm as CaCO3 (i.e., 5 mg/L),
  • Magnesium hardness: 82.4 ppm as CaCO3 (i.e., 20 mg/L).

As a reminder, general hardness (GH) is the sum of calcium and magnesium hardness. I suspect their calcium and magnesium hardness tests could also be stretched and reversed !

Hopefully this post will make water measurement simpler and clearer for some of you !