A V60 Pour Over Video

Today I decided to release publicly one of the V60 videos from my Patreon. I plan to make a better quality video eventually for my blog, but in the meantime I thought this would be interesting to a wider audience. Please view this recent post I made about what is going on with Patreon if you are worried that I’m making some of my content access-restricted, and this previous blog post explains the method I use here in more details.

A video of me brewing a V60 pour over

You can find a higher-resolution version of this video here, but be warned that it is 1.2 GB large !

In this video I’m brewing Gardelli’s Ethiopian natural Chiriku with a 1:17 ratio and 22 grams dose. I used grind setting 6.8, slightly finer than my usual 7.0@700RPM, because last time I brewed this coffee, it felt a bit watery. Turns out I preferred it at 6.8, and I rarely get astringency at that grind setting. I’ll tell you more about that in a different post, but I now suspect that the average mass of coffee particles is an important factor that determines channeling, because it has a lot to do with the structural integrity of the coffee bed. I therefore suspect that there is a lower limit in grind size that will get you some astringency very easily for a fixed brew technique; it corresponds to the point where channels are being dug by water. Most of the time I still use a 7.0 grind setting, just to be sure.

You will notice in this video that I spin a bit harder than I used to. This is partly because I was being too gentle especially in the first video, but it is also because I realized I have so little fines with the EG-1 grinder + SSP ultra low-fines burrs that fines migration is not as much of an issue compared to other grinders. I suggest to start very gentle, and then try more brews (with the same coffee) where you gradually spin a bit harder. If your brew time goes up significantly, then you might want to go a little easier to avoid fines migrating to the bottom of the brewer. You’ll have to find your own pace, as I suspect it depends (slightly) on the grinder you’re using; in general, a higher quality grinder should allow you to spin a bit harder.

I had pre-rinsed my Hario tabless filter before starting the video, first with a lot of tap water and then with a bit of warm brew water. The kettle was also preheated at 189F before I started the video. My pours will all be made with boiling water, but preheating it at 189F means I’ll have to wait less when I’m ready to pour. The first thing I do is weight and grind my beans – I weighed 22.3 grams and ground 1-2 beans to make sure nothing was stuck on the grinder burrs from yesterday’s brew. I then cleaned up my blind shaker and placed it back on for the main grind.

You’ll notice that during the bloom pour, I don’t concentrate too much on my pour technique: I move horizontally a bit too fast and I also move vertically which I ideally shouldn’t. Instead, I make sure I have high flow and to stop at the right amount. My goal here is to wet everything quickly rather than immediately getting a perfectly level bed. I’m also giving it a much more thorough spin after that pour because I found that helps with getting everything wet at once.

You can see that I spent a short amount of time removing the high & dry grounds with my pours, but otherwise I described a very slow flower pattern that hits the center more often than the sides. I’m trying to get the whole bed agitated by doing that, with more focus on the center because there’s more layers of coffee there. I move very slowly because I want the water to fall very straight (this helps getting a flat coffee bed), and I don’t move vertically. I try to get a very steady flow too, but that’s the part I’m still the worst at without the ability to measure it on-the-go.

When I spin after the first pour to 200 grams, you can see that two bubbles appeared. That is generally not a good sign, as it means some brew water just touched dry coffee. The fact that it happens while I was spinning tells me that I probably just destroyed a channel and forced the water to flow through dry coffee. That doesn’t mean the brew will necessarily be bad, but it means I could have done a better job during the bloom phase. It happens to me 5 times in the last 14 brews, so about a third of the time. This is one reason why I’d really like to have a plastic V60 brewer with a steep & release mechanism (I know about the Clever but I don’t like its shape); it would allow me to stop water from flowing during the bloom, and probably give me enough time & control that I would be comfortable with mixing the bloom with a small spoon.

If you wonder why I tap the cork lid before putting it on the V60, it’s not from an obsessive compulsive disorder, but rather to make sure that there’s no coffee grounds on it (or at least I like to tell myself that).

Notice how clear the water is at the end of the drawdown. This is because the EG-1 with SSP burrs produces a crazy small amount of fines at the optimal V60 grind size. It reminds me of when I experimented with the Melodrip, but now I get even after having agitated the coffee bed. If you pay attention at the end you’ll see that I stop the scale’s timer exactly when the reflection of light from water above the coffee bed ceases because water just went below the height of the coffee bed. I like to use this cue because it’s very repeatable, and it might help you compare your own brew times with mine more precisely.

At the end of the brew, I let the V60 drip a bit more into the beverage, then I place the V60 on top of a small glass and gently move it up and down to get a few more drips and measure the approximate TDS of the slurry at the end of the brew.

After that, I clean up the refractometer and measure the beverage TDS, but I make sure to taste it before looking at the TDS measurement, otherwise I found that it can affect my taste perception. Also notice how I mix the brew with a spoon before sampling it; this is better at mixing up all coffee layers than just spinning the brew.

I know the ending is a bit abrupt, sorry about that – my iPhone ran out of storage ! You just missed the brew TDS measurement. I’m starting to be more satisfied with this angle of view, so I’ll start thinking about how I can make a more complete brew video that I can eventually publish on my blog. I’ll make sure I don’t wear slippers for that one.

Here are the relevant measurements for this brew:

  • Coffee: Gardelli Chiriku Ethiopian Natural
  • Grind size: 6.8 at 700 RPM
  • Ratio: 1:17
  • Dose: 22 grams
  • Water: 374 grams
  • Bloom water weight: 77 grams
  • Time at the end of first pour (200 grams): 1:08
  • Time where second pour is initiated: 1:45
  • Time at the end of second pour (374 grams): 2:20
  • Total brew time: 4:03
  • Beverage weight: 319.9 grams
  • TDS of last drops: 0.94 %
  • TDS of beverage: 1.43%
  • Approximate EY (percolation equation): 20.8 %
  • More accurate EY (general equation): 22.4%

I hope you enjoyed it, and don’t hesitate to leave a comment if you have questions.

I’d like to thank Scott Rao for his tremendous help in improving my pour over technique. The method above is also strongly inspired from his method !

About Patreon

I realize I haven’t talked a lot about my Patreon page on this blog yet, so I thought I’d update you all about it in a short blog post. I might remove it later, if it becomes irrelevant, and because I am trying to make this blog a repository of useful resources rather than updates on my whereabouts (for that, you can see my Instagram).

The reason I created a Patreon is to buy some expensive equipment that will allow me to push my coffee posts further, but rest assured I have no intention for these posts to remain only accessible on Patreon. You can find more about these future plans in one of the public posts I wrote on Patreon “Some Future Projects I Have in Mind“. There are a few more posts directly on my Patreon that are public and won’t make it to this blog, because I they are not directly relevant to it, or I don’t feel their content is best explained there.

I don’t want anyone to feel forced to contribute to my Patreon, rather I’d like it to be only for the more “hardcore” fans who really want to contribute regardless. I do offer some benefits to my backers following the Patreon model with tiered donations, but these benefits are either not refined enough for being on my blog yet, or they are things that I never planned to share publicly on this blog. For example, I share multiple Patreon-only videos that are “in development”, either because the quality is not there yet, or because the content is not final. Stay tuned for such a video to be released here later today.

An example of something I do not plan to share publicly is my (almost) live-updated personal coffee log (although I will share some stats about it), and my running list of experience with different roasters. I do eventually share publicly the roasters that I prefer, but I don’t share publicly those that I didn’t like – I feel like this would be a bit too hostile. So in conclusion, I view my Patreon as a “backstage” access to the stuff that is in development, rather than anything that should replace the blog posts that I will keep making public here.

I’m hoping this will address some fears I read about online, and stay tuned for a V60 brew video later today !

Seasoning Grinder Burrs and Grind Quality

[Edit May 28, 2019: The violin plot and two of the other plots below had an axis that stated micron squared and should have been millimeters squared; those are now fixed. Thanks to Mark Burness for noticing !].

As some of you know, I recently decided to make the move and get the Lyn Weber EG-1 grinder with the SSP Ultra low fines burrs. I took this decision mainly because I heard this combination generates the lowest amount of fines other than industrial roller mill grinders, but also because of its design focused on single dosing and low grind retention. I like to switch coffee every brew, so those are very nice features for me. I’ll make a more detailed post where I compare the EG-1 with my previous Baratza Forté, but in the mean time I’d like to talk about burr seasoning.

The EG-1 grinder by Lyn Weber. This is what all grinders should look like.

The Seasoning

If you never heard the term seasoning, it refers to the habit of grinding a large quantity of roasted coffee (or even rice) to break in grinder burrs which initially have harsh angles and corners. It is often said that this is done to prevent grind size from changing with use, and to obtain a more uniform grind distribution, which maximizes the average extraction yield of good-tasting espresso or pour over brews. When I seasoned my Baratza Forté, I did it with 12 pounds of roasted coffee at espresso grind size. Back then, I didn’t have a good way to measure the particle size distribution of my grinder, and I just supposed that I was done.

It is possible to diagnose whether you are done seasoning your grinder with a refractometer, by actually brewing coffee and noting the maximum average extraction yield you are able to reach without getting astringent taste. This typically takes me a couple of brews, and giving the limited time I had to do this I just ground a large amount of coffee and called it a day.

Now that I wrote an application to actually measure grind size distributions, I decided to take a sample of coffee every few pounds while I was seasoning the SSP burrs of my EG-1 grinder. I zeroed the grinder position at grind setting 0.0, which means that this is the point where burrs touched (I can hear burrs that start to rub against each other at grind setting 1.5). I initially started seasoning 2 pounds with a 700 RPM (revolutions per minute) motor speed and grind setting 8.5, which means that the burrs were 425 microns apart; turns out this was closer to a V60 grind size, so I went down to grind setting 5.0 (250 micron burr spacing) and 800 RPM after that. The slightly higher motor speed made sure that the motor didn’t stop from time to time as I fed a lot of coffee in the grinder. After 12 pounds, I even went a bit faster (1000 RPM) for the same reason. I sticked with this setting all the way to 24 pounds; I went all the way to 24 pounds because I heard the SSP burrs were particularly hard to break in.

I used a collection of beans from bad roast batches at my local roaster to do the seasoning, so they consisted in mix of roast profiles and bean varietals. However, after every 2 pounds of seasoning with the mixed coffee, I always took a small ~10 grams sample of the same bean, a washed Bourbon from Burundi (roasted by my friend Andy Kires at the Canadian Roasting Society), which came from a single roast batch. I always made sure to purge the grinder of any grounds from the seasoning before collecting the sample, and I ground and threw away a small amount  of the Burundi just before grinding the actual sample to make sure none of the seasoning coffee was left in. I always collected the Burundi samples at grind setting 8.5 (425 micron burr spacing) with a 700 RPM motor speed.

The Particle Size Distributions

I decided to measure the particle size distribution of half the samples (every 4 pounds), because this takes a crazy amount of work; for each sample, I took 12 images that I analyzed and combined with my grind size application. I didn’t count exactly how many hours this took, but it was about 2 seasons of The Office.

In this figure, the thickness of the horizontal band represents the total mass of the particles at each particle surface (this is called a violin plot and it’s great to compare several distributions together). This allowed me to see for the first time how the particle distribution moves to coarser particles as the burrs are breaking in. It makes a lot of sense that the distribution moves to coarser sizes, as the more rounded edges of the burr’s teeth should allow slightly coarser particles to pass through.

In the figure above, I show how the average particle surface changed with the total seasoning weight. The error bars are based on small number statistics (for the statistics geeks, they are based on Poisson distributions), and represent the fundamental limit in precisely measuring the average particle surface from the limited number of particles that I analyzed (typically approximately 15,000 particles, which is what 12 photos on a standard white sheet of paper gets you).

Notice how the 16 pounds data point seems off from the general trend. I strongly suspect this was caused by me forgetting to set the motor speed to 700 RPM when taking the Burundi sample – leaving the motor speed at the 1000 RPM I used for seasoning would make the particle spread distribution finer on average. This is an effect I also observed with my app, but this will be for another blog post.

One thing that I found particularly interesting is the fact that, even when the particle distribution stabilizes and stops moving to coarser particle sizes, it kept becoming more uniform. I can’t say for sure that this happens on all burrs and all grinders, but this is a good thing ! I found it amusing that I stopped seasoning within 4 pounds of where the shifting of the particle size distribution stopped being detectable with high statistical confidence with my 12 photos. One thing that hit me when I saw this figure is that it resembles a relation that exponentially approaches an asymptote, like a lot of other things in life; another example of such a relation is the concentration of water versus time in an immersion brew.

Grinder Quality Factors

Another interesting relation to look at is how the width of the particle size distribution evolves with seasoning weight. I did this by looking at its standard deviation:

In the figure above, the error bars are similarly based on small number statistics. What we see here is a little different; the distribution initially becomes wider, but then it starts becoming narrower (more uniform). In my experience, particle size distributions that are centered on coarser particle sizes always seem to be wider. This is what led me to define something called the Q-factor (for “quality” factor) in my grind size app, which is simply the ratio of the average particle surface divided by the standard deviation of the particle surface distribution. This ratio seems to be relatively constant across grind sizes (at least in the neighborhood of filter brews), and it also seems to go up with grinder quality. I’ll get back to this in more detail in a future blog post, but here are typical Q-factors that I started compiling for different grinders:

  • A friend’s Mahlkonig EK43* after aligning with shims: 1.45 ± 0.02
  • Baratza Forté BG*: 1.53 ± 0.01
  • My EG-1 with SSP burrs before seasoning: 1.59 ± 0.02
  • An older EK43* model that another friend carefully aligned: 1.61 ± 0.01
  • My EG-1 with SSP burrs after seasoning: 1.76 ± 0.02

An asterix indicates a grinder with its original stock burrs.[Update May 17 2019: Stay tuned for a more complete list of Q-factors that will evolve over time and be accessible to Honey Geisha-tier patrons.]

Gathering these data takes a tremendous amount of work, but I’m gradually building up a library of quality factors for different grinders that I managed to get my hands on. My Patreon followers can already access that partial list as I build it up, but I will eventually release it to the public; it will take a while for me to finish this up however.

This led me to think that a more interesting way to look at how my particle distributions evolve through seasoning is to look at their Q-factor versus seasoning weight:

As you can see, the Q-factor didn’t change much at first while the particle distribution shifted to coarser grind sizes (it hovered around ~1.55, similar to a re-aligned EK43), but then it started increasing by quite a lot.

Eventually, I will map out precise particle size distributions for several different grind sizes with my fully seasoned EG-1. This will allow me to compare each of the particle size distribution above with a fully seasoned distribution at the same average grind size, and thus to say more precisely how the distribution narrowed versus seasoning weight, without having to make the assumption that the Q-factor is perfectly independent of grind size. But this will also take many more seasons of The Office 🙂

More seasoning and an Interesting Observation

When I left my friend’s roaster place after having seasoned the EG-1 with 24 pounds of coffee, I grabbed a bit more of his Burundi and put it in a sealed opaque bag with a 1-way valve and an oxygen absorbing pad (see my other blog post about keeping your coffee fresh for why this is good practice). I did this with the plan to eventually season the grinder a bit more with a 4 pounds bag of bad coffee I had at home. It took me 23 days (and 3.5 pounds of filter pour over coffee that I actually drank) before I had the time to do so. Fortunately, I had the good idea to take a sample before this additional seasoning, as well as after. The effect of this additional seasoning on the particle size distribution was very small, as expected:

There is one thing that really surprised me however; if I compared the grind size distribution right before seasoning again to that right after my first seasoning, it actually became much finer and slightly wider, as you can see in this next figure !

It is highly unlikely that this was caused by the additional 3.5 pounds of coffee that I ground at filter size, because (1) grinding this coarse has a much smaller effect on breaking in the burrs; and (2) this goes exactly the other way than what seasoning does (as we saw above, it makes the particle distribution coarser and narrower, not finer and wider !).

My best hypothesis for what happens here is this: I think that the coffee beans de-gassed and dried as they aged, and the cellulose structure of the beans may also have weakened form the aging. All of these effects will make it easier for the beans to shatter, which will produce more fines, therefore shifting the particle distribution to finer average sizes and widening it. This is exactly what happens with decaffeinated coffee, which requires grinding at slightly larger grind sizes than regular coffee. This will be the subject of a different post, but some extensive blind-tasting dialing in had me select an optimal grind size of 7.5 (375 micron burr spacing) for Heart‘s Colombian decaffeinated coffee, whereas I selected 7.0 (350 micron  burr spacing) for several different caffeinated beans; you can also see this nice coffee tip of the day from Scott Rao about brewing decaffeinated coffee.

I found this possible explanation so interesting that I plan to do more experimentation about it, to determine exactly how particle size distributions shift with aging. Imaging knowing exactly how coarser your optimal grind will change versus the age of your coffee, without needing to dial in again. I would definitely love that !

Disclaimer: Doug Weber generously offered me the SSP Ultra low fines burrs when I bought the EG-1 (under no obligations). I decided to get this grinder based on my friend Mitch’s recommendation and my own research on available grinders, and I receive no benefits from Lyn Weber.

Grind Quality and the Popcorning Effect

I often heard worries in the coffee community about a difference of quality in the coffee grind size distribution when grinding with a full hopper versus a single dose of coffee in an otherwise empty hopper.

The idea behind this is that coffee beans forced through the rotating grinder burrs have no choice but to go through whatever openings they encounter between the burrs. In contrast to this, a single bean in an empty hopper will bounce around and may end up passing through a larger hole between the burrs when the opportunity arises, because nothing is forcing it to pass through very fast. This bouncing around of a bean is what “popcorning” refers to.

As a result of this effect, beans that popcorn will end up getting ground somewhat coarser on average. Grinding a single dose of coffee in an otherwise empty hopper will therefore generate two kinds of coffee grounds: a first batch of slightly finer grounds resulting from beans forced through the burrs, plus a smaller batch of slightly coarser grounds resulting from the last beans that popcorned. The result will be a distribution of coffee particles slightly wider than what you would have obtained if you ground a small dose of coffee with a full hopper.

This more uneven distribution of grinds will cause an increase in the amount of coffee particles larger than average, sometimes called boulders. As I mentioned before on this blog (e.g. see this article), only the surfaces of coffee particles extract efficiently when you brew the coffee, and this larger amount of boulders will limit the amount of coffee compounds that you are able to extract quickly and evenly. Several people therefore suggested that it is best practice to grind with a full hopper, and even to grind one bean at a time if you are extremely patient and want to grind a single dose of coffee at a time.

Now that I built a tool to measure particle size distributions, I decided to test all of these claims. They all make sense, but none of these arguments are really telling us how important this effect is. To do this, I ground 3 different doses of 10 grams each on my Baratza Forté BG grinder. I used the same coffee for all these tests, which is important (especially the roast profile may affect how the coffee shatters). In this particular case, I used a relatively light roast of an Ethiopian Guji by Saint-Henri roasters in Montreal. I ground them on setting 6L with the factory-set zero position. In my case, this means burrs would only touch if I went 3-4 ticks finer than 1A. The first dose was ground with a hopper full of beans, the second one was ground by dropping just 10 grams of coffee beans in an empty hopper, and the third one was ground one bean at a time. This last batch bored me to hell.

I measured the particle distribution of each dose by taking 12 different samples sprayed on a 8.5″ by 11″ sheet of paper and combining them together. I took that many samples to make sure that I would have good statistics to be able to resolve minute differences in particle size distributions. I decided on the number 12 because I noticed that comparing the first 6 data sets combined together looked similar to the last 6 combined together when binning the particle size distributions in 16 distinct particle surfaces, so having double that amount of data seemed conservatively good enough.

As a first test, we can ask ourselves how important the popcorning effect is, i.e. how much coarser do the grounds come out compared to beans forced into the burrs ? To do this, we need to compare the full hopper versus the bean-by-bean doses.

What I call the “fraction of available mass” in this figure is the mass of coffee that is available for extraction if you assume that only outer shells of 100 micron are extracted in each coffee particle. This is just an approximation, but it is already more meaningful than just looking at the total mass of coffee particles. For more information, I suggest reading this previous blog post where I discuss a very interesting experiment carried by Barista Hustle to explain why this approximation makes sense. Basically, we want our particle size distributions to contain some information about how the coffee will extract, so we don’t care about weighing the cores of coffee particles that will never be extracted. I also talked about this more here and here. Another thing to notice in the figure above is that the horizontal axis indicating particle surfaces is in logarithmic scale. This means that every shift of e.g. 60 pixels to the right corresponds to a particle size twice as large. On top of each distribution, there is also a single data point with horizontal error bars, that respectively indicate the average particle surface and the spread of the distribution on each side.

As you can see, we are able to see a difference, albeit a small one: the beans ground one at a time are indeed about 0.08 mm² coarser than those ground with a full hopper. To get a better sense of how coarser they are, I compared the bean-by-bean dose to other full-hopper grind sizes on my Forté, and determined that the closest match was to setting 6Q:

Another interesting part of this is that grinding bean by bean generates a slightly tighter distribution, therefore mimicking a higher quality grinder. It might seem tempting to adopt this practice, but do it once and you’ll see why no one does it. It is also possible that this is just an effect of having started the grinder motor before the first bean hits it; this means the motor was rotating at the same exact speed during the full grind. The “full hopper” and “empty hopper” data sets were taken with coffee already dropped on the burrs before the grinder was started, therefore the start of the dose was grinder at a slower motor speed while it was speeding up. I am under the impression that this doesn’t entirely explain “bean by bean” doing so much better, but I will be isolating out this effect very soon to test that hypothesis 🙂

As we saw, popcorning beans are ground approximately 5 clicks coarser on the Baratza Forté BG. A strategy suggested by Scott Rao to grind a single dose was to start at your desired grind setting, and then change your grind size to something slightly finer when you see that your beans start popcorning. This figure above tells us that, if you wanted to do this, it would be appropriate to grind exactly 5 clicks finer when the beans start popcorning. That may require mastery of the on-the-spot Forté fine controls.

However, let’s first ask ourselves another interesting question; does popcorning affects enough beans to have any effect at all on the particle size distribution of a full 10 grams dose ? The smaller the dose, the bigger the effect will be, as the number of last beans bouncing around will always be the same. To answer this, let’s compare the particle size distributions of the full hopper versus empty hopper doses:

As you can see on this figure, the two distributions are virtually undistinguishable. This means that the popcorning affects such a small fraction of the 10 grams dose that I was not able to see any difference with this analysis. As you can imagine, the effect will be even smaller on typical doses which tend to be around 15 to 25 grams. As we saw previously, the last few popcorning beans were clearly affected and they were ground coarser, so there has to be a difference between the two particle size distributions even though it is a very small one. But to put this in perspective, the overall difference on a 10 grams dose has to be much smaller than one click on the Forté, and also much smaller than the difference in grind quality between all different brands of grinders I have ever tested. This includes Comandante’s C40, Orphan Espresso’s Lido 3, Mahlkonig’s EK43, Lyn Weber’s EG-1 and Baratza’s Forté BG. All of these grinders generate particle distributions that a similar analysis can easily distinguish.

This distribution illustrates how well the grind size app can distinguish between the quality of the grind distribution of the Lido 3 hand grinder versus the Baratza Forté BG grinder. The difference between an empty and full hopper seen in the last figure is minuscule compared to these cross-grinder differences.

The take out message that I got from this experiment is that popcorning has a non-negligible effect on grind size, but it affects a negligible amount of beans in any reasonable dose of coffee. You should therefore not be afraid to grind single doses at a time, because any degradation that results in your particle size distribution will be much smaller than any difference between brands of grinders. I’m hoping this post will alleviate the admittedly first-world and very geeky problem of single-dosing anxiety.

For those geek enough to ask or even to make it this far in the post, I did make my data public on Github.

I’d like to thank Douglas Weber for useful comments, and Victor Malherbe for proofreading.

An App to Measure your Coffee Grind Size Distribution

[Edit April 25 2019: Please note this is not an iPhone or Android app, and I have no plans to release it as such. You can use your phone or any other camera to take pictures of your ground coffee, but then you need to install the application on either OS X or through Python (on any operating system) to analyze the data. Download the application package here.]

Today I would like to present an OS X application I have been developing for a few months. It turns out writing Python software for coffee is a great way to relax after a day of writing Python software for astrophysics.

When I started being interested in brewing specialty coffee a few years ago, one of the first things that irritated me was our inability to recommend grind sizes for different coffee brewing methods, or to compare the quality of different grinders in an objective way. Sure, some laboratories have laser diffraction equipment that can measure the size of all particles coming out of a grinder, but rare are the coffee geeks that have access to these multi-hundred thousands of dollars kinds of equipment.

At first, I decided to take pictures of my coffee grounds spread on a white sheet, and to use an old piece of software called ImageJ, developed by the National Institutes of Health mainly to analyze microscope images, to obtain a distribution of the sizes of my coffee grounds. This worked decently well, and allowed me to start comparing different grinders. Then Scott Rao made me realize that a stand-alone application that doesn’t need a complicated installation and that is dedicated to coffee would be of interest to many people in the coffee industry. Probably just the 10% geekiest of them, but that’s cool.

I’m hoping that this application will help us understand the effects of particle size distributions on the taste of coffee. I don’t think the industry really kept us in the loop with all the laser diffraction experiments, so hopefully we can help ourselves as a community.

If you are interested in measuring the particle size distribution of your grinder, then this app is for you ‒ and it’s free. I placed it as “open source” on GitHub, so if you are a developer, you are welcome to send me suggestions in the form of push requests (the developers will know what that means).

If you would like to get started, I suggest you read this quick installation guide, which will explain how to download the app and run it even though I am not a registered Apple Developer. Then, you can choose to either read this quick summary that will get you running with the basics, or this very detailed and wordy user manual that will guide you through all the detailed options the application offers you.

I would like to show you an example of what can be done with the software. Below, I am comparing the particle size distribution of the Baratza Forté grinder, which uses 54 mm flat steel burrs, with that of the Lido 3 hand grinder, which uses 48 mm conical steel burrs. I set both grinders in a way that produces a similar peak of average-sized particles with diameters around 1 mm, but as you can see, the particle size distributions are very different ! The Forté generates way less fines (with diameters below 0.5 mm) and slightly less boulders (with diameters of approximately 2 mm), which is indicative of a better quality grinder.

An example of figure that can be generated with the coffee grind size software. Each red bar corresponds to one kind of particle diameter generated by the Lido 3 grinder (smallest particles correspond to the leftmost bar, largest ones correspond to the rightmost bar), and their heights correspond to the total contribution of these kinds of particles, by mass. As you can see, the particles that contribute to the largest amount of mass have sizes just above 1 mm. The red circle shows the average particle diameter for the Lido 3, and the horizontal bars show the “characteristic width” of the distribution – this corresponds to the width that covers approximately 68% of all the distribution. Similarly, the blue line and the blue circle describe the distribution of the Forté grinder.

For now, the app is only intended to be used on OS X computers. But if you are running any other kind of system and know your way around Python, you can always download it directly from GitHub and run it with your own installation of Python 3.

This is an example of how to use the coffee grind size application.

I would like to thank Scott Rao for his excitement when I shared this project idea with him, and for beta testing the software. I would also like to thank Alex Levitt, Mitch Hale, Caleb Fischer, Francisco Quijano and Victor Malherbe for beta testing the software.

Some Strategies to Keep your Coffee Fresh

[Edit October 28 2019: I now strongly recommend against using oxy-sorbs or any oxygen scavenger bags with coffee. I will eventually write a detailed post about this, but it can impart a really bad taste to coffee; for now you can find more information here.]


There are few things more annoying than discovering some of your favorite coffee beans are getting stale and taste much worse than when you first opened the bag. I experimented with various methods to keep coffee beans fresher in the last six months, and I would like to share some of my findings in the form of different options you could adopt.

In short, there are four things you want to keep your coffee away from: oxygen, humidity, heat and UV light. All of those can damage coffee over time. The various tools and approaches described below are therefore designed to protect coffee from one or more of these factors.

Original Packaging

Freshly roasted coffee degases a lot of CO2 for a short amount of time. If it is quickly sealed in a bag with a one-way valve by your roaster, this will contribute to expel some of the oxygen that was initially present in the bag, therefore creating an even better protection against oxygen. When you open the bag for the first time however, all of this CO2 immediately leaves the bag and gets replaced again by the average air composition, with all its oxygen. The coffee does not have that much more CO2 to release anymore, and therefore this newly added oxygen will be free to slowly damage the coffee with oxidation. This effect becomes even more marked when the bag gets almost empty, because then you have more air (and therefore oxygen) in your bag that is free to attack a smaller amount of coffee.

These considerations explain why you can get a great cup from a freshly opened bag roasted a month ago, but within just a week or so after you opened that bag, the taste will quickly degrade unless you take proper care to protect your coffee against oxygen.

Generally, it’s a good idea to open the original bag only when you brew your first cup of coffee with it. You still need to keep the bag away from heat, and it’s good not to shake it too much and store it upright so as to keep its CO2 reserve intact.

Opaque zip-locks with valves

One of the easiest ways to quickly store coffee in a safe place is to use opaque and hermetic zip-lock bag with one-way valves such as these ones. They will protect coffee from humidity, UV light, and also from oxygen up to a point. A lot of roasters already sell their coffee in high quality bags like this, but I find it useful to have a few of them for the moments where I discover a roaster did not add a zip-lock to their bags for example (I’m looking at you with an angry face, all roasters who don’t). Just make sure that you get as much of the air out of the bag every time you close it.

Inert Gases

Bottles of inert gases (typically CO2 and nitrogen) can often be bought in wine stores for about fifteen dollars a bottle. They are a bit hard to order online, but it is worth getting a few of them as a nice addition to zip-lock bags; you can just push the oxygen out of the bag by adding a small amount of inert gases in the bag just before sealing it.

Here’s how I use inert gas bottles: Put the bottle’s straw through the zip-lock, and almost close the zip-lock bag except for a small opening where the straw is. Get most air out of the bag by pressing on it with your other hand, and with your hand still pressed on the bag, give a small 1-second push of inert gases (you can make it 2 seconds if you have a very large and almost empty bag). Immediately remove the straw and close the rest of the zip-lock bag. A typical inert gas bottle will last for a bit more than a hundred uses like that.

You will need to do this every time you open and close the bag however, so this is not a particularly great solution if you constantly brew coffee. In these cases, oxygen pads (more below) are a better solution.

An inert gas bottle.

Vacuum Sealers

Vacuum sealers require a bit more work on your part, but in my opinion they provide one of the best ways to keep your coffee fresh, especially in the long term when combined with other methods. I had amazing coffee vac-sealed months ago with no hint of oxidation whatsoever.

I only tried one vacuum sealer yet, and it turns out that it works pretty well for me. It’s probably not appropriate for industrial use, but I have been using it almost every day for more than three months and I had no issues with it. There are two annoying things about it, but I’m not sure which vacuum sealers don’t have these issues, if any. (1) The plastic bags are way too large and need to be cut, which is a bit more work than I’d like; (2) the vacuum chamber is a bit far from the edge of the sealer, so you always need to leave a bit more than an inch empty in the plastic bag.

Here’s how I recommend using this sealer; take one of the unnecessarily huge plastic bags with a ruler and a pen. Make two marks on the bag at 1/3 and 2/3 of its width (excluding the sealed margins). Do this near the bottom and near the top of the bag. With the ruler, use these marks to draw a vertical line at 1/3 the width and another one at 2/3. Use scissors to cut along only one of these lines (if you cut out both right away, the rest will be more of a hassle). Use the vacuum sealer in heat-seal mode (not vac-seal) to seal both sides that you just cut out. Cut along the second line, and heat-seal both sides again. You now have three thin and long bags; those are much more useable sizes in my opinion, and they will save you plastic in the long term because you will minimize the empty portion of the that the vac sealer forces you to leave.

When you fill a vacuum bag with coffee, I recommend using a relatively large funnel – just make sure the mouth will let beans through without clogging first. As I mentioned earlier, make sure you leave enough space at the top of the bag to seal it properly. I suggest marking what coffee this is and the roast and seal dates at the bottom (not the top) of the bag. The thing that is really neat with this format is that you can easily cut the bag open, weight a single dose out of it, then immediately re-seal it with your vac-sealer. Because you are freeing up more space than you are cutting out every time, you will even be left with a smaller but re-usable bag.

It’s always good to leave your vac-sealed coffee bag on the counter for a dozen minutes after you vac-sealed it, especially when you use a plastic bag for the first time. This will allow you to quickly notice if you didn’t properly heat-seal one of the bags or if it was otherwise damaged, because it will become loose. There is also one thing your vac-seal bags won’t protect against: UV light. It is therefore good precaution to either store them in a dark closet or in an opaque bag.

Make sure you store vac-sealed bags of coffee in a dark place, or in an opaque bag.

Freezing Coffee

There is one device in most people’s houses that is great at long-term preservation: freezers. Keeping coffee in the freezer has sometimes been feared by the coffee community. This is probably mostly true because careless storage in the freezer can quickly destroy your coffee. Remember that humidity is one principal enemy of your coffee; this is one that your freezer alone will not protect against.

To protect your coffee against the humidity and potential odors in your freezer, you simply need to seal it carefully. This is very easy to do with vac-sealed coffee, but I recommend putting vac-sealed bags in a large plastic container, because otherwise it’s easy to poke a hole or tear a bag with the other food you store in the freezer. Cheap zip-locks or plastic containers often do not provide a good seal and are at risk of letting humidity in your bag of coffee. I suggest using slightly more expensive sturdy bags with double zip-locks, or tupperwares with a rubber gasket and clips. You can also use something like the Airscape, but I tend to prefer bags because they take up less space as you use up the coffee. But hey, maybe you don’t store 25 different coffees in your freezer like I do.

There’s another subtlety in using the freezer to store your beans. When you take something cold out of the freezer and leave it in contact with air, the ambient humidity will quickly condense on its cold surface. This means your coffee beans will come in contact with water if you open a sealed bag of coffee that is still cold. This is probably not bad for a dose of coffee you’re about to use, but it is really bad for the rest of coffee you’re about to re-seal. I recommend only using the freezer for medium to long term use: when you decide to drink one of your bags of coffee, just take it out of the freezer a dozen minutes before breaking its seal, and then store it outside the freezer.

If you’re motivated enough to single-dose your coffee in the freezer, you won’t have this problem as much, but vac-sealing them will be really annoying. There is a study showing that cold coffee beans shatter a bit more therefore creating slightly more fines in your grind distribution at typical freezer temperatures, so you might still want to let them thaw a little before grinding them.

I read some baristas refraining from using the freezer because they were afraid that the humidity inside the coffee beans would freeze into crystals. However, I have seen Scott Rao mention that this water is trapped in cellulose cells and cannot crystallize as a consequence – I have not seen studies on this, but my taste buds informed me that coffee vac-sealed and properly stored in the freezer for more than a year still tastes great. I also read that un-freezing and re-freezing coffee is bad; I am not sure why and I never tried, but my guess would be that this is either based on bacterial build up or a gradual weakening of the cellulose cells inside the coffee. One last consideration; I read testimonies about how great ultra-low temperature freezers are for preserving coffee, but those are very expensive and I never tried it.

Oxygen Absorbers

Thanks to Matt Perger who recently shared something about this on Instagram, more recently I decided to use oxygen absorber pads instead of inert gases. They are relatively cheap, can be shipped easily, and they won’t get immediately spoiled every time you open a bag. If you would like to know more technical details on oxygen bags, I recommend reading this page. Here’s a summarized version of useful facts: (1) typical 100cc bags can be used to store up to about a pound of material; (2) it takes several hours for the pad to absorb all oxygen from your bag. Oxygen also won’t attack your coffee extremely fast, so this is good because it also means you don’t have to be in a total rush to seal your bag; (3) when the oxygen pad is completely spent, you can feel through the bag that the materials inside it will are clumpy and crystallized.

If you order some of them, you’ll notice that they come vac-sealed. This should not surprise you, because they would otherwise already be spent (it surprised me for 5 seconds). When you break the package open, I recommend placing them all in a large sealed zip-lock (see above). You can then just open and close the zip-lock every time you need one. Just don’t forget the zip-lock bag open.

My Gold Standard for Storing Coffee

Now that we discussed all the tools that I like to use, here’s my gold standard of how I store coffee that I care about:

  • I leave it in the original bag until I first brew it, unless I want to keep it for long term use, in which case I open it right away.
  • When I open the bag, I transfer it to a few thin vac-sealed bags, each with one oxygen absorber at the bottom.
  • I typically keep one of them in an opaque bag at room temperature and store the rest in a plastic tupperware in the freezer.
  • When I want to use a coffee stored in the freezer, I just transfer it to the opaque bag at least a dozen minutes before I use it (sometimes the night before).
  • When I want to brew coffee, I cut open a room-temperature vac-seal bag, get the dose out, and immediately seal it again.

When I don’t care about a coffee as much, I simply put it in a sealed zip-lock if the original bag lacks it, and put an oxygen absorber pad in the bag. Before closing the bag, you can whisper “you should have been roasted better” in the bag.

A Recipe to Brew High Extraction Coffee with the Siphon

Today I am finally sharing a recipe for the siphon brewer. I will use a bit of technical jargon at times in this blog post. If you encounter a word you’re not familiar with, I recommend you consult Mitch Hale’s glossary.

The siphon method is far from being appropriate for most people’s daily routines, as it is harder to execute correctly, and it takes time and requires more maintenance and cleaning. But the siphon has a really neat advantage: it does not rely on gravity to drive water through the bed of coffee. Instead, air inside the lower chamber is heated, causing it to expand and push water in the upper chamber. When the heat source is interrupted and the air contracts back to normal, this creates a large pressure difference between the lower and upper chambers, and sucks back water through the coffee bed, even if the coffee is ground fine enough to completely clog a V60 brew.

These siphon mechanics open up a very interesting door: you can play with much finer grind sizes and much higher average extraction yields than a V60. The method I want to present you today leverages this to reach much higher extraction yields than what can typically be done with a V60 brew (think of extraction yields 24% and above), while still getting a very clean cup free of oil or fine coffee particles.

I am re-discovering a lot of the coffee stored in my freezer with this recipe; not all of them react very well to these very high extraction yields however. I recommend using this recipe only with coffee beans with very well developed roasts. Otherwise, some woody or astringent tastes may appear much more dominantly than they would in a ~20-22% extraction yield V60. When I receive a new bag of coffee I never tried, I won’t go directly try this siphon recipe on it. Instead, I’ll make a V60 with my usual Rao-style recipe, and if I notice that I can reach higher extraction yields than most other coffee (e.g., 22%), then I will try brewing it with this siphon recipe.

There are a few coffees that yielded great to amazing results with this method for me: The Buufata Konga Ethiopian and the Gesha Village 2018 Wet Processed both roasted by Passenger; the Mamuto AA Kenyan roasted by George Howell; the Kayon Mountain Ethiopian roasted by 49th Parallel; the Kiandu AB Kenyan roasted by Heart; and the Karogoto Kenyan roasted by Tim Wendelboe. All of them produced average extraction yields between 23.2% and 24.6% with this recipe and my Forté grinder, when calculated with the simple percolation equation (I’ll discuss this more below).  However, in my experience very few roasters that I have tried were able to pull amazing roasts several times in a row even when I ordered the same bag of coffee a month later. I’ve only tried a single batch of each of these coffees and they turned out great, but I don’t know whether they will always be great. I also don’t know either of these roasters well enough to make any prediction about this.

I have been trying to use finer grind sizes with the siphon for four months now, with the goal of achieving higher extraction yields, but I was struggling to avoid fines getting in the beverage and to obtain good-tasting brews. As you will see, it turns out that I was failing because I was not being bold enough; there is a valley in grind sizes, from espresso to a bit finer than V60, where coffee fines can sneak through the Hario filter holders. And this led me to believe I could not use a grind size as fine as I hoped with the siphon.

You might be wondering why I would even want to try grinding finer in the first place. To understand the thought process, I recommend reading my post on The Dynamics of Coffee Extraction. It explains why grinding finer will allow you to reach higher average extraction yields and also more even extractions – this is something Matt Perger has been shouting from the rooftops for a while now, but I only recently heard about it.

I have also recently been experimenting Dan Eils’ Vac60 prototype which he generously sent me, and this led me to explore and think about finer grind sizes again. My friend Mitch Hale had also been taunting me with his 26+% extraction yield Turkish brews made with his fantastic EG-1 grinder that everyone is jealous of. All of this led me to try grinding Turkish style for my siphon – Mitch also helped me figure out whether my Forté could actually do it, and turns out it can. These experiments finally allowed me to brew the first high-extraction yield siphon that I really enjoyed, about 60 siphon brews later. If you are a regular siphon user, you may find this recipe somewhat weird or shocking, but please try it before judging 🙂 I especially recommend it for very soluble coffee that doesn’t taste roasty (i.e., try it on great roasts).

The basic idea behind this recipe is that we want to grind as fine as we can and prepare an immersion-style brew similar to Turkish and quickly reach a very high extraction yield, then filter out the oil and coffee fines to get a clean beverage. To achieve this, regular cloth filters are not great because they don’t filter out all the oil. Plus, they are a total nightmare to clean and maintain – they quickly develop a rancid taste, and they can even easily develop mold from their constant contact with water. If you insist on using them, please read the Cloth Filters section at the end of this article, but I honestly have not made a great brew with them even after going through a pack of 10 brand new Hario cloth filters.

Fortunately, paper filters can be used with this recipe, and I find that they produce better coffee. They are also way easier to maintain: you just need to pre-rinse them, brew your coffee, then carefully throw them in the garbage. Unfortunately, the design of the Hario paper filter holder is not too optimal in my opinion. The central screw (WHY ?) allows fines to go through if you grind just a bit finer than V60 brews. But as mentioned earlier, what I recently discovered is that if you grind much finer, you don’t get this problem – the coffee bed becomes much more cohesive (probably because of the surface tension of water) and forms some kind of paste that stops getting around the edges of the filter holder.

One key point of this recipe is that the coffee bed itself is used as a filter, in addition to the paper filter. As long as a small layer of coffee grounds deposit on the paper filter fast enough before the start of the drawdown phase, the rest of the beverage will be filtered out of coffee fines by the bed of coffee itself. The paper filter is still important to absorb coffee oils, an effect that you would not get with a metal filter – I also suspect fines may pass through the metal filter, but I have not tried.

List of Required Material

  • A siphon brewer: I use the Hario Next 5-cup, but I recommend the Hario Technica three-cup if you never brew large batches.
  • A bamboo paddle or any other kind of similar stirring object (ideally food grade and not thermally conductive).
  • The Hario paper filter holder and some paper filters.
  • A heat source: I use the Hario beam heater, but butane heaters should also work. I do not think alcohol heaters would be powerful enough. The “Hario Smart” beam heater works too and is sometimes the only one available, but it is way more expensive.
  • A bead temperature probe. This may seem overkill, but it is not. I find it extremely hard to brew a consistent siphon without it, and without it is also very easy to brew coffee at 180℉ without noticing. A kitchen heat thermometer can work, but it will be much more of a hassle, and much slower because you won’t be able to put the plastic lid on top of the upper chamber to keep the heat in.
  • Some brew water – I like to use the Rao/Perger water recipe. For more detail, see my blog post on brew water.
  • Any clean kettle. If you are extremely patient you could do without by heating water directly with your beam heater or butane heater.
  • A brewing scale. I use the Acaia Pearl scale which is neat because it’s large enough to put the Siphon on, but I’m sure you can get around with other less expensive scales, just make sure you get one precise at 0.1g or better.
  • A timer, unless your scale can act as one.
  • A grinder that can grind extremely fine. I use the Baratza Forté BG grinder which can grind fine enough only when the zero position is well calibrated (more on that below). I suspect the Encore and Virtuoso could also do it, but I don’t know. The more expensive grinders like Mahlkonig EK43 or Lyn Weber’s EG-1 can do it better than my Forté (for a LOT more money), but with the EK43 you will typically needs to align the burrs for that. Going this fine with a manual grinder would certainly be a huge pain when it’s even possible.
  • A small container to hold your dose of coffee beans or ground coffee.
  • Tabbed and bleached Hario V60 filters (not crucial but they can be useful, more information below).
  • Some OxiClean for the occasional maintenance of your siphon glass. Soap can work but it is not as effective.
  • I suggest always keeping a kitchen towel near your brewing space in case the lower chamber of the siphon loosens when it is hot.
  • A bottle brush to clean up the lower chamber of the siphon.
  • A lens blower to help clean up your grinder. This is facultative but it really helps especially when grinding Turkish style.
  • A “not a flamethrower” by the Boring company.

Grind Size

You will want to grind extremely fine for this recipe. The grind size should be finer than espresso and similar to Turkish brews. It should look and feel like flour, and form clumps. You should be able to see your fingerprint if you press a finger on a bunch of ground coffee (see picture below). On my Forté, 1A was not fine enough with its initial calibration so I had to re-zero it with the burr calibration tool. Please follow the Baratza user manual if you do this. Here’s where I set mine: turn the calibration screw until you just very barely start hearing the burrs touching at setting 1M, then use setting 1I (letter i) for this recipe. When the calibration tool is not inserted, I do not hear the burrs touching at 1M, but I do hear them touch at 1I. Please be aware that doing this will mess up all your previously cataloged grind size setups, sorry. The only reason I don’t go finer is because I don’t want to put too much strain on my Forté’s motor; Mitch has been grinding much finer than I do with great results (and higher extraction yields), so I suspect that there is no grind size that is “too fine” for this recipe.

Your grind size should be fine enough that it spontaneously forms clumps.
Your grind size should also be fine enough that it starts retaining low-resolution fingerprints if you press your index finger against the dry coffee. This requires an even finer grind size than the formation of clumps. If you see clearer fingerprints than those on this image, congratulations, your grinder is better aligned than mine ! This is good for you, and you will probably be able to reach higher average extraction yields.

Grinding this fine means that your grinder may retain a lot of ground coffee, depending on its internal structure. For example, my Baratza Forté retains about 1.5 grams with this setting. Because of this, I recommend either grinding about 2 grams of the coffee you are about to use and throwing away what comes out – it’s mostly coffee from the last time you brewed. Another way to do this even better is to turn on the (empty) grinder, move the grind size up to the coarsest setting, and wait for the old coffee to come out. You can then move the grind size back to the desired setting while the grinder is running, and then turn the grinder off.

Preparing the Filter and Workspace

First, make sure you have some un-cluttered space on your counter in front of the beam heater. You’ll need to quickly remove the siphon from the beam heater at some point, and put it down somewhere nearby. If you use the butane burner, it might be easier to move the burner instead, so this might not apply to you. I once had the genius idea to try using a thick cork pot stand and put it on the beam heater right after I turn it off to avoid having to move the siphon at all, but it turns that out even once it’s turned off, the beam heater is crazy hot, and it completely burned the cork stand. I suggest you don’t try this and set your house on fire. If this is your first time using the siphon, I suggest checking that your counter is level; otherwise it could affect channeling in the drawdown phase.

Check that the lower chamber of the siphon is well attached to its stand. I have the Hario Next model, and this part is a little finicky – pushing too much on the upper chamber can cause the lower chamber to come off from the stand, which could be a huge problem when it’s full of hot water. Now that you did that, fill the bottom chamber of the siphon with hot tap water. This will help speed up the brewing process.

Make sure the plastic lid of your siphon (also used as a stand for the upper chamber) is clean and free of water or old coffee. One of the quickest ways to turn your brew to shit is to forget some old coffee at the bottom of the lid and put it on top of your siphon. One way to be sure you don’t do this is to put the lid upside down on the counter after you cleaned it. Also make sure all parts of your siphon are clean and free of coffee oil stains. See the section on clean up and maintenance otherwise.

Mount the paper filter on the filter stand, and gently fold the paper up without straining the edges. Make sure the holes on both sides of the filter holder are aligned by looking through the filter toward a bright source of light. If you can’t see them you can gently wet the filter a little bit. This step is crucial because mis-aligned holes will allow less flow of water and this will increase channeling around the filter holder. Place it in the upper chamber of the siphon – the paper should be folding up, not down.

Pull the hook and put it in place, then make sure the filter stand is centered by looking at the bottom of the siphon chamber and making sure the holes in the filter holder are centered. You can also look sideways and make sure the filter holder is level. You can move it around by holding on the middle pole of the filter holder. Do this before wetting the filter completely to avoid damaging the paper filter when you are installing it in the brew chamber. Run some tap water on the filter to make sure it’s free of paper taste. At this point I recommend putting the upper chamber upside down on your counter until you use it – Don’t use the plastic lid as a stand for now, to ensure the lid is still upside down and entirely clear when you put it on top of the upper brew chamber later on.

The paper filter initially tends to extend downward.
Gently fold the dry paper filter upward with your fingers.
The paper filter should be folding upward in the siphon chamber.
Look from the bottom of the chamber and center the filter holder.
The plastic lid of the Hario siphon also serves as a stand for the upper chamber, but don’t do it for now.

Now determine how much coffee you want to make, as well as your brew ratio. In his Everything but Espresso book, Scott Rao recommends making a batch of coffee that takes up at least 2/3 of your siphon capacity to get a good vacuum during drawdown, and I tend to follow this recommendation. With this method I recommend starting with a ratio 1:17 and go up from there next time (e.g. 1:18) if you find that your brew was too strong. This may be needed for very well developed roasts, which are more easily soluble, or with an extremely well-aligned grinder. For example, you might decide to go with 400g of water and 23.5g of coffee, for a 1:17 ratio. I’ll use these numbers from now on. If your batch is too strong, you can always add some hot brew water to your beverage to lower down the concentration, and it should still taste good. The other way around is not fun, because there is no easy way to increase the concentration of your beverage.

You might notice that you prefer higher concentrations when you brew at very high extraction yields – this seems to be the case for me. I used to prefer TDS concentrations of about 1.3% when I brewed V60s at 19-20% extraction yields, and moved to preferring brews at 1.4% to 1.45% TDS with V60s at 20-22% extraction yields. Now, these siphon brews that get me extraction yields around 24% seem to be more enjoyable at a concentration around 1.5% TDS. Maybe it’s just me 😛

Brewing the Coffee

Boil some of your brew water in a kettle. Empty the warm tap water form the bottom chamber of the siphon, place it on your brew scale and tare, then pour 400g of hot water in it. If you happen to have 1mL glass pipettes, I find it easier and faster to go just a tad over 400g and adjust by removing water with the glass pipette. Place the lower chamber on the heat source and turn the heat up to its maximum level; here I’ll assume you are using a beam heater; I’m not familiar with the butane heater so I’ll let you decide on the exact heat level if that’s your case. You can now preheat the upper chamber a bit by pouring warm tap water on it. This is not required but it will shorten the preheat time.

Place the temperature probe in the bottom chamber, turn on the thermometer and gently put the upper chamber on top without sealing it on. This will allow you to heat up the water for a bit without having it immediately climb up the upper chamber. When the temperature reaches 210℉ or more, remove the temperature probe and place the upper chamber on top to seal it on. Do not press down too much because this could cause the lower chamber to fall down from the stand in which case I wish you luck. Make sure your upper chamber is level; I find it easier to tell when looking at the rubber joint.

It is important not to leave the lower chamber heating up without having anything in contact with the water, whether it is the temperature probe or the tube of the upper chamber. Otherwise, the water could get superheated and “explode” everywhere when you put the upper chamber on. This is caused by the very smooth surface of the siphon – water can go slightly above boiling point without actually boiling if it’s not perturbed in any way and does not have access to a nucleation site (see this Wikipedia article if you want to know more).

Once you sealed the upper chamber, place the temperature probe inside the upper chamber and close it with the plastic lid to get better heat retention. You can now turn the temperature reader on. Now you’ll need to wait for a good 6-8 minutes for the upper chamber to reach 202℉ (94℃) – this will allow you to actually brew at approximately 198℉ (92℃). If just used your siphon, it will be a bit faster; otherwise I find that the temperature stagnates at around ~185℉ (85℃) for a little while as the siphon’s upper chamber glass is sucking up heat and warming up. Take this time to meditate on the idea that the self is an illusion. Also make sure that the brew water is perfectly transparent in the upper chamber, otherwise it means that something was not cleaned up properly and your brew will probably taste like dirt.

You can also use this time to weigh your dose of coffee, set up your grinder and grind the coffee. If you single dose, start the motor before you put the coffee in so that you will be grinding at a uniform motor rotation rate. Single dosing is often recommended against because pop-corning beans get ground coarser than those pushed through the burrs, but in my experience this affects such a small fraction of the dose that it virtually does not affect the particle size distribution (this will be a future blog post).

Make sure your small coffee container is perfectly dry, otherwise ground coffee might stick in it and this will be very annoying. Weigh the exact dose of ground coffee in the container. You can wait a bit to grind if you are afraid that the coffee loses freshness, but I was unable to taste a difference even when I ground right when I turned the beam heater on. Just put something on top of your ground coffee container to limit contact with air. I like to put the grounds in a container smaller than the width of the upper siphon chamber to help pouring it more quickly and precisely later on. Just make sure you use a clean container. I also like to use a tea spoon to transfer ground coffee from my Forté’s bin to the container to avoid spilling coffee.

The bead-type temperature probe allows to accurately measure the upper chamber of the siphon with the plastic lid on. This will allow you to brew more precisely and reach the proper temperature more quickly.
Bubbles will form because of small gaps at the edges and the center of the paper filter holder – this is normal, and surprisingly coffee fines won’t be able to sneak through these same gaps if you grind fine enough.

When the temperature reaches 202℉ (94℃), it is time to start brewing. If you missed this window, turn off the heat (leave the plastic lid on to avoid evaporation) and wait for the temperature to go back to about 195℉ (91℃) then turn the heat back on. If you see temperature going up so fast that you will probably miss the window, it’s ok to turn down the heat to approximately 1/3 of the maximum while you finish your preparatives, and then turn it back on to maximum heat. This will cause the temperature to go up much more slowly, without losing enough pressure for the water to fall back in the bottom chamber.

Once you reached 202℉ (94℃), lower down heat to approximately 2/3 of the maximum (on my Hario beam heater this is at the logo with two flames), and immediately remove the lid. Remove the temperature probe now if you don’t want to have to clean it up. Start your timer and quickly put the coffee in the upper chamber. To minimize spilling, place your coffee container centered straight above the siphon, turn it upside down and gently tap it with your other hand (this takes me approximately 4 seconds). Make sure that no grounds are stuck in the container, it’s worth tapping a bit more vigorously if they stick. Use the bamboo paddle to vigorously stir in up-down and left-right linear motions to create a lot of turbulence and maximize extraction, until the timer hits 0:15. This is a good time to hum some Fleshgod Apocalypse.

I find that holding the bamboo paddle straight vertically and moving your arm in a back and forth motion, rather than keeping your hand in the middle and paddling with your wrist, will minimize splatter on the glass walls. This should allow you to quickly break down any clump of dry coffee. If some grounds stick to the glass walls, try to quickly drag them back in. End the initial stirring phase with a very short and vigorous rotation motion (e.g. 1 turn) when the timer hits 0:15.

I also tried using a kitchen whisk instead of a bamboo paddle, but I was not satisfied with the results. The width of the paddle makes it easier to induce a strong flow in the slurry, which creates a lot of turbulence – I trust turbulence more than the many branches of a whisk to break down clumps of dry coffee.

I don’t think you need to be too obsessed about the repeatability of your stir, because this recipe aims to quickly reach very high extraction yields, which also quickly saturates the concentration of the slurry. As a result, extraction will become very slow after the first few seconds of immersion, so differences in how much you stir will only marginally affect your final extraction, as long as you break all clumps of dry coffee. Mitch and I were able to produce subsequent brews at the same concentration (down to 0.01%) with this method, when we used the same coffee beans.

When the timer hits 0:35 (yes, just 20 seconds later), turn off the heater and very carefully remove the siphon from the beam heater and put it on the counter. You don’t want to impart any acceleration that could agitate the coffee bed. You can make the drawdown phase faster by wetting a rag with cold water and immediately wrapping it around the bottom siphon chamber (I recommend wetting it during the 20 seconds wait and already have it in your hand). This is especially useful if you are brewing smaller batches. If you do this, you will need to remove the rag as soon as it feels hot because at that point it’s working against you by insulating the siphon chamber from cooler ambient air. Whether you decided to use a cold rag or not, immediately start inducing a slightly vigorous rotation motion in the slurry with the bamboo paddle. I do something like 5 full rotations in about 3 seconds total. Don’t put the paddle too deep because you don’t want to agitate the coffee bed that started depositing at the bottom; I put it about half the slurry deep. Note your total brew time when the beverage in the lower chamber starts bubbling heavily. Try to have a total brew time shorter than 3:00.

Scott Rao recommends that the shape of the coffee bed should resemble a slight parabola at the end of the drawdown, which goes slightly above the central pole of the filter holder. The reason for this is to avoid channeling and ensure an even extraction. This recipe allows to reach very high extraction yields during the immersion phase (before drawdown), so I think that this requirement a bit is less critical. I tried measuring the concentration of the upper chamber (with syringe filters and the VST refractometer) immediately before drawdown and I obtained exactly the same concentration as the final beverage, down to a precision of 0.01%. Extreme levels of channeling could lead to the paper filter breaking or fines passing around the filter holder in the beverage, but in my experience this did not happen even when the coffee bed was flat.

Scott convinced me that I should still care about channeling despite these considerations, and after a bit of experimentation I did produce some relatively astringent brews when channeling was significant, so I still recommend that you care about it too. Keep in mind that it is much harder to obtain a nice dome-shaped coffee bed with an extremely fine grind, in part because the drawdown phase takes more time, but I also suspect that the bed crumbles more easily. If your coffee bed is too flat, it means you didn’t cause rotate the slurry enough with the paddle, or you rotated it too early before drawdown. If the the center of the coffee bed is too high and is shaped like a dome, it means that you rotated too much.

The ideal shape of the coffee bed (in red) and my closest approach with such a fine grind size. As you can see, the bed cracked on its sides at the very end of drawdown.
This coffee bed has too much curvature and resembles a dome, indicating that I applied too much rotation with the paddle, and that the percolation phase channeled. You know you had channeling if the depth of the coffee bed is almost zero along the edges of your siphon chamber.

Once the drawdown phase is over, carefully remove the upper chamber (it will be hot), and place it upright in the siphon plastic lid, which also serves as a stand for the upper chamber. Once the foam around the coffee bed disappears, look at it from above; a non uniform color would indicate an uneven extraction due to channeling. The coffee bed will often crack, this is normal with such a fine grind.

A view of the coffee bed from above. The color of the coffee grounds is uniform, except for the light reflections caused by the glass walls.

Brews produced with the siphon are initially much hotter than most other brew methods. It will be hard to taste all the subtleties of the coffee when it is still very hot, and while it looks, the bottom chamber of the siphon won’t allow the coffee to cool down very quickly. For this reason, I like to pour the coffee out into a different vessel. You will automatically lose around 10℉ (5℃) when pouring out the coffee this way. If you have the Melodrip, you can pour your coffee through it to get it to an enjoyable temperature faster. James Hoffmann suggests to do this promptly, because the very high temperature may actually degrade the coffee flavors after a while. I never tried blind tasting to confirm this, but I follow the recommendation because it’s an easy thing to do and I prefer drinking coffee when it’s a bit cooler anyway.

If you look at your siphon beverage from under a bright light, you should see a clear beverage free of fines. The shades in this image are due to a deformed image of the lamp behind it.

More Beverage Clarity

I found that sometimes, these siphon brews left a small bit of coffee oil in the beverage, which probably passed through the edges of the filter mechanism. This is a small amount that is not always obvious to taste, but I found that filtering the siphon brew again with a pre-rinsed V60 paper filter will remove any potential oil left in there, so I recommend it if you are not low on paper filters.

To do this, simply put a V60 paper filter in any type of V60, pre-rinse it and pour the siphon beverage through it. I try to pour it uniformly across the walls of the V60 paper to use as much paper surface as I can, and absorb as much oil as possible. This is one rare occurrence where you don’t need to care about what material the V60 is made of, because it’s not actually a bad thing to cool down the beverage a bit more.

If you want to measure your beverage weight to get a more accurate measurement of your average extraction yield (see below for more details), this is a great moment to do it. Simply place your vessel and V60 on your brew scale, tare it, and pour the coffee. Just make sure you note the weight measurement before removing the V60, because you tared the scale with the V60 on.

Filtering the brew a second time with a pre-rinsed V60 paper filter will ensure that no coffee oil remains in the beverage. Pouring the brew on the sides of the V60 walls will allow it to absorb more oil.
The V60 filter will become slightly stained by coffee oils, but as you can see no coffee fines are visible on the paper filter.

Summary of the Brew Steps

Here is a short summary of the recipe detailed above. Here, I will assume that you are using the 5-cup sized siphon to brew an approximately 400g beverage and that you are brewing with a 1:17 ratio.

  • Boil more than 400g of brew water with a kettle.
  • Mount the paper filter on the filter holder.
  • Install the filter holder in the upper siphon chamber.
  • Make sure the paper filter is centered.
  • Rinse the paper filter with tap water.
  • Place the lower siphon chamber on your brew scale and tare.
  • Pour 400g hot water in the lower siphon chamber.
  • Place the lower siphon chamber on the beam heater and turn it on at maximum heat.
  • Immediately place the upper chamber on to seal it. Don’t press down too hard. Make sure it’s level.
  • Place the thermometer probe in the upper chamber and turn the thermometer on.
  • Make sure the plastic lead is dry and clean and place it on the upper chamber.
  • Weight about 25g of beans (or 23.5g plus whatever your grinder retains).
  • Grind the beans, weigh a dose of exactly 23.5g, and cover the grounds.
  • Wait for the temperature to reach 202℉ (94℃).
  • Lower the heat source temperature to 2/3 of the maximum.
  • Remove the plastic lid and temperature probe.
  • Start the timer and put in the coffee dose.
  • Stir vigorously in linear motions until 0:15 then make one vigorous rotation.
  • Wait 20 seconds. Prepare a rag wetted with cold water and keep it in your hand.
  • At 0:35, turn off the heat source.
  • Immediately and gently remove the siphon from the beam heater.
  • Place the cold wet rag around the lower siphon chamber.
  • Immediately impart a slightly vigorous rotation (5 turns in 3 seconds) while placing the paddle approximately half as deep as the slurry.
  • At any point where the rag becomes hot, remove it.
  • Note the drawdown time when the beverage starts bubbling. Aim for less than 3 minutes.
  • Inspect the shape and color of the coffee bed.
  • Facultative: pre-rinse a V60 paper filter and filter the beverage through it.
  • Facultative: use the Melodrip if you want to cool down your brew faster.
The siphon brewer in all its hellish might.

Clean up and Maintenance of the Siphon

Cleaning up the siphon takes a bit more time than other brew methods. I suggest first unhooking the filter holder while the upper chamber is upright, then place yourself above the sink, reach for the central pole of the filter holder with your other arm and gently lean the chamber on its side until it is upside down. Make sure you are holding the filter holder while you do this, and carefully remove it. Be careful as the coffee may still be hot. The filter holder and coffee bed are a bit heavy, and this will avoid damaging the glass walls of the siphon chamber. Rinse the filter holder and throw the paper filter in the thrash. Rinse both siphon chambers, the bamboo paddle and the plastic lid thoroughly, and leave them to dry. It’s good to wipe your beam heater clean too.

From time to time, I recommend cleaning up the glass parts (both chambers) of the siphon with a single drop of soap and a wet cloth. Make sure you rinse the soap thoroughly. You will need a bottle brush to clean up the inside of the lower chamber.

More rarely, or if you need a very thorough clean up of your siphon, you can put a solution of OxiClean (up to Line 1 of the provided spoon) in 500g of water in the lower chamber, and heat it up with the upper chamber attached and sealed. Do not put the bamboo paddle or filter holder in when you do this. Wait for a few minutes, then throw away the water and rinse all parts very thoroughly.

Calculating the Extraction Yield of your Beverage

The typical immersion equation (concentration times water-to-dose ratio) will not give you accurate results at all when calculating the extraction yield of a siphon beverage, and will tend to over-state your extraction yield by as much as ~2%. For more details about this, I recommend reading my blog post about how to calculate extraction yield for different brew methods.

The siphon method is special because the pressure differential during drawdown sucks out most of the interstitial water from the spent coffee bed. In the brews I have experimented with, the liquid retained ratio (mass of retained water divided by mass of coffee dose) was between 1.3 and 1.5, which is lower than the usual liquid retained ratio (about 2.0) for V60 brews. As a consequence, it will be more accurate to weight your beverage and simply use the percolation equation for extraction yields (detailed in my blog post on extraction). This assumes that all of the water retained in the coffee bed is absorbed in coffee cells rather than interstitial, and therefore does not participate to extraction. A slightly more precise way would be to assume that the coffee bed absorb its own weight in water (fabs = 1), that your slurry concentration is the same as your beverage concentration, and enter your beverage weight, water weight and dose in Mitch Hales’ online extraction yield calculator.

If you have a refractometer and want to verify whether coffee fines made it to your brew independent of your palate (fines would flatten the taste and leave a silky feeling on your tongue), you can use VST syringe filters and compare your concentration with or without using the syringe filters. If you followed these steps properly and have a well aligned grinder, both readings should be within 0.01% of each other. I highly recommend reading Mitch Hale’s post on how to measure extraction yields accurately.

This method allowed me to reach extraction yields up to 24.6% on a few coffees now, and I suspect I may be able to reach even higher extractions with very well developed roasts. Mitch has reached crazy extraction yields of 26% with his Turkish brews but I suspect this has more to do with his EG-1 grinder producing less “boulders” (the term is very relative here).

I tried this method on about 8 different coffee beans now, with really great results. Contrary to my expectations, I brewed an amazing Gesha Village that retained a lot of its floral and honey notes, and made great Kenyans and Ethiopians too. I did however try one particularly under-developed roast and it tasted awful and astringent. I suspect that this brew method will be best when you use well-roasted, high-quality beans, and it’s possible it won’t play well with some types of coffee. As Mitch put it, “you should use this when you actually want to extract everything from the beans”.

Cloth Filters

This is an adaptation of this comic at pbfcomics.com, reproduced with permission of the author.

For a reason that escapes me, siphons are traditionally sold with cloth filters. I have found that brews I made with cloth filters tasted less good than those I made with paper filters, even if I used a brand new clean and well-rinsed cloth filter, and post-filtered the brew with a V60 paper filter. However, the worst part about cloth filters is the rancid taste that they very quickly develop, even after just one brew.

If you insist on using them, here are some recommendations for you. When you install them on the filter holder, always do a simple loop knot, not a double one, otherwise you will hate yourself later on. Also buy a bunch of them, at least 10. After every brew, immediately clean up the top chamber of the siphon – leaving the coffee bed on will stain the filter more. Clean up as much of the coffee as you can by running tap water in the upper siphon chamber before you remove the filter holder, otherwise coffee fines could get stuck in the string seams of the filter.

Immediately wash the filter thoroughly under hot tap water and rub it gently with the palm of your hand until it looks white. Twist out as much water out as you can, and immediately store it in a sealed plastic container with something that absorbs humidity (raw rice would work). Place the plastic container in the freezer. You can keep a single plastic container filled with rice with all of your dirty cloth filters.

The only way I found to get the rancid taste off from cloth filters is to boil them with unscented, dye-free OxiClean (yellow label) after every brew. This is why I suggest doing this in batches once you accumulated at least a couple dirty filters. Once you have accumulated enough dirty filters, take the plastic container out of the freezer and let it thaw for the rice to separate from the cloth. Shake down the filters and throw away and replace the rice in your plastic container. Boil all filters in a solution of OxiClean and water (follow the package recommendations for the dose; fill spoon to line 1 per approx. 500 mL water) in a large pot. I recommend putting a lid on the pot and turning on your stovetop fan to the maximum, because this stuff stinks.

You can dry tumble your freshly washed cloth filters in a meshed cloth bag like this one.

Boil the filters for about 15 minutes, or until all filters look white, stirring occasionally. Throw away the dirty water, and rinse the filters with cold water. If they do not look perfectly white, do another OxiClean boil and rinse them again. Put them back in the pot with clean water and boil them again for 15 minutes, stirring occasionally. Do this again if they still smell like OxiClean; using a larger pot with more water will make this more efficient. When they don’t smell bad anymore, place them in a strainer and rinse them with cold water for a minute or so. Twist the water out of each filter and place them back in your plastic rice container in the freezer. If you don’t want to wait until you have no filters left before cleaning them, you can use two plastic containers, one for dirty filters and one for clean ones. Another option is to dry your freshly cleaned cloth filters in the tumbler. I tried placing the filters in a meshed cloth bag, similar to what you would use to buy fruits at the grocery store.

Welcome to Hell.

Some Acknowledgements

I would like to thank Mitch Hale for useful discussions and Dan Eils, who re-invigorated my interest in fine grind vacuum brews by generously sending me his Vac60 prototype. Thanks to Scott Rao for providing a lot of help to minimize channeling with this method. Thanks to Victor Malherbe for suggesting the use of a lens blower to clean up coffee grounds. I’d also like to thank Matt Perger and Barista Hustle for sharing a considerable amount of their knowledge about coffee extraction in general. Without this knowledge, I would not have cared about grinding finer in the first place.

Testing a Model of Extraction Dynamics

In a recent post, I presented a mathematical model for the dynamics of coffee extraction that is based on a few simple hypotheses. One of these is that the rate of extraction decreases exponentially. The rate at which it decreases can depend on many things: the type of coffee, the roast, the amount of agitation and the brew method would be examples that affect how fast the rate of extraction goes down. For example, in an immersion brew, water becomes gradually more concentrated, and it thus becomes less efficient at dissolving coffee. We should thus expect the rate of extraction to decrease faster in an immersion brew, compared to a percolation, even with the same exact coffee. How fast the rate of extraction goes down is what we technically call a free parameter of the model: the model itself makes no prediction on its value, and can accommodate any number.

One thing that the model crucially lacked until now is being tested against real-world data. As a scientist, I don’t like to leave something floating without testing it, and I want to see how many teeth it loses when it faces the real world. Hence, I didn’t lose too much time and tried to apply it to the data recently gathered by Barista Hustle. As mentioned earlier, an aspect I did not explicitly include in the equations of the model is the effect of fines. The maths behind them is boring, because I make they get extracted to the maximum as soon as they touch water, but I do include their effect to model real-life data. My model makes no prediction about how many fines there are in a given brew, so their proportion is yet another free parameter.

For all those of you new to this, what I’ll attempt here is called fitting a model to experimental data. It’s a game we often play in science: build a model based on some assumptions, which has a few free parameters (think of them like knobs you can adjust to get a different result). Take a set of real-life data, and try to reproduce exactly the same data by playing with your free parameters. If the model is good, you’ll be able to adjust the free parameters such that the model looks a lot like the data. If you have a really large number of free parameters, you will be able to reproduce any kind of data, and your model becomes very poor at making any kind of predictions – in that situation you will even be unable to test whether your starting hypotheses were good or not.

This might all seem a bit abstract, but I think it has the potential to unlock some important understanding about coffee brewing, which I hope will inform us on new ways we can experiment with brew methods and recipes.

Because we will now explicitly work with the Barista Hustle data, I want to remind readers of what their experiment was. They ground some coffee, and sifted it with a Kruve sifter set up with the 250 micron and 500 micron sieves. They thus ended up with three groups of coffee grounds; those that went through the 250 micron sieve (i.e., with diameters smaller than 250 micron along at least one axis); those that went through the 500 micron sieve but not the 250 micron sieve; and those that couldn’t pass even the 500 micron sieve. Depending on how long they sieved, and how much static electricity was present in the grounds, we should expect that some grounds fine enough to pass a given sieve might not always have passed it, but the global result will still be that they end up with three piles of grounds, which average sizes will be (1) smaller than 250 micron, (2) between 250 and 500 micron, and (3) above 500 micron.

They then weighed the exact same amount for each kind of grounds, and placed them in three distinct cupping bowls. They added hot water, and took samples out of the cupping bowls at distinct times. They then measured the concentration of their samples, and worked out the corresponding average extraction yield in the usual way. They got the following result:

Extraction yield versus time for three cupping bowls with large (blue), medium (orange) and fine (green) coffee particles.

So now, the question is – can my model fit this data ? What we have here are 18 data points, but those at t = 0 are really a given (no water = no extraction), so we really have 15 data points to model. When you play the game of model fitting, it is very important that you have less free parameters than the amount of data points. In my case, the model has 9 free parameters, here’s what they are:

  • The average characteristic time scale of extraction (τ)
  • The characteristic depth that water reaches in coffee particles (λ)
  • The maximal extraction yield that this coffee can reach
  • The average diameter of particles in the first bowl
  • The average diameter of particles in the second bowl
  • The average diameter of particles in the third bowl
  • The mass fraction of fines in the first bowl
  • The mass fraction of fines in the second bowl
  • The mass fraction of fines in the third bowl

The first three parameters are required to be the same for all three cupping bowls, because I assumed they use the exact same brew method, and the same agitation in all cases. We also know they used the same coffee and water in all cases. Now, I want to remind you of the hypotheses my model relies on. Any one of these being false will potentially hurt the model’s ability to represent the data.

  • The rate of extraction decreases exponentially for each coffee cell.
  • The rate of water contact decreases exponentially with the depth of a coffee cell.
  • All coffee particles are perfect spheres (the model actually doesn’t change much if you remove that hypothesis).
  • Coffee cells are cubic with a side of 20 micron.
  • Each bowl contains some fines plus a set of perfectly uniform coffee particles.
  • All available chemical compounds get immediately extracted from fines.

We in fact know that some of these must be false (e.g. spherical particles), but it will be very informative to see how well the model fares despite this. Make no mistake: making simplifying assumptions is a very powerful tool in science. It allows you to verify which aspects of an experiment are most important in explaining the outcome, and which aspects have a lesser impact. It is important to bear in mind that the model is a simplified version of reality, but it does not make it useless.

Now, we need to use a recipe to adjust the 9 knobs (the free parameters) of our model in a way that makes it look the most like the data. There are several ways to do this, and this is one aspect of science that I have a decent amount of experience with – a method I really like for this is called a Markov Chain Monte Carlo sampler. The details of it are quite technical, so I won’t go into them. Instead, I will provide you with a simple analogy, that I think is kind of accurate. Imagine the model is a blackbox machine with 9 knobs, and the goal is to adjust each knob individually to reproduce the data the best you can.

A Markov Chain Monte Carlo sampler is a bit like having a hundred monkeys with their own respective blackboxes, randomly tweaking the knobs and judging if the data fits the model. When the model of one monkey starts resembling the data, it starts getting agitated and yells, which gathers the attention of some other monkeys. They get jealous and notice how the successful monkey has adjusted its knobs, and they try to adjust theirs in a similar way, but they don’t do it perfectly. Some monkeys are harder to distract and they keep exploring very different combinations of knobs, until there is a turnaround point where so many monkeys are yelling that really all monkeys are starting to converge on a similar set-up. This moment is called the end of the burn-in phase in technical terms. Once it is reached, you can let the monkeys keep playing with the knobs for a while, and carefully pay attention to what knob combinations they try. At that point, most of what they try will be very close to the best solution. If you gather enough observations despite all the yelling, you will be able to tell what their average knob setting was, and how much they swung each knob around after the burn-in phase. From mathematical considerations, these two aspects will correspond to your best parameters as well as measurement errors for each parameter value. This method is very powerful at exploring all different combinations of knobs, while paying more attention to the combinations that produce better results.

So, I let my hundred computational “monkeys” try 4000 combinations each – the point where all of them were yelling loudly was reached well before the first 1000 combinations, so I paid attention to the last 3000 combinations to determine what the best parameters were. Here’s what the best combination generated:

Best fitting model (black lines) to the three cupping bowl experiments of Barista Hustle (red circles).

In all honesty, I was really surprised at this – I expected the model to do much more poorly. You can see that the first cupping bowl with the coarser particles (lower average extraction yields) does not fit as well as the other ones, so whatever effect makes the model imprecise is more pronounced for coarser particles. I suspect this may be related to the fact that each cupping bowl has a distribution of particle sizes and shapes, rather than a very uniform set of particles. Now, let’s have a look at what the best values were for the free parameters, and their measurement errors:

  • Maximum extraction yield: 24.0 ± 0.1 %
  • Characteristic extraction time: 5 ± 2 s
  • Characteristic depth reached by water: 35 ± 8 micron
  • Average diameter of particles in bowl 1: 1400 ± 300 micron
  • Average diameter of particles in bowl 2: 420 ± 70 micron
  • Average diameter of particles in bowl 3: 140 ± 60 micron
  • Mass fraction of fines in bowl 1: 49 ± 4 %
  • Mass fraction of fines in bowl 2: 50 ± 10 %
  • Mass fraction of fines in bowl 3: 50 ± 30 %

The characteristic depth reached by water is smaller than what Matt Perger estimated (100 micron), but I am using an exponentially decreasing reach of water, while he assumes that water accesses the coffee cells equally well down to 100 micron, and then not at all in deeper layers. Matt’s estimation of a 100 micron depth corresponds to the layer where only 6% of water reaches in my model.

The characteristic extraction time is very short, at 5 seconds. This means that, if you were to leave intact coffee cells in a cupping bowl in direct contact with water (i.e., each coffee particle would not have any coffee cell hidden under a surface), you would extract ~63% of all available compounds in just 5 seconds ! This is illustrates of how important it is to consider the effect of coffee cells being hidden under the surface of a coffee particle, as we need much more than 5 seconds to be satisfied with a cupping bowl.

To me, the most surprising parameters were the mass fractions of fines in each cup. They are huge, and almost constant across particle sizes ! I was tempted to make the assumption that each coarse coffee cell has a thin layer, half a cell thick, with broken coffee cells that act like fines. But what we have here is something entirely different: a whopping 50% of all coffee mass seems to be trapped in fines that extract immediately, even in the cupping bowl with coarse particles ! Here’s a hypothesis that I think could possibly explain this – I did not come up with this, but saw a comment that Scott Rao made somewhere about this: a lot of fines may be sticking to the surfaces of coarser coffee particles, possibly by static electricity. It’s also possible that some fines did not have time to migrate through the sieves during Matt’s experiment, even though they were freely hanging out among the coarser particles.

Now, it would be unfortunate to just stop here. The main driver behind why I started worrying about the dynamics of extraction was the question of flavor profile. Since we do not have sensory data to play with, the best we can do is approximate it with a profile of extraction yields. Now that I have a model in which I know how many particles there are of each size, and how fast each layer extracts, it becomes possible to build a distribution of extraction yields for each coffee cell. An even more interesting thing we can look at is the extraction yield profile of each drop of coffee in the resulting cup. To obtain this, we just have to look at how much mass was extracted from each coffee cell, and what its corresponding extraction yield was. We can do this for each cup separately, and at each moment where Matt sampled the cup. Here’s what you get at the first sample collection (15 seconds):

Distribution of extraction yields that end up in the coffee cup, after 15 seconds of immersion.

Don’t be surprised that nothing shows up at 0% extraction yield ! This is a distribution of what actually landed in the cup of coffee. While a lot of coffee cells were extracted at 0% because they were near the core of a coffee particle, the “brew” that is 0%-extracted just did not contribute to the cup of coffee. What you are seeing here is a combination of two things: (1) a profile peaking at ~21% extraction yield with a long tail to the lower extraction yields, which corresponds to the stuff that was extracted by diffusion (you might recognize these shapes from my last blog post); and (2) a large peak at ~24% extraction yield, corresponding to the fines which immediately extracted by erosion.

There is something shocking to me about this distribution: between 70 and 80% of the liquid in the cup comes from coffee cells that were fully extracted ! This lends a lot of credence to Matt Perger’s claim that high extractions do not necessarily taste bad, as well as Scott Rao’s comment that fines play a crucial role even if you sift your coffee. This is however confusing to me for one reason: where does all the bitterness and astringency come from in over-extracted brews, if a cup mostly extracted at ~25% tastes good ?

While I do not yet have an answer that I find satisfactory to this question, here are a few hypotheses that I’d like to throw out here:

  • Maybe the bitter and astringent compounds have a much slower extraction speed and account for a very small fraction of the mass.
  • Maybe the presence of bitter and astringent compounds is explained by something else than high extraction yields. That something else would need to correlate with extraction yield, because we know that astringent and bitter cups have either a higher average extraction yield, or a more uneven extraction yield which caused a larger fraction of the brew to be highly extracted. This is closer to Matt’s explanation that you get bitterness when you “beat up” your coffee too much. No offense to Matt, but I’d really like to find a more precise and scientific description of this process 🙂
  • Maybe something is flat wrong with my model, and the fines are not the entire explanation for the very quick rise in average extraction yield in the first sample at t = 15 seconds. It would then be surprising that the model reproduces the data quite well.

Whatever the answer is, I think we need to do more experiments like this one. Having a much finer time sampling especially at the start in our data collection, and going to much longer times especially for the coarse cupping bowl, would be super useful to get better constraints on what’s going on here. This was extremely illuminating to me, but as usual in science, it brought a few answers and many more questions !

If you’re curious about the extraction yield distributions at later times, here they are:

I’d like to thank Mitch Hale, Can Gencer, Mark Burness and Scott Rao for useful discussions.

The header image is by Alexandre Bonnefoy at Issekinicho Editions, Strasbourg, France.

A More Accurate Way to Calculate Average Extraction Yield

I recently wrote about the detailed explanations and calculations behind the equations we use to estimate average extraction yield from coffee concentration (often called TDS) measured with refractometers. If you have not seen this discussion, I highly recommend reading it before you start reading this blog post, as it introduces a lot of the concepts I will discuss here.

As Scott Rao and Dan Eils pointed out a while ago now, we have almost certainly not been calculating average extraction yield in a very accurate way. They describe in this blog post how (1) retained liquid in a V60 brew does not really have a zero concentration, as the standard percolation equation assumes, and (2) the retained liquid should really be divided in two categories. The first category, which they termed interstitial liquid, is the water between coffee particles, which concentration at the exact moment where the brew ends we want to count in our average extraction yield calculation.

In my last post, I suggested measuring the concentration of the last few drops to estimate the concentration of this interstitial liquid. I think this is more accurate than sampling the grounds after a brew, because there is a risk that the interstitial liquid concentration keeps going up after the brew ended, in a way that has no effect at all on the taste profile of the beverage. Remember that the taste profile correlates with average extraction yield because how aggressive the extraction was will dictate the relative abundances of different chemical compounds in the beverage. Therefore we want to calculate by how much the coffee particles were extracted, exactly when the brew ends, regardless of where the concentrated liquid ends up.

Scott and Dan termed the second category of water retained in the coffee bed absorbed water; it consists of water that penetrated the coffee cells inside a coffee particle, but never made it out carrying dissolved coffee solids with it. Hence, this liquid should not be counted in our average extraction yield equations, because by definition it has not extracted any coffee compounds.

The direct effect of this absorbed water will be to slightly decrease the average extraction yields calculated for immersion brews, or the immersion term (the one that goes as W/D, i.e., brew water over dose) in the general equation. If we knew the weight of water that remains trapped in coffee particles (let’s call it Wabs for absorbed water), then implementing it in the general equation would be relatively straightforward.

To do this, we would need to link the concentration of retained water (which we call Clast because we measure it through the concentration in the last few drops) with the mass of interstitial water (let’s call it Wint) and the mass of coffee liquids dissolved in that retained water (Mret), instead of that of all retained water, like this:

and then reversing this equation using some algebra would result in:

The fact that we are now counting only part of the retained water in our equation would also change the relation between beverage mass (B) and the mass of brew water (W). Remember that this relation also included the mass of coffee solids dissolved in the beverage (Mbev). That equation now becomes:

and now we can use this relation to express Mret as a function of more readily measurable quantities. Skipping some of the detailed algebra, we can then express our general equation for the extraction yield (E) as:

Remember that D is the mass of the coffee dose, and Cbev is the beverage concentration (sometimes called the beverage TDS). In this equation, we also introduced a term fabs, which I’ll call the absorbed liquid ratio. It is defined in a way similar to the retained liquid ratio, but counts only the part of the liquid that is absorbed by coffee particles and does not count interstitial liquid in the spent coffee bed:

We already know that fabs must be smaller than 2 for V60 and most other percolation brews, because the liquid retained ratio is approximately 2 and includes both absorbed and interstitial liquid retained in the spent coffee bed.

Now what we need is a bit of experimentation before we can really use the equations above. We should either come up with an easy way for anyone to directly measure fabs, or otherwise hope that it does not strongly depend on roast, brew method and particle size distribution. I suspect that using an aeropress or siphon might generate a scenario where Wint is close to zero because of the suction. If this is the case, then we would be in a pretty ironic situation where the percolation equation would become more accurate for such methods, while possibly not being accurate at all for V60 brews.

If you’d like to view the detailed algebraic calculations leading to the generalized average extraction yield equation above, you can find it in PDF format here.

Mitch has also just updated his universal extraction calculator to include this new fabs term.

I’d like to thank Scott Rao and Mitch Hale for useful discussion.

Measuring and Reporting Extraction Yield

Since I received the VST Coffee Lab III refractometer thanks to Vince Fedele’s generosity, I started logging the concentration (in % TDS) of every coffee I brew. This allows you to calculate the average extraction yield of your brew, which represents the fraction of your coffee beans by mass that was dissolved in your brew water. This is a super useful measurement because it correlates very well with taste. Many of you might know that finding a good brew recipe is like navigating a thick forest at night – I would argue that using a refractometer is as useful as having a compass in that situation. I discuss coffee concentration and average extraction yield a bit more in this post.

I often compare the average extraction yield of my brews with other coffee geeks. While it’s an extremely useful measurement, I came to realize that we need to be careful when we compare numbers, because people use several different methods to estimate it. I’d like to review some of those methods here, and discuss precautions I think we should take when communicating our measurements.

A summary of this post is available for download here in the form of a cheat sheet with the relevant equations only. I also added it to the resources menu.

VST labs provides phone and computer applications to calculate extraction yields, which takes away the need to do the calculations yourself, but even in this scenario it’s really useful to understand how to properly use it, and to understand what the calculations rely on when you use different modes in the application. If you compare your numbers with people not using the VST application, one immediate difference will be that the application accounts for moisture and CO2 contained in the bean. Those using simpler approximations of average extraction yield will most likely not be including these correction factors, and as a result your average extraction yields will seem approximately a full percent higher than those of others.

Because of this, I like to set moisture and CO2 to zero in the application when I compare numbers with other people. It’s important to keep in mind that this makes the calculation less realistic, but it’s also important to compare apples with apples when you communicate with someone not using the app.

Coffee brews can generally be split between two big categories: percolation and immersion. We’ll discuss these two categories separately, and then we will discuss mixed methods last.

Percolation

In a percolation brew, fresh water is being continuously added on top of a coffee bed, resulting in an aggressive extraction because fresh water is a great solvent. The coffee bed also acts as a filter, which prevents a lot of very fine coffee particles to end up in the beverage, and therefore results in a brew with less body and more clarity of taste. Brew methods such as the V60, the Kalita Wave, the Chemex, the moka pot, espresso and batch brewers fall in this category. Espresso is the only one among these where it is not just gravity that is forcing water through the bed of coffee, but it is still a percolation brew.

Calculating the average extraction yield is most straightforward for percolation brews, but it requires an additional measurement on your part if you want to be precise. Typically, brew recipes are designed with a coffee dose in grams (we’ll call it D below), and a mass of brew water also in grams (we’ll call it W). A refractometer allows you to measure the concentration of coffee in %, let’s call this C. This is often referred to as the total dissolved solids, or TDS. The concentration of your brew is by definition the amount of coffee mass that made it into your beverage (we will call this Mbev), divided by the total beverage mass (we’ll call it B):

There are two reasons why we divided by B, and not by the mass of water W which was poured over the coffee. First, this quantity B also includes the mass of coffee compounds. But most importantly, a lot of water actually never made it into the cup of coffee, and instead remained trapped in the spent coffee bed. The mass of water in grams that each gram of coffee can retain is called the liquid retained ratio (often called LRR, we will call it just L). Typically, a coffee particle retains twice its weight in water, so in other words its liquid retained ratio is approximately two. By now, we can write the relation between the total beverage mass B and the other variables:

The first term on the right hand is the total amount of water poured, to which we subtract the amount of water retained in the spent coffee bed, and to which we add the mass of dissolved coffee solids. In this discussion, we will ignore the effects of CO2 and moisture in the coffee bean.

The quantity we want to measure is the average extraction yield (we will call it E), and from its definition you might have foreseen that it will be given by:

If that’s what you expected, you are kind of right. In reality, we should include the coffee compounds that were dissolved in all of the water at the exact moment where the brew ended, because this is the quantity that informs us on the profile of chemical compounds that were extracted from the beans. Whether these compounds ended up in the cup of coffee or in the spent coffee bed, we must count them if we want the average extraction yield to correlate with flavor profile as best as it can.

I know this is counter-intuitive, so let me offer a thought experiment to settle this. Imagine you brew yourself a V60, place the spent bed in a glass, and immediately pour half of your coffee cup in the spent bed. This will artificially bump up your liquid retained ratio artificially, and half both Mbev as well as B. Did you just change the flavor profile of your cup ? You didn’t, but the equation above would tell you that you just halved it, so we know it’s wrong.

The reason why I called this extraction equation kind of right is because we assume the water retained in the coffee bed in a percolation brew has almost no dissolved coffee solids in it, at the moment where the brew ended. The key words here are in a percolation brew, because you are constantly pouring fresh water on the coffee, and near the end of the brew there won’t be a lot more stuff that comes out from the coffee particles and into the fresh water. What happens if you wait 15 more minutes bears no impact on the flavor profile of your cup, this is why we are worrying about the concentration of retained water at the moment where the brew ended. The complete equation for the average extraction yield should be:

where Mret is the mass of coffee solids dissolved in the retained water exactly when the brew ended. But as we just discussed, Mret is approximately zero in a percolation brew.

We already know the dose of coffee because that’s something we specify when we build a brew recipe and (hopefully) actually measure before brewing. What we must now deduce is this mass of coffee dissolved in the brew water, Mbev. The clue we have to figure it out is the concentraction C which we measured with the refractometer. If we combine the first two equations in this blog post, we get:

and we now want to revert this equation to obtain Mbev as a function of the concentration C. This takes a bit of algebra, which I’ll spare you. The result is this:

And we can now directly calculate the extraction yield, by substituting Mbev using the equation above:

Please note that average extraction yield and concentrations are all defined as fractional numbers between zero and one (so, 1.4% TDS would be 0.014). This is true throughout this blog post, but the cheat sheet available in the Resources menu has a version of this equation with everything in %.

The 1/(1-C) factor on the right-hand side of the equation has a very small effect on the calculated extraction yield for filter coffee, typically smaller between 0.2% and 0.4%. What this term represents intuitively is the contribution of extracted coffee mass to the beverage weight, so it is more important when C is high.

The equation above is useful if you know the liquid retained ratio, or want to approximate it. But in practice it’s more precise and easier to actually weigh the mass of your brewed coffee B (just note the mass of your empty mug before brewing). Look how much easier the extraction yield equation becomes, and it’s not an approximation:

Measuring the mass of your brewed coffee makes the calculation of average extraction yield much easier, and more precise ! It’s a win-win, so I really recommend that you always do it. I recommend this even if you use the VST application, because then you don’t need to assume any liquid retained ratio. Make sure the application is in percolation mode, and then you can directly adjust your beverage weight to your measured B in the application, instead of adjusting the amount of brew water (which we called W here).

Unless you use an unusually fine grind size and filter papers with unusually large pores, syringe filters should not be needed when you measure the concentration of a percolation brew, with the very important exception of espresso (see a recent awesome experiment by Mitch Hale about that). If you want to be sure your particular set-up does not require syringe filters, I recommend measuring your concentration with and without for a few brews, and determine whether they affected the measurement.

In my first blog post, I made the mistake of ignoring water retained in the spent coffee bed when I build a coffee control chart that is useful for V60 brews. As a result, my fixed ratio (W/D) curves were offset (this should now be corrected in the post).

Here’s an updated coffee control chart that assumes a liquid retained ratio of 2, which is much more appropriate for percolation brews than the one I had posted in my first blog post:

Coffee control chart for percolation brews (assuming a liquid retained ratio of 2.0) with a slightly updated “ideal” range more consistent with high extractions that can be achieved with well-developed roasts and high-quality grinders. The Brix degrees follow Alan Adler’s relation and are useful for handheld optical refractometers.

Immersion

An immersion brew consists of plunging coffee beans in water (or the reverse) and leaving the same water with the coffee until the end of the brew. Extraction happens a bit more slowly because as water becomes more concentrated, its power as a solvent goes down. The spent coffee is then typically gently separated from the water to avoid drinking it, but typically a lot of fine coffee particles end up in the beverage, resulting in more body and less flavor clarity. Cupping and the french press fall in this category. You may be tempted to think that other brew methods like the aeropress, vacuum pots (also called siphons or syphons) and the Clever Dripper also fall in this category, but they don’t exactly – we’ll discuss these in the next section.

In an immersion brew, most of the technical discussion we already had in the Percolation section still holds. The main difference is that you cannot ignore the mass of coffee solids dissolved in water retained by the spent coffee bed anymore, and the approximation that the liquid retained ratio is near 2 can become very inaccurate depending on the brew method. Let’s go back to our full equation for the average extraction yield E:

We must now calculate Mret, and to do this it is useful to recall that, at the precise moment where the brew ended, the concentration of coffee that will end up in the cup or in the spent coffee bed is the same. We can therefore calculate Mret with the following equation:

which can also be inverted with a bit of algebra:

Now if we put together our equations for Mret and Mbev in the extraction yield equation and do a bit more algebra, we end up with:

Please note that average extraction yield and concentrations are all defined as fractional numbers between zero and one (so, 1.4% TDS would be 0.014). This is true throughout this blog post, but the cheat sheet available in the Resources menu has a version of this equation with everything in %.

As you can see, all terms with the liquid retained ratio L disappeared ! This means you do not need to weight your beverage or make a supposition about L, which makes it easier to calculate the average extraction yield of an immersion brew. Again, the term in 1/(1-C) on the right-hand side of the equation is a small correction that has an effect of 0.2% to 0.5% on the calculated extraction yield.

The fact that beverage weight disappeared in the equation above should tell you something about how to use the VST application in immersion mode: you’ll want to adjust “BW” directly (here we call it water weight W), rather than the beverage weight, to achieve a better precision.

Syringe filters are needed to measure the concentration of an immersion brew. They all let enough fine coffee particles in the beverage which cannot be dissolved in water, so you will get very imprecise and inaccurate measurements if you don’t use syringe filters in this scenario.

The coffee control chart appropriate for immersions doesn’t need to assume any liquid retained ratio:

Coffee control chart for immersion brews with a slightly updated “ideal” range more consistent with high extractions that can be achieved with well-developed roasts and high-quality grinders. The Brix degrees follow Alan Adler’s relation and are useful for handheld optical refractometers.

Mixed Phases Methods

There are a few methods that cannot be simply categorized as percolation or immersion, and that are instead better described by an initial immersion phase, followed by a percolation phase where the already concentrated brew water passes through the partly spent coffee bed and typically also a filter to end up in the cup of coffee. Coffee brewed with these methods shares the properties of both: extraction is a bit more aggressive than an immersion brew alone because of the final percolation phase, but not as aggressive as a pure percolation method, because the percolation phase is done with water already concentrated with coffee, that is therefore a worse solvent. Depending on the details of where the filter is placed and what force pushes the coffee through the filter, a varying amount of fine coffee particles, smaller than typical immersion brews, ends up in the cup. Similarly, the liquid retained ratio will strongly depend in this force. The brew methods that fall in this category are the aeropress, the siphon and the Clever Dripper.

The main difference between these mixed methods and regular immersions in how they affect the calculation of extraction yield lies in the fact that the concentration in the spent coffee bed is not necessarily the same as in the coffee cup, but it is not zero either. Instead, it is somewhere in between, and will be close to the concentration of water at the end of the immersion phase, just before the percolation phase. Accurately measuring the extraction yield of these methods is more cumbersome and twice as expensive if you use a brew method that allows enough fines in the beverage that syringe filters are needed. Basically, you need to measure the weight of your beverage B, the concentration of your beverage (let’s call it Cbev), and the concentration of your spent bed (let’s call it Clast). You can measure the latter by keeping the few last drops of your brew in a different container. Make sure to keep at least a dozen drops if you need a syringe filter.

You can calculate Mbev and Mret with the exact same equations as those in the sections above, by just replacing the concentration C with the respective Cbev and Clast concentrations. There is just one step that is easy to miss, where you estimate the total weight of retained water (let’s call it Wret) from the water and beverage weights, make sure you don’t forget the contribution of coffee solids that were dissolved in the beverage:

This will allow you to properly write down the equation linking the concentration to the dissolved mass in the retained liquid:

Add to this a little bit of algebra, and you get the following equation:

Please note that average extraction yield and concentrations are all defined as fractional numbers between zero and one (so, 1.4% TDS would be 0.014). This is true throughout this blog post, but the cheat sheet available in the Resources menu has a version of this equation with everything in %.

Note how setting Clast = Cbev will simplify it to the immersion equation, and setting Clast = 0 will simplify it to the percolation equation, as it should. In other words, the equation above is more general, and includes both of the immersion and percolation cases.

If you are interested to view the detailed calculations leading to this more general equation, you can find them in PDF format here.

This particular equation is not currently supported by the VST application. The closest you can do is assume that Clast = Cbev and use the immersion equation. In fact, there are some recipes for which this approximation will be very good; I encourage you to verify this for your particular recipe, and see the difference you get from this equation versus the immersion equation. If you find out that the difference is small, then just use the immersion equation for that particular recipe.

This equation is a bit large, and clumsy to use, so Mitch Hale gracefully created a web tool so that you can use it way more easily ! Please have a look at it here.

Here’s a way to tell if the immersion equation is accurate enough, in one equation:

If that constraint is verified, then you can just use the immersion equation.

Determining whether these mixed brew methods require syringe filters or not will require experimentation on your part. Try measuring your concentrations with and without them for five or six brews, and notice if the syringe filters had an effect or not. With my very limited trials, it seems that a regular aeropress method requires a syringe filter, even if you use two filters. With the siphon, I noticed syringe filters were also needed, at least with the relatively fine grind size I tested and the Hario paper filters. Combining aeropress with the thick aesir filters and the prismo valve with a grind size slightly coarser than typical V60 brews did not seem to require syringe filters. Do not take these as absolute recommendations, but more as an illustration that whether syringe filters are required will depend on several parameters.

Sharing Extraction Yields

As Mitch Hale pointed out recently on his Instagram account, when using a scale precise at 0.1 grams or worse to measure your coffee dose, it doesn’t make sense to report average extraction yields with more than one digit. This is true because effect of your 0.1 grams measurement error on your coffee dose will impact your calculated average extraction yield by about 0.1%, depending on your exact recipe.

When sharing extraction yields, I recommend that you also report all the variables that are required to use the relevant equation, plus the water/dose ratio. In the example of a percolation brew, this means reporting your coffee dose, brew water ratio, beverage weight and beverage concentration.

Some Parting Thoughts

While this blog post summarizes the concepts behind equations currently used for calculating extraction yields, it is likely not the final answer to how we should calculate them. More than a year ago, Scott Rao posted a very interesting discussion about the limitations of our current assumptions, and how he thinks that the retained liquid in percolation brews are in fact not completely devoid of dissolved solids. I really recommend you read his post, especially if you just went through all of this blog post with a fresh memory of how things are currently calculated. I’ll definitely do some experiments in the future and think about how we can implement Scott’s and Dan Eil’s suggestions.

[EDIT 2019 March 25: I wrote a follow-up discussion to this post here].

Disclaimer: I was offered the VST Coffee Lab III refractometer for free by Vince Fedele, but I do not have any financial interest related to any coffee equipment.