The Effects of Varieties, Origin and Processing

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

The Physics of Kettle Streams

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Dan Eils’ Vac60 brewer.

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

The Melodrip, photo by Elika Liftee.

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

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

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

The coffee station I assembled to brew consistent V60s.

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

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

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

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

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

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

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

My kettle pours compared during the grind size experiment.

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

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

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

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

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

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

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

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

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

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

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

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

Measuring Brew Water Properties

Photo header credit: Kathy Gagné

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

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

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

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

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

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

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

Colorimeters and Photometers

A Hanna Instruments HI-775 colorimeter for total alkalinity.

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

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

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

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

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

Aquarium Titration Kits

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

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

What is Titration ?

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

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

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

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

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

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

Stretched Titration with Aquarium Kits

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

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

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

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

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

Back Titration with Aquarium Kits

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

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

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

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

Scale-Assisted Back Titration with Aquarium Kits

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

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

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

Testing the Different Methods

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

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

When to Use Which Method

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

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

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

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

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

Red Sea Titration Test

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

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

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

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

The Physics of Fines Migration

Jesse Lambeth, a member of my Telegram channel, recently drew my attention to a really interesting physics paper by Knight, Jaeger & Nagel that I now think may be relevant to our understanding of fines migration in coffee brewing. There are some considerations that could complicate its application to coffee, but I’ll come back to those at the end of the post.

First, let me summarize my view of fines migration before I had read that paper. You might remember that when we discuss the size distribution of coffee particles, we often call the smallest ones fines and the largest ones boulders. When we use the word migration in this context, we usually refer to fines preferentially moving towards the bottom of the coffee bed during a brew. This is also sometimes called the Brazil nut effect because fragments will tend to accumulate at the bottom of a container where dry food is stored.

The Brazil nut effect observed in a jar of cat food. The smaller fragments tend to accumulate at the bottom of the jar.

This phenomenon of fines moving toward the bottom of the coffee bed is mostly relevant to percolation methods, i.e.those where we pour water on top the coffee and use the grounds themselves as part of the filtration system. We worry about this because the accumulation of fines near the filter that holds the coffee bed together might get enough of its pores blocked to cause clogging. When a filter clogs, it will slow down the flow of water in a poorly controlled way, and this will also result in a less evenly distributed flow and therefore extraction through the coffee bed. If you’re interested to read more about this, see this previous post I wrote on the subject.

My previous understanding of how fines migration occurred was that, in the presence of vibrations strong enough to lift coffee boulders, some larger gaps between them would appear momentarily and allow fines to fall through toward the bottom. This video shows this mechanism in action:

While it can still happen, the key finding of the Knight paper is that this effect is not required for fines migration to occur ! Even if the vibrations are too weak to lift the boulders or cause them to jump around, a whole different mechanism can still cause the fines to clog the pores of your coffee filter, and its details are incredibly interesting.

To figure this out, the team of scientists led by Knight placed identically shaped glass balls in a cylindrical container, and added a single larger ball with the same mass density as the other ones. They painted the large ball and a couple of small ones with a bright color so that they could easily track their displacements, and they attached a device to the container that can imitate a short tap in a very controllable way. This way, they could artificially tap the container as many times as they like, always with the same exact force and duration. All they then had to do is turn on the tapping and observe how the colored balls moved around. To the scientists’ surprise, no ball actually needed to be lifted for anything to happen. Instead, the balls near the edges of the container were pushed down by the frictional forces of the walls vibrating against the balls, which started a cyclic flow in the whole container that looks very much like convection in a hot liquid.

The image above displays a temporal sequence of motion in a cylindrical container as it is tapped many times. Some of the balls were colored to keep track of their motion. The balls against the walls flow downward, and push the central particles upward, starting a convection-like cyclical flow. Source: Knight et al. (1993), modified.

As you can see in the image above, the glass balls that are incoming from above are pushing the bottom layer of balls toward the center of the container, and those are in turn pushing the bottom center balls upward. When they reach the surface, they migrate outward to the container walls, and that completes the cycle where they get pushed down again by the vibration.

In nature, convection can be observed in all kinds of places where a fluid is quickly heated; at the surface of the Sun, in a boiling pot of water, in a hot Miso soup especially just after you remove its lid, or even in a small cup of tea in a cool environment with the right lighting. I don’t think the motion of the balls can be called proper convection, but they sure move in a very similar way.

The image above shows convection cells at the surface of the Sun. Large bubbles of (brighter) hot gas flow upward and thin layers of (fainter) cool gas flow downward between the bubbles. The cyclical nature of this flow is similar to what is observed in a tapped cylinder of glass balls. Source: University of Wisconsin, Madison.

In the image above, you can see convection taking place at the surface of the Sun; the cells of upward-moving hot fluid and the interstices of slightly cooler, downward-moving fluid look similar to the container of glass balls. In fact, the latter almost looks like a large convection cell going up at the center of the cylinder.

If all balls had the same size, the result would be a slow and cyclical motion around the container. But something goes haywire when the larger glass ball gets near the cylinder wall at the top layer. It is too large to fit in the downward flow, and it gets stuck at the top layer. If you had many large balls, they would eventually all get trapped near the top of the container. If you imagine a large amount of large nuts with some amount of nut fragments, you would end up with all the powder at the bottom after a little while, and therefore observe the Brazil nut effect.

To verify their hypothesis that the whole flows were driven by friction with the container walls, they repeated the experiment using a container with a rough and a smooth side. The rough side provides a lot more friction, and as they expected it drove a much more important downward flow near the rough edge of the container wall:

In the image above, the right side of the cylindrical container is rough and provides friction, whereas the left side is smooth and provides almost no friction. As a consequence, the downward flow only happens on the right side. Source: Knight et al. (1993).

The team of scientists did not stop there. They decided to test another container shape, and quite amazingly they chose to study the behavior of a cone, making this very relevant to V60 brews ! They repeated the same experiment, and observed something shocking. The flow reversed entirely, and this caused the larger balls to get trapped at the bottom, instead of the surface !

A conical container reverses the direction of the convection-like flows, causing the larger balls to get stuck at the bottom. Source: Knight et al. (1993), modified.

They also observed that the thickness of the upward edge flow was a bit larger compared to the cylinder case. In principle, this could mean slightly larger boulders can complete the full cycle rather than get stuck somewhere, compared to the cylinder case.

Another science paper by Hejmady and co-authors showed this beautiful sequence of how layers of colored spheres evolve with vibration to bring out the details of the flow structure:

Top layer of photos: temporal sequence of how vibrations affect layers of colored balls. Bottom layer: similar with the exception of a large ball that is included to show how it gets trapped at the surface. Source: Hejmady et al. (2012).

They also show a nice visualization of the flow direction in a superposed image similar to a long-exposure photo:

This video shows a great visualization of the phenomenon:

With all these results in mind, you might think that a V-shaped container might work against clogging a filter because it would concentrate the boulders at the bottom instead of the fines, but that’s besides the point because the fines circulating around the edges of the paper filter would contribute to clogging it, regardless of the direction in which they are flowing. Fines small enough to penetrate the pores of the paper filter will get stuck in it, so any kind of motion that brings more fines in the vicinity of the filter will contribute to reach the point where the filter clogs. In other words, tapping either a conical or cylindrical filter will contribute to clogging the filter, even if they trap the boulders in different regions of the coffee bed.

These concepts could also be applied to coffee brewing in a couple more ways:

  • A vibrating cylinder could be used to lift boulders to the top of a dose of ground coffee to scoop it out and make the particle distribution narrower. This would be similar to sifting, but might be a bit more convenient and faster. It would probably share some of the disadvantages of sifting however, for example it would probably be messy and hard to replicate exactly.
  • The wall angles of a conical brewer could probably be chosen to minimize any displacement of particles in the brewer even in the presence of vibrations. As the Knight paper discusses, the fact that the very slanted walls reverse the flow probably means that there is one geometry with less slanted walls, in between this particular cone and a cylinder, which would stop all ball motion. Very smooth edges could also achieve this, but remember that the edges of a coffee bed are the paper filter itself, and it seems implausible to have a very smooth, frictionless paper filter.
  • If we had a filter that cannot be clogged, we could use this re-organization of the coffee bed particles to make extraction more even. Normally, the bottom of the coffee bed only comes in contact with concentrated water, and this causes the bottom to extract less and in a different way: it will preferentially extract chemicals that are not already dissolved in water, see this previous post I wrote for more about this. Conical brewers partly make up for this by having a larger amount of water pass through the bottom of the coffee bed because of their geometry, and this prevents the bottom from being under-extracted. This is an imperfect solution however, because the bottom of the coffee bed still extracts in a different way. Using vibration to re-organize the coffee bed without clogging the filter would be amazing, but a real challenge: it would require both the coffee particle distribution to be very even, and the filter pores to also be very even and smaller than the coffee particles.

It’s important to keep in mind that these papers are based on idealized scenario which could make their application to coffee brewing less straightforward. Here are some caveats I could think about, but there may be more:

  • The presence of water in the brewer provides additional forces (dragging along the currents and upward buoyancy) that may introduce different displacements of the coffee particles. In the presence of vibration, the flow of particles described above probably still happens, but it could be washed away by stronger effects.
  • The scientists that carried these experiments used a very specific type of tapping with a single 30 Hz frequency vibration. It’s possible that changing that frequency might affect the strength or even the direction of particle flows. In practice, finger tapping might cause vibrations of different and even varying frequencies.
  • The shapes of coffee particles are far from being spherical, which could affect these results. Similarly, in real scenarios we have a wide variety of particle sizes, not just uniform small particles and one large particle. I think those effects are less likely an issue, because convection-type flows are observed even in containers of unevenly shaped and sized nuts and dry foods.
  • It’s important to keep in mind that strong vibrations or upward forces can still lift boulders and cause fines to fall in between the cracks. This means that even in a scenario where the geometry of a container prevents convection-like displacements, this alternative type of fines migration could still happen.

I’m hoping you found these results as interesting as I did ! I’d like to thank Jesse Lambeth for digging up these papers too ! The header image is from Gsrdzl at Wikimedia Commons.

The Struggle for a Steady Kettle Flow

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Recently, I started attempting to modify my Brewista Artisan 0.6L gooseneck kettle to reduce its maximum flow rate and make it easier to keep a constant kettle flow of about 5 grams per second during my brews.

The Brewista Artisan 0.6L has a gooseneck with an inner opening diameter of 5.5 mm at the exit spout, which results in a maximum flow rate of about 24 g/s. This is a bit more than I would like, as I rarely ever need to pour that fast, and this makes it a bit harder to pour consistently at only 20% of the maximum flow rate. I asked someone on Instagram who has a Fellow Stagg EKG 0.6L to measure its maximum flow rate, and it is apparently a bit lower, at 20 g/s. This is quite nice, and adds another reason why I would have chosen the latter kettle if I was to do this again. Other reasons include not having to press re-heat again every time I pick up the kettle; the nice counter-weight in the handle which makes it easier to pour consistently; and the much better and more responsive customer service of Fellow versus Brewista.

If you wonder why I’m worrying about kettle flow speed, the consistency of my swirls and my accuracy in measuring TDS, it’s all because I’m trying to make every aspect of my brewing as consistent as I can from one brew to the next. There are a lot of things I would like to investigate which will require my brews to be very stable, including but not limited to:

  • The effect of filters on flow and clogging.
  • The effect of coffee aging and hardness on flow (through its shattering properties during grind).
  • The effects of bed preparation (nest shape, etc.) on extraction evenness.

Those are just a few examples in a very long list; it’s possible that getting a Decent espresso machine in pour over mode may allow me to do this, but saving up Patreon funds for it will take a long time, so I think it’s worth improving my manual repeatability in the meantime. I want to improve my repeatability not only in terms of extraction yield, but also brew time and flow of coffee coming out of the V60.

The first way in which I attempted to reduce my kettle flow was to obtain a Mandritech Hand Drip Water Valve, which you may have seen on my Instagram or Telegram channel. This product is only sold in Asia, but Edo Rezaprasga, a reader of my blog, was kind enough to mail me one so that I could test it.

The device ended up working very well in terms of getting a more constant kettle flow, as can be seen on this graph where I tried to maintain my flow at 5 g/s with and without the valve:

There is however one big drawback in using the Mandritech valve; it ended up making the flow of water much less cohesive, which resulted in less agitation, and lower average extraction yields. You can see how the flow breaks up after a very small vertical distance at 5 g/s in this video:

I could even see that the layer of water on top of my coffee bed was super clean, which is a telltale sign that very little agitation is being applied on the slurry:

For example, brewing Passenger’s Los Yoyos 2017 Harvest coffee without the Mandritech with my usual recipe (22 g dose with 1:17 ratio, Rao/Perger boiling water, 5 g/s kettle flow) yielded 1.49% TDS with a 320 g beverage weight for an average extraction yield of 21.7%, whereas brewing the same coffee in the same way with the Mandritech yielded a 328 g beverage weight at 1.28% TDS for an average extraction yield of 19.1%. This is quite a heavy loss !

Fancy gooseneck kettles are slightly twisted near the tip, and that makes their flow much more cohesive. As a result, the stream of water falling from the kettle can penetrate deeper in the slurry, and can introduce turbulence all the way down to the coffee bed instead of having all its kinetic energy dispersed near the surface of the water. This is an important aspect of gooseneck kettles that I had not appreciated before.

This does not mean it’s impossible to brew well with the Mandritech, but optimal pour over recipes for its use would resemble those that work well with the Melodrip, and this is yet another rabbit hole I have not explored well. Judging from Ray Murakawa’s brews, it seems that Melodrip-style pour overs work better with smaller doses, much finer grind size and more agressive blooms to get rid of dry pockets despite the finer grind size.

Just when I was about to leave the possibility of flow restriction aside, Edo wrote back to me and told me he found a set of flow restrictor plugs small enough to fit my Artisan 0.6L model ! Those are very similar to the Brewista-branded ones that only work with the larger 0.9L kettles, except with way more choices in terms of outer and inner diameter sizes. This is yet again another piece of geeky coffee gear that is only available in Asia, so I could only get my hands on one thanks to Edo again ! I asked him to get the model that has an inner diameter closest to 4.1 mm, as I calculated that this would result in a reduced flow of about 10 grams per second according to Sampson’s law. Edo found one with an inner diameter of 4.0 mm (the Bonavita number 5) and sent it to me.

We were not sure if the outer diameter of the restrictor would correspond to whatever the inner diameter of my kettle is at the base of the gooseneck, but I couldn’t verify whether it was 5.5 mm wide with my digital caliper anyway, because it doesn’t fit inside the kettle. We thus decided to try ordering it anyway and see what would happen.

When I received it, I measured its outer and inner diameters with my caliper and found that the inner diameter was 4.0 mm as expected, but the outer diameter was 6.9 mm, a bit larger than the exit spout of the Brewista Artisan gooseneck. Fortunately, 6.9 mm seems to be exactly the size of the gooseneck entrance opening at the base, because the flow restrictor fitted perfectly ! I have no idea why they are called Bonavita flow restrictors; I am not aware of a Bonavita kettle with such a small gooseneck diameter.

The maximum flow rate with the restrictor installed turned out to be about 9.5 g/s, very close to the expected value ! When I saw that, I went ahead and tried brewing with it, which resulted in a total mess. I realized mid brew that the boiling water seemed to create some bubbles near the smaller entrance of the flow restrictor, resulting in a very uneven flow. The kettle flow kept interrupting, or spitting high-velocity drops of water which created bad splattering. Here’s an example of such uneven flow in this video:

In order to test whether the boiling water was really the cause of this messy flow, I tried pouring with water at 210°F instead of 212°F, and the result was much better ! Here’s what this looked like:

So I went ahead and made another brew at 210°F, and this time the kettle flow went super well. Here’s a video of that brew:

I was brewing Heart’s Colombia Decaf Platino which is a washed mix of Caturra & Castillo decaffeinated with the ethyl acetate process. I used a grind setting of 7.3 at 700 RPM on the EG-1 with a 22 g dose, Rao/Perger water, a kettle flow of 5 g/s with a 1:17 ratio that resulted in a brew time of 4:50, a 323 g beverage weight and a 1.31% TDS concentration. This corresponds to a 19.2% average extraction yield. Decaffeinated coffee always extracts much lower than usual so that number is quite normal. Decaf coffee beans are also very brittle and therefore generate a lot of fines. This is why I used a coarser grind size than usual, and still ended up with a long drawdown time of 4:50; I should have ground even coarser than that. The brew tasted quite good; Heart are one of the rare roasters that roast their decaf coffee well. It was not astringent despite the long drawdown time, but I actually never tasted an astringent decaf, so I would not be surprised if the ethyl acetate process also removes whatever is responsible for astringency. Note that both of my swirls were quite bad in this brew.

You might think that it’s not worth losing 2 degrees Fahrenheit to get a more reliable kettle flow, but I actually think it’s worth it especially in my case; the loss of temperature in the slurry will be smaller than 2 degrees because water temperature drops faster from 212°F than it does from 210°F, but more importantly it will make my brews more repeatable which is very useful for science !

Here’s a comparison of how steadily I was able to pour with the non-upgraded Brewista Artisan kettle, with the Mandritech valve, and with the flow restrictor:

You can see that either the Mandritech or the flow restrictor allowed me to reach a much better stability, but only the flow restrictor allowed me to do this without losing the cohesion of my kettle flow ! To quantify this a bit better, here are the averages and standard deviations of the kettle flows I obtained during these practice pours:

  • Default kettle: 4.6 ± 0.6 g/s
  • Mandritech: 5.0 ± 0.2 g/s
  • Flow restrictor: 5.1 ± 0.2 g/s

Thanks again to Edo Rezaprasga for making all of this possible ! I’m super happy I finally found a solution to make my kettle flow more stable.

The Hardness of Green Coffee

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When reading about coffee roasting, I have often seen bean density being discussed as an important variable that affects how beans will behave during the roasting process. The term bean density is a slightly vague way to refer to its volumetric mass density. One thing I have rarely seen discussed is the green coffee’s surface hardness and how it may affect roasting. In this post, I’ll show that the hardness of different green coffee beans can vary quite a lot, and even seem to vary more than their mass densities across different origins and processing methods.

Lately, I obtained a Shore D durometer with the goal to investigate how roasted coffee hardness correlates with pour over drawdown time through the beans’ shattering dynamics when grinding. This is an experiment I’m in the process of completing, but while I was doing this my friend Andy Kires at Canadian Roasting Society generously sent me samples of seven different green coffee beans so that I could try measuring the surface hardness of green coffee directly. So, I decided to compare the hardness and volumetric mass densities of each of these beans. Here are quick descriptions of the seven coffees:

  • Washed Colombian
  • Natural Colombian
  • Washed Ethiopian
  • Natural Ethiopian
  • Natural Brazilian
  • Washed Kenyan
  • Ethyl acetate decaffeinated washed Colombian

A more detailed description for each of them is given at the end of this post.

To measure the mass density, I followed this method by Green Farm Coffee where 25.0 grams (let’s call this M) of green coffee is placed in a 100 mL glass graduated cylinder pre-filled with 50 mL of water (this is V1).  After gently knocking the graduated cylinder to remove all air bubbles, I measured the new total volume V2 which can be used to determine how much displacement the green coffee caused. The volumetric mass density can then be calculated from this simple equation:

In the image above, some green coffee beans are floating because I have not yet gently knocked the graduated cylinder to remove air bubbles

The measurement errors on both volumes were about 0.5 mL as my cylinder is graduated to the mL, and the measurement error on the mass was 0.1 grams given the scale that I used. The results surprised me quite a bit: after knocking out the air bubbles which could seriously alter the results, all coffee beans had almost exactly the same volumetric mass density !

The next step was to measure the surface hardness of the green beans with the durometer. I tried both a Shore A and Shore D durometers, and it turns out that only the Shore D had the correct range to take useful measurements. I placed the green coffee beans on a flat surface, with the flat side of the bean down, and placed the durometer’s needle near the center of the rounded back of the coffee bean. I then pushed straight down on the durometer relatively hard until the measurement stabilized, and noted it down.

As I expected hardness to vary from one bean to the other, or even with the exact needle placement, I repeated this on at least 17 beans for every origin. One could look at the average of these measurements as the most interesting variable, but in these situations I tend to prefer looking at the median because it’s more robust against outlier measurements. The statistics geeks will know that the measurement error on the average is the standard deviation divided by the square root of the number of beans; an analogy can be made for the median, but the median absolute deviation is what must be used instead of the standard deviation. This is very convenient because the median absolute deviation is also more robust against outlier measurements.

Here are the median Shore D measurements I obtained for the seven different green coffees:

The individual raw measurements are available here. The Shore D hardness is a number that goes from 0 to 100 and goes up with hardness.

I still do not know exactly how these will affect roast behavior, but this seems very interesting to me, because we can tell different origins apart much easier than with the volumetric mass density. It might be worth the effort of taking 17 hardness measurements on green coffee beans if that allows a roaster to predict how hard a bean might crash, especially if it’s an expensive microlot !

Furthermore, it’s nice to see that the washed Kenyan lived up to the common folklore which says that Kenyans are particularly hard coffees ! However, it did not show any noticeable increase in mass density. This may be further evidence that mass density, or at least the Green Farm Coffee method of measuring it, are not that useful to characterize how green coffees behave during roasting !

If any of you roasters out there try measuring your green coffee hardness and notice a correlation with roast behavior, I’d love to hear about it; otherwise, Andy and I will keep you posted about what we find out.

For those interested, here’s the detailed information about each of the green coffees:

Washed Colombian: Laderas Del Tapias

Location: Neira, Caldas, Colombia
Altitude: 1650 to 1950 m.a.s.l.
Varietals: Caturra, Castillo, Bourbon, Catiopes
Owner: Rodrigo Alberto Pelaez

Natural Colombian: Villa Clabelina Natural

Cropster link
Location: Ciudad Bolivar, Antioquia, Colombia
Altitude: 1510 to 1800 m.a.s.l.
Varietals: Colombia, Caturra
Owner: Finca Villa Clabelina

Washed Ethiopian: Suke Quto

Trabocca link
Location: Guji, Ethiopia
Altitude: 1800 to 2200 m.a.s.l.
Varietals: Kurume, Welicho
Owner: Suke Quto farm

Natural Ethiopian: Chelelektu Natural

Ally Coffee link
Location: Yirgacheffe, Ethiopia
Altitude: 1800 m.a.s.l.
Varietals: Kurume, Dega, Wolisho
Owner: Chelelektu washing station (several small farms)

Natural Brazilian: Pedro Humberto Veloso

Location: Carmo do Paranaiba, Santa Cecília, Santa Catarina, Brazil
Altitude: 980 to 1120 m.a.s.l.
Varietals: Catucaí 785-15
Owner: Pedro Humberto Veloso

Washed Kenyan: Kii AA

Cropster Link
Location: Kirinyaga, Kenya
Altitude: 1600 to 1800 m.a.s.l.
Varietals: SL28, SL34
Owner: Smallholders at Kii wet mill factory

Decaffeinated Washed Colombian: Cauca Inzá 

Location: Cauca Inzá, Colombia
Altitude: 1750 to 1900 m.a.s.l.
Varietals: Caturra, Colombia, Castillo
Decaffeination: Ethyl Acetate
Owner: Asorcafe

I’d like to thank Andy Kires from Canadian Roasting Society for allowing me to run this cool experiment by providing the green coffee !

What is Astringency ?

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If you have been reading about coffee extraction lately, you might be familiar with the term astringencywhich is often used to describe poorly extracted coffee. This is a descriptive for which the meaning is generally not well known, and it’s worth pondering why we dislike it so much in the specialty coffee world, contrary to other communities like wine and beer.

First, let me try to describe what astringency feels like. Astringency creates a dryness sensation in your mouth, and generally mutes a lot of other flavors, especially when it’s strong. It can often be found in fruits or vegetables, especially unripe ones − in fact, the astringency that is present in wine comes from the grape’s seeds and skin (Mattivi et al. 2009). If you ever ate an unripe banana and could not resist from grinning as the dryness sensation invaded your whole face, then you might know what I’m talking about. The one thing I recall being the most astringent I have experienced was the spongy white pulp separating seeds inside of a pomegranate. Another good example of astringency is over-extracted green tea, but it also contains other strong flavors so it’s not the most precise example.

Astringency is not seen as a necessarily bad thing by wine experts, and I suspect that this is due at least in part to red wine being much more concentrated than filter coffee; typically wine has total dissolved solids concentrations between 1.7% and 3.0% compared to 1.3% to 1.5% for filter coffee (e.g. see Schopfer & Lipka, 1973 and this French post), so astringency won’t easily mask everything else, and can instead be in balance with the global taste profile. In tea and coffee, we try to get rid of astringency as much as we can because it can easily become the dominant sensation, and can even come close to being the only perceptible one.

The astringency sensation is caused by soluble organic compounds that belong to a class called polyphenols, which includes tannins but also other complex molecules. They are often produced by plants as a defense mechanism against insects and other predators. These complex and large molecules can attach to proteins like Lego blocks, and form large clusters that can precipitate out of solution. This is what happens when we feel astringency; the polyphenols bind to proteins in our saliva and precipitate, forming clumps that inhibit our taste buds’ ability to taste the coffee properly, and cause this rugged and dry sensation.

Fortunately for us, polyphenols are mostly very large molecules, which makes them heavier and harder to extract from coffee particles, compared to smaller molecules like caffeine and the various acids we enjoy. This makes it possible to extract the good molecules, while avoiding the polyphenols that cause this astringent sensation. If you prepare coffee by immersion, you likely won’t often encounter astringency, because the immersion method extracts coffee solubles more gently, especially if you grind coarse and don’t agitate too much. Furthermore, the extraction happens in a relatively uniform way, in other words you won’t have large amounts of fresh water extracting a small amount of coffee particles. Remember that fresh water is a much more potent solvent compared to concentrated water. However, immersion brews also never achieve a very good filtration of coffee fines and other insoluble compounds.

Above: ellagitannin, an example tannin molecule from raspberries (source: Wikimedia Commons).
Above: malic acid (top) and caffeine (bottom), two examples of smaller, more easily extracted chemical compounds found in coffee (sources: Wikimedia Commonsand PubChem).

If you happen to prefer the taste of coffee without these insoluble particles as I do, you might prefer coffee prepared by the percolation method, where fresh water is poured on a bed of coffee, filtering out any undissolved solids (see this previous post for a more detailed discussion of the differences between percolation and immersion). Percolation brews are however much more prone to causing astringency, because channels can form where a larger fraction of water passes through preferential paths, which can over-extract some small regions of the coffee bed (we sometimes call this local over-extraction). This allows heavier polyphenols to be extracted, and makes the resulting brew astringent.

One thing I do not know is whether polyphenols can be filtered out by the coffee bed itself. When preparing percolation brews, the coffee bed filters out a lot of undissolved solids, and prevents them from getting in your brew. This is why a coffee bed full of fines will only clog the paper filter if you agitate it a lot; if you don’t agitate it, then the coffee bed acts as a filter and retains these fines, preventing your paper filter from clogging. This is also why a V60 has much less undissolved solids than a typical Aeropress brew; the depth of the coffee bed in the latter is typically much smaller, so it lets more fines in your cup.

Based on experience, I know that a typical V60 coffee bed can filter out a lot more compounds than just a paper filter, even in the extreme case of Whatman Grade 5 paper filters that have average pore sizes of 2.5 micron; I’ll talk more about this in a future post. However, I’m not sure if coffee beds are such good filters that they would remove polyphenols that were extracted and dissolved in the slurry. I think it is unlikely, because although polyphenols can be much larger than other coffee compounds, they are still much smaller than a micron. For example, some wine tannins have sizes in the range 50−70 Å (McRae et al., 2014). If you are not familiar with these units, 1 micron is equivalent to 10,000 Angstrom (Å), so a coffee bed would need to be a much better filter filter than a Whatman Grade 5 paper filter to remove polyphenols.

Ifthe coffee bed is able to filter out polyphenols however, the presence of a large channel could provide an additional reason why they let polyphenols in our beverage, because they would create localized regions of bad filtration, where polyphenols and undissolved solids pass through. That is an interesting question to me, because it would mean that over-extraction could potentially be fixed by a good non-channeling coffee bed, even if polyphenols are getting extracted in the slurry for other reasons.

This whole idea of channels causing localized poor filtration made me want to directly measure the amount of fines and other undissolved solids in my coffee brews in an objective way. This can be done with the help of turbidity meters, but until about a month ago, I though those were extremely expensive equipment only built for labs. When I saw Ray Murakawa using what seemed like a portable turbidity meter on Instagram, I got very excited and he told me they are actually portable and affordable ! Thanks to the help of my Patreon supporters, I promptly ordered one and started measuring some of my brews. Turbidity meters work through a different principle than refractometers, but they give us information that is a bit similar. Refractometers inform us about the concentration of dissolved solids, and turbidity meters inform us about the concentration of undissolved solids.

Above: a turbidity meter with its calibration solutions and a coffee sample.

When taking turbidity measurement, I realized something really interesting; the cloudiness goes up pretty fast as a brew cools down and stales. This was not too surprising at first because cooler water is less good at dissolving things, so we can expect the total amount of undissolved solids to increase as the brew cools. What I found really surprising is that even if you quickly cool down filter coffee to room temperature, its cloudiness keeps going up quite quickly for about half an hour, and then very slowly for more than 12 hours (I haven’t tried measuring older coffee). I’m not sure about this, but I suspect this increasing cloudiness at room temperature might be caused by polyphenols binding to some proteins that were also extracted from the coffee. Undoubtedly there are many other things that affect the absolute cloudiness of a coffee beverage, especially in an immersion, but it’s possible that the rate of increase at room temperature correlates with astringency. Furthermore, given that most of this turbidity increase happens within half an hour or so, I now wonder whether it’s related at all with coffee tasting bad after a brew stales.

Reading about polyphenols also made me realize there may be a reason beyond channeling why filter brew methods with very finely ground coffee (i.e., finer than espresso) often come out tasting astringent. For example, the high-extraction siphon method I posted a while ago only worked well with some specific roasts (some of which I listed in the post), and others came out very astringent regardless of whether channeling seemed to occur or not. Other examples include a few finely ground Aeropress and Buchner siphon brews I made that came out very astringent. I think some coffees may simply naturally contain a much lower amount of polyphenols, whether it is because of their varietal, terroir, processing or roasting. Grinding so fine ends up breaking most coffee cells, and therefore the chemical compounds are washed out by water (a process sometimes called erosion) rather than having to diffuse through the small pores in the cellulose walls of coffee cells. In that situation, polyphenols may very easily end up in the slurry, and if the coffee bed can’t filter them out, they may end up in the final brew even in the absence of channeling.

You may think that the same should happen with espresso and Turkish coffee, and you’re probably right. Assuming I’m not entirely mistaken about polyphenols extracting easily from broken coffee cells even in the absence of channels, I think either the high concentration or other things present in these types of brews (oils, suspended solids, etc.), may balance out the presence of polyphenols and make them less overwhelmingly astringent. That might also explain why it’s hard to take an evenly, highly extracted shot of espresso and dilute it into an amazing filter brew.

I think that one promising avenue may be to precipitate the polyphenols post-brewing by adding proteins in the finished coffee beverage, much like is routinely done in wine or beer making. Some things that are used for precipitation include egg whites, gum arabic, silica gels and a product called Polyclar (e.g., see this article about beer filtration) − these things are all rich in proteins. One potential major problem is that those compounds are typically left in the beer or wine for several hours to allow the precipitation to happen; if we wait this long with brewed coffee, it will probably taste very bad even if we remove all polyphenols from it.

I know this post probably opens up more questions than it answers, but I’m hoping it will help us think more clearly about what makes a brew taste astringent and how to avoid it !

I’d like to thank Scott Rao and Sylvain Mussigmann for useful comments.

What Affects Brew Time

[Edit October 23, 2019: Kevin Moroney pointed out to me that the slurry getting concentrated as it passes through the coffee bed also makes it more viscous as it reaches the bottom layers in a V60. This is indeed a very valid point, so I added a paragraph about this in the viscosity discussion below. In practice, this effect depends on the more direct variables of (1) brew recipe, (2) dripper geometry, and (3) the type of coffee, so it doesn’t change the final list of directly controllable variables that affect brew time. I still thought it is valuable to discuss it. ]

If you are used to pulling shots of espresso, measuring shot time might be a tool you use often to determine whether your grind size was dialed in appropriately for that coffee and set-up. This may lead you to believe that total brew time is also a very useful concept in the context of pour over filter coffee, for example to communicate your preferred grind size.

I think that this is really not the case, and I’d like to lay down the reasons why. I’m even slightly skeptical that shot time in the context of espresso is that useful especially when communicated online to different baristas that live in different conditions, but I don’t have any strong opinion about espresso making, because my lifetime cumulated number of shots pulled is currently a grand total of 1. So, at least for now, let’s focus on pour over coffee, as I usually do.

The reason why I think brew time is not that useful is simple: I think there are way too many variables that affect it, several of which are almost never measured, and some of which would be hard to always measure accurately. I’m personally striving at eventually measuring all of them such as to make my brews as repeatable as I can, but I’m not even sure yet whether that’s a realistic goal at all.

If we want to understand what affects drawdown time, it’s very useful to turn to Darcy’s law; this is an empirical equation that describes how a liquid percolates through a porous solid medium. In other words, it was originally deduced directly from observations and experiments rather than from fundamental concepts of physics. We now know enough about the physics of hydrodynamics that it can be derived from more fundamental principles, but it is only valid under some specific conditions which are almost always respected in coffee brewing, and more generally in daily life.

For the more mathematically inclined, let’s have a look at Darcy’s law applied to a liquid flowing down through a cylindrical medium, and then I’ll explain it with words:

In the equation above, Q represents the discharge, which is the volume of water coming out from the percolation medium in units of volume per time (e.g., mL/s). k is the permeability of the porous medium, which can also be though of as the inverse of its resistance. A more permeable medium will let more water pass through in a fixed amount of time; the physical unit of k is a surface (e.g., m²). A is just the surface area of the medium (remember we are applying Darcy’s law in a cylindrical geometry, so A is the same at all depths). μ is the dynamic viscosity of the liquid; it is low for most of the pure liquids we encounter in daily life such as distilled water and alcohol, but can get very large for more complex or heterogeneous stuff like honey or olive oil.

Dynamic viscosity is sometimes also called absolute viscosity, and it represents how much a liquid resists to deformations. The more viscous a liquid is, the harder it will be to pass it through small pores. L is the total length of the percolation medium, which in our case usually corresponds to the depth of a coffee bed. ρ is just the mass density of the liquid (for example in kg/m³), g is how fast objects accelerate when falling freely at the surface of the Earth (approx. 9.8 m/s²) and h is the total height of the column of liquid that is percolating (also counting the liquid above the surface of the solid medium). Δp refers to the difference in pressure below versus above the percolation medium (in physics and maths, we use this weird upward triangle to represent a variation or a change). In the context of espresso, this would be the atmospheric pressure (below the puck) minus the pressure a machine applies on top of the coffee bed. There’s a minus sign here because of how we defined the pressure differential, such that a downward pressure results in a positive flow of liquid.

Those of you who have worked with Darcy’s law may not have encountered it in the form above: it is often shown in a simpler form where the ρgh term is ignored, because it is often applied in a context where the pressure applied on the fluid is much more important than the fluid’s weight (as is the case with espresso). But for pour over coffee, we are in a context of gravity-driven flow, and therefore this more general form of Darcy’s law is useful.

Now that we defined all the terms in Darcy’s law, let’s explain it with words in the context of coffee. Basically, it says that any of these changes will make water flow come out from under the at a faster rate:

  • A more permeable coffee bed;
  • A wider coffee bed;
  • A shallower coffee bed;
  • Water that is less viscous;
  • Water that is denser;
  • Brewing from the surface of a denser planet;
  • Applying pressure on top of the coffee bed.

These changes are combined and independent of each other, and they are also linear, which means that doubling any of the things mentioned above will double the flow, for example. The geometry of the brewer won’t change all of these relations, and will only add a constant of proportionality (i.e., a number) in front of the right-hand side of the equation.

As you can imagine, the faster water flows through the coffee bed, the shorter your brew time will be. Therefore, we can look at all of these terms in Darcy’s law as potential variables that will affect brew time. You can already start seeing that there are quite a few of them, but it’s even worse than that; some of the terms above hide more than one variables that are combined together. The most dramatic one is permeability; in the context of pour over, it is affected by the following variables:

  • The grind size (coarser coffee is more permeable, finer coffee is more resistive);
  • The permeability of the coffee filter (affected by its pores and thickness);
  • The ridges on the inside of the brewer’s wall and filter creping (they allow air to flow upward outside the filter and increase permeability);
  • The saturation of the coffee bed (a coffee bed saturated with water increases its permeability, which is probably the most important reason why we bloom).

If you though this was starting to look like a rabbit hole, brace yourselves, because grind size also hides several other variables:

  • The grind size that you set your grinder to (which will differ even between two units of the same grinder model);
  • The grinder rotation rate (a faster rotation will generally produce finer grounds);
  • The grinder design;
  • The grinder burr size, geometry, material and alignment;
  • The bean temperature when you grind them (here’s a paper about that and another interesting discussion, but I want to discuss this more in the future). 
  • The bean terroir, varietal, processing, roast development, and aging − all of these variables affect the bean hardness and density, which will make it shatter less or more during grinding. I will talk more about this in a future post, but if a coffee shatters more, it will generate more fines and result in a less permeable coffee bed. The exact defects and variations in green coffee bean ripeness, humidity will also likely have an effect on roast and shattering.

The width and depth of the coffee bed can be expressed as being dependent on more intuitive and practical variables:

  • The dripper geometry;
  • The dose of coffee (in grams);
  • The mass density of coffee (less dense coffee will result in a deeper bed for the same dose).

Ah, finally… we listed all the variables.

Nope ! We are far from being done. You might think that the viscosity and density of water are known, fixed quantities, but they are not: they depend on its temperature ! The effect of changing water density is very small in the context of coffee brewing, as this data illustrates:

At sea level atmospheric pressure, the difference of room temperature vs boiling water density is just about 4%. The change in viscosity, however, is not small. I talked about this a little bit in an earlier Instagram post, where I built this graph of water viscosity (in red) from literature data (specifically, IAPWS 2008 and Engineers Edge Machinery’s Handbook):

The difference between room and boiling water viscosity is therefore about 70% ! In the figure above, I also marked some typical slurry temperatures I obtained with a glass or plastic V60, and how the flow is affected if everything else is kept constant (in blue).

As you can see, warmer water is significantly less viscous, and it will therefore flow faster through the coffee bed. And please do not go thinking I am talking about kettle temperature here ! I don’t only say this because kettle thermometer readings are not reliable (which they are not in my experience), but also because kettle temperature is only one of the variables that will affect the temperature of water as it percolates through the coffee (i.e., in the slurry); these additional variables will also significantly affect the slurry temperature:

  • The dripper material (i.e. both its thermal mass and conductivity);
  • The room temperature;
  • Any air flow in the room;
  • The temperature of your ground coffee;
  • The moment during the brew (temperature will typically fluctuate);
  • How many pours your recipe has (more pours tend to result in cooler slurries).

The viscosity of water is also affected by its hardness and total alkalinity (I talk about these concepts in detail here), but the effect is very small unless you have very unusual water. Let’s quantify that a bit more. According to this scientific publication, the viscosity of water does depend on its bicarbonate content:

To put this into a bit more context, the addition of Na2CO3 at a concentration of 1 mol/L would result in a total alkalinity of 2000 meq/L, or in units we are more familiar with, about 100,000 ppm as CaCO3. That much bicarbonate is needed to almost double the viscosity of water. Given that brew water recipes for coffee are almost mostly below 80 ppm as CaCO3, we can safely ignore the effect of total alkalinity on viscosity.

The viscosity of water is similarly affected by its general hardness, here’s an example of how it increases as Calcium Chloride is added to water:

Yet again, we are talking about a 10% concentration (by mass) for a doubling in water viscosity, which is insanely larger than typical water hardness we use for coffee: even achieving the “Hard AF” Barista Hustle hardness with calcium chloride would necessitate less than 0.03% concentration by mass. We can thus also safely ignore the effect of water’s total hardness on its viscosity, and only care about its temperature.

There is another thing that affects viscosity in the slurry; the concentration of coffee compounds being extracted from the coffee particles. In espresso brewing, the high concentration of the beverage can cause it to become 2 to 3 times as viscous as the input water (e.g., see Clarke & Vitzthum 2001). For filter coffee, we can expect the effect to be much smaller, about a 30% increase if we assume it is a linear function of concentration. This is still not negligible, and means that the viscosity of water near the bottom of the coffee bed will flow a bit slower because if this higher concentration, therefore making the global flow slightly slower than one would expect based on pure water. However, for a fixed brew method, dripper geometry, and coffee type, the profile of concentration versus depth and time should be the same every time the coffee is brewed, so this effect can be categorized under the umbrella of these three more direct variables.

We have still not unwrapped most of the variables between the big parentheses of Darcy’s law, and those are the ones that make pour over timing much nastier than espresso timing. In the case of espresso, the Δp term is much larger than the ρgh term, and this means that repeating the exact same pressure profile every time will ensure that the shot time only depends on the variables we already studied above.

As we won’t be brewing coffee on the surface of Mars (that would suck), there is only one other variable we haven’t considered, and it’s not a fun one: the height of water in your V60, or h, is what makes pour over timing much harder. This is true mostly because it depends on one input variable that we control and measure only rarely: the rate at which we pour water from the kettle. Someone that pours a lot of water very fast in a single pour will build a very tall column of water in the V60, and it will flow much faster, and finish brewing much before, a barista who pours slower or in several smaller pours. Similarly, the bloom length will obviously affect the total brew time because it’s a period where no water is being poured from the kettle. The geometry of the dripper also has an effect on the height of the water column, but that’s much easier to control or keep constant.

There are also circumstances during pour over brewing where Darcy’s law fails, although typically only momentarily. Darcy’s law is valid only for a fixed porous medium, and there are a few things that can change the structure of the coffee bed, which is our porous medium:

  • The preparation of the coffee bed (distribution, bloom, swirling after bloom, and tamping in the context of espresso);
  • The amount of agitation: water poured faster or from higher up will lift the upper parts of the coffee bed and temporarily make it shallower, increasing flow (using devices like the Gabi B or Melodrip will eliminate most or all agitation);
  • Channeling: the appearance of a large channel can increase the coffee bed’s permeability;
  • Erosion, also called fines migration: finer coffee particles being displaced to the bottom by water can decrease the permeability of the coffee bed. This can also cause the filter to clog, which will decrease the permeability even more.

Another possibly important factor that may affect brew time is how much coffee particles swell during the bloom phase. As coffee swells, it slightly closes the gaps between particles, effectively making the coffee bed less porous. I’m not sure what properties of coffee affect how much they swell, but it’s possible that beans of varying hardness or particle porosity may swell differently.

There’s also one final thing that can easily be forgotten: the exact way in which we choose to define the start and end of a brew is also a factor. For pour over coffee, an obvious choice of when the timer starts is when kettle water hits the dry grounds, but the moment where the brew ends is a bit less obvious. I personally choose to stop the timer when the level of brew water just passed the surface of the coffee bed and I can see ambient light first reflecting on the surface of the wet coffee; I mostly choose this moment because it’s easily repeatable.

I think we have now finally detailed all of the most important variables that affect brew time ! But hey, maybe I forgot about others. If you think about more, I’d love to hear about it, but please don’t send me suggestions about light speed travel and kiloGauss magnetic fields lol.

It would be useful to regroup all of the important variables that we discussed above, in terms of what we can control directly when brewing (i.e., not viscosity), so here’s an extensive list:

  • Grinder setting, rotation rate, model, zero point, burrs and alignment;
  • Coffee terroir, varietal, processing and other bean characteristics (defects, drying, ripeness etc.), exact roasting process and development and bean aging;
  • Dripper model (geometry, material, inner wall ridges);
  • Exact brew recipe (bloom length and efficiency, number of pours, pressure or suction devices, coffee dose, pressure profile in the context of espresso);
  • Brew temperature;
  • Kettle flow speed and height, or anything else that affects agitation;
  • Room, bean and grinder temperature;
  • Exact filter model (e.g., Hario tabbed and tabless are different);
  • Air currents in the room;
  • Coffee bed distribution and preparation (and tamping if applicable);
  • Channeling and erosion;
  • How the brew start and finish are defined.

Now, if you want to have a consistent brew time, you need to measure and control and fix all of the things above, which is no easy feat. If you want to use brew time to communicate grind size, not only the two persons talking need to have the same grinder, zero point, and burr alignment, they must also be drinking the same coffee, have the same exact dripper, filters, water temperature, recipe down to the pour rate, etc. You can see why I think brew time is not that useful for communication ! If you are insane like I strive to be and measure most of the things above, then and then only changes in brew time will inform you that something is going on.

I’d like to thank Kevin Moroney for useful comments.