Measuring and Reporting Extraction Yield

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

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

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

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

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

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

Percolation

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Immersion

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

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

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

which can also be inverted with a bit of algebra:

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

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

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

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

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

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

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

Mixed Phases Methods

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

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

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

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

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

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

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

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

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

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

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

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

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

Sharing Extraction Yields

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

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

Some Parting Thoughts

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

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

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

Measuring the Concentration of Espresso Shots

If you are wondering why it is widely recommended to use syringe filters when you measure the concentration of espresso shots with a refractometer, or how bad exactly a measurement would be if you don’t use such a filter, I highly recommend this recent blog post by my friend Mitch Hale. And while you’re there, you should also check out his new blog, which already has a method on how to align your grinder, and much more geeky stuff to come.

Mitch ran a very dedicated experiment to compare the precision and accuracy of filtration by VST syringe filters as well as centrifuging espresso samples. The results are very clear: while the precision inherent to syringe filters is as good as the internal precision of the VST Coffee lab III refractometer (0.01% total dissolved solids), not using the syringe filters will give you concentration measurements around 0.38% too high, and way less precise on top of that.

This graph shows a kernel density estimation of the frequency of measurements that are discrepant by a given number. Mitch obtained two samples and two measurements with three different methods, for a total of 20 espresso shots. These curves tell you how often the two measurements disagreed by a given quantity. You can see that the unfiltered samples can disagree by a larger number on average, and even worse, the disagreement is highly unpredictable. This means unfiltered espresso samples are not reliable for measurements of concentration.

Even in a best-case scenario where all coffee roasts and origins have the same amount of oils and suspended solids (this is most likely false), deciding not to filter your espresso sample, and instead subtract 0.38% from it, would result in a very degraded precision of about 0.1%, instead of 0.01%. Hence, I highly recommend that you always filter your espresso shots before you measure their concentration.

Another thing that Mitch concluded from his experiment is that centrifuging allows to obtain measurements as accurate as the VST syringe filters, and that contrary to some popular worries, the syringe filters do not bias measurements by filtering out some of the coffee’s dissolved solids.

This graph is very similar to the last one, but this time it shows the difference between measurements taken with unfiltered samples and those taken with filtered samples. You can see that unfiltered samples will on average seem 0.38% too high in concentration, but that difference is rather unpredictable – this difference itself varies by about 0.11%, so it is not possible to just correct your unfiltered measurement by subtracting 0.38% to it.

Please have a look at his blog post for much more detailed results, and a very detailed description of his experiment.

Another perk from Mitch: he had the very ingenious idea to add a full glossary section to his website, which I will now link to in the Resources menu of my blog. This way, every time you encounter a weird geeky word that you’re not sure about, you can consult his glossary !

The Dynamics of Coffee Extraction

[Edit January 30, 2019:

After receiving some feedback about this post, I would like to address a few things that I think were not clear enough. I’d like to thank Scott Rao for his comments.

First, when I use language such as “over-extraction”, and “under-extraction”, I don’t mean that the associated extraction numbers are necessarily undesirable. What I really mean is “more extracted” and “less extracted” – the actual level of extraction that is desirable depends on several things, one of which is the subjective sensory factor. Another is the narrowness of the particle size distribution generated by a grinder, as I mentioned in the post. So, it would be wrong to say that an “optimal extraction is at 21%” for example; the exact number that someone finds optimal will depend on preference, roast development and quality, and evenness of extraction. The extraction yield numbers that I give in the text are just examples that I threw around, please don’t take them as absolutes.

It also came to my attention that the evidence for fast-extracting compounds being more on the “vegetal and sour” side of taste is speculative, so please take this claim with a big grain of salt. Instead, it would be more careful to say that low extraction yields will generally produce a less balanced overall taste, because only some fraction of all available chemical compounds get extracted. Think of it like listening to music with a very agressive equalizer turned on. The evidence seems to be stronger on the other side of average extraction yields, in the sense that bitterness and astringency are part of the slow-extracting compounds, and they tend to take a lot of space in the perceived taste profile of a cup. 

I did not do a detailed consideration of the process of erosion in this post, but it still plays a role even in filter coffee. It is simpler to model because the fines just immediately extract completely in contact with water, so I did not include erosion in this discussion without some data to play with. The amount of fines present in a particle distribution will definitely have a strong effect on the flavor profile of the cup, on top of the size of particles – I will talk more about it in the near future !

Finally, please do take this whole model with a grain of salt – It was not yet tested against real data, I assumed spherical particles, and based all of it on the assumption that chemical compounds extract at a rate that decreases exponentially. My hope is that it will be useful to understand some aspects of extraction dynamics, but it is in no way a perfect model.

]

Coffee extraction is a subject I’ve touched a few times on this blog. Today I want to have a more profound discussion on this subject, because I recently realized I had a very simplified view of what’s happening during coffee extraction. I’ll go over the basic principles first, and then gradually deeper and deeper in this rabbit hole. This is one of those times where I will be posting some equations, but I will try to translate them in words and figures as we go along, so please don’t feel bad if you don’t know anything about maths. I hope to be able to describe them well enough that you won’t need to have a degree in maths or physics to follow the big picture. The value of equations is that they allow me to see what arises from just a few fundamental suppositions.

Specialty coffee brewers often talk about total dissolved solids (TDS) and average extraction yield (EY) when they describe a method or a coffee they brewed. As I briefly described earlier on this blog, the first concept of TDS really describes the concentration of your beverage: espresso typically has 7% to 12% TDS, and filter coffee typically has 1.3% to 1.45% TDS. The second concept of average extraction yield describes what fraction of the coffee beans were dissolved in your beverage. This number is typically between 19 and 23%, and can never go above ~ 30% because the remaining 70% of the coffee beans is just not dissolvable in water.

At first glance, knowing the average extraction yield might seem to be just another, more convoluted way of describing the concentration of your coffee. But it’s not ! Average extraction yield was found to correlate very well with the taste profile of a brew. If you make three brews with the same coffee, and reach 18%, 22% and 27% average extraction yields, then add the appropriate amount of water such that they all have the same concentration (e.g. 1.3% TDS), the three cups will taste very different. The first one will tend to be more vegetal and sour, the second one will be more well-balanced, complex and enjoyable, and the third one will be more bitter and astringent.

Why does average extraction yield correlate so well with flavor profile ? Ultimately, this is due to different chemical compounds extracting at different rates. Some of the compounds that we typically don’t like to taste are very slow to extract (thankfully !), so they will start to become apparent only when you reach high extraction yields. Other components that extract very fast are enjoyable, but if they’re not balanced with other stuff they produce a less interesting cup. In other words, our goal is to extract as much of the good stuff (the compounds that extract at average and fast speeds) as we can, while avoiding the nasty stuff (the compounds that extract at slow speeds).

The concept of an average extraction yield is useful, but it’s not at all the ultimate descriptor of a coffee cup’s flavor profile. Imagine a situation where some of your coffee grounds extract faster than others – the resulting coffee cup might be composed of some grounds extracted at 18%, and others extracted at 28%, and you could still get an average extraction yield around 23% in the cup. If you were to compare this with a cup where all coffee grounds extracted at 23% exactly, you would most likely find the second cup more enjoyable (this is not the one they sell at Second Cup). Basically, the second cup has extracted a lot of the “good stuff”, and very little of the bitter, astringent taste. The first cup however has a lot of coffee grounds that reached a 28% extraction yield, so they will be contributing some of the less desirable taste in the cup.

One practical result that arises from this is that lower quality equipment or brew methods that produce a wider range of extraction yields will only allow you to reach average extraction yields around 20-21%. If you go any higher than this, then you will start getting too much of the bitter taste. If you manage to produce a brew where the extraction of individual coffee particles is much more uniform, then you will be able to reach higher average extraction yields, about 22-24%, without getting too much of the bad stuff.

An illustration of why extraction yield uniformity matters. If you brew two coffees with the same average extraction yield (green dashed line), but one (blue) is much more uniform than the other (red), then you will be able to extract a larger amount of the good flavors (white background) without extracting a significant amount of the bad flavors (orange region). This illustrates why dialing in your coffee with better equipment or a better method that produce more uniform extractions will have you reach higher extraction yields. In this illustration, the red curve would have tasted bad because it extracted too much of the bad stuff, so you would have been forced to use a much lower average extraction yield.

One thing that can explain why your coffee particles may not all extract at the same rate is the fact that they may have different sizes. As Scott Rao explains nicely in this blog post, there are two completely different physical processes by which coffee extracts: erosion and diffusion. Erosion happens when a coffee cell is broken and water can very easily wash away all of the dissolvable compounds that it contains. As coffee cells are very small (around 20 microns), this happens only at the surface of coffee particles, where some broken cells are exposed, or in coffee particles so small that all coffee cells are broken up. In this scenario, water dissolved the full ~30% of anything that can be dissolved very fast. As you may have guessed, erosion is the dominant process in espresso or Turkish brews, because those use very fine grind sizes.

Diffusion is the process that dominates in filter brews. In this scenario, water has to enter the tiny pores of the coffee cell walls, dissolve the flavors, and come back through the tunnels. As you might expect, diffusion is much slower than erosion. In this post I will focus more on diffusion, because filters brews are my bigger focus at the moment.

Cells inside a coffee bean. Their size is typically around 20 micron. Royal Photographic Society, Kacie Prince @ Pinterest.

Now comes the part I did not understand very well until very recently. One thing I mentioned earlier on this blog was that smaller coffee particles extract faster than the larger particles. This was actually kind of true, but my reasoning was not. I was really confusing the extraction of a single coffee particle with that of a population of coffee particles. If you have a collection of very coarse coffee particles, they will collectively extract much slower than a collection of very fine coffee particles, because the finer particles are presenting much more total surface for the same total mass of coffee.

If you look at a single coarse particle and a single fine particle however, and measure how fast they provide flavor compounds, then the picture is quite different. The single fine particle is much lighter, and has a much smaller total surface than the single coarse particle, so it is actually the coarse particle that would win the race to higher concentrations. Assuming the fine particle is large enough that we are still within the regime of diffusion, each cell at the surface of the fine coffee particle is extracting at the exact same speed as each cell at the surface of the coarse particle.

The last paragraph is really key to understanding why I have been thinking a lot about this lately. It’s worth reading it again and make sure you understand it well. Once you do, something might become clear to you: a population of finer grinds will reach higher beverage concentrations faster, because you have a large number particles and they collective provide coffee compounds faster than a collective of coarse particles, because of their larger total surface area. Our picture of how TDS depends on grind size is quite clear.

BUT, once you accept that each coffee cell at the surface of each coffee particle extracts the same way and at the same speed regardless of the particle size, then it becomes entirely mysterious why different grind sizes or different particle distributions would produce different uniformities of extraction yield, and different taste profiles ! If this was the whole picture, then the only thing we would ever care about would be the beverage concentration (in % TDS), and all coffee cells would always be providing us with the same flavor profiles whether they are attached to a large or a small coffee particle.

I think the key to understand the link between the distribution of particle size and the distribution of extraction yield is something else: deeper layers of coffee cells extract slower than surface layers. Imagine you had only two layers of coffee cells that can be reached by water, and the deeper layer extracts much slower than the surface layer. Now imagine you have two spherical coffee particles, one that is just as large as two layers of coffee cells, and one that contains thousands of layers of cells. Let’s draw this:

Comparison of layer sizes for different coffee particle sizes. For a very large coffee particle, the two layers (black and blue) have almost exactly the same total size, and therefore contain the same amount of coffee cells. For the very small coffee particle, its more pronounced curvature means that the second layer (in blue) is much smaller than the first layer (in black). As a result, there will be a smaller amount of deep coffee cells that extract slower.

It might become obvious from this drawing that the amount of second-layer cells is much smaller than the amount of surface cells in the small coffee particle. In the case of the very large particle, they’re almost equal ! This immediately provides a way to understand how different-sized coffee particles are providing different flavor profiles. The small particle will be producing a more uniform extraction yield, because it is composed of one surface layer extracting uniformly, plus a small contribution of a deeper layer that extracts slowly. The combination will be a little bit non-uniform. It will be skewed slightly on the low extraction side, because of the small contribution of these second-layer coffee cells. The larger coffee particle will produce a much less uniform extraction, because the contribution from the slowly extracting second-layer cells is as big as that of the surface cells. Once again, I think this might be easier to understand with a figure:

An example of extraction yield distributions provided by four different sizes of coffee particles. The under-extracted coffee cells will contribute a vegetal, sour taste, and the over-extracted coffee cells will contribute a bitter, astringent taste. Each type of particle size produces an average extraction yield marked with the open circle. We will revisit this figure in more detail later in this post.

In real life, water is able to reach a bit deeper than two layers of coffee cells. In one of my earlier posts, I discussed a recent experiment carried out by Barista Hustle, which demonstrated that water can reach down to approximately the 5th layer of coffee cells on average. If you haven’t watched their video, it’s worth it – this is what made me realize that I was misunderstanding the details of extraction.

So now, we saw that each size of coffee particle produces a distinct profile of extraction yield, and therefore a distinct flavor profile. We also saw that coarser particles inevitably produce less uniform extractions. You can now see why using a grinder that produces a very wide distribution of coffee grind sizes might be a problem: you are mixing up lots of different flavor profiles. However, this new way of thinking about extraction might also have you realize that a perfectly uniform particle distribution will not produce a perfectly uniform distribution of extraction yield !

Instead, such a perfectly uniform particle distribution would just produce exactly the same extraction yield distribution than a single coffee particle would – and it is not uniform. Still, the final extraction yield distribution will be tighter if your particle size distribution is also tighter, which is desirable. It still came as a shock to me that even with a light years-wide roller mill grinder, you will not obtain a perfectly uniform distribution of extraction yields, unless you also use coffee particles that all contain exactly 2 x 2 x 2 intact coffee cells (OK, maybe you can do this if you have such a large grinder).

There’s also another consideration about grind size which I did not touch in this discussion: coffee waste. The coarser you grind, the larger will be the total mass of coffee that is inaccessible to water. This means that, in addition to changing the taste profile, grinding coarser is in some way similar to also using a smaller coffee dose. I won’t discuss this more in this post, but it’s worth remembering it.

Now let’s do some maths

Now that I’ve tried to lay out the concepts with hand waving explanations and drawings, I’d like to attempt formalizing it with equations. Those not too versed or interested in maths may find the rest of this post anywhere between boring to insufferable. I find it really interesting to be able to write down equations to describe a system and see where it leads me. Often, this is a way to realize some consequences that you may not have foreseen, and I think some of you will find value in the figures below (or even in the equations).

The first assumption I will base this formalism on is that each of the chemical compounds in a coffee cell gets extracted at an exponentially decreasing rate:

In this equation, m_i is the amount of mass extracted from a chemical compound that we would call “compound number i“, t is the amount of time since the beginning of the extraction, and τ_i is the characteristic time needed to extract the compound: it’s larger for the more slowly extracting compounds. The left-side of the equation is a time derivative, which means that it describes the rate of mass extraction per unit of time. This might seem like I pulled this equation out of nowhere, but it’s something that arises quite often in these kinds of problems: there is initially a lot of different ways for water to enter in contact with large amounts of the solvable compound, and the least of it remains in the coffee cell, the slower the extraction rate becomes. I’m not convinced this is the ultimate way to characterize this problem, but I think it’s at least a good one.

This figure shows the evolution of the extraction rate over time. The characteristic time τ corresponds to the moment where the extraction rate has fell to ~37% of its starting value. After more than about 5 times the characteristic time, nothing much is extracting.

This equation tells us about the rate of increase of the compound, but what we really want to know is the amount of extract that ends up dissolved in water as a function of time. To obtain this, we need to solve the equation above (which I won’t do in detail here). The solution is:

where a new constant M_i was introduced, representing the total mass of this particular compound inside the coffee cell. At this point, it would be worth visualizing what this equation looks like:

The fraction of total available mass that is extracted from a coffee cell over time. Most of the mass is extracted after a few times the characteristic time τ.

Now, given that each chemical compound extracts at its own speed, obtaining the total mass of everything extracted requires you to take the sum of the equation above, for all available compounds:

Now, what does the sum of lots of different extraction equations like those look like ? It’s really hard to tell if you make no assumption at all about the collective properties of the extraction rates τ_i. One way to go around that is to do it numerically, or something else we can do is ask what the result looks like if all the extraction rates τ_i are close to one another, and thus close to an average extraction rate τ. A mathematical way to express this is:

Here, τ is the average characteristic extraction time, and ε_i is just a symbol I decided to use to express the small deviations around the average, for each compound. It may seem weird that I defined this equation with respect to the inverse of the extraction times, but it will make the subsequent maths easier. Now, I need to make the approximation that the deviations are very small with respect to the average:

And this will allow me to simplify the equation for the total extracted mass as a function of time, with a neat trick that physicists love, called a Taylor expansion around ε_i = 0:

The technical term for what I just did there, besides annoying most of my readers, is a first-order approximation (literally, not just figuratively). You might notice that the first term on the right part of the equation is very similar to the equation we had for a single species, with τ_i replaced by the average τ. This is very neat, because it tells us that this very similar equation is a zeroth-order approximation of the real solution. This means that it captures the largest portion of the answer, as long as the ε_i factors are small like we first assumed.

The second term, which looks a bit more complex and still has this big Σ symbol that represents the sum of many terms (i.e., all the chemical compounds), is a first-order perturbation. If you add it, your answer will be more precise. There is an infinite number of smaller and smaller terms that you could add, which would make your answer more and more precise. If you added this infinity of terms, the solution would be valid regardless of whether all the ε_i are small or not. It turns out that the zeroth-order approximation is quite good (to 1% precision) if you have a lot of chemical compounds (at least 100) even if some extract ~15% faster or slower than the average:

An example simulation with 100 individual chemical compounds (blue lines) that have standard differences of 15% in characteristic extraction time, and individual masses that can differ by up to 40%. Even with this relatively modest number of compounds that have very different properties, the zeroth-order approximation for the total extracted mass is (black curve) accurate to within 1% of the true sum (red dashed line).

Now that we have described more formally what happens to a single layer of cells, we can turn our attention to the more general case where there are more than one layers. Without any detailed experiment, we need to make an assumption about the rate at which water is able to access the deeper layers. Intuitively, I see water diffusing in the coffee particle like an ensemble of small creatures that walk around randomly, and have a very small chance of getting through a door which leads to a deeper level of cells, or one that leads to a shallower level. In order for water to grab some compounds from the deeper layers and bring them back out, it will need to be able to pass back and forth through several doors.

What this kind of scenario tends to produce is also an exponentially decreasing access to the deep layers (by this point, do you think I love exponentials ?). To be sure about that, I decided to actually run such a simulation, where I took a million “droplets” of water that have a 0.1% probability of crosser over to a deeper or shallower layer at every step of time. I ran the simulation for ten thousand time steps, and every time a droplet came out of the coffee particle, I asked it how deep it reached. I then made a figure with the distribution of depths that each droplet reached:

Distribution of depths reached by randomly moving droplets of water (also called “walkers“), for a million droplets and ten thousands units of time. At each unit of time, each droplet had a 0.1% chance to go either to the deeper or shallower layer of coffee cells. The result is an exponentially decreasing distribution (the vertical axis is displayed in logarithmic scale).

In other words, the deeper layers will extract exponentially slower. If we decide to call s the coordinate that points inward to the deeper layers, then the characteristic time of extraction τ_i will be the combination of an intrinsic rate τ’_i dependent on chemistry, and the depth x:

The new parameter λ represents a characteristic depth before which most of the extraction happens. If it’s small, then the extraction will only happen in a very thin shell of the coffee particle, and if it’s very large, some extraction may happen deep into its core. In reality, thanks to Barista Hustle we know that this parameter λ is probably of the order of 100 microns.

A demonstration of how layers extract slower with cell sizes of 20 microns and a characteristic depth of 100 micron. Extracting 90% of the available mass per cell takes 3 times longer at the seventh layer compared to the surface layer !

This new level of complexity means that we need to sum the extraction equations over all depths, each having their own extraction speed. The result is:

In that equation, R is the radius of a spherical coffee particle and s is the typical length of a coffee cell (about 20 microns, we assumed the cells are cubes). The index k is representative of the layer, where k = 0 is the surface. There’s a term in (R – k s) squared that appeared, which is due to the geometry of a spherical particle; each level deeper has a smaller amount of cells in it. For non-spherical particles, this term would be a bit less steep. The spherical case scenario is the most dramatic one, in the sense that the deep layers are slowest to extract (this is because spheres have maximal curvature).

There is a way to make the equation above a bit more easier to deal with, by assuming that the cell layers are continuous instead of discrete. This is not true in real life because there are no “half-cells”, or fractions of cells, extracting in their own particular ways. However, I believe a continuous model would be more realistic because it would be more similar to the results of irregular layers of cells, where not all cells in a given layer are exactly at the same depth, or have exactly the same number of entry ports. These small random deviations in the exact extraction speed within a given layer of cells will produce a similar effect to the assumption of continuous layers of cells. Thus, here is the continuous version of the equation above:

In this equation, x represents the depth inside the coffee particle (expressed in the same units as the radius R). One may be tempted to solve this integral and find a form for the extracted mass versus time that is easier to work with, but don’t go there – you would encounter a dreadful beast that has many names, one of which is the confluent hypergeometric function. It takes three sets of arguments, and is a real nightmare to deal with (to all readers that are thinking right now “what the hell is this guy rambling about“, I apologize).

Now that we re-framed m_i in this way, it now represents the full extraction output of chemical compound number i for a full coffee particle, instead of just a coffee cell. Let’s see how its rate of extraction would be affected by the fact deep layers extract slower:

Comparison of extracted mass for a 1 millimetre radius spherical particle (red) versus a uniform layer of the same total mass (black), assuming a characteristic depth of 100 microns for the particle. The particle extracts slower because of deep layers lagging behind, but it eventually reaches full extraction after several times the characteristic time of the chemical compounds. In this situation, the outer layers will get over-extracted much before the deep layers are well extracted.

Basically, the overall extraction for the spherical coarse particle is slower, and reaches an inflection point where this become really slow near 3 times the characteristic extraction time τ. If you’re willing to, go re-watch the Barista Hustle experiment, you might notice this red curve looks a lot like the cupping bowl that contains coarse coffee grounds !

Now, another interesting aspect is to estimate the contributing fraction of a fast-extracting compound to the beverage, as a function of time. Obviously, the concentration of this compound will be at its highest when the brew just started, because it had a head-start from being a fast-extracting compound. Here’s what this would look like:

Mass fraction versus time for a 15% faster-than-average chemical compound, for a uniform layer versus several particle radii with the same total mass. The larger particles lag behind in terms of flavor balance. In this example, the total available mass for the chemical compound in question was set to 20%.

… and if we did the same thing for a slow-extracting compound:

Mass fraction versus time for a 15% slower-than-average chemical compound, for a uniform layer versus several particle radii with the same total mass. The larger particles lag behind in terms of flavor balance. In this example, the total available mass for the chemical compound in question was set to 20%.

The figures above show how even this contribution of a given chemical compound to the cup’s flavor evolve differently for particles of different sizes. One way to go even deeper is to look at the distribution of extraction yields per coffee cell, in terms of their contribution to the total beverage by weight. Let’s look at the result for four different particle sizes, and three different brew times:

Distributions of extraction yield per coffee cell for a brew time of 1 average characteristic extraction time τ, for four different coffee particle radii. The average of each curve is represented with an open circle. The larger particles provide flavours at lower average extraction yield, because of the long tail of under-extracted cells deep under the surface. Non-spherical particles would have a shorter tail at low extractions, and a slightly higher average extraction yield.
Same distributions for a brew three times longer than the average characteristic extraction time τ.
Same distributions for a brew five times longer than the average characteristic extraction time τ.

There are a few things we can learn from these figures:

  • Different-sized coffee particles provide different flavor profiles to the cup.
  • The highest extraction yields are always the top contribution, regardless of particle size (they correspond to the collective outermost layer of all particles).
  • A coffee cup made with a perfectly even distribution of particle sizes will not taste like a set of perfectly evenly extracted coffee cells.
  • Differences in flavor profiles for different particle sizes should be more stark for shorter brew times.

There’s another interesting thing we can look at with this model: How does the average extraction yield depend on particle size ? To do this, I ran a simulation at ten different brew times, from one to ten times the average extraction yield:

Average extraction yield for particles of different radii and different brew times.

There’s nothing shocking here: smaller particles extract faster, and longer brew times lead to higher average extraction yields. The more interesting part is that these curves are not easy to reproduce with a simple equation. Even if we were to look at the speed at which each particle extracts as a function of its radius, or surface, the result is not a nice power law, like the naive “extraction speed” = “1 / particle surface” assumption I once made in the past. Rather, it’s a relatively complex functional form, that needs to be modelled properly instead of approximated !

The models I developed in this post will be used to translate particle size distributions into distributions of flavor profiles (via the distribution of extraction yields), in a future application I will release soon. To the hardcore geeks that made it this far in the blog post, I congratulate you two !

I’d like to thank Mitch Hale for a discussion that helped me put some order to these thoughts, and Noé Aubin Cadot for helping me with figure out some Mathematica stuff.

Statistics and Blind Tasting

A friend recently asked me a really interesting question about the statistical significance of blind tasting. He asked: “How many times should I successfully identify an intruder coffee by blind tasting among a set of three cups, in order for my experiment to be statistically significant ?

As you might imagine, I really like this scientific-minded approach to blind tasting. Unfortunately, the answer to this question is not that simple, and we must plunge into combinatory statistics if we want to answer it. I won’t do this here, but I will provide you with a way to get an answer without caring about combinatory statistics. I’m sure most of do not care about the long, detailed equations.

Even if you don’t care about maths, I would like you to read a few paragraphs below that I think are super important to understand, so please bear with me for a bit longer. I promise you won’t encounter any more equations.

A common theme to all problems of mathematics and physics is that a question must be posed very precisely before we can answer it. This is often the hardest part of a problem: formulating it precisely and correctly. The way we posed the question earlier is not precise enough to start doing maths with it, because we need to specify what we mean exactly by “statistically significant”. To do this, we also need some reference point. We want our experiment to be better than something, but better than what ?

One neat way of setting up the problem is by adopting the frame of mind of classifiers. A person trying to identify the intruder coffee among three cups can be called a classifier: it can be a very efficient classifier, succeeding every blind taste, or a very bad classifier, randomly selecting a cup because it is unable to taste anything different in the three cups. There’s also a third possibility, which is more rarely interesting: someone could be a misguided classifier, by always identifying one of the two wrong cups as the intruder. If you think about it, this is even worse than a random classifier, because the random classifier will be right at least a fraction of the time.

Now that we talked about classifiers, it becomes easier to ask the question more precisely. As a first step toward this, we can ask instead something like “Am I better than a random classifier at this ?“. This is a step in the right direction, but it is slightly incomplete. Let’s take a simple example: You did the blind tasting test three times, and successfully identified the intruder coffee twice. The third time, you chose the wrong cup, and therefore you failed. Is the random classifier better than you ? Well, it will be sometimes. If you ask a random classifier to repeat this experiment of three tastings a dozen times, it may beat you a few times by identifying the correct cup at least twice, and then it may do worse the rest of the time.

An even better way to pose the question is thus: “What fraction of the time will I beat a random classifier ?” This is now a question posed precisely enough that statistics can answer. Obviously, you will want this fraction to be high ! For example, if statistics tell you that you are better than a random classifier 99.9% of the time, you should be happy about it. If you are better than it only 50% of the time, this is not great news. You might now realize that there is a subjective aspect to the way we interpret this score. There is no universal laws of nature that tell you: “You must be better than 99.9% of random classifiers in order to be a good taster“. What does “good” mean ?

This is a problem we must embrace, because we are stuck with it. Physics, Chemistry and all other fields of science are also stuck with it. How confident do you need to be before you think something is probably true ? This is a fundamental question, and different fields of science adopted different goals of confidence. As an example, the field of astrophysics decided that a confidence of 99.7% is cool. The field of particle physics decided to be more conservative, and decided they want to be at least 99.99994% confident before they change their minds. There is probably some sociology playing a role in this decision, but it is also certainly in part related to how precisely we are able to measure stuff. Particle physicists have big labs and can design experiments in them – astrophysicists are stuck lightyears away from their experiment, and all they can do is watch.

Talking in terms of probabilities like 99.7% or 99.99994% is a bit impractical, unless you really enjoy counting decimals. Fortunately, there is another way to describe this, with a very simple number that you can view as a score. In technical terms, this is called an “N-sigma significance“, but you can now safely forget I ever said that. Just think of it as a score, and you want it to be as high as possible. Let’s visualize a few different scores in a table, and translate them to % of confidence:

ScoreConfidence
168.3%
295.4%
2.498.4%
399.7%
499.994%
599.99994%

Here’s what I suggest: let’s try to reach a score of at least 2 when we do blind cupping experiments. This means we will draw wrong conclusions only 4.6% of the time, and it will not take a crazy amount of tasting ability or repetition to reach this. Obtaining a Q-grader license requires correctly identifying an intruder cup amongst three in at least five out of six trials, and this corresponds to a score of 2.4. I won’t suggest that everyone should aim at Q-grader level scores all the time. 🙂

Now, let’s talk about designing a blind cupping experiment. Choose a number of identical cups, maybe you would like to do three cups like my friend. Now fill all but one cups with the same coffee, and fill the last cup with a coffee that is different in some way. Maybe you want to see if you are able to recognize this different origin, something new you tried with roasting, or a different type of brew water. The more cups you use, the harder the challenge will be, and you will thus get to higher scores faster when you succeed. Mark the bottom of each cup with what they are, ask someone else to swap them around, and then try to identify the intruder cup without looking at the tags. Once you think you found it, look at the tag underneath, and mark on a sheet whether you succeeded and failed. Do this a dozen times and log or results; I’ll help you decide what your score is.

One thing you absolutely cannot do when you design this experiment, is to decide after 5 tastings, you are failing too many times, let’s just start from scratch. This is how the fields of psychology and biology got themselves into a crisis where a large number of their experiments were false. Erasing your failed tastings is cheating. If your score is never getting high after you are exhausted, here’s what happened: the experiment informed you that you were unable to distinguish the intruder cup from the other ones. You can start again if you change something about the variable you are testing. For example, if you failed to differentiate two types of water, you can restart the experiment with an entirely different type of water in your intruder cup. This would not be cheating, because it is now a different experiment.

For those of you who do not want to think further about maths or science, I built a Wolfram Alpha widget for you. Just enter how many cups you are using in each tasting (“Number of Cups“), how many times you tasted (“Number of Trials“), and how many times you failed to identify the intruder cup (“Number of Failures“). Then press “Submit“. You will get then see some ugly equation stuff that I was unable to remove from Wolfram’s output, but just focus on the number at the end. This is your score.

For the default values (3 trials, 3 cups, 1 failure), your score would be 1.1. This is a really bad score – you will certainly need to do more than 3 tastings if you want to be confident about your experiment. If you reach 8 tastings with only one failure (with 3 cups each time), then you will reach a score of 2. If you however fail twice, you will need even more successful tastings to reach a score of 2.

If you never heard about Wolfram Alpha, it’s a wonderful website. It’s like a robot version of Wikipedia that can do maths. You can ask it really silly stuff, like “What is the average life span of donkeys ?” and it knows the answer surprisingly often. The kind of questions you probably ask yourself every day.

I’d like to thank Victor Malherbe of the Montreal Coffee Academy for asking me this question on statistics, and for the information on Q-grade requirements.

Extracting Shells of Coffee

Recently, Barista Hustle posted a very interesting video on YouTube. I am embedding it to this blog post to make sure that you view it before reading more.

I found this video rather illuminating, but at first the last minute or so was a bit confusing to me, so I decided it may be worth discussing it here.

The most illuminating part for me was the fact that a very long immersion brew with coarse grounds never reached the higher extraction yields that the finer coffee grounds reached. Here, this higher extraction yield is about ~25%. To those wondering why the fraction of extracted coffee is not higher, it is because ~30% roughly corresponds to the fraction of coffee beans by mass that can be dissolved in water (this is discussed a bit more in a previous post). The rest consists of cellulose walls and other stuff that cannot be dissolved, and remains in the coffee bed. This maximum extraction yield will depend on the type of coffee beans and the roast profile you used, so you may sometime hear slightly different numbers.

When you think about it, it makes a lot of sense that water is just unable to reach the core of each coffee particle when you use a coarse grind setting. The typical size of a cell inside a coffee bean is about ~20 micron (see image below), and when we brew for a V60 filter coffee we typically have a majority of particles with diameters around 500 micron. This means that water would need to diffuse through 12-13 layers of cells to reach the core of each coffee particle. What this video from Barista Hustle demonstrated is that water only penetrates about 100 micron, or ~5 layers of coffee cells.

An electron microscopy image of coffee beans. This image was obtained by Rebeckah Burke at University of Rochester. Please have a look at her very interesting project here.

This means that we are calculating average extraction yields wrong when we grind anything coarser than ~200 micron in diameter, as Matt also mentions in the video. This is because we assume that the full mass of our coffee dose is being extracted when we calculate extraction from total dissolved solids, but in reality there is a large portion of the core of each coffee particle that is still intact, and effectively wasted. This is interesting, but does not provide an easy way to correctly measure the extraction yield – to do this, you would need to know the full distribution of coffee particle sizes you are brewing with.

The part I found a bit more confusing was the end of the video. There, Matt mentions that their cupping bowl made with the coarser grounds was actually a high extraction. When I first heard this, I thought he meant that it was a maximally extracted thin shell around the coffee bean, which would mean that the nasty bitter and astringent chemical compounds should have come with it. But when I thought a bit more about this, I realized this is not what he meant. The red (higher-extraction) curve in the video is actually a good representation of how much every part of accessible coffee is being extracted. If the fine grounds (red curve) are not reaching extractions high enough to obtain bad-tasting compounds, then none of the accessible coffee mass is. The shells of coarser grounds will get extracted exactly to the same extent as the fines are, you will just be wasting a lot of intact coffee in their core.

At the end of the video, Matt briefly mentions that it is still possible to over-extract coffee and obtain a bitter and astringent cup. This problem will certainly arise if you are trying to extract the full core of a coarsely ground coffee. When you do this, the outer surface of the coffee ground will be super over-extracted by the time you start extracting the core. In effect, this lone, coarse coffee ground will be producing an uneven extraction, a lot like a wide distribution of particle sizes would !

Suppose you had a large number of spherical coarsely ground coffee particles, all with exactly the same size. You would be faced with a choice: either you extract a thin shell of coffee around each particle in a relatively uniform way and waste a lot of coffee in the cores, or you minimize waste and produce a very uneven extraction.

An example of what an extraction profile as a function of radius inside a 500-micron coffee particle could look like. The red curve would represent the case where more coffee is wasted (everything inside of ~400 micron) so that the outer shell is not over-extracted, and the other curves would be cases where the brewer tries to minimize waste, but ends up over-extracting the outer shell. These numbers and curves are not drawn from data, they are just used as illustrative examples.

All this discussion made me appreciate more why cuppings produce very balanced flavor profiles, typically produced by even extractions – finer grind sizes are used for cuppings, compared to other immersion methods like the siphon and french press. This makes me want to experiment with my siphon at a much finer grind size !

[EDIT January 23, 2019: For those interested, you can calculate the mass fraction of a coffee particle that is accessible to water with the following equation, which was derived from simple geometry:

where the [depth] was demonstrated by Barista Hustle to be approximately 100 micron. For example, if you have a coffee particle with a diameter of 1 millimetre (or 1000 micron), the fraction of mass available to water is 48.8%.]

I would like to thank Mitch Hale and Caleb Fischer for the long and very interesting discussion that led to some of the thoughts shared here. I also want to thank Barista Hustle for their very illustrate experiment !

Why Spin the Slurry ?

Today I’m excited to release a joint blog post with Scott Rao, the one who helped me so much understand the basics of brewing, and also helped me to improve my cups of coffee significantly. I was never really proud to serve my friends a cup of V60 coffee that I made, and this is finally changing. After some discussions on my very unstable brew times, Scott made me realize that I needed to be a bit more careful with my execution of the spin. This led to a discussion about the physics of why that was. Scott asked me to try to explain this simply in a few paragraphs for a joint blog post – this is what came out of this idea. This blog post is also available on his own website, scottrao.com.

A preamble from Scott:

The history of The Spin is murky…although it’s often called the “Rao Spin,” I did not invent the spin. It’s likely that James Hoffmann was the first person to spin the slurry. Almost everyone to whom I’ve shown The Spin has immediately adopted it. It’s easy to execute well and it works, pretty much every time.

Jonathan Gagné, an astrophysicist based in Montreal, came to my roasting masterclass this past November. I’ve been fortunate to befriend Jonathan, as I’ve always wanted to have an astrophysicist on speed dial to call when I have a question about how things work :). I’ve been helping Jonathan with his coffee making and he’s been providing some great coffee-analysis resources, some of which I hope appear on this blog.

I asked Jonathan to explain why he believes The Spin works and we decided to publish his answer as a guest post here.

A Summary

In this post we will discuss the physics behind why spinning the V60 during a brew is a useful method to obtain a more uniform extraction. While spinning is helpful, it’s important not to overdo it – it can cause fine coffee grounds to migrate to the bottom of the slurry and clog the filter, slowing the drawdown and imitating a brew made with a lower-quality grinder.

So, why spin the slurry ?

Spinning the slurry during a V60 brew is useful to minimize the channeling of water that can lead to an uneven extraction. The reason why this is true can be understood with the help of physics.

A rotating slurry will experience a centrifugal force*, which means that every drop of water and every particle of coffee will suffer a force that pushes them outwards. In physics, the strength of centrifugal force is more important for heavier objects, and because of this, water will tend to migrate outward more than coffee because water is heavier.

When you brew coffee, the main cause for channeling is that dry coffee repels water more than wet coffee does. The physics behind this effect are not fully understood: they are related to the fact that molecules of water bond with each other, and dry coffee doesn’t bond with them in the same way. At first pour, water might begin travelling through a tiny hollow on the surface of the dry coffee, and then it will prefer to keep traveling through that same tunnel, because the rest of the coffee bed is still dry and repels water. In practice, a coffee bed will often develop several channels if you don’t take steps to avoid it.

When you rotate a channeled coffee bed, the water flowing down the narrow tunnels is forced out of them by the centrifugal force, and the water will wet some of the dry coffee. This horizontal re-mixing of the slurry will cause channeling to decrease overall.

There is, however, a drawback if you spin too much. As we mentioned earlier, heavier things are more affected by the centrifugal force. The largest coffee particles will thus experience a stronger pull toward the walls of the V60. In a slurry where coffee is mixed with water, this effect will be slightly reduced by water friction. Think of trying to run in the sea – the friction water exerts on you will slow down your movement, especially if you present it with a large surface, for example by wearing saggy pants. The friction is however not strong enough to completely stop the migration of particles based on their size, and the larger coffee particles will be sent outwards.**

This whole situation presents the smallest particles with an opportunity: the larger ones having moved out of the way, fines will sink down to the bottom of the V60, where they will be free to do their worst at clogging the paper filter. This will significantly slow down the flow of your brew.

As an illustration of this, I recently brewed a few V60s with a prewet-plus-two-pours method, performing a spin right after the prewet, and right after each of the two pours. At first I did not pay too much attention to how long or how strong I was spinning, and I experienced large inconsistencies in my brew time (up to ~20 seconds), which led to inconsistencies in average extraction yields by about 0.7%. I was controlling everything else, including the height from which I poured, the flow rate, timing, grind size, slurry temperature, etc.

I then tried timing my spins, and found that using seven-second spins resulted in a 5:18 drawdown time, while two-second spins resulted in a much shorter 4:28 drawdown time! This is a nice demonstration that fines can migrate and clog your filter if you spin too much. Adjusting the grind size appropriately to maximize extraction yield and avoid astringency, I found that the two-second spins resulted in a brighter and more enjoyable cup.

In summary, you want to spin just enough to break the up channels, but not so much that fines clog the bottom of the filter.

Spinning the slurry in the prewet phase

* I can already hear the interwebs shouting “CENTRIPETAL NOT CENTRIFUGAL”. Both concepts are valid and useful tools: when you stand outside of a rotating system and want to describe forces acting on that system that keep it together, the concept of a centripetal force (directed toward the center) is appropriate. It describes the external force that allows the system to keep going in this rotating motion without splattering everywhere around. In our case, this force is provided by the walls of the V60, preventing the slurry from flying around and messing up your counter. If, however, you take the point of view of the things rotating (the water and coffee), then the concept of a centrifugal force becomes very useful. You can then describe the system as if it was not rotating, by just adding a slight modification: you add an artificial “pseudo force”, also called an “inertial force” that points toward the outside, in our case the “centrifugal force”.  It is often called an “inertial force” because it arises from the fact that your frame of reference (the V60 in this case) is rotating (in technical terms, it is “not inertial”). A “pseudo force” is by no means a false thing or an invalid concept, as long as you understand where it arises from and use it carefully — in fact, one can even see gravity as a pseudo force (Einstein realized that), yet it is very useful in everyday situations to view gravity as just a normal force.

** The mass of a coffee particle is proportional to its volume, which is itself proportional to the cube of its size. The water friction that the particle experiences is proportional to its surface, or to the square of its size. As a consequence, a particle three times larger will be nine times more massive and will feel nine times more centrifugal force, but only six times more water friction. If you combine the two effects, it will therefore be pushed outwards 9 – 6 = 3 times more.

Water for Coffee Extraction

[Edit August 23, 2019: Please have a look at this new blog post if you are interested to craft your own brew water recipes ! ]

Introduction

In my first post, I mentioned how the water you use to extract coffee has a significant impact on the taste profile of your cup, in a way that does not necessarily depend on the taste of the water by itself. If you were using water just to dilute a cup of espresso (e.g., when making an americano), then your only worry would be that the water tastes good.

The key difference comes when you use water to extract coffee from the ground beans. In that situation, you want to have some potent mineral ions like magnesium (Mg+2) and calcium (Ca+2) that can travel inside the bean’s cellulose walls and come back with all the compounds that give the great taste to a cup of coffee. According to the Specialty Coffee Association (SCA), sodium (Na+) also plays a role, but a somewhat less important one. If you are wondering whether this is also true about tea – yes it is. If you live in Montreal, you might have noticed that you are unable to brew tea as good as the one you can drink at Camellia Sinensis, and your tap water is one main reason (they use mineralized water at Camellia Sinensis).

The Recommended Water Properties

In this post, I’d like to discuss extraction water a bit more, and give some practical tools for everyone to improve their brew water without necessarily needing fancy equipment. Let’s start by listing some of the SCA recommendations for brew water (I ordered them in my perceived order of importance):

  • No chlorine or bad smell
  • Clear color
  • Total alkalinity at or near 40 ppm as CaCO3
  • Calcium at 68 ppm as CaCO3 , or between 17–85 ppm as CaCO3
  • pH near 7, or between 6.5–7.5
  • Sodium at or near 10 mg/L
  • Total Dissolved Solids (TDS) at 150 mg/L, or between 75–250 mg/L

The first two are more widely known, but it’s always good to keep in mind if you start creating your own mineral recipes (more on that later). If your resulting water is milky or has visible precipitation of minerals, it’s not good ! If this happens, you probably added way too much minerals for some reason. You can also easily get rid of chlorine by letting water sit on the counter for an hour or so.

Total alkalinity is often confounded with pH, but it’s not the same thing. pH measures the (logarithm) ratio of free OH ions to H+ ions in a solution, with pH = 7 corresponding to a unit ratio (neutral). A larger amount of H+ ions produces a more acidic solution, with a lower pH, and a larger amount of OH ions produces a more alkaline solution, with a higher pH. This is why total alkalinity is often confused with an alkaline solution, which is kind of understandable given this poor choice of terms.

Total alkalinity typically measures the amount of HCO3 ions, which are able to capture any free H+ ions that are added to the solution, and prevent them from making the solution more acidic by forming carbonic acid: 

For this reason, HCO3 is termed an alkaline buffer in this context. A high total alkalinity will therefore make a solution more stable against pH changes. This bears some importance in coffee making, but there is a big problem with having a total alkalinity that is too high; it can react with the aromatic acids that were extracted from the coffee beans, and mask some of these important flavors. This is why the SCA recommends a very narrow range in total alkalinity near 40 ppm as CaCO3.

You may sometimes hear total alkalinity referred to as carbonate hardness. It’s a slightly different concept, but for coffee extraction water it’s almost always equal to total alkalinity (technically, this is true when the total hardness of water is higher than its total alkalinity).

At this point you may thinking “what the hell is this unit of measurement involving this random molecule CaCO3 ?”. Turns out scientists love to create large collections of weird measurement units, and this is yet another example of that (like measuring the energy of stars in ergs…). These ppm as CaCO3 basically ask “how many parts per million CaCO3 would you need to produce the observed HCO3 concentration ?”, which relates to this chemical reaction: 

The next recommendation is to have calcium hardness between 17–85 ppm as CaCO3, with the units again relating to the same chemical reaction above. Magnesium is also widely used in the specialty coffee association, and is believed to extract slightly different flavors, but to my knowledge there are not yet any lab tests to back this up (there might be some blind testing backing it up, but I’m not aware of them). As a consequence, most people use a mix of magnesium and calcium as the extracting agents. I already explained the logic behind this recommendation above; you basically just want enough of these cations to do the extraction job properly, but not too much as to completely throw off balance the flavor of the coffee or to cause massive corrosion or scaling in your equipment.

Both of the magnesium and calcium cations are related to the total hardness of a solution, defined as the summed concentration of many cations (positively charged ions), among them calcium, magnesium, iron, strontium and barium. In coffee extraction applications, only magnesium and calcium are typically present, so total hardness is just taken as their sum. A more widely used recommendation would therefore be to keep total hardness in the SCA range, rather than just calcium hardness.

The next two recommendations are often not focused on too much in the specialty coffee community. I often see water recipes with pH in the range 8.0–8.2 (slightly alkaline), and the resulting coffee tasted great. I haven’t done extensive tests comparing pH~7 water to these recipes, as it’s typically hard to play with pH without affecting the other variables above. I also have not experimented much with the effect of sodium, so that could be the subject of a future blog post; for now, I just try to follow the SCA recommendation, but I don’t put too much focus on it.

A lot of people use tap water through a Brita to brew coffee. This is not bad in principle, but all such a carbon filter does is remove chlorine and other undesirable components, and soften the water (it decreases total hardness and total alkalinity). If this lands you in a good zone for brewing, that’s great, but it is rarely the case for typical tap water.

Visualizing the Water Options

At this point, it would be useful to visualize the water properties of different cities, bottled waters and some recipes of coffee professionals: 

Water properties for various recipes, cities and bottles. The dashed line represents a 1:1 relation, and color lines correspond to different recommended ranges.

In the figure above, you can see the range recommended by the SCA (green bar), the region recommended by the Colonna-Dashwood & Hendon (2015) Water for Coffee book (this mythical book is now pretty much impossible to find, but it is said by the ancient ones to go much deeper in the chemistry of coffee extraction than what I could ever write in this blog post), and the more constrained region recommended by the Specialty Coffee Association of Europe (SCAE), which is mainly based on avoiding regions of significant scaling (upper right) or corrosion (upper left), two aspects that are mostly important to the delicate internal parts of espresso machines. The Third Wave Water (TWW) classic and espresso profiles are little bags of pre-weighted minerals that you can dump in a gallon of distilled water to get easy water for coffee brewing.

The dashed line on the figure corresponds to a 1:1 total alkalinity and total hardness. Most naturally occurring water will fall near this line because of how water acquires its minerals by dissolving limestone. The widely used process of water softening by de-carbonization also moves the composition along this region (toward the origin of the figure). This is why a lot of city tap waters (triangles) and bottled waters (stars) fall along that line. I can’t believe that I lived for 3 years in Washington D.C. without ever knowing about any of this (and I Brita’d my water out of this great spot like a fool). You would be surprised how many of the city or bottled waters that fall completely outside of the range of this figure.

All other circles on the figure correspond to mineral recipes used or recommended by different professionals (e.g.,  the Leeb & Rogalla book, Scott Rao, Matt Perger, Dan Eils, the World of Coffee Budapest championship, the 2013 Melbourne World Barista Championship, and several recipes from Barista Hustle), the stars correspond to bottled waters, and the triangles correspond to different cities.

Practical Implementations

Now that we talked about the theory behind extraction water, we should focus on practical applications. You would be surprised how many specialty coffee shops have very expensive water filtration systems based on reverse-osmosis to rid the water of all its contents, and re-mineralization resins to achieve something close to these recommendations (try asking your favorite coffee shop).

At home however, none of this is really practical, as these devices typically cost several thousands of dollars, and still require you to monitor your tap water and adjust their setting from time to time. Unless you have the incredible luck of living somewhere with great brew water (the only example I know is Washington DC, at least in 2018), you have these types of choices (ordered by increasing effort required):

  • Get a magnesium re-mineralizing water pitcher (e.g., the BWT).
  • Order some third wave water minerals and dissolve them in a gallon of distilled water.
  • Mix a pre-determined combination of bottled water brands.
  • Buy distilled water and re-mineralize it yourself. This requires a bit more work but gives you incredible flexibility.

The BWT Pitcher

The first option has the merit of being simple, but you have almost no control over the final result. A BWT pitcher will soften your water and then add in some magnesium, which will move you toward (0,0) and then upward in the figure above. I don’t know to what extend it moves the composition around, so ideally you’ll want to test the result with some aquarium water hardness and alkalinity kits. I suspect the result would be decent in cities with similar compositions to Montreal.

Third Wave Water

I found that third wave water (the “classical profile”) produces a really good result for very little effort. You do have to buy a gallon of distilled water, which is a bit of effort, but they are extremely cheap and will last for a dozen cups of coffee. The “espresso profile” of third wave water is useful if you are worried about scaling and corrosion in your espresso machine, so I recommend only using it for espresso, not for filter coffee. I compared a Colombian coffee (the Ignacio Quintero from Café Saint-Henri) extracted with third wave water, the Rao/Perger and Dan Eils water recipes (discussed more below) by blind tasting, and I found the third wave water to be a bit overwhelming in term of resulting acidity.

My guess is that this is due to third wave water being much higher than the other recipes in terms of total water hardness. I preferred the Rao/Perger recipe, but in all honesty all three cups were very good, and way better than what you get with Montreal tap water. I think third wave water is also a good option for traveling, as it comes in a little sealed package with the composition marked on it, so that might not cause problems at TSA (although I have not tested this yet). You would still need to buy a gallon of distilled water though, so depending on the nature of your trip this could be a non-ideal solution.

I must confess, I am not sure I placed the Third Wave Water points on the right position of the “total alkalinity” axis. This is because they use a less usual component called “calcium citrate”, or Ca3(C6H5O7)2 in their mix of minerals. Once dissolved in water, each of these molecules will liberate three Ca+2 cations and two C6H5O7-3 citrate anions (negatively charged ions). I treated each of these citrate anions as an alkaline buffer that can capture three H+ cations each, and assumed that they are stable enough as citrate acid (C6H8O7) to prevent a significant pH change. This is a lot of assumptions, and I also needed to assume that citrate acid is as efficient at actually capturing the H+ cations as are the HCO3 anions. Once I made these assumptions, I just calculated what amount “ppm as CaCO3” of HCO3 would have the ability to capture the same amount of H+ cations. It is quite interesting that the classic profile falls quite close to other brew water recipes in total alkalinity when making all these assumptions.

[Update, January 3 2019: I have now tested the total alkalinity of Third Wave Water (classic profile) with a Hanna Instruments photometer, and obtained a measurement of  43 +/- 5 ppm as CaCO3 total alkalinity; this is very close to the ~ 50 ppm as CaCO3 that I had predicted ! It could be slightly lower because citrate anions may be slightly slower or worse at capturing H+ cations, but this is almost within the measurement error so I would not deduce too much from this measurement alone. The main point is: citrate anions do act as an alkaline buffer, and third wave water is exactly at the SCA-recommended value for total alkalinity !]

Water properties for various bottles. The dashed line represents a 1:1 relation, and color lines correspond to different recommended ranges. Orange dashed lines show all possible combinations of Montclair/Distilled or Compliments/Distilled ratios, and stars A, B and C represent the corresponding water bottle recipes described in the text.

Bottled Water

Mixing water bottles or distilled water is another viable option. If you use a combination of two bottled waters, you can imagine a line drawn between the two stars that correspond to each of the bottled water properties in the figure above, and different mixing ratios will place you at different spots along that line. Using three bottled waters instead of two will allow you to move on a triangle-shaped surface that connects the three bottles in the chart. A problem with a lot of bottled waters is that they are not far above the 1:1 total alkalinity vs total hardness line (the dashed line in the chart), making it harder to fall anywhere in the Colonna-Dashwood & Hendon (2015) region. The lack of bottled waters high in total hardness and low in total alkalinity limits the use of three-bottled combinations.

From the little data gathering I have done yet, I found that using a water really high in both total alkalinity and total hardness (like Montclair water) mixed with much softer water is a good way to go. Here are a three bottled water recipes that seem to work great (with their designated letter on the next figure):

(A) The Montclair/Smart recipe

Right now, the best 2-bottled combination I could find is 10 parts Smart Water to 1.6 parts Montclair. This will place you at a total alkalinity of 40 ppm as CaCO3, and a total hardness of 69 ppm as CaCO3, nicely split between calcium (17 mg/L) and magnesium (6 mg/L). It will even include 5 mg/L of sodium, falling a bit short but not that far from the SCA recommendation.

(B) The Montclair/Distilled recipe

Another great option is to mix 10 parts distilled water with 2.05 parts Montclair water. This is very similar to the last recipe, but slightly softer (67 ppm as CaCO3), and with a bit more sodium (9 mg/L), extremely close to the SCA recommendation in sodium.

(C) The Smart/Compliments recipe

If you can’t get your hands on Montclair water, try this one: 10 parts Smart Water with 1.6 parts Compliments. This will get you something a bit softer in total hardness (57 ppm as CaCO3), still with a mix of calcium (14 mg/L) and magnesium (5 mg/L), but without sodium. 

I have not tried tastings with these bottled water recipes yet; this was determined just from calculations. Let me know if you try them before I do !

If you would like to experiment with some more mixes of bottled water, I created a Google Sheet here, which I will keep updating in the future. You can do File/“Make a Copy”, and then you’ll be able to add in some more bottled water and create new recipes. You can also find many more mixed bottle water recipes that I fiddled with in there.

Another viable option may be to mix your tap water with distilled water, but this will only allow you to move along a line connecting (0,0) to your city in the first figure, and you would ideally need to monitor seasonal variations in your tap water hardness and alkalinity. I added a few tap water compositions (Montreal, Laval and Washington DC) in the bottled water spreadsheet.

Mineral Recipes

If you want to take things to the next level, you can get yourself some minerals, a scale precise at 0.1 g or better (mg-precision scales are not too expensive; I use this one and I really like the small plastic dishes that come with it), some mason jars, and a pipette or a small kitchen plastic spoon. There are a total of five minerals you will need if you want to do all of the recipes below, but the simpler ones can be done with just the first two in this list. For the less common items, below I will give you some Amazon links that I used to buy them.

Please make sure you always buy food-grade ingredients, not the pharmacy-grade or lab-grade ones. The latter two may be more pure than food grade is, but the rare impurity could be much worse for your health (e.g., heavy metals). Barista Hustle mention that pharmacy-grade epsom salt is probably ok to consume at these low concentrations, but I consider the key word here to be “probably”, especially if you’re going to drink this every morning. Once you opened a bag of minerals, always keep them in a cool, dry place in a hermetic jar, especially those in anhydrous form.

    • Epsom salt (MgSO4•7H2O) [Amazon]
    • Baking soda (NaHCO3) – This is not baking powder. [Amazon]
    • Magnesium chloride hexahydrate (MgCl2•6H2O) [Amazon]
    • Calcium chloride anhydrous (CaCl2) [Amazon]
    • Potassium bicarbonate (KHCO3) [Amazon]

Notice that epsom salt is not simply MgSO4, but rather its heptahydrate form MgSO4•7H2O, which makes it look like a clear crystal. MgCl2 and CaCl2 can be found both as hydrates or anhydrous (no water) forms. Some vendors don’t specify what form they are providing, which can be annoying, but in general if you have little white spheres of CaCl2 they are probably anhydrous, and if you have milky clear crystals of MgCl2 they are probably of the hexahydrate from (see pictures below). It’s ok if you don’t get the exact hydrate form, but you’ll need to adjust the weights to get the same amount of Ca+2 or Mg+2 cations. 

Anhydrous CaCl2 (top) and hexahydrate MgCl2 (bottom). The pale, milky crystalline structure is a good indication that you have a hydrate form of MgCl2.

After doing some research on the web, I could get my hands on a dozen mineral water recipes. I have not tried them all yet, but I will comment those that I did try. I modified all recipes below to make them more uniform. In all cases, you’ll need to put the specified weights of minerals in a jar that can hold 200 mL of water (ideally slightly more). A glass jar such as a regular mason jar is good for this, and I would avoid metallic containers because of potential corrosion.

Once you put the required minerals in the jar, add in some distilled water until you hit a total weight of 200 g. This will be your concentrate; a solution often white that will initially degas some CO2 and will easily precipitate solid minerals. I recommend keeping such a concentrate in a cool dark place for a few hours with the mason jar lid just slightly screwed, to allow for the outgassing to complete. You might even stir it up a few times to help things get going. You will also get a much faster reaction and outgassing if you use warm or hot distilled water, but I am not sure if this affects the resulting composition (I don’t think it does, and my first trial with the Rao/Perger recipe and hot distilled water turned out great).

Your concentrate will be good for 50 liters of water. This is a lot of water. Think of it like this: you can fill a very big bath with amazing coffee with that much water. In other words, I highly recommend (1) not going crazy and starting up 8 different 200 mL concentrates when you first read this, and (2) keep them tightly closed in the fridge after they degassed. If you want to compare several water recipes, you can create downsized versions of the concentrates without problem (use a rule of three to downsize both the concentrate volume and mineral weights by the same factor).

Once you have a concentrate, I recommend putting it on your scale, taring the scale, and using the pipette or small plastic spoon to scoop out 16 g and put it in a 4 L of distilled water (or 4 grams per liter). Congratulations, this is your mighty brew water. Make sure you keep it in the fridge, especially when it is almost empty, and always smell it before using it. As I mentioned in my last post, if it smells like an old rag, so will your coffee. In my experience, a gallon of distilled water will turn bad after approximately a week out of the fridge, or a month in the fridge. This is a much slower staling process than what you would get with tap water, as distilled water starts out free of any bacteria. I also don’t really recommend letting the water sit in your boiler for more than a few hours, but this is definitely less an issue when you started with distilled water as a base.

Now, here are the recipes !

The Rao/Perger Recipe

    • 5 g epsom salt (MgSO4•7H2O)
    • 2 g MgCl2•6H2O (hexahydrate) or 1 g anhydrous MgCl2
    • 1.5 g anhydrous CaCl2 or 2 g CaCl2•2H2O (dihydrate)
    • 1.7 g baking soda (NaHCO3)
    • 2 g bicarbonate potassium (KHCO3)

Reference: Scott Rao

Comments: So far this is my favorite recipe from blind testing.
It produces a bright and well-balanced cup.

[Edit May 11, 2019: Please note that this concentrate (and perhaps others on this page) will precipitate some white salts. This is perfectly normal and it will not precipitate once diluted with distilled water into your brew water. Just make sure that you mix the concentrate thoroughly until no deposit is left at the bottom every time before you use it.]

The Dan Eils Recipe

    • 5 g MgCl2•6H2O (hexahydrate) or 2.3 g anhydrous MgCl2
    • 3.8 g anhydrous CaCl2 or 5 g CaCl2•2H2O (dihydrate)
    • 5 g bicarbonate potassium (KHCO3)

Reference: Scott Rao’s Instagram post

Comments: This is a great and simple recipe.
So far, my 2nd best favorite from blind testing.

The Matt Perger Recipe

    • 10 g epsom salt (MgSO4•7H2O)
    • 3.4 g baking soda (NaHCO3)

Reference: This website.

Comments: I have not tried this one yet.

The Rao 2013 Recipe

    • 4 g MgCl2•6H2O (hexahydrate) or 1.9 g anhydrous MgCl2
    • 3 g anhydrous CaCl2 or 4 g CaCl2•2H2O (dihydrate)
    • 3.4 g baking soda (NaHCO3)

Reference: I deduced this one from other recipes above.

Comments: I have not tried this one yet.

The Melbourne Recipe

    • 2.9 g epsom salt (MgSO4•7H2O)
    • 1.0 g baking soda (NaHCO3)

Reference: The Barista Hustle simple DIY recipes.

Comments: I have not tried this one yet.

The World of Coffee Budapest Recipe

    • 6.2 g epsom salt (MgSO4•7H2O)
    • 3.4 g baking soda (NaHCO3)

Reference: The Barista Hustle simple DIY recipes.

Comments: I have not tried this one yet.

The Barista Hustle-Simplified SCA Optimal Recipe

    • 8.4 g epsom salt (MgSO4•7H2O)
    • 3.4 g baking soda (NaHCO3)

Reference: The Barista Hustle simple DIY recipes.

Comments: I have not tried this one yet.

The Barista Hustle Recipe

    • 9.8 g epsom salt (MgSO4•7H2O)
    • 3.4 g baking soda (NaHCO3)

Reference: The Barista Hustle simple DIY recipes.

Comments: I have not tried this one yet.

The Barista Hustle-Simplified Rao 2008 Recipe

    • 9.2 g epsom salt (MgSO4•7H2O)
    • 4.2 g baking soda (NaHCO3)

Reference: The Barista Hustle simple DIY recipes.

Comments: I have not tried this one yet.

The Barista Hustle-Simplified Hendon Recipe

    • 12.2 g epsom salt (MgSO4•7H2O)
    • 2.6 g baking soda (NaHCO3)

Reference: The Barista Hustle simple DIY recipes.

Comments: I have not tried this one yet.

The Barista Hustle Hard Recipe

    • 15.4 g epsom salt (MgSO4•7H2O)
    • 2.9 g baking soda (NaHCO3)

Reference: The Barista Hustle simple DIY recipes.

Comments: I have not tried this one yet.

The Barista Hustle Hard “AF” Recipe (i.e., “Hard as Falcon”)

    • 21.5 g epsom salt (MgSO4•7H2O)
    • 3.8 g baking soda (NaHCO3)

Reference: The Barista Hustle simple DIY recipes.

Comments: I have not tried this one yet.

I also collated all of these recipes in another Google sheet, which you can also play with if you do File/“Make a Copy”. That one will estimate the resulting total hardness and alkalinity from the input recipes, as well as other detailed quantities. You can also use the Aqion website to get the same outputs for the simpler recipes (maximum 3 minerals, and the calcium citrate present in Third Wave Water cannot be included). A nice aspect of the Aqion website is that it also gives you the electric conductivity (EC), in the units of μS/cm (microSievens per centimeter) often measured by cheap TDS-meters (TDS is for total dissolved solids). This is a great way to double-check that you didn’t mess up your brew water, but always make sure you measure it at 25°C. Even when TDS-meters say they do a temperature correction, it’s a bad one. I would also not trust the TDS reading itself, because these instruments make important assumptions on the actual composition of your water to translate the EC to a TDS.

Happy brewing ! In BOTH senses 😀

Special thanks to Alex Levitt and fungushumungous for proofreading.

[Edit May 30 2019: If you’d like to read more about brew water recipes, head over here to Mitch Hale’s blog !]

References

Improving the Bloom of your V60 Coffee

Thanks to some research done recently by Matt Perger & friends at Barista Hustle, a nice improvement to preparing V60 coffee just came to my attention (see my first blog post for the full details on preparing V60 coffee).

In my previous post, I indicated that one of the steps in preparing the coffee bed when brewing coffee with a V60 is to dig a small trench with a finger so as to quickly wet the coffee bed more uniformly at the bloom phase. I mentioned that I had not seen a convincing demonstration that this actually helped, but because it’s easy and makes sense, I ended up adopting the practice.

The small trench I used to dig when preparing my coffee bed

Well, thanks to Barista Hustle, now the tests have been done, and it turns out it does help. They even found a better way to do it, which they describe as preparing the coffee bed into a “nest” shape, in other words into quite a deeper and larger trench.

A nest-shaped coffee bed

They achieved this roughly with a spoon, but I find it easier by sticking a chopstick in the center of the coffee bed through the bottom of the filter (taking care not to pierce it), and rotating it around in circles that slowly increase in radius. I posted a video of this method below.

[Edit January 11, 2019: Scott Rao pointed out to me that the chopstick method I present here could potentially be compressing some coffee, which could lead to more channeling. I think this is a valid worry. However, the alternative (used by Perger and Rao) is to dig the nest with your fingers – I remain agnostic as to whether one compresses the coffee bed more than the other, but I find it harder to replicate the same nest shape every time using my fingers. Please keep this in mind if you use the chopstick method, and use a chopstick that has a pointy end if you can (make sure you don’t poke a hole in your paper filter). I will explore this more in a future blog post.]

Here you can find a PDF with an updated version of my first blog post, that includes this “nest” technique.

A video of the method I use to create a nest-shaped coffee bed with a chopstick

References

Barista Hustle

How to Brew Better Coffee with a V60

Table of Contents

    1. Introduction
    2. Percolation and Immersion
    3. Why use a V60 ?
    4. The Brew Variables
      1. Coffee to Water Ratio
      2. Coffee Beans Quality and Roast Profile
      3. Grind Size and Uniformity
      4. Coffee Freshness
      5. Filter quality
      6. Brew temperature
      7. Water Agitation
      8. Uniformity of Extraction and Repeatability
      9. Water quality
      10. Contact Time
    5. Determining Grind Size
    6. Practicing the Rao Spin
    7. How To Actually Make a Better V60
      1. The Gear I Use
      2. The Water I Recommend
      3. A V60 Pour Over Workflow
    8. References

1. Introduction

I’ve been interested in specialty coffee for a few years now, and never went to a telescope run without my V60, manual grinder and some fresh beans. I had not actually read that much about the details of how to brew a great cup of coffee; rather, I just bought some good beans and read some reviews for the most useful and easily transportable gear to brew coffee when traveling. A few months ago, I decided to get a little deeper into this, and to read as much as I could on the subject. This all started because I was regularly buying fresh, great tasting beans from the Saint-Henri coffee shop in Montreal, and I really wasn’t satisfied about my brews, especially when I compared them with what the Saint-Henri staff brewed with the same coffee.

This turned out to be a massive rabbit hole, but I actually love it when it happens. During the past few months, I changed a lot of things about my brew method and equipment. Having great coffee beans is definitely required to brew a great coffee, but I learned there are countless obstacles that can ruin your cup pretty drastically, regardless of your bean’s quality. I was quite surprised how hard some of this information was to find on the web, and in the past months I have done a lot of experiments and huge mistakes that took me a while to figure out. So, I decided that I’ll write a blog post to distill some of this information, and to guide any interested readers through the steps to improve their coffee cups, either at home or when traveling. I might write a few more posts like this in the future when I discover better methods, or when I familiarize myself with new tools and recipes.

Another thing that really frustrated me when researching this topic is the amount of pseudoscience one encounters on the subject of specialty coffee. Surprisingly enough, some of the main culprits include previous world champion baristas (hmmmmmm Kasuya, are you using these magical self-transmuting beans again ?)

For those who don’t know me, this blog post is not intended to be a quick-and-approximate guide of how to make a V60 – there are plenty of videos already doing that on the web. Rather, it’s intended for the more geeky-minded people that are interested in all the technical details. I’m fully aware that half of the things I’ll discuss here will only produce an increase in cup quality of a few % (e.g., “now let’s sift our coffee grounds for 5 minutes and remove 5 g of the grinds with a 350 μm sieve to achieve a tighter particle size distribution”), but there are some other aspects of brewing that, despite sounding insane, can have a profound impact on your cup quality (e.g., “oh no, this tap water has 200 ppm alkalinity”).

If you are inclined to think that bringing a scientific mindset and sophisticated tools to improve a coffee is not justified, here’s something to consider – most other products that we consume went through much more technical assessment, and we understand more of the science behind them (e.g., wine, beer, cheese, scotch). Somehow, coffee almost never gets this treatment. I’m not sure why, but maybe its being used for caffeine rather than taste might have something to do with it. But that’s a whole other topic about which I know very little.

For now I’ll focus on the V60 method, because it’s cheaper, simpler and easier to carry around for travel. For those not familiar with the V60, it’s basically minimalism for baristas: a conic shaped object, often made of plastic or ceramic, that you put on top of your coffee mug (or other fancy container) to manually brew coffee. It also serves as a convincing argument when a TSA agent asks you what the hell is all that other equipment making up half of your luggage.

2. Percolation and Immersion

Because you constantly pour fresh water over coffee with the V60 method, it’s part of the many methods under the umbrella of percolation brewing. Other examples include batch brewers and moka pots. This is in contrast to immersion brewing, where coffee grounds remain in contact with the same water during the totality of the brew process. Some examples of immersion brews include a french press or a siphon brewer. The main difference between percolation and immersion arise from the fact that fresh water is a better solvent than coffee. In an immersion brew, the speed of extraction decreases as the coffee gets more concentrated, but in a percolation brew, the water in contact with the coffee grounds is almost fresh, so the extraction is always very efficient. You can achieve similar results with either types of brews, but how you navigate across brewing recipes can be very different, and each method has its particularities.

3. Why use a V60 ?

Some people might ask “What’s the point of using a V60 if I have a great batch brewer at home ?” This is a fair question; I think it’s possible to achieve great results with a clean and high-quality batch brewing machine (some coffee experts like Scott Rao also do), but personally I haven’t often tasted great cups of coffee made with batch brewers.

In theory, I don’t see why an automated household brewer couldn’t mop the floor with cups made from manual V60s, but in practice they are often cheap, and never made with the question “how to produce a great extraction ?” in mind, but rather to simply dump water on coffee with the cheapest materials. They are often made of plastic, which I find can impart a bad taste to the cup (this probably depends on the type of plastic). It’s also easy to forget to clean them from time to time, which will eventually make your coffee disgusting (just try to smell the basket of a batch brewer in a hotel room if you want an idea – but really, don’t). They also often come with a relatively large container for water which will go stale if you don’t use it intensely, and that will also result in really bad coffee. Some of these machines also come with a heating element under the coffee jar, which is a great way to burn your coffee and make it even worse.

At most specialty coffee shops, they have decent batch brewers and they seem to clean them often. The problem I most often encounter in that scenario is how long the cup of coffee stayed in the hermetic jar before the staff served it. If it’s fresher than about half an hour, the resulting cup can be super good, but wait much longer than that and it will start to gradually taste like each subsequent level of hell. It will never reach the final level of hell however, this one is only accessible to the worst lattes.

Uh oh (I won’t say where I got this latte). It tasted as good as the latte art looks.

A V60 offers the advantage of potentially producing a great cup (if you do  things correctly) with a ~20$ piece of equipment. Plus, it’s really easy to clean and carry for travel, and it’s more flexible than a cheap batch brewer if you want to experiment with your brew recipes.

In my view, there are three challenges when using a V60: (1) being consistent from one brew to the next, (2) not losing a lot of heat from your brew water, and (3) having several of your brew variables intertwined (we’ll come back to what these variables are), leaving you with less control over individual variables.

4. The Brew Variables

There are several brew variables that will affect the taste (e.g., extraction yield) and strength of your cup:

    • Coffee to water ratio
    • Coffee beans quality
    • Roast profile and quality
    • Grind size
    • Grind uniformity
    • Coffee freshness
    • Filter quality
    • Brew temperature
    • Water agitation
    • Uniformity of extraction and repeatability
    • Water quality
    • Contact time between water and grounds

These are not listed in order of importance, but rather in order that we will discuss them.

4.1 Coffee to Water Ratio

It is useful to describe two relevant ways in which a coffee cup can vary: (1) strength, and (2) extraction yield. Strength is basically just how concentrated your coffee is, e.g. an espresso is much stronger than a filter coffee. The clarity of a coffee cup will be affected by this (but also by many other things). The strength is typically measured in “total dissolved solids” (TDS). It corresponds to the fraction mass of all non-H2O stuff in your cup, and can be measured with refractometers. Some Brix optical refractometers can be bought for ~20 $; (I use this one), but their precision is not great at ~0.1–0.2% TDS (you also need to convert °Brix to TDS with Alan Adler’s relation; TDS = 0.85 x °Brix). Refractometers can be very useful for diagnostics and communication, even though they shouldn’t replace your taste buds. More precise refractometers cost a few hundred dollars. Ideally, you want a brew strength high enough without overwhelming your senses, so that you can appreciate the full aromatic complexity of your cup of coffee. This is a matter of personal preference (and context; an espresso will be perceived as very weak if it has double the strength of a filter coffee), but most people prefer a strength of 1.15–1.35% TDS.

Extraction yield is the total mass fraction of your coffee grounds that was dissolved into your cup of coffee. This variable will mostly affect the taste profile of your cup, because different compounds dissolve in water at different rates. You can use less coffee with a higher extraction yield and end up with the same strength, but you will have a larger extraction yield, and your taste profile will contain more of the aromatic compounds that are longer to extract from the coffee bean. Typical preferred extractions are in the range 18–22%, and a higher extraction yield will correspond to more bitter and astringent taste. If you’re not sure what astringent means, try an unripe pear, or infuse some green tea at 100°C for 5 to 10 minutes. It should be hard not to grin, and will leave a dry taste in your mouth afterward: this is astringency. Lower extraction yields will tend to be sour and have a more vegetal taste. If you know the strength of your cup and the mass ratio of coffee grounds to water that you used for your brew, you can estimate your extraction yield with a simple rule:

There are more precise ways to calculate extraction yield, by accounting for  the fact that each coffee ground retains approximately double its mass in water, but in general you don’t need to worry about this.

These variables can be nicely visualized in a 2D graph called the “coffee control chart” (CCC):

Coffee Control Chart. Degrees Brix units are used by handheld optical refractometers.

[Edit February 17, 2019: The original coffee control chart that I published here did not account for liquid retained in the coffee bed, and was inappropriate for percolation brews like the V60. Please have a look at my more recent blog post on extraction here, which explains this in much more details. The control chart above has been rectified.]

You can already see from this chart that, if you aim at the typically preferred strength and extraction, you’ll want to use a coffee-to-water mass ratio between 1:15 and 1:17. If you establish that you prefer a strength of 1.3% TDS for example, using a 1:15 ratio will produce a 19.5% extraction yield (more sour), whereas a 1:17 ratio will produce a 22% extraction yield (more bitter). What you prefer will depend on your taste, but also on your brew method and the quality of your equipment. I tend to prefer a 1:16 ratio (I really enjoy 1.25% TDS with 20% extraction), whereas a local coffee shop (Saint-Henri Coffee) uses 1:15 (they prefer higher TDS) and Scott Rao uses 1:17 (he likes higher extraction, and I suspect he also likes a TDS close to 1.25%, being a fan of batch brews). I suspect I might appreciate 1:17 more (with higher extraction yields) if I had a very high quality grinder (more on this in Section 4.3 “Grind Size and Uniformity” below).

[Edit Jan 11, 2019: Please note that all TDS-to-extraction yield calculations in this post are very approximate. I used a simple rule of three while approximating that the beverage weight is just the total amount of water that you poured (see equation above). A much more accurate way to calculate extraction yields for a percolation brew such as a V60 is to assume that dry coffee will retain twice its weight in water. Because of this, your beverage weight will be smaller. A quick way to get a better approximation is to shift two lines to the left in the CCC chart above, only for percolation brews. If you made a V60 with a 1:17 ratio, following the 1:15 line in the graph above will give you a much better estimation of your extraction yield. Thanks to Louis Brickman for pointing this out.]

4.2 Coffee Beans Quality and Roast Profile

Obviously enough, the quality of the coffee bean is important to brew a great cup of coffee. But don’t let yourself think that things end here; there are countless ways to prevent your GREAT and possibly very expensive coffee bean from resulting in a good cup at all, if you neglect any one of the other variables.   

I personally prefer lighter roasts, as they retain more of the flavor character in the original coffee bean. Darker roasts correspond to coffee that was roasted for a longer time, and where the aromatic organic compounds were caramelized for a longer time. This will produce a cup with much more body, but you might also notice that different beans will taste more similar with darker roasts. Dark-roasted coffee is also more tricky to preserve without it going stale. When you buy mass-produced cheap coffee, they will tend to be roasted dark because lower quality beans can be used without a noticeable difference after roast. When you buy specialty coffee from hipster coffee shops, they will tend to be lightly roasted to highlight the differences between different coffee origins. If you happen to prefer dark roasts, it doesn’t mean you have poor taste, but you won’t need to worry as much about the origin of your coffee beans. Dark roasts also somehow seem to produce a lot more static electricity, which means your coffee grounds will easily stick to the sides of your grinder (and even V60 !), and they will also de-gas a lot more CO2 when you start brewing. They kind of give me nightmares.

Another aspect to consider is the quality of roasting. It is totally possible to have a great coffee bean and a roast profile that you enjoy (e.g., a light roast in my case), and not to be able to brew a great coffee with it whatever you do. One example would be baked coffee, which I suspect happens more often that we may think. The only recommendation that I can give here, is try notice who roasted your coffee, try different roasters, and remember those that you liked the most.

Particle size distribution of the Lido 3 grinder (red bars; upper panel), the Porlex grinder (red bars; lower panel) versus the Mahlkonig EK grinder (blue bars; both panels), with respective setups that produces the same average particle size (blue and red circles; error bars represent the standard deviation). I built these figures by spreading grounds over a white sheet, taking a picture and measuring the projected 2D surface of each grain with the ImageJ software. Notice how the Lido 3 produces more small particles than the Mahlkonig EK at similar average particle size, and the Porlex similarly produces more small particles than the Lido 3.

4.3 Grind Size and Uniformity

Grind size will have a significant effect on your brew, because finer grounds have more surface area in contact with water, and will therefore extract much more rapidly. When you do filter coffee, the dominant extraction happens by diffusion, meaning that water needs to diffuse into the pores of the coffee bean cells, grab some stuff, and diffuse out through the same pores. We’ll talk more about this when we discuss water quality. If you go as fine as espresso-type grinds however, you enter a new regime of extraction where individual cells in the coffee bean are broken. This allows water to easily dissolve everything inside the coffee cells very easily (this extraction regime is called erosion).

Interior of a coffee bean cell. Royal Photographic Society, Kacie Prince @ Pinterest.

Here we’ll focus on filter brews, and we’ll think specifically about diffusion (although some erosion still happens in filter brews because of the finest coffee particles in the distribution, often called fines). A larger surface area will allow the water to enter the coffee cells more efficiently, therefore extraction will happen faster. When brewing a V60, finer grinds will result in a stronger cup with a higher extraction yield, at a fixed contact time between water and coffee. However, finer grinds will also slow down your flow of water through the coffee bed (water has to take a longer path through the coffee, and coffee fines will clog some pores in your paper filter), which means that your cup will have even more strength and a higher extraction yield. In other words, the resulting brew in a V60 is very sensitive to your grind size.

Another important aspect of grind size is that most grinders are unable to produce a very narrow range of particle sizes. Higher quality grinders will produce a narrower particle size distribution, which is desirable because it will allow you to extract each coffee particle at the same extraction yield.   Below you can see an example of a particle size distribution for a Lido 3 hand grinder, compared to the (just slightly) more expensive Mahlkonig EK43 automatic grinder. I chose a grind setting that produces the same average grain size, but you can see that the Lido 3 has a much larger number of fines and a flatter distribution of particles of diameters ~0.6–1.5 mm. I also show a similar distribution for an entry level Porlex manual grinder, where the difference in particle size distribution is quite stark. It would make more sense to compare the total mass of grinds at each surface, rather than the number of grinds at each diameter, and to compare distributions of the same weight-averaged surface rather than the average diameter, but I still find these figures illustrative to make my point, as we won’t be actually tasting the resulting coffee.

You can imagine the resulting coffee cup as a collection of smaller coffees, each of which is made with particular grind size on these histograms, and the resulting cup will be the average of all of these grind sizes (so, all smaller coffees are poured inside your mug). In other words, the resulting brew will be an average of several different points on the coffee control chart; some will be over extracted, and some will be under extracted. In a case like the Porlex grinder, you won’t be able to reach average extraction yields of ~20–22% without a significant amount of fines over-extracting, which would result in an unpleasant astringent cup. You will therefore be forced to stay in the range 18–19%, and you won’t be able to taste the full potential of your coffee beans. I used the Porlex for a while, and although I didn’t have a refractometer at the time, I’m pretty sure my extraction yields were around ~18%. I still enjoyed my pour over cups, but I did puzzle over why it tasted so much better at the coffee shop (water quality was an even bigger reason for this however, as we’ll discuss later).

Generally you will get a narrower particle size distribution by using burr grinders; the larger the burrs, the more uniform the particle size distribution will be. An alternative way to improve your particle size distribution is to use a sifter, like the Kruve. However, filtering out fines takes at least 2–3 minutes, and will require you to throw away a non-negligible fraction of your coffee. Still, you won’t be getting a perfectly uniform particle size distribution, because electrostatic forces cause a significant amount of fines to stick on the surface of larger grinds, and those won’t be filtered out. I am tempted to experiment with pressured CO2 (to avoid oxidizing grinds) or statically charged balloons to get rid of these sticking fines, but that’s for another day.

I will eventually experiment with the Kruve and report whether I find something interesting, but doing so is not as simple as the manufacturer wants you to think – they suggest to just filter out fines and brew as you would. If you do this, the water flow through your V60’s coffee bed will be much faster, and your resulting cup will be extremely weak and under-extracted. I had a few good laughs watching YouTube videos of baristas comparing a Kruve vs non-Kruve cup of coffee using the same brew recipe in a blind tasting, and choose the non-Kruve cup every time, to their dismay and confusion.

One last note on grinders – avoid blade grinders at all costs. They will create an extremely wide particle size distribution, but they will also overheat the grounds, which will deteriorate the taste of your coffee. James Hoffmann actually made an interesting video of some tricks to follow if you were stuck on an island with just a blade grinder and some coffee.

4.4 Coffee Freshness

One of the difficulties in brewing good coffee is to keep your beans fresh. From personal experience, I find that it’s best to buy your coffee beans between ~1 and ~3 weeks after their roasting date (assuming they are properly packaged). If the coffee is fresher than this, it will degas a crazy amount of CO2 (accumulated during roasting), making it harder to brew it properly, and if it is too old, the coffee will develop noticeable defects in its flavor.

The four big enemies of coffee beans are oxygen, humidity, heat and UV light, so you should make sure that you shield your beans from these four elements. When you buy a bag of fresh beans, make sure that the package is not transparent, and that it has a one-way valve on it; this will allow the coffee to de-gas CO2 (accumulated during roasting), and prevent oxygen from entering the bag. Ideally, the bag would also have a ziplock-type re-closable opening, and it would be small enough that you will finish it within a week or so. Avoid buying pre-ground coffee; it will go stale orders of magnitude faster, making it hard to even run back to your house and brew a single good cup of coffee. Keep the bag of coffee upside if you can; this will keep the most amount of CO2 inside the bag, which acts as a protection against oxygen.

Once the seal on a bag of beans is broken, it will start going stale much faster. This is because all CO2 will immediately leave the bag (usually with a great smell of volatile aromatics), leaving the beans without protection against oxygen. Most of the CO2 de-gassed in the first few days, so the coffee beans won’t build back that protective chamber of CO2. Every time you re-open and re-close the bag, a little more oxygen will enter, accelerating the staling process. One way to get around this is to split your coffee beans in pre-weighted doses, in small individual ziplock bags. Just make sure these bags are food grade ! Currently I use plastic-aluminum bags of 7.7×10 cm which can store up to 22 g of coffee. Some other non-food grade bags have a very foul plastic smell. It is best to use bags as small as possible to minimize the amount of air enclosed, and to get as much air as possible outside the bag before closing it. I recommend doing this right after you open your bag of fresh coffee.

[Edit January 7, 2019: I found that the ziplock bags described above do not keep a good seal. They are also too small for any dose above 22g. I switched to using these plastic-only ziplocks now, or a cheap-ish vacuum sealer which is by far the best option.]

Do not forget to put these ziplock bags in an opaque container, and to keep them away from heat and humidity (I often put then back in their original coffee bag). If you want to go one step more crazy (welcome to the club), you can buy pressurized inert gases (typically CO2, nitrogen and argon) and shoot some of those inside the bag before closing it back, to vent out all oxygen. The cheapest way to get these is to find a local specialty wine store near you, because shipping can be quite expensive for this type of item. A worse but simpler alternative is to simply inject inert gases in the original coffee bag (or a better container like the Planetary design Airscape), but every time the bag is opened, the staling will happen a bit faster until you close the bag back up with its inert gases, because the beans will have lost all ability to de-gas CO2. Just using an Airscape without any extra complication is already much better than leaving your beans in your coffee bag, unless it has a zip-lock closing and a hermetic inner lining (in the latter case, make sure you press any air out from the bag each time before closing it).

If your goal is to keep some coffee fresh for later use (say, more than one month), I recommend freezing your beans. You have to do so carefully however; freezing will slow down the staling process, but could also expose your beans to foul odors and humidity if not done properly. As a first step, I highly recommend pre-dosing your beans in small zip-lock bags, because you really want to avoid de-freezing and re-freezing your beans. Put the zip-locks in a hermetic container (e.g. a hermetic Tupperware), and take out only the amount of doses that you need at a time (quickly putting back the container in the freezer). Take them out of the freezing at least a few hours before using them, don’t open the zip-lock, and store them somewhere away from the 4 aforementioned elements. The reason for waiting a few hours is (1) to avoid water condensing on the cold beans (remember, humidity is bad), and (2) to avoid grinding the beans while cold. The latter is because grinding beans while cold causes shattering (Uman et al. 2016), resulting in a larger amount of fines. This is ok for espresso, but it is not great for filter brews. You can take out a few days worth of zip-lock bags and store them in your opaque coffee bag, especially if you sealed your zip-locks with inert gases.

It would be even more awesome to store coffee beans under positive pressure (especially in a CO2-filled container), because it would prevent them from de-gassing, and therefore they would still have their ability to de-gas their CO2 as protection after opening the container. Sadly, I haven’t found an easy solution to do this at home.

Figure 4 from Uman et al. 2016. Colder temperatures produce more small-sized coffee particles.

4.5 Filter quality

The filter that you use with your V60 will have a strong effect on your extraction time. This surprised me quite a bit, but it makes sense if you realize that different types of paper have different pore sizes. I don’t think that the pore size matters that much in the end, as long as you find a recipe you like and don’t change filters after this.

Another aspect that coffee filters can have is their impact on the taste of the coffee. As we will see later, there is a pre-infusion step in V60 recipes where you pre-wet the filter with hot water, to get out the paper taste as much as possible from your coffee cup. In order to verify the impact of paper filters on the cup, I tried to infuse four different filters in hot water, wait for the water to cool down and tasted them. Specifically, I tried the  Hario V60 natural filters, the Hario V60 bleached filters, the Hario natural siphon paper filters, and the small circle-shaped Aeropress bleached filters. Only the first two can be used in a V60 pour-over, but I was curious. I found that both the bleached filters imparted less taste in the final cup, and that the Hario V60 natural filter had the strongest taste, which reminded me of wet cardboard. However, I tried this experiment a second time after having pre-washed each filter with hot water (as we do when we prepare a V60), and I could barely notice any difference. In other words, as long as you pre-wet your filter correctly, it shouldn’t have a strong effect; note that this might not be true of other V60 filter brands (you’d have to experiment yourself). I still chose the bleached Hario V60 filters, just in case.

There are also cloth filters available for V60, and more often with siphon brewers. I’ll talk about siphons in a separate post, but after a lot of experimentation, I highly recommend never to use cloth filters. They can produce a great cup the first time or two (with more body than paper filters), but then keeping them clean of coffee oils and bacteria quickly becomes a nightmare. I think using them properly would require boiling them with a strong odorless chemical (e.g. Oxiclean) after every use, then abundantly rinsing them with fresh water (potentially boiling them a few times in clean water), and storing them in a closed container in distilled water, in the fridge (for no more than a week or so before their next use). Just a bit extreme.

4.6 Brew temperature

As I mentioned earlier, achieving a high enough and stable temperature is one of the main challenges in V60 brewing. To my knowledge, currently there isn’t much solid data backing up brew temperature recommendations, but most people recommend 91–94°C (for you weirdos in the USA; 196–202°F). Matt Perger, an influential coffee expert, suggested that the highest possible brew temperature is great too, because you can’t burn your coffee. While I agree with this (roast temperatures are much higher than brew temperatures), you need to keep in mind that different chemical compounds in the coffee bean extract at different rates, and these rates depend on the water temperature. If you brew at much different temperatures, the relative extraction rates of different compounds will be very different, and the resulting flavor profile of your cup will also be very different. Ever wondered why cold brew coffee has absolute zero acidity ? It’s because acidic compounds only extract at high temperatures. I experimented a bit with brew temperatures higher than 94°C in a siphon brewer (it’s very hard to get there with a V60), and I didn’t really enjoy the result, but I need to explore this more before having an informed opinion.

When brewing in a V60, I recommend achieving the highest possible water temperature in your kettle, because you will lose a lot of heat in the pouring process, and it’s easy to fall much below 91°C, which will result in a more under-extracted sour cup. The coffee grounds, V60 and surrounding air are all much cooler than your water even if you pre-heated everything, so your brew temperature will hardly ever exceed 93°C. For this reason, Scott Rao recommends using a plastic V60 because it is a better insulator than ceramic or glass V60s, and I fully agree with that. I experimented brewing with different V60s and different methods holding a thermometer in my slurry (the mix of brewing water and coffee grounds), and found that the plastic V60 is indeed much better.

Another cause of worry with brew temperature is the amount of water you pour at a given time. If you pour your V60 water in a lot of tiny batches (WHAT IS THIS ?), you can be sure that the extraction temperature will be much below 91°C. Maybe you will like the resulting taste, but you should also try a higher brew temperature before deciding.

I’ve heard baristas recommend me to bring water to 99°C instead of boiling it to get a better cup of coffee. However, I haven’t seen much convincing evidence or reasoning behind this. I remember reading somewhere that most oxygen is lost at boiling point, and that it may be somehow important for brewing, however given the amount of pseudo-science going on in the coffee brewing game, I don’t think I could recommend this with a straight face.

Simulated turbulence generated by a Kelvin-Helmholtz instability at the boundary of two rapid flows of opposite directions (white to the right; black to the left).

4.7 Water Agitation

The efficiency of extraction depends a lot on the contact surface between water and grounds. The speed at which you pour water over your coffee grounds will determine the amount of turbulence that ensues (faster flows will cause more turbulence). One thing turbulence is great at is balance everything out by creating gigantic, fractal-like contact surfaces. Balancing out water and coffee is another way of saying extracting the hell out of your coffee. I don’t think that high or low levels of turbulence are necessarily a problem in themselves, but I suspect that using reasonably high turbulence is a way to facilitate the repeatability of your brew.

The beak of your kettle will have an effect on turbulence (you might notice some vortices at the surface of your slurry while you pour), the height from which you pour, and any up-and-down motion (i.e., accelerations in your flow) will also affect turbulence. You should avoid up-and-down motions because they are hard to exactly reproduce every time, and you should try to pour water from the same height every brew. I personally like kettles with a long, small-opening beak because they create a nice amount of turbulence without splashing the coffee grounds everywhere.

I recently ordered a product called Melodrip which allows you to imitate the flow of a batch coffee brewer, but I still have not experimented much with it. The instruction manual comes with a brew recipe that will probably produce something more similar to a cold brew cup (with more than 10 tiny pour stages…), but using the Melodrip without any modification to my current recipe creates so little turbulence, or agitation of the coffee bed, that it produces an awful sour cup. I suspect it’s possible to improve repeatability if I experiment with it and come up with an adapted brew method.

4.8 Uniformity of Extraction and Repeatability

Having a uniform extraction is somewhat analogous to having a uniform particle size distribution; you don’t want to over-extract some of your coffee grounds, while under-extracting others. There are several problems that can prevent you from achieving a uniform and easily repeatable extraction.

Channeling is one of the biggest problems for a uniform V60 extraction. Dry coffee grounds tend to repel water, so when water has created a path across the coffee bed, all subsequent water will tend to keep passing there, and a significant fraction of your coffee grounds might stay dry. One way to avoid this is to agitate the slurry with a spoon or stick (which Scott Rao recommends in his V60 pour-over video), but unless you are a well-oiled robot, this method has a draw back: repeatability. Playing with the slurry will have a very strong effect on the efficiency of extraction (by creating a lot of turbulence, and presenting the coffee grounds with higher-velocity fluid), and it will be extremely hard to agitate the slurry by the exact same amount every V60 you brew. Therefore, this will result in a more even extraction by virtually eliminating channeling, but from one cup to another you might be producing weaker and stronger brews. Scott Rao told me that he now does his famous Rao spin instead of agitating the slurry, as a more repeatable way to decrease channeling. I’ll discuss this more below with Scott Rao’s latest recommended method to brew a V60.

In a cone-shaped V60, one region that tends to remain dry often is a small cone-shaped region near the center of the coffee bed, slightly below the surface. I’ve seen some baristas online dig a small trench with their finger or finger joint before pouring water to avoid this. I haven’t really experimented enough with this yet to be sure that it does help, but I think there is good reason to think it might, and it’s easy enough to do.

[UPDATE: See my second blog post for an update on the trench method].

Another common problem in V60 brews are “high-and-dry” grounds, i.e. grounds that stick to the walls of your filter. If you try to knock them off just by pouring water over them, you’ll end up pouring most of your water around the edges of your coffee bed, where you want to distribute your water as uniformly as possible across the coffee surface. The brew method developed by Scott Rao deals with this by spinning the V60 around at various stages of the brew. Using a regular kettle without a precision beak can vastly worsen the problem of high-and-dry grounds.

4.9 Water quality

The last, but not the least: water makes all the difference. I’m not talking about the taste of your water, or whether you put it through a Brita filter (although all of this is good). I’ve heard too many times “coffee is made of 99% water, so your water better taste great”. This is true, but beside the point. Some water tastes great and makes shitty coffee (looking at you, Montreal.) The main problem here is extraction. Water has to get inside the cells of the coffee beans, and come back with the good stuff, and not all types of water will be able to do this efficiently. Specifically, the content in alkaline buffers and minerals (specifically, calcium and magnesium) are of paramount importance.

This might sound crazy, or too complicated to care about, but if you have any doubt, here is an experiment for you to try: buy a bottle of water (e.g. Evian), and brew a coffee with it. Using the same exact coffee and recipe, brew another coffee with your local tap water. Then, do the exact same thing with distilled water. Try the three cups of coffee, and see what I’m talking about. The one made will distilled water will be the weakest cup you have ever seen, and the one made with the bottled water should be quite strong, and have a dull taste without acidity (assuming you use a typical brand – they almost always have crazy high amounts of minerals and alkaline buffers). Depending on where you live, the tap water coffee might be located anywhere in between these two cups. Most of the time, it will be a bit weak and lack body, something that you won’t be able to compensate by playing with the other variables.

This is something I discovered only recently, and I was always astounded at how the coffee tasted different when I traveled, and even how much my V60 extraction time changed when I used bottled versus tap water. All of this felt like total chaos, but it wasn’t. Even coffee experts started realizing this relatively recently (as you can see in this 2016 conference) – before that, some world barista champions fine-tuned their recipes at home, traveled to the championship country, and crafted the worst cup of coffee in their life, then came back home only to have never-ending nightmares to cope with their perception that truth doesn’t exist and nothing makes sense. Now, world barista championships use tightly controlled water composition, which is announced much before the competition. Even your favorite hipster-y specialty coffee shop probably has a refined water filtration, reverse osmosis and/or re-mineralization system, depending on the local tap water composition.

There are 4 aspects of water that you want to control when brewing coffee, in order of importance:

    1. Total alkanility
      This is the total mass fraction of HCO
      3 negatively charged ions (anions) in your water. This is often called a buffer because it will react with free H+ radicals, and prevent water to become more acid (smaller pH). Therefore a high total alkalinity makes your water more stable against changes toward more acidity. If this is too high however, the HCO3 anions will start to react chemically with your coffee flavors, and it will taste dull and flat. The Specialty Coffee Association (SCA) recommends ~ 40 mg/L total alkalinity.
    2. Total hardness
      This is the total mass fraction of Ca
      + and Mg+ calcium and magnesium positive ions (cations) in your water. These cations are responsible for bonding with coffee flavors inside the coffee cells, and bring those back in your coffee cup. They don’t bring the exact same stuff back, so it’s good to have both, but this is still an active area of research. The SCA recommends a total hardness of 17 to 85 mg/L.
    3. pH
      You don’t want free H
      + or OH radicals to start reacting with your coffee, so you’ll want to have a pH near 7 (neutral). The SCA recommends a pH between 6.5 and 7.5.
    4. Cleanliness and good taste
      This shouldn’t pose any problems if you started from distilled water, but basically you don’t want chlorine, too much sodium (SCA recommends less than 10 mg/L sodium), or more generally dirt in your water.

I’ll describe some ways to get water with properties in this range below, in Section 7.2 “The Water I Recommend”.

[Update: See my third blog post for a much more detailed discussion of water quality]

4.10 Contact Time

As you might have guessed, contact time between the water and grounds will have an effect on both the strength and extraction yield of your brew. However, in a V60 you don’t really have a direct control over this variable, so you can view it as more of a consequence of all other variables. Your filter type, water composition, roast profile, type of beans, and pour over method can all have surprisingly significant effects on your contact time. It is a good habit to note them down from one brew to another, as it is a good way to see how well you were able to reproduce your last brew.

The grind size I use for a V60.

5. Determining Grind Size

It is typically quite hard to communicate grind size efficiently, so you will have to experiment a bit to figure out what works best for you. Scott Rao recommends grinding finer and finer (not changing any other variable) until you can taste astringency, and then go back one notch on your grinder. Another way to do this is to brew a few cups and blind taste them.

With the Lido 3 grinder and a 1:16 ratio, I found that I prefer what I call mark 9 (see picture below for guiding). To identify it, unscrew the grind size screw all the way up, and count the marks from the right to the left, starting from the reference symbol on the Lido 3 (it looks like this: O|o).   

The first mark is located very near the reference symbol and counts as mark 1. When you counted all the way to 9, place the blue marker there and tighten back the grind size screw carefully. Always make sure you hold the bottom of the metal part of the Lido 3 before you unscrew the plastic recipient containing the ground coffee, otherwise you might shift the grind size.

The resulting particle size distribution (upper panel) and surface area mass contribution (lower panel) for grind size 9 of the Lido 3 grinder. These were built with the ImageJ software from the picture above. Please note that particles with diameters smaller than 0.4 mm cannot be detected accurately with this method.
The Lido 3 setup corresponding to grind sizes 1 (up) and 9 (bottom), counted after the O|o symbol. Note that the markings must be counted when the adjustment screw is completely up; the pictures above show the resulting position when the adjustment screw is secured back down.

6. Practicing the Rao Spin

I recommend viewing this video to get a grasp of what the Rao Spin method is. Even after having watched it, you might find that you end up just shaking the slurry back and forth in a back-and-forth linear motion rather than circular motion when you try it. Here’s a way you can practice your spin without producing a lot of sub-par cups.

Grind 10 g of coffee with the finest grind that won’t damage your grinder. Place your V60 on a mug, put a filter in, and pre-wet it with your faucet. Put the fine coffee in the filter and slowly fill it with tap water. The coffee grounds should quickly start clogging the filter, which will allow you to have a good amount of water in the V60 without it draining efficiently. Now you can start trying to do the Rao spin. Try it with both hands, or with just your left or right hand. Try to do a clockwise or counter-clockwise spin. There’s a good chance that one of these combinations will work best for you (I find it easier to do a clockwise spin with just my right hand). You will find that there’s a small lag between the small circular motion that you induce in the V60 and the motion of the fluid itself; I found that getting used to this lag and adjusting it properly is key to getting a circular motion rather than a back-and-forth linear motion. It’s also important to not spin when the V60 is too full, and to adjust the amplitude of motion so that you don’t get a spillover.

Something that can prevent you from easily doing the Rao spin is a non-circular mug, a mug with an opening that is too small, or a non-conic shaped V60-type brewer. Using a ceramic V60 will complicate things because, being a great conductor of heat (and a bad insulator), the V60 will get very hot to the touch.

7. How To Actually Make a Better V60

After having explained all of the considerations behind making a good cup of V60, here’s a list of actual practical recommendations for brewing a better cup of coffee.

7.1 The Gear I Use

Here’s a list of the V60 gear I use. I’m tentatively ordering them in decreasing order of importance in terms of impact on the cup quality.

  1. The Lido 3 grinder (I think the Comandante grinder might be as good). If you are rich and have counter space, get yourself a Mahlkonig EK.
  2. A plastic Hario V60 (plastic is better for heat retention).
  3. The Brewista Artisan gooseneck kettle (it’s very precise, and very beautiful; the Bonavista or Hario precision kettles are very good too).
  4. Hario V60 bleached paper filters (they’re the white ones).
  5. Acaia scale (a Hario scale would be as good for this, but Acaia can be programmed to yell at you if you pour too much water).
  6. A mug. To pour the coffee in. A Hario pour over recipient would be more practical when making coffee for two.
  7. Gorgeous Bodum double-walled degustation glasses.

7.2 The Water I Recommend

Right now I use Dan Eil’s water recipe. It doesn’t require too many hard-to-get minerals, and it’s just great for coffee. I haven’t experimented enough with different water recipes, so this might change with time.

First, you need to get yourself some magnesium chloride (MgCl2), calcium chloride (CaCl2), potassium bicarbonate (KHCO3), distilled water and a regular-sized mason jar or something similar to it (don’t worry, we won’t be preparing triple distilled mason water). Please make sure you are ordering food-grade (not lab-grade, of pharmaceutical-grade) products. Lab-grade may seem great because it has less contaminants, but the contaminants can be very nasty for your health (e.g., heavy metals). You’ll also need a pipette (a small spoon might do) and a precise scale, ideally something at 0.1 g precision or better (e.g. this thing). Another perfectly acceptable buffer would be the more readily accessible sodium bicarbonate (NaHCO3) – you’d need to use 1.68 g instead of 2 g of it  to get the same amount of HCO3 anions (see below), but then you’ll be adding sodium (Na+; ~23 mg/L of it) as well as HCO3 buffer in your water, which could affect the taste of your coffee (the SCA recommends having less than 10 mg/L sodium in your water). On the other hand, adding potassium (K+) in your water is not a problem because coffee beans are already full of it, so it’s unlikely that will affect the taste of your cup.

First pour 194 g distilled water in the mason jar (you can use a Hario scale to weight that if your precise scale cannot go this high). Add 2 g CaCl2, then 2 g MgCl2, and 2 g KHCO3. Bubbles should appear and the solution should become white and opaque. Shake it well with a spoon, then let it sit still for a few hours, with the mason jar top on it, but not screwed (so it can degas without blowing). After a few hours, you can stir it up to make sure it doesn’t degas too much anymore, then screw up the lid. This is your minerals concentrate, which you can keep in your fridge.

Now get yourself a large container of distilled water (e.g. 4 liters), and put 10 g/L of concentrate in it (for a 4 liter container, you need 40 g of concentrate). To do this properly, you can place the concentrate on a scale, tare it, then take out some concentrate with the pipette until the scale reads -40 g. Every time you take out some concentrate, you should stir the solution with the pipette, because it quickly precipitates, and you don’t want to preferentially draw concentrate that is too diluted. Shake the water container well, and keep it in your fridge. This is your water for brewing coffee.

If you find that this is too complicated, then a simpler and good option is to get yourself some Third Wave Water (filter coffee profile) and dissolve a bag in a gallon of distilled water. Distilled water is very cheap and can be found in most grocery stores or pharmacies. Third Wave Water is not too expensive, lasts for several gallons of water, and it’s quite decent for extraction. I like to have a few extra bags of it for in-depth diagnostics when something’s wrong with my coffee, to eliminate as many variables as possible.

A somewhat simpler approach would be to mix some bottled water with distilled water, although you need to choose a brand of bottled water that already has the right ratio of total hardness to total alkalinity. I won’t make recommendations right now, but I plan to write something up about this soon.

Whichever method you choose, make sure to always keep your gallon of “coffee water” in your fridge, and don’t let it stand there for more than 2 to 3 weeks (assuming you started from distilled water, which you should have), especially if it’s almost empty. Always smell the water before using it; if it smells like an old rag, it means bacteria built up in your gallon, and your coffee will taste like an old rag.

[Update: See my third blog post for more explanations and more water recipes]

7.3 A V60 Pour Over Workflow

Here are the steps I follow when I brew a V60 cup of coffee:

  1. Get a bag of coffee that was roasted between ~1 and ~3 weeks ago.
  2. When you’re ready to brew your first coffee, open the bag and split the beans in pre-dosed batches of 22 g in small ziplocks. Make sure to expel as much air as possible before closing. Put ziplocks back in the opaque coffee bag. Put your coffee bag away from heat and moisture. Optional: Put some inert gases in there before closing the bag. Optional: Put some of them in the freezer (see Section 4.4 “Coffee Freshness”).
  3. Start boiling some water to 100°C. Use high-quality water with the right alkalinity and mineral content (see Section 4.9 “Water Quality”).
  4. Drop a pre-dosed 22 g of coffee in your grinder. If you don’t have a pre-dosed bag, weight 22 g of coffee now and drop it in your grinder.
  5. Place the plastic V60 on your cup, on the balance.
  6. Take a V60 filter, fold its thick side and place it on your V60.
  7. When the water boils, thoroughly pre-wet the filter.
  8. Place your kettle back on its base and re-heat it.
  9. Drop the pre-wet water in the sink.
  10. Grind your coffee.
  11. If your grinder tends to retain some coffee grounds, gently tap it with the palm of your hand.
  12. Tare your scale.
  13. Pour your coffee in the V60. Do you measure 22 g ? If not, your grinder retained some coffee grounds.
  14. Gently shake your V60 to get an even flat bed of coffee grounds.
  15. Dig a small shallow trench with a finger in the middle of the coffee bed.
  16. Tare your scale again.
  17. Start a timer.
  18. The bloom phase: pour 66 g of water with a uniform and somewhat fast flow, starting from inside the small hole. Wet all beans uniformly across  the surface by preferentially hitting the darker regions with fresh water. If your coffee is fresh, it will release lots of CO2. If you are brewing a very dark roast, it will form a large dome of hell that will summon Ihsahn, who will ensure that your coffee tastes like the devil.
  19. Immediately after you hit 66 g, gently hold the V60 and do the Rao spin (see description above). If you haven’t yet seen bubbles, your coffee might be stale.
  20. Ideally you will have completed steps 18 and 19 within 15 – 20 seconds. Wait until the timer reads 30 seconds (you might see bubbles coming up from the slurry).
  21. Start gently pouring water, hitting the dark regions preferentially, and describe spirals over the surface of the slurry so as to hit every part of the surface as evenly as possible. Stop around ~200 g, or when the water is near the top of the V60’s height. If you see bubbles starting from now, it means your bloom wasn’t perfectly executed (it does take some practice).
  22. When water is not too high (~2/3 of the V60 height), do a Rao spin.
  23. When the water height approaches that of the coffee bed, start pouring again, until you hit 352 g (for a 1:16 ratio). Don’t let the coffee bed appear (to avoid having the temperature drop too much).
  24. When water is low enough again, do another Rao spin.
  25. Wait for the water to flow through completely and note your full extraction time. This time will depend on a lot of things (see Section 4.10 “Contact Time”), but typically it will range from ~2:30 to ~3:30. Higher-quality grinders will typically have you converge on shorter extraction times. You can compare your future brew times to see how consistent you are.
  26. Swirl your coffee around if you’re going to share it or drink it directly from the recipient you poured it in. The coffee will have sedimented a bit, so this will even things out.

I highly recommend looking at this V60 pour-over video by Scott Rao for initial guidance. The method that I recommend above is very similar, except (1) he does one pour instead of two; (2) during bloom, he agitates the slurry with a spoon instead of a Rao spin; (3) he doesn’t dig a small hole in the coffee bed; and (4) he uses a 1:17 ratio; and (5) he gently induces a rotation at the surface of the water after the full pour. Modifications (1) and (2) were also designed by Scott Rao who told me about them after he published his V60 video. Modification (3) is discussed in Section 4.8 “Uniformity of Extraction and Repeatability”; modification (4) is discussed in Section 4.1 “Coffee to Water Ratio”; and modification (5) is just based on my noticing that I did not need to do it to avoid having “high-and-dry” grounds. I suspect this is a consequence of doing two pours instead of one (and therefore two Rao spins).

If you’d like to dig deeper in some of these methods, I highly recommend Scott Rao’s books.

Special thanks to Sandie Bouchard for proofreading.

8. References

Folding the filter (step 6)
The pre-wet phase (step 7)
Digging the trench (step 15)
Spinning the bloom (step 19)
Initiating the first pour (step 21). Notice the darker regions; you should hit those next with the fresh water.
End of the first pour (step 21). The color is now much more uniform.
The Rao spin (step 22).
The coffee bed is about to be visible – it’s time to pour again (step 23). Also note some “high-and-dry grounds”, which I’ll hit with my next pour.
The resulting coffee bed after full extraction. It should be flat and have as little “high-and-dry” grounds as possible.