Extraction Uniformity and Channeling

For a while now I’ve been trying to understand the details of channeling in pour over coffee, and I found it very difficult to find a convincing description of why channeling (and thus astringency) happens suddenly when we grind a bit too fine, even if the surface of the coffee bed looks flat at the end of a brew.

Yesterday I finally found a scientific paper about percolation in non-uniform porous media that I think may be the missing piece to how we think about channeling.

Before I get into it, I’d like to briefly try to explain why a Google search for percolation returns a lot of stuff not obviously related to water penetrating a porous medium. It happens that the maths which are useful to describe water traversing a porous medium are also very useful to describe many other systems in physics. This edifice of mathematics called “percolation theory” turns out to be extremely useful in describing large statistical systems like those often encountered in quantum physics, and therefore most of what you’ll tend to find online is specifically centered around quantum or particle physics rather than brewing coffee.

So, back to the scientific paper above – the authors used a computer simulation to model the details of how a fluid flows in a disordered set of obstacles, which is exactly what happens when we brew coffee. Water flows around our coffee particles, and because they have variable shapes and sizes, the voids between them (which we can loosey call “pores”) are also very disordered. Water will flow faster where the voids are larger, and slower where the voids are small.

This is a consequence of two things: the “no-slip boundary condition” that states the layer of water immediately touching a solid surface must have zero velocity; and the fact that water is viscous means that subsequent layers of water can’t easily have extremely large differences in velocity. The no-slip boundary condition is a consequence of the adhesion between water molecules and solids being larger than the cohesion of water molecules within themselves; it is true in most typical real-life conditions, and coffee brewing is one of those.

In other words, if you imagine a small “tube” of spacing between coffee particles with water flowing in it, the thin layer of water on the sides of the tube that touches a coffee particle is not moving, and the layer immediatey on top of it (toward the center of the tube) can only move slowly. The next layer of water on top of all that can move a bit faster than the last one, and this trend goes on until you reach the layer in the center of the tube. You can imagine that a wider tube will have a larger central flow, and therefore also a larger average flow.

Here’s what this looks like in a computer simulation:

In the figure above from Stanley et al. (2003), the white pixels are obstacles to the flow of water (much like coffee particles) and redder colors correspond to regions where water flows more rapidly. I rotated the figure to make it more similar to coffee percolation where water flows downward. The simulation above would correspond to a V60 that drips at an extremely slow rate of 5 mg/s.

You can see how the flow of water is not very uniform, and some clumps of particles tend to be isolated from most of the flow (in the blue regions). In the context of coffee brewing, these particles will get under extracted. But now let’s see what happens if we pump up the flow of water, by applying more pressure on it:

The figure above is also a simulation from Stanley et al. (2003) with a thousand times more overall flow. It would correspond to a V60 that drips at a slightly rapid flow rate of 5 g/s.

If you look carefully at the second image, you’ll notice that there are now much less clumps of particles that are isolated from the flow of water, which is now overall a bit more uniform than before (although it is still not perfectly uniform). The authors decided to characterize this global flow uniformity in an objective way – this is great for us, because it directly impacts the uniformity of extraction. To do this, they simply measured the standard deviation or water’s kinetic energy (its energy of motion) across the pixels in the simulations, and they called the inverse of that quantity π. Larger values of π mean that the flow is more uniform, and smaller values mean that it’s very non-uniform, or “localized” in only a few paths as they call it. A perfectly uniform flow would have π = 1 (this can’t happen even with perfectly uniform spherical particles, because water still has to get around them), and an extremely non-uniform flow would have π close to zero. The authors parametrized the flow velocity in terms of the “Reynolds number” (Re), which we don’t need to get into here; we just need to realize that a higher Reynolds number corresponds to a faster overall flow.

As you can see, very slow dripping rates correspond to a “flat” regime with very poor uniformity that doesn’t depend much on overall flow rate, but above the threshold of Re ~ 0.6 (or log Re ~ -0.25) you start getting more uniformity as you have more overall flow. Now the question is: what Reynolds numbers correspond to realistic V60 preparations ? Are we in the regime where flow has an effect on uniformity or not ?

To answer this, I used the geometry of Hario’s plastic V60 with my typical 22 grams dose of coffee and the properties of water at a typical V60 slurry temperature of 90°C (194°F – this corresponds to a kettle set to boiling) to translate this into a V60 dripping rate instead, in grams per second. The threshold below which flow has very little effect on uniformity (Re ~ 0.6) corresponds to a V60 dripping rate of ~ 0.2 g/s, which is extremely slow. If we transform the x-axis of the figure above to talk about V60 dripping rate, and plot it in linear rather than log space, we get this:

I removed a few data points in the “low flow” regime for visibility because they were very crowded.

If you want to measure your V60 dripping rate you need to use a brew stand and weigh your beverage rather than the total water, and see how fast it goes up with time during your brew. To do this I use two Acaia scales (a Pearl and a Lunar) and a Hario brew stand (make sure your server is not too tall; I use the 400 mL Hario Olivewood one; apparently it’s only on Canadian Amazon) which allow me to build detailed brewprints like this one:

If you focus on the dark purple dashed line, you’ll see that my flow rate went from ~ 3 g/s when the V60 had the most water in it, down to ~ 1 g/s when it was almost empty, placing me right in the regime where flow rate affects flow uniformity, and therefore extraction uniformity, quite a lot.

Here’s why I think this is really interesting: this could explain why brews suddenly become astringent when we grind too fine, even if no channels were physically dug into the coffee bed by the flow of water. I think it would be confusing to call this effect of low-velocity non-uniform flow “channeling”, and I’d rather keep this word for situations where the coffee bed is eroded by water and coffee particles are pushed away to form a channel. Rather, I’d prefer to speak about this as “flow uniformity”, or its direct consequence “extraction uniformity”.

Speaking of which, there is one major limitation to the computer simulation these authors made: it treats the bed of coffee as a fixed and immovable object. Therefore, no bed erosion can occur, and no channels can be dug by water. This is why their simulation tells us that “the fastest flow is always best”, which may have you want to apply 150 bars of pressure on your pour over. If you did this however, you’d find that your coffee bed would quickly erode and channel pretty badly, resulting in a super astringent brew (and probably an exploded coffee server). Espresso brewing often faces this challenge: you don’t want flow to clog, but you also don’t want to destroy your coffee puck by eroding it with a very large flow and pressure. This is partly why puck preparation became very important in espresso brewing, as a way to make the coffee bed structurally more robust against erosion.

That’s a lot of information, so I think it would be good to remind ourselves of all the possible sources of non-uniform flow can be:  

  • Classical channels, i.e. water pushed away coffee particles to form a void space. Those channels will appear more easily if coffee particles are lighter (therefore smaller), and may be visible from the formation of hollows at the surface of the coffee bed. This will also happen more easily if the global flow of water is too intense by applying a lot of pressure on it, and can be mitigated by compressing the coffee bed with puck preparation like we do when pulling espresso shots.
  • The uniformity of your grinder’s particle size distribution will directly affect flow uniformity because it governs the uniformity of void spaces between the particles.
  • A flow that is too slow, either from filter clogging or a coffee bed resistance that is too high, will make the flow of water less uniform even in the absence of classical channels.
  • Clogging your filter will also likely not happen everywhere at once on the filter, causing the flow to be even less uniform because it will only pass where the filter wasn’t clogged.
  • Poor blooming that leaves dry spots in your coffee bed will also make your flow less uniform, because the coffee bed will have more resistance in these dry spots (dry coffee is more hydrophobic than wet coffee).

This realization made me think that maintaining a more stable flow of water through the coffee bed is crucial to get a good, uniform extraction. Here are a few predictions I think I can make based on the considerations above:

  • Applying a gentle pressure (or suction) on a pour over would allow us to grind a little bit finer without astringency, and therefore reach higher extraction yields, more particle size uniformity and better brews overall. I think this is only true up to a point, because if you apply too much pressure or grind too fine, then you need to care about puck preparation like for espresso.
  • Using James Hoffman’s continuous pour method rather than the two-pour method might produce more evenly extracted brews, because it eliminates a moment of slow water flow between the two pours where less water in the V60 is providing downward pressure. This is completely independent of temperature stability.
  • Using a warmer slurry temperature will make water less viscous, which will make it flow faster and therefore more uniformly.
  • Using too much water and cutting off the brew at the desired beverage mass may allow us to eliminate that final moment of slow water flow, and further improve extraction uniformity.
  • Using many pours will produce a less uniform extraction unless you compensate with a coarser grind setting. This is doubly true not only because less water in the V60 will be pressing down on the coffee bed, but also because the slurry temperature will be lower and water will be more viscous.

As you can imagine, I’ll now definitely try James Hoffman’s pour over method, and I will also investigate whether cutting off a brew produces a better coffee ! I’ll also pay a lot more attention to my V60 dripping rate and the coffee bed resistance that I calculate for my brews.

14 Replies to “Extraction Uniformity and Channeling”

  1. That’s an amazing post!
    I always thought very confusing to talk about “channeling” on a v60 that’s full of water and has only a small amount of coffee at the bottom. Your explanation makes much more sense, and the terminology you suggested “flow uniformity” seems to capture the phenomenon way better.

    Like

  2. Great post! How much more water will you be adding? I sort of started doing this while ago cos I didnt always like those last few drops when I tasted them separately so I made a conclusion that they may sometimes ruin my brews. So now I go with 1:17 plus (only) 2 grams more and remove the V60 right after water hits coffe bed level and I see first naked ground. With more water it would be difficult for me to recognize the right moment when to remove the V60 with my standard one scale setup tho.

    I am also thinking nowadays whether I should dose 19,2g or 20,8g with my Comandante cos with bigger dose and coarser grind there are less fines but also less grind uniformity. I know that you dont have a direct experience with Comandante but what would you recommend in general?

    Like

    1. Oh I would probably add much more water and use a second scale plus a drip stand to remove the V60 when I reach my desired beverage weight. I used to have the Lido 3 grinder and I still did 22 grams, but more often 1:16 rather than 1:17. I never tried smaller doses with Lido, but in my limited experience the recipe I use doesn’t play that well with small dose, they seem to require less agitation.

      Like

    1. I’m not sure I understand the question; if you’re asking how pressure can be applied on pour over, one method is to create a vacuum under the coffee bed with e.g. a vacuum pump and a Buchner funnel.

      Like

      1. Yes i mean that, sorry for unclear question. So the prrssure applied still need a buchner funnel (which is not practical i think for regular home brewer). How about turbulence from kettle flow ? While its disturb the coffee bed how its affect the “water flow uniformity” ?

        Like

      2. Turbulence from the kettle will help extraction and temporarily make the coffee bed shallower, which results in (1) faster flow and (2) less filtration during that short time. Having a taller water column on top of the coffee bed (even if you pour gently with eg a Melodrip and close to the coffee bed, but fast) is also a way to get more flow without necessarily having more agitation.

        Like

  3. That’s exactly what I used to do in my graduate school days in the 1980s. I ground my coffee with a huge mortar & pestle (so not very uniform!) and then used a Buchner funnel and a filter flask powered by running water (do they still have those?) 🙂

    Nice post! There is nothing more powerful than the literature and a good brain. I later learned that a sign you are getting old is when you look at the references of a paper and instantly know that the citation is not the original work and you do indeed remember the original work. 🙂

    Finally – I love how you took a very common name and created a unique name from it!

    Like

  4. Jonathan, so very interesting. Thank you for the insight. I sent this article to my mechanical engineering friend (Chris) who also enjoys coffee brewing. He had a thought. What about lightly vibrating the grounds mechanically during brewing (or at least during the bloom phase)? This might close the larger channels and move particles to more positions in the flow path, causing greater (or more even) extraction. My friend told me this is how he uniformly mixed nanoparticles when he was doing research at Washington University. Interesting thought.

    Like

  5. On a side note, this seems to have a big impact in brewing with an Aeropress.

    Do you think the pressure applied on an Aeropress is sufficient to impact the flow uniformity and EY, and also to classify it as a percolation method (or at least immersion + percolation)?

    Like

    1. I think it has an impact for Aeropress yes. A friend or mine (Alex Levitt) says that pressing more gently also makes the brews less turbid, I have yet to verify that, but the same flow vs EY uniformity considerations should apply for Aeropress. However, the first immersion stage means that less extraction must be occurring during the percolation phase, so the effect of constant flow on evenness might be less dramatic than the case of a V60. I would classify Aeropress as immersion + percolation

      Like

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s