Restraining Flow to Mitigate Channels – Coffee ad Astra

In the past weeks, I have been experimenting with the AeroPress combined with the Prismo attachment, and I tried one small hack that produced a surprising result. I inserted a pasta strainer mesh like the one I described in my Stagg [X] recipe to increase the total open surface area under the AeroPress filter. As shown on the photo below, placing a paper filter directly on top of the Prismo’s metal filter will only allow water to flow through the tiny and sparse holes of the metal filter, and therefore more pressure will be required to achieve the same drip rate.

The reason why I tried to increase the total free surface below the paper filter is simply to reduce the amount of pressure required to push the AeroPress plunger. As James Hoffmann described in his recent AeroPress video, more pressure usually leads to a coffee that tastes more astringent, which reduces the perceived complexity and sweetness of the beverage. It is possible that this could be explained by an increase of channels at the microscopic level when more pressure is applied — to my knowledge this has never been demonstrated very clearly, but so far this the best hypothesis to explain the results, and there are plausible physical mechanisms that could cause channels to occur when more pressure is applied (i.e., a faster localized flow of water that causes larger drag forces on the coffee particles).

In fact, I have often assumed that it was desirable to have as much free surface as possible below a paper filter, even with gravity driven brews, in order to get the fastest drip rate for a given pressure. As I discussed in a previous blog post, physics-based simulations showed that faster drip rates are expected to correspond to a more even flow of water through a percolation medium, everything else being equal. Applying the results of these simulations, however, requires us to assume that the coffee bed acts as an immovable percolation medium, which probably breaks down under high pressures.

When I tried brewing with the pasta strainer mesh placed between the Prismo metal filter and the AeroPress paper filter, I was surprised to find that the brews were noticeably more astringent. When this happened, I stored this observation in the “confusing things” drawer in my brain, and left it there. But more recently, something else happened that caused me to think about this again.

A small company called Del Creatives generously sent me a prototype of one of their inventions. It is basically a modified espresso basket with a thick metal filter that is inserted below the coffee bed in order to provide both more resistance and a truly immovable percolation medium. This allows us to brew coarsely ground and untamped coffee and produce a beverage that resembles filter coffee, with a few important differences.

Some parts of the prototype seem like temporary hacks, but I was immediately impressed by the device: the physics of it make a lot of sense to me, and the design seems to prevent any possible bypass of water around the metal mesh. They sent me 4 different filters with average pore sizes of 2, 5, 15 and 60 microns, and recommended that I use about 7 grams of untamped coffee with it.

Experimenting with this device has been quite eye-opening to me. I think this does more than transform the Decent DE1 espresso machine into a tiny and fast batch brewer; in fact, it also adds something valuable because it de-couples the extraction medium (the coffee) from the filtration medium (the thick metal filter). Navigating brew parameters with this device felt like navigating a new kind of beverage, because the brew defects that I encountered were completely different. Instead of encountering astringency when I ground too fine, I encountered rancidity when I brewed too hot (the DE1 can achieve ridiculously high slurry temperatures compared with pour over), and a really strong flavor and odor that reminds me of burnt rubber when the shots stayed at high pressures (i.e. not counting preinfusion and blooming) for more than about 30 seconds. This latter defect happened at very high extraction yields (in the 25-27% range) and only seemed to depend on how much time was spent at high pressure. This characteristic burnt rubber odor reminded me a lot of the what my kitchen smelled like and what my shots tasted like when I pulled blooming shots with 1:10 ratios to understand how the puck resistance evolves a few months back (some of my findings were discussed in this post).

This reminded me of a discussion I had with Zurich University coffee scientist Samo Smrke where he told me that very-long espresso shots might be able to extract some chemical compounds of the coffee bean (arabinogalactans and galactomannans) that are usually insoluble in water. These large chemical compounds can become soluble if they are broken down into smaller parts, which instant coffee manufacturers have often done by extracting with acids or using pressurized water at temperatures of 130—160°C, above the boiling point at atmospheric pressure.

I think what is happening with this device from Del Creatives has something to do with my earlier observations that liberating more surface area under the AeroPress filter caused more astringent beverages. The 2 micron filter that I used actually acts as a source of resistance under the coffee bed, as well as a filtration medium. I suspect that this limits the maximum flow rate that can take place in any pores between the coffee particles, which will reduce the rate at which channels may form, and perhaps more importantly, will prevent the local flow of water to become too fast where channels form. I think this is why all my brews with the Del Creatives filter had quite high extractions yields (about 24-27%) and none of them had much detectable astringency.

When I got a hold of what caused these brew defects specific to this device, I finally landed on some recipes that tasted really great, even at surprisingly high extraction yields. This is actually the first method by which I have really enjoyed brews in the 25.5-26.5% range of average extraction yields. The beverages that it generates can easily reach 3-4% TDS concentrations (which I now love without dilution), with intense sweetness, acidity and very discernable origin characteristics. I think this is really impressive for a brew method that takes barely a minute and requires no intervention from the barista, and this tastes so nice to me that it may become my daily driver (although I’d love it if it was able to brew larger doses).

Here’s my current recipe with the Del Creatives basket, I expect this to keep evolving:

• I use 7—8 grams of untamped coffee with a grind size slightly coarser than espresso. As a reference point, I pull espresso at about 5.5 on my EG-1 with SSP Ultra Low Fines burrs, and allongés at 6.5. I can hear my burrs start touching when the grinder is running at 8.0 or finer, and my burrs fully lock at around 3.0. My V60 brews would be located around 14.5 (which would read 4.5 after a full turn of the EG-1 dial).
• I grind directly in the Del Creatives basket and I use a WDT tool to even out the coffee bed.
• I place a Flair 58 Puck Screen on top of the basket to make the water distribution as even as possible because I have noticed my DE1 shower screen is not always very even.
• I use a profile that I built on the DE1 with the following steps: (1) a fast preinfusion with 16—20 grams of water at 95°C; (2) a 23-seconds phase with a slow refilling of 90°C water at 0.5 grams per second to compensate for some water dripping; (3) a 30-seconds blooming phase without flow; and (4) a smooth transition to a 1.0 mL/s flow of 90°C water until about a beverage weight of about 73 grams has been reached. I initially use hot water because the cooler coffee bed will immediately lower the temperature of the incoming water, and then I stick with 90°C water because I have found hotter temperatures to produce a rancid taste even with light roasts.
• I place a Stagg [X] Dripper on top of my coffee glass with an AeroPress filter inside of it to filter out a bit of the coffee oils, because the Del Creatives basket is not conveniently sized to add a paper filter to it.

You can find an example DE1 profile file here, or use the screenshots below to build it yourself:

One example of a typical brew that I really enjoyed with this recipe was with 8.0 grams of Heart’s Honduran Extreberto Caceres (a washed Pacas and Catimor grown at 1750 m.a.s.l. in Santa Barbara), with a beverage weight of 71.0 grams, a concentration of 2.9% TDS and an average extraction yield of 25.7% (not syringe filtered). This is a ratio of about 1:9 if you speak espresso language (beverage weight over dose) or about 1:11.5 if you speak pour over language (total water weight over dose).

This is what this the graphs looked like for this particular beverage:

You can also explore this graph’s data here with Miha Rekar’s Decent shot visualizer.

As some of my supporters noted when we were discussing this on my Patreon Discord channel, you may expect that these Del Creatives shots are somewhat similar to Scott Rao’s famous Allongés, or the turbo shots described elsewhere (e.g. this paper and this video by Lance Hedrick) that also use coarser grounds but shorter ratios.

However, these Del Creatives shots do not necessarily rely on a fast drip rate, even if the coffee is ground coarser, and they taste distinctly different. I actually prefer them to turbo shots, and I usually prefer them to Allongés. Qualitatively, I found that turbo shots almost always have some level of astringency that annoy me a bit. I really enjoy well-dialed in Allongés, but I find it a bit annoying to have to dial them in separately from espresso shots, and they actually taste like a completely different beverage compared with the Del Creatives shots. The Allongés tend to have a bright, crisp acidity, and the Del creatives shots are thick, viscous with intense sweetness. The downside with Del creatives shots is that the burnt rubber taste can appear easily if the ratio is stretched just a bit too much.

Maybe more importantly for me, this device has added another layer to my understanding of the physics in filter coffee, which I didn’t expect would happen so early after having released my book on this topic. Namely, I now think that adding a source of resistance below a coffee bed can be beneficial to mitigate or even eliminate the impact of channels. This is particularly powerful when a pump is available, because it allows to de-couple the extracting medium from the filtration medium. I would certainly like to see more experimentation and data on this topic, but so far all my observations line up with this potential explanation.

These new observations also change what I would do if I were to design an “ideal” dripper; I would actually want it to have a valve to control the drip rate, but this source of resistance would have to be distributed uniformly across the bottom of the coffee bed, much like the small but evenly distributed pores of the Prismo metal filter. This might be a more difficult engineering challenge. Maybe two punctured circles that can rotate with respect to each other in order to gradually open or close the holes could achieve this.

I am also quite excited to see this basket from Del Creatives being fully released. It is currently a bit hard to clean thoroughly between shots (which is usually needed in its current form), and I would love to see it made to accommodate larger doses. I also found that the thick metal filters needed to be flushed very thoroughly before brewing with them (the water that came through was initially gray and smelled metallic), and it remains to be seen whether they will eventually clog after many shots (I have now done about 30 shots using the 2-micron filter without problems).

TL;DR

• I recently realized that purposely limiting the maximum drip rate by adding a source of resistance below a coffee bed can be desirable to limit the amount and the impact of channels.
• The source of resistance must not make the flow of water uneven through the coffee bed, so it can be made either of many small holes, or a single valve placed under a metal filter with even holes.
• A new prototype basket manufactured by Del Creatives matches this new understanding and also provides a thick metal filter that acts as an immovable porous medium below the coffee bed, which de-couples the extraction medium (coffee) from the filtration medium (the metal filter).
• Combining the Del Creatives basket with the DE1 espresso machine allowed me to brew some of my favorite coffee beverages so far, with very high concentrations (3–4% TDS) and high average extraction yields (25.5—26.5%). They taste juicy, vibrant and preserve origin characteristics.
• Pushing the ratio too far with these brews will bring a very specific awful taste that reminds me of burnt rubber, and does not appear to cause much astringency.

Added Note: Since I published this post on Patreon back on September 10, I have come to realize that the Del Creatives matrix filters eventually started clogging for me and required increased levels of pressure after about 50 shots. After talking with Omri at Del Creatives, I realized that cheap ultrasonic baths can be used to declog them (I used this exact model – not an Amazon Affiliates link). This device is also great to clean up the Flair 58 puck screen that can accumulate some oils over time.