How a Paper Filter Below an Espresso Puck Affects Hydraulic Resistance

In one of my latest posts, I investigated the effect of puck preparation, and in particular the addition of a dry paper filter above the espresso puck, affects the hydraulic resistance of the system during an espresso shot. While I have not yet tested its effect on average extraction yield, I did not see an obvious effect of the top paper filter on shot repeatability, although it increased the hydraulic resistance by about 6% on average. This is a small effect, and is about the same as my shot-to-shot variations of 5% caused by my imperfect puck preparation if I exclude the significant outlier shots that happen 15–25% of the time.

One of the next logical steps was to test the effect of adding a paper below the puck, which was also popularized by Scott Rao a little while ago as a method to increase the average extraction yield of espresso.

[Edit: I initially said that Scott introduced the idea of using a bottom paper filter, but thanks to Robert McKeon Aloe for pointing out that others had been doing this a long time ago on Home-Barista. Mark J. Burness also pointed out that Sang Ho Park may have been the first person to use the technique. As far as I know, this idea had remained quite obscure until Scott talked about it on Instagram.]

While the paper filter on the top might help dispense the water more evenly across the puck and potentially prevent some structural damages from the impacting water, I believe that the role bottom paper filter is quite different. One of biggest revelations I had while working on my book The Physics of Filter Coffee was related to this: lifting a paper filter that sits directly on the bottom of a dripper is often a really good thing, because it liberates all of the filter’s pores for coffee to flow through more evenly, and then the fluid can flow through the exit holes very fast.

There is one subtlety here that had prevented me from fully appreciating this fact: the hydraulic resistance of a paper filter blocked everywhere except for a dripper’s exit holes is often way higher than the same system where you just lift the paper filter slightly. This is true because water can flow very fast at the center of an unobstructed dripper hole, whereas it will flow at the same velocity everywhere through the exit hole if a paper filter sits directly on it. This is why liberating the full surface of the paper filter, by lifting it, is what dominates the end result: once the paper filter is lifted, the exit holes of any dripper on the market really don’t offer much resistance at all.

Because of this consideration, my hypothesis for why the bottom paper filter appears to produce higher average extraction yields (as observed by Scott Rao, Stéphane Ribes and Socratic) is that it simply allows water to flow through more paths across the coffee puck. This means that there are probably less regions of the coffee puck that remain under extracted, and on top of that, the lower overall hydraulic resistance that this results in should allow one to grind slightly finer and gain a bit of accessible surface of coffee particles to extract solubles more quickly.

This is what I set out to test with a small experiment. I pulled 10 shots with a new batch of the washed Mas Morenos Honduras coffee roasted by my friend Andy Kyres (owner of Color/Full Coffee Corp), the same green coffee I used in my last experiment. The coffee was roasted on 2020 December 12, and I opened the sealed two-pounds bag on the day of the experiment, on 2020 December 22. I decided to pull 10 shots, alternating between the use of a paper filter at the bottom only versus no filter at all. In this experiment, I also did not measure average extraction yields to maximize the number of shots I pulled in the short amount of time I had.

Yet again, the reason why I alternated between the two methods is to minimize the effect of the espresso machine or grinder getting gradually warmer, or my puck preparation slowly changing. I used the Niche Zero on grind size 13.0 (at factory zero-point) for this experiment, with the DE1 Decent Espresso machine’s “Best Pressure Profile”, much like last time. I also used the same ground distribution method, and opted for the “deep WDT” puck preparation because it allowed me to achieve more repeatable results in my last experiments. I used Levercraft’s WDT tool in its default configuration, the Force tamper at its default pressure setting, Cafelat Robot 58mm paper filter, and I recommend reading my last blog post if you would like to get more details about any of these considerations; it also includes videos of my puck preparation routine. Yet again, I pulled 3 shots before starting the experiment to ensure everything was warm enough.

It is interesting to note that I needed to grind 1.0 dial finer on the Niche compared to the last batch of the same green coffee. This is probably related to either differences of aging, or slightly different roast profiles. Because I consider Andy very good at replicating roasts, I would favor the hypothesis of either the fact that the coffee had been more freshly roasted, or that the green aged more which could have changed the bean moisture and how it shatters, i.e. how many fines it generates, when ground.

I pre-wetted the bottom paper filter by flushing the DE1 into the dry filter, and then carefully pressed on its edges with my finger to get them to stick properly, taking care not to displace the filter. I placed the creped side of the filters up and toward the coffee puck, because I want to maximize the surface of contact between the coffee particles and paper filter to get as much of an even flow as I can. I placed a video of this here.

One of the first things that became immediately obvious during this experiment is how the use of a bottom paper filter completely fixed the issue I was discussing in my last experiment where my spent pucks had a slight hollow near the center. All five brews without a paper filter still clearly showed this central hollow at the center of the spent puck, while none of those with the bottom paper filter did.

There is something about this initial observation that I found really surprising. The fact that the top filter did not fix the hollows, but the bottom paper filter did, leaves me with only two hypotheses to explain it, and both surprise me. The first hypothesis is that there is really almost no flow of water far from the center of the puck unless you use a paper filter at the bottom. The second one is that some coffee particles are able to pass through the portafilter holes near the center of the puck, even with the Decent Espresso baskets which have even basket hole sizes compared to other manufacturers (except VST baskets which are also very even).

A recent experiment carried by Stéphane Ribes on the Decent Diaspora forum makes me think the first hypothesis is more likely. Stéphane had the ingenious idea of cutting out spent espresso pucks and measuring how much solubles were left on the edges versus center with a subsequent immersion in clean water. His experiment clearly demonstrated that the outer edges of espresso pucks are under extracted when no paper filter is placed under the puck.

All of these observations point in the same direction, as Stéphane already noted way before me: current espresso baskets do not seem optimal at all for even extractions, because the basket holes do not extend close enough toward the edges of the basket. I suspect there are engineering reasons for that; such baskets may be too fragile to sustain high pressures for very long, and may break more easily. If this is the case, then using disposable paper filters may still be the best solution for more-even, home espresso, even though this is definitely not a great option for heavy use in a cafe.

Now, let’s shift our focus to what I actually intended to measure during this experiment: how the hydraulic resistance of my espresso shots were affected by the use of a paper filter at the bottom of the puck. Below, I shot DE1 graphs of the 5 shots without paper filters, followed by the 5 shots in which I used a paper filter at the bottom.

Once again, these graphs contain a lot of information, which I explained in great detail in my last post. One important point I want to mention again is how I calculated the puck resistance; the DE1 usually displays them as the pressure drop (green curve) divided by the square of the DE1-estimated flow rate of water at the shower head (blue curve). This is actually an estimate of the square of the puck resistance, from which the changes in bed depth and porosity versus pressure (due to puck compression) are removed. It is useful to remove these effects because they are both reversible, and this allows you to only see how other variables like grind size, fines migration, and channels, may affect your puck resistance. Note that, as I also detailed in my last post, I believe that the initial rise in puck resistance is still due to un-corrected effects in puck compression, and the subsequent fall in most profiles that don’t have a blooming phase (as is the case here) are due to the puck gradually becoming fully saturated with water.

In the graph above and in all of the remainder of this post, I am showing the square root of the pressure curve divided by the flow, to obtain the puck resistance not squared. This makes it easier to talk about puck resistance and relate it to bed depth and other variables as per Darcy’s law. As a reminder, the orange resistance curve is calculated similarly, but using the output weight of espresso measured by the Acaia Lunar scale which I connected on the DE1 using Bluetooth. Once again, I slightly modified all resistance curves by less than 1% to account for small variations in my exact doses (all shots here have doses between 17.8 and 18.0 grams).

It is quite clear in the figures above how adding a paper filter at the bottom of the coffee puck decreased the hydraulic conductivity, making the water flow faster and the shots faster to reach similar beverage weights. In the figure below, I show only the (shower head flow-based) resistance curves compared with each other:

This reduced hydraulic resistance really fits well with the results of Stéphane’s experiment discussed above, and the observation that the use of a bottom paper filter completely removes the problem of hollows at the center of spent pucks. To quantify this a bit further, here’s a comparison of the peak values of these resistance curves, as well as the hydraulic resistances near the end of the shots (when the beverage weights reach 40.0 grams).

As shown above, the bottom paper filter reduced the hydraulic resistance by a significant factor 1.9 ± 0.2, i.e. almost reduced it by half. The standard deviations as well as median absolute deviations of both samples were also reduced when using a paper filter at the bottom, but I believe the more relevant quantity is the fractional variation in resistance, not the absolute variation. If this is what we look at, both samples have a standard deviation of about 18% versus 19% of the average hydraulic resistance, which I believe is not significant here.

Now, let’s look at similar graphs but for the stabilized hydraulic resistance, using either the DE1-estimated values based on flow rate at the shower head, and those using the output drip rates as measured by the Acaia Lunar scale.

In my last experiment, I explained how I think that the peak value of the resistance curves is particularly sensitive to preinfusion because it happens when the coffee puck has not yet been entirely saturated with water. As a consequence, I think that looking at the hydraulic resistance near the end of a shot is a better indicator of what is going on. The hydraulic resistance values calculated from the Acaia Lunar scale are also probably more accurate, because the DE1 flow rates at the shower head are estimated based on a complicated physical model of the machine that depends on many factors such as the properties of the electrical grid the machine is used with.

Therefore, I think the most informative graph is the one showing the stable scale hydraulic resistance (the last one above). This graph shows that using a paper filter at the bottom of the puck decreased the stable hydraulic resistance by a factor 1.43 ± 0.04, and possibly reduced the shot-to-shot variation slightly: I’m getting variations of 4 ± 1 % with the paper filter and 5 ± 1 % without it. This is similar to my previous experiment, and probably not a significant difference between the two samples. I do not think the dramatically reduced median absolute deviation (blue bars) is particularly informative in the “no paper” case because of the small sample with three tightly grouped data points.

In other words, using a paper filter at the bottom of the coffee puck did not affect the variability in peak resistance much, but it significantly reduced the hydraulic resistance by about 43 ± 4%. I find this quantification really interesting, because we can compare it to the surface coverage of holes in the Decent baskets. The hole pattern of the Decent baskets have an outer diameter of about 50 mm, 8 mm smaller than the full 58 mm diameter of the basket. Therefore, if the decreased hydraulic resistance is only caused by the hole pattern not reaching the edges of the basket, we would expect a change in resistance of only (50/58)² = 35%. The value that I found, 43 ± 4%, is a bit larger than this, and might suggest that even within the central pattern of basket holes, flow might not be perfectly even because of the spacings between the holes. Adding a paper filter below the puck may therefore make the flow about 6% more even even within the central region covered by basket holes, although this number is quite imprecise.

As a result of this experiment, I will definitely be using a paper filter at the bottom of my puck more often. It is a bit more trouble, but I now believe it is really worth it. I plan to eventually measure the effect on average extraction yield myself, and I would love it if anyone could try assessing its effect with blind tasting. 

I also noted during this experiment that all shots with a paper filter at the bottom showed their first droplets of espresso in a ring shape at the bottom of the portafilter. Although this is not conclusive evidence, it may suggest that the overall flow was still not perfectly even, and that the addition of a paper filter may have over-compensated and allowed for a bit more flow than we want near the edges of the basket. If this is true, then we may benefit from using slightly smaller paper filters, perhaps something in the range 55–57 mm. I think that comparing the average extraction yields and resistance curves of shots taken with paper filters of different diameters may turn out to be very interesting, and we might find that there is an optimum filter diameter that is slightly smaller than the full basket size.

Another explanation for the outer ring of espresso appearing first under the basket could simply be related to the fact that all of the espresso near the edges has nowhere else to escape, and therefore pools at the outer basket holes, giving us a false impression that more fluid is flowing there. If this is the case, the 58mm paper filters will probably be optimal for an even extraction. If you go back to Stéphane’s slide above, this interpretation seems likely because the edges of the puck were very slightly under extracted even when he used a paper filter below the puck.

You can find the log of my shots here, as well as the DE1 shot files and the profile I used here.

Disclaimer: I receive no financial benefits from any of the companies mentioned above, and I have no business ties to them. Decent Espresso generously offered me a 25% discount on their DE1 machine, and Weber Workshops offered me a set of SSP Ultra-low-fines burrs and their glass cellars, without obligations or expectations. All my impressions of the gear that I use are my own and never financially motivated. The owner of color/full is a personal friend.

I would like to thank Johanna “Mimoja” Amélie Schander for having coded the required Bluetooth communication codes on the DE1 and making it possible to pair the DE1 with Acaia scales.