Published: May 17, 2019

Espresso With Aeropress Filter Papers

Above and below the coffee bed

This week, Scott Rao made espresso with filter papers above and below the coffee bed yielding over 25%. What’s he playing at? If you follow Scott Rao’s series of daily coffee tips on Instagram, you’ll have seen one recent post where he describes using paper filters cut to fit the espresso basket, placed above and below the puck. By doing this, he is able to reduce channeling, and push extractions to 25% and beyond, and get tasty espressos even at these high extraction percentages.

‘Not only does each filter seem to increase extraction, but the top filter seems to mitigate channeling — a lot,’ Scott writes. ‘This is a very big deal.’

While people have already been experimenting with Aeropress filters in espresso baskets for some years, it seems there’s something pretty interesting happening if it’s possible to achieve such high extractions without ‘over-extraction’ flavours. So what part of this approach is new, and why do paper filters have such a big effect on espresso? And perhaps most importantly, why don’t these shots taste over-extracted?


How Does This Work?

First – let’s look at what we already knew. If you’ve ever experimented with putting a paper filter inside your espresso basket, you might have been surprised to find the shots run considerably faster, even though you’re putting an extra layer in the way of the flow. During an espresso shot, coffee particles, especially fines, ‘migrate’ – moving with the flow of liquid towards the bottom of the puck. In a traditional espresso basket, these particles partially block the holes in the bottom, slowing the flow. With the paper filter at the base, the particles can’t pass through to reach and block the holes, so the flow is faster. The leap here is remembering that a faster flow allows you to grind finer, giving higher extraction. In espresso, any method that allows higher extraction is effectively giving you more even extraction, which leads to better flavour and more sweetness, and allows you to use less coffee to achieve the same strength.

What we at BH hadn’t seen before is the step of adding a paper filter above the puck as well, which further increases extraction. “As with many things in coffee, we know it works, but the “why” is mostly speculative,” Scott says. “I assume the filter on top helps the dispersion of water on top of the puck… and perhaps helps keep the top of the puck smoother and more intact.” This would be similar to the reason cold drip towers often use a paper filter on top of the coffee bed, to prevent the drips carving a channel through the bed.


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What About Over-Extraction?

When combined with other methods to increase extraction, Scott is making espressos at even higher extractions – over 27% – and with that, approaching the limit of coffee’s solubility. Yet he claims that the shots taste better than ever, without the bitter and dry flavours we’re taught to associate with over-extraction. How is this possible?

The answer is all down to channelling. Those dry and bitter flavours are due to tannins and other large molecules, which can only dissolve in areas of extremely high localised extraction — in other words along the path of a channel. Over to Professor Steven Abbott to explain why this is:

“There’s a law of physics called the no slip boundary condition which means that the velocity of a liquid flowing past a surface is exactly zero at the surface itself. This means that the only way for molecules to escape from the surface is via diffusion, and for large molecules like the bitter tannins this is very slow – a good thing because we don’t want them. The only way for these molecules to escape is to get turbulent flow coming as close as possible to the no slip boundary – and the only way to do this is via the Forcheimer, rather than Darcy flows mentioned in the previous post. Channelling is a sort of Matthew effect – the more flow starts down one path, the more turbulent it gets and the more it flows, creating more turbulence and dragging out more tannins. So anything which reduces channelling will help reduce the bitter extraction.”

In other words, it’s not just over-extraction:  tannins and other large molecules need the turbulent flow created by channels to extract into the coffee. Because our tongues are so sensitive to bitterness and dryness, it doesn’t take much of these molecules to ruin an espresso. However, if we’re able to reduce channeling, then we can push extraction higher before these flavours start to become apparent. “‘Over-extraction’ is an overused term,’ Scott explains. ‘These days I never use the term except to describe localized over-extraction due to channeling.’

The SCAA recommended range of 18-22% was based on established equipment and techniques, and stood for many years. There’s been a steady trend however in recent years towards aiming for higher extractions — enabled by better equipment, better roasting, and better barista techniques. This began with the use of better grinders like the EK43, allowing extractions at perhaps 24% before over-extraction flavours became apparent, but has continued since as more techniques to promote even extraction have emerged.

“Mind you, not all extractions over 26% taste great,” Scott says. “I see this is a new frontier where step one is to raise the ceiling on the extraction level and step two is to begin optimizing coffee at these higher extractions.”

As to how to keep raising that ceiling? We at BH have been discussing an idea with Professor Abbott that could take this a step further: “A proposal for a drastic solution to the channelling problem is to have such a loose bed that there is nothing to channel, with the hot water free to just extract all the good stuff,” he explains. “But what about back pressure? First, why do we need it? Partly because without it the CO2 that explodes out of the beans can get in the way of the water (which is why we bloom open beds of coffee). And partly to get the crema that we all love. So let’s get the pressure via a different means – a filter. Not the usual “easy flow” filters we are used to. But the very fine filters used routinely in the bio-world for filtering out, say, bacteria. A look at the data sheets suggests that with a 9 bar pressure, the flows would deliver a shot in similar times to regular espresso. And by controlling the flow independently of the coffee, we are free to, for example, over-grind the coffee with plenty of fines. But won’t fines block the filter? We might call them fines, but to a bio-grade filter, these are huge boulders, the filters won’t even know they are there. That’s the theory. All we now need is to test it.”


  1. leonardo.delgado

    Late to the party here, I just cut my Aeropress paper filter to fit my VST basket and pull a shot for the very first time (only at the bottom, though). I have been using the same Ethiopian beans for a week, regularly pulling 37g shots at ~25s. This morning using the paper filter (and same prep process and inputs) I got the same amount of espresso but at ~31s. I honestly was expecting it to be a longer shot, even before I pulled it, because in my mind adding the filter obstructs flow. So I was a bit surprised to read in this blog that the opposite was supposed to happen due to “fines obstructing the basket holes”. Maybe a combination of both should happens? Adding the paper filter a) reduces flow due to the added extra layer of porous material and at the same time b) favors increasing it due to preventing fines obstructing the basket holes. But I think a>b. Any thoughts? I will keep pulling shots with paper filter to see if I stay around the ~31s mark.

    • BHLearn

      Hi Leonardo, thanks for commenting. That’s interesting, please do share if you get more results. Can I ask what type of machine and grinder you’re using? I wonder if it’s something specific to the type of pressure/flow profile your machine generates? Perhaps we should test this with a wider variety of machines, although we’ve seen this result replicated many times.

  2. sunneuro

    What do you do when you have a inconsistent grind? Your brew pressure is fluctuating because of fines getting stuck in the hole of filter basket? Answer: use a Aeropress filter paper below the coffee bed in you porta basket. 🙂

    I got my Bartza Encore malfunction due to problem with burr holder. I could not get the spare parts due to international shipping restrictions in US. So I had to resort to using the blade grinder which off course gave inconsistent grind with lot more fines. It resulted in my brew pressure fluctuating a lot between 8-14 towards the middle of the shot and making the flow very restricted some times almost stopping. Suddenly, my life felt like hell. I searched and found the problem could be that the fines may have been blocking the holes. So I simply had to find a way to stop the fines from getting the holes blocked. I just stumbled upon the idea to use Aeropress filter papers for that purpose since I had read long ago some people using double Aeropress filters below and above the fine grind in the Aeropress to get a stronger coffee – very much like espresso. For the first time after trying shots with inconsistent grind, using this method made the brew pressure quite consistent throughout. Although I do have to try more shots to validate it. But it seems Scot and some others have already used the method. Therefore I am very much hopeful that this method would certainly solve my problem of the fluctuating brewing pressure due to inconsistent grind (fines blocking the holes). And I guess it’s also a great news for those who may have to use blade grinders for some reasons that gives a lot more fines with inconsistent grind. Now I am back to the heaven of decent espresso shots at my home. 🙂

  3. Jeremy

    If the filter is the main source of pressure drop, rather than the bed, aren’t you just describing a pressurized portafilter?

  4. myleswilliammmj

    Im curious, what type of filters do i need to buy to be able to do this? And you need one for the bottom and top? Does the size matter to fit a specific portafilter?

  5. Jekabs

    I don’t have an espresso machine, so I will attempt this with my Bialetti stove top.

    • Debs

      Hello Jekabs, it is not strictly necessary to own any coffee equipment to complete our courses. But of course it’s great to have some equipment to practice on. You might find it fun to read through some user reviews websites like or (it’s not just for home baristas.)
      At BH we have an online ‘support’ forum where any users can ask us questions about the course. Just check the ‘support’ menu on the website. 
      All the best, 

  6. tth.hientranthu

    May I ask how do you perform this correctly? Do you just place the dry paper filter under and on top of the coffee bed?

    • BHLearn

      That’s right tth.hientranthu.

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