Coffee Data Science

A Possible Explanation for Paper Filters in Espresso

A short experiment for understanding

Robert McKeon Aloe
3 min readSep 30, 2022

While progressing in experiments for steam pre-infusion, I started to think about paper filters. Paper filters have become popular additions for espresso enthusiast in the past few years. They have been shown to allow someone to grind finer and extract more coffee in the same volume. I think I have found an unlikely reason why they function well: steam pre-infusion.

I did my fair share of testing with paper filters and eventually, I found cloth filters worked the best in performance but the worst in terms of preparation. They allowed an incredible flow, but they were difficult to cut, hard to get the right size, and then they shrunk over time. At one point in the summer of 2021, I decided to take a break from using them to run a few experiments on simple variable changes.

In the past few months, I have discovered steam pre-infusion was the key reason behind my Kim Express lever machine outperforming my Decent Espresso (DE) machine. As I explored how to do steam pre-infusion on the DE, I noticed some common effects as paper filters.

Theory: a paper filter helps trap steam in the bottom of the basket as hot water flows into the basket, and this causes steam pre-infusion.

To test this theory, I used some spent coffee grounds. I wanted to see how water flow and steam flow would occur with and without a paper filter. I ran two shots with the same profile which used 10 seconds of steam pre-infusion and 10 seconds of pre-infusion. The beginning of the shot was the most important.

Water came out using the paper filter in the first 10 seconds, and the flow was higher overall. Usually, as pre-infusion starts, steam is pushed out into the cup, fogging the cup. This happened for the paper filter, but a lot of the steam in the first 10 seconds condensed into water.

Looking at the data charts, the flow for both was about the same, but for the paper filter, the pressure increased as did the temperature. Both indicate steam being trapped in the filter basket.

The caveat in this discussion is that not every uses super hot water. Some people use water below 90C especially in the case of Flair or Robot which has the water temperature drop as you first fill the machine.

There is potential that paper filters could help cause steam pre-infusion, but if that’s their main benefit, then we can instead run shots using steam pre-infusion to get the same extraction benefits. I will definitely try paper filters once I finalize my steam pre-infusion profile, but I seem to be in unknown territory at the moment.

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Further readings of mine:

My Future Book

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Collection of Espresso Articles

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Robert McKeon Aloe

I’m in love with my Wife, my Kids, Espresso, Data Science, tomatoes, cooking, engineering, talking, family, Paris, and Italy, not necessarily in that order.