Coffee Data Science
Espresso Pre-wetting as Opposed to Pre-infusion
Residuals of a transparent portafilter experiment on pre-infusion
I was experimenting with a transparent portafilter for espresso, and I published some results across a few experiments. In talking with Jonathan Gagné, he wondered if I could see a relationship between flow and time based on capillary action. So I applied my image processing skills to a video to help see if the relationship was visible, and it was observed!
Pre-wetting follows right in line with capillary action!
This is important because some times, we mistake pre-infusion for pre-wetting, and the result is that the puck is not fully wet which could result in a bad espresso shot. Pre-infusion is really the key to great espresso, so we ought to get that right every time.
Capillary Action
The general theory (see the heading “Liquid transport in porous media) is that when liquid being absorbed by a porous material, the rate of absorption decreases over time. So the vertical distance liquid has penetrated a volume of porous material is proportional to the square-root of the time the material has been in contact with the liquid.
Vertical Distance = constant*sqrt(time)
Experimental Setup
I used a make shift transparent portafilter (aka Kompresso), and I put some coffee in it. Initially, I poured hot water in, and I didn’t apply any pressure. The paper filter at the top reduced channeling based on flow input, so it acted like a shower screen.
I captured a video at 4K resolution at 60 FPS. In processing the video, I focused on one area of the video.
I then summed the pixels across each row. I aimed for the simplest, quickest way to compute the average coffee line. I figured if there was a relationship, I should have to do anything super fancy. I then applied a simple threshold.
From here, I was able to extract the average coffee row value per frame. I plotted it below, and then I plotted it against the square root of time.
When comparing to the square root of time, there is a very strong correlation which confirms the way water goes through a puck during pre-wetting follows capillary action predictions.
A theory is testable, and I quite enjoy seeing this theory on capillary movement explain why pre-wetting isn’t the same as pre-infusion. This is most important for machines like Flair or Kompresso where you pour the water in, and you can decide if you want to do just pre-wetting or a 1 to 2 bar pre-infusion.
This is particularly important to point out for new espresso users to help distinguish the much needed pre-infusion from the optional pre-wetting.
If you like, follow me on Twitter and YouTube where I post videos of espresso shots on different machines and espresso related stuff. You can also find me on LinkedIn. You can also follow me on Medium.
Further readings of mine:
Collection of Espresso Articles
A Collection of Work and School Stories
An Affordable and Simple Transparent Portfilter for Espresso
Fines don’t migrate during an Espresso Shot
The Diminishing Returns of Tamping for Espresso
Can Espresso be Turbo Charged?
Can Kompresso get 9 Bars of Pressure for Espresso?
Experiments with a Transparent Portafilter for Espresso
Deconstructed Coffee: Split Roasting, Grinding, and Layering for Better Espresso
Spicy Espresso: Grind Hot, Tamp Cold for Better Coffee
Staccato Espresso: Leveling Up Espresso
Staccato Tamping: Improving Espresso without a Sifter
