Coffee Data Science
Pre-Infusion for Espresso
The calm before the storm
I didn’t realize how spoiled I was with a lever machine until I found out pump machines didn’t pre-infuse, at least for the cheaper ones. Pre-infusion is a key parameter that allows one to grind finer for espresso because it helps reduce channeling.
For those who don’t know, pre-infusion is letting water into the coffee puck at a lower pressure than normal. Typically, pre-infusion pressure is between 0.5 and 4 bars. 1 to 2 bars is typical.
I didn’t know much about pre-infusion when I started regularly using the Kim Express, but I ended up in a routine with 10 seconds of pre-infusion. I started exploring this variable a bit more in the past two years as I found a longer pre-infusion led to a better tasting shot.
For this article, I used three pre-infusion parameters to show how they affect the shot. This is not a recommendation for a specific parameter but rather for people to explore variables to find what works best for them. For all three shots, they were pulled on a Kim Express with the same coffee roast, same grinder, and same puck preparation. All the shots used pressure pulsing during infusion.
Each column of images is one parameter while each row is 5 seconds of time with the pre-infusion times of:
30 seconds ……………….20 seconds………………………..10 seconds.
The first thing that is clear is that the lower pre-infusion shots produce much more crema. Let’s check out some metrics of performance.
Metrics of Performance
I use two metrics for evaluating the differences between techniques: Final Score and Coffee Extraction.
Final score is the average of a scorecard of 7 metrics (Sharp, Rich, Syrup, Sweet, Sour, Bitter, and Aftertaste). These scores were subjective, of course, but they were calibrated to my tastes and helped me improve my shots. There is some variation in the scores. My aim was to be consistent for each metric, but some times the granularity was difficult.
Total Dissolved Solids (TDS) is measured using a refractometer, and this number combined with the output weight of the shot and the input weight of the coffee is used to determine the percentage of coffee extracted into the cup, called Extraction Yield (EY).
Performance
For TDS and EY, a shorter pre-infusion time wins, but oddly enough, the taste improves only a little with a longer pre-infusion. The biggest changes are sweetness increasing and syrup (or mouthfeel) decreasing.
Time metrics were largely driven by pre-infusion time because infusion time remains roughly the same.
A shorter pre-infusion time creates more crema. This can be seen in the time it takes to get to a particular volume.
We can plot them together to see a general trend.
I’m not sure what that says about crema. Often people think crema is a good qualitative metric of espresso performance, but most of my shots have very little crema due to a long-pre-infusion.
While I prefer a longer pre-infusion, the data suggests there is still room to explore. Usually, my pre-infusion time is tied to Time to Cover Filter (TCF) because I have previously found the best pre-infusion time to be around 3*TCF.
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 and Subscribe.
Further readings of mine:
Collection of Espresso Articles
A Collection of Work and School Stories
Measuring Coffee Grind Particle Distribution using Image Processing
A Summary of the Staccato Lifestyle
Measuring Coffee Grind Distribution
Espresso Baskets and Related Topics
