Coffee Data Science
Thermal Pre-infusion for Espresso: Say Less
A deeper dive into thermals
Previously, I found reducing the temperature differential between the coffee and brew water improved extraction and evenness of extraction. I did this by locking in the portafilter with the puck for a few minutes before the shot to apply thermal pre-infusion. This brings the temperature of the coffee closer to the grouphead and the brew water.
To better understand why this works, let’s take a look at a few experiments.
Previous Data
Another group did temperature investigation of the top and bottom of the puck. The coffee in-between these two show the range of temperatures. This means that the extraction efficiency will vary. Even the starting temperature shows how much the coffee on top warms up by the grouphead. Coffee is starting to exit the filter basket at 4 seconds.
By the end of the shot, the bottom coffee only got up to 70C. So if the aim is 85C, the brew temperature falls far below that goal.
Temperature Data
I collected some temperature data for the puck to see how quickly the coffee warms up.
I looked at three points:
- Center Top
- Bottom Center
- Bottom Side
I checked every minute for the first 5 minutes, then every 5 minutes. The grouphead was set at 83C, and measuring water in the blind basket came in at 68C.
The coffee temperature at the bottom center trailed the side which shows how the thermal energy went down the side of the filter basket faster than through it.
I was surprised that the temperature had a limit, and at a certain point, the metal basket was emitting heat to the environment. So even when pulling a shot, the thermal energy is affected by the outside environment and the basket material.
Pressure Experiments
I also looked at the show profile on a flat flow of 1 ml/s to see how pressure is affected. Pressure stays low around 1 bar after thermal soaking, but otherwise, it rises to 2.5 bars.
Pressure is primarily a function of the gas being released, so something is going on inside the puck that is not well known.
To study this, I tried heating and cooling:
- Lock in the portafilter
- Heat soaked it for 30 minutes
- Removed the portafilter
- Let cool to room temperature
- Pull the shot
The pressure of this test shot didn’t go back up to the baseline, which indicates that simply heating up the coffee degasses it.
In terms of extraction, this test shot performs between the baseline and the 30 minute thermal soak.
Flow Experiments
I did a short flat flow experiment with 5 minutes of thermal pre-infusion, and the slower the flow, the higher the extraction.
These experiments helped me better understand how much thermal heat and evenness impacts extraction. This is primarily driven by the coffee having a more even temperature as well as degassing the coffee, which will help the extraction process.
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