Coffee Data Science
Side Channeling in Espresso: Thermal Impacts
Examining a theory for side channeling
When discussing channeling, side channeling is the first thing that comes to my mind. For a long time, I thought side channeling was caused mainly by water distribution from lever machines and E61 machines which pushes water to the sides of the basket. This water dispersion causes water to start going through the coffee on the sides sooner than the center. Maybe there was some coffee packing issue with how water flowed against metal vs coffee. However, one variable I hadn’t considered was thermal energy.
Over the course of random shots, I started tracking the temperature of the basket because on a few occasions, I left the portafilter in the machine with the coffee and the flow changed to come from the center more. It was hard to pinpoint why, but when thinking about the dynamics of the basket, thermal energy seems to be key.
Theory: side channeling is partially caused by the higher heat conductivity of a metal basket compared to the coffee.
If the side of the basket heats up faster, then it will heat the coffee nearest to the sides before the inside of the puck. Previous data shows coffee solubles extract faster at higher temperatures, so then with a higher grounds temperature, the rate of extraction on the sides of a puck will be higher than the center of the puck.
To better understand these thermal effects, I collected data with a thermal camera (FLIR). I am aware that thermal imaging off of shiny metal doesn’t work as well as other surfaces. For espresso baskets, it seemed to work well enough considering I was looking for temperature differences relative not absolute.
First Test: Portafilter
I tested the original instance that caused me to think about these variables by locking a portafilter into the machine and waited. I filled it with spent coffee grounds, and then I took a timelapse video.
The filter basket eventually heated to a more even temperature, but over the span of a few minutes, you could see the inside of the basket took longer to heat because of the coffee grounds.
I took an image of the top of coffee puck after pulling it out of the portafilter:
Even the heat had a donut effect.
Second Test: Moka with Filter Basket on Top
The trouble with a metal basket is that thermal cameras don’t work very well off of metal. I wanted to see what was happening to the coffee. So I put a basket of coffee on the bottom of a Moka. I put some tape on one side to help with the thermal imaging.
Then I captured a video. There was a slight unevenness that caused steam to go up one side on the top left of the basket. This caused an island of heat to expand from that side.
Extra Credit
This basket contained spent coffee, and at the time, the shot used to produced these spent grounds was around 20% EY. So there were still some solubles in the coffee. I decided to test how steam affected the location of the solubles. Just to note, I remixed the spent coffee before preparing the basket, so I assume an even distribution of solubles in the grounds.
I took cores and measured the grounds TDS (gTDS).
I was surprised to find the solubles seemed to be pushed to the top of the puck. I have seen this effect in steam going through a coffee puck.
This study provides initial evidence that the thermal properties of the coffee basket compared to the coffee is one variable that plays into side channeling. I’m not sure how to adjust accordingly, but it is hard to solve a problem without identifying the variable in the first place.
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