Coffee Data Science
Silicone Rings to Reduce Side Channeling in Espresso?
This exploration did not intend to go as far as it did, but here we are. I explored adding a silicone ring on top of my puck. I found through some experimentation (on this test coffee) that a silicone ring reduced side channeling and increased extraction efficiency. I also found some good examples to show that the water column initially develops vertically with little horizontal spread until the basket is pressurized.
The second discovery was more helpful than the first, and this all started because of the Blue Kim Express.
I bought a blue colored Kim Express two years ago with the intention of taking it to Italy and leaving it with family so that I had a good machine while I was there. I ended up buying another red Kim Express from Germany that was shipped to Italy last year. However, I decided to bring the blue one since it was 220V.
In preparation, I attempted to replace the shower screen with my IMS center cut shower screen, but it didn’t fit.
It was just a little too small, and this one had screws holding the screen in place.
So I thought maybe I could get the same effect cutting a silicone ring to the same size.
However, this caused major center channeling as you can see below. What’s amazing is that there wasn’t any side channeling. That suggests that water water flow over the top of the puck was blocked by the silicone ring.
Part of this center channeling is caused by my restricted grouphead, but I guess some water travels across the shower screen on the puck. Above my shower screen, I had installed this thick silicone ring to restrict flow to the middle of the shower screen. There is still some movement horizontally across the shower screen, but not much.
So I tested a few sizes of donuts and some weird stuff. I wanted to push the limits.
I ended up with a kit of rings in part because they were easy to cut to the right shape.
I also tried a bunch of weird shapes.
These shapes ended up with a spiral.
The spiral as well as these other shapes confirmed this phenomena where water came straight down through the coffee. I would have expected the small gaps of the spiral to be filled in by the time it got to the bottom, but that was not the case.
I did another test with a smaller paper filter just to test, but the result wasn’t too interesting.
Coffee Grinder: Zerno
Coffee: Home Roasted Coffee, medium (First Crack + 1 Minute)
Shot Preparation: Staccato Tamped
Pre-infusion: Long, ~25 seconds
Filter Basket: 20 Wafo Spirit
Metrics of Performance
I focused on metrics for coffee extraction.
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).
Intensity Radius (IR) is defined as the radius from the origin on a control chart for TDS vs EY, so IR = sqrt( TDS² + EY²). This metric helps normalize shot performance across output yield or brew ratio.
I plot these test shots here as part of the feasibility discussion. It appears the widest ring performed the best. This ring barely covered the edge of the filter basket.
EY and IR had similar conclusions.
More testing is needed on bigger data sets, with more coffees, and with taste as a metric.
This data helps explain the flow of water through coffee. It also shows that side channeling is the result of water rushing along the top of the puck to the sides, and this reinforces the idea of using minimal headspace for espresso. The beauty of this ring is that a silicone ring could be added in the design of espresso machines with relative ease. So while a home enthusiast can run these experiments, the design is straightforward to build in if it is found to be useful.
I’m collected more data on other coffees to understand how well a silicone ring can improve espresso.
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