Coffee Data Science
Revising Dialing in Coffee
Another look at optimal grinding
I received a Zerno grinder to do some coffee grinder testing, and they sent two extra burr sets. My main aim has been to understand which burr set provides better espresso extraction, if any. But I ran into something unexpected. So I did a side quest to understand an unexpected result in my experiments, and I was pleasantly surprised by the data.
In an initial experiment across burr sets, I noticed something interesting with adjusting the grind: multiple local maximums for peak extraction yield. This means that depending on where you start dialing in coffee and depending on the ratio, you may hit a local maximum that may not be the global maximum.
Dialing-in refers to adjusting the grinder to get peak performance. For many decades, this was measured in shot time and taste. Over the past decade, extraction yield has played a larger role in determining optimal performance. The assumption being that wherever one starts, they should end up at the optimal grind setting using Newton’s method. This means the optimal grind should be found in O(log(n)) time.
Metrics
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).
Experimental Design
My original experiment involved 3 grind settings across 3 burr sets, and I pulled salami shots measuring extraction yield. I measured extraction yield at the 1:1, 1.5:1, and 2:1. I didn’t go higher because I was using thermal pre-infusion, so the shots were already extracting very high.
The first thing I noticed is that at 100um (Zerno has grind settings labeled as the burr gap in microns), extraction was not optimal for the 1:1 ratio, but for the 2:1, it could have been. This phenomena occurred for all three burrs, so I focused on the SSP Espresso burrs.
This finding bothered me because I would assume the best dial-in point should be the same regardless of ratio.
So I took a two more grind settings. These results left me with more questions.
So I took 4 more grind settings, and what appeared was not expected.
For the First Cup, which is the 1:1 shot, there is a clear global maximum at 225um. However, for the second (1.5:1) and third cup (2:1), the cumulative EY had multiple peaks. It appears the best setting had a wide variance for longer rations.
Here is another way to view this data:
The EY for 1.5:1 ratio across 75um to 200um is between 24% and 25%, which makes it very unclear where the best setting is. This is the same for the 2:1 ratio.
I don’t know how the results would differ without thermal pre-infusion, but there are two take-aways for me:
- Dialing-in greatly depends on output ratio
- Dialing-in should be done based on the 1:1, a shorter shot than what you normally pull, or using a salami shot because it is more sensitive to grind setting changes.
These results don’t take into account taste, but I suspect there is a blurring of taste when you aim for a longer ratio.
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