Coffee Data Science
Initial RPM Exploration for Coffee Roasting
Spin it faster or slower?
I looked at how fast the drum spins, and in this small study, I’m searching for signal on how to optimize drum RPM. Previous studies of mine suggested a higher RPM would be interesting, so I did a little bit more with a lower RPM closer to what I use now as a contrapositive:
- 45 RPM
- 55 RPM (baseline)
- 65 RPM
My theory is that if going to a higher RPM is better then going to a lower RPM should be worse.
Post-Roast Metrics
The roasts were allowed to go 1 minute past the first crack, but that resulted in some variance. So the 65 RPM was lighter. This trend in metrics was most notable for the Indian coffee.
More data should aim for the same ending coffee color, which I have been using bean temperature to help regulate.
Metrics of Performance
I used two sets of 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).
Shot Data
Looking over the data, 65 RPM seems to be an improvement.
This is a little clearer with re-ordering the data.
A scatter plot helps especially to establish that 45 RPM decreases performance.
There wasn’t much change for EY.
The data suggests 45 RPM is not good, and this agrees with the previous data showing 20 RPM was not good. 65 RPM might be better than 55 RPM, but with the caveat that the roast color was lighter for 65 RPM on the Indian roast. So more data is required with a closer match on coffee color.
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Further readings of mine:
My Second Book: Advanced Espresso
My First Book: Engineering Better Espresso
