Coffee Data Science

Variable RPM for Coffee Roasting: Feasibility

Surfing Drum RPM

3 min readSep 9, 2025

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Previous data suggested 65 RPM was better than 55 RPM as a global setting for my roast profile. I wanted to better understand if different phases of the roast would benefit from a different drum RPM.

I made four roasts:

Baseline at 65 RPM
45 RPM for Dry phase only (bean temperature < 150 C)
45 RPM for Maillard phase only (150 C < bean temperature < 200 C)
45 RPM for Development phase only (bean temperature > 200 C)

Post-Roast Metrics

All the roasts ended close to one another in their bean metrics.

The coffee color was close enough even though they were all dropped at 212 C.

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

I pulled four shot pairs for feasibility. The 45 RPM Dry was slightly worse, but both the 45 RPM Maillard and Development roasts outperformed the baseline.

Another way to look at the data.

EY was not affected by these changes.

These roasts showed more could be learned by decreasing the drum RPM at different phases. From this feasibility experiment, I’m going to do two more feasibility experiments:

This experiment repeated with 25 RPM vs 45 RPM.
45 RPM for Maillard only, Development only, and both Maillard and Development.

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