Coffee Data Science

Fan Speed Exploration for Coffee Roasting

Another day, another variable

Robert McKeon Aloe
4 min readApr 19, 2024

Today’s variable for coffee roasting is fan speed. The fan and the drum RPM control how well the hot air is mixed into the beans. To get some intuition on fan speed, I looked at a few fans speeds as a percentage of the maximum fan speed.


I used Scott Rao’s roast profile on the Roest that uses bean temperature as turning points for inlet temperature. This is my baseline profile.

Roasting Data

The bean temperatures all followed a similar path, but a lower fan speed took a little longer.

The Rate of Rise (RoR) followed the same curve, but the peak of the curve was flattened out a bit.

The development could be best seen with the first crack, but the total number of cracks were similar.

Post-Roast Metrics

The weight loss wasn’t affected much by fan speed.

Moisture also didn’t see variation.

Roast density had a small variation.

Roast color also had a little variation, but I’m not sure if it has a big effect. Possibly between 50% and 75% fan power, the difference might be noticeable.

I used these data points to make three roasts for taste testing.

Tasting Equipment/Technique

Espresso Machine: Decent Espresso Machine, Thermal Pre-infusion

Coffee Grinder: Zerno

Coffee: Home Roasted Coffee, medium (First Crack + 1 Minute)

Pre-infusion: Long, ~25 seconds, 30 second ramp bloom, 0.5 ml/s flow during infusion

Filter Basket: 20 Wafo Spirit

Other Equipment: Acaia Pyxis Scale, DiFluid R2 TDS Meter

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 roasted three roasts of the Colombia test coffee, and I pulled three shots on each roast.

I didn’t notice a taste difference across fan speeds. These roasts may taste differently if brewed as something other than espresso.

The same was true for EY, not much variation.

Fan speed is an interesting variable, and I suspect when combined with other variables, it could be helpful. I didn’t see much difference in taste, but hopefully there is more nuance in fan speed that I could learn.

If you like, follow me on Twitter, YouTube, and Instagram where I post videos of espresso shots on different machines and espresso related stuff. You can also find me on LinkedIn. You can also follow me on Medium and Subscribe.

Further readings of mine:

My Second Book: Advanced Espresso

My First Book: Engineering Better Espresso

My Links

Collection of Espresso Articles

A Collection of Work and School Stories



Robert McKeon Aloe

I’m in love with my Wife, my Kids, Espresso, Data Science, tomatoes, cooking, engineering, talking, family, Paris, and Italy, not necessarily in that order.