Coffee Data Science

Drum vs Fan: Coffee Roasting

Heat variants

4 min read5 days ago

Coffee roasting can be done using conduction, convection, or radiation. The Roest uses conduction and convection by using a fan to blow hot air on a rotating drum. The drum provides conduction by touching the beans, and the fan provided convection heating to both the beans and the drum. Let’s separate some heat elements to better understand them.

I made four roasts to try to explore the options:

Baseline
Fan only (high fan speed, low drum speed)
Drum only (low fan speed, high drum speed)
Drum & Fan Max (DF Max): maximum setting for both

Due to a mistake, I dropped the drum only roast too soon, but it had not quite hit the first crack, so I threw it back in and continued immediately. So there is a slight caveat.

For the Rate of Rise (RoR), the curves were similar except for Fan Only, which matches similar tests with low drum RPM.

The bean cracks happened at reasonable times and didn’t say much.

Post Roast Metrics

These roasts looked very close to one another except for Drum Only. It lost more weight.

The pattern was similar for moisture.

Water activity is still unknown, but I keep including it until I better understand it’s use post-roast.

Drum only was less dense.

Not surprisingly, drum only was darker but just slightly. I would normally expect a loss in density and weight to cause a much darker roast.

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).

The Shots

I pulled four shots per roast, and as usual, these are just feasibility shots. It’s hard to make a great deal of sense unless all of one roast are much better than the others. Looking at shot by shot, it seems unclear.

If we re-sort by roast, Drum/Fan Max seem to be around the baseline if not slightly better. It’s a 2–1–1 split in favor of Drum/Fan Max. However, fan and drum only seemed to have underperformed.

In terms of EY, each set of shots were similar. There was some variation in dose between the sets of shots which caused some changes in EY.

This experiment makes me want to explore using the maximum setting for the Drum and Fan. Does a high level of mixing allow heat to be better distributed?

Drum only seemed closer to baked, and that might have been due to the mistake. It might need retested, but my suspicion is that a higher drum RPM would benefit from a high fan RPM. With a low fan speed, the air pressure in the chamber should increase, which might also be impact the roast in some ways.

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