Coffee Data Science

High Input Fan Speed for Coffee Roasting

Continued testing

4 min read1 day ago

The Roest coffee roaster has two fans: inlet and exhaust. The inlet fan is held at a constant rate in the settings at 3400 RPM. You can adjust this setting. The exhaust fan speed is set by a user’s profile. One can create positive pressure inside the roast chamber by increasing the inlet fan (to 4400 RPM) and decreasing the exhaust fan. Some initial tests were interesting, so I did more testing in this area across a few roasts.

I used three beans:

Kenya
Tanzania
Nicaragua

The Kenya bean had a different baseline profile, but I then ran the Slow Maillard profile I used as a baseline for the other two roasts.

Coffee Roast Metrics

There was a lot of variation for the end weight loss.

The moisture seemed off for only the Kenya Slow Maillard.

For roast color, I saw a mix of colors, and I was particularly concerned that the high internal fan roasts had a 9 to 10 point color drop, which I would have thought to bias the taste data. This is foreshadowing.

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

This dataset started with just the Kenya roast. The results seem mixed on which tasted better.

Even in a re-sorting of the columns, there doesn’t seem to be a clear winner.

So I roasted the Tanzania and Nicaragua beans to double check. Now with 13 pairs of data, I could plot this taste data for the final score as a scatter plot to look for a pattern.

Baseline refers to the Slow Maillard roast

The results seemed murky, which was similar for TDS/EY.

When looking at the average individual taste components, the high internal fan was slightly higher.

From a statistical level, none of the differences were statistically significant.

While the taste differences aren’t statistically significant, the color differences are pretty large. The high internal fan roasts were 10 points lower in color which means they were darker roasts. They also all were 1% lower in roasted weight divided by green weight. Even though they were more developed, they maintained flavor.

The next experiment in this domain should be to try to get similar colors which seems to be ending the roast a little early for the high internal fan. All roasts had a 1 minute post first crack development time.

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