Coffee Data Science
Metal Ball Bearings in Coffee Roasting Drum
Another impractical variant of coffee roasting
I have been curious about how coffee beans retain heat and lose heat, and I was thinking about what role the drum played in gaining and losing heat plays into coffee roasting. So I added metal ball bearings into the roast to see how the roast would change.
I added 82 g of ball bearings that are about the size of the coffee beans. The batch size of beans was 120 g.
The experiment ended up with three variants:
- Baseline
- Cold ball bearings added to the green beans before the roast
- Ball bearings added to the roaster before the green beans. Those ball bearings are heated to the charge temperature of the roaster.
Post-Roast Metrics
The roasts were different across the metrics. The roasts were ended at 212 C bean temperature, but the hot metal ball bearings went a bit darker (lower Agtron color).
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 pairs of shots, but only included the hot ball bearings for the first two. It was clear by the taste that the hot ball bearings were too developed for a good comparison.
For the cold ball bearings, there was not much of a difference to the baseline, positive or negative.
EY didn’t show many differences.
I was looking for a catalyst that could be easily removed and reused during the roast, but the metal ball bearings were not what I was looking for. They also showed that they did not have much of an effect. This could simply be due to batch size, metal ball size, or the roaster.
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Further readings of mine:
My Second Book: Advanced Espresso
My First Book: Engineering Better Espresso
