Coffee Data Science
Expanding a Double Roasting Coffee Dataset
An accident turned into an experiment
Occasionally, I have dropped a roast too early. I don’t like waste, so I always double roasted in those scenarios. I found the double roast did alright with respect to taste. I don’t think double roasting is efficient or something to do regularly, but there is something to learn from it. So I pulled a few more roasts trying to get the color to match closer between the baseline and the double roast.
For these double roasts, I dropped the coffee at 200 C, let the coffee cool for two minutes, and put it back into the roaster. Usually, the end bean temperature is around 212 C.
Post-Roast Metrics
As expected, the double roast was drier, but the density did not have a huge impact.
Moisture was not so useful. I was able to get a coffee color within a few points of the baseline.
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).
Intensity Radius (IR) is defined as the radius from the origin on a control chart for TDS vs EY, so IR = sqrt( TDS² + EY²). This metric helps normalize shot performance across output yield or brew ratio.
Shots
I pulled 14 shots across 4 roasts pairings. I missed two shot pairs on the Colombia roast because of a shot error.
The taste comparison seemed inconclusive.
Across components, the double roast didn’t have a positive or negative impact.
For TDS and EY, there was a clearer pattern which was a decrease in performance.
Statistically, using a two-tail paired t-test, only TDS/EY/IR had significant differences. However, taste did not have a statistically meaningful result.
Even though double roasting didn’t improve taste, it didn’t make taste worse. So it is reasonable to double roast when you drop a roast too soon. This is valuable to a home roaster like me who doesn’t want to waste coffee.
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Further readings of mine:
My Second Book: Advanced Espresso
My First Book: Engineering Better Espresso
