Coffee Data Science
Searching for the Worst Coffee Roast
Part 2: 175 C Hold
A few months ago, I made a stupid roast by putting beans into a sourdough loaf, and they turned out to be the worst coffee I have ever had. As a contrapositive to finding optimizations for the best coffee roast, I am also very curious to understand why this roast was so bad.
The first experiment is to hold the beans at 175 C for 14 minutes compared to the baseline profile that took 8 minutes and ended at 212 C. This roast was pretty bad, but it didn’t make me want to quit coffee.
Out of curiosity, I re-roasted the coffee, and I found it wasn’t too bad.
Post-Roast Metrics
These metrics are wildly different because 175 C Hold is unconventional as well as a double roast.
In terms of coffee color, they were all close. What is weird is that the 175 C Hold color is the same as the re-roast, which means the color development on the outside was unaffected by the re-roast. I should have measured coffee color of the coffee grounds, but I forgot to.
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 only pulled one shot for 175 C Hold because it was so bad. The difference between that shot and the baseline was clearly not noise, so then I re-roasted using the same profile as the baseline which ended up close in taste.
EY was unaffected in all these cases. I thought it might have been affected by the 175 C Hold.
This experiment is strange because the bean color was developed in the 175 C Hold, but the full taste development happed a bit too close in the re-roast compared to the baseline. I think this experiment is a clue into better insights for coffee roasting and helps to promote the idea that coffee bean color is to coffee roasting as TDS is to coffee brewing. Both give hints for the right direction but tell an incomplete story.
More testing is necessary.
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Further readings of mine:
My Second Book: Advanced Espresso
My First Book: Engineering Better Espresso
