Coffee Data Science
Vacuum Freezing for Post-Roast Coffee Processing
More data on an interesting technique
Of the post-roast cooling techniques I investigated, one had an improvement on taste that I wanted to investigate further: vacuum freeze.
Vacuum freezing involves a vacuum chamber sitting in an ice bath. This cools the coffee as the vacuum pump pulls out air and hopefully CO2 gas. I added ice because as the pressure drops in the chamber, the temperature of the gases goes up, and I wanted the coffee to cool down.
This technique is not particularly simple, and I’m not sure it is something I would regularly do. However, I’m curious what it can teach about how beans cool and roast.
Post Roast Metrics
I added two more sets of beans to the original Honduras roast. There is some offset in the roast metrics.
However, the moisture was pretty close, and the roast color was very close. This is the most important for consistency.
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 12 paired shots, and the vacuum freeze roasts always did better.
The initial sharp taste component didn’t have an improvement, but the sweet, sour, and bitter had the larger improvements.
In terms of TDS/EY, they were about the same.
Statistically, using a two-tailed paired t-test, the differences in the taste metrics are significant.
While this technique is not something I would do all the time, one could for special roasts. Maybe this is something one does for special coffees or maybe a better system could be designed to make the process easier.
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Further readings of mine:
My Second Book: Advanced Espresso
My First Book: Engineering Better Espresso
