Coffee Data Science
Post-Roast Processing Experiment: Frig vs Counter?
An experiment on bean processing
Adding moisture to freshly roasted coffee has accelerated degassing, but I have concerns about long term flavor impacts. I often found the roasts where I add moisture, peak after 2 or 3 weeks rather than 4 to 5 weeks. So I did an experiment where I modified part of my process by using a refrigerator (frig) for part of it.
Previously, I follow these steps to add moisture:
- Roast coffee
- Add 4% weight in water
- Let sit for 12 hours or until absorbed.
- Put into vacuum chamber
- Re-vacuum 2x daily for 1 week
The new thought was along the lines of reducing the amount of flavor loss or at least maintaining temperature during the process. Putting the coffee in the frig would slow degassing, but it would also slow oxidation.
So between steps 4 and 5, I put the jar in the refrigerator (frig). The jar itself has a vacuum seal on it, so the low pressure is maintained.
Equipment/Technique
Espresso Machine: Decent Espresso Machine, Thermal Pre-infusion
Coffee: Home Roasted Coffee, medium (First Crack + 1 Minute)
Shot Preparation: Staccato Tamped
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).
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.
Data
I collected data over three roasts and 15 shot pairs. I didn’t see a change in shot performance in taste.
This was true for individual components too.
Broken down by roast, each roast saw similar performance between both.
This result is confirmed by TDS, EY, and IR.
Even zoomed in, there seems to be a slight trend on EY and TDS.
At a statistical level, the decrease in EY is statistically significant, but none of the other changes are statistically significant.
It is possible that more data points would have shown something different, but the trend was towards a decrease in performance. While this experiment didn’t find an improvement, it answered the question as I try to refine my post-processing of roasted coffee.
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Further readings of mine:
My Second Book: Advanced Espresso
My First Book: Engineering Better Espresso
