Coffee Data Science
Same Starting Drum Temperature for Coffee Roasting
Consistency in Batch-to-Batch
Previously, I looked at taste differences across a few batches of the same coffee with the same profile. I wanted to repeat this experiment with a particular emphasis on the same starting drum temperature. I wanted to start the roast at the same drum temperature to see how charge affected the roast. Within this test, there was still some variability on roast color.
The target was for the drum to be 138 C at the start of the roast. All roasts were done on the Roest coffee roaster.
Post-Roast Metrics
There was some variation in total roast time (6:00, 6:08, 6:22, and 6:37), but all were dropped 1 minute after the first crack.
This led to some variations in the post-roast metrics.
There is more variation in moisture than I would have expected.
Water activity is off, but I think this trend may be related to roast order. It seems water activity is low right after a roast but then normalizes to some higher level closer to the green bean water activity level.
Density also had a lot of variation for how I thought it should have turned out.
The same variation showed up in the color. The range is nearly 8 points, which should be noticeable in tasting.
Tasting Equipment/Technique
Espresso Machine: Decent Espresso Machine, Thermal Pre-infusion
Coffee: Home Roasted Coffee, medium (First Crack + 1 Minute)
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).
Data
I pulled four shots per roast, and I pulled each series of shots across all roasts within a day of each other. None of these saw a particular improvement.
Another look by roast didn’t suggest otherwise. Shot #1 on Roast #3 seems like an outlier.
There was some variation in EY, but I’m not sure that’s connected to any changes in taste.
Overall, this experiment ended similar to a previous test I did for quality control to see how variations batch to batch affect taste. I suspect starting drum temperature is not super important in the initial charge of the roast in terms of being at an exact temperature every time. Another experiment with slight variations in starting drum temperature would help examine the question.
If you like, follow me on Twitter, YouTube, and Instagram where I post videos of espresso shots on different machines and espresso related stuff. You can also find me on LinkedIn. You can also follow me on Medium and Subscribe.
Further readings of mine:
My Second Book: Advanced Espresso
My First Book: Engineering Better Espresso
