Coffee Data Science
RoR Gap in BT/IT Coffee Roast Profiles
Aiming for optimizations
Traditional coffee roasting is done using a inlet temperature profile. As sensors have improved, bean temperature sensors were added to better understand how bean temperature was affected during the roast. This has led to a new kind of roast profile that modifies inlet temperature (IT) based on the bean temperature (BT): BT/IT profiles.
The BT/IT profile does a better job controlling the Rate of Rise (RoR) than a regular profile on the Roest because there is a relationship between the two temperatures. So what if we aim to change the gap and have a tighter control over RoR? If RoR is too low, than the bean temperature won’t rise enough to get to the next step in the BT/IT profiles, but what exactly is too low?
Beans are either absorbing or losing heat. The way coffee beans absorb heat is still being studied because the heat transfer inside the bean is still being understood.
Roast Profile
My baseline profile is Scott Rao’s roast profile on the Roest that uses bean temperature as turning points for inlet temperature. This is my baseline profile where the inlet temperature is around 120 C higher than the bean temperature.
Test Coffee
For the first test, I used a test coffee. The Inlet Temperature (IT) was set at a number of degrees above the Bean Temperature (BT). 50C higher means the inlet temperature is 50C higher than the current bean temperature. This profile was different than the baseline because the baseline has a curve to taper at the end, and these profiles were direct temperature gap that stayed the same through the end.
I also looked at coffee for tasting, which I will discuss afterwards.
Higher temperature differences completed their development much faster, which is expected.
For the Rate of Rise (RoR), the higher temperatures had a higher starting peak.
We can look at the RoR at a few points in time that overlap. Having a RoR closer to 5 makes the roast take a long time. Part of this challenge is that each step in the BT/IT profile relies on the bean temperature to hit a point before the inlet temperature is increased.
As the temperature gap decreases, the number of loud cracks also decreases.
Tasting Coffee
I looked back at the Baseline, and I offset the baseline profile by -20C and -40C to get closer to these profiles above but not so slow.
The profiles were pretty similar, but the Rate of Rise (RoR) was more affected than anything as well as the length of the roast.
This was more apparent in terms of the number of audible bean cracks.
Post-Roast Metrics
There was more weight loss for the tasting, but it was the opposite for the test coffee.
Moisture didn’t show so much except in the test at 50C higher.
Roast color was very close for the test, but it showed the 50C higher was a lighter roast which could explain the higher moisture content.
Density wasn’t so helpful, but it does mean that for the tasting coffees, the differences are not easily observable.
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).
Shot Data
I looked at three shots for each of the tasting coffees, and I’m not looking for statistical significance. My main aim is to look for some signal or hint, and if there is a major difference, explore that pathway more.
In this case, the -20C tasted better on average, but the -40C did not until the last shot.
The coffees all had similar EY’s, which is also a good constant variable.
These tests showed a small amount of evidence that moving to a smaller RoR difference in a BT/IT profile may improve taste. I will explore this variable again in some other experiments related to thermal pulsing.
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Further readings of mine:
My Second Book: Advanced Espresso
My First Book: Engineering Better Espresso
