BT RoR guidance for popcorn poppers and small fluid bed roasters? - Page 3

Discuss roast levels and profiles for espresso, equipment for roasting coffee.

#21: Post by Trjelenc »

GDM528 wrote:In case a drum roaster pilot is reading this thread: how important is RoR during the drying phase?
For drum roasters, I've been going off of the guidance that the minute details of the drying phase don't really matter as long as you're not doing anything to induce roasting defects onto the coffee, such as having drum too hot and scorching the beans. Allegedly no discernable flavor reactions occur prior to the color change to yellow. So the idea is to run the RoR at dry in a way that helps set up the rest of the roast. For example, if you're trying to go quicker through the middle phase, you might not want to start off with a slow momentum through to yellow.

minus1psi (original poster)

#22: Post by minus1psi (original poster) »

This may or may not be forward progress, but it is at least an incremental improvement in knowledge.

Following insights from @GDM528 and @Trjelenc I reviewed graphs of old roasts and came to the theory that the rapid drop in RoR always has an inflection point where the curve becomes more horizontal. Roasts with higher temperatures of that inflection point naturally had more vertical room to decline without flattening out.

I am pursuing the idea that the drying phase is less critical. Raising the temperature of the inflection point without worrying about the shape of the BT RoR curve pre-dry end is the current theory. I am trying to get the inflection point to happen right around the end of the drying phase. The hope is that at DE the inflection point of the BT RoR has been increased and has more room to decline gracefully.

I tried a couple of other approaches but the only way to bump the voltage just before DE (G/Y) was to have headroom at that moment. The voltage (effectively the heat) was increased when BT was at 300°F. For my typical roast DE happens closer to 320°F. It sort of worked. I chose to approach DE at 110V and increase to 120v at around 300°F. I do not love that the middle phase is so short.

On the graph below the RoR curve is less flat than in many previous roasts. This roast was allowed to go for a long time; I like for a portion of cold brew beans to go well into 2C and regard those roasts as excellent opportunities for this sort of experiment. 25% DTR would have been pretty close to the end of 1C.

minus1psi (original poster)

#23: Post by minus1psi (original poster) »

This is progress. I increased the sampling interval from 3s to 4s, reduced the drying phase voltage, and futzed with curve settings a little bit. This is looking more manageable. Maybe the software changes are cheating in some sense but the information on the graph feels like a good reflection of the mechanical changes during the roast. The target phase percentages were 40%/35%/25%. In this case the beans roasted through to 33%DTR based on color. Weight loss was 16.7%; in retrospect I could have pulled them earlier.

Current questions:
  • How low can the voltage (heat) go for the drying phase while maintaining a drying phase percentage of 40% or less
  • What factors influence the timing of the BT RoR inflection point -- how does the roast move it left or right on the x-axis
  • If the nuanced shape of the BT RoR curve under the drying phase doesn't make much difference, how much does the length of it matter -- essentially the length of the drying phase. The horizontal length of the RoR curve is a component of it's shape, right


#24: Post by GDM528 »

An interesting property of a fluid-bed roaster with aggressive airflow, is the consistency of the shape of the temperature readings. Empty chamber / full chamber, whatever - the curve is essentially the same. Chart below shows the temperature setpoints and the temperature readings in the chamber with and without a load of greens (thermocouple located in the spinning bean mass when loaded).

The RoR line is from the beans-in-chamber readings - but could've been from the setpoints and still look the same. I was able to prototype the RoR curve on a spreadsheet to get the initial profile, and it only took a couple tweaks to account for thermal inertia. Key to avoiding sudden drops in the RoR curve was making uniform adjustments to the temperature rise. In my case I could only make six adjustments, which was barely enough.

An essential component to my process was constant airflow. Changing both airflow and temperature is much more complex, so perhaps the constant-radius principle doesn't apply. Would've been handy if the Wearever had separate power cords for the fan and heater ;)

FWIW I don't use the RoR profile because it spends so much time below caramelization temperatures (320F-ish). I theorize that over-dries the greens before they hit the caramelization/Mailliard phase. As precedence, I cite drum roasters that are preheated prior to dropping in the greens, and even the Pro version of my roaster does a similar pre-heat. Lately I've been running drying phase down to about 15%.