Turning Point

We need more POWER, Cap’n!

11 December 2021

What I’m doing: Ten roasts - 0018-0027 (Honduras)

What I’m drinking: Stage Light Roasting Kenyan Gatomboya PB as a V60; really interesting sweetness and a slightly acidic finish

I’m continuing to research and study my way through better coffee roasting and stumbled across an interesting article by Scott Rao. (For those unfamiliar with Scott - he’s a master coffee roaster and coffee consultant who has literally written the book on coffee roasting.)

When you plot those numbers, it looks something like this kind of exponential curve.

I’m not great at math, but based on Rao’s numbers, my little 300g drum ought to be spinning at 136rpm….. Which is wicked fast.

When I first started looking into the Kaldi roaster, I chose it in part because of how simple it is - bare metal, a cheap thermometer, and a small motor. Because it’s so stripped down, it’s very easy to modify the roaster to accommodate additional tools, monitors, and expansions.

On day one, for example, I removed the cheap thermometer that came standard and replaced it with a digital bbq thermometer which is more responsive and allows me to record two temperatures simultaneously.

The second modification I made was to use a 3-24V adjustable DC power supply. This allows me to adjust my drum speed by decreasing and increasing the voltage supplied to the motor. Since starting this roasting journey, my voltage was set at 10V or approximately 60rpm (based on my unscientific "beans-and-a-stopwatch" technique). But after reading Rao’s article, I decided to increase the voltage to 17V or approximately 100rpm.

This little change had significant impacts. My TP shifted right to about 1:51 (from 1:25 at 10V). My yellowing and browning phases shifted left though to about 4:45 and 6:34 (vs.5:05 and 6:47 at 10V). The roasts haven’t degassed, so I’m not sure about these impacts on the flavor profile, but I think this is worth additional investigation.*

I wonder if this is because there was an increase in convection heat as the beans tumbled more frequently.

But I’m curious if you’ve experienced this phenomenon. Have you played with the drum speed of your roasts? What did you find? Email me your experiences. And if you’ve tried one of these roasts (0018-0027) let me know what you thought!

*Humidity was also a factor for these roasts at about 73% (vs 40% last week).