More about the process...
The Aillio Bullet I use to roast coffee has a few features that help make those of limited skill (that would be me!) appear to be smarter than they are. As I mentioned before, the 4 determining sensory inputs that guide the roast are sound (1C and 2C), sight (mostly color), smell (which can be illusory) and temperature.
One of the reasons I wanted to buy the Bullet is that it has what I think is a good temperature measuring system. The Bullet provides...
- Bean Temperature from a thermocouple
- Drum Temperature from an infrared sensor
Bean Temp (smooth green line at left) originates from a thermocouple probe located in what the designer hopes is the center of the bean mass as the beans tumble inside the drum. The user can defeat that carefully chosen geometric location simply by choosing a different batch size than what was intended. As a user I don't know if my typical batch size of 550 gm of green beans causes any errors or not, but being consistent from batch to batch should cancel those errors (I hope!).
Infrared (IR) temperature sensing (red line to the left; named Drum Temp) was thought to be the new best thing in coffee roasting. The reality is that IR data has a number of artifacts that relate to batch size, bean color & texture, drum speed, sampling rate and electrical noise in the measuring circuits. Also, the temperature calculation relies on the emission characteristics of beans that are changing color and texture, both of which affect emissivity correction factors. Still, it provides thermal emission data in the infrared light range, that data can be converted to temperature without having contact with the beans. In reality the IR sensor seems to get contaminated with 'stuff' and that 'stuff' blocks some of the IR emissions which then causes an error in calculating Drum Temp (temperature of either the drum or beans depending if it's before or after beans have been added).
Rate of Rise (RoR) is the ragged green line above and displays the rate of change of Bean Temp. In mathematical terms it's approximately the first derivative of Bean Temp if that's in your vocabulary. It tells the user how fast Bean Temp is changing. If you can see the RoR curve, Bean Temp is rising.
In the Bullet, the Drum Temp data (IR data) is really important as it's used to determine the temperature of the drum at the start of the roast. The user selects the Pre-heat temp and the Bullet firmware controls the inductive heaters for the drum to reach that set point. Bean Temp gets used too, but only to the extent it must be stable. More about that later.
With the Bullet, roasting amounts to
- selecting a Pre-heat temp, then
- let the roaster warm up to that temperature
- add the green beans at the appropriate time (the Bullet has an audio cue for that)
- listen and watch for the sound and sight cues as well as the displayed temperature
- adjust the power, fan and drum speed to manage the temperature as the beans roast
- open the exit door at the proper time to eject the beans
- start the fan in the cooling tray
Presto! A fresh batch of home roasted coffee.
Of course the devil is in the details. There's timing involved as well as knowing the temps to expect for 1C & 2C and when a given variety of bean needs to be ejected in order to get something special that the coffee variety offers.
Coffee fresh out of the roaster may not taste all that great. Just-roasted coffee needs to 'rest' for 24-48 hours or even longer. Most coffee snobs (I'm probably in that group!) want the beans refrigerated while they rest. And experience with a certain variety of bean may dictate that as much as 5 or 6 days are needed.
Then there are those roasts that can't be saved no matter how long they sit around! Those beans need to be remembered so you never make that mistake again. To do that, each roaster has to adopt a scoring system for the roasted coffee. I use a 3-point system for rating my results-
- +1 = pretty good
- 0 = drink it anyway
- -1 = yuk!
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