A problem I’ve had with my chaff collector hood is that it retains too much heat thus allowing the beans to heat up faster than they ought to. When I set the PID to 400 and let ‘er rip I get roasts that look like this:
Notice the mottled color, uneven roasting, and the fissures. The coffee is bitter with no depth.
Edit: I’ve recently learned that the effects of over-heating the beans is called “tipping”, because the small ends of the bean tend to get burned first.
Tonight I did a batch following a more gentle curve and got a much better-looking roast:
They already smell better than the bad roast, and they’re 10 minutes old. I managed to keep the temperature right inline with this recommended roast profile by setting each minute’s taget temperature at the start of the previous minute. For example if at 4 minutes I was suppoesd to be at 180 I would start adjusting my PID at 2:50 which takes about 10 seconds to increase by 20 degrees. Then over the next minute the PID will gradually bring it up to the correct temp. They key here is that when the PID is close to its target temperature it won’t go full power. If you’re 300 degrees away from your target it’s going to apply heat 100% of the time which apparently leads to crappy roasts like you see here.
Having an open top bread machine coffee roaster has two drawbacks, the first being that chaff flies all over your workspace with the second being there’s a lot of wasted heat. I made a lid/exhaust vent/chaff collector in an attempt to solve these problems.
My bread machine came with a lid for the bread pan which would be used for its rice cooking mode. I put two holes in this, one for the heat gun and the other for the exhaust pipe. The pipe is a piece of galvanized ducting rolled into a tube, secured with pop rivets. On one end I cut a series of slits to make tabs, these were used to pop-rivet the pipe onto the lid. The tube is 2.25″ in diameter.
For the flexible tubing I found some 3″ corrugated aluminum ducting at Lowe’s.
The tube/duct join needs some cleaning up! The end of the heat gun snout is slightly smaller that the rest so it nicely rests in the lid. You can also see how the plastic body of the heat gun is melted, this is from when I would simply stick the heat gun into the pan. With the lid in place the heat gun won’t melt anymore and there’s no chaff to get sucked into the gun!
Some of the chaff collected, about halfway through a roast. There was a lot more at the end.
The most obvious drawbacks of this setup is that one can’t see the beans and they’re more difficult to hear. I do plan on making a window on top with a small piece of tempered glass but the sound part is harder to tackle. 1st crack is audiable but I’m pretty sure 2nd crack is totally muffled.