converting a stovetop popcorn popper for coffee roasting

When I began roasting coffee experimentally I considered the two standard low-end devices: a hot-air popcorn popper and a hand-cranked stovetop popcorn popper. I couldn't find a suitable model of the former in a nearby store or thrift shop, and I also liked the idea of being a little more hands-on, so I went with the latter.

My first attempt was with an aluminum Whirley-Pop Stovetop Popcorn Popper. I won't go into the modification details, other than that I followed Kenneth Davids' instructions in Home Coffee Roasting, because I don't think it's worth bothering with. The agitators are flimsy, the bottom deforms constantly so the agitators get stuck, and worst of all the gears are plastic. It didn't take long before I was highly annoyed, and just a bit longer before it didn't work at all. Don't use one of these.

The Back to Basics Stainless Steel Stove-Top Popper was a little more expensive but of far higher quality. There are two basic modifications you must make: install a thermometer, and replace the plastic viewing window. The tee-nut method described at Sweet Maria's is perfect. I think this is a much sturdier technique than using a spring clip, which I found unsatisfying on the Whirley-Pop. I used this Update International 550 thermometer.

The popper has a plastic viewing window that I imagine is nice for popcorn but which is rapidly rendered opaque when roasting coffee beans, so it's functionally useless. The first time I roasted, this window began to sag a bit. The second time, while the popper was pre-heating (perhaps for a few minutes longer than necessary), it melted entirely leaving a mess more difficult to remove than Tyrone Willingham.

I had on hand some thin galvanized sheet steel, which was readily snipped into the necessary semicircular shape. This must be folded over so that it can be held in place by the restraining tabs of the popper lid. One way to do this is to rest it on a workbench so that it hangs over the edge by the desired amount and gently tap it with a hammer to create a 90 degree bend. Flip the piece over and continue to tap from one side to the other until a fold is complete. Fit it to the lid, drill a hole through it for the screw that holds the wooden handle in place, and you're in business with a somewhat less transparent but significantly more heat-resistant viewing window.

I've now used the finished product for a couple of years and dozens of roasts with nary a hitch. I would highly recommend this approach if you're considering the stovetop method.