Roasting coffee in a popcorn popper produces a large amount of smoke and should only be done in a well ventilated space (e.g. the garage with the door open or under a very strong extractor fan). Chaff is also blown out of the popper in the first stages of the roasting and while much of this can be collected, some will escape and drift around.
You will need a specific type of hot air popcorn popper where the popcorn chamber has a solid bottom and the air enters through vents in the side of the chamber.
Start with around 100g of green coffee beans - you will need to work out the right amount for your popper. There needs to be enough beans to retain sufficient heat for roasting, but few enough so that they are still moving at all times. We tend to put in a little more than we should and stir them with a wooden spoon to keep them moving until they start to reduce in weight and popper's fan can maintain the movement.
Put cold water in the butter dish, then close the plastic hood, placing the butter dish on top. The cold water helps to stop the plastic melting. Once you become more experienced, you may also want to extend the shute of the popper with some mesh - this helps to keep the beans hotter and prevent them falling out. To collect the chaff, place a large bowl under the chute. Turn the popper on.
The beans will start to agitate a little at first, then begin to move more freely as they heat, changing colour and emitting a fragrant "grassy" smoke and chaff. Shortly the audible loud popping of "first crack" will be heard.
When "first crack" has completed the "second crack" will occur a minute or so later. This can be delayed by opening the lid for a while. Extending the time between first and second crack will increase the caramelisation. Second crack is a much lighter sound, very similar to the "snap and crackle" when milk is poured onto rice bubbles. At first only a few occasional "snaps" will be heard but the rate will increase quickly as the beans get hotter. The smoke will increase in volume and smell quite caramelly.
Stop the roast at your desired level. Roasts can develop quickly so keep an eye and an ear on it - you will also find the smell changes. Switch off the popper and pour out the beans for cooling as quickly as possible.
Roast times are dependent on a variety of factors (one being the weather!) and so it is impossible to specify timeframes. We have used poppers with identical power ratings and seen quite different roast times with them but your roast should have completed within 15 minutes..
Finally, the poppers have an internal thermal cut out that will trip after the unit is switched off and so it will not immediately start again. You will need to wait approximately 10 minutes for the popper to cool down before starting again or the thermal cut out may trip during a roast.
Your beans will be at their best 2-5 days after roasting.
Disclaimer: popcorn poppers are not designed to roast coffee.