Tips for using a popcorn air popper to roast coffee
"At first, I did it because I could, and wanted to learn. And I learned I could make better coffee."
— Dave of Zolo Coffee Roasters.
Why roast my own?
It's fun. You hear people tell of roasting to save money, as green coffee is cheaper per pound than roasted. That may be true, but the real reason, I suspect, is that we roast because we enjoy making coffee at home, possibly coffee has become a hobby and we want to learn more about it, and get closer to our goal of better understanding our coffee. We get to make our own blends, roast them to our taste. It's fun and it's satisfying as heck.
What you'll need
I use an electric air popcorn popper to roast, which took a massive investment of $7 from a local thrift shop. Other machines are available, but most of them are relatively cheap. Mine is a 1000 watt unit and can comfortably roast 80 grams (roughly ⅖ cup) of green coffee in about 7½ or 8 minutes. More powerful machines will roast faster and possibly darker. Look for a machine in which the air is blown through the beans through radial vents inside the popper, which moves the beans in a circular motion whilst working. This results in a faster and more even roast than a popper which simply blows air in from the bottom. You'll also need a scale to weigh the beans before and after, oven gloves or similar, two colanders or metal bowls, a couple of sieves, a notebook and something to time the process.
Preparation
Plug the popper into the mains, preferably near the stove's extraction fan. If you can, avoid using extension cords as this will slightly reduce the power available to the machine when it's working. Some poppers will draw up to 1500 watts, and even being further away from the mains distribution panel can have a negative effect on roast times.
The process will produce some smoke and smell, so I advise you to roast near an open window, or under the extraction fan in your kitchen. I keep the fan on low, and it's enough to clear the air. The other thing this process will produce is chaff. This is the remaining parchment skin left after processing the beans. It manifests as small papery scraps that, if unchecked, will cover the whole kitchen. I direct the output of the popper toward a colander in order to collect the mess. Afterwards, either dump it on your garden or into the compost.
The Process
Switch the unit on a couple of minutes before adding the beans in order to warm everything up and give the process a running start. Once the beans are added, start a stopwatch and stand ready so you can a) take notes and b) be on hand in the unlikely event that Something Goes Wrong (i.e. fire). The latter is unlikely as poppers all have a thermal cutout switch to prevent overheating and thus, fires. I strongly advise you to take notes on the process, so you can track what happens and when. I note the start of yellowing, the time of first crack, total roast time and the weight of beans at the outset and the finished weight. This helps me to track what works (and what doesn't) when it comes to tasting the coffee.
After a minute or so, you will notice the beans turning from green to yellow as they start losing moisture. Soon after this they will begin to turn a pale brown as the Maillard reaction starts to work. Soon after the beans begin to darken, you will start to hear little cracking noises. This is known as "first crack", which happens as the interior of the beans starts to really heat up, driving the moisture out in tiny explosions. After first crack, keep a close eye on the colour of the beans, and stop the roast when the majority of them are as dark as you want. If you leave the beans longer, you may hear a second series of cracks, the "second crack" which is caused by escaping CO2. At this point you may notice the beans becoming glossy. This is because the oils are being forced to the surface of the beans by the offgassing. At this point you have a dark, possibly even a "French" roast. It's generally thought unwise to roast to this level in a popcorn popper as the thermal stress may be beyond the ability of the machine's designed limits. It's also possible that you'll go beyond second crack to "third crack" The crack is the sound the beans make as they the beans begin to combust. ("At some point it just turns to carbon and then catches fire. I don't recommend that"—James Hoffmann)
Once the beans have hit the level you want, off with the power, on with the gloves. Dump the beans into a metal colander or sieve, pour it from one container to another until it's at room temperature. You might want to do this outdoors because there' likely to still be some chaff in those beans. Seriously, you don't want that stuff invading your house. That done, record the weight of the finished beans in your notebook, then pop the beans into a Ziplock bag or single-wall coffee bag and leave it in a cool, dark place for a couple of days.
The reason you're leaving it is to allow the CO2 to escape and the flavours to finish developing. I generally leave mine for at least 48 hours, and still notice improvements if I leave it for a while longer, up to about a week. Be patient, and don't forget to take some notes of how the flavour changes.
How Long?
There seem to be two schools of thought as to how long to roast. One says "roast until they look about right", which is fine as long as you know exactly how you want them to look. The other school says to be more scientific and drop the beans at a particular temperature. Judging colour can be tricky, so I prefer to go by both colour and temperature. Sweet Maria's Coffee have a good colour/temperature guide here. But if you're taking notes, a little experimentation will guide you in the right direction. FOR GOODNESS' SAKE, I CAN'T STRESS THIS ENOUGH, TAKE NOTES.
Modifying your roaster
If you want to measure your bean temperature, you'll need to modify your popper. This is managed by drilling a small hole in the side to accept a temperature probe (which could be as simple as a candy thermometer or as complex as a thermocouple and display). The probe should be in the midst of the bean mass as they're roasting. You may also want to disable or remove the thermal cutout switch attached to most poppers as it will not allow the temperature to get high enough for darker roasts (if that's your preference!). It goes without saying that you should be careful with any modification you make (there are many websites and videos dedicated to this, so do your research well, and don't attempt it if you're unsure. Safety First! ☺. Your mileage may vary, of course; I've not felt the need to remove the thermal switch as I can get to a good medium roast with it in place.
Final notes
Be more careful with decaffeinated coffee - the beans roast very differently, often being more done inside than appears on the outside. More on that soon.
You may also want to go a step further and calculate the water loss from the roasting process. To calculate this, take the weight of the green beans, subtract the final weight and multiply by 100. This gives the percentage weight loss, and can be a good guide to how well roasted your beans are. I drop my beans at about 425°C, with a weight loss of about 12.5% for a medium-dark roast. Again, YMMV according to taste and the beans' origin.
Next up will be on how to store your coffee.
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