Standard Electric or Gas Convection Oven... WORKS GREAT!!!
First off, some avid home coffee roasters are bit too staid and stiff for their own good in many forums online, to tell you how to do this. Some are just too picky and touchy or proud to even try this method or at least one that doesn't cost them money (it's the "I've Got To Keep Up With the Jones syndrome I guess). Yet it's so simple, even your grandmother likely home roasted coffee in her standard oven. Sometimes you'll find a few members in these forums that want to keep it expensive and complicated so they feel better about their skills and knowledge. Answering your rather simple request doesn't get them points or make them look "Cool" in these sometimes rather highfalutin forum memberships. The other side of these self professed coffee roasting connoisseurs are those that pride themselves on being "Coffee Red Necks". They'll just tell you "Why he.., I just go out to my garage/shop, get my blow torch fired up, throw some beans in my dog's stainless water bowl and roll 'em around till they turn brown to my liking's"! lol.... but mainly most members just don't have the time or desire or maybe aren't adventurous enough to try a slew of different methods and then explain them to others online. Some? ....er well... the wife/husband just flat out won't let them do their roasting in her/his/their kitchen oven!
So back to roasting coffee in your "standard convection oven" right now in your kitchen (just let me say I've owned some expensive fancy roasters, barbecue drum setups, etc that don't do any better of a job roasting coffee than my convection oven has done after experiencing a few screw ups. If your a stickler for precision and flaunting your coffee roasting skills, Top Brand Names, etc, then this isn't the method for you to try). Don't be afraid to make mistakes though, smell up your house/kitchen with smoke, burn your fingers a little, and maybe eat pizza afterwards with a hint of Coffee Bean flavor and you'll be just fine (actually Coffee and Pizza flavors go quite well together). But just to let you know, I do prefer to roast coffee in my fancy roasting machines/barbecue for a variety of reasons, yet I still occasionally (in winter when I don't want to go out into my frozen shop, heat it up, etc) will throw some beans into my standard "Jenn Air Range" convection oven to roast. Like you say... in reality done properly, their extremely consistent temperatures and air circulation actually make it so you don't need to really stir them much (if at all) and NO.... there is no chaff flying around in your oven either.
Just remember there are 3 kinds of heat transfer/generation used to roast coffee beans. Conduction is transfer of heat directly by contact with a hot surface and this means of heat transfer is what roasts beans in a pan on a stove top. It's the one where you really have to be concerned with keeping beans moving. Second; is Convection Heat and it's the transfer of heated air in circulation (heat rises, fans, etc). This is the primary method of heat transfer in most common home coffee roasters, heat guns, popcorn poppers, etc. Last but certainly not least... is Radiant Heat (non-heat transfer generation method). Which is where the solid molecules of water/oils within the beans are excited by waves of energy to produce their own heat from within. A pure example would be roasting coffee beans in your Microwave Oven. There are even roasters (Black & Decker makes one) that use this method to roast beans. I've tried them and although they do work great for very small batches you really have no control for consistent bean roasts/batches.
But the reality is that all methods of roasting coffee, utilize Radiant Heat to a greater or lesser degree. This is what's actually roasting your coffee beans if they're done properly. Conduction, convection are just indirect forms of heat transfer to get the beans actually roasting themselves from the inside out w/ their oils and water. Rather than depending on the beans to solely create their own radiant heat in a microwave . The whole object is to transfer as much heat without burning/scorching the outside of the beans and your Standard Home Range Convection Oven, can most likely provide a near perfect means to do it, without buying any additional roasters, drums, popcorn poppers, heat guns, etc!
Bean choice is important for your first time, depending on your present coffee tastes. If you are one that loves Bold and rich coffee flavors, then you most likely won't be happy with the predominantly more subtle fruit, flowery flavors of Dry Processed coffees out of Ethiopia for instance. Stick with more traditional American tastes in Central and South American coffee beans instead then. Read your cupping's and reviews concerning the beans you buy to roast.... first. Because they can make your first try good or bad and you may never want to try roasting coffee again, if bad. If you don't really know what you like and aren't just the typical Costco Kirkland Bold coffee lover, I'd be a little more adventurous and try a North African Dry Processed Coffee to roast then. Near anything from Ethiopia, Kenya, etc should work well for you.Some people say you should use a solid thicker perforated cookie sheet in your home oven and those are fine, but provide more conductive heat, that could scorch or burn spots on beans. So just for trying it out and/or roasting occasionally, like I do in my convection oven, I use aluminum foil (or pizza screen) and actually... in some ways they're better.
The reasons are that you don't have to ruin the wife's cookie sheets or baking pans drilling/punching holes in 'em for one. But mostly because you're using less Conductive Heat Transfer to beans instead. Naturally most well designed Convection Ovens will be really even in heat distribution throughout your entire oven. But I've found a level or two up from center works best. Form your make shift aluminum foil pan in your cookie/baking pan with turned up and doubled edges. Pull it back out of it and punch a slew of holes for convective air flow with it sitting on your oven rack or if you do have a fine wire mesh pizza screen they work great instead of a foil pan too. The whole idea is that the less contact with thick conductive heat source you have the more even the roast. Just think if you could roast coffee beans suspended in mid air currents, that would be ideal. So you pre heat your oven for your first attempt, to between 425*F and 475*F. Have a large wire strainer and large bowl ready for cooling and getting the chaff off the beans. Mittens naturally to handle everything after roasting and one of your oven racks to set the formed foil baking pan or pizza screen on.
Now I will usually make ready enough beans to cover the pan or screen's surface with beans, just touching each other prior to roasting. Depending on the beans, set your timer for 10 min (if using 475*F temp, 15min for 450*F and 20min for 400-425*F). Just remember these are only initial guidelines. So for a Harrar Longberry for instance (that's better at a lighter roast), you may or may not use these times. At 475*F I will generally get 1st Crack starting at around 4-5min in. Hopefully your oven has a glass front so you can watch while listening for signs of cracking and when it stops, etc. You will see some slight movement of the beans as the chaff pops off on cracking. But I've never had chaff flying around in my convection oven... ever.
Next you'll start to see some smoke out the vent at around 7 to 8 min at this higher temp. Now just be listening for cracking and whether it slows or stops and this should get you a nice city roast somewhere within the final 2 min. Which is preferred for these sweeter, fruity high grown African dry processed beans. If you like coffee closer to Full City, then you should see that roast level happen at around 11 to 12 min into roasting (at 475*F). Look for when 2nd crack begins or slightly into 2nd crack. With experience you'll find out just how your particular oven works. So these are just initial guidelines remember. But also remember that for the time they're roasting you can't go off and read a book, etc either. When you determine that they're done, quickly open the oven, pull the perforated foil pan (or pizza screen) and fold the foil up to make it easier to pour into the strainer over the bowl. Have a screen or piece of foil ready to cover the strainer (crimp edges), so you can take it outside and shake the beans till cool (do this fast as you can). Now use your hand to mix the beans around in the strainer w/ bowl underneath to further clean chaff from them. Hopefully you don't have too much chaff outside the range/oven, but it's easy to clean up.
That's it.... you'll find that your freshly roasted coffee beans should look fairly consistent in roast over the whole bean. Although because you'll most likely have beans of different sizes if Harrar, there could be some color variation due more to that and origin, greener, etc than due to our roasting method!You'll want to naturally sample some right then, but they will be far better after you let them air and degas for at least a day or two. Generally I do this in an open cardboard box or in the bag left open topped. My local market allows me to take some coffee bags from the fresh coffee bean section I use to store them in after that. When your spending $100's of dollars a month there, they can't complain. If they do, threaten to take your business elsewhere. I've never had to. From here you'll just grind 'em up to your liking and enjoy your first batch of Home Convection Oven Roasted Coffee! *_^ ......your grandma would have loved to have one of these new-fangled Convection Ovens. Don't you think?
