Nice! Another black hole hobby I have also fallen down - although its just espresso, I havent roasted my own yet...whats the process?
Process is easy to grasp but difficult to master. I have just went down this rabbit hole this past week, from what I have learned so far.....
Apply heat source to green beans...3 phrases.
Phase 1 - Dehydration
Phase 2 - Browning (Maillard reaction)
Phase 3 - Development (What taste you prefer, this is where you get the term medium roast, dark roast etc)
The metric to consider before you start....the science.
1 - You want to aim for a dehydration of between 11.5 -16% dehydration for a medium roast. So if you put in 100g of green beans, if it comes out at 85g at the end. That's a good number. If it's like 90g/10% it is very likely it could still be a bit green, aka raw.
2 - Like cooking meat, but unlike a steak, you don't want a raw center, you want it roasted all the way through the bean. But too much heat at the start will mean it will be black before the inside is roasted. But you don't want to start too low and too long because you end up with a "baked" bean, rather than a "roast". How much heat you can apply depends on the type of bean, higher density beans can "take" higher heat, and more resilient to heat before it will burn. Generally higher altitude coffee will produce higher density beans. You can measure it against the same volume of water. Get a cup, say a 500ml glass. If you fill it with green beans, weigh it. If it weights 400g then you have a density of 0.8. In coffee, I think anything over 0.7 is consider a higher density bean. Mine was 0.766
Typical Phase 1 time is about 40-50% of the total time, depending on density, starting temp. Then Phase 2 is about 30-40% and then Phase 3 is like 10-20%.
3 - There are a lot of terms in coffee and lots of temperatures to roast them. Some people start off at like 160c (this would be for like a lower density coffee, I started at 200c straight off (because I was mostly winging it and I didn't want a baked bean). The way in which how long and what temp you use is call a "profile".
4 - After roasting, it is said you need to wait 3-5 days before drinking it to let the CO2 from the roasting built up inside to be released.
5 - Speaking of the terms used. The key words I've come to learn is "first crack" and "2nd crack". After the millard phase in phase 2, towards the end of that you will start to hear cracking noises, and if you watch it, the chaff from the skin of the bean starts to come off. This is call the First Crack, the first crack happens when the water moisture inside the coffee builds up too much and bursts open. So this is a indicator that the coffee is now "cooked" and you can stop right there and it will be drinkable. I wouldn't personally stop at the first sound, I would wait for the popping sound to stop, so there is some time in Phase 3, to get that little more brown colour. The 2nd crack will happen quite soon after if you don't stop it though, like as little as 90seconds after the first crack stops. The 2nd crack is when the oils in the bean now burst open. This is why dark beans looks shiny due to the oils coming up to the surface and it is at this stage where you will definitely start tasting bitterness. Almost all store-bought, definitely Starbucks coffee are at this stage. If you like your coffee a little dark, stop at the VERY first sign of the 2nd crack.
Then once you stop, you need to cool it to room temperature in less than 5mins. The stopping part is call The Drop, I believe it originates from big commercial roasters where in order to stop roasting, the open up the roasting chamber and all the beans drop onto a large cooling pan.. From the drop to beans at room temp in 5mins. My machine has a cool fan cycle, if you are doing it in a pan, you need to put it out on a large metal colander and then keep moving it around. The whole process from pressing the power button to the drop can be done in about 8mins for 200g of coffee. Not long at all.
Then you wait 5 days and then start drinking.
edit - another important term is "rate of rise", it is the rate of which the temperature is rising inside the roaster/beans. You want the rate of rise to be lower as you go. So you don't want to start at 160c then go to 170c then 220c. You want to go from like 160c to 200c then 220c. I am not sure as to why but that's what people who knows says.
It is quite a rabbit hole but it is rewarding, especially if you like real coffee and with coffee prices going up, this way I can get it cheaper too. I can get a bag of green Geisha (a type of coffee), for as little as £15 for 500g. If I were to buy a roasted Geisha, it would be double that for half the amount, like £30-40 for 225g bag. The stuff that most coffee shop sell for like £12 a bag, I can now get the same amount for half that. Of course, this is very much dependent on my skill as the "cook" of the coffee. There is a saying "The darker the coffee is, the less you taste of the farmer." or something like that. So to get the best out of these coffee, I would avoid going too dark a roast, as the darker it is, the less I am tasting of the farmer, the origin of the plant. I guess this is where the art of it is and what people are paying for in a store bought coffee. There is a balance between the taste of the farmer and the taste of the cook. You don't want a grassy raw taste, nor do you want a black burnt cup.