So you have decided to home roast; congratulations! The quality of your roasts may be variable however the roasts should improve over time, and you will surely gain a better understanding of coffee itself, which will contribute to your enjoyment of the beverage. This post is intended to teach you what you will need to know about the process, and how to learn from your experiences. Roasting coffee is no more difficult than baking bread, however anyone who has ever baked bread knows there is a world of difference between good and outstanding results.
Choice of Roaster: There are a lot of them out there. You will pick yours based upon price, batch size, and various convenience features. There are many reviews of various roasters, and all the affordable home ones are a collection of compromises. Search engines are your friend, and by reading a lot of reviews you should be able to pick a roaster whose features are appealing (to you) and whose compromises don't bother you too much. You will need to concentrate on those parts of the roasting process you can control with your given roaster, which will not be the same for every roaster out there. Some roasters, usually air roasters that roast small batches of coffee, have the ability to roast very rapidly, in fact too rapidly, producing an inferior result unless the heat is controlled. Others will struggle to roast the coffee fast enough to produce good results. Know what you have and what your roaster can do, will guide you in how to control it to minimize its limitations.
The Roasting Process (simplified): When heated up the green beans will lose most of their internal moisture, they will turn brown, and their structure will change. Sounds, smells, and appearances will change. Some of these changes will enable you to monitor, and to alter, the course of the roast. At first, moisture departs the bean during the so-called "drying phase." You only need to worry about this with a roaster having the tendency to roast "too fast, " in which case you might choose to apply less heat during the first 3 or 4 minutes of the roast. Roasters that have trouble roasting "fast enough" don't need any attention given to the drying phase, as it will happen, all on its own. Next you will have what is called "First Crack," which is a loud series of pops that start slowly but rapidly coalesce together. Finally, if you roast long enough, you will get into "Second Crack," where you get a sound like milk being poured on Rice Crispies cereal, or like crinkling plastic sheeting. It is not as loud as 1st crack, but rapidly starts to pick up steam as the beans get darker very rapidly and more and more smoke is produced.
Variables You Can Control: These are basically few in number: (1) the amount of green beans you attempt to roast in each batch; (2) how hot the roaster is before you put in the green beans, e.g. how much preheating you do; (3) the amount of heat applied during the roast; (4) the amount of ventilation, either passively by opening up a vent or door, or actively by controlling a fan; (5) how dark you let the beans roast; and (6) the time course over which #s 3, 4, and 5 occur. That's basically it.
For most roasters, the larger the quantity of beans you roast, the slower the roast will go, at least initially. You can increase the "batch size" up to a point in a way to slow down your roasts, but beyond a certain point the beans will roast unevenly, which will be obvious when you look at the roast product, and when you taste the coffee. Conversely, if your roasts go too slowly you can reduce the batch size. The exception is small roasters using hot air as their sole heating mechanism; for these roasters, the opposite is true, up to a point, where reducing batch size will SLOW DOWN the roast, and increasing batch size speeds it up.
The more heat you apply, the faster will go your roast, however after a certain point you will cause the roast to accelerate and you can lose control over it. This is especially important as the beans begin First Crack, since beans that are roasting too fast will not have an adequate time interval between the onset of First Crack and the end of the roast, giving inferior results (see below under "General Parameters"). The more ventilation you apply the slower the roast will progress, however too much ventilation will cause the roast to progress too slowly and may cause the roasting process to stop altogether, or to "stall," which will surely give bad results. The more you preheat your roaster, the faster the roast will go, but if the roaster is too hot when the beans are put in, they will be damaged both visually and in how they taste. Obvious signs of having loaded the beans in when the roaster was too hot are the presence of scorch marks and divots/defects in the roasted beans when you look at them after roasting.
Ways to Monitor the Roast: Get a minute timer or stopwatch and have it start counting from the moment the beans are introduced into the roaster. Follow the general directions for your roaster. If your roaster has built in thermometry, then follow it. If the roaster does not have built in thermometry, then you might be able to add it in by following suggestions given by other people who have written about ways they have modified your roaster. If you do put in your own thermometry, make sure that it gives consistent readings that correlate with what you can observe in the beans as they roast; if not, then what you are monitoring is not useful. You will want to use your senses of sight, hearing, and smell during the roast, and correlate them with the timing on your stopwatch or timer. If you are able to correlate the information you get from thermometry with what is going on with the beans (especially the cracks) then you will know that your thermometry is useful.
First Roasting Experience: Buy some simple washed beans (generic wet processed Colombians would be a good choice) and using the directions that came with your roaster, run 2 or 3 batches through, making no adjustments to the standard process, to see how your roaster behaves. Observe the sights and sounds and smells as the coffee turns browner, starts to smoke and to crack, and take the roast well into 2nd crack (but not far enough to have a fire!) to appreciate what the roasting process is like and how the cracks sound in your roaster. Toss these beans in the trash since they were just for practice, unless you really need a fix!
General Parameters: You want to roast the beans in a way that preserves their flavors and produces a pleasant beverage that isn't either too "bright" (or loud) or too "flat" (or boring/uninteresting). This requires you to roast "fast enough" but not "too fast," and to allow "enough time" between the beginning of the 1st Crack and the end of your roast to enhance flavors. You must also avoid damaging the beans in other ways, principally by burning or scorching them, or by not applying enough heat so that the roast proceeds through without stopping in mid process, which is referred to as "stalling" the roast, which can make the coffee taste "baked" as opposed to "roasted," the latter being what you want. You will be using your senses of sight, smell, and sound to monitor the roast, and perhaps also thermometry. Remember that what you can see, smell, and hear, reflect the reality of what is going on in the beans. The thermometry might give you more precise information, but it also might give you false or confusing information, so if in doubt, rely much more on the stuff you can see and hear, than on the thermometry, which if confusing might indicate that your thermometry probe is not located in a perfect spot.
As a general rule, small home air roasters will roast faster than home drum roasters, especially drum roasters with electrical heating elements, so adjust these times somewhat with experience to account for your particular roaster. First Crack should begin by 9 or at most 10 minutes if at all possible, and if it starts before 6 or 7 minutes, it probably indicates a roast that going too fast to have good results. The first crack should start slowly then pick up speed, then appear to peter out slowly. There will likely be a period towards the end where there are no audible sounds at all, but if this happens early on in first crack, then the roast is probably stalling due to not having enough heat applied, too much ventilation, or perhaps both. The duration of first crack is VERY VERY VERY important if you want to get a good end result. In my own experience, if Second Crack starts quickly after the beginning of 1st Crack, the results will be poor. I generally try to get about 4-5 minutes in between the beginning of 1st Crack and the end of the roast. When I have had only 2.5 minutes or so, the coffee that was roasted was nearly undrinkable. In order to extend the interval between onset of First Crack and the onset of 2nd Crack, you will probably need to reduce the amount of heat input or increase the ventilation, (or both) before the roast begins first crack. Successful overall roast times, start to finish, will generally run from about 11 to 15 minutes.
Finally, consider how you will use the coffee you are roasting. Coffee roasted faster, which will be "brighter" may be perfectly acceptable when used for brewed coffee, but unpleasant for espresso. Up to a certain point a longer overall roast time can produce pleasing flavors in an espresso but seem flat or boring for drip coffee. You may also prefer certain roast levels more for certain types of coffee preparation, so how dark you choose to roast a given coffee should take into consideration how you will use it.
Tying this all Together: This is the part that only you can do for yourself and that no one else can help you with. In order to become a successful home roaster, you are going to have to roast a lot of coffee, including many different types. You are going to have to roast a very large number of batches, and to play around with the variables, listed above, that you can control. You will have to look at the beans after you roasted them for any obvious flaws you may have caused, and most importantly, you are going to have to taste the coffee that you roasted, trying to correlate the results you get in your cup with the variables you experimented with on that particular roast or set of roasts. The specifics of exactly how you will need to modify these variables to get the results that you want will only become clear with your own hard work and experience. No one can give you more than general guidelines unless they personally have a lot of experience using the same roasting device that you are using, and in addition, know your taste.
There are books and articles written on the subject of what is going on chemically and physically inside the beans during the roast process. My suggestion is to avoid getting very deep into this stuff as it is most unlikely that any of it will truly help you to become a better roaster, until very long after virtually everything in this post has become second nature to you. Rather, I suggest that you spend your time roasting coffee and learning from your own experiences, good and bad. Avoid the use of jargon that is often confusing and seldom really leads to any improvement in your roast product. If and when you conclude that you are no longer able to make progress using your current equipment, you can give consideration towards buying a "better" roaster, if your interest and finances allow it.
What, me worry?
Alfred E. Neuman, 1955