I started my coffee roasting career as a home roaster. Getting started was easier than I thought. And as I did so, it occured to me how a relatively simple concept, roasting coffee beans, has been made to appear to be an arcane art, with a variety of roast types held out as arcane knowledge. How many different names have you run across for different types of coffee roasts? Light, Medium, Dark? Espresso? Continental? Vienna, French, Italian, Spanish? City? Full-City? C'mon, who's thinking up these things?
Well, dark secret (pardon pun) of coffee industry is that, well, there really isn't full agreement on which roast is which. So basically, we all pretty much get to hunt around, try different coffees from different sources and pick one(s) we like. In this article, I'll try to use standard nomenclature, and map it to a process of observing color and texture anyone can judge for himself.
The roasting adventure begins with green coffee beans. These are stored at room temperatures, at 12-15% moisture content. Roasting is done at temperatures of up to 450+ degrees F. Duration and temperature determine roast.
A coffee bean will take on heat until internal temperature of bean reaches approximately 212-240 deg F. At this point, outer layer of bean(s) will discolor, turning a nice cinnamon color. Here, steam will start being released from bean.
As bean heats up further (approx 250-300 degrees F, again depending on variety), external membrane of bean will dry up and start separating from bean itself. At approximately 350 degrees F, continuing heating of bean forces a 'first crack.' This cracking occurs as moisture within is released through existing seam in bean. This essentially blows this small crack open, forcing separation of remaining bean 'chaff'.
Coffee at this stage is a light brown color; entering 'light City Roast' stage. City Roast is usually achieved at a slightly higher temperature (above 370 deg F), where sugars within bean start melting or ‘carmelizing’. This gives distinctive 'coffee brown' color. City Roasts are usually stopped around 400 deg. or so. At this point, sugars are not fully carmelized, and flavor of beans at this stage are very much determined by their origin; not by degree of roast. The 'Full City Roast' stage occurs at higher temperatures, just as bean reaches 'second crack' stage. This stage happens at different temperatures for different beans based on variety. The second crack comes as temperatures of bean reach point where cellular composition of bean starts breaking down. To obtain Full City roast, roasting is stopped just at point where this second crack starts (approx 425-435 deg F.) At this point bean is darker brown, but 'dry' looking, as oils of bean have not started to emerge through molecular breakdown of bean.
Going into second crack, we reach Vienna, Continental, French and/or Italian roast stages. These are sometimes also referred to as "Espresso Roast", although strictly speaking, there's no such thing. Italian espresso blends actually vary - northern blends are typically roasted to 'Vienna' stage, well into second crack, where sugars within bean are almost fully carmelized and many beans within roast will appear dark brown with hints of fissures. Espresso blends in southern Italy are usually roasted into "French Roast" stage, where almost all of beans will be about one shade removed from black and oils will start emerging from some beans. Beyond this point, beans will start releasing oils and their soluble compounds - mainly as a lot of smoke; but beans will be left quite dark with a very oily sheen. Assuming they have not fully burnt yet, this can be specified as "Italian Roast". I've observed different temperatures (within roaster) for all of these stages depending on bean variety - so as my roasts reach second crack, I tend to trust my eyes and ears more than I trust my probe thermometer.