There are two approaches when comes to roasting coffee: Either roast to extract the flavour inherent in the beans or create distinct flavours from roasting. Which makes for better coffee?
Each green, unroasted coffee bean comes with a sense of terroir and inherent quality much like a wine grape. These can either be known or unknown, good or bad. Assuming the roastery has bought decent coffee, it’s up to the roaster to find those flavours.
First, a foray into a little science, and I’ll try not to bore you too much. The basic flavour molecules in coffee, the flavonoids or volatiles, are developed in a process known as Maillard reaction after French chemist Louis-Camille Maillard, who first described it in 1912. Essentially we are talking about the caramelisation of flavours, much like what’s involved with the browning of food. The blackening of both is due to pyrolysis, when those carbohydrates begin to burn.
Now a great wine has around 150 different flavonoids. Coffee has a few more – last time they checked they lost count at around 1500! Each of them develops at a different rate. First getting enhanced by heat, then reaching their zenith, then burning off.
So in essence too light a roast or too slow an application of heat will result in the volatile not developing; too dark or too fast will result in it burning off and turning bitter! And this process is independent for each type of molecule! The maths becomes a bit tricky as you can imagine.
So back to the real world. Under- roasting will cause green and grassy and astringent flavours. Over-roasting will cause the coffee to lose body and flavour with the end-result weak, watery and bitter!
The only way through this minefield is to bring hard-won experience to bear. This depends on many hours of roast development and coffee cupping (the universal tasting method for formal evaluation of coffee flavours). Looking at coffee beans will tell you nothing.
During all my years in the coffee trade, I’ve never discovered the optimum in a very dark roast. Any visible oils or black beans are evidence of utter destruction of potential. No blending before roasting will allow optimal flavour development. You would not roast a quail and a turkey in an oven at the same temperature for the same period of time.
As for the second approach mentioned above, many roasters deliberately attempt to create a roast flavour, burning a bit of caramel and bitterness into the coffee. I believe this is a fault, and an excuse for roasters to buy lesser coffees, or what comes from refusing to go down the rabbit hole of empirical testing. There are those who disagree with me, but the world’s leading speciality coffee roasters tend to side with my opinion.
The washed process Ethiopian Yirgacheffe on my cupping table this morning bore out my beliefs once again – a melon-like fragrance before a delightful sweetness offset by side-of-tongue acidity. Bitterness should never be a dominant flavour when it comes to coffee.