What is coffee roasting?

Roasting is the controlled cooking of green coffee in a drum or fluid-bed roaster, at temperatures between roughly 180 °C and 230 °C, for 8 to 18 minutes. It turns a hard, grassy, almost odourless seed into an aromatic brown bean through a cascade of chemical reactions — drying, Maillard, caramelisation, pyrolysis — which create the hundreds of volatile compounds that make up coffee as we drink it.

A green coffee bean enters the roaster with around 10-12 % moisture. In the opening minutes — the drying phase — internal bean temperature climbs slowly from 20 °C to roughly 150 °C while water content drops below 5 %. The bean turns yellow, releasing aromas of cut grass, then lightly toasted bread. From 150 to 190 °C, Maillard reactions kick in: free amino acids react with reducing sugars, producing brown melanoidins along with hundreds of aromatic molecules (pyrazines, furans, thiols) that drive the hazelnut, chocolate and bread notes found in the cup.

Around 196-205 °C comes 'first crack', an audible pop very similar to popcorn, caused by steam and CO2 fracturing the bean's cellular structure. This marks the beginning of the development phase — the linchpin of modern roasting. The Development Time Ratio (DTR, development time divided by total time) is a central profile metric: 18-22 % for a light filter roast, 22-28 % for an espresso-oriented roast, higher for darker profiles. Past 224 °C, a drier, metallic 'second crack' signals advanced pyrolysis and the migration of oils to the bean surface — French and Italian roast territory.

Craft roasters work off continuous thermal curves: bean probe, air probe and Rate of Rise (RoR, temperature gain per minute) are logged live on software such as Cropster or Artisan. A good profile shows a smoothly declining RoR after the first third — no kinks, no sudden flat spots. Industrial builders like Probat (Germany, founded 1868), Giesen (Netherlands), Loring (USA, high-efficiency convection) and Diedrich (USA) equip most European roasteries. Specialty batches typically run from 500 g to 30 kg; beyond 60 kg, operations shift to industrial equipment where precision often yields to throughput. In Belgium, the specialty scene took root in Brussels, Ghent and Antwerp during the 2010s around micro-roasters of 6 to 15 kg.

Typical roast phases (5 kg batch, 12 min)