What is a roast curve?
A roast curve (or roast profile) is a graphic representation of how bean temperature evolves over time during roasting. It is the 'fingerprint' of a roast: analysing it allows the roaster to reproduce results precisely, diagnose defects and optimise the final sensory profile of the coffee.
The roast curve is built on two axes: time on the horizontal axis (in minutes, from 0 to the end of the roast, typically 8 to 14 minutes) and temperature on the vertical axis (in degrees Celsius, generally from about 100 °C at charge to 195–230 °C at drop). It is captured in real time by one or more thermocouple probes placed inside the drum in contact with the beans. Modern software tools (Cropster, Artisan, RoastPath) simultaneously calculate the Rate of Rise (RoR) — the change in temperature per unit of time — displayed as a derivative curve overlaid on the main profile.
A well-managed roast curve presents several characteristic phases. First, the charge phase (or drying phase): cold beans absorb heat from the drum and the measured temperature dips before rising again (the 'turning point', usually at 60–90 seconds). Then a drying and yellowing phase where the surface of the beans transitions from green to straw yellow, evaporating residual moisture. The Maillard and caramelisation phase then begins, characterised by accelerated browning. First crack typically occurs around 180–205 °C depending on the bean, after which the development phase (DTR) runs through to the final drop — discharge into the cooling tray.
The shape of the curve — its slope, concavity, accelerations and decelerations — matters as much as absolute temperatures. A RoR that flattens or stalls during the Maillard phase produces a 'baked' defect; a consistently high RoR throughout produces an 'underdeveloped' coffee with raw acidity. Mastering the roast curve is what distinguishes the skilled craft roaster from the semi-automatic machine operator. A surprising fact: at constant weight, coffee loses between 12 and 22 % of its mass during roasting (mainly water, but also volatile compounds) — a light roast generates a smaller weight loss, a dark roast a larger one.
Phases of a roast curve
| Phase | Indicative temperature | Key phenomena |
|---|---|---|
| Charge (drop-in) | 180–220 °C (drum temp) | Thermal shock, turning point |
| Drying / yellowing | 100–150 °C (bean) | Water evaporation, Strecker reaction |
| Maillard | 150–185 °C | Browning, primary aroma formation |
| First crack | 180–205 °C | Decompression, bean expansion |
| Development (DTR) | 190–225 °C | Caramelisation, acidity/sweetness balance |
| Drop (discharge) | Defined target | Reaction stop, rapid cooling |