Roasting is an exact science. Roasting requires a constant temperature of 200oC (392oF) for 6 - 8 minutes of roasting time, continuous movement of the beans and fast cooling. During this process, a chemical reaction occurs that see the soluble oils rise to the surface, appearing as the sheen on high roast coffee beans.
At its simplest, the longer the roast, the darker and the stronger the coffee.
However, the darkening of the bean occurs in stages.
- The green coffee bean takes the first five minutes to heat through becoming beige in colour;
- Next it turns brown with a loud snap. This is Medium Roast.
- After the coffee bean has snapped, there is only 1 - 2 minutes between Medium Roast' and French Roast.
- When it reaches the second stage of releasing oils, it becomes darker and more concentrated. This is Italian Roast.
Roasting too fast does not allow the flavours to develop, while if the temperature is too high the body of the coffee appears to burn away. However, if roasting takes over 10 minutes, even at low temperatures, the coffee develops a baked, biscuity flavour.