Turning Point

The turning point is a pivotal moment in the roasting process, representing the transition from the initial heat-absorption phase to the active roasting phase. When cool green coffee is charged into a hot roaster, the temperature of the beans and the air inside the drum drops rapidly. The turning point is the exact moment this temperature decline stops and the bean temperature begins to rise again.

On most commercial drum roasters, the turning point typically occurs 60 to 90 seconds after the charge, usually at a temperature between 75°C and 100°C. This moment is highly significant because it indicates that the beans have absorbed enough energy to overcome the initial thermal shock and are now actively undergoing the chemical changes required for roasting. A turning point that arrives too early or too late can be a sign of an improper charge temperature or an issue with the roaster's energy input, both of which can negatively impact the final cup quality.

For the roaster, the turning point is a key data point for monitoring consistency. By logging the time and temperature of the turning point for every batch, a roaster can ensure that their roast profile is repeatable. If the turning point varies significantly from one batch to the next, it suggests that the roaster is not starting from the same thermal baseline, which will inevitably lead to inconsistencies in the final product. It is a diagnostic tool that helps roasters understand how their machine is performing under different conditions.

In practice, the turning point is the 'bottom' of the roast curve. It is the moment of truth where the roaster can see if their initial energy application was sufficient. By carefully managing the conditions leading up to the turning point, a roaster can set the trajectory for the rest of the roast, ensuring that the coffee is developed evenly and that the unique characteristics of the origin are preserved and highlighted in the final cup.