The ideal time to drop a batch depends on exactly what flavour and roast colour you are trying to achieve. Once you have a target in mind, there are three main ways to decide if your roast has reached that target:
- Bean colour
- Bean temperature
- Progression through first (or second) crack
Each of these methods has its limitations, so Scott recommends using a combination of all three factors to decide when to finish the roast. Using all three factors to guide your decision-making will allow much more consistent results than relying on any one factor alone.
Colour
The flavour of coffee is closely linked to the bean colour. This would make colour the ideal way to decide when to drop a batch — the only problem is that it is very difficult to accurately gauge the colour of the beans.
Automated systems for measuring colour objectively during the roast exist, but they are currently financially out of reach of most roasters. Instead, to assess the colour, a roaster must rely on a quick glance at a small sample of beans in the trier — and it is very hard to spot small differences in colour between samples.
Comparing roast colour with a previous batch. Colour is the most important measure of roast level, but it is hard to determine colour precisely in real time, so roasters must use additional factors to help them decide when to end the roast.
Colour is a useful indicator of when to drop a batch, but is not precise enough to be the only factor you consider. Furthermore, the colour of the beans also depends on factors such as the processing method, and thus the same level of development in different beans will result in different shades.
Bean Temperature
Many roasters rely heavily on the bean temperature as a guide to when to drop a batch.