The roasting process aims to transform the green coffee's potential by developing its intrinsic flavours, body and aromatic characteristics. It is specific to the brewing method being applied.
- Each coffee type should be roasted separately to allow for differences in size, density, moisture and the specific profile.
- The application of heat (radiant, conductive and convective) and the airflow throughout the roast process creates a unique heat imprint that is applied to the bean.
- It is crucial to ensure the environment temperature within the roaster drum is greater than that of the bean temperature (endothermic). If the opposite occurs (exothermic), the bean development and its sweetness are compromised.
- During the first phase of roasting (4min – 4min 30sec) the moisture in the green beans evaporates. This stage is imperative in order to achieve optimal development of flavour and aroma compounds.
- Following this, the bean takes on a straw-like colour which develops further as the process continues. The bean’s principle sugar, sucrose, begins to break down.
- At an external bean temperature of approximately 194 degrees Celsius, stresses within the internal cell walls cause it to burst. This is known as ‘first crack’. The remaining silverskin also comes off at this time.
- As development of aroma, flavor and body continues, it is the relationship between the roasting time and the bean temperature that helps determine the coffee’s profile.
- Shorter roast times (eg. under 13 minutes) often accentuate a coffee’s astringency, whereas extended roast times (eg. 15 minutes and over) can bake the coffee and mute the flavour profile.
- The bean temperature at the end of the roasting process (the ‘drop temp’) heavily determines the degree of development. The higher the temperature, the greater the roast character imparted. This often results in the masking of inherent characteristics of the coffee.
- The final stage of the process is the cooling of the beans, during which the beans are ideally brought back to an ambient temperature in less than 4 minutes. This is done to stop any further development of the bean and to enhance the natural sweetness of the coffee.
Next time, we’ll look at the espresso brewing method.