PRE-HEATING
Roast Phases
In order to activate the chemical and physical processes that lead to the development of the required aromas, it’s vitally important to start with a hot drum before feeding in the green coffee beans. The charge temperature depends on the size of the machine, the amount of coffee introduced as well as its density and moisture.
DRYING PHASE
This is the moment where the coffee enters the roasting machine’s drum and is where and when the real roasting starts. During this phase the beans, which are at room temperature, absorb the drum’s heat, and actually reduce it by half. It is this endothermic phase where the coffee absorbs heat, beginning to lose some characteristics but gaining others. Among the elements that degrade the beans are: water, chlorogenic acids and trigonelline. The main transformational changes in this phase are: an increase in bean volume and internal pressure (due to the transformation of water into steam), loss of mass and density as well as a colour change from green to intense yellow. This last chromatic transformation marks the end of this first phase with this stark visual stimulus.
DEVELOPMENT OF THE AROMAS
It is the phase where we can extract the most complex aromas by managing the time and the temperature that make up the roasting “curve”. Even here, having collected information on the raw material will help us to understand how to modulate the heat so that the potential hidden between the fibres of the bean can best be expressed. This is the phase in which we slow down the acceleration of the temperature – its rate of rise – in order to allow the triggering of the “Maillard reaction” and its subsequent development. If the triggering of these reactions (given their high number, it is more correct to use the plural) is determined by the yellow phase, then the end of their development takes place thanks to an equally evident, though this time acoustic, signal. It is the famous “first crack” – a sound similar to pop-corn crackling in which the water inside, having now become steam, presses on the inner walls of the bean coming out through small crevices near the surface – the sulcus. Here too the search for balance is essential because if we make this phase last too long we would “boil” the aromas and lose the complexity of the coffee (baked defect). On the other hand, an acceleration that is too fast at this stage would not allow an adequate development of these aromatic components. If we wanted to numerate all this, we would say that this central phase should constitute around 40% of the whole roasting process.
CARAMELIZATION
From the first crack onwards, most of what is produced is the caramelization of simple sugars, which is activated at much higher temperatures (about 160°C). This is the so-called “development time” and is the final part of the roasting process. The more we extend this phase the more we find hints of caramel and a buttery body, however sacrificing some sweetness.
COOLING
Cooling is the last step and takes place outside the roasting drum in the cooling tank. It is important that the coffee beans are brought to room temperature as quickly as possible (4-5 minutes at the most) so that roasting does not continue bringing bitterness and hints of burning. To preserve the quality and shelf life of the product, it is preferable to air-cool instead of water-cool (quenching).
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