Roasting is an art that you learn by putting into practice - and not putting aside! There are some basic notions that are important to know for greater awareness of what is occurring during the many transformations the roasting process triggers: chemical, physical, structural and sensorial. But first, have you read our post about the 5 steps of green coffee preparation? If not, check it out here before continuing: the 5 steps before roasting.
So now that we are perfectly "calibrated", let's see what the basic stages of the roasting process are: after gathering as much information as possible on the raw material (green coffee beans) to be used, let's start with:
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. So it’s important to measure the amount of beans first. In general, the starting temperature is a little under 200°C but this always depends on both the type of roasting and the characteristics of the coffee required. So, how do we find the ideal situation? Well, familiarity with the machine makes things much easier to predict, but the best way is to do many different tests and monitor and record what happens. At this point one needs to “pre-heat” their senses too, since the roasting phase is also a multisensory experience and requires a lot of concentration. This is why it is always better to use software that monitors each step so you can then devote yourself entirely to what is happening inside the machine at any given moment. Now, let the show begin!
The curtain is drawn, the actors enter stage right, and the first act begins – the exposition! 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. How long should it last, ideally? Generally, no less than 30% and no more than 50% of the duration of the whole roasting phase. A drying phase that is too fast will result in an uneven heat distribution inside the bean with a greater risk of external burning or scorching, while a drying phase that is too slow leads to under developed roasting resulting in a bitter, earthy taste.
DEVELOPMENT OF THE AROMAS
The second act begins, the complication and the conflict! Here we focus on our nose and ear – the smell and the sound. 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. The interaction between simple sugars and amino acids exposed to high temperatures allows the formation of a multitude of very complex aromatic compounds. This transformation takes its name from the French chemist who first described it, Louis Camille Maillard, and who - with his thick characteristic moustache - is considered to be one of the very first “bloomers”! 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. From here on the coffee assumes characteristics making it suitable to drink. If we stop the roasting at the first crack, we would get what’s called a “City Roast”. The nose and the ear therefore guide us during this phase, assisted by a good dose of intuition and experience.
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. This phase ends with the pouring of the roasted beans into the cooling tank. Here a second quieter crack occurs leading to the breaking of most of the remaining fibres in the bean and leading to the escape of CO2 together with other organic material. Here we are at the height of the tension and close to the third and final act with results that vary between a “Full City Roast” and a “Full City+ Roast” depending on whether you stop before or during the second crack. The caramelization represents 10%-20% of the roasting phase depending again on the results we want to obtain.
Here we are well within the third act. The concluding, climactic act - the denouement! And after the drum roll comes the release of all the protagonists. 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). Here the performance ends and another begins that leads to further transformation: the drink in the cup.
Applause? Encore? Your audience shall certainly decide!