After having already spoken about physical changes, let's now take a closer look at the most important, and extremely delicate, chemical transformations that take place during the roasting phase.
When coffee is roasted it undergoes significant physical and chemical changes: today we focus on the chemical mutations that occur inside the coffee bean which alters its composition, weight, taste and volume.
Let’s zoom into the three main processes: Drying, the Maillard reaction and Caramelization of sugars.
Drying is the phase in which the coffee bean absorbs heat, reducing the temperature inside the roasting drum. It is an endothermic process causing the beans to lose some characteristics but allowing it to also gain others. In this initial phase the water inside the bean evaporates, transforming its cellular structure and causing it to expand. Consequently there is a loss of weight - as we saw earlier when talking about physical reactions. This in turn leads to an increase in volume and a decrease in density of the bean.
Drying too quickly distributes the heat unevenly, with the risk of burning the bean’s outer surface. Conversely, drying too slowly under-roasts the bean, producing a bitter taste.
The Maillard reaction
This is a rather well-known chemical reaction in the world of chemistry that occurs in daily life, named after the scholar who first analysed it: Louis Maillard. With respect to coffee, this process generally begins when the bean reaches a temperature of 150°C, while the heat is still being absorbed endothermically and the exothermic phase begins.
The increasing temperature causes a reaction between the amino acids and the carbohydrates contained in the bean, which consequently varies the colour, flavour and nutritional content, producing melanoidins.
This is the phase in which most of the aromatic compounds are created, so it is important that it takes place within a correct timeframe and temperature range. Too high temperatures within a too short time will not allow these compounds to develop, resulting in a coffee that is much poorer in terms of aroma, and thus more acidic. Temperatures that are too low, on the contrary, will "boil" the bean, inhibiting these reactions since there is too little inertia necessary to carry out these processes correctly.
For your information, these are the very same reactions that occur when we prepare bread, a cake and even a grilled steak!
Caramelization of sugars
Above 170°C, complex carbohydrates break down into smaller sugar molecules. This process causes even more steam to be released as the temperature increases, causing the sugars to caramelize. This manifests itself audibly with the "first crack". It is precisely thanks to the caramelization phase that the perceived level of sweetness of the bean will increase giving the grains sweet notes of almond, hazelnut and caramel.
This phase is also called "development" (development time), it is measured from the first crack to the end of the roasting process and greatly characterizes the colour of the roasted bean as well as the final result we taste in the cup. This is why it is monitored very carefully - it will enhance the more acidic and fruity notes when it is shorter and will instead be sweeter and more balanced when it is extended a little longer.
There is no right or wrong development time per se, everything will depend on the final result desired and the extraction method that will be used to prepare the coffee being drunk.
In general, a light roast is used for filter extractions, a medium one (with development of 1:30 or 2:00) for cupping and a darker one for espresso. As a guide the longer the extraction times, the shorter the development times.
Have fun experimenting while not exaggerating with the times since the caramelization will be followed by carbonization and subsequently combustion! So stay off the phone or any other distraction and keep water and fire extinguishers handy!