Coffee Roasting: How Does it Work?

Coffee roasting is the act of cooking, heating, and grilling the green coffee bean to transform it into a roasted coffee bean. The device used for this process is called a roaster. If, at one time, families roasted their coffee themselves, this basic operation is now a matter for the specialists. Today, the roaster – or master roaster – heats the green coffee bean in order to transform it and achieve a brown bean consisting of over 800 aromatic molecules.

What’s the point of roasting?

Roasting is without a doubt the most important step in coffee production. The green coffee beans are, in effect, flavourless, but heat will have several impacts on the beans:

  • They will lose some of their water and inflate slightly;
  • Their colour will gradually transform;
  • The thermal breakdown of certain chemical compounds will help the bean develop its flavour.






The result (the taste of the coffee, in other words) will depend, however, on the following parameters:

  • The variety and the geographical origin of the coffee (altitude, soil type, the angle of the ground, how much sun it gets, climate, humidity);
  • The post-harvest processing conditions;
  • The roasting;
  • The final preparation.


Without roasting, none of these steps will let you obtain that flavour that we’re familiar with.

This chemical transformation in the chain – roasting – was discovered by two chemists: Maillard and Strecker.


The Maillard chemical reaction:

It’s the interaction under the influence of heat between the sugar and the acids naturally contained in the coffee that leads to caramelization. This caramelization is characterized by a change in pigmentation/colour and has the effect of giving the coffee all its flavours.

The Strecker degradation reaction:

This is the chemical change that occurs in cascade after the Maillard reaction. It leads to a release of CO2 and is responsible for the foam that results from preparing the coffee using the espresso method, also called the crema.

Technical process


The green coffee enters the oven through a hopper located above the machine. The roasting is carried out in a cylindrical broiler that consists of blades called the roaster. The drum that surrounds the roaster is heated by electricity, wood, or gas, and remains constantly in rotation (the same principle as a washing machine). This operation allows for a continuous heat transfer without the beans directly touching a hot surface for prolonged periods of time.


The heat in the oven should be between 180°C and 220°C when the beans enter. Their “coolness” then drops the temperature in the roaster to 120°C, but this slowly rises to around 200°C in the equivalent of 15 minutes. This heat transfer operation is complex, since it is accompanied by physical and chemical transformations leading to the problems of friction and compression inside the beans. This heat transfer is based on an important characteristic of the coffee beans: their thermal conductivity.



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During this period, the master roaster analyzes and regulates the heat source based on the reactions of the coffee. It’s the roaster’s responsibility to determine the progress of the roasting (duration, temperature, air circulation, etc.). Often, the control over the roasting is done automatically during the first 17 minutes. Once this quarter of an hour has passed, the roaster judges the results achieved with a window and a probe, with which they can remove a few coffee beans. Their analysis – which is at once visual, olfactory, and auditory – lets them evaluate the roasting. If it seems “correct,” they open the metallic door on the roaster to let the coffee beans flow onto a cooling tray. Blades and a powerful fan then spring into action to quickly cool the beans. Without this step, the coffee beans will continue to heat up, and the roasting will then exceed the desired level.

Roasting and taste

As with other products such as wine, aroma plays a major role in the pleasure one finds when drinking a cup of coffee. This aroma is perceived by the nasal mucosa either directly, through the nose, or retronasally through the pharynx when the volatile compounds rise up towards the olfactory mucosa.

Roasting also has an impact on the colour of the bean: from white or green, it becomes blonde, brown, dark brown, or black. The longer the beans are roasted, the darker they become and the stronger their flavour will be. We thus speak of brown, amber, medium-black, or black coffee.

Once roasted, the coffee can be sold in whole beans or already ground. Certain coffee makers include an option to grind the beans, which helps you achieve an excellent brew because it’s so fresh. Ground coffee can be just as good, but for that, it has to be conditioned in an opaque, airtight bag.

Once the green coffee is delivered to our workshop, it will already have been subjected to several taste tests, based on the different roasting levels. Our first goal is to analyze the coffee bean and see how its flavours develop through roasting.

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