Transformations during roasting

The transformation of the raw bean, i.e. green coffee, into a finished product is undoubtedly one of the most complex stages of coffee production.

When you smell green coffee beans, they have practically none of the olfactory, taste, and appearance characteristics that we generally associate with roasted coffee. Once roasted, the raw bean's raw materials undergo a major transformation that reveals more than 800 new identifiable organoleptic components. Nearly a third of these are aromatic components essential to aromatic complexity.

Green coffee has a moisture content varying from 10 to 12.5%. This significantly decreases after the roasting process. Generally, the roasting mechanism is characterized by three important phases, each with several degrees of roasting color (we use the Agtron reference measurement scale): golden coffee, first crack, and second crack. Coffee reacts to the following chemical reactions: pyrolysis, Strecker reaction, caramelization, and Maillard reaction, which we describe here.


Summary of transformations during roasting

Golden coffee

When lightly roasted, green coffee turns golden but its texture remains the same. The roasting degrees are between 80 and 70 on the Agtron scale. The end of the roasting process occurs before the first crack.

First crack

When we begin roasting, we observe a significant drop in temperature as the beans quickly absorb ambient energy. This first roasting phase is completely endothermic. At this stage, heat allows water to evaporate.

At the beginning of roasting, colored plant components, such as chlorophyll and anthocyanins, begin to decompose, and the bean changes from a green to a golden color. This transformation is generally accompanied by subtle changes in aroma, shifting from herbaceous to more roasted notes.

As the temperature increases, it tends to stabilize temporarily around 100°C (212°F); the water in the bean begins to form steam and a pressure gradient is created within the bean. As the pressure increases, it ruptures the cells of the bean to create an audible, repetitive sound, known as the "first crack."

At this stage, it is not uncommon for the bean to double in volume and begin to develop specific coffee aromas. Depending on the extent of roasting, the moisture levels in the bean drop from the initial level – 10 to 12% – to about 3 to 5% and a corresponding Agtron value of 75 to 65.

Second crack

The second roasting phase is accompanied by another endothermic stage, shorter than the first phase, and a short exothermic stage. During this phase, much of the water is entrained and the decomposition of sugars, proteins, and lipids begins to take shape. Unlike the first crack, which occurs mainly through the formation of steam, the second crack occurs through the formation of CO, CO2, NOx, and various other gases.

The exact temperature that causes this transformation can normally be observed between 225° and 230°C (437°-446°F). As with the first crack, a new increase in internal pressure occurs, and the beans begin to take on a shiny appearance due to the presence of coffee oils pushed to the surface. The Agtron values associated with these levels are 45 to 35.

To this day, we continue to discover physical and chemical reactions occurring during roasting.

Chemical reactions

Pyrolysis, or thermolysis, is the chemical decomposition of an organic compound created by a significant increase in its temperature to develop other products (gas and matter) that it did not originally contain. The operation is carried out in the absence of oxygen to prevent oxidation and combustion. This is the first stage of thermal transformation after dehydration.

The Strecker synthesis, named after Adolph Strecker who first discovered and published it in 1850, is a series of chemical reactions allowing the synthesis of an amino acid from an aldehyde or a ketone. These reactions concern the change in bean pigmentation.

Caramelization is an organic chemical reaction resulting from the dehydration of sucrose and the formation of fructose anhydrides. It is also a culinary technique that consists of polymerizing the sugar contained in a food, so that it acquires a nutty taste and browns without being burned or carbonized.

This is the process by which caramel is produced, as well as caramel-colored dyes. Caramelization is the mechanism during which several identical molecules assemble to form a single, larger one.

The Maillard reaction

The Maillard reaction (MRx) is one of the most important reactions occurring during roasting. The MRx occurs through the binding of an amino acid with a sugar, leading to the formation of a number of determining aromatic and colored components.

Examples of the Maillard reaction in other products: toasted bread and roasted meat flavor. MRx is non-enzymatic, meaning it requires an external energy source such as heat to trigger the reaction.

The AGTRON system

Developed by Agtron Corporation (Reno, NV), the Agtron scale is the most commonly used reference scale for classifying roasting colors.

The scale ranges from 25 to 95. It measures the reflected light from roasted coffee, whether whole bean or ground. The lower the number, the darker the roasted coffee – meaning less light is reflected – while higher numbers refer to lighter roasts. The image below illustrates typical color degrees.

References

Baumlin S (2006) Thermal cracking of biomass pyrolysis-gasification vapors in a perfectly self-agitated reactor by gas jets [archive] (Doctoral dissertation, Institut National Polytechnique de Lorraine)

(en) F. A. Ishitani, R. E. Reddy et al., « Asymmetric strecker synthesis using enantiopure sulfinimines: A convenient synthesis of α-amino acids », Tetrahedron Lett., vol. 35, no 50,‎ 1994, p. 9351–9354 (ISSN 0040-4039, DOI 10.1016/S0040-4039(00)78540-6).

(en) Hervé This, « Solution to Maillard and grilled steak challenge », Analytical and Bioanalytical Chemistry, vol. 407, no 27,‎ 2015, p. 8173-8174 (DOI 10.1007/s00216-015-9001-y)

Jacques Defaye, José Manuel Garcia Fernández, Valérie Ratsimba, « Les molécules de la caramélisation : structure et méthodologies de détection et d’évaluation », L’Actualité chimique, no 240,‎ novembre 2000, p. 24-27