The pvrolysis or roasting is an operation which consists in raising the coffee beans
at high temperature to develop the sensory qualities of green coffee. Generally the roasting temperature is between 190 and 230 ° C. During this period the coffee undergoes several important physical and chemical modifications :
- The bean water is largely eliminated, since the green coffee passes from one humidity of around 12% to 2% for roasted coffee
- The sugars are partially degraded and turn brown giving color characteristic of roasted coffee
- Carbon dioxide is released but part remains trapped in the grain which causes the cells to explode.
- Depending on the varieties or ecological zones where the coffee grows, the rate of swelling can vary from 40 to more than 100%
- The average weight loss is around 17% but the differences range from 12-23% depending on the degree of dryness of the coffee and the method of roasting
- Volatile acids are evaporated
- The fat is altered
- The aromatic compounds develop thanks to the Maillard reaction. The carbohydrates and amino acids break down and / or combine with each other. The degradation of Strecker would allow the formation of aldehydes and pyrazines which are volatile compounds.
There are currently two techniques for roasting coffee, one to indirect heating or conduction, the other with direct heating or fluid bed.
During conduction heating the coffee is placed inside an animated ball of a rotary movement which is heated below by electric resistances (small roasters under 12 kg) or by gas or fuel.
With the fluid bed heating – also called high-yield – the coffee is roasted in a current of hot air whose temperature can reach 350 ° C. This method allows a better heat exchange therefore a roasting time reduced to 2 or 3 minutes. The rise in temperature promotes the production of carbon dioxide which causes a bursting and a more important degradation of the cells.
The coffees produced by this mode of roasting give finer grinds and larger. This saves up to 15% in weight of coffee for the manufacture of the drink compared to the conventional system by conduction.
Temperature and roasting time play a key role in quality final product. Nowadays there are roasters who praise the qualities of slowly roasted coffee in 20 – 25 minutes. This procedure persists because the material is obsolete or unsuitable. In our opinion the roasting time should be
between 6 and 12 minutes depending on the equipment used.
Roasting parameters vary according to habits of consumer countries and the origin of the coffee used. We can globally distinguish three types of roasting according to the shade of the bean obtained:
- Light: USA, Germany
- Average: France
- Dark: Italy
We thus name the set of reactions that occur during heating or roasting.
These reactions involve sugars, amino acids, peptides and proteins during heat treatment. This reaction is maximum when humidity is low, ph slightly alkaline and temperatures high. These
constitute a very complex set of chemical reactions, simultaneous or successive. They are linked to the degradation of Strecker which leads to the formation of aldehydes by decarboxylation of amino acids.
Maillard reactions start with a condensation reaction between a aldehyde function of a sugar (ose) and an amine function of a free amino acid or constituent of a protein.
At first, an imine called the Schiff base is formed, which rearranges itself (transformation of Amadori) into a ketosamine or an aldosamine.
In a second step, we observe the degradation of the amino acid by the reductone with the formation of a host of aromatic compounds: aldehydes and heterocycles of all kinds, oxygenated, nitrogenous, sulfur, dioxygenated, dinitrogenous, etc. These specific reactions of the Maillard reaction are known as degradation of Strecker.