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Degradat

Roasting

 

The most important process in the technology of coffee is roasting, because, during this process, is when the coffee beans develop their organoleptic qualities (smell and taste), as a result of physicochemical changes that take place inside the bean.

 

There are two groups of roasted coffees, the natural one and the “torrefacto”, which is done by adding up to 15 kg of sugar per 100 kg of green coffee. After caramelisation, the coffee beans acquire a bitter stronger flavour, change colour and become darker. This variety of roasting is only produced in Spain, Portugal, Argentina and some Southeast Asian countries. In our country it is now in recession, because it does not add any positive value to natural coffee.

 

In Cafès Cornellà, roasting usually takes place before blending, because each different origin provides significant characteristics, so, time and temperatures should vary and the process has to be different depending on each type.

 

Physical changes carried out during roasting:

  • Volume increases, because the breakdown of carbohydrates produces carbon dioxide, which causes the expansion of the cells, and even occasional structural fractures.
  • Weight of loss, due to evaporation of the water accumulated in the beans during the post-harvest processes (pulping, drying and storage). The density of the bean changes from 550-700g / L (green coffee) to 300 - 450g / l (roasted coffee).
  • Colour changes. It turns from the green-golden yellow to dark brown. This factor depends on the concentration of melanin substances, depending on the roasting cycles.

 

Chemical changes. Changes and variations challenged by the different components of the coffee bean:

  • Decreasing of water content. Due to the evaporation produced by the exposure to high temperatures.
  • Increasing of fatty substances, due to the loss of weight. It may also arrive that oil inside the bean removes to the surface.
  • Reduction of carbohydrate because of decomposition of sucrose by high temperatures.
  • Reduction of chlorogenic acids, which means a reduction of the acidity of the infusion. So, short cycles involve less acidity.
  • Loss of trigonelline, which involves an improvement of the taste and digestibility.
  • Loss of protein, and increasing of free amino-acids thermally stable, which are involved in many reactions that produce coffee aroma: degradation of hydroxylamine acids, Maillard reaction, Strecker degradation, degradation of sulfur-amino acid and degradation of the proline and hydroxyproline, among others.

 

 

Roasting process consists of three stages:

 

  • Dehydration: The purposes are drying coffee beans and evaporate water. The temperature reached is about 100 °C. The beans change their colour to yellow because of the degradation of some green pigments.
  • Roasting: The temperature reaches 220°C - 250°C. First, the process is exothermic, and pyrolytic reactions take place as well as temperature increases. Then the process becomes endothermic and the volatile compounds appear.  Last stage is again exothermic. To obtain a darker coffee it is necessary to increase the temperature without turning to carbonization, which takes place up about 250°C. During this last exothermic phase, the coffee beans crackle due to the rupture of the cells, when the inner pressure goes up, releasing a substantial quantity of energy.

The first gases are bluish and turn to dull greyish. They first spread the characteristic smell of fresh bread and later smell spicy and sour. Finally they become more dense and whitish.

  • Cooling: The last phase must stop roasting with rapid cooling of the beans to avoid excessive cooking.

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