Roasting
With this article we will make a first overview of coffee roasting starting from a question what is the roasting process?
Roasting is the first phase of the coffee production process that takes place in consumer countries and is the operation that allows us to consume coffee. From a chemical-physical point of view, roasting is a thermal process that allows heat to be transferred from the surrounding environment to the green coffee bean, which increases its temperature.
There are different styles of roasting therefore roasting processes and they depend on time and temperature.
- Fast Roasting less than 5 minutes
- Regular Roasting less than 20 minutes
- Slow Roasting more than 20 minutes
We can broadly divide the roasting process into three phases
- Drying of the grain
- Roasting
- Cooling
The different types of heat transfer
The heat from the external environment to the coffee bean can occur in several ways:
- Conduction: by contact with the hot body of the roaster
- Radiation: from hot surfaces which can be flames, gases, walls
- Convention where a gaseous fluid at a high temperature between 370 and 480 degrees surrounds the grain.
What happens in the toaster drum?
The heat transfer takes place in the toaster drum in this mode
- Chicco - grain
- Drum - grain
- Indoor environment - grain
- Outside air - grain
The transfer based on contact is immediate and consists in the direct heat transfer between the red-hot drum and the grain.
The Roasters
Roasters can be classified on the basis of multiple criteria. Here are some examples:
Machine with batch process is characterized by a discontinuous process by inserting a certain quantity of green coffee a certain quantity of roasted coffee comes out
Continuous cycle machine, green coffee is inserted in a continuous cycle and roasted coffee is released in a continuous cycle
The different types of heat transfer, as we anticipated before, can be conduction and convention:
- In conduction roasters, the temperature is controlled by adjusting the burner flame
- In the convection roaster, the temperature can be adjusted by acting on the air
PHASES OF THE ROASTING PROCESS
Roasting generates chemical reactions in the bean
• 20-130 ° C: Liquid - vapor transition
• 130-140 ° C: Endothermic reaction.
- It lasts about half of the total cycle
- The grains lose 6-8% of their weight
- Caramelization of sugars is activated
- The coffee takes on the cinnamon color.
140-160 ° C: Endothermic and exothermic reactions
- The coffee turns light brown
- The so-called "Maillard reactions" that contribute to giving life to hundreds of aromatic substances (they occur thanks to the heat brought to the combination of sugars / proteins, with the formation of brown compounds with the characteristic smell of bread crust)
- FIRST CRACK: Characterized by the escape of gases and Co2 from microscopic cracks in the grain caused by internal pressure. These gases are generated by chemical reactions (Maillard, caramelization, etc.). After this phase the grain becomes more fragile
- 160-190 ° C: The reactions described above involve the innermost structure of the grain
- 190-220 ° C SECOND CRACK: Purely exothermic reactions - Presence of evident micro cracks in the grain - Leakage of fumes - Leakage of oils - Pigmentation "like a monk's tunic" • 220-230 ° C: Brown phase. Darker color and abundant oils • 240 ° C: Black phase. Oils envelop the coffee. The culmination of the exothermic reaction, afterwards it can start to burn
DEGASING
Before tasting the coffee, it is necessary to rest for at least 24 hours (less for the filter)
Roasted coffee will emit gas for a long time, even a month The degassing speed depends on factors such as: Roasting time and color of the bean, Storage temperature, Packaging
The quality of roasted coffee is compromised by: Oxygen, Temperature, Humidity, Light.
FINAL TEMPERATURE AND TASTE OF COFFEE
The final temperature, with the same raw material and process conduction, together with the roasting curve, determines the degree of color and therefore the perceived bitter / acid ratio.
The correlation between the degree of roasting and the bitter / acid ratio is practically linear.
INCREASING THE ROASTING
- Organic acids are degraded
- Bitter substances are created or decomposed 15
- Light roasting: Between 170 and 190 ° C, the taste of the coffee is sweeter and more acidic than a more intense roasting.
- Medium roasting: From 200 ° to 220 ° C the balance of the taste and aroma of the espresso is best expressed.
- Dark roast: It ranges from 230 ° to 240 ° C. Above 230 ° C, the roaster's skill lies in avoiding the carbonization of the coffee, stopping the process an instant before it burns. It has a very strong and bitter coffee.