The roasting curve

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Coffee is not born as we are used to seeing it, but it is born green.

The beans are the seeds of the fruit of the coffee plant called Drupa.

Only with roasting they become brown in color.

We have already seen how belonging to the Arabica and Robusta varieties influences the organoleptic profile of coffee >>

We have already seen how the harvesting method >> and the processing method >> affect the final quality of the coffee.

With this article, instead, let's see how the roasting profile changes, and from which phases it is composed. All with the aim of understanding how fundamental this process is to develop the more than 800 volatile substances that make up the aroma.

The roasting curve is a line that is created on a graph, to represent the roasting profile used for a given coffee at a given moment, based on the time (minutes) taken and the temperature (° C) detected.

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For the roaster it is essential to know how to read it because it is the only way to obtain a register of all the roasts and thus be able to check them in case the coffee is perfect or defects are found. In the first case in order to replicate the roasting and in the latter to modify and improve the roasting.

It is also very important to know that there is no single roasting profile for each coffee. It can also change for the same species, based on humidity, density, size of the beans, the temperature of the external environment and so on.

But let's start from the beginning.

The roasting curve

The roasting curve consists of several phases:

  1. Pre-heating
  2. Drying
  3. Aromatic development
  4. Development phase
  5. Cooling down

Phase 1 - The pre-heating

In this phase, the drum, that is the roasting chamber, is heated, a large tub similar to a washing machine, with a basket that rotates inside it, keeping the beans always in motion.

It is important to start with a hot car in order to activate the chemical and physical processes that lead to the development of aromas.

The initial temperature depends on the size of our machine, on the mass of coffee that I will introduce and on its density and humidity.

A skilled roaster knows his machine well and knows how to do the first tests to evaluate the correct starting temperature.

In fact, let us remember that toasting is an art and the roaster is a mix between artist and scientist.

Phase 2 - Drying

The coffee enters the green roaster, dense and humid, about 12% if it is fresh coffee, and above all at room temperature, so in the first few minutes it lowers the temperature of the roaster.

The temperature continues to drop until the TP, that is the Turning Point, that moment when the temperature begins to rise again and therefore the roasting curve changes direction and becomes positive again.

In this phase, the coffee absorbs heat from the environment in which it is located. This is why it is also called the endothermic phase.

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Thus we begin to see the first chemical-physical changes of the grain:

  • Loss of water, therefore a decrease in humidity;
  • Degradation of chlorogenic acids and trigonellin;
  • Increased pressure inside the grain;
  • Color change from green to yellow.

And it is precisely when the coffee turns yellow that this second phase ends.

It should last no less than 30% of the roasting process and no more than 50%, to avoid common roasting mistakes.

We will deal with this topic in another ABCoffee article.

Phase 3 - The aromatic development

From here on, the roaster must implement sight, smell and hearing.

We entered the main act! This is precisely the moment in which we build the aromatic profile of our coffee.

During this phase we slow down the acceleration of the temperature, while countless chemical reactions are triggered. Given the countless number of reactions, we will dedicate another article to this topic.

The main chemical-physical modifications of the beans in this phase are:

  • Volume increase;
  • Weight loss;
  • Change in color from yellow to cinnamon;
  • Aroma development.

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This third phase ends with an auditory signal: the “Crack”.

Phase 4 - Development phase

This phase, as we have just said, begins with the Crack.

The "Crack" is a crackle, very similar to what popcorn does.

But how does it happen?

Until now, the grain has absorbed heat from the external environment and has lost all its moisture. The degradation of water has formed steam and gases that push outwards, pressing on the walls of the grain.

The pressures have reached such a force that they split the surface of the grain to be able to exit, thus generating this sound, the “Crack”, as well as an increase in the volume of the grain.

Here the exothermic phase begins, since now for the first time it is the bean that gives heat back to the drum, generating a rapid rise in temperature, which the roaster must be able to control in a skilful and immediate way.

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This is the phase that lasts the least in the whole process and it is in this momente that you choose which level of roasting of your coffee:

  • If the coffee is removed near the first crack we will have a light roast;
  • If you remove it a few seconds or a minute from the crack, you will have a medium roast;
  • If you take it off on the second crack we will have a dark roast.

In general, the less the coffee is roasted, the more it will have hints of freshness and a well-developed acidity. A medium roast maximizes all the qualities and aromas of carefully selected raw coffee, acquiring sweetness and body. As the roasting proceeds, the coffee becomes more bitter to the detriment of sweetness and acidity.

To learn more about the differences between the different roasting levels, read The Medium Roasting >>

Phase 5 - Cooling down

Once the coffee roasting level has been chosen, the drum opens to let the coffee flow into the cooling tank.

This phase is also fundamental, as if the coffee is not cooled immediately, roasting could proceed and create problems or defects in the extracted coffee.

This phase must last a maximum of 5 minutes to be effective.

Now that you understand how important roasting is and which phases the roasting curve is made up of, you just have to try a medium roast coffee! >>

If you have any questions, I am available to answer on shop@caffeernani.com or on whatsapp at +39 3756879940. 

Martina Mazzoleni 

The best coffees, roasted to perfection and delivered where you want

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