The roasting curve

I maestri della tostatura media
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 drupe.

Only with roasting do they turn brown.

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

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

With this article, let’s see instead how the roasting profile changes and what stages it consists of. All with the aim of understanding how crucial this process is in developing the more than 800 volatiles that make up 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 time, based on the time (minutes) taken and temperature (°C) measured.

It is essential for the roaster to be able to read it, as well as create it, because it is the only way to obtain a history of all roasts and thus be able to check them in case, during tasting, the coffee turns out to be perfect or in case it has defects. In fact, only in this way will it be possible to replicate the roasting, or avoid defects by modifying and improving it.

Most important then is to know that there is no single roast profile for every coffee. It can change even for the same species, depending on humidity, density, grain size, outdoor environment temperature and so on.

But let’s start from the beginning.

The roasting curve

The roasting curve consists of several stages:

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

Step 1 – The pre-heating

At this stage, the drum, or roasting chamber, a large, washing machine-like tank with a basket that rotates inside, keeping the beans moving at all times, is heated.

It is important to start with the machine hot in order to activate the chemical and physical processes that lead to flavor development.

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

A skilled roaster knows his machine well and knows how to do initial tests to assess the correct initial temperature.

Indeed, let us remember that roasting is an art, and the roaster is a mix of artist and scientist.

Step 2 – Drying

The coffee enters the roaster green, thick and moist, about 12 percent if it is fresh coffee, and mostly at room temperature, so in the very first few minutes turn down the roaster temperature.

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

phase the coffee absorbs heat from its environment. That is why it is also called the endothermic phase.

Thus we begin to see the first chemical and physical changes in the grain:
  • Loss of water, thus decrease in humidity;
  • Degradation of chlorogenic acids and trigonelline;
  • Increased pressure inside the grain;
  • Color change from green to yellow.

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

It should take no less than 30 percent of the roasting process and no more than 50 percent to avoid common roasting errors.

We will cover this topic in another ABCoffee article.

Stage 3 – The aromatic development

From here on, the roaster must put sight, smell and hearing into action.

We have entered the main act! This is precisely the time when we build the flavor profile of our coffee.

During this phase we slow the acceleration of temperature while triggering countless chemical reactions. Given the countless number of reactions we will devote another article to this topic.

The main chemical and physical changes in the grains at this stage are:
  • Increased volume;
  • Weight loss;
  • Color change from yellow to cinnamon;
  • Aroma development.

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

Phase 4 – Development Phase

This phase as we just mentioned begins with the Crack.

“Crack” is a popping sound, very similar to what popcorn makes.

But how does this happen?

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

The pressures have reached such a force that they crack the surface of the grain in order to get out, thus generating this sound, the “Crack,” as well as an increase in the volume of the grain.

Here the exothermic phase begins,

for now for the first time it is the bean that re-heats the drum, generating a rapid surge in temperature, which the roaster must be able to skillfully and immediately control.

This is the absolute least time-consuming stage in the entire process, and it is here that you choose what level of roast to give your coffee:
  • If the coffee is removed near the first crack we will have a light roast;
  • If you take it off a few seconds to a minute after cracking you will have a medium roast;
  • If you take away at the second crack, then a second crackle, we will have a dark roast.

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

To learn more about the differences between the different levels of roasting read The Medium Roast >>

Step 5 – Cooling

Once the coffee roasting level is chosen, the drum is opened to let the coffee drop into the cooling tank.

Also crucial is this step, as if you do not cool the coffee immediately, roasting may proceed and create problems or defects in the extracted coffee.

This phase should last no more than 5 minutes to be effective.

Now that you understand how important roasting is and what stages the roasting curve is composed of, you just have to try a medium roast coffee!

If you have any questions I am available to answer them at [email protected] or on whatsapp at +39 3756879940.

Marketing, E-commerce e Social Media Manager
Coffee Lover


Martina Mazzoleni

Marketing, E-commerce e Social Media Manager Coffee Lover

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