What is coffee roasting?
It's a word that we hear mainly in reference to coffee , hazelnuts or almonds. In the collective imagination we know that it vaguely implies a source of heat and the act of cooking seeds and cereals.
But what's the difference between baking and roasting? Why use another word to say the same thing? And conversely, why use the same word to say two different things (roaster the profession vs. roaster the machine)?
Definition - nf
Roasting – nf – From the Latin Torre (to dry) and Facere (to make). A cooking method that uses dry heat to evenly heat food to develop distinctive flavors
Roasting coffee isn't just a simple oven bake like you'd do with a cake or roast chicken. Decades of roasting and billions of cups of coffee have elevated the practice to a true art form.
Tools - nm
Not so long ago (30 years ago), most coffee roasting was controlled using the 5 senses:
- The appearance : The color of the coffee bean reveals a lot about the roasting stages. First green, then yellow, light brown, and increasingly darker.
- Smell: During certain phases, notably the Maillard reaction, a characteristic smell is released from the beans and allows the roaster to determine the moment of roasting.
- Hearing: Coffee has the particularity of signaling the approach of its optimal cooking, by what is called the first crack. This is water vapor that will be released from the bean while it cracks, a bit like popcorn.
- Taste: This sense is not used during roasting, but afterward. Tasting coffees is essential for assessing the quality of the roaster's work and making corrections if necessary.
- Touch: The intensity of a roast can be judged by the texture of the roasted bean. A light roasted bean will be harder to crack than a dark roasted bean.
Technological progress has allowed for greater precision and, above all, consistency. Because if the taste of the same coffee differs from one roast to another, it can be complicated to explain this to the public.
- Roasting time
- The size of the spit (between 5kg and 15kg of green coffee)
- The loading temperature of green grains
- The outlet temperature of the green beans to the cooling tank
- The gas power, which will vary during roasting (programmed, ad hoc or a bit of both)
- The airflow, which must also be adjusted during roasting
- The rotation speed of the drum (usually constant during roasting)
The roasting parameters are very different depending on the type of extraction you are aiming for: when you want to roast for filter coffee , you use small roasts and much shorter roasting times than for espresso. To simplify, for filter coffee we are looking to reveal as much fruit as possible, while for espresso we need roundness and more body, even if each coffee is unique and this is not applicable to all beans.
Progress - nm
The beans rise in temperature throughout the roasting process. We also carefully monitor what we call the Rate of Rise (ROR), which allows us to precisely track the rate of change in this temperature. This data, generally expressed in °C/minute, is key to understanding the dynamics of a spit (or batch).
Concretely, roasting takes place in this way:
- Preheating : as with an oven with certain foods, you need to reach a certain thermal inertia in the machine and therefore start a preheating program, for at least 30 minutes (sometimes more, depending on the roaster model)
- Loading the green coffee : Pour the green beans into the hopper at the top of the machine (the large funnel). Once the hopper is open, the beans will fall into the drum and heat up while being stirred quickly to avoid conduction as much as possible (in other words, prolonged contact between the beans and the walls of the drum).
Tipping point : as you can see on the roasting curves, the temperature drops sharply at the beginning. This is simply due to the fact that you are loading 15kg of material at 20°C into an environment at over 200°C. The temperature of this environment will drop, before in turn changing the temperature of these 15kg of green coffee. The tipping point is the moment when the temperature stops dropping and starts to rise again.
- Drying phase : This drying phase allows the moisture (between 9 and 12%) of the green bean to evaporate. The water vapor released on the surface will change the color of the beans, which will go from pale green to a deeper green. This phase generally lasts at least half of the roasting.
- Maillard reaction : easily identifiable by the color of the beans which changes from green to yellow and the characteristic odor which is released (hay aromas), the Maillard reaction (around 160°C) marks the end of the drying phase and a series of reactions between sugars and amino acids to give a distinctive color and aromas. It is not exclusive to coffee and also gives their brown color to bread, chocolate and even meat.
- Caramelization : Following the Maillard reaction, caramelization begins and observes similar effects: browning and development of new aromatics. Unlike the Maillard reaction, caramelization does not require amino acids to occur, and in the case of coffee occurs a little later (170°C).
- The first crack: as mentioned above, this is the moisture contained in the coffee beans, which has transformed into water vapor, some of which had remained trapped within the beans. As the temperature rose, this water vapor expanded and tried to escape from the beans. It does so during this first crack, cracking the beans and emitting that recognizable, audible crack. This generally occurs around 200°C.
- Development phase : A key moment in coffee roasting that does not tolerate imprecision. From the first crack, it will be necessary to monitor both the development time in seconds and the development ratio (as a percentage of development time over the total roasting time). During this phase, the beans will continue to brown, and the aromatic characteristics will be defined: acidity, body, bitterness. The roasting style will be defined here: a light roast, rather widespread in Anglo-Saxon countries, will see the roaster remove the beans from the drum almost during the first crack, while a dark roast, common in Italy and France, will extend several minutes after.
NB: At Celsius, but this is just our opinion, we seek to enhance each coffee on a case-by-case basis, so there are no set rules other than avoiding excess. We therefore try to avoid underdeveloping the coffees as much as possible: a coffee taken out of the drum at the moment of the first crack will reveal aromas that are too vegetal for our taste, while a coffee taken out too late will reveal too many smoky and roasted aromas that will be more the mark of the roasting and not of the bean itself.
- The second crack : The absolute scarecrow of roasting when we talk about specialty coffee, the second crack occurs when the oils contained in the coffee (which have resisted until then, unlike the humidity) in turn want to extract themselves from the bean with a new cracking of the latter. At this stage, the coffee beans are dark brown, almost black, and especially shiny (due to the oils on the surface). On the nose, the beans will be rather enticing, on vanilla and tonka, but on the palate they will have no balance: no acidity, a lot of body and an extreme bitterness.
- Cooling : the most Instagrammable stage of roasting, the one where you see the blades spinning amidst the beautifully cooked beans. Yet it's a subject too often overlooked, cooling is just as important as the previous stages. Much like a hard-boiled egg being removed from the pan, it will continue to cook if you don't slow it down with cold water. It's the same for coffee: without a powerful chiller, the coffee will stay hot for over an hour and won't have the desired aromatic profile. Therefore, it must be done in 3-4 minutes maximum thanks to a powerful suction system.
Then - adv.
After roasting, we have several ways to analyze it. The roasting curve and data are important, but not sufficient.
- Cupping : As mentioned earlier, we need to taste all our coffees. We taste according to the standardized protocol commonly called cupping, which consists of lining up bowls and tasting with a spoon. This has the advantage of being able to taste a certain number of different coffees and compare them with each other.
- Weight : by weighing each spit after roasting, we can deduce the weight loss (in %), closely linked to the roasting level, from light to dark.
- Color : The human eye is insufficient to discern roasting differences with the naked eye, except in extremes. We therefore use a colorimeter, which allows us to accurately measure the color of ground coffee and thus the degree of roasting. The unit of measurement is the Agtron, ranging from 0 to 150. 0 is the darkest and 150 is the lightest.
For the bravest among us who have managed to read this far, let's go further and talk a little physics. Coffee roasting is nothing more or less than a heat transfer, or rather heat transfers, because there are three of them, more or less present depending on the roasting technology:
-
- Conduction : This is the transfer of heat within a material, without the movement of matter. It occurs through the vibration of atoms and the transmission of energy from one to another, therefore with physical contact. In the case of roasting, this conduction will mainly exist between the roaster drum and the coffee beans. This is a heat transfer present on most roasters on the market: Probat, Giesen, Diedrich.
-
- Convection : This is the transfer of heat by the movement of a fluid (liquid or gas). In the case of roasting, the hot air inside the drum will be the fluid that will transfer the heat to the coffee beans. This type of transfer exists on the 3 roaster brands mentioned above, and almost exclusively on the IMF and Loring brands.
-
- Radiation: This is the transfer of heat in the form of electromagnetic waves (infrared), without the need for material support (unlike the other two modes). A minority in coffee roasting, this transfer exists, to our knowledge, on certain models, via halogen lamps such as those of the Stronghold brand.
We believe that convection is the most interesting heat transfer method, as it will be balanced and homogeneous throughout the drum. This is why we chose a forced-air roaster, which has a remote heat source, to then send the air inside the drum. This way, the walls of the drum are not really hotter than the air inside, and we avoid conduction and the famous scorching as much as possible. We have no opinion on radiation as a heat transfer for coffee roasting, as we have used this type of roaster very little.
,
Choice - nm
With all these variables, you can imagine how difficult it is for a roaster to choose their machine. That's why, at Celsius, after more than two years of testing different brands of roasters, we decided to equip ourselves with a model from the Italian brand IMF , for its many qualities:
- Precision: Nearly 100% convection provides excellent consistency. Furthermore, the vortex system, which mixes air heated to 650°C with the ambient air, provides exceptional responsiveness.
- Consistency: Regardless of the outside temperature and humidity, our roaster is not affected, and we can reproduce the same curves with the same coffee. This allows us to optimize our roasting profiles to get the best out of them without having to adjust for external factors.
- Low NOX emissions: thanks to an integrated flue gas burner, NOX (nitrogen oxides) emissions are considerably reduced at the chimney outlet.
- Reduced gas consumption: Despite this system, this roaster does not consume more energy than an equivalent model from another brand. In addition, roasting times are 30 to 40% shorter, which optimizes energy consumption and therefore greenhouse gas emissions.
- Efficiency: The aforementioned features give our roaster great efficiency and a production capacity worthy of much larger models. This is why we are able to offer Co-Roasting, i.e., sharing this machine with roasters who do not yet have their own machine.
It's a word that we hear mainly in reference to coffee , hazelnuts or almonds. In the collective imagination we know that it vaguely implies a source of heat and the act of cooking seeds and cereals.
But what's the difference between baking and roasting? Why use another word to say the same thing? And conversely, why use the same word to say two different things (roaster the profession vs. roaster the machine)?
Definition - nf
Roasting – nf – From the Latin Torre (to dry) and Facere (to make). A cooking method that uses dry heat to evenly heat food to develop distinctive flavors
Roasting coffee isn't just a simple oven bake like you'd do with a cake or roast chicken. Decades of roasting and billions of cups of coffee have elevated the practice to a true art form.
Tools - nm
Not so long ago (30 years ago), most coffee roasting was controlled using the 5 senses:
- The appearance : The color of the coffee bean reveals a lot about the roasting stages. First green, then yellow, light brown, and increasingly darker.
- Smell: During certain phases, notably the Maillard reaction, a characteristic smell is released from the beans and allows the roaster to determine the moment of roasting.
- Hearing: Coffee has the particularity of signaling the approach of its optimal cooking, by what is called the first crack. This is water vapor that will be released from the bean while it cracks, a bit like popcorn.
- Taste: This sense is not used during roasting, but afterward. Tasting coffees is essential for assessing the quality of the roaster's work and making corrections if necessary.
- Touch: The intensity of a roast can be judged by the texture of the roasted bean. A light roasted bean will be harder to crack than a dark roasted bean.
Technological progress has allowed for greater precision and, above all, consistency. Because if the taste of the same coffee differs from one roast to another, it can be complicated to explain this to the public.
- Roasting time
- The size of the spit (between 5kg and 15kg of green coffee)
- The loading temperature of green grains
- The outlet temperature of the green beans to the cooling tank
- The gas power, which will vary during roasting (programmed, ad hoc or a bit of both)
- The airflow, which must also be adjusted during roasting
- The rotation speed of the drum (usually constant during roasting)
The roasting parameters are very different depending on the type of extraction you are aiming for: when you want to roast for filter coffee , you use small roasts and much shorter roasting times than for espresso. To simplify, for filter coffee we are looking to reveal as much fruit as possible, while for espresso we need roundness and more body, even if each coffee is unique and this is not applicable to all beans.
Progress - nm
The beans rise in temperature throughout the roasting process. We also carefully monitor what we call the Rate of Rise (ROR), which allows us to precisely track the rate of change in this temperature. This data, generally expressed in °C/minute, is key to understanding the dynamics of a spit (or batch).
Concretely, roasting takes place in this way:
- Preheating : as with an oven with certain foods, you need to reach a certain thermal inertia in the machine and therefore start a preheating program, for at least 30 minutes (sometimes more, depending on the roaster model)
- Loading the green coffee : Pour the green beans into the hopper at the top of the machine (the large funnel). Once the hopper is open, the beans will fall into the drum and heat up while being stirred quickly to avoid conduction as much as possible (in other words, prolonged contact between the beans and the walls of the drum).
Tipping point : as you can see on the roasting curves, the temperature drops sharply at the beginning. This is simply due to the fact that you are loading 15kg of material at 20°C into an environment at over 200°C. The temperature of this environment will drop, before in turn changing the temperature of these 15kg of green coffee. The tipping point is the moment when the temperature stops dropping and starts to rise again.
- Drying phase : This drying phase allows the moisture (between 9 and 12%) of the green bean to evaporate. The water vapor released on the surface will change the color of the beans, which will go from pale green to a deeper green. This phase generally lasts at least half of the roasting.
- Maillard reaction : easily identifiable by the color of the beans which changes from green to yellow and the characteristic odor which is released (hay aromas), the Maillard reaction (around 160°C) marks the end of the drying phase and a series of reactions between sugars and amino acids to give a distinctive color and aromas. It is not exclusive to coffee and also gives their brown color to bread, chocolate and even meat.
- Caramelization : Following the Maillard reaction, caramelization begins and observes similar effects: browning and development of new aromatics. Unlike the Maillard reaction, caramelization does not require amino acids to occur, and in the case of coffee occurs a little later (170°C).
- The first crack: as mentioned above, this is the moisture contained in the coffee beans, which has transformed into water vapor, some of which had remained trapped within the beans. As the temperature rose, this water vapor expanded and tried to escape from the beans. It does so during this first crack, cracking the beans and emitting that recognizable, audible crack. This generally occurs around 200°C.
- Development phase : A key moment in coffee roasting that does not tolerate imprecision. From the first crack, it will be necessary to monitor both the development time in seconds and the development ratio (as a percentage of development time over the total roasting time). During this phase, the beans will continue to brown, and the aromatic characteristics will be defined: acidity, body, bitterness. The roasting style will be defined here: a light roast, rather widespread in Anglo-Saxon countries, will see the roaster remove the beans from the drum almost during the first crack, while a dark roast, common in Italy and France, will extend several minutes after.
NB: At Celsius, but this is just our opinion, we seek to enhance each coffee on a case-by-case basis, so there are no set rules other than avoiding excess. We therefore try to avoid underdeveloping the coffees as much as possible: a coffee taken out of the drum at the moment of the first crack will reveal aromas that are too vegetal for our taste, while a coffee taken out too late will reveal too many smoky and roasted aromas that will be more the mark of the roasting and not of the bean itself.
- The second crack : The absolute scarecrow of roasting when we talk about specialty coffee, the second crack occurs when the oils contained in the coffee (which have resisted until then, unlike the humidity) in turn want to extract themselves from the bean with a new cracking of the latter. At this stage, the coffee beans are dark brown, almost black, and especially shiny (due to the oils on the surface). On the nose, the beans will be rather enticing, on vanilla and tonka, but on the palate they will have no balance: no acidity, a lot of body and an extreme bitterness.
- Cooling : the most Instagrammable stage of roasting, the one where you see the blades spinning amidst the beautifully cooked beans. Yet it's a subject too often overlooked, cooling is just as important as the previous stages. Much like a hard-boiled egg being removed from the pan, it will continue to cook if you don't slow it down with cold water. It's the same for coffee: without a powerful chiller, the coffee will stay hot for over an hour and won't have the desired aromatic profile. Therefore, it must be done in 3-4 minutes maximum thanks to a powerful suction system.
Then - adv.
After roasting, we have several ways to analyze it. The roasting curve and data are important, but not sufficient.
- Cupping : As mentioned earlier, we need to taste all our coffees. We taste according to the standardized protocol commonly called cupping, which consists of lining up bowls and tasting with a spoon. This has the advantage of being able to taste a certain number of different coffees and compare them with each other.
- Weight : by weighing each spit after roasting, we can deduce the weight loss (in %), closely linked to the roasting level, from light to dark.
- Color : The human eye is insufficient to discern roasting differences with the naked eye, except in extremes. We therefore use a colorimeter, which allows us to accurately measure the color of ground coffee and thus the degree of roasting. The unit of measurement is the Agtron, ranging from 0 to 150. 0 is the darkest and 150 is the lightest.
For the bravest among us who have managed to read this far, let's go further and talk a little physics. Coffee roasting is nothing more or less than a heat transfer, or rather heat transfers, because there are three of them, more or less present depending on the roasting technology:
- - Conduction : This is the transfer of heat within a material, without the movement of matter. It occurs through the vibration of atoms and the transmission of energy from one to another, therefore with physical contact. In the case of roasting, this conduction will mainly exist between the roaster drum and the coffee beans. This is a heat transfer present on most roasters on the market: Probat, Giesen, Diedrich.
- - Convection : This is the transfer of heat by the movement of a fluid (liquid or gas). In the case of roasting, the hot air inside the drum will be the fluid that will transfer the heat to the coffee beans. This type of transfer exists on the 3 roaster brands mentioned above, and almost exclusively on the IMF and Loring brands.
- - Radiation: This is the transfer of heat in the form of electromagnetic waves (infrared), without the need for material support (unlike the other two modes). A minority in coffee roasting, this transfer exists, to our knowledge, on certain models, via halogen lamps such as those of the Stronghold brand.
We believe that convection is the most interesting heat transfer method, as it will be balanced and homogeneous throughout the drum. This is why we chose a forced-air roaster, which has a remote heat source, to then send the air inside the drum. This way, the walls of the drum are not really hotter than the air inside, and we avoid conduction and the famous scorching as much as possible. We have no opinion on radiation as a heat transfer for coffee roasting, as we have used this type of roaster very little.
,
Choice - nm
With all these variables, you can imagine how difficult it is for a roaster to choose their machine. That's why, at Celsius, after more than two years of testing different brands of roasters, we decided to equip ourselves with a model from the Italian brand IMF , for its many qualities:
- Precision: Nearly 100% convection provides excellent consistency. Furthermore, the vortex system, which mixes air heated to 650°C with the ambient air, provides exceptional responsiveness.
- Consistency: Regardless of the outside temperature and humidity, our roaster is not affected, and we can reproduce the same curves with the same coffee. This allows us to optimize our roasting profiles to get the best out of them without having to adjust for external factors.
- Low NOX emissions: thanks to an integrated flue gas burner, NOX (nitrogen oxides) emissions are considerably reduced at the chimney outlet.
- Reduced gas consumption: Despite this system, this roaster does not consume more energy than an equivalent model from another brand. In addition, roasting times are 30 to 40% shorter, which optimizes energy consumption and therefore greenhouse gas emissions.
- Efficiency: The aforementioned features give our roaster great efficiency and a production capacity worthy of much larger models. This is why we are able to offer Co-Roasting, i.e., sharing this machine with roasters who do not yet have their own machine.
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