It's a word we mainly hear when talking about coffee , hazelnuts, or almonds. In the collective imagination, it vaguely implies a source of heat and the act of cooking seeds and grains.
But what is the difference between cooking and roasting, ultimately? Why use a different word for the same thing? And conversely, why use the same word for two different things (roaster the craft vs. roaster the machine)?
Definition of roasting
Roasting – nf – From the Latin Torre (to dry) and Facere (to make). A cooking method that uses dry heat to evenly heat food and develop particular aromas.
Coffee roasting is not simply baking in an oven like you would a cake or a roast chicken. Decades of roasting and billions of cups consumed have elevated this practice to a true art form.
Tools
Not so long ago (30 years ago), most coffee roasting was controlled using the 5 senses:
- Visually , the color of the coffee bean reveals a lot about the roasting stages. First green, then yellow, light brown, and increasingly darker.
- The sense of smell: During certain phases, particularly the Maillard reaction, a characteristic odor is released from the beans, allowing the roaster to determine the roasting stage.
- Hearing: Coffee has the unique characteristic of signaling its approach to optimal roasting with what is called the first crack. This is water vapor that is 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 to assess the quality of the roaster's work and make adjustments if necessary.
- Touch: The intensity of a roast can be assessed by the texture of the roasted bean. A light roast bean will be harder to break than a dark roast bean.
Technological progress has led to greater precision and, above all, consistency. Because if the taste of the same coffee differs from one roast to another, it can be difficult 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 temperature at which the green grains exit the cooling tank
- The power of the gas, which will vary during roasting (programmed, ad hoc, or a bit of both)
- The airflow, which also needs to be adjusted during roasting.
- The rotation speed of the drum (generally constant during roasting)
Roasting parameters vary greatly depending on the type of extraction desired: when roasting for filter coffee , we use smaller roasting times and much shorter roasting times than for espresso. To simplify, for filter coffee we aim to bring out maximum fruitiness, while for espresso we want roundness and more body, although every coffee is unique and this doesn't apply to all beans.
The roasting process
The beans heat up throughout the roasting process. We also closely monitor what we call the Rate of Rise (ROR), which allows us to precisely track the rate at which this temperature changes. This data, generally expressed in °C/minute, is key to understanding the dynamics of a batch.
In practice, the roasting process unfolds as follows:
- Preheating : as with an oven with certain foods, a certain thermal inertia must be reached in the machine and therefore a preheating program of at least 30 minutes (sometimes more, depending on the roaster model)
- Loading the green coffee beans : Pour the green beans into the hopper at the top of the machine (the large funnel). Once the hopper is opened, the beans will fall into the drum and heat up while being stirred rapidly to minimize conduction (in other words, prolonged contact between the beans and the drum walls).
Tipping point : As seen on roasting curves, the temperature drops sharply at the beginning. This is simply because you are loading 15 kg of raw material at 20°C into an environment exceeding 200°C. The temperature of this environment will drop, before in turn affecting the temperature of these 15 kg of green coffee beans. The tipping point is the moment when the temperature stops dropping and begins to rise.
- Drying phase : This drying phase allows the moisture (between 9 and 12%) of the green beans to evaporate. The water vapor released at 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 process.
- Maillard reaction : easily identifiable by the color of the beans, which changes from green to yellow, and the characteristic odor they release (hay-like aroma), the Maillard reaction (around 160°C) marks the end of the drying phase and triggers a series of reactions between sugars and amino acids to produce a distinctive color and aroma. It is not exclusive to coffee and also gives bread, chocolate, and meat their brown color.
- Caramelization : Following the Maillard reaction, caramelization begins and exhibits similar effects: browning and the development of new aromas. Unlike the Maillard reaction, caramelization does not require amino acids to occur, and in the case of coffee, it happens slightly later (170°C).
- The first crack: as mentioned earlier, this is due to the moisture inside the coffee beans, which has turned into water vapor, some of which remained trapped within the beans. As the temperature rises, this water vapor expands and tries to escape from the beans. It succeeds during this first crack, splitting the beans and emitting that audible and identifiable cracking sound. This generally occurs around 200°C.
- Development Phase : A crucial moment in coffee roasting that leaves no room for error. From the first crack, it's essential to monitor both the development time in seconds and the development ratio (as a percentage of the total roasting time). During this phase, the beans will continue to brown, and their aromatic characteristics will develop: acidity, body, and bitterness. The roasting style is defined here: a light roast, common in English-speaking countries, involves removing the beans from the drum almost at the first crack, while a dark roast, prevalent in Italy and France, will continue for several minutes afterward.
Note: At Celsius, and this is just our personal opinion, we strive to bring out the best in each coffee on a case-by-case basis, so there are no set rules other than avoiding excess. We therefore avoid under-developing the coffees as much as possible: a coffee taken from the drum at the first crack will reveal aromas that are too vegetal for our taste, while a coffee taken too late will reveal too many smoky and roasted aromas, which are more indicative of the roasting process than the bean itself.
- The second crack : The absolute nightmare of roasting when it comes to specialty coffee , the second crack occurs when the oils in the coffee (which have resisted until now, unlike the moisture) try to escape from the bean, causing it to crack again. At this stage, the coffee beans are dark brown, almost black, and especially shiny (due to the surface oils). On the nose, the beans will be quite enticing, with hints of vanilla and tonka bean, but on the palate, they will lack balance: no acidity, a lot of body, and extreme bitterness.
- Cooling : the most Instagrammable step in roasting, the one where you see the blades spinning amidst the beautifully roasted beans. Yet, it's a subject too often neglected; cooling is just as important as the previous steps. Much like a hard-boiled egg taken out of the pan, it will continue to cook if you don't stop the process with cold water. It's the same for coffee. Without an efficient cooler, the coffee will stay hot for over an hour and won't have the desired aromatic profile. Therefore, it should be done in a maximum of 3-4 minutes using a powerful extraction system.
Afterwards
Following roasting, we have different ways to analyze the process. The roasting curve and data are important, but insufficient.
- Cupping As mentioned earlier, we need to taste all our cups. We taste them according to the standardized protocol commonly called cupping, which consists of lining up bowls and tasting with a spoon. This has the advantage of allowing us to taste a number of different coffees and compare them.
- Weight : by weighing each skewer after roasting, we can deduce the weight loss (in %), closely linked to the level of roasting, from light to dark.
- Color : The human eye is insufficient to accurately discern differences in roasting levels with the naked eye, except at extremes. Therefore, we use a colorimeter, which allows us to precisely measure the color of the ground coffee and thus its degree of roasting. The unit of measurement is the Agtron, ranging from 0 to 150, with 0 being the darkest and 150 the lightest.
For the most intrepid among us, who have managed to read this far, let's delve deeper and talk a little about physics. Coffee roasting is nothing more than a heat transfer, or rather heat transfers, as there are three, more or less pronounced depending on the roasting technology:
- - Conduction : This is the transfer of heat within a material, without any physical movement of matter. It occurs through the vibration of atoms and the transmission of energy from one atom to the next, thus requiring physical contact. In the case of roasting, this conduction occurs primarily between the roaster drum and the coffee beans. This type of heat transfer is present in most roasters on the market: Probat, Giesen, Diedrich.
- - Convection : this is the transfer of heat through the movement of a fluid (liquid or gas). In the case of roasting, the hot air inside the drum is the fluid that transfers heat to the coffee beans. This type of heat transfer is present in all three roaster brands mentioned previously, and almost exclusively in the IMF and Loring brands.
- - Radiation: This is the transfer of heat in the form of electromagnetic waves (infrared), without the need for a physical medium (unlike the other two methods). While less common in coffee roasting, this transfer method exists, to our knowledge, in some models using halogen lamps, such as those from the Stronghold brand.
Convection is, in our opinion, the most efficient heat transfer method because it ensures balanced and even distribution throughout the drum. That's why we chose a forced-air roaster, which uses a separate heat source to circulate air inside the drum. This way, the drum walls don't get significantly hotter than the air inside, minimizing conduction and the dreaded scorching. We have no opinion on radiation as a heat transfer method for coffee roasting, as we have used this type of roaster very little.
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Choosing a coffee roaster
With all these variables, you can imagine how difficult it is for a coffee 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:
- Note: Nearly 100% convection ensures excellent uniformity. Furthermore, the vortex system, which mixes air heated to 650°C with ambient air, provides exceptional responsiveness.
- Consistency: regardless of the outside temperature and ambient humidity, our roaster remains unaffected, and we can reproduce the same roasting curves with the same coffee. This allows us to optimize our roasting profiles to achieve the best results 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. Furthermore, roasting times are 30 to 40% shorter, which optimizes energy consumption and therefore greenhouse gas emissions.
- Efficiency: The aforementioned features give our roaster high efficiency and a production capacity comparable to much larger models. This is why we are able to offer co-roasting, meaning the sharing of this machine with roasters who do not yet own their own.