It’s a word we essentially hear when talking about coffee, hazelnuts, or even almonds. In the collective imagination, we know it vaguely involves a heat source and the cooking of seeds and cereals.
But what is the difference between cooking and roasting, ultimately? Why use a different 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 of roasting
Roasting – n.f. – From the Latin *Torre* (to dry) and *Facere* (to make). A cooking method that uses dry heat to uniformly heat food to develop specific aromas.
Roasting, when applied to coffee, is not simple oven-cooking as one would apply to a cake or a roast chicken. Decades of roasting and billions of cups consumed have elevated this practice to the status of a true art.
Tools
Not so long ago (30 years ago), the majority of coffee roasts were controlled using the 5 senses:
- Sight: the color of the coffee bean says a lot about the roasting phases. First green, then yellow, light brown, and darker and darker.
- Smell: During certain phases, notably the Maillard reaction, a characteristic odor is released from the beans, allowing the roaster to gauge the progress of the roast.
- Hearing: Coffee has the particularity of signaling that it is nearing its optimal roast through what is called the first crack. This involves water vapor being released from the bean while it cracks, a bit like popcorn.
- Taste: This sense is not used during roasting, but after. Tasting coffees is essential for estimating the quality of the roaster's work and making corrections if necessary.
- Touch: The intensity of a roast can be evaluated by the texture of the roasted bean. A bean in a light roast will be harder to break than a bean in a dark roast.
Technological progress has allowed for greater precision and, above all, consistency. Because if the taste of the same coffee differs from one roast to the next, it can be complicated to explain it to the public.
- Roasting time
- The batch size (between 5kg and 15kg of green coffee)
- The charging temperature of the green beans
- The discharge temperature of the beans into the cooling tray
- The gas power, which will vary during the roast (in a programmed manner, ad hoc, or a bit of both)
- The airflow, also to be adjusted during the roast
- The drum rotation speed (generally constant during the roast)
Roasting parameters vary greatly depending on the intended extraction method: when roasting for filter coffee, we perform smaller batches and much shorter roasting times than for espresso. To simplify, for filter coffee, we aim to reveal a maximum amount of fruitiness, while for espresso, we need roundness and more body, even if every coffee is unique and this is not applicable to every bean.
The Roasting Process
The beans rise in temperature throughout the roasting process. We also monitor closely what we call the Rate of Rise (ROR), which allows us to precisely track the rate of this temperature change. This data, generally expressed in °C/minute, is key to understanding the dynamics of a batch.
Concretely, the roasting unfolds in this way:
- Preheating: just like an oven with certain foods, it is necessary to reach a certain thermal inertia in the machine and therefore launch a preheating program, for at least 30 minutes (sometimes more, depending on the roaster models).
- Loading the green coffee: you must pour the green beans into the hopper located at the top of the machine (the large funnel). Once the hopper is opened, the beans fall into the drum and heat up while being stirred rapidly to minimize conduction (in other words, the prolonged contact between the beans and the drum walls).
Turning point: one can see it 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 causing the temperature of those 15kg of green coffee to rise. The turning point is the moment when the temperature stops falling and begins to climb back up.
- 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 more intense green. This phase generally lasts at least half of the roast.
- Maillard reaction: easily identifiable by the color of the beans, which changes from green to yellow, and the characteristic odor 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 provide a distinctive color and flavor. It is not, by the way, exclusive to coffee and also gives their brown color to bread, chocolate, or even meat.
- Caramelization: following the Maillard reaction, caramelization begins, observing similar effects: browning and the development of new aromatics. Unlike the Maillard reaction, caramelization does not require amino acids to occur, and in the case of coffee, it happens a bit later (170°C).
- The first crack: as mentioned previously, this concerns the moisture contained in the coffee beans, which has transformed into water vapor, a portion of which remained trapped inside the beans. As the temperature continued to rise, this water vapor increased in volume and sought to escape the beans. It succeeds during this first crack, by fissuring the beans and emitting this audible, identifiable cracking sound. This generally occurs around 200°C.
- Development phase: A key moment in coffee roasting that does not tolerate imprecision. From the 1st crack, it is necessary to monitor both the development time in seconds and the development ratio (as a percentage of the development time over the total roast time). During this phase, the beans will continue to brown, and the aromatic characteristics will be defined: acidity, body, bitterness. The style of the roast will be defined here: a light roast, more common in Anglo-Saxon countries, will see the roaster pull the beans from the drum almost during the first crack, while a dark roast, common in Italy and France, will continue for several minutes after.
NB: At Celsius, though this is just our own perspective, we seek to sublimate each coffee on a case-by-case basis, so there are no defined rules other than avoiding excesses. We therefore avoid, as much as possible, under-developing coffees: 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 unveil too many smoky and toasted aromas, which would be the hallmark of the roast and not the bean itself.
- The second crack: The absolute bogeyman of roasting when talking about specialty coffee, the second crack occurs when the oils contained in the coffee (which have resisted until then, unlike the moisture) want in turn to extract themselves from the bean with a new fissure of the latter. At this stage, the coffee beans are dark brown, almost black, and especially shiny (due to the oils on the surface). To the nose, the beans are rather alluring, with notes of vanilla and tonka, but on the palate, they lack balance: no acidity, a lot of body, and extreme bitterness.
- Cooling: the most Instagrammable stage of roasting, where one sees the paddles turning amidst the beautifully cooked beans. Yet, this is a topic that is too often neglected; cooling is just as important as the previous steps. Much like a hard-boiled egg that you take out of the pot, it continues to cook if you don't stop it with cold water. It's the same thing for coffee; without a high-performance cooler, the coffee will remain hot for more than an hour and will not have the sought-after aromatic profile. Thus, it must be done in a maximum of 3-4 minutes thanks to a powerful suction system.
Afterward
Following the roast, we have different ways of analyzing it. The roasting curve and data are important, but insufficient.
- Cupping: as mentioned previously, we must taste all of our batches. 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 number of different coffees and being able to compare them with each other.
- Weight: by weighing each batch after roasting, we can deduce the weight loss (in %), which is closely linked to the roast level, from light to dark.
- Color: the human eye is insufficient to reliably discern differences in roast levels, except at the extremes. We therefore use a colorimeter, which allows us to precisely measure the color of ground coffee and thus the 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 bravest among us who have managed to read this far, let's go a step further and talk a little physics. Coffee roasting is nothing more than heat transfer, or rather heat transfers, as there are three types, present to varying degrees depending on the roaster technology:
- - Conduction: This is the transfer of heat within a material without any displacement of matter. It occurs through the vibration of atoms and the transmission of energy from one particle to another, requiring physical contact. In the case of roasting, this conduction exists mainly between the roaster drum and the coffee beans. It is a heat transfer method present in most roasters on the market: Probat, Giesen, Diedrich.
- - Convection: this is heat transfer 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 transfer exists in the three brands of roasters mentioned above, and almost exclusively in IMF and Loring machines.
- - Radiation: This is heat transfer in the form of electromagnetic waves (infrared), without the need for a material medium (unlike the other two modes). A minor method in coffee roasting, to our knowledge it exists on certain models using halogen lamps, such as those from the brand Stronghold.
In our opinion, convection is the most interesting heat transfer method because it is balanced and homogeneous throughout the entire drum. This is why we chose a forced-air roaster, which uses an external heat source to then send air into the drum. In this way, the walls of the drum are not significantly hotter than the air inside, and we minimize 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 roaster
With all these variables, you can imagine how difficult it is for a roaster to choose their machine. That is why, at Celsius, after more than two years of testing different roaster brands, we decided to equip ourselves with a model from the Italian brand IMF for its many qualities:
- Precision: Near 100% convection allows for excellent homogeneity. Furthermore, the vortex system, which mixes air heated to 650°C with ambient air, allows for extraordinary responsiveness.
- Consistency: regardless of the outside temperature or ambient humidity, our roaster is unaffected, and we can reproduce the same curves with the same coffee. This allows us to optimize our roast profiles to get the best out of the coffee without having to adjust for external factors.
- Low NOX emissions: thanks to an integrated fume burner, NOX (nitrogen oxide) emissions are considerably reduced at the chimney outlet.
- Reduced gas consumption: despite this system, this roaster consumes no more energy than an equivalent model from another brand. Additionally, roasts are 30 to 40% shorter, which optimizes energy consumption and therefore greenhouse gas emissions.
- Efficiency: The previously mentioned characteristics 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 own their own equipment.