The process of roasting coffee is remarkably simple, yet intensely complex. Feeling a little confused?
At first glance the process is simple, heat up a drum, throw in the beans, let the roaster heat them up to a desired temperature, drop them out of the roasting drum. What could be simpler?
Well, true that is the process, an oversimplification of the roasting process, but the roasting process none-the less.
Yet, to be an Artisan Roaster requires far more than such simplicity. It requires an understanding of the chemical reactions taking place in the bean during the roasting process. From that base understanding, experience is needed to critically evaluate each phase of the roast. Additionally each individual batch of beans (critical to note that a batch of beans means "every roast going into the roaster", as green coffee beans can vary greatly, even from the same lot, bag-to-bag) prior to roasting to determine the best way to roast the beans to accentuate all the possible character and qualities.
After being evaluated by the Roast Master, the green coffee beans are weighed and placed in the hopper atop the Roaster, poised for roasting. It is important to keep the beans at room temperature until the point that roasting begins so they should not sit in this hopper for more than a few seconds.
Once the Roast Master has brought the drum temperature to the desired Charge temperature, anywhere from 350F to 450F, the beans are “dropped”, or “charged” into the roaster.
Because of introducing a large volume of room temperature beans to the charged roaster, the temperature will dramatically decrease by as much as 200F, this is when the roasting process begins.
The beans drink in the heat as they absorb the latent heat from the drum without any external extra heat being added. This initiates a chemical process. First there is the "drying phase", much of the free moisture in the bean evaporates under the heat. Then a sudden change in the molecular structure of the coffee begins in which the amino acids, sugars and moisture will caramelize, moisture in the bean will dissipate and the bean will begin to brown, giving off a scent similar to that of baked bread. This stage of the roast is called “Endothermic” (internally heating or absorbing heat), as the beans “absorb” the heat, the fiber structure of the bean is rapidly changing. As the newly caramelized particles and Carbon Dioxide combine in the bean contained by a thin wall of moisture. All of these chemical reactions have one thing in common: expansion, until they have nowhere to go. Somewhere between 350F and 395F (many factors determine this including your altitude etc.... perhaps we will address this in a future FAQ) the beans will finally burst through that wall of moisture, creating the characteristic “cracking” sound of roasting coffee. This is called the first crack.
It is imperative that the Roast Master control the rate of rise of temperature as the beans approach this point and care must be taken to not let it happen too quickly otherwise the carefully created caramelized structure of the beans will burst and the oils will rush to the surface of the bean. Oil on coffee is always - ALWAYS - a sign of bad roasting.
Between the first and second cracks, is where the skill of the roaster is tested. Care is taken to carefully match the bean category, evaluated earlier, to the necessary process between cracks. That may be to speed up the process with more heat or to slow it down with less. Mistakenly doing one instead of the other, can have majorly negative consequences to the quality in the cup.
After the first crack, the beans can literally not absorb any more heat and the way the beans roast changes, and this second phase of roasting becomes “Exothermic” (externally heating or giving off heat). The beans literally start to create their own heat in this phase.
As the ambient air temperature approaches 400F a different type of chemical reaction is taking place. Again, care must be taken to ensure this phase does not happen too quickly as this is the phase in which all the final flavour characteristics which will be present in the cup are developed. Without sufficient time to develop , an unpleasant acidity and bitterness can occur - this is one of the prime reasons a fluidized bed roaster (or Air Roaster) lacks the ability to fully extract flavours from coffee. Too much time in this phase and the beans will taste baked and starchy (like baked potato). Conversely, too little time will deliver a lack of sweetness, and acidity that is an afront to your senses.. By carefully extending the time between the first and second cracks (whether or not the roaster takes the coffee all the way to the second crack - something we RARELY do) to the optimum length, the flavour has time to properly develop, accentuating the natural character of the bean.
Around 420-440F, the beans become exhausted and brittle due to dehydration and a carbonization process begins causing a second crack.
Depending on the desired roast degree, beans may be dropped (removed from the roasting drum) before, during or after the second crack. All coffee should at LEAST reach the first crack.
Beans are dropped into a cooling tray. It is very important to stop the roasting process as quickly as possible. Much effort is put into cooling them as quickly as possible. In some instances like dark roasted coffees (something we never do), beans are removed from the roaster only moments before they would otherwise burst into flames, yet within only a few minutes, they are cool enough to touch.
Because the roasting process ignites several hundred chemical processes within the bean, after roasting it remains very chemically active and requires 24 hours to stabilize and for the Carbon Dioxide levels reduce to a level sufficient for brewing.
After 24 hours, every care should be taken to stop the Oxidization process by shielding the beans from its enemies: moisture, air and light.
Of course, we would be remise to not add the caveat that roasting is an art and science taking years of experience to fully comprehend, hence to presume that a short FAQ could describe in any detail all the nuances the Roast Master understands and deals with every day would be absurd. Yet, maybe it sparked your interest in learning more and joining the conversation. We would love to hear from you with your questions...