Roasting coffee involves several significant stages, and the pace at which a specific batch of coffee progresses through each stage is referred to as its roast profile. Coffee roasters meticulously monitor and record their roast profiles to ensure precise replication within narrow temperature and time parameters.
Stage 1: Drying
In its raw form, coffee beans contain about 7-11 percent water distributed evenly through their dense structure. Coffee won't turn brown in the presence of water, and in fact this is true of browning reactions when cooking anything.
When the coffee beans are loaded into the roaster, they require a certain period to absorb enough heat before the water within them begins to evaporate. This initial stage of roasting demands a significant amount of heat and energy for the drying process. The coffee barely changes in look or smell for these first few minutes of roasting.
Stage 2: Yellowing
After the moisture has been expelled from the beans, the initial browning reactions commence. During this phase, the coffee beans remain compact and emit an aroma reminiscent of basmati rice, with a subtle hint of breadiness. Gradually, the beans begin to expand, shedding their delicate, papery skins known as chaff. The chaff is separated from the roasting beans by the air flowing through the roaster and is collected and safely removed to prevent the risk of fire.
These first town roasting stages are very important: if the coffee is not properly dried then it will not roast evenly during the next stages and while the outside of each is well roasted the inside will essentially be undercooked. Consequently, such coffee would yield an unpleasant taste, combining outer bitterness with an underdeveloped, sour, and grassy flavour. Merely slowing down the roasting process thereafter would not rectify this issue, as different sections of the coffee beans would continue to progress at different rates.
Stage 3: First Crack
As the browning reactions gain momentum, gases, primarily carbon dioxide and water vapor accumulate within the coffee bean. Once the pressure inside reaches a critical point, the bean fractures, producing a distinct popping sound and nearly doubles in size. From this point onward, the familiar and desired flavours of coffee start to develop, and the roaster can choose to end the roast at any point.
A roaster will see a decrease in the rate at which the coffee is increasing in temperature at this point, despite the fact that they may be adding a similar amount of heat. Failure to add enough heat can stall the roast and 'bake' the coffee, resulting in poor cup quality.
Stage 4: Roast Development
After the first crack stage, the coffee beans undergo a transformation wherein their surface becomes notably smoother, albeit not entirely. This particular stage of the roasting process plays a crucial role in determining the end colour of the beans and the roast degree. It is during this phase that the roaster possesses the ability to influence the balance between acidity and bitterness in the final product. The acids within the beans degrade rapidly at this point, while the level of bitterness progressively intensifies as the roast advances.
Stage 5: Second Crack
During this stage, the coffee beans undergo another round of cracking, characterized by a quieter and snappier sound. Reaching the second crack signifies the expulsion of oils to the surface of the beans. At this point, much of the acidity present in the beans has dissipated, giving rise to a distinct flavour often referred to as the generic "roast" flavour. This flavour doesn't depend on the kind of coffee used as it is a result of essentially charring or burning the coffee, rather than working with its intrinsic factors.
Continuing the roast beyond the second crack typically leads to the beans igniting, posing an extremely hazardous situation, particularly with large commercial roasting machines.
In coffee roasting, terms like "French Roast" or "Italian Roast" are employed to indicate very dark roasts, which are typically rich in body and bitterness but have lost many of the unique characteristics of the raw coffees. While many enjoy coffees roasted in this manner, these kinds of roasts are not suitable for exploring the flavours and characteristics of high-quality coffees from different origins.
Information provided by: The World Atlas of Coffee, James Hoffmann, 2nd Edition