1. What is coffee roasting?
You can understand that coffee roasting is the heating process that helps “green coffee” become “ripe coffee” that we brew and drink.
Before roasting, coffee beans are green in color with an odor similar to beans or grass; only when roasted do flavor and aroma substances begin to increase in the bean. All such flavor creation is controlled under two mechanisms: one part science with physicochemical reactions, the other part art & inspiration. So the flavor we get from coffee doesn’t come from the tree, the farm, or the processing station entirely; it comes from a short journey of a few dozen minutes to convert from element to flavor, with a multitude of qualitative variations including:
- The seed color will change from green to yellow, brown, and black.
- Nearly doubling in size and almost halving in mass, the seeds become more spongy and brittle.
- Forms 800 to 1000 flavor compounds, decreasing dramatically if roasting for too long.
- Rack as they release steam & other gases.
2. Roast segment
2.1. Drying Phase
Raw coffee beans, after processing, have a moisture content of 8 to 12%. This stage usually lasts 4 to 8 minutes with a traditional roaster. At the end of the drying period, the temperature is usually 140 – 160⁰C. This is a necessary time to gather the heat needed for the delicate flavor development stage of the coffee beans.
When familiar with the roasting process, you will know that we only add coffee when the roaster has reached the required temperature; the roasting industry term calls this “The charge temperature.” And to determine what the inlet temperature is, you must consider the origin & processing method of that coffee, the size of the roast, more importantly, the type of roaster, the total expected roasting time. …
If the heat input is too high, the coffee will burn. But if the temperature is low, the coffee will not receive enough heat for flavor. You are then forced to increase the temperature or extend the roasting time longer than expected, making the appliance hotter more difficult to control (the temperature difference between the core and the outer surface). Again, coffee fails to its potential flavors.
2.2. The Middle
A transition (although not quite a phase) is called ‘The Middle’ while the drying process continues; green coffee will turn yellow or light brown, so some documents call this “browning phase.”
From this stage, the natural sugars in the seeds will participate in a chemical reaction, along with the release of water vapor that causes the roots to begin to expand and give off a pleasant aroma. The changes in color and aroma are essentially the product of the Maillard reaction – they accelerate when the grain temperature reaches about 121°C – 149°C. At about 171°C, Caramelization begins, depleting the natural sugars so the Maillard reactions will slow down. The caramelization process will profoundly change the color of the nut and create a fruity, toasted, or nutty flavor…
Both the Maillard and caramelization reactions reduce the sweetness of the already low sugars in the coffee to dramatically increase the bitterness. Physically, the swelling of the beans and the high temperature also helps to release the silver skin. At the same time, the roasting process to this stage will remove a large amount of smoke; the operator must ensure that the airflow is high enough to suck all the dust and smoke out.
2.3. Development Time
Many roasters have mentioned that the time from the start of the first crack, until the coffee is released from the machine is considered “Development Time.”
According to Scoot Rao, most roasters usually adjust the development time separately from the rest of the roasting process. Still, such an approach will often result in the coffee not achieving the complete flavor formation. Its delicate taste makes this stage proportional to the whole roasting process a complex problem.