For any coffee species or coffee blend, roasting the coffee beans is the most important process in determining the flavor and intensity of the coffee. Before roasting, coffee beans are green. During the coffee roasting process, the coffee bean nearly doubles its size, and changes to a dark coffee color. The coffee roasting process is finished by removing the coffee beans from the heat source, and cooling them. The method and rate at which the coffee beans are cooled will also effect the flavor of the coffee.
The premier coffee beans (such as Java, Jamaican Blue Mountain, and Hawaiian Kona) are roasted lightly in order to preserve their innate coffee flavor, which result from the coffee species itself, as well as the soil quality and conditions under which the coffee plants were grown.
For darker coffee roasts, the coffee's innate flavor (due to the coffee species and growing conditions) is less distinguishable. The roasting flavor itself dominates the flavor of the finished coffee. Since espressos tend to be darker roasts, this explains why Robusta coffee is suitable for inclusion is some espresso blends.
There are various degrees of roasting the coffee beans. In the coffee market, these coffee roasts are described by terms like "Light Cinnamon Roast Coffee" or "French Roast Coffee." Many experts consider "Full City Roast Coffee" the best choice, as it preserves part of the coffee's original flavor, and combines it nicely with the roasting coffee flavor.
In the nineteenth century, coffee beans were roasted in frying pans. While this method for roasting coffee is still used by some, it is a tricky proposition that requires experience.
To pan-roast coffee beans, the green coffee beans are placed in a dry frying pan over medium heat. The coffee beans should be roasted for about 15 minutes. As the coffee beans roast, they must be stirred and turned constantly to avoid burning them. A stronger coffee will be obtained by prolonging the roasting for another 5-7 minutes.
After the coffee roasting process is done, the coffee beans should be moved to a plate (so that they don't continue to roast), and covered with a towel to remove any excess oil formed during roasting until the coffee beans have reached room temperature. The roasted coffee beans should be stored in airtight coffee canisters. In the first two days after roasting, the beans will "outgas" aromatic byproducts of the roasting process. Allow the beans to vent each of those days by opening the canister breifly so as to avoid a pressure buildup.
Home roasting is regaining popularity. Instead of pan-roasting coffee (which can certainly be learned), there are several coffee roasting machines available for the home market. These machines make the coffee roasting process much easier, and will provide a repeatable and uniform roast coffee.
For industrial coffee roasting, the scale is much larger, so the process is more complex. Industrial coffee roasting machines have a large hopper into which the green coffee beans are placed. Coffee debris (coffee husks, etc.) is removed in the hopper, and the sorted coffee beans are weighed. The precise weight of the coffee should be known to accurately control the coffee roasting process.
The coffee beans are then transferred to the coffee roaster - usually a large drum. The coffee roaster operates between 700 and 1000 degrees (Fahrenheit), while constantly moving the coffee beans as they are roasted. The coffee roasting time and temperature are determined by the weight of the coffee, the ambient humidity, and other factors.
When the coffee roasting process has finished, the coffee beans are sprayed with water to cool them. Any remaining coffee debris is removed at this point. The coffee beans are then transferred to a another hopper, where the coffee beans are brought to room temperature and the water is removed. When the coffee beans have stabilized and dried, they are ground (or not), and vacuum sealed for the market.
...written by your friends at The Coffee Brewers