Updated: Jul 7, 2021
Coffee roasting can be best described as the cooking of coffee beans. As the coffee beans are heated up they produce chemical and physical changes that give coffee its flavor and distinctive smell. In the industry this is known as the Maillard Reaction, named after a French scientist that discovered this in 1912. This browning effect is the reaction between amino acids and sugars when the right amount heat is reached. When roasting green coffee beans the internal sugars caramelize and fats convert into oils producing the smell and look of the coffee bean.
Initially, the process is absorbing heat, but at around 347 °F it becomes exothermic or starts to give off heat. This means that the beans are heating themselves and an adjustment of the roaster's heat source might be required. At the end of the roasting cycle, the roasted beans are dumped from the roasting chamber and are typically cooled by air or water-quenching.
The majority of coffee is roasted commercially on a large scale, but small-scale commercial roasting has grown significantly with the trend toward single origin coffees. Some coffee drinkers even roast their own coffee at home as a hobby in order to both experiment with the flavor profile of the beans and ensure the freshest possible roast.
During the roasting process, coffee beans lose about 15% of their mass due mainly to the loss of water and some compounds. Although the beans experience a weight loss, the size of the beans double during the roasting process due to the physical expansion caused by an increase in internal pressure from vaporized water.
The most common roasting machines are of two basic types: drum and hot-air, although there are others including packed-bed, tangential and centrifugal roasters. Roasters can operate in either batch or continuous modes. Home roasters are also available.
Timing is extremely important, roast too short and you will end up with a bitter, doughy flavored bean. Roast too long you will end up with a brew that tastes burnt, bitter, and just bad. This is why most say roasting coffee is an art form, relying on instincts and experience knowing when coffee is ready.
But simply speaking the best way to know when a coffee bean roast is ready is to use both time and sound. Around 8 minutes of roasting they will make a popping sound, known as the first crack. The bean ruptures and doubles in size and turns a light brown. Between 11-14 minutes the beans will have a second crack where they suddenly expand heat. The roast will be medium brown color and start to have an oil sheen on the bean. Light and medium coffee roasts are produced between the first and second cracks. After 15 minutes the beans will start to smoke having caramelized and the surface is very oily. The beans will darken as the oils surface. When a roast reached 20 minutes the beans will turn very dark brown, almost black. This is the darkest roasts like the French and Italian roasts.
“What goes best with a cup of coffee? Another cup.” - Henry Rollins