In larger operations, bags of green coffee beans are hand or machine-opened, dumped into a hopper, and screened to remove debris. The green beans are then weighed and transferred by belt or pneumatic conveyor to storage hoppers. From the storage hoppers, the green beans are conveyed to the roaster. Roasters typically operate at temperatures between 370° and 540°F (188° and 282°C), and the beans are roasted for a period of time ranging from 3 to 30 minutes. Roasters are typically horizontal rotating drums that are heated from below and tumble the green coffee beans in a current of hot gases. The heat source can be supplied by natural gas, liquefied petroleum gas (LPG), electricity or even wood. These roasters can operate in either batch or continuous modes and can be indirect- or direct-fired.
Indirect-fired roasters are roasters in which the burner flame does not contact the coffee beans, although the combustion gases from the burner might contact the beans. Direct-fired roasters are roasters in which the flame contacts the beans. At the end of the roasting cycle, the roasted beans are cooled using forced air; sometimes, in large commercial roasters, the beans are first quenched with a fine water mist. Following roasting and cooling, the beans are rested to allow for degassing of CO² prior to packaging. The roasted beans are then packaged, usually in light-resistant foil bags fitted with a one-way valve to allow gasses to escape while protecting the beans from moisture and oxygen. Roasted whole beans can be considered fresh for up to one month if kept cool. Once coffee is ground it is best used immediately.
Roastmasters often prefer to follow a "recipe" or "roast profile" to highlight certain flavor characteristics. Any number of factors may help a person determine the best profile to use, such as the coffee's origin, variety, processing method, or desired flavor characteristics. A roast profile can be presented as a graph showing time on one axis and temperature on the other, which can be recorded manually or using computer software and data loggers linked to temperature probes inside various parts of the roaster.
Coffee roasters use names such as City Roast and French Roast for the internal bean temperatures found during roasting called degrees of roast. This is similar to other arbitrary naming systems; for instance, for steaks, "Well Done" is hotter on the inside than "Medium Rare."
The most popular, but probably the least accurate, method of determining the degree of roast is to judge the bean's color by eye (the exception to this is using a colorimeter to measure the ground coffee reflectance under infrared light and comparing it to standards such as the Agtron scale). As the beans absorb heat, the color shifts to yellow and then to increasingly darker shades of brown. During the later stages of roasting, oils appear on the surface of the bean, making it shiny. The roast will continue to darken until it is removed from the heat source. Beans will also darken as they age, making color alone a poor roast determinate. Most roasters use a combination of bean mass temperature, smell, color, and sound to monitor the roasting process.
Sound is a good indicator of bean temperature during roasting. There are two temperature thresholds called "cracks" that roasters listen for. At about 400–405°F, beans will emit a cracking sound much like popcorn does when it pops, only much quieter. This point is called "first crack", marking the beginning of light roasts. When the beans are at about 435–440°F, or a medium roast, they emit a "second crack". This is the dividing point between medium and dark roasts.
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DEGREES OF ROAST
Green Beans
Drying Phase
Cinnamon Roast
New England Roast
American Roast
City Roast
Full City Roast
Vienna Roast
French Roast
Italian Roast
Spanish or Turkish Roast
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