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. Drum machines consist of horizontal rotating drums that tumble the green coffee beans in a heated environment. The heat source can be supplied by natural gas, liquefied petroleum gas (LPG), electricity, or even wood. The most common employ indirectly heated drums where the heat source is under the drum. Direct-fired roasters are roasters in which a flame contacts the beans inside the drum; very few of these machines are still in operation. Hot-air roasters force heated air through a screen or perforated plate under the coffee beans with sufficient force to lift the beans. Heat is transferred to the beans as they tumble and circulate within this fluidized bed. [edit]Degree of roasting Coffee roasters use names for the various degrees of roast, such as City Roast and French Roast, for the internal bean temperatures found during roasting. 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 temp rature probes inside various parts of the roaster. [edit]Determining degree of roast 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. 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 determinant. 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 205Ц207 ∞C (401Ц405 ∞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 224Ц227 ∞C (435Ц441 ∞F), or a medium roast, they emit a "second crack." This is the dividing point between medium and dark roasts. [edit]Degree of roast pictorial These images depict samples taken from the same batch of a typical Brazilian green coffee at various bean temperatures with their subjective roast names and descriptions.