Roasting coffee transforms the chemical and physical properties of green coffee beans into roasted coffee products. The roasting process is what produces the characteristic flavor of coffee by causing the green coffee beans to change in taste. Unroasted beans contain similar if not higher levels of acids, protein, sugars, and caffeine as those that have been roasted, but lack the taste of roasted coffee beans due to the Maillard and other chemical reactions that occur during roasting.
The vast 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 served at specialty shops. Some coffee drinkers even roast coffee at home as a hobby in order to both experiment with the flavor profile of the beans and ensure the freshest possible roast.
Some 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 known as a "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, moisture content, bean density, 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.
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 spectrophotometer to measure the ground coffee reflectance under infrared light and comparing it to standards such as the Agtron scale). As the coffee absorbs 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. Coffee also darkens as it ages, making color alone a poor roast determinant. Most roasters use a combination of temperature, smell, color, and sound to monitor the roasting process.
Sound is a good indicator of temperature during roasting. There are two temperature thresholds called "cracks" that roasters listen for. At approximately 196 °C (385 °F), the coffee will emit a cracking sound. This point is referred to as "first crack," marking the beginnings of a "light roast". At first crack, a large amount of the coffee's moisture has been evaporated and the beans will increase in size. When the coffee reaches approximately 224 °C (435 °F), it emits a "second crack", this sound represents the structure of the coffee starting to collapse. If the roast is allowed to progress further, the coffee will soon fully carbonize, and eventually combust.
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.
|22 °C (72 °F) Green Beans
Green coffee as it arrives at the dock. They can be stored for approximately 12-18 months in a climate controlled environment before quality loss is noticeable.
|165 °C (329 °F) Drying Phase
During the drying phase the beans are undergoing an endothermic process until their moisture content is evaporated, signifying first crack.
|196 °C (385 °F) Cinnamon Roast
A very light roast level which is immediately at first crack. Sweetness is underdeveloped, with prominent toasted grain, grassy flavors, and sharp acidity prominent.
|205 °C (401 °F) Light Roast
Moderate light brown, but still mottled in appearance. A preferred roast for some specialty roasters, highlights origin characteristics as well as complex acidity.
|210 °C (410 °F) American Roast
Medium light brown, developed during first crack. Acidity is slightly muted, but origin character is still preserved.
|219 °C (426 °F) City Roast
Medium brown, common for most specialty coffee. Good for tasting origin character, although roast character is noticeable.
|225 °C (437 °F) Full City Roast
Medium dark brown with occasional oil sheen, roast character is prominent. At the beginning of second crack.
|230 °C (446 °F) Vienna Roast
Moderate dark brown with light surface oil, more bittersweet, caramel flavor, acidity muted. In the middle of second crack. Any origin characteristics have become eclipsed by roast at this level.
|240 °C (464 °F) French Roast
Dark brown, shiny with oil, burnt undertones, acidity diminished. At the end of second crack. Roast character is dominant, none of the inherent aroma or flavors of the coffee remain.
|245 °C (473 °F) Italian Roast
Nearly black and shiny, burnt tones become more distinct, acidity nearly eliminated, thin body.