The aromatic and gustatory qualities of coffee are developed by the high temperatures to which they are subjected during roasting or broiling. Temperatures are raised progressively to about 204–217 °C. This releases steam, carbon dioxide, carbon monoxide, and other volatiles from the beans, resulting in a loss of weight between 14 and 23 percent. Internal pressure of gas expands the coffee beans by 30 to 100 percent. The beans become a deep, rich brown, and their texture becomes porous and crumbly under pressure. But the most important phenomenon of roasting is the appearance of the characteristic aroma of coffee, which arises from very complex chemical transformations within the bean. Roasting too long can destroy volatile flavour and aroma compounds. For this reason, Robusta beans are often over-roasted (as in the dark French and Italian roasts) to rid the coffee of its natural harshness.
In the oldest method of roasting, a metal cylinder, or sphere, containing the coffee is rotated above a source of heat such as charcoal, gas, or electricity. In modern roasters, hot air is propelled by a blower into a rotating metal cylinder containing the coffee. The tumbling action of rotation ensures that all beans are roasted evenly.
Regardless of the method used, the coffee, after leaving the industrial roasters, is rapidly cooled in a vat, where it is stirred and subjected to cold air propelled by a blower. Good-quality coffees are then sorted by electronic sorters to eliminate those seeds, either too light or too dark, that roasted badly and whose presence depreciates the quality.