Coffee Roasting Process

As air dried green beans, coffee has an earthy smell and is different in composition from the coffee we drink. The beans need to be roasted to burn off some oils and waxes, and to develop the flavors and properties of others. Traditionally, roasting was done using a pan over a fire. Some modern home roasters have even successfully used popcorn makers. Commercially, coffee roasters have mostly been either gas or electric.

Early gas roasters operated at up to 1000 degrees F and tended to heat unevenly and to char beans, with the result that the coffee tasted burned. Later machines operated at about 500 degrees F and retained important flavor constituents without the charring. However, the smoke, chemicals and chaff given off by large gas powered coffee roasters can be polluting.

More recent coffee bean roasting technologies use electric powered hot air. The beans are kept in motion by the hot air and absorb the heat more evenly with less risk of burning. These roasting machines can be made and operated on a smaller scale.

During roasting the beans lose weight, from 10% to 20%. The sugars in the beans are caramelized to give the dark colors under the influence of the heat. The beans become more brittle, the more so in the case of darker roasts. This effect is related to the strength of the coffee, as darker, more brittle roasts allow more water penetration of the coffee grounds to draw out more of the flavors of the bean.

Darker roasts release the chaff in the crevice of the bean, which becomes more pronounced.

Different lengths of roasting result in different chemical compositions as acids and oils are burned off or released at each stage. Accordingly, each stage leads to different coffee flavors. Coffee roasted for a longer period tends to be less acidic, or to be duller or flatter in its taste within your mouth, but stronger in flavor. The roasting process has a great influence on the quality of the final product.

As the bean heats further, the surface breaks down so that further oils are released from inside the bean. Therefore, darker roasts tend to be oilier in appearance than the dry mild roasts. Darker roasts also lose some of their caffeine, but this difference is not great.

After the roasting has been completed some gas machine roasters will use water to cool the beans. This also adds weight! It may also wash away flavor. Hot air roasters use cold air to cool the beans, which avoids these issues.

After roasting, beans give off carbon dioxide. A small (up to 1%) reduction of weight may result, and some flavor is lost. In effect the roasted beans become stale through this process after a couple of weeks unless they are packaged or frozen to minimize the effect.