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Coffee roasting is a step in the processing of coffee which is designed to bring out certain traits in the beans. Roasting is the last step before the coffee reaches the consumer, and some consumers actually do their own coffee roasting to have greater control over their coffee. There are several different ways in which coffee beans can be roasted, and there are a range of what are known as “roast profiles” which determine how long the beans should be roasted, and in which conditions.
At harvest, raw coffee needs to go through some processing including drying and removal of the hulls before it will be suitable for roasting. Once the raw coffee has been milled, hulled, polished, cleaned, and sorted, it is known as green coffee. Green coffee is ready for roasting, but roasting does not necessarily happen right away.
There are several reasons to wait on coffee roasting. The first is that green coffee is more stable than roast coffee. For transit, it makes sense to keep the coffee green because it is less likely to be damaged, and to roast as close to the consumer as possible so that the coffee retains the complex chemical profile which makes it such a popular drink. As people who have consumed old roasted coffee know, the coffee loses much of its flavor if it is allowed to sit too long.
The other reason to wait on coffee roasting is that the flavor profile of coffee varies depending on how soon it was roasted after processing. Some people agree that green coffee peaks at about one year of curing, after which point it starts to lose the oils which give coffee its flavoring. Others may age coffee much longer, for seven or eight years. People have acquired a taste for coffee which has been aged before roasting, and may not appreciate the flavor of unaged roasted coffee.
In the coffee roasting process, the beans are essentially toasted. Roasted coffee is often graded by color, from light to dark roasts, based on how dark the beans turn out. Roasting can be accomplished in a drum roaster which turns the coffee, on roasting beds, and in other types of roasters, depending on the preference of the producer. Depending on how long the beans are roasted and at what temperature, different traits will be developed in the bean, leading to a radically different flavor profile when the coffee is brewed.