The green coffee beans have no flavour or aroma and are just a pale green shadow of their future dark brown selves. All of the flavour and aroma that we enjoy in coffee is created by roasting the beans.
Green coffee beans are heated to between 180ºC and 240ºC for 8 to 15 minutes, depending on the degree of roast required. The longer the coffee is roasted the darker it becomes. During the roasting process moisture is lost and the bean "pops" audibly rather like popcorn. A chemical reaction takes place: starches are converted into sugar, proteins are broken down and the whole cellular structure of the bean is altered. The heating process precipitates the release of coffee oil, or what is called "caffeol", which is the essence of coffee.
This essence of coffee is what we enjoy in the cup. It is also volatile and water soluble, so once the coffee beans have been roasted until dark, the flavour can be damaged by moisture, light and especially by oxygen
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