The green coffee bean, like the other nuts, kernels, and beans we consume, is a combination of fats, proteins, fiber, and miscellaneous other substances. The aroma and flavor that make coffee so distinctive are present only potentially until the heat of roasting simultaneously forces much of the moisture out of the bean and draws out of the base matter of the bean fragrant little beads of a volatile, oily substance variously called coffee essence, coffee oil, or coffeol. This substance is not properly an oil, since it (fortunately) dissolves in water. It also evaporates easily, readily absorbs other less desirable flavors, and generally proves to be as fragile a substance as it is tasty. Without it, there is no coffee, only sour brown water and caffeine; yet it constitutes only one two-hundredth of the weight of the bean.
The roasted bean is, in a sense, simply a dry package for this oil. In medium- or American-roasted coffee, the oil gathers in little pockets throughout the heart of the bean. As the bean is held in the roaster for longer periods and more moisture is lost, the oil develops further and some begins to rise to the surface of the bean, giving dark roasts their characteristic lightly slick to oily appearance.
Beneath the oil, the hard matter of the bean begins to develop a slightly burned flavor while the sugars caramelize, which together help create the bittersweet tones so attractive to dark-roast aficionados. Eventually, the sugars are burned off almost entirely and the woody matter of the bean turns dry and brittle. This ultimately roasted coffee is variously called dark French, Italian, or Spanish, and tastes thin and charred.
Dark roasts also contain a touch less caffeine than lighter roasts, and lack the dry snap coffee people call acidy. Some dark-roast coffees may taste unpleasantly bitter, but this bitterness is the result of poor quality coffee or clumsy roasting technique. This disagreeable bitterness or sharpness should not be confused with either the dry-wine bite of a good, medium-roasted acidy coffee or the rich bitter-sweetness of a good dark roast.