The key to excellent coffee is the roasting process, to which we owe the delicately flavored oils that speak to the palate as eloquently as caffeine does to the nervous system.
The chemistry of coffee roasting is complex and still not completely understood. This is owing to the variety of beans, as well as to the complexity of the coffee essence, which still defies chemists' best efforts to duplicate it in the laboratory.
Much of what happens to the bean in roasting is interesting, but irrelevant. The bean loses a good deal of its moisture, for instance, which means it weighs less after roasting than before. It loses some protein, about 10 to 15 percent of its caffeine, and traces of other chemicals. Sugars are burned or caramelized, which contributes color and some body to the cup.
Roasting is simple in theory: The beans must be heated, kept moving so they don't burn or roast unevenly, and cooled, or quenched, when the right moment has come to stop the roasting. Coffee that is not roasted long enough or hot enough to bring out the oil has a pasty, nutty, or bread-like flavor. Coffee roasted too long or at too high a temperature is thin-bodied, burned, and industrial-flavored. Coffee roasted too long at too low a temperature has a baked flavor.
Most roasting equipment uses a rotating drum above a heat source, usually a gas flame. The drum rotates, tumbling the beans to ensuring an even roast. The air temperature inside the drum is usually controlled at about 500 degrees F; the precise temperature depends on the intentions and philosophy of the operator. Eventually, the deep "bound" moisture is forced out, expanding the bean and producing a snapping or crackling noise. Then, when the interior temperature of the bean reaches about 400 degrees F, the oil suddenly begins developing. This process is called pyrolysis, and it is marked by darkening in the color of the bean.
This is the moment of truth for the coffee roaster, because the pyrolysis, or volatilization, of the coffee essence must be stopped at precisely the right moment to obtain the flavor and roast desired. They are quickly dumped into a metal box, where pyrolysis continues until the beans are quenched with either cold air or a light spray of cold water. Most specialty roasters air-quench their coffee.