Fergus Clydesdale, head of the Food Science Department at the University of Massachusetts at Amherst, provides this answer:
"First, some background. Coffee is the second most popular beverage in the world, after tea. Historians believed the use of coffee as a stimulant originated in ancient Abyssinia (Ethiopia). Caffeine is the component of coffee that is responsible for its mild stimulatory effect on the central nervous system. A six-ounce cup of coffee typically contains approximately 50 to 75 milligrams of caffeine, although the amount varies considerably depending on the method of preparation and the type of coffee; Robusta coffee contains nearly twice as much caffeine as Arabica, for instance. For people who are sensitive to caffeine, even 10 milligrams can cause discomfort. That is why nearly all decaffeinated coffees contain less than 10 milligrams of caffeine (typically two to five milligrams) per serving. Today decaffeinated coffee accounts for approximately 12 percent of total worldwide coffee consumption, or nearly 1 billion pounds per year.
"The first process for decaffeinating coffee was invented by Ludwig Roselius in 1905. Roselius's method used benzene, a potentially toxic hydrocarbon, to remove caffeine from premoistened, green coffee beans. Modern decaffeination processes are much more gentle; many make that point by claiming to be 'naturally decaffeinated.'
"There are three main decaffeination processes currently in use. They have some basic similarities. In all three approaches, the green or roasted beans are first moistened, making the caffeine soluble so that it can be drawn out. Also, they all decaffeinate green coffee at moderate temperatures, typically ranging from 70 to 100 degrees Celsius (160 to 210 degrees Fahrenheit).
"One method is water processing. As you might expect, this process employs water as the solvent to remove caffeine from the green coffee beans. Typically a battery extraction process using eight to 12 vessels is employed; each vessel contains green coffee at a different stage of decaffeination.
"A mixture of water and green-coffee extract that has already been reduced in caffeine is circulated around the coffee beans within the extraction battery (oils in the coffee extract aid in the decaffeination process). After a predetermined time, the vessel that has been exposed to the low-caffeine extract is isolated and emptied. The decaffeinated coffee beans are then rinsed and dried, and a vessel containing fresh green coffee is put on stream. The caffeine-rich extract that was drawn off from the vessel containing the fresh, green coffee is passed through a bed of activated charcoal, which absorbs the caffeine. This charcoal has been pretreated with a carbohydrate, typically sucrose, that helps it absorb caffeine without removing other compounds that contribute to the flavor of the coffee. The sucrose blocks carbon sites that would normally absorb sugars from the liquid, green-coffee extract. The caffeine-reduced extract can then be reused to begin the process anew. The water process is natural (that is, it does not involve any chemicals), but it is not very specific for caffeine; it removes 94 to 96 percent of the caffeine.
"A second decaffeination method is the direct solvent method. These days this technique usually employs methylene chloride (used predominately in Europe), coffee oil or ethyl acetate to dissolve the caffeine and extract it from the coffee. Ethyl acetate is an ester that is found naturally in fruits and vegetables such as bananas, apples and coffee. The liquid solvent is circulated through a bed of moist, green coffee beans, removing some of the caffeine; the solvent is then recaptured in an evaporator, and the beans are washed with water. Residues of the solvent are removed from the coffee to trace levels by steaming the beans. Often this process utilizes batch processing--that is, solvent is added to the vessel, circulated and emptied several times until the coffee has been decaffeinated to the desired level. Solvents are used because they are generally more precisely targeted to caffeine than is charcoal, leaving behind nearly all the noncaffeine solids. The more caffeine-specific solvents, such as methylene chlorides, can extract 96 to 97 percent of the caffeine.