How is caffeine removed to produce decaffeinated coffee?

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.

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