Decaffeination removes about 97% or more of the caffeine in coffee beans. A typical cup of decaf coffee has about 2 mg of caffeine, compared to a typical cup of regular coffee, which has about 95 mg of caffeine.
Like regular coffee, decaf coffee begins as green, unroasted beans. The hard beans are warmed and soaked in liquid to dissolve and remove the caffeine in one of four ways: using water alone, using a mixture of water and solvents (most commonly methylene chloride or ethyl acetate) applied either directly or indirectly, or using water and “supercritical carbon dioxide.”
All four methods are safe, and once the caffeine is removed (well, at least 97% of it), the beans are washed, steamed, and roasted at temperatures that evaporate all the liquids used in decaffeination.