Decaffeinated coffee, some see it as a necessity, some see it as an abomination, some as a convenient luxury, some a necessary evil. Decaffeinated coffee is in significant demand and the methods of decaffeination, while still not what I would call adequate, are improving. The key to coffee decaffeination is using a solution that selectively dissolves caffeine into solution while leaving behind the other coffee flavor and aroma compounds. Unfortunately, none of the decaffeination methods or chemicals are totally selective for caffeine and therefore they all do affect the coffee flavor. The methods primarily take advantage of the fact that the caffeine is one of the most easily solublized compounds in coffee. Methylene chloride, ethyl acetate, and carbon dioxide liquid are commonly used as these agents and can produce coffee with only 2-3% of its original caffeine left. When talking coffee decaffeination one will usually hear of three methods, the European Process (EP), the Swiss Water Process (SWP), and the Natural Processes (NP).
In the European process, the unroasted green beans are steamed and then washed with methylene chloride, rinsed and dried. The methylene chloride removes most of the caffeine and some of the other flavor compounds. The same process using ethyl acetate is sometimes referred to as a "Natural Process" because ethyl acetate can be acquired from some fruits and is thus "natural". Other Natural processes are the CO2 method and the Sparkling Water Process. In the CO2 method, the beans are soaked in highly compressed carbon dioxide, removing the caffeine. The CO2 is then passed through activated carbon filters which remove the caffeine from the CO2. The sparkling water process is similar, but instead of using carbon filters to remove the caffeine from the CO2, the CO2 is washed free of caffeine with water in a separate tank.
The Swiss water process is a little more difficult to understand. Beans are soaked in hot water to remove all compounds in the coffee. These beans (which are little more than cellulose walls) are discarded. The flavor-charged water, is then passed through activated carbon filters to remove the caffeine. Then a new batch of green beans are soaked in this caffeineless flavor-charged water. Because all the compounds in the bean except caffeine, are in equilibrium with the caffeineless flavor-charged water, there is no concentration difference and they stay in the bean. For the caffeine, however, there is a concentration difference, and thus (to achieve equilibrium) the caffeine diffuses out of the bean into the water. The beans have thus been decaffeinated.