All decaffeination processes are performed on green beans, but the methods vary somewhat. They generally start by steaming the beans. The beans are then rinsed in some solvent that contains as much of the chemical composition of coffee as possible without also containing the caffeine in a soluble form. The process is repeated anywhere from 8 to 12 times until it meets either the international standard of having removed 97% of the caffeine in the beans or the EU standard of having less than 0.10% caffeine by mass in the coffee at the end of the process. Coffee contains over 400 chemicals important to the taste and aroma of the final drink; this effectively means you can not find a chemical reaction that will remove only caffeine while leaving the other 400 chemicals at their original concentrations.
Coffea arabica normally contains about half the caffeine of coffea robusta, and a coffea arabica bean containing a tenth as much caffeine as a normal bean has been found by some Brazilian scientists. This may change how low caffeine coffee is produced in the future. But for now, one of several methods is employed.
The first commercially successful decaffeination process was invented by Ludwig Roselius and Karl Wimmer in 1905. It involved steaming coffee beans with a brine (salt water) solution and then using benzene as a solvent to remove the caffeine. Coffee decaffeinated this way was sold as Cafe sanka in France and later as Sanka® in the US. Today benzene is considered so unsafe that in the 1990's the US military reformulated Napalm to reduce the benzene level. It's safe to assume this process is no longer employed to make Sanka®.
In the direct method the coffee beans are first steamed for 30 minutes and then repeatedly rinsed with either methyl chloride or ethyl acetate for about 10 hours. The solvent is then drained away and the beans steamed for an additional 10 hours to remove any residual solvent. In the direct method the caffeine reacts with the solvent to form a chemical that is not soluble in the solvent. Methyl chloride is considered a superior solvent since it can function at a lower temperature and react with fewer secondary chemicals, but ethyl acetate can be extracted from various fruits and vegetables and so allows the beans to be labeled as "Natural Decafs".
In the water method (also called indirect method) beans are first soaked in hot water for several hours. Then the water is removed and either methyl chloride or ethyl acetate is used to remove the caffeine from the solution. The solution is then heated to evaporate the methyl chloride or ethyl acetate. Finally beans are soaked again in water after the caffeine is removed. The first few batches are tossed out and the water reused so that the water actually has a similar composition to the beans' liquids, except for the lack of caffeine.
With the CO2 process, pre-steamed beans are soaked in a liquid bath of carbon dioxide at 73 to 300 atmospheres. After a thorough soaking, the pressure is reduced allowing the CO2 to evaporate, or the pressurized CO2 is run through either water or charcoal filters to remove the caffeine. The carbon dioxide is then used on another batch of beans. This same process can also be done with oxygen (O2). These liquids work better than water because they are kept in supercritical state near the transition from liquid to gas so that they have the high diffusion of gas and the high density of a liquid.
Filtered water method
This is the same as the Water Method, except a charcoal filter is used instead of methyl chloride or ethyl acetate to remove the caffeine from the water. The charcoal is generally bathed in some carbohydrate, such as sucrose. This prevents the charcoal from absorbing many of the similar molecules in the water.