The process is usually performed on unroasted (green) beans, and starts with the steaming or soaking of the beans. They are then rinsed with a solvent that extracts the caffeine while leaving the other essential chemicals in the coffee beans. Coffee contains over 400 chemicals important to the taste and aroma of the final drink: It is, therefore, challenging to remove only caffeine while leaving the other chemicals at their original concentrations. Most major producers use one of the following decaffeination techniques.
The Direct Method: The coffee beans are first steamed for 30 minutes and then repeatedly rinsed with either dichloromethane or ethyl acetate for about 10 hours. The solvent is then drained away and the beans steamed for an additional 10 hours to remove residual solvent. Sometimes coffees that are decaffeinated using ethyl acetate are referred to as naturally processed because ethyl acetate can be derived from various fruits or vegetables, but because of the impracticality of gathering natural ethyl acetate, the ethyl acetate used for decaffeination is synthetic.
The Indirect Method: The beans are first soaked in hot water for several hours, in essence making a strong pot of coffee. Then the beans are removed and either dichloromethane or ethyl acetate is used to extract the caffeine from the water. As in other methods, the caffeine can then be separated from the organic solvent by simple evaporation. The same water is recycled through this two-step process with new batches of beans. An equilibrium is reached after several cycles, wherein the water and the beans have a similar composition except for the caffeine. After this point, the caffeine is the only material removed from the beans, so no coffee strength or other flavorings are lost. Because water is used in the initial phase of this process, sometimes indirect method decaffeination is referred to as “water-processed” even though chemicals are used.
Supercritical Fluid Extraction: Pre-steamed beans are immersed in supercritical carbon dioxide in a pressure chamber at 73 to 300 atmospheres. After a thorough soaking for around ten hours, the pressurized CO2 containing dissolved caffeine is removed from the chamber which is returned to atmospheric pressure, allowing the CO2 to evaporate. The caffeine is removed from the CO2 using charcoal filters and is recycled for use on another batch of beans. This fluid works better than water because it is kept in a supercritical state, which displays lower diffusivity than gasses and smaller density than liquids. This process has the advantage that it avoids the use of potentially harmful substances.
Its important to keep in mind that not all beans are created equal, for example coffea arabica normally contains about half the caffeine of coffea robusta. In recent years their has been progress toward growing coffee beans that do not contain any caffeine at all. The naturally caffeine-free Coffea charrieriana has a deficient caffeine synthase gene, leading it to accumulate theobromine instead of converting it to caffeine. Either this trait could be bred into other coffee plants by crossing them with C. charrieriana or an equivalent effect could be achieved by knocking out the gene for caffeine synthase in normal coffee plants.