Coffee is a natural product that contains over 400 different chemcials and compounds that give it its unique flavour and aroma. Caffeine is just one of these substances.
It is an extremely difficult process therefore to remove only the caffeine and leave the other chemicals intact. To all intents and purposes this is an impossible task and even the best and most carefully produced decaffeinated coffees will have its taste and aroma altered by the decaffeinating process.
Coffee comes in two major varieties: Arabica coffees and Robusta coffees. The Arabicas contain about half the amount of caffeine as Robustas.
So how do manufacturers go about removing the caffeine from the bean?
There are several different methods in use today, all of which treat the green or raw coffee bean before roasting.
A popular method is known as "Water Processing" that was originally developed by the Swiss in the 1930's.
This process involves soaking the green coffee beans in hot water to extract all the soluble and semi soluble compounds (including the caffeine) into the solution. The first batch of 'used' coffee beans are then removed and thrown away. The solution containing all the coffee compounds is then treated to remove the caffeine by passing through carbon filters in solution. These carbon filters are designed to only remove the caffeine molecules and to allow the other compounds in solution to pass through.
A new batch of raw coffee beans is now introduced into the 'filtered solution' which has been kept at the same temperature throughout the process. The 'clever' part of this process is that since the hot water solution has reached equilibrium and already has the maximum amount of coffee compounds suspended in solution, then when the new beans are introduced only the caffeine is dissolved out from the new beans and the other compounds remain in the bean as they are unable to be extracted by the hot solution that already contains them from the original batch!
The coffee beans are then removed and sent away for drying. They are then stored and shipped to market.
This process is repeated many times using the original solution, which is continually treated to remove the caffeine.
Other processes used to remove caffeine rely on similar principles as water processing but use different ways to soak the beans and different ways to extract the caffeine. For example some processes use steam instead of soaking the beans and use solvents to remove the caffeine. Others use carbon dioxide and oxygen gases to achieve a similar thing.
It should be noted that there are different criteria for labelling coffee as decaffeinated: The International Standard demands 97% of the caffeine to have been removed from the beans and the European EU Standard of having 99.9% of the caffeine removed by mass.
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