Decaf is coffee that has had its caffeine — the naturally occurring compound largely responsible for giving you the energy boost you feel after drinking coffee — removed by some sort of decaffeination process.
The decaffeination method starts with green coffee beans — seeds of coffee cherries that have been separated from their fruit and dried, but not yet roasted. Legend goes that decaf coffee was discovered when coffee beans fell overboard from a ship and became less caffeinated by the salt water soak. From there, the first patented decaffeination methods used a chemical solvent like ammonia and benzene.
Decaffeination involves the breakdown of the structure of a coffee bean in a way that will affect its flavor when it’s roasted. It might also affect its appearance, as the breakdown of cell walls in decaffeination can make it easier for oils to escape the beans even in medium roasts (while, in caffeinated coffee, oils are something we don’t usually see unless it’s dark roast coffee). Finally, while caffeine isn’t the main compound that makes coffee bitter, it does contribute to that flavor, so its removal can actually reduce bitterness.
While no decaffeination processes offer a 100 percent caffeine-free product, most decaf coffee has a negligible amount of caffeine. Swiss Water’s website promises 99.9 percent caffeine reduction, while studies of other methods commonly used in specialty found results close to 97 percent. It’s likely that decaffeinated coffee won’t give you any sort of caffeine buzz, but for those with extreme intolerance to caffeine consumption, it might still be wise to ingest decaffeinated coffee with caution.