An alkaloid compound which has a physiological effect on humans, and a slight, bitter flavour. It is found throughout the coffee plant but is more concentrated in the seed / coffee bean. Arabica ranges from 1.0 to 1.6% caffeine and Robusta (Coffea Canephora) from 1.6 to 2.2% caffeine. It is highly water soluble. The amount of caffeine in brewed coffee is directly proportional to how much ground coffee was used to make the cup. See Decaffeinated Coffee.