Furthermore, changes take place in the chemical composition of coffee during the making of the beverage. This is due to the fact water-soluble substances are extracted during processing. The caffeine content varies according to the species. Contrary to what is generally thought, the organoleptic characteristics.of bitterness in coffee are only marginally due to its caffeine content. Caffeine is the pharmacologically active substance causing coffee to provoke in the human body those well-known and strongly marked physiological effects that have made this beverage so widespread and so popular.
Caffeine is the substance that stimulates the central nervous system.
Gastrointestinal caffeine-absorption is rapid and complete. Caffeine circulates in the body s water and its hematic concentration reaches the highest level 15-45 minutes after ingestion. Caffeine s half-life in a human being varies according to age, sex, hormone condition, drugs taken, and effects of tobacco. It increases up to 18-20 hours during the third quarter of pregnancy, and it is about 13 hours long in the case of women taking oral contraceptives. Caffeine is biotransformed by the liver and metabolites are expelled almost completely by the kidneys and the urinary tract.