Both tea and coffee, but especially coffee, rely on caffeine for their effect. Caffeine has been accused of causing sleepless nights, indigestion, bad breath, raised cholesterol levels and even cancer. 'It now appears to cause none of these,' says Dr Euan Paul of the Coffee Science Information Centre in Oxford. This was the conclusion that emerged from the International Caffeine Workshop, held in Greece last month, and attended by 175 scientists from around the world. 'As more and more data has been collected and analysed, the scares about caffeine are being laid to rest,' Dr Paul claims.
More than 60 plant species produce caffeine, which probably serves to protect them against insects. Two plants in particular have become economically important because of their caffeine content - tea and coffee. Tea is mainly drunk in the countries in which it is grown, especially China, but a few countries are large importers, such as Britain. Coffee, on the other hand, is mainly grown as a crop for export in countries such as Brazil, Colombia, Indonesia and Kenya. Trade in the coffee bean exceeds dollars 7bn a year, second in value only to oil as a traded commodity.
The coffee bush is indigenous to Ethiopia, and was cultivated there as long ago as 1000AD. It reached Europe around 1600AD, probably via Turkey, where it got its name, kahveh. The first coffee house in the UK opened in Oxford in 1650. Tea has a much longer tradition; it was being drunk in China in 2500BC, but it too did not reach Europe until the 17th century. The third type of caffeine-rich drink is fizzy cola, which originated in North America about 100 years ago and has since spread to all parts of the world.
The amount of caffeine in a drink varies: a cup of freshly ground coffee provides 85mg, instant coffee 60mg; a cup of tea 40mg; and a can of cola 40mg. Most caffeine is now taken through instant coffee. Caffeine is also put in painkillers, asthma treatments and diet aids. These make use of its effect of stimulating metabolism and relaxing the bronchial nerves. Chemically, caffeine is a white powder with the formula C8 H10 N3 O2 . It was first isolated in 1820 by a German chemist called Runge, but it was not until 1897 that its molecular structure was deduced.
Worldwide consumption of caffeine is now estimated to be more than 120,000 tonnes per year, or 60mg per person per day. Scandinavians have the highest intake, generally from coffee, with more than 400mg per day; Britons consume about 300mg per day, mainly as tea; and the Americans, long regarded as big coffee and cola drinkers, take a surprisingly low 200mg per day. The half-life of caffeine in the body is about four hours and it is metabolised by the liver.
Caffeine affects us in many ways. It increases the heart rate and blood pressure when we first take it, so, not surprisingly, it is blamed for heart disease. However, regular drinkers are not affected this way. A report in 1973 suggested that the risk of thrombosis doubled if you drank six or more cups of coffee a day. But a 1990 study of 45,000 men failed to find any connection. Coffee drinkers suffered no increased risk of coronary heart disease.
Caffeine is popularly believed to make us more alert, keep us awake at night and sober us up. It probably does none of these things, unless we drink a lot of it. Despite earlier reports, we cannot become addicted to it and the withdrawal symptoms of a headache and nausea after two days without caffeine are a myth.
It is also a myth that the scum that sometimes forms on the top of a cup of tea is due to caffeine - it is in fact a thin film of scale, mainly calcium carbonate, from hard water. 'To prevent scum forming you need to make the tea slightly more acidic, either by adding lemon or by brewing stronger tea,' says Professor Michael Spiro, of Imperial College, one of Britain's leading researchers into the chemistry of tea and coffee. His studies have shown that the speed with which caffeine is extracted from a tea leaf depends on many factors, such as temperature, the size of leaf and the salts present in the infusing water. His work has also shown that the extraction of the caffeine follows a strict mathematical formula.
Removing caffeine without affecting the taste of coffee proved to be relatively simple. Caffeine is soluble in organic solvents such as chloroform, which was once used to extract it from the raw beans, but this left traces of solvent. Now manufacturers of decaffeinated coffee are using liquid carbon dioxide and superheated water. This decaffeinates just as well, but leaves no unacceptable residues. Tea has proved harder to decaffeinate, says Professor Spiro, because its delicate aromas are also partly extracted, although these can be recovered and returned later. Colas, of course, can simply be made without adding caffeine.
When Coca-Cola was first introduced it contained cocaine- which gave it its name - caffeine, sugar, phosphoric acid and flavourings. First the cocaine went, then the sugar, then the caffeine. At least the phosphoric acid still remains.
John Emsley is science writer in residence at the Department of Chemistry, Imperial College, London.