CAFFEINE is a small, bitter-tasting alkaloid. High-quality Arabica coffee beans (the source of most specialty coffees) are typically 1 percent caffeine by weight, whereas cheaper and more bitter Robusta beans have twice that amount.

Spurred by the belief that excessive coffee drinking had poisoned his father, the German chemist Ludwig Roselius, in about 1900, found a number of compounds that dissolved the natural caffeine in coffee beans without ruining the drink's taste. Chloroform and benzene did the job but were toxic, so for 70 years methylene chloride became the solvent of choice. 

When it was discovered in the 1980s to be a suspected carcinogen, the chemical was abandoned by all the big U.S. coffee labels. The Food and Drug Administration continues to permit the use of methylene chloride if the residues in the coffee are below 10 parts per million. Processing for specialty decafs still often uses it because it perturbs other flavorings so little. 

Many other solvents can serve to debuzz coffee. An "all-natural" label may mean that ethyl acetate is the solvent in use, because that chemical occurs naturally in fruit. Water also works as a means of decaffeination. The so-called Swiss water process soaks green coffee beans in a solution that contains the chemical components of beans dissolved from a previous batch, except for the caffeine. Because the water is already saturated with sugars and peptides, only the caffeine passes from the beans into the water. 

Another process, illustrated here, uses supercritical carbon dioxide as a solvent; in this state, the carbon dioxide is intermediate between a gas and a liquid. The variety of caffeine extraction methods demonstrates that a lot of sleepless nights have gone into helping the world get a good night's rest. 

2. CAFFEINE REMOVAL occurs in an extraction vessel, which may be 70 feet high and 10 feet in diameter, suffused with carbon dioxide at roughly 200 degrees Fahrenheit and 250 atmospheres. Caffeine diffuses into this supercritical carbon dioxide, along with some water. Beans enter at the top of the chamber and move toward the bottom over five hours. To extract the caffeine continuously, the beans lower in the column are exposed to fresher carbon dioxide, which ensures that the caffeine concentration inside beans is always higher than in the surrounding solvent. Caffeine therefore always diffuses out of the beans. 

3. DECAFFEINATED BEANS at the bottom of the vessel are removed, dried and roasted. 

4. RECOVERY of dissolved caffeine occurs in an absorption chamber. A shower of water droplets leaches the caffeine out of the supercritical carbon dioxide. The caffeine in this aqueous extract is then often sold to soft-drink manufacturers and drug companies. The purified carbon dioxide is recirculated for further use.