caffeine

On 3 October 1819 Friedlieb Ferdinand Runge isolated a stimulant from coffee beans Goethe had given him and called it the Kaffeebase. He had no way to know he had found the same molecule that coffee, tea, cacao, kola, guarana, yerba mate and citrus all make on their own: six plant families across six orders, none of them close relatives, all producing C₈H₁₀N₄O₂ down to the last atom. A comparison of the enzymes across five of those lineages found the identical molecule arriving by different biosynthetic routes, built from parts each plant already had lying around for other jobs. In this library caffeine appears in four recipes, traced to black tea in three and to coffee in one.

The molecule is a purine alkaloid, and for over a century after Runge nobody knew what it did in the body, only what to call it: Kaffeebase in 1819, guaranine in 1826, theine in 1827, each treated as a distinct substance until 1838 and 1840 showed all three were caffeine wearing another name. The mechanism came in 1981, when Solomon Snyder's group ranked ten methylxanthines by how tightly each bound the brain's adenosine receptor and separately by how strongly each stimulated a mouse, and the two rankings matched. Caffeine occupies the receptor without activating it, and Bertil Fredholm later compressed the finding into one line: of caffeine's known biochemical actions, only adenosine receptor blockade happens at concentrations reached by normal human consumption.

Caffeine's own taste is bitter, and oddly sensitive to temperature: cooling the tongue from thirty-six degrees down to twenty measurably reduced the bitterness of a caffeine solution while sourness and saltiness held steady, one reason iced coffee is not actually more bitter for being cold. What the dose buys a non-drinker is contested: a 2010 trial of 379 people found caffeine did not raise alertness above baseline in people who were not habitual consumers, while withdrawal in habitual drinkers cost them alertness that caffeine only restored, and a 2005 crossover study in genuine non-consumers found real gains in reaction time and working memory. Two harms are not contested: 400 milligrams within six hours of bedtime measurably disturbed sleep, and withdrawal, a recognized clinical syndrome, can start after as little as 100 milligrams a day. An eight-ounce cup of brewed coffee runs about 95 milligrams against black tea's 47; this library's Turkish coffee, unfiltered and brewed strong, carries roughly 90 in a 70 milliliter cup.