citric acid

Citric acid takes its name from citrus, but for more than a century now it has mostly come from somewhere else. In 1784 the Swedish chemist Carl Wilhelm Scheele worked out how to extract it from lime juice, and that stayed the source for over a hundred years, until 1917, when the American food chemist James Currie found that the black mould Aspergillus niger would sit in a sugar solution and excrete the acid on its own; Pfizer took the process commercial soon after. More than 90 per cent of the world's citric acid is now made by fermentation rather than by squeezing fruit, with world production reaching 2.39 million tonnes in 2020. In this library, though, it still arrives the old way: squeezed from lemon in a grape mint wine and a harak osbao, released from strawberry and kiwi as they ripen, and carried as a secondary acid behind tamarind and pomegranate molasses.

The molecule is a tricarboxylic acid, C₆H₈O₇, with three acid groups visible in its own structure, each able to give up a proton. Sourness itself is not read by a dedicated receptor the way sweetness is: it runs through OTOP1, a channel shown to let hydrogen ions directly into the taste cell, so what the tongue is measuring is free protons in solution, and citric acid is described, against the other kitchen acids, as bright and short, one that gets out of the way. It also reaches for calcium directly: a slice of lemon cuts the film that forms on a cup of hard-water tea by two routes at once, dropping the pH and tying up the calcium ions the film needs.

The one health claim in this territory that holds up is a real one: raising urinary citrate is a recognised way to blunt calcium kidney stone formation, and a Wisconsin urology group measured citric acid across 21 juices and three fresh fruits by ion chromatography for exactly that reason, to tell patients what to drink. In this library the acid is traced in five recipes, always in the tart or fruity register: the lift under a grape mint wine and a harak osbao, the sourness that needs no added lemon in a kiwi smoothie, the secondary sharpness behind a tamarind drink, and the dominant tartness of a strawberry banana blend.