lemon

Citrus limon is itself a hybrid: genetic work in 2016 traced the citron, Citrus medica, as the male parent behind every lemon and lime lineage the study looked at, crossed in turn with a papeda, a mandarin or a sour orange to produce the different subgroups. In this library the fruit shows up as juice only, in two recipes: about three-quarters of a lemon in the grape mint wine, and one lemon's worth, roughly two and a half tablespoons, squeezed into harak osbao's pomegranate-molasses lentils.

What that juice actually is, chemically, is citric acid, C₆H₈O₇, at a concentration measured around 4.9 grams per 100 millilitres. Citric is also, among the four, the one that gets out of the way fastest on the tongue: it lacks acetic acid's volatile pungency, which reaches the nose before the tongue does, and it lacks malic acid's lingering sourness, the quality that keeps a green apple tart long after the bite. That combination, bright and then gone, lets a squeeze of lemon sharpen harak osbao's lentils or the grape mint wine without dominating either the way vinegar or a tart apple would.

Almost none of the citric acid in a bottle actually comes from a lemon anymore: industrial production has run on fermenting a mould, Aspergillus niger, since 1917, and today supplies over 90 per cent of the world's citric acid. But the citrate in an actual lemon does one well-documented thing once it is in the body: it inhibits the crystallisation behind calcium kidney stones, and raising urinary citrate is an established clinical target for people who form them. A Wisconsin urology group ran real juices through ion chromatography specifically to give stone patients a shopping list, which is also, incidentally, why the citric acid content of lemon juice is measured so precisely in the first place.