The acid of most sours, and a fickle one: about 6% acid, roughly two parts citric to one malic, and at its best a few hours after squeezing — not a week later.
Heavy, thin-skinned limes that give a little when pressed. Squeeze on the day; bottled juice tastes cooked.
Worth knowing
Lime juice changes within hours — many bartenders think it's best between two and four hours old. When it's scarce or bitter, super juice or acid-adjusted orange does the job.