In biochemistry, it is an intermediate in the citric acid cycle, which occurs in the metabolism of all aerobic organisms.

Lemons and limes have particularly high concentrations of the acid; it can constitute as much as 8% of the dry weight of these fruits (about 47 g/L in the juices[12]).

Citric acid was first isolated in 1784 by the chemist Carl Wilhelm Scheele, who crystallized it from lemon juice.

[16] However, microbial production of citric acid did not become industrially important until World War I disrupted Italian Citrus exports.

In 1917, American food chemist James Currie discovered that certain strains of the mold Aspergillus niger could be efficient citric acid producers,[17] and the pharmaceutical company Pfizer began industrial-level production using this technique two years later, followed by Citrique Belge in 1929.

In 1977, a patent was granted to Lever Brothers for the chemical synthesis of citric acid starting either from aconitic or isocitrate (also called alloisocitrate) calcium salts under high pressure conditions; this produced citric acid in near quantitative conversion under what appeared to be a reverse, non-enzymatic Krebs cycle reaction.

In consequence, the hydroxyl group can be deprotonated, forming part of a more stable 5-membered ring, as in ammonium ferric citrate, [NH+4]5Fe3+(C6H4O4−7)2·2H2O.

Citrate can be transported out of the mitochondria and into the cytoplasm, then broken down into acetyl-CoA for fatty acid synthesis, and into oxaloacetate.

High concentrations of cytosolic citrate can inhibit phosphofructokinase, the catalyst of a rate-limiting step of glycolysis.

Citrate acts by augmenting the inhibitory effect of high concentrations of ATP, another sign that there is no need to carry out glycolysis.

Citrate salts of various metals are used to deliver those minerals in a biologically available form in many dietary supplements.

Citric acid can be added to ice cream as an emulsifying agent to keep fats from separating, to caramel to prevent sucrose crystallization, or in recipes in place of fresh lemon juice.

Citric acid can be used in food coloring to balance the pH level of a normally basic dye.

[citation needed] Citric acid is an excellent chelating agent, binding metals by making them soluble.

A solution with a six percent concentration of citric acid will remove hard water stains from glass without scrubbing.

Illustrative of its chelating abilities, citric acid was the first successful eluant used for total ion-exchange separation of the lanthanides, during the Manhattan Project in the 1940s.

Used in foods and dietary supplements, it may be classified as a processing aid if it was added for a technical or functional effect (e.g. acidulent, chelator, viscosifier, etc.).

[37] Citric acid is used as one of the active ingredients in the production of facial tissues with antiviral properties.

Sodium citrate is a component of Benedict's reagent, used for both qualitative and quantitative identification of reducing sugars.

[40] Citric acid can be used as a lower-odor stop bath as part of the process for developing photographic film.

[41] Citric acid is an excellent soldering flux,[42] either dry or as a concentrated solution in water.