Within its molecular structure, citric acid has three carboxyl groups and for this reason it has three dissociation stages as well (and 3 pKa values). Considered a weak acid, citric acid can be found naturally in citrus fruits like lemons, limes and oranges. It is also naturally found in humans as its conjugate base, citrate, is the well-known intermediate that serves as a key component in the Krebs (or citric acid) cycle, which is what all aerobic organisms use in order to yield energy. In its anhydrous form, citric acid is crystallized from a source of hot water, while the opposite can be said for its monohydrate form (cold water). These two forms of citric acid exist as white, hygroscopic (water-holding and –attracting) crystalline powder in room temperature. Citric acid is categorized as a bulk chemical as approximately two millions tonnes of it is produced on an annual basis and used for various industries on a global scale.
In 1784, the Swedish pharmaceutical chemist, Carl Wilhelm Scheele, was the first known scientist to isolate citric acid from lemon juice. Producing citric acid through fermentation, however, wasn’t discovered until 1893, when it was found that Penicillium mould in conjunction with sugar yielded the weak acid. Less than three decades later, American food chemist James Currie discovered a more efficient citric acid producer in the Aspergillus niger mould, which led pharmaceutical company, Pfizer, to initiate macro-scale production. Slight deviations of this proven technique are still used today with a carbohydrate-based medium of either sucrose or glucose.