Most of the world's citric acid has never been near a lemon

In 1784 the Swedish chemist Carl Wilhelm Scheele worked out how to get citric acid out of lime juice, and for the next hundred and thirty years that is where it came from. Then in 1917 an American food chemist named James Currie found that a black mould, Aspergillus niger, would sit in a sugar solution and excrete the stuff. Pfizer took the process commercial. ¹

Today more than 90 per cent of the world's citric acid is made by fermentation rather than squeezed out of fruit, about 80 per cent of it in submerged tanks and the remaining fifth by the older surface method Pfizer started with. World production reached 2.39 million tonnes in 2020 and is expected to pass 2.9 million by this year. ¹ The label on a soft drink says citric acid and you picture a lemon. What it means is a fungus and a vat of sugar.

That gap between the name of an acid and its actual source runs through this whole subject, and it is worth holding onto while we go through the four that matter in a kitchen.

One acid usually leads

Fruits and ferments are not sour in a general way. Each one leads with a particular acid, and the others sit underneath in smaller amounts. Citric leads in citrus. Malic leads in apples, and gets its name from them: malum, the Latin apple. Tartaric leads in grapes, and the crust in an old wine bottle is its potassium salt. It leads again in tamarind, a long way from any vineyard.

Tamarind is the interesting one because it does not lead cleanly. An HPLC survey of 96 tamarind genotypes grown across Karnataka separated five acids from the pulp at once: tartaric, malic, acetic, citric and oxalic. ⁶ So the intuition that tamarind is a tartaric fruit that also happens to carry citric acid is right, and it is unusual. Most sour fruits are a solo with backing. Tamarind is closer to a chord.

How much tartaric acid is in it, though, is a question the literature has not settled as firmly as you would expect for a crop this old. Figures of 8 to 12 per cent and of 12 to 18 per cent both circulate in the same paper's own introduction. ⁶ I have left the range wide in the table below rather than pick the number I liked best.

What they do to flavour

All of them taste sour, and they taste sour by the same route. Sourness is not detected by a receptor in the way sweetness is. It is detected by a channel that lets protons into the taste cell directly: OTOP1, identified as a proton channel and shown to be required for the taste response to acids. ⁵ The tongue is measuring free hydrogen ions, so anything that releases them registers, and the differences between acids are differences of degree and of what else they bring.

The degree matters more than cooks tend to assume. A weak acid holds most of its protons back, so two solutions of equal concentration can taste quite unequally sour. And the rest of the molecule carries flavour of its own. Acetic acid arrives with a volatile pungency that goes up the nose before it reaches the tongue, so a splash of vinegar reads as sharp from across the kitchen and a squeeze of lemon does not. Tartaric is the hardest and cleanest of the four, an edge without much fruit behind it. Malic is slower and rounder and lingers, the quality that makes a green apple taste sour for longer than the sourness lasts. Citric is bright and short and gets out of the way, and that combination is exactly why the food industry likes it.

Vinegar, which is not legally a thing

Vinegar is the only one of these that arrives as a whole ingredient rather than a component, and its regulatory position in the United States is stranger than it looks. There is no standard of identity for vinegar under the Federal Food, Drug, and Cosmetic Act. ³ What exists instead is a compliance policy guide, a set of labelling conventions the agency treats as current policy.

Those conventions define vinegars entirely by what was fermented. Cider vinegar comes from the juice of apples, wine vinegar from the juice of grapes, malt vinegar from an infusion of barley malt without distillation, sugar vinegar from syrup or molasses, and spirit or distilled vinegar from dilute distilled alcohol. Each should carry 4 grams of acetic acid per 100 millilitres at 20 degrees, and natural vinegars off the generator normally run above that. ³ Note what the definitions do not mention: acetic acid strength alone never makes something vinegar. Dilute acetic acid sold as vinegar is a substitution, not a formulation.

The Supreme Court settled a version of this in 1924, in a case with one of the better names in American law: United States v. 95 Barrels, More or Less, Alleged Apple Cider Vinegar. The vinegar had been made from dried apples rather than fresh ones. The Court held that the product was not the same as one made from apples without dehydration, and that calling it apple cider vinegar did not represent the article to be what it really was. ³

What the acids are actually good for

The health claims around food acids are mostly thin, but one is unusually well grounded, and it is the reason the best measurements of citric acid in juice exist at all. Citrate inhibits the crystallisation that produces calcium kidney stones, and raising urinary citrate is a clinical target in managing them. A urology group at Wisconsin ran 21 juices and three fresh fruits through ion chromatography specifically to tell patients what to drink. ² The paper is a stone-clinic handout that happens to be the cleanest citric acid dataset in the literature.

The other genuine use is duller and older: these acids preserve things. Dropping the pH far enough stops most spoilage organisms, which is what a pickle is, and acetic acid is particularly good at it. That is the whole of the benefit, and it is enough.

Malic acid has an odd afterlife as a fraud detector. Apples make only the L form. Malic acid synthesised industrially comes out as a mixture of L and D in equal parts, because a chemical synthesis has no reason to prefer one hand over the other. So there is an official analytical method, AOAC 993.05, whose entire job is measuring the ratio of L-malic to total malic acid in apple juice. ⁴ If the D form is there, something was added. The same industrial shift that moved citric acid from lemons to a fungus is why that method needed to exist.

The numbers, such as they are

Four acids, four different jobs, and in three of the four cases the version you are most likely to eat was grown in a tank rather than on a tree.

One last thing from the vinegar guidance, which I did not expect to find in a regulatory document. Vinegar generators can develop a population of nematodes called vinegar eels. The agency's position is that finding them in a firm's bulk tanks is not objectionable, on the grounds that some information indicates the eels actually help vinegar production. They only become a problem in the bottle, where their presence renders the product adulterated. ³ Somewhere in a federal file there is a settled policy that the worms in the vat are working.