Citric Acid: the Invisible Preservative in Almost Everything

The lemon molecule made by a mould

Citric acid (E-330, 2-hydroxypropane-1,2,3-tricarboxylic acid) was first isolated from lemon juice in 1784 by the Swedish chemist Carl Wilhelm Scheele. For more than a century it was obtained only from citrus fruit, until in 1917 the microbiologist James Currie discovered that the mould Aspergillus niger could produce it in industrial quantities by fermenting sugar.

Today more than 99 % of the world's citric acid is made by Aspergillus niger fermentation, around 2.8 million tonnes a year. Lemons contribute less than 0.1 % of global production. It is one of the most elegant ironies of the chemical industry: the "citrus flavour" of practically every processed food comes from a black mould growing on sugar.

Structure: three carboxyls and a hydroxyl

The citric acid molecule has three carboxyl groups (–COOH) and a hydroxyl group (–OH) on its central carbon, an architecture that directly explains its four technological functions:

Geometry. It is a symmetrical tricarboxylic acid. Its central carbon carries an –OH, a –COOH and two identical –CH₂COOH groups, so the molecule is achiral. It is, however, prochiral: enzymes can tell its two arms apart, and in the Krebs cycle aconitase always acts on the same one.

Three stepwise pKa values. The three carboxyl groups lose their protons one after another:

• pKa₁ = 3.13 (first –COOH)
• pKa₂ = 4.76 (second –COOH)
• pKa₃ = 6.40 (third –COOH)

This wide range (pH 3 to 6.4) makes citrate one of the most versatile biological and industrial buffers: it can hold the pH of a system steady anywhere in that range very effectively.

Four functions in one molecule

1. Acidulant

Citric acid is the most widely used acidulant in the food industry, ahead of tartaric, malic and phosphoric acids. Its sourness is mild, clean and of medium persistence, without the astringent notes of phosphoric acid or the harshness of tartaric acid, and with a fruity profile.

In drinks, citric acid lowers the pH to 2.5–3.5, adds freshness and balances the sweetness of sugar and high-intensity sweeteners.

2. Preservative

Citric acid is not a primary preservative: it does not kill microorganisms directly. Its preserving effect is indirect: it lowers the food's pH into ranges where most pathogenic bacteria cannot grow (notably Clostridium botulinum, which does not grow below pH 4.6). It also boosts primary preservatives such as potassium sorbate and sodium benzoate, whose antimicrobial activity depends on being in their undissociated acid form.

In cut fruit and vegetables, citric acid slows enzymatic browning by acting as a chelator of the Cu²⁺ ions that polyphenol oxidase (PPO) needs, the same enzyme behind browning during cacao fermentation.

3. Metal chelator

This may be citric acid's most underrated property. Fully deprotonated citrate (the trianion, at pH above 6.4) is a powerful chelating agent for divalent and trivalent cations: Fe²⁺, Fe³⁺, Cu²⁺, Ca²⁺, Mg²⁺, Al³⁺.

The chelation geometry is elegant: several of its carboxylate groups and the hydroxyl on the central carbon can bind the same metal at once, forming very stable complexes. With Fe³⁺ in particular it forms remarkably stable complexes for such a simple ligand.

This chelating ability has important practical consequences:

• In oils and fats: citrate binds the trace Fe²⁺ and Cu²⁺ ions that catalyse lipid oxidation, acting as a synergistic antioxidant (it boosts BHA, BHT and tocopherols)
• In milk and dairy: citrate (naturally present in milk at ~1.7 g/L) helps stabilise casein micelles by keeping Ca²⁺ bound and evenly distributed
• In detergents: trisodium citrate replaces sodium tripolyphosphate (STPP) as a sequestering agent for calcium and magnesium in hard water, with the advantage of being biodegradable

4. pH buffer

In products whose pH must stay stable throughout their shelf life (jams, drinks, liquid medicines), the citrate buffer system combines citric acid with trisodium citrate in proportions calculated with the Henderson-Hasselbalch equation. Thanks to its three stepwise pKa values, the citrate system has an exceptionally high buffering capacity between pH 3 and 7.

Citric acid in the body: the Krebs cycle

Beyond its technological role, citric acid is a central molecule in human biochemistry. It is the first intermediate of the citric acid cycle (Krebs cycle), the mitochondrial pathway that oxidises the acetyl-CoA produced from carbohydrates, fats and proteins.

In the mitochondrion, citrate synthase condenses acetyl-CoA (2 carbons) with oxaloacetate (4 carbons) to form citrate (6 carbons). Each turn of the cycle releases 2 CO₂ and generates 3 NADH, 1 FADH₂ and 1 GTP, feeding the respiratory chain that makes most of the cell's ATP.

Dietary citric acid is quickly metabolised to CO₂ and water and provides about 2.5 kcal/g. As a free acid, its net effect on the body's pH is practically nil; it is its salts, such as potassium citrate, that make urine more alkaline when metabolised, which is why they are used to prevent certain kidney stones.

Industrial production: Aspergillus niger as a living factory

Production by submerged fermentation is fascinating biochemically. Aspergillus niger accumulates citric acid when grown in a medium with:

• High sucrose or glucose (15–20 %)
• Manganese deficiency, which changes the fungus's shape and metabolism: glycolysis runs flat out while the Krebs cycle cannot keep up, so citrate "overflows" and is excreted into the medium
• pH below 2.5, which stops the fungus from making other acids (oxalic, gluconic) and reduces contamination by other microorganisms
• High aeration, because the respiratory chain (including an alternative oxidase found in the fungus) needs O₂ to reoxidise NADH

Industrial yields reach 100–150 g of citric acid per litre of broth in 5–8 days, one of the most efficient bioconversions in industry. Purification involves precipitating calcium citrate, treating it with sulfuric acid and crystallising the product.

The next article explores food colour through molecular chemistry: carotenoids, why carrots are orange, tomatoes red and salmon pink, and what the length of a conjugated system has to do with the colour we perceive.