The lemon molecule made by mold
Citric acid was first isolated in 1784 by Carl Wilhelm Scheele from lemon juice. Today, over 99% of world citric acid is produced by fermenting molasses with Aspergillus niger — approximately 2.8 million tonnes per year. The "citrus flavor" in virtually all processed food comes from a black mold grown on sugar.
Structure: three carboxyls, one hydroxyl
Citric acid (2-hydroxypropane-1,2,3-tricarboxylic acid) has three carboxyl groups (–COOH) and one hydroxyl (–OH) at the central carbon. Its three staggered pKa values (3.13, 4.76, 6.40) make it one of the most versatile buffers in industrial and biological chemistry, capable of stabilizing pH anywhere in the 3–6.4 range.
Four functions in one molecule
Acidulant: The most-used food acidulant worldwide. Its mild, clean sour taste (from stepped dissociation) potentiates sweetness perception and enhances effervescence in carbonated drinks.
Preservative: Reduces food pH below 4.6 where Clostridium botulinum cannot grow. Inhibits enzymatic browning by chelating the Cu²⁺ cofactor of polyphenol oxidase (PPO) — the same enzyme that browns cacao during fermentation.
Metal chelator: The trianion citrate is a potent chelating agent for Fe²⁺, Fe³⁺, Cu²⁺, Ca²⁺. Chelation stability constant for iron(III): log K = 11.85. This prevents metal-catalyzed lipid peroxidation (antioxidant synergism) and stabilizes casein micelles in dairy by keeping Ca²⁺ evenly distributed.
pH buffer: Combined with trisodium citrate, the citrate system provides exceptional buffering capacity across pH 3–7 for the full shelf life of products.
The Krebs cycle connection
Citric acid is the first intermediate of the citric acid (Krebs) cycle — the mitochondrial pathway that oxidizes acetyl-CoA to CO₂ and H₂O, feeding the respiratory chain that produces the majority of cellular ATP.