Citric acid (C6H8O7) imparts the characteristic tartness of citrus fruits such as lemons and limes. Beyond its culinary role, it is a key intermediate in the citric acid cycle—an essential metabolic pathway in nearly all living organisms.
In modern industry, citric acid is predominantly manufactured via the fermentation of the filamentous fungus Aspergillus niger. The process is highly engineered and requires controlled conditions; it is not suitable for a typical home laboratory.
Commercially available high‑yield strains of Aspergillus niger are inoculated into a sterile fermentation vessel. These strains have been selected for their robust citric acid production and are sold through scientific and agricultural suppliers.
The fungal culture is supplied with a readily metabolizable carbon source, commonly sucrose or a glucose‑rich medium such as hydrolyzed corn starch, corn syrup, or molasses. The inexpensive sugar stream fuels fungal metabolism, producing citric acid and releasing carbon dioxide as a by‑product.
Once the intracellular citric acid concentration reaches its peak, the mycelial biomass is separated from the broth by filtration or centrifugation. The clarified liquid contains a high concentration of citric acid.
Calcium hydroxide (Ca(OH)₂) is added to the filtrate, reacting with citric acid to form insoluble calcium citrate (Ca₃(C₆H₅O₇)₂). The reaction proceeds as follows:
3 Ca(OH)₂ + 2 C₆H₈O₇ → Ca₃(C₆H₅O₇)₂ + 3 H₂O
The calcium citrate precipitate is then collected by filtration.
To recover pure citric acid, the calcium citrate is treated with sulfuric acid (H₂SO₄). The stoichiometric reaction is:
3 H₂SO₄ + Ca₃(C₆H₅O₇)₂ → 2 C₆H₈O₇ + 3 CaSO₄
Resulting in the release of citric acid and the formation of calcium sulfate, which can be separated and disposed of in accordance with environmental regulations.
Following acid recovery, the solution is typically purified further—often via ion exchange chromatography or recrystallization—to meet food‑grade or pharmaceutical specifications.
