Citric acid is made almost entirely through industrial fermentation, where a specific mold feeds on sugar and converts it into acid. About 99% of the world’s citric acid supply comes from this process, not from lemons. It’s also possible to extract small amounts of citric acid directly from citrus fruit juice using a series of chemical steps, though the yield is tiny compared to fermentation.
The Fermentation Process
Commercial citric acid production relies on a mold called Aspergillus niger, a common black fungus. When this mold is grown in a sugar-rich environment with very limited nutrients (particularly low nitrogen and certain minerals), its metabolism shifts. Instead of using sugar efficiently for growth, it begins accumulating citric acid as a byproduct. Under optimized conditions, roughly 50 to 85 grams of citric acid can be produced from every 100 grams of sugar consumed.
The mold can feed on a range of sugar sources. Factories typically use beet or cane molasses, corn starch that has been broken down into simple sugars, or pure glucose solutions. Corn and potato starch yield around 65 to 93 grams of citric acid per liter, depending on the mold strain and starch concentration. Mutant strains bred specifically for acid production consistently outperform wild strains by 10 to 50%.
Two main fermentation setups are used. In submerged fermentation, the mold grows in large liquid-filled tanks with controlled aeration, temperature held near 28°C, and continuous stirring. This method accounts for most global production. In surface fermentation, an older technique, the mold grows as a mat on top of shallow trays of sugar solution. Submerged fermentation runs for several days and produces higher, more consistent yields, which is why it dominates.
Turning Fermentation Broth Into Pure Citric Acid
The liquid that comes out of fermentation is a murky broth containing citric acid along with leftover sugars, mold fragments, and other organic compounds. Purifying it into the white crystalline powder you’d recognize on a store shelf takes several steps.
First, the broth is filtered to remove solid debris. Then calcium carbonate (essentially chalk) is added to the liquid. This reacts with the citric acid to form calcium citrate, a solid that precipitates out and can be collected by filtration. The calcium citrate is then treated with sulfuric acid, which breaks the bond between calcium and citrate. The calcium binds to the sulfuric acid to form calcium sulfate (gypsum), which is filtered off as a solid waste. What remains in solution is free citric acid.
This liquid citric acid is further purified using activated carbon to remove color and ion exchange resins to strip out trace metals and impurities. Finally, the solution is concentrated by evaporation. The temperature during crystallization determines which form of citric acid you get: cooling below 36.6°C produces monohydrate crystals (the kind with one water molecule attached), while crystallizing from a hot concentrated solution above 36.6°C produces the anhydrous, water-free form. The dried crystals are food-grade citric acid, typically 99.5% pure or higher.
The calcium sulfate waste generated during this process is a significant environmental concern. It accumulates in large quantities and is expensive to dispose of, which has driven research into alternative purification methods, including processes that use compressed carbon dioxide instead of sulfuric acid.
Extracting Citric Acid From Lemons
Before industrial fermentation took over in the early 20th century, all citric acid came from citrus fruit, primarily Italian lemons. You can still extract it at home or in a lab, though the process is slow and produces very small amounts.
The basic procedure follows the same chemical logic as industrial purification. Squeeze your lemons and dilute the juice with an equal volume of water. Neutralize the solution by slowly adding a dilute sodium hydroxide (lye) solution until it’s no longer acidic, checking with pH paper. Filter the mixture to remove pulp and solids.
Next, add calcium chloride solution to the filtered liquid. This converts the dissolved citrate into calcium citrate, which appears as a white precipitate. Gently boil for a minute or two to help the precipitation complete, then filter again. The white solid on your filter paper is calcium citrate. Discard the liquid.
To free the citric acid from the calcium, add dilute sulfuric acid (around 2%) slowly to the calcium citrate. This creates calcium sulfate, which precipitates out, leaving citric acid in solution. Filter once more to remove the calcium sulfate. The clear liquid that passes through is your citric acid solution. Concentrate it by gently heating in an evaporating dish on low heat, then dry the resulting crystals in an oven at 80°C.
The yield from this process is modest. A lemon contains roughly 5 to 8% citric acid by weight of its juice, and losses at each filtration step mean you’ll recover only a fraction of that as dry crystals.
Why Fermentation Replaced Fruit Extraction
The shift happened because fermentation is dramatically more efficient. A single fermentation tank can produce tens of grams of citric acid per liter of broth in a matter of days, using cheap sugar feedstocks. Genetically engineered strains of the yeast Yarrowia lipolytica have pushed laboratory yields as high as 200 grams per liter with a conversion efficiency of 85%. Extracting the same amount from lemons would require enormous orchards, seasonal harvests, and far more processing.
Yarrowia lipolytica has gained attention as a potential replacement for the traditional Aspergillus niger mold. The yeast tolerates low pH better, handles high sugar concentrations without trouble, and is classified as “generally recognized as safe” for food and pharmaceutical use. It also avoids a key problem with mold-based production: Aspergillus niger releases airborne spores during cultivation that can cause respiratory illness in factory workers. Yeast-based processes are easier to scale, automate, and keep clean. Despite decades of research, though, Aspergillus niger still dominates commercial production worldwide.
What Home Users Actually Need to Know
If you want citric acid for canning, cleaning, or cooking, buying food-grade citric acid powder is far more practical than making it. It’s inexpensive, widely available, and certified to 99.5% purity or higher under food chemical codex standards. The anhydrous form is the most common in retail packaging and stores well in a sealed container away from moisture.
If you’re interested in the extraction process as a chemistry project, the lemon method works and is a satisfying demonstration of precipitation and filtration. Just be aware that dilute sulfuric acid requires careful handling: use gloves, eye protection, and work in a ventilated area. The fermentation route, while fascinating, requires microbiological equipment, sterile technique, and access to Aspergillus niger cultures, putting it well outside typical home chemistry.