Nearly all of the world’s citric acid starts as sugar fed to a mold called Aspergillus niger, which converts it into citric acid through industrial fermentation. Before the twentieth century, citric acid was physically extracted from citrus fruits, mainly Italian lemons, but that method was expensive and couldn’t keep up with demand. A pivotal discovery in 1917 shifted production to fermentation, and the fungal method has dominated ever since, supplying food, pharmaceutical, and chemical industries worldwide.
From Lemon Juice to Mold Vats
For centuries, citric acid meant citrus fruit. Italian processors controlled most of the global supply by pressing lemons and concentrating the juice until crystals of calcium citrate could be separated out. The process was labor-intensive, geographically limited, and vulnerable to price manipulation by citrus cartels. Chemists knew that citric acid existed in many living cells, not just lemons, but they lacked a practical alternative source.
That changed in 1917, when American food chemist James Currie discovered that essentially any strain of Aspergillus niger would produce high concentrations of citric acid when grown in sugar medium. Within two years, industrial-scale fermentation using the fungus was underway, launching what many historians consider the birth of the modern biochemical fermentation industry.1PubMed Central. How a fungus shapes biotechnology: 100 years of Aspergillus niger research The transition was dramatic: by the mid-twentieth century, citrus-based extraction had been almost entirely replaced, and today fermentation accounts for more than 99 percent of global citric acid output.
How the Fungus Actually Makes Citric Acid
At its core, citric acid production by A. niger exploits the same metabolic pathway that operates in your own cells. Sugar is broken down and fed into a cycle of chemical reactions (often called the Krebs cycle or TCA cycle), and citric acid is one of the first intermediates produced in that cycle. Under normal conditions, citric acid would be quickly converted into other compounds and used up. The trick is getting the fungus to accumulate citric acid instead of burning through it.
How this accumulation happens has been debated for decades. One long-standing theory held that certain enzymes downstream of citric acid, particularly aconitase, were simply shut down during high-producing fermentations. But careful laboratory work comparing the ratios of citric acid to its breakdown products inside the fungal cells found that aconitase remained active under citric acid-producing conditions, strongly arguing against simple enzyme inhibition as the explanation.2PubMed Central. Aconitase and citric acid fermentation by Aspergillus niger
A more nuanced picture has emerged. Rather than blocking the cycle outright, the fungus appears to ramp up the rate at which it pumps citric acid out of its cells before other enzymes can consume it. A key player is a transport protein called CexA, which sits in the cell membrane and actively exports citric acid into the surrounding liquid. Researchers have also explored an alternative approach involving dicarboxylate transporters that compete with aconitase for access to citric acid inside the cell’s mitochondria, allowing the acid to be shuttled outward instead of being further metabolized.3PubMed Central. An overview of key industrial product citric acid production by Aspergillus niger and its application In effect, the fungus doesn’t so much stop making downstream products as it diverts the traffic of citric acid toward the exit before it can be used internally.
The Manganese Connection
One of the stranger quirks of citric acid fermentation is that it depends on starving the fungus of a specific trace metal: manganese. When manganese levels in the growth medium are kept extremely low, A. niger develops a distinctive compact, pellet-shaped morphology with stubby, swollen filaments. This stressed growth form correlates tightly with high citric acid output.4PubMed. Influence of manganese on morphology and cell wall composition of Aspergillus niger during citric acid fermentation
The mechanism connects back to the CexA transporter. Under manganese-limiting conditions, the gene encoding CexA is turned up, meaning the fungus produces more of the protein that exports citric acid out of the cell. When manganese is added back, expression of that gene drops, and citric acid secretion falls accordingly. In experiments where researchers disconnected CexA from its normal regulatory switches and forced it to stay on, the fungus secreted more citric acid even with manganese present.5PubMed Central. Manganese(II) ions suppress the transcription of the citrate exporter encoding gene cexA in Aspergillus niger
The link between manganese and fungal shape is mediated partly by a manganese transporter protein called DmtA. Deleting the gene for this transporter makes the fungus behave as though it’s manganese-starved even when plenty of the metal is available, causing defective growth but maintaining high citric acid yields. Overexpressing the transporter has the opposite effect: normal growth but poor acid production.6PubMed Central. The effects of external Mn2+ concentration on hyphal morphology and citric acid production are mediated primarily by the NRAMP-family transporter DmtA in Aspergillus niger Industrial producers therefore go to great lengths to purify their sugar feedstocks and remove trace metals, because even small amounts of manganese can slash yields.
Submerged Fermentation vs. Solid-State Fermentation
Most large-scale citric acid plants use submerged fermentation, where A. niger grows suspended in a liquid broth inside large steel tanks called bioreactors. Operators control the sugar concentration, temperature, pH, and dissolved oxygen to keep conditions optimal. One study identified the sweet spot for a typical process as around 12 percent sucrose, 30 °C, pH 5.0, and about 5 percent inoculum by volume, which yielded roughly 59 grams per liter of citric acid with almost all of the sugar consumed.7PubMed Central. Process evaluation for enhanced production of aflatoxin-free citric acid by food-grade Aspergillus niger These parameters vary by strain and feedstock, but they illustrate the precision involved: small shifts in pH or temperature can swing output considerably.
Solid-state fermentation offers an alternative in which the fungus grows on a moist solid substrate rather than in liquid. This approach is especially attractive for converting agricultural waste into value-added products, since the waste itself serves as both the growth surface and the carbon source.8PubMed Central. Valorization of agro-industrial waste through solid state fermentation: Mini review Fruit processing leftovers like apple pomace, banana peels, grape pomace, and orange peels have all supported significant citric acid production, with yields depending largely on how much sugar the waste material contains.9Applied Biosciences. Citric Acid Production by Aspergillus niger Using Solid-State Fermentation of Agricultural Processing Coproducts
Solid-state methods use less water, generate less liquid waste, and can sometimes deliver higher concentrations of product per unit of substrate. They’re harder to scale up, though, because controlling temperature and oxygen distribution inside a bed of moist solids is trickier than in a well-mixed liquid tank. Most of the world’s commercial citric acid still comes from submerged fermentation for this reason, while solid-state processes occupy a growing niche focused on waste valorization and smaller-scale production.
What Goes In: Feedstocks and Raw Materials
The traditional feedstock for citric acid fermentation is molasses, a byproduct of sugar refining that supplies cheap, abundant sugar. Beet molasses and cane molasses both work, though their trace metal profiles differ, and some batches need pre-treatment to strip out metals like manganese and iron that would interfere with production.
Researchers have also explored a wide range of alternative carbon sources. The yeast Yarrowia lipolytica, an organism that can also produce citric acid, has been tested on rapeseed oil, glucose, glycerol, ethanol, and waste glycerol from biodiesel manufacturing. High citric acid concentrations of 100 to 140 grams per liter were achieved using rapeseed oil, ethanol, and raw glycerol as feedstocks.10Fermentation. Citric Acid Production by Yarrowia lipolytica Yeast on Different Renewable Raw Materials Raw glycerol is a particularly appealing option because biodiesel plants generate large volumes of it as a byproduct with limited uses, turning a disposal problem into a feedstock opportunity.
Starch-based substrates, such as corn starch or cassava, are common in regions where these crops are cheaper than sugar. In these cases, the starch is usually enzymatically broken down into simple sugars before fermentation begins, since A. niger works fastest on readily available sugars.
Purification: Turning Fermentation Broth Into Pure Crystals
Once fermentation is complete, the broth is a messy mixture of citric acid, leftover sugars, fungal biomass, proteins, and various metabolic byproducts. Getting food-grade or pharmaceutical-grade citric acid out of this requires several steps of downstream processing.
The dominant industrial method is calcium citrate precipitation. The broth is first filtered to remove the fungal mass, then treated with slaked lime (calcium hydroxide). This converts the dissolved citric acid into solid calcium citrate, which precipitates out and can be collected. The calcium citrate is then re-acidified with sulfuric acid, which breaks it apart and releases pure citric acid back into solution while generating calcium sulfate (gypsum) as a byproduct. The citric acid solution is further purified through carbon treatment, ion exchange, and evaporation before being crystallized.
This method works, but it has environmental downsides. The precipitation and re-acidification steps consume significant quantities of lime and sulfuric acid and generate large volumes of gypsum waste. Alternative downstream approaches including adsorption, liquid-liquid extraction, and membrane separation have shown lower environmental impact in research settings, but their technological readiness for large-scale production remains limited.11PubMed. Citric acid bioproduction and downstream processing: Status, opportunities, and challenges For now, the precipitation route still handles most of the world’s tonnage.
Genetic Engineering and the Push for Higher Yields
Industrial strains of A. niger have been improved through classical mutation and selection for a century, but modern genetic tools are opening new possibilities. The arrival of CRISPR/Cas9 gene editing has made it possible to precisely modify the fungus’s genome, enabling researchers to test targeted changes to metabolic pathways rather than relying on random mutations.12PubMed Central. Systems metabolic engineering for citric acid production by Aspergillus niger in the post-genomic era
Some of the most striking results have come not from tweaking the enzymes that make citric acid, but from boosting the proteins that export it. Overexpressing the CexA citrate exporter gene increased citric acid output by roughly three to five times, depending on the promoter system used. In contrast, overexpressing the gene for citrate synthase, the enzyme that actually produces citric acid inside the cell, did not improve yields at all.13Biochemical Society Transactions. Something old, something new: challenges and developments in Aspergillus niger biotechnology The bottleneck, in other words, isn’t making the acid. It’s getting it out of the cell fast enough.
Another creative strategy has targeted the supply of a key building block called acetyl-CoA in the cell’s cytoplasm. By engineering A. niger with a bacterial pathway (a phosphoketolase-phosphotransacetylase system) that generates extra acetyl-CoA, one research group achieved a 96 percent increase in citric acid titer and an 88 percent improvement in yield in bioreactor fermentation compared to the parent strain.14PubMed Central. Engineering a Phosphoketolase Pathway to Supplement Cytosolic Acetyl-CoA in Aspergillus niger Enables a Significant Increase in Citric Acid Production Results like these suggest there is still considerable room for improvement in a production system that has been operating for over a century.
Yarrowia lipolytica as an Alternative Producer
A. niger dominates commercial production, but it isn’t the only microorganism that can make citric acid at industrial-relevant levels. Yarrowia lipolytica, a yeast rather than a mold, has attracted attention because it thrives on oily and glycerol-based substrates that A. niger handles less efficiently. Under nitrogen-limited conditions, this yeast excretes citric acid as a way to dump excess carbon it can’t use for growth.15PubMed. Yarrowia lipolytica as a potential producer of citric acid from raw glycerol
Mutant strains of Y. lipolytica have been developed specifically for citric acid production. In batch fermentation on raw glycerol, one strain reached 124.5 grams per liter with a yield of 0.62 grams of acid per gram of glycerol consumed.16Chemical Papers. Citric acid production from raw glycerol by acetate mutants of Yarrowia lipolytica In continuous fermentation with cell recycling, another mutant sustained a steady output of 116 grams per liter in the outflow.17Chemical Papers. Continuous production of citric acid from raw glycerol by Yarrowia lipolytica in cell recycle cultivation These numbers are competitive with A. niger on conventional sugar feedstocks, making the yeast especially appealing for biodiesel-producing regions looking to add value to their glycerol waste streams.
Safety and the Mycotoxin Question
Since citric acid is a food additive consumed by millions of people daily, the safety of the production organism matters. A. niger has a long history of safe industrial use and is generally recognized as safe by regulatory agencies. But “generally safe” is not the same as “incapable of producing toxins.” Some strains of A. niger can produce ochratoxin A and fumonisins, two classes of mycotoxins that raise food safety concerns.
Research on industrial A. niger strains used in the Chinese food industry found that a meaningful fraction of them could produce ochratoxin A when grown on natural substrates like corn, rice, and wheat bran, with the proportion of toxin-producing strains varying by substrate and incubation time.18PubMed Central. Dynamic Ochratoxin A Production by Strains of Aspergillus niger Intended Used in Food Industry of China Other work has recommended using strains with inactive or deliberately inactivated mycotoxin gene clusters, or switching to closely related non-toxigenic species like A. tubingensis or A. brasiliensis for biotechnological applications.19PubMed Central. Fumonisin and ochratoxin production in industrial Aspergillus niger strains
In practice, the risk to consumers is managed through strain selection, process controls, and testing of the final product. Mycotoxin production in A. niger is strain-specific and environment-dependent, and the conditions inside a citric acid fermentation tank (acidic, sugar-rich, carefully controlled) differ from the conditions that typically favor toxin production on grains. Careful evaluation of each production strain’s genetics and biochemistry is nonetheless recommended.20PubMed. Production of toxic metabolites in Aspergillus niger, Aspergillus oryzae, and Trichoderma reesei: justification of mycotoxin testing in food grade enzyme preparations derived from the three fungi
The Gypsum Waste Problem
For every ton of citric acid produced through the standard calcium citrate precipitation method, a substantial quantity of calcium sulfate (citrogypsum) is generated as waste. This gypsum is contaminated with organic residues from the fermentation broth, making it unsuitable for most of the same applications that natural gypsum serves without further treatment.
Disposal has traditionally meant landfilling or stockpiling, which takes up space and can leach into soil and groundwater. Researchers have proposed several recycling routes, including using citrogypsum in building materials, extracting rare earth metals it may contain, and blending it with animal manure to create organic-mineral fertilizers. The fertilizer approach is especially appealing because it addresses two waste streams at once and can potentially be used to rehabilitate damaged land.21IOP Conference Series: Earth and Environmental Science. Recycling and disposal of gypsum-containing waste generated in the production of citric acid Widespread adoption of these alternatives, however, has been slow, and gypsum waste remains one of the largest environmental liabilities of conventional citric acid manufacturing.
Where All That Citric Acid Ends Up
Global citric acid production exceeds two million metric tons per year, and demand keeps growing. The compound’s versatility explains why. In the food and beverage industry, it serves as an acidulant, flavor enhancer, and preservative. In pharmaceuticals, it adjusts pH in syrups, solutions, and effervescent tablets, and acts as a chelating agent that binds unwanted metal ions.22Future Journal of Pharmaceutical Sciences. Pharmaceutical applications of citric acid Cosmetics use it for pH adjustment and as part of antioxidant formulations. It also shows up in detergents, where it replaces phosphates as a water softener, and in industrial cleaning products that dissolve mineral scale.
The sheer breadth of applications across food, pharmaceuticals, cosmetics, chemicals, and cleaning products is part of what drives ongoing research into cheaper feedstocks, higher-yielding strains, and cleaner downstream processing.23PubMed Central. Citric Acid: Properties, Microbial Production, and Applications in Industries Each improvement in the fermentation process, even a few percentage points of better yield, translates into real savings at the scale of millions of tons per year. And as the economics of waste-derived feedstocks and genetically optimized strains continue to improve, the century-old practice of growing mold in sugar water shows no sign of being replaced by anything fundamentally different anytime soon.