Nearly all of the citric acid in your food, drinks, and household products is made using Aspergillus niger, a fungus that does indeed appear black. Roughly 99% of the world’s manufactured citric acid has been produced this way since 1919, making it one of the oldest and largest-scale examples of industrial biotechnology.1PubMed Central. Potential role of the common food additive manufactured citric acid in eliciting significant inflammatory reactions contributing to serious disease states: A series of four case reports But the connection between citric acid and “black mold” is both more mundane and more nuanced than the alarming phrase suggests.
How a Fungus Replaced Lemons
Citric acid was originally extracted directly from citrus fruits, particularly lemons and limes. For decades, Italy dominated the global supply, squeezing the acid out of fruit for use in food and beverage production. That changed in 1917, when an American food chemist named James Currie discovered that Aspergillus niger could produce large amounts of citric acid when fed sugar under the right conditions. Within two years, industrial-level production using the fungus had begun, and the biochemical fermentation industry was effectively born.2PubMed Central. How a fungus shapes biotechnology: 100 years of Aspergillus niger research
The economic logic was straightforward. Growing a fungus on cheap sugar feedstocks was far less expensive than importing millions of lemons, and the yields were more consistent. By the mid-twentieth century, microbial fermentation had almost entirely replaced fruit extraction. Today, the citric acid you encounter on ingredient labels is overwhelmingly the manufactured version, not something squeezed from a lemon.
Is This the Same “Black Mold” People Worry About?
When most people hear “black mold,” they picture the dark patches growing on damp drywall after a flood, and they think of the health scares associated with it. That mold is typically Stachybotrys chartarum, a completely different species from the Aspergillus niger used in citric acid production. The two share a dark coloring, but they belong to different genera and behave very differently.
Aspergillus niger is extraordinarily common in soil, on decaying vegetation, and in household dust. You have probably inhaled its spores many times without incident. It can cause problems for people with severely weakened immune systems, and it is a known allergen, but it is not the mycotoxin-heavy villain that Stachybotrys represents in public health discussions.1PubMed Central. Potential role of the common food additive manufactured citric acid in eliciting significant inflammatory reactions contributing to serious disease states: A series of four case reports Calling it “black mold” is technically accurate in the sense that the fungal colonies are black, but the label invites a misleading association with the much more feared household mold.
How the Fungus Makes Citric Acid
The process starts with sugar. When A. niger breaks down sugar through its normal metabolism, it produces a molecule called pyruvate, which enters the same energy-production cycle that runs in your own cells. Inside the fungal mitochondria, pyruvate gets converted into a compound called acetyl-CoA, which then combines with another molecule called oxaloacetate to form citrate, the salt form of citric acid.3PubMed Central. An overview of key industrial product citric acid production by Aspergillus niger and its application
In most organisms, citrate is just a waypoint that quickly gets broken down further for energy. What makes A. niger special is that under certain conditions, particularly when certain trace metals are scarce and sugar is abundant, the fungus accumulates citric acid instead of processing it further. A key enzyme that would normally break citrate down gets inhibited, so the acid builds up and the fungus essentially excretes it. Industrial producers exploit this quirk by carefully controlling the growth environment to maximize citric acid output.
Does the Final Product Contain Any Mold?
The manufactured citric acid that ends up in your soda or your cleaning spray is a purified chemical compound. After fermentation, the liquid broth containing citric acid is separated from the fungal biomass through filtration, then the acid is precipitated out, purified, and crystallized. The white powder or clear solution that reaches the market is citric acid in the same chemical sense that citric acid from a lemon is citric acid. It has the same molecular structure and the same chemical properties.
The fungus itself is removed during processing. You are not eating mold when you consume a product containing manufactured citric acid. The question that lingers, though, is whether trace residues from the fungal production process, proteins or other biological fragments too small to see, could remain in the final product and cause problems for sensitive individuals. That question has proven harder to settle definitively.
The Mycotoxin Question
One genuine concern about using A. niger at industrial scale is that certain strains of the fungus can produce mycotoxins, specifically fumonisins and ochratoxins. A study examining industrial strains found that several of those used for citric acid production were actually among the best producers of fumonisins when grown on agar in lab conditions. The toxins accumulated primarily in the fungal biomass rather than in the liquid broth, which raises questions not so much about the citric acid itself but about what happens to the leftover fungal matter.4PubMed Central. Fumonisin and ochratoxin production in industrial Aspergillus niger strains
That leftover biomass is sometimes repurposed as animal feed, which is where the mycotoxin concern becomes practical. The same study recommended that producers either use strains with inactive gene clusters for fumonisin and ochratoxin production, or switch to closely related non-toxigenic species such as A. tubingensis or A. brasiliensis that do not produce these toxins at all.4PubMed Central. Fumonisin and ochratoxin production in industrial Aspergillus niger strains For the citric acid product itself, the purification process is designed to isolate the acid and leave behind biological material, mycotoxins included. But the fact that toxigenic strains are still in use underscores that strain selection matters.
Can Manufactured Citric Acid Trigger Allergic Reactions?
A small but persistent body of anecdotal and clinical reports suggests that some people react poorly to manufactured citric acid even though they tolerate natural citric acid from fruits. A 2018 case series described four patients who experienced repeated inflammatory reactions after consuming foods, beverages, or vitamins containing manufactured citric acid. Their symptoms included respiratory problems, joint pain, irritable bowel symptoms, and muscle pain.1PubMed Central. Potential role of the common food additive manufactured citric acid in eliciting significant inflammatory reactions contributing to serious disease states: A series of four case reports
The authors of that case series hypothesized that trace fungal proteins or other residues from A. niger fermentation might persist in the finished product at levels high enough to trigger immune responses in susceptible people. Because A. niger is a known allergen, this is at least biologically plausible. Four case reports, however, is an extremely small evidence base and cannot establish a causal link on its own. Millions of people consume manufactured citric acid daily without any apparent reaction. Still, for the small number of individuals who do seem to react, the connection to fungal-derived production is worth knowing about, because it is not something most people would ever think to investigate.
How Regulators View It
In the United States, manufactured citric acid is classified as Generally Recognized as Safe (GRAS) by the FDA. That designation has been in place for decades. The authors of the case series mentioned above criticized this classification, noting that the GRAS designation was made without dedicated research into the specific safety profile of citric acid produced via A. niger fermentation, as opposed to citric acid extracted from fruit.1PubMed Central. Potential role of the common food additive manufactured citric acid in eliciting significant inflammatory reactions contributing to serious disease states: A series of four case reports
This criticism deserves some context. The GRAS framework was designed in the late 1950s, and many substances received the designation based on a long history of use rather than on targeted safety studies. Citric acid had been used in food for well over a century by that point. The purified compound is chemically identical regardless of its source, and no large-scale epidemiological evidence has emerged linking manufactured citric acid to widespread health problems. That said, the question of whether trace residues from microbial production should be studied more rigorously is a fair one, particularly as production volumes have grown enormously.
The Scale of Production
Global citric acid production has grown from under 500,000 tonnes to more than 2 million tonnes over the past two decades, making it the single largest chemical produced through biomass fermentation.5PubMed Central. Citric acid: emerging applications of key biotechnology industrial product It is the most widely used organic acid in the world, showing up in food, beverages, pharmaceuticals, cosmetics, and cleaning products.3PubMed Central. An overview of key industrial product citric acid production by Aspergillus niger and its application About 70% of it goes into food and drinks as a flavoring, preservative, or acidity regulator.1PubMed Central. Potential role of the common food additive manufactured citric acid in eliciting significant inflammatory reactions contributing to serious disease states: A series of four case reports
The majority of new production capacity is in China, which has become the dominant global supplier. The sheer volume tells you something about how deeply embedded this ingredient is. If you check the labels in your kitchen, you will find citric acid in soft drinks, canned goods, candy, frozen meals, sauces, jams, and many supplements. It is also in bath bombs, face creams, and dishwasher detergent. All of it, with vanishingly rare exceptions, came from a fungus.
Could Anything Replace the Fungus?
Researchers have spent decades looking for alternatives to A. niger, and the most promising candidate is a yeast called Yarrowia lipolytica. This yeast naturally secretes large amounts of organic acids, including citric acid, when it has excess carbon to feed on but limited nutrients for growth.6PubMed. Citric acid production from sucrose using a recombinant strain of the yeast Yarrowia lipolytica Yeast-based production offers some practical advantages over mold: the organisms grow faster, the fermentation is easier to control, and the risk of mycotoxin contamination is essentially eliminated.7FEMS Yeast Research. Yarrowia lipolytica: a model yeast for citric acid production
Despite those advantages, A. niger still dominates commercial production. The infrastructure, the optimized strains, and the accumulated know-how built over a century are hard to displace. On the genetic engineering front, CRISPR-based tools have recently been applied to A. niger to make its genome easier to edit, which could lead to strains that produce even more citric acid, with fewer unwanted byproducts like mycotoxins.8PubMed Central. Systems metabolic engineering for citric acid production by Aspergillus niger in the post-genomic era The future of citric acid production may not be a wholesale switch away from the fungus so much as a more precise version of the same organism.
Industrial Fermentation Methods
Two main fermentation techniques are used at commercial scale. In surface culture, the fungus grows as a mat on top of a shallow tray of nutrient liquid, and citric acid accumulates in the broth below. In submerged culture, the fungus grows dispersed throughout a deep tank of aerated liquid, which allows for higher volumes and more automation.9PubMed Central. Biotechnological production of citric acid Most large modern plants use the submerged method because it scales more easily and is simpler to keep sterile.
The sugar feedstock can come from a range of sources. Molasses, a byproduct of sugar refining, is one of the most common. But researchers have also demonstrated citric acid production from agricultural waste, including orange peels and apple processing waste.9PubMed Central. Biotechnological production of citric acid Using waste feedstocks has obvious appeal from a sustainability standpoint, though it introduces more variability in the quality and trace-element content of the growth medium, which can affect yields.
What Happens to the Leftover Fungus
At the scale of millions of tonnes of citric acid, the amount of waste fungal biomass is substantial. One estimate put the annual generation of A. niger mycelium waste at around 340,000 tonnes, with that figure growing at roughly 5% per year.10Industrial Crops and Products. Integrated process for fungal citric acid fermentation using apple processing wastes and sequential extraction of chitosan from waste stream The default disposal method has been incineration, which is neither economically productive nor particularly environmentally friendly.
Some producers have explored using the waste biomass as animal feed, but the mycotoxin issue described earlier complicates that. Other researchers have worked on extracting chitosan, a commercially valuable polymer, from the fungal cell walls. The idea is to turn the waste stream into a co-product rather than a disposal problem. Whether these approaches gain traction at industrial scale depends partly on economics and partly on how the mycotoxin risks are managed through strain selection.
Reading Ingredient Labels
If you pick up a packaged food product and see “citric acid” on the label, there is no requirement for the manufacturer to disclose that it was produced via fungal fermentation. The ingredient is simply listed by its chemical name. For the vast majority of consumers this is irrelevant, since the purified compound is chemically identical to the citric acid in a lemon. But if you are among the small number of people who suspect they react to manufactured citric acid, the labeling gap means you have no easy way to distinguish fermentation-derived citric acid from the fruit-extracted kind.
Some specialty suppliers do sell citric acid explicitly marketed as derived from non-GMO sources or from specific production methods, but “fruit-derived” citric acid at commercial scale is essentially nonexistent at this point. The economics of extraction from lemons simply cannot compete. If you want to avoid manufactured citric acid entirely, you would need to avoid most processed foods and drinks, which is a significant dietary restriction. For perspective, even many “natural” and organic products use manufactured citric acid, since organic standards generally permit it as a processing aid.