Citric acid is a colorless, weak organic acid that occurs naturally in many fruits and vegetables, lending them their characteristically sour taste. While it is abundant in citrus fruits like lemons and limes, the vast majority of the world’s commercial supply is not extracted from plants. Instead, industrial citric acid is produced through microbial fermentation, overwhelmingly utilizing the fungus Aspergillus niger. This manufactured compound is chemically identical to its natural counterpart and is one of the most widely used additives in the food, beverage, and pharmaceutical industries.
Why Aspergillus niger is Used
Large-scale citric acid production originally involved extracting the acid directly from lemon juice, a method that proved vulnerable to disruptions. In 1917, American food chemist James Currie discovered that specific strains of the mold Aspergillus niger could produce high yields of the acid. The switch to microbial fermentation transformed citric acid from a seasonal commodity into an inexpensive, high-volume industrial chemical, with global annual production now exceeding two million tons.
The fungus is favored due to its efficiency, ability to tolerate highly acidic conditions, and capacity to use a wide range of inexpensive sugar sources. A. niger converts crude sugars, such as molasses or corn starch, into citric acid. This high conversion rate and the fungus’s resilience make it cost-effective compared to traditional extraction from fruit.
The organism’s metabolic pathway is manipulated to accumulate citric acid, which is an intermediate product in the tricarboxylic acid (TCA) cycle. Under specific conditions, such as high sugar concentration and restricted levels of trace metals, the fungus is metabolically driven to overproduce and excrete citric acid. This controlled imbalance forces the fungus to halt the TCA cycle early, causing the citric acid to accumulate in the fermentation broth rather than being fully metabolized for energy.
The Fermentation and Purification Process
Industrial production relies on a carefully managed fermentation process. This begins by preparing a sterile, nutrient-rich substrate, typically a diluted solution of sugar or molasses. This liquid medium is then inoculated with productive spores of Aspergillus niger in large bioreactors.
The two main industrial techniques are surface culture and submerged fermentation. Submerged fermentation, which involves growing the mold throughout the liquid in large tanks, is the predominant method today due to its superior efficiency and ease of scaling. During fermentation, the fungus converts the sugar into citric acid, which is released into the liquid broth, causing the pH to drop significantly.
After fermentation, a downstream process separates the pure citric acid from the fungal biomass and liquid. First, the broth is filtered to remove the fungal mycelium, which is the solid residue. The remaining liquid is then treated with calcium hydroxide, which reacts with the dissolved citric acid to form a solid, insoluble precipitate called calcium citrate. This precipitation step separates the citric acid from other soluble impurities.
The calcium citrate precipitate is collected and reacted with sulfuric acid, which regenerates the free citric acid and produces calcium sulfate (gypsum) as a byproduct. The resulting weak acid solution undergoes extensive purification steps, including adsorption decolorization, filtration, and demineralization using ion-exchange resins. These steps remove residual pigments, metal ions, and other trace impurities. Finally, the highly purified solution is concentrated through evaporation, allowing the pure, colorless citric acid to crystallize.
Consumer Safety and Allergy Concerns
The final product, food-grade citric acid, is a highly purified, crystalline substance. Due to the intense purification steps, the finished product contains no mold, spores, or fungal biomass. Regulatory bodies, such as the U.S. Food and Drug Administration (FDA), have granted manufactured citric acid the status of “Generally Recognized as Safe” (GRAS) for its intended use in food.
Despite the high purity, some consumer anxiety exists related to the product’s mold origin, particularly for individuals with mold sensitivities. Rare reports suggest that certain individuals may experience inflammatory reactions, such as joint pain or abdominal cramping, after consuming manufactured citric acid. It is theorized that these reactions may be caused by trace amounts of mold residues or metabolic byproducts, rather than the citric acid molecule itself.
The presence of mycotoxins, toxic compounds produced by some fungi, is not a concern because the Aspergillus niger strains used are non-toxigenic. Furthermore, the stringent purification process eliminates all biological material. For the vast majority of the population, manufactured citric acid poses no unique risk and is considered safe for consumption.
Roles of Citric Acid in Everyday Products
In the food and beverage industry, which accounts for about 70% of its use, citric acid’s primary function is as an acidulant, providing a sharp flavor profile to soft drinks, candies, and fruit-flavored products. Its low pH also makes it an effective preservative, inhibiting the growth of bacteria, mold, and yeast, which extends the shelf life of items like canned goods and jams.
Citric acid also functions as an antioxidant and a chelating agent, helping stabilize food products and improve their appearance. As a chelating agent, it binds to trace metal ions, preventing undesirable reactions like oxidation. This metal-binding property is also utilized in household and industrial applications, where it is added to detergents and cleaning products to soften water and remove mineral deposits like limescale. Additionally, it is used in pharmaceuticals and cosmetics to adjust pH, act as a preservative, and improve the absorption of mineral supplements.