Citric acid is a common compound found throughout the natural world, recognized for the sharp, tart flavor it imparts to citrus fruits. Commercially, it acts as a preservative and an acidity regulator in countless products globally. The vast majority of citric acid used today is not extracted from fruit but is manufactured through a bio-industrial process involving a fungus, specifically a strain of the mold Aspergillus niger. This microbial production method is the standard for meeting the massive global demand for this versatile organic acid.
The Transition to Microbial Production
For over a century, the primary source of citric acid was extraction directly from citrus fruits, such as lemons and limes. This industrial production began in the late 19th century, relying heavily on the Italian citrus industry. The process involved treating lemon juice with hydrated lime (calcium hydroxide) to precipitate calcium citrate, which was then filtered and converted back into the pure acid using diluted sulfuric acid.
This fruit-based extraction was expensive, inefficient, and dependent on seasonal agricultural yields, limiting the scale of production. The method could not keep pace with the growing demand from industries like food and pharmaceuticals. The transition to microbial production accelerated when World War I disrupted the supply of Italian citrus exports, creating a significant shortage. This accelerated the adoption of a fermentation process discovered in 1917, providing a reliable, year-round, and high-volume alternative.
Aspergillus niger and the Fermentation Process
The fungus responsible for nearly all modern commercial citric acid production is Aspergillus niger, a mold classified within the Aspergillus genus. Industrial strains are carefully selected and controlled to be efficient producers. This organism is favored for its ability to grow rapidly and excrete the acid under specific, highly acidic conditions.
Production takes place in large, sterile fermentation tanks, typically using a method called submerged fermentation, which accounts for approximately 80% of the world’s output. The fungal culture is fed a growth medium rich in carbohydrates, such as molasses, refined sugar, or glucose syrup. A. niger metabolizes these sugars to generate energy, but under specific limiting conditions—such as a lack of nitrogen or trace metals—it over-produces and excretes citric acid into the liquid medium.
Fermentation is carried out in an extremely acidic environment, often with a pH maintained between 2.5 and 3.5, which A. niger tolerates well. The process is highly temperature-controlled, usually between 27 and 33 degrees Celsius, and requires constant aeration to supply oxygen. Over several days, the fungus converts the sugar into the organic acid, which accumulates to high concentrations in the fermentation broth.
Ensuring Purity and Regulatory Safety
Following fermentation, the liquid medium contains crude citric acid mixed with fungal biomass (mycelium) and other byproducts. Purification is required to ensure the final product is chemically pure and free of residue. The initial step involves filtering the fermentation broth to separate the liquid, which contains the acid, from the solid fungal mass.
The next step, known as the lime-sulfuric acid method, involves adding calcium hydroxide to the filtered liquid, which causes the citric acid to precipitate out as an insoluble salt called tri-calcium citrate. This solid salt is then treated with sulfuric acid, converting the calcium citrate back into a highly pure citric acid solution while forming a calcium sulfate precipitate that is easily removed.
Further purification involves treating the solution with activated carbon to remove color and organic impurities, followed by ion-exchange resins to remove trace metal ions. The final, colorless solution is concentrated under vacuum, allowing the pure citric acid to crystallize, resulting in a product that is typically \(geq 99.5%\) pure. Due to this rigorous process, regulatory bodies like the U.S. Food and Drug Administration (FDA) classify the finished acid as “Generally Recognized As Safe” (GRAS) for use in food, as it is chemically identical to the acid found in fruit.
Essential Applications of Citric Acid
Commercial citric acid is a versatile ingredient used across numerous industries due to its unique chemical properties. In the food and beverage industry, its primary function is as a flavor enhancer, providing a sour or tart taste to soft drinks, candies, and jams. It also works as a preservative by inhibiting the growth of microorganisms and lowering the pH of food products, which increases shelf life.
Citric acid also functions as a chelating agent, meaning it binds to trace metal ions that can catalyze undesirable chemical reactions like rancidity or color change. This metal-binding property makes it valuable in non-food applications, such as in detergents and cleaning agents, where it softens water and helps remove limescale and rust. It is also a common ingredient in pharmaceuticals and cosmetics, used for pH adjustment, as a stabilizer, and for its mild exfoliating properties in skin care products.