Citric acid is a weak organic acid naturally present in many fruits and vegetables. Characterized by its sour taste and ability to act as a chelating agent (binding to metal ions), it is widely used commercially. It serves as a preservative and flavoring agent in food and beverages, and in industrial and domestic applications, it adjusts acidity levels and acts as an environmentally benign cleaning ingredient.
Extraction from Natural Sources
Historically, citric acid was obtained by extraction from fruits where it naturally concentrates. Carl Wilhelm Scheele first isolated and crystallized the compound from lemon juice in 1784. This discovery led to initial commercial production starting in the late 19th century, primarily centered around the robust citrus industry in Italy.
The process involved crushing fruits, such as lemons and limes, to extract the juice, which contained high concentrations of citric acid (five to eight percent by weight). The extracted juice was then mixed with calcium hydroxide (lime). This initiated a chemical reaction, causing the citric acid to precipitate out of the liquid as the solid salt, calcium citrate.
The calcium citrate was filtered and treated with diluted sulfuric acid to convert it back into the soluble, free citric acid form. While effective, this method proved inefficient and costly for large-scale industrial demand due to several inherent limitations. Production relied solely on fruit juice, making it subject to seasonal harvest variations, unpredictable crop yields, and high logistical costs. As global demand for the versatile acid grew, a more stable, higher-yield production method became apparent.
The Modern Method: Fungal Fermentation
The limitations of fruit extraction led to the development of a highly efficient biological process that now accounts for the vast majority of global citric acid production. This modern method is a form of industrial fermentation, relying on the mold Aspergillus niger to synthesize the acid in large bioreactors. The process was industrialized in the early 20th century after it was discovered that certain strains of this fungus could efficiently convert sugars into citric acid.
The Aspergillus niger mold is cultivated in a liquid medium containing a concentrated carbon source, typically inexpensive, readily available substrates like molasses, corn syrup, or hydrolyzed corn starch. The success of this method hinges on carefully manipulating the mold’s environment to force it into a state of “overproduction.” Scientists achieve this by creating conditions that stress the organism, diverting its normal metabolic pathway.
A primary step involves tightly controlling the concentration of trace elements, particularly iron, zinc, and manganese, which must be kept at extremely low levels in the fermentation broth. Iron is a necessary cofactor for the enzyme aconitase, which is involved in the tricarboxylic acid (TCA) cycle. By limiting iron, aconitase activity is suppressed, causing the citric acid to accumulate and be secreted into the surrounding medium.
Furthermore, the process is maintained at a highly acidic pH, often in the range of 2.5 to 3.5, which naturally occurs as the acid is produced. This low pH not only inhibits the activity of unwanted enzymes but also suppresses the growth of most contaminating microorganisms. The fermentation is carried out using submerged fermentation, where the A. niger is suspended throughout the liquid medium, allowing for precise control of temperature, aeration, and nutrient flow.
Isolation and Purification Processes
Once the fermentation batch is complete, the resulting broth contains a high concentration of crude citric acid mixed with the fungal biomass and residual medium components. The first step in recovery is to separate the fungal mycelium from the liquid broth, usually accomplished through filtration or centrifugation. The clear broth is then ready for a series of chemical engineering steps designed to isolate and purify the acid.
The classical and still widely used purification method involves a chemical precipitation step, similar to the historical fruit extraction process. Calcium hydroxide is added to the broth, causing the dissolved citric acid to react and form the insoluble salt, tri-calcium citrate. This solid material is then filtered out from the remaining liquid impurities, separating the desired product from the majority of the fermentation byproducts.
The purified calcium citrate is then treated with sulfuric acid, a stronger acid, to liberate the citric acid from its salt form. This reaction regenerates the soluble citric acid while simultaneously forming insoluble calcium sulfate, commonly known as gypsum, as a byproduct. The gypsum is easily removed by a second filtration step, leaving a solution of crude citric acid.
For commercial-grade purity, the solution is further refined through decolorization, often by passing it over activated carbon to remove colored organic impurities. Ion-exchange resins are also used to remove lingering inorganic ions, a process called demineralization. Finally, the highly pure citric acid solution is concentrated through evaporation and subjected to controlled cooling to induce crystallization, yielding the final product in either its anhydrous (water-free) or monohydrate form.