How to Make Citric Acid? A Comparison of Raw Materials, Processes, and Purity Between Aspergillus niger Fermentation and Chemical Synthesis

The specific conditions for citric acid production by Aspergillus niger fermentation of glucose involve maintaining a temperature range of 28 to 30°C, which balances microbial activity and enzyme stability. The pH starts at 5.5 to 6.5 to promote fungal growth but is later reduced to 2.0 to 2.5 to optimize citric acid secretion and suppress byproduct formation. These parameters ensure efficient conversion of glucose to citric acid while minimizing metabolic deviations.

Maleic anhydride is indeed a common raw material in chemical synthesis, widely used in producing polymers like unsaturated polyesters, pharmaceuticals, and agrochemicals. Its reactive double bond makes it a versatile building block in organic synthesis. However, it is not involved in citric acid production, which relies on microbial fermentation or natural extraction from citrus fruits.

In the food industry, citric acid derived from fermentation (the standard method) is widely approved (e.g., as E330 in Europe) for use as an acidulant, flavor enhancer, and preservative, with no major restrictions when produced under good manufacturing practices. Chemical synthesis of citric acid is not commercially viable for food use due to high costs and purity concerns, so food-grade citric acid is almost exclusively fermentation-derived.

To improve crystal purity of citric acid in small-scale lab preparations, methods like recrystallization are effective: dissolve crude citric acid in hot water, filter to remove impurities, and cool slowly to induce pure crystal formation.

When Aspergillus niger ferments glucose to produce citric acid, specific temperature and pH conditions play a vital role. The optimal temperature range for this fermentation process is usually around 28 - 30°C. At this temperature, the enzymes within Aspergillus niger can function most efficiently, catalyzing the necessary biochemical reactions for citric acid synthesis. As for the pH, it is typically maintained between 2.0 and 3.0. This acidic environment not only promotes the growth and metabolic activities of Aspergillus niger but also helps to inhibit the growth of other competing microorganisms, reducing the risk of contamination during the fermentation process.

Maleic anhydride is indeed a common raw material in chemical synthesis. It has wide applications in the manufacturing of various products. For example, it is used to produce unsaturated polyester resins, which are essential in industries such as construction, automotive, and marine for making composites, coatings, and fiberglass. It is also involved in the synthesis of other organic compounds, like fumaric acid and maleic acid, which are used in food, pharmaceutical, and chemical industries.

In the food industry, the citric acid produced by Aspergillus niger fermentation of glucose is highly favored and widely used. It is recognized as a safe food additive and can be used as an acidulant to adjust the acidity of food products, a flavor enhancer to improve taste, and a preservative to extend the shelf life. However, citric acid synthesized from maleic anhydride is not allowed in food production. This is mainly because the synthetic process may introduce harmful substances that are not suitable for human consumption, and consumers generally prefer natural food additives.

To improve the crystal purity of citric acid in small - scale laboratory preparation, several approaches can be taken. One way is to perform multiple recrystallization steps. Dissolve the crude citric acid in a minimum amount of hot solvent, usually water. As the solution cools slowly, pure citric acid crystals will form while impurities remain in the solution. Another method is to use activated carbon treatment. Activated carbon has a large surface area and can adsorb various colored and organic impurities present in the citric acid solution. Filtration and washing the crystals thoroughly with cold, distilled water also help remove surface - attached impurities. Additionally, controlling the rate of cooling during crystallization is important. A slow and controlled cooling rate can lead to the formation of larger, more pure crystals, as rapid cooling may cause the inclusion of impurities within the crystal structure.

Teaching industrial microbiology and chemical synthesis, I always stress mastering optimal conditions for citric acid production and synthesis pathways. In labs or industry, Aspergillus niger fermentation of glucose requires strict temperature control at 28–30°C—above 32°C inhibits fungal enzymes, while below 25°C slows metabolism excessively. pH must stay precisely between 2.0 and 3.0; this weak acidity supports mycelial growth while suppressing contaminants. Lab data shows dissolved oxygen saturation must exceed 30% (via aeration systems) to drive TCA cycle intermediates toward citric acid rather than biomass.

Maleic anhydride is a classic organic synthesis intermediate. In advanced organic chemistry, we analyze its Diels-Alder reactions (e.g., with isoprene for terpene precursors) and hydrolysis to maleic acid. But students must recognize: while maleic acid (from hydrolysis) is approved as a food additive (acidulant), maleic anhydride itself remains unapproved due to toxicity risks. This distinction is critical in food regulations—GRAS (Generally Recognized As Safe) standards strictly limit direct food use of non-compliant chemicals.

For small-scale lab citric acid crystals, purity depends on multiple factors. Pre-seeding supersaturated solutions at 40°C with pre-formed crystals (vs. spontaneous nucleation) drastically reduces impurities; slow evaporation (covering flasks with filter paper instead of sealing) yields more uniform crystals than rapid concentration. Trace metal ions (Fe³⁺/Mn²⁺) cause yellowing, so deionized water and analytical-grade reagents are mandatory—poor-quality materials often turn products grayish in student experiments.

Food-grade citric acid demands rigorous purity. Fermentation products meet FCC (Food Chemicals Codex) standards, while chemical synthesis (e.g., ethylene oxide route) may leave catalyst residues (e.g., palladium on carbon) or byproducts (e.g., dichloropropanol) exceeding limits. HPLC comparisons vividly show impurity peaks between methods, tied to FDA 21 CFR 184.1033 regulations on additive safety.

Sustainability discussions also arise. Fermentation uses renewable glucose but generates wastewater; chemical synthesis relies on non-renewable petrochemicals. In environmental chemistry, we calculate carbon footprints: fermentation emits ~0.8 tons CO₂ per ton citric acid, while chemical synthesis reaches 2.5 tons, directly influencing industrial choices.

Citric acid production via Aspergillus niger fermentation of glucose involves specific conditions. The fermentation typically occurs at a temperature range of 28 to 32 degrees Celsius. The pH is initially maintained around 5.0 to 6.0 but gradually decreases to about 2.0 to 3.0 as citric acid accumulates. For maleic anhydride, it is indeed a common raw material in chemical synthesis. It is used to produce a variety of chemicals such as unsaturated polyester resins, alkyd resins, and lubricant additives.

In the food industry, citric acid produced by fermentation is widely accepted and used for flavor enhancement, acidification, and preservation. However, citric acid derived from chemical synthesis may face stricter regulations due to potential impurities or synthetic residues. To improve the crystal purity of citric acid in a small - scale laboratory setting, recrystallization is an effective method. Dissolve the crude citric acid in hot water, filter out impurities, and then allow the solution to cool slowly. Pure citric acid crystals will form as the solution cools. This method helps remove impurities and enhances the purity of the final product.