Citric acid can be produced through both natural and synthetic methods, but the majority of global production relies on microbial fermentation. The natural method involves extracting citric acid from citrus fruits like oranges, lemons, and limes, although this process is costly and cannot meet the high demand for citric acid. The more prevalent method is microbial fermentation, primarily using the fungus Aspergillus niger or yeast such as Candida species. In this process, the microorganisms are fed with a carbon source, typically molasses, sugar solutions, or starch-based media derived from sources like wheat, corn, or cane sugar. The fermentation process takes place under specific conditions, including high sugar concentrations, low pH, and adequate aeration. The microorganisms metabolize the sugars and produce citric acid after their growth has ceased, often under nitrogen and phosphate limitation[1][2][3].
The fermented broth is then processed to isolate the citric acid. The biomass is separated by filtration, and the liquid is treated with calcium hydroxide to precipitate calcium citrate. This precipitate is then washed and dissolved in dilute sulfuric acid to yield citric acid and calcium sulfate. The resulting citric acid is further purified through demineralization, decolorization with activated carbon, and finally crystallized to produce either anhydrous or monohydrate citric acid. This process is commonly carried out in bioreactors using batch, fed-batch, or continuous fermentation techniques to optimize yield and efficiency[4][1][3].
Citric acid is generally considered safe for human consumption in food and beverages. It is of low acute oral toxicity, with a median lethal dose (LD50) in rats ranging from 3000 to 12000 mg/kg body weight, indicating that it would take a very high dose to cause severe harm[1]. In humans, ingestion of large doses can lead to vomiting, acidosis, and calcium deficiency, but these effects are typically seen at relatively high doses. Chronic oral studies in animals have shown no significant tissue abnormalities, with a No Observed Adverse Effect Level (NOAEL) of 1200 mg/kg body weight per day in rats[1]. However, it is important to note that citric acid can harm tooth enamel if consumed in large amounts, and rinsing the mouth after consumption is recommended to protect dental health[2].
While citric acid itself is not typically a source of contamination, there are several potential risks associated with its production and use. One concern is the use of microbial fermentation, particularly with mold, to produce citric acid. This method can lead to the presence of remnants of black mold, which may trigger allergic reactions or inflammatory responses in individuals with a genetic predisposition to black mold allergies[2]. Additionally, citric acid used in food preservation or other applications may be contaminated with heavy metals if the raw materials or extraction processes are not properly managed. For instance, citric acid is effective in extracting heavy metals from contaminated sludge, which suggests it could also potentially leach heavy metals from other sources if not handled correctly[3]. Furthermore, the cultivation of crops used in the production of citric acid could involve pesticide use, although this is more relevant to the agricultural practices rather than the citric acid itself. There is no direct evidence linking citric acid production to pathogen risks, but general food safety practices should always be followed to minimize any potential microbial contamination[4].
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