Citric acid is a compound naturally present in many fruits and vegetables, particularly citrus fruits like lemons and oranges. Known for its tangy taste and beneficial properties, it plays a central role in numerous fields, from the food industry to medicine and cosmetics. This article offers an in-depth exploration of its properties, its diverse uses, and the scientific methods used to analyze it.
1. Identification and chemical properties
Definition and chemical formula
Citric acid, with the chemical formula C6H8O7 , is a tricarboxylic organic acid with a hydroxyl group (OH). It takes its name from the lemon, the fruit in which it is particularly concentrated. In chemistry, it is classified as a weak acid due to its ability to release hydrogen ions (H+) gradually.
Molecular structure and characteristics
The molecular structure of citric acid consists of three carboxyl groups (-COOH) and one hydroxyl group attached to a carbon chain. This structure gives citric acid unique properties, notably its ability to act as a chelator, effectively binding metal ions. In aqueous solution, it exists in different ionized forms depending on the pH, making it an excellent chemical buffer.
Key physicochemical properties
Citric acid possesses physicochemical characteristics that explain its many uses. It is highly soluble in water, with a solubility reaching 592 g/L at 20 °C. It is also soluble in ethanol, but remains insoluble in organic solvents such as benzene or chloroform. Its three pKa values (3.13, 4.76, and 6.40 at 25 °C) make it an ideal compound for regulating acidity in various formulations. In its pure form, it appears as colorless or white crystals and decomposes at approximately 175 °C, releasing carbon dioxide (CO2) and water.
Importance in biochemistry: the Krebs cycle
Citric acid plays a fundamental role in the Krebs cycle , a metabolic pathway essential for energy production in aerobic cells. This cycle begins with the formation of citrate, a product of the condensation of acetyl-CoA and oxaloacetate by citrate synthase. Citrate is then converted into other intermediate compounds to produce ATP, the primary source of cellular energy. This process explains why citric acid is present in all living organisms.
2. Production and sources
Natural abundance
Citric acid occurs naturally in many fruits and vegetables, but citrus fruits, such as lemons and oranges, contain the highest concentrations. In lemon juice, it can represent up to 8% of the dry weight, or about 47 g/L. This high concentration explains its characteristic tangy taste. Besides citrus fruits, citric acid is also produced by the roots of some plants, which release it to improve phosphorus uptake in the soil.
Industrial production: historical processes
Citric acid was first isolated in 1784 by the Swedish chemist Carl Wilhelm Scheele, who extracted it from lemon juice. Until the early 20th century, production was primarily based on natural extraction. Citrus juices were treated with lime (calcium hydroxide), forming calcium citrate, which was then converted back into citric acid using sulfuric acid.
Modern production methods
Today, the industrial production of citric acid relies primarily on fungal fermentation. This method was discovered in 1917 by James Currie, who demonstrated that molds, particularly Aspergillus niger , could produce citric acid from sugary substrates such as glucose or molasses. This process is not only more economical but also more environmentally friendly, as it utilizes byproducts of the food industry. Fermentation is followed by purification through precipitation and recrystallization to obtain a pure product.
Environmental impact of production
The industrial-scale production of citric acid is generally considered environmentally friendly. Fungal fermentation, utilizing sugar-rich residues, helps reduce waste from the food industry. Furthermore, citric acid is biodegradable, minimizing its environmental impact at the end of its life cycle. However, increasing global demand necessitates responsible resource management to avoid long-term sustainability issues.
3. Uses and applications
Agri-food
In the food , citric acid is widely used under the code E330 as a food additive. Its main role is to act as an acidifier , giving a slightly tangy taste to products such as sodas, candies, and jams. It is also used as a pH regulator to stabilize foods sensitive to acidity fluctuations, which helps extend their shelf life.
Another key property of citric acid is its ability to act as a chelating agent , binding metal ions to prevent food oxidation and protect its color and flavor. For example, in carbonated beverages, it prevents interactions between metal ions and active ingredients, thus preserving their stability.
Cosmetics and pharmaceuticals
cosmetic products , citric acid is used to adjust the pH of formulations, thus ensuring their compatibility with the skin. It is also included in skincare products such as chemical peels, where its gentle exfoliating action helps promote cell renewal.
Industrial applications
Citric acid is also a key player in various technical industries. It is commonly used as a descaling cleaner , dissolving limescale deposits on household appliances such as coffee makers. In construction, it acts as a cement additive , slowing the setting time to allow for better handling in hot conditions.
In the chemical industry, it serves as a basis for the manufacture of plasticizers used in plastic materials. Furthermore, its antimicrobial and antialgal properties make it useful as a fungicide and bactericide , particularly in the treatment of surfaces and cooling systems.
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4. Analysis and control of citric acid
Importance of laboratory analysis
Analytical testing of citric acid is essential in the food, cosmetics, and pharmaceutical industries. Laboratories verify its purity , concentration , and the presence of contaminants to ensure product safety. For example, in beverages and food products, precise measurement of citric acid concentration ensures optimal pH regulation and prevents product degradation.
Furthermore, these analyses make it possible to comply with international standards such as ISO 17025 and regulations specific to each sector, thus ensuring total conformity.
Analytical methods
Laboratories use several methods for citric acid analysis, each tailored to the specific requirements of the samples.
HPLC (Liquid Phase Chromatography)
Liquid chromatography is one of the most widely used methods for quantifying citric acid. It offers exceptional accuracy and allows detection even at low concentrations. This method is ideal for food and cosmetic products, where traceability and reliable results are essential.
Enzymatic method
This approach relies on the use of specific enzymes to measure citric acid. The enzymatic reaction is rapid, and detection is performed by spectrophotometry. This type of analysis is particularly well-suited for laboratories seeking a simple and rapid method for food and beverage testing.
UV-Vis spectrophotometry
Used for rapid measurements, this technique detects changes in the absorbance of solutions containing citric acid. Although less precise than HPLC, it is an economical method for preliminary testing.
Quality standards and protocols
The laboratories operate according to validated protocols , ensuring the accuracy and reproducibility of the analyses. Limits of detection (LOD) and limits of quantification (LOQ) are carefully defined to meet client and regulatory requirements. For example, the limit of quantification for citric acid in cosmetic or food samples can range from 0.1 mg/L to 10 mg/L , depending on the method used.
5. Ecological role and sustainability
Impact on phosphorus bioassimilation
Plants secrete citrates into the soil, including citric acid, to improve the bioavailability of phosphorus, a nutrient essential for their growth. This process is particularly crucial in phosphorus-poor soils, where citrate ions released by the roots help solubilize insoluble phosphate compounds, thus facilitating their absorption. For example, some wheat varieties naturally secrete citrates, allowing them to thrive in challenging agricultural environments.
Contribution to agroecology
Within the framework of agroecological practices, citric acid plays a role in the sustainable management of resources. By acting as a natural agent for solubilizing minerals, it reduces the need for chemical fertilizers, thus limiting the environmental impact of intensive agricultural practices. Furthermore, its role in regulating soil microorganisms promotes greater biodiversity and increased productivity.
Biodegradability and low environmental impact
Citric acid is fully biodegradable, making it an environmentally safe component. After use, it naturally decomposes into carbon dioxide and water without leaving any toxic residues. This characteristic makes it ideal for applications such as eco-friendly detergents and cleaning products. Unlike many industrial chemicals, its use does not lead to persistent accumulation in the environment.
Life cycle analysis
A comprehensive life cycle assessment of citric acid shows that it has a relatively low environmental footprint. Production via fungal fermentation utilizes by-products from the food industry, contributing to a circular economy. Efforts to reduce carbon emissions during manufacturing and optimize purification processes have further improved its environmental profile.
Evidence of its harmlessness
Citric acid is one of the most studied and well-documented food additives. It is approved by regulatory bodies such as the European Food Safety Authority (EFSA) and the Food and Drug Administration (FDA) . These institutions confirm that citric acid is safe for use in food, cosmetics, and pharmaceuticals when used in recommended concentrations.
Furthermore, due to its simple chemical structure and its natural presence in human metabolism via the Krebs cycle, citric acid is efficiently metabolized and eliminated by the body. This biodegradability, combined with its non-toxic profile, makes it a safe choice for many applications.
Safety measures in industry
Despite its general harmlessness, certain precautions are taken in industry to ensure proper handling and use. In its concentrated form, citric acid can irritate the skin and mucous membranes. Safety Data Sheets (SDS) classify this compound as an "irritant," and personal protective equipment (gloves, goggles) is recommended for large-scale handling.
In the cosmetics industry, citric acid is often used to adjust the pH of formulations. Skin tolerance tests are routinely performed to ensure that products are safe for end users. Furthermore, its GHS and WHMIS classification helps regulate its use and raise awareness among operators regarding good safety practices.
