C₆H₈O₇ — Citric Acid

Citric acid is a naturally occurring weak organic acid found predominantly in citrus fruits such as lemons, limes, oranges, and grapefruits. It is one of the most important organic acids used in food and pharmaceutical industries due to its excellent flavoring, preservative, and buffering properties. The chemical formula of citric acid is \(C_6H_8O_7\). It is classified as a tricarboxylic acid because it contains three carboxyl (–COOH) groups and one hydroxyl (–OH) group in its structure.

Citric acid plays a central role in biochemistry as an intermediate in the Krebs cycle (citric acid cycle), which is the key metabolic pathway in all aerobic organisms for energy production. Industrially, citric acid is mainly produced through microbial fermentation using the fungus Aspergillus niger. It is non-toxic, biodegradable, and environmentally safe, making it highly valuable in multiple applications.

The structure of citric acid can be represented as:

This molecule has three carboxyl groups (–COOH) and one hydroxyl group (–OH) attached to a three-carbon backbone. The central carbon atom is asymmetric, making the molecule chiral. Citric acid is highly polar and forms multiple hydrogen bonds with water molecules, accounting for its high solubility and crystalline nature.

The strong hydrogen bonding network in the solid state gives citric acid its stability, while in aqueous solution, partial ionization of the carboxylic acid groups provides its weakly acidic properties. Each carboxyl group can donate a proton, making citric acid a triprotic acid with three dissociation constants.

Citric acid naturally occurs in all aerobic organisms as part of the citric acid cycle (Krebs cycle), where it plays a vital role in the metabolism of carbohydrates, fats, and proteins. It acts as an intermediate compound during the oxidation of acetyl-CoA to carbon dioxide and water, releasing energy in the process.

Natural sources of citric acid include citrus fruits such as lemons (about 5–8% citric acid), limes, oranges, and pineapples. It is also found in smaller quantities in berries, tomatoes, and other fruits. Industrially, citric acid is extracted or synthesized via microbial fermentation using sucrose or glucose as substrates.

1. Fermentation Process:

The industrial production of citric acid primarily relies on the fermentation method using the fungus Aspergillus niger. In this process, carbohydrate sources like molasses, corn starch, or sugarcane juice are used as substrates:

After fermentation, citric acid is precipitated as calcium citrate using calcium hydroxide and then converted to pure citric acid by acid treatment with sulfuric acid:

2. Extraction from Citrus Fruits:

Earlier, citric acid was obtained directly from lemon and lime juices by precipitation with calcium hydroxide followed by crystallization, but fermentation has largely replaced this method due to higher yield and lower cost.

Physical Properties:

• Appearance: White crystalline solid or fine powder.
• Solubility: Highly soluble in water and alcohol, slightly soluble in ether.
• Taste: Strongly sour and refreshing.
• Hygroscopic: Absorbs moisture from air.

Chemical Properties:

• Acidic Behavior: Citric acid can donate three protons, acting as a triprotic acid.

\(C_6H_8O_7 \rightleftharpoons C_6H_7O_7^- + H^+\)

• Formation of Salts: Reacts with bases to form citrate salts like sodium citrate and calcium citrate.

\(C_6H_8O_7 + 3NaOH \rightarrow Na_3C_6H_5O_7 + 3H_2O\)

• Esterification: Reacts with alcohols in presence of acid catalysts to form citric esters used in flavorings and plasticizers.
• Oxidation: On oxidation, it produces carbon dioxide and water.

• Food and Beverage Industry: Citric acid is used as a flavoring agent to provide tartness, as a preservative to inhibit microbial growth, and as an acidity regulator in beverages, candies, and jams.
• Pharmaceuticals: Used as an excipient, stabilizer, and pH regulator in medicines, effervescent tablets, and syrups.
• Cosmetics and Personal Care: Acts as a mild acidulant in shampoos, creams, and exfoliating products for pH adjustment and skin brightening.
• Chemical Industry: Used to prepare citrate salts, chelating agents, and biodegradable detergents.
• Cleaning Agents: Its chelating ability allows it to dissolve metal oxides, making it effective in descaling, rust removal, and household cleaners.
• Industrial Applications: Used in electroplating, photography, and as a buffering agent in textile processing.

Citric acid plays a vital role in the Krebs cycle (also known as the citric acid cycle), which is a key metabolic pathway that occurs in mitochondria. The cycle begins when acetyl-CoA combines with oxaloacetate to form citric acid:

This reaction initiates the energy production process in aerobic organisms. Citric acid also serves as a natural antioxidant and helps regulate mineral absorption by chelating metal ions. In the human body, it contributes to maintaining acid-base balance and detoxifying heavy metals.

Citric acid is non-toxic, biodegradable, and generally recognized as safe (GRAS) for human consumption. It is a common ingredient in food and pharmaceutical formulations. However, prolonged exposure to concentrated citric acid solutions can cause mild skin or eye irritation. In the environment, citric acid poses no ecological risk and degrades quickly in soil and water.

Due to its eco-friendly properties, citric acid and its derivatives are increasingly being used as sustainable alternatives to synthetic chemicals in cleaning agents, food preservation, and bioplastics production.