Citric Acid
Citric acid is a weak organic acid that plays a pivotal role in both biological and industrial contexts. Naturally occurring in citrus fruits such as lemons, oranges, and limes, it is known for its sour taste and its importance in metabolism as a key intermediate of the citric acid cycle (Krebs cycle). Chemically represented as C₆H₈O₇, citric acid is a tricarboxylic acid containing three carboxyl (-COOH) groups and one hydroxyl (-OH) group. Beyond its natural occurrence, citric acid has immense industrial, pharmaceutical, and environmental significance, making it one of the most versatile organic acids utilised worldwide.
Chemical Structure and Properties
Citric acid is a crystalline, colourless compound that belongs to the family of carboxylic acids. Structurally, it consists of a three-carbon backbone with three carboxylic acid groups and one hydroxyl group attached. The systematic IUPAC name is 2-hydroxy-1,2,3-propane-tricarboxylic acid.
Key properties include:
- Molecular formula: C₆H₈O₇
- Molecular mass: 192.12 g/mol
- Melting point: 153°C
- Solubility: Highly soluble in water, alcohol, and ether.
- Acidity (pKa values): 3.13, 4.76, and 6.40 for its three acidic protons.
Due to its three carboxylic groups, citric acid exhibits triprotic behaviour, allowing it to donate up to three protons in solution. This makes it an effective buffering agent capable of maintaining pH stability in numerous chemical and biological systems.
The anhydrous form and monohydrate form are the two common crystalline variants, both widely used depending on industrial requirements.
Natural Occurrence and Sources
Citric acid is naturally found in the tissues of all aerobic organisms since it is a central intermediate of the tricarboxylic acid (TCA) cycle. In plants, especially citrus fruits, it accumulates in large quantities in the vacuoles, contributing to their characteristic sour taste. Fruits such as lemons, grapefruits, and limes contain up to 8% citric acid by dry weight.
In microorganisms, citric acid is produced during carbohydrate metabolism. Certain species of Aspergillus niger and Candida lipolytica are known for their high citric acid secretion ability, which is exploited industrially for large-scale production.
Industrial Production
Although citric acid was historically extracted from citrus fruits, the global demand today is primarily met through microbial fermentation, as it is more cost-effective and sustainable.
Microbial Fermentation Process:
- Microorganism used: Aspergillus niger is the most common fungus employed.
- Substrates: Sucrose, glucose, molasses, or corn steep liquor act as carbon sources.
- Conditions: The process is carried out aerobically at a pH around 2.0–3.0 and a temperature between 28°C and 32°C.
- Precipitation: After fermentation, calcium citrate is precipitated using calcium hydroxide and later converted into citric acid using sulphuric acid.
This biotechnological process accounts for over 90% of global citric acid production, yielding a highly pure product suitable for food and pharmaceutical applications.
Role in Metabolism – The Citric Acid Cycle
Citric acid plays an essential metabolic role as the first product of the citric acid cycle, also known as the Krebs cycle or tricarboxylic acid (TCA) cycle, which takes place in the mitochondria of eukaryotic cells.
The cycle begins when acetyl-CoA, derived from carbohydrates, fats, or proteins, combines with oxaloacetate to form citric acid, catalysed by the enzyme citrate synthase.
Simplified reaction: Acetyl-CoA + Oxaloacetate + H₂O → Citrate + CoA-SH
Citric acid then undergoes a series of enzymatic transformations, resulting in the release of carbon dioxide and the generation of energy-rich molecules such as NADH, FADH₂, and ATP. These molecules feed into the electron transport chain to produce the majority of cellular energy.
The citric acid cycle is thus the central hub of metabolism, linking carbohydrate, fat, and protein catabolism. The formation and breakdown of citric acid ensure the continuous supply of metabolic energy essential for cell survival and function.
Applications in the Food and Beverage Industry
Citric acid is one of the most widely used additives in the global food industry. Its acidic, preservative, and chelating properties make it an indispensable ingredient.
Major applications include:
- Flavouring and acidulant: Adds a pleasant tartness to soft drinks, candies, jams, and confectionery.
- Preservative: Inhibits microbial growth by lowering pH and binding metal ions that promote oxidation.
- pH control: Acts as a buffering agent to stabilise the acidity in processed foods and beverages.
- Antioxidant synergist: Enhances the effectiveness of antioxidants like ascorbic acid by chelating trace metals.
Citric acid is also commonly used to prevent crystallisation of sucrose in syrups and to improve texture in dairy products and frozen foods.
Pharmaceutical and Medical Uses
In medicine, citric acid has a range of applications due to its biocompatibility and physiological importance.
- Effervescent formulations: Combined with sodium bicarbonate to create effervescent tablets that dissolve rapidly in water.
- Anticoagulant: Citrate salts such as sodium citrate and trisodium citrate are used in blood transfusion bags to prevent clotting by binding calcium ions.
- Urinary alkaliniser: Citric acid salts are prescribed to prevent kidney stones by reducing urinary acidity and increasing citrate excretion.
- Dentistry: Used in root canal treatments and as a chelating agent to clean root surfaces.
- Pharmaceutical stabiliser: Prevents degradation of active ingredients by maintaining appropriate pH levels.
Industrial and Technical Applications
Beyond food and medicine, citric acid finds broad applications across industries due to its chelating, cleaning, and environmentally friendly characteristics.
- Cleaning agents: Citric acid effectively removes limescale, rust, and mineral deposits, making it a common ingredient in household and industrial cleaners.
- Metal treatment: Acts as a chelating agent in electroplating, preventing oxidation of metals.
- Cosmetics and personal care: Used in skin-care products for pH adjustment, mild exfoliation, and as a preservative.
- Textile industry: Serves as a pH regulator and softening agent during dyeing processes.
- Biodegradable plastic production: Citric acid is used as a crosslinking agent in environmentally friendly polymer synthesis.
Environmental and Green Chemistry Relevance
Citric acid’s biodegradability and non-toxic nature make it a cornerstone in green chemistry. It replaces environmentally harmful substances like phosphates in detergents, reducing water pollution. In agriculture, citric acid is applied to enhance nutrient uptake by chelating metal ions in the soil.
In wastewater treatment, it aids in the removal of heavy metals by forming soluble complexes, thus promoting environmental sustainability.
Chemical Reactions and Derivatives
Citric acid participates in numerous chemical reactions that yield important derivatives:
- Citrate salts: Such as calcium citrate, potassium citrate, and sodium citrate—used in medicine, food processing, and buffering solutions.
- Acetylation and esterification: Produces citric acid esters like triethyl citrate, which serve as plasticisers in pharmaceutical coatings and biodegradable plastics.
- Dehydration: When heated, citric acid forms aconitic acid and subsequently itaconic acid, compounds valuable in polymer and resin industries.
These derivatives greatly expand citric acid’s industrial versatility.
Safety and Toxicological Aspects
Citric acid is generally recognised as safe (GRAS) for human consumption when used within prescribed limits. However, excessive ingestion or contact may cause mild irritation. Industrial exposure to concentrated citric acid can lead to skin or eye irritation due to its acidity. Ingestion of large quantities can also disturb gastric balance, though such cases are rare.
Biotechnologically produced citric acid is free from harmful residues and approved by food safety authorities such as the FDA, FAO, and WHO for global use.
Historical Background and Discovery
Citric acid was first isolated in 1784 by Carl Wilhelm Scheele from lemon juice. For over a century, it was produced by crystallising lemon juice, but this method was labour-intensive and costly. The shift to microbial fermentation in 1917 by James Currie marked a major milestone in industrial biotechnology. Using Aspergillus niger, Currie’s process revolutionised production efficiency, making citric acid one of the first commercially successful microbial products.
Today, global production exceeds two million tonnes per year, with China, the United States, and Europe as leading producers.
Economic and Commercial Significance
Citric acid represents a multi-billion-dollar global market due to its broad range of applications. Its demand continues to rise owing to increasing consumption of processed foods, beverages, pharmaceuticals, and eco-friendly cleaning agents. The global citric acid market is projected to grow further with trends toward sustainability and natural additives.
Producers are also exploring waste valorisation, converting agricultural residues and molasses into fermentation substrates, thereby reducing environmental impact and enhancing economic viability.
Biological and Ecological Significance
Citric acid’s role in nature extends beyond metabolism. It serves as a natural chelating agent in soils, aiding plants in nutrient absorption by forming soluble complexes with iron, calcium, and magnesium. In aquatic systems, it helps maintain pH stability and influences metal ion availability, indirectly affecting ecosystem health.
Furthermore, certain bacteria and fungi use citric acid as a carbon source, contributing to organic matter decomposition and nutrient cycling.