Antioxidant

Antioxidants are chemical compounds that slow or inhibit oxidation, a redox process capable of generating reactive species that degrade organic materials, including those found in living organisms. By limiting autoxidation and free-radical formation, antioxidants play significant roles in biology, food preservation, industrial chemistry and pharmaceuticals. Their functions span natural cellular defence, commercial stabilisation of consumer goods and the maintenance of product quality in a wide range of materials.

Chemical Nature and Mechanisms of Action

Oxidation involves the transfer of electrons, which can produce reactive oxygen species or free radicals. These species initiate chain reactions that damage cellular components or degrade industrial materials. Antioxidants act by interrupting these reactions, either by supplying electrons to reactive molecules or by decomposing peroxides formed during oxidation.
In biological systems, antioxidants include molecules such as glutathione, mycothiol and bacillithiol, as well as enzymatic defences like superoxide dismutase. These systems protect cells from oxidative stress by scavenging reactive oxygen species before they can damage lipids, proteins or nucleic acids. Dietary antioxidants such as vitamins A, C and E are known for similar protective roles, although in vitro antioxidant activity does not necessarily translate to significant antioxidant effects within the human body. Evidence from clinical studies shows that antioxidant supplements generally do not prevent disease or improve health outcomes.

Historical Development

The concept of antioxidants originated in industrial chemistry, where the term first referred to substances that prevented oxygen consumption in chemical processes. Early studies focused on preventing the corrosion of metals, inhibiting the polymerisation of fuels and aiding the vulcanisation of rubber. In the early twentieth century, research expanded into food science, examining methods to prevent rancidity in fats by limiting oxidation.
The identification of vitamins C and E as biological antioxidants marked a turning point, integrating antioxidant chemistry into cellular biochemistry. As research progressed, antioxidants were recognised as reducing agents that could prevent oxidative injury by undergoing oxidation themselves.

Antioxidants in Food Preservation

Antioxidants are widely employed in foods to limit spoilage caused by exposure to oxygen and ultraviolet light. Preserving conditions may involve restricting light or air, although plant materials require a controlled oxygen level to avoid undesirable flavours or discolouration. Antioxidants remain crucial preservatives in frozen or refrigerated foods where microbial spoilage is slowed but oxidation continues.
Common food antioxidants include:

• Natural compounds: ascorbic acid (E300) and tocopherols (E306)
• Synthetic preservatives: propyl gallate (E310), tert-butylhydroquinone (TBHQ), butylated hydroxyanisole (BHA, E320) and butylated hydroxytoluene (BHT, E321)

Unsaturated fats present in oils are prone to rancidification, producing discolouration and off-flavours. Foods containing such fats are often protected by smoking, salting or fermenting, and in some cases by applying sulfur-based antioxidants prior to drying. Oils like olive oil contain natural antioxidants that provide some protection against photooxidation. Metal ions can accelerate oxidative damage; therefore, controlling metal exposure is another strategy used in food preservation.

Pharmaceutical and Cosmetic Applications

Antioxidants serve as excipients in pharmaceutical formulations to prevent oxidation of active compounds. Sequestrants such as disodium EDTA can bind metal ions, reducing metal-catalysed oxidation. In cosmetics, antioxidants stabilise oils and other lipids in products such as moisturisers and lipsticks. Phenolic antioxidants and related compounds absorb ultraviolet radiation through chromophore structures, reducing photo-induced degradation of ingredients and minimising oxidative stress from sunlight.

Industrial and Polymer Uses

Antioxidants are essential for maintaining the stability of fuels, lubricants and polymers. In fuels such as petrol, compounds derived from p-phenylenediamine inhibit gum formation caused by oxidation. Lubricants similarly employ antioxidants to prevent polymerisation and deposit formation.
In polymer chemistry, antioxidants preserve the integrity of materials like natural rubber, polybutadiene and adhesives. These polymers are vulnerable to oxidation and ozonolysis, leading to loss of strength and flexibility. Additives used for stabilisation include:

• Primary antioxidants, which scavenge free radicals at the onset of oxidation
  • Hindered phenols, e.g., butylated hydroxytoluene and related substituted phenols
  • Aromatic amines, used selectively due to regulatory limitations
  • Hindered amine light stabilisers (HALS), effective against UV-induced degradation
• Secondary antioxidants, which decompose peroxides into non-reactive products
  • Phosphites, such as tris(2,4-di-tert-butylphenyl)phosphite
  • Thiosynergists, particularly thioesters derived from thiodipropionic acid

Multifunctional antioxidants incorporate both primary and secondary antioxidant groups. Hydroxylamines, for example, can scavenge a broad range of radicals and offer enhanced stabilisation. Lactone-based antioxidants and acrylated bisphenols serve similar roles, especially in polymeric materials exposed to heat or ultraviolet light.
A notable pharmaceutical example is probucol, originally designed as a polymer stabiliser before being repurposed for lowering LDL cholesterol.

Environmental and Health Considerations

Synthetic phenolic antioxidants (SPAs) and aminic antioxidants are widely detected in indoor dust, surface waters, wastewater and environmental sediments. These compounds, including BHT, BHTQ and related phenols, may pose risks such as hepatotoxicity, endocrine disruption or genotoxicity. Concerns about environmental persistence and bioaccumulation have prompted ongoing assessment of their safety profiles and the development of alternative stabilisers with reduced ecological impact.