either preventing the loss of electrons from fats and oils by donating electrons of their own, thus removing reactivefree radicals which are needed for the propagation step. or reducing the availability of transition metal ions in solution.
Types of antioxidantsReducing Free radical Chelating agents scavengers agents
Are electron donors and they remove or reduce the concentrations of oxygen. Reducing agents are oxidized in preference to fats or oils. Rancidity of fats and oils is an oxidation reaction, which occurs by a free- radical mechanism, and is catalysed by transition metal ions. Examples include ascorbic acid(vitamin C) and carotenoids.
Vitamin C is readily oxidized on exposure to air, to form dehydroascorbic acid.
Form stable and less reactive free radicals: ROO• +AH → ROOH + A• Examplesinclude BHA, BHT, TBHQ and tocopherols.
BHA, BHT, TBHQ and tocopherols (which we will represent as AH) acts as antioxidants as they form relatively stable and unreactive free radicals when they lose a hydrogen atom to the reactive free radicals through homolytic fission. R+ AH → RH + A RO + AH → ROH + A ROO + AH → ROOH + A
Thefree radicals produced, can also remove free radicals in termination steps: R+ A → RA RO + A → ROA
Reduce the concentration of free metal ions in solution. Examples include salts of EDTA and plant extracts (rosemary, tea, ground mustard) EDTA forms especially strong complexes with Mn(II), Cu(II), Fe(III), Pb (II) and Co(III).
A chelate is a chemical compound composed of a metal ion and a chelating agent. A chelating agent is a substance whose molecules can form several bonds to a single metal ion. In other words, a chelating agent is a polydentate ligand. EDTA is a versatile chelating agent. It can form four or six bonds with a metal ion, and it forms chelates with both transition-metal ions and main-group ions.
The polydentate ligandEDTA can take the place ofsixmonodentate ligands as ithas six lonepairs available. Metal-EDTA
The removal of the free metal ion from solution as a chelate, means that it is not free to catalyse oxidation reactions. EDTA forms chelates with many metal ions and is widely used as a food additive.