Lipid Oxidation In Foods

Lipid oxidation is a major cause of quality deterioration in foods, especially those containing fats and oils. It’s a complex process where unsaturated fatty acids react with oxygen, producing a range of compounds that can negatively affect the taste, smell, colour, and nutritional value of food.

🧪 Stages of Lipid Oxidation (Autoxidation)

1. Initiation:

   • Free radicals (highly reactive molecules with unpaired electrons) are formed when lipids are exposed to heat, light, or metals. We often denote these as R•.
   • Unsaturated fatty acids are especially prone to oxidation because of their double bonds.
   • When hydrogen is abstracted from an unsaturated lipid, the remaining electron can move along the chain in the form of a rearrangement which is possible with a conjugated diene.

2. Propagation:

   • The free radicals (R•) react with oxygen (O₂), forming peroxyl radicals (ROO•).
   • These radicals  (ROO•) pull hydrogen atoms from nearby lipids (RH), forming lipid hydroperoxides (ROOH) and generating more free radicals (RO•) and HO•, creating a chain reaction.
   • Lipid peroxides often react with ferrous iron complexes to form ferric iron complexes.

3. Termination:

   • Eventually, free radicals interact with each other or with antioxidants, forming non-reactive products and stopping the chain reaction. These are sometimes called non-radical products.
   • Typical reactions include:-
     • R• + R• ——> RR
     • R•  + ROO• —–> ROOR
     • ROO•  + ROO•  —->ROOR + O₂ 

Reactive Oxygen Species

A number of reactive oxygen species are generated:-

• triplet oxygen
• singlet oxygen
• superoxides
• hydrogen peroxides
• ozone
• hydroperoxyl radical
• hydroxyl radical

The other types of reactive radicals are peroxyl radicals, alkoxyl radicals, iron-oxygen complexes (ferryl and perferryl radicals). Some radicals are formed from sulphur including thiyl radicals and nitric oxide.

🌬️ Key Factors Influencing Lipid Oxidation

• Degree of Unsaturation: Polyunsaturated fats (like omega-3s) oxidize faster than monounsaturated or saturated fats.
• Oxygen Exposure: The more oxygen present, the faster oxidation occurs.
• Light and Heat: UV light and high temperatures accelerate free radical formation.
• Metal Ions: Transition metals (like iron and copper, magnesium and nickel) catalyze oxidation by breaking down lipid hydroperoxides. Iron is often involved in peroxidation. The types of iron involved include loosely bound and tightly-bound iron. Haem (heme) iron, iron-complexes and hematin are also contributors of iron. The iron chelating agents are phosphate, EDTA, citric acid and desferrioxamine.
• Water Activity: Intermediate moisture levels can increase oxidation, while very low or high water activity may slow it.
• Antioxidants: Natural (e.g., vitamin E, carotenoids) or synthetic antioxidants can delay oxidation by neutralizing free radicals.

😖 Effects on Food Quality

• Off-flavours and Rancidity: Breakdown products like aldehydes and ketones create undesirable odours and flavours.
• Nutrient Loss: Fat-soluble vitamins (A, D, E, K) can degrade during oxidation.
• Colour Changes: Oxidation can cause pigment degradation — for example, in meat, it leads to brown discoloration.
• Texture Alterations: In baked goods, oxidized fats can harden or soften textures abnormally.

🛡️ Prevention and Control

• Packaging: Use of vacuum-sealing, nitrogen flushing, or oxygen barrier materials.
• Storage: Keeping foods cool, dark, and dry reduces oxidation rates.
• Antioxidants: Adding tocopherols, ascorbic acid, or synthetic stabilizers like BHT (butylated hydroxytoluene).
• Processing Methods: Minimizing metal contact and reducing heat/light exposure during production.

The Addition of Antioxidants

By far and and a way the most popular method of stabilising foods from lipid oxidation is the addition of antioxidants (Nawar, 1985). Antioxidants can be added directly as in the case of bulk addition to oils or they can be added in the form of vesicles, micelles and emulsions.

References

Nawar, W.W. (1985) Lipids in: Food Chemistry (2nd edn) (Fennema, O.R., ed.), pp. 139-254, Marcel Dekker.