Oxidation reactions happen when chemicals in the food are exposed to oxygen in the air. In natural conditions, animal and plant tissues contain their own antioxidants but in foods, these natural systems break down and oxidation is bound to follow.

Oxidation of food is a destructive process, causing loss of nutritional value and changes in chemical composition. Oxidation of fats and oils leads to rancidity and, in fruits such as apples, it can result in the formation of compounds which discolour the fruit.

Roll over the picture to see the effects of oxidation on cut apple.

Antioxidants are added to food to slow the rate of oxidation and, if used properly, they can extend the shelf life of the food in which they have been used.

photo of apple

Move the mouse over the picture to see how oxidation makes cut apples go brown

Oxidation of Fats

Fats and oils, or foods containing them, are the most likely to have problems with oxidation. Fats react with oxygen and even if a food has a very low fat content it may still need the addition of an antioxidant. They are commonly used in:

  • vegetable oil
  • snacks (extruded)
  • animal fat
  • meat, fish, poultry
  • margarine
  • dairy products
  • mayonnaise / salad dressing
  • baked products
  • potato products (instant mashed potato)

Fats are broken downwhen they react with oxygen. This makes the food containing the fats go off

As the fat decomposes and reacts with oxygen, chemicals called peroxides are produced. These change into the substances characteristic of the smell and soapy flavour of a rancid fat.

Antioxidants prevent the formation of peroxides and so slow the process of the food 'going off'. Some antioxidants react with oxygen itself and so prevent the formation of peroxides.

Air-tight packaging, using inert gases like nitrogen, vacuum packing and refrigeration can all be used to delay the oxidation process. However, these can still be inefficient and adding antioxidants can be an effective way of extending the shelf life of a product.

Find out more about why antioxidants are used in food