Oxidation of food • Oxidation reactions can occur when food is exposed to oxygen in the air. • Foods containing fats or oils are at the greatest risk of oxidation.
Effects of oxidation on food When fats react with oxygen they are broken down, causing: • deterioration of flavour (rancidity) • loss of colour • loss of nutritional value • a health risk from toxic oxidation products.
Fat breaking down Fat Fat Oxygen
Fat Fat molecules Radicals attack near the double bond (NB ‘R’ represents the remainder of the fat molecule)
Antioxidants • Antioxidants are chemicals that are added to food to prevent the food from ‘going off’. • An antioxidant is a substance that slows down or prevents the oxidation of another chemical.
Oxidative damage • Oxidation reactions can produce free radicals. • A free radical is a highly reactive species containing an unpaired electron. • Free radicals can damage food by removal of an electron. • Antioxidant molecules ‘mop up’ free radicals to protect the foodstuff.
Radical now in a stable pair Damaging free radical Neutralised free radical Electron transferred Antioxidant converted to a stable free radical Antioxidant
How does an antioxidant cancel out a free radical? • The antioxidant molecule donates an electron to the potentially damaging free radical. • A stable electron pair is formed, stabilising the free radical. • The antioxidant itself becomes oxidised (loses an electron).
Antioxidants in action Oxidation occurs when the apple is left exposed to air The apple is protected when dipped in orange juice containing the antioxidant vitamin C
Vitamin C (ascorbic acid) • The antioxidant vitamin C can act as a reducing agent (electron donor), preventing oxidation (electron loss) from the foodstuff. C6H8O6 C6H6O6 + 2H+ + 2e- Ascorbic acid Dehydroascorbic acid