Rancidity is brought about by the action of air (oxidative rancidity) or by microorganisms (ketonic rancidity) in oil. In oxidative rancidity oxygen is taken up by the fat with the formation of peroxides. The degree of peroxide formation and the time taken for the development of rancidity differ among oils.
Peroxide Value is a measure of the peroxides contained in the oil. The peroxides present are determined by titration against thiosulphate in the presence of KI. Starch is used as indicator.
Solvent Mixture - Mix two volumes of glacial acetic acid with one volume of chloroform.
5% Potassium Iodide Solution.
1% Starch Solution.
N/500 Sodium Thiosulphate Solution. Prepare N/10 solution and dilute to N/500 on the day of use.
1. Weigh 1g of oil or fat into a clean dry boiling tube and add 1g of powdered potassium iodide and 20mL of solvent mixture.
2. Place the tube in boiling water so that the liquid boils within 30 seconds and allow to boil vigorously for not more than 30 seconds.
3. Transfer the contents quickly to a conical flask containing 20mL of 5% Potassium iodide solution.
4. Wash the tube twice with 25mL water each time and collect into the conical flask.
5. Titrate against N/500 Sodium thiosulphate solution until yellow color is almost disappeared.
6. Add 0.5 mL of Starch Solution, shake vigorously and titrate carefully till the blue color just disappears.
7. A blank should also be set at the same time.
Peroxide Value ( milliequivalent peroxide / kg Sample ) = [ S x N x 1000 ] / [ Weight of Sample (g) ]
S = mL of Na2S2O3 ( Test - Blank ) and N = Normality of Na2S2O3
1. Cox, H E and Pearson, D (1962) The Chemical Analysis of Foods Chemical Publishing Co Inc New York p 421.