SOP: Peroxide Value of Oils and Fats

GIFSQ — Global Institute for Food Safety & Quality

Principle

The peroxide value of an oil or fat is the amount of peroxides present, expressed as milli-equivalents of peroxide per 1000 g of sample. The sample is dissolved in a solvent, treated with potassium iodide, and the iodine liberated by the peroxides present in the rancid fat or oil is titrated with sodium thiosulphate solution. Often the number of millimoles of peroxide oxygen is reported, and the result is then half that of the peroxide value; in this case the term “Lea-value” is frequently used.

Apparatus required

• Conical flask with stopper

Reagents

a. Solvent: mix 2 volumes of glacial acetic acid and one volume of chloroform.

b. Saturated potassium iodide solution: dissolve 4 parts of pure potassium iodide in 3 parts of distilled water; keep the solution in a brown bottle.

c. 0.1 N sodium thiosulphate solution.

d. 0.5% starch indicator.

e. CO₂ (for displacing the air above the liquid).

f. Distilled water.

Procedure

Weigh an aliquot of the extracted fat of the sample into a conical flask

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Add 25 ml of the solvent and displace the air above the liquid with CO₂

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Add 1 ml of the potassium iodide solution; stopper the flask and allow it to stand for 1 min (with shaking)

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Add 35 ml of water and titrate the liberated iodine with 0.1 N sodium thiosulphate solution, using starch as indicator; shake vigorously at the end to remove the last traces of iodine from the chloroform layer

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Carry out a blank determination simultaneously (the thiosulphate consumption should be negligible)

Calculation

Peroxide value (milli-equivalents per 1000 g) = (Sample titre − Blank titre) × Normality of sodium thiosulphate solution × 1000 / Wt of fat taken

Note: the source’s printed calculation block is garbled in the available text; the expression is carried here as the source structures it (the “Lea-value” is half the peroxide value).

References

Ranganna, S., Handbook of Analysis and Quality Control for Fruit and Vegetable Products, 2nd edition.