SOP: Oxidation Level of Fats by Thin-Layer Chromatography

GIFSQ — Global Institute for Food Safety & Quality

Principle

When an oil is used for frying, a progressive deterioration takes place — mainly the build-up of oxidised material — and the oil has to be rejected at some stage. Physical signs of deterioration are colour, increased viscosity, tendency to foam and lowering of the smoke point. The progressive oxidation can be followed by measuring the oxidised glycerides using thin-layer chromatography (TLC): unoxidised triglycerides have an Rf value of about 0.4, while the oxidised glycerides are held close to the base line.

Apparatus required

• TLC plates, 20 × 20 cm, coated with 0.25 mm layer of Silica Gel G
• Hot air oven, at 130°C (for activating plates) and 180°C (for developing)
• Scanning densitometer (optional, for quantitative determination)

Reagents

a. Chloroform.

b. Benzene and diethyl ether (developing solvent: 99 parts benzene to 1 part diethyl ether).

c. 50% v/v chromic acid: prepared by adding potassium chromate to conc H₂SO₄ until the colour becomes deep red, and then diluting with an equal volume of water.

Procedure

Prepare TLC plates of size 20 × 20 cm coated with 0.25 mm layer of Silica Gel G; air-dry the plates and activate by heating at 130°C for 30 min

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Prepare a 10% solution of the sample in chloroform; spot 10 µl

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Develop the plates using 99 parts of benzene and 1 part of diethyl ether to a length of 15 cm; allow the solvent to evaporate in the air

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Spray with 50% v/v chromic acid; heat the plates in a hot air oven at 180°C for 15 min

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Compare the spots visually; quantitative determination may be made using a scanning densitometer

Interpretation

Unoxidised triglycerides have an Rf value of about 0.4, while the oxidised glycerides are held close to the base line. The oxidised fat spot will also contain partial glycerides and certain non-saponifiable components, but the amount of these is small and affects the results only at low concentration levels.

Changes in oil during frying (oxidation level, %):

• Groundnut: 6 before use — unsuitable for further use at 30–35
• Palm: 9 before use — unsuitable at 34
• Soyabean: 5 before use — unsuitable at 36
• Sunflower: 5 before use — unsuitable at 32–38

References

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

Freeman, I.P., Chem. & Ind., p. 623, 1974 — cited in the book.