SOP: Ketone Rancidity Test

GIFSQ — Global Institute for Food Safety & Quality

Principle

Butter fat, coconut oil and palm kernel oil develop a distinctive odour and taste quite different from those of oxidative rancidity. The taste and odour are due to ketones rather than aldehydes and esters, and arise from the action of moulds, mainly Penicillium and Aspergillus, on the fat in the presence of moisture and nitrogenous materials. Detection is based on the development of a red colour when salicylaldehyde reacts with the ketones in the presence of H₂SO₄.

Apparatus required

• 200 ml glass-stoppered distilling flask fitted with a short condenser
• 50 ml glass-stoppered cylinder
• Centrifuge

Reagents

a. Pure salicylaldehyde.

b. Concentrated H₂SO₄.

c. Distilled water.

Procedure — Blank

Take 160 ml of water in the 200 ml glass-stoppered distilling flask fitted with a short condenser; distil 25–30 ml into a 50 ml glass-stoppered cylinder

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Add 0.4 ml of pure salicylaldehyde; shake vigorously, centrifuge and allow to settle

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Pour off the supernatant until about 4 ml of the residue remains; to the residue, add 2 ml of conc H₂SO₄ from the sides of the tube

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Shake the mixture vigorously and allow to stand; the salicylaldehyde which separates as the upper layer should show only a pale yellow colour, or at the most a faint pink colour

Procedure — Sample

Do not use apparatus having rubber or cork parts; use purified salicylaldehyde

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Add 5–10 g of the oil or melted fat to the water remaining in the distilling flask and distil; collect 25 ml of the distillate

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Test the distillate with salicylaldehyde and H₂SO₄ as in the blank; the aldehyde layer will be coloured pink to deep red in the presence of ketones

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If the colour is only faint, immerse in boiling water for 15 min, when the colour intensifies

References

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

Official and Tentative Methods, 3rd edn., American Oil Chemists’ Society, Vol. 1, Cd 12-57 (1975) — cited in the book.

Van Der Vet, A.P., in Quality Control in the Food Industry, (Ed.) Herschdoerfer, S.M., Academic Press, London and New York, Vol. 2, p. 355 (1968) — cited in the book.

Williams, K.S., Oils, Fats and Fatty Foods, 4th edn., Churchill Ltd., London, p. 55, 1966 — cited in the book.