Two Cooking Oils, One Sunny Shelf: Peroxide Values and Rancidity
Two 5-litre tins of cooking oil are opened in a school canteen in Chennai in June. One contains refined sunflower oil and the other coconut oil. Both are decanted into clear PET bottles and kept on a shelf beside a window for six weeks. At the end of this period the sunflower oil smells sharp and painty, while the coconut oil smells normal. The chemistry teacher tests both samples. The sunflower oil has a peroxide value of 18 mEq O2 per kg (a fresh sample gave 3), an iodine value of 132 and a saponification value of 191. The coconut oil has a peroxide value of 6, an iodine value of 9 and a saponification value of 255. The canteen manager wants to filter the sunflower oil through charcoal, blend it with fresh oil and continue using it, arguing that "frying kills the bacteria and the smell goes off". A student argues that the coconut oil must be the more unsaturated oil because it has the higher saponification value.
1. Use the iodine and peroxide values to decide which oil has undergone oxidative rancidity, and explain the chemical basis of the change.
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Which oil? The sunflower oil. Its peroxide value has risen from 3 to 18 mEq O2 per kg, a six-fold increase, and it has developed the sharp, painty odour typical of secondary oxidation products. Coconut oil rose only to 6 and remains odourless.
Why the iodine value matters here
- Iodine value measures the grams of iodine absorbed by 100 g of fat, i.e. the number of carbon–carbon double bonds.
- Sunflower oil (132) is highly unsaturated; coconut oil (9) is predominantly saturated (lauric and myristic acids).
Mechanism
- Oxygen, light and heat initiate free-radical attack at the allylic methylene groups next to double bonds.
- The resulting lipid hydroperoxides are the species measured as peroxide value.
- Hydroperoxides are unstable and decompose into aldehydes, ketones and short-chain acids, which give the painty smell and taste.
Conclusion: the sunflower oil has undergone oxidative rancidity, and its high unsaturation plus clear-bottle storage beside a window explains why it deteriorated faster than the coconut oil.
2. Evaluate the student's claim that the higher saponification value proves coconut oil is more unsaturated.
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The claim is false. Saponification value and iodine value measure different things.
- Saponification value is the mass of potassium hydroxide needed to saponify 1 g of fat. It depends on the number of ester bonds per gram, and therefore on the average molecular mass of the fatty acids, not on their unsaturation.
- Coconut oil contains short-chain fatty acids (mainly C12 lauric and C14 myristic), so there are more fatty acid residues per gram and more ester bonds to hydrolyse. Hence its high value of 255.
- Sunflower oil contains mainly C18 acids, so fewer moles of ester bonds per gram, hence a lower value of 191.
- The approximate mean molecular mass can be checked from SV = 3 × 56,100 / M. For coconut oil, M ≈ 660; for sunflower oil, M ≈ 880. These values are consistent with short-chain versus long-chain acids.
- Unsaturation is measured by iodine value, and here sunflower oil (132) is far more unsaturated than coconut oil (9).
So the student has used the wrong index. If anything, the saponification values support the opposite structural conclusion about chain length, while the iodine values settle the unsaturation question.
3. The manager proposes charcoal filtration, blending with fresh oil and continued frying. Give your judgement with reasons.
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Judgement: the proposal should be rejected.
- Charcoal filtration may adsorb some coloured compounds and volatile aldehydes, but it does not reverse the oxidation that has already occurred. The peroxide value is only an index; secondary products and polymers remain in the oil.
- Blending with fresh oil dilutes the rancid material but does not remove it. The mixture still contains oxidised products and will oxidise faster because fresh oil is being seeded with radicals.
- Frying adds heat and oxygen, the two conditions that accelerate lipid oxidation. It may drive off some volatile odour, but it generates more aldehydes and polymerised products. "Killing bacteria" is irrelevant to a chemical spoilage problem.
- Sensory and safety point: a painty smell indicates that the oil is already unacceptable for repeated high-temperature use.
Better practice: discard the affected oil; store oils in dark glass or opaque containers, closed, cool and away from light, with minimal headspace; use within the recommended period; and consider antioxidants such as tocopherols for long storage. The correct control measure is prevention, not salvage.