Oxidative stability of oils: iodine value, peroxide value, induction period, and how to estimate shelf life

Predicting rancidity instead of waiting for it

Every formulator has smelled it: the odour, which in our experience people describe as crayon-like or cardboard-like, that means a facial oil has gone off, usually noticed in a sample that was supposed to prove the product would last. The smell is the end of a chemical process that began the day the oil was pressed, and the frustrating thing about it is that it was foreseeable. The numbers on the supplier's certificate of analysis describe the oil's identity and its condition on the day of testing; a handful of laboratory tests show how far oxidation has gone; and product-specific stability evidence is what supports the durability decision that ends up on the jar. This article, a practical spoke of our series on plant oils by fatty-acid group, is about those three things. It explains what the standard measures actually measure, where the accelerated tests mislead, and how a UK brand turns the results into a defensible period-after-opening.

What oxidation is, in the two sentences you need

Lipid oxidation begins when a carbon next to a double bond loses a hydrogen atom, leaving a lipid radical; oxygen adds to form a peroxyl radical, which takes a hydrogen from another chain to become a hydroperoxide and start the next cycle. The hydroperoxides are the primary oxidation products. Their decomposition forms volatile aldehydes, ketones and short-chain acids, the secondary products, which include the compounds associated with rancid odour. Two consequences drive everything below. The more double bonds an oil has, and especially the more carbons that sit between two double bonds, the more starting points the first step has; and because the primary products are not the ones you smell, an oil can be some way into oxidation before the nose notices, which is why measurement beats the nose.

The numbers on the certificate: what each one measures and what it does not

The Codex Alimentarius standard for named vegetable oils (CXS 210-1999) defines both the reference ranges for authentic oils and the methods behind them. Four figures matter for stability.

Iodine value: how much there is to oxidise

The iodine value is an analytical measure related to the degree of unsaturation: how much iodine a hundred grams of oil will take up across its double bonds (Codex specifies the Wijs method, ISO 3961 or AOCS Cd 1d-1992). It primarily characterises unsaturation and is not a reliable measure of the oil's oxidation state. What it does tell you is the oil's susceptibility. Codex authentic-sample ranges run from 6.3–10.6 for coconut, through 78–90 for high-oleic sunflower, to 118–141 for conventional sunflower, 128–150 for grapeseed and 136–148 for safflower. Two cautions. First, the iodine value counts double bonds, not their arrangement, and the arrangement matters: one carbon flanked by two double bonds (a bis-allylic carbon) is far more reactive than two isolated bonds, so an oil rich in linoleic or linolenic acid is more fragile than its iodine value alone would suggest. Second, the iodine value is an identity check as much as a stability one: a "sunflower oil" at 85 is consistent with the Codex high-oleic range rather than the conventional one, and the specification deserves a second look.

Peroxide value: how far it has already gone

The peroxide value measures the primary products, expressed as milliequivalents of active oxygen per kilogram of oil (Codex: AOCS Cd 8b-90 or ISO 3960). In our incoming-material method it is the first number we look at on a delivery, read alongside the other measurements and the odour of the sample. Codex quality factors set maximums of 10 for refined oils and 15 for cold-pressed and virgin oils; olive oil, under its own standard (CXS 33), allows up to 20 for virgin grades, 5 for refined olive oil and 15 for the blend sold as "olive oil". A value near the maximum is not a fault under the standard, but it is a snapshot of an oil in which primary oxidation is already measurable. The peroxide value also has a well-known limitation: because hydroperoxides decompose into secondary products, it is a measure of primary oxidation at the time of testing, not a cumulative record. A low peroxide value in an oil that smells wrong is a reason to look at the secondary products, not a reason to relax.

Anisidine value: a later stage of oxidation

The anisidine value is a measure of the amount of aldehydes present in an oil, principally the alpha, beta-unsaturated aldehydes (ISO 6885). Those aldehydes are among the secondary products formed as hydroperoxides decompose, so the anisidine value looks at a later stage of oxidation than the peroxide value does. In our experience suppliers do not report it unless asked; for an oil that will carry a product's shelf life, we ask. Food-oil researchers routinely report peroxide value and anisidine value side by side when following an oil through storage, and reading the two together gives a fuller picture than either alone.

Acid value: a different failure

The acid value (or free acidity) measures free fatty acids released by hydrolysis of the triglycerides (ISO 660 or AOCS Cd 3d-63). It is not an oxidation measure, but in our incoming-quality method it sits on the same checklist as a general indicator of the oil's condition. Codex maximums are 0.6 mg KOH per gram for refined oils and 4.0 for cold-pressed and virgin oils.

The number that is not on the certificate

Codex Table 4 lists the natural tocopherol content of authentic oils, and it is worth knowing because tocopherols are the oil's own antioxidants. Sunflower oil carries 440–1520 mg/kg of total tocopherols, of which alpha-tocopherol is 403–935 mg/kg; soybean 600–3370 mg/kg, of which gamma-tocopherol is 89–2307 mg/kg; maize 330–3720; safflower 240–670; grapeseed 240–410; coconut from not detectable to 50. Two oils with a similar iodine value can therefore start life with very different natural antioxidant content, which is one of the inputs to an antioxidant plan, not the plan itself.

Accelerated tests: what the induction period means and where it lies

The standard laboratory shortcut is the accelerated oxidation test, best known through the Rancimat instrument and standardised as the oil stability index (AOCS Cd 12b-92) and as ISO 6886. In the AOCS method a stream of air is passed through the oil held in a thermostatic bath, typically at 98 °C, and the effluent air is bubbled through water whose conductivity is monitored as volatile acids are carried over. The oil stability index is the point of maximum change in the rate of oxidation, taken from the second derivative of the conductivity curve; in everyday use it is called the induction period. A longer induction period means a more stable oil under those conditions, and the test is well suited to what it was designed for: comparing oils, and comparing antioxidant treatments of the same oil, side by side.

It is much less good at telling you the shelf life of a product on a bathroom shelf. Kochhar and Henry (International Journal of Food Sciences and Nutrition, 2009) measured induction periods of eight culinary oils at 90, 100, 110 and 120 °C and extrapolated to storage temperature using Arrhenius plots and Q10 factors. For rice bran and hazelnut oil the predicted shelf life compared well with the producers' stated storage times. For the other oils, with the exception of macadamia, the predicted shelf lives were significantly lower than the storage times, and for walnut oil, the most oxidation-prone of the set, the prediction was 15–20 times lower than the best-before storage life. Whichever side of that gap the truth lies on, the lesson is the same: an extrapolated induction period is a comparison tool and a rough forecast, not a number to print. The same study reported that, for the high-monounsaturated oils, 100 divided by the induction period was linearly related to total unsaturation expressed as %C18:2 + 0.08 × %C18:1 + 2.08 × %C18:3, while the polyunsaturated oils gave an exponential relationship.

There is a second reason to be careful with high-temperature tests. Antioxidants behave differently hot. Hwang, Winkler-Moser and Liu (Journal of Food Science, 2019), working in soybean oil at 180 °C, refer to the pro-oxidant activity of alpha-tocopherol at high concentrations as known behaviour, and Chapman, Kim and Min (same journal, 2009) showed that alpha-tocopherol that has itself been oxidised acts as a pro-oxidant when added to oils, more strongly in soybean and corn oil at about 55 percent linoleic acid than in oils at about 12 percent. Both are food-oil studies under food-oil conditions. They are a reason for caution about reading high-temperature results across to a shelf: in our method, accelerated results are screening evidence, and the confirmation is done at the temperature the product will actually live at.

Antioxidants: evidence, not folklore

Three points from the food-oil literature are worth carrying into a cosmetic stability programme, with the caveat that none of them was measured in a cosmetic.

• Tocopherol is not a linear safety margin. The reported pro-oxidant behaviour of alpha-tocopherol at high concentrations, and of oxidised alpha-tocopherol, means that more is not automatically better. In our method the concentration comes out of the stability study, not out of a rule of thumb.
• Rosemary extract is a serious tool. Guo and colleagues (Foods, 2023) found that 100 mg/kg of rosemary extract in rapeseed oil gave a shelf life equivalent to 50 mg/kg of the synthetic antioxidant TBHQ, that the optimum addition at room temperature was 50–200 mg/kg, and that the extract slowed the loss of the oil's own alpha-tocopherol. Those are starting points for a study in your own oil phase, not specifications.
• The oil's own tocopherols are part of the picture. Sunflower oil arrives with 403–935 mg/kg of alpha-tocopherol on Codex ranges; grapeseed with 240–410 mg/kg of total tocopherols. We treat that difference as one input to the antioxidant plan for each oil phase, to be confirmed by testing, not as a dose calculator.

From measurement to the label: the UK rules

Under the Cosmetics Regulation as retained in Great Britain (Regulation 1223/2009, Article 19), a product with a minimum durability of 30 months or less carries its date of minimum durability. Where the minimum durability is more than 30 months, indicating that date is not mandatory; instead the label carries the period after opening for which the product is safe, shown by the symbol in Annex VII followed by the period in months and/or years, except where the concept of durability after opening is not relevant. Either way, the figure has to be supported. The government's guidance on making cosmetics available in Great Britain lists product stability among the information in Part A of the Cosmetic Product Safety Report, and requires the Responsible Person to keep a product information file, in English, for ten years after the last batch was made available. Oxidation data on the oil phase are one input to that stability evidence, alongside microbiology, packaging and foreseeable use; the durability decision itself sits with the Responsible Person and the safety assessor who writes Part B.

What that evidence should look like is not prescribed line by line, which is both a freedom and a responsibility. The framework that follows is the school's method, not a regulatory checklist; the design and acceptance criteria for a given product are yours to justify.

Our working method for an oil-based product

1. Start from the certificate. Record iodine value, peroxide value, acid value and, where reported, anisidine and tocopherol values for every oil in the phase, per batch. Place each oil in its fatty-acid group and identify the most fragile one; that oil sets the pace for the whole product.
2. Build the oil phase around the fragile oil, not around the label. If a linoleic or omega-3 oil is in the formula for a reason, keep its share to what the reason requires, surround it with oleic or saturated oils, and add the antioxidant with that oil in mind. If it is there for marketing, this is the moment to ask whether the shelf life is worth it.
3. Screen with an accelerated test. Use the induction period to compare antioxidant options and oil blends against each other under identical conditions. Do not extrapolate the winner's induction period to a shelf life.
4. Run real-time stability in the final pack. Store the finished product in the packaging the customer will buy, at room temperature and at a warm condition of your choosing, and follow peroxide value, odour and appearance over the intended life. Oxidation needs oxygen and is accelerated by light and warmth, so a clear dropper bottle and an airless pump are two different products and must be tested as such.
5. Bring the evidence to the durability decision and write down why. In our method the acceptance criteria, the raw results, the pack, the batch certificates and the reasoning are filed with the product information file so that the Responsible Person and the safety assessor can rely on them. If a supplier changes variety, origin or process, the fragile oil may have changed with it, and the study may need repeating.

Reading the whole certificate at once

None of this is difficult, and all of it is cheaper than a shelf of returned stock. The certificate tells you how fragile an oil is before you buy it, the tests tell you how far it has gone before you use it, and the stability evidence on the finished product in its pack is what supports the durability decision. Read the numbers in that order and rancidity stops being a surprise.

Sources

• Codex Alimentarius. Standard for Named Vegetable Oils, CXS 210-1999 (amended 2015): Table 1 fatty-acid composition, Table 2 iodine values, Table 4 tocopherols, Appendix quality factors and methods of analysis. FAO PDF
• Codex Alimentarius. Standard for Olive Oils and Olive Pomace Oils, CXS 33-1981 (revised 2015): peroxide value by grade. FAO PDF
• ISO 6885:2016. Animal and vegetable fats and oils. Determination of anisidine value. iso.org
• ISO 6886:2006. Animal and vegetable fats and oils. Determination of oxidative stability (accelerated oxidation test). iso.org
• AOCS Official Method Cd 12b-92. Oil Stability Index (OSI). AOCS
• Kochhar S. P., Henry C. J. Oxidative stability and shelf-life evaluation of selected culinary oils. International Journal of Food Sciences and Nutrition, 60 (Suppl 7), 289–296, 2009. PubMed 19634067
• Chapman T. M., Kim H. J., Min D. B. Prooxidant activity of oxidized alpha-tocopherol in vegetable oils. Journal of Food Science, 74(7), C536–C542, 2009. PubMed 19895457
• Hwang H. S., Winkler-Moser J. K., Liu S. X. Study on Antioxidant Activity of Amino Acids at Frying Temperatures and Their Interaction with Rosemary Extract, Green Tea Extract, and Ascorbic Acid. Journal of Food Science, 84(12), 3614–3623, 2019. PubMed 31769515
• Guo M. et al. Antioxidant Efficacy of Rosemary Extract in Improving the Oxidative Stability of Rapeseed Oil during Storage. Foods, 12(19), 3583, 2023. PubMed 37835236
• Regulation (EC) No 1223/2009 on cosmetic products, Article 19 (labelling), as retained in UK law. legislation.gov.uk
• Office for Product Safety and Standards. Making cosmetic products available to consumers in Great Britain. GOV.UK