Iodine Value

Definition: Iodine value (IV), also called iodine number, is a measure of the degree of unsaturation of a fat or oil — the number of carbon–carbon double bonds in its fatty acids. It is defined as the grams of iodine absorbed by 100 g of the oil under standard conditions, because iodine (or an iodine reagent) adds across each double bond. A high IV means a more unsaturated oil (rich in polyunsaturated or monounsaturated fatty acids, and more prone to oxidation and drying); a low IV means a more saturated oil (harder, more stable). Each oil has a characteristic IV range, so the value is used to identify oils, assess stability and detect adulteration.

Iodine value across oils (self-drawn) — coconut lowest, linseed highest

What the iodine value measures

The iodine value answers one question: how unsaturated is this oil? Fatty acids can be saturated (no carbon–carbon double bonds) or unsaturated (one double bond for monounsaturated, several for polyunsaturated). The double bonds are chemically reactive, and iodine adds across them — so if you expose a measured amount of oil to an iodine reagent, the amount of iodine that reacts is proportional to the number of double bonds present. Reported as grams of iodine per 100 g of oil, the result is therefore a direct, single-number index of unsaturation. It does not tell you which fatty acids are present, only the overall level of double bonds — but because different oils have characteristic unsaturation, that single number is remarkably informative about the oil's nature and behaviour.

What iodine value measures (self-drawn)

How it is measured

The classic method reacts the oil with an iodine reagent — historically the Wijs reagent (iodine monochloride) — added in excess, left to react with the double bonds, after which the unreacted iodine is back-titrated to find how much was consumed. Standardised procedures are published by bodies such as AOCS and ISO, and because the result depends on the exact method, an iodine value is only comparable when the same method is used. Modern labs can also calculate IV from the fatty-acid composition measured by gas chromatography, since the contribution of each fatty acid's double bonds is known. Either way the output is a number that can be checked against the expected range for the declared oil — see the quality-testing guide for how it sits among the other parameters.

What high and low iodine values mean

The single number carries a lot of meaning. A low iodine value (say, the range of coconut and palm kernel oil) signals a highly saturated oil: harder or semi-solid at room temperature, and stable against oxidation. A high iodine value (linseed, safflower, sunflower) signals a highly unsaturated oil: liquid, and more prone to oxidation and rancidity because those double bonds are where oxygen attacks. Very high IV oils are also "drying" oils that harden into a film in air, which is why they are used in paints and coatings. So IV predicts, at a glance, whether an oil will be solid or liquid, stable or perishable, and food-grade or industrial-drying in character — all from the level of unsaturation it reflects.

Indicative iodine value ranges by oil

Every oil sits in a characteristic band, and although exact figures vary by variety and source, the relative order is consistent. As indicative context (not a specification), the most saturated food oils — coconut and palm kernel — have the lowest iodine values; palm oil is moderate; the common liquid seed oils like rapeseed/canola, sunflower, soybean and safflower run higher; and the drying oils like linseed are the highest of all. High-oleic versions of sunflower and safflower have a lower IV than their standard forms, reflecting their shift toward monounsaturated fat. The point of the ranges is comparison and identity — a measured IV should fall within the known band for the declared oil.

Iodine value in identity and adulteration

Because each oil has its own characteristic band, iodine value is a classic tool for identity and authenticity. If a sample labelled as one oil returns an IV well outside that oil's expected range, it suggests the oil has been blended, substituted or mislabelled. A cheaper, more unsaturated oil blended into a premium one, for example, will pull the measured IV toward the cheaper oil's value. On its own IV cannot prove exactly what was added — it is one line of evidence — but combined with saponification value and the full fatty-acid profile it forms part of an oil's chemical fingerprint. This is why product standards for named oils specify an acceptable IV range, and why the test remains a staple of trade and regulatory checking.

Iodine value and oxidation stability

IV also predicts how well an oil keeps and how it behaves under heat. More double bonds mean more sites for oxygen to attack, so a high-IV, polyunsaturated oil oxidises faster — it has a shorter shelf life and is more prone to developing rancid, off flavours, which is why such oils are better kept cool and dark and used sooner (see storing edible oil). A low-IV, saturated oil is more resistant to oxidation and more stable for repeated high-heat use. This is a large part of why high-oleic crops were bred: lowering the IV (fewer polyunsaturated bonds, more monounsaturated) makes an oil more stable for frying without fully saturating it. So the iodine value is not just a lab curiosity but a practical predictor of stability, storage life and frying suitability.

Iodine value and drying oils

At the high end of the scale, iodine value defines a whole class of industrial oils. Oils with a very high IV — above the food-oil range — are "drying oils": their many double bonds let them react with oxygen in air and polymerise into a tough, solid film. This is exactly the property exploited in oil paints, varnishes, wood finishes and linoleum, where linseed (flax) oil and tung oil are traditional examples. Oils in the middle of the range are semi-drying, and low-IV oils are non-drying. So the same number that tells a food chemist an oil is perishable tells a coatings chemist it will cure into a film — two sides of the same underlying fact, that unsaturation makes an oil chemically reactive toward oxygen.

Iodine value and hydrogenation

Because iodine value measures double bonds directly, it is the natural way to track hydrogenation — the process that adds hydrogen across double bonds to harden an oil. As hydrogenation saturates the fatty acids, it removes double bonds, so the iodine value falls: a fully hydrogenated fat has a very low IV, while a partially hydrogenated one sits somewhere between the starting oil and full saturation. Processors therefore use IV to target and monitor the degree of hydrogenation, stopping the reaction when the oil reaches the intended value for the product. This connects the parameter to the wider world of fat modification: hydrogenation lowers IV by changing the fatty acids, whereas interesterification rearranges fatty acids without touching the double bonds and so leaves the iodine value essentially unchanged. So a simple before-and-after IV can tell you which kind of modification a fat has undergone — a drop points to hydrogenation, an unchanged value is consistent with interesterification or blending. That diagnostic use, alongside identity and stability, is part of why IV remains such a workhorse number.

Iodine value alongside other parameters

Iodine value belongs to the group of characterisation parameters that describe an oil's nature rather than its freshness. It pairs naturally with saponification value, which measures average fatty-acid chain length: together, IV (unsaturation) and SV (chain length) sketch the shape of an oil's fatty acids and are the two classic identity numbers. They are distinct from freshness tests like acid value and peroxide value, which change as an oil ages, whereas IV and SV are largely intrinsic to the oil type. Reading them together — a characteristic IV and SV, an acceptable acid and peroxide value — is how a lab confirms both what an oil is and how fresh it is. See the quality-testing guide for the full set.

Quick facts

Attribute	Value
Also called	Iodine number
Symbol / unit	IV · g iodine per 100 g oil
Measures	Degree of unsaturation (double bonds)
Method	Wijs (ISO/AOCS) or calculated from FA profile
High IV	Unsaturated, liquid, less stable, drying
Low IV	Saturated, harder, more stable
Used for	Oil identity, adulteration, stability, drying-oil class

⚠️ Indicative reference; actual values vary by oil, process and context. No fabricated numbers.

Quality testing guide AOCS / ISO test methods Codex oil standards Oil smoke points

FAQ

What is the iodine value of an oil?

It is a measure of the oil's unsaturation — the grams of iodine absorbed by 100 g of oil, which is proportional to the number of carbon–carbon double bonds in its fatty acids. A high value means a more unsaturated (and less stable) oil; a low value means a more saturated, stable one.

What does a high iodine value mean?

A high iodine value means the oil is highly unsaturated — rich in double bonds — so it tends to be liquid, oxidises and goes rancid faster, and at very high values behaves as a drying oil that hardens in air (as in paints). Low-iodine-value oils are more saturated and stable.

How is iodine value used to check oil quality?

Each oil has a characteristic iodine-value range, so a value outside that range suggests blending, substitution or mislabelling. Combined with saponification value and the fatty-acid profile, it helps confirm an oil's identity and detect adulteration; product standards specify acceptable ranges.

What is the difference between iodine value and saponification value?

Iodine value measures unsaturation (the number of double bonds); saponification value measures average fatty-acid chain length (molecular weight). Together they characterise the type of oil, while acid and peroxide values instead track freshness.

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