How does the iodine number measure unsaturation in fats and oils?

The iodine number, also known as the iodine value, measures the degree of unsaturation in fats, oils, and waxes. It is defined as the number of grams of iodine (or an equivalent halogen) absorbed by 100 grams of the substance.12

Unsaturated compounds contain molecules with double or triple bonds, which are highly reactive toward iodine. Saturated substances do not react with iodine and thus have an iodine value of zero.2

To perform the measurement, a known excess of iodine is reacted with a specific weight of the substance. The amount of iodine that remains unreacted is then determined through titration. A higher iodine value indicates a greater degree of unsaturation, which generally makes the substance more reactive, less stable, and more susceptible to oxidation and rancidification.2