Structure indices in FA chemistry. How relevant is the iodine value?

Most common analytical methods in FA chemistry can be categorized as yielding either structure or quality indices. Quality indices often describe components in fats and oils arising from processing, storage, and naturally occurring, nonfatty materials. Common structure indices are the iodine value (IV), the saponification value, and the hydroxyl value. Although modern analytical methods yield more detailed and reliable information, structure indices are still widely used. The IV, which indicates total unsaturation, has even been included in some standards for industrial products such as biodiesel. However, the IV index is too general to allow the correlation of physical and chemical properties with FA composition. The IV is treated in a theoretical fashion regarding biodiesel- and oxidative stability-related issues. That the concept of IV as a structure index is unsatisfactory is shown by the development of a quaternary composition diagram (QCD). The QCD demonstrates the derivation of a specific IV from varying FA compositions. Improved correlations are possible among the structure indices. Alternative indices for the IV are developed. Possible alternatives are the allylic position equivalent (APE) and the bis-allylic position equivalent (BAPE), which better relate structure and amount of common component FA in vegetable oils to observed properties. The APE and BAPE indices are based on the number of reactive positions in oxidation.