Unsaponifiables

Boil an oil with a strong alkali and the triglycerides split into soap and glycerol. Whatever refuses to react is the unsaponifiable fraction, and the name has outlived the test that produced it. What sits in there: plant sterols, tocopherols, squalene and related hydrocarbons, pigments and traces of other small molecules.

It is a small share of the material by weight, and much of an oil's character lives in it. The tocopherols are the oil's built-in defence against oxidation. The sterols and the rest are usually what the interest in a specific botanical oil is really about, which is why two oils with near-identical fatty acid profiles are still discussed differently — and why some materials are sold as the concentrated fraction rather than as the oil it came from.

Processing decides how much survives. Refining for colour, smell or shelf life takes part of this fraction away along with what it was meant to remove. That is a trade rather than a scandal: a refined oil is paler, blander and steadier, and an unrefined one carries more of this and a shorter life.

The caution is the usual one. That a fraction contains interesting molecules is a statement about composition, not about what reaches skin. How much arrives from a minor fraction of an oil that is itself a minor part of a formula is not something an ingredient list lets anyone work out.