Rancidity

Oils go off by reacting with oxygen. The reaction begins at a double bond, produces fragments that start it again elsewhere, and speeds up as it runs; light, warmth and traces of metal all help it along. The products are small, volatile molecules, which is why the first sign is a smell — flat, waxy, faintly like old nuts or crayons — rather than any change in how a product looks.

Two things follow for a formula. The more unsaturated an oil is, the sooner this happens, so an oil chosen for a high share of the most reactive acids is also the one with the shortest working life. And it is why oil-rich products carry antioxidants, chelating agents to bind stray metal ions, opaque or airless packaging and a shorter recommended period once opened.

For a user the signal is the smell, and it deserves to be acted on rather than masked. Oxidation products are reactive in their own right, so an oil that has turned is a reasonable thing to suspect when a product you got on with starts to sting or leaves skin looking irritated.

The habits that help are unglamorous. Buy sizes you will finish, keep oil-rich products off a sunny windowsill and out of the steam of a bathroom, close them properly, and treat a change in smell as the real expiry rather than the date printed on the box.