I have done a few posts on oils already if you want to check them out...
This beautiful molecule to the left is a triglyceride (castor oil to be exact). It is a molecule with a glycerol (or glycerine) backbone and three fatty acids attached to it. If you look at this molecule - around the middle, before the OH bonds - you'll see a double line. This is a double bond, which means this is an unsaturated molecule.
In a saturated triglyceride, the carbons are single bonded, which are hard to break. They are stable over long periods of time because there isn't going to be oxidation. Most of these are buttery fats like coconut oil, babassu oil, palm oil, and animal oils. Oils like beeswax and candelilla wax are also saturated (yep, I don't think of beeswax as an oil either, but it fits the description!) Jojoba is another saturated triglyceride, which explains its long shelf life.
You can tell a single bond by the name "-ane". Squalane, for example, contains only single bonds, which means it is more resistant to rancidity. "-ene" means there are double bonds in the molecule. And "-yne" means triple bonds! These are going to have shorter shelf lives!
In an unsaturated triglyceride, these double bonds can be broken easily and oxidation occurs. The more double bonds, the more potential for oxidation. This explains the shelf life of something like grapeseed oil. It has 3 double bonds in the chain (it is a C18:3 triglyceride, meaning is has 18 carbon bonds and 3 double bonds), which means it has three places where the bonds can be broken and the oxidation can occur!
Most of the oils we use are "class 5: plant derived products, C18, unsaturated" meaning they contain 18 carbon atoms in those long chains. The unsaturated part means they have double bonds. Some of our oils have more than 18 carbon molecules - like meadowfoam (C20:1) and jojoba - but what's really important is the number of double or triple bonds when it comes to rancidity.
Sweet almond oil is a C18:1 triglyceride, meaning it has 18 carbon molecules and 1 double bond. Other C18:1 oils are olive oil, hazelnut oil, avocado oil, rice bran oil, and cocoa butter. So we know these oils are going to last longer than the C18:2 oils like soybean, sunflower, safflower, and wheat germ. And these oils will last longer than the C18:3 oils like grapeseed and borage.
Join me tomorrow for the chemistry of fatty acids!