This beautiful molecule to the left is a triglyceride (castor oil, to be exact). It is a molecule with a glycerol or glycerin 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.
Yes, this is the glycerin we know and love as a humectant in our product.
In a saturated triglyceride, the carbon chains are single bonded, which are hard to break. They are stable over long periods of time because it's hard for an oxygen to insert itself into the chain in the place where the bond is broken.
When a molecule has double bonds, that double bond can be broken and the fatty acid now reacts chemically with oxygen to produce all kinds of molecules we don't want in our lotions that have horrible smells. This is called oxidation!
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!
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!
If you're a lotion maker, you're familiar with stearic acid as a thickener. Stearic acid is a C18 fatty acid, which means it contains a chain of 18 carbon atoms connected together without any double bonds, so it's called a long chain saturated fatty acid without any double bonds. If we put three of these fatty acids together with a glycerol molecule, we'd have a saturated glyceride, and one with a great shelf life!
oleic acid). C18:2 means it has 18 carbons in the chain and two double bonds (linoleic acid). A C18:3 chain has 18 carbons in the chain and three double bonds (gamma linoleic acid). A C18 chain is called stearic acid. A C18:1 chain is oleic acid. And C18:2 is linoleic acid.
But it's fairly uncommon for an oil to have three of the same fatty acids. They tend to have at least 2 different kinds, and sometimes three, as you'll see below with sunflower and olive oil.
For instance, it looks like this triglyceride is composed of a C16 chain, a C18:1 chain, and a C18:3 chain. I know C16 is palmitic acid. C18:1 is called oleic acid. And C18:3 is linolenic acid. This could be a corn, cottonseed, or palm oil molecule. The polyunsaturated chain (the C18:3 or gamma linolenic fatty acid has more than 1 double bond, which means it is unsaturated, and because there's more than 1, it's called polyunsaturated!) can go rancid quite easily!
What this means in terms of making lotions or other creations is this molecule has THREE double bonds on that last fatty acid, so it may go rancid more quickly than something like olive oil below. (I honestly can't think of something off the top of my head with triple bonds, so it's not really an issue. But we do see three double bonds in gamma linoleic acid!)
Olive oil has between 55 and 85% oleic acid, 4.6% linoleic acid, 6.9% palmitic acid, and 2.3% stearic acid. In this sample molecule, we see a triglyceride with an oleic fatty acid (C18:1 - 1 double bond), linoleic acid (C18:2 - 2 double bonds), and palmitic acid (C16 - no double bonds). If oleic acid makes up the bulk of the fatty acids with its 1 double bond, we are going to see an oil that is less likely to go rancid than one that is filled with linoleic acid (2 double bonds).
Most of the oils we use are "class 5: plant derived products, C18, unsaturated" meaning they contain 18 carbon atoms in those long chains. How do we interpret this sentence? This means that our oils are plant based and contain mostly (if not all) C18 fatty acids. (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.
grapeseed oil - 'cause that was the question that started all of this - consider the fatty acids. Grape seed oil is a light oil containing 7% palmitic acid (C16), 4% stearic acid (C18), 16% oleic acid (C18:1), and 72% linoleic acid (C18:2). It contains very low levels of Vitamin E (265 ppm). It has about 11% saturated fatty acids and 89% unsaturated fatty acids. One of the fatty acids - oleic acid - has one double bond that can be broken, but the majority of this oil contains linoleic acid, which has two double bonds that can be broken. We can predict that this will have a shorter shelf life than an oil that is mainly oleic acid (like olive oil). We can add anti-oxidants to the mix to increase the shelf life and retard rancidity.
How can something like soy bean oil contain a ton of those unsaturated fatty acids - about 53% linoleic acid and 29% oleic acid, for a total of 82% unsaturated fatty acids - and stay good for up to a year? It's thanks to the high levels of Vitamin E found in this oil, about 700 ppm. The Vitamin E is an anti-oxidant that will retard the rate of rancidity.
Have we answered the question - is it possible for grapeseed oil to have a one year shelf life? I think we have. No, it is not possible for grapeseed oil to have a one year shelf life when used normally.
Hydrogenation and fatty acid shapes
Cis and trans configurations
Mechanisms of rancidity
An in depth look at anti-oxidants