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 is a long chain (C18) fatty acid without any double bonds, so it's a long chain saturated fatty acid. 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!
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.
In the picture to the left, the three fatty acids attached to the glycerol backbone are different. One is a single bonded fatty acid, one has 1 set of double bonds, and the other has three sets of double bonds!
The fatty acids connected to the glycerol backbone determine what the kind of oil or butter. The fatty acids can have differing carbon chain lengths and different types of bonding. They can also have different configurations (trans fats - you've heard about those!) that determine if an oil is liquid or solid.
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 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.
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).
(Interesting site on olive oil chemistry here!)
Okay, so this is fascinating and all, but what does this mean for bath & body makers? Olive oil is a liquid oil that is unlikely to go rancid quickly, but it will go rancid eventually, as indicated by the double bonds. It also indicates it's a liquid oil, which I'll go into tomorrow!
This is a high oleic sunflower oil molecule. I love sunflower oils in my lotions and other creations, but it tends to go rancid far too quickly for my tastes!
Normal sunflower oil will have about 25% oleic acid (C18:1), 66% linoleic (C18:2), 2% stearic (C18), and 5.6% palmitic (C16). What this means is about 91% of the sunflower oil is composed of fatty acids with 1 or 2 double bonds, so it's going to go rancid quickly. A high oleic sunflower oil is composed of 80-92% oleic acid (C18:1) and 3 to 10% linoleic acid (C18:2), with some stearic and palmitic fatty acids thrown in (less palmitic - about 5.6%).
The high oleic sunflower oil will last longer than the regular sunflower oil because there are fewer double bonds to break. It will still go rancid more quickly than a saturated oil, but fewer polyunsaturated fatty acids and more monounsaturated fatty acids (ones with 1 double bond) means you're going to have a longer shelf life oil.
The down side to the high oleic sunflower oil is you are losing the linoleic fatty acid that can be beneficial to your skin, but I'll get into that in the sunflower oil post shortly!
So what does this all mean? By looking at a oils and butter property chart, you can figure out if an oil will go rancid quickly! And you can look for specific properties the fatty acids can offer in the profile to see which one you want to add for what properties. Soapmakers have long known how to choose oils to increase or decrease certain characteristics - you can figure out which ones you want to use in your bath & body creations by looking at the oil profile as well!
Join me tomorrow for fun with molecule configurations (it really is interesting, I promise you!)