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.
Hydrogenation is the process of breaking those double bonds in advance and inserting hydrogen into the open spaces. This makes an oil less likely to go rancid because you've pre-oxidized it, as it were. You've turned an unsaturated fat (one with at least 1 double bond) into a saturated fat (one with no double bonds).
Oils can be saturated without the help of processing - animal fats, shea butter, and illipe butter are all naturally occurring saturated fats.
Saturated fats lie in a straight line (more about this below), so they pack together more easily. When triglyceride molecules are packed together well, they become a solid oil with a higher melting point. And they have longer shelf lives because the double bonds have been eliminated, so they are more resistant to rancidity. You can see why this has some appeal, eh?
FATTY ACID SHAPES
FATTY ACID SHAPES
Double bonds in a fatty acid shape can make it go from the lovely straight line we see in the saturated fatty acids like stearic and palmitic acid, to the kinky line you see in the oleic acid (C18:1 - 1 double bond) and erucic acid (C22:1 - 1 double bond, found in rapeseed and mustard seed). When we add another double bond, it can make the fatty acid contort in all kinds of ways, as we see in the linoleic acid (C18:2) or arachidonic (C20:4 - 4 double bonds, an omega-6 fatty acid).
So what does this mean for us? A double bond makes the fatty acid kinky, which means it can't pack in as tightly as those without double bonds. So we get a liquid oil. If you look at the oils with more double bonds - grapeseed, sunflower, or safflower oils - you'll notice they are considered "light" oils. Whereas oils like olive oil, avocado oil, and rice bran oil have fewer double bonds and are heavier.
So the heavier oils have fewer double bonds, which means they go rancid less quickly than the lighter oils. (I'm not talking about fractionated oils here...more on this in another post.) The butters will go rancid in the distant future because they don't have any double bonds at all!
Okay, I want to acknowledge there are other ways for fatty acids to go rancid other than the whole breaking of the double bonds thing, but that's Thursday's post!
Join me tomorrow for cis and trans fats (eek!)