The double bonds of the fatty acid react chemically with oxygen. This turns the fatty acid molecules into other molecules that smell awful!
This can be a result of photo-oxidation or auto-oxidation.
The double bond interacts with a singlet oxygen (1O2), which is produced by the light. It is highly reactive with unsaturated lipids. The process is even quicker when you introduce sensitizers like chlorophyll and various other organic substances (like blood, bile, and riboflavin, but those shouldn't be issues for lotion makers - I hope!)
This is why we try to keep our oils away from strong light - bright light can produce more oxygen in the bottle, which can increase the process of photo-oxidation. This is a much faster process than auto-oxidation. Various carotenoids in our oils can slow this process - derivatives of lycopene, like lutein, violaxanthin, and neoxanthin, as well as beta-carotene - and many oils contain these ingredients. These are natural anti-oxidants found in our oils, and most of them contain at least a few to fight rancidity!
Even in the absence of air, we find oxygen. Oh oxygen, you are so necessary but so annoying! Through the breaking of the double bonds, the oxygens helps the the fatty acids break down into hydrocarbons (the H-C-H chains you see, which can be methane - 1 carbon, 4 hydrogens - or ethane - 2 carbons, 6 hydrogens), ketones, aldehydes, epoxides, and alcohols, some of which are smelly ingredients!
This process is a slow one when anti-oxidants are found in the oils. When the anti-oxidants are gone, it's a really fast process and takes very little time at all if the oil is heated as well. (Which gives you a good reason to get some Vitamin E into your oils when you get them from the supplier!)
Metal ions in the water at low levels can promote auto-oxidation. This is why we use chelating agents (also called sequestering agents) like citric acid and EDTA to bind the metals so they won't be a nuisance in our lotions. (And why we use distilled water that should not contain these metals!) The main culprit is iron, and the process can be speeded up by exposure to light.
Hydrolysis is a chemical reaction in which a molecule is cleaved into two parts by the addition of a water molecule ("hydro" is water, "lysis" is splitting"). The fatty acids are split away from the glycerol backbone, and the water is split into H (hydrogen) or OH (hydroxide) ions. When this happens, our lovely fatty acid molecules are morphed into a new molecule and we have rancidity.
Interestingly enough, this process - the hydrolysis, not the rancidity part - is saponification or soap making. Saponification is the hydrolysis of a fat and an aqueous base like sodium hydroxide or potassium hydroxide. Glycerol (or glycerine) is formed as the fatty acids are removed from the triglyceride form and converted into salts! So hydrolysis can be a benefit for soap makers and a bane to lotion makers!
Yes, our little beastie friends can cause rancidity in our lotions! (Which is why you must ALWAYS use a preservative!) Microorganisms use their little enzymes (usually lipases) to break down the chemical structure in the fat. Which, again, results in rancidity.
Wow, when you consider how many ways a lotion can go wrong - between rancidity and flocculation and separation and so on - it's a wonder we can make them at all. Fortunately, we have anti-oxidants ready and waiting to stave off rancidity as long as possible!
Join me tomorrow for fun with anti-oxidants!