Friday, October 16, 2009

Mechanisms of rancidity

There are different ways by which our oils can become rancid, most of them involving oxygen. I know we need it to live, but it's such a nuisance!

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!


Esmée said...

Hi Susan!
It's like baby's! When you know how much can go wrong you still find it a miracle that there are so many 'normal' children!
Thanks again for this explanation!

FG said...
This comment has been removed by the author.
FG said...

Hello Susan!
Thank you for explaining the mechanisms of oxidation; it's super useful. I have a few butters in my workshop and I'm not very familiar with how they smell when fresh (I bought them then never used them), and I'm having a hard time figuring out if they oxidized or not. There's illipe, kokum, mango and shea butters (all refined except the shea. There's cocoa too but I'm pretty sure that's still good; it smells delicious and I know that it stays good pretty long. Other than smelling, how can we tell if they've oxidized a lot? Are there other hints and clues that can help me figure out if they're still good to use?

P.S. My name is Farah but my Google account is only using my initials for a strange reason; I know you like comments to have names :-)

Theraisa K said...

Hi. I can't seem to find a straight answer online. Are you suppose to use anti-oxidants AND a preservative or is it just one or the other depending on whether your ingredient base contains water or not? Most of my products are waterless (no water, mostly just coconut oil, exfoliants and extracts). Thanks.

Susan Barclay-Nichols said...

Hi Theraisa! Have you looked at the newbie section? I know this answer is in there!

Carolina said...


Some of my soaps changed their color to an yellow-orange and you can barely smell the fragrance. This wasn't an immediate noticeable change. They looks gine for 2 weeks after the 3rd week they started to look different. I was wondering if this are signs of oxidation and what could have done wrong.