Skip to content

So which of my fats will go rancid first?

December 4, 2018

I’m asked so often about oil rancidity that I thought it best if I explain as best as I can here, with a blog post.

So the question that comes up is always a variation on the theme of ‘what is the shelf life of my vegetable oil/ oil blend?’

You would have maybe thought that to be one of the simplest questions for me to answer but no, like most things it’s actually quite complicated to understand the science.

Well, when I say complicated, I mean that there is a lot to consider.

So for those of you who are bored already I’ll give you the answer now to what is the most stable oil and work backwards from there:


Or, even better and easier to use.

Fractionated Coconut Oil (MCT Oil – Medium Chain Triglycerides) AKA liquid coconut oil.

The detail as to why these are the most stable is due to their structure.  Coconut, palm and it’s by-product MCT oil are saturated fats so there are no double bonds around to come under the influence of that naughty little shelf-life sucker-outer OXYGEN.

Want to know more?    Well brace yourself because here we go.

Most of the oily stuff we use in cosmetics these days is alive – based on non-fossilised carbon. For example, here are two beautiful plants that give us interesting oils to play with – Poppy and Passionfruit:

In the recentish old days we mostly made emulsions with mineral oils and mineral oil is dead carbon so being already dead it tended not to go off unless you contaminated it with water or something else.

Live carbon sources which include both animal and vegetable oils still have some fight left in them and they will interact with the environment and go off – turn rancid – end up stinking – however you wish to describe it.

We have to make sure we hold back that tide like Moses and the Red Sea 🙂


It turns out, rather unsurprisingly that these ‘live’ oils are not one-in-the-same.  They each have their own personalities and preferences.

We can group the oils in families of similar features to make it easier for us to make assumptions about them.  Scientists do this sort of thing all the time as part of their ‘top down’ approach to sorting.  It’s a good first step but where non-scientists and career scientists deviate is that the career scientist never, ever (and I mean never) gives up in looking to make either more and more groups or make the groups match in more and more ways (increasing level of similarity between items in the same group).

An analogy that came to mind just then was this.  When we look at a patch of sand from a distance we might see just yellow.

If we stand above it we start to see speckles of colour.

On our hands and knees we see more shades of colour pop out and we also see shapes of particles and can make out individual grains.

If we had a magnifying glass we could more clearly start to discern the shapes and sizes of the grains.

With a microscope we could determine whether the grains were porous or solid, sharp or smooth etc.

The pro scientist will bore you to death with the number of ways they can find to make smaller and smaller distinctions here just like I could with the veggie oils.

So basically the more closely we look, the more we start to understand the parts that go together to make the whole.

In cosmetic science it remains of key importance that we don’t focus on the parts to the detriment of the whole – i.e: that we don’t start becoming way too theoretical and stop making sense in the real world.

So back to living oils.

Here are some of the things that affect the chemical stability of living oils such as your veggie oils or animal fats.

  • Saturated vs not.

Saturation means that the carbon chains in the fats are all full with no double bonds.  This is the most stable formation for a fatty acid as there is no easy window or door for oxygen to get in.  Also, the electronic charges that exist around each element in the molecule (each carbon and hydrogen) are evenly settled so there is no massive electronic weak spot or focal point. In this drawing I’ve highlighted with green the hydrogens that I think might be vulnerable to oxygen attack. I might be wrong on this but in general it is the hydrogen on the carbon next to the one with the double bond.  It’s just for a general idea.

When fatty acids are unsaturated they contain double bonds, maybe one (mono-unsaturated), maybe more.  These double bonds do provide a window or door for oxygen to get in.  They do this by affecting the electrical force field around the molecule.  Double bonds tend to pull charge towards them which then leaves elements further down the chain with less electrical protection – think of it like your partner or pet hogging the bed clothes at night leaving your toes exposed to the monsters.  Anyway, those exposed parts become targets for the oxygen which is seeking a needy partner to hold onto. Before long you have your oxygen attached and oxidation starts.

While oils like coconut and palm are fully saturated, most oils contain a mixture of both saturated and unsaturated parts.  Looking at the ratio of saturated to non can help give a bit of an idea of how vulnerable an oil might be.

For example, if we look at a range of common oils we get these figures, the lower the number the more stable the oil as the higher the influence of the saturated part.

Camelia 1:4

Canola 1:16

Evening Primrose 1:11

Grape Seed 1:7

Hemp Oil 1:21

Olive:  1:5

Sunflower 1:7

Safflower 1:10

Ricebran: 1:4

Rosehip 1:14

  • The size and shape of the fatty acids as this helps determine how resilient it is to double bonds.

Think of your house and now think of how much more work it would be to secure your house if you doubled the number of windows and doors it had.   Now think of your house and then double it’s size without doubling the number or size of the doors and windows.  That’s kind of what’s going on here with the fatty acids.  Long chains will deal with the door and window created by a double bond or two better than small chains.  Small numbers of double bonds are easier to deal with than many BUT it could be that one double bond on a very small chain is more damaging than two double bonds on a very long chain.  The ability to influence the electronic force field is a little complicated.

I found a study that ranked the oxidative stability of some fatty acids in water and that ranked them like this:

Docosahexaenoic acid (from fish, omega 3) > Eicosapentaenoic acid (As before) > Arachidonic acid (similar to that found in cupachu butter) > alpha Linolenic acid (Flax, Perila, Soy, Walnut, Canola) > Gamma Linolenic (borage, Evening Primrose, Blackcurrant, Hemp) > Linoleic (Corn, Cotton, Grapeseed, Evening Primrose, Passionfruit, Poppy, Prickly Pear, Rosehip, Safflower, Sunflower, Watermelon).

Fatty acid structure is a big determinant in how vulnerable the oil will be to oxidation but that doesn’t mean that a vulnerable oil is a useless oil.  As you may see from the list (and that isn’t exhaustive), many of the oils commonly used in cosmetics contain Linoleic fatty acid, indeed these oils can be useful as cost-effective lubricants for the skin and pleasant, natural additives to hair, body and household care products.  However, we would be wise to take steps to manage the oxidative stability of the products we make using Linoleic rich oils so that we don’t end up with oxidative damage.


  • The presence of antioxidants including the unsaponifiable fraction. 

Antioxidants help to stop oxidation in its tracks so it is a good thing to have an antioxidant strategy whenever you are using ‘living’ ingredients.  Often the unsaponifiable fraction of an oil has some antioxidant protective features.

Plant antioxidants come in many forms with the best known being vitamin E (tocopherols).  Some methods of production are better for protecting these trace antioxidants than other. It is somewhat over-simplistic to try to hone in on a universal best method and apply that to all oils, rather it is best to research what is the optimal processing for the oil that is in question.  This may need to take into consideration yield values, price point, trace impurities, colour, odour etc.  Remember what I said about not losing sight of the end goal- the cosmetic.  The cosmetic chemist can add back antioxidants and use packaging methods to secure their choice of oil and price point rather than having to gain every solution inside the oil its self. Some plant antioxidants such as the chlorophyll and carotenoids are highly coloured chemicals and while that may be appropriate for some formulations, it may make for an unattractive proposition in others.   In addition chlorophyl rich oils, while naturally well protected from oxidation in some conditions, may turn pro-oxidative in others as the chlorophyl breaks down and catalyses other reactions.

So, something to ponder on is whether it is best, all things being equal, to buy a refined oil and add in a known quality of antioxidant or buy a virgin oil and manage the potential pro-oxidation caused by photo-catalysed break down via packaging?  I’m in no doubt that there are several right answers to that question.


  • The quality of the oil in general.

Oil quality starts at the farm and may not even be in the hands of the farmer (weather, soil quality that year, shade etc).  What happens to the crop, the seeds, during the squishing, the drumming, storage, transportation, further decanting and then manufacturing of your product can all impact the oil quality.  We can only control what we have within our power to influence so take a step back and work out what you can and can’t do then take formulation steps to address the gap.

There are some oils such as Olive, Canola, Grapeseed and Sunflower that have been widely studied, are grown all over the place and have quite tight quality specifications.  On the other hand there are some exotic and exciting oils like Raspberry and Apple Seed, Seabuckthorn and Papaya that are more boutique and are  less well understood.  Be sure to set your expectations in alignment of what the supply chain can provide.

Free Oleic Acid is something I’ve blogged about before as that is a skin penetration active, however high level of free oleic can also mean the oil is more unstable.  Levels of free oleic can be just a natural feature of the oil or a could be caused by poor growing or pressing conditions or could happen over time through bad storage. Check the number and the context.


  • Your manufacturing.

I grouped this into the step above but just to re-iterate that how you treat and manage the oil will also affect its shelf life. Do you take 2 years to get through a big drum thus leaving it for most of the time with a large head space full of oxygen?  Do you pack into open-mouthed jars?  Do you use permeable packaging?  Do you leave the oil heating for a long time?  Do you store in a garage that gets super hot in summer and freezing in winter?  All of these things can stress an oil and affect your shelf life.


  • Your formula.

I put this last because I didn’t want people thinking ‘well I’m not a chemist so I can’t do any of this’.   The chemistry of the oil can be found online generally and you can get an idea of how likely it is to oxidise there.  Remember that information is just ‘top level’ info and doesn’t mean your beloved oil will be useless just because it looks to be a fast-oxidiser.  It just means you need to take precautions.

So precautions could be blending your faster oxidising oil in with a base oil that is much more stable so as to reduce the overall oxidation potential of the product.  This can work well, especially when you really want to use a particular oil in a formula for marketing or access reasons.  Another option is to add in more antioxidants.  You can go too far with anti-oxidants and make a product too expensive, too irritating or even more likely to oxidise so my advise is this:  A blend of antioxidant strategies is better than lots of one,  do some stability testing to ascertain the size of the problem and the effectiveness of the solution (oil testing for rancidity is currently well under $100 per sample which is, in most cases cheaper than having failed batches).  Be sure to think about other ingredients that you are adding to your formula and make those as stable as you can. In some cases (emulsions etc) that would include adequate preservation, in other cases (where there are salts and /or sugars, extracts or clays)  it would include steps to avoid the dry ingredients attracting water into the mixture (dry isn’t always quite so dry).  Don’t forget to lean on your packaging for some support, store your ingredients well and manufacture with stability in mind as per above.

Useful oil-soluble antioxidants include:


Rosemary Antioxidants.


Coenzyme Q10

Some lycopene extracts (some of which can even be provided without the colour – Hydropom lycopene by Tri K. Lycopene helps make tomatoes red).

Specialised antioxidant blends such as Antiox GT from Hallstar or Phytocide Elderberry from Active micro technologies.

And if you are making a water and oil product.

Don’t forget that it will be at the interface between the oil and water where the maximum oxidation will take place or will at least start.  Take care to add some antioxidant to the water phase as well as keeping your oil phase protected.  Things like vitamin C,  Alpa Lipoic Acid and varying plant polyphenols from your extracts can help here but take care of adding trace metal ions as they catalyse oxidation.  Using a chelating agent is good where possible as this mops up the metal ions that can be brought in my plant-based herbs and extracts.  Chelating agents are things like EDTA, EDDS, Citric Acid,  Sodium Citrate, Sodium Phytate and the like.


OK so now we’ve saturated our minds with that can we answer the question?

Which of my oils will go rancid first?

On a purely fatty acid profile basis you have to start by considering Linoleic acid content (Omega 6).

Following that, look at saturated vs unsaturated ratio then antioxidant concentration / antioxidant stability, then free fatty acid then other things.

It might be that the oil that you can source the freshest is best for you.  It seems silly to use Rice bran because it is more stable on paper than Canola when you can get Canola fresh from up the road but the rice bran takes 6 months of shipping to arrive.

The bottom line is that the oil that ACTUALLY goes rancid first will be the oil that you fail to protect and preserve because you didn’t understand it or treat it right.  Hopefully now this epic article has put at least some of that power into your hands.

Useful References:

Auto-oxidation of fats – an oldie but a goodie for getting to grips with where oxygen attacks.

Linoleic acid – just some info that people considering eating the oil might stumble across. Don’t forget that ingestion and topical application are not the same. In fact I’d rather us use the oils that we can easily grow but can’t eat on our skin so we don’t compete and push up food prices.

Oil table – I wrote this a while ago and it took ages so I like to refer to it whenever I can.

Effect of fatty acids and tocopherol on oil stability.  


8 Comments leave one →
  1. sukimarmelaide permalink
    December 6, 2018 12:14 pm

    Dear Amanda, i’ve been thinking about this subject a lot lately..pretty much since your post on a particular formula that caused a reaction & your suspicion that it was actually the Vitamin E, in the library with a candlestick.
    Also, i’ve read that levels above 0.1% tocopherol(s) can actually induce a pro-rancidity reaction?
    All of which has made me reluctant to continue using E at all, especially as the sole defender of my oil fraction.
    So, i’ve instead, as of late, been using a complex of 2.5% Alpha-Lipoic Acid (the racemic form) & 0.25% Rosemary Oleoresin Extract, mixed into my oils &/or my emulsions at cool-down. The only flexible aspect being whether i’m working with the A-L A as just 1 of my anti-oxidants, or whether i’m utilizing it as an active also, in which case, i would not use less than 5% (based on 1 semi-shaky study that had a rather small sample where 5%A-L A & if i remember correctly, 2.5%synthetic caffiene were used in combination to achieve some possibly squishy results, of lessening the depth? & severity? of some facial furroughs, maybe neck too?)..for some reason, i’m under the impression that this study was one of, or The linchpin of Perricone’s business plan..
    Can you tell that i’m feeling quite insecure about all of this? Is it, by chance, obvious to you that i’m royally screwing the pooch here?
    If, in your enormous generosity, you feel inclined to pen a few words tilting me in the right direction, i would be forever grateful.
    Thank you so much for all you do.
    Muchlov, suki

    • RealizeBeautyEd permalink*
      December 7, 2018 7:04 am

      HI there,
      Thanks for the comment.
      With regards to the vitamin E thing I know I have written about vitamin E being a common thread in skin irritation but as for pro-oxidation (I know you didn’t write it that way) I doubt it.
      So the writing about anything above 0.1% of tocopherols being potentially pro oxidant has persisted but is most likely complete rubbish in my opinion. I say that because vitamin E is naturally found in vegetable oils to protect them and it is typically found at much higher quantities in seeds. I have a feeling that the pro oxidant comment originated from some research about metabolism in an airless environment rather than a demonstrated link in a cosmetic product. I could absolutely be wrong but this hunch I have seems to be born out by the experience I have of making and testing vitamin E containing products.
      Vitamin E (mixed tocopherols) consistently out performs all other types of simple antioxidant, it may not always be as good as proprietary blends but on a cost basis it is still very good. So, I’d not rush to throwing vitamin E out unless you are particularly sensitive to it. It is wise to create a blend of antioxidants to secure the shelf life of a product that is likely to undergo complex oxidation. There is more than one oxidation reaction to quench. So your strategy seems sound enough.
      I don’t know the Perricone paper you are referring to but I’d generally advise in taking any single scientific paper as a starting point and then seeking comment from as many sources as you can to build up a picture from all angles. Have other people replicated their findings/ expanded on them/ corroborated the results in any other way? Even then it can sometimes be that you have stumbled upon a new way to solve a problem that hasn’t been discussed in this context before. It’s always possible. Oh but what I would do is check to make sure a combination isn’t patent protected as sometimes that can happen.
      With regards to you feeling quite insecure my advise is don’t. The best advise I can give anyone in business and especially in the business of science is to challenge your feelings rather than holding onto them. So my question would be how do we get evidence for this? What tests can we carry out? What observations would support/ refute my hypothesis?
      I can’t really say more than that as I don’t know what you are trying to achieve but as long as the active you are working with is properly solubilised and is safe at the level you are using it I see no problem in continuing your experiment.

  2. sukimarmelaide permalink
    December 6, 2018 12:23 pm

    Please forgive postscript: how often, if at all, do silicone oils go off? Are these actually considered oils? Are they like mineral oil, kind of benign in their inertness?
    Thanks again! Have a lovely day!
    Sincerely, suki

    • RealizeBeautyEd permalink*
      December 7, 2018 7:06 am

      Silicones are made from dead oil (sand and mineral oil) so unless they have been made with double bonds or other vulnerable points in their structure they shouldn’t really go off. That said there are lots of blended silicones – silicone emulsions and gum resins etc – that could come under micro attack or separate over time. So it is more correct to say that dimethicone fluids shouldn’t go off.

  3. sukimarmelaide permalink
    December 7, 2018 9:11 am

    Awwww!!!! You are wonderful!!!
    Thank you so very much for your quick & astute & generous response!
    For my own edification, i’m going to ferret out all the little threads of data i referred to so i can verify my failing memory, at least for myself.. (the Perricone comment was a wisecrack though, (i think)..i thought it was an independent study & secretly always suspected it became the touchstone research that inspired the Dr. to lauch an A-L A-centric product line, but i’ve not an iota of proof of this..).
    Regardless, i know i have every study & article (& innumerable blog-posts of yours) book-marked on my poor, imposed-upon tablet that was nevernever built for retaining this many links (i can hear it creaking when i attempt to lever it onto my lap (it weighs at least 1000 metric tons now))..
    That is such an apt point RE: the mixed tocos in seeds & oils & thank you for the good advice on seeking to establish whether there might be a patent on any particular combo of anti-oxidants!
    “..challenge your feelings rather than holding onto them..”! & “..What observations would support /refute my hypothesis? ..”! Thisthisthis is why i’m obsessed with your blog!
    Thank you for always reminding me there is a greater objective to guide me through uncertainty; the search for actual, irrefutable truth, which is the best gift for everyone pursuing knowledge in this or any other science/art/industry! Because each piece of the truth we find & freely offer to others builds on the greater infrastructure of information that leads to the next groundbreaking discovery-all of which benefits everyone so much more than a bunch of greedy little secret bits of ego-hoarding!
    Thank you! Thank you! For your excellent, inquisitive mind & your impeccable integrity & open-source philosophy! i so appreciate these all too rare qualities,in these unfortunately empirically-retarded days in which we now live.
    Muchlove, suki

  4. RealizeBeautyEd permalink*
    December 7, 2018 10:37 am

    That is very sweet, I am glad that I could help make your world a little less chaotic 🙂


  1. So which of my fats will go rancid first? | Pretty Random Health and Beauty Blog
  2. So which of my fats will go rancid first? | Pretty Random Health and Beauty Blog

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.

%d bloggers like this: