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Empirical evidence that O-6 oils are commonly rancid?

Commented on January 06, 2014
Created January 04, 2014 at 8:44 PM

A very common paleo claim is that mainstream vegetable oils are inevitably rancid (or at least so commonly rancid that they're dangerous to eat). There's also much speculation about the oxidation of omega-3s.

Can anyone point me to empirical evidence for these concerns? I have not been able to find any data on actual rancidity assays of supermarket vegetable oils or point-of-sale fish oil. And yet, the test is so simple and common that I can't imagine it hasn't been done. I'll take a scientific paper, a trade journal, a blog post, anything... as long as it's based on actual testing and not just theory.

edit: To clarify, I'm talking specifically about oxidative rancidity, not about other potential health effects of various kinds of oil.

96440612cf0fcf366bf5ad8f776fca84

(19483)

on January 06, 2014
at 05:44 PM

+1 excellent points. I'd add, you could make some piping and heat some hydrogen peroixide and bubble the vapor from that through the oil to see if it will oxidize faster (or as you mentioned, use iodine or some other oxidizer.) As to point 2, depends on the oil, some seeds will readily release oils, others will not.

72cf727474b8bf815fdc505e58cadfea

on January 06, 2014
at 04:38 AM

It looks like testing for oxidation at home wouldn't be too tough -- there are a couple of options mentioned in this earlier question. Anyone with a couple of years of undergraduate chemistry could probably do it. Automatic pipettes and titration apparatus are pretty darn expensive, but maybe you could titrate by weight instead... Anyway, thank you for your thoughtful answer.

72cf727474b8bf815fdc505e58cadfea

on January 06, 2014
at 03:58 AM

This is a good start, but there are still so many questions. Like:

1) What compounds produce the characteristic smell of rancidity? Are they the same ones that harm your health? If so, then does "deodorizing" also remove them, and once that's done is there a reason to care?

2) It's possible to get low-temperature, expeller-pressed oils without chemical processing. Should we assume that if they don't smell bad, then they're not substantially oxidized?

3) How effective are added antioxidants? Assuming they're not dodgy ones like BHA/BHT, is there any reason not to add them?

72cf727474b8bf815fdc505e58cadfea

on January 05, 2014
at 03:58 AM

Thanks! That does look interesting. I like that they have both pre- and post-heating reactivity values.

Next I need to find out what those values actually mean, and whether either the pre- or the post- values are high enough to be of practical concern.

543a65b3004bf5a51974fbdd60d666bb

(4493)

on January 04, 2014
at 09:56 PM

is this any good to you; Lipid peroxidation in culinary oils subjected to thermal stress

(have not read it myself...yet)

found the link from ref#3 here, wiki-Auto-oxidation

32f5749fa6cf7adbeb0b0b031ba82b46

(41747)

on January 04, 2014
at 09:39 PM

Good question! The same goes for cooking with anything but coconut oil really. Folks would have you believe that 100% oxidation occurs instantaneously when heating oils. Nonsense really, but I'll let others do the Googling today.

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2 Answers

0
Medium avatar

(10601)

on January 06, 2014
at 11:54 AM

A few years ago I was looking for inexpensive replacements for paraffin used to make agricultural wet pack boxes. Waxes made by hydrogenating vegetable oils were marginally acceptable (more dense and brittle than paraffin), waxes made from animal fats less so (greasy and hard to solidify). We found this out in a number of field trials, and depending on oil prices paraffin/veg wax blends were used successfully in the field.

None of this involved oxidizing the oils, just hydrogenating to make them more saturated. But at one point I ran a test using soy oil to saturate box samples followed by an oven bake to oxidize the oil, akin to drying linseed oil in paint drying. Unfortunately the soy oil didn't dry to a hard waterproof state, leaving the samples as sticky as adhesive tape. Worse than that, the oven treatment gave off the strong smell of drying paint. I had only succeeded in making a PUFA go rancid in a very short time.

I don't know if this helps regarding your question. But if a PUFA has gone rancid I would expect it to have the sharp smell of drying paint when you open the bottle. I would also expect that the worst oxidative effects on the oils would occur in hot cooking (350F or higher), such as deep fat or stir frying.

0
96440612cf0fcf366bf5ad8f776fca84

(19483)

on January 05, 2014
at 02:51 PM

Any PUFA can be oxidized. So sticking to oils that are low in PUFAs is the key.

The question is how do you know when it has been oxidized, when it has been subjected to an industrial extraction process analogous to a diesel refinery. The oils are heated to high temperatures, deodorized, degummed, and clarified, some are extracted with hexane - even though they may be highly oxidized, we can't just smell them to be able to tell. Part of the processing involves adding antioxidants, such as BHA/BHT in the cheaper stuff.

http://wellnessmama.com/2193/why-you-should-never-eat-vegetable-oil-or-margarine/

http://www.ifc.org/wps/wcm/connect/2cd27f0048865863b5b2f76a6515bb18/vegoil_PPAH.pdf?MOD=AJPERES

This mentions how to test for rancidity, but doesn't appear to have much you could test at home:

http://www.oilsfats.org.nz/Oxidation%20101.pdf

Perhaps you could try a test at home: seal one cup of oil in a dark container, place it next to a one cup container that's open to the air with a cheesecloth over it (to prevent contaminants from the air from falling it), wait a few days, smell both. If the oil you're testing hasn't oxidized and you know it to be high PUFA, the question is what was done to prevent oxidation, or was the oil already oxidized and deodorized? i.e. you'd want to detect it going rancid after some time as this means the oil in the bottle hasn't oxidized.

Some of the funk of rancidity is from sulfur or iron, perhaps repeat the experiment and add a bit of iron in the open container.

72cf727474b8bf815fdc505e58cadfea

on January 06, 2014
at 03:58 AM

This is a good start, but there are still so many questions. Like:

1) What compounds produce the characteristic smell of rancidity? Are they the same ones that harm your health? If so, then does "deodorizing" also remove them, and once that's done is there a reason to care?

2) It's possible to get low-temperature, expeller-pressed oils without chemical processing. Should we assume that if they don't smell bad, then they're not substantially oxidized?

3) How effective are added antioxidants? Assuming they're not dodgy ones like BHA/BHT, is there any reason not to add them?

72cf727474b8bf815fdc505e58cadfea

on January 06, 2014
at 04:38 AM

It looks like testing for oxidation at home wouldn't be too tough -- there are a couple of options mentioned in this earlier question. Anyone with a couple of years of undergraduate chemistry could probably do it. Automatic pipettes and titration apparatus are pretty darn expensive, but maybe you could titrate by weight instead... Anyway, thank you for your thoughtful answer.

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