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Fish oil, is susposed to suffer from rancidity(i.e. oxidation) when stored improperly , for example in hot temperature.

I'm interested in buying fish oil from abroad(usa) it will be shipped by air and won't be kept in a cold chain,i live in a mediternean nation(climate wise).

How should i choose my fish oil , such that when i recieve it ,it will be OK and not rancid ? is it even possible ?

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    $\begingroup$ Encapsulated fish oil in softgels inhibits oxidation so that storage on the shelf (even in warm climates) is possible. Some companies will ship it on ice if you request it (it may add $50 to the order). $\endgroup$ – LDC3 May 23 '15 at 13:56
  • $\begingroup$ The recent fish oil capsules that i've bought , locally ,are rancid(fishy smell even without opening the capsule) - and they use gelatin capsule even include topotocrels (which are anti-oxidants). How come? $\endgroup$ – r.bar May 23 '15 at 18:22
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    $\begingroup$ Unfortunately, the gelatin shells are not impervious. I know that if they are heated, the oil inside will leak out. It might be that some of the oil leaked out and oxidized, that is causing the smell (it doesn't take a lot). Also the amount of tocopherols and tocotrienols added is small (~50 mg in to a gram of oil). $\endgroup$ – LDC3 May 23 '15 at 19:20
  • $\begingroup$ Ah ok. reading more about it - seems that they don't work well below 15C or above 30C. drugs.com/cdi/cyclosporine-soft-gelatin-capsules.html $\endgroup$ – r.bar May 23 '15 at 21:43
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First off, a "fishy" smell and rancidity are two different things. A fishy smell is likely due to impurities, primarily small volatile amines such as trimethylamine. These impurities are unrelated to oxidative degradation of the fish oil.

Rancidity in fish oil is the result of oxidation. Oxidation leads to the formation of lipid hydroperoxides and short-chain aldehydes, which can have "off" tastes and give rise to unpleasant odors, but in a very different way than the classic fishy smell.

Fish oil's primary components are docosahexaenoic acid and eicosapentaenoic acid. They are more sensitive to oxidation than many other fats because they contain many double bonds.

The best bet is to look for oil that is freshly prepared and highly purified. Some fish oils are esterified with ethanol to form ethyl esters right "on the boat". These ethyl esters can be distilled to remove most impurities. Optionally, the purified esters can be converted back to triacylglycerols to give "molecularly distilled" fish oil in the triglyceride form.

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