Are polyunsaturated fat cooking oils dangerous?
February 28, 2012 2:35 AM   Subscribe

Do cooking oils containing polyunsaturated fats peroxidize or go rancid before the smoke point? Common advice says that the smoke point is when cooking oils go bad, but sites like and other websites for saturated fat and against polyunsaturated fat imply that free radicals are formed from sitting on the shelf in a clear bottle, heating it to less than the smoke point, and the refining process(which heats the oil). What exactly is the truth here?
posted by vash to Food & Drink (10 answers total) 3 users marked this as a favorite
Response by poster: sorry it should be "sites like this and..."
posted by vash at 3:40 AM on February 28, 2012

Apparently it depends on the oil and the temperature. EVOO is not recommended for cooking because there are solids left over after processing (part of where the enhanced flavour comes from) and these solids burn at low temps.

Refined olive oil is a better step, as it has had the solids to a large degree filtered, first by the EVOO pressing and then by the refining process itself, thus refined olive oil is better for cooking-temp heat but will still burn, producing carcinogens.

Vegetable/seed oils are both heavily filtered/refined as well as capable of standing more heat, thus they produce less carcinogens than olive oil itself.

As far as free radicals being formed on a shelf in a clear bottle, I've never heard that. Like anything else, if it's in a plastic bottle, chances are the liquid inside is absorbing either the plastic or the coating over time, and sunlight/heat will certainly help that process. But I don't think that has as much to do with what's in the bottle as it does with the packaging and storage itself.
posted by nickrussell at 4:06 AM on February 28, 2012

The relevant process here is radical-mediated oxidation of the olefin groups in the oil. The diagram shows bromination of benzene, but oxidation of an unsaturated fat actually goes by pretty much the same mechanism. Light catalyzes the reaction by a somewhat complicated mechanism I don't understand well enough to explain (look up photocatalysis and molecular orbital theory if you have the background for it; I don't), and heat accelerates the reaction just because heat accelerates most kinetically-limited reactions.

This is a standard topic in any intro organic chemistry course, so you should be able to find plenty of discussion online.

I doubt cooking oils can peroxidize to any appreciable extent, just because peroxides tend to be explosive and I would expect more people to suffer small kitchen explosions if cooking oils could peroxidize. For comparison, ethers are well known to peroxidize in storage, and every now and then even trained chemists will accidentally distill to dryness and blow something up.
posted by d. z. wang at 5:02 AM on February 28, 2012 [2 favorites]

Sorry, so, in case I was unclear, yes, oils definitely go rancid by sitting in a warm, clear bottle. They probably don't peroxidize.
posted by d. z. wang at 5:02 AM on February 28, 2012

This may not count as data, but I have helped with the cooking often enough to know that many of my friends and neighbors and relatives have clear bottles of vile, rancid oil in the pantry. And sometime rancid butter in the fridge.

I do not know what peroxidized would smell like. Possibly I would happily drink peroxidized oil.
posted by Lesser Shrew at 9:26 AM on February 28, 2012

Yeah, the stuff does not last forever, no matter what the bottle.
As for cooking with low smoke point oils such as olive oil, I tend to avoid this, using them instead as a final dressing as the food comes off the heat. I use high smoke point oils for frying, pan frying etc. Nice side effect is the kitchen doesn't end up all smoky.
posted by chosemerveilleux at 9:33 AM on February 28, 2012

Response by poster: I thought lipid peroxidation was the process in which free radicals were formed. I hadn't heard of it in the context of explosives.

The answers have been helpful, but I haven't seen anyone address whether or not polyunsaturated fats(to a greater extent than saturated fats) produce free radicals before the smoke point and whether or not the refining process, during which oils are usually heated to over 400F cause the oil to create free radicals that are in them when you buy them at the store.
posted by vash at 12:51 PM on February 28, 2012

Best answer: You're right, cooking oils tend not to form explosive peroxides; I was giving a counterfactual there.

Yes, polyunsaturated fats produce more free radicals than unsaturated fats (because saturated fats don't really produce any). Since you know about lipid peroxidation, think about the mechanism here: because unsaturated fats have the olefin groups, they can form allyl radicals, and the delocalization of the allyl group stabilizes the radical. Saturated fats don't have the olefins, so they don't get this stabilization. I don't have the numbers, but I would expect saturated fats (essentially, straight-chain alkanes) either not to form radicals at all, or to form such short-lived radicals that the reaction doesn't proceed.

Radical production happens even under the smoke point. The reaction is favorable at room temperature, which is why sufficiently old oil will go rancid even if it's not heated. Additional heat accelerates the reaction, but is not necessary for the reaction to occur.

I don't know about the refining process. I imagine (not an industrial chemist here) you actually can heat oils to 400F (assuming smoke point is above that) without creating a lot of free radicals by putting a lot of antioxidants into the system, basically to break the propagation chains early. I don't know what's done in practice.

Does that help?
posted by d. z. wang at 6:56 AM on February 29, 2012

Response by poster: That helps a lot. Can you explain what you mean by "The reaction is favorable at room temperature"? To be honest your first paragraph is mostly over my head, but you basically answered my questions.
posted by vash at 4:29 PM on February 29, 2012

If you want to simplify chemistry to the point of incorrectness (but not unhelpfulness), everything is either thermodynamics or kinetics. Thermodynamics looks at the start state and the end state and says whether the reaction should happen; kinetics looks at the path from start to end and says how fast it happens.

For oil going rancid at room temperature, I say the reaction is favorable to mean that thermodynamics says the reaction should happen. Fortunately, kinetics says it's going to take a while. If you heat the oil, the kinetics become more favorable and the oil goes rancid faster.

If you want an example of oil going rancid beneath the smoke point, look at a drying oil such as linseed oil. You have to store it in an opaque bottle in a refrigerator with antioxidants (e.g., vitamin E) mixed in, and even then it goes bad within a few months.
posted by d. z. wang at 9:53 PM on March 4, 2012

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