February 19, 2009

You Ask, I Answer: Polyunsaturated Oils

Have you ever heard of any danger in polyunsaturated vegetable oils from the way they are refined?

Mainly that they are already rancid [and bad for you] before they leave the factory?

What do you think?

-- Jade (last name withheld)

Istanbul, Turkey

Conventional refined oils undergo lengthy processes involving heat and chemical solvents.

In the case of polyunsaturated oils, this is mostly done to make them suitable for high-heat cooking (i.e.: deep frying) as they are easily degraded by heat in their unrefined state.

Several nutrition extremists online -- none of whom appear to have any nutrition credentials -- claim refined polyunsaturated oils are "cancer-causing."

Let's return to reality.

The issue with refined oils isn't so much that they are already rancid -- at some point in the processing, ascorbic acid (vitamin C) or tocopherols (vitamin E) are added to lengthen their shelf life -- but that they are nutritionally inferior to unrefined oils due to high amounts of phytonutrients and antioxidants being lost in the refining process.

This is why, when it comes to polyunsaturated fats, the recommendations are mainly to consume whole foods containing these heart-healthy oils (i.e.: nuts, seeds, fatty fish) as opposed to the oils themselves.

Unrefined polyunsaturated oils consumed at room temperature (i.e.: drizzling walnut oil over a salad) contain more antioxidants and phytonutrients than refined oils, but are still somewheat nutritionally inferior to whole foods.

Some Internet health extremists (the type who believe nutrition experts are part of a giant "conspiracy") go as far as saying that polyunsaturated fats are inherently unhealthy and unnecessary for human health. Quite a laughable claim, since Omega-3 and Omega-6 fatty acids (which we must get through diet since our our bodies can not produce them) are both polyunsaturated fats.

So while an unopened bottle of corn oil is not cancer-causing and rancid, there are better ways of adding polyunsaturated fats to your diet.


John Serrao said...


Doesn't this argument go a bit further into smoke points? For example, we know oxidation of the oil (hence rancidity) occurs at far lower temperatures for mono-unsaturated oils, like olive oil, due to the chemical construction of their molecules.

So wouldn't it make sense that polyunsaturated or even fully saturated oils, like coconut oil or butter, would hold up to heat the best - hence they would be best for frying?

Seems like the whole rancidity argument hangs on the temperature at which oxidation occurs. Whats your take?

Andy Bellatti said...


Unrefined polyunsaturated oils actually have lower smoke points than unrefined monounsaturated oils (unrefined corn oil, for example, has a lower smoke point than extra virgin olive oil.)

However, refined polyunsaturated oils have higher smoke points than their monounsaturated counterparts.

You are correct in identifying heat as one factor behind oxidation (along with light, time sitting in storage, and exposure to air.)

Keep in mind, too, that during the deodorization process (in which unrefined polyunsaturated oils are heated past their smoke point, citric acid is usually added in to prevent oxidation.)