Non-hydrogenated peanut oil becomes hydrogenated when heated?

1. Jul 1, 2011

Flatliner

Can't find a reliable source on the internet for the answer to this question. Some claim that heating peanut oil at high enough temps to fry food causes the oil to become partially hydrogenated therefore causing it to become an "unhealthy" oil. Is this true?

2. Jul 1, 2011

Staff: Mentor

That's just a guess on my side, but I don't see how it can become hydrogenated when there is no hydrogen. However, with plenty of atmospheric oxygen around, oxidation of the double bonds (no matter what the exact products are) seems quite possible.

3. Jul 1, 2011

Flatliner

Is there hydrogen already naturally in peanut oil that separates from molecules at high temps and hydrogenates the oil?

That's unhealthy?

4. Jul 1, 2011

Staff: Mentor

No, there is a hydrogen, but it doesn't separate.

Yes, that's more or less how fats get rancid.

5. Jul 1, 2011

Moonbear

Staff Emeritus
The claim I've heard isn't that heating it to high temperatures does it, but the rapid temperature drop along with introduction of water and then reheating when you drop whatever frozen food you're frying into it. I'm also not sure if this is a legitimate claim or an internet myth, but perhaps with those additional details, Borek will have more insight if it's possible.

6. Jul 1, 2011

Pythagorean

The way Moonbear heard it seems more reasonable, since theoretically: $$E + 2H_2 O = 2H_2 + O_2$$

where E is energy.

But then there's at least two important questions to ask:
1) how much Energy is required; does a fryer produce it?
2) how much water is required to produce enough hydrogen to saturate the food (in a dilution of peanut oil?) to a significant level?

7. Jul 2, 2011

Staff: Mentor

Hi Moonie

No way, thermal decomposition of water requires insane temperatures - over 2000 deg C for a single digit percent decomposition.

On the other hand, water doesn't have to decompose first, such thing could be a several step process, yielding a similar final result. But I find it highly unlikely.

This is the simplest part. Assuming oil is a trigliceryde with three monounsaturated fatty acids, each 18 carbons long, its molar mass is 879 g/mol. There are three double bonds, so in theory it can react with up to three moles of water. In other words - you would need 56 g of water to fully saturate 879 g of oil. That's not exact, as oil is not just a single trigliceryde, but IMHO it is a quite good estimate

I can be wrong, but I don't like this explanation. Water can be added to the double bond, that's a well known reaction, but it doesn't yield alkane and oxygen - it yields secondary alcohol. Besides, water has much better place to attack - it can hydrolise the ester, yielding glycerin and fatty acid itself. The latter reaction is catalyzed by bases and acids, so it won't be very fast in the fryer, but seems much more likely to me.

8. Jul 2, 2011

Pythagorean

I am now hungry for beer battered halibut

9. Jul 2, 2011

SW VandeCarr

The partial or complete hydrogenation of unsaturated vegetable oils is an industrial process called "sparging". It involves treating the oil under high temperature and pressure in the presence of a catalyst such as powered nickle together with molecular hydrogen.

http://www.tutorvista.com/science/vegetable-oil-saturated

Last edited: Jul 2, 2011
10. Jul 2, 2011

Proton Soup

most store-bought peanut butter already has hydrogenated oil added so that it doesn't separate. natural peanut butter will separate with a layer of light oil on top if you leave it out, meaning you've either got to stir it with a butter knife regularly or leave it in the fridge.

so, for most of us, worrying about cooking temps is a bit like trying to shut the barn door after the horse has escaped.