What does it mean when an oil oxidizes?
I've read in various sources that when cooking certain oils oxidize faster or slower. From what I understand it means that when they reach a certain temperature they become chemically unstable and unhealthy. But from what I read oxidizing means combining with oxygen which seems counterintuitive when cooking, because I would imagine if anything it should lose oxygen when in a very high temperature.
What does oxidizing exactly mean? What are antioxidiants then? And does the boiling point of a given oil = the point where it oxidizes?
From what I understand, it has to do with rancidity. Oxygen compounding results in a different chemical resultant.
This article describes what happens to lipids when they oxidize in your body, but the chemistry is the same for fatty acids outside your body. Lipid peroxidation - Wikipedia, the free encyclopedia There are reactive oxygen species (free radicals) everywhere, not just in your body so when your cooking oil gets up to a certain temperature, the fatty acids can react with free radicals. This is why things like ghee and coconut oil, which are fully saturated fats, are much more stable at high temps than things like olive oil and (gasp, don't ever use this) canola oil.
This is even more relevant: Rancidification - Wikipedia, the free encyclopedia
I guess there are other ways that fatty acids in cooking oil can go rancid, other than oxidation.
The worst of all is flaxseed oil (aka linseed oil), which is used straight as a furniture finish. It oxidizes so fast that if you leave used oil rags in a closed area the rate of the reaction will cause so much heat to form that it will start a fire.
Originally Posted by diene
Wow, I didn't know that. Yeah, flaxseed oil is mostly alpha-linolenic acid, which is polyunsaturated. Pretty unstable.
Originally Posted by eKatherine
Didn't know that it could spontaneously combust though. According to Wikipedia: "In 1991, One Meridian Plaza, a high rise in Philadelphia was severely damaged and three firefighters perished in a fire caused by linseed oil-soaked rags. In 2011, a garage in Sacramento also caught fire due to the spontaneous combustion of linseed oil-soaked rags."
I had an acquaintance who burned her porch down that way.
Originally Posted by diene
I always thought that the oils began breakdown at the smoke point when cooking. I know coconut oil has one of the highest smoke points of the oils, lard too, I think.
Originally Posted by DisappearingOne
In practice, it basically means "choose low-polyunsaturate fats". Solid fats (tallow, lard, ghee, coconut, palm kernel) are by nature more saturated and safe at most temperatures and techniques.
Monounsaturated liquid (olive, avocado, macadamia) are suitable for cold dressings or short sauté.
Polyunsaturate liquid (seeds and nuts) are often excluded due to their omega-6 content regardless of how they're used--it's true that baking or deep frying them makes them worse but there are plenty of reasons to skip them in the first place.
Ever taken a bottle of vegetable oil out of the cabinet and seen a tacky yellowed coating where dripped oil has gelled and/or hardened on the outside of the bottle? That's oxidation at work. It's something you don't want to happen inside your body.