When an oil oxidizes, it reacts with oxygen and creates an assortment of harmful compounds. This generally can happen at room temperature and is one of the possible ways oils can go bad, but this process is enhanced when oils are heated.
An oil's susceptibility to oxidative damage typically depends on two main things:
As outlined above, Olive Oil is low in polyunsaturated fatty acids (about 11%) and high in antioxidants.
A handful of studies have exposed Olive Oil in high heat areas for long periods of time and measured how it affects the quality and nutritional properties of the oil. A high percentage of these studies used a high temperature for a very long period of time. But even under these extreme conditions, Olive Oil did pretty good.
One study actually deep fried a variety of different types of Olive Oil for 24 hours and noted that it was highly resistant to oxidation. Extra Virgin Olive Oil, which is higher in antioxidants, did great.
It has been found that Olive Oil does not oxidize much when used for cooking, while vegetable oils such as sunflower oil do oxidize and actually form harmful compounds. One study displayed that eating a meal with heated Olive Oil increased oxidative markers in the blood when compared to a meal with unheated Olive Oil.
Additionally, in this study, the Olive Oil was not Extra Virgin Olive Oil and it was cooked for 8 hours.
Furthermore, it is also a myth that heating Olive Oil leads to the formation of trans fats. In one study, frying Olive Oil 8 times in a row only increased the trans fat content from 0.045% to 0.082%, still a negligible amount.
Overall. it appears that Olive Oil is very stable, even under extreme conditions such as deep frying for extended periods of time.
Each fat molecule (triglyceride) consists of a glycerol molecule linked to three fatty acids.
All glycerol molecules are very identical, but there are hundreds of different fatty acids in nature and the health effects vary between them.
For the most part, fatty acids can be either:
Saturated fatty acids have no double bonds, monounsaturated have one (mono = one) and polyunsaturated fatty acids have a variety of double bonds (poly = many).
Here's the essential aspects. The double bonds are unstable when heated and they tend to react with oxygen.
Therefore, the more double bonds a fatty acid molecule has, the more unstable it will be when used for cooking. This is the reason saturated fats (zero double bonds) such as coconut oil are very resistant to heat.
Although most vegetable oils contain polyunsaturated fatty acids with a handful of double bonds, Olive Oilcontains mostly monounsaturated fatty acids with one double bond. As a result, having one double bond in the fatty acid molecule is not such a horrible thing. It's only the polyunsaturated fatty acids (such as those in soybean and canola oils) that can become harmful.
Oils are usually a mix of a variety of fatty acids. Olive Oil, for example, is 73% monounsaturated, 11% polyunsaturated and 14% saturated. In other words, the heat resistant monounsaturated and saturated fats make up 87% of olive oil.
Olive Oil contains mostly monounsaturated fatty acids, which are actually pretty resistant to heating. Damage-prone polyunsaturated fats make up only about 11% of Olive Oil.
Each fat molecule (triglyceride) consists of a glycerol molecule connected to 3 fatty acids.
All glycerol molecules are quite identical, but there are dozens and dozens of different fatty acids in both nature and the health effects vary among them.
For the most part, fatty acids can be either:
