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:
- Its concentration of polyunsaturated fatty acids, which usually tend to oxidize (react with oxygen).
- The presence of antioxidants, which counteract the oxidative damage (that's why they're called anti-oxidants).
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.