Does cooking with olive oil change the oil and turn it carcinogenic?
We asked oil expert Laurence Eyres, who is Business Development Director, Food and Nutrition at Auckland University, to clarify this one. He explained not all olive oils are equal:
"If you heat an imported olive oil, which will typically have high acidity, to the point where it smokes, you will cause some degradation of the oil, which is not ideal, but has not been shown to be dangerous to humans. On the other hand, if you use a local NZ extra virgin olive oil (which has low acidity), you'll find this much more stable to heat, it won't smoke, and you won't spoil the nutritional properties of the oil." So for normal cooking, like frying onions or stir-frying vegetables, extra virgin olive oil is a good and healthy choice.
However, with any unsaturated oil, including olive oil, heating it repeatedly to a high heat (say for deep frying) will cause the oil to develop compounds that have been found to have carcinogenic properties when tested on rats. So it's not a good idea to use any oil more than once, or to heat it to the point where it starts to smoke.
The bottom line: Mostly myth
Use a good-quality, local olive oil, use it only once, and don't heat it to the 'smoke point', and you will not destroy the oil's healthy properties. The same applies to all cooking oils.