Olive oil is not afraid of heat.
When oleic acid is subjected to high heat, more and more trans oleic acid will be produced as time goes on. That's right. However, trans fatty acids are not just trans oleic acid. As long as the oil is rich in unsaturated fatty acids, including soybean oil, sunflower oil, corn oil and so on. Theoretically, cis-trans isomerization can be carried out after heating to form trans fatty acids.
Moreover, trans fatty acids are not the most terrible thing produced by cooking waste oil. There are many products worse than it. For example, fatty acids containing multiple double bonds are particularly prone to form cyclization products, oxidative polymerization products and thermal decomposition products after heating. These products are more toxic than trans fatty acids.
As the saying goes, "flies don't bite seamless eggs." Double bonds on fatty acids, or unsaturated bonds, are like "cracks", which will bend molecules, easily let oxygen in and easily lead to polymerization. Generally speaking, the more double bonds in fatty acid molecules, the lower the oxidation stability and the worse the stability to heat. In the case of heating, the oxidation rate is faster.
Monounsaturated fatty acids are the main components in olive oil, that is, there is only one "seam" in the molecule. Soybean oil contains more than 50% linoleic acid with two double bonds and even linolenic acid with three double bonds, and its heat resistance is not as good as olive oil. The fatty acid composition of corn oil and soybean oil is quite similar, and the linoleic acid content of safflower oil and grape seed oil is higher, reaching more than 70%. Health oils such as flaxseed oil and perilla seed oil rich in α -linolenic acid, and walnut oil with more arachidonic acid, because of their large proportion of fatty acids containing 3-4 double bonds, are particularly delicate and easily oxidized and polymerized, and are completely unsuitable for heating. In contrast, peanut oil and rice bran oil are slightly "solid" because the proportion of saturated, monounsaturated and polyunsaturated fatty acids is relatively balanced.
Saturated fatty acids do not contain double bonds. Unless heated at high temperature for a long time, oxygen is "seamless and drillable" and has relatively good heat resistance. Therefore, the higher the saturated fatty acid content in oil, the more suitable for frying and frying. Instant noodle factories and fast food restaurants like to fry food with palm oil, precisely because palm oil contains nearly half of saturated fatty acids and the content of polyunsaturated fatty acids is very low. In the past hundreds of years, people have been frying food with lard, butter and butter rich in saturated fatty acids, which is also the reason. The food they fried is not easy to change color and tastes crisp.
Among the oils rich in unsaturated fatty acids, olive oil is still relatively "solid". It is very suitable for all kinds of ordinary cooking, and its stability is stronger than soybean oil. The virgin olive oil made of immature olives is not refined, not only rich in free fatty acids, but also yellow-green, containing chlorophyll, a photosensitive substance that promotes oxidation, so it is not suitable for high temperature and long-term heating. It is best used as a cold dish or a dish that only needs to be slightly heated. In addition, the special fragrance of olives will disappear with heating, just as sesame oil wastes its fragrance when cooking.