When oil is heated to a certain temperature, it will be decomposed into glycerol and free fatty acid, and produce bluish smoke, which is the so-called smoke point. Then glycerin is broken down into acrolein, which is an unsaturated aldehyde, which irritates the eyes and throat, and also destroys the nutritional value of oil. Therefore, the smoking point of cooking oil can be said to be the highest temperature at which the oil can be used. As the oil is refined and purified, the smoking point will increase. When the grease is heated repeatedly, the free fatty acids in it will accumulate and increase, resulting in a decrease in the smoking point. In particular, intermittent frying has a much greater impact on oil quality than continuous frying. And this is why it should not be used more than twice as oil for frying.
The reference values for smoking points are often different, varies from different sources, to explain why, we have to review the manufacturing process of common vegetable oils. The general production process of vegetable oil can be divided into three categories. 1) Cold-pressed oil: harvesting→cleaning→baking dehydration→cold pressing (for example, cold-pressed olive oil, sesame oil, peanut oil, coconut oil). 2) Refined oil: harvesting→cleaning→flakes→chemical solvent extraction→distillation→degumming→refining (for example, refined coconut soybean oil, corn oil). 3) Hydrogenated oil: harvesting → cleaning → flakes → chemical solvent extraction → distillation → degumming → refined bleaching → deodorization → preservatives → hydrogenation (such as hydrogenated palm oil, milk marin, ghee).
Take olive oil and coconut oil as example, some may suggest extra virgin olive oil or extra virgin coconut oil is healthier because it’s least processed form, it retains antioxidants and vitamins.
Due to factors such as different extraction methods, different grades, and different mixing ratios, even the so-called "same" oil has different ingredients and different natural smoke points, such as extra virgin olive oil versus the common one. The main differences may due to different origins thus different raw materials. Using the physical cold pressing method and the heating cooking method, the oil composition obtained is different, and the smoke point is also different. Even if the oil made from the same raw material, different grades of raw materials have different production processes, such as the first extraction, heating and then pressing, and the third pressing, the composition is also different․ Different parts are extracted after oil extraction, such as upper oil, bottom oil, residue-separated oil and non-separated oil. ․ Many commercial products are sold as a mixture of oils of the same but different grades. For example, 70% cold-pressed virgin oil is mixed with 30% heavy-pressed oil to form "another" oil. ․ It is sold as a mixture of different oils, such as sunflower oil and olive oil.
Therefore, the smoking points are often different. The smoking point is highly concerned, therefore some manufacturers advertise the smoking point of their products is and how suitable for cooking.