Raw oil is the oil we usually buy from supermarkets or shopping malls, and cooked oil is the oil obtained by heating raw oil. Generally, there is little difference between raw oil and cooked oil in the family, but in the catering industry, many merchants use cooked oil in cooking, mainly by putting some seasonings in the heating process of raw oil, and then cooking with cooked oil will be more delicious. This has caused repeated heating and use of edible oil. But from a nutritional point of view, such edible oil is very unhealthy. In the process of repeated heating, especially in the process of frying food, edible oil will undergo a series of chemical and physical changes with water in food and oxygen in the air, such as hydrolysis, thermal oxidation, thermal polymerization, thermal cracking, isomerization and cyclization. , producing free fatty acids, unsaponifiable substances, cyclic compounds, dimers, trimers, polymers, hydrocarbons and cyclizations. At the same time, from the quality index, the acid value, peroxide value, iodine value, carbonyl value, saponification value, polar components and fatty acid composition of edible oil will also change accordingly. Intuitively, there are also obvious color and viscosity changes.