Oxygen in the air let the oil change quickly, especially as soybean oil, sunflower oil, a relatively high degree of unsaturated edible oil, more easily oxidized once exposed will have a “restless factor” – free radicals, it series of reactions will occur, so that further oil oxidation. Once this process occurs, it got out of hand, its index oxidation process is peroxide value. Initial segment of oil oxidation is quiet, colorless and tasteless. After intermediate oxidation and decomposition, will see a slight sniff “does not taste fresh.” Continued reaches a certain level, it becomes aldehydes, ketones, acids and other substances with a clear “rancid taste” of small molecules. This is called oil “rancid.” The higher the degree of rancidity, peroxide value level will be higher.