It is important for vegetable oils that the value of peroxide index remains within the limits specified not to cause any major changes in food quality. Indeed, when the oil is heating in the frying process, a complex series of reaction produces many degradation compounds. In the course of the reactions, the functional qualities of sensory and nutritional properties are modified. Than, when the food is deep in hot oil, it is exposed to water from the food itself, oxygen and the high temperature at which the process occurs, resulting in thermal changes such as isomerisation and fission reactions, producing aldehydes and ketones to form many degradation products such as epoxides and peroxides