Comparison of physico-chemical properties of different types of vegetable oils prior to and post deep frying

Deep frying is one of the most commonly used methods of food preparation. An essential factor in the frying process is the choice of a suitable oil – it plays a role in the heat transfer and as a product impregnation medium. Repeated use at high temperature leads to a number of reactions of oxidation, polymerization, and thermal destruction, which lead to changes in oils’ physical, chemical, nutritional, and sensory properties. Since the changes result from different processes, they cannot be evaluated by a single method. The processes differ in their extent for the different oil types. Therefore, the aim of the present study is to investigate the quality changes in three vegetable oils, namely refined sunflower oil, high oleic acid sunflower oil (HOSO), and rapeseed oil using different physical and physico-chemical methods. General chemical parameters, such as acid value and fatty acid composition were measured by standard methods, namely titrimetry and gas chromatography. The oxidation stability and oxidation degree along with the dynamics of the oxidation processes during frying were assessed by Rancimat method along with UV spectroscopy and electric impedance spectroscopy. The thermal behaviour of the oils and more precisely the amount of polar compounds formed during frying and their effect on the recrystallization phenomena are examined by differential scanning calorimetry.