Use of separative techniques to evaluate the influence of thermal oxidation in the phenolic composition and antioxidant activity of extra-virgin olive oils

Virgin olive oil (VOO) may be consumed raw in toast, salads, and other foodstuffs, but it is also consumed after domestic heating, such as frying, boiling, and microwave heating. VOO is considered to be excellent for applications involving high temperatures, as it fulfills all the fatty acid criteria of the stable healthful frying oils, i.e., being rich in monounsaturated fatty acids, low in saturated and polyunsaturated fatt acids, very low in linolenic acid and containing practically no trans fatty acids. During the thermal treatments, due to the high temperature and the absorpion of oxigen and water, triacylglycerols in the oil suffer a series of reactions, namely hydrolysis, oxidation, isomerization, and polymerization. Furthermore, the thermal treatments are influenced by a large number of variables among which the type of process, i.e., continuous or discontinuous, the surface-to-oil volume ratio, the food (in the case that there is some food in contact with the oil), the addition of fresh oil or not, the temperature and the oil selected, are of particular relevance. Even though many researches have studied polyphenols in raw and stored VOOs, their influence on the stability and organoleptic properties of the oil, and nutritional benefits, there are not many data that report the availability of these compounds after domestic heating of the oil. In theses cases, the phenolic compounds have been determined colorimetrically and also by using HPLC. The aim of this work was to study the changes in the phenolic compounds present in different extra-VOO during thermal oxidation at frying temperatures and during different periods of time by using two separative techniques (HPLC and CE). The concentration of several compounds belonging to four families of phenols (simple phenols, lignans, complex phenols and phenolic acids) was determined in the samples after the thermal treatment by both techniques, as well as the concentration of several “unknown” compounds which appeared after the treatment. The peroxide value trend during heating was evaluated and changes in the antioxidant capacity of the phenolic fraction of each virgin olive oil after different time intervals of treatment was indirectly measured by the OSI test.