Stability of virgin olive oil and behaviour of its natural antioxidants under medium temperature accelerated storage conditions

The aim of the present research is to advance the study of virgin olive oil stability, in particular the behaviour of its natural antioxidants during long term storage. The changes undergone by tocopherols and complex and simple phenolic compounds and their antioxidant activity were studied under medium temperature (50 °C) accelerated oxidation conditions over a storage period of 8 months. Different oxygen availability (open and closed bottles, OB and CB) and four different monovarietal Tunisian virgin olive oils (VOO; Chemlali, Chétoui, El Hor and Oueslati) which vary in their fatty acid profile and content of natural antioxidants were employed. Oueslati VOO showed the lowest initial oxidation rate (4.6 PV/week) and as a consequence the highest induction period (IP; 16 days) as compared to the other VOOs studied (initial rate from 7.7 to 12.1 PV/week and IP from 9 to 12 days for El Hor and Chemlali, respectively, in both cases). In all cases the complete depletion of α-tocopherol corresponded with the observed induction period. Since the oxidation rate in the varieties studied is not sufficiently different to explain the great differences in the initial degradation rate observed in the phenolic compounds (mainly between Chétoui VOO, 163 μmol/kg/week, as compared to the other VOO oils, e.g. 6.1 and 2.5 μmol/kg/week, respectively, in Chemlali and El Hor VOOs), a concentration dependent degradation rate of complex phenols is suggested.