Kinetics of quality degradation during storage of monovarietal extra virgin olive oils

Introduction. Lipophilic derivatives of olive secoiridois (namely, the dialdehydic form of decarboxymethyl oleuropein aglycone, the isomer of oleuropein aglycone, the dialdehydic form of decarboxymethyl ligstroside aglycone, and the isomer of ligstroside aglycone) are present in extra virgin olive oil (EVOO) due to the hydrolysis of their glycosidic precursor catalysed by endogenous glycosidases during the crushing of olive fruit. These compounds as well as alpha-tocopherol have received considerable interest due to their potential health effects, and could be used as indices to evaluate EVOO quality in addition to those established by the EU Regulations (1). Here we report on the kinetics of autoxidation and hydrolytic processes that occur during storage of EVOO, and their effects on the antioxidant content. The aim of this study was to determine whether the potential properties of EVOO antioxidants are maintained or lost during the commercial life of the product. Materials and methods. EVOOs of the Picual, Arbequina, Taggiasca, and Colombaia cultivars were obtained from industrial oil mills. EVOOs were stored under dark, in closed-bottles (10% head-space), at 25-40ºC for a period of 8 months. For each cultivar six bottles were taken from the incubator at scheduled times (monthly) for the analysis of acidity, peroxide number, spectroscopic indices K232 and K270, alpha-tocopherol, phenolics, and antioxidant activity (2). Results. Four monovarietal EVOOs with different fatty acid compositions and antioxidant contents were chosen to define the extent of antioxidant degradation during storage. Secoiridoids, alpha-tocopherol, and the antioxidant activity decreased in all EVOOs following pseudo-first order kinetics. Oleuropein derivatives were less stable than the corresponding ligstroside derivatives and alpha-tocopherol. Hydroxytyrosol and tyrosol increased during storage as a result of secoiridoid hydrolysis. However, the increase of simple phenolics was lower than the decrease of their precursors, indicating that degradation occurred via both hydrolysis and oxidation. Free acidity, peroxide value, and the spectroscopic indices in the UV region increased following pseudo-zero order kinetics. The rate constants for quality indices degradation were obtained, and the time to reach the limits established by EU Regulation was calculated. In all EVOOs K232 was the first parameter that exceeded the legal limit. It was found that when EVOOs attained the this latter threshold the residual antioxidant activity was more than 60% with respect to the initial value. These data led to the conclusion that the beneficial properties of EVOOs due to antioxidant activity can be maintained throughout their commercial lives. References 1. V. Lavelli (2002) J Agric. Food Chem. 50, 26, 7704-7708. 2. V. Lavelli, G. Fregapane, M.D. Salvador (2006), J Agric. Food Chem. 54, 8, 3002-3007.