Monitoring the phenolic compounds of Greek extra-virgin olive oils during storage.

Extra virgin olive oil (EVOO) samples, of five Greek olive varieties, were stored in dark glass bottles (headspace 0.5%) in a basement without central heating for 24 months. Quantitative variations of the phenolic compounds and their degradation products were monitored over time. The differences observed in the initial total phenolic compounds concentration (ranging between 250.77 and 925.75 mg/kg) were attributed to extraction system, olive variety, and maturity stage. Even after 24 months, the degree of reduction in total phenolic compounds did not exceed 31%. The reduction was more pronounced in dialdehydic forms of oleuropein and ligstroside aglycones (DAFOA and DAFLA), indicating a more active participation in the hydrolysis and oxidation processes of the more polar secoiridoids. The initial total phenolic content was the main factor correlated to the degradation rate of the phenolic compounds. The decrease in secoiridoid derivatives, gave rise to hydroxytyrosol and tyrosol content and to the formation of four oxidized products.