Antioxidant capacity of individual and combined virgin olive oil minor compounds evaluated at mild temperature (25 and 40 °C) as compared to accelerated and antiradical assays

The individual and combined antioxidant and antiradical capacity of the main minor compounds of virgin olive oil (α-tocopherol, hydroxytyrosol, tyrosol and oleuropein aglycone) spiked in Purified Olive Oil (POO) as the lipid matrix model is described. The antioxidant activity was assessed under mild temperature conditions (25 and 40 °C) to mimic the autoxidation process during real storage conditions. These results were compared with accelerated (Rancimat Induction Period) and antiradical (DPPH) tests. The higher concentration of o-diphenols (hydroxytyrosol or oleuropein aglycone) in olive oil led to a lower oxidation rate under the conditions studied, resulting in a strong antioxidant effect. Remarkably α-tocopherol acted as a pro-oxidant at 25 and 40 °C, in particular during the first oxidation stage. In contrast, this compound behaved as an antioxidant under Rancimat and DPPH conditions. The oxidation rate constant as a function of the concentration of spiked compound fit an exponential decay model very well and therefore the progress of the oxidation reaction could be predicted. No synergistic or antagonistic effects were generally observed when combined antioxidant compounds were assayed.