Various luminescent methods applied to evaluate olive oil Total Antioxidant Capacity

Oxidation damages in foods and biological systems are directly related with free radicals attack. Recently, the methods used to determine activity of foods and biological antioxidants have evolved to complex assays to measure total antioxidant capacity (TAC). A rapid method for determining the potential TAC in foods could be a useful tool to make a selection among different species, varieties, maturation degree and culture conditions, in order to obtain high content of natural antioxidants, i.e. good quality of the food as supplement to organism antioxidant status. Being the activity of antioxidants in foods dependent on a multitude of factors there cannot be a short-cut approach to determining these activities. Several methods should be used to obtain chemical information that can be related directly to the antioxidant potential of the investigated sample. In particular, the antioxidant capacity of olive oil can be ascribed to both hydrophilic and lipophilic compounds, then a single test cannot be adequate to make an appropriate evaluation of both. Following this idea we started to apply and compare various luminescent based assays, able to work in aqueous or lipophilic environment, with the aim to collect enough information to assess which of them are actually useful to evaluate olive oil stability and quality, in terms of antioxidant capacity. Several samples of olive oil, produced from different cultivars, were directly analysed or separated, by extraction, in their hydrophilic and lipophilic fractions, then analysed independently. After evaluation of the spontaneous light emission of olive oil and of its fractions, the TAC of the hydrophilic fraction was determined by the luminol/H2O2/HRP inhibition assay. The content of the compounds considered responsible of the main part of olive oil antioxidant capacity, the hydrophilic polyphenols, was determined spectrophotometrically on the same fraction. On the lipophilic fraction, as well as on the whole olive oil, the content of hydroperoxides was evaluated by luminol emission in presence of cytocrome c, acting as heme catalyst of hydroperoxides degradation. The H2O2 scavenging ability of olive oil and of the lipophilic fraction was determined by measuring the peroxyoxalate chemiluminescence, using 9,10 diphenylantracene as fluorophore. Another parameter, evaluated in hydrophobic environment, was the light emission induced in the oil samples by the addition of the strong oxidant potassium superoxide. It was possible to observe a lack of correspondence between the polyphenols content and the respective TAC values. An interesting direct relationship was found between the intensity of the emitted light and the unsaturated fatty acids/polyphenols content ratio in the samples.