Evaluating the reliability of specific and global methods to assess the phenolic content of virgin olive oil: Do they drive to equivalent results?

Abstract

Despite the huge number of different published methodologies, there is an open debate regarding which one is the most convenient analytical strategy for the determination of phenolic compounds from virgin olive oils. Diverse technical issues together with the disparity of criteria regarding results expression cause a lot of confusion. Herewith, a systematic comparison between specific (a powerful and LC-MS method) and global methodologies (the Folin-Ciocalteau (FC) colorimetric assay, the International Olive Council (IOC) method and hydrolysis plus HPLC-DAD) has been carried out. Thus, these strategies have been applied to the analysis of 50 extra virgin olive oils (covering all the possible quantitative ranges of these substances). This is the first time in which the individual LC-MS quantification of so many phenolic substances is included in this kind of comprehensive comparison. The outcomes of all the strategies have been thoroughly confronted and their equivalence (or divergence) has been carefully evaluated, establishing possible correspondence factors. The LC-MS individual determination with the pure standard of every analyte represented the ideal situation; when only the commercially available standards were used, a drastic change was observed in the absolute concentrations of oleuropein derivatives (in terms of hydroxytyrosol). Total phenolic content (summing individual levels) proved to be higher (1.9-3.0 times when data was expressed in mg/kg) than the values given by the three non-specific methods (with R² from 0.84 to 0.90). In any case, the IOC method, the FC assay and the hydrolysis approach could be considered as feasible strategies when a global value is pursued. Good correlations between their results were found (R² > 0.89), with the following equivalence factors: FC_{(mg caffeic acid/kg)} ≈ 0.60 IOC_{(mg TY/kg)}; IOC_{(mg TY/kg)} ≈ 1.27 Sum acid hydrolysis_{(mg TY+HTY/kg)}; FC_{(mg HTY/kg)} ≈ 1.04 Sum acid hydrolysis_{(mg TY+HTY/kg)}.

Keywords: Folin-Ciocalteu reagent; Hydrolysis; Liquid chromatography; Mass spectrometry; Phenolic compounds; Virgin olive oil.