Rapid determination of hydrophilic phenols in olive oil by vortex-assisted reversed-phase dispersive liquid-liquid microextraction and screen-printed carbon electrodes

Abstract

A novel approach is presented to determine hydrophilic phenols in olive oil samples, employing vortex-assisted reversed-phase dispersive liquid-liquid microextraction (RP-DLLME) for sample preparation and screen-printed carbon electrodes for voltammetric analysis. The oxidation of oleuropein, hydroxytyrosol, caffeic acid, ferulic acid and tyrosol was investigated, being caffeic acid and tyrosol selected for quantification. A matrix-matching calibration using sunflower oil as analyte-free sample diluted with hexane was employed to compensate matrix effects. Samples were analyzed under optimized RP-DLLME conditions, i.e., extractant phase, 1M HCl; extractant volume, 100µL; extraction time, 2min; centrifugation time, 10min; centrifugation speed, 4000rpm. The working range showed a good linearity between 0.075 and 2.5mgL (r = 0.998, N = 7) for caffeic acid, and between 0.075 and 3mgL (r = 0.999, N = 8) for tyrosol. The methodological limit of detection was empirically established at 0.022mgL for both analytes, which is significantly lower than average contents found in olive oil samples. The repeatability was evaluated at two different spiking levels (i.e., 0.5mgL and 2mgL) and coefficients of variation ranged from 8% to 11% (n = 5). The applicability of the proposed method was tested in olive oil samples of different quality (i.e., refined olive oil, virgin olive oil and extra virgin olive oil). Relative recoveries varied between 83% and 108% showing negligible matrix effects. Finally, fifteen samples were analyzed by the proposed method and a high correlation with the traditional Folin-Ciocalteu spectrophotometric method was obtained. Thereafter, the concentrations of the fifteen oil samples were employed as input variables in linear discriminant analysis in order to distinguish between olive oils of different quality.

Ancient olive trees as a source of olive oils rich in phenolic compounds

“…Olive oil is one of the most important components of the Mediterranean diet, due to its organoleptic characteristics, nutritional properties, and cultural influences (García-Vico, García-Rodríguez, Sanz, & Pérez, 2017;Fernández, Vidal, & Canals, 2018;Polari, Garcí-Aguirre, Olmo-García, Carrasco-Pancorbo, & Wanga, 2018). Compared to other vegetable oils, olive oils greatest richness is mainly due to differences in the production process (Reboredo-Rodríguez et al, 2017).…”

Laccase-Based Biosensor Encapsulated in a Galactomannan-Chitosan Composite for the Evaluation of Phenolic Compounds

“…As can be observed, lower concentrations were systematically found with the biosensor compared to those obtained with the colorimetric method. This outcome could be explained considering the following: firstly, the Folin-Ciocalteu method estimates the total polyphenol content whereas the biosensor detects the concentration of pyrocatechol and other monophenols; moreover, the Folin-Ciocalteu reagent is considered a non-specific reagent by many authors since it can be reduced by non-phenolic compounds [25]. Thus, the Folin-Ciocalteu method could also reflect the presence of other oxidizable species present in the sample extract.…”

Electrochemical Sensor-Based Devices for Assessing Bioactive Compounds in Olive Oils: A Brief Review

“…In fact, voltammetric E-tongues were previously successfully applied to assess, in a single assay, the bitterness index of EVOO as well as to quantify the contents of total phenols and phenolic compounds in EVOO, which are directly related to positive sensory attributes [91,92,94,95,97]. Furthermore, the E-tongue combined with an E-nose allowed assessing the levels of total polyphenols, flavonoids, phenolic acids, and other phenolic fractions [96], extracted from EVOO. Indeed, the voltammetric E-tongues quantitative accuracy was similar to the standard spectrophotometric and liquid chromatography techniques.…”

“…(iv) Classifying the quality commercial grade of table olives considering the intensity of the sensory defect predominantly perceived (DPP), to quantify the DPP intensity as well as to evaluate table olives' positive gustatory notes (acid, bitter and salty sensations) using a potentiometric E-tongue [88][89][90]. (v) Quantifying total polyphenolic, flavonoids, and phenolic acids contents in olive oils using voltammetry [91][92][93][94][95][96][97], potentiometry [98,99] or amperometry [100,101] as electroanalytical methods.…”