Rapid assessment of fatty acids alkyl esters in extra virgin olive oils by Time Domain Reflectometry (TDR)

Fatty acids alkyl esters (FAAEs) in extra virgin olive oils (EVOOs) are recognized markers of the quality and genuineness of the product. As a consequence of sugar fermentation in damaged olives stored under bad conditions or pectin enzymatic demethylation in overripe olives, ethyl and methyl derivatives of fatty acids are produced. Concentrations of FAAEs out of the legal limit for EVOO (EU Reg. 61/2011) can be accompanied by sensory defects as fusty, winey and musty, linked to the low quality of processed olives. The legal limit of FAAEs in EVOOs is fixed to 75 mgkg-1 and up to 150 mgkg-1 if the ratio between ethyl and methyl esters is less or equal to 1.5. Since the illegal practice of "mild deodorization" is able to remove volatile molecules responsible for the sensory defects without producing clear chemical markers, FAAEs assessment is recognized, at date, as an effective method to make this fraud useless and assure the technological authenticity EVOO. Given the commercial importance of the FAAEs assessment, a rapid and environmental friendly method based on the multivariate analysis of Time Domain Reflectometry (TDR) signals was proposed for a fast screening of the total amount of FAAEs. TDR is one of the techniques used to measure the dielectric properties of different kind of liquids. In the present research, Principal Component Analysis (PCA) and PLS (Partial Least Squares) regression were respectively performed in order to identify the TDR regions that better describe the variances of EVOO samples and to estimate FAAEs content. Main results indicated that PLS models are able to predict FAAEs content in terms of sum of fatty acid methyl esters and fatty acids ethyl esters, with good values of the coefficient of determination R2 (higher than 0.920 in test set validation). The ratio between ethyl and methyl esters was also statistically correlated with the TDR signals, but the prediction was not so good, so when the content of FAAEs is between 75 and 150 mgkg-1, at the time, the official chromatographic determination is mandatory.