A capacitive technique to assess water content in extra virgin olive oils

The present research investigated the correlations between capacitance and water content of extra virgin olive oils (EVOO). A commercial capacitor probe for radio applications and an LCR meter were used for electric tests in the frequency range from 500 Hz to 512 kHz. Seventeen samples of different EVOO with a moisture content ranging from 178 to 1321 mg/kg oil were selected for study. To assess the influence of moisture only, the oil with the maximum water content was filtered down to 288 mg/kg oil and five samples with intermediate water contents were prepared and submitted to electrical measurements. Subsequently, the capacitance of all 17 EVOO samples was measured at selected frequencies. Water content and capacitance for filtered oil were linearly correlated, showing R2 values up to 0.959 and a root mean square error (RMSE) of 48 mg/kg oil at 2 kHz. When the main oil composition together with the moisture changed, the trend was always linear, but the correlation (R2) decreased to 0.818 with an RMSE value of 123 mg/kg oil (at 8 kHz), so that the system remains suitable for screening of water content. Strong correspondences between capacitance and impedance suggest that these measurements can be carried out with very simple instruments for measuring voltage and current. Some novel linear correlations also emerged between water content, capacitance and fatty acid composition.