Determination of the main phenolic compounds of olive (Olea europaea L.) leaves by near infrared spectroscopy (NIR)

The main phenolics compounds in Olea europaea L. leaves are oleuropein, hydroxytyrosol, hydroxytyrosol hexoside, verbascoside and apingenin-7-glucoside, being oleuropein the most abundant. In the literature, there are different methods to determine some of these compounds, which imply an analytical preparation for their determination. In this line, the use of techniques based on infrared spectroscopy is presented as a promising alternative. Therefore, a near-infrared spectroscopy (NIR) calibration, validation and prediction to determine the main metabolites in olive leaves has been developed. To achieve this, olive leaves samples were analysed by HPLC-DAD to quantify the concentrations of the main phenolic metabolites. Subsequently, a predictive model was developed through NIR calibration, achieving a good R² value in all cases, being higher than 0.9 and close to 1 in all calibrations, as the same for R²_cv in all validations, with an excellent value for oleuropein (R²_cv = 93%). The RPD values were close to 4 for oleuropein and verbascoside and higher than 4 for hydroxytyrosol hexoside and apingenin-7-glucoside, indicating that the model has good reliability. So, good predictive models have been developed to determine the content of the main phenolic compounds in Olea europaea L. leaves, not only for the most abundant, but also for others with important bioactive properties.