Shining light on olive trees: Hydroxytyrosol screening in leaves by fluorescence spectroscopy

An innovative, non-destructive, and rapid method was introduced to quantify hydroxytyrosol content in olive leaves by means of advanced photonic technologies and chemometrics. At the heart of this approach is a custom- made, pocket-sized fluorometer with LED excitation at 375 nm and a miniaturized spectrometer for emission detection in the visible range, designed to measure intact olive leaves with ease and precision. Four Italian olive cultivars, namely Frantoio, Leccino, Leccio del Corno, and Moraiolo, were studied, with nine leaf samplings conducted over the 2022–2024 period. Simultaneously, hydroxytyrosol concentration was measured using HPLC-DAD-MS on the same samples to establish the reference data. Chemometric analysis was applied to the spectroscopic and HPLC-DAD-MS results. The Orthogonal Partial Least Squares (OPLS) regression method was successfully used to build a predictive model to quantify hydroxytyrosol concentrations ranging from 200 to 7000 mg/kg. The model achieved very good regression coefficients of 0.9 for calibration and 0.83 for cross- validation, with root mean square errors of 600 mg/kg and 720 mg/kg, respectively. This non-destructive method, combined with the portability of the pocket-sized fluorometer, is an innovation for high-throughput screening of olive leaves. It offers significant potential for evaluating olive leaves before their use in tea production or as raw material for dietary supplements, making it highly appealing for both the agriculture and health industries