Combining remote sensing, SPAD readings, and laboratory analysis for monitoring olive groves and olive oil quality

The color of olive oil is influenced by chlorophyll pigments, which is one of the basic quality characteristics of olive oil. Furthermore, the concentration of chlorophyll pigments in olives and oil is related to the ripening stage of the fruit at harvest. In this study, the chlorophyll content (CC) of olive groves was estimated using vegetation indices. The field datasets include CC measurements made in the laboratory using a destructive method, as well as SPAD readings taken in the field from different olive groves. Thus, Sentinel-2 time-series data were collected during the years 2021 and 2022, processed and used in to evaluate three vegetation indices (VIs), namely the Transformed Chlorophyll in Reflectance Index and the Optimised Soil-Adjusted Vegetation Index (TCARI/OSAVI), the normalised difference vegetation index (NDVI) and the Green Chlorophyll Index (CIgreen). The results demonstrated that the TCARI/OSAVI index is an accurate predictor of the phenological stage of olive trees. The CIgreen, using bands at 10 m spatial resolution, is highly correlated with SPAD-meter readings. The CC in olive oil is positively correlated with SPAD-meter readings in olive fruit in early stages of maturity (R 2 = 0.93), but there are inversely proportional in the late stages (R 2 = 0.67). Olive oil quality during the ripening process showed a slight decrease of the specific extinction coefficient K232, insignificant change of K270 coefficient and free acidity. In general, a decrease of CC and increase of oil content were observed in the late stages of maturity of olive fruit.