Impact of heat treatment and granulometry of olive leaf powder (Moroccan picholine) on its phenolic compounds, antioxidant activity and physical–chemical properties

The olive tree (Olea europaea) holds great sociocultural significance in Mediterranean countries, particularly in Morocco, where the olive industry generates a large amount of by-products annually. Our goal is to convert this waste into therapeutic products that are rich in bioactive compounds. This study aimed to determine the impact of thermal activation (ranging from 25 to 100 °C) and the particle size of olive leaf powder (> 125 µm, between 50 and 125 µm, between 25 and 50 µm, or < 25 µm) on its phenolic profile and physical–chemical properties. The results revealed that exposing the powder to a thermal treatment of 80 °C and selecting particle sizes of < 25 µm resulted in the highest polyphenol content, antioxidant activity, and levels of pigments. Identification of phenolic compounds by chromatography (HPLC) showed the presence of hydroxytyrosol, catechin, caffeic acid, vanillin, naringin, oleuropein, quercetin and kaempferol at concentrations that varied according to the heat treatment and particle size. Furthermore, the analysis showed that oleuropein is the primary molecule found in olive leaves, with an average concentration ranging from 6 to 7.7%, depending on the temperature and particle size.