Microwave-Assisted Extraction of Olive Leaf from Five Italian Cultivars: Effects of Harvest-Time and Extraction Conditions on Phenolic Compounds and In Vitro Antioxidant Properties

The aim of this study was to determine the total phenolic content (TPC), flavonoids (TFC), phenolic composition, and in vitro antioxidant activity of olive leaf extract of five cultivars (Caiazzana, Frantoio, Leccino, Ortice, and Coratina) from the Campania Region (Italy) during three harvest-times (March, June, and November). Different microwave powers, treatment-times, and three solvents (methanol, ethanol, and water) in different proportions were tested. Our results show that the TPC is not affected by the different MAE conditions but by the cultivar and the harvest-time. Instead, the TFC content also depends on the solvent used; with EtOH:H2O (70:30) and MetOH:H2O (80:20) giving higher yields than H2O alone. The Caiazzana and Coratina cultivars harvested in March showed the highest TPC compared with other cultivars, and they were also particularly rich in oleuropein. These cultivars showed the highest antioxidant activity, as indicated by the DPPH and hemolysis tests. In conclusion, cultivar and harvest time influence the polyphenol and flavonoid content of the olive leaf.