Effect of cultivar and drying methods on phenolic compounds and antioxidant capacity in olive (Olea europaea L.) leaves

Up to 5% of the total olive weight arriving at the mill is discarded as leaves. Interest in the possible uses of these residues is growing, because they constitute a potential cheap and abundant source of compounds with high total antioxidant capacity (TAC) associated with total phenolic content (TPC) and biophenols such as hydroxytyrosol (HC) and oleuropein (OC), which could be used as nutraceuticals or as natural substitutes for synthetic antioxidants. However, studies that characterize specific cultivars, interannual variability, and different drying methods are lacking. This work investigates the TAC, TPC, HC and OC in olive (Olea europaea L.) leaves under four drying methods (vacuum-drying, oven-drying, freeze-drying and air-drying). Leaves were collected from cultivars 'Arbequina' grown under organic methods and from 'Arroniz', 'Empeltre', 'Arbosana', 'Picual' and 'Arbequina' grown under conventional systems. Among fresh samples, 'Arbosana' leaves presented the highest TPC (34.0 ± 1.1 mg gallic acid equivalents/g dry weight (DW)) and TAC (146 ± 20 μmol Trolox equivalents/g DW) and the lowest interannual variability of the TPC (3.2%). The four tested drying methods were also compared as the effect on TPC, TAC, HC and OC. Freeze-drying and air-drying best preserved TPC and TAC in olive leaves. However, air-drying maintained greater OC (14–40 mg/g DW) than freeze-drying (3–20 mg/g DW). Air-dried ecological 'Arbequina' leaves exhibited the highest TPC and TAC. Consequently, this cultivar presented more valorization opportunities as a source of nutraceuticals or natural antioxidants.