Drying at high temperature for a short time maximizes the recovery of olive leaf biophenols

- Extracts of oven dried leaves at 105 °C showed the highest total phenol recovery and increased oleuropein recovery up to 38 fold compared with fresh olive leaves.
- Post-harvest drying can show dramatic changes in the phenol recovery that varies based not only on the plant but also on the tissue involved.
- Lack of specificity of the chemical methods for determination of “total” phenol content is undervalued in the literature.
- Chromatographic determination of the phenolic composition is essential for accurate estimation of the phenolic content.

In the current study, recovery of phenolic compounds from fresh, air-dried, freeze-dried and oven-dried (at 60 °C and 105 °C) olive leaves was investigated. The phenol content and antioxidant activity were assessed by gross quantitative methods such as total phenol content (Folin-Ciocalteu's method), total flavonoid content, total o-diphenol content and total antioxidant capacity using ABTS+° and DPPH° scavenging assays. In addition, the phenolic composition of extracts was determined by high performance liquid chromatography (HPLC) equipped with diode array detection (DAD) with tandem mass (MS/MS) and the contribution of individual phenolic components to the antioxidant activity of extracts were evaluted by online ABTS scavenging assay. Extracts obtained from oven-dried leaves at 105 °C showed the highest phenol recoveries and antioxidant activities, whereas extracts obtained from oven-dried leaves at 60 °C had the lowest values. Oven drying of olive leaves at 105 °C for three hours increased oleuropein recovery up to 38 fold as compared with fresh olive leaves. Our results stress the paramount importance of sample pre-treatment in the preparation and analysis of herbal medicines. Futhermore, we highlight the limitations of sole dependence on gross assessment of total phenolic composition and total antioxidant activity in studying plant samples.