Influence of Drying Temperature and Harvesting Season on Phenolic Content and Antioxidant and Antiproliferative Activities of Olive (Olea europaea) Leaf Extracts

Abstract

Interest in plant compounds has increased, given recent evidence regarding their role in human health due to their pleiotropic effects. For example, plant bioactive compounds present in food products, including polyphenols, are associated with preventive effects in various diseases, such as cancer or inflammation. Breast and colorectal cancers are among the most commonly diagnosed cancers globally. Although appreciable advances have been made in treatments, new therapeutic approaches are still needed. Thus, in this study, up to 28 olive leaf extracts were obtained during different seasons and using different drying temperatures. The influence of these conditions on total polyphenolic content (measured using Folin–Ciocalteu assays), antioxidant activity (using Trolox Equivalent Antioxidant Capacity and Ferric Reducing Ability of Plasma assays) and antiproliferative capacity (using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide, MTT assays) was tested in breast and colorectal cancer cells. Increased phenolic composition and antioxidant and antiproliferative capacity are noted in the extracts obtained from leaves harvested in autumn, followed by summer, spring and winter. Regarding drying conditions, although there is not a general trend, conditions using the highest temperatures lead to the optimal phenolic content and antioxidant and antiproliferative activities in most cases. These results confirm previously published studies and provide evidence in support of the influence of both harvesting and drying conditions on the biological activity of olive leaf extracts.

Oleuropein in olive leaf, branch, and stem extracts: stability and biological activity in human cervical carcinoma and melanoma cells

“…Generally, the content of OLE in LE spreads over a wide range of concentration values from 0.073 to 210.6 mg/g DW as was recently reviewed (13), and the reason for this wide range is numerous factors affecting the content of phenolics, as was reported throughout the literature (10)(11)(12)(13)(14)(15). Our results for OLE in LE of 1.48 mg/g DW (RT) and 2.88 mg/g DW (+4 °C) fall within the range reported in the literature (13), but closer to lower values.…”

“…Olive leaves and leaf extract (LE) preparations are already used by some industries to obtain natural products rich in bioactive compounds that could be adequate food additives, dietary supplements, or cosmetic ingredients, mostly owing to their antioxidant and antimicrobial properties (6)(7)(8)(9). Those properties are mainly attributed to olive leaf phenolics, the composition of which heavily depends on the cultivar, harvesting season, drying method, and extract preparation (10)(11)(12)(13)(14)(15). Olive leaf is particularly rich in oleuropein (OLE), the main polyphenol found in LE, renowned for its antioxidant, anti--inflammatory, antidiabetic, anti-atherogenic, antihypertensive, anti-obesity, neuroprotective, cardioprotective, anticancer and many other effects (2,(16)(17)(18)(19)(20).…”

“…Anticancer effects of LE and OLE have been investigated in many different cancer cell lines and in animal models as well (11,17,24,25). Anticancer effects of OLE in human cervical carcinoma (HeLa) cells include reduction of cell viability and induction of apoptosis via the JNK signaling pathway (26,27).…”