Changes in Phytochemical Profiles and Biological Activity of Olive Leaves Treated by Two Drying Methods

Abstract

Olive leaves, which are the most abundant byproducts of the olive industry, offer multiple health benefits. The investigation of the phytochemical profiles and relevant biological activities is an essential step toward transforming these low-value byproducts into value-added ones. This study systematically investigated the phytochemical profiles, antioxidant capacity, and inhibition rates of olive leaves from four cultivars on the α-glucosidase, α-amylase, and angiotensin-converting enzyme (ACE). The leaves were prepared using two common drying methods, namely, hot air-drying and freeze-drying. A total of 33 bioactive compounds were identified in the olive leaves, namely, 19 flavonoids, 2 phenylethanoids, 2 coumarins, 2 hydroxycinnamic acids, 2 iridoids, and 6 triterpenic acids. Quantification of the bioactive compounds revealed high amounts of polyphenols, especially flavonoids [2,027–8,055 mg/kg dry weight (DW)], iridoids (566–22,096 mg/kg DW), and triterpenic acids (13,824–19,056 mg/kg DW) in the olive leaves. The hot air-dried leaves showed significantly (P < 0.05) higher iridoid (oleuropein and secoxyloganin) content than the fresh leaves, while freeze-drying resulted in significantly (P < 0.05) higher flavonoid aglycone and hydroxytyrosol content. Additionally, freeze-drying led to samples with the highest radical scavenging, α-amylase, α-glucosidase, and ACE inhibition abilities. The flavonoid (e.g., quercetin, luteolin, eriodictyol, kaempferol-7-O-glucoside, and luteolin-7-O-glucoside), hydroxytyrosol, and oleanolic acid contents in the olive leaves were positively correlated (P < 0.05) with their bioactive potentials.

Exploring the Antioxidant, Neuroprotective, and Anti-Inflammatory Potential of Olive Leaf Extracts from Spain, Portugal, Greece, and Italy

“…According to literature, harvesting season has been shown to modulate the TPC and TFC, with higher values for leaves cultivated between March and April [ 22 , 26 ]. Similarly, the drying method and the solvent used for OL extraction have been related to changes in the phenolic and flavonoid content [ 21 , 27 ]. Therefore, these procedures were standardized for the 49 samples, with all samples collected between November and February.…”

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).…”

“…These processes should ensure the removal of moisture from plant material to avoid bacterial infestation and preserve valuable nutrients (4). In the case of olive leaves, drying methods mostly include air-drying at different temperatures (usually from 25 °C to above 100 °C) and freeze-drying under vacuum pressure (10,12). The effect of various drying methods, storage time, and temperatures on OLE stability in olive leaves was examined recently by Feng et al (23).…”

Antioxidant Capacity of Free and Bound Phenolics from Olive Leaves: In Vitro and In Vivo Responses

“…Olive leaves, which are low-value byproducts generated from olive tree cultivation and olive oil processing, contain a considerable quantity of antioxidant-active substances. Phenolic acids, flavonoids, secoiridoids, hydroxycinnamic acids, simple phenols, and triterpenic acids, which have been shown to be have the potential to scavenge reactive oxygen radicals and resist oxidative stress in the body, are the main sources of the antioxidant properties of olive leaves [13,14]. However, phenolic compounds in plants are usually present in free and bound forms [15,16].…”