Abstract
The phenolic compound contents and antioxidant activities of the leaf extracts of nine olive genotypes were determined, and the obtained data were analysed using chemometric techniques. In the crude extracts, 12 compounds belonging to the secoiridoids, phenylethanoids, and flavonoids were identified. Oleuropein was the primary component for all genotypes, exhibiting a content of 21.0 to 98.0 mg/g extract. Hydroxytyrosol, verbascoside, luteolin 7-O-glucoside, and luteolin 4′-O-glucoside were also present in noticeable quantities. Genotypes differed to the greatest extent in the content of verbascoside (0.45–21.07 mg/g extract). The content of hydroxytyrosol ranged from 1.33 to 4.03 mg/g extract, and the aforementioned luteolin glucosides were present at 1.58–8.67 mg/g extract. The total phenolic content (TPC), DPPH• and ABTS•+ scavenging activities, ferric reducing antioxidant power (FRAP), and ability to inhibit the oxidation of β-carotene-linoleic acid emulsion also varied significantly among genotypes. A hierarchical cluster analysis enabled the division of genotypes into three clusters with similarity above 60% in each group. GGE biplot analysis showed olive genotypes variability with respect to phenolic compound contents and antioxidant activities. Significant correlations among TPC, FRAP, the values of both radical scavenging assays, and the content of oleuropein were found. The contents of 7-O-glucoside and 4′-O-glucoside correlated with TPC, TEAC, FRAP, and the results of the emulsion oxidation assay.
Highlights
The evergreen olive tree (Olea europaea L.) is native to coastal Mediterranean areas and is one of the oldest crops in this region
The total phenolic content (TPC) of olive leaf extracts ranged from 110 mg gallic acid equivalents (GAE)/g to 268 mg GAE/g (Table 1) and decreased in the following order of genotypes: ‘Esek Zeytini’ > ‘Ayvalik’
The hot-water extracts obtained from leaves of the same genotypes were characterised by 1.4- to 2.6-fold lower TPC, but as in our research, ‘Esek Zeytini’ and ‘Uslu’ extracts were the most and least abundant in phenolic compounds, respectively [13]
Summary
The evergreen olive tree (Olea europaea L.) is native to coastal Mediterranean areas and is one of the oldest crops in this region. Large amounts of by-products are generated by olive oil production processes, including olive leaves [1,2]. This cheap agro-industrial material is generally used as animal feed or energy biomass, but recently, interest in the potential use of olive leaves and olive leaf extracts in the pharmaceutical, food, and cosmetics industries is growing [2,3,4,5]. The biological activities of olive leaf compounds have been reported. These compounds have cholesterol lowering effects, antiviral and antibacterial activity against a wide range of microorganisms, radioprotective effect, in vitro and in vivo antioxidant activity, and antiproliferative effect against. The potential of olive leaves in the prevention of hypertension, cardiovascular and neurological diseases, diabetes, and hyperlipidaemia has been shown [5]
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