Screening Olive Leaves from Unexploited Traditional Greek Cultivars for Their Phenolic Antioxidant Dynamic

An Eco-Friendly Supercritical CO2 Recovery of Value-Added Extracts from Olea europaea Leaves

An eco-friendly approach towards the recovery of value-added extracts from olive tree leaves with the aid of supercritical CO2 at 30 MPa was carried out. The impact of extraction temperature (35-90 °C) and presence of co-solvents (ethanol, water, and aqueous ethanol) on the total phenolic, flavonoid, and pigment content, as well as oleuropein, hydroxytyrosol, tyrosol, and α-tocopherol content was determined. In addition, the antioxidant activity of extracts from tree leaves using DPPH, ABTS, and CUPRAC assays was investigated. The results of the study showed that the most effective supercritical CO2 extraction was at 90 °C with an addition of ethanol, which enabled the separation of extract with the highest content of tested compounds. Some of the highest recorded values were for oleuropein 1.9 mg/g, for carotenoids 5.3 mg/g, and for α-tocopherol 2.0 mg/g. Our results are expected to contribute to the efforts towards the valorization of olive leaves as a sustainable source of valuable compounds, and boost local economies as well as the interest of pharmaceutical, food, and cosmetic industries for novel food by-product applications.

Redox State Modulatory Activity and Cytotoxicity of Olea europaea L. (Oleaceae) Leaves Extract Enriched in Polyphenols Using Macroporous Resin

The food products derived from Olea europaea are a fundamental part of the Mediterranean diet, and their health-promoting effects are well known. In this study, we analyzed the phytochemical characteristics, the redox state modulatory activity, and the cytotoxic effect of an olive leaf aqueous extract enriched by macroporous resin on different tumor and normal cell lines (LNCaP, PC3, HFF-1). HPLC-DAD analysis, the Folin-Ciocalteu and aluminum chloride methods confirmed the qualitatively and quantitatively high content of phenolic compounds (130.02 ± 2.3 mg GAE/g extract), and a DPPH assay (IC50 = 100.00 ± 1.8 μg/mL), the related antioxidant activity. The biological investigation showed a significant cytotoxic effect, highlighted by an MTT test and the evident cellular morphological changes, on two prostate cancer cell lines. Remarkably, the extract was practically non-toxic on HFF-1 at the concentrations (100, 150, 300 µg/mL) and exposure times tested. Hence, the results are selective for tumor cells. The underlying cytotoxicity was associated with the decrease in ROS production (55% PC3, 42% LNCaP) and the increase in RSH levels (>50% PC3) and an LDH release assay (50% PC3, 40% LNCaP, established necrosis as the main cell death mechanism.

Exploring the Bioactive Content of Liquid Waste and Byproducts Produced by Two-Phase Olive Mills in Laconia (Greece): Is There a Prospect for Added-Value Applications?

The use of a two-phase decanter (TwPD) for olive-oil extraction produces wastes and byproducts (a small volume of water from oil washing, olive leaves from the defoliator, and a high moisture pomace which can be destoned) that contain valuable bioactive compounds, such as phenolics and/or triterpenic acids. So far, there is no (water) or limited information (leaves and the destoned pomace fraction) on their content of bioactives, especially triterpenic acids. To contribute to the characterization of such streams from cultivars of international interest, in the present study, samples obtained from five mills from the region of Laconia (from one or two harvests) in Greece, where Koroneiki cv dominates, were screened for phenols and/or triterpenic acids. The leaves and pomace were dried at two temperatures (70 °C and/or 140 °C), and the pomace was also destoned before analysis. The liquid wastes contained low amounts of total (TPC) phenols (<140 mg gallic acid/L), hydroxytyrosol (<44 mg/L), and tyrosol (<33 mg/L). The olive leaves varied widely in TPC (12.8-57.4 mg gallic acid/g dry leaf) and oleuropein (0.4-56.8 mg/g dry leaf) but contained an appreciable amount of triterpenic acids, mainly oleanolic acid (~12.5-31 mg/g dry leaf, respectively). A higher drying temperature (140 vs. 70 °C) affected rather positively the TPC/oleuropein content, whereas triterpenic acids were unaffected. The destoned pomace TPC was 15.5-22.0 mg gallic acid/g dw, hydroxytyrosol 3.9-5.6 mg/g dw, and maslinic 5.5-19.3 mg/g dw. Drying at 140 °C preserved better its bioactive phenols, whereas triterpenic acids were not influenced. The present findings indicate that TwPD streams may have a prospect as a source of bioactives for added-value applications. Material handling, including drying conditions, may be critical but only for phenols.

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

The leaves of the olive tree (Olea europaea L.) are one of the major solid wastes from the olive industry. Globally, the European Union is the largest producer of olive by-products, with Spain, Italy, Greece, and Portugal accounting for almost the entire production. Many questions remain to be solved concerning olive leaves (OL), including those related to possible differences in composition and/or biological activities depending on their geographical origin. In the present work, OL from Spain, Italy, Greece, and Portugal have been characterized according to their phytochemical profile, antioxidant capacity, neuroprotective activity, and anti-inflammatory effects. The Spanish and Italian OL samples presented the highest antioxidant and neuroprotective activities, while the Greek OL showed the lowest. These results were strongly associated with the content of oleoside methyl ester and p-hydroxybenzoic acid for the Spanish and Italian samples, respectively, whereas the content of decarboxymethyl elenolic acid dialdehyde form (hydrated) was negatively associated with the mentioned biological activities of the Greek samples. No country-related effect was observed in the anti-inflammatory activity of OL. Comprehensively, this work could provide a useful tool for manufacturers and R&D departments in making environmentally friendly decisions on how OL can be used to generate nutraceutical products based on the composition and origin of this by-product.

Antioxidant and Antimicrobial Activity of Hydroethanolic Leaf Extracts from Six Mediterranean Olive Cultivars

Phenolic profiles, antioxidant, and antimicrobial activities of hydroethanolic olive leaf extracts from six Mediterranean olive cultivars (Croatian: Lastovka, Levantinka, Oblica; Italian: Moraiolo, Frantoio, Nostrana di Brisighella) were investigated. As expected, various distributions of phenolic levels were observed for each cultivar and the total phenolic content showed high variability (ranging from 4 to 22 mg GAE/g of dry extract), with the highest amount of phenolics found in the Oblica sample, which also provided the highest antiradical (ORAC) and reducing activity (FRAP). The screening of individual compounds was performed by HPLC-PDA-ESI-QTOF-MS and the main detected compounds were oleuropein, hydroxytyrosol, oleoside/secologanoside, verbascoside, rutin, luteolin glucoside, hydroxyoleuropein, and ligstroside. While the antioxidant activity of the samples was relatively high, they showed no bactericidal and bacteriostatic activity against E. coli and S. Typhimurium; weak activity against Staphylococcus aureus, Bacillus cereus, and Listeria innocua; and inhibitory effects against Campylobacter jejuni at 0.5 mg dry extract/mL. The obtained results support the fact that olive leaf extracts, and especially those from the Oblica cultivar, could potentially be applied in various industries as natural preservatives and effective and inexpensive sources of valuable antioxidants.

Effect of Ohmic Heating on the Extraction Yield, Polyphenol Content and Antioxidant Activity of Olive Mill Leaves

This study examined the influence of ohmic heating (OH), compared to the conventional heating (Conven) and Control (solvent) methods, on the extraction of olive mill leaves. The main extraction parameters were: (i) solvent ratio (aqueous ethanol; 40%, 60%, and 80%, v/v), and (ii) extraction temperature; 45 °C, 55 °C, and 75 °C (for OH and Conven), and room temperature (for Control). The selected response variables were extraction yield (%), total phenolic content (TPC), and antioxidant activity (ABTS and DPPH). The ohmic system, compared to Conven and Control, exhibited the greatest effects (p < 0.001) on increasing (i) extraction yield (34.53%) at 75 °C with 80% ethanol, (ii) TPC at 55 °C (42.53, 34.35, 31.63 mg GAE/g extract, with 60%, 40%, and 80% ethanol, respectively), and (iii) antioxidant potency at 75 °C detected by DPPH and ABTS, in the range of 1.21–1.04 mM TE/g, and 0.62–0.48 mM TE/g extract, respectively. Further, there were relatively similar trends in TPC and antioxidant activity (both methods), regardless of solvent ratios, p < 0.001. These findings demonstrate the potential of ohmic heating, as a green processing tool, for efficient extraction (15 min) of olive leaves. To date, no literature has described ohmic application for olive leave extraction.

Production of hydroxytyrosol rich extract from Olea europaea leaf with enhanced biological activity using immobilized enzyme reactors

As olive leaves constitute the main by-product of the olive oil industry with important environmental and economic impact, there is an increasing demand for its valorization. In the present work, we report the development and application of immobilized enzyme batch bioreactors for the chemo-enzymatic treatment of an aqueous Olea europaea leaf extract rich in oleuropein to produce an extract enriched in hydroxytyrosol and other oleuropein hydrolysis products. To this end, a robust biocatalyst was developed through the immobilization of β-glucosidase on chitosan-coated magnetic beads which exhibited high hydrolytic stability after 240 h of incubation at 37 °C. The biocatalyst was successfully used in both a rotating bed-reactor and a stir-tank reactor for the modification of the olive leaf extract leading to high conversion yields of oleuropein (exceeding 90%), while an up to 2.5 times enrichment in hydroxytyrosol was achieved. Over 20 phenolic compounds (from different classes of phytochemicals such as flavonoids, secoiridoids, and their derivatives) were identified, in the extract before and after its modification through various chromatographic and spectroscopic techniques. Finally, the biological activity of both extracts was evaluated. Compared to the non-modified extract, the modified one demonstrated 20% higher antioxidant activity, seven-fold higher antibacterial activity, and enhanced cytotoxicity against leiomyosarcoma cells.

Innovative Extraction Technologies for Development of Functional Ingredients Based on Polyphenols from Olive Leaves

Olive tree (Olea europea L.) leaves represent around 10% of the total weight of olives arriving at any given mill, which are generally discarded, causing economic and environmental issues. However, these are rich sources of natural bioactive compounds (i.e., polyphenols), which have health-promoting potential. Thus, the valorization of olive leaves by recovering and reusing their components should be a must for food sustainability and circular economy. This review provides an insight into the principal polyphenols present in olive leaves, together with agronomic variables influencing their content. It also summarizes the recent advances in the application of novel extraction technologies that have shown promising extraction efficacy, reducing the volume of extraction solvent and saving time and cost. Moreover, potential industrial uses and international patents filed in the pharmaceutic, food, and cosmetic sectors are discussed.

Antioxidant Capacity of Free Volatile Compounds from Olea europaea L. cv. Oblica Leaves Depending on the Vegetation Stage

Previous research on specialized metabolites of olive leaves has focused on the phenolic components and their biological role. The research in this article focuses on the metabolites that form free volatile compounds (FVCs). The composition of FVCs is divided into compounds isolated in the oil phase (essential oils; EO) and in the aqueous phase (hydrosols; Hy) from leaves of Olea europaea L. cultivar Oblica. Plant material was collected from the same olive tree over a six-month period, from December to May, and analyzed by gas chromatography-mass spectrometry (GC-MS). The compounds β-caryophyllene, α-humulene, allo-aromadendrene, docosane, hexadecanoic acid and oleic acid were identified in all EO study periods. In the Hy in all studied periods, the major compounds are α-pinene, β-ionone, myristicin, docosane, 1-hexanol, oleic acid and (E)-β-damascenone. The differences in the qualitative composition of FVC are directly related to the phenological development of the leaves. Antioxidant capacity of the EOs and hydrosols was measured with two methods, ORAC and DPPH. Hydrosol extracts showed higher capacity than the EOs in all methods.

Development and validation of an HPLC-DAD method for determination of oleuropein and other bioactive compounds in olive leaf by-products

BACKGROUND

Oil mills could benefit by preparing their own aqueous extracts from olive leaves. Accordingly, the present study aimed to measure the bioactive compounds richness of such extracts, especially oleuropein. A water-based microwave extraction procedure was developed and a selective and precise high-performance liquid chromatography with diode array detection (HPLC-DAD) method was validated for the determination of oleuropein and others bioactive compounds from olive leaves.

RESULTS

The water solubility of oleuropein was determined to be 9.5 g L^-1 . The extraction procedure was optimized in terms of power, olive leaf weight/water volume ratio and time of extraction, and the results revealed that 2 mg mL^-1 and a microwave irradiation at 800 W for 30 s resulted in the greatest efficiency. Oleuropein was determined by the new validation method, which showed good linearity (r² = 0.996), precision (% relative standard deviation < 10%), recovery (118.6%), and limits of detection (17.48 mg L^-1 ) and quantification (21.54 mg L^-1 ). Good correlation (r² = 0.979) was obtained between oleuropein of the olive leaf extracts determined by HPLC-DAD and by UV-visible spectrophotometry.

CONCLUSION

A simple extraction method was developed and validated to obtain aqueous extract from olive leaves by microwave extraction, determining for the first time oleuropein water solubility. Validation of the method showed that oleuropein in olive leaves could be quantified when it is at least 1% of dry weight by means of HPLC-DAD. UV-visible spectrophotometry can be useful in oil mills because it enables the content of oleuropein and other bioactive compounds content to be determined in situ in such leaf aqueous extracts. © 2020 Society of Chemical Industry.

Bio-Based Active Packaging: Carrageenan Film with Olive Leaf Extract for Lamb Meat Preservation

Carrageenan-based active packaging film was prepared by adding olive leaf extract (OLE) as a bioactive agent to the lamb meat packaging. The OLE was characterized in terms of its phenolic compounds (T.ph), antioxidant activity (AA), oleuropein, and minimum inhibitory concentration (MIC) against Escherichia coli. The film’s formulation consisted of carrageenan, glycerol as a plasticizer, water as a solvent, and OLE. The effects of the OLE on the thickness, water vapor permeability (WVP), tensile strength (TS), elongation at break (EB), elastic modulus (EM), color, solubility, and antimicrobial capacity of the carrageenan film were determined. The OLE had the following excellent characteristics: the T.ph value was 115.96 mg_GAE∙g⁻¹ (d.b), the AA was 89.52%, the oleuropein value was 11.59 mg∙g⁻¹, and the MIC was 50 mg∙mL⁻¹. The results showed that the addition of OLE increased the thickness, EB, and WVP, and decreased the TS and EM of the film. The solubility was not significantly affected by the OLE. The color difference with the addition of OLE was 64.72%, which had the benefit of being a barrier to oxidative processes related to light. The film with the OLE was shown to have an antimicrobial capacity during the storage of lamb meat, reducing the count of psychrophiles five-fold when compared to the samples packed by the control and commercial films; therefore, this novel film has the potential to increase the shelf life of lamb meat, and as such, is suitable for use as active packaging.

Target and Suspect HRMS Metabolomics for the Determination of Functional Ingredients in 13 Varieties of Olive Leaves and Drupes from Greece

The huge interest in the health-related properties of foods to improve health has brought about the development of sensitive analytical methods for the characterization of natural products with functional ingredients. Greek olive leaves and drupes constitute a valuable source of biophenols with functional properties. A novel ultra-high-performance liquid chromatography-quadrupole time of flight tandem mass spectrometry (UHPLC-QTOF-MS) analytical method was developed to identify biophenols through target and suspect screening in Greek olive leaves and drupes of the varieties: Koroneiki, Throumbolia, Konservolia, Koutsourelia, Kalamon, Petrolia, Amigdalolia, Megaritiki, Mastoeidis, Agouromanakolia, Agrilia, Adramitiani and Kolovi. The method's performance was evaluated using the target compounds: oleuropein, tyrosol and hydroxytyrosol. The analytes demonstrated satisfactory recovery efficiency for both leaves (85.9-90.5%) and drupes (89.7-92.5%). Limits of detection (LODs) were relatively low over the range 0.038 (oleuropein)-0.046 (hydroxytyrosol) and 0.037 (oleuropein)-0.048 (hydroxytyrosol) for leaves and drupes, respectively For leaves, the precision limit ranged between 4.7 and 5.8% for intra-day and between 5.8 and 6.5% for inter-day experiments, and for drupes, it ranged between 3.8 and 5.2% for intra-day and between 5.1 and 6.2% for inter-day experiments, establishing the good precision of the method. The regression coefficient (r²) was above 0.99 in all cases. Furthermore, the preparation of herbal tea from olive leaves is suggested after investigating the optimum infusion time of dried leaves in boiling water. Overall, 10 target and 36 suspect compounds were determined in leaves, while seven targets and thirty-three suspects were identified in drupes, respectively.

Content of phenolic compounds and mannitol in olive leaves extracts from six Spanish cultivars: Extraction with the Soxhlet method and pressurized liquids

Olive leaves are considered a promising source of bioactives such as phenolic compounds and mannitol. The extraction of high added value products is an issue of great interest and importance from the point of view of their exploitation. However, the content of these compounds can differ between cultivars and extraction methods. In this work, six olive leaves cultivars, including three wild cultivars, and two extraction processes (an innovative and alternative technique, pressurized liquid extraction, and a conventional Soxhlet extraction) were evaluated and compared towards the selective recovery of bioactive compounds. The wild cultivars showed the highest content of phenolic and flavonoid compounds, being oleuropein the compound present in higher amount. Findings also revealed that the highest mannitol content in the extracts was observed with the commercial cultivars, specifically in Arbequina. It is thus possible to decide which cultivars to use in order to obtain the highest yield of each bioproduct.