Abstract
Olive leaves (OLL), an agricultural waste by-product, are considered a significant bioresource of polyphenols, known as bioactive compounds. This study evaluates the pulsed electric field (PEF) technique for the extraction of polyphenols from OLL. The study parameters included a series of “green” solvents (ethanol, water as well as mixtures of them at a 25% step gradient) and different input values for the pulse duration of PEF. The phytochemical extraction degree was evaluated using total phenol concentration (Folin–Ciocalteu method) and high-performance liquid chromatography (HPLC) analyses, while the antioxidant activity was assessed using differential scanning calorimetry (DSC). The results obtained from the PEF extracts were compared with those of the extracts produced without the PEF application. The highest PEF effect was observed for aqueous ethanol, 25% v/v, using a pulse duration of 10 μs. The increase in the total polyphenols reached 31.85%, while the increase in the specific metabolites reached 265.67%. The recovery in polyphenols was found to depend on the solvent, the pulse duration of treatment and the structure of the metabolites extracted.
Highlights
Olive (Olea europaea L.) leaves (OLL) have a dual identity, both as waste material from the olive oil production or the olive tree-pruning season and as aromatic and medicinal herbs that are beneficial for human health [1,2]
The significance of the results presented above are further illustrated based on the fact that in the current study, the 71.87% increase, for the pulsed electric field (PEF) condition with pulse duration 10 μs, in the case of the basic metabolite luteolin-7-O-glucoside, led to an amount of 0.82 mg g−1 dw in the extract
This study aimed to extract polyphenols from olive leaves using the PEF technique and to suggest a method that would substantially reduce the use of the organic solvent ethanol, replacing it with a technology that would allow us to isolate phenolic compounds in an economically feasible way
Summary
Olive (Olea europaea L.) leaves (OLL) have a dual identity, both as waste material from the olive oil production or the olive tree-pruning season and as aromatic and medicinal herbs that are beneficial for human health [1,2]. Based on available data from the literature [3,4,5], the global volume of wasted olive leaves reaches 12 Mt/year. This volume comes from pruning and olive oil production waste (~2 Mt/year), with most of it produced in Europe (~50%). OLL contain a considerable amount of bioactive compounds belonging to the group known as polyphenols, such as phenolic acids, phenolic alcohols (hydroxytyrosol and tyrosol), flavonoids (luteolin-7-O-glucoside, rutin, apigenin-7-O-glucoside, luteolin-4-Oglucoside), and secoiridoids (oleuropein) [5,6,7,8]. Dietary oleuropein from OLL resulted in hypo-cholesterolaemic activities [11]. The above effects of hydroxytyrosol and oleuropein are possibly related to the decreases in the levels of plasma cholesterol [13]
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