Abstract
A fast, clean, energy-saving, non-toxic method for the stabilization of the antioxidant activity and the improvement of the thermal stability of oleuropein and related phenolic compounds separated from olive leaf extract via salting-out-assisted cloud point extraction (CPE) was developed using Tween 80. The process was based on the decrease of the solubility of polyphenols and the lowering of the cloud point temperature of Tween 80 due to the presence of elevated amounts of sulfates (salting-out) and the separation from the bulk solution with centrifugation. The optimum conditions were chosen based on polyphenols recovery (%), phase volume ratio (Vs/Vw) and concentration factor (Fc). The maximum recovery of polyphenols was in total 95.9%; Vs/Vw was 0.075 and Fc was 15 at the following conditions: pH 2.6, ambient temperature (25 °C), 4% Tween 80 (w/v), 35% Na2SO4 (w/v) and a settling time of 5 min. The total recovery of oleuropein, hydroxytyrosol, luteolin-7-O-glucoside, verbascoside and apigenin-7-O-glucoside, at optimum conditions, was 99.8%, 93.0%, 87.6%, 99.3% and 100.0%, respectively. Polyphenolic compounds entrapped in the surfactant-rich phase (Vs) showed higher thermal stability (activation energy (Ea) 23.8 kJ/mol) compared to non-entrapped ones (Ea 76.5 kJ/mol). The antioxidant activity of separated polyphenols remained unaffected as determined by the 1,1-diphenyl-2-picrylhydrazyl method.
Highlights
The health promoting properties of plant polyphenolic antioxidant compounds [1,2,3,4], as well as their potential application as natural food additives [5] have led to a great scientific and commercial interest
The aim of this work was to develop a salting-out-assisted cloud point extraction for a sufficient separation of oleuropein and related phenolics from olive leaf extract in a single CPE step, as well as monitoring the antioxidant activity and thermal stability of these nutraceuticals entrapped in the surfactant-rich phase
CPE can be operated without heating up the olive leaf extract at elevated temperatures, which could lead to the thermal degradation of the phenolic compounds
Summary
The health promoting properties of plant polyphenolic antioxidant compounds [1,2,3,4], as well as their potential application as natural food additives [5] have led to a great scientific and commercial interest. A lot of effort has been expended on the extraction, isolation and separation of those natural secondary metabolites [9] For this purpose, several extraction techniques (liquid-solid phase, liquid-liquid phase, supercritical fluid, accelerated pressurized, ultrasound and microwave-assisted extraction) have been developed [10]. Several extraction techniques (liquid-solid phase, liquid-liquid phase, supercritical fluid, accelerated pressurized, ultrasound and microwave-assisted extraction) have been developed [10] Most of those methods are characterized either by the use of large solvent volumes and long extraction times or by high energy consumption and expensive facilities. Difficulties in their application have emerged for analytical purposes or even more for the industrial production of natural phenolic antioxidants, especially for food applications [11]. Solid phase extraction (SPE) shows a lower recovery of phenolic compounds, while supercritical fluid extraction (SFE) using liquid CO2 requires expensive, high-pressure equipment [12]
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