Abstract
Laurus nobilis L. is an evergreen Mediterranean shrub whose leaves have been known for various health-promoting effects mainly attributed to polyphenols. Microwave- (MAE) and ultrasound-assisted extraction (UAE) are green extraction techniques that enable effective isolation of polyphenols from plant material. Therefore, the aim of this research was to optimize the extraction conditions of MAE (ethanol percentage, temperature, extraction time, microwave power) and UAE (ethanol percentage, extraction time, amplitude) of polyphenols from Laurus nobilis L. leaves and to assess their polyphenolic profile by ultra performance liquid chromatography- tandem mass spectrometry (UPLC-MS/MS) and antioxidant capacity by oxygen radical absorbance capacity (ORAC) assay. Optimal MAE conditions were 50% ethanol, 80 °C, 10 min and 400 W. Optimal UAE conditions were 70% ethanol, 10 min and 50% amplitude. Spectrophotometric analysis showed the highest total phenolic content in the extracts was obtained by MAE, compared to conventional heat-reflux extraction (CRE) and UAE. The polyphenolic profile of all obtained extracts included 29 compounds, with kaempferol and quercetin glycosides being the most abundant. UPLC-MS/MS showed the highest total phenolic content in the extracts obtained by CRE. ORAC assay showed the highest antioxidant capacity in extracts obtained by CRE, which is in agreement with the polyphenolic profile determined by UPLC-MS/MS.
Highlights
Laurus nobilis L., a representative of the family Lauraceae, is an evergreen shrub native to the Mediterranean area
This study examined the influence of different extraction parameters of MAE and ultrasound-assisted extraction (UAE)
The total phenolic content of the obtained extracts was measured by the Folin–Ciocalteu spectrometric method (Table 1) and the optimal extraction conditions were determined by statistical analysis
Summary
Laurus nobilis L., a representative of the family Lauraceae, is an evergreen shrub native to the Mediterranean area. Establishing an optimal methodology for the isolation of polyphenols is a crucial step for the utilization of their beneficial properties, and various techniques can be applied in order to achieve their effective recovery Conventional extraction techniques, such as heat-reflux, applicable, are often time-, energy- and solvent-consuming with difficulties when it comes to scale-up processes [13]. The distribution of ultrasonic waves is not homogenous and the wave power decreases with the increase in distance between the sample and radiating surface, which is why shaking and agitation are useful during UAE [23] The efficiency of both MAE and UAE depends on their parameters (e.g., extraction time, temperature, solvent type, microwave power (MAE), frequency and amplitude of ultrasonic waves (UAE)), which should be chosen with respect to the properties of the plant material and the targeted compounds. ORAC has been established as an assay of choice for determining the antioxidant capacity of plant material and food
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