Polyphenolic Characterization and Antioxidant Capacity of Laurus nobilis L. Leaf Extracts Obtained by Green and Conventional Extraction Techniques

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.

Isolation of Laurus nobilis Leaf Polyphenols: A Review on Current Techniques and Future Perspectives

“…The influence of the irradiation power and temperature must not be excluded, since Rincon et al [ 70 ] performed the extraction at 90 °C and 500 W, which might have caused degradation of thermosensitive phenolic compounds over a prolonged time. In accordance, Dobroslavić et al [ 68 ] reported that the increase of temperature from 40 to 80 °C resulted in higher TPC; however, with an irradiation time prolonged from 10 to 15 min, a stagnation of the TPC was observed, which was brought by the authors into connection with possible thermal degradation. The authors have also observed a decline in the TPC when an irradiation power higher than 400 W was applied.…”

“…However, in their study, the TPC after 15–30 min was lower than the yield that Muniz-Marquez et al [ 44 ] achieved after 6 min with 50% ethanol. This indicates that use of 50% ethanol under MAE conditions of Muniz-Marquez et al [ 44 ] is more time-efficient, which can be substantiated by results recently reported by Dobroslavić et al [ 68 ] where 10 min was optimal during the extraction or L. nobilis leaf polyphenols with 50% ethanol. The presence of water in ethanol increases the dielectric constant of the system, which could result in an increased extraction yield by improving the swelling of the plant material and therefore increasing the surface contact of the matrix and solvent [ 95 , 96 ].…”

“…In all of them, aqueous solutions of ethanol in different concentrations were used as the solvent. Muniz-Marquez et al [ 44 ] reported that ethanol concentration was the most significant influencing factor for TPC, contrary to the results reported by Dobroslavić et al [ 68 ] where ethanol concentration had no significant influence on the TPC. At lower ethanol concentrations, Muniz-Marquez et al [ 44 ] reported that irradiation time had very little effect on yield, while at a concentration of 50%, the TPC increased proportionally with prolonged irradiation time.…”

“…Muniz-Marquez et al [ 11 ] reported that there was no significant difference in the TPC between using 35% and 70% ethanol for heat-reflux extraction, which implies that the concentration of 35% is sufficient to obtain maximum TPC. In addition, Dobroslavić et al [ 68 ] reported that no significant difference was observed in the TPC when 50% and 70% ethanol were used during heat-reflux extraction. In contrast to their conclusions, the highest TPC out of all included studies (expressed as mg GAE g −1 leaves) was the one obtained using 80% ethanol during 60 min of extraction [ 75 ].…”

“…This technique was previously used in three studies [ 44 , 68 , 70 ], where phenolic compounds were extracted from L. nobilis leaves. In all of them, aqueous solutions of ethanol in different concentrations were used as the solvent.…”