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Molecules 2021, 26, 1516 10 of 13 3.3. Characterization of the Curcuma longa L. Oil The Curcuma longa L. oil samples obtained by MAE in the optimum point, and by Soxhlet extraction were characterized in terms of chemical structure and composition as well as their total phenolic content and antioxidant properties. 3.3.1. Attenuated Total Reflection-Fourier Transform Infrared Radiation (ATR-FTIR) Attenuated Total Reflection-Fourier Transform Infrared Radiation (ATR-FTIR) was employed to determine the chemical structure of the Curcuma longa L. oil. For this purpose, a spectrum-two FTIR spectrometer (Perkin Elmer Inc., Waltham, MA, USA) with a Univer- sal Attenuated Total Reflection accessory was used. The transmittance range was from 600 to 4000 cm−1 using 32 scans and 4 cm−1 of resolution. 3.3.2. GC/MS Analysis The chemical composition of the extracted Curcuma longa L. oil was assessed using gas chromatography and mass spectrometry (GC/MS, GC 7890A, MS 5975C, Agilent, Santa Clara, CA, USA). First, 0.01 g of Curcuma longa L. oil was dissolved in 1 mL of ethyl acetate, and, then this solution was injected into a HP-5MS capillary column (5%- phenyl)-methylpolysiloxane, 30 m × 0.25 mm). The conditions used to carry out the GC/MS analysis were as following described: the injector was at 280 ◦C with split/splitless method in split mode (10:1); the carrier gas was helium with a flow rate of 0.7 mL/min; the temperature ramp started at 50 ◦C and was increased by 10 ◦C/min until reaching 120 ◦C and maintained for 5 min; in the second stage of the ramp, the temperature was increased by 10 ◦C/min until 280 ◦C and maintained for 8 min; finally, in the third step, the temperature increased 10 ◦C/min until 300 ◦C and kept constant for 2 min. The identification of the different compounds was performed through the Library of the National Institute of Standards (NIST). 3.3.3. Determination of the Total Phenolic Content (TPC) The total phenolic content of the Curcuma longa L. oil was measured according to the method described by Fernández-Marín et al. [35] with slight modifications. Briefly, 0.025 g of each extract was dissolved in 3 mL of methanol and centrifuged at 450 rpm for 24 h. The supernatant was then collected and a 300 μL solution of each extract was used. The measurement was carried out at 760 nm using an UV/Vis spectrophotometer (V-630, Jasco, Pfungstadt, Germany). Gallic acid was used as the reference standard. The results were expressed as mg gallic acid equivalents/g of extracted Curcuma longa L. oil (mg GAE/g). Each sample was analysed in triplicate. 3.3.4. Antioxidant Activity The antioxidant activity of the extracted Curcuma longa L. oil was evaluated using three different assays: 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging assay; ferric ion reducing antioxidant power (FRAP) assay; and 2,2′-azinobis(3-ethylbenzothiazoline-6- sulfonic acid) (ABTS) radical scavenging assay. The samples were prepared by dissolving 0.025 g of Curcuma longa L. oil in 3 mL of methanol (MeOH) and centrifuged at 450 rpm for 24 h. The supernatant was then collected for use in the antioxidant assays. DPPH Radical Scavenging Assay The antioxidant activity of Curcuma longa L. oil was evaluated by the DPPH (2,2- diphenyl-1-picrylhydrazyl) free radical elimination assay according to the method de- scribed by Fernandez-Marin et al. [35] with slight modifications. Three hundred μL of supernatant (prepared as described before) from each sample was mixed with 3 mL of a DPPH solution (6 × 10−5 M) in MeOH. After 15 min, the decrease in absorbance was measured at 515 nm with UV/Vis spectrophotometer (Jasco V-630, Pfungstadt, Germany). Trolox was used as standard and the results were expressed in mg Trolox equivalent/g of Curcuma longa L. oil (mg TE/g). Each sample was measured in triplicate.PDF Image | Microwave-Assisted Extraction of Curcuma
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