2.3. DPPH Radical Scavenging Assay

The DPPH radical scavenging assay was performed on an EL800 Universal Microplate reader (Bio-Tek Instruments, Inc.) using multi-well plates as a previously published method described [32]. Firstly, different concentrations of positive control BHT (1.5, 0.75, 0.375, 0.1875, and 0.094 mg/mL) and C. wenyujin Y.H. Chen et C. Ling essential oil (3.36, 1.68, 0.84, 0.42, and 0.21 mg/mL) were prepared. Then the DPPH solution was diluted by methanol to 32 µM as a working solution. The reaction was initiated by mixing 20 µL of sample solution with 180 µL of DPPH working solution and incubated in dark at room temperature for 30 min. A monitoring of the absorbance at 570 nm was carried out after the reaction was completed. The scavenging capacity of samples were calculated by experimental scavenging capacity (ESC) using Equation (1) as follows [33]:

where Abs_sample is the absorbance value of the sample (DPPH solution plus antioxidant) at each time interval, Abs_blank is the absorbance value of the blank (methanol plus antioxidant(s)). Abs_control is the absorbance value of control (methanol plus DPPH solution).

The value of 50% inhibition (IC₅₀) was calculated by the graph plotting sample concentration and inhibition percentage.