Volatile compound analysis
The volatile compounds extracted using SPME with DVB/CAR/PDMS (Supelco, Inc.) fiber following to previously described (Yang et al., 2017). Individual oil samples (5g) were chosen and placed into a sealed bottle and water bath at 50°C immediately. The aroma compounds were extracted using SPME method for 30 min. The fiber was then removed and transferred into the GC/MS injector For the GC–MS measurements, GC HP 6890 (Calif, USA) attached to an HP5973MSD detector with a DB–1 column (60 m×0.25 mm×0.25 μm, Agilent, Palo Alto, CA, USA) was used. Helium was operated at flow rate of 1 mL/min. The ionization potential used was 70 eV, and the temperature of the ion source was at 230 °C. The condition of the oven temperature was as follow: I) 40 °C as initial temperature, then increased to 120 °C (speed of 3°C/min) and II) then set to a rate of increase to 200 °C (speed of 5°C/min), the temperature which then was held for 10 min. The linear retention indices (RIs) were calculated from the retention times of n–alkanes (C5–C25), and the volatiles Identification by matching the RI with data found other literature (Yeh et al., 2014). The quantities of volatile compounds were indicated using the peak area.