2.6. GC–MS Analysis of Volatile Compounds

Headspace solid-phase microextraction (HS-SPME) was used to prepare the volatile compounds before and after fermentation [12]. The volatile compounds were analyzed by a QP2010 Ultra gas chromatograph–mass spectrometer (Shimadzu, Japan) with a BR-SWAX (30 m × 0.32 mm × 0.50 μm) quartz capillary column. Briefly, the R–B mixed juice before and after fermentation was poured into sample bottles and heated at 55 °C for 30 min, followed by treating the SPME fiber assembly (PDMS/DVB 65 μm) at 250 °C for 30 min in the inlet, and then the adsorption of the gas by the fiber for 30 min. Subsequently, the SPME fiber was analyzed at 250 °C for 15 min, and the volatile compounds were further analyzed for 30 min after removing the fiber. In the GC conditions, the sample was split in split mode with a 10:1 split ratio, and the carrier gas was high-purity helium at 1 mL/min, while the inlet temperature was 240 °C. The initial temperature of the column was maintained at 50 °C for 4 min and then increased to 150 °C at 4 °C/min, with each temperature point maintained for 3 min. Finally, the temperature was raised to 250 °C at 8 °C/min, with each temperature point held for 5 min. In the MS conditions, the ionization mode was EI, the electric energy was 70 eV, the heating current of the filament was 0.25 mA, the voltage of electronic multiplier was 1000 V, the temperature of the ion source was 220 °C, the transmission line temperature was 220 °C, and the scanning range was 40–500 (m/z).

The results of determination obtained were cross-referenced with the NIST 05 library, and the relative content of each volatile compound was measured by the peak area normalization method. The volatile components with the highest peak area in the top 70 and a high matching degree were selected.