## 3 Results and Discussion
### Volatile Compounds in Foxtail millet Determined by HS-GC-IMS

Table 2: Volatile compounds tentatively identified in the 20 foetal millet varieties determined by HS-GC-IMS. Figure 2: Conf. Figure 2: _Conf._  
Figure 2: 2D topographic photographs (**A**), 3D topographic photographs (**B**), and contrast difference spectra (**C**) of volatile compounds in 20 varieties of total millet. To reflect the differences in volatile compounds of different foxtail millet varieties, the characteristic non-targeted volatile fingerprints were conducted and analyzed based on the whole spectral information. All the signals were extracted to form the fingerprints (Figure 3), in which each column and row indicated one volatile compound and the sample, respectively. The total signal intensities in Samples 1-8 were obliviously higher than in Samples 9-20, and there were significant differences in volatile compound content between Samples 1-8 and Samples 9-20, which was consistent with the E-Nose results. Methyl-5-hepten-2-one and butyl acrylate reached the maximum concentration in Sample 1 (77-322) (as shown in region A). The intensities of volatile compounds in Sample 3 (Jingu 26) were higher than those in other samples, especially dihydro-2(3H)-furanone and butyl acrylate (as shown in region B). Samples 3 (Jingu 26), 4 (Jingu 28), and 5 (Jingu 34(1)) in region C had a high content of volatile compounds, and were dominated by alcohols and aldehydes, such as 1-pentanol, 2-octenal, 2-heptenal, and 2-hexenal. The contents of 2-methylbutanal and 3-methylbutanal were highest in Sample 16 (Jingu 59), as shown in region D. The highest content of 2-butanone was found in Sample 18 (Jingu 62) (as shown in region E). Hexanal with strong signal intensity was one of the main volatile compounds in 20 foxtail millet varieties, which is consistent with [PERSON]'s results [32] (as shown in region F). Figure 3: The volatile compounds fingerprints of the 20 foxtail millet varieties. To better understand the differences in volatile compounds in the 20 foxtail millet varieties, PCA was carried out based on the signal intensity of the compounds (Figure 4). The accumulative variance contribution rate of the first PC (58%) and the second PC (15%) was 73%, which could well represent the characteristic differences in the original variables. The samples were well dispersed in four different quadrants. Samples 9 to 20 and Sample 7 (Jingu 34()) were clustered on the left side of the \(x\)-axis, and other samples were on the right side. Samples 1 (77-322), 3 (Jingu 26), 4 (Jingu 28), 5 (Jingu 34()), and 8 (Jingu 41) were clustered together and distributed in the fourth quadrants; Samples 2 (Jingu 21), 6 (Jingu 34()), and 7 (Jingu 34()) were grouped together. Aside from Sample 16 (Jingu 59), other samples gathered together and distributed in the third quadrant. The results showed that the different varieties of foxtail millet could be clearly discriminated from one another based on the volatile compounds by HS-GC-IMS. The differences in volatile compounds across different foxtail millet varieties were analyzed, and the volatile profile in foxtail millet is specificity-dependent on variety. As a variety widely cultivated and highly favored by consumers in northern China, Jingu21 (Sample 2) dominated in aldehydes and esters, and the typical volatile compounds were benzaldehyde (bitter almond, cherry, nut), 2-methylbutyrate (fruity, green), ethyl acetate (fruity, sweet), 2-methylbutanal (musty, nutty), and propyl acetate (raspberry-like). These typical volatile compounds combine together to give Jingu 21 a unique aroma.