## 3 Results and Discussion
### Volatile Compounds of Foxtail Millet Analyzed by HS-SPME/GC-MS
The odor of hexanal is described as being grassy in low concentrations and an unpleasant sour smell at high concentrations [36]. The contents of heptanal and octanal were all higher than 0.22 \(\upmu\)g/g (Figure 5B, Table S2), and they were all accompanied with a grassy and fatty odor [37; 38]. Nonanal, which is generally expressed as a fragrant and fatty odor [39], accounted for 2.93-9.20% of the total volatile compounds and constitutes a common volatile substance in cereals. An earlier piece of work on the volatile profiles of foxtail millet reported that nonanal was the major volatile compound [15]. Ketones are generally considered to have a fatty and burnt aroma as well as an enhanced floral aroma with an increase in carbon chains [39]. The relative contents of ketones in foxtail millet were 8.09-29.46% (Figure 5A), of which 2-heptanone was the most  
Figure 5: Composition (**A**) and relative content (**B**) of volatile compounds in foxtail millet. abundant in Sample 8 (Jingu 41), accounting for 10.7% of the total volatile compounds (Figure 5B, Table S2). In addition, 2-heptanone can be produced by the oxidation of linoleic acid [40] with a banana and slightly medicinal odor [41], and is detected in sorghum [42], black rice [43], and other cereals. Alcohols accounted for 18.28-23.71% of the total volatile compounds. Unsaturated alcohols have a lower threshold value and make a greater contribution to the characteristic aroma of foxtail millet. 1-octene-3-ol is an unsaturated alcohol with a mushroom and earth aroma [17], which is considered to make an important contribution to the characteristic aroma of rice (Figure 5B, Table S2). Although a low content of 1-octene-3-ol was detected in foxtail millet (0.76-1.44%) (Figure 5A), it contributes to the characteristic aroma to foxtail millet due to its low threshold value [31]. The relative content of alkane compounds ranged from 2.66% to 3.89% (Figure 5A); however, their effects on the odor quality of the different foxtail millet were insignificant due to the high threshold. Compared with other volatile compounds, heterocyclic compounds accounted for the smallest proportion of total volatile compounds (1.12-2.13%) (Figure 5A). Although the composition of volatile compounds in different varieties of foxtail millet was similar, the contents of volatile compounds in foxtail millet were different (Figure 5B). The contents of hexanal in Sample 12 to Sample 20 were higher than those in the other samples. Differences in the composition and content of volatile compounds contributed to the distinctive aroma of the different foxtail millet varieties. The PCA was carried out to understand the differences in volatile compounds among 20 foxtail millet varieties (Figure 6). The cluster and separation of samples on the score plot are related to the distribution of volatile compounds on the load plot in Figure 6B. Different samples can be better resolved by presenting score plots combined with loading plots. In this way, correlations between the 18 volatile compounds and the samples can be easily observed. The score plot and loading plot revealed the volatile compounds responsible for the differences among the foxtail millet. The accumulative variance contribution rate was 93% of the two principal components. We found that 20 varieties of foxtail millet clustered into four different groups (Figure 6A), indicating the differences in the volatile compounds of the different varieties. It was observed in the load plot that the volatile compounds with significant contributions to PC1 were hexanal and acetophenone. These compounds were characterized by green, grass,  
Figure 6: PCA analysis: score plot (**A**) and load plot (**B**). and sweet aromas. The volatile compounds with a clear contribution to PC2 were heptanal, 2-heptanone, 1-pentanol, and nonanal, which have aroma profiles of fruity, grassy, fatty, and vanilla (Table 3).