## 3 Results
### HS-GC-IMS Analysis of Smoke-Flavored Sea Bass in Different Production Steps
#### 3.2.2 Changes in the Volatile Compounds of Different Smoke-Flavored Sea Bass  
To further analyze the effect of different processing stages on smoke-flavored sea bass, a fingerprint comparison of volatile compounds was conducted for each stage. Different columns represent various volatile compounds, and different rows represent samples at different stages (Figure 5). The color depth of signal points represents the concentration of the substance. As shown in Figure 5, the different stages show different fingerprint plots of volatile compounds. In region A, pentanal, acetic acid, 3-methylbutanal monomer, 2-methyl-1-propanol, ethyl acetate both monomer and dimer were present at each stage, while the contents of compounds except 3-methylbutanal dimer did not change significantly. Compounds in region B increased significantly during fish processing. They contain aldehydes such as butanal, ketones such as 3-hydroxy-2-butanone and alcohols such as propanol, which play an important role in the volatile compounds of smoke-flavored sea bass. The content of compounds in area C increased significantly in the dried sample, but these substances disappeared in the smoked sample. In area D, the content of compounds was reduced in the heated sample, including 3-pentanone, propanal, 3-methylbutanol. Volatile compounds in region E include 2-methylpropanal, furfural, 2-acetylfuran, and 2,3-butanedione, which were not found in the fresh, salted and dried samples, and the highest content was in smoked sample. With the increase of temperature, the content of these substances decreased. The characteristic volatile compounds of each stage help to distinguish smoke-flavored sea bass in different production steps. Figure 4: Two-dimensional (2D)-topographic subtraction plots based on the signal intensity of smoke-flavored sea bass with different production steps via HS-GC-IMS. The horizontal axis represents the ion migration time (ms), and the vertical axis represents the retention time (s). PCA analysis could clearly highlight the volatile compounds differences in various processing stages based on the volatile compounds area signal intensity (Figure 6). The larger the sample distance in the figure, the more significant the difference between samples. The score chart shows that three principal components (PC1, PC2 and PC3) were obtained which accounted for 86.7% of the total variation. PC1, PC2, and PC3 explained 43.3%, 29.8%, and 13.6% of the variation, respectively. The distance between the samples in the heated and the others was significantly different, which indicated that volatile compounds in the heated sample was significantly different from other samples. Furthermore, the flavor curves of the dried and smoked samples were also different from those of other samples. Aldehydes were the most prevalent volatile compounds in all phases of the samples, according to the classification diagram (Figure 7) of volatile compounds of smoke-flavored sea bass. The relative content of aldehydes was significantly higher in the dried and smoked samples than in the other groups (\(p\) < 0.05), and the concentration of aldehydes was significantly lower in the heated samples (\(p\) < 0.05). The relative contents of heptanal, 2,4-heptadienaland, hexanal, pentanal and 3-methylbutanal were the highest in the dried samples, while furfural and propanal were the highest in the smoked samples. The contents of 3-methylbutanal, furfural and propanal decreased significantly after high-temperature  
Figure 5: Gallery plot based on the signal intensity of smoke-flavored sea bass with different production steps via HS-GC-IMS. * Dimers formed in the IMS drift tube were represented by symbol “*”. Figure 6: PCA based on the signal intensity of smoke-flavored sea bass with different production steps via HS-GC-IMS. a: fresh fish; b: salting; c: drying; d: smoking; e: heating. heating (\(p<0.05\)). Alcohols were the lowest in the dried samples.