## 3 Results
### HS-SPME-GC-MS Analysis of Smoke-Flavored Sea Bass in Different Production Steps
#### 3.1.2 Changes in the Volatile Compounds in Different Production Steps  
To further understand the differences in volatile flavor of smoke-flavored sea bass at different stages of processing, heatmap and hierarchical cluster analysis were used to visualize the data based on the concentrations of 63 volatile compounds initially identified (Figure 2), and analysis of ANOVA was conducted to analyze the initially identified volatile compounds.  
\begin{table}
\begin{tabular}{c c} \hline \hline \multicolumn{2}{c}{**Gas Phase-Ion Mobility Spectrometry Unit**} \\ \hline Analysis time & 30 min \\ Column type & MXT-5 (15 m \(\times\) 0.53 mm) \\ Column temperature & 60 \({}^{\circ}\)C \\ Carrier gas/drift gas & N2 (99.99\%) \\ IMS temperature & 45 \({}^{\circ}\)C \\ \multicolumn{2}{c}{Automatic headspace sampling unit} \\ Injection volume & 500 \(\mu\)L \\ Incubation time & 20 min \\ Incubation temperature & 40 \({}^{\circ}\)C \\ Syringe temperature & 85 \({}^{\circ}\)C \\ Incubation speed & 500 rpm \\ \hline \hline \end{tabular}
\end{table}
Table 1: Experimental conditions for smoke-flavored sea bass analysis by HS-GC-IMS.  
In the cluster heat map, the distribution of dark red lattices was different, indicating that different samples have different characteristic volatile compounds. In the salted samples, the contents of hexanal, heptanal, nonanal, palmitic acid, undecane, dodecane, 1-octen-3-ol, tridecane and heptadecane decreased, and some alcohols (benzaldehyde, octanal, nonanal) and aldehydes (4-ethylcyclohexanol, 2-octen-1-ol, 2-ethylcyclohexanol) were not detected. The content of undecane, dodecane, tridecane, heptadecane, pristane, almitoleic acid, trans-2-undecen-1-ol and (Z)-7-hexadecenal were significantly higher in the dried samples. Aldehydes, ketones, phenols, and furans species all increased significantly after the fish was exposed to the smoke flavors, except for 15 phenolic compounds (4-ethylphenol, phenol, o-cresol, p-cresol, guaiacol, 2,4-dimethylphenol, 2-ethylphenol, 2-methoxy-5-methylphenol, 2,4,6-trimethylphenol, 4-propylphenol, 4-ethyl-2-methoxyphenol, 2,6-dimethoxyphenol, eugenol, dihydroeugenol, and ethylhydroquinone), and the other compounds all changed to different degrees. In the heated samples, fifteen volatile compounds (5-methyl furural, 5-ethyl-2-furraldehyde, 3-furaramethanol, 4-methoxybenzhydrol, 3-methyl-2-cyclopenten-1-one, phenol, 2,4-dimethylphenol, 4-propylphenol, eugenol, ethylhydroquinone, 2-acetylfuran, 5-methyl-2-acetylfuran, palmitic acid, 3,5-dimethylpyrazole, and 3,4,5-trimethylpyrazole) disappeared and twelve new substances (1-methylcyclohexanol, 2,3-dimethylcyclohexanol,  
Figure 2: The heat map and clustering results of 63 volatile compounds in different production steps. Color shading from red to blue indicates abundances of compounds from high to low.  
2-phenyl-2-norbornanol, geraniol, 2-methyl-2-cyclopentene-1-one, 4-hexen-3-one, methyl acetate, 2-methoxycarbonylimidazole, 1-octylformate, 3,5-dimethylphenol, 2-ethyl-5-methylfuran, and o-xylene) were formed.  
To deeply explore the difference between volatile compounds in different production steps, the principal components were classified by PLS-DA score map (Figure 3a), and the total contribution rate of CP1 and CP2 reached 98.6%. Variable importance in projection (VIP) scores of various volatile compounds in smoke-flavored sea bass at various processing stages are shown in Figure 3b. The results show that there are eight substances with significant contribution (VIP > 1), including six phenols (guaiacol, o-cresol, 2-methoxy-5-methylphenol, p-cresol, 3, 5-dimethylphenol, and 4-ethyl-2-methoxy-phenol), one alcohol (3-furaldehyde) and one ketone (2-methyl-2-cyclopentene-1-one). These distinctive volatile compounds set the fresh, salted, dried, smoked, and heated samples apart to varying degrees.