## 3 Results and Discussion
### Volatile Compound Analysis  
The detection of volatile compounds is an important method for evaluating the quality of foods and TCMs during storage link. For example, the content of esters generally decreased and the content of aldehydes, ketones, alcohols, acids, and pyrazines generally increased during storage link [34; 35; 36; 37]. Therefore, it is necessary to explore the changes in volatile compounds before and after rancidity of ZSS samples. In this study, HS-SPME technology was used to extract volatile components, and GC-MS technology was used to separate and identify volatile compounds. A total ion chromatogram is shown in Figure 2. The result showed that there was a significant difference between normal and rancid samples regarding their volatile components. The results of volatile compounds are shown in Table 3. These identified compounds showed that mass spectrum and retention times were identical to standards under the given chromatographic/mass spectrometric conditions. For example, compound 2 (3.46 min) yielded a parent ion at \(m/z\) 170 and fragment ions at \(m/z\) 155, 112, 99, and 57, and was identified as 2,2,4,6,6-pentamethylheptane according to the NIST database. Compound 14 (19.48 min) displayed a parent ion at \(m/z\) 60 and fragment ions at \(m/z\) 45, 43, 29, and 15, and was identified as acetic acid. Compound 21 (24.48 min) produced a parent ion at \(m/z\) 90 and fragment ions at \(m/z\) 75, 57, 45, and 29, and was identified as (\(R\),\(S\))-2.3-butanediol. Figure 2: GC-MS total ion chromatogram of volatile components from normal and rancid ZSS samples. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline
**No. ** & **Rt (min)** & **Compound** & **MF** & **CAS** & **MW** & **Fragment (w/o)** & **Normal (v)** & **Randi (v)** \\ \hline
1 & 3.21 & Ethanol & CH\({}_{4}\)O & 64-127.5 & 46 & 45/31/29/27 & 4.26 \(\pm\) 0.12 & 0.75 \(\pm\) 0.12 ** \\
2 & 3.46 & 22,466-Pentamethylheptane & CH\({}_{4}\)O\({}_{3}\) & 13,475-52.6 & 170 & 155/12/99/57 & 4.93 \(\pm\) 0.08 & 1.35 \(\pm\) 0.07 \\
3 & 4.46 & -x-Pure & CH\({}_{4}\)O\({}_{3}\) & 2407-95.8 & 136 & 90/77/9/27 & - & 9.21 \(\pm\) 0.18 \\
4 & 7.59 & 3-Carne & CH\({}_{4}\)O\({}_{3}\) & 13,466-78.9 & 136 & 121/10/59/59 & - & 2.74 \(\pm\) 0.12 ** \\
5 & 8.20 & \(\beta\)-Mycene & CH\({}_{4}\)O\({}_{3}\) & 12,235-3.5 & 136 & 90/94/12/7 & - & 2.53 \(\pm\) 0.06 \\
6 & 9.24 & \(\lambda\)-Limonon & CH\({}_{4}\)O\({}_{3}\) & 13  
As a result, 39 volatile compounds were identified from the normal and rancid ZSS, including 20 in normal and 38 in rancid samples, and covering acids, alcohols, alkenes, aldehydes, ketones, etc. Then, they were selected and used to plot a heat-map and hierarchical cluster analysis (HCA) to find relatively homogeneous clusters of samples and to illustrate the content changes in volatile compounds from normal and rancid samples. As shown in Figure 3, normal and rancid samples were divided into two groups under HCA, and the different content levels of these compounds in the normal and rancid samples could be intuitively and clearly observed, suggesting that there was a significant difference between normal and rancid samples and enabling further analysis of common and unique compounds. Both normal and rancid samples had 19 common compounds. As shown in Figure 4A, the content of 10 compounds was decreased but that of nine compounds was increased in rancid samples. For 10 decreased compounds, the relative content of \(\delta\)-limonene had the largest decrease from 46.43 to 15.97%, followed by (\(R\),\(R\))-2,3-butanediol from 13.22 to 4.92%, and (\(R\),\(S\))-2,3-butanediol from 10.40 to 5.40%. \(\delta\)-Limonene, a principal active compound of limonenes, was frequently used as a dietary supplement and fragrance ingredient for cosmetics products [38; 39].