## 3 Results and Discussion
### Volatile Compound Analysis
They were determined as odor-active compounds with unpleasant odors such as rancid, cheese-like, and fermented, which could be important contributors to flavor changes during fermentation and storage [58; 59; 60]. In addition, it was found that butanoic acid could be converted from lipids by intracellular enzymes in lactic acid bacteria [61]. Branched-chained amino acids such as valine, leucine, and isoleucine can be converted into branched-chained organic acids such as 2-methyl-butanoic acid and 3-methyl-butanoic acid by the fermentation and rancidity process during storage [62]. ZSS, as a seed foods or TCM, is rich in lipids and contains several amino acids [63]. High-temperature and -humidity conditions promoted the transformation of these compounds during storage. Figure 4: Increase (green) and decrease (red) in common volatile compounds after rancidity (**A**). The content of unique volatile compounds in rancid samples (**B**). Compared with the normal samples, as shown in Figure 4B, 19 unique compounds were detected and only one compound, \(\delta\)-cadinene, was not detected in rancid samples. \(\delta\)-Cadinene might be decomposed or transformed in the rancidity process. Nineteen compounds were produced in the rancidity process. Among them, the relative content of \(\beta\)-phellandrene was the highest at 10.46%, followed by \(\alpha\)-pinene at 9.21%, and 3-carene at 7.74%. Related studies showed that \(\beta\)-phellandrene had a pleasant odor, \(\alpha\)-pinene had a pine odor, 3-carene had a fruity odor, and \(\beta\)-myrrene had a pleasant and terpene odor [64; 65]. In theory, the large increase in these olefin contents resulted in a stronger fragrance of rancid samples. On the one hand, the decreased level of the common volatile compound \(\delta\)-limonene was more than the increased level of those unique compounds, so they failed to increase the fragrance of rancid samples. On the other hand, olefins generally have a higher odor threshold and do not contribute much to the overall odor [66]. Moreover, the production of \(\sigma\)-cymene and \(\beta\)-myrcene was proved to be closely related to lactic acid bacteria, and the increase in their content could promote lactic acid bacteria to become the dominant bacteria in ZSS during storage and an increase in butanoic acid content [46]. In addition, benzaldehyde is a product of oxidative degradation of fat. The relative content was not high in ZSS, but the threshold value of benzaldehyde was low, which contributed to the overall odor degree of rancid samples. Therefore, the increase in benzaldehyde still resulted in a stronger irritating odor of rancid samples [67]. The contents of both 5-ethyldihydro-2(3H)-furanone and 6-methyl-3,5-heptadiene-2-one were increased, but they did not smell and had little contribution to the overall odor of rancid ZSS samples. The content of 2-heptanol was also increased, which was consistent with previous studies on the storage of foods such as fragrant rice and roasted almonds [68; 69]. Notably, the content of \(\delta\)-limonene and anethole was decreased but that of \(\alpha\)-pinene, \(\sigma\)-cymene, \(\beta\)-myrcene, terpinen-4-ol, benzaldehyde, and bornyl acetate was increased in cooked mutton meatballs at a later stage of storage. As a result, the pleasant odor was gradually weakened and the unpleasant odor was more prominent [66]. This was consistent with the change in corresponding volatile compounds in this study, which further confirmed the credibility of the results. The content of 11 organic acids in rancid samples was increased, including seven common and four unique compounds. They were a series of short-chain acids such as acetic, propanoic, butanoic, pentanoic, hexanoic, heptanoic, octanoic, and nonanoic acids. Their total relative content was from 13.63% in normal samples to 22.39% in rancid samples. These acids had some pungent and sweaty odors and could cause unpleasant odors when they were combined in rancid samples.