## 3 Results and Discussion
### GC-MS Analysis Results
#### 3.2.2 Analysis of Volatile Substances in Oysters with Different Freshness  
Alcohols are the most representative volatile organic compounds produced during the storage of oysters. The types and relative percentages of alcohols in oysters vary at different stages of spoilage. According to GC-MS analysis of oysters stored at various temperatures (4, 12, 20, and 28 \({}^{\circ}\)C), 11, 20, 20, and 34 different alcohols were detected, respectively. During storage, the content of alcohols remained relatively high. Among these alcohols, Styrene, 1-Hexadecanol, Ethanol, 2-(tetradecyloxy)-, 1-Octadecanol, and methyl ether were only present in the early stages of storage. In contrast, 1-Ethynylcyclododecanol, Cyclohexanol, 5-methyl-2-(1-methyletenyl)-, 6-Pentyltetrahydro-2H-pyran-2-ol, and others were formed in the later stages of storage. Myrtenol, which has a grass, wood, and camphor-like aroma, exhibited particularly noticeable changes at 4 \({}^{\circ}\)C, with both its absolute and relative contents increasing significantly. At 12 \({}^{\circ}\)C, the absolute content of Myrtenol also increased markedly; however, due to the rise in other more volatile substances, the relative content of Myrtenol stabilized. Alcohols are generally formed through the reduction of aldehydes and ketones, as well as the oxidation of lipids, with a typically high threshold. Among these, unsaturated alcohols, such as 1-octene-3-ol, contribute to the fishy taste commonly associated with aquatic products. An increasing body of research indicates that unsaturated alcohols are produced through the oxidation of unsaturated fatty acids, imparting a mushroom-like flavor to fresh seafood [26]. The odor threshold of unsaturated alcohols is generally lower than that of saturated alcohols, significantly influencing the odor properties of oyster meat. Aldehydes and ketones exhibit relatively high concentrations and low thresholds during storage, which significantly contributes to flavor [27]. Aldehydes are primarily formed [28] from unsaturated fatty acid hydroperoxides through the action of lipoxygenase, a crucial enzyme in food production that generates a variety of oxidative flavors. These aldehydes possess strong aromas reminiscent of grass, fat, and fruit [29]. Ketones, on the other hand, are unstable intermediate compounds typically produced by the degradation of amino acids and the thermal oxidation of unsaturated fatty acids. They can subsequently be oxidized or reduced into corresponding alcohols, which impart unique fruit flavors [30]. At storage temperatures of 4, 12, 20, and 28 \({}^{\circ}\)C, a total of 19, 29, 16, and 23 types of aldehydes, as well as 15, 10, 8, and 11 types of ketones, were detected in oysters at different storage durations. Notably, compounds such as benzaldehyde, 4-ethyl-, 2, 4-decadienal (E,Z)-, bicyclo [6.1.0]non-4-ene-9-carbaldehyde, 1-cyclohexene-1-acetaldehyde, and \(\alpha\),2-dimethyl- were identified exclusively in the later stages of storage, along with ketones like 2-tridecanone and 2-nonanone. Fatty acids are often characterized by their aromatic and sweet notes; however, their impact on overall flavor is minimal due to their higher thresholds [31]. The primary substances identified during storage include acetic acid, caprylic acid, dodecanoic acid, tetradecanoic acid, pentadecanoic acid, and palmitic acid, among others. Acetic acid is the principal contributor to rancidity, which becomes particularly pronounced when its concentration exceeds 1%. Octanoic acid imparts a greasy and creamy taste, detectable at all storage temperatures, and is typically produced during the later stages of storage. Composed of saturated fatty alcohols and lower levels of saturated fatty acids, most esters emit fruity and floral notes that effectively mask the bitter taste of amino acids and the pungent odor of fatty acids. A total of 28 esters were detected at 4 \({}^{\circ}\)C, with the relative content of esters being highest after 168 h of storage.