## 3 Results and Discussion
### GC-MS Analysis Results
#### 3.2.2 Analysis of Volatile Substances in Oysters with Different Freshness
At 12 \({}^{\circ}\)C and 20 \({}^{\circ}\)C, 21 and 13 types of esters were detected, respectively, while at 28 \({}^{\circ}\)C, a total of 27 esters were identified. At 12 \({}^{\circ}\)C, the absolute content of esters exhibited an increasing trend, although the relative content began to decline after the fifth day due to the production of additional substances. Conversely, at 20 \({}^{\circ}\)C and 28 \({}^{\circ}\)C, the absolute content of esters remained low, and the trends in relative content were not clearly defined, likely due to interference from microorganisms and changes in pH. At all temperatures, the types and relative contents of phenolic and ether compounds are minimal. At 28 \({}^{\circ}\)C, no phenols or ethers were detected. Various kinds of hydrocarbons are present during the storage process, but their relative content remains low. At temperatures of 4, 12, 20, and 28 \({}^{\circ}\)C, 30, 46, 31, and 34 types of hydrocarbons were detected, respectively. At 4 \({}^{\circ}\)C and 20 \({}^{\circ}\)C, the content of hydrocarbon compounds initially increased and then decreased, reaching peak levels at 192 h and 60 h, respectively. At 12 \({}^{\circ}\)C, the content of hydrocarbons showed a trend of first decreasing and then increasing. Conversely, at 28 \({}^{\circ}\)C, the content of hydrocarbons fluctuated significantly and the trend is irregular. Among these hydrocarbons, compounds such as Styrene, 1,3-Cyclooctadiene, and Allylidenecyclohexane were only detected in the early stages of storage. In contrast, substances like Pentadecane, 1H-Indene, 1-ethylideneoctahydro-, trans-Cyclohexene, and 3-ethyl appeared in the later stages of storage. Due to their higher odor threshold, hydrocarbons exert less influence on food odor [32]. However, under certain conditions, hydrocarbons can be converted into aldehydes and ketones, which may be responsible for the fishy odor observed in aquatic products [33]. Under all temperature conditions, the relative content of nitrogen-containing compounds remains low, while sulfur-containing compounds are only detected at 4 \({}^{\circ}\)C. At temperatures of 4, 12, 20, and 28 \({}^{\circ}\)C, a total of 6, 4, 5, and 7 types of nitrogen-containing compounds were identified, respectively. The primary nitrogen-containing compounds include oxime-, methoxy-phenyl-, and 2-(E)-hexen-1-OL, as well as (4S)-4-amino-5-methyl-. The decomposition of proteins predominantly generates nitrogen-containing compounds through the action of microorganisms and enzymes. Notably, only one sulfur-containing compound was detected in the volatile substances of oysters at 4 \({}^{\circ}\)C. These compounds were not present in particularly fresh oyster samples but became apparent with extended storage. Most sulfur compounds, which are characterized by aromas reminiscent of onion, cabbage, boiled sulfur, or rotten eggs, have low thresholds that significantly influence the overall flavor of the food. The other compounds identified primarily include furan compounds, pyridine compounds, and various additional compounds. GC-MS analysis of oysters stored at different temperatures--4, 12, 20, and 28 \({}^{\circ}\)C--revealed the detection of 19, 17, 12, and 21 types of nitrogen compounds, respectively. Among the furan compounds, 2-ethyl-, cis-2-(2-pentenyl)furan was found to be the most abundant, characterized by a strong fleshy, burnt, and sweet aroma, as well as a very low aroma threshold, which contributes to an appealing fragrance. The Maillard reaction, which occurs between amino acids and reducing sugars, is one of the key processes responsible for the development of meat flavor. This complex reaction yields numerous important flavor compounds, including furan, pyrazine, pyrrole, and other heterocyclic compounds [34]. Furan, 2-pentyl- is primarily generated through the oxidation and degradation of lipids during the heating process. It is commonly found in cooked meat and meat products, imparting a baked flavor along with notes of beans, fruit, green, and similar vegetable flavors [35].