## 3 Results and Discussion
### Differences in Volatile Compounds of Large Yellow Croaker Under Different Storage Conditions
#### 3.5.2 Lipid Oxidation Pathway  
The lipid oxidation pathway during storage of the large yellow croaker is shown in Figure 4. During the storage of large yellow croaker, fat degradation and oxidation are the primary factors influencing its odor, leading to the production of characteristic volatile compounds [8]. In the present study, the primary compounds associated with fat degradation and oxidation included aldehydes, ketones, and alcohols, including benzaldehyde (D1), nonanal (D2), toluene (H1), Benzene,1,3-dimethyl (H2), naphthalene (H3), dodecane (H4), cyanophyllene (H6), pentadecane (H7), hexadecane (H8), and 2-(formyloxy)-1-phenyl(K1). Among these, nonanal was the predominant aldehyde during the storage of large yellow croaker and was a key factor affecting its odor. Nonanal, primarily derived from the oxidation of unsaturated fatty acids like EPA and DHA, showed high VIP (variable importance in projection) values in PLS-DA analysis, making it a reliable marker for assessing the  
Figure 4: Brief scheme representation of the formation pathways of characteristic volatile compounds identified in large yellow croaker.  
quality of large yellow croaker. Lipid oxidation and degradation are the primary causes of rancid volatiles in fish during storage [27]. Additionally, benzaldehyde, which imparts an almond-like aroma, is generated through a deacidification reaction. Specifically, lipid oxidation triggers the volatile compounds of linoleic acid, resulting in benzaldehyde as a final product (Figure 4). Given its low odor threshold, benzaldehyde significantly influences the overall odor profile of the fish [28].  
Additionally, fat oxidation is accompanied by the production of small organic molecules, such as ketones, hydrocarbons, and alcohols, which significantly contribute to the odor of meat [29]. In this study, acetone was identified as a key ketone, primarily resulting from the oxidation of fatty acids (e.g., linoleic acid). This process produces peroxides that subsequently break down into acetophenone, contributing to a distinct sweet aroma. In addition, only 1,2-propanediol was detected among the alcohol compounds during storage, indicating relatively low levels of lipid degradation products. This suggests that the three treatments applied effectively slowed the oxidation of large yellow croaker. Box plot analysis (Figure 5) revealed that five hydrocarbons--toluene (H1), Benzene,1,3-dimethyl (H2), n-dodecane (H4), pentadecane (H7), and hexadecane (H8)--played a role in shaping the odor profile of large yellow croaker, primarily through lipid oxidation. Notably n-Dodecane (H4), pentadecane (H7), and hexadecane (H8) levels gradually decreased after 12 days of storage due to ongoing fat degradation and nutrient depletion (see Table 2), with these changes being less pronounced in the L-treated group. Additionally, free radicals and hydroperoxides generated during fat oxidation can serve as precursors or promoters for amino acid degradation [30].  
Figure 5: Box plots of 12 example volatile compounds showing significant differences during storage of these nine groups. Normalized intensities are presented on the \(y\)-axis.