## 3 Results and Discussion
### Proteolytic Degradation Analyzed by SDS-PAGE  
The proteolytic degradation of different bacterial groups after being stored for 18 days was analyzed by SDS-PAGE. Twenty to thirty visual bands of total soluble protein were shown in Figure 3A, and the molecular weight ranged from 0 kDa to 230 kDa. No significant difference was observed among some main proteins of different bacteria inoculated groups such as myosin heavy chain (MHC, 230 kDa), \(\alpha\)-actinin (100 kDa), actin (43 kDa), and tropomyosin (35 kDa). However, there were still different characteristics of the total soluble proteins among the different bacteria inoculated groups. The intensity of the bands with molecular weight of approximately 33, 25, 17, 15, and 10 kDa decreased _Shewanella baltica_ group, _Pseudomonas fluorescens_ group, and _Pseudomonas mandeli_ group. The proteins of 15 kDa and 25 kDa were also degraded by _Shewanella putrefaciens_, _Pseudomonas deceptionensis_, and _Pseudomonas fragi_. Meanwhile, the intensity of the bands with molecular weight of approximately 22 kDa and 50 kDa in _Pseudomonas fluorescens_ and _Pseudomonas mandeli_ groups were higher than that of other bacteria groups. This may be the intermediate or oligomer of small protein molecules formed after protein degradation. Interestingly, compared with the total soluble proteins of other groups, a new protein band with the molecular weight less than 10 kDa was observed only in _Shewanella_ species inoculated groups. Changes of water-soluble proteins and salt-soluble proteins of different bacterial groups are shown in Figure 3B and Figure 3C, respectively. The bands of water-soluble proteins had the largest increase while salt-soluble proteins with similar molecular weight almost disappeared in the _Pseudomonas fluorescens_ group. Meanwhile, as is shown in Figure 3B, the intensity of bands with the molecular weight of approximately 90 kDa became higher in different bacterial groups when compared with the control; the intensity of bands in approximately 40 kDa had a slight decrease in _Pseudomonas fluorescens_ group and _Pseudomonas deceptionensis_ group, while the proteins with similar molecular weight disappeared in _Pseudomonas mandelli_ group. MHC and actin degradation by _Pseudomonas mandelli_ was observed in Figure 3C. An increase of intensity in bands with the molecular weight of approximately 25-40 kDa and 80-100 kDa was found in _Pseudomonas mandelli_ group. As for other bacteria inoculated groups, proteins with molecular weight of 25 kDa to 30 kDa showed difference. cohols, and hydrocarbons varied greatly among different inoculated samples. To better visualize, a heat-map of the 35 compounds was constructed for different samples at different periods (Figure 4E). Each row represents a separate volatile substance, and each column represents the group used for analysis. In addition, different colors indicated the relative abundance of volatile species, in which high abundance were depicted in red, and conversely, low-abundant were marked in green. The results show that in all fish samples, hexanal, octanal, nonanal, decanal, dodecanal, hexanol, and ethyl hexanoate content decreased. A corresponding decrease of most of the aldehydes was also found in all spoiled samples in this study when compared with the control. An increasing trend of 2-undecanone, 6, 10-dimethylundeca-5, 9-undecadien-2-one, and 1, 3-dichloro-benzene was shown in the spoiled samples, which implied that these compounds might be responsible for off-odors in spoiled sturgeon fillets. Some polycyclic aromatic hydrocarbons and tetramethyl-pyrazine might be the spoilage indicators of the samples inoculated with _Pseudomonas fluorescens_, polycyclic aromatic hydrocarbons were the off-odor contributors in fish previously. Pyrazine compounds were considered as the empyreumatic-odors source, and were possible to be produced by the condensation reactions between amino acid (ammonia) and dicarbonyl compounds or Maillard reaction.