Quality Changes And The Specific Spoilage Organisms Of Grass Carp(Ctenopharyngodon Idellus) Fillets During Storage

Grass carp (Ctenopharyngodon idella) is one of the most important farmed species of freshwater fish in China for its large output, low prices, and rich nutritional value. Grass carp is extremely perishable and has a short shelf-life. This present thesis is mainly foucus on those problems mentioned above. Initially, we investgated the quality changes of grass carp during storage and mainly analysed the changes of biogenic amines consentrations and ATP and its related compounds. And then, the investigation on the microbial succession of grass carp fillets during storage at 4 ℃ was carried out. For identification,16S rRNA genes of the isolated pure strains were sequenced and analyzed. In order to to identify the specific spoilage organisms (SSO) of grass carp, the spoilage potential of the predominant bacteria isolated from spoiled grass carp fillets was evaluated. The behavior of SSO of grass carp was monitored and modelled at different storage temperatures from 0 to 20℃. Finally, two types of natural preservatives were used to evaluate their effect on the growth of SSO in grass carp. The results show as follows:(1) The effect of different salt concentrations (2% and 10%) on the quality changes of grass carp fillets were evaluated in terms of biogenic amines, ATP and its related compounds, sensory attributes, total volatile basic nitrogen (TVB-N), and total viable counts (TVC) during 4 ℃ and 20 ℃ storage. The unsalted carp fillets were used as controls. Putrescine and cadaverine were the predominant biogenic amines for the spoiled grass carp. Tempreture has an impact on the formation of tryptamine, histamine, and cadaverine. The content of putrescine in grass carp filltes stored at 4 ℃ and 20 ℃ was 66.9 mg/kg and 74.5 mg/kg at the end of storage, respectively. During storage at 20 ℃, brining with salt could accelerate the degregation of ATP, while extend the degregation of HxR. Higher salt concentration had better inhibitory effect on the accumulation of some biogenic amines, such as tryptamine, 2-phenylethylamine, putrescine and cadaverine. Tryptamine,2-phenylethylamine, putrescine and cadaverine showed a significant (p< 0.05) correlation with TVB-N for the control group at 20℃ and 4℃.(2) Investigation on the microbial succession of grass carp fillets during storage at 4 ℃ was carried out. For identification,16S rRNA genes of the isolated pure strains were sequenced and analyzed. Acinetobacter was dominant in fresh grass carp (33% in total). Species from the genera of Kocuria, Chryseobacterium, Aeromonas, Brevundimonas, Empedobacter, Pseudomonas, Microbacterium, Flavobacterium, Moraxella, Shewanella and Soonwooa were also detected at the initial day. Aeromonas and Acinetobacter dominated the communities after 6 days. Aeromonas followed by Pseudomonas was the predominant genera at the end of shelf-life of grass carp, represented 43% and 37% of the total number of isolates, respectively.(3) The growth of Aeromonas hydrophila, Pseudomonas jessenii and Shewanella putrefaciens in sterile grass carp fillets stored at 4 ℃ was evaluated, as well as their effect on physicochemical and sensory spoilage. The bacteria showed different spoilage characteristic. The highest producers of the total volatile basic nitrogen (TVB-N) were Aeromonas hydrophila and Pseudomonas jessenii, which reached a maximum level around 30 mg/100g after 16 days. Aeromonas hydrophila and Pseudomonas jessenii were active producers of biogenic amines in grass carp fillets and Pseudomonas jessenii was the only bacterum that produced histamine in the present study. All the inoculated samples exhibited a large production of cadaverine, reaching 51.5,54.8 and 37.8 mg/mL for Aeromonas hydrophil, Pseudomonas jessenii, and Shewanella putrefaciens, respectively. A total of more than 100 volatile compounds were detected and 28 compounds were analysed by SPME-GC-MS. The majority of volatile compounds produced during spoilage of grass carp were alcohols and aldehyde. Shewanella putrefaciens produced the highest amount of 1-octen-3-ol, while Pseudomonas jessenii produced high amount of ketones such as 2-nonanone and 2-undecanone.(4) The Gompertza model based on Belehradek-type model for the growth of Aeromonas hydrophila、 Pseudomonas jessenii and total viable counts of grass carp at 0-20 ℃ fitted to the following equations: N(t)=N0+(8.63-N0)×exp{-exp [2.718×(0.016T+0.1116)2/8.63-N0×(1/(0.0181T+0.1062)2-t)+1]} N(t)=N0+(8.94-N0)×exp{-exp [2.718×(0.0119T+0.133)2/8.94-N0×(1/(0.0111T+0.0785)2-t)+1]} N(t)=N0+(9.22-N0)×exp{-exp [2.718×(0.0234T+0.2013)2/9.22-N0×(1/(0.0227T+0.0712)2-t)+1]}(5) The antimicrobial effectiveness of chitosan and garlic oil on the growth of spoiled bacteria of the grass carp was evaluated. Grass carp fillets were coated with the solution of 2% chitosan (T1),5% garlic oil (T2) and 5% garlic oil+ 2% chitosan (T3), and the untreated group was control. Microbiological analyses (TVC, Pseudomonas, H2S-producing bacteria and Aeromonas) were carried out during the storage. A reduction of initial bacteria for the three groups was observed. Both 2% chitosan, and 5% garlic oil group inhibited the growth of SSO.