Study On Spoilage Bacterial Flora And Biological Preservative Of Iced Fresh Large Yellow Croaker (Pseudosciaena Crocea)

Large yellow croaker (Pseudosciaena crocea) is an important commercial marine fish of China. Large yellow croaker is sold as iced whole fresh fish in markets in order to meet the demand for freshness, rather than frozen fish. The objectives of this dissertation were to investigate the spoilage microbial ecology of large yellow croaker and develop some preservation technologies to extend the shelf-life of the fish.The main research results are listed as follows:As the traditional indicators of freshness of fish, TVB-N (TotalVolatile Base Nitrogen) and TVC (Total Viable Count)in fish fleshwere less sensitivethan sensory evaluation. The research showed TVB-N and TVC in gills and intestines were qualified indicators, whoseconclusionswere consistent with sensory evaluation.It was suggested that the diversity of bacterial communities of fish could be used to discriminate and predict fishfreshness. The bacterial flora changed significantlyduring storage, and it was showed that 2-3d and 5-6d of storage were key time pointsin the change of bacterial flora and freshness.The diversity of bacterial communities of fish based on 16S rDNA PCR-DGGE analyzedwithprincipal component analysis (PCA) showed the abundantbacterial flora, featuredby Psychrotrophic and Shewanelladuring early storage time, and during later storage time,the bacterial flora were simple, featured by Lactobacillusplantarum, PsychrotrophicShewanella and Pediococcus. Fish intestine spoiled fastest. Intestinalbacterial flora was most stable, featured byPseudomonas. Pseudomonas in the fish intestine diffused to fish flesh and gills, and Shewanella in fish skin and gills spread to the intestine, resulted in freshness decay absolutely.20 genuses of bacteria were identified from scale, skin, flesh, gills and intestine of fish based on 16S rDNA PCR-DGGE, purification and identification with multiple medium and fatty-acid identification technology. The results showed that Shewanella, Psychrotrophic, Pseudomonas, Lactobacillus, Pediococcus and Chryseobacteriumwere dominant flora.Shewanella putrefaciens, Pseudomonas fluorescens and Psychrobacter immobilis were confirmed to be spoilage bacteria by inoculation test The strainsgrew faster and spoilage capacity was enhancedas temperature increased from 4℃ to 25 ℃.Shewanella putrefaciensstrain O showed greaterability of spoilageat lower inoculum concentration, and Pseudomonas fluorescensshowed higher ability of spoilage at higher inoculum concentration. Self-inhibition of TMAwere found at high bacterial concentration, particularly withShewanella putrefaciensstrain A. GC-O method was used to distinguish the freshness of fish and determine the type of spoilage strains.Bacillus subtilis BS08 hadbroad spectrum antibacterial activity against spoilage strains isolated from large yellow croaker. The antibacterial substances wereidentifiedasBacillus subtilis surfactinwithHPLC-MS and IR. The surfactin had good thermal stability, pH stability and hemolytic activity.Fermentation process wasoptimizedfor the yield ofsurfactin, and the biological preservative was used to prevent and control spoilage in large yellow croaker.