Microflora Analysis And Preservation Of Taihu White Prawn (Exopalaemon Modestus) During Low Temperature Storage

The Taihu white prawn(Exopalaemon modestus)was a widely aquacultured freshwater shrimps which had good flavor and high nutritional value.However,the captured shrimps were vulnerable to bacteria spoilage because of high protein and moisture content,resulting in a short shelf life,even the risk of pathogenic bacteria contamination.In view of this,in the article,the Taihu white prawn(Exopalaemon modestus)purchased from a local market of aquatic products in Nanjing,China was taken as the research object,microbiological and chemical changes of the white prawn during different storage temperature were evaluated,PCR-DGGE and Illumina high-throughput sequencing technology were used to reveal the dynamic changes of spoilage bacteria,and then three kinds of preservatives with excellent properties were screened.On this foundation,the compound preservatives were optimized by response surface Box-Behnken design,then the fresh-keeping effect of compound preservatives was verified and compared combined with modified atmosphere packaging technology to provide a theory evidence for ensuring the flavor quality,prolonging shelf life and saving energy.The main results were as follows:1.Total viable counts(TVC),TVBN value and BAs were evaluated to analyse the quality changes and preliminarily determine the end point of the shelf life,at the same time,the total bacterial DNA in shrimp was extracted directly,and the succession of dominant bacteria during storage was studied by PCR-DGGE.The results showed that the initial TVC and TVBN value were 6.17 log CFU/g and 7.23 mg/100g respectively,which were all conformed to fresh standards.The initial concentrations of BAs were almost Omg/100kg,except SPD and SPM(SPD:2.97-6.81mg/kg;SPM:3.22-8.97mg/kg)while the contents of PUT showed the most significant change.The TVC、TVBN value and PUT content continually increased to 7.08 log CFU/g、31.21mg/100g and 95.46±2.17 mg/kg after 8 days storage.The predominant bacteria at the early stage of storage were Uncultured bacterium isolate DGGE gel,Chryseobacterium sp.,Acidovorax sp.,Curvibacter sp.,Pelomonas sp.and Kinneretia asaccharophila strain.The dominant spoilage bacteria at the end of the shelf life were Acidovorax,Curvibacter,Pelomonas and Lactococcus garvieae.2.Based on the methods and results of the first chapter,the changes of freshness of Taihu white shrimp under different storage temperatures were further analyzed.Meanwhile,the composition of dominant bacteria in Taihu white shrimp under different storage temperatures was analyzed by Illumina high-throughput sequencing technology.The results showed that both the TVC value and the TVBN value were increasing with the prolongation of the storage time under the three kinds of storage temperatures.The shelf life of Taihu white shrimp during storage at 0,4 and 15 ℃ was 8d,4d and 36h,respectively,indicating that ice storage could significantly inhibit the growth of spoilage bacteria and prolong the storage period of Taihu white shrimp.A total of 626758 high quality sequences were obtained through Illumina high-throughput sequencing technology.The drawing of the sparse curve showed that the sequencing of this experiment had reached saturation and could represent the diversity of the original community.The Chao1 index and the Shannon index were calculated to find that the bacterial community at three storage temperatures had higher richness.Analysis of sample community structure showed that the predominant bacteria under 0℃ were Acinetobacter,Ralstonia,Lactobacillus,Ochrobactrum,Burkholderia and Pseudomonas,the predominant bacteria under 4℃ were Acinetobacter,the predominant bacteria under 15℃ were Lactococcus,Psychrobacter,Ochrobactrum,Burkholderia and Achromobacter.3.With the consideration of bacteriostasis,the flavor quality of the prawn and the prime cost,Nisin、ε-PL and PLA with excellent properties were selected to conduct immersion experiments considering,the optimum concentration of a single preservative was Nisin80mg/100ml,epsilon-PL1g/L and 2g/100mlPLA,respectively.Using Box-Behnken center combination test design and response surface analysis to constructed a quadratic polynomial regression model with the total number of colony as the response value.The results indicated that the optimal concentrations was composed of 71.75mg/100ml Nisin,1.2g/L ε-PL and 1.5g/100ml PLA.Nisin and PLA had a significant interactive effect on the quality of Taihu white prawn during storage.It was found that the interaction between Nisin and PLA was significant,and their combination could synergistically inhibit the growth of spoilage bacteria during storage at 4℃.Response surface optimization results combined with 80%CO2/10%N2/10%O2 modified atmosphere packaging technology,contrasted with lactic acid fresh-keeping liquid at the same time was carried out to evaluate the preservation effect of compound preservative combined with MAP.The results showed that:compound preservative combined with MAP could significantly inhibit the growth of microorganisms,delay the generation of TVBN and biogenic amines.The TVC increased to 7.011og CFU/g after 10 days of storage which just exceeded the national regulations,and the shelf life was longer than 5-6d in the control group.