| The fenneropenaeus chinensis,a major farmed shrimp,is an important part of the fishery economy and has become a popular choice for many consumers because of its firm,tasty meat and high nutritional content.Nevertheless,as aquatic products are susceptible to spoilage by microorganisms,the problem of how to address the deterioration in quality of Fenneropenaeus chinensis due to microbial proliferation is one that requires attention.In order to solve the above problems,the optimal ratio of chitosan composite preservative was determined by response surface experiment,the dominant putrid bacteria were isolated and identified from refrigerated penaeus chinensis,and the rot causing ability was analyzed.The antibacterial activity and antibacterial mechanism of chitosan composite preservative against the strain with the strongest rot causing ability were discussed.Finally,chitosan composite preservative was applied to refrigerated and preserved penaeus chinensis.In order to effectively prolong the shelf life of penaeus chinensis.The main research contents and conclusions are as follows.（1）The chitosan is selected as the base of the coating solution,andε-polylysine with good antibacterial properties and sodium D-isoascorbate with good antioxidant properties are added to prepare a biocomposite preservative.To identify the optimum ratio of the chitosan composite preservative,single-factor experiments are conducted to determine the optimum concentration range of each single preservative,and then a response surface experimental design is used to determine the optimum addition of each component of the chitosan composite preservative at a concentration of 1.41%for chitosan,0.14%forε-polylysine and0.15%for sodium D-isoascorbate.（2）Four strains of spoilage bacteria are finally obtained by isolation and purification of Fenneropenaeus chinensis terminal spoilage bacteria under storage conditions at 4℃,using colony morphological characteristics,Gram staining and other methods,and are identified by physiological and biochemical experiments combined with 16S r DNA technology as follows:Bacillus licheniformis,Shewanella putrefaciens,Moellerella wisconsensis and Pseudomonas fluorescens.Four strains of spoilage bacteria are quantified for p H,TVC,TVB-N and putrefactive capacity.The results shows that the p H values of Pseudomonas fluorescens and Shewanella putrefaciens shows the fastest increase,and that the TVC and TVB-N values of shrimp inoculated with Pseudomonas fluorescens are significantly higher than those of the other groups.The final experimental results show that the corruption-causing ability is from high to low:Pseudomonas fluorescens>Shewanella putrefaciens>Moellerella wisconsensis>Bacillus licheniformis,with Pseudomonas fluorescens having the highest spoilage capacity.（3）Subsequently,the bacterial inhibition activity and mechanism of inhibition of chitosan freshness inhibitors are investigated using Pseudomonas fluorescens as the experimental strain.The inhibition activity of the preservative against Pseudomonas fluorescens is investigated by measuring the minimum inhibitory concentration（MIC）,the inhibition circle and the bacterial growth curve,and the changes of OD260value,ATPase and AKPase activities.The results demonstrated that:the MIC concentration of chitosan composite preservative is 2.24 mg/m L,and the chitosan composite preservative had significant antibacterial activity.The chitosan composite preservative led to the increase of cell wall membrane permeability,the destruction of structure and integrity,the inhibition of ATP and AKP enzyme activities in the bacterium,and the decrease of total protein content in the bacterium,which eventually led to the death of the bacterium and achieved the purpose of antibacterial.（4）The effect of chitosan composite preservative on the quality of Fenneropenaeus chinensis is studied in four aspects:sensory index,microbial index,chemical index（TVB-N,p H,Ki value,K value）and physical index（colour difference,texture,electronic nose）.The results of the study shows that the sensory score of Fenneropenaeus chinensis coated with chitosan composite preservative is higher than that of the single chitosan group and the control group,and the microbiological indicators shows that the TVC value of chitosan composite preservative is 4.83±0.04 lg（CFU/g）at the 12th day.In the control group,the TVC value is 7.92±0.1 lg（CFU/g）,and the growth rate of the total number of colonies of Fenneropenaeus chinensis treated with chitosan composite preservative is effectively slowed down.The rate of increase of both TVB-N and p H index of Fenneropenaeus chinensis coated with chitosan composite preservative is significantly lower than that of the control group（P<0.05）.Meanwhile,the freshness Ki and K values also indicate that the preservation agent could effectively slow down the loss of fresh taste substances such as IMP and AMP in shrimps and slow down the increase of Hx and Hx R bitter taste substances content,and the effect of composite preservation agent coating is better than single chitosan coating in maintaining the flavour and quality of Fenneropenaeus chinensis.In the e-nose experiment,PCA,LDA and radar plot analyses demonstrated that the maintenance of Fenneropenaeus chinensis odour by the chitosan composite preservative is closest to that of fresh samples at the same storage time.Loading analysis reflected that R（7）W1W represented the most significant effect of hydrogen sulphide in the first principal component,followed by R（9）W2W for organic aromatic sulphides and R（3）W3C for sensitivity to aromatic components and the most significant effect on the second principal component.It is effectively demonstrated again that the preservatives had good flavour retention properties for Fenneropenaeus chinensis.Meanwhile,the results of the colour difference and textural characteristics studies proves that the chitosan composite preservative had a good effect of Fenneropenaeus chinensis such as colour,hardness,elasticity and stickiness. |
