Effect Of Plant Source Fresh-keeping Agent On The Quality Change Of Large Yellow Croaker(Pseudosciaena Crocea) With Ice During Logistics Process And Its Antibacterial Mechanism

Large yellow croaker?Pseudosciaena crocea?is generally distributed in Yellow Sea,East China Sea and Taiwan Strait,which was accustomed to be known as one of the main marine fishery resources for high economic interest along with Larimichthys polyactis,Trichiurus haumela and Sepia in China.Large yellow croaker has a good market in Southeast Asia due to its delicious taste and high nutritional value.In addition,large yellow croaker also contains a wealth of trace elements-selenium,can remove the body's free radicals produced,anti-aging,and also have a good effect on prevention of various cancers.In this study,effects of different circulation methods?cold chain and broken-off cold chain?on the quality and microbial diversity of large yellow croaker?Pseudosciaena crocea?with ice were researched;the antimicrobial mechanism of Ginkgo biloba leaves extracts?GBLE?against specific spoilage bacteria in large yellow croaker?Pseudosciaena crocea?-Shewanella putrefaciens and Saprophytic staphylococcus was studied.The optimum ratio of compound plant source preservative was optimized by using response surface methodology,and based on this,the plant fresh ice was made and its effect on the quality of large yellow croaker during logistics process was studied.Through the study on the quality change of large yellow croaker with ice in cold chain and broken chain circulation,it was found that the sensory score of large yellow croaker was increasing with the extension of circulation time,the quality of the large yellow croaker was changed,the number of microbes increased,the rate of Ca²⁺-ATPase activity and total sulfydryl content in the two groups were higher than those in the cold chain group.The rate of the descending group was higher than that of the cold chain group.The distribution and migration of water in different states were analyzed by Low field Nuclear Magnetic Resonance.It was found that the rate of free water growth in chain breaking group was higher than that in cold chain group.In addition,the sensory scores and the total number of colonies reached 2.35 ± 0.13 and 6.50 ± 0.07 lg CFU/g respectively,and the TVB-N value was 31.04 ± 0.06 mg N/100 g,which were beyond the acceptable range,while the cold chain group samples exceeded the shelf life limit at 347 h.The shelf life of the cold chain group is 275h-347 h,while the shelf life of the broken chain is only203h-275 h.It has been shown that temperature fluctuations allow fish samples to accelerate corruption.The number and species changes of microorganism in samples of two different circulations were analyzed by Polymerase Chain Reaction-Denaturing Gradient Gel Electrophoresis?PCR-DGGE?fingerprint technology.The DNA of bacteria in different flow stages was extracted directly.After PCR amplification,DGGE electrophoresis analysis was carried out,and the separation and sequencing of the better and bright bands on the DGGE map were carried out.The changes in microbial species and dominant bacteria could be obtained in two circulations.The results of PCR-DGGE demonstrated that the number of Staphylococcus saprophyticus,Pseudoalteromonas sp.,Vibrio Parahemolyticus decreased,but the number of Pseudomonas sp.and Psychrobacter sp.increased gradually in later circulation,Acinetobacter sp.and Shewanella sp.were also occupied a certain proportion.Pseudomonas sp.and Psychrobacter sp.were the special spoilage organism of large yellow croaker in the later of cold-chain and broken-off cold chain process.The optimum concentration of Ginkgo biloba extract,Bamboo vinegar solution and tea polyphenol solution on TVB-N value was 0.5%,0.5% and 1.5%,respectively.The optimal ratio of three kinds of plant preservatives was 1.38% of Ginkgo biloba extract,0.92% of bamboo vinegar and 0.85% of tea polyphenols.At this time,the predicted value of TVB-N was 47.17 mg N/100 g,the measured value was 50.80 mg N/100 g.Therefore,the response surface regression model was reliable.Effect of the plant fresh ice on the quality of large yellow croaker during logistics process was studied.The results showed that the sensory score of the common ice control group and the fresh ice treatment group increased with the prolongation of the circulation time.The number of microbes and TVB-N value were significantly increased,and the growthrate of the control group was higher than that of the treatment group.In the control group,the sensory scores and the total number of colonies reached 2.34 ± 0.09 and 6.24 ± 0.13 lg CFU/g respectively,and the TVB-N value was 30.87 ± 1.56 mg N/100 g,which was beyond the acceptable range,and the samples in fresh ice treatment group did not reach the shelf life limit at 395 h.The shelf life of the control group was275h-347 h,while the fresh ice treatment group could extend the shelf life of more than 48 h.The changes of T-AOC,SOD and MDA in the two groups were higher than those in the treatment group.The rate of MDA increase was higher than that of the treated group.It was proved that the fresh ice treatment could exert the antibacterial and antioxidation of the compound preservative,inhibit the growth of microorganisms,improve the antioxidant capacity of fish,slow down the rate of protein degradation and prolong the shelf life.The antimicrobial mechanism of Ginkgo biloba leaves extracts?GBLE?against specific spoilage bacteria in large yellow croaker?Pseudosciaena crocea?,minimal inhibitory concentration?MIC?of GBLE against Shewanella putrefaciens and Saprophytic staphylococcus was measured by method of broth dilution combined with plate counting in this paper.The impact of GBLE on the growth of bacteria,the permeability of cell membrane and cell wall were also investigated by the methods,such as growth curve,content of alkaline phosphatase?AKP?,electrical conductivity and content of protein.The structure of the Shewanella putrefaciens and Saprophytic staphylococcus cells was observed by scanning electron microscope?SEM?.The results showed that the MIC of GBLE against the two kinds of microorganisms was both100mg/mL.The inhibitory rates of MIC and 2MIC concentration of GBLE against Shewanella putrefaciens were 36.11% and 100%,while27.78% and 62.22% for Saprophytic staphylococcus.The growth curve,results of alkaline phosphatase?AKP?and electrical conductivity content showed that growth of bacteria was inhibited and the permeability of cell membrane increased significantly when compared with untreated group.The results of SEM revealed that the structure of Shewanella putrefaciensand Saprophytic staphylococcus cells was destroyed and the limit of cells became blur,which suggested that the inhibition effect of GBLE against Shewanella putrefaciens and Saprophytic staphylococcus were related with the damage of cell membrane and cell wall.