Mechanisms Of Salmonella Enteritidis And Listeria Monocytogenes Inactivation And Application In Tilapia Of Cold Plasma

As a new non-thermal sterilization technology,cold plasma has been drawing more and more attention because of its simple operation and effective sterilization at low temperature.Foodborne pathogens have strong pathogenicity,and contaminated food is easy to cause foodborne diseases,causing great damage to human health and economic property.Therefore,reducing microbial contamination is an important step in food production,especially for foods that are not suitable for thermal sterilization.It is very important to research the non-thermal sterilization technology to improve the safety of food.To study the bactericidal effect of S.enteritidis and L.monocytogenes by different plasma treatment time and voltage,to explore the mechanism of the difference of the effects of cold plasma on the two different bacterial.The effects of the change of treatment time on sterilization effect and quality of tilapia were studied,and the changes of the microbial growth of tilapia during cold storage were studied,which provided experimental basis and theoretical guidance for the application of cold plasma in food industry.（1）The effects of different treatment voltages and treatment time on the bactericidal effect were studied with S.enteritidis and L.monocytogenes;the activities of membrane-bound aerobic respiratory enzymes,the contents of reactive species were measured immediately after cold plasma treatment;the relationships among these parameters were assessed by Pearson’s correlation coefficient analysis.Colony numbers of the two bacterial species decreased with increasing voltage and treatment time.When samples were treated at80 k V for 180 s,numbers of S.enteritidis and L.monocytogenes decreased by 1.82 and 0.75log CFU/m L,respectively.The bactericidal effect of cold plasma on S.enteritidis was better than that of L.monocytogenes.The activities of five membrane-bound aerobic respiratory enzymes（Na⁺/K⁺-ATPase,succinate dehydrogenase,malate dehydrogenase,pyruvate kinase and hexokinase）were significantly decreased after cold plasma treatment.After 80k V cold plasma treatment for 180 s,the activities of succinate dehydrogenase（S.enteritidis）and pyruvate kinase（L.monocytogenes）decreased most significantly,which decreased to undetectable.The content of bactericidal reactive species（·OH,H₂O₂,¹O₂,NO_x,O₃）increased with the increase of treatment voltage and time.The bactericidal effects of plasma on S.enteritidis and L.monocytogenes were most strongly correlated with·OH and H₂O₂,respectively.（2）To study the effects of cold plasma treatment time on the microbiological and quality of peeled tilapia fillet.The fish fillets were treated at 70k V for 0,60,120,180,240and 300 s after sealed in a tray,and the changes of surface colonies number were measured to analyze the bactericidal effect.The colonies number of fillets were decreased as the increase of treatment time.When samples were treated for 300 s,S.enteritidis and L.monocytogenes were reduced by 2.34 and 1.69 log CFU/g,respectively;while the reduction of S.enteritidis and L.monocytogenes of samples inoculated with bacteria mixture was 1.84and 1.36 log CFU/g,respectively.With the treatment time extending,the hardness of fish fillet decreased,and the elasticity was not changed.By low-field NMR analysis,no significant change was found in the content of combined water compared with control,and the content of non-flowing water was decreased slightly,while the content of free water was increased significantly.The total chromatism of fish increased with the increase of treatment time.The TBARS value increased from 0.13 MDA eq/kg to 1.83 MDA eq/kg which treated for 300 s,still within acceptable limits.Sarcoplasmic protein of tilapia was slightly oxidized after treatment,but myofibrillar protein had no significant change.（3）To study the changes of main microflora（total viable count,Psychrophilic bacteria count,Pseudomonas spp.count,Lactobacillus count,Enterobacteriaceae and H2S-producing bacteria count）and microbial diversity of tilapia fillets during storage at 4℃,after tilapia fillets were treated with cold plasma for different time.Different plasma treatment time had great antibacterial effect on the total viable count,Psychrophilic bacteria count,Pseudomonas spp.count,Lactobacillus count,Enterobacteriaceae and H2S-producing bacteria count.The treatment group of 300 s had the best bactericidal effect,and the growth rate of bacteria was the slowest.High-throughput sequencing results showed that the microbial diversity of cold plasma treated tilapia fillets fluctuated with the increase of storage time,species richness decreased from 0 to 2 d,then increased to the peak from 3 to9 d,and then decreased again.The dominant bacteria in storage for 0 days were Acinetobacter,Macrococcus,Pseudomonas and Lactococcus.The abundance of Acinetobacter and Macrococcus decreased gradually in the stage of storage,while the abundance of Lactococcus increased after 3 days and then decreased gradually.After 15 days of cold storage,the dominant bacteria were transformed into Pseudomonas,Arthrobacter and Kurthia.