Salmonella Typhimurium Cold Plasma Inactivation And Strawberry Preservation Application

Plasma activated water(PAW)produced by cold plasma discharge at the gas-liquid interface contains a large amount of bactericidal active substances,which has huge application potentials in the sterilization and preservation of fruits and vegetables.This project took the common food-borne pathogenic bacteria—Salmonella typhimurium(S.typhimurium)as the research object,and used the reactive oxygen species(ROS)and reactive nitrogen species(RNS)as the starting point generated by dielectric barrier discharge(DBD)cold plasma.This research explored the influence of different treatment conditions of plasma on the bactericidal effect,revealed the mechanism of bacterial cell membrane oxidative damage,and further studied the effect of PAW on the sterilization and preservation of strawberries,which will provide the oretical foundation and technical support for the engineering application of sterilization and preservation.The main findingswere as follows:(1)Analysis of the inactivation effect of DBD plasma on the gas-liquid interface on S.typhimurium.The results showed that the inactivation effect of DBD plasma on S.typhimurium was positively correlated with the treatment time and the treatment voltage.When the input voltage was 40 V,the total number of colonies was reduced by 3.3 log CFU/mL after treatment for 5 min;when the input voltage was 50 V and the treatment time was 5 min,S.typhimurium was not detectable(the total number of surviving colonies was less than 2 log CFU/mL);when the input voltage was higher than 60 V,plasma treatment produced filamentary discharge at the gas-liquid interface,and S.typhimurium was undetectable after 2 min of treatment.A mathematical model was used to fit the inactivation curve of S.typhimurium with different input voltages,and it was found that the Weibull model had a good degree of fit(RMSE<0.15,R2>0.99)to the sterilization effect of 40 V and 50 V input voltage.(2)The mechanism of DBD plasma oxidative damage to S.typhimurium cell membrane.Comparing the changes of cell membrane integrity and permeability before and after treatment showed that DBD plasma treatment caused S.typhimurium cell membrane damage and leakage of cell contents;gas chromatography-mass spectrometer(GC-MS)experimental data showed that the ratio of unsaturated fatty acids of the cell membrane dropped from 0.310 to 0.089 after treatment.The cell membrane lipids were oxidized,and the malondialdehyde(MDA)content of plasma-treated S.typhimurium increased from 0.219 nmol/mL to 0.658 nmol/mL.The catalase(CAT)activity of S.typhimurium bacterial solution increased from 751 U/mL to 2542 U/mL,and the total superoxide dismutase(SOD)activity increased from 3.076 U/mL to 4.54 U/mL,which confirmed that DBD plasma treatment had the oxidative damage to the cell membrane of S.typhimurium.(3)Analysis of the physical and chemical properties of PAW produced by DBD plasma.The DBD plasma was directly applied to deionized water to prepare PAW.The pH of the PAW prepared by DBD plasma treatment for 1 min decreased to 3.32,and the conductivity increased from 24 μs/cm to 930 μs/cm,indicating that the plasma treatment was in the liquid phase.A large number of active particles were accumulated horizontally,and these active particles reacted with water molecules to produce various acid radical ions such as NO2-and NO3-.When the treatment time was 5min,the concentration of H2O2 rose from 0 μmol/L to 5372.69 μmol/L,which showed that the PAW has both acidic and strong oxidizing properties,and has a theoretical basis for sterilization and preservation of fruits and vegetables.(4)The S.typhimurium removal effect of PAW treatment on the surface of strawberry and its influence on the quality of the strawberry itself.PAW had a significant inactivation effect on S.typhimurium on the surface of strawberries,and it reduced S.typhimurium on the surface of strawberries by about 2 log CFU/g,and the killing rate reached 99%,indicating that PAW could effectively remove the S.typhimurium on the surface of strawberries,and the surface S.typhimurium of the strawberry did not grow at 4℃ after 6 days storage.In addition,the multivariate statistical analysis showed that there were no significant changes in the main sensory and nutritional indicators of the strawberries before and after the PAW treatment,indicating that the PAW treatment would not significantly damage the quality of the strawberry itself.On the contrary,PAW treatment delayed the degradation of strawberry quality during the storage period.The PAW 2 treatment group showed best preservation effect,which confirmed the broad applications of PAW in the preservation of fruits and vegetables.