Microorganisms And Polyphenol Oxidase Inactivation In Fresh-cut Apple By Dielectric Barrier Discharge Plasma And The Mechanisms For Its Action

Cold plasma is an innovative non-thermal food processing technology,which has broad application prospect in the food processing and safety control fields for its various advantages such as high efficiency,low processing temperature,no pollution and no residue.However,the research on the effect and mechanism of cold plasma on fresh-cut fruits and vegetables are limited,which restricts its practical application.This study aimed to investigate the inactivation of microorganisms and polyphenol oxidase(PPO)of the fresh-cut apples by dielectric barrier discharge(DBD)plasma,and evaluate its influences on the quality of the tested samples.In addition,the underlying mechanisms of DBD plasma-induced inactivation of microorganism and enzyme were elucidated in this research.The obtained results provide theoretical basis for the application of cold plasma technology in fresh-cut fruits and vegetables.The main results are as follows:(1)Effects of DBD plasma treatment on microorganisms,enzyme inactivation and quality of fresh-cut apples.DBD plasma could significantly inactivate the microorganism and PPO of the fresh-cut apples,and the inactivation effect were enhanced with the increase of treatment time and discharge power.Compared with the control group,the populations of Listeria monocytogenes(L.monocytogenes)and Escherichia coli O157:H7(E.coli O157:H7),two common foodborne bacteria,on fresh-cut apples surface decreased by 2.43 and 2.68 log₁₀CFU/g after 120 s treatment with DBD plasma at 32.6 W,respectively.Compared with the control group,PPO activity of fresh-cut apples decreased by 41.7%after DBD plasma treatment at 20.8 W for 180 s.Meanwhile,no significant changes were observed in the firmness,titratable acid,soluble solids,vitamin C,total polyphenol,total flavonoid and antioxidant capacity of fresh-cut apples treated with DBD plasma treatment(p<0.05).(2)Inactivation effects and mechanisms of L.monocytogenes by DBD plasma.The results showed that the populations of L.monocytogenes decreased significantly after DBD plasma treatment.Compared with the control group,the populations of L.monocytogenes decreased from 8.26 log₁₀ CFU/g to 1.30 log₁₀ CFU/g after DBD plasma treatment for 80 s(p<0.05).After DBD plasma treatment at 20.8 W for 80 s,L.monocytogenes cell morphology was damaged,and the integrity of cell membrane was destroyed,the membrane potential levels and intracellular reactive oxygen species levels were dramatically increased.These results suggest that DBD plasma may inactivate L.monocytogenes by disrupting cell membrane and inducing oxidative stress.(3)Kinetics and underlying mechanisms of PPO inactivation induced by DBD plasma.DBD plasma could significantly inactivate PPO in solution,and the inactivation effect was heightened with the increasing treatment time and discharge power.After DBD plasma treatment(32.6W)for 3 min,PPO activity decreased by 94.2%(p<0.05).The PPO inactivation curves were well fitted by the Weibull and Logistic models.Circular dichroism and intrinsic fluorescence spectra analysis indicated that DBD plasma caused the disruption of its second and tertiary structures of PPO.DBD plasma also induced the oxidative degradation and carbonylation of PPO.In addition,After DBD plasma exposure at 26.4 W for 3 min,the surface hydrophobicity and sulfhydryl content of PPO decreased by 60.9%and 31.2%,respectively(p<0.05).In summary,DBD plasma induced significant changes in the structure and induced oxidative degradation and carbonylation of PPO,which might be responsible for the loss of its enzymatic activity.