Study On The Effect Of Cold Plasma On The Browning Inhibition Mechanism And Polyphenol Oxidase Properties Of Agaricus Bisporus

Within recent years,as one of the most efficient and economical methods,thermal processing,is widely used to inactivate enzymes.However,under many processing conditions,products often lose natural flavor and some of their nutritional value.Atmospheric cold plasma（ACP）,a promising nonthermal preservation technology,has been extensively applied to foodborne pathogens elimination,toxin removal,food packaging modification and so on.ACP possess several advantages,such as inexpensive operation costs,short treatment times at low temperatures and free of chemicals.It is so attractive that cold plasma can trigger conformation changes and inactivate undesirable enzymes without major quality loss.It enables this nonthermal technology a potential alternative to others.The undesirable enzymatic browning would significantly decrease organoleptic and nutritional quality of food,which would reduce the acceptance of consumers,thus lead to serious loss of market value.Little work has been done on button mushroom（Agaricus bisporus）PPO inactivation after ACP treatment.The scarcity of information concerning PPO inactivation promotes the investigations of the present work,which further complicates the scaling up of cold plasma on food processing.The main conlusions are as follows:（1）The effect of ACP on the activity and kinetic changes of mushroom（Agaricus bisporus）polyphenol oxidase（PPO）was evaluated with catechol as the substrate.Results demonstrated that the dielectric barrier discharge（DBD）based plasma technology could inactivate PPO（up to 69%）at 50 k V with the increased concentrations of H₂O₂ and NO_x.The results of K_m indicated that the affinity of PPO for the catechol substrate decreased with increasing treatment time.The catalytic efficiency(V_max/K_m ratio)was considered to evaluate the substrate specificity.The ratios were calculated to be 46.51,18.53,5.05,3.19,1.41 and 1.07 U/min/m M,respectively,for native PPO and that after different treatment time（0,2,4,6,8 and 10min）.It was inferred that ACP treatment might affect the binding of the enzyme to the substrate of catechol.（2）This study investigated the structural changes to mushroom（Agaricus bisporus）polyphenol oxidase affected with cold atmospheric plasma reactive species.In Native-PAGE images,the color of the PPO active staining bands gradually became lighter with increasing treatment time,and the staining bands almost completely disappeared after treated for 10 min at 50 k V.After ACP treatment for 10 min,the content of the vast majority of amino acids decreased,except for proline（Pro）and cysteine（Cys）.An obvious enhancement of surface hydrophobicity was observed,whereas a gradual reduction of total sulfhydryl content was recorded with the increasing exposure time.Data from circular dichroism,atomic force microscopy,particle size distribution and fluorescence spectra displayed the rearrangement of secondary structure and disruption of the tertiary structure.Red shifts of fluorescence spectra showed positive correlations with the inactivation rate of PPO.Therefore,ACP treatment could be served as an alternative approach to inactivate undesirable enzymes to minimize the loss of food nutrition and quality.（3）Using Agaricus bisporus as experimental material,after treating with atmosphere cold plasma（ACP）directly and indirectly,the color change,hardness and,p H of the direct treatment（DBD）group,plasma-activated water（PAW）group,pure water（PW）group,and control group were determined.The results showed that the p H of pure water decreased from 5.90±0.03 to 5.16±0.03 after cold plasma treatment,and the temperature increased significantly（p<0.05）.During the storage period,the browning index（BI）and（?）E value were the lowest in PAW group and maintained the best hardness and sensory evaluation.Neither the p H nor water activity changed significantly during the storage period in all groups.The polyphenol oxidase（PPO）activity decreased faster after ACP treatment.It is speculated that indirect treatment with ACP will inhibit PPO activity and delay mushroom browning.