Mechanism Of Cold Plasma Restraining Enzymatic Browning And Application In Fresh Apple Juice

Polyphenol oxidase(PPO)and peroxidase(POD)are widely existed in fruits and vegetables,they can catalyze the oxidation of various phenolic substances,producing black or brown pigments,and destroying the sensory and the nutritional quality of foods.As an emerging non-thermal technology for food processing,cold plasma has been gradually applied in the food sterilization.However,mechanisms of cold plasma in inactivating PPO,POD and inhibiting the enzymatic browning of fruits and vegetables are still not fully understood.Therefore,this research is concentrated on the mechanism of cold plasma on the activity and conformation of PPO and POD from the view of plasma physics,food enzymology and molecular biology technology.In particular,inactivation kinetic dynamics,conformational analysis and application of fresh apple juice were presented.This study would provide technical support for the inactivation of PPO and POD as well as the control of enzymatic browning with cold plasma technique in apple juice processing.The main conclusions are shown as follows:(1)Characterization analysis of reactive oxidation effect of cold plasma.The variation of p H and conductivity as well as the optical emission spectrum indicated that a number of reactive oxygen species(ROS)and reactive nitrogen species(RNS)were generated from cold plasma.(2)Effects of cold plasma on the activity of PPO and POD.Activity of PPO and POD can be reduced by ROS and RNS effectively.Weibull model was proved as the most suitable dynamic model to illustrate the inhibitory effect of PPO with dielectric barrier discharge(DBD)plasma.(3)Effects of cold plasma on the conformation of PPO and POD.Circular dichroism showed that secondary structures of PPO and POD were destroyed by active species,namely,the ?-helix structures were reduced,and the ?-sheet structures were increased.Results of UVVis spectra and intrinsic fluorescence emission spectra suggested that the unfolding of POD induced by plasma species,and the hydrophilicity of the microenvironment around aromatic amino acids risiduals was enhanced.In addition,the degradation of heme in the active site of POD was also tested.The synchronous fluorescence spectrum indicated that the fluorescence intensity of PPO was quenched by plasma,and the active species tend to attack tryptophan residuals.3D fluorescence spectroscopy further revealed the polypeptide chain skeleton of PPO were destroyed by plasma.AFM analysis showed that POD molecules were depolymerized at first and then repolymerized by micro-plasma jet,whereas PPO molecules were redepolymerized firstly and then depolymerized by DBD plasma,indicating the different mechanisms of cold plasma on PPO and POD at the molecular level.The aforementioned results indicated that plasma can reduce the activity of PPO and POD by destroying their conformation.(4)Application of cold plasma pretreatment in maintaining the quality of fresh apple juice.The browning of apple juice can be retarded obviously when prolonging the plasma treatment time(130 s and 170 s).The total phenol content and antioxidant activity of apple juice were enhanced,and slight changes in soluble solid content were shown.The declination of p H and increase of titratable acid were tested as well.Pearson correlation analysis showed that the inhibition of enzymatic browning positively correlated with the improvement of the sensory and nutritional quality of apple juice,suggesting cold plasma is a promising,practical and efficiency technology in inhibiting enzymatic browning and maintaining the quality of apple juice.