Preparation Of Antifogging And Antibacterial Polyethylene Film And Its Application In Lettuce Preservation

The traditional plastic film packaging of fruits and vegetables will produce atomization phenomenon in the packaging film due to the metabolism of fruits and vegetables and the change of the external environment.This will not only reduce the visibility of fruits and vegetables products,but also affect the sales of products,and the water vapor generated by dropping on the surface of fruits and vegetables will accelerate the deterioration of fruits and vegetables products and shorten the shelf life of products.Antifogging and antibacterial packaging film combines antifogging function with antibacterial activity,which can not only effectively prevent fogging problem,but also inhibit the growth of microorganisms and improve the quality of products.In this study,low-density polyethylene(LDPE)was used as the matrix,tween 80(T-80)and coconutt diethanol amide(CDEA)were used as antifogging agents,and nano-ZnO was used as antibacterial agent to prepare blend films with antifogging and antibacterial functions.Firstly,the influence of different concentrations of SF on the basic properties of modified LDPE antifogging film was studied;Secondly,the effects of different concentrations of nano-ZnO on the properties of modified LDPE antibacterial and antifogging and antibacterial films were investigated;Then the fresh-keeping effect of different antifogging and antibacterial films on lettuce at 4 ℃was studied;Finally,the antibacterial effect and principle of antifogging and antibacterial film on Pseudomonas fluorescens,the dominant putrefactive bacteria in lettuce,were analyzed.The main research contents and results are as follows:(1)The antifogging films were prepared by extrusion casting.T-80 and CDEA were added to LDPE resin to obtain antifogging films with SF concentration of 0%,1%,3% and 5% respectively.The effects of different SF additions on the properties of the modified films were studied.The results showed that the films showed a slight yellow-trend with the addition of SF.There was physical interaction between SF and LDPE.SF was uniformly dispersed in LDPE matrix as a whole,but SF tended to aggregate in LDPE matrix with the increase of SF concentration.Excessive SF had negative effects on the mechanical and thermal properties of the films.The hydrophilicity of the modified films have been greatly improved.The antifogging films prepared in this study has good antifogging performance,and its antifogging duration increased with the increase of SF,where the antifogging duration of SF-5%film reached 216 h.(2)The antifogging and antibacterial films were prepared by extrusion casting.Nano-ZnO and SF(T-80,CDEA)were added to LDPE resin to obtain a series of antifogging and antibacterial films,and the microstructure and properties of the modified films were characterized.The results showed that the films became white and showed UV resistance after doped with nano-ZnO.The addition of SF had no obvious effect on the color of the films,and to some extent,it improved the ultraviolet resistance of the films.Nano-ZnO was uniformly dispersed in LDPE matrix,but nano-ZnO tended to aggregate with the increase of nano-ZnO content.However,the addition of SF improved the dispersion of nano-ZnO in LDPE matrix.With the incorporation of nano-ZnO and the increase of its content,the TS and Young’s modulus of the films first increased and then decreased,and the EAB,OP,WVP of the films continued to decrease.The addition of SF increased the TS,EAB and Young’s modulus of the film,so SF played a role in toughening and plasticizing.The addition of SF slightly reduced the gas barrier properties of the modified film.The addition of SF and nano-ZnO made the film have good antifogging and antibacterial properties.The antifogging duration of LDPE/SF/5% ZnO film was 166 h,and the bacteriostatic rate of E.coli and S.aureus reached 96.47% and 97.93%.(3)LDPE/SF/ZnO series blended films were used to preserve lettuce,and the changes of relevant preservation indexes during storage were investigated.The results showed that LDPE/SF/5% ZnO film had the best fresh-keeping effect on lettuce and the lettuce packaged by this film still had good quality after 15 days of storage.On this basis,the effect of LDPE/SF/5% ZnO film on the antioxidant activity of lettuce during storage was further studied.The results showed that the level of reactive oxygen species(ROS),the content of malondialdehyde(MDA)and the activities of polyphenol oxidase(PPO)and peroxidase(POD)of lettuce in LDPE/SF/5% ZnO film packaging group were lower than those in LDPE film packaging group.At the same time,LDPE/SF/5% ZnO film packaging maintained a high total phenol content and the activities of antioxidant enzyme in lettuce.This showed that LDPE/SF/5% ZnO film packaging could reduce the production and accumulation of ROS,inhibit the oxidation of phenolic compounds in tissues and delay the browning of lettuce by maintaining high antioxidant enzyme activity and reducing the activity of browning related enzymes.(4)In this chapter,the antibacterial effect and principle of LDPE/5% ZnO and LDPE/SF/5% ZnO film on the dominant putrefactive bacteria(Pseudomonas fluorescens)in lettuce were investigated.The results showed that LDPE/5% ZnO and LDPE/SF/5% ZnO films had obvious antibacterial effect against Pseudomonas fluorescens.Compared with the LDPE film packaging group,the cell wall,cell membrane and cell morphology of Pseudomonas fluorescens packaged by LDPE/5%ZnO and LDPE/SF/5% ZnO films were significantly damaged,nucleic acid and protein leakage were obvious,and the intracellular ROS level was significantly increased,resulting in a large number of cell death.At the same time,it was found that the antibacterial property of LDPE/SF/5% ZnO film was significantly better than that of LDPE/5% ZnO film,mainly because the initial Nano-ZnO content of LDPE/SF/5% ZnO film was higher,and more Nano-ZnO migrated out,which greatly improved the utilization of Nano-ZnO,thus improving the antibacterial property of the LDPE/SF/5% ZnO film.