Synthesis Of Nano-Silver From Cinnamomum Camphora Leaves And Its Preservation Of Nanfeng Tangerine

Fresh fruits and vegetables have high water content and high sugar content,which are prone to problems such as dehydration and microbial growth after harvesting.And the vigorous physiological metabolism of harvested vegetables will also lead to poor quality changes,resulting in huge waste of food resources and economic losses.Therefore,it is of great significance to properly preserve the fruits and vegetables after harvest.The composite antibacterial coating containing Ag nanoparticles(AgNPs)combines antibacterial,degradable and barrier properties,and is a promising way to preserve fruits and vegetables.The synthesis methods of AgNPs can be divided into physical method,chemical method and biological method according to the principle.Due to the high energy consumption and high cost of preparing AgNPs by physical methods,and the chemical synthesis methods involve reagents and reaction by-products that are toxic to the environment,a green synthesis method was explored to prepare AgNPs.In this study,the green synthesis of AgNPs was carried out by using the extract of Cinnamomum camphora leaves,and the physical and chemical properties of the AgNPs and their inhibitory effects on microorganisms were investigated.The natural biomacromolecule composite film loaded with AgNPs was prepared,and the preservation effect and safety of the film on Nanfeng tangerine were preliminarily evaluated,to provide a basis for the application of Ag NP in the field of food preservation.The contents of each chapter are as follows:In the first chapter,the biosynthesis of AgNPs and the application research progress of AgNPs in the preservation of fruits and vegetables were reviewed.In the second chapter,in order to solve the application limitation of AgNPs prepared by physical and chemical methods,a scheme for green synthesis of AgNPs based on Cinnamomum camphora leaves was designed,and the physicochemical properties and antibacterial properties of AgNPs were studied.Using the extract of Cinnamomum camphora leaves as reductant and stabilizer,the spherical or elliptical AgNPs particles with an average particle size of 19.57 nm were synthesized at room temperature.The AgNPs have a face-centered cubic lattice,good stability,and good antibacterial activity against Gram-positive bacteria[Gram(+)],Gram-negative bacteria[Gram(-)]and fungi.The Minimum inhibitory concentration(MIC)against Gram(-)was 3μg/m L.Through the analysis of the apparent morphology of bacterial,the leakage level of bacterial contents and the level of reactive oxygen species,the antibacterial mechanism of AgNPs on typical Gram(+)and Gram(-)was preliminarily explored.the results showed that:compared with the blank control group,the bacteria treated with AgNPs showed the following phenomena:The morphology of bacterial cells was damaged;Leakage of proteins and reducing sugars from bacteria;Increased levels of Reactive oxygen species(ROS).It was speculated that the antibacterial mechanism of the synthesized AgNPs on bacteria was that AgNPs acted on the surface of bacteria,causing damage to the bacteria and causing the contents to flow out;in addition,the bacteria were subjected to oxidative pressure from silver ions(Ag ion,Ag~+)produced by AgNPs,which increased the level of ROS in the bacteria.When the amount of AgNPs used exceeds the threshold that the bacteria can bear,the physiological balance in the bacteria will be broken,resulting in irreversible damage to the bacteria,or even death.In the third chapter,in order to expand the application range of natural macromolecular films,the silver-loaded composite antibacterial film was prepared by using Sodium alginate(SA)and Gelatin(GL)as the main materials,glycerol as the plasticizer and AgNPs as the antibacterial agent.With the mechanical property of the film as the main index,the ratio of each component of the film was optimized,and the best film ratio of the mechanical property was SA:GL=1:1,5%glycerol,20%100μg/m L AgNPs(v/v).The analysis of the film showed that the interaction between the components of the silver-loaded composite antibacterial film was dominated by hydrogen bonds(between biomacromolecules)and metal coordination bonds(between silver atoms and biomacromolecules),and the components were closely connected.The degradation rate of the silver-loaded composite antibacterial film can reach 70%at the temperature of 600℃,and the degradation rate of the silver-loaded composite antibacterial film can reach 40%after being buried under dry soil to simulate natural environment degradation for 15 days(d).In addition,due to the addition of AgNPs,the film showed a good inhibitory effect on Staphylococcus aureus(S.aureus)and Escherichia coli(E.coli).In the fourth chapter,in order to explore the feasibility of the degradable silver-loaded composite film for the preservation of fruits and vegetables,this study used the silver-loaded composite antibacterial film-forming liquid as the coating material,and analyzed the effect of the coating film on the preservation of Nanfeng tangerines.The migration of silver elements in the film into the tangerine was evaluated.The results showed that compared with the blank control group,the tangerines treated with the silver-loaded composite coating film had a longer shelf life,more glossy surfaces,less water loss,and more stable internal nutrients.In addition,the migration of silver elements was detected by inductively coupled plasma atomic emission spectrometry(ICP-AES).The experimental results show that no matter in the short-term(3 d)or long-term(15 d)storage period,the detected amount of silver elements in the tangerines treated with the silver-loaded composite antibacterial coating film is lower than the drinking water standard.The migration of silver elements during the period will not bring security risks.In the fifth chapter:the conclusions of this study are summarized,and its future research directions are prospected from the perspectives of the preparation and application scope of silver-loaded composite films.In conclusion,AgNPs with good microbial inhibitory effect can be synthesized by using the extract of camphor leaves,and the AgNPs can be loaded into the biomacromolecular film,which can improve the performance of the film.The silver-loaded composite antibacterial film-forming liquid is used as the coating material for the preservation of Nanfeng tangerines,which can prolong the shelf life of the tangerines.