| Since food is easily contaminated by various pathogenic microorganisms,which leads to spoilage,nanomaterials have attracted more and more attention as a new type of food packaging antibacterial material.Silver nanoparticles are an inorganic antibacterial agent with good effect and low toxicity.As a long-acting antibacterial agent,they can be easily added to polymers to prepare composite active packaging.However,because silver nanoparticles are prone to agglomeration and oxidation during the extrusion-casting thermal processing of packaging material,their application in food active packaging has been restricted to a certain extent.Therefore,in this study,chitosan-gelatin antibacterial microcapsules modified with green-synthesized silver nanoparticles were prepared by complex coacervation and spray drying and applied to polylactic acid(PLA)food active packaging.Firstly,the core material of oily silver nanoparticles was prepared by using Ginkgo biloba essential oil as reducing agent by green synthesis method,and chitosan-gelatin antibacterial microcapsules were prepared by complex coacervation and spray drying method.Then PLA containing chitosan-gelatin antibacterial microcapsules modified with green-synthesized silver nanoparticles were prepared by extrusion-casting.Finally,the feasibility of the films in the field of food active packaging was investigated,and it was applied to the preservation of grass carp fillets.The main findings are as follows:(1)The fourier transform infrared spectroscopy(FTIR)confirmed that the phenols and flavonoids in Ginkgo biloba essential oil successfully reduced silver nanoparticles.UV-visible spectrophotometry(UV-vis)showed that the surface plasmon resonance of the silver nanoparticles produced a peak centered near 436 nm,implying the transformation of Ag~+into Ag~0.The fluorescence spectrophotometer(PL)found strong fluorescence emission peaks at 280 nm and 557 nm when excited by 280 nm of silver nanoparticles.The transmission electron microscope(TEM)images of silver nanoparticles showed a typical spherical and ellipsoidal morphology,and the average particle size was about 17 nm.The sample’s X-ray photoelectron spectroscopy(XPS)Ag 3d of this sample showed two individual peaks at 368.20 and 374.20 e V with a spin-orbit separation of 6 e V,showing the typical XPS spectra of silver nanoparticles.(2)Chitosan-gelatin antibacterial microcapsules were prepared by complex coacervation and spray drying.FTIR spectra showed interactions between chitosan and type B gelatin in microcapsules A(chitosan solution,type B gelatin solution and surfactant Triton X-100),microcapsules B(chitosan solution,type B gelatin solution,Ginkgo biloba essential oil and surfactant Triton X-100)and microcapsules C(chitosan solution,type B gelatin solution,silver nitrate solution,Ginkgo biloba essential oil,and surfactant Triton X-100).The components of Ginkgo biloba essential oil were present in microcapsules B and C.The microcapsules A showed an irregular appearance and uneven size distribution,the surface of microcapsules B became smoother than that of microcapsules A,and microcapsules C exhibited a partially wrinkled and porous structure that can be attributed to the presence of silver nanoparticles.Moreover,the average sizes of microcapsules A,B and C were approximately 1.75,2.17,and 2.25μm,respectively.The energy dispersive X-ray spectrometer(EDX)and XPS spectroscopy of microcapsules C showed that the chitosan-gelatin antibacterial microcapsules(microcapsules C)modified by green synthetic silver nanoparticles were successfully prepared,and the silver nanoparticles were uniformly distributed throughout the microcapsules.All microcapsules(A-C)showed no obvious weight loss below 165°C by thermal stability,indicating that microcapsules A,B and C could all be blended with PLA resin to form films at temperatures below 165°C.(3)The thickness of the four films prepared by extrusion-casting method is 50±3μm with PLA resin as matrix.After adding 2%microcapsules A,B and C(corresponding to PLA-A,PLA-B and PLA-C,respectively),the transparency and tensile properties of the pure PLA films were significantly reduced.The water vapor permeability(WVP)and the water vapor permeability coefficient(WVP coefficient)(p<0.05)of PLA-A were significantly increased relative to pure PLA films.The cross-sectional microscopic morphology showed that the addition of microcapsules(A-C)made the pure PLA films some cracks and pores,which can also explain the decrease in tensile strength.By treating Escherichia coli and Staphylococcus aureus with 6 mm different film discs,and then using thermal field emission scanning electron microscope(TFE-SEM)to observe the microscopic morphological changes of the bacteria,the antibacterial properties of the films against food-borne pathogens were investigated.The results of the antibacterial activity of the films against food-borne pathogens(Escherichia coli and Staphylococcus aureus)were PLA-C>PLA-B>PLA-A>control films.(4)PLA-A,PLA-B,PLA-C and control films(corresponding to A,B,C and control group,respectively)were applied to the fresh-keeping experiment of grass carp fillets,and their effects were investigated.The juice loss rate(JLR)of experimental group D during storage was lower than that of other groups(p>0.05).With the extension of storage time,all groups showed an increase in the value of total volatile basic nitrogen(TVB-N).After the 6th day,the TVB-N values of the grass carp fillet samples encapsulated in PLA-C were significantly lower than that of the other groups during storage for 6-10 days(p<0.05).The total viable counts(TVC)in all groups increased at different rates with the increase in storage time.The control group and group A reached the upper limit of 7.0 log CFU/g after 5.42 days and 5.57 days,respectively.However,both groups B and C did not exceed the limit on day 6,which were 6.70 and 5.25 log CFU/g,respectively,and the TVC in group C was significantly lower than that in group B(p<0.05).In addition,group C only exceeded the 7.0 log CFU/g limit in 7.69 days.These results showed that PLA films(PLA-C)containing chitosan-gelatin antibacterial microcapsules modified with green-synthesized silver nanoparticles can extend the shelf life of grass carp fillets by 2-3 days compared with the control group(pure PLA films encapsulation). |
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