Green Synthesis Of Nano Zinc Oxide And Its Application In Coating Preservation And Evaluation Of Zn Migration

Zinc oxide nanoparticles(ZnO NPs)are widely introduced into food packaging materials due to their good UV-blocking properties and broad-spectrum antimicrobial activity.ZnO NPs can inhibit the growth of microorganisms on the food surface by contacting in food or forming coatings on the food surface,so as to reduce spoilage and prolong shelf life.At present,the synthesis of ZnO NPs is still based on traditional physical and chemical methods,but these methods are difficult to mass-produce and often involve the use of toxic reagents,resulting in environmental issues and toxic residues.Therefore,green synthesis of ZnO NPs using extracts of non-toxic natural plants instead of toxic chemical reagents has attracted more attention in recent years.Citrus is one of the most productive fruits in the world.Citrus peel,as the main by-product of citrus,is rich in phenolic acid,flavonoids,ascorbic acid and other antioxidant substances,which can be used as stabilizer and dispersant for green synthesis of ZnO NPs.In addition,many studies have confirmed the migration of nanoparticles from composite packaging film in food or food simulation solution,but whether the nanoparticles in the nano composite coatings for postharvest fruits and vegetables preservation will migrate on the surface of fruits and vegetables has not been studied.Based on the above background,in this paper,ZnO NPs synthesised by orange peel extract(SNP)first,and compared the structure and performance with commercial nanoparticles(CNP).Then ZnO NPs composited with carboxymethylcellulose sodium(CMC)was used for coating preservation of postharvest tangerine.The concentration of Zn in the peels and pulp of the coated fruit was measured to preliminarily explore whether the Zn in the coatings migrated,and the effect of the composite coatings on the storage quality and antioxidant enzyme activity of the fruit was also studied.The main research contents and results are as follows:(1)Green preparation and characterization of ZnO NPs.SNP particles were prepared from orange peel extract and zinc nitrate solution.The crystal structure,morphology,particle size distribution,composition and optical properties of SNP and CNP were characterized by XRD,FT-IR and TEM.The results showed that there was almost no impurity in SNP,and it had a crystal structure similar to CNP,but the crystallinity of SNP was higher,and the particle size was about 33.1 ± 11.7 nm.(2)The antibacterial activity and cytotoxicity of SNP and CNP were compared.The minimum inhibitory concentration(MIC)and minimum bactericidal concentration(MBC)of SNP and CNP against two common foodborne pathogens(Escherichia coli and Staphylococcus aureus)were determined.The antifungal activity of SNP and CNP against Botrytis cinerea was compared by growth rate method,and the cytotoxicity against human umbilical vein endothelial cells(HUVECs)was measured.The results showed that the inhibition rate of CNP to Botrytis cinerea was higher than that of SNP when the concentration was lower than 400 mg/L,and the cytotoxicity of SNP against HUVECs was similar to that of CNP when the concentration was lower than 25 mg/L,and the MIC of SNP and CNP to Escherichia coli and Staphylococcus aureus was similar,but the MBC of SNP to Escherichia coli was lower than that of CNP.(3)Migration behavior of Zn in ZnO NPs/CMC composite coatings.ZnO NPs with different concentrations(3%,6% and 10%)and different particles sizes(17,50 and 90 nm)were coated on the surface of tangerine fruits respectively with CMC,and the fruits were stored at different temperatures(4,12 and 20℃).The variation of Zn concentration in the peel and pulp of each group of fruits was detected by atomic absorption spectrum(AAS),so as to explore the migration behavior of Zn from the coatings to the fruits.The results showed that the Zn content in the tangerine peels of groups treated by composite coatings increased rapidly within 8 h after coating,and the maximum value reached to 15.05 mg/(kg,dry weight),which indicated that Zn element in the coatings did migrate to the fruit.In the following 1～13 d,the Zn concentration in the peels of each group increased slowly.The Zn concentration in the peels of the fruits stored at 12℃ and 20℃ had no significant difference,but they were significantly higher than that of 4℃;the particle size had little effect on the Zn concentration in the peels;and the migration of Zn increased with the increasing of the concentration of ZnO NPs.During storage,there was no significant difference of Zn concentration in the pulp between the composite coating groups and the control group,indicating that the migration of Zn was limited to the peels and not to the pulp.Therefore,nanocomposite coatings on citrus have a great safety.(4)The effect of ZnO NPs/CMC composite coatings on storage quality and antioxidant enzyme activity of postharvest tangerine.The treatment of ZnO/CMC with the highest Zn concentration in peels(ZnO NPs with the concentration of 10% and the particle size of 17 nm,and stored at 20℃)was chosen to test the the storage quality,antioxidant enzyme activity and total antioxidant capacity of tangerine fruits.The results showed that compared with CK group,CMC and ZnO/CMC coatings could significantly reduce the rotting rate of tangerine in the later stage of storage.Compared with CMC group,ZnO/CMC group had a lower rotting rate from the 7th to 9th day,indicating that ZnO NPs had a limited inhibition effect on the rotting rate of tangerine.However,in the ZnO/CMC group,the total soluble solids(TSS)and ascorbic acid(AA)content were higher at the later stage of storage,and the decline of titratable acid(TA)content was delayed.The coatings could enhance the activities of peroxidase(POD),superoxide dismutase(SOD),phenylalanine ammonia lyase(PAL)and the total antioxidant activity(T-AOC)in the peels,but the migration of Zn from the composite coatings had little effect on the defense system of the peels.