Preparation Of Carvacrol/ε-polylysine Antibacterial Nanoemulsion And Its Application In Mango Preservation

In recent years,more and more attention has been paid to food safety,and more and more studies related to natural plant-derived antimicrobial agents have been conducted [1].As a natural and safe phytogenic antimicrobial active agent,carvacrol suffers from low bioavailability and poor water solubility,which restrict its bioactivity and thus reduce its practical application.ε-polylysine is also a natural preservative with broad-spectrum antimicrobial properties and safety,and is widely used in food products.The nanoemulsion delivery system can be used as a carrier for carvacrol andε-polylysine to guarantee their antimicrobial properties and bioactivity.Therefore,in this study,whey protein-based carvacrol/ε-polylysine nanoemulsions were prepared and used for mango preservation to lay the foundation for the development and utilization of this emulsion as a food preservative.The research of this paper is as follows:1.Carvacrol/ε-polylysine nanoemulsions(CA/ε-PL NEs)were prepared,and the optimal water-oil ratio of 95:5 in the loaded carvacrol nanoemulsions(CA-NEs)was derived to synthesize the optimal CA/ε-PL NEs.The ratios of various composition parameters of CA/ε-PL NEs: whey protein(WPI),ε-polylysine(ε-PL)and carvacrol were also optimized,and finally the average particle size(217 nm),polydispersity index(0.391)and zeta potential(+41%)of CA/ε-PL NEs were obtained when containing 0.3 wt% of WPI,0.5% v/v of carvacrol and 0.3 wt% of ε-PL The rheological properties of CA/ε-PL NEs were determined,and the results showed that CA/ε-PL NEs had typical non-Newtonian fluid properties;the microstructure of CA/ε-PL NEs was determined with the aid of laser confocal microscopy and transmission electron microscopy,and it was observed that the nanoemulsions consisted of protein-coated oil droplets forming droplets,and the droplets were adsorbed with ε-PL molecular layer was adsorbed on the outside of the droplet;the whiteness index of the nanoemulsion was further determined,which showed that CA/ε-PL NEs were bright white.Finally,by measuring the storage stability of the emulsions at different temperatures,it was shown that CA/ε-PL NEs were more stable than CA-NEs during storage.2.The antibacterial activity and cytotoxicity of carvacrol/ε-polylysine nanoemulsions were tested.The minimum inhibitory concentrations(MICs)of CA/ε-PL NEs against Staphylococcus aureus ATCC 29213 and Enterohemorrhagic Escherichia coli O157:H7 were determined,and the results showed that the MICs of CA/ε-PL NEs against both strains were 32 μg/m L.By measuring the growth curves of pure carvacrol,ε-PL,MCT emulsion and CA/ε-PL NEs against the two strains,the results showed that CA/ε-PL NEs could inhibit the growth of both strains better.The results showed that CA/ε-PL NEs could better inhibit the growth of both bacteria.The anti-biofilm activity of CA/ε-PL NEs against the two bacteria was further measured and microscopically observed by fluorescence microscopy,and the results showed that CA/ε-PL NEs at a concentration of MBIC(64 μg/m L)had a significant effect on the bacterial biofilm with a clearance rate of 65.9%(P < 0.05),and when the concentration of CA/ε-PL NEs reached 2 MBIC(128 μg/m L),the anti-biofilm activity of CA/ε-PL NEs was significantly reduced.(128 μg/m L),the clearance rate reached72.6%(P < 0.05),and the fragmentation of biofilm was obvious under fluorescence microscope observation.The results showed that the MIC and MFC of the nanoemulsion were 200 μg/m L and 400 μg/m L,respectively,and could inhibit the growth of Aspergillus niger hyphae well.Further,the toxic effects of the nanoemulsions on Hep G2 cells were measured,and the survival rate of Hep G2 cells reached 97% at the concentration of CA/ε-PL NEs of 512 μg/m L,which indicated that CA/ε-PL NEs were non-toxic to Hep G2 cells under the MIC of bacteria,MBIC of bacterial biofilm and MFC of mycobacteria.3.The changes in appearance and quality of mangoes soaked in CA/ε-PL NEs at sterile distilled water,parsley phenol solution,MIC and MFC concentrations,respectively,were investigated in a natural decay model.The results showed that at the end of 7 d of storage,the weight loss of mangoes in the blank control group reached 30.7% and the hardness decreased significantly to 27.4 g;the weight loss of the pure parsley phenol treated group reached 27.4% and the hardness decreased to 29g;the weight loss of the CA/ε-PL NEs treated group at MIC concentration was 14.3%and the hardness decreased to 32 g;while the weight loss of the CA/ε-PL NEs treated group at MFC concentration was 12.7%.The softening rate of the flesh was significantly lower than that of the blank control group,the pure carvacrol-treated group and the CA/ε-PL NEs-treated group at MIC concentration,with a weight loss rate of 12.7% and a hardness decrease to 39 g.Not only that,the mass fraction of titratable acid and ascorbic acid decreased and the mass fraction of soluble solids and malondialdehyde increased as the storage period of mangoes was prolonged,but the mass fraction of titratable acid and ascorbic acid in the CA/ε-PL NEs treatment group was higher than that in the blank control group and the group treated with pure parsley phenol,and the mass fraction of soluble solids and malondialdehyde was lower than that in the blank control group and the group treated with pure parsley phenol,and the MFC The best results were obtained for the CA/ε-PL NEs treatment group at the MFC concentration.Therefore,CA/ε-PL NEs were effective in preserving mangoes,improving water retention and maintaining the firmness of flesh,providing a good prospect for the actual preservation of mangoes.