Encapsulation of combined natural antimicrobial compounds for improving food safety

Abstract

The aim of this study was to improve the antimicrobial activity and stability of natural antimicrobials by combination and encapsulation technology. The minimum inhibitory concentrations (MICs) of natural antimicrobials were initially screened against two Gram-positive bacteria (Listeria monocytogenes and Staphylococcus aureus) and two Gram-negative bacteria (Escherichia coli and Salmonella typhimurium). Combinations of natural antimicrobials were then investigated using the checkerboard method to calculate fractional inhibitory concentration (FIC) indices. The clove oil mixture with thymol (ClTh) and the clove oil mixture with α-tocopherol (ClTo) had additive effects. The ClTh-loaded nanoparticles (ClTh NPs) and ClTo-loaded nanoparticles (ClTo NPs) were prepared by ionic gelation using chitosan and poly-γ-glutamic acid (γ-PGA). The characteristics of the nanoparticles (NPs), including the particle size, polydispersity index (PDI), derived count rate, and zeta-potential were investigated. In addition, the effects of combination and encapsulation of ClTh NPs and ClTo NPs on antimicrobial activity were demonstrated by time-kill assay. The time-kill results clearly demonstrated the improved antimicrobial activity and stability of ClTh NPs. Finally, antimicrobial activities of ClTh NPs against L. monocytogenes and E. coli in the fresh-cut apples and minced beef were demonstrated. The results revealed that ClTh NPs were able to more effectively inhibit the growth of the tested pathogens than free ClTh during the longer storage time. This study suggests the potential of ClTh NPs as a natural preservative in food.