Nanodispersed eugenol has improved antimicrobial activity against Escherichia coli O157:H7 and Listeria monocytogenes in bovine milk

Abstract

There has been great interest in intervention strategies based on plant essential oils to control pathogens such as Escherichia coli O157:H7 and Listeria monocytogenes (Lm). However, the poor solubility of essential oils in water makes it difficult to disperse evenly in food matrices, impacting food quality and antimicrobial efficacy. In the present study, eugenol was dispersed in nanocapsules prepared with conjugates of whey protein isolate (WPI) and maltodextrin (MD, of various chain lengths). When eugenol was encapsulated in the conjugate made with MD40 at a WPI:MD mass ratio of 1:2, the nanodispersion was transparent and was characterized for antimicrobial efficacy against E. coli O157:H7 strains ATCC 43889 and 43894, and Lm strains Scott A and 101 in tryptic soy broth (TSB) and milk with different fat levels (whole, 2% reduced fat, and skim) at 35 or 32°C, with comparison to the same levels of free eugenol. In TSB, antimicrobial efficacy of nanodispersed eugenol against E. coli O157:H7 and Lm was not improved, with minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) values being 0.25g/L higher than those of free eugenol. Free eugenol performed better in TSB because there was no interfering compound and the MIC and MBC were below the solubility of eugenol. In milk, nanodispersed eugenol was consistently observed to be more effective than free eugenol, with MIC and MBC values above the solubility limit of eugenol. The nanodispersed eugenol was speculated to be evenly distributed in food matrices at concentrations above the solubility limit and supplied the antimicrobial locally when the binding caused eugenol level below the inhibition requirement. Nanodispersed eugenol thus provides a novel approach for incorporation in foods to improve antimicrobial efficacy without changing turbidity.