No abstract

The antimicrobial activity of eugenol microemulsions (eugenol encapsulated in surfactant micelles) in ultrahigh-temperature pasteurized milk containing different percentages of milk fat (0, 2, and 4%) was investigated. Antimicrobial microemulsions were prepared from a 5% (wt) aqueous surfactant solution (Surfynol 485W) with 0.5% (wt) eugenol. Two strains each of Listeria monocytogenes and Escherichia coli O157:H7 previously shown to be the least and most resistant to the microemulsion in microbiological media were used to inoculate sterile milk (10(4) CFU/ml). Samples were withdrawn and plated at 0, 1, 3, 6, 12, and 24 h for enumeration. Microemulsions completely prevented growth of L. monocytogenes for up to 48 h in skim milk and reduced both strains of E. coli O157:H7 to less than detectable levels in less than 1 h. Similarly, in 2% fat milk, eugenol-Surfynol combinations reduced both strains of E. coli O157:H7 to less than detectable levels in less than 1 h but only increased the lag phase of both strains of L. monocytogenes. In full-fat milk (4% fat), microemulsions inhibited growth of the least resistant strains of L. monocytogenes and E. coli but were ineffective against the two resistant strains. Unencapsulated eugenol was slightly more or as inhibitory as microemulsions against target pathogens. Results were attributed to diffusional mass transport of antimicrobials from microemulsions to the macroemulsion (milk). Results suggest that food composition, especially fat level, may affect the efficiency of targeting of foodborne pathogens with surfactant-encapsulated antimicrobials.

Growth inhibition of four strains of Escherichia coli O157:H7 (H1730, F4546, 932, and E0019) and Listeria monocytogenes (Scott A, 101, 108, and 310) by essential oil components (carvacrol and eugenol) solubilized in nonionic surfactant micelles (Surfynol 465 and 485W) was investigated. Concentrations of encapsulated essential oil components ranged from 0.02 to 1.25% depending on compound, surfactant type, and surfactant concentration (0.5 to 5%). Eugenol encapsulated in Surfynol 485W micelles was most efficient in inhibiting growth of the pathogens; 1% Surfynol 485W and 0.15% eugenol was sufficient to inhibit growth of all strains of E. coli O157:H7 and three of four strains of L. monocytogenes (Scott A, 310, and 108). The fourth strain, L. monocytogenes 101, was inhibited by 2.5% Surfynol and 0.225% eugenol. One percent Surfynol 485W in combination with 0.025% carvacrol was effective in inhibiting three of four strains of E. coli O157:H7. Strain H1730 was the most resistant strain, requiring 0.3% carvacrol and 5% surfactant for complete inhibition. Growth inhibition of L. monocytogenes by combinations of carvacrol and Surfynol 465 ranged between 0.15 and 0.35% and 1 and 3.75%, respectively. Generally, the antimicrobial activity of Surfynol 465 in combination with eugenol was higher than that for the combination with carvacrol. The potent activity was attributed to increased solubility of essential oil components in the aqueous phase due to the presence of surfactants and improved interactions of antimicrobials with microorganisms.

Phytophenols were solubilized in nonionic surfactant micelles to form antimicrobially active and thermodynamically stable microemulsions. Formulation of phytophenols in microemulsions has previously been shown to improve their antimicrobial activity in model microbiological and food systems. Carvacrol and eugenol were incorporated in micellar solutions of two nonionic surfactants (Surfynol® 485W and Surfynol® 465) by mixing at room temperature. Particle size of formed microemulsions was determined by dynamic light scattering, and structural information about the mixed micellar system was obtained by nuclear magnetic resonance spectroscopy (NMR). Uptake of carvacrol and eugenol in surfactant micelles as determined by ultrasonic velocity measurements was very rapid, e.g., below the maximum additive concentration, the phytophenols were completely solubilized in the micelles in less than 30 min. Depending on the surfactant-phytophenol combination, the self-assembled surfactant-phytophenol aggregates had mean particle diameters between 3 and 17 nm. Elucidation of the structure of aggregates by 1H NMR studies indicated that micelles had a "bracket-like" structure with phytophenols being located inside the palisade layer of the micelle in direct contact with adjacent surfactant monomers. Encapsulation of phytophenols in surfactant micelles enables the incorporation of large amounts of hydrophobic antimicrobials in aqueous phases. Formulation of antimicrobial microemulsions may thus offer a means to deliver high concentrations of phytophenols to the bacterial surfaces of foodborne pathogens to affect kill.

Liposome entrapment may improve activity of protein or polypeptide antimicrobials against a variety of microorganisms. In this study, ability of liposomes to withstand exposure to environmental and chemical stresses typically encountered in foods and food processing operations were tested. Liposomes consisting of distearoylphosphatidylcholine (PC) and distearoylphosphatidylglycerol (PG), with 0, 5, or 10 μg/ml of the antimicrobial peptide nisin entrapped, were exposed to elevated temperatures (25-75°C) and a range of pH (5.5-11.0). Ability of liposomes to maintain integrity was assessed by measuring the encapsulation efficiency (EE), z-potential, and particle size distribution of liposomes. Distearoylphosphatidylcholine, PC/PG 8:2, and PC/PG 6:4 (mole fraction) liposomes retained between~70-90% EE despite exposure to elevated temperature and alkaline or acidic pH. Particle size of liposomes averaged between 100 and 240 nm depending on liposome preparation. Liposomal surface charge depended primarily on phospholipid composition and changed little with inclusion of nisin. Surface charge was not affected by temperature for PC and PC/PG 8:2 but decreased for PC/PG 6:4 liposomes. Our results suggest that liposomes containing nisin may be suitable for use as antimicrobial-active ingredients in low-or high-pH foods subjected to moderate heat treatments.