Physicochemical and Antimicrobial Properties of Antimicrobials Encapsulated in Surfactant-based Nanoparticles

Phytophenols, components of essential oil extracts, are antimicrobials that may inhibit the growth of foodborne pathogens. However, their antimicrobial activity in food systems is low because of low water solubility and association with other food components. Incorporation of phytophenols in nanoscale surfactant micelles may offer a potential solution to increase solubility and improve activity in food systems. The objectives of this study were to determine the stability and physicochemical and antimicrobial properties of micellar antimicrobial-surfactant systems.

Carvacrol and eugenol containing micelles were prepared by dispersing Surfynol^® 485W and Surfynol^® 465 in water at room temperature. Stability was determined by UV-visible spectroscopy, particle size was determined by dynamic light scattering and, structural information about the mixed micellar systems was obtained by nuclear magnetic resonance spectroscopy (NMR). The antimicrobial activity was determined using a microbroth dilution assay.

Incorporation of carvacrol and eugenol to surfactant-based nanoparticles was very rapid. Depending on the surfactant-antimicrobial combination, the particles produced varied in size between 5 to 20 nm. The phytophenolic-based nanoparticles were effective against Escherichia coli O157:H7 and Listeria monocytogenes at low surfactant and essential oil component concentrations, i.e., 1 % of 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). A fourth strain, L. monocytogenes 101 was inhibited with 2.5 % Surfynol^® and 0.225 % eugenol. 1 % 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 requiring 0.3 % of carvacrol and 5 % of surfactant for complete inhibition. Growth inhibition of L. monocytogenes by combinations of carvacrol and Surfynol^® 465 varied between 0.15 and 0.35 % and 1 and 3.75 % , respectively. Nanocapsules composed of Surfynol^® 485W were generally more inhibitory than those containing Surfynol^® 465. E. coli O157:H7 was more sensitive to the antimicrobial system than L. monocytogenes.

All nanoparticles were stable over a wide range of pHs; however, temperature and essential oil component concentration influenced the stability of the mixed micelles, i.e., micelles destabilized at lower temperatures with increasing encapsulated essential oil component concentration. For example, 0.9 % of eugenol encapsulated in Surfynol^® 485W exhibited turbidity (cloud point) at 55°C, while at 0.5 %, 70 °C was required to reach the cloud point. At temperatures optimal for microbial growth, micelles were stable and retained activity.

The study indicated that encapsulation of essential oil components in surfactant-based nanoparticles offers an excellent means to incorporate large amounts of these compounds in an aqueous system and thereby increase antimicrobial activity.