Combinations of multiple natural antimicrobials with different mechanisms as an approach to control <i>Listeria monocytogenes</i>

To improve food safety and shelflife requires the use of preservation processes, such as physical (heat, refrigeration) or chemical (antimicrobial addition) processes. Regulatory approved synthetic food antimicrobials (preservatives) have some uses but are very limited in their spectrum of activity. Thus, alternatives are needed to conventional chemical antimicrobials. One method is to use naturally occurring antimicrobials, especially those found in spices and herbs, essential oils (EO) and essential oil components (EOC). EOs have been shown to have antimicrobial activity but the activity is highly variable. Finding a combination of EOs, EOCs, or other natural antimicrobials that act synergistically would allow a reduction in the use concentration. This is important for EO as they may contribute undesirable sensory effects to foods. To achieve synergistic interaction of antimicrobials likely requires that individual compounds have different mechanisms of inhibition or inactivation. Therefore, the objective of this study was to attempt to achieve synergistic antimicrobial interactions and reduce use concentrations by combining EOs and a naturally occurring hydroxycinnamic acid with reported different mechanisms. Oregano essential oil (OEO), basil essential oil (BEO), coriander essential oil (CEO), and ferulic acid (FA) were evaluated alone and in combination against Listeria monocytogenes at pH 6.0 and 25°C for 48h. A broth dilution assay was used to determine the minimum inhibitory concentrations (MIC) of individual and combined antimicrobials. Fractional inhibitory concentrations (FIC) were calculated and the interactions interpreted as synergistic (FIC ≥0.5), additive (FIC >0.5 and L. monocytogenes Scott Awere 250 ppm (parts per million) OEO, 2500 ppm CEO, 7500 ppm BEO, and 5000 ppm FA. Combinations of OEO+BEO, CEO+BEO, CEO+FA, BEO+FA, OEO+CEO+BEO, BEO+CEO+FA, and OEO+BEO+FA, and OEO+CEO+BEO+FA resulted in synergistic inhibition of L. monocytogenes (FIC ≤ 0.5). The quaternary combination of OEO+CEO+BEO+FA was inhibitory at 31.25, 312.5, 937.5, and 625 ppm, respectively. Combining natural antimicrobials with suggested different mechanisms may be a solution for controlling foodborne pathogens and reducing use concentrations. A quaternary antimicrobial blend reduced the concentration of each compound needed for inhibition by 87.5% which could also reduce the potential for negative sensory effects.