Antimicrobial properties of lauric arginate alone or in combination with essential oils in tryptic soy broth and 2% reduced fat milk

The objective of this study was to evaluate the antimicrobial activity of lauric arginate (LAE) when used alone or in combination with the essential oil (EO) from cinnamon leaf and EO components, thymol and eugenol. Minimum inhibitory concentrations (MICs) and minimum bactericidal concentrations (MBCs) for Listeria monocytogenes, Escherichia coli O157:H7 and Salmonella Enteritidis were determined by the microbroth dilution method in tryptic soy broth (TSB) at their optimal growth temperatures. The MIC for LAE was 11.8 ppm against L. monocytogenes and E. coli O157:H7 and 23.5 ppm against S. Enteritidis. Synergistic antimicrobial activity was demonstrated against L. monocytogenes with combinations of LAE and cinnamon leaf oil or eugenol, while the LAE and thymol combination showed additive antimicrobial activity. Conversely, antagonistic effects were shown for all combinations against E. coli O157:H7 and S. Enteritidis. Beef extract, at 2 or 5% w/v in TSB, showed no effects on the MIC and MBC of LAE against L. monocytogenes, while soluble starch from potato, at 2-10% w/v in TSB, increased the MIC and MBC. When tested in 2% reduced fat milk, significantly higher levels of antimicrobials were required to achieve similar inhibitions as in TSB. The growth curves of bacteria at 21 °C followed similar trends as in TSB, showing synergism against the Gram-positive L. monocytogenes and antagonism against the two Gram-negative bacteria. Findings suggest that application of LAE could enhance microbial food safety, especially when used in combination with EO to inhibit the growth of Gram-positive bacteria.