Highly enhanced bactericidal effects of medium chain fatty acids (caprylic, capric, and lauric acid) combined with edible plant essential oils (carvacrol, eugenol, β-resorcylic acid, trans-cinnamaldehyde, thymol, and vanillin) against Escherichia coli O157: H7

Medium chain fatty acids (MCFAs) and essential oils (EOs) are known to be natural antimicrobials, but their combined effects have not been fully investigated. The objective of the present study was to examine the bactericidal effects of various combined treatments of MCFAs [caprylic (CA), capric (CPA), and lauric acid (LRA)] and EOs [carvacrol (CAR), eugenol (EUG), β-resorcylic acid (RA), trans-cinnamaldehyde (TC), thymol (TM), and vanillin (VNL)]. Escherichia coli O157:H7, was treated with 1) control (2% ethanol), 2) MCFA alone, 3) EO alone, and 4) different combinations of MCFAs and EOs at 37 °C for 5 and 10 min. Synergistic bactericidal effects were observed with combined treatments; the log reduction in viable bacteria in response to the combined treatments was much greater than the sum of the effects of the two compounds applied individually. For example, individual treatment with 0.2 mM CPA (0.004%) and 0.4 mM RA (0.006%) for 5 min resulted in a negligible reduction in bacterial load (0.25 and 0.21 log reduction, respectively), whereas combined treatment at the same concentrations and for the same time reduced the bacterial population in the test sample to an undetectable level (initial population: 7.51 log CFU/ml; detection limit: 1 CFU/ml). The ranking of EOs showing the highest bacterial killing activities when combined with MCFAs was generally RA, CAR, TM > EUG > TC > VNL. All the antimicrobials used in this study are natural compounds that have been widely used in industry, so they are both consumer- and user-friendly. Combined treatment can overcome the disadvantages of MCFAs and EOs such as unpleasant odor and high cost because the required concentrations can be reduced. Our results indicate that the combined treatments used here could be successfully used to eliminate foodborne pathogens, significantly improving the microbiological safety of foods.