Synergism between carvacrol or thymol increases the antimicrobial efficacy of soy sauce with no sensory impact

• Carvacrol (CV) and thymol (TM) showed the greatest synergy with soy sauce (SS).
• SS containing CV and TM (< 0.0157%) killed > 7 log CFU/ml of foodborne pathogens.
• At 4 °C, L. monocytogenes was slightly more resistant than the other bacteria.
• High NaCl levels in SS may act synergistically with membrane attack by CV and TM.
• CV and TM may improve the safety of various marinated foods.

Here, we examined the antimicrobial effects of soy sauce containing essential oils (EOs) against Escherichia coli O157:H7, Salmonella Typhimurium, and Listeria monocytogenes at 22 °C and 4 °C. To screen a variety of combined effects, soy sauce was mixed with six different EOs (carvacrol, thymol, eugenol, trans-cinnamaldehyde, β-resorcylic acid, and vanillin), each at a concentration of 1 mM for 10 min. None of the oils showed bactericidal activity when used alone. Soy sauce combined with carvacrol and thymol induced the greatest antibacterial activity against all tested bacteria; therefore, these oils were further tested at 0.25, 0.5, and 1 mM (0.0039%, 0.0078%, and 0.0157%) for 1, 5, and 10 min at 4 °C and 22 °C. In addition, sensory evaluation of soy sauce containing each EO at 0.25, 0.5, 1, and 2 mM was performed using the nine point hedonic test. Carvacrol or thymol (1 mM) eliminated all the test bacteria (initial population, 7.0–7.5 log CFU/ml) in 1–5 min at 22 °C and within 10 min at 4 °C. L. monocytogenes was slightly more tolerant at 4 °C, which may be attributable to the ability of the cell membrane to adapt to low temperatures. The sensory scores for soy sauce containing EOs were not significantly different from that of soy sauce without EOs (P > 0.05). The stability of EO efficacy in soy sauce was also verified. These results suggest that carvacrol and thymol act synergistically with other factors present in soy sauce to increase antimicrobial activity against major foodborne pathogens at both 4 °C and 22 °C. The synergism may be attributable to the combination of factors (mainly high salt concentration and low pH imparted by organic acids) present in soy sauce and the membrane attacking properties of carvacrol and thymol. This method will facilitate the production of microbiologically safe soy sauce, soy sauce-based marinades, and various marinated foods.