Combination of phenolic acids and essential oils against Listeria monocytogenes

- Essential oils (EOs) and phenolic acids (PAs) inhibited Listeria monocytogenes.
- EOs and PAs interacted synergistically at pH < 6.0.
- Allyl isothiocyanate and ferulic acid were synergistic at pH 6.0.

Listeria monocytogenes is a psychrotrophic pathogen widely distributed in meat processing plants. Listeriosis presents very low morbidity, but very high mortality rates. Several outbreaks involving this bacterium have been reported due to the consumption of refrigerated meat and dairy products. The objective of the present study was to evaluate the in vitro antimicrobial activity of 10 phenolic acids (PAs) and 4 essential oil compounds (EOCs) as natural alternatives to control L. monocytogenes growth. The minimum inhibitory concentration (MIC) of all natural compounds was determined at pH 5, 6 and 7. The 4 most potent PAs (ρ-hydroxybenzoic acid, ferulic acid, o-coumaric acid and syringic acid) were selected and tested in combination with the 2 most active EOCs (allyl isothiocyanate and carvacrol), giving a total of 8 combinations. Lower pH levels enhanced the antimicrobial activity of both PAs and EOCs, with the exception of geraniol, which had the same MIC at all pH levels. Most combinations of essential oils and phenolic acids showed synergistic effects at pH 5, whereas only ferulic acid plus allyl isothiocyanate showed synergism at pH 6. The combined use of these natural compounds can improve their antimicrobial activity and may eliminate L. monocytogenes from mildly acid food products.