This study assessed the occurrence of an enhancing inhibitory effect of the combined application of Origanum vulgare (OV) and Rosmarinus officinalis (RO) essential oils against bacteria associated to minimally processed vegetables using the determination of Fractional Inhibitory Concentration (FIC) index, kill-time assay in vegetal broth and application in vegetable matrices. Moreover, it was determined chemical composition of the essential oils and their effects alone and in mixture on sensory characteristics of minimally processed vegetables. Carvacrol (66.9 g/100 g) was the most prevalent compound in OV essential oil, while for RO was 1.8-cineole (32.2 g/100 g). OV and RO essential oil showed MIC in a range of 1.25-5 and 20-40 μL/mL, respectively. FIC indices of the combined application of the essential oils were 0.5 against Listeria monocytogenes, Yersinia enterocolitica and Aeromonas hydrophilla suggesting a synergic interaction. Only for Pseudomonas fluorescens FIC index was 0.75 purposing additive effect. Application of the essential oils alone (MIC) or in mixture (¼ MIC + ¼ MIC or ¼ MIC + ½ MIC) in vegetable broth caused significant decrease (p b 0.05) in bacterial count over 24 h. Mixture of essential oils reduced (p b 0.05) the inocula of all bacteria in vegetable broth and in experimentally inoculated fresh-cut vegetables. Similar efficacy was found to reduce the autochthonous microflora in vegetables. Sensory evaluation of vegetables sanitized with essential oils revealed that the scores of the most evaluated attributes fell between like slightly and neither like nor dislike. The combination of essential oils at sub-inhibitory concentrations could mean an interesting approach to sanitize minimally processed vegetables.

ABSTRACTOvernight exposure ofSalmonella entericaserovar Typhimurium to sublethal amounts ofOriganum vulgareessential oil (OV) and carvacrol (CAR) did not result in direct and cross-bacterial protection. Cells subcultured with increasing amounts of OV or CAR survived up to the MIC of either compound, revealing few significant changes in bacterial susceptibility.

The capacity of Origanum vulgare L. essential oil (OVEO) and its majority compound, carvacrol (CAR), to induce direct tolerance and cross-tolerance in Staphylococcus aureus against high temperature (45 °C), lactic acid (pH 5.2) and NaCl (10 g/100 mL) was assessed. Overnight exposure of S. aureus to sublethal concentrations (1/2 MIC, 1/4 MIC) of either OVEO or CAR in meat broth revealed no induction of direct protection. S. aureus cells pre-adapted to OVEO or CAR showed no induction of cross-protection to high temperature, lactic acid or NaCl. Cells subjected to 24 h cycles of adaptation in increasing amounts (1/2 MIC to 2 × MIC) of OVEO or CAR showed no increase in direct tolerance. These results revealed a lack of induction of direct protection or cross-protection in S. aureus exposed to sublethal amounts of OVEO or CAR in meat-based broth, as determined by monitoring cell survival and growth behavior.