Synergistic Antibacterial Activity of Irradiated and Nonirradiated Cumin, Thyme and Rosemary Essential Oils

The study aimed to evaluate the efficacy of plant essential oils and their mixtures and to investigate the effect of γ‐irradiation on their antibacterial activity. About 2 kGy (C2) and 4 kGy (C4) in cumin essential oil, 4 kGy (T4) in thyme essential oil, and 6 kGy (R6) in rosemary essential oil were the effective antibacterial dose against Escherichia coli, Salmonella typhimurium, Bacillus cereus and Staphylococcus aureus. The checkerboard method was used to quantify the efficacy of mixtures made from cumin, thyme and rosemary. Fractional inhibitory concentrations were calculated and interpreted as synergy, addition, indifference or antagonism. Most of the mixtures showed indifference efficacy against selected strains. (R6 × T4) and (C4 × R6) represented additive efficacy against B. cereus and S. typhimurium, respectively. Synergistic effect was observed against B. cereus and E. coli in (C0 × R0). (C2 × T4) represented a higher minimum inhibitory concentration than (C0 × T0). The mixtures of essential oils either irradiated or nonirradiated could be used to minimize essential oil concentrations and to reduce any adverse sensory impact on food. Practical Application Essential oils would have to be used in considerable concentrations to achieve an antibacterial potential. Thyme essential oil is individually defined as a higher antibacterial agent against wide spectrum of pathogenic bacterial strains, while cumin and rosemary showed limited antibacterial activity compared to thyme essential oil. The levels of essential oils necessary to inhibit microbial growth are higher in foods than in culture media, thus the quantity of essential oil to be used has a distinctive bitter flavor. The changes made by gamma irradiation increased the antibacterial activity of essential oil at lower doses. The finding of our study clearly indicates that the possibility of using mixtures of essential oils, either irradiated or nonirradiated, would have a synergistic effect and that lower concentrations of the oils could therefore be used.