Many essential oils, secondary metabolites derived from a variety of plant materials including flowers, buds, roots, seeds, herbs and bark, possess antimicrobial and antioxidant properties in addition to their other physical characteristics. Previously used as flavoring or in perfumes, now the food industry is looking at these botanical volatile oils for preserving meat products against contamination with food-borne pathogens and food spoilage bacteria.
Mith et al. (2014) used standard bacteriological investigations to characterize the antimicrobial abilities of a range of fifteen commercially available essential oils including cinnamon, oregano and thyme1. The researchers also tested individual constituents such as carvacrol, eugenol and thymol, found in essential oils. They established antimicrobial activity, quantifying results with a paper disk diffusion test in culture. The team then selected the essential oils and constituents showing good antibacterial activity for further testing, to calculate mean inhibitory concentrations (MIC) and minimum bactericidal concentrations (MBC) in culture.
To assess antibacterial activity, Mith et al. placed paper disks impregnated with increasing dilutions of the test products onto the surface of Muller-Hinton agar plates (Oxoid, Thermo Scientific) inoculated with pathogens (Listeria monocytogenes, Salmonella typhimurium, enterohemorrhagic Escherichia coli O157) or food spoilage organisms, Brochothrix thermosphatica and Pseudomonas fluroscens. They incubated the plates for 24 hours before reading results.
The team found varying levels of antimicrobial activity among the essential oils and constituents tested. Some oils such as cinnamon and thyme, showed effective inhibition over a wide range of dilutions whereas galangal oil showed no antimicrobial activity. The researchers also noted that activity varied according to bacterial species; for example, Indian Lemongrass was more active against Gram-positive strains whereas Sweet Marjoram was more effective against Gram-negative bacteria. Although essential oil treatment inhibited growth in all the bacteria tested, P. fluorescens was most resistant, showing a response to only 10 of the volatile oils.
Mith et al. then established MICs and MBCs for seven essential oils and five constituents. The team calculated MICs and MBCs ranging between 0.13 µL/mL and 1 µL/mL, and 0.125 and >1.5 µL/mL respectively, for all bacteria except P. fluorescens , which was less susceptible as before (MIC and MBC = 1-1.5 µL/mL). Coincidentally, data from the constituents tested reflected those obtained from the essential oils themselves thus indicating the probable source of antimicrobial activity.
Although not tested on food products themselves, the authors conclude that essential oils show potential as a natural preservative for meats, with the added bonus as desirable natural food additives for improved consumer acceptance.
1. Mith, H. et al. (2014) “Antimicrobial activities of commercial essential oils and their components against food-borne pathogens and food spoilage bacteria“, Food Science & Nutrition 2(pp. 403–416) doi: 10.1002/fsn3.116
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