Essential Oils’ Antimicrobial Efficacy Studied

Researchers from the Dublin Institute of Technology recently studied the antimicrobial impact of several essential oils, in various combinations, to determine their potential effects on common foodborne pathogens. The results of this research were published in the May 10 edition of the International Journal of Food Microbiology.

The essential oils studied were basil, lemon balm, marjoram, oregano, rosemary, sage and thyme. These were combined in various permutations to determine their impact on pathogens like Bacillus cereus, Escherichia coli, Listeria monocytogenes and Pseudomonas aeruginosa. The researchers also studied the effects of various concentrations of essential oils on E. coli.

Tests showed promising results for oregano in combination with basil, thyme or marjoram. The researchers note that “all oregano combinations showed additive efficacy against B. cereus, and oregano combined with marjoram, thyme or basil also had an additive effect against E. coli and P. aeruginosa. Mixtures of marjoram or thyme, when combined with basil, rosemary or sage, also displayed additive effects against L. monocytogenes.

However, the researchers found that the actual degree of efficacy was dependant on the concentrations of various ingredients in the food system. For instance, they note that starch and oil concentrations of 5% and 10% had a negative impact on efficacy. On the other hand, the essential oils were more effective in the presence of high protein concentrations and at pH 5 (versus a higher, more alkaline pH of 6 or 7).

Overall, the Irish researchers concluded that combinations of essential oils “could minimize application concentrations and consequently reduce any adverse sensory impact in food. However, their application for microbial control might be affected by food composition...” Therefore, they suggest that careful selection of essential oils “appropriate to the sensory and compositional status of the food system is required.” They suggest that essential oils might prove most effective against foodborne pathogens “when applied to ready-to-use foods containing a high protein level at acidic pH, as well as lower levels of fats or carbohydrates.”