Essential oil combinations offer shelf-life prolongation

"This study suggests that combinations of EOs could minimise application concentrations and consequently reduce any adverse sensory impact in food," wrote the researchers in the International Journal of Food Microbiology .

"However, their application for microbial control might be affected by food composition, therefore, careful selection of EOs appropriate to the sensory and compositional status of the food system is required."

The tide is currently turning against chemical-based bactericides for food use, opening up opportunities for alternatives from natural sources.

The reasons for this are manifold and include general consumer preferences for natural foods, legislative changes, and the isolation of antibiotic resistant pathogens.

Researchers from Dublin Institute of Technology report that combinations of EOs at low concentrations show strong potential to be used instead.

Paula Bourke and co-workers studied the efficacy of essential oil combinations, including rosemary, thyme, basil, oregano, sage, basil, lemon balm, and marjoram, against food-borne pathogens such as Bacillus cereus, Escherichia coli, Listeria monocytogenes and Pseudomonas aeruginosa .

Promising results were obtained from these initial experiments for oregano in combination with basil, thyme or marjoram, with an additive efficacy of the oregano combinations against B. cereus , E. coli and P. aeruginosa .

In terms of L. monocytogenes , the researchers report that marjoram or thyme in combination with basil, rosemary or sage showed additive protection.

"Although none of the combinations showed clear synergistic effects, combining EOs selected in this study, at lower concentrations than required for the EOs alone, has potential for practical application to extend the shelf-life of selected foods," wrote the researchers.

This result is important, they suggest, since a current limitation to the use of EOs is the potential change in the organoleptic and textural quality of the food.

Questions over interactions with other food components are also quoted as a limiting factor.

Effects of other food ingredients To test the potential of using low-concentration essential oil combinations in actual foods, Bourke and co-workers evaluated the efficacy of plant essential oils (EOs) in combination and to investigate the effect of food ingredients on their efficacy.

"The antimicrobial efficacy of the EOs in this study was found to be a function of ingredient manipulation," they stated.

Higher protein concentrations and moderately acidic conditions, for example, were found to boost the antimicrobial activity of oregano and thyme, stated the researchers.

On the other hand, higher concentrations of potato starch or sunflower oil reduced the efficacy of the oils.

"This work shows that EOs might be more effective against food-borne pathogens and spoilage bacteria when applied to ready to use foods containing a high protein level at acidic pH, as well as lower levels of fats or carbohydrates," stated the researchers.

The study was supported financially by the Irish Department of Agriculture and Food.

Source: International Journal of Food Microbiology (Elsevier) Published online ahead of print, doi:

10.1016/j.ijfoodmicro.2008.02.028

"The antimicrobial efficacy of plant essential oil combinations and interactions with food ingredients" Authors: J. Gutierrez, C. Barry-Ryan and P. Bourke