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Abstract
The aim of this study was to optimise the antimicrobial efficacy of plant essential
oils (EOs) for control of Listeria spp. and spoilage bacteria using food model media
based on lettuce, meat and milk. The EOs evaluated were lemon balm, marjoram, oregano
and thyme and their minimum inhibitory concentrations (MIC) were determined against
Enterobacter spp., Listeria spp., Lactobacillus spp., and Pseudomonas spp. using the
agar dilution method and/or the absorbance based microplate assay. MICs were significantly
lower in lettuce and beef media than in TSB. Listeria strains were more sensitive
than spoilage bacteria, and oregano and thyme were the most active EOs. EO combinations
were investigated using the checkerboard method and Oregano combined with thyme had
additive effects against spoilage organisms. Combining lemon balm with thyme yielded
additive activity against Listeria strains. The effect of simple sugars and pH on
antimicrobial efficacy of oregano and thyme was assessed in a beef extract and tomato
serum model media. EOs retained greater efficacy at pH 5 and 2.32% sugar, but sugar
concentrations above 5% did not negatively impact EO efficacy. In addition to proven
antimicrobial efficacy, careful selection and investigation of EOs appropriate to
the sensory profile of foods and composition of the food system is required. This
work shows that EOs might be more effective against food-borne pathogens and spoilage
bacteria when applied to foods containing a high protein level at acidic pH, as well
as moderate levels of simple sugars.