Date of Award
Master of Science
Food Science and Technology
Faith J. Critzer
P. M. Davidson, Irene B. Hanning
Efficacy of plant essential oils against spoilage lactic acid bacteria was examined using two different study methods with the goal of determining minimum inhibitory (MIC) and minimum lethal concentrations (MLC) of the essential oils. The initial study included the incorporation of the essential oils, or their major constituents, into agar to allow uniform dispersion of the substance throughout an agar surface. Individual cultures of nine lactic acid bacteria species (Pediococcus acidilactici, Pediococcus damnosus, Lactobacillus fermentum, Lactobacillus fructivorans, Lactobacillus brevis, Lactobacillus buchneri, Lactobacillus plantarum, Leuconostoc mesenteroides, and Leuconostoc citrovorum) were spot inoculated onto de Man, Rogosa, and Sharpe (MRS) agar containing the essential oils and incubated under ambient conditions (as determined independently per organism) for 4 days. The plates were examined for the evidence of growth, with the lowest concentration adequate to suppress growth being identified as the MIC. The most antimicrobial compounds were thymol and carvacrol, both of which had MICs of 0.1% w/v or v/v, respectively. Cinnamaldehyde, cinnamon bark oil, eugenol, thyme oil, and clove bud oil had MICs of 0.2% v/v. Cinnamic acid had an MIC of 0.5% w/v, while no MIC was determined for allyl isothiocyanate up to concentrations of 0.75% v/v.
Minimum lethal concentrations were examined using a broth dilution assay for 72 h. Carvacrol, thymol, eugenol, and cinnamaldehyde were dissolved in 95% ethanol to create a 50% stock solution. This solution was then added to MRS broth and mixed thoroughly. Individual cultures of P. acidilactici, L. buchneri, and L. citrovorum were added to the broth at concentrations of 4 log CFU/mL. The broth was spiral plated at 0 (immediately after exposure), 6, 12, 24, 48, and 72 h, and lethality was determined according to log reduction at these time points. Carvacrol was lethal to against all species at 0.2% (v/v) to the limits of detection (0.95 CFU/mL, while thymol at 0.2% and 0.1% (w/v) prevented recovery of L. buchneri and L. citrovorum, respectively. Concentrations of cinnamaldehyde at 0.2% were lethal against L. buchneri and L. citrovorum, and at 0.25% (v/v) against P. acidilactici. Eugenol required concentrations in excess of 0.3% (v/v) for universal lethality.
Gann, Laurel Dunn, "Antimicrobial Activity of Essential Oils and Their Components Against Lactic Acid Bacteria. " Master's Thesis, University of Tennessee, 2013.