Effectiveness of various solvent-produced thyme (Thymus vulgaris) extracts in inhibiting the growth of Listeria monocytogenes in frozen vegetables

The objective of this study was to evaluate the efficiency of thyme extracts in preventing the growth of Listeria monocytogenes in frozen vegetables. In order to investigate the microbiological properties of thyme extracts, Listeria strains were isolated from frozen vegetables and identified, as well as the minimum inhibitory concentration (MIC) of thyme extracts (in vitro study) and their activity in vegetable samples (in vivo study) were determined. Thyme extracts were prepared using ethanol, acetone, and water (cold and hot) from whole plants and different morphological parts of the thyme plant (seeds, leaves, and stems). Selected thyme extracts were assessed for their content of bioactive compounds (total phenolic compounds, flavonoids, carotenoids, and chlorophylls). Furthermore, the profiles of phenolic acids and flavonoids were analyzed using high-performance liquid chromatography (HPLC). The study results showed that 67.2% of the vegetable samples were contaminated by L. monocytogenes strains, with carrots and stir fry mixtures being the most infected samples. The ethanolic and acetone extracts from thyme leaves and seeds were effective bactericidal agents at a concentration of 0.5 of the mean MIC values and proved effective in reducing L. monocytogenes at <2 log CFU/g. The strongest antibacterial activity was found for the acetone extracts from leaves, while the ethanolic extract from seeds exhibited the lowest activity. However, the ethanolic extract from leaves was more effective at a lower concentration. The greatest number of bioactive compounds (including phenolic compounds, carotenoids, and chlorophylls) were determined in the ethanolic and acetone extracts from thyme leaves.