Development of thyme essential oil-based coatings and assessment of their antimicrobial activity

• Coatings containing thyme essential oil on polypropylene film were designed. • The antimicrobial properties of coatings were investigated by vapor diffusion assay. • The coatings demonstrated antibacterial effect againts Escherichia coli and L. monocytogenes . In this study, coatings composed of acrylic binder, starch sodium octenylsuccinate and essential oil (EO), as a volatile antibacterial agent were developed and applied to polypropylene films. Among various EOs tested, thymol exhibited the highest antimicrobial activity in agar diffusion assays, resulting in inhibition zone of 12.3 ± 1.2 to 40.0 ± 1.0 mm with Escherichia coli and 11.3 ± 0.6 to 88.0 ± 1.0 mm with L. monocytogenes . Based on these results, thymol was selected and incorporated in the coatings, and the antimicrobial activity of films with coatings containing various amounts of thyme EO (585 ± 30 – 2159 ± 112 µg/cm 2 ) were assessed using both agar diffusion and vapor diffusion assays. Films with the highest amount of thyme essential oil (2159 ± 112 µg/cm 2 ) exhibited significant antimicrobial activity in the agar diffusion test. In vapor diffusion assays the active films demonstrated a significant antibacterial effect, producing a log reduction of 2.3 log E. coli and 3.8 for L. monocytogenes . Studies on the release of thymol, the main antibacterial component of thyme EO, revealed a continuously release with a maximum concentration of 21.85±2.11 µg/cm 3 achieved after 10 days. The results further indicated that only a very small proportion of the thymol in the coating was released during the storage period.