Inhibition of Listeria monocytogenes by Melaleuca alternifolia (tea tree) essential oil in ground beef

The food industry has been valuing the quest for natural substances for use in food preservation aiming to meet consumer demand for safer, more natural foods with preserved nutrients. This study aimed to assess the antimicrobial potential of essential oil of Melaleuca alternifolia (EOMA) in the inhibition of Listeria monocytogenes (ATCC 7644) (L. monocytogenes) in ground beef. An in vitro screening in solid phase was performed and the minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) were determined through microdilution test. The time of action of EOMA was assessed through the death-time curve at 2 x MIC and MBC. The effects of MBC on bacterial morphology were verified under scanning electron microscopy (SEM). The meat samples were inoculated with four different suspensions of L. monocytogenes (1.5 x 10(8) CFU/mL, 4.6 x 10(4) CFU/mL, 9.2 x 10(3) CFU/mL, and 1.2 x 10(2) CFU/mL) and stored at 4 +/- 1 degrees C for up to 14 days. The test samples were added with 1.5% v/w EOMA. The test of diffusion in solid medium showed L. monocytogenes ATCC 7644 was extremely sensitive to EOMA. MIC and MBC values were 0.10 mu L/g and 0.15 mu L/mL, respectively. The death-time curve revealed a reduction of viable cells after 1 h of contact with the oil. SEM showed that the treated cells had wrinkled surface and some cells had lower size and diameter when compared to control ones. The food matrix test indicated EOMA had antimicrobial activity in all samples except for the one inoculated with the suspension at 1.5 x 10(8) CFU/mL. Thus, the use of essential oil of Melaleuca alternifolia as a potential natural antimicrobial agent to preserve ground beef was promising as it was effective at low concentration. The data lay bases for new tests to be carried out in other food matrices.