Determination of the effect of plant essential oils obtained by supercritical fluid extraction on the growth and viability of Listeria monocytogenes in broth and food systems using flow cytometry

The antimicrobial properties of oregano, rosemary and laurel extracts obtained by supercritical fluid extraction were investigated by examining their influence on the growth and viability of Listeria monocytogenes in laboratory medium and broccoli juice at 30 and 8 degrees C. Important decreases in the L. monocytogenes population were shown in presence of all the extracts obtained from rosemary and one oregano extract. The counts were reduced below the level of detection after 4 h of exposure at 30 degrees C in laboratory medium. A bactericidal effect was observed also when L. monocytogenes was exposed to rosemary at 30 degrees C and 8 degrees C in broccoli juice. Significant reductions in growth rate and an increase in lag phase of L. monocytogenes were observed in presence of some of the laurel and oregano extracts at both temperatures. Flow cytometry was used as a rapid method to determine the antibacterial effect of supercritical extracts and the physiological state of L. monocytogenes. Bacterial viability performed by dual staining of L. monocytogenes with SYTO 9 and propidium iodide revealed three different cell populations, specifically, living, dead and compromised cells. Live cell percentage decreased with the time of exposure, whereas the percentage of compromised cells remained constant and the dead cells increased in the same period. (C) 2008 Swiss Society of Food Science and Technology. Published by Elsevier Ltd. All rights reserved.