Antibacterial Activity of the Lactoperoxidase System on Listeria monocytogenes in Trypticase Soy Broth, UHT Milk and French Soft Cheese

Abstract

The antibacterial activity of the lactoperoxidase system on the growth and survival of Listeria monocytogenes in trypticase soy broth, UHT milk and French soft cheese was determined. Several levels of Listeria cells, ranging from ca. 10 to 10(4) or 10(7) CFU/ml were studied. A comparison was made between the behavior of L. monocytogenes in these media with or without the lactoperoxidase system at two storage temperatures, 4 and 15°C. In trypticase soy broth and UHT milk, presence of the lactoperoxidase system either inhibited growth or completely inactivated inoculated cells. Complete inactivation occurred at different times depending on initial inoculum concentration, culture medium, and storage temperature. In UHT milk, the D-values were determined from the regression lines of log cell populations versus time. The linear correlations are significant, and D-values were the same (ca. 5 d at 15°C and ca. 8 d at 4°C) whatever the initial cell level. The efficiency of the lactoperoxidase system is dependent on temperature, which directly determines the antibacterial activity. The addition of the lactoperoxidase system to the surface of soft cheese led to elimination of viable Listeria cells from cheeses previously inoculated to contain 10 to 10(6) CFU/g. The results of this study suggest that the lactoperoxidase system may be effective in dairy products as a safety factor to assist in the inhibition of L. monocytogenes .