Inactivation of Listeria monocytogenes by hydrogen peroxide addition in commercial cheese brines

Abstract

Commercial cheese brines are used repeatedly over extended periods, potentially for years, and can be a reservoir for salt-tolerant pathogens, such as Listeria monocytogenes. The objective of this study was to determine the inactivation of L. monocytogenes in cheese brines treated with hydrogen peroxide (H2O2) (0, 50, and 100 ppm) at holding temperatures representing manufacturing conditions. In experiment one, four fresh cheese brines were prepared with 10 or 20% salt and pH 4.6 or 5.4 (2x2 design; duplicate trials). Brines were inoculated with L. monocytogenes, treated with H2O2, and stored at 10 and 15.6°C. For experiment two, seven used commercial brines (representing 5 cheese types, 15-30% NaCl, pH 4.5-5.5; three seasonal trials) were inoculated with L. monocytogenes or S. aureus, treated with H2O2, and stored at 12.8°C (both L. monocytogenes and S. aureus), 7.2 and 0°C (L. monocytogenes only). Each treatment was assayed on Day 0, 1, and 7 for microbial populations and residual H2O2. Data revealed that pathogen populations decreased <1 log in cheese brines with no hydrogen peroxide stored for 7 days, regardless of the storage temperature. In fresh brine treated with 50 or 100 ppm of H2O2, populations of L. monocytogenes were reduced to less than the detectable limit by 7 days at 10 and 15.6°C (> 4 log reduction). For unfiltered used brines, H2O2 had no effect on L. monocytogenes populations in Brick J (pH 5.4, 15% NaCl) likely due to rapid inactivation of H2O2, likely by indigenous yeasts (~3-log CFU/ml). For the remaining brines, the addition of 100 ppm H2O2 killed >4 log L. monocytogenes when stored at 7.2 or 12.8°C for one week, but only 3-4 log reduction when stored at 0°C. The addition of 50 ppm H2O2 had similar lethal effects at 12.8°C but was less effective at 7.2 or 0°C. Inactivation rates of S. aureus were similar to that of L. monocytogenes. This study confirmed that high salt, warmer temperature, and 100-ppm H2O2 accelerated the inactivation of L. monocytogenes in cheese brines. Data also suggests that the presence of catalase-positive indigenous microorganisms may neutralize the effect of H2O2.