Inactivation of Listeria monocytogenes by Hydrogen Peroxide Addition in Commercial Cheese Brines

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 five 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 Days 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) 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 1 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 suggest that the presence of catalase-positive indigenous microorganisms may neutralize the effect of H₂O₂.