Enhancing Bactericidal Efficacy of High-Pressure Processing (HPP) Against Foodborne Pathogens: The Role of pH and Mechanistic Insights

This study investigated the influence of pH on the efficacy of high-pressure processing (HPP) in inactivating foodborne pathogens in juice products, using phosphate-buffered saline as a standardized medium. Although previous studies have suggested that lower pH levels can enhance HPP efficiency, the present study provides a comprehensive analysis of this effect across different pathogens and pH levels. Additionally, we examined bactericidal mechanisms, specifically focusing on the response of bacterial cell structures to HPP treatment. The efficacy of HPP at pH 3.5, 5.5, and 7.5, was measured by reductions in pathogen count, with the results indicating that lower pH levels considerably increased the bactericidal efficacy. Transmission electron microscopy revealed that after HPP treatment, Listeria monocytogenes cells retained their cell membrane structure despite internal aggregation. In contrast, Escherichia coli O157:H7 cells showed cell membrane disruption. Additionally, an increase in propidium iodide uptake was observed in both pathogens across all pH levels, indicating compromised membrane integrity. This increase was more pronounced in E. coli O157:H7, highlighting a greater degree of membrane damage than that in L. monocytogenes. The inactivation of respiratory chain dehydrogenase, indicated by a decrease in iodonitrotetrazolium formazan conversion value, demonstrated the significant effect of HPP on the metabolic activity of cells.