Mechanistic insights into the photodynamic inactivation of foodborne pathogens by UVA-mediated protocatechuic acid.

This study investigated the potential of protocatechuic acid (PCA), a natural polyphenolic compound, as a photosensitizer for antimicrobial photodynamic inactivation (PDI) and elucidated its underlying mechanism. The synergistic effect of PCA combined with UVA LED irradiation (365 nm) was evaluated against major foodborne pathogens, including Escherichia coli O157:H7, Salmonella Typhimurium, Listeria monocytogenes, and Staphylococcus aureus, in peptone water. When applied at doses ranging from 20 to 40 J/cm2, the combined treatment of UVA irradiation and 30 mM PCA (UVA + PCA) resulted in markedly enhanced bacterial inactivation. Reductions of 5-7 log CFU/mL were achieved at 20 J/cm2 for E. coli O157:H7, S. Typhimurium, and L. monocytogenes, while S. aureus required a dose of 40 J/cm2 to achieve a similar reduction, whereas UVA or PCA alone produced minimal effects (< 1 log CFU/mL). Intracellular reactive oxygen species (ROS) quantification and scavenger assays revealed that PCA acted as an effective photosensitizer under UVA exposure, generating singlet oxygen (1O2)-a Type II ROS-as the primary reactive species responsible for oxidative damage. The resulting oxidative stress compromised cellular structures, including the membrane and nucleic acids, ultimately leading to cell death. Furthermore, when applied to grape juice, a low-transmittance food matrix, the UVA + PCA system effectively inactivated microorganisms without significant alterations in pH, color, or anthocyanin content, demonstrating its potential applicability in real food systems. Collectively, these findings highlight PCA as a promising natural photosensitizer that enables efficient and quality-preserving microbial control, offering an eco-friendly approach for use in juice sterilization and broader food industry applications.