Intense pulsed light (IPL) and UV-C treatments for inactivating Listeria monocytogenes on solid medium and seafoods

The inactivation effects of intense pulsed light (IPL) on Listeria monocytogenes surface-inoculated on solid medium and on seafoods such as flatfish, salmon, and shrimp fillets were investigated for various light doses (0.11-1.75mJ/cm² per pulse), number of pulses (0-9800 pulses, treatment time of 0-1960s), and total fluences (0-17.2J/cm²), and also the inactivation characteristics of UV-C irradiation on L. monocytogenes were evaluated for treatment time of 0-1960s. Besides, any structural damage to the treated cells after IPL and UV-C treatments was evaluated by transmission electron microscopy (TEM). On the solid medium, approximately 4.0- and 6.0-log reductions of L. monocytogenes cells were achieved with UV-C irradiation for 1000s and with IPL treatment for 180s (900 pulses) at a fluence of 1.75mJ/cm² per pulse, respectively, with a negligible temperature rise (<2.0°C) during treatment. On the seafood products, IPL treatment at 1.75mJ/cm² per pulse produced approximately 2.2-, 1.9-, and 1.7-log reductions of L. monocytogenes cells inoculated onto shrimp, salmon, and flatfish fillets, respectively, for 3600 pulses (720s, total fluence of 6.3J/cm²) and approximately 2.4-, 2.1-, and 1.9-log reductions, respectively, for 6900 pulses (1380s, total fluence of 12.1J/cm²), with a slight temperature rise (<5.0°C) and no observable effect on the food color. Meanwhile, UV-C treatment on the inoculated fish fillets did not show significant effect on irradiation time of 0-1960s. TEM observations clearly indicated destruction of the cell wall, cytoplasm shrinkage, and leakage of the cellular contents from the cytoplasm in IPL-treated L. monocytogenes cells, unlike UV-C treated cells.