Elevated Inactivation Efficacy of a Pulsed UVC Light-Emitting Diode System for Foodborne Pathogens on Selective Media and Food Surfaces
UVC light, a strong surface disinfection technology, is used worldwide to ensure not only environmental safety but also food safety. Several drawbacks associated with the use of mercury-containing UV lamps, especially human and environmental health risks, led to the Minamata Convention on Mercury, which prohibits the manufacture and import/export of products containing mercury. Therefore, light-emitting diode (LED)-based UVC irradiation, a new technology that is ecofriendly and represents an effective UV light source, has been researched recently. To date, however, there has been no report describing pulsed UVC-LED irradiation for improvement of inactivation of foodborne pathogens, although much research regarding conventional pulsed xenon lamps has been published. In this investigation, we evaluated the enhanced bactericidal effect of a pulsed UVC-LED system, compared to continuous irradiation, and optimum conditions for maximizing the effect were determined. Also, the differences in inactivation between pulsed and continuous UVC-LED irradiation were determined by inactivation mechanism analyses. The combination of 20-Hz frequency and 50% duty ratio for pulsed UVC-LED irradiation achieved 4- to 5-log-unit reductions of Escherichia coli O157:H7, Salmonella enterica serovar Typhimurium, and Listeria monocytogenes; this combination showed the greatest bactericidal effect among various treatment conditions using 2 or 5 mJ/cm2. In mechanism assessments, membrane integrity (propidium iodide uptake) was not affected by UVC-LED treatment but membrane potential [bis-(1,3-dibutylbarbituric acid)trimethine oxonol [DiBAC4(3)] accumulation] showed significantly different values when pulsed and continuous treatments were compared. Changes in membrane lipid peroxidation and respiratory enzyme activity were attributed to generation of more reactive oxygen species by pulsed UVC-LED irradiation.
IMPORTANCE In 2013, the United Nations Environment Programme convened the Minamata Convention on Mercury, which prohibits trade in mercury-containing products in order to ensure human health. It will be effectuated in 2020; use of low-pressure mercury lamps will be discontinued and a new UV light source selected to replace the conventional technology. In this regard, UVC-LEDs have been developed and the fundamental inactivating effect has been researched. However, a pulsed UVC-LED system has not been studied, because of the difficulty of generating a UVC-LED pulse wave. An optical chopper system that physically divides the light with an adjustable blade, with personalized frequency and duty ratio settings, was introduced for generation of pulsed UVC-LED irradiation. This study elucidated the efficacy of a pulsed UVC-LED system and investigated its enhanced bactericidal effect in mechanism analyses.