Inactivation of Pathogenic Bacteria in Ice Using an Ultraviolet C Light-emitting Diode

Introduction:   Ice is widely used in the food industry; however, it can be contaminated with microbes, and thus can cause food poisoning. Ice-contaminating microorganisms originate from poor quality source water and/or a lack of hygiene during production or handling. Direct microbial inactivation of ice could ensure its microbiological safety. Thus, we studied direct inactivation of microorganisms in ice using an ultraviolet C light-emitting-diode (UVC-LED).

Purpose: The purposes of this study were to compare the performance of UVC-LEDs with conventional UV lamps and to investigate the inactivation effect of UVC-LED on pathogenic bacteria in ice.

Methods:  Four strains of Escherichia coli O157:H7, five strains of Salmonella Typhimurium, and six strains of Listeria monocytogenes were used. One milliliter of each bacterial suspension was added to 24 ml of distilled water in ice cube tray (length: 30 mm, width: 30 mm, thickness: 30 mm) and frozen at −80°C for five to six hours. The ice samples were irradiated by UVC-LED or conventional UV lamp at −30°C. Following UVC treatment, the samples were melted at room temperature, and appropriate serial dilutions (1/10) were performed using sterile 0.1% peptone water. Dilutions were plated on duplicate tryptic soy agar plates, and then incubated at 37°C for 24 to 48 h.

Results: The UVC-LED inactivated the pathogenic bacteria in ice more efficiently than the conventional UV lamp. Pathogens contained in ice at a concentration of 10⁶ to 10⁷ CFU/ml were inactivated by UVC-LED at a UV dosage of 160 mJ/cm², regardless the species of pathogen. UVC-LED irradiation from two different directions more efficiently inactivated bacteria than two irradiation cycles from the same direction.

Significance: The results presented here suggest that UVC-LED could contribute to ensuring the microbiological safety of ice.