Inactivation of Listeria monocytogenes , Escherichia coli O157:H7, and Pre-existing Bacteria on Spinach by Combined Treatment of Cudrania tricuspidata Leaf Extract Washing and Ultraviolet-C Irradiation

Sodium hypochlorite has been used in the food industry to reduce microbial contamination in food; however, the use of sodium hypochlorite is insufficient considering the human health hazards and environmental pollution. To overcome this issue, plant-derived antimicrobial agents have been developed. In our previous study, Cudrania tricuspidata leaf extract (0.4%) was used to wash fresh vegetables for 3 min; however, Cudrania tricuspidata leaf extract was not effective in inhibiting gram-negative pathogens. Therefore, to enhance microbial decontamination by Cudrania tricuspidata leaf extract, ultraviolet-C irradiation (1.8 kJ/m2) was used in combination on spinach as a model system. After the combined treatment, the microbial count of Listeria monocytogenes, Escherichia coli O157:H7, and pre-existing bacteria on spinach were reduced by 1.94, 1.93, and 2.10 log CFU/g, respectively. The log reduction levels of the combined treatment against Listeria monocytogenes, Escherichia coli O157:H7, and pre-existing bacteria on spinach were higher than those of 200 ppm sodium hypochlorite washing (1.46, 1.26, and 1.44 log reduction). Moreover, an increased amount of cellular constituents was released from the pathogens due to the combined treatment compared with the single treatment of Cudrania tricuspidata leaf extract washing or ultraviolet-C irradiation. The distorted and ruptured pathogens after the combined treatment were observed via scanning electron microscopy. Furthermore, combined treatment did not affect the color, texture, and bioactive compounds in spinach. These results suggest that combination of Cudrania tricuspidata leaf extract washing and ultraviolet-C irradiation used in the present study can be an effective method to replace sodium hypochlorite washing.