Inactivation and potential reactivation of pathogenic Escherichia coli O157:H7 in bovine milk exposed to three monochromatic ultraviolet UVC lights

The ultraviolet (UVC) light irradiation has been recently studied as a novel non-thermal treatment method for milk. However, the potential reactivation of microorganisms following exposure to UVC light in milk medium was not studied yet. Therefore, the present study was conducted to determine the inactivation and reactivation of pathogenic Escherichia coli O157:H7 following exposure to UV light at three monochromatic wavelengths (222, 254 and 282 nm) in bovine milk. The results showed that inactivation of E. coli O157:H7 following exposure to the UV light at 254 nm was higher (P < 0.05) than that following exposure at 222 and 282 nm at the same UV fluence of 5, 10 and 20 mJ/cm2. The reactivation of E. coli O157:H7 was increased as the incubation time and temperature increased regardless of the UV light sources under dark incubation phases. The evaluated reactivation ratios of E. coli O157:H7 following exposure to the UV light at 254 nm in milk were lower (P < 0.05) than that following exposure at 222 nm after 1 to 6, 2 to 5 and 5-6 h incubation at 4, 20 and 37 °C, respectively. Furthermore, at most incubation time points, the reactivation ratio of E. coli O157:H7 following exposure to these three UV light sources were lower (P < 0.05) than that of non-UV treated cells regardless of the incubation temperature. The lowest reactivation ratios of E. coli O157:H7 were observed after milk exposure to the UV light at 254 nm at 4 °C incubation when compared to that following exposure to the UV light at 222 and 282 nm.