Ultraviolet bactericidal irradiation of ice.

We investigated the germicidal activity of 2,537 A ultraviolet (UV) radiation on bacteria in ice cubes of varying thickness and in aqueous suspensions beneath an ice layer. The test bacteria used were Escherichia coli, Serratia marcescens, Bacillus subtilis, and Sarcina lutea; aqueous suspensions of the selected organisms were frozen into ice cubes, 2 mm to 30 mm thick, at -20 C. The cubes were irradiated for 1 min, whereas the suspensions of bacteria were placed beneath an ice block (19 cm thick) and were irradiated for 0.5 to 15 min. In both groups of experiments, the standard plate count method was used to compare the number of bacteria surviving the UV treatment with the number of bacteria in the untreated controls. The results showed that 1 min of UV treatment killed as many as 97% of the gram-negative and at least 60% of the gram-positive test bacteria (freezing survivors) frozen in ice cubes 30-mm thick. Within 15 min, UV light transmitted through a 19-cm thick ice block inactivated 98% of the bacteria suspended in the buffer solution. We concluded that the UV rays were able to penetrate at least 19 cm of ice and still retain enough energy to kill bacteria. However, the UV penetration depended greatly on the optical quality of the ice. Although it was not the purpose of these experiments to find a practical method for sanitizing ice, the results of this study and of our other unpublished experiments indicate that UV light has adequate penetrating power to be considered practical in certain selected applications.