Kinetics of ultraviolet light inactivation of Escherichia coli O157:H7 in liquid foods

The effects of pH, depth of food medium and ultraviolet (UV) light dose on the inactivation of Escherichia coli O157:H7 in UV-opaque products such as apple juice (pH 3.5) and egg white (pH 9.1) were investigated. The applied UV dose ranged from 0 to 6.5 mW min cm^-2, while the depths of the medium were 1, 3.5, 5 and 10 mm. The pH of the medium did not affect the inactivation of E coli O157:H7, since similar inactivation characteristics were obtained for both apple juice and liquid egg white. As expected, decreasing the depth of the medium increased the inactivation of E coli O157:H7. More than a 5-log reduction was obtained when the fluid depth and UV dose were 1 mm and 6.5 mW min cm-2 respectively. However, less than a 1-log reduction was obtained when the fluid depth was 10 mm. A two-phase kinetic model was used to model the inactivation of E coli O157:H7. This model indicated that at higher fluid depths the inactivation rate was controlled by the second, slower inactivation phase, resulting in a lower overall inactivation. The visual appearance of the treated apple juice and egg white did not show any discolouration changes during 4 weeks of storage at ambient temperature (25 °C).