WATER ACTIVITY AND THE INACTIVATION OF ENTEROBACTER CLOACAE INOCULATED IN CHOCOLATE LIQUOR AND A MODEL SYSTEM BY PULSED ELECTRIC FIELD TREATMENT

Effects of water activity (a w ) on the inactivation of Enterobacter cloacae inoculated in chocolate liquor and in a model system of 0.1 % (w/v) peptone water and glycerol by pulsed electric field (PEF) treatment were investigated. An electric field strength of 24.5 kV/cm, a total treatment time of 320 μs, a pulse duration time of 4 μs, a pulse delay time of 15 μs, and a pulse cycle time of 15 s were selected for PEF treatment. The inactivation of E. cloacae by PEF increased significantly as a w increased (P < 0. 05). As a w of chocolate liquor increased from 0.48 to 0.89, the log reduction ofE. cloacae increased from 0.1 to 1.3. The measured temperature change inside the PEF treatment chamber was 0.4C when the log reduction was 1.3. Similarly, as a w increased from 0.51 to 0.91 in the model system, the log reduction increased from 0.4 to 1.3. E. cloacae surviving a low a w environment had high resistance to PEF. PEF inactivated E. cloacae in the chocolate liquor with a w of 0.85 by 1 log at 0 h incubation. However, the log reduction was only 0.1 when PEF treatment was applied to E. cloacae which was incubated for 2 h in the chocolate liquor with a w of 0.85 before PEF treatment. E. cloacae surviving the low a w environment might have resistance not only to the low a w but also to PEF. The resistance to low a w environment may need to be considered when the inactivation of microorganisms by PEF is evaluated.