Effects of pulse polarity and pulse delaying time on inactivation of E. coli O157:H7 inoculated into apple juice and skim milk samples were determined. Inactivation effect was observed as inoculated food samples were subjected to mono and bipolar square wave pulses. Bipolar pulses with different pulse delaying times were also applied. There was no significant difference between mono (2.56 log(10) cfu/ml) and bipolar (2.63 log(10) cfu/ml) pulses on the inactivation of E. coli O157:H7 inoculated into apple juice (P > 0.05). However. results in skim milk yielded a significant difference between mono (1.27 log(10) cfu/ml) and bipolar (1.96 log(10) cfu/ml) pulses with bipolar pulses being significantly more efficient (P < 0.05). Among different pulse delaying times, 20 mus caused a significantly higher inactivation than the others (P < 0.05) in apple juice and skim milk. (C) 2004 Elsevier Ltd. All rights reserved.