Effects of pulse polarity and pulse delaying time on pulsed electric fields-induced pasteurization of E. coli O157:H7

Abstract

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 log₁₀ cfu/ml) and bipolar (2.63 log₁₀ 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 log₁₀ cfu/ml) and bipolar (1.96 log₁₀ cfu/ml) pulses with bipolar pulses being significantly more efficient (P < 0.05). Among different pulse delaying times, 20 μs caused a significantly higher inactivation than the others (P < 0.05) in apple juice and skim milk.

Introduction

Effectiveness of pulsed electric fields (PEF) when compared to thermal processing is well established for the inactivation of different microorganisms as well as minimization of losses in volatile compounds and color in the course of processing and storage (Barbosa-Canovas, Gongora-Nieto, Pothakamury, & Swanson, 1999; Qin, Zhang, Swanson, & Pedrow, 1994; Zhang, Monsalve-Gonzalez, Qin, Barbosa-Canovas, & Swanson, 1994). The PEF processing in both bench and pilot plant scale has been successfully applied to various liquid products including apple, orange and cranberry juices, and whole, 2%, skim and chocolate milk (Evrendilek, Streaker, Dantzer, Ratanatriwong, & Zhang, 2001; Evrendilek et al., 2000; Evrendilek, Zhang, & Richter, 1999; Reina, Jin, Zhang, & Yousef, 1998; Qiu, Sharma, Tuhela, Jia, & Zhang, 1998).

The effectiveness of pulsed electric fields on the inactivation of microorganisms depends upon several processing conditions, electric field strength, treatment time, pulse duration, pulse frequency, flow rate, pulse shape, and number of pulses per chamber. The electric field pulses may be applied in the form of exponentially decaying, square-wave, bipolar, and instant-charge-reversal. An exponential decay pulse is a unidirectional voltage that rises rapidly to a maximum value and decays slowly to zero (Barbosa-Canovas et al., 1999). Square-wave pulse can be described as the rapid increase of the voltage, application of the constant voltage for a specified period of time, and then, rapid decays of the voltage application. Bipolar pulses can be exponentially decay or square-wave. In both cases they contain one positive and one negative consecutive pulse. Instant-charge-reversal pulse can be described as partially positive at first and partially negative immediately thereafter (Ho & Mittal, 1997).

Several studies were conducted to determine the effect of pulse wave shape on microbial inactivation by PEF. Zhang et al. (1994) reported that square wave pulses resulted in a 60% more inactivation of Saccharomyces cerevisiae than did exponential decay pulse wave. Qin et al. (1994) determined that compared to exponential decay, oscillatory decay pulses were not efficient on the inactivation of Escherichia coli.

The effect of the pulse polarity on E. coli and Bacillus subtilis cells inoculated into simulated milk ultra filtrate (SMUF) was investigated with a monopolar pulse of the exponentially decaying waveform. Bipolar pulse pair possessed a waveform with exponential decay pulses of one positive and one negative. Bipolar pulses led to a more efficient inactivation of microorganism. Similar results were obtained for B. subtilis inactivation (Qin et al., 1994).

Previous studies with square wave bipolar pulses focused on inactivation of different microorganisms such as Listeria monocytogenes, Salmonella dublin, S. Enteritidis, B. subtilis spores, and E. coli O157:H7 (Evrendilek et al., 1999, Evrendilek et al., 2000; Hermawan, 1999; Reina et al., 1998; Sensoy, Zhang, & Sastry, 1997; Su, 1996). Inactivation kinetics of these microorganisms was studied with bipolar pulses. However, literature lacks information on the effect of pulse polarity of square wave pulses and pulse delaying time on the inactivation of different microorganisms. Therefore, the objectives of the study were to determine the effects of pulse polarity and pulse delaying time on the inactivation of E. coli O157:H7 inoculated into apple juice and skim milk samples.