Effects of pulse polarity and pulse delaying time on pulsed electric fields-induced pasteurization of E. coli O157:H7
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.
Section snippets
Materials and methods
E. coli O157:H7 serotype 35150 was cultured from tryptone glucose yeast (TGY) (Difco, Detroit, MI) agar slants obtained from the Department of Microbiology at the Ohio State University (Columbus, OH). The initial culture was activated by transferring into Brain Heart Infusion (BHI) (Difco, Detroit, MI) tubes and incubated at 37 °C for 12 h prior to inoculation of the samples. Overnight grown cultures of E. coli O157:H7 were inoculated into 500 ml of apple juice and skim milk samples for a final
Results and discussions
Apple juice and skim milk samples had different pH and conductivity values. Apple juice samples had pH and electrical conductivity of 3.70 ± 0.24 and 0.23 S/m. These values for skim milk samples were 6.7 ± 0.65 and 0.62 ± 0.34 S/m, respectively. Since physical properties of the samples were different, different PEF processing parameters were applied to apple juice and skim milk samples. In order to determine PEF processing parameters, a dosage study was performed to determine treatment conditions.
Conclusion
Evaluation of the efficacy of the current bench scale PEF pulse generators and the treatment conditions revealed that square wave bipolar and monopolar pulses were all effective in the inactivation of E. coli O157:H7 cells in both apple juice and skim milk (P < 0.05). Bipolar pulses appeared more efficient in skim milk, protein rich media (P < 0.05). Efficiency of the PEF processing increases as the pulse delaying times decreases from 20 to 3 μs. Similarly, longer pulse delaying times from 20 to
Acknowledgments
We would like to thank Dr. C. Wang for building the circuit diagram and Dr. S. Li for building OSU-4A treatment chambers used in the study.
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