Membrane permeabilization due to pulsed electric field (PEF) treatment of gram-positive Lactobacillus cells was investigated by using propidium iodide uptake and single-cell analysis with flow cytometry. Electric field strength, energy input, treatment time, and growth phase affected membrane permeabilization of Lactobacillus plantarum during PEF treatment. A correlation between PEF inactivation and membrane permeabilization of L. plantarum cells was demonstrated, whereas no relationship was observed between membrane permeabilization and heat inactivation. The same results were obtained with a Lactobacillus fermentum strain, but the latter organism was more PEF resistant and exhibited less membrane permeabilization, indicating that various bacteria have different responses to PEF treatment. While membrane permeabilization was the main factor involved in the mechanism of inactivation, the growth phase and the acidity of the environment also influenced inactivation. By using flow cytometry it was possible to sort cells in the L. plantarum population based on different cell sizes and shapes, and the results were confirmed by image analysis. An apparent effect of morphology on membrane permeabilization was observed, and larger cells were more easily permeabilized than smaller cells. In conclusion, our results indicate that the ability of PEF treatment to cause membrane permeabilization is an important factor in determining inactivation. This finding should have an effect on the final choice of the processing parameters used so that all microorganisms can be inactivated and, consequently, on the use of PEF treatment as an alternative method for preserving food products.A high-voltage pulsed electric field (PEF) can inactivate microorganisms under reduced-temperature conditions. Consequently, food products have a fresher appearance and lose less flavor and other functional food components, factors that are currently in high demand by consumers (1,16,27). PEF treatment is the application of pulses with very high field strength for a short time (microseconds) to foods placed between two electrodes. Due to technical and technological developments during the last few years, it is now possible to perform PEF treatment in a continuous-treatment chamber. This has increased the efficiency of the treatment process and offers more possibilities for scaling up the technology (4, 26, 40), which has enhanced interest by the food industry.Recently, microbial inactivation kinetics were systematically studied under a range of conditions in continuous-PEF systems (8,29,39). Furthermore, inactivation kinetics were determined under close-to-isothermal conditions to study the effect of field strength and energy input independent of heat (12). It was concluded that electric field strength and the amount of energy input, (i.e., the number of pulses) were important in determining the inactivation level. Other important process factors were pulse length and inlet temperature (39). Product factors (pH and conductivity) and the physiological s...
Aims: The effect of critical pulsed electric field (PEF) process parameters, such as electric field strength, pulse length and number of pulses, on inactivation of Lactobacillus plantarum was investigated. Methods and Results: Experiments were performed in a pH 4Á5 sodium phosphate buffer having a conductivity of 0Á1 S m À1 , using a laboratory-scale continuous PEF apparatus with a co-linear treatment chamber. An inactivation model was developed as a function of field strength, pulse length and number of pulses. Based on this inactivation model, the conditions for a PEF treatment were optimized with respect to the minimum energy required to obtain a certain level of inactivation. It was shown that the least efficient process parameter in the range investigated was the number of pulses. The most efficient way to optimize inactivation of Lact. plantarum was to increase the field strength up to 25Á7 kV cm À1 , at the shortest pulse length investigated, 0Á85 ls, and using a minimum number of pulses. The highest inactivation of Lact. plantarum at the lowest energy costs is obtained by using the equation: E ¼ 26Á7s 0Á23 , in which E is the field strength and s the pulse length. An optimum is reached by substituting s with 5Á1. Conclusions: This study demonstrates that the correct choice of parameters, as predicted by the model described here, can considerably improve the PEF process. Significance and Impact of the Study: The knowledge gained in this study improves the understanding of the limitations and opportunities of the PEF process. Consequently, the advantage of the PEF process as a new option for non-thermal decontamination can be better utilized.
The effects of pulsed electric field (PEF) treatment and processing factors on the inactivation kinetics of Listeria innocuaNCTC 11289 were investigated by using a pilot plant PEF unit with a flow rate of 200 liters/h. The electric field strength, pulse length, number of pulses, and inlet temperature were the most significant process factors influencing the inactivation kinetics. Product factors (pH and conductivity) also influenced the inactivation kinetics. In phosphate buffer at pH 4.0 and 0.5 S/m at 40°C, a 3.0-V/μm PEF treatment at an inlet temperature of 40°C resulted in ≥6.3 log inactivation of strain NCTC 11289 at 49.5°C. A synergistic effect between temperature and PEF inactivation was also observed. The inactivation obtained with PEF was compared to the inactivation obtained with heat. We found that heat inactivation was less effective than PEF inactivation under similar time and temperature conditions.L. innocua cells which were incubated for a prolonged time in the stationary phase were more resistant to the PEF treatment, indicating that the physiological state of the microorganism plays a role in inactivation by PEF. Sublethal injury of cells was observed after PEF treatment, and the injury was more severe when the level of treatment was increased. Overall, our results indicate that it may be possible to use PEF in future applications in order to produce safe products.
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