Pulsed Electric Field Inactivation Research Articles

Disentangling the effects of electroporation and heat during pulsed electric field (PEF) processing of oat-based milk alternative: A case study on Lactiplantibacillus plantarum inactivation

The study aimed to quantify the resistance of two L. plantarum strains and estimate the contribution of temperature during the pulsed electric field (PEF) processing of oat beverage. PEF and thermal inactivation kinetic experiments of L.plantarum strains were performed separately. Log10 reductions ≥5.7 of L. plantarum was achieved when 10–24 kV/cm, 110–115 kJ/L, 80–522 μs, and 35 °C inlet temperature were applied, while strain variability was evident in the performed assessments. Accurate monitoring of the temperature profiles for PEF allowed the estimation of the thermal microbial inactivation which was separated from electroporation effect. The electroporation impact was found to increase with increasing electric field strengths. Temperature measurements in the treatment zone confirmed the presence of hot spots. At higher specific energy inputs, there was significant contribution of temperature to the total observed inactivation. This contribution became smaller when the flow rate was increased, as shorter residence time reduced the exposure to high temperature peaks. Industrial relevanceThe findings demonstrated the potential of PEF as a promising technology for the microbial decontamination of oat-based milk alternative. Knowledge of strain variability in response to emerging food processing technologies is critical during process optimisation and quantitative microbial risk assessment. New developments in PEF equipment and process should ideally aim to maximize inactivation while minimizing the impact of high temperature on sensitive food constituents.

Pulsed Electric Field Inactivation of Microorganism Using Chambers With Different Electrode Topologies

The effects of electrode topology on pulsed electric field (PEF) inactivation of microorganisms was investigated in the present study. Three PEF treatment chambers with different electrode topologies, which were parallel-plane, coaxial, and co-field respectively, were designed and built. PEF inactivation of yeast Saccharomyces cerevisiae suspension was conducted using these chambers with different electric field strength and a number of impulses. The performance of PEF inactivation was assessed by comparison of the population density of treated and untreated yeast samples. The results showed that the sterilization rate, as a function of the specific energy density, followed the same tendency regardless of the electrode topology, electric field strength, or the number of impulses. This strong correlation suggested that the specific energy density (energy per unit mass) could play a dominant role in the PEF inactivation of microorganisms once the threshold field strength was reached.

Pulsed electric field inactivation of microorganisms: from fundamental biophysics to synergistic treatments.

The growth of antibiotic resistant microorganisms and the increasing demand for nonthermal antimicrobial treatment in the food and beverage industry motivates research into alternative inactivation methods. Pulsed electric fields (PEFs) provide an athermal method for inactivating microorganisms by creating nanometer-sized membrane pores in microorganisms, inducing cell death when the PEF duration and intensity are sufficient such that the pores cannot reseal after the PEFs through a process referred to as irreversible electroporation. While PEF inactivation has been studied for several decades, recent studies have focused on extending the technique to various liquids in the food industry and optimizing microorganism inactivation while minimizing adverse effects to the treated sample. This minireview will assess the biophysical mechanisms and theory of PEF-induced cellular interactions and summarize recent advances in applying this technology for microorganism inactivation alone and synergistically in combination with other technologies, including temperature, pressure, natural ingredients, and pharmaceuticals.

Engineering of operation condition in milk pasteurization with PEF treatment

We propose a new continuous pasteurization technique for milk that involves pulsed electric field (PEF) treatment with a voltage of 40 kVp and a frequency of 100 Hz. Escherichia coli with an initial concentration 107 cells/mL that was suspended in commercial milk was used as the target microorganism. Without control of the milk temperature, an average holding time in PEF treatment of 20 s should be necessary for the efficient inactivation of E. coli suspended in milk. Thermal control was incorporated in our system to improve the productivity and stability of the pasteurization process. Sufficient inactivation of E. coli cells was found to be possible even with an average holding time in PEF treatment of 10 s at a milk temperature of 50 °C or higher. We proposed a system for the PEF pasteurization of milk, in which a preheating unit and postholding unit were incorporated. When the milk temperature at the inlet of the PEF treatment chamber was controlled at 60 °C using preheating unit and the milk temperature after the PEF treatment was held at 70 °C for 20 s using the postholding unit, a reduction in the CFU of E. coli cells suspended in milk of at least seven orders of magnitude was achieved in our continuous PEF inactivation system.

Kinetic models for pulsed electric field and thermal inactivation of Escherichia coli and Pseudomonas fluorescens in whole milk

Inactivation of Escherichia coli ATCC 11775 and Pseudomonas fluorescens ATCC 948 in UHT whole (4% fat) milk during thermal processing at 56–62 °C and pulsed electric field (PEF) processing at 30 or 35 kV cm−1 at approximately 30, 40 or 50 °C was investigated. E. coli ATCC 11775 was more heat-resistant than P. fluorescens ATCC 948, but more susceptible to PEF processing. All inactivation kinetics showed strong deviations from log-linearity. Thus, a simplified logistic (log-decay) regression model was used to accurately predict thermal and PEF inactivation of E. coli ATCC 11775 and P. fluorescens ATCC 948 under various treatment conditions. This is a useful tool for identifying processing conditions to inactivate pathogenic and spoilage microorganisms in whole milk at sub-pasteurisation temperatures.

Synergistic effect of thermal and pulsed electric field (PEF) treatment on the permeability of soya PC and DPPC vesicles

The permeability change of vesicles composed of soya phosphatidylcholine (PC) and dipalmitoyl phosphatidylcholine (DPPC) encapsulated with 5(6)-carboxyfluorescein (CF) suspended in isotonic sucrose solution under separate or combined thermal and continuous pulsed electric field (PEF) treatments was investigated, respectively. The electric field intensity applied was in the range from 20 to 40kV/cm with total treatment time of 1.2ms, and thermal treatments were conducted at temperatures from 15 to 65°C. Results indicated that the permeability behavior between DPPC vesicles and soya PC vesicles were quite different under the experimental conditions. Compared to soya PC vesicles, DPPC vesicles were more sensitive to thermal treatments. As the temperature increased to 41°C, the 5(6)-CF release percentage (R) reached almost 100% abruptly. On the other hand, the synergetic effects of PEF and temperature on permeability of two vesicles were apparently different. For soya PC vesicles, the increment degree of R value was almost the same under various combination treatment conditions. However, for DPPC vesicles, it was demonstrated that the lipid bilayers membrane was almost impermeable to CF when the PEF strength was lower than 30kV/cm at 20°C and 20kV/cm at 30°C. As the temperature increased to 38°C, the permeability of DPPC vesicles was significantly increased with increasing PEF intensity, reaching 25.25% at 40kV/cm. Results from the current study could be helpful for better understanding of the underlying mechanism on PEF inactivation as affected by the lipid composition of cell membrane and its phase transition temperature.

TOWARD 6 log10 PULSED ELECTRIC FIELD INACTIVATION WITH CONDUCTIVE PLASTIC PACKAGING MATERIAL

ABSTRACTGenerally, high grade products such as pulsed electric field (PEF) treated fruit juices are packaged after their preservative treatment. However, PEF treatment after packaging could avoid recontamination of the product and becomes feasible when electric field pulses of sufficient magnitude can be generated inside closed food containments. Microbial inactivation of 2 log10 was proved by using an electrically conductive plastic packaging material, and in this contribution it is shown that inactivation toward pasteurization level is achievable. Therefore, a series of microbial experiments with Lactobacillus plantarum at different energy levels were performed. The obtained microbial data with the plastic material have been compared with inactivation models from literature as well as with reference experiments. All treatments were done at ∼2.3 kV/mm, with treatment times between 10–600 µs, and the thermal load was below 38C. The maximum obtained microbial inactivation with the plastic packaging material was 5.9 log10 with a specific energy of 255 J/mL.PRACTICAL APPLICATIONSIn order to further minimize the risk of contaminating microorganisms, e.g., introduced during the packaging process, this new technology of pulsed electric field (PEF) treatment after packaging can be beneficial in reducing food safety incidents. This technique also makes hygienic packaging machines superfluous, which will reduce operational and investment costs. By proving that PEF inactivation through a plastic packaging material is possible, the first step toward PEF treatment after packaging has been taken. Important next steps toward industrial realization are extensive research on food grade conductive packaging films, PEF in‐pack processing methodologies and microbial investigation of different microorganisms.

Pulsed electric field inactivation of E. coli O157:H7 and non-pathogenic surrogate E. coli in strawberry juice as influenced by sodium benzoate, potassium sorbate, and citric acid

Current FDA regulations require that juice processors achieve a 5 log CFU/ml reduction of a target pathogen prior to distributing products. Whereas thermal pasteurization reduces the sensory characteristics of juice, pulsed electric field (PEF) treatments can be conducted at lower temperatures and may preserve sensory characteristics.Escherichia coli O157:H7 (ATCC 43895) and a non-pathogenic E. coli (ATCC 35218), respectively, were inoculated into single-strength strawberry juice with or without 750 ppm sodium benzoate (SB), 350 ppm potassium sorbate (PS), and 2.7% citric acid (CA). Juice was treated at outlet temperatures of 45, 50 and 55 °C at a field strength of 18.6 kV/cm for 150 μs with a laboratory-scale PEF unit. Inactivation of surrogate E. coli at 45, 50, and 55 °C were 2.86, 3.12, and 3.79 log CFU/ml, respectively, in plain juice (pH 3.4), and 2.75, 3.52, and 5.11 with the addition of benzoic and sorbic acids (pH 3.5). Inactivation of E. coli O157:H7 under the same conditions were 3.09, 4.08, and 4.71 log CFU/ml, respectively, and 2.27, 3.29, and 5.40 with antimicrobials. E. coli O157:H7 in juice with antimicrobials and 2.7% CA (pH 2.7) treated with PEF was reduced by 2.60, 4.32 and 6.95 log CFU/ml at 45, 50 and 55 °C while the surrogate E. coli decreased by 3.54, 5.69, and 7.13 log under the same conditions. When juice (pH 2.7) was held for 6 h without PEF treatment, higher numbers of E. coli 35218 (7.17 log CFU/ml) were inactivated than of acid-resistant E. coli O157:H7 (3.89 log). Slightly greater PEF inactivation of E. coli O157:H7 than of the surrogate bacterium indicates that E. coli ATCC 35218 provides a margin of safety when used as a surrogate for O157:H7 in plain strawberry juice or in juice + SB + PS at 45–50 °C, or with SB + PS and CA at 55 °C.

GERMINATION AND SUBSEQUENT INACTIVATION OFBACILLUS SUBTILISSPORES BY PULSED ELECTRIC FIELD TREATMENT

ABSTRACT The use of high-intensity pulsed electric field (PEF) as a nonthermal method for triggering spore germination was investigated. Spores suspended in 0.01% NaCl solution (ca. 108cells/mL) were treated with an exponential-decay pulse (20–50 kV/cm) at selected temperatures. The inactivation resulting from the direct effect of PEF treatment was less than 0.5 log cycles, even with a relatively high electric field intensity of 40 kV/cm. Most of the surviving PEF-treated spores germinated after inoculation into selected germination media (0.85% NaCl solution, nutrient broth [NB], or tryptic soy broth [TSB]) and subsequent incubation at 37C for 40 min. The extent of germination increased with the treatment temperature and electric field intensity. More than 98% of surviving spores in TSB germinated during incubation after PEF treatment with an electric field strength of 40 kV/cm for 1,000 µs at 50C. The rate of germination in nutrient-rich TSB medium was higher than that in NB. PRACTICAL APPLICATIONS High-intensity pulsed electric field (PEF) treatments have been studied recently as an alternative method of conventional thermal sterilization. High-intensity PEF inactivation of microorganisms can be carried out at ambient temperature with a short treatment time of seconds or less without inducing a significant temperature increase. Considerable research has been conducted on the inactivation of microbial cells using PEF. However, a major obstacle to the application of PEF as a technology for the preservation of foods and pharmaceuticals is that it is inefficient at inactivating bacterial spores. Bacterial spores are also resistant to inactivation by adverse physical and chemical agents. Methods to overcome this problem have been studied extensively with the aim of establishing reliable sterilization procedures for the food and pharmaceutical industries.

Critical Comparison between the Pulsed Electric Field and Thermal Decontamination Methods of Hospital Wastewater

Filtered wastewater samples were inoculated with Enterococcus faecium and exposed to different pulsed electric field (PEF) treatment energies at 40, 50 and 60◦C. A lethal effect of heat treatment on E. faecium can be observed for T > 56◦C and a heating duration of 4 min. A treatment with 60◦C, 4 min, provokes a bacterial reduction of 4 log. A combined treatment of inoculated wastewater samples with pulsed electric field and heat (40, 50 and 60◦C) reduced the bacterial contamination considerably. At a suspension temperature of 60◦C pulsed electric field inactivation resulted in a complete bacterial decontamination (8 log reduction rate). Moreover, it was demonstrated that naturally occurring nuclease activities were not changed by the pulsed electric field treatment. In contrast to a thermal treatment with temperature over 72◦C, for 4 min., the nuclease activity was reduced up to 90%. In this case, inactivation is only 4 log. The same inactivation rate can be obtained by a combination of heat treatment (60◦C) and pulsed electric field treatment (30 J/ml).

Inactivation System Using Silver Electrode by High Voltage Pulsed Electric Field

The inactivation of Escherichia coli K-12 was investigated in a system using a pulsed electric field (PEF) and a spiral silver wire electrode. The survival ratios of E. coli K-12 decreased with increasing voltage. The sterilization effect was greater with a high-voltage silver wire electrode than with a stainless steel electrode. This result was attributed to the enhancement of PEF inactivation by the lethal effect of silver. The PEF inactivation effect with electrochemically eluted silver was higher than that with silver nitrate solution, though the silver concentrations were almost the same. These results suggest that simultaneous treatment of E. coli cells with PEF inactivation and silver electrochemical elution improves inactivation efficiency.

Inactivation of Bacteria in Oil-Field Reinjection Water by Pulsed Electric Field (PEF) Process

Microbial pollution commonly causes serious pipe corrosion in oil-field reinjection water systems. As an alternative and environmentally friendly inactivation technology, the pulsed electric field (PEF) process is employed to inactivate microorganisms in oil-field reinjection water. The effects on inactivation of process parameters including electric field strength, process temperature, initial bacteria concentration, and conductivity were investigated. The results showed that electric field intensity is the most important parameter in PEF inactivation. After a 20-min treatment, the inactivation efficiency was a 2.8 log reduction for saprophytic bacteria, a 3.6 log reduction for iron bacteria, and a 3.9 log reduction for sulfate-reducing bacteria. Transmission electron microscopy observations of the bacteria confirmed that PEF technology can lead to severe surface damage and rupture to the cells. Moreover, a possible mechanism of inactivation is discussed and used to explain the influence of process param...

Inactivation of spores using pulsed electric field in a pressurized flow system

Pulsed electric field (PEF) inactivation is a very effective method to kill bacteria in liquid while avoiding thermal damage. However, only a limited inactivation effect on spores has been reported to date and the possible mechanisms are still unclear. Therefore, a study of inactivation of spores using PEF in a pressurized flow system is reported in this paper. PEF with a maximum magnitude higher than 110 kV/cm and a pulse width of 100 ns has been applied to a carefully designed treatment chamber through which a suspension fluid of 0.5 MPa continuously flows. Using the proposed PEF inactivation method, maximum 6.7 log reductions were achieved for B. subtilis spores that were investigated. These reductions were much greater than those obtained by a heat inactivation approach. Through frequency analysis using the frequency components of the applied pulses and the frequency response of the equivalent circuit of the spore, it was found that most voltage is applied to the outside of the core in the lower frequency and to the inside in the upper frequency. Also, transmission electron microscope micrographs of B. subtilis spores were taken in order to verify the effect of the PEF treatment.

Development of Textile Electrode for Microbial Inactivation with Pulsed Electric Field

We have been studying pulsed electric field (PEF) inactivation of various bacteria, which is based on the destruction of biological membrane due to applying pulsed electric field. In this study, we report a novel treatment system based on the textile electrode, combination of polyester fiber and 0.2 mmφ metal wire. The efficiency of PEF inactivation of Escherichia coli cells depended on the peak voltage and the conductivity of the target solutions. Higher efficiency of PEF sterilization could be achieved with 7 kV of peak voltage and lower solution conductivity. When the solution conductivity was lower, some materials, which seemed to be bacterial residue, were sedimented on high voltage electrode.

Synergistic effect of temperature and pulsed electric field on inactivation of Escherichia coli O157:H7 and Salmonella enteritidis in liquid egg yolk

The effects of temperature, treatment time and electric field strength on inactivation of Escherichia coli O157:H7 and Salmonella enteritidis in liquid egg yolk by pulsed electric field (PEF) processing were examined using the reaction kinetics approach. Egg yolk, inoculated with ∼10 8 CFU mL −1 of E. coli O157:H7 or S. enteritidis, was treated in a continuous flow process at 20, 30 and 40 °C in combination with pulsed electric field intensities of 20 and 30 kV cm −1. A biphasic instant reversal PEF waveform with a 2 μs pulse width was used and a maximum of 105 pulses were applied. Increasing the applied electric field intensity, treatment time and process temperature resulted in increased bacterial inactivation. At 30 kV cm −1 and 40 °C, the populations of E. coli O157:H7 and S. enteritidis were reduced by ∼5 logs. The inactivation rate constants increased from 0.004 to 0.098 μ s −1 for S. enteritidis whereas for E. coli O157:H7 the constants increased from 0.009 to 0.039 μs −1 as processing temperature increased from 20 to 40 °C. S. enteritidis was more resistant to PEF inactivation than E. coli O157:H7 at lower processing temperatures.

Microbial inactivation by new technologies of food preservation

The increasing consumer demand for 'fresh-like' foods has led to much research effort in the last 20 years to develop new mild methods for food preservation. Nonthermal methods allow micro-organisms to be inactivated at sublethal temperatures thus better preserving the sensory, nutritional and functional properties of foods. The aim of this review is to provide an overview of the microbiological aspects of the most relevant nonthermal technologies for microbial inactivation currently under study, including irradiation, high hydrostatic pressure, pulsed electric field and ultrasound under pressure. Topics covered are the mechanisms of inactivation, sensitivity of different microbial groups and factors affecting it and kinetics of inactivation.

Pulsed electric field inactivation of spoilage microorganisms in alcoholic beverages

In recent years, a number of new applications have emerged where pulsed power is being used in the treatment of waste and effluent, foodstuffs and beverages. One of these emerging applications is pulsed electric field (PEF) inactivation of microorganisms in liquid media. This involves the generation of electric fields of the order of 30 kV/cm across liquids contaminated with microorganisms. This induces a relatively large transmembrane potential that can lead to irreversible electroporation and consequently cell lysis. The nature of the PEF pulse profile is the subject of extensive study, and it has been reported that bipolar square waves provide superior inactivation when compared to monopolar pulses. A previous study, however, has challenged this view, and results will be presented demonstrating that more effective inactivation of bacteria can be achieved using the monopolar pulse. Results will also be given on the effect of monopolar pulse PEF applied to alcoholic beverages containing known spoilage microorganisms. This will highlight an apparent synergistic inactivation effect when microorganisms in alcoholic beverages are exposed to PEF.

Weibull Distribution Function Based on an Empirical Mathematical Model for Inactivation of Escherichia coli by Pulsed Electric Fields

The pulsed electric field inactivation kinetics of Escherichia coli suspended in orange juices with three different concentrations of carrot juice (0, 20, and 60%) was studied. Electric field strengths ranged from 25 to 40 kV/cm, and treatment times ranged from 40 to 340 μs. Experimental data were fitted to Bigelow, Hülsheger, and Weibull distribution functions, and the Weibull function provided the best fit (with the lowest mean square error). The dependency of each model's kinetic constant on electric field strength and carrot juice concentration was studied. A secondary model was developed to describe the relationship of Weibull parameters a and n to electric field strength and carrot juice concentration. An empirical mathematical model based on the Weibull distribution function, relating the natural logarithm of the survival fraction to treatment time, electric field strength, and carrot juice concentration, was developed. Parameters were estimated by a nonlinear regression. The results of this study indicate that the error rate for the model's predictions was 6.5% and that the model was suitable for describing E. coli inactivation.

Inactivation and injury of Escherichia coli O157:H7 and Staphylococcus aureus by pulsed electric fields

Two pathogenic microorganisms Escherichia coli O157:H7 and Staphylococcus aureus, suspended in peptone solution (0.1% w/v) were treated with 12, 14, 16 and 20 kV/cm electric field strengths with different pulse numbers up to 60 pulses. Pulsed electric field (PEF) treatment at 20 kV/cm with 60 pulses provided nearly 2 log reduction in viable cell counts of E. coli O157:H7 and S. aureus. S. aureus cells were slightly more resistant than E.coli O157:H7 cells. The results related to the effect of initial cell concentration of E. coli O157:H7 on the PEF inactivation showed that more inactivation was obtained by decreasing initial cell concentration. Any possible injury by PEF was also investigated after applying 20 kV/cm electric field to the microorganisms. As a result, it was determined that there was 35.92 to 43.36% injury in E. coli O157:H7 cells, and 17.26 to 30.86% injury in S. aureus cells depending on pulse number. The inactivation results were also described by a kinetic model.

Membrane permeabilization in relation to inactivation kinetics of Lactobacillus species due to pulsed electric fields.

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.