Effect of electric and flow parameters on PEF treatment efficiency

Decay of Trichomes of Arthrospira platensis After Permeabilization Through Pulsed Electric Fields (PEFs) Causes the Release of Phycocyanin

The cyanobacterium Arthrospira platensis is a promising source of edible proteins and other highly valuable substances such as the blue pigment-protein complex phycocyanin. Pulsed electric field (PEF) technology has recently been studied as a way of permeabilizing the cell membrane, thereby enhancing the mass transfer of water-soluble cell metabolites. Unfortunately, the question of the release mechanism is not sufficiently clarified in published literature. In this study, the degree of cell permeabilization (cell disintegration index) was directly measured by means of a new method using fluorescent dye propidium iodide (PI). The method allows for conclusions to be drawn about the effects of treatment time, electric field strength, and treatment temperature. Using a self-developed algorithm for image segmentation, disintegration of trichomes was observed over a period of 3 h. This revealed a direct correlation between cell disintegration index and decay of trichomes. This decay, in turn, could be brought into a direct temporal relationship with the release of phycocyanin. For the first time, this study reveals the relationship between permeabilization and the kinetics of particle decay and phycocyanin extraction, thus contributing to a deeper understanding of the release of cell metabolites in response to PEF. The results will facilitate the design of downstream processes to produce sustainable products from Arthrospira platensis.

Advantages and limitations of various treatment chamber designs for reversible and irreversible electroporation in life sciences

The fundamental mechanisms of pulsed electric fields on biological cells are not yet fully elucidated, though it is apparent that membrane electroporation plays a crucial role. Little is known about treatment-chamber-specific effects, and systematic studies are scarce. Thus, the present study evaluates the (dis-)advantages of various treatment chamber designs for liquid applications at differing scales. Three chambers, namely parallel plate microfluidic (V̇: 0.1 ml/min; titanium electrodes), co-linear meso (V̇: 5.0 ml/min; stainless steel electrodes), and co-linear macro (V̇: 83.3 ml/min; stainless steel electrodes) chambers, were studied. Electroporation effects on Escherichia coli in media with 0.1-10.0 mS/cm were evaluated by plate counts and flow cytometry at 8, 16, and 20 kV/cm. For the microfluidic chamber, predominantly irreversible electroporation (2.5 logs₁₀ reductions) was seen at 0.1 mS/cm, while high irreversible electroporation (4.2 logs₁₀ reductions) at 10.0 mS/cm was observed for the macro chamber. The meso chamber indicated a similar trend towards increased conductivity, even though only low inactivation levels were present. Variation in conductivity and electrode configuration or area likely induces effects resulting in distinct electroporation levels, as observed for the micro and macro chamber. Suitable application scenarios, depending on targeted electroporation effects, were suggested.

Reversibility of membrane permeabilization upon pulsed electric field treatment in Lactobacillus plantarum WCFS1

Pulsed electric field (PEF) treatment, or electroporation, can be used to load molecules into cells. The permeabilizing effect of the PEF treatment on the cellular membrane can be either reversible or irreversible depending on the severity of the PEF treatment conditions. The influence of PEF on the reversibility of membrane permeabilization in Lactobacillus plantarum WCFS1 by two different fluorescent staining methods was investigated in this study. Whereas staining with propidium iodide (PI) before and after PEF treatment indicated small reversible permeabilized fractions of maximum 14%, the use of a double staining method with PI and SYTOX Green suggested larger reversible permeabilized fractions up to 40% of the population. This difference shows that the choice for a fluorescent staining method affects the conclusions drawn regarding reversibility of membrane permeabilization. Additionally, the effect of PEF treatment conditions on membrane integrity was compared, indicating a relation between critical electric field strength, cell size and membrane permeabilization. Overall this study showed the possibilities and limitations of fluorescent membrane integrity staining methods for PEF studies.

Modular Serial Flow Through device for pulsed electric field treatment of the liquid samples

In biotechnology, medicine, and food processing, simple and reliable methods for cell membrane permeabilization are required for drug/gene delivery into the cells or for the inactivation of undesired microorganisms. Pulsed electric field treatment is among the most promising methods enabling both aims. The drawback in current technology is controllable large volume operation. To address this challenge, we have developed an experimental setup for flow through electroporation with online regulation of the flow rate with feedback control. We have designed a modular serial flow-through co-linear chamber with a smooth inner surface, the uniform cross-section geometry through the majority of the system’s length, and the mesh in contact with the electrodes, which provides uniform electric field distribution and fluid velocity equilibration. The cylindrical cross-section of the chamber prevents arching at the active treatment region. We used mathematical modeling for the evaluation of electric field distribution and the flow profile in the active region. The system was tested for the inactivation of Escherichia coli. We compared two flow-through chambers and used a static chamber as a reference. The experiments were performed under identical experimental condition (product and similar process parameters). The data were analyzed in terms of inactivation efficiency and specific energy consumption.

Mechanisms of inactivation of Candida humilis and Saccharomyces cerevisiae by pulsed electric fields

AIMS

This study aimed to determine how electric field strength, pulse width and shape, and specific energy input relate to the effect of pulsed electric fields (PEF) on viability and membrane permeabilization in Candida humilis and Saccharomyces cerevisiae suspended in potassium phosphate buffer.

METHODS AND RESULTS

Cells were treated with a micro-scale system with parallel plate electrodes. Propidium iodide was added before or after treatments to differentiate between reversible and irreversible membrane permeabilization. Treatments of C. humilis with 71kV/cm and 48kJ/kg reduced cell counts by 3.9±0.6 log (cfu/mL). Pulse shape or width had only a small influence on the treatment lethality. Variation of electric field strength (17-71kV/cm), pulse width (0.086-4μs), and specific energy input (8-46kJ/kg) demonstrated that specific energy input correlated to the membrane permeabilization (r²=0.84), while other parameters were uncorrelated. A minimum energy input of 3 and 12kJ/kg was required to achieve reversible membrane permeabilization and a reduction of cell counts, respectively, of C. humilis.

CONCLUSIONS

Energy input was the parameter that best described the inactivation efficiency of PEF.

SIGNIFICANCE AND IMPACT OF STUDY

This study is an important step to identify key process parameters and to facilitate process design for improved cost-effectiveness of commercial PEF treatment.

Selective extraction of intracellular components from the microalga Chlorella vulgaris by combined pulsed electric field-temperature treatment

The synergistic effect of temperature (25-65 °C) and total specific energy input (0.55-1.11 kWh kgDW(-1)) by pulsed electric field (PEF) on the release of intracellular components from the microalgae Chlorella vulgaris was studied. The combination of PEF with temperatures from 25 to 55 °C resulted in a conductivity increase of 75% as a result of cell membrane permeabilization. In this range of temperatures, 25-39% carbohydrates and 3-5% proteins release occurred and only for carbohydrate release a synergistic effect was observed at 55 °C. Above 55 °C spontaneous cell lysis occurred without PEF. Combined PEF-temperature treatment does not sufficiently disintegrate the algal cells to release both carbohydrates and proteins at yields comparable to the benchmark bead milling (40-45% protein, 48-58% carbohydrates).

Protein Extraction by Means of Electroporation from E. coli with Preserved Viability

Extracting proteins by means of electroporation from different microorganisms is gaining on its importance, as electroporation is a quick, chemical-free, and cost-effective method. Since complete cell destruction (to obtain proteins) necessitates additional work, and cost of purifying the end-product is high, pulses have to be adjusted in order to prevent total disintegration. Namely, total disintegration of the cell releases bacterial membrane contaminants in the final sample. Therefore, our goal was to study different electric pulse parameters in order to extract as much proteins as possible from E. coli bacteria, while preserving bacterial viability. Our results show that by increasing electric field strength the concentration of extracted proteins increases and viability reduces. The correlation is reasonable, since high electric field destroys bacterial envelope, releasing all intracellular components into surrounding media. The strong correlation was also found with pulse duration. However, at longer pulses we obtained more proteins, while bacterial viability was not as much affected. Pulse number and/or pulse repetition frequency at our conditions have no or little effect on concentration of extracted proteins and/or bacterial viability. We can conclude that the most promising pulse protocol for protein extraction by means of electroporation based on our experience would be longer pulses with lower pulse amplitude assuring high protein yield and low effect on bacterial viability.

Inactivation of Escherichia coli and Staphylococcus aureus by pulsed electric fields increases with higher bacterial population and with agitation of liquid medium

Inactivation of Escherichia coli, E. coli O157:H7, and Staphylococcus aureus in liquid media by pulsed electric fields (PEF) was conducted at varying bacterial populations with and without sample agitation. A laboratory-scale PEF batch unit with a rectangular electric pulse was used, operating under the following conditions: 25 kV/cm (E. coli, E. coli O157:H7) and 30 kV/cm (S. aureus) electric field strengths, 1-μs pulse width, 1-Hz pulse repetition rate, and 20 to 350 pulses for all samples. Not surprisingly, bacterial inactivation (for all three strains) increased with increasing pulse number, achieving the highest reduction at 350 pulses. Log CFU per milliliter microbial inactivation increased commensurately with increasing bacterial population (P < 0.05) but only when samples were treated with more than 200 pulses. For example, when E. coli was treated with 200 pulses at 10(5) CFU/ml, inactivation was only 3.0 Log versus 4.8 Log at the 10(10) inoculation level. When E. coli O157:H7 was treated with 200 pulses at 10(5) CFU/ml, inactivation was only 2.5 Log versus 4.6 Log at the 10(10) inoculation level. When S. aureus was treated with 200 pulses at 10(6) CFU/ml, inactivation was only 2.6 Log versus 4.8 Log at the 10(10) inoculation level. Inactivation of populations was also found to be statistically greater (P < 0.05) when liquid samples were agitated, in comparison to nonagitated samples. Because PEF inactivation activity is influenced by bacterial population and sample agitation, future studies should carefully consider these factors in experimental designs and/or scaled-up industry application.

Effect of pulsed electric fields on microbial inactivation and physico-chemical properties of whole porcine blood

The objective of this study was to determine the lethal effectiveness of pulsed electric fields on the inactivation of the porcine blood endogenous microflora. Furthermore, the impact of pulsed electric field application on physico-chemical and sensory properties in this medium should be proved. Blood samples from a commercial abattoir in Germany were processed by a continuous pilot plant-pulsed electric field system at electric field strength of 11 kV/cm for treatment times of 163 and 209 µs. The applied pulse frequencies of 134 and 175 Hz correspond to an energy input of 91 and 114 kJ/kg, respectively. In these conditions, the effectiveness of pulsed electric field processing on microbial inactivation was limited: 1.35 log10 CFU/mL reduction of total aerobic plate count ( p < 0.05), 1.0 log10 CFU/mL for Pseudomonas spp. ( p < 0.05), 0.97 and 0.66 log10 CFU/mL reduction for Enterobacteriaceae and sulfite-reducing anaerobic bacteria, respectively. However, the storage experiment (14 days at +3 ℃) showed a significant reduced growth of total aerobic plate count ( p < 0.05) and Pseudomonas spp. ( p < 0.05) in the pulsed electric field-treated blood samples. Pulsed electric field processing leads to a complete hemolysis of the red blood cells, in addition significant decreased L* (lightness), a* (redness) and b* (yellowness) values ( p < 0.0001) were observed. Furthermore, changes in the sensory attributes color (changed from red to dark brown) and odor (changed from fresh to musty and tangy) were noticed.

Inactivation of Pseudomonas putida by pulsed electric field treatment: a study on the correlation of treatment parameters and inactivation efficiency in the short-pulse range

An important issue for an economic application of the pulsed electric field treatment for bacterial decontamination of wastewater is the specific treatment energy needed for effective reduction of bacterial populations. The present experimental study performed in a field amplitude range of 40 > E > 200 kV/cm and for a suspension conductivity of 0.01 = κ(e) > 0.2 S/m focusses on the application of short pulses, 25 ns > T > 10 μs, of rectangular, bipolar and exponential shape and was made on Pseudomonas putida, which is a typical and widespread wastewater microorganism. The comparison of inactivation results with calculations of the temporal and azimuthal membrane charging dynamics using the model of Pauly and Schwan revealed that for efficient inactivation, membrane segments at the cell equator have to be charged quickly and to a sufficiently high value, on the order of 0.5 V. After fulfilling this basic condition by an appropriate choice of pulse field strength and duration, the log rate of inactivation for a given suspension conductivity of 0.2 S/m was found to be independent of the duration of individual pulses for constant treatment energy expenditure. Moreover, experimental results suggest that even pulse shape plays a minor role in inactivation efficiency. The variation of the suspension conductivity resulted in comparable inactivation performance of identical pulse parameters if the product of pulse duration and number of pulses was the same, i.e., required treatment energy can be linearly downscaled for lower conductivities, provided that pulse amplitude and duration are selected for entire membrane surface permeabilization.