Pulsed electric field (PEF) technology applied to food processing was firstly used in the late 1960s. The currently available systems use either conventional Blumlein generators or generators similar to those found in radar power sources to produce the required high voltage pulses. The liquid to be processed is passed through a number of treatment chambers or cells which each contain a pair of electrodes in contact with the liquid. An electric field is thereby applied to the liquid, leading to the technology being termed invasive and it can be used only with liquid food. A novel and non-invasive PEF technology for use in the food processing industry is introduced and investigated in this thesis. The technology represents a novel way of performing PEF treatment. A proof of concept arrangement uses two ceramic cylinders mounted inside the non-invasive PEF cell with a gap of 3 mm between them. A displacement current of the order of mA passes through the non-invasive PEF cell during treatment, as compared with the kA of current usually produced during an invasive treatment. The low current is not only economic in electric energy but also maintains a low food temperature, which implicitly maintains food flavour. In the thesis the electro-optic Kerr effect technique is used to perform accurately the PEF measurement and convincingly prove that strong electric fields are present. Two Kerr water cells were designed and used to determine the Kerr constant for water, since the data presented in the literature is unreliable. The first Kerr water cell uses a pair of Bruce profile stainless steel electrodes and the second a pair of parallel plate stainless steel electrodes. An electro-static solver (Maxwell software) was used to determine the electric field distribution and to calculate the electric field integral to accurately determine the Kerr constant for water. Water samples containing the E-coli bacteria were prepared and filled in the non-invasive PEF cell by the Flavometrix Company. Eight PEF experiments were successfully performed during this research programme and the results show unequivocally that the novel noninvasive technique is effective in significantly reducing the initial concentration of E-coli bacteria. This opens the door for the future design of an industrial prototype.

In this study conducted at Food Science Australia (FSA) and Berlin University of Technology (BUT), the effects of pulsed electric field (PEF) treatment, a novel non-thermal processing technology on bovine milk microflora and native enzymes and on the rheological and textural properties of rennet-induced milk gels was investigated. The PEF treatments were conducted at field intensities of 25-37 kV cm-1 (up to 50 kV cm-1)and temperature range of 30°C to 75ºC. Native milk enzymes selected for the study included alkaline phosphatase, lipase, xanthine oxidase and plasminand microbiological study included determining Total Plate Count (TPC) and Pseudomonas and Enterobacteriaceae counts in skim milk. At 30ºC PEF treatment at maximum field intensity inactivated AlP by 42% while at 60oC inactivation was higher (67%). Under these treatment conditions less than1 log reduction in TPC and Pseudomonas count and 2.1 logs reduction in the Enterobacteriaceae count was achieved at 30oC while at 60ºC TPC dropped by 2.4 logs and Pseudomonas and Enterobacteriaceae counts were reduced by 5.9 and 2.1 logs, respectively to below the detection limit of 1 CFU mL-1. Combining PEF treatment with heat increased the inactivation level of all enzymes which showed an increasing trend with increasing field intensity and temperature. Treatment time (4.8, 9.6, 19.2, 28.8 and 38.4 µs) was controlled by either changing the pulse frequencies (100-400 Hz) or product flow rate (30-240 mL min-1) at a constant field intensity of 31 kV cm-1 and it was found that changing the flow rate was a more effective way of enzyme inactivation than changing the frequency due to longer exposure time of enzymes to heat and field intensity. The size of casein micelles and fat globules was not affected by PEF treatment while severe heating of milk at 97oC for 10 min decreased both micelle and fat globule sizes marginally. The coagulation time of rennet-induced gels made from PEF-treated (35 to 50 kV cm-1) milks (whole and skim) increased as the treatment intensity increased, but remained shorter than gels made from pasteurised milk. The PEF treatment of milk at various field intensities and temperatures adversely affected the G′, G′′ and firmness of gels, but the effects were less pronounced than in gels made from pasteurised milks. This study concludes that for successful application in milk processing the PEF treatment needs to be combined with mild heat treatment. This approach could achieve safer milk with less damage to milk functionality. However, the quest for a suitable quality assurance indicator enzyme will need more extensive studies.

The PEF treatments were conducted at field intensities of 25-37 kV cm-1 (up to 50 kV cm-1)and temperature range of 30°C to 75ºC. Native milk enzymes selected for the study included alkaline phosphatase, lipase, xanthine oxidase and plasminand microbiological study included determining Total Plate Count (TPC) and Pseudomonas and Enterobacteriaceae counts in skim milk. At 30ºC PEF treatment at maximum field intensity inactivated AlP by 42% while at 60oC inactivation was higher (67%). Under these treatment conditions less than1 log reduction in TPC and Pseudomonas count and 2.1 logs reduction in the Enterobacteriaceae count was achieved at 30oC while at 60ºC TPC dropped by 2.4 logs and Pseudomonas and Enterobacteriaceae counts were reduced by 5.9 and 2.1 logs, respectively to below the detection limit of 1 CFU mL-1. Combining PEF treatment with heat increased the inactivation level of all enzymes which showed an increasing trend with increasing field intensity and temperature. Treatment time (4.8, 9.6, 19.2, 28.8 and 38.4 µs) was controlled by either changing the pulse frequencies (100-400 Hz) or product flow rate (30-240 mL min-1) at a constant field intensity of 31 kV cm-1 and it was found that changing the flow rate was a more effective way of enzyme inactivation than changing the frequency due to longer exposure time of enzymes to heat and field intensity. The size of casein micelles and fat globules was not affected by PEF treatment while severe heating of milk at 97oC for 10 min decreased both micelle and fat globule sizes marginally. The coagulation time of rennet-induced gels made from PEF-treated (35 to 50 kV cm-1) milks (whole and skim) increased as the treatment intensity increased, but remained shorter than gels made from pasteurised milk. The PEF treatment of milk at various field intensities and temperatures adversely affected the G′, G′′ and firmness of gels, but the effects were less pronounced than in gels made from pasteurised milks. This study concludes that for successful application in milk processing the PEF treatment needs to be combined with mild heat treatment. This approach could achieve safer milk with less damage to milk functionality. However, the quest for a suitable quality assurance indicator enzyme will need more extensive studies.

The use of high-voltage pulsed electric fields in biotechnology and medicine has lead to new methods of cancer treatment, gene therapy, drug delivery, and non-thermal inactivation of microorganisms. Regardless of the application, the objective is to open pores in the cell membrane and hence either facilitate the delivery of foreign materials inside the cell or to kill the cell completely. Pulsed power supplies are needed for electroporation, which is the process of applying pulsed electric fields to biological cells to induce a temporary permeability in the cell membrane. The applications of pulsed electric fields are dependent on the output pulse shape and pulse parameters, both of which can be affected by the circuit parameters of the pulsed power supply and the conductivity of the media being treated. <br /><br /> In this research, two Metal Oxide Field Effect Transistor (MOSFET)-based pulsed power supplies that are used for electroporation experiments were designed and built. The first used up to three MOSFETs in parallel to deliver high voltage pulses to highly conductive loads. To produce pulses with higher voltages, a second pulsed power supply using two MOSFETs connected in series was designed and built. The parallel and series MOSFET-based pulsed power supplies are capable of producing controllable square pulses with widths of a few hundred nanoseconds to dc and amplitudes up to 1500 V and 3000 V, respectively. The load in this study is a 1-mm electroporation cuvette filled with a buffer solution that is varied in conductivity from 0. 7 mS/m to 1000 mS/m. The results indicate that by controlling the circuit parameters such as the number of parallel MOSFETs, gate resistance, energy storage capacitance, and the parameters of the MOSFET driver gating pulses, the output pulse parameters can be made almost independent of the load conductivity. <br /><br /> Using the pulsed power supplies designed in this work, an investigation into electroporation-mediated delivery of a plasmid DNA molecule into the pathogenic bacterium <em>E. coli</em> O157:H7, was conducted. It was concluded that increasing the electric field strength and pulse amplitude resulted in an increase in the number of transformants. However, increasing the number of pulses had the effect of reducing the number of transformants. In all of the experiments the number of cells that were inactivated by the exposure to the pulsed electric field was measured.

The use of high-voltage pulsed electric fields in biotechnology and medicine has lead to new methods of cancer treatment, gene therapy, drug delivery, and non-thermal inactivation of microorganisms. Regardless of the application, the objective is to open pores in the cell membrane and hence either facilitate the delivery of foreign materials inside the cell or to kill the cell completely. Pulsed power supplies are needed for electroporation, which is the process of applying pulsed electric fields to biological cells to induce a temporary permeability in the cell membrane. The applications of pulsed electric fields are dependent on the output pulse shape and pulse parameters, both of which can be affected by the circuit parameters of the pulsed power supply and the conductivity of the media being treated. <br /><br /> In this research, two Metal Oxide Field Effect Transistor (MOSFET)-based pulsed power supplies that are used for electroporation experiments were designed and built. The first used up to three MOSFETs in parallel to deliver high voltage pulses to highly conductive loads. To produce pulses with higher voltages, a second pulsed power supply using two MOSFETs connected in series was designed and built. The parallel and series MOSFET-based pulsed power supplies are capable of producing controllable square pulses with widths of a few hundred nanoseconds to dc and amplitudes up to 1500 V and 3000 V, respectively. The load in this study is a 1-mm electroporation cuvette filled with a buffer solution that is varied in conductivity from 0. 7 mS/m to 1000 mS/m. The results indicate that by controlling the circuit parameters such as the number of parallel MOSFETs, gate resistance, energy storage capacitance, and the parameters of the MOSFET driver gating pulses, the output pulse parameters can be made almost independent of the load conductivity. <br /><br /> Using the pulsed power supplies designed in this work, an investigation into electroporation-mediated delivery of a plasmid DNA molecule into the pathogenic bacterium <em>E. coli</em> O157:H7, was conducted. It was concluded that increasing the electric field strength and pulse amplitude resulted in an increase in the number of transformants. However, increasing the number of pulses had the effect of reducing the number of transformants. In all of the experiments the number of cells that were inactivated by the exposure to the pulsed electric field was measured.

Pulsed electric field (PEF) processing has been demonstrated to be an effective non-thermal pasteurization method for food-treatment applications. With this method, high voltage, short-duration pulses are applied to a chamber through which liquid food is passed. If the voltage applied and the corresponding electric field develops a potential higher than a critical trans-membrane potential, the pores expand, and the membrane of the living cell is ruptured. Due to the lower amount of energy consumed during a PEF process, the temperature of the liquid is kept much lower than as opposed to conventional pasteurization. The PEF method thus kills bacteria and other microorganisms while preserving the nutrition and taste of the liquid foods. Although the parameter responsible for inactivation is the voltage applied, for any given voltage, the conductivity of the liquid defines a current through the liquid that causes the temperature to rise. Therefore, preventing excessive heating of the liquid requires the application of an efficient waveform. According to the literature, the most efficient waveform is a square wave since the entire energy applied would be used for the inactivation process. Although some power supplies are capable of generating such a waveform, the generation of an efficient waveform that satisfies all the requirements for producing a viable product for PEF applications is still a challenging problem. In this research, a cascadable pulse generator, based on a Marx generator design, was designed and implemented in order to generate a pulsed waveform for the treatment of liquid food. IGBT switches were used to charge capacitors in parallel and to discharge them in series as a means of generating a high voltage at the output. The design was implemented and tested for two stages, generating up to 6 kV and 1.6 kA square pulses with a controllable pulse width from 1 µs to 10 µs. Up to 3 switches were connected in parallel to enhance the current capability of the system. Also investigated are ways to improve the transient time by enhancing the IGBT driver circuit. The effect of design parameters such as pulse width, voltage, and current on the temperature rise in the liquid was also studied. A variety of liquid foods with different conductivities were tested in order to confirm the functionality of the system.

Pulsed electric field (PEF) processing has been demonstrated to be an effective non-thermal pasteurization method for food-treatment applications. With this method, high voltage, short-duration pulses are applied to a chamber through which liquid food is passed. If the voltage applied and the corresponding electric field develops a potential higher than a critical trans-membrane potential, the pores expand, and the membrane of the living cell is ruptured. Due to the lower amount of energy consumed during a PEF process, the temperature of the liquid is kept much lower than as opposed to conventional pasteurization. The PEF method thus kills bacteria and other microorganisms while preserving the nutrition and taste of the liquid foods. Although the parameter responsible for inactivation is the voltage applied, for any given voltage, the conductivity of the liquid defines a current through the liquid that causes the temperature to rise. Therefore, preventing excessive heating of the liquid requires the application of an efficient waveform. According to the literature, the most efficient waveform is a square wave since the entire energy applied would be used for the inactivation process. Although some power supplies are capable of generating such a waveform, the generation of an efficient waveform that satisfies all the requirements for producing a viable product for PEF applications is still a challenging problem. In this research, a cascadable pulse generator, based on a Marx generator design, was designed and implemented in order to generate a pulsed waveform for the treatment of liquid food. IGBT switches were used to charge capacitors in parallel and to discharge them in series as a means of generating a high voltage at the output. The design was implemented and tested for two stages, generating up to 6 kV and 1.6 kA square pulses with a controllable pulse width from 1 µs to 10 µs. Up to 3 switches were connected in parallel to enhance the current capability of the system. Also investigated are ways to improve the transient time by enhancing the IGBT driver circuit. The effect of design parameters such as pulse width, voltage, and current on the temperature rise in the liquid was also studied. A variety of liquid foods with different conductivities were tested in order to confirm the functionality of the system.

Increasing consumer demand for new products with high nutritional qualities has spurred a search for new alternatives to food preservation. Pulsed electric field (PEF) is an emerging technology for non thermal food pasteurisation. Using this technology, enzymes, pathogenic and spoilage microorganisms can be inactivated without affecting the colour, flavour, and nutrients of the food. PEF treatment may be provided by applying pulsed electric field to a food product in a treatment zone between two electrodes at ambient , or slightly above ambient temperature. Exposure of microbial cells to the electric field induces a transmembrane potential in the cell membrane, which results in electroporation (the permeabilization of the membranes of cells and organelles) and/or electrofusion (the connection of two separate membranes into one) of the cells. An innovative pulsed electric field (PEF) unit was designed and constructed in the University of Auckland using modern IGBT technology. The system consists of main equipments, the high voltage pulse generator and the treatment chambers. The main focus of this work was to design an innovative PEF treatment chamber that provide uniform distribution of electric field, minimum increase in liquid temperature, minimum fouling of electrodes and an energy efficient system. Four multi pass treatment chambers were designed consisting of two stainless steel mesh electrodes in each chamber, with the treated fluid flowing through the openings of the mesh electrodes. The two electrodes are electrically isolated from each other by an insulator element designed to form a small orifice where most of the electric field is concentrated. Dielectric breakdown inside the chambers was prevented by removing the electrodes far from the narrow gap. The effect of the chambers different geometries on the PEF process in terms of electric parameters and microbial inactivation were investigated. Electric field intensity in the range of (17-43 kV/cm) was applied with square bipolar pulses of 1.7 µs duration. The effect of PEF treatment on the inactivation of gram-negative Escherichia coli ATCC 25922 suspended in simulated milk ultra-filtrate (SMUF) of 100%, 66.67% and 50% concentration was investigated. Treatments with the same electrical power input but higher electric field strengths provided larger degree of killing. The inactivation rate of E coli was significantly increased with increasing the electric field strength, treatment time and processing temperature. Morphological changes on E coli as a result of PEF treatment were studied under transmission electron microscopy (TEM). Significant morphological changes on E coli after PEF treatment were observed. The TEM studies suggested that the microbial inactivation was a consequence of electroporation and electrofusion mechanisms. Kinetic analysis of microbial inactivation due to PEF and thermal treatment of E coli suspended in SUMF were also studied. Comparison between measured (experimental) and predicted (theoretical) variation of E coli concentration with time following the PEF treatment was discussed, taking into consideration the recirculation mode of the PEF treatment. The treated liquid was circulated more than once through the treatment chamber to provide higher microbial inactivation. Arrhenius constants and activation energies of E coli inactivation using combined PEF and thermal treatment were calculated and generalized correlation for the inactivation rate constant as a function of electric field intensity and treatment temperature was developed. / Fonterra Research Institute (NZ) and the Foundation for Research Science and Technology (NZ)

Increasing consumer demand for new products with high nutritional qualities has spurred a search for new alternatives to food preservation. Pulsed electric field (PEF) is an emerging technology for non thermal food pasteurisation. Using this technology, enzymes, pathogenic and spoilage microorganisms can be inactivated without affecting the colour, flavour, and nutrients of the food. PEF treatment may be provided by applying pulsed electric field to a food product in a treatment zone between two electrodes at ambient , or slightly above ambient temperature. Exposure of microbial cells to the electric field induces a transmembrane potential in the cell membrane, which results in electroporation (the permeabilization of the membranes of cells and organelles) and/or electrofusion (the connection of two separate membranes into one) of the cells. An innovative pulsed electric field (PEF) unit was designed and constructed in the University of Auckland using modern IGBT technology. The system consists of main equipments, the high voltage pulse generator and the treatment chambers. The main focus of this work was to design an innovative PEF treatment chamber that provide uniform distribution of electric field, minimum increase in liquid temperature, minimum fouling of electrodes and an energy efficient system. Four multi pass treatment chambers were designed consisting of two stainless steel mesh electrodes in each chamber, with the treated fluid flowing through the openings of the mesh electrodes. The two electrodes are electrically isolated from each other by an insulator element designed to form a small orifice where most of the electric field is concentrated. Dielectric breakdown inside the chambers was prevented by removing the electrodes far from the narrow gap. The effect of the chambers different geometries on the PEF process in terms of electric parameters and microbial inactivation were investigated. Electric field intensity in the range of (17-43 kV/cm) was applied with square bipolar pulses of 1.7 µs duration. The effect of PEF treatment on the inactivation of gram-negative Escherichia coli ATCC 25922 suspended in simulated milk ultra-filtrate (SMUF) of 100%, 66.67% and 50% concentration was investigated. Treatments with the same electrical power input but higher electric field strengths provided larger degree of killing. The inactivation rate of E coli was significantly increased with increasing the electric field strength, treatment time and processing temperature. Morphological changes on E coli as a result of PEF treatment were studied under transmission electron microscopy (TEM). Significant morphological changes on E coli after PEF treatment were observed. The TEM studies suggested that the microbial inactivation was a consequence of electroporation and electrofusion mechanisms. Kinetic analysis of microbial inactivation due to PEF and thermal treatment of E coli suspended in SUMF were also studied. Comparison between measured (experimental) and predicted (theoretical) variation of E coli concentration with time following the PEF treatment was discussed, taking into consideration the recirculation mode of the PEF treatment. The treated liquid was circulated more than once through the treatment chamber to provide higher microbial inactivation. Arrhenius constants and activation energies of E coli inactivation using combined PEF and thermal treatment were calculated and generalized correlation for the inactivation rate constant as a function of electric field intensity and treatment temperature was developed. / Fonterra Research Institute (NZ) and the Foundation for Research Science and Technology (NZ)

Increasing consumer demand for new products with high nutritional qualities has spurred a search for new alternatives to food preservation. Pulsed electric field (PEF) is an emerging technology for non thermal food pasteurisation. Using this technology, enzymes, pathogenic and spoilage microorganisms can be inactivated without affecting the colour, flavour, and nutrients of the food. PEF treatment may be provided by applying pulsed electric field to a food product in a treatment zone between two electrodes at ambient , or slightly above ambient temperature. Exposure of microbial cells to the electric field induces a transmembrane potential in the cell membrane, which results in electroporation (the permeabilization of the membranes of cells and organelles) and/or electrofusion (the connection of two separate membranes into one) of the cells. An innovative pulsed electric field (PEF) unit was designed and constructed in the University of Auckland using modern IGBT technology. The system consists of main equipments, the high voltage pulse generator and the treatment chambers. The main focus of this work was to design an innovative PEF treatment chamber that provide uniform distribution of electric field, minimum increase in liquid temperature, minimum fouling of electrodes and an energy efficient system. Four multi pass treatment chambers were designed consisting of two stainless steel mesh electrodes in each chamber, with the treated fluid flowing through the openings of the mesh electrodes. The two electrodes are electrically isolated from each other by an insulator element designed to form a small orifice where most of the electric field is concentrated. Dielectric breakdown inside the chambers was prevented by removing the electrodes far from the narrow gap. The effect of the chambers different geometries on the PEF process in terms of electric parameters and microbial inactivation were investigated. Electric field intensity in the range of (17-43 kV/cm) was applied with square bipolar pulses of 1.7 µs duration. The effect of PEF treatment on the inactivation of gram-negative Escherichia coli ATCC 25922 suspended in simulated milk ultra-filtrate (SMUF) of 100%, 66.67% and 50% concentration was investigated. Treatments with the same electrical power input but higher electric field strengths provided larger degree of killing. The inactivation rate of E coli was significantly increased with increasing the electric field strength, treatment time and processing temperature. Morphological changes on E coli as a result of PEF treatment were studied under transmission electron microscopy (TEM). Significant morphological changes on E coli after PEF treatment were observed. The TEM studies suggested that the microbial inactivation was a consequence of electroporation and electrofusion mechanisms. Kinetic analysis of microbial inactivation due to PEF and thermal treatment of E coli suspended in SUMF were also studied. Comparison between measured (experimental) and predicted (theoretical) variation of E coli concentration with time following the PEF treatment was discussed, taking into consideration the recirculation mode of the PEF treatment. The treated liquid was circulated more than once through the treatment chamber to provide higher microbial inactivation. Arrhenius constants and activation energies of E coli inactivation using combined PEF and thermal treatment were calculated and generalized correlation for the inactivation rate constant as a function of electric field intensity and treatment temperature was developed. / Fonterra Research Institute (NZ) and the Foundation for Research Science and Technology (NZ)