Patrick D. Pedrow

Inactivation of microorganisms by pulsed electric fields of different voltage waveforms

Processing foods with HV pulsed electric fields (PEF) is a new technology to inactivate microorganisms and denature enzymes with only a small increase in temperature. Introduction of this new technology will replace or complement conventional thermal processing methods. It will also provide consumers with safe, nutritious foods with fresh quality. For a given peak value of field intensity and amount of electric energy input, PEF inactivation of microorganisms is closely related to the waveform of applied pulses. Inactivation of microorganisms was studied with different waveshapes including exponential decay, oscillatory decay, square waves, and bipolar pulses. Microbial inactivation was tested in a parallel-plate static treatment chamber. Treatment field intensity ranged from 12 to 40 kV/cm while pulse length ranged from 30 to 180 /spl mu/s. From the microbial test results, bipolar square-wave pulses are the most efficient in terms of microbial inactivation for commercial PEF pasteurization. >

Inactivating microorganisms using a pulsed electric field continuous treatment system

High-voltage pulsed electric fields (PEFs) can be used to inactivate microorganisms in liquids. Applying PEF technology to food pasteurization is a promising nonthermal method, which may radically change food preservation processes and provide consumers with microbiologically safe, minimally processed, fresh-like products. A continuous-flow system in a laboratory-size prototype was constructed for the nonthermal pasteurization of liquid foods with PEF technology. Major components in the prototype include a high-voltage repetitive pulse generator, a coaxial liquid food treatment chamber, a fiber-optic temperature sensing instrument and a data acquisition system. Microbial inactivation tests were conducted in the continuous PEF treatment system. Repetitive high-voltage pulses with an exponential decaying waveshape were applied to the liquid food which was pumped through the treatment chamber. Test microorganisms selected for inactivation were Escherichia coli, Staphylococcus aureus and Saccharomyces cerevisiae. Over 6-order-of-magnitude reductions in the viability of selected microorganisms were achieved while the food temperature was maintained below 40/spl deg/C.

Pulsed Electric Field Treatment Chamber Design for Liquid Food Pasteurization Using a Finite Element Method

High voltage pulsed electric field (PEF) treatment is a promising nonthermal processing method that may radically change liquid food preservation technology. One of the key components in the PEF pasteurization process is a treatment chamber. In this work, the chamber was designed with the aid of an electric field optimization technique. The finite element method was used in the numerical analysis of electric fields and for optimization of the electrode geometry. Using the optimized electrodes in the coaxial treatment chamber, a prescribed field distribution in foods was achieved without points of electric field enhancement. Two types of treatment chambers, parallel plate and coaxial electrode, were constructed and tested. Microbial inactivation tests were conducted using exponential decaying and square waveforms. Electric field intensity ranged from 20 to 50 kV/cm, while pulse length ranged from 2 to 20 ms. More than seven log cycles of microbial inactivation of Escherichia coli and Saccharomyces cerevisiae were achieved when the food temperature was maintained below 30° C.

Energy analysis of liquid whole egg pasteurized by pulsed electric fields

Abstract Non-thermal preservation of liquid whole egg (LWE) with pulsed electric fields (PEF) is an attractive alternative to thermal processing where protein coagulation is of concern. The objective of this study was to evaluate the energy applied under different PEF processing conditions and its effect on the microbial quality of LWE during refrigerated storage. The LWE was stabilized with citric acid (CA) at 0.15% and 0.5% to prevent color darkening. Inside a pilot plant-size PEF chamber, the peak values of the electric field traces at the high-voltage electrode, middle gap, and low-voltage electrode were 37, 30, and 25 kV/cm, respectively. The pulse width was 1.84 μs, with energy density at 11.9 J/ml per pulse. The total treatment time varied from 54 to 478 μs (corresponding with 30–266 slightly underdamped pulses). The microbiological quality of the LWE was monitored weekly while under refrigerated storage at 4 °C. The CA not only acted as a color stabilizer but also increased the effectiveness of PEF treatment. The maximum shelf-life sustained at 4 °C of LWE with 0.15% CA was 20 days, with PEF treatments up to 489 μs (266 pulses) at an average electric field of 30 kV/cm. The total processing energy delivered to this product was 6331 J/ml. LWE with 0.5% CA had a shelf-life of almost 30 days at 4 °C, using a maximum PEF energy expenditure of 357 J/ml (30 pulses or 55 μs of 30 kV/cm).

Impact of air bubbles in a dielectric liquid when subjected to high field strengths

Abstract The dielectric breakdown of gas bubbles entrapped in liquid food flowing through the cavity of a pulsed electric field treatment chamber has been a limiting factor in this non-thermal food preservation technology. Prediction of electric field enhancement due to gas bubbles is an important tool in the design, modification, and optimization of the treatment chambers electrode geometry and pressurization. Simulation of the electrostatic characteristics of a coaxial treatment chamber with specified voltage of 25 kV, filled with dielectric material with conductivity of 0.6 S/m containing gas bubbles, evidenced a significant perturbation in the electric field. The magnitude of electric fields generated inside the bubbles was almost two times higher than in the homogeneous food. Without pressurization (atmospheric conditions), the dielectric breakdown strength of the gas-filled bubbles was exceeded, thus indicating the risk of arcing. A system pressurization of approximately 8 atm could be sufficient to limit arcing when small gas bubbles (∼1 mm) are present. The presence of gas bubbles caused the field magnitude to decrease significantly near the boundary of the bubble, thus threatening the uniformity of the PEF treatment across the chamber gap. This perturbation in the electric field was more significant when more than one bubble was present or when smaller gaps were used. The dielectric breakdown threshold at a given pressure is more likely to be exceeded by bigger bubbles (>1 mm) entrapped in a fluid processed in smaller treatment chamber gaps (3 mm), than by smaller bubbles (

Corona onset as a design optimization criterion for high voltage hardware

Hardware for use on HV systems is designed to be corona free. Recent hardware design activity has been centered around attempts to increase the corona onset voltage by optimizing the electric field distribution on its surface. Corona onset, however, is not only a function of the surface electric field, but also its rate of decay away from the surface, and the temperature and pressure of the gas in which it is immersed. While corona onset is a better criterion, formulas for predicting it have been validated only for simple geometries. The first goal of this work, then, is to validate corona onset conditions for more practical electrode geometries. The second goal is to determine whether the use of corona onset rather than surface electric field as an optimization criterion can result in hardware with a measurably higher corona onset voltage. To test this idea, two electrodes were designed, one using electric field optimization and the second using corona onset optimization. The corona onset voltage of each electrode was then measured and the results compared with predicted values. It was found that the measured results compared favorably with the predicted values and that the use of corona onset optimization can result in a modest but measurable increase in corona onset voltage.

Cold Atmospheric-Pressure Plasmas Applied to Active Packaging of Apples

Active packaging of fruits and vegetables uses films that absorb molecules from or contribute molecules to the produce. The pilot application developed in this paper has resulted in the deposition of film to apples. A prospective application relates to replacing hot wax that is expensive and that lowers the textural quality of the apple. This was the early motivation of this paper. Moreover, the focus of this paper will be mostly on the reactor design and film evaluation. The cold-plasma zone was obtained by increasing the voltage on a needle-to-needle electrode structure until the electric field in the feed material (argon + monomer) was sufficiently high to yield electron avalanches and self-propagating streamers. The ¿corona onset criterion¿ was used to design the cold-plasma reactor. The apple was placed in a treatment chamber downstream from the activation zone (cold-plasma zone). Selected physical properties of the film were measured. Environmental scanning electron microscopy and Fourier transform infrared studies of samples were also performed to determine the presence of the film. Electromagnetic modeling was applied to the design of the cold-plasma reactor, and those results are presented in this paper.

Electrical environment surrounding microbes exposed to pulsed electric fields

Inactivation of microbes by the application of intense pulsed electric fields (/spl sime/10 to 40 kV/cm) could result in low-temperature pasteurization of liquid foods. Advantages over conventional heat pasteurization include longer shelf-life, better flavor, and less enzyme damage. Numerical modeling of electrical parameters near the microbe during exposure to these intense electric fields is described. The continuity equation describes movement of positive and negative ions while Gausss law yields the electric field after movement of the ions. One negative ionic species and one positive ionic species are assumed to be in the suspension fluid and protoplasm of the microbe. The microbe membrane is modeled as a nonconducting dielectric. With application of unidirectional electric fields, free volume and free surface charge densities form along the membrane. Comparison is made with a uniform conductivity model and it is shown that significant differences exist in parameters such as ion concentration, free surface charge density, free volume charge density, heat sources due to conduction current, and ionic injection at membrane surfaces.

Plasma-enhanced metal-organic chemical vapor deposition (PEMOCVD) of catalytic coatings for fuel cell reformers

Fuel cells have the potential to solve several major challenges in the global energy economy: dependence on petroleum imports, degradation of air quality, and greenhouse gas emissions. Using catalyst-based reformer technology, hydrogen for fuel cells can be derived from infrastructure fuels such as gasoline, diesel, and natural gas. Platinum is one catalyst that is known to be very effective in hydrogen reformers. Reformer size can be reduced when there is more efficient catalyst loading onto the substrate. In this experimental work, platinum was loaded onto /spl gamma/-alumina coated substrates by plasma-polymerization followed by heat treatment. Vapor from a platinum-containing organic precursor was converted to plasma and deposited onto the substrate. The plasma-polymerized film was then calcined to drive off organic material, leaving behind a catalyst-loaded substrate. The plasma-polymerized organic film and the final heat-treated catalyst-loaded substrate surface were characterized by scanning electron microscopy (SEM) and impedance spectroscopy. Energy dispersive spectroscopy (EDS) was used to detect the presence of the catalyst on the substrate.

A continuous treatment system for inactivating microorganisms with pulsed electric fields

High voltage pulsed electric fields (PEF) can be used to inactivate microorganisms in liquids. Applying this PEF technology to food pasteurization is a promising nonthermal method, which may radically change food preservation processes and provide consumers with microbiologically safe, minimally processed, fresh-like products. A continuous-flow system in laboratory-size prototype was constructed for the nonthermal pasteurization of liquid foods with PEF technology. Major components in the prototype include a high voltage repetitive pulse generator, a coaxial liquid food treatment chamber, a fiber optic temperature sensing instrument, and a data acquisition system. Microbial inactivation tests were conducted in the continuous PEF treatment system. Repetitive high voltage pulses with an exponential decaying waveshape were applied to the liquid food which was pumped through the treatment chamber. Test microorganisms selected for inactivation were: Escherichia coli; Staphylococcus aureus; and Saccharomyces cerevisiae. Over six order-of-magnitude reductions in the viability of selected microorganisms were achieved when the food temperature was maintained below 40/spl deg/C.