R.A. Fouracre

Inactivation of food-borne enteropathogenic bacteria and spoilage fungi using pulsed-light

The lethality of high-intensity pulsed-light emissions from low and high ultraviolet (UV) light sources on predetermined microbial populations has been investigated. Prior to treatment, the bacterial enteropathogens Bacillus cereus, Escherichia coli, and Salmonella enteritidis and the food-spoilage fungi Aspergillus niger and Fusarium culmorum were seeded separately onto the surface of either tryptone soya yeast extract or malt extract agar plates. Prescribed microbial population densities were applied to the test media and these samples were exposed to one of two light sources. These were low-pressure, xenon filled, flash lamps that produced either high or low UV intensities. They were operated in pulsed mode, being driven by a stacked Blurnlein table generator. Microbial samples were treated by exposure to different numbers of light pulses. The treated bacterial populations were reduced by /spl sim/8 log orders after 1000 light-pulses of the higher UV intensity light and the fungal counts had a corresponding reduction of 4.5 log orders. The fungus, Aspergillus niger, was shown to be significantly more resistant in spore form to the intense UV light compared with Fusarium culmorum. This resistance has been attributed to the high level of UV absorbance associated with the dark pigment present in A. niger. The pulsed light source of lower UV intensity was shown to be significantly less effective in reducing microbial populations.

Light inactivation of food‐related pathogenic bacteria using a pulsed power source

The effects of high intensity light emissions, produced by a novel pulsed power energization technique (PPET), on the survival of bacterial populations of verocytotoxigenic Escherichia coli (serotype 0157:H7) and Listeria monocytogenes (serotype 4b) were investigated. Using this PPET approach, many megawatts (MW) of peak electrical power were dissipated in the light source in an extremely short energization time (about 1 μs). The light source was subjected to electric field levels greater than could be achieved under conventional continuous operation, which led to a greater production of the shorter bacteriocidal wavelengths of light. In the exposure experiments, pre‐determined bacterial populations were spread onto the surface of Trypone Soya Yeast Extract Agar and were then treated to a series of light pulses (spectral range of 200–530 nm) with an exposure time ranging from 1 to 512 μs. While results showed that as few as 64 light pulses of 1 μs duration were required to reduce E. coli 0157:H7 populations by 99·9% and Listeria populations by 99%, the greater the number of light pulses the larger the reduction in cell numbers (P < 0·01). Cell populations of E. coli 0157:H7 and Listeria were reduced by as much as 6 and 7 log10 orders at the upper exposure level of 512 μs, respectively. Survival data revealed that E. coli 0157:H7 was less resistant to the lethal effects of radiation (P < 0·01). These studies have shown that pulsed light emissions can significantly reduce populations of E. coli 0157:H7 and L. monocytogenes on exposed surfaces with exposure times which are 4–6 orders of magnitude lower than those required using continuous u.v. light sources.

Inactivation of pathogenic and spoilage microorganisms in a test liquid using pulsed electric fields

Experiments have been carried out to investigate the effect of pulsed electric fields (PEFs) on the inactivation of microbial populations suspended in liquids using nonflowing and continuous flowing test chambers. Electric fields of /spl sim/30 kV/cm, and a pulse duration of 500 ns, were generated from a coaxial table Blumlein pulse forming network (PFN) and applied to a parallel plate, circular electrode test configuration. Sample microorganisms were grown under standardized conditions and were introduced into test liquids in order to produce known population densities within the treatment celt. The organisms investigated include the mold Aspergillus niger, the yeast Sacckaromyeces cerevisiae, and the bacterial pathogens Bacillus cereus, Staphylococcus aureus, and Pseudomonas aeruginosa. The PEF studies were undertaken at a sample temperature range of 25/spl deg/C-30/spl deg/C, and the effect of the number of pulses on the test microbial population was studied. The results of this investigation showed that the greater the number of pulses applied, the larger the corresponding reduction in microbial cells/spores obtained. With the exception of dormant fungal spores, all of the test organisms were reduced by -3 to 4 log orders after 3000 pulses. The number of B. Cerus cells was reduced by -7.5 log orders after 15 000 pulses, of which 10 000 pulses were applied in a flowing system followed by 5000 pulses in a static system.

Pulsed electric field inactivation of diarrhoeagenic Bacillus cereus through irreversible electroporation.

The physical effects of high‐intensity pulsed electric fields (PEF) on the inactivation of diarrhoeagenic Bacillus cereus cells suspended in 0·1% peptone water were examined by transmission electron microscopy (TEM). The levels of PEF‐induced microbial cell death were determined by enumeration on tryptone soy yeast extract agar and Bacillus cereus‐selective agar plates. Following exposure to lethal levels of PEF, TEM investigation revealed irreversible cell membrane rupture at a number of locations, with the apparent leakage of intracellular contents. This study provides a clearer understanding of the mechanism of PEF‐induced cellular damage, information that is essential for the further optimization of this emerging food‐processing technology.

The influence of pulse duration on the inactivation of bacteria using monopolar and bipolar profile pulsed electric fields

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. There are several ways in which PEF can be applied, including oscillatory, double exponential, and square wave pulses. Of these, the square-wave pulse is considered to be the most energy-efficient form of PEF delivery. It has been reported that the bipolar square-wave pulse, involving polarity reversal half way through the pulse, provides superior inactivation when compared to the monopolar pulse. However, results from the authors have shown that this is not always the case and that monopolar PEF is at least as effective for bacterial inactivation under the conditions investigated (Beveridge, et al., 2004). Further results are presented here on the effect of changing the pulse duration of the monopolar and bipolar pulse, using total pulse durations of 1, 2, 3, and 4 /spl mu/s. These results, obtained from an improved system, demonstrate how the relative effectiveness of inactivation is a function of the pulse duration. When inactivation is plotted against energy delivered, they show the superiority of monopolar over bipolar pulses for 1- and 2-/spl mu/s pulse profiles. For a pulse duration of around 3 /spl mu/s, there is no significant difference, and with still longer pulses, bipolar pulses appear to be superior to monopolar. It is postulated that electric-field-induced orientation may, in part, be responsible for this effect. These comparative experiments are conducted with complete control of the fluid temperature, which is maintained below 30/spl deg/C.

The effects on polyetheretherketone and polyethersulfone of electron and /spl gamma/ irradiation

In this study, the effects of ionizing radiation on the aromatic polymers polyethersulfone (PES) and polyetheretherketone (PEEK) were investigated. UV light, /spl gamma/ rays and a high-energy electron beam were used as radiation sources. A range of techniques has been utilized to investigate changes in PES and PEEK as a result of exposure to these sources. Electron spin resonance (ESR) measurements showed the creation of radicals in PES by UV or /spl gamma/ radiation. These radicals decayed, but some were still present 24 h after irradiation at room temperature. In PEEK, radicals were formed as a result of UV at room temperature and by UV or /spl gamma/ irradiation at 77 K. After irradiation at room temperature, these radicals decayed totally within 24 h. UV-visible absorption measurements of PES samples showed a dose related change at the absorption edge; no such change was observed for PEEK. Phosphorescence after /spl gamma/ irradiation increased in intensity for PES at low temperature but decreased at higher temperatures. For PEEK, no such changes were observed after irradiation. The differential scanning calorimeter (DSC) measurements showed changes in the crystallization, glass transition and melt-point temperatures of both PES and PEEK after electron irradiation. Electrical changes induced by irradiation were investigated using thermally stimulated discharge current (TSDC) and low frequency dielectric responses, derived from transient current (TC) measurements. Dielectric changes were observed for PES exposed to either /spl gamma/ rays or an electron beam. For PEEK only minor changes in the TSDC spectrum were observed after exposure to /spl gamma/ radiation. More significant changes were observed after exposure to the electron beam. Correlations were found between the production of radicals and the changes in the structural and electrical properties of the materials. Since significant changes were observed in the dielectric spectra of PEEK and PES at low frequencies, dielectric spectroscopy may provide a measure of aging in both insulation systems.

Transient electrical field across cellular membranes - pulsed electric field treatment of microbial cells

The pulsed electric field (PEF) treatment of liquid and pumpable products contaminated with microorganisms has attracted significant interest from the pulsed power and bioscience research communities particularly because the inactivation mechanism is non-thermal, thereby allowing retention of the original nutritional and flavour characteristics of the product. Although the biological effects of PEF have been studied for several decades, the physical mechanisms of the interaction of the fields with microorganisms is still not fully understood. The present work is a study of the dynamics of the electrical field both in a PEF treatment chamber with dielectric barriers and in the plasma (cell) membrane of a microbial cell. It is shown that the transient process can be divided into three physical phases, and models for these phases are proposed and briefly discussed. The complete dynamics of the time development of the electric field in a spherical dielectric shell representing the cellular membrane is then obtained using an analytical solution of the Ohmic conduction problem. It was found that the field in the membrane reaches a maximum value that could be two orders of magnitude higher than the original Laplacian electrical field in the chamber, and this value was attained in a time comparable to the field relaxation time in the chamber. Thus, the optimal duration of the field during PEF treatment should be equal to such a time.

Hydrodynamic modelling of transient cavities in fluids generated by high voltage spark discharges

Application of a voltage pulse having a rise time of tens of nanoseconds to electrodes immersed in water results in streamer development and the formation of a highly conductive plasma channel between the electrodes. The electrical resistance of such channels decreases rapidly from a few ohms to a few tens of milliohms due to Joule heating resulting from the high current which flows through the plasma. The dynamics of the plasma resistance depend on the parameters of the discharge circuit and the medium in which the discharge takes place. The resistance of the channel reaches a minimum value approximately at the moment of the peak current for under-damped current oscillations. During the resistance collapse, the pressure inside the channel rises to several GPa, causing a rapid expansion of the channel which forms a cavity in the liquid resulting in a high power ultrasound pulse. The cavity expands to a maximum size which is dependent on the circuit driving the discharge and the properties of the plasma discharge channel. The cavity then collapses producing a second acoustic pulse. In this paper the dynamic resistance of the spark channel is described using a phenomenological model based on the plasma channel energy balance equation used by Braginskii. The model which links the hydrodynamic characteristics of the channel and the resulting cavity with the parameters of the electric driving circuit allows the development of the plasma channel and cavity to be predicted. The peak high-power ultrasound (HPU) pressures calculated using this approach are compared with the pressure values estimated by an analytical model which uses a constant value of the spark channel resistance derived from experimental data. Comparisons are also made with direct measurements of HPU output made using a Pinducer sensor. Although the model is based on a phenomenological description of the plasma channel dynamics and its resistance and requires the value of the spark constant, the results obtained using this approach provide a reasonable agreement with experimental measurements and could therefore be used for the estimation of HPU pulse characteristics in practical applications of spark discharges in water.

Partial discharge current pulses in SF6 and the effect of superposition of their radiometric measurement

The practical advantages of employing non-contact radio frequency (RF) methods for detecting partial discharges (PDs) in high voltage equipment have led to significant effort being focused on the diagnosis of electrical plants using RF techniques. This has particularly been the case for gas insulated substations, which use sulphur hexafluoride (SF6) as an insulating medium. One of the most important challenges facing RF diagnostics is the problem of relating the RF emissions to some measure of severity of the PD. Previous work has established that the amplitude or energy of RF signals radiated from a PD source is strongly dependent on the rate of change of current in the PD pulse. In this paper, measurements of PD current pulses in SF6 are presented for a point-plane configuration using an extremely wide bandwidth (13 GHz) measurement system. By this means, PD pulse shapes have been recorded with better resolution than has previously been possible and rise times have been measured with a high degree of accuracy. The results show a considerable variation in pulse shape, with the minimum rise time measured being 35 ps. With this high time-domain resolution, we have been able to distinguish features within the PD pulses that will affect the energy of the radiated RF signal. In particular, the current pulses tend to occur in bursts of up to ten individual pulses in as little as 1 ns, which will excite multiple RF signals in rapid succession. The effect of superposition of RF waveforms has been investigated by studying the variation in detected RF energy with respect to the time delay between PD pulses. It was found that when two PDs occur within a short period (< 150 ns) the combined energy of the resulting RF pulse has the potential to vary by ±30% of that resulting from two equivalent PD pulses with a wider pulse spacing ( 150 ns). In terms of a practical monitoring system concerned with order-of-magnitude variations; this is not considered to pose a major problem for the RF technique.

The influence of chemical structure on the dielectric behavior of polypropylene

An initial study has been made of radiation-induced oxidation of polypropylene (PP) in both its additive-free form and one containing discrete amounts of stabilizer (HALS). Samples of PP were irradiated in a /sup 60/Co /spl gamma/ source to various doses. This took place in either room air to give in-source oxidation or in vacuum at 77 K to render post-irradiation oxidation when the material was exposed to room air following irradiation. Studies were made of the chemical structure of the material and its concomitant dielectric behavior in terms of loss at low frequencies and thermally-stimulated discharge current (TSDC). The polar functional groups induced via irradiation were observed to affect the dielectric response of the material, with the introduction of HALS reducing the concentration of gamma-induced oxidation products and the magnitude of the corresponding electrical signals.