Inactivation of microorganisms using pulsed high-current underwater discharges
Introduction
Food safety is a major concern of the food industry, consumers and authorities. Present methods to prevent food spoilage and food poisoning of non-frozen or dehydrated products include, among others, thermal treatments, chemical additives and irradiation. While heating inactivates spoilage microorganisms and enzymes it may affect negatively food quality characteristics such as nutrients, vitamins and flavor (Gould, 1995). One of the most common organoleptic changes due to thermal processing, is the appearance of a ‘cooked flavor’. Chemical additives and irradiation are not universally accepted by consumers (Olson, 1998). The demand for microbiologically safe, shelf stable and high quality food products requires the development of novel preservation methods. Consequently, preservation methods which provide less extreme treatments and no addition of chemicals are very much in demand.
High hydrostatic pressure (HHP) technologies have been proposed to inactivate bacteria and enzymes for the preservation of commercial food products (Mertens & Knorr, 1992). However, HHP requires expensive equipment and takes considerable time in order to be effective (Gould, 1995, Mertens & Knorr, 1992). The application of HHP treatments increases the temperature of samples by approximately 3 °C per 100 MPa, depending on their composition. For example, if the food contains a significant amount of fat, such as butter or cream, the temperature rise can be larger. Regarding HHP as a food-processing technology, the greater the pressure and time of application, the greater the potential for changes in the appearance of selected food. This is especially true for raw, high-protein foods where pressure-induced protein denaturation will be visually evident. HHP can also cause structural changes in fragile foods such as strawberries. Cell deformation and cell membrane damage can result in softening and serum loss. These changes are undesirable because the food will appear to be processed and no longer fresh (FDA, 2000). Patterson and Kilpatrick (1998) used HHP for the inactivation of E. coli O157:H7 NCTC 12079 and S. aureus NCTC 10652 in milk and poultry. Their findings showed the necessity for the combined use of pressure and elevated temperatures.
Zuckerman and Solomon studied the hydrodynamic pressure (HDP) wave technology in order to address texture issues associated with meat products (Zuckerman & Solomon, 1998). The HDP method involves underwater detonation of a high-energy explosive to generate shock waves in the water in a containment vessel. Applying HDP to meat affected indigenous microflora and caused disruption of myofibrillar proteins which improved the texture of the meat (Zuckerman & Solomon, 1998, Williams-Campbell & Solomon, 2000). However, this process has complex safety as well as ecological problems.
We suggest another method of food preservation namely, the application of pulsed shock waves (PSW), which are generated by a powerful electrical discharge in liquids. It is well known that powerful electrical discharges in a liquid are accompanied by the generation of PSW (Vitkovitsky, 1987). These PSW are generated due to the formation of extremely high pressures in the discharge plasma channel. Fast plasma expansion due to large plasma density and temperature gradients together with non-elastic properties of the liquid lead to the formation of PSW. By changing parameters of the electrical system one can control the parameters of the generated PSW. Thus, this method provides a way of very rapidly attaining extreme conditions of pulsed high pressures, which affect a sample during a short period of time.
The PSW method is an innovative non-thermal technology that has the potential of becoming a microbial preservation technique (Zuckerman, Krasik, & Felsteiner, 2001). Water discharge has been previously explored and patented for treating meat (US patent 6,120, 818) and for preventing Zebra mussel (Dreissena polymorpha) in water (US Patent 5,432, 756). However, no information or experiments are found in the literature.
The cost-effective, shock wave system has the ability to increase product safety, shelf life and quality. Therefore, this method provides a novel approach to solving several food safety and quality-related issues. In addition, an interesting problem that has not yet received much consideration, is the response of biomaterials to shock waves. This is especially true with materials having a complicated microstructure, such as proteins, membranes, cells and tissues (Davis & Brower, 1996).
The purpose of this study was to evaluate the efficiency of a new system, that creates pulsed shock waves, to inactivate microorganisms of different types. E. coli ATCC 3110, Staphylococcus aureus ATCC 49444, Lactic acid bacteria (an undefined strain) and Saccharomyces cerevisiae ATCC 7560 were chosen as model microorganisms.
Section snippets
Electrical setup and diagnostics
The experimental setup (see Fig. 1) consists of two Maxwell low-inductance capacitors 0.5 μF, 50 kV, which are charged in series by a high-voltage power supply up to ±40 kV. The total energy stored in the capacitors is ≤800 J. Discharge occurs by two gaseous spark gap switches having a distortion electrode to which a high-voltage pulse (up to 60 kV) is applied. Thus, at the output one obtains a high-voltage pulse with an amplitude ≤80 kV. This high-voltage pulse is applied to a stainless steel
Sample preparation and experimental procedure
We applied PSW to four selected microbial strains that included one strain of Gram-negative bacteria E. coli ATCC 3110, and two strains of Gram-positive bacteria Staphylococcus aureus ATCC 49444 and the combination of Lactococcus sp. and Lactobacillus sp. (no ATCC available; isolated in our lab on MRS agar (Difco). The yeast Saccharomyces cerevisiae, ATCC 7560, was also treated. Bacterial cultures of E. coli and Staphylococcus were grown at 30 °C and maintained on tryptone soya agar (Oxoid,
Electrical diagnostics
A typical waveform of the discharge current for the case of short circuit between the electrodes is presented in Fig. 2a. One can see that the amplitude of the discharge current reaches Im=38 kA at an output voltage of U0=80 kV. Also, one can estimate the self-inductance L and the impedance Z of the total electrical circuit as L=[T/2π]2C−1≅0.95 μH and Z=(L/C)1/2≅2 Ω. Here the total capacitance of the two capacitors is C=0.25 μF, and the period of the current oscillations is T=3.4 μs. From Fig. 2
Conclusions
The new Pulsed Shock waves system was shown to create high pressures on a sub-microsecond time scale with no temperature rise being detected. Although, it is known that E. coli can grow at high static pressures, up to 55 MPa, the response to high-pressure pulses within a very short time has a different effect. However, the exact mechanism of microbial death due to heat or pressure is still not fully understood. It is known that microorganisms can be killed by a static pressure of approximately
Acknowledgements
This research was supported by the Smoler Research Fund and the Elite Food Engineering Research Fund.
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