Effect of short HV pulses on bacteria and fungi

The survival of three kinds of microorganisms under strongpulse electric field conditions was investigated with a possible application of the electric pulse method for sterilization of consumable liquids. The results of the investigations of survival ratio of Gram-negative (Escherichia coli, Yersinia enterocolitica) and Gram-positive (Staphylococcus aureus, Listeria monocytogenes) bacteria and yeastlike fungi (Candida albicans) are presented. The HV pulses with peak voltage U = 0 to 100 kV and rise time t, = 0.5 to 1.2 ps were applied. The microorganisms were suspended in an NaCl solution with 7 = 6 to 13 mS/cm conductivity and pH = 7.2. The experimental setup and the dependency of the microorganism survival ratio on the rise time, peak voltage and on the number of pulses applied, are presented. It has been found that the lethal effect on microorganisms caused by HV pulses depends on the pulse parameters as well as on the kind of microorganism being treated

Effect of Water conductivity on its Pulse Electric Strength

The dependence of the impulse electrical strength of water on conductivity was investigated in non-uniform electric field - point-plate electrode system. The voltage pulses, with the rise time t,=l ps, and time to half-decay tuz= 30 ps, were generated by a Marx bank. Conductivity was varied by the use of different CuSO4 concentrations in distilled water. The voltage and current during the electrical breakdown were recorded by using a pulse voltage divider and current shunt. The dynamic resistance calculated from current and voltage waveforms during electrical breakdown, is also presented. On the basis of these waveforms, the dependencies of the time to breakdown and breakdown voltage on the conductivity for two polarizations of the point electrode are plotted. Investigations made by means of static photography were carried out. It was found that the electrical strength of water depends strongly on the polarity of point electrode and the conductivity of the water. The electrical strength of water decreases, and time to breakdown increases, as water conductivity is increased

Pulsed corona for removing volatile impurities from drinking water

Drinking water can be contaminated with hydrocarbons through industrial waste and also as consequence of disinfecting water with chlorine. Trihalomethens (THMS) are found in drinking water and are believed to be carcinogens. Many of these compounds are soluble and can not be removed by common techniques. Air stripping by itself is used as means of reducing the amount of volatile substances in the water. The paper reports a combination of air stripping and pulsed corona to remove THMS from water. A specially designed pulsed generator was used where the rise time of the pulse is around 30 ns. Point to plane geometry, where nitrogen bubbles are introduced at the point was used. Voltage level, gap spacing, electrode position, water type and the frequency of applied voltage are the main parameters varied in this study. In addition to the existence of the bubbles, voltage level and water type are the controlling parameters of the water treatment process..IEE

Inactivation of Yersinia enterocolitica Gram-negative bacteria using high voltage technique

High voltage pulses of peak voltages U=5-75 kV and rise times tp=500-1300 ns were applied with repetition frequency f=lHz in order to cause the irreversible electroporation of Gram negative bacteria Yersinia enterocolitica. The bacteria were suspended in NaCl solution of pH=7.2 and conductivity y ~0.8-1.3 S/m. The suspension was placed in glass tube immersed in the cylindrical electrode system gap filled with distilled water. Such an electrode system will protect the bacteria suspension from the chemical processes at the electrode-liquid interface due to conduction and pre-breakdown phenomena. The current chopping electrode system was connected in parallel to the sample in order to avoid heat generation from direct discharge of the pulse through the suspension. The dependence of the survival ratio s=N/No (the number of bacteria per cm3 after pulse treatment, N, divided by the number of bacteria per cm3 before treatment, NJ of Y. enterocolitica on peak voltage of the pulse, number of pulses applied and on various rise times of pulses have been measured. The reduction by 6 orders of magnitude of Y. enterocolitica living cells per cm3 was achieved. The results show that considerable inactivation of microbes can be achieved by the application of short (tp 4000 ns) high voltage pulses for bacteria suspension without directly exposing the bacteria suspension to the electrodes. It is therefore possible to use the electrode system proposed as a means for sterilization of liquid foods