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ow through the food object for a very short period of time, on the order of microseconds. Therefore, a capacitor is needed to generate pulses. The capacitor slowly charges and then quickly discharges its stored electrical energy. The microbial inactivation in foods due to an imposed electrical eld depends on the length of time the eld is applied and the number of pulses. It is recommended that high electric eld and short time pulses be used to minimize heat generation due to Joule heating effect. Otherwise, the bene ts of PEF are lost due to thermal degradation of the treated food. There are operational problems with the use of monopolar pulses (7). Because many constituents of a food material, such as electrolytes, protein, and living cells, have a net electric charge, they tend to accumulate on the charged electrode surfaces. A shielding layer is therefore created on the electrodes that make the electric eld nonuniform. The undesirable shielding layers are prevented when bipolar pulses are used (7). Several technical issues that are important in the industrial application of PEF have been noted (7, 15). These include:
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determining the optimum electric eld strength for inactivating bacteria; provision to cool the food material that heats up due to Joule heating effect; dielectric breakdown in foods; proper selection of power and ow rates; operational safety issues.
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A simpli ed schematic design of a PEF system is shown in Fig. 8.1. The main components are the high-voltage generator, switch, capacitor, and electrodes. A cooling system with recirculating cold water is often used to dissipate the heat generated from electric currents and keep treated foods at low temperature (8). Two types of pulses have been considered for PEF applications, namely exponential decay and square pulses. In square pulses, the voltage increases instantaneously to a peak value, where it is held for some time before decreas-
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Figure 8.1 A simpli ed general design of a PEF apparatus (15).
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ing to zero almost instantly. With exponential pulses, the long tail section of the pulse is not effective in killing bacteria. On the other hand, it generates excess heat. Square pulses can maintain their peak voltage for a longer time than exponential pulses, and they generate less heat. Although the generation of square pulses needs more complex circuits, it is preferred for its advantages in food applications. While PEF is desirable for microbial inactivation, it causes undesirable arcing or dielectric breakdown in a material. Arcing occurs when the applied eld strength becomes equal to the dielectric strength of the material. When a liquid food is subjected to PEF, any presence of vapor bubbles causes arcing. Gases or vapors have a much lower dielectric strength than do pure liquids. Any roughness of electrode surface also causes dielectric breakdown of the food material. Zhang et al. (7) recommend considerations of the following points to avoid arcing:
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using electrodes that are smooth; carefully designed treatment chambers to provide uniform electric eld strength; degassing; pressurizing the liquid in the treatment chamber to prevent bubble formation.
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Several different designs of PEF treatment chambers have been investigated (9). A static chamber used at Washington State University is shown in Fig. 8.2. The disk-shaped electrodes (area 27 cm2) are made of stainless steel polished to mirror-like surface, with a gap that could be set at either 9.5 or 5.1 mm. Electric eld strengths of up to 70 kV/cm could be used. Electrodes contain built-in jackets that allow circulation of water to maintain low temperatures. A modi ed version of this static cell has been used for continuous application, as shown in Fig. 8.3. To continuously pump a liquid food through the cell, baf ed ow channels were added inside the treatment chamber. A pulse width of 2 to 15 s with a repetition rate of 1 Hz has been tested, and the
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