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Design of PEF Processing Equipment
CHAPTER 2
Design of PEF ProcessingEquipment
I.
Introduction
Pulsed electric field processing of food involves the application of short pulses (duration of micro- to milliseconds) of high electric field intensity. Food may be processed at ambient or refrigerated temperatures. In a continuous process the food is subjected to electric field pulses while being pumped through the system. The residence time of the food in the treatment chamber is adjusted so that it is subjected to the required number of pulses. The number of pulses depends on the type of microbes to be inactivated so that the food is safe for consumption. The PEF processing system is composed of a high voltage repetitive pulser, a treatment chamber(s), a cooling system(s), voltage- and currentmeasuring devices, a control unit, and a data acquisition system. A pulsed power supply is used to obtain high voltage from low utility level voltage, and the former is used to charge a capacitor bank and switch to discharge energy from the capacitor across the food in the treatment chamber. Treatment chambers are designed to hold the food during PEF processing and house the discharging electrodes. After processing the product is cooled, if necessary, packed aseptically, and then stored at refrigerated or ambient temperatures depending on the type of food (Qin et al., 1995a; Zhang et al., 1997).
II.
High-Voltage Pulsers
A typical repetitive high voltage pulser (RP1292) manufactured by Physics International in San Leandro, California features a repetitive capacitor discharge modulator (Fig. 2.1) that is designed to drive load resistances 20
21
II. High-Voltage Pulsers Stray
Series Resistor
Switch
Inductance
'W
Output Cables
I
TT
~
I
()
i i ouut Load
High Voltage .. Trigger Generator IR! I~ V Output
Fig. 2. I tances.
T h e circuit of a repetitive pulser with various resistances, capacitances, and induc-
between 2.5 and 15 1~. Some of the other components of the pulser include (a) a 16-kJ/sec switching power supply, (b) an adjustable shunt resistor assembly, (c) a series-limiting resistor that can be bypassed if desired, (d) temperature sensors on the high-power resistor, (e) a fully enclosed interlocked shield cabinet, and (f) an interactive computer control for operating the pulser. The pulsed power supply converts voltage at a normal utility level to a high voltage. The pulser is designed with a set of five capacitors and two shunt resistors to allow adjustment of energy per pulse and pulse shape. Energy stored in the capacitors is discharged almost instantaneously (in a millionth of a second) at power levels higher than 40 kV. The pulser is designed to allow a widely adjustable operating voltage, repetition rate, pulse duration, and pulse energy. The pulse repetition rate f(Hz) depends on the number of pulses desired (n), the treatment chamber volume (v), and the volumetric flow rate of the food (F) (Zhang et al., 1995): nF f = --.
(2.1)
The pulse repetition has an overall limit of 10 Hz and is limited at the higher energy capacitances by the average power rating of the power supply. To expose the food to the electric field pulses, the storage capacitor is charged to a preselected voltage. The full-rated charge voltage is 40 kV. One of the two output monitors is for current and the other for voltage, both of which can be measured with an oscilloscope. The high voltage involved in PEF processing guarantees definite safety measures. The RP1292 pulser is provided with a series interlock loop, which disables the high-voltage output of the power supply and activates the safety crowbar system. The modulator cabinet doors are included in the interlock loop. Opening the power supply cabinet doors disables the high-voltage output and abort switches that are located on the modulator cabinet and
22
Fig. 2.2 system.
2. Design of PEF Processing Equipment
A control panel consisting of an oscilloscope, modulator cabinet, and computer
front control panel (Fig. 2.2). Pushing either of these will activate the safety crowbar system and disable the power supply. The lab scale pulser at Washington State University (WSU) is a repetitive capacitor discharge modulator that supplies energy by switching into an output cable by a pair of triggered series-operated ignitions (Fig. 2.3). Two power supplies arranged in a master-slave configuration provide the highvoltage charging current. In this system the voltage driven into the treatment chamber or load is dependent on the direct current (DC) charge voltage, whereas the pulse width is determined by the R C (resistance-capacitance) decay constant where R is a parallel combination of a shunt resistor and the load resistance. The R C decay constant can be adjusted within the range of 2-30 ~sec with a capacitor combination. This system has been used successfully to study microbial and enzyme inactivation (Qin et al., 1995b, 1996; Mart~n-Belloso et al., 1997; Vega-Mercado et al., 1995, 1996; Pagfm et al., 1998). The GeneZapper (commercial electroporator) manufactured by IBI/ Kodak (or equivalent electoporators) can be used as a bench-top pulsed power supply for basic or preliminary studies of electric field food processing (Fig. 2.4). These units provide pulses with a maximum voltage of 2.5 kV. The instruments consist of a capacitor (7/~F), charge and discharge switches, and a wave controller that can be used to process small volumes of model liquid food. Appropriate voltage and current monitors may be attached to the electoporator to evaluate the energy delivered to the chamber.
II. High-Voltage Pulsers
23
Fig. 2.3 (a) The pilot plant scale pulser used at WSU for PEF inactivation of microorganisms and enzymes. (b) The main cabinet of the WSU PEF pulser. (c) The current setup of the PEF facility at WSU in which the pulser has a 16-kJ/sec charging power supply, 40-kV peak charging voltage, and 10-Hz pulse repetition rate: C, storage capacitor; D, power supply protection diode; Rc, charging resistor; Rs, series resistor; Rsh, shunt resistor; Rp, voltage-measuring resistor; Imon, current monitor; and Vmon, voltage monitor (adapted from Qin et al., 1995d).
24
2. Design of PEF Processing Equipment
Trigger Rc
lm n
Crowbar
y't)
/~46,
_TD
Power Supplies
--~C
•-Jm--EarthGround
() Treatment Chamber
V,m
f
Fig. 2.3 Continued.
Ho et al. (1995) have demonstrated the effectiveness of a low-cost pulse generator with a maximum power of 30 kV DC. The voltage of the supplier line is transformed and rectified before directed to a 12-/xF capacitor through a series of 6-M l) resistors, and the energy is delivered to the treatment chamber or load by discharging the capacitor through a thyratron switch. In this pulse generator a trigger circuit converts 5-V pulses to 500-V pulses using a silicon control rectifier. A pulse with a 2-/.~sec width and a 2-sec period (Fig. 1.7) characterizes the pulse waveform of this system. A versatile pulse-forming network, capable of delivering square, bipolar and exponential wave-shape pulses has been used successfully by Zhang et al. (1997). The pulse generator includes a 40-kV/8-kV c o m m a n d power supply that charges the system with two 100-1) resistors. A 50-kV/5-kA hollow anode thyratron tube switch delivers the energy at a maximum repetition rate of 1000 Hz. In an effort to uniform the PEF process, leading groups (EPRI, 1998) have established five main characteristics that a laboratory pulse generator should have in order to contain the cost of the system, reduce its complexity, and limit temperature rises above 70~ 9 9 9 9 *
a laboratory pulser power range between 1.5 and 2 kW and a flow rate of 20 l i t e r / h r output voltages from 20 to 30 kV pulse widths and repetition rates defined by power pulse rates and treatment chamber resistances small pulse widths from 1 to 3 txsec repetition rates up to 2000 Hz, determined by the energy into the load and pulse shape flexibility
III. Switches
25
Fig. 2.4 (a) The GeneZapper (IBI, Kodak) used for preliminary studies. (b) Major components of the commercial electroporator GeneZapper (reprinted from Food Technol., Vol. 49(12), Qin et al., "Food pasteurization using high-intensity pulsed electric fields," pp. 55-60, 1995, with permission from Elsevier Science).
III.
Switches
Generally, two basic circuit configurations are available for the generation of high-power pulses: those that require " o n " and "off" switches and those that need only " o n " switches. This distinction is important because it is rather difficult to turn a machine off at high-power levels. An example of a circuit which requires both " o n " and "off" switches is the partial discharge of a capacitor, which results in a square-shaped wave. Only " o n " switches are n e e d e d for the full discharge of capacitors and pulse-forming networks to generate exponentially decaying and square-like wave-shape pulses, respectively. The switching elements available for this purpose are vacuum tubes, high-power transistors, ignitrons, triggered vacuum gaps, triggered spark gaps, thyratrons, tetrodes, semiconductors, and force-commutated thyristors
26
2. Design of PEF Processing Equipment
TABLE 2. I Switching Devices and Typical Expectations (EPRI, 1997)
Mode Anode U0 Peak current Repetitionrate Ignitron on Gas spark gap on Thyratron on Tetrode on/off Semiconductor on/off
20 kV 40 kV 50 kV 20 kV 1.2 kV
10-100 kA 10-20 kA 5-10 kA 5-10 kA 1 kA
Life(number of pulses)
singleshot to 10 Hz to 10 Hz to 10 Hz to 10 Hz
104 10 6
108 1010
1012
(in which a c o u n t e r c u r r e n t is injected to create zero current). T h e m i n i m a l desirable operation conditions of these switches such as operational modes, m a x i m u m peak voltages, c u r r e n t a n d pulse repetition ranges, as well as a p p r o x i m a t e lifetimes are p r e s e n t e d in Table 2.1. Ignitrons have some restraints, allow just single shots, have the shorter lifetime a m o n g the other switches, a n d p r e s e n t thermal conditioning problems due to the m e r c u r y content. Physics International C o m p a n y developed a spark gap switch (Fig. 2.5) a n d a m a t c h e d series-injection trigger g e n e r a t o r that is suitable for high-energy capacitor b a n k applications (Bhasavanish et al., 1991). Some of the advantages of this switch are its long life, low r e q u i r e d m a i n t e n a n c e , a n d superior p e r f o r m a n c e in terms of p e a k currents. T h e c o m p a n y p u t special care into its design in o r d e r to maximize the life of the switch: a two-electrode gap that operates without a trigger electrode to r e d u c e energy losses, coaxial c u r r e n t returns whose magnetic forces are b a l a n c e d to minimize vibration a n d mechanical loads, graphite electrodes so
Fig. 2.5 A schematic drawing of a spark gap switch (Bhasavanich et al., 1991).
27
III. Switches TABLE 2.2 ST-300 Demonstrated Operating Parameters (Bhasavanich et al., 199 I) operating voltage range peak current charge transfer inductance electrode tip life action integral switching gas size and weight
0-55 kV 280 kA 700 Cb < 200 nH 150,000 Cb (replaceable tips) 120 MJ/I~ air 9 in. dia.• 11 in. high, 20 lbs.
that buildup and pitting are diminished, and insulated housing placed far from the spark channel to lessen heat exposure as much as possible. In addition, the switch has a low prefire pressure and a probability of prefire as low as 10 -5. Because the switch has no separated trigger electrodes and must be directly overvolted, Physics International selected a spiral generator that is conveniently placed at the location of the switch that simplifies cabling and offers ruggedness and low impedance. Table 2.2 lists the operating parameters of this switch. The trigatron gap switch (Fig. 2.6) consists of high-voltage spherical electrodes, a grounded main electrode of spherical shape, and a trigger electrode through the main electrode. The trigger electrode is a metal rod with an annular clearance and is fitted into the main electrode through a bushing. The trigatron is polarity sensitive and requires a proper polarity pulse for correct operation (Naidu and Kamaraju, 1996).
H.V. Electrode
Earthed electrode Bushing
k
Anular gap,
[ii#iiiiiiiii!iiii!ili!il
iJ
li!iiii
Trigger Electrode
Main Gap Fig. 2.6 A schematic drawing of a trigatron gap switch. (Reproduced from Naidu and Kamaraju. "High Voltage Engineering," 2nd Ed, 1996, with permission from The McGraw-Hill Companies.)
28
IV.
2. Design of PEF ProcessingEquipment
Treatment Chambers
A traditional treatment c h a m b e r consists of two electrodes held in position by insulating material to form an enclosure containing the food to be treated. Parallel plate (Figs. 2.7 and 2.8), parallel wire, concentric cylinder
Fig. 2.7
An unassembled (a) and assembled (b) parallel plate static treatment chamber.
IV. Treatment Chambers
Fig. 2.8
29
A parallel plate continuous treatment chamber.
(Fig. 2.9), concentric cone, cofield tube, r o d - r o d , n e e d l e - p l a t e , and r o d - p l a t e are some of the possible electrode configurations. Parallel plates produce uniform electric field strength distribution in a large usable area and are the most practical choice. Concentric cylinders, however, provide smooth and uniform product flow and are attractive in industrial applications. One of the newest electrode configurations includes an e n h a n c e d electric field zone in the t r e a t m e n t c h a m b e r (Fig. 2.20), the implementation of which has led to more configurations (Figs 2.21-2.23) that allow a series of treatment zones with e n h a n c e d electric fields without significantly increasing the complexity of the PEF system. This c h a m b e r design is important to the development of PEF pasteurization technology. Food can be processed using PEF in a batchwise or continuous mode. Laboratory-scale studies of electric fields on foods have been conducted using both, but for industrial-scale operations a continuous m o d e is more economical and efficient. Static chambers are typically used for batchwise operations, whereas continuous and coaxial chambers were designed to facilitate a continuous flow of food during processing. Four major aspects have to be considered in the design of a t r e a t m e n t chamber, including 9 9 9 9
simulation for shape optimization electric field uniform distribution geometry and dimensions mechanical construction and materials
The geometry and dimensions of a c h a m b e r influence the design, construction, and price of the pulsing system in which it will be used. The chamber must permit ease of sterilization, the electrodes have to withstand
30
2. Design of PEF Processing Equipment
Fig. 2.9 An unassembled (a) and assembled (b) coaxial continuous treatment chamber.
the clean in place (CIP) or autoclaving process, a n d the m e c h a n i c a l characteristics n e e d to allow work at pressures of at least 7 bars. Features for cooling the electrodes should be included to prevent t e m p e r a t u r e rises above a target level typically assigned at 70~
IV. T r e a t m e n t Chambers
31
A. ElectrodeShape Optimization The aim in designing a high-voltage PEF treatment chamber is to attain high-intensity, spatially uniform electric fields in the treatment region for maximum microbial inactivation. To obtain high-field intensifies, the electrodes should be designed to minimize local field enhancements as these increase the probability of dielectric breakdown. Nonuniformity less than 10% is highly desirable. Adequate shaping of the electrode configuration is an essential task in high-voltage engineering design. Numerical electric field optimization enables a uniform field distribution within the treatment volume. Qin et al. (1995c) proposed a m e t h o d to optimize electrode configurations by correcting electrode contours to provide a uniform electric field. As presented in Fig. 2.10, point R a o n the electrode surface A moves to R'a in the normal direction while the potential of the electrode Va is kept constant and the field intensity (E a) i n c r e a s e s to E'a or Eta > E a,
where E'a =
Va [Rb -- R'a[
and E a
(2.2)
Va [Rb -- Ra["
Vb and R b are the potential voltage and field points of the electrode with surface B, respectively. Clearly, by moving the electrode contour points in a normal or opposite direction, the field intensity at the electrode surface can be increased or decreased. Electrode shape change is performed by moving the contour points in proportion to the difference between calculated and desired values of field intensity: if A E m _< k oEd
Ed - Ei Agi--- ~ g d - E i AEm
~
( k 0Ed )
~
a
"'",
otherwise
(2.3)
Vb
ER
AIII.I.R,R i >E'a
Ra
Ra
B Fig. 2. l0 Electrode shape optimization to obtain uniform electric fields at electrode surfaces A and B (Qin et al., 1995c).
32
2. Design of PEF Processing Equipment
where the maximum difference between the two electric fields is defined as AE m
=
max{IE a
-
Eil}, i = a, 2 . . . . Arc
(2.4)
and E d, Ei, ko, and Arc are the desired value of electric field intensity, field intensity at the i th point of the electrode contour (a factor that limits the moving distance of the node point), and the total n u m b e r of node points at which the correction is made, respectively. In this case E d was held constant in order to achieve a uniform electric field, and the factor k 0 chosen such that it provided an acceptable finite element mesh. If a node point moves more than the longest side of the triangular element, the meshing information of the element will be incorrect and the finite element m e t h o d (FEM) analysis will cease. Correction of the node points on the electrode contours is given by the following correction vector (Misaki et al., 1982): AEi) rex = r i 1 +
Ei
.
(2.5)
In the optimization based on finite elements, rci and r i are, respectively, the vectors defining the i th node point on the corrected and initial contours. E i was obtained by finite element computation. To simplify electrode optimization, the net space charge due to the movement of charged species (ions, protein, and living cells) in liquid foods may be ignored. The algorithm for electrode shape optimization consists of the following steps (Qin et al., 1995c): 1. Use a FEM to solve for the electric field inside a chamber with an initial contour design and a high-resolution mesh in the region of highintensity electric fields to reduce the numerical error. 2. Move the coordinates of the selected boundary nodes at the surface of the electrode within the treatment region in proportion to the difference in electric fields as described in Eq. (2.2). (This step results in a piecewise linear description of the electrode shape, which may contain n u m e r o u s spikes and edges.) 3. Obtain a smooth approximation of the piecewise linear shape using a spline routine. (Note that the electrode contour change should be conducted in the treatment region only, where only the FEM mesh will be affected.) 4. Regenerate the triangular mesh in the treatment region based on the approximated electrode contour and solve for the electric field intensity on the corrected electrode contour using FEM. The optimization criterion is to find an electrode contour that provides a uniform electric field at the electrode surfaces in the treatment region. If the calculated field distribution is not close enough to the uniform one, return to Step 2. This iteration procedure may end when the optimization criterion is satisfied.
IV. Treatment Chambers
B.
33
Static Chambers
Materials selected to construct a t r e a t m e n t c h a m b e r n e e d to be washable and autoclavable. Polysulfone and stainless steel are r e c o m m e n d e d for the insulation and electrodes, respectively. However, Bushnell et al. (1993) suggested using electrochemically inert materials such as gold, platinum, carbon, or metal oxides to construct the electrodes or electrode surfaces. Parallel plate electrodes with gaps sufficiently smaller than their electrode dimensions can achieve uniform electric field strength. Disk-shaped, roundedged electrodes can minimize electric field e n h a n c e m e n t and reduce the possibility of dielectric breakdown in fluid foods (Zhang et al., 1995). Designing t r e a t m e n t chambers to facilitate sample filling and removal adds to the complexity of their construction. Since a gas bubble is a potential trigger of dielectric breakdown, the filling ports need to facilitate a complete expulsion of air during filling (Zhang et al., 1995). Sale and Hamilton (1967) were a m o n g the earliest researchers to study the inactivation of microorganisms with PEF. At this time carbon electrodes supported on brass blocks were used and placed in a U,shaped polythene spacer as illustrated in Fig. 2.11. Using different spacers regulated the electrode area and a m o u n t of food that could be treated. The m a x i m u m electric field that the c h a m b e r could withstand was limited to 30 k V / c m due to the electrical breakdown of air above the food. The temperature of the food was controlled by the circulation of water through the brass blocks. The chamber designed by D u n n and Pearlman (1987) consists of two stainless-steel electrodes and a cylindrical nylon spacer. The c h a m b e r is 2 cm high with an inner diameter of 10 cm and an electrode area of 78 cm 2 (Fig. 2.12). The c h a m b e r is intended for treating liquid foods, which are introduced through a small aperture in one of the electrodes. The aperture can also be used for temperature m e a s u r e m e n t during the t r e a t m e n t of foods with high-intensity electric fields. The static t r e a t m e n t c h a m b e r designed at WSU is presented in Fig. 2.13. Two round-edged, disk-shaped stainless-steel electrodes were polished to mirror surfaces. Polysulfone or Plexiglas was used as the insulation material. The effective electrode area is 27 cm 2 and the gap between electrodes can
Fig. 2.11 The static chamber designed by Sale and Hamilton (1967).
34
2. Design of PEF Processing Equipment
Fig. 2.12 A cross section of the static chamber designed by Dunn and Pearlman (1987).
be selected at either 0.95 or 0.5 cm. Electric field strengths up to 70 k V / c m have been tested. Cooling of the c h a m b e r is provided by circulating water at preselected temperatures through jackets built into electrodes. An acoustic pressure pulse is observable while PEF is applied. Since a completely sealed t r e a t m e n t chamber is dangerous because of possible sparking and high pressure that could subsequently develop and cause the c h a m b e r to break apart, a pressure release device must still be included to ensure safe operation (Zhang et al., 1995). T r e a t m e n t chambers with parallel plate electrodes offer a uniform electric field distribution along the gap axes and electrode surfaces, but create a field e n h a n c e m e n t problem at the edges of the electrodes. The FEM was used to determine the o p t i m u m position of the insulating spacer in the WSU design so that the food to be treated could be held in the region of the uniform electric field. Section A in Fig. 2.14 represents the region of the uniform electric field, and section B the spacer. The o p t i m u m position for the boundary between sections A and B was d e t e r m i n e d using the FEM. Equipotential lines from the analysis using 4992 elements and 2651 nodes are presented in Fig. 2.15. In an effort to avoid p r o d u c t contact with the electrode wall, Lubicki and Jayaram (1997) proposed the use of a glass coil surrounding the anode
Fig. 2.13 The static chamber designed at WSU.
IV. Treatment Chambers
35
Fig. 2.14 Boundaries of electric field regions in a treatment chamber with parallel plate electrodes (the electric field is symmetrical about the center line, and only the top portion of the configuration is shown) (Qin et al., 1995c).
(Fig. 2.16). T h e s a m p l e v o l u m e o f t h e i r static c h a m b e r is 20 c m ~, w h i c h n e c e s s i t a t e s a filling l i q u i d with h i g h c o n d u c t i v i t y a n d similar p e r m i t t i v i t y to t h e s a m p l e ( m e d i a NaC1 s o l u t i o n tr = 0 . 8 - 1 . 3 S / m , filling l i q u i d w a t e r ~ 10 -a S / m ) u s e d b e c a u s e t h e r e is n o i n a c t i v a t i o n with a n o n c o n d u c t i v e m e d i a (i.e., t r a n s f o r m e r silicon oil). T h i s c o n f i g u r a t i o n m u s t also a d d r e s s t h e q u e s t i o n o f h o w efficiently t h e p u l s e e n e r g y c a n b e t r a n s f e r r e d i n t o t h e sample.
C.
Continuous Chambers
Static c h a m b e r s a r e m a i n l y suitable f o r l a b o r a t o r y use. F o r l a r g e r scale o p e r a t i o n s , c o n t i n u o u s c h a m b e r s a r e m o r e efficient. T o suit this p u r p o s e , D u n n a n d P e a r l m a n (1987) d e s i g n e d a c h a m b e r c o n s i s t i n g o f two p a r a l l e l
20-
z
15-
J
5
. . . . . . . . . .
!
0
!
5
!
!
15
- ~ . /
!
!
J
!
22
!
28
!
j
J
!
!
32
!
!
40
r(mm) ---'-0
& 100 . . . . .
80 - - - 6 0 . . . .
40 - - 2 0 I
Fig. 2.15 Calculated equipotential lines for the half-field region in a parallel plate treatment chamber (the potential distribution is given as a percentage of the electrode voltage) (Qin et al., 1995c).
36
2. Design of PEF Processing Equipment
Fig. 2.16 A treatment chamber with no food exposed directly to electrodes (adapted from Lubicki and Jayaram, 1997).
plate electrodes and a dielectric space insulator (Fig. 2.17). The electrodes are separated from the food by ion conductive m e m b r a n e s m a d e of sulfonated polystyrene and acrylic acid copolymers, but fluorinated hydrocarb o n polymers with p e n d a n t groups would also be suitable. An electrolyte is used to facilitate electrical c o n d u c t i o n between electrodes and ion p e r m e able m e m b r a n e s . Suitable electrolyte solutions include sodium carbonate, sodium hydroxide, potassium carbonate, and potassium hydroxide. These
Fig. 2.17 A continuous chamber with ion-conductive membranes separating the electrodes and food (adapted from Dunn and Pearlman, 1987).
IV. Treatment Chambers
37
Fig. 2.18 A continuous chamber with electrode reservoir zones (adapted from Dunn and Pearlman, 1987). are circulated continuously to remove the products of electrolysis a n d replaced in the event of excess c o n c e n t r a t i o n or depletion of ionic c o m p o nents. A n o t h e r c o n t i n u o u s c h a m b e r described by D u n n a n d P e a r l m a n (1987) is c o m p o s e d of electrode reservoir zones instead of electrode plates (Fig. 2.18). Dielectric spacer insulators that have slot-like openings (orifices) in between where the electric field is c o n c e n t r a t e d a n d liquid food is i n t r o d u c e d u n d e r high pressure. T h e average residence time in each of these reservoir zones is less than 1 min. T h e WSU static parallel plate electrode c h a m b e r was modified by a d d i n g baffled flow channels inside to m a k e it operate as a c o n t i n u o u s c h a m b e r (Fig. 2.19). Two stainless-steel disk-shaped electrodes separated by a polysulfone spacer f o r m the c h a m b e r . T h e designed operating conditions are:
Fig. 2.19 The continuous chamber with baffles designed by WSU: (a) cross section view and (b) top view (reprinted from Food Technol., Vol. 49(12), Qin et al., "Food pasteurization using high-intensity pulsed electric fields," pp. 55-60, 1995, with permission from Elsevier Science).
38
2. Design of PEF Processing Equipment
Fig. 2.20 A cofield treatment chamber (Sensoy et al., 1997).
c h a m b e r volume, 20 or 8 ml; electrode gap, 0.95 or 0.51 cm; a n d food flow rate, 1200 or 6 m l / m i n (Qin et al., 1996; Zhang et al., 1995). T h e c o n c e p t of e n h a n c e d electric fields in the t r e a t m e n t zone was applied by Yin et al. (1997) for the d e v e l o p m e n t of a continuous cofield flow PEF c h a m b e r (Fig. 2.20) with conical insulator shapes to eliminate gas deposits within the t r e a t m e n t volume. T h e conical regions were designed so that the voltage across the t r e a t m e n t zone could be almost equal to the supplied voltage. O t h e r configurations with e n h a n c e d electric fields are p r e s e n t e d in Figs. 2.21 a n d 2.22. In these devices the flow c h a m b e r s can have several crosssection geometries that may be u n i f o r m or n o n u n i f o r m . In this type of c h a m b e r configuration the first electrode flow chamber, insulator flow c h a m b e r , second electrode flow c h a m b e r , c o n d u c t i n g insert m e m b e r s , a n d
Fig. 2.21 A treatment chamber with different electrode geometries and enhanced electric fields in the insulator channel (adapted from Yln et al., 1997).
IV. Treatment Chambers
39
Fig. 2.22 A treatment chamber with enhanced electric fields in the insulator channel and tapered electrodes (adapted from Y'ln et al., 1997).
insulating insert m e m b e r are f o r m e d and configured such that the electrode flow c h a m b e r and insulator flow c h a m b e r form a single tubular flow chamber t h r o u g h the PEF t r e a t m e n t device (Yin et al., 1997). Figure 2.23 is a variant of the flow c h a m b e r configuration where the uniformity of the liquid p r o d u c t flow velocity is improved. In reality this configuration can be a plurality of c o n d u c t i n g m e m b e r s and at least one insulator m e m b e r . W h e n the voltage pulse signal is applied across high and g r o u n d voltage electrodes, an electric field is f o r m e d in the electrode flow channels as well as the insulator channel where the electric field strength is strongest. Therefore, the bactericidal effect of the PEF t r e a t m e n t process of this device occurs primarily in the liquid p r o d u c t flowing t h r o u g h the insulator flow channel. The electrodes of these chambers are of food grade stainless steel and the insulators of policarbonate, but can also be ceramic, glass, or plastic.
Fig. 2.23 A treatment chamber with improved flow characteristics and enhanced electric fields (adapted from Yin et al., 1997).
40
2. Design of PEF Processing Equipment
Coaxial configurations, which provide the advantage of a uniform fluid flow and simple chamber structure, can be easily manufactured and provide well-defined electric field distributions. The field intensity (E) between coaxial electrodes is given by (Zhang et al., 1995):
E = 1//[ r l n ( R 2 / R 1)],
(2.6)
where r is the radius at which the electric field is measured and R 2 and R 1 are the radii of the outer and inner electrodes, respectively. The uniformity of the electric fields in the t r e a t m e n t chambers with coaxial electrodes can be improved when R 2 - R 1 < R 1. Coaxial chambers are basically composed of an inner cylindrical electrode surrounded by an outer annular cylindrical electrode that allows food to flow between them (Bushnell et al., 1993). It is r e c o m m e n d e d that the length of the fluid flow path not be too small or too long c o m p a r e d to the c h a m b e r diameter. The electrical energy (W) consumed in each pulse is given by
W= EZv~'/p,
(2.7)
where E is the electric field (volts/cm), ~- is the pulse duration, v is the volume (ml), and p is the electrical resistivity of the food sample (ohm-cm). The coaxial chamber designed at WSU is based on a modified coaxial cylinder a r r a n g e m e n t (Fig. 2.24). A p r o t r u d e d outer electrode surface enhances the electric field within the treatment zone and reduces the field intensity in the remaining portion of the chamber. The electrode configuration was obtained by optimizing the electrode design with a numerical electric field computation. Using the optimized electrode shape, a pre-
Fig. 2.24 A cross-sectional view of the modified coaxial treatment chamber designed, constructed, and tested for microbial inactivation at WSU (Mart~n-Belloso et al., 1997).
V. Cooling System
41
Fig. 2.25 Electric field distribution in the food region between two electrodes of a coaxial treatment chamber (Qin et al., 1995c).
scribed field distribution along the fluid path without electric field enhancement points was determined. The outer electrode has a p r o t r u d e d contour surface that was obtained by numerical electric field optimization. Figure 2.25 illustrates the electric field distribution of the chamber in its treatment region; to allow for a clear view, the field inside the dielectric spacer is not presented. For numerical simplicity, a 100-V applied voltage was used in the field calculation in order to verify electric field uniformity. In the treatment region between the two electrodes, the potential drop is nearly uniform so a strong electric field is generated. Since outside the treatment region most of the potential drop occurs inside the spacer, the electric field is quite weak. The gap between the chamber's electrodes is adjustable within 2 to 6 m m or more by changing the inner electrode to different diameters. Cooling jackets are built into both electrodes to maintain low temperatures. The whole treatment chamber has an outer diameter of around 13 cm and an approximate height of 20.3 cm. The PEF system in which this chamber is used can handle flow rates from 30 to 120 l i t e r / h r (Qin et al., 1995c).
V.
Cooling System
When no cooling is provided, the increase in temperature (AT) of the fluid subjected to electric field pulses is given by
Q AT =
pfCp
E2n~'o =
pfCp
,
(2.8)
where Q is the energy input and pf a n d Cp are the density and specific heat of the fluid being treated, respectively. The energy input can be defined by
42
2. Design of PEF Processing Equipment
the product of the square of the electric field (E), the n u m b e r of pulses (n), and electrical conductivity (~r) of the fluid being treated. The food temperature is maintained by circulating constant temperature water through the cooling jackets built into the electrodes. A rise in temperature over 70~ must be avoided to preserve natural attributes of food products and to claim a n o n t h e r m a l treatment. Zhang et al. (1994) proposed a one-dimensional finite difference heat transfer model to predict the fluid temperatures in a static chamber. The fluid food is divided into circular disks as illustrated in Fig. 2.26. A disk layer of fluid with thickness Ad, radius r, and volume AV represents each node in the one-dimensional model. The t r e a t m e n t chamber is divided into 2-m nodes, where m is selected to be 20. Using an energy balance, the temperature of each element is related to its adjacent elements as Tr
pfCpAV
- T( =
At
Ad
T?_I - Ti" + ha
Ad
'
(2.9)
where pf is the density, Cp is the specific heat, k is the thermal conductivity of the fluid, T is the temperature, A is the area of heat transfer, At is the time interval of each time step, d is the electrode gap, V is the c h a m b e r volume, and Ad - d / 2 m and AV = V / 2 m are the thickness and volume of fluid for each node, respectively. (Superscripts denote the time step, and subscripts the space coordinate.) The food in contact with the electrodes in the model by Zhang et al. (1994) is assumed to have their same temperature and is therefore considered the boundary condition of the design. Energy input into the food should be uniform within the bulk and is only present at the time of pulse application. Energy input was treated as the initial condition of the model and proved to generate no heat after pulsing was generated. The fluid
/•
_
. ~ .
ermocoupleLocation
TmcontactBottomElectrode /
I
CenterAxis
Fig. 2.26 A one-dimensional finite difference grid of a fluid food inside a parallel plate static treatment chamber (Zhang et al., 1994).
VI. Typical Measurements in a PEF System
43
temperature is measured with a thermocouple inserted 2 m m into the food and halfway between the electrodes, and a thermocouple attached to the cathode gauges the electrode temperature. Figure 2.27 illustrates the predicted fluid center temperatures of apple juice subjected to 20 pulses at 30 sec intervals for energy input levels of 100, 300, and 600 J / p u l s e in the heat transfer model of Zhang et al. (1994). These energy levels correspond to electric fields of 12, 20, and 28 kV/cm, respectively. When repetitive pulses are applied, the fluid center temperature is significantly higher than the electrode temperature. By controlling the interval of time between pulses to longer than 30 sec, the maximum fluid temperature is expected to remain below 25~
VI.
Typical Measurements in a PEF System
Electrical parameters such as voltage and current pulse waveforms applied during PEF treatments should be recorded via a digital acquisition system. Outputs from voltage and current monitors can be recorded by a digital oscilloscope into a computer for future analysis, but it is important that each be placed in a shielded area to minimize electromagnetic interference. To achieve accurate measurements of voltages and currents, all important frequency components must be recorded, which includes specification of the bandwidth of each transducer. With these measurements, further evaluation of the applied energy per volume and spatial average of the electric fields in the treatment region can be computed in order to determine if the process is under its control limits. Direct measurement of the electric fields by
25.00 o
20J30 84 15s IO.O0-
5.oo
0.00
I 100
I 200
I 300
I 400
I 500
600
Time(s) Fig. 2.27 Model-predicted fluid temperatures of apple juice subjected to 20 pulses with energy inputs of 100, 300, and 600 J / p u l s e (the electrode temperature was maintained at 4~ (Zhang et al., 1994).
44
2. Designof PEF ProcessingEquipment
optical fibers in the treatment gap of the chamber would be highly desirable, but this anticipates complexity and increased cost. It is definitely advantageous to take oscilloscope readings with a wellvalidated measuring system. This is especially evident when the frequency content of pulses is desired, as change may indicate a partial discharge that could lead to sparks. The frequency content of pulses is obtained by applying the fast Fourier transform algorithm to the voltage and current as a function of time [v(t), i(t)]. The analog/digital converter is also useful to warn of missed or weak pulses. Because it is important to maintain a treated food temperature at low values to minimize thermal effects, measurements should be made with a high-precision thermocouple, (placed outside the processing chamber) or even the more sophisticated fiber optic transducer. The latter has a precision of up to 0.1~ and a response time of 0.2 sec. Ideally, about four temperature monitoring optical fibers in the interelectrode region, inlet, and outlet of the treatment chamber should be installed. The flow rate and pressure in PEF chambers are critical quantities in the control process and should be measured throughout the system as a function of time and space, respectively. The applied dosage will d e p e n d directly on the flow rate when the pulse frequency and treatment chamber volume are fixed. Although the pressure will not affect the inactivation rate or applied dosage, it will prevent arcing, and m e a s u r e m e n t will detect any plug or leakage in the system.
VII.
Packaging and S t o r i n g
After PEF treatment, foods are cooled if necessary and aseptically packaged. Aseptic technology has been used for liquid milk and fruit juices for more than 30 years and is also known as an effective m e t h o d of producing shelf-stable preserved products that have quality advantages over their conventionally canned or jarred counterparts. Some of these include packaging in a range of materials without limitations on container size and the use of thin plastic membranes or plastic/paper-laminated materials that do not have to withstand high temperatures as in conventional thermal processing, as no retorting of the product is applied after packaging. Additional benefits of such an operation include less damage to the product, shorter processing periods, uniform and improved quality, reduced energy consumption, and utilization of new packaging materials (Singh and Nelson, 1992). After packaging, the bagged foods may be stored at refrigerated or ambient temperatures depending on the type of product. For example, milk may have to be stored at refrigerated temperatures, whereas apple juice has a reasonable long shelf-life even when stored at room temperature. The effectiveness of aseptic packaging has been demonstrated by the extended
References
45
shelf-life of several PEF products (Qin et al., 1995a,d; Zhang et al., 1997). Because c o n s u m e r s d e m a n d freshness, aseptic packaging is the perfect c o m p l e m e n t for PEF to provide a cost-effective way for processors to deliver what markets expect.
VIII.
Final Remarks
This c h a p t e r reviewed key elements for the design of a PEF system a n d the description of the main c o m p o n e n t s that are involved in the p r o d u c t i o n of high-voltage pulses. T h e i m p o r t a n c e of the high-voltage repetitive pulser, switches, t r e a t m e n t chamber(s), cooling system(s), voltage a n d c u r r e n t measuring devices, control units, data acquisition system, a n d packaging system were addressed based on specific system r e q u i r e m e n t s , t r e a t m e n t convenience, a n d future needs. T h e efforts of leading groups a n d institutions to obtain the best processing conditions are p r e s e n t in the d e v e l o p m e n t of new t r e a t m e n t c h a m b e r s a n d PFN configurations. Mathematical simulation used in the design of t r e a t m e n t c h a m b e r s represents a safe a n d m o r e cost-effective tool in the r e f i n e m e n t of new configurations a n d minimization of trial-error steps, so the future i m p l e m e n t a t i o n of m a t h e m a t i c a l software is encouraged. A l t h o u g h there is increasing interest in PEF technology, we detect the n e e d for commercially available high-voltage pulsers a n d switching devices. T h e r e are m a n y suppliers of high voltage i n s t r u m e n t a t i o n but few are involved in the m a n u f a c t u r i n g of complete PEF systems. T h e next c h a p t e r reviews how a n d by which m e c h a n i s m s the pulses p r o d u c e d by the PEF systems discussed in this chapter affect biological materials such as cell m e m b r a n e s , microorganisms, a n d proteins.
References Bhasavanich, D., Hitchcock, S. S., Creely, P. M., Shaw, R. S., Hammon, H. G., and Naff, J. T. (1991). International Pulsed Power Conference, San Diego, California. Bushnell, A. H., Dunn, J. E., and Clark, R. W. (1993). High pulsed voltage systemsfor extending the shelf life of pumpable food products. U. S. Patent 5,235,905. Dunn, J. E., and Pearlman, J. S. (1987). Methods and apparatus for extending the shelf life of fluid food products. U. S. Patent 4,695,472. EPRI (1997). EPRI/Army PEF Workshop II, Chicago, Illinois, 10-11 October. EPRI (1998). Pulsed electric field processing in the food industry: A status report on PEF. Report CR-109742. Industrial and Agricultural Technologies and Services, Palo Alto, California. Ho, S. Y., Mittal, G. S., Cross, J. D., and Griffiths, M. N. (1995). Inactivation of Pseudomonas fluorescens by high voltage electric field pules. J. Food Sci. 60(6), 1337-1343. Lubicki, P., and Jayaram, S. (1997). High voltage pulse application for the destruction of the Gram negative bacterium Yersinia enterocolitica. Bioelectrochem. Bioenergetics 43, 135-141. Mart~n-Belloso, O., Vega-Mercado, H., Qin, B. L., Chang, F. J., Barbosa-C~novas, G. V., and Swanson, B. G. (1997). Inactivation of Escherichia coli suspended in liquid egg using pulsed electric fields. J. Food Proc. Pres. 21, 193-208.
46
2. Design of PEF Processing Equipment
Misaki, T., Tsuboi, Itaka, K., and Hara, T. (1982). Computations of three dimensional electric field problems by a surface charge method and its application to optimum insulator design. 1EEE Trans. Power Appar. Sys. 101(3), 627-634. Naidu, M. S., and Kamaraju, V. (1996). "High Voltage Engineering," 2nd Ed. McGraw-Hill, New York. Pagan, R., Esplugas, S., G6ngora-Nieto, M. M., Barbosa-Canovas, G. V., and Swanson, B. G. (1998). Inactivation of Bacillus subtilis spores using high intensity pulsed electric fields in combination with other food conservation technologies. Food Sci. Technol. Int. 4(1), 33-44. Qin, B. L., Pothakamury, U. R., Vega-Mercado, H., Martin, O., Barbosa-C{movas, G. V., and Swanson, B. G. (1995a). Food pasteurization using high-intensity pulsed electric fields. Food Technol. (Chicago) 49(12), 55-60. Qin, B. L., Vega-Mercado, H., Pothakamury, U. R., Barbosa-C{movas, G. V., and Swanson, B. G. (1995b). Application of pulsed electric fields for inactivation of bacteria and enzymes. J. Franklin Inst. 332a, 209-220. Qin, B. L., Zhang, Q., Barbosa-C{movas, G. V., and Swanson, B. G. (1995c). Pulsed electric field treatment chamber design for liquid food pasteurization using a finite element method. Trans. ASAE 38(2), 557-565. Qin, B. L., Chang, F. J., Barbosa-C~novas, G. V., and Swanson, B. G. (1995d). Nonthermal inactivation of Saccharomyces cerevisiae in apple juice using pulsed electric fields. Lebensm. -Wiss. Technol. 28, 564-568. Qin, B. L., Pothakamury, U. R., Barbosa-C{movas, G. V., and Swanson, B. G. (1996). Nonthermal pasteurization of liquid foods using high intensity pulsed electric fields. Crit. Rev. Food Sci. Nutr. 36(6), 603-627. Sale, A.J.H., and Hamilton, W. A. (1967). Effects of high electric fields on microorganisms. I. Killing of bacteria and yeasts. Biochim. Biophys. Acta 148, 781-788. Sensoy, I., Zhang, Q. H., and Sastry, S. K. (1997). Inactivation kinetic of Salmonella dublin by pulsed electric fields. J. Food Proc. Eng. 20, 367-381. Singh, R. K., and Nelson, P. E. (1992). "Advances in Aseptic Processing Technologies." Elsevier Science, London. Vega-Mercado, H., Powers, J. R., Barbosa-C{movas, G. V., and Swanson, B. G. (1995). Inactivation of plasmin using high voltage pulsed electric fields. J. Food Sci. 60, 1150-1154. Vega-Mercado, H., Pothakamury, U. R., Chang, F.J., Zhang, Q., Barbosa-C~novas, G. V., and Swanson, B. G. (1996). Inactivation of E. coli by combining pH, ionic strength and pulsed electric field hurdles. Food Res. Int. 29(2), 117-121. Ym, Y., Zhang, Q. H., and Sastry, S. H. (1997). High voltage pulsed electric field treatment chambers for the preservation of liquid food products. U. S. Patent 5,690,978. Zhang, Q., Monsalve-Gonz{tlez, A., Barbosa-C~novas, G. V., and Swanson, B. G. (1994). Inactivation of E. coli and S. cerevisiae by pulsed electric fields under controlled temperature conditions. Trans. ASAE 37(2), 581-587. Zhang, Q., Barbosa-C~movas, G. V., and Swanson, B. G. (1995). Engineering aspects of pulsed electric field pasteurization. J. Food Eng. 25, 261-281. Zhang, Q. H., Qiu, X., and Sharma, S. IL (1997). Recent developments in pulsed electric processing. In "New Technologies Yearbook" (D. I. Chandrana, ed.), pp. 31-42. National Food Processors Association, Washington, D.C.
Design of PEF ProcessingEquipment
I.
Introduction
Pulsed electric field processing of food involves the application of short pulses (duration of micro- to milliseconds) of high electric field intensity. Food may be processed at ambient or refrigerated temperatures. In a continuous process the food is subjected to electric field pulses while being pumped through the system. The residence time of the food in the treatment chamber is adjusted so that it is subjected to the required number of pulses. The number of pulses depends on the type of microbes to be inactivated so that the food is safe for consumption. The PEF processing system is composed of a high voltage repetitive pulser, a treatment chamber(s), a cooling system(s), voltage- and currentmeasuring devices, a control unit, and a data acquisition system. A pulsed power supply is used to obtain high voltage from low utility level voltage, and the former is used to charge a capacitor bank and switch to discharge energy from the capacitor across the food in the treatment chamber. Treatment chambers are designed to hold the food during PEF processing and house the discharging electrodes. After processing the product is cooled, if necessary, packed aseptically, and then stored at refrigerated or ambient temperatures depending on the type of food (Qin et al., 1995a; Zhang et al., 1997).
II.
High-Voltage Pulsers
A typical repetitive high voltage pulser (RP1292) manufactured by Physics International in San Leandro, California features a repetitive capacitor discharge modulator (Fig. 2.1) that is designed to drive load resistances 20
21
II. High-Voltage Pulsers Stray
Series Resistor
Switch
Inductance
'W
Output Cables
I
TT
~
I
()
i i ouut Load
High Voltage .. Trigger Generator IR! I~ V Output
Fig. 2. I tances.
T h e circuit of a repetitive pulser with various resistances, capacitances, and induc-
between 2.5 and 15 1~. Some of the other components of the pulser include (a) a 16-kJ/sec switching power supply, (b) an adjustable shunt resistor assembly, (c) a series-limiting resistor that can be bypassed if desired, (d) temperature sensors on the high-power resistor, (e) a fully enclosed interlocked shield cabinet, and (f) an interactive computer control for operating the pulser. The pulsed power supply converts voltage at a normal utility level to a high voltage. The pulser is designed with a set of five capacitors and two shunt resistors to allow adjustment of energy per pulse and pulse shape. Energy stored in the capacitors is discharged almost instantaneously (in a millionth of a second) at power levels higher than 40 kV. The pulser is designed to allow a widely adjustable operating voltage, repetition rate, pulse duration, and pulse energy. The pulse repetition rate f(Hz) depends on the number of pulses desired (n), the treatment chamber volume (v), and the volumetric flow rate of the food (F) (Zhang et al., 1995): nF f = --.
(2.1)
The pulse repetition has an overall limit of 10 Hz and is limited at the higher energy capacitances by the average power rating of the power supply. To expose the food to the electric field pulses, the storage capacitor is charged to a preselected voltage. The full-rated charge voltage is 40 kV. One of the two output monitors is for current and the other for voltage, both of which can be measured with an oscilloscope. The high voltage involved in PEF processing guarantees definite safety measures. The RP1292 pulser is provided with a series interlock loop, which disables the high-voltage output of the power supply and activates the safety crowbar system. The modulator cabinet doors are included in the interlock loop. Opening the power supply cabinet doors disables the high-voltage output and abort switches that are located on the modulator cabinet and
22
Fig. 2.2 system.
2. Design of PEF Processing Equipment
A control panel consisting of an oscilloscope, modulator cabinet, and computer
front control panel (Fig. 2.2). Pushing either of these will activate the safety crowbar system and disable the power supply. The lab scale pulser at Washington State University (WSU) is a repetitive capacitor discharge modulator that supplies energy by switching into an output cable by a pair of triggered series-operated ignitions (Fig. 2.3). Two power supplies arranged in a master-slave configuration provide the highvoltage charging current. In this system the voltage driven into the treatment chamber or load is dependent on the direct current (DC) charge voltage, whereas the pulse width is determined by the R C (resistance-capacitance) decay constant where R is a parallel combination of a shunt resistor and the load resistance. The R C decay constant can be adjusted within the range of 2-30 ~sec with a capacitor combination. This system has been used successfully to study microbial and enzyme inactivation (Qin et al., 1995b, 1996; Mart~n-Belloso et al., 1997; Vega-Mercado et al., 1995, 1996; Pagfm et al., 1998). The GeneZapper (commercial electroporator) manufactured by IBI/ Kodak (or equivalent electoporators) can be used as a bench-top pulsed power supply for basic or preliminary studies of electric field food processing (Fig. 2.4). These units provide pulses with a maximum voltage of 2.5 kV. The instruments consist of a capacitor (7/~F), charge and discharge switches, and a wave controller that can be used to process small volumes of model liquid food. Appropriate voltage and current monitors may be attached to the electoporator to evaluate the energy delivered to the chamber.
II. High-Voltage Pulsers
23
Fig. 2.3 (a) The pilot plant scale pulser used at WSU for PEF inactivation of microorganisms and enzymes. (b) The main cabinet of the WSU PEF pulser. (c) The current setup of the PEF facility at WSU in which the pulser has a 16-kJ/sec charging power supply, 40-kV peak charging voltage, and 10-Hz pulse repetition rate: C, storage capacitor; D, power supply protection diode; Rc, charging resistor; Rs, series resistor; Rsh, shunt resistor; Rp, voltage-measuring resistor; Imon, current monitor; and Vmon, voltage monitor (adapted from Qin et al., 1995d).
24
2. Design of PEF Processing Equipment
Trigger Rc
lm n
Crowbar
y't)
/~46,
_TD
Power Supplies
--~C
•-Jm--EarthGround
() Treatment Chamber
V,m
f
Fig. 2.3 Continued.
Ho et al. (1995) have demonstrated the effectiveness of a low-cost pulse generator with a maximum power of 30 kV DC. The voltage of the supplier line is transformed and rectified before directed to a 12-/xF capacitor through a series of 6-M l) resistors, and the energy is delivered to the treatment chamber or load by discharging the capacitor through a thyratron switch. In this pulse generator a trigger circuit converts 5-V pulses to 500-V pulses using a silicon control rectifier. A pulse with a 2-/.~sec width and a 2-sec period (Fig. 1.7) characterizes the pulse waveform of this system. A versatile pulse-forming network, capable of delivering square, bipolar and exponential wave-shape pulses has been used successfully by Zhang et al. (1997). The pulse generator includes a 40-kV/8-kV c o m m a n d power supply that charges the system with two 100-1) resistors. A 50-kV/5-kA hollow anode thyratron tube switch delivers the energy at a maximum repetition rate of 1000 Hz. In an effort to uniform the PEF process, leading groups (EPRI, 1998) have established five main characteristics that a laboratory pulse generator should have in order to contain the cost of the system, reduce its complexity, and limit temperature rises above 70~ 9 9 9 9 *
a laboratory pulser power range between 1.5 and 2 kW and a flow rate of 20 l i t e r / h r output voltages from 20 to 30 kV pulse widths and repetition rates defined by power pulse rates and treatment chamber resistances small pulse widths from 1 to 3 txsec repetition rates up to 2000 Hz, determined by the energy into the load and pulse shape flexibility
III. Switches
25
Fig. 2.4 (a) The GeneZapper (IBI, Kodak) used for preliminary studies. (b) Major components of the commercial electroporator GeneZapper (reprinted from Food Technol., Vol. 49(12), Qin et al., "Food pasteurization using high-intensity pulsed electric fields," pp. 55-60, 1995, with permission from Elsevier Science).
III.
Switches
Generally, two basic circuit configurations are available for the generation of high-power pulses: those that require " o n " and "off" switches and those that need only " o n " switches. This distinction is important because it is rather difficult to turn a machine off at high-power levels. An example of a circuit which requires both " o n " and "off" switches is the partial discharge of a capacitor, which results in a square-shaped wave. Only " o n " switches are n e e d e d for the full discharge of capacitors and pulse-forming networks to generate exponentially decaying and square-like wave-shape pulses, respectively. The switching elements available for this purpose are vacuum tubes, high-power transistors, ignitrons, triggered vacuum gaps, triggered spark gaps, thyratrons, tetrodes, semiconductors, and force-commutated thyristors
26
2. Design of PEF Processing Equipment
TABLE 2. I Switching Devices and Typical Expectations (EPRI, 1997)
Mode Anode U0 Peak current Repetitionrate Ignitron on Gas spark gap on Thyratron on Tetrode on/off Semiconductor on/off
20 kV 40 kV 50 kV 20 kV 1.2 kV
10-100 kA 10-20 kA 5-10 kA 5-10 kA 1 kA
Life(number of pulses)
singleshot to 10 Hz to 10 Hz to 10 Hz to 10 Hz
104 10 6
108 1010
1012
(in which a c o u n t e r c u r r e n t is injected to create zero current). T h e m i n i m a l desirable operation conditions of these switches such as operational modes, m a x i m u m peak voltages, c u r r e n t a n d pulse repetition ranges, as well as a p p r o x i m a t e lifetimes are p r e s e n t e d in Table 2.1. Ignitrons have some restraints, allow just single shots, have the shorter lifetime a m o n g the other switches, a n d p r e s e n t thermal conditioning problems due to the m e r c u r y content. Physics International C o m p a n y developed a spark gap switch (Fig. 2.5) a n d a m a t c h e d series-injection trigger g e n e r a t o r that is suitable for high-energy capacitor b a n k applications (Bhasavanish et al., 1991). Some of the advantages of this switch are its long life, low r e q u i r e d m a i n t e n a n c e , a n d superior p e r f o r m a n c e in terms of p e a k currents. T h e c o m p a n y p u t special care into its design in o r d e r to maximize the life of the switch: a two-electrode gap that operates without a trigger electrode to r e d u c e energy losses, coaxial c u r r e n t returns whose magnetic forces are b a l a n c e d to minimize vibration a n d mechanical loads, graphite electrodes so
Fig. 2.5 A schematic drawing of a spark gap switch (Bhasavanich et al., 1991).
27
III. Switches TABLE 2.2 ST-300 Demonstrated Operating Parameters (Bhasavanich et al., 199 I) operating voltage range peak current charge transfer inductance electrode tip life action integral switching gas size and weight
0-55 kV 280 kA 700 Cb < 200 nH 150,000 Cb (replaceable tips) 120 MJ/I~ air 9 in. dia.• 11 in. high, 20 lbs.
that buildup and pitting are diminished, and insulated housing placed far from the spark channel to lessen heat exposure as much as possible. In addition, the switch has a low prefire pressure and a probability of prefire as low as 10 -5. Because the switch has no separated trigger electrodes and must be directly overvolted, Physics International selected a spiral generator that is conveniently placed at the location of the switch that simplifies cabling and offers ruggedness and low impedance. Table 2.2 lists the operating parameters of this switch. The trigatron gap switch (Fig. 2.6) consists of high-voltage spherical electrodes, a grounded main electrode of spherical shape, and a trigger electrode through the main electrode. The trigger electrode is a metal rod with an annular clearance and is fitted into the main electrode through a bushing. The trigatron is polarity sensitive and requires a proper polarity pulse for correct operation (Naidu and Kamaraju, 1996).
H.V. Electrode
Earthed electrode Bushing
k
Anular gap,
[ii#iiiiiiiii!iiii!ili!il
iJ
li!iiii
Trigger Electrode
Main Gap Fig. 2.6 A schematic drawing of a trigatron gap switch. (Reproduced from Naidu and Kamaraju. "High Voltage Engineering," 2nd Ed, 1996, with permission from The McGraw-Hill Companies.)
28
IV.
2. Design of PEF ProcessingEquipment
Treatment Chambers
A traditional treatment c h a m b e r consists of two electrodes held in position by insulating material to form an enclosure containing the food to be treated. Parallel plate (Figs. 2.7 and 2.8), parallel wire, concentric cylinder
Fig. 2.7
An unassembled (a) and assembled (b) parallel plate static treatment chamber.
IV. Treatment Chambers
Fig. 2.8
29
A parallel plate continuous treatment chamber.
(Fig. 2.9), concentric cone, cofield tube, r o d - r o d , n e e d l e - p l a t e , and r o d - p l a t e are some of the possible electrode configurations. Parallel plates produce uniform electric field strength distribution in a large usable area and are the most practical choice. Concentric cylinders, however, provide smooth and uniform product flow and are attractive in industrial applications. One of the newest electrode configurations includes an e n h a n c e d electric field zone in the t r e a t m e n t c h a m b e r (Fig. 2.20), the implementation of which has led to more configurations (Figs 2.21-2.23) that allow a series of treatment zones with e n h a n c e d electric fields without significantly increasing the complexity of the PEF system. This c h a m b e r design is important to the development of PEF pasteurization technology. Food can be processed using PEF in a batchwise or continuous mode. Laboratory-scale studies of electric fields on foods have been conducted using both, but for industrial-scale operations a continuous m o d e is more economical and efficient. Static chambers are typically used for batchwise operations, whereas continuous and coaxial chambers were designed to facilitate a continuous flow of food during processing. Four major aspects have to be considered in the design of a t r e a t m e n t chamber, including 9 9 9 9
simulation for shape optimization electric field uniform distribution geometry and dimensions mechanical construction and materials
The geometry and dimensions of a c h a m b e r influence the design, construction, and price of the pulsing system in which it will be used. The chamber must permit ease of sterilization, the electrodes have to withstand
30
2. Design of PEF Processing Equipment
Fig. 2.9 An unassembled (a) and assembled (b) coaxial continuous treatment chamber.
the clean in place (CIP) or autoclaving process, a n d the m e c h a n i c a l characteristics n e e d to allow work at pressures of at least 7 bars. Features for cooling the electrodes should be included to prevent t e m p e r a t u r e rises above a target level typically assigned at 70~
IV. T r e a t m e n t Chambers
31
A. ElectrodeShape Optimization The aim in designing a high-voltage PEF treatment chamber is to attain high-intensity, spatially uniform electric fields in the treatment region for maximum microbial inactivation. To obtain high-field intensifies, the electrodes should be designed to minimize local field enhancements as these increase the probability of dielectric breakdown. Nonuniformity less than 10% is highly desirable. Adequate shaping of the electrode configuration is an essential task in high-voltage engineering design. Numerical electric field optimization enables a uniform field distribution within the treatment volume. Qin et al. (1995c) proposed a m e t h o d to optimize electrode configurations by correcting electrode contours to provide a uniform electric field. As presented in Fig. 2.10, point R a o n the electrode surface A moves to R'a in the normal direction while the potential of the electrode Va is kept constant and the field intensity (E a) i n c r e a s e s to E'a or Eta > E a,
where E'a =
Va [Rb -- R'a[
and E a
(2.2)
Va [Rb -- Ra["
Vb and R b are the potential voltage and field points of the electrode with surface B, respectively. Clearly, by moving the electrode contour points in a normal or opposite direction, the field intensity at the electrode surface can be increased or decreased. Electrode shape change is performed by moving the contour points in proportion to the difference between calculated and desired values of field intensity: if A E m _< k oEd
Ed - Ei Agi--- ~ g d - E i AEm
~
( k 0Ed )
~
a
"'",
otherwise
(2.3)
Vb
ER
AIII.I.R,R i >E'a
Ra
Ra
B Fig. 2. l0 Electrode shape optimization to obtain uniform electric fields at electrode surfaces A and B (Qin et al., 1995c).
32
2. Design of PEF Processing Equipment
where the maximum difference between the two electric fields is defined as AE m
=
max{IE a
-
Eil}, i = a, 2 . . . . Arc
(2.4)
and E d, Ei, ko, and Arc are the desired value of electric field intensity, field intensity at the i th point of the electrode contour (a factor that limits the moving distance of the node point), and the total n u m b e r of node points at which the correction is made, respectively. In this case E d was held constant in order to achieve a uniform electric field, and the factor k 0 chosen such that it provided an acceptable finite element mesh. If a node point moves more than the longest side of the triangular element, the meshing information of the element will be incorrect and the finite element m e t h o d (FEM) analysis will cease. Correction of the node points on the electrode contours is given by the following correction vector (Misaki et al., 1982): AEi) rex = r i 1 +
Ei
.
(2.5)
In the optimization based on finite elements, rci and r i are, respectively, the vectors defining the i th node point on the corrected and initial contours. E i was obtained by finite element computation. To simplify electrode optimization, the net space charge due to the movement of charged species (ions, protein, and living cells) in liquid foods may be ignored. The algorithm for electrode shape optimization consists of the following steps (Qin et al., 1995c): 1. Use a FEM to solve for the electric field inside a chamber with an initial contour design and a high-resolution mesh in the region of highintensity electric fields to reduce the numerical error. 2. Move the coordinates of the selected boundary nodes at the surface of the electrode within the treatment region in proportion to the difference in electric fields as described in Eq. (2.2). (This step results in a piecewise linear description of the electrode shape, which may contain n u m e r o u s spikes and edges.) 3. Obtain a smooth approximation of the piecewise linear shape using a spline routine. (Note that the electrode contour change should be conducted in the treatment region only, where only the FEM mesh will be affected.) 4. Regenerate the triangular mesh in the treatment region based on the approximated electrode contour and solve for the electric field intensity on the corrected electrode contour using FEM. The optimization criterion is to find an electrode contour that provides a uniform electric field at the electrode surfaces in the treatment region. If the calculated field distribution is not close enough to the uniform one, return to Step 2. This iteration procedure may end when the optimization criterion is satisfied.
IV. Treatment Chambers
B.
33
Static Chambers
Materials selected to construct a t r e a t m e n t c h a m b e r n e e d to be washable and autoclavable. Polysulfone and stainless steel are r e c o m m e n d e d for the insulation and electrodes, respectively. However, Bushnell et al. (1993) suggested using electrochemically inert materials such as gold, platinum, carbon, or metal oxides to construct the electrodes or electrode surfaces. Parallel plate electrodes with gaps sufficiently smaller than their electrode dimensions can achieve uniform electric field strength. Disk-shaped, roundedged electrodes can minimize electric field e n h a n c e m e n t and reduce the possibility of dielectric breakdown in fluid foods (Zhang et al., 1995). Designing t r e a t m e n t chambers to facilitate sample filling and removal adds to the complexity of their construction. Since a gas bubble is a potential trigger of dielectric breakdown, the filling ports need to facilitate a complete expulsion of air during filling (Zhang et al., 1995). Sale and Hamilton (1967) were a m o n g the earliest researchers to study the inactivation of microorganisms with PEF. At this time carbon electrodes supported on brass blocks were used and placed in a U,shaped polythene spacer as illustrated in Fig. 2.11. Using different spacers regulated the electrode area and a m o u n t of food that could be treated. The m a x i m u m electric field that the c h a m b e r could withstand was limited to 30 k V / c m due to the electrical breakdown of air above the food. The temperature of the food was controlled by the circulation of water through the brass blocks. The chamber designed by D u n n and Pearlman (1987) consists of two stainless-steel electrodes and a cylindrical nylon spacer. The c h a m b e r is 2 cm high with an inner diameter of 10 cm and an electrode area of 78 cm 2 (Fig. 2.12). The c h a m b e r is intended for treating liquid foods, which are introduced through a small aperture in one of the electrodes. The aperture can also be used for temperature m e a s u r e m e n t during the t r e a t m e n t of foods with high-intensity electric fields. The static t r e a t m e n t c h a m b e r designed at WSU is presented in Fig. 2.13. Two round-edged, disk-shaped stainless-steel electrodes were polished to mirror surfaces. Polysulfone or Plexiglas was used as the insulation material. The effective electrode area is 27 cm 2 and the gap between electrodes can
Fig. 2.11 The static chamber designed by Sale and Hamilton (1967).
34
2. Design of PEF Processing Equipment
Fig. 2.12 A cross section of the static chamber designed by Dunn and Pearlman (1987).
be selected at either 0.95 or 0.5 cm. Electric field strengths up to 70 k V / c m have been tested. Cooling of the c h a m b e r is provided by circulating water at preselected temperatures through jackets built into electrodes. An acoustic pressure pulse is observable while PEF is applied. Since a completely sealed t r e a t m e n t chamber is dangerous because of possible sparking and high pressure that could subsequently develop and cause the c h a m b e r to break apart, a pressure release device must still be included to ensure safe operation (Zhang et al., 1995). T r e a t m e n t chambers with parallel plate electrodes offer a uniform electric field distribution along the gap axes and electrode surfaces, but create a field e n h a n c e m e n t problem at the edges of the electrodes. The FEM was used to determine the o p t i m u m position of the insulating spacer in the WSU design so that the food to be treated could be held in the region of the uniform electric field. Section A in Fig. 2.14 represents the region of the uniform electric field, and section B the spacer. The o p t i m u m position for the boundary between sections A and B was d e t e r m i n e d using the FEM. Equipotential lines from the analysis using 4992 elements and 2651 nodes are presented in Fig. 2.15. In an effort to avoid p r o d u c t contact with the electrode wall, Lubicki and Jayaram (1997) proposed the use of a glass coil surrounding the anode
Fig. 2.13 The static chamber designed at WSU.
IV. Treatment Chambers
35
Fig. 2.14 Boundaries of electric field regions in a treatment chamber with parallel plate electrodes (the electric field is symmetrical about the center line, and only the top portion of the configuration is shown) (Qin et al., 1995c).
(Fig. 2.16). T h e s a m p l e v o l u m e o f t h e i r static c h a m b e r is 20 c m ~, w h i c h n e c e s s i t a t e s a filling l i q u i d with h i g h c o n d u c t i v i t y a n d similar p e r m i t t i v i t y to t h e s a m p l e ( m e d i a NaC1 s o l u t i o n tr = 0 . 8 - 1 . 3 S / m , filling l i q u i d w a t e r ~ 10 -a S / m ) u s e d b e c a u s e t h e r e is n o i n a c t i v a t i o n with a n o n c o n d u c t i v e m e d i a (i.e., t r a n s f o r m e r silicon oil). T h i s c o n f i g u r a t i o n m u s t also a d d r e s s t h e q u e s t i o n o f h o w efficiently t h e p u l s e e n e r g y c a n b e t r a n s f e r r e d i n t o t h e sample.
C.
Continuous Chambers
Static c h a m b e r s a r e m a i n l y suitable f o r l a b o r a t o r y use. F o r l a r g e r scale o p e r a t i o n s , c o n t i n u o u s c h a m b e r s a r e m o r e efficient. T o suit this p u r p o s e , D u n n a n d P e a r l m a n (1987) d e s i g n e d a c h a m b e r c o n s i s t i n g o f two p a r a l l e l
20-
z
15-
J
5
. . . . . . . . . .
!
0
!
5
!
!
15
- ~ . /
!
!
J
!
22
!
28
!
j
J
!
!
32
!
!
40
r(mm) ---'-0
& 100 . . . . .
80 - - - 6 0 . . . .
40 - - 2 0 I
Fig. 2.15 Calculated equipotential lines for the half-field region in a parallel plate treatment chamber (the potential distribution is given as a percentage of the electrode voltage) (Qin et al., 1995c).
36
2. Design of PEF Processing Equipment
Fig. 2.16 A treatment chamber with no food exposed directly to electrodes (adapted from Lubicki and Jayaram, 1997).
plate electrodes and a dielectric space insulator (Fig. 2.17). The electrodes are separated from the food by ion conductive m e m b r a n e s m a d e of sulfonated polystyrene and acrylic acid copolymers, but fluorinated hydrocarb o n polymers with p e n d a n t groups would also be suitable. An electrolyte is used to facilitate electrical c o n d u c t i o n between electrodes and ion p e r m e able m e m b r a n e s . Suitable electrolyte solutions include sodium carbonate, sodium hydroxide, potassium carbonate, and potassium hydroxide. These
Fig. 2.17 A continuous chamber with ion-conductive membranes separating the electrodes and food (adapted from Dunn and Pearlman, 1987).
IV. Treatment Chambers
37
Fig. 2.18 A continuous chamber with electrode reservoir zones (adapted from Dunn and Pearlman, 1987). are circulated continuously to remove the products of electrolysis a n d replaced in the event of excess c o n c e n t r a t i o n or depletion of ionic c o m p o nents. A n o t h e r c o n t i n u o u s c h a m b e r described by D u n n a n d P e a r l m a n (1987) is c o m p o s e d of electrode reservoir zones instead of electrode plates (Fig. 2.18). Dielectric spacer insulators that have slot-like openings (orifices) in between where the electric field is c o n c e n t r a t e d a n d liquid food is i n t r o d u c e d u n d e r high pressure. T h e average residence time in each of these reservoir zones is less than 1 min. T h e WSU static parallel plate electrode c h a m b e r was modified by a d d i n g baffled flow channels inside to m a k e it operate as a c o n t i n u o u s c h a m b e r (Fig. 2.19). Two stainless-steel disk-shaped electrodes separated by a polysulfone spacer f o r m the c h a m b e r . T h e designed operating conditions are:
Fig. 2.19 The continuous chamber with baffles designed by WSU: (a) cross section view and (b) top view (reprinted from Food Technol., Vol. 49(12), Qin et al., "Food pasteurization using high-intensity pulsed electric fields," pp. 55-60, 1995, with permission from Elsevier Science).
38
2. Design of PEF Processing Equipment
Fig. 2.20 A cofield treatment chamber (Sensoy et al., 1997).
c h a m b e r volume, 20 or 8 ml; electrode gap, 0.95 or 0.51 cm; a n d food flow rate, 1200 or 6 m l / m i n (Qin et al., 1996; Zhang et al., 1995). T h e c o n c e p t of e n h a n c e d electric fields in the t r e a t m e n t zone was applied by Yin et al. (1997) for the d e v e l o p m e n t of a continuous cofield flow PEF c h a m b e r (Fig. 2.20) with conical insulator shapes to eliminate gas deposits within the t r e a t m e n t volume. T h e conical regions were designed so that the voltage across the t r e a t m e n t zone could be almost equal to the supplied voltage. O t h e r configurations with e n h a n c e d electric fields are p r e s e n t e d in Figs. 2.21 a n d 2.22. In these devices the flow c h a m b e r s can have several crosssection geometries that may be u n i f o r m or n o n u n i f o r m . In this type of c h a m b e r configuration the first electrode flow chamber, insulator flow c h a m b e r , second electrode flow c h a m b e r , c o n d u c t i n g insert m e m b e r s , a n d
Fig. 2.21 A treatment chamber with different electrode geometries and enhanced electric fields in the insulator channel (adapted from Yln et al., 1997).
IV. Treatment Chambers
39
Fig. 2.22 A treatment chamber with enhanced electric fields in the insulator channel and tapered electrodes (adapted from Y'ln et al., 1997).
insulating insert m e m b e r are f o r m e d and configured such that the electrode flow c h a m b e r and insulator flow c h a m b e r form a single tubular flow chamber t h r o u g h the PEF t r e a t m e n t device (Yin et al., 1997). Figure 2.23 is a variant of the flow c h a m b e r configuration where the uniformity of the liquid p r o d u c t flow velocity is improved. In reality this configuration can be a plurality of c o n d u c t i n g m e m b e r s and at least one insulator m e m b e r . W h e n the voltage pulse signal is applied across high and g r o u n d voltage electrodes, an electric field is f o r m e d in the electrode flow channels as well as the insulator channel where the electric field strength is strongest. Therefore, the bactericidal effect of the PEF t r e a t m e n t process of this device occurs primarily in the liquid p r o d u c t flowing t h r o u g h the insulator flow channel. The electrodes of these chambers are of food grade stainless steel and the insulators of policarbonate, but can also be ceramic, glass, or plastic.
Fig. 2.23 A treatment chamber with improved flow characteristics and enhanced electric fields (adapted from Yin et al., 1997).
40
2. Design of PEF Processing Equipment
Coaxial configurations, which provide the advantage of a uniform fluid flow and simple chamber structure, can be easily manufactured and provide well-defined electric field distributions. The field intensity (E) between coaxial electrodes is given by (Zhang et al., 1995):
E = 1//[ r l n ( R 2 / R 1)],
(2.6)
where r is the radius at which the electric field is measured and R 2 and R 1 are the radii of the outer and inner electrodes, respectively. The uniformity of the electric fields in the t r e a t m e n t chambers with coaxial electrodes can be improved when R 2 - R 1 < R 1. Coaxial chambers are basically composed of an inner cylindrical electrode surrounded by an outer annular cylindrical electrode that allows food to flow between them (Bushnell et al., 1993). It is r e c o m m e n d e d that the length of the fluid flow path not be too small or too long c o m p a r e d to the c h a m b e r diameter. The electrical energy (W) consumed in each pulse is given by
W= EZv~'/p,
(2.7)
where E is the electric field (volts/cm), ~- is the pulse duration, v is the volume (ml), and p is the electrical resistivity of the food sample (ohm-cm). The coaxial chamber designed at WSU is based on a modified coaxial cylinder a r r a n g e m e n t (Fig. 2.24). A p r o t r u d e d outer electrode surface enhances the electric field within the treatment zone and reduces the field intensity in the remaining portion of the chamber. The electrode configuration was obtained by optimizing the electrode design with a numerical electric field computation. Using the optimized electrode shape, a pre-
Fig. 2.24 A cross-sectional view of the modified coaxial treatment chamber designed, constructed, and tested for microbial inactivation at WSU (Mart~n-Belloso et al., 1997).
V. Cooling System
41
Fig. 2.25 Electric field distribution in the food region between two electrodes of a coaxial treatment chamber (Qin et al., 1995c).
scribed field distribution along the fluid path without electric field enhancement points was determined. The outer electrode has a p r o t r u d e d contour surface that was obtained by numerical electric field optimization. Figure 2.25 illustrates the electric field distribution of the chamber in its treatment region; to allow for a clear view, the field inside the dielectric spacer is not presented. For numerical simplicity, a 100-V applied voltage was used in the field calculation in order to verify electric field uniformity. In the treatment region between the two electrodes, the potential drop is nearly uniform so a strong electric field is generated. Since outside the treatment region most of the potential drop occurs inside the spacer, the electric field is quite weak. The gap between the chamber's electrodes is adjustable within 2 to 6 m m or more by changing the inner electrode to different diameters. Cooling jackets are built into both electrodes to maintain low temperatures. The whole treatment chamber has an outer diameter of around 13 cm and an approximate height of 20.3 cm. The PEF system in which this chamber is used can handle flow rates from 30 to 120 l i t e r / h r (Qin et al., 1995c).
V.
Cooling System
When no cooling is provided, the increase in temperature (AT) of the fluid subjected to electric field pulses is given by
Q AT =
pfCp
E2n~'o =
pfCp
,
(2.8)
where Q is the energy input and pf a n d Cp are the density and specific heat of the fluid being treated, respectively. The energy input can be defined by
42
2. Design of PEF Processing Equipment
the product of the square of the electric field (E), the n u m b e r of pulses (n), and electrical conductivity (~r) of the fluid being treated. The food temperature is maintained by circulating constant temperature water through the cooling jackets built into the electrodes. A rise in temperature over 70~ must be avoided to preserve natural attributes of food products and to claim a n o n t h e r m a l treatment. Zhang et al. (1994) proposed a one-dimensional finite difference heat transfer model to predict the fluid temperatures in a static chamber. The fluid food is divided into circular disks as illustrated in Fig. 2.26. A disk layer of fluid with thickness Ad, radius r, and volume AV represents each node in the one-dimensional model. The t r e a t m e n t chamber is divided into 2-m nodes, where m is selected to be 20. Using an energy balance, the temperature of each element is related to its adjacent elements as Tr
pfCpAV
- T( =
At
Ad
T?_I - Ti" + ha
Ad
'
(2.9)
where pf is the density, Cp is the specific heat, k is the thermal conductivity of the fluid, T is the temperature, A is the area of heat transfer, At is the time interval of each time step, d is the electrode gap, V is the c h a m b e r volume, and Ad - d / 2 m and AV = V / 2 m are the thickness and volume of fluid for each node, respectively. (Superscripts denote the time step, and subscripts the space coordinate.) The food in contact with the electrodes in the model by Zhang et al. (1994) is assumed to have their same temperature and is therefore considered the boundary condition of the design. Energy input into the food should be uniform within the bulk and is only present at the time of pulse application. Energy input was treated as the initial condition of the model and proved to generate no heat after pulsing was generated. The fluid
/•
_
. ~ .
ermocoupleLocation
TmcontactBottomElectrode /
I
CenterAxis
Fig. 2.26 A one-dimensional finite difference grid of a fluid food inside a parallel plate static treatment chamber (Zhang et al., 1994).
VI. Typical Measurements in a PEF System
43
temperature is measured with a thermocouple inserted 2 m m into the food and halfway between the electrodes, and a thermocouple attached to the cathode gauges the electrode temperature. Figure 2.27 illustrates the predicted fluid center temperatures of apple juice subjected to 20 pulses at 30 sec intervals for energy input levels of 100, 300, and 600 J / p u l s e in the heat transfer model of Zhang et al. (1994). These energy levels correspond to electric fields of 12, 20, and 28 kV/cm, respectively. When repetitive pulses are applied, the fluid center temperature is significantly higher than the electrode temperature. By controlling the interval of time between pulses to longer than 30 sec, the maximum fluid temperature is expected to remain below 25~
VI.
Typical Measurements in a PEF System
Electrical parameters such as voltage and current pulse waveforms applied during PEF treatments should be recorded via a digital acquisition system. Outputs from voltage and current monitors can be recorded by a digital oscilloscope into a computer for future analysis, but it is important that each be placed in a shielded area to minimize electromagnetic interference. To achieve accurate measurements of voltages and currents, all important frequency components must be recorded, which includes specification of the bandwidth of each transducer. With these measurements, further evaluation of the applied energy per volume and spatial average of the electric fields in the treatment region can be computed in order to determine if the process is under its control limits. Direct measurement of the electric fields by
25.00 o
20J30 84 15s IO.O0-
5.oo
0.00
I 100
I 200
I 300
I 400
I 500
600
Time(s) Fig. 2.27 Model-predicted fluid temperatures of apple juice subjected to 20 pulses with energy inputs of 100, 300, and 600 J / p u l s e (the electrode temperature was maintained at 4~ (Zhang et al., 1994).
44
2. Designof PEF ProcessingEquipment
optical fibers in the treatment gap of the chamber would be highly desirable, but this anticipates complexity and increased cost. It is definitely advantageous to take oscilloscope readings with a wellvalidated measuring system. This is especially evident when the frequency content of pulses is desired, as change may indicate a partial discharge that could lead to sparks. The frequency content of pulses is obtained by applying the fast Fourier transform algorithm to the voltage and current as a function of time [v(t), i(t)]. The analog/digital converter is also useful to warn of missed or weak pulses. Because it is important to maintain a treated food temperature at low values to minimize thermal effects, measurements should be made with a high-precision thermocouple, (placed outside the processing chamber) or even the more sophisticated fiber optic transducer. The latter has a precision of up to 0.1~ and a response time of 0.2 sec. Ideally, about four temperature monitoring optical fibers in the interelectrode region, inlet, and outlet of the treatment chamber should be installed. The flow rate and pressure in PEF chambers are critical quantities in the control process and should be measured throughout the system as a function of time and space, respectively. The applied dosage will d e p e n d directly on the flow rate when the pulse frequency and treatment chamber volume are fixed. Although the pressure will not affect the inactivation rate or applied dosage, it will prevent arcing, and m e a s u r e m e n t will detect any plug or leakage in the system.
VII.
Packaging and S t o r i n g
After PEF treatment, foods are cooled if necessary and aseptically packaged. Aseptic technology has been used for liquid milk and fruit juices for more than 30 years and is also known as an effective m e t h o d of producing shelf-stable preserved products that have quality advantages over their conventionally canned or jarred counterparts. Some of these include packaging in a range of materials without limitations on container size and the use of thin plastic membranes or plastic/paper-laminated materials that do not have to withstand high temperatures as in conventional thermal processing, as no retorting of the product is applied after packaging. Additional benefits of such an operation include less damage to the product, shorter processing periods, uniform and improved quality, reduced energy consumption, and utilization of new packaging materials (Singh and Nelson, 1992). After packaging, the bagged foods may be stored at refrigerated or ambient temperatures depending on the type of product. For example, milk may have to be stored at refrigerated temperatures, whereas apple juice has a reasonable long shelf-life even when stored at room temperature. The effectiveness of aseptic packaging has been demonstrated by the extended
References
45
shelf-life of several PEF products (Qin et al., 1995a,d; Zhang et al., 1997). Because c o n s u m e r s d e m a n d freshness, aseptic packaging is the perfect c o m p l e m e n t for PEF to provide a cost-effective way for processors to deliver what markets expect.
VIII.
Final Remarks
This c h a p t e r reviewed key elements for the design of a PEF system a n d the description of the main c o m p o n e n t s that are involved in the p r o d u c t i o n of high-voltage pulses. T h e i m p o r t a n c e of the high-voltage repetitive pulser, switches, t r e a t m e n t chamber(s), cooling system(s), voltage a n d c u r r e n t measuring devices, control units, data acquisition system, a n d packaging system were addressed based on specific system r e q u i r e m e n t s , t r e a t m e n t convenience, a n d future needs. T h e efforts of leading groups a n d institutions to obtain the best processing conditions are p r e s e n t in the d e v e l o p m e n t of new t r e a t m e n t c h a m b e r s a n d PFN configurations. Mathematical simulation used in the design of t r e a t m e n t c h a m b e r s represents a safe a n d m o r e cost-effective tool in the r e f i n e m e n t of new configurations a n d minimization of trial-error steps, so the future i m p l e m e n t a t i o n of m a t h e m a t i c a l software is encouraged. A l t h o u g h there is increasing interest in PEF technology, we detect the n e e d for commercially available high-voltage pulsers a n d switching devices. T h e r e are m a n y suppliers of high voltage i n s t r u m e n t a t i o n but few are involved in the m a n u f a c t u r i n g of complete PEF systems. T h e next c h a p t e r reviews how a n d by which m e c h a n i s m s the pulses p r o d u c e d by the PEF systems discussed in this chapter affect biological materials such as cell m e m b r a n e s , microorganisms, a n d proteins.
References Bhasavanich, D., Hitchcock, S. S., Creely, P. M., Shaw, R. S., Hammon, H. G., and Naff, J. T. (1991). International Pulsed Power Conference, San Diego, California. Bushnell, A. H., Dunn, J. E., and Clark, R. W. (1993). High pulsed voltage systemsfor extending the shelf life of pumpable food products. U. S. Patent 5,235,905. Dunn, J. E., and Pearlman, J. S. (1987). Methods and apparatus for extending the shelf life of fluid food products. U. S. Patent 4,695,472. EPRI (1997). EPRI/Army PEF Workshop II, Chicago, Illinois, 10-11 October. EPRI (1998). Pulsed electric field processing in the food industry: A status report on PEF. Report CR-109742. Industrial and Agricultural Technologies and Services, Palo Alto, California. Ho, S. Y., Mittal, G. S., Cross, J. D., and Griffiths, M. N. (1995). Inactivation of Pseudomonas fluorescens by high voltage electric field pules. J. Food Sci. 60(6), 1337-1343. Lubicki, P., and Jayaram, S. (1997). High voltage pulse application for the destruction of the Gram negative bacterium Yersinia enterocolitica. Bioelectrochem. Bioenergetics 43, 135-141. Mart~n-Belloso, O., Vega-Mercado, H., Qin, B. L., Chang, F. J., Barbosa-C~novas, G. V., and Swanson, B. G. (1997). Inactivation of Escherichia coli suspended in liquid egg using pulsed electric fields. J. Food Proc. Pres. 21, 193-208.
46
2. Design of PEF Processing Equipment
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