effect of high pulsed electric field in liquid food treatment

Journal of Research in Electrical and Electronics Engineering (ISTP-JREEE)
EFFECT OF HIGH PULSED ELECTRIC FIELD IN
LIQUID FOOD TREATMENT
S.Priya, Department of EEE, AMET University
Abstract
Processing foods with HV pulsed electric fields is a new
technology to inactivate microorganisms and denature
enzymes with only a small increase in temperature. For a
given peak value of field intensity and amount of electric
energy input, PEF inactivation of microorganisms is closely
related to the waveform of applied pulses. This paper presents
the effect of process parameters used in liquid food treatment.
It is performed by using fixed R and C values and by varying
L values in RLC network. From the measured output voltage
and current waveforms the appropriate value of inductance is
chosen which make the cell survivability minimum.
Comparison is made out for the different types of liquid foods
such as orange, Apple and Tomato juice respectively. By
using this approach, the energy efficiency required for
electrical sterilization can be improved.
Index Terms – High Voltage, Pulsed electric fields (PEF),
Sterilization, Transmembrane Potential (TMP)
I. Introduction
Pasteurization by heat is the conventional method used to
inactivate microorganisms in liquid food to extend their shelf
life. However, a heat treatment has several side effects like
causing an irreversible loss of taste, colour, flavor and the
nutrional value of the food. Therefore, there is a growing
interest in non-thermal pasteurizing methods to treat liquid
food. Use of the pulsed electric fields (PEFs) is considered to
be one of the most promising non-thermal food-treatment
methods for this application. PEF processing offers high
quality fresh-like liquid foods with excellent flavor,
nutritional value, and shelf-life. Since it preserves foods
without using heat, foods treated this way retain their fresh
aroma, taste, and appearance. It can be used for processing
liquid and semi-liquid food products.
PEF method involves the application of a high electric field
(15–80 kV/cm) across a certain electrode geometry that
contains the liquid food [1].The effect of high electric field on
biological cells causes a structural change of cell
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membranes. Especially, with higher field strength and longer
stimulation period, the membrane irreversibly breaks down
which results in inactivation of the cell. This phenomenon is
known as 'electrical sterilization' of biological cells. The highvoltage pulse width is very short (nanoseconds to
microseconds) so that the thermal effect can be minimized
[2].The PEF treatment causes damage of the cell membrane,
therefore primarily lethal effect is caused by the formation of
pores in the cytoplasmic membrane. These pores are reversible
or irreversible depending on the level of the damage, which is
dependent on the treatment condition [3]. This phenomenon is
known as electroporation which occurs in living cells when
the transmembrane potential (TMP–potential difference
between inner and outer surfaces of cell membrane) is within a
range of 0.5-1.2V and it is dependent on kind and size of a cell.
The inactivation of microbes is due to charging and
subsequent breakdown of the cell membrane. The optimum
efficiency of killing is achieved for pulses with duration
slightly longer than the charging time of the membrane [4].
Microbial inactivation in liquid foods by PEF depends on
many factors such as process parameters, product parameters,
and microbial characteristics [5].It is well known that the
application of PEF to biological cells can be examined using
equivalent circuit models [6] where membrane and cytoplasm
are represented by equivalent capacitances and resistances.
However, under sub-microsecond PEF it is also necessary to
consider intracellular organelles including nucleus,
mitochondria and other sub-cellular structures.
In this paper, various types of liquid foods are used as
characteristic loads. The two factors, process and product
parameters, significantly influence the wave shape of the pulse
which is generated and applied to the medium to be treated.
II. Mechanism of microbial cell
In activation
Microbe processing includes food sterilization, waste
treatment, pollution control and medical diagnostics treatment.
The applied electric field induces an additional voltage across
the cell membrane which leads to a rupture of the membrane,
resulting in a disorder in the membrane structure. Membrane
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Journal of Research in Electrical and Electronics Engineering (ISTP-JREEE)
damage and deterioration are direct causes of cell inactivation.
Inactivation of a microorganism exposed to PEF is related to
the electro-mechanical instability of the cell membrane.
The cell membrane protects the microbe from its
surrounding environmental conditions and if disrupted,
intracellular contents leak out and the cell metabolic activities
are lost. Exposure of the cell membrane to an electric field
leads to an increase in TMP which leads to a reduction in the
membrane thickness.
The TMP of the cell membrane is given as:
U(t) = 1.5 r E cosθ
where
U(t)
r
E
θ
(1)
= transmembrane potential (V)
= radius of the cell (mm)
= applied electric field strength (V/mm)
= angle between a given membrane site
and the field direction (degrees)
The cytoplasm has a conductivity of 1S/m with a dielectric
constant of 80.The membrane has a thickness of 2µm, a
conductivity is 10-5 S/m, and a dielectric constant of 2.5. The
resistivity of cytoplasm and nuclear plasma is assumed to be
identical as 100Ω.cm. Similarly the resistivity of the medium
is 1kΩ.cm. The specific capacitance of the outer membrane is
1μF/cm2. The capacitance of the nuclear membrane (Cn) was
assumed to be half of the outer membrane (Cm). In general,
the electric pulses do not affect the intracellular membranes
because the outer membrane shields the interior from the
influence of electric fields. However, if the pulse duration
becomes very short and consequently, the cutoff frequency of
its Fourier spectrum becomes very high, the electric field can
penetrate the outer membrane and affect intracellular
membrane. Fig. 3 shows the electroporation processing of a
cell.
A. Model of cell
Biological cell exist in a wide variety of shapes and sizes,
which is dependent on their function in the organism. One of
the most useful geometries that describe a wide range of
cellular properties is that of a spherical shel[4]l. The cell
structure is formed by two layers (cytoplasm and
membrane), which is immersed in a continuous medium
formed by electrolytes.
Fig. 1 shows the equivalent model of a cell inside
suspension, which describes about the concept of electric field
and cell interactions.
Figure 1. Equivalent model of a cell inside suspension
Figure 2. Electrical model of a cell
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Figure 3. Electroporation processing of a cell
The electric field in cytoplasm is given as
E(t)=Eoexp(-t/τm)
(2)
The pulse duration required to reach a critical voltage across
the membrane is determine by the charging of the outer
membrane. The charging time constant of the membrane is
τm = (ρ1/2 + ρ2) Ca
(3)
where ρ1 is the resistivity of suspending medium,ρ2 is the
resistivity of the cytoplasm is the capacitance of the membrane
per unit area and a is the radius of the cell. The electric field
required to charge the membrane so as to reach a critical
electric field Ec can be expressed as
Ec=Vc/fa
(4)
A typical value of the critical membrane voltage is 1V and for
spherical cells form factor (f) is 1.5.The theoretical lower limit
for the required peak power at the chamber terminals can be
calculated from
Pmin = σ E2Vch
(5)
where Vch is the volume of the treatment chamber, E is the
field strength and σ is the specific conductivity of the food.
The volume of the treatment chamber is
Vch = π/4D2L
(6) where
D is the diameter of the cylindrical chamber and
L is
the chamber length.
III. Electrical sterilization using RLC
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Discharging circuit
A Very intense, short-duration electric field can be
applied to achieve sterilization of liquid foods. Ohmic heating
and disintegration of food particles by shock waves, which are
the main drawbacks of electrical treatment methods, are
eliminated through the use of short duration pulses. A RC
circuit, which consists of a capacitive energy storage source
and a resistive suspension chamber, has been widely used for a
discharging network. The oscillating waveforms obtained by
adding inductor L to the RC circuit are used to investigate the
effect of the multiple peak waveforms on the liquid medium.
By choosing appropriate circuit parameters or discharging
waveforms the efficiency is optimized.
The process parameters include electric field intensity
which strongly influences microbial inactivation, treatment
time defined as the product of the number of pulses and the
pulse duration which increases the processed food
temperature, pulse waveshape in which electric field pulses
may be applied in the form of exponential decaying, squarewave, oscillatory, bipolar, or instant reverse charges and also
temperature maintained at (~50-60°c) had synergistic effect on
the inactivation of microorganisms.
In product factor, there exist many parameters such as
electrical conductivity of a medium (σ, Siemens/m) which is
an important parameter in PEF applications. Food with high
conductivity needs a powerful power supply to maintain
required electric field intensity which is not feasible with PEF
treatment. The electrical conductivity of a medium, which is
defined as the ability to conduct electric current, is an
important variable in PEF. An increase in the difference
between conductivity of the medium and microbial cytoplasm
weakens the membrane structure due to an increased flow of
ionic substance across the membrane. Table 1 shows electrical
parameters for various mediums.
Table 1 Electrical parameters for various mediums
Type of
Medium
Orange Juice
Apple Juice
Tomato Juice
Liquid food
Resistance
5.85Ω
8.00Ω
1.25Ω
Relative
permittivity
84
72.5
90
IV. Materials and methods
Conductivity
(S/m)
0.335
0.18
1.58
serves as a discharge current limiting resistance to a dc
power source (20KV) while inductance are varied from micro
Henry to milli Henry. The load resistance is mainly due to the
liquid food sample and the value of resistance depends on the
type of juice used and the electrode geometry of the test
chamber. Due to the presence of R, the oscillations of charge,
current, and voltage continuously decrease in
amplitude
giving a damped sinusoidal waveform. The
differential
equation that governs such an RLC series circuit is given by
L
d 2q
dq L
+R
+ q=0
2
dt
dt C
(7)
which represents the damped oscillations.
Figure 4. Circuit diagram for the Oscillatory pulse (Orange
Juice)
The solution to the above equation is
-Rt
Q0 =Qe 2L Cos( t   )
d
(8)
in which  ( 2  (R 2L)2 ) , and because of the values of
d
L and R it is possible to approximate the relation as
    1 LC .This equation describes how the charge
d
on the capacitor oscillates in a damped RLC circuit with
exponentially decaying amplitude Qe-Rt 2L . Thus, the
energy of the electric field oscillates accordingly transferring
the electromagnetic stored energy from L and C into the loss
component R (thermal energy). When the R value is
sufficiently large, the oscillations can be critically damped.
V. Results and discussion
The inductance
parameter is varied for the generation of high voltages pulses
with a desired peak voltage and pulse width. If that inductance
is not that much fruitful to get a desired survivability value of
bacteria[2], then a new inductance is chosen so that a different
value of the peak voltage and the pulse width has to be
generated with the given load resistance [Fig.4] and the output
measured across the load is shown in Fig.6-8.Since the
capacitance C and the initial charging voltage Vo (20kV) are
Fast rise time high-voltage short pulses in the kilovolt
range are produced using a capacitor bank, which is charged
by a high-voltage dc source and discharged directly into the
test chamber using a fast switching mechanism.Fig.4
illustrates the circuit diagram for the oscillatory pulses used in
PSPICE simulation. A capacitor C(20nF) is charged to a dc
voltage through a charging resistance (200kΩ) which also
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Journal of Research in Electrical and Electronics Engineering (ISTP-JREEE)
constant, the initial energy (4J) stored in the capacitor
(Eo=CVo2/2) is identical for all voltage waveforms.
A. Effect of inductance
In the case of a constant input voltage from the capacitor,
the voltage applied to the treatment chamber depends on
electrical parameters and temperature of the medium, as
temperature influences the medium conductivity[4]. However,
with very short pulses, the effect of temperature rise can be
ignored, unless the repetition rate is high. Fig. 5 shows the
simulation with cell model. The resistance in Fig. 4 is
replaced with the bacteria cell model and the output
voltage across the bacterium is measured. As expected with
the increased inductance in the circuit, and the low
resistance offered by tomato juice, amplitude (peak to peak),
levels of oscillation, and the rise time have all increased
[Fig.8]. Also, when the circuit inductance is high the pulse has
a slow rising waveform in the medium orange and
tomato
juices respectively [Fig.6 and Fig.7]
Figure 6. Output Voltage waveform for Orange juice L=3μH,
Rise time =685.7ns
Figure 6. Output Voltage waveform for Orange juice L=5mH,
Rise time =12.7s
Figure 5. Simulation model with cell
Table 2. Effect of inductance for various juices
Load
Inductance
3µH
6µH
10µH
0.2mH
5mH
Orange
Ip
(Amps)
906.65
729.71
596.24
157.35
31.58
Apple
Ip
(Amps)
832.11
676.74
562.49
154.73
31.48
Tomato
Ip
(Amps)
1.11k
870.89
690.58
163.19
31.80
Figure 7. Output Voltage waveform for Apple Juice L=3μH, Rise
time =688.1ns
Table 3. Electrical Parameters of biological cell
Rs
Load
Cs
(Ω)
Orange 4.205pF 5.85
Apple 3.629pF 8
Tomato 4.506pF 1.25
Cm
1nF
1nF
1nF
Rc1
(Ω)
100
100
100
Rc2 (Ω) Cn Rn (Ω)
79.57k 0.5nF 33.33
79.57k 0.5nF 33.33
79.57k 0.5nF 33.33
Figure 7. Output Voltage waveform for Apple Juice L=5mH,
Rise time =12.79s
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threshold voltage which illustrates the rupture of the outer
membrane.
Figure 8. Output Voltage waveform for Tomato Juice L=3μH,
Rise time =681.1ns
Figure 9. Effect of inductance on peak current (Simulated)
Figure 8. Output Voltage waveform for Tomato Juice L=5mH,
Rise time=12.79s
The peak current (Ip) are measured for various mediums are
shown in Table 2.The rise time is less and so the system settles
down faster (micro sec) for a low value of inductance. The
high value of peak voltage shows the better inactivation rate in
the liquid food treatment [7]. Increase in inductance value
causes more oscillations and high damping time. Hence the
system performance is improved by choosing a very low value
of inductance. Table 3 shows the equivalent electrical
parameters of biological cell where Rs and Cs is the resistance
and capacitance of the suspension medium. The effect by the
oscillatory pulses on inactivation is not same because these
pulses have decrease in voltage[6]. Also the low inductance
pulse has longer duration time which leads to considerable
increase in temperature of the food. So the minimum value of
inductance is the optimum one. Thus it can be useful in
improving the energy efficiency of the technique.
For low conductivity juices like apple and orange, the
voltage pulse has considerable amount of oscillations even
with the low inductance, whereas in high-conductivity tomato
juice, large oscillations are observed. Fig.9 shows the effect
of inductance on output current. The model of the cell in Fig.
2 is illustrated in the oscillatory circuit in order to analysis the
membrane potential. Fig.10-11 shows the voltage plot across
bacterium for orange and Tomato juice. The threshold level
for the inactivation of the cell is 1V. In this analysis the
voltage developed across the bacteria for both orange and
apple juices are 7.67V which is large when compared with the
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Figure 10. Voltage Plot across Bacterium (Orange)
Figure 11. Voltage Plot across Bacterium (Tomato)
B. Experimental setup
The following test procedure has to be adopted during
treatment of substrate with high voltage pulses. The
experimental set-up for the impulse oscillatory waveform with
varying inductor is depicted in Fig 12. A capacitor C (100nF)
is charged to a dc voltage through a charging resistance
(2.5MΩ) which also serves as a discharge current limiting
resistance to a dc power source (20kV) with inductance varied
from micro Henry to milli Henry.
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VI. Conclusion
From the table 4, it is inferred that for the higher number of
pulses the inactivation rate is higher. The lower value of
inductance (10µH) increases the microbial inactivation rate
when compared to all other inductances[9]. Based on the
above experimental results, a further experiment on orange
juice was conducted by choosing the extreme value of the
inductances namely 10µH and 1.5mH respectively.
Table 4. Effect of Number of pulses on the reduction of
microorganisms in Apple Juice
Number of colonies in the controlled Sample (No) =910
colonies
No
% of
of
Bacterial
Peak
No of cells Reduction
growth
voltage/Inductance Pulses
reduction
20kV(1.5mH)
50
818
92
10.11
100
742
168
18.46
50
689
221
24.29
20kV(10 µH)
100
491
419
46.04
Table 5. Effect of Number of pulses on the reduction of
microorganisms in Orange Juice
Number of colonies in the controlled Sample (No) =924
colonies
Peak
voltage/Inductance
20kV(1.5mH)
20kV(10 µH)
No of
Pulses
50
100
50
100
No
of
cells
Reduction
801
715
648
456
123
209
276
468
% of
Bacterial
growth
reduction
13.31
22.62
29.87
50.65
By comparing the table 4 and 5, experimental results on
different liquid food samples shows that the inactivation rate is
more for orange juice because of higher conductivity. The
conductivity of the suspending medium affects the
inactivation kinetics of the microorganisms.
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50
% of microbial growth
reduction
Figure 12. Experimental set-up for microbial inactivation
The effect of pulse waveshape on the viability of bacteria is
important for the application of the PEF method. From the
simulation results, it is concluded that shorter rise time of the
oscillatory waveform has the better inactivation rate. The
lower value of inductance (10µH) increases the microbial
inactivation rate when compared to all other inductances.
Comparing the experimental results on orange and apple juice,
cell destruction is more efficient in orange than in the apple
juice for the same inductance value and the number of applied
pulses[8]. The experimental result clearly shows that, the
percentage reduction of bacterial growth in liquid food sample
increases proportionally with the number of pulses applied for
a given inductance
40
30
50 pulses
20
100 pulses
10
0
10
50
200
1500
Inductance in m icro Henry
Figure 13. Effect of inductance on % of bacterial growth
reduction (For Apple juice)
Fig 13 shows the effect of inductance on percentage reduction
of bacterial growth in apple juice for the different number of
pulses. The results revealed that at the applied field of
20kV/cm, significant killing of microorganisms was found for
the higher number of pulses (100 pulses).
The following conclusions were drawn based on the
explored effect of successive pulses for various inductance
values,
Simulation Results
1. During the simulation taking the treatment chamber filled
with orange juice as a resistive load, the difference in
percentage reduction in peak current for 10μH and 200μH is
73.61%. Hence the inductance produces higher peak current
with faster rise time oscillatory wave has a better
inactivation rate.
2. Similarly simulation is extended by replacing the resistive
load with the exact electrical equivalent model of the cell
which is suspended in the liquid foods. For the input voltage
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of 20kV, the peak voltage developed across the bacterium to
produce the induced transmembrane potential of 1V is
higher for orange juice.
3. For the same stress level of 20kV/cm and same number of
pulses (100) in orange liquid food sample, there is a 50.65% of
bacterial growth reduction is achieved for 10μH whereas
10.11% of reduction is obtained for 1.5mH. This shows that
the minimum inductance has more influence in the liquid food
treatment using pulsed electric field.
Experimental Results
1. For the same stress level (20kV/cm) and number of pulses
(100), bacterial growth reduction achieved for 10μH is 50.65%
whereas for 1.5mH it is 10.11%.
2. As stated in the literature [4], higher the conductivity of
the liquid food samples higher will be the microbial
inactivation rate. It was proved here by conducting experiment
on different liquid food samples. Compared to the apple juice
of conductivity 0.18 S/m, experiment resulted in 9.1% more
bacterial growth reduction for orange juice with conductivity
of 0.335 S/m.
References
[1]
[2]
[3]
[4]
[5]
[6]
[7]
Roodenburg, J.Morren and H.Prins, “Technology for
Preservation of food with Pulsed Electric Field”, IEEE
2002.
Karl.H.Schoenbach, Robert H.Stark and Jingdong Deng,
“Biological / Medical Pulsed Electric Field
Treatments” ,IEEE 2000.
Tsai-Fu Wu, Sheng-Yu Tseng and Jin-Chyuan Hung,
“Generation of Pulsed Electric Fields for Processing
Microbes”, IEEE Transactions on Plasma Science,
Vol.32, N0.4, August 2004.
Sesha H.Jayaram, “Sterilization of Liquid Foods by
Pulsed Electric Fields”, IEEE 2000.
Karl H. Schoenbach, Frank E. Peterkin and Stephen J.
Beebe, “The Effect of Pulsed Electric Fields on
Biological Cells: Experiments and Applications”, IEEE
Transactions on Plasma Science Vol.25, No.2,
April1997.
Bai-Lin Quin, Qinghua Zhang and Patrick D. Pedrow
“Inactivation of Microorganisms by Pulsed Electric
Fields of Different Voltage Waveforms”, IEEE
Transactions
on
Dielectric
and
Electrical
Insulation,Vol.1 No.6 December 1994
V. Gowrisree, K. Udayakumar and P. Gautam,
“Application of
High Voltage Pulses in the
Preservation of Orange Juice”, IEEE Indicon 2005
Conference, pp. 502-503, Dec 2005.
ISSN: 2321-2667
[8]
[9]
Hee-Kyu Lee, Junya Suehiro, Masanori Hara and DuckChul Lee, “Energy Efficiency Improvement of
Electrical Sterilization Using Oscillatory Waveforms
from a RLC Discharging Circuit”, IEEE Transactions
on Dielectrics and Electrical Insulation Vol.7 No.6,
December 2000.
R. M. Campbell, B. H. Crichton, R. A. Fouracre and M.
D. Judd, “Simulated Pulse Response of Intracellular
Structures in Biological Cells Exposed to HighIntensity Sub-Microsecond Pulsed Electric Fields”,
IEEE Transactions on Dielectric and Electrical
Insulations, Vol 30, No.5, 2005.
Acknowledgments
I wish to thank editor in chief for their valuable suggestions.
Finally I wish to thank entire team of ISTP Journal for the
support to develop this document.
Biographies
S.PRIYA received the B.E. degree in Electrical and
Electronics Engineering from the University of Madras,
Chennai, Tamil Nadu, in 2001 and M.E. degree in High
voltage Engineering from the Anna University,
Chennai,
Tamil Nadu, in 2007 respectively. Currently, She is an
Assistant Professor of Electrical and Electronics Engineering
at Amet University. Her teaching and research areas include
electrical machines, industrial applications in high voltage
engineering, powers system and Insulation materials.
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