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 ISSN: 2321-2667 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 Volume 3, Issue 3, May 2014 1 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 ISSN: 2321-2667 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 Volume 3, Issue 3, May 2014 2 Journal of Research in Electrical and Electronics Engineering (ISTP-JREEE) 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 ISSN: 2321-2667 Volume 3, Issue 3, May 2014 3 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 =12.7s 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 =12.79s ISSN: 2321-2667 Volume 3, Issue 3, May 2014 4 Journal of Research in Electrical and Electronics Engineering (ISTP-JREEE) 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=12.79s 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 ISSN: 2321-2667 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. Volume 3, Issue 3, May 2014 5 Journal of Research in Electrical and Electronics Engineering (ISTP-JREEE) 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. ISSN: 2321-2667 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 Volume 3, Issue 3, May 2014 6 Journal of Research in Electrical and Electronics Engineering (ISTP-JREEE) 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. Volume 3, Issue 3, May 2014 7
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