2348 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 40, NO. 10, OCTOBER 2012
Comparison Between Monopolar and BipolarMicrosecond Range Pulsed Electric Fields in
Enhancement of Apple Juice ExtractionP. S. Brito, H. Canacsinh, J. P. Mendes, L. M. Redondo, Member, IEEE, and M. T. Pereira
Abstract—The effect of monopolar and bipolar shaped pulsesin additional yield of apple juice extraction is evaluated. The ap-plied electric field strength, pulsewidth, and number of pulses areassessed for both pulse types, and divergences are analyzed. Vari-ation of electric field strength is ranged from 100 to 1300 V/cm,pulsewidth from 20 to 300 μs, and the number of pulses from 10to 200, at a frequency of 200 Hz. Two pulse trains separated by 1 sare applied to apple cubes. Results are plotted against referenceuntreated samples for all assays. Specific energy consumption iscalculated for each experiment as well as qualitative indicators forapple juice of total soluble dry matter and absorbance at 390-nmwavelength. Bipolar pulses demonstrated higher efficiency, andspecific energetic consumption has a threshold where higher in-puts of energy do not result in higher juice extraction when electricfield variation is applied. Total soluble dry matter and absorbanceresults do not illustrate significant differences between applicationof monopolar and bipolar pulses, but all values are inside the limitsproposed for apple juice intended for human consumption.
Index Terms—Bipolar pulses, juice extraction, monopolarpulses, pulsed electric fields (PEFs).
I. INTRODUCTION
THE EXPOSURE of a living cell to short high-intensitypulsed electric fields (PEF), in the range of nanoseconds
to milliseconds, can cause the disruption of the cell membrane.This phenomenon is called electroporation, and its exploitationin the food and drug industry has been largely researched.Applied PEFs into a cell cause the charging of the lipid bilayer
Manuscript received December 2, 2011; revised February 24, 2012 andMay 22, 2012; accepted July 4, 2012. Date of publication August 27, 2012;date of current version October 5, 2012. This work was supported by thePortuguese National Strategic Reference Framework, QREN, under Project1600/A2P2/2008.
P. S. Brito is with the Instituto Superior de Engenharia de Lisboa, 1959-007Lisbon, Portugal (e-mail: [email protected]).
H. Canacsinh is with the Instituto Superior de Engenharia de Lisboa, 1959-007 Lisbon, Portugal. He is also with the Nuclear Physics Center, Lisbon Uni-versity (CFNUL), 1649-003 Lisbon, Portugal, and also with Instituto SuperiorTécnico, Universidade Técnica de Lisboa, 1049-001 Lisbon, Portugal (e-mail:[email protected]).
J. P. Mendes is with the Instituto Superior de Engenharia de Lisboa, 1959-007 Lisbon, Portugal. He is also with the Nuclear Physics Center, Lisbon Uni-versity (CFNUL), 1649-003 Lisbon, Portugal, and also with the Faculdade deCiências e Tecnologia, New University of Lisbon (UNL), 2829-516 Caparica,Portugal (e-mail: [email protected]).
L. M. Redondo is with the Instituto Superior de Engenharia de Lisboa,1959-007 Lisbon, Portugal, and also with the Nuclear Physics Center, Lis-bon University (CFNUL), 1649-003 Lisbon, Portugal (e-mail: [email protected]; [email protected]).
M. T. Pereira is with Lusoforma, 2725-393 Mem Martins, Portugal (e-mail:[email protected]).
Digital Object Identifier 10.1109/TPS.2012.2209444
membrane that rapidly rearranges its structure as a responseto the electric field. A great increase in ionic and moleculartransport takes place, and it is commonly accepted that thiscauses a transition to a localized water-filled structure alsocalled “aqueous pathways” or “pores” [1]. Every living cell hasa local transmembrane voltage at each point of its membrane,and for a specific pulse duration and applied external field, thereis a threshold for the electroporation phenomenon be able totake place [2]. It is generally accepted that, for an average cellsize of 100 μm, field strengths of 100–1000 V/cm, together withpulsewidths of 10–100 μs, trigger a reversible electroporationeffect (where the pores are able to reclose and the cell life is notcompromised) if the transmembrane charge is smaller than thethreshold value. Similarly, if the value is higher than the criticalvalue, the cell death may occur in an irreversible electroporationeffect [3].
Continuous technological research has been carried outthrough the last decades in developing flexible solid-statepulsed-power modulators [4], [5]. In fact, the ability to controlthe pulsewidth, electric field amplitude, number of pulses, fre-quency, and polarity of the applied field associated with solid-state-based modulators is of main importance regarding thesuccess of food and drug treatments, and today, this technologyhas proven to be surpassing numerous traditional methodsof fruit juice extraction [6]–[8], fruit juice quality [9]–[11],food nonthermal sterilization [12]–[15], and medical treatments[16]–[22].
Apples are a rich source of antioxidant compounds andfibers. Apple polyphenol extracts can lead to reduction in LDLand total cholesterol, and their fiber content is particularly im-portant in prevention of heart disease through healthy regulationof blood fat levels. The apple juice industry produces over1.5 million tons per year [23], making the production increaseand cost reduction an attractive field.
Application of PEF for the enhancement of apple juiceextraction has obtained good results with effective increasesin yield from applying electric field in 100–520 V/cm range,100-μs pulse duration, and 50 pulses with 100-Hz repetitionrate; the pulse shape was rectangular and monopolar [7].
Another experiment was carried out varying electric fieldsfrom 100 to 1000 V/cm, pulsewidth from 10 to 100 μs, and thenumber of pulses from 100 to 1000 with 100-Hz repetition rate;the pulse shape was also rectangular and monopolar [8].
However, very few studies have been done using bipolarpulses. Pasteurization of apple juice using PEF with bipolar
BRITO et al.: COMPARISON BETWEEN MONOPOLAR AND BIPOLAR PULSED ELECTRIC FIELDS 2349
Fig. 1. Microscope image of apple cells. Courtesy of Geoff Whiteway, teacherat the Marine Institute, Canada. Magnification: 100×.
Fig. 2. Electric field effects on the differently shaped and directed apple cells.Transmembrane induced voltage is higher on sharper points, and electropora-tion occurs if the threshold value of membrane surface charge is surpassed.Bipolar pulses seem to increase the odds to permeabilize some nonsphericalcells, increasing the amount of disrupted membranes. The arrows represent thepoints where electroporation may take place.
pulses was compared to the traditional method [24], [25], sugarbeet tissue damage degree was studied with PEF application(although together with thermal treatment) [26], and a com-parative study between the effects of monopolar and bipolarpulses on the contamination of metallic ions in a cell suspensionduring electroporation [27] and a study of electroporation ofmuscle fiber cells showed interesting results [28]. The hypoth-esis behind these results lies in the fact that living cells arenot perfectly spherical and the position of the cell membranewith respect to the direction of the applied electric field causesthe critical membrane surface charge to change [29]–[31], ifthe interior of a cell has a negative potential; concerning itsexterior, its transmembrane potential will be higher at the polefacing the positive electrode and vice versa. Figs. 1 and 2 show amicroscope image of apple cells and a schematic of the conceptabove described, respectively.
Apple juice PEF assisted enhanced extraction with separatebipolar rectangular pulses in comparison to monopolar rectan-gular pulses of equal amplitude and duration is described in thispaper, using a solid-state modulator capable of delivering eitherpulses. The aim of this work is to conjecture about the efficiencyof monopolar and bipolar pulses, comparing the yields ofapple juice extraction with both pulse types with referenceblank samples as well as the measurement of the absorbanceand total solid content. This is critical as bipolar generatorsare more complex and expensive than monopolar ones, sincein average, two switches are used, which in general doublesthe associated electronics. Pulsewidth, number of pulses, andapplied electric field are also studied with both pulse types,in order to evaluate the mechanisms of electroporation and toimprove the performance of PEF application.
II. EXPERIMENTAL PROCEDURES
Fig. 3 shows the simplified schematic of the experimentalprocedure used for the apple juice PEF assisted extraction.
Golden-type apples of caliber 60/65 mm were cut in cubes(1–1.5-cm side) at room temperature and immediately placedinside a cylindrical treatment chamber, with 9-cm diameter,1–5-cm height, and a maximum volume of 300 cm3. Thegood electric contact between apple cubes is obtained withthe superposition of the material until the desired electrodedistance is achieved. In order to suppress yield differences,due to different fruits and ages, every assay had a referenceblank sample collected and analyzed. Both blank and testedsamples were equivalent; a mixture of fruits was divided intotwo parts, one for the blank and the other for the tested samples.Considering the relative low voltages involved in the laboratoryexperiments, an H-bridge topology was used, which is capableof delivering monopolar and bipolar pulses into the load v0,where the Si switches hold off the voltage of the power supplyUdc, as shown in Fig. 4(a).
In general, in a direct capacitor discharge topology, thechanges of load impedance do not affect the voltage pulseshape, as long as the energy stored is much higher than theenergy delivered in each pulse.
During monopolar experiments, Ni positive voltage pulseswith ton width were applied to the load. On the other hand, forbipolar experiments, Ni pulses were also applied, each com-prising a positive and a negative pulse, where the pulsewidthton is divided in half for positive (tonP) and negative (tonN)pulses, such that ton = 0.5tonP + 0.5tonN. Fig. 4(b) showstypical voltage and current waveforms of a bipolar pulse appliedinto the load, where the relaxation time between positive andnegative pulses was fixed at 100 μs.
Each tested sample was submitted to the exact same variationof studied parameters. First, electric field E was changed from100 to 1300 V/cm, with constant pulsewidth ton = 100 μsand frequency f = 200 Hz, for Ni = 75 pulses (pulse train).Second, E was fixed at 600 V/cm, Ni was maintained at 75,frequency was kept at 200 Hz, and ton was studied with avariation from 20 to 300 μs. Finally, the number of pulsesper train was changed from 10 to 200, keeping constant thefollowing: f (200 Hz), E (600 V/cm), and ton (100 μs). For
2350 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 40, NO. 10, OCTOBER 2012
Fig. 3. Schematic of the process with application of monopolar and bipolar impulses in apple samples for juice extraction.
Fig. 4. (a) H-bridge generator topology used in experiments. (b) Typicalbipolar voltage (v0; 1 kV/div) and current (i0; 5 A/div) waveforms on the load,with 50 μs(div).
all assay, two pulse trains were applied with 1-s separation.The two trains simulate the practical industrial line application,where the product being treated passes through a cylindricalchamber, with the center electrode at HV isolated from the twogrounded side electrodes.
Application of the desired electric fields was calculatedaccording to the formula E = V/d, where d is the distancebetween electrodes and V is the selected tension in the pulsegenerator. Adjustment of the distance between electrodes wasused to achieve the electric field for each assay, setting theapplied voltage to the appropriate values. Table I shows theelectrode distances and applied voltages.
Energy per unit mass (specific energy) delivered per pulsetrain Ec (in joules per kilogram) can be calculated according to
Ec =Niv0i0ton
M(1)
TABLE IELECTRODE DISTANCES AND APPLIED VOLTAGES FOR EACH
TESTED ELECTRIC FIELD
where M (in kilograms) is the initial apple mass, Ni is the pulsenumber, v0 is the applied voltage, and i0 is the current across.
The PEF treatment was applied, and the apple cubes werecollected. The same treatment chamber was used for the com-pression stage, and a Sigma Aldrich filter cloth with a perme-ability factor of 0.012 m3/s was placed inside it before thecompressing of the apple treated cubes for the juice collection.
A 500-kPa pressure was applied for 5 min, and the juice wasinstantly collected and weighted. Experiments have been doneonce for each assay.
All absorbance measurements were made in a UNICAMUV2 UV/Vis spectrophotometer in plastic cuvettes at roomtemperature and at 390-nm wavelength. Dissolved total solidcontent (◦Brix) was measured in an ATAGO 3T refractometer;samples were kept at a constant temperature of 23 ◦C with awater bath incorporated in the refractometer. All samples werestored in the fridge at 5 ◦C for no more than 24 h before theirchemical analysis. Standard deviations in the data representthree assays for each experiment.
III. RESULTS AND DISCUSSION
A. Effect of Applied Electric Field on Additional Juice Yield
Comparison between the effects of applying different electricfields with pulse shape is shown in Fig. 5. Values in percentagecorrespond to the difference between the treated apple cubes
BRITO et al.: COMPARISON BETWEEN MONOPOLAR AND BIPOLAR PULSED ELECTRIC FIELDS 2351
Fig. 5. Additional juice yield versus applied electric field for (a) monopolarand (b) bipolar shaped pulses. f = 200 Hz, ton = 100 μs, and Ni = 75.
Fig. 6. Additional juice yield versus pulsewidth for (a) monopolar and(b) bipolar shaped impulses. f = 200 Hz, E = 600 V/cm, and Ni = 75.
and reference samples (without any PEF treatment), i.e., plotsare based on additional yield percentage of the untreated applesand not on the total yield of treated samples.
For an applied electric field of 200 V/cm onward, the positiveeffect of the application of PEF in apple samples for bothpulse shapes is clear. In the two pulse shapes studied, higherelectric fields applied result in more additional extracted juice,being in agreement with other studies with apples [8], [11]and electroporation general theory. From 600 V/cm onward,saturation is reached where increasing the applied electric fieldstrength does not result in higher yields for both pulse types.For this reason, an electric field of 600 V/cm was chosen forthe subsequent assays of variation of pulsewidth and number ofpulses. Bipolar pulses seem to be more effective than monopo-lar ones with a clear tendency for higher extraction of juiceunder the same conditions: 20% increase with bipolar pulses,in average, against 10% increase with monopolar pulses.
B. Effect of Pulsewidth on Additional Juice Yield
Pulsewidth is one important factor that can determine theirreversibility of the pore formation in cells, and generally,when PEF is applied for pasteurization purposes, the pulsewidthis proportional to the number of bacterial and microbial cells’death [33]. Similar tendency is expected for juice extraction.Pulsewidth influence on additional juice yield is shown inFig. 6, for the two pulse shapes considered.
Fig. 7. Additional juice yield versus number of pulses for (a) monopolar and(b) bipolar shaped impulses. f = 200 Hz, E = 600 V/cm, and ton = 100 μs.
For each pulse shape and pulsewidth, the increase in juiceyield regarding untreated samples is clear. Increase in additionalyields is seen up to 300 μs for monopolar and bipolar pulses.The point at 50 μs for bipolar pulses reveals higher additionalyield but not significantly, and the general tendency for bothpulse shapes is higher additional yields for higher pulsewidthsin the juice extraction at all studied pulsewidths.
C. Effect of the Number of Pulses on Additional Juice Yield
Simultaneously with the pulsewidth, the number of appliedpulses in the PEF treatment is directly related to the energyconsumption of the system. Fig. 7 shows the comparison ofmonopolar- and bipolar-pulse additional yields when the num-ber of pulses is changed.
There is a tendency of achieving better performances forincreasing the number of pulses, as can be seen in Fig. 7.However, above the presented results, due to the stored energylimitation in the chosen Cdc capacitor, which imposed a highvoltage droop in the voltage pulse (i.e., the electric field de-creased during the pulse), it was not possible to conclude ifthere is a tendency to saturation when increasing the numberof pulses. Nevertheless, additional yield in the juice extractionis higher when bipolar shaped pulses are applied, being thisa common tendency with all tested PEF parameters in thiswork. Further research is needed for adequate conclusions onthe effect of the number of pulses on electroporation.
D. Energy Consumptions of Monopolar and Bipolar Impulses
Duration of PEF treatment is one of the main parameters thatdetermine the amount of consumed energy in the process. Forjuice extraction application in industry, it is worth to deepenthe effect of pulsewidth and number of pulses and find opti-mal points between energy consumption and additional yields.For PEF successful industrial applications, it is essential tounderstand and find the balance between energy consumptionand additional product achieved. As concluded in the previoussection, bipolar shaped pulses are able to deliver higher ad-ditional apple juice yields. Nonetheless, pulse generators ableto produce bipolar pulses are more expensive to manufacture,and food industry investments for higher productions musttake into account all these combined aspects. Figs. 8–10 showthe specific energy consumed during application of PEF for
2352 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 40, NO. 10, OCTOBER 2012
Fig. 8. Specific energy for (a) monopolar and (b) bipolar pulses versusadditional yield with electric field variation. Values of electric fields applied,in volts per centimeter, are specified next to each point. f = 200 Hz, ton =100 μs, and Ni = 75.
Fig. 9. Specific energy for (a) monopolar and (b) bipolar pulses versusadditional yield with pulsewidth variation (ton). Values of electric pulsewidths,in microseconds, are specified next to each point. f = 200 Hz, E = 600 V/cm,and Ni = 75.
Fig. 10. Specific energy for (a) monopolar and (b) bipolar pulses versusadditional yield with the number of pulses (Ni) variation. Values of the numberof pulses are specified next to each point. f = 200 Hz, E = 600 V/cm, andton = 100 μs.
both shape type pulses toward the variation of electric field,pulsewidth, and number of pulses and additional juice yield.
The observations in Figs. 8–10 show that, only for variationsof pulsewidths, bipolar impulses have higher specific energyconsumptions, although delivering significant higher additionalyields. For the other studied parameters, bipolar pulses havesimilar energy inputs for lower electric fields and pulse numbersuntil 400 V/cm and 200 pulses, respectively. Higher electricfields for bipolar pulses clearly demand less energy than themonopolar equivalents.
Fig. 11. (Left) Total soluble dry matter and (right) absorbance for monopolarand bipolar pulses and control samples versus applied electric field (E), with75 pulses of 100-μs width and 200-Hz frequency. The error bars represent thestandard deviations in the data.
Fig. 12. (Left) Total soluble dry matter and (right) absorbance for monopolarand bipolar pulses and control samples versus pulsewidth (ton), at 600 V/cm,with 75 pulses and 200-Hz frequency. The error bars represent the standarddeviations in the data.
Nonetheless, when electric field is increasing, there is clearlythe presence of a threshold, in both unipolar and bipolar shapes,where increased specific energy inputs do not result in higheradditional extraction yields.
Energy consumptions are as low as 1.8 kJ/kg, for an ad-ditional yield of 18%, when bipolar pulses are applied at a600-V/cm electric field, with a pulsewidth of 100 μs and125 impulses. These results confirm the strong applicabilityof PEF in the food industry with great increase in the processefficiency.
E. Juice Quality Evaluation
The composition of the extracted apple juice, coloration, andconcentration of soluble dry matters affect important visualand qualitative perceptions of the final consumers. Hence, thepossible PEF impact is shown in Figs. 11–13.
Evaluation of both qualitative parameters suggests that theshape of applied pulses in PEF treatment does not have asignificant influence on the release of soluble dry matter andcoloration of extracted juice. Bipolar pulses may influencethe number of electroporated cells, but in fact, pore size andselective permeability of the membranes after being submittedto reversible or irreversible electroporation do not change withthe amount of electroporated cells. Hence, quality parametersrequired by European Union directives [34] are kept, and it is
BRITO et al.: COMPARISON BETWEEN MONOPOLAR AND BIPOLAR PULSED ELECTRIC FIELDS 2353
Fig. 13. (Left) Total soluble dry matter and (right) absorbance for monopolarand bipolar pulses and control samples versus number of pulses (Ni), at600-V/cm electric field, 100-μs pulsewidth, and 200-Hz frequency. The errorbars represent the standard deviations in the data.
possible to establish that, for all the studied parameters in thiswork, the application of PEF in the extraction of apple juice isin conditions to be widely applied in the food industry.
IV. CONCLUSION
Differences on apple juice extraction additional yield andqualitative parameters have been analyzed. Application of PEFon apple precut cubes has been investigated with monopolarand bipolar shaped impulses with variation of pulsewidth,number of pulses, and electric field.
Bipolar pulses revealed higher additional yields for all stud-ied parameters. Increasing the electric fields, pulsewidths, andthe number of impulses leads to higher additional yields, butresults suggest that there is a tendency for reaching a saturationpoint when electric fields and pulsewidths are increased. Pulsenumber influence needs to be further studied since results werenot consistent regarding tendency to reach a threshold.
Disregarding the pulse shape, electric field variation showeda threshold where increased energy inputs do not imply in-creased additional yields; considerably low energetic consump-tions (1.8 kJ/kg) are able to produce over 18% more juicethan their corresponding untreated samples. Differences inadditional yields between monopolar and bipolar pulses reachvalues as high as 8% more extracted juice for bipolar pulsesthan corresponding monopolar pulses at 800 V/cm, 100-μspulsewidth, and 75 pulses.
Evaluation of qualitative parameters and comparison of re-sults between monopolar and bipolar shaped pulses do notreturn significant differences between total soluble matter andabsorbance values of treated apples, and all values achievedare in conformity with the European Union directives for applejuice for human consumption.
Bipolar pulses demonstrated better performances in juiceextraction yields, but pulse generators able to deliver bipolarshaped impulses are more complex and, consequently, theireconomic viability for large-scale juice production is dependenton the additional production in respect to more simple pulsegenerators. Other considerations such as the influence of thepulse shape in the electrode and its reaction with the materialmust be taken into consideration in order to have the fulleconomic assessment of the use of either shape pulses.
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P. S. Brito was born in Lisbon, Portugal, in 1971.She received the B.Sc. degree in chemical engineer-ing from the Instituto Superior de Engenharia deLisboa (ISEL), Lisbon, in 2005 and the Ph.D. degreein electrochemistry and biosensing from CranfieldUniversity, Cranfield, U.K., in 2010.
She is currently a Postdoctorate Researcher withthe Electrotechnical Engineering Department, ISEL.Her current interests include the study of pulsed-power system effects in living animal and vegetablecells.
Dr. Brito has been an Associate Member of the Royal Society of Chemistrysince 2006.
H. Canacsinh was born in Inhambane,Mozambique, in 1974. He received the B.Sc. degreein electrical engineering from the Instituto Superiorde Engenharia de Lisboa (ISEL), Lisbon, Portugal,in 1999 and the M.Sc. degree in electrical andcomputer engineering from Instituto SuperiorTécnico, Universidade Técnica de Lisboa, Lisbon,in 2008, where he is currently working toward thePh.D. degree in electrical engineering and computerscience.
Since 2000, he has been an Assistant Professorwith ISEL. He is also a Collaborator with the Nuclear Physics Center, LisbonUniversity (CFNUL), Lisbon. His topics of research include power electronics,pulsed-power systems, and solid-state Marx generator.
Mr. Canacsinh is a Collaborator of the Portuguese Engineering Society.
J. P. Mendes was born in Souto, Portugal, in 1975.He received the B.Sc. degree in electrical engineer-ing from the Instituto Superior de Engenharia deLisboa (ISEL), Lisbon, Portugal, in 2003. He is cur-rently working toward the Ph.D. degree in electricalengineering and computer science in the Faculdadede Ciências e Tecnologia, New University of Lisbon(UNL), Lisbon.
Since 1999, he has been a Superior TechniqueResearcher with ISEL. He is also a Collaboratorwith the Nuclear Physics Center, Lisbon University
(CFNUL), Lisbon. His topics of research include power electronics, pulsed-power systems, solid-state Marx generator, and Blumlein lines.
Mr. Mendes is a Collaborator of the Portuguese Engineering Society.
L. M. Redondo (M’06) was born in Lisbon,Portugal, in 1968. He received the B.Sc. andDipl.Ing. degrees in electrical engineering from theInstituto Superior de Engenharia de Lisboa (ISEL),Lisbon, in 1990 and 1992, respectively, the M.Sc.degree in nuclear physics from the Faculdade deCiências, Universidade de Lisboa, Lisbon, in 1996,and the Ph.D. degree in electrical and computer en-gineering (pulsed-power electronics) from InstitutoSuperior Técnico, Universidade Técnica de Lisboa,Lisbon, in 2004.
He is currently a Coordinator Professor with ISEL, teaching power elec-tronics and digital systems. He is a Member of the Nuclear Physics Center,Lisbon University (CFNUL). His current research interests include pulsed-power systems for industrial applications, nuclear instrumentation, and ionimplantation.
Dr. Redondo is a member of the Portuguese Engineering Society and thePulsed Power Science and Technology Standing Technical Committee of theNuclear and Plasma Science Society of IEEE.
M. T. Pereira was born in Sintra, Portugal, in 1965.He is currently with Lusoforma, Mem Martins, Portugal.
