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Journal of Food Engineering 80 (2007) 639–644

Plasmolysis of sugarbeet: Pulsed electric fields and thermal treatment

N.I. Lebovka a,b, M.V. Shynkaryk a,b, K. El-Belghiti a, H. Benjelloun a, E. Vorobiev a,*

a Departement de Genie Chimique, Universite de Technologie de Compiegne, Centre de Recherche de Royallieu, B.P. 20529-60205 Compiegne Cedex, Franceb Institute of Biocolloidal Chemistry named after F.D. Ovcharenko, NAS of Ukraine, 42, blvr. Vernadskogo, Kyiv 03142, Ukraine

Received 3 May 2006; received in revised form 12 June 2006; accepted 13 June 2006Available online 18 September 2006

Abstract

The influence of pulsed electric fields (PEF) and heating on the cell damage (plasmolysis) of sugarbeet tissue was studied. The PEFtreatment was applied using bipolar pulses of near-rectangular shape in near-isothermal regime. The thermal treatment temperatureinterval was within T = 20–70 �C. The tissue damage degree was characterized by the electrical conductivity disintegration index Z.The Arrhenius form of the characteristic damage time – temperature dependences was observed both for the electrical and thermal pro-cesses. The corresponding activation energies W � 149–166 kJ mol�1 were responsible for the damage of cell membranes. The PEF treat-ment noticeably accelerates the diffusion process, which requires �1 h with mild heating at 40 �C. The results shown open new prospectsfor low heating processing in sugar industry.� 2006 Elsevier Ltd. All rights reserved.

Keywords: Plasmolysis; Pulsed electric field; Thermal treatment; Electrical conductivity; Diffusion coefficient; Sugarbeet

1. Introduction

The thermally induced damage of sugarbeet cells (some-times called as plasmolysis) is widely used for diffusionenhancement in the sugar-making industry. Industrial dif-fusion requires high temperatures (�70–75 �C) and longtimes (�1 h) (Van der Poel, Schiweck, & Schwartz, 1998).The thermal plasmolysis can be accompanied by degrada-tion of the product properties, cell tissue alteration andpectin release into the juice, thus creating a need in costlyand complicated juice purification.

There were previously some efforts to intensify diffusionprocesses by the electric field treatment. The early worksshow that application of the low-gradient alternating elec-tric fields (<100 V/cm) can enhance the juice extraction(Zagorulko, 1958) and increase the soluble matter diffusioncoefficient in the sugarbeet (Jemai, 1997; Katrokha &Kupchik, 1984). More recently the efficiency of the indus-trial scale pulsed electric field (PEF) generators for sugar-beet processing was demonstrated (Frenzel, Michelberger,

0260-8774/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.jfoodeng.2006.06.020

* Corresponding author.E-mail address: eugene.vorobiev@utc.fr (E. Vorobiev).

& Witte, 2004; Sack, Schultheiss, & Bluhm, 2005; Schulthe-iss et al., 2002).

The PEF treatment at high electric field strength ofE = 500–1000 V/cm and treatment time within 10�4–10�2 s allows to damage tissue effectively without any sig-nificant temperature increase (Lebovka, Shynkaryk, &Vorobiev, 2006; Vorobiev, Jemai, Bouzrara, Lebovka, &Bazhal, 2005) due to electroporation mechanism (Fincan& Dejmek, 2002; Lebovka, Bazhal, & Vorobiev, 2002).Higher electric fields (104–105 V/cm) of microsecond dura-tion are promising for the microbial inactivation purposes(Barbosa-Canovas & Vega-Mercado, 1996). Efficiency ofthe electroporation damage can be appreciably enhancedby higher temperature treatment (Lebovka, Praporscic,Ghnimi, & Vorobiev, 2005a). The moderate electric fields(MEF) treatment at field strength below 100 V/cm can alsoinduce the tissue damage through electroporation mecha-nism (Wang & Sastry, 2002), but here a noticeable damagerequires longer treatment time (tens of seconds) and can beaccompanied by significant increase of temperature andenergy consumption (Lebovka, Praporscic, Ghnimi, &Vorobiev, 2005b Praporscic, Lebovka, Ghnimi, & Voro-biev, 2006). The MEF treatment applications in order to

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enhance extraction and expressing processes in differentfood materials were discussed previously (Wang & Sastry,2002; Zhong & Lima, 2003).

The previous studies showed that PEF-treatment canintensify sugar extraction from the sugarbeets (El Belghiti& Vorobiev, 2004; El-Belghiti, Rabhi, & Vorobiev, 2005;Jemai & Vorobiev, 2003) and is advantageous for industrialapplications (Eshtiaghi & Knorr, 2002; Wawro, Iciek, &Gruska, 2004). However, the electric field treatment isnot yet adopted for industrial sugar technology. The PEFapplication in the sugar industry requires thorough scien-tific, engineering and economical justification and optimi-sation of the regimes of thermal and electrical treatment.The electric field application to the sugarbeet can resultin increase of temperature (through ohmic heating mecha-nism) and electrically stimulated damage (through electro-poration mechanism) (Weaver & Chizmadzhev, 1996).Both temperature elevation and tissue damage result inincrease of the electrical conductivity, which is used as ameasure of tissue damage degree. So, in practice, it is verydifficult to separate two mechanisms influencing electricalconductivity of a tissue and to make conclusions aboutthe real contribution of the electric field treatment intothe damage efficiency (Lebovka et al., 2006).

The aim of this investigation was to compare the effectsof thermal and PEF treatments in relation to the sugarbeetplasmolysis. In this study we compared the degrees of dam-age induced by the thermal and PEF treatments, and esti-mated the treatment time and sugar diffusivity insidesugarbeet slices, as well as juice purity of the untreatedand PEF pretreated tissues.

Fig. 1. Scheme of the e

2. Materials and methods

2.1. Materials

Field-grown roots of the commercial sugarbeet (Beta

vulgaris L.), taken from a field near Compiegne, France(November–December 2005), were used in all the experi-ments. Different tissue replicates were sampled from thecentral part of the roots.

2.2. PEF-treatment experiments

Fig. 1 shows a schematic diagram of the experimentalsetup. Electric field treatment was applied using the PEFgenerator. The PEF generator, 400 V–38 A (Service Elect-ronique UTC, Compiegne, France) provided the bipolarpulses of near-rectangular shape. Bipolar mode of thePEF treatment allows to avoid asymmetry of electropora-tion at the poles of the cells. The trains of pulses were usedfor PEF treatment. An individual train consists of n pulseswith pulse duration ti and pulse repetition time Dt (Fig. 1).There was a pause of Dtt after each train. In this work, theinter-train pause was always put Dtt = 30 s. Owing to thelong inter-train pause, the ohmic temperature elevationDT during one train application was rather small(DT � 1 �C). Therefore, the system relaxed to the initialtemperature during the inter-train period and the PEFtreatment was done practically under isothermal condi-tions. The total time of electrical treatment during thePEF experiments was calculated as tt = nNti, where N isthe number of trains.

xperimental setup.

N.I. Lebovka et al. / Journal of Food Engineering 80 (2007) 639–644 641

The temperature inside the samples under the PEF treat-ment was recorded in online mode in the inter-train periodby a teflon-coated K-type thermocouple (±0.1 K), con-nected to the data logger thermometer Center 305/306(JDC Electronic SA, Yverdon-les-Bains, Switzerland).The electrodes were connected to the PEF generator andthe electrical conductivity of samples was measured in theinter-train period at frequency 0.5 kHz, selected as optimalfor purposes of removing the polarizing effects on the elec-trodes and tissue sample.

All the output data (current, voltage, electrical conduc-tivity, and temperature) were collected using a data loggerand a special software, adapted by Service ElectroniqueUTC, Compiegne, France.

The PEF treatment cell consisted of a polypropylenecylindrical glass with 30 mm inner diameter and an elec-trode (Fig. 2). The treatment cell, filled with fresh sugarbeetjuice, was placed inside the water thermostat. The PEFtreatment was applied to the sample through the mesh elec-trodes at different electric field strengths E and differentfixed temperatures T.

Fresh juice was chosen as a natural medium in order toreduce the sample degradation and for improvement of theelectric contacts between the electrodes and the sample.The cylinder-shaped samples were 30 mm in diameter (d)and 10 mm in height (h).

The tissue damage degree was estimated from the electri-cal conductivity disintegration index Z (see e.g., Lebovkaet al., 2002):

Z ¼ ðr� ruÞ=ðrd � ruÞ ð1Þwhere r is the measured electrical conductivity value andthe subscripts u and d refer to the conductivities of un-treated (intact) and completely damaged tissue,respectively.

The maximally damaged sugarbeet tissues (Z � 1) wereobtained by their PEF treatment at the electric fieldstrength E = 400 V/cm and the total time of the PEF treat-ment tt = 0.1 s. After such treatment, the electrical conduc-

Fig. 2. The PEF treatments and diffusion cells. See text for details.

tivity of a tissue attained its maximal value rd and wasapproximately the same as for the freeze-thawed tissue.Application of the above equation gives Z = 0 for an intacttissue and Z = 1 for a maximally disintegrated material.

For study of damage degree Z versus treatment time tt

the PEF protocol parameters were as follows: pulse dura-tion ti = 9.9 ms, pulse repetition time Dt = 10 ms, numberof pulses n = 4, and number of trains N = 500.

2.3. Diffusion experiments

The cylindrical diffusion cell with distilled water waspreheated during 30 min at the desired temperature(Fig. 2). The slices sized 1.5 mm · 10 mm · 10 mm wereprepared from the sugarbeet tissue (untreated or PEFpre-treated) and placed into the diffusion cell. The liquid-solid ratio was fixed at the level 3. The PEF-pretreatedsamples with high disintegration index (Z � 1) were used(E = 400 V/cm, tt = 0.1 s).

The concentration of soluble matter content (�Brix) wascontrolled by a digital Atago refractometer PR-101(Sciencelab.com, Houston, USA) every 2 min at room tem-perature. A careful agitation was provided using the mag-netic mixer 94360 (Heidolph, Kehlheim, Germany). Inorder to avoid any evaporation, the diffusion cell wasclosed during the extraction process.

Sucrose concentration was analysed by the high-perfor-mance liquid chromatography (HPLC) using the refrac-tometer ionising detector Refractometer-10 A (ShimadzuFrance, Champs sur Marne, France) and a calcium ligandexchange column, PL Hi-plex Ca column 300 · 7.7 mm(Polymer Laboratories, Marseille, France), with water asan eluent. Analyses were made at the elution rate of0.6 mL/min and the oven temperature of 80 �C. Each sam-ple was centrifuged for 10 min at 8000 · g. The 20 ll of the0.2 lm filtered supernatant were then injected. Retentiontimes of sucrose, glucose and fructose were 11.67, 13.86,and 16.95 min, respectively, and the peak areas were deter-mined using the CR-5 A integrator (Shimadzu France,Champs sur Marne, France). The purity P was calculatedas the percentage ratio of sucrose to the total soluble solids(or �Brix) in a diffusion solution.

2.4. Statistical analysis

All the experiments were repeated, at least, three timesand means of the data and their standard deviations werecalculated.

3. Results and discussion

Fig. 3a presents the damage degree Z versus PEF-treat-ment time tt. The damage degree was estimated from Eq.(1).

The damage degree Z increased during the PEF treat-ment, and effect was more pronounced at higher tempera-tures. At elevated temperatures (T > 50 �C), the thermally

Fig. 3. Damage degree Z versus time of treatment tt. Treatments weredone in the near-isothermal conditions at different temperatures T andfixed electric field strength E = 100 V/cm (a) as well as at different electricfield strength E and fixed temperature T = 60 �C (b). Error bars are thestandard deviation.

Fig. 4. The Arrhenius plots of electrical (sE) and thermal (sT) character-istic damage times. Error bars (of the order of symbol size) are thestandard deviation.

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induced damage was also important and the damage degreeZ increased with thermal treatment time tt even in caseswhen PEF was not applied (Fig. 3b). The electrical (sE)and thermal (sT) characteristic damage times, shown byarrows in Fig. 3b, were estimated as the treatment timerequired for Z to attain one-half of its maximal value,i.e., Z = 0.5 (Lebovka et al., 2002).

Dependencies of sE and sT versus the inverse tempera-ture 1/T are presented in Fig. 4 in the Arrhenius form:

sE;T ¼ s1E;T expðW E;T;s=RT Þ ð2Þ

where W is the activation energy, s1 is the limiting time,R = 8.314 J K�1 mol�1 is the universal gas constant, T isthe absolute temperature, WE,s = 149 ± 8 kJ mol�1,

s1E � 1:4� 10�25 s and WT,s � 166 ± 15 kJ mol�1, s1T �3:7� 10�23 s. Note that activation energies WE,s, WT,s areclose in values and correspond to the process of membranedamage in the sugarbeet tissue.

Fig. 5 shows data of the extraction kinetics at differenttemperatures (T = 30–80 �C) presented as B versus time t,where B is the normalized �Brix of the sugarbeet juicedefined as

B ¼�Brix�� Brixi

�Brixf �� Brixi

ð3Þ

Here, �Brixi and �Brixf are the initial and the final solu-ble matter content, respectively. Value of �Brixi was deter-mined immediately after placing slices into diffusion cell,and �Brixf corresponds the final value of saturation afterthe infinite time of extraction.

Both temperature increase and the PEF pretreatmentaccelerated the extraction kinetics (Fig. 5a and b). For pur-poses of simplicity, it was assumed that the slices were thinslabs of a uniform thickness (Fig. 2), and solution of theFick’s second law (Crank, 1975) was used for estimationof the effective sugar diffusion coefficient Deff in asugarbeet:

B ¼ 1� 8

p2

X1n¼0

1

ð2nþ 1Þ2exp �ð2nþ 1Þ2p2Deff t

4h2

!ð4Þ

where h � 1.5 mm is the thickness of a slab.This summation series converges rapidly for large values

of time t, so, only the first five leading terms were takeninto account for Deff estimation by the least square fittingof the experimental data to Eq. (4).

The Arrhenius plots of the effective sugar diffusion coef-ficient Deff in the untreated and PEF-pretreated sugarbeet

Fig. 5. The normalized �Brix of the sugarbeet juice B versus extractiontime t for the untreated (a) and PEF-pretreated (b) slices at differenttemperatures. The PEF pretreatment was done at the electric field strengthE = 400 V/cm and corresponds to the higher damage disintegrationdegree, Z = 0.93 ± 0.05. Error bars are the standard deviation.

Fig. 6. The Arrenius plots of the effective diffusion coefficient Deff for theuntreated and PEF-pretreated sugerbeet slices. Error bars are the standarddeviation.

Fig. 7. Purity of the diffusion solutions at different temperatures for theuntreated and PEF-pretreated slices.

N.I. Lebovka et al. / Journal of Food Engineering 80 (2007) 639–644 643

slices are presented in Fig. 6. At 70 �C, the values of Deff

were nearly the same for both untreated and PEF-pre-treated slices (1–1.5 · 10�9 m2/s) and were close to valuespreviously reported for the same temperature (Schwartz-berg & Chao, 1982). The activation energy for the PEF-treated slices was WE,D = 21 ± 2 kJ mol�1, and it was closeto the activation energy of sugar in the aqueous solutions,WD � 22 kJ mol�1 (Lysjanskii, 1973). The activationenergy for the untreated slices was noticeably higherWT,D = 75 ± 5 kJ mol�1, reflecting, possibly, interrelationsof the restricted diffusion and thermally induced damageeffects in the untreated sugarbeet tissue.

Fig. 7 presents the purity P of the diffusion solutions forthe untreated and PEF-pretreated slices at different temper-atures. The final �Brix values of juices (cooled to the roomtemperature) were approximately the same, 5.8 ± 0.5. ThePEF treatment seems to have no noticeable effect on purityat T = 70 �C, although it resulted in purity increase whensamples were treated at the room temperature T = 20 �C.

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4. Conclusions

The results of this work clearly evidence benefits of thePEF application for purposes of diffusion improvementin the sugarbeet processing. The PEF treatment at moder-ate electric field strength (E � 100 V/cm) and time (tt � 1 s)allows to disintegrate efficiently the biological cells andnoticeably decrease the diffusion temperature (up to 40–50 �C) without any loss of solution purity. The power con-sumption and the product quality arguments require moreaccurate temperature and PEF-treatment protocol optimi-sation in future investigations. The PEF-enhanced plas-molysis apparently, opens new horizons for the powerconsumption sugarbeet processing technologies.

Acknowledgements

The authors would like to thank the ‘‘Pole RegionalGenie des Procedes’’ (Picardie, France) for providing finan-cial support. Authors also thank Dr. N.S. Pivovarova forher help with preparation of the manuscript.

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