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Food Engineering and Physical Properties

Textural Changes in Apple Tissue During Pulsed Electric Field Treatment M.I. B AZHAL , M.O. N GADI , G.S.V. R AGHAVAN , AND D.H. N GUYEN

Introduction The major interest in pulsed electric field (PEF) treatment of cellular materials is derived from its nonthermal applications in inducing increased cell permeability (Knorr and Angersbach 1998; Barbosa-Canovas and others 2000). Dielectric breakdown (Zimmermann and others 1976) or electroplasmolysis (McLellan and others 1991; Gudmundsson and Mafsteinsson 2001; Fincan and Dejmek 2002) of biological cells during PEF treatment is generally due to electroporation. This is the formation and growth of pores in biological membranes resulting from their polarization under external electric field (Winterhalter and Helfrich 1987; Teissie 1999). In food technologies, PEF techniques have been used for nonthermal pasteurization of liquid products (Barbosa-Canovas and others 2000; Barsotti and others 2001) and for intensification of mass-transfer processes in biological tissues (Gulyi and others 1994; AdeOmowaye and others 2001). For solid foods, PEF treatment is usually a supplemental method for enhancing certain processes such as pressing (Bazhal and Vorobiev 2000), decanter centrifugation (Beveridge 1997; Knorr and others 2001), diffusion (Jemai 1997), osmotic dehydration (Rastogi and others 1999), and drying (Ade-Omowaye and others 2000). Efficiency of these processes may be deduced by monitoring textural quality changes of the treated materials. Therefore, PEF-induced textural changes in solid tissues are important attributes for optimizing PEF treatment either as independent treatment or in combination with other food treatment methods © 2003 Institute of Food Technologists

(Ade-Omowaye and others 2001; Bazhal and others 2001). Recently, several investigations have been conducted to extend application of PEF in food processing (Knorr and others 2001; Ngadi and others 2001). The nature of electroplasmolysis and localization of pores during PEF are not yet well understood. The generally accepted mechanism of electropermeabilization of suspended cells such as microorganisms and erythrocytes is formation of pores in the lipid matrix of the cell membranes (Weaver and Chizmadzhev 1996). For solid cellular materials, such as plant and animal tissue, cell plasmolysis after PEF treatment may be explained by electroporation of the cellular membranes (Ade-Omowaye and others 2001) and reduction in the mechanical integrity of the cell wall (Rojas and others 2001). Mechanisms of the electrical damage of cell walls during PEF treatment have not been well studied and remain largely unknown. After plasmolysis by different pretreatment methods, tissue tends to be more viscous and firmness is reduced (Krokida and others 2001; Rojas and others 2001). Matvienko (1996) reported a decrease in the elastic modulus of sugar beet from 12.5 to 6.5 MPa after electropermeabilization, depending on treatment conditions. Taiwo and others (2001) reported similar results, that maximum compressive force significantly decreased for samples treated by PEF. However, the relationship between degree of tissue destruction and the viscoelastic behavior of electrically treated material has not yet been studied. The objectives of this study were to in-

vestigate the influence of electroplasmolysis on textural properties of plant tissue and determine the relationship between failure stress and degree of electroplasmolysis in plant tissue.

Materials and Methods Sample preparation The Cortland apple variety, purchased from a local grocery, was used in the study. Samples were cut from the middle portion of the apples into disks of 57 ± 0.5-mm dia and of 15 ± 0.5-mm thickness. Initial moisture contents of the samples were within 81% to 84% (wet basis).

Experimental setup and PEF treatment The experimental setup for PEF treatment and compression test is shown in Figure 1. Compression tests were achieved by using an Instron Testing Machine (model 1011, Instron Corp., Canton, Mass., U.S.A.) with a 20-kN transducer. Each apple sample was placed in a cylindrical press cell and compressed at the crosshead speed of 10 mm/min. The laboratory press cell consisted of an insulator cylinder, steel piston, and steel disk. The piston and disk were perforated to allow juice drainage and they also served as 2 parallel electrodes for the PEF treatment. A single slab of apple disk was placed between the electrodes during the compression test. Electrical treatment of the sample was carried out without filter cloth. However, the filter cloth was used during the compression test. PEF treatments were

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ABSTRACT: The effects of pulsed electric field (PEF)–induced electroplasmolysis on mechanical and structural properties of apple tissue were investigated. Properties such as porosity, pore distribution, particle density, bulk density, electrical conductivity, and compression modulus were evaluated after different degrees of electroplasmolysis. PEF treatment decreased bulk density, decreased volume shrinkage, and increased porosity of air-dried apple tissue. The overall average mean size of the PEF-induced pores was 5.86 µm, lower than 7.81 µm obtained for the untreated samples. By determining electrical conductivity, disintegration index, and failure stress of apple samples, a linear dependency was observed between failure stress and degree of electroplasmolysis. Failure stress decreased with intensification of electrical treatment. Keywords: pulsed electric field, electroplasmolysis, pores, apple, structural properties

Textural changes during PEF treatment. . .


with electric field intensity of 1000 V/cm, pulse widths of 300 µs, and pulse frequency of 1 Hz. The number of pulses varied from 1 to 60. The PEF treatment protocols were selected to avoid increase in sample temperature no more than 3 °C. This was to prevent the “jamming” of kinetic curves of the tissue breakage that may result due to successive phases of acceleration of tissue electroplasmolysis at lower electric field strength or higher temperature changes at higher number of pulses (Lebovka and others 2000). The temperature was checked during PEF treatment using a PC-based Hotmux thermocouple data acquisition system (Hotmux, DCC Corp., Pennsauken, N.J., U.S.A.). A high-voltage pulse generator (Velonex model 350-12, Pulse Engineering, Inc., (Santa Clara, Calif., U.S.A.) was used to provide rectangular-shaped monopolar pulses. Pulse voltage and current were measured using a high-voltage probe (P6015A, Tektronix Inc., Beaverton, Oreg., U.S.A.) and a pulse current transformer (model 411, Pearson Electronics, Inc., Palo Alto, Calif., U.S.A.), respectively. The voltage and current data were recorded on a digital 2-channel oscilloscope (Tektronix TDS 3012, Tektronix Inc.) Pulse width and frequency were controlled via an external TTL with a frequency trigger. Cut apple samples were treated in about 180 to 240 s after cutting to avoid the transition time of electrical conductivity increasing due to juice migration inside the tissue from freshly cut cells at the outer boundary of the sample (Lebovka and others 2001). Conductivity of apple sample was measured using an alternating current of 800 mA and frequency of 60 Hz, which were selected as optimal for minimizing polarization effects (Bazhal 2001). Voltage and current through the apple sample (for determination of electrical conductivity)

Table 1—Bulk (rb) and skeletal density (rs), porosity (e), average pore diameter (ádñ), and volume ratio (sv) for apple samples dried at 45 °C. PEF parameters were E = 1000 V/cm; ti = 300 ms; n = 60; f = 1 Hz Samples

rb (kg/m3)

rs (kg/m3)

e

ádñ (mm)

sv

Control PEF

0.46 ± 0.05 0.38 ± 0.03

1.39 ± 0.06 1.52 ± 0.05

0.67 ± 0.02 0.75 ± 0.03

7.81 ± 0.29 5.86 ± 0.16

0.20 ± 0.02 0.26 ± 0.01

were measured using a multimeter (Fluke model 187, Fluke Corporation, Everett, Wash., U.S.A.). Electrical conductivity of the PEF-treated samples was measured within 10 s after the PEF was turned off. Conductivity disintegration index, z, was calculated thus (Lebovka and others 2002):

(1) where s is effective electrical conductivity at different PEF parameters (S/m), si is the conductivity of intact tissue, and sd is the sample conductivity corresponding to maximum electropermeabilization obtainable by PEF treatment (electric field = 1000 V/cm; pulse width = 300 µs; number of pulses = 60; and pulse frequency = 1 Hz).

Omowaye and others 2000). A penetrometer (type 5 cc-solid) was filled with the dried apple slabs and positioned in the porosimeter. Pore size measurements for the apple samples using the porosimeter covered pore diameters ranging from about 0.015 to 72 µm. Porosity e is defined as the ratio of the total volume of air pores Vp and volume of the dry sample V:

(2) Pore distribution in a sample was determined as the ratio of the volume Vp(d) of pores with mean size d and the dry sample volume V (that is partial porosity) ed :

Measurement of porosity and density Mercury porosimeter (Autopore III 9400, Micromeritics Instrument Corp., Norcross, Ga., U.S.A.) was used to analyze pore, pore distribution, porosity, bulk density, and skeletal density of the apple tissues. Cylindrical apple samples (with and without PEF treatment) were sliced into cubic slabs of about 3- to 5-mm-thick sizes. The slabs were dried in a laboratory oven (model 750F, Fisher Scientific, (Pittsburgh, Pa., U.S.A.) set at 45 °C for 2 d. This temperature was selected as optimum for preventing thermal plasmolysis (Jemai 1997; Ade-

(3) since, (4)

Combining Eq. 2 and 4, overall porosity can be expressed as:

(5)

Bulk density of sample was also measured with the porosimeter. Shrinkage of samples due to drying was determined as the ratio of the sample volumes, as follows: (6) where V and Vi are the volumes of sample after and before drying, respectively. The volume of sample was determined by using the water-displacement method in a pycnometer (with a precision of ± 0.05 mL). Immersion time did not exceed 30 s.

Statistical analysis Figure 1—PEF treatment and compression test principle of apple samples 250

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Experimental results for PEF treated and untreated (control) samples were com-

Textural changes during PEF treatment. . .

Results and Discussion Porosity measurements Table 1 shows mean values of some physical properties of air-dried apple samples that were either pretreated or untreated with PEF. There was no difference between the final moisture contents of both dried control and pulsed samples. As shown in Table 1, treating the samples with PEF significantly (at the 5% level) affected bulk density, skeletal density, porosity, pore size, and volume shrinkage. Control samples had higher volume shrinkage than treated samples. Treating apple slices with PEF resulted in a 30% less volumeshrinkage than control samples. Volume shrinkage is related to structural collapse during drying (Zogzas and others 1994; Karathanos and others 1996). There was apparently a more severe collapse of structure in the control apple samples than in the PEFtreated samples. This can be attributed to weakening of the cell walls in the PEF-treated tissue as a result of induced pores. Table 1 also shows mean bulk and skeletal densities of control samples as 0.46 and 1.39 kg/m3, respectively. Corresponding values of bulk and skeletal densities for the PEF-treated samples were 0.38 and 1.52 kg/ m3, respectively. Zogzas and others (1994) reported bulk and solid densities of apple samples air-dried at 70 °C as 0.51 and 1.60 kg/m3, respectively. Krokida and Maroulis (2001) obtained bulk density (apparent density) of apple samples that were completely dried using different methods to be in the range of 0.12 to 0.73 kg/m3. The authors also reported skeletal density (true density) of the apple samples that were also completely dried using different methods to be in a range from about 1.40 to 1.65 kg/ m3. Differences in the value of densities may arise due to differences in drying methods and measurement approach used. The mean bulk and skeletal densities obtained in this study for the control samples were in the range of the data reported in literature. PEF treatment of samples resulted in lower bulk densities due to the resulting less shrinkage during drying of the treated materials. The higher mean skeletal density obtained with PEF-treated samples is attributed to the increased porosity resulting from electroplasmolysis. With increased pores and with no moisture occupying the pores, skeletal density of the tissue was

largely influenced by the densities of the various constituents of tissue such as carbohydrate with density in the range of 1.5 to 1.6 kg/m3 (French 1984). Pore size distributions in the PEF-treated and control (untreated) samples are shown in Figure 2. PEF-treated samples had larger pore volume resulting from pores with mean pore sizes less than 5 mm compared with the untreated samples. This is attributed to the formation of new pores with smaller mean sizes due to PEF electroplasmolysis as shown in Table 1. PEF treatment resulted in decrease of mean pore diameter from 7.81 to 5.86 mm. Karathanos and others (1996) reported 2 peaks in the pore size distribution of dried plant materials such as apple, potato, and cabbage samples. For air-dried apple samples, the peaks corresponded to maxima at 2.1 and 18 mm. For other vegetable tissues, the maxima of size distribution function were reported to be in the range varying from 0.04 to 24 mm. In general, the majority of the regular pores in air-dried vegetable tissues are of sizes greater than 10 mm (Karathanos and others 1996). The mean thickness of apple cell walls has been reported to be in the range of 0.1 to 10 mm for average parenchyma cells (Mauseth 1991; Jackman and Stanley 1995). The cell walls are covered by biological membrane called plasmalemma with a thickness between 7 and 10 nm (Stanier and others 1970; Mauseth 1991). Treating the apple tissue with an electric field resulted in the generation of more pores of sizes in the order of cell wall thickness, thus suggesting that the generated pores may be located at the cell wall. Therefore, electroplasmolysis affects

Figure 2—Distribution of pores in untreated and PEF-treated apple samples with conductive disintegration index in the range of 0.9 to 1 (n = 60 pulses; E = 1000 V/cm; ti = 300 ms; f = 1 Hz). Points represent averaged data from 3 determinations. Lines were obtained with least square polynomial fitting of experimental data.

not only cell membranes but also cell wall integrity.

Compression test on apple samples Figure 3 shows typical stress–strain curves for control and pulsed samples of apple tissue. The samples represented in Figure 3 exhibited the characteristic elastic behavior at very low strain levels (up to about 0.15 strain), beyond which there was a nonlinear strain response to stress leading to a bioyield failure point. The bioyield point may be associated with achieving external pressure and cell turgor pressure equilibrium and the beginning of cell rupture (Sajnin and others 1999; Wu and Pitts 1999); Jackman and Stanley 1995). Exposing the apple tissue to PEF affected the bioyield point. Increasing the number of applied pulses decreased the bioyield stress values as shown in Figure 3. The slope of the stress curve for the initial range of small strain reveals the elastic properties of the tissue (Blahovec 2001). Viscous behavior of the tissue was observed from nonlinear stress–strain dependency for the larger strain. Figure 4 presents the compression modulus determined from the slope of the stress–strain curves up to the strain of 0.15 for the samples with different conductive disintegration index z calculated from Eq. 1. Figure 4 reveals a sharp decrease in the compression modulus with an increase in disintegration index up to z » 0.25. The turgor pressure apparently reduced because of the electric field–induced pores in cell walls, thus explaining the decrease in compression modulus. Therefore, bioyield stress decreased with intensification of elec-

Figure 3—Typical stress–strain curves from the compression test on apple samples treated by different number n of electric field pulses (E = 1000 V/ cm; ti = 300 ms; f = 1 Hz); curve with n = 0 corresponds to untreated sample.


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pared. All experiments were performed in triplicate. Regression and statistical analysis were performed using Sigma Plot (SigmaPlot 2000, Ver. 6, SPSS Inc., Chicago, Ill., U.S.A.).

Textural changes during PEF treatment. . . troplasmolysis. Apple tissues become more compressible after PEF treatment, even at a low tissue destruction degree.

Estimation of dependencies between rupture force, conductive disintegration index, and tissue damage degree Dependencies of the failure stress P from the compression test against disintegration index are presented in Figure 5. The experimental data were fitted by the following equation: P = Pmax – kzb

(7)


where Pmax is the maximal failure stress corresponding to the untreated sample (z = 0), k is a regression parameter, and b is an index. The parameters Pmax, k, and b in Eq. 7 were estimated as 1.26 ± 0.04, 0.71 ± 0.04, and 0.46 ± 0.06, respectively. The correlation coefficient was 0.944. The disintegration index z was related to tissue damage degree (electroplasmolysis degree) p according to Archie’s equation (Archie 1942; Lebovka and others 2002): z = pm

(8)

The nonlinear relation between tissue disintegration index and damage degree is attributed to the dependency of electrical conductivity on heterogeneous structure of tissue (Angersbach and others 2000; Wang and others 2001). Besides these, different transition phenomenon inside the tissue, such as mass transfer, moisture, and air redistribution among the microscopic pores in

Figure 4—Changes in compression modulus compared with conductivity disintegration index z for apple tissue plasmolyzed at different PEF-treatment intensities: n = 0 to 60 pulses; E = 1000 V/cm; ti = 300 ms; f = 1 Hz). Error bars represent standard deviations from 3 determinations. 252

the cell wall matrix can be observed during food processing by PEF (Aguilera and others 2000; Lebovka and others 2001). Combining Eq. 7 and 8 yields P = Pmax – kps

(9)

where s = mb. Taking into account that m = 1.8 – 2.5, evaluated earlier for apple tissue treated by PEF with strength in the range of 100 to 1500 V/cm (Lebovka and others 2002), the s value may be estimated as s = 0.83 – 1.15 with mean value s » 1. This result reveals the practically linear relation between failure stress and degree of tissue damage.

Conclusions

T

HE INFLUENCE OF PEF TREATMENT ON

textural properties of apple tissue has been studied. The current investigation showed that PEF treatment changes apple tissue porosity from 67 to 75%. Sizes of the induced pores were mostly smaller than the average sizes of pores in the untreated samples. Sizes of the PEF-induced pores were less than the cell wall thickness. The analysis of textural parameters indicated that turgor pressure and failure stress tended to decrease with PEF treatment intensity. This suggests that electroplasmolysis affects not only plasmalemma membranes but also cell wall integrity of samples. Failure stress compared with conductivity disintegration index function was determined as an exponential relation. Experimentally estimated exponent is 0.46 for investigated apple tissue. This result reveals the linear dependency between failure stress and tissue damage degree. The data

Figure 5—Relationship between failure stress and conductive disintegration index for apple tissue plasmolyzed at different PEF-treatment intensities: n = 0 to 60 pulses; E = 1000 V/cm; ti = 300 ms; f = 1 Hz.

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can be applied to design suitable applepressing technology.

References Ade-Omowaye BIO, Angersbah A, Eshtiaghi NM, Knorr D. 2000. Impact of high intensity electric field pulses on cell permeabilisation and as pre-processing step in coconut processing. Innov Food Sci Emerg Technol 1(3):203-9. Ade-Omowaye BIO, Angersbah A, Taiwo KA, Knorr D. 2001. Use of pulsed electric field pretreatment to improve dehydration characteristics of plant based foods. Trends Food Sci Technol 12(8):285-95. Aguilera JM, Stanley DW, Baker KW. 2000. New dimensions in microstructure of food products. Trends Food Sci Technol 11(1):3-9. Angersbach A, Heinz V, Knorr D. 2000. Effects of pulsed electric fields on cell membranes in real food systems. Innov Food Sci Emerg Technol 1:135-49. Archie GE. 1942. The electrical resistivity log as an aid in determining some reservoir characteristics. Trans AIME 146:54-62. Barbosa-Canovas GV, Pierson MD, Zhang QH, Schafner DW. 2000. Pulsed electric fields. J Food Sci 65(suppl.):65-79. Barsotti L, Dumay E, Mu TH, Diaz MDF, Cheftel JC. 2001. Effects of high voltage electric pulses on protein-based food constituents and structures. Trends Food Sci Technol 12:136-44. Bazhal MI. 2001. Etude du mécanisme d’électroperméabilisation des tissus végétaux. Application à l’extraction du jus des pommes [Dphil thesis]. France: Univ. de Technologie de Compiègne. Bazhal MI, Lebovka NI, Vorobiev EI. 2001. Pulsed electric field treatment of apple tissue during compression for juice extraction. J Food Eng 50(3):129-39. Bazhal MI, Vorobiev EI. 2000. Electrical treatment of apple slices for intensifying juice pressing. J Sci Food Agric 80:1668-74. Beveridge T. 1997. Juice extraction from apples and other fruits and vegetables. Crit Rev Food Sci Nutr 37(5):449-69. Blahovec J. 2001. Improved rate controlled model for stress relaxation in vegetable tissue. Int Agrophys 15:73-8. Fincan M, Dejmek P. 2002. In situ visualization of the effect of a pulsed electric field on plant tissue. J Food Eng 55(3):223-30. French D. 1984. Organization of starch granules. In: Whistler RL, Bemiller JN, Paschall EF, editors. Starch: chemistry and technology. 2nd ed. New York: Academic Press. p 183-247. Gudmundsson M, Mafsteinsson H. 2001. Effect of electric field pulses on microstructure of muscle foods and roes. Trends Food Sci Technol 12:122-8. Gulyi IS, Lebovka NI, Mank VV, Kupchik MP, Bazhal MI, Matvienko AB, Papchenko AY. 1994. Scientific and practical principles of electrical treatment of food products and materials. Kiev: UkrINTEI (in Russian). 64 p. Jackman RL, Stanley DW. 1995. Perspectives in the textural evaluation of plant foods. Trends Food Sci Technol 6(6):187-94. Jemai AB. 1997. Contribution a l’étude de l’effet d’un traitement électrique sur les cossettes de betterave a sucre. Incidence sur le procède d’extraction[Dphil thesis]. France: Univ. de Technologie de Compiègne. Karathanos VT, Kanellopoulos NK, Belessiotis V.G. 1996. Development of porous structure during air drying of agricultural plant products. J Food Eng 29(2):167-83. Knorr D, Angersbach A. 1998. Impact of high intensity electric field pulses on plant membrane permeabilization. Trends Food Sci Technol 9:185-91. Knorr D, Angersbach A, Eshtiaghi MN, Heinz V, Lee DU. 2001. Processing concepts based on high intensity electric field pulses. Trends Food Sci Technol 12(4):129-35. Krokida MK, Maroulis ZB. 2001. Structural properties of dehydrated products during rehydration. Int J Food Sci Technol 36:529-38. Krokida MK, Oreopoulou V, Maroulis ZB, MarinosKouris D. 2001. Effect of pre-treatment on viscoelastic behaviour of potato strips. J Food Eng 50:11-7. Lebovka NI, Bazhal MI, Vorobiev E. 2000. Simulation and experimental investigation of food material breakage using pulsed electric field treatment. J Food Eng 44:213-23.

Textural changes during PEF treatment. . . Rojas AM, Castro MA, Alzamora SM, Gershenson LN. 2001. Turgor pressure effects on textural behaviour of honeydew melon. J Food Sci 66(1):111-7. Sajnin C, Gerschenson LN, Rojas AM. 1999. Turgor pressure in vegetable tissues: comparison of the performance of incipient plasmolysis technique using mannitol and polyethylenglycol. Food Res Int 32(8):531-7. Stanier RY, Doudoroff M, Adelberg EA. 1970. The microbial world. Englewood Cliffs, N.J.: PrenticeHall Inc. p 325-6. Taiwo KA, Angersbach A, Ade-Omowae BIO, Knorr D. 2001. Effects of pretreatments on the diffusion kinetics and some quality parameters of osmotically dehydrated apple slices. J Agric Food Chem 49:2804-11. Teissie J, Eynard N, Gabriel B, Rols MP. 1999. Electropermeabilization of cell membranes. Advanced Drug Delivery Rev 35:3-19. Wang CS, Kuo SZ, Kuo-Huang LL, Wu JSB. 2001. Effect of tissue infrastructure on electric conductance of vegetable stems. J Food Sci 66(2):284-8. Weaver JC, Chizmadzhev YA. 1996. Theory of electroporation: a review. Bioelectrochem Bioenerg 41:135-60. Winterhalter M., Helfrich W. 1987. Effect of voltage on pores in membranes. Phys Rev A 36(12):5874-6.

Wu H, Pitts MJ. 1999. Development and validation of a finite element model of an apple fruit cell. Postharvest Biol Technol 16:1-8. Zimmermann U, Pilwat G, Beckers F, Rieman F. 1976. Effects of external electric fields on cell membranes. Bioelectrochem Bioenerg 3:58-83. Zogsas NP, Maroulis ZB, Marinos-Kouris D. 1994. Densities, shrinkage and porosity of some vegetables during air drying. Drying Technol 12(7):165366. MS 20020292 Submitted 5/10/02, Revised 9/11/02, Accepted 10/8/02, Received 10/11/02 The authors acknowledge the financial support from the National Science and Engineering Council (NSERC) of Canada.

Authors Bazhal, Ngadi, and Raghavan are with the Dept. of Agricultural and Biosystems Engineering, McGill Univ., Macdonald Campus, 21111 Lakeshore Road, Ste-Anne-de-Bellevue, Quebec, Canada H9X 3V9. Author Nguyen is with the Inst. de Recherche d’Hydro-Québec, 1802 Boul. LionelBoulet, Varennes, Quebec, Canada J3X 1S1. Direct inquiries to author Ngadi (E-mail: [email protected]).


Lebovka NI, Bazhal MI, Vorobiev E. 2001. Pulsed electric field breakage of cellular tissues: visualization of percolative properties. Innov Food Sci Emerg Technol 2(2):113-25. Lebovka NI, Bazhal MI., Vorobiev E. 2002. Estimation of characteristic damage time of food cellular materials in pulsed electric fields. J Food Eng 54: 33746. Matvienko AB. 1996. Intensification of the extraction of soluble substances by electrical treatment of plant materials and water [ Thesis]. Kiev, Ukraine: Ukrainian State Univ. of Food Technologies (in Russian). Mauseth JD. 1991. Botany: an introduction to plant biology. Philadelphia: Saunders College Pub. 800 p. McLellan MR, Kime RL, Lind LR. 1991. Electroplasmolysis and other treatments to improve apple juice yield. J Sci Food Agric 57:303-6. Ngadi MO, Arevalo P, Raghavan GSV, Nguyen DH. 2001. Pulse electric fields in food processing. Paper nr 01-016071, presented at the ASAE annual international meeting, 29 July–1 August 2001, Sacramento, Calif. Rastogi NK, Eshtiaghi NM, Knorr D. 1999. Accelerated mass transfer during osmotic dehydration of high intensity electrical field pulse pretreated carrots. J Food Sci 64(6):1020-3.


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