Pulsed electric field enhanced drying of potato tissue

Journal of Food Engineering 78 (2007) 606–613 www.elsevier.com/locate/jfoodeng

Pulsed electric field enhanced drying of potato tissue Nikolai I. Lebovka a

a,b

, Nikolai V. Shynkaryk

a,b

, Eugene Vorobiev

a,*

Departement de Ge´nie Chimique, Universite´ de Technologie de Compie`gne, Centre de Recherche de Royallieu, B.P. 20529-60205 Compie`gne Cedex, France b Institute of Biocolloidal Chemistry named after F.D. Ovcharenko, NAS of Ukraine, 42, blvr. Vernadskogo, Kyiv 03142, Ukraine Received 27 April 2005; accepted 28 October 2005 Available online 10 January 2006

Abstract The influence of pulsed electric field (PEF) treatment on convective drying of potato tissue was investigated. The drying experiments for potato disks were done in the interval of drying temperatures 30–70 C. The effects of pre-treatment before drying and PEF treatment in the course of the drying are discussed. The temperature dependencies of moisture effective diffusion coefficient Deff for intact, PEFtreated and freeze-thawed potato tissues are compared. It is shown that Deff is a function of the conductivity disintegration index Zr. The moisture effective diffusion coefficient Deff is sensitive to the pre-treatment procedure, and the highest values of Deff are always observed for freeze-thawed pretreated samples. The data show that thermal pre-treatment of samples at high temperatures (T = 70 C) has practically no beneficial effect on the drying rate, though the same thermal pre-treatment at mild temperatures (T = 50 C) increases the moisture effective diffusion coefficient Deff and gives an effect that is comparable with that for the PEF pre-treated samples.  2005 Elsevier Ltd. All rights reserved. Keywords: Drying; Temperature dependence; Pulsed electric field; Diffusion coefficient; Potato

1. Introduction Moisture removing from the food materials allows to minimize substantially the microbial activity and deterioration chemical reactions (Barbosa-Canovas & VegaMercado, 1996). But commonly used freeze-drying or conventional drying techniques are limited by high energy consumption and long drying times. Moreover, drying at elevated temperatures can produce undesirable changes in pigments, vitamins and flavouring agents (Aguilera, Chiralt, & Fito, 2003; Chou & Chua, 2001; Jayaraman & Gupta, 1995). In general, the drying processes consume an appreciable part of the total energy used in food industry and so, it is very important to develop the new hybrid drying technologies for energy consumption reduction and preserving of food qualities (Chou & Chua, 2001). Different pre-treat*

Corresponding author. Tel.: +33 3 4423 5273; fax: +33 3 4423 1980. E-mail address: [email protected] (E. Vorobiev).

0260-8774/$ - see front matter  2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2005.10.032

ment drying techniques, such as the microwave heating (Beaudry, Raghavan, & Rennie, 2003; Datta & Anantheswaran, 2001; Kostaropoulos & Saravacos, 1995), ohmic heating (Lima & Sastry, 1999; Salengke & Sastry, 2005; Wang & Sastry, 2000; Zhong & Lima, 2003), electrohydrodynamic drying (Bajgai & Hashinaga, 2001; Cao, Nishiyama, Koide, & Lu, 2004; Chen & Barthakur, 1994; Hashinaga, Kharel, & Shitani, 1995; Isobe, Barthakur, Yoshino, Okushima, & Sase, 1999; Li, Li, Sun, & Tatsumi, 2005) and drying by chemical reagents or osmotic pretreatment (Chua, Chou, Mujumdar, Ho, & Hon, 2004; Salas & Labuza, 1968; Saravacos, Marousis, & Raouzeos, 1988) were reported. Recently, the pulsed electric field (PEF) treatment have been proposed for food dewatering enhancement in the pressing and drying processes (Ade-Omowaye, Rastogi, Angersbach, & Knorr, 2003; Fincan, DeVito, & Dejmek, 2004; Lebovka, Praporscic, & Vorobiev, 2003; Vorobiev, Jemai, Bouzrara, Lebovka, & Bazhal, 2005). It was shown that the effective plant tissue disintegration under the PEF

N.I. Lebovka et al. / Journal of Food Engineering 78 (2007) 606–613

607

Nomenclature d Deff D1 E h M m n P R t ti tPEF Dt T DU Zr

sample diameter (mm) moisture effective diffusion coefficient (m2/s) limiting diffusion coefficient at infinitely high temperature (m2/s) PEF intensity (V/cm) sample height (mm) moisture content in the sample Archie’s exponent number of pulses damage degree universal gas constant, 8.314 (J K1 mol1) time (s) pulse duration (ls) time of PEF treatment (s) pulse repetition time (ms) temperature (C) activation energy (kJ/mol) electrical conductivity disintegration index

Subscripts b bound d maximally destroyed e equilibrium i intact 0 initial Greek symbols q correlation coefficient r electrical conductivity (S m1) x dimensionless moisture ratio Abbreviation PEF pulsed electric fields

treatment can be achieved at moderate electric fields of 200–1000 V/cm and short treatment time within 104– 102 s (Lebovka, Bazhal, & Vorobiev, 2000, 2001, 2002). The PEF treatment at mild thermal conditions further improves the effectiveness of PEF-induced damage (Lebovka, Praporscic, & Vorobiev, 2004a, 2004b). The PEF treatment seems to be a promising non-thermal method that gives the interesting advantages for drying enhancement of thermally sensitive food materials. The PEF cause electroporation and complete damage of the tissue cells (Weaver & Chizmadzhev, 1996), facilitate moisture diffusion and can enhance drying. However, relations between the drying rate and disintegration degree in food tissues, practically, were not studied yet. The objective of this study was to determine the effect of PEF treatment on the conventional drying rate of potato tissues. 2. Materials and methods 2.1. Materials A potato tissue was chosen as an object for investigation. Potatoes (Agata) of good and uniform quality were purchased from the local market. 2.2. Experimental setup Fig. 1 shows a schematic diagram of the experimental setup. An air flow rate was adjusted by the GFC47 Controller (Aalborg, USA). The air heating was performed in an electrical flow heating quartz tube with a power of 2.3 kW. The temperature of air was adjusted by the heating controller Horst HT 30 (Laborgerate GmbH, Germany) with the help of the K-type (NiCr–Ni) thermocouple (1),

Fig. 1. A scheme of the setup for PEF-enhanced drying experiments. See text for details.

installed into the drying chamber (Fig. 2). The drying data were obtained by continuous weighting of the drying chamber at an analytical balance CP6201 (Sartorius, Germany, sensitivity 0.1 g). The PEF-drying chamber was constructed in a form of two concentric polypropylene rings (Fig. 2). The potato disk was put in the cavity compartment between two stationary wire gauze electrodes (stainless steel 316 L) with square holes of 0.2 · 0.2 mm2. Both the bottom and top rings were tightly closed from both sides by steel covers and fixed by cramp in a steel frame. The filter cloth was

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where M is the moisture content in a sample, and the subscripts ‘0’ and ‘e’ refer to the initial and equilibrium (final) moisture contents, respectively. For estimation of the damage degree after the PEF treatment, the electrical conductivity disintegration index Zr was used (see e.g. Lebovka, Bazhal, & Vorobiev, 2002) r  ri ; ð2Þ Zr ¼ rd  ri

put at the bottom of the chamber in order to make airflow more homogeneous. The temperatures inside the geometric center of the sample and near its surface were measured by K-type thermocouples 2 and 3, respectively, connected to the data logger thermometer Center 305/306 (JDC Electronic SA, Switzerland). Two electrodes were connected to the PEF generator. The PEF generator, 1500V-20A (Service Electronique UTC, France) provided the monopolar pulses of nearrectangular shape (Lebovka et al., 2003) the electric voltage U and current not exceeding 1500 V/cm and 20 A, respectively, the number of pulses n = 1–30,000, pulse duration ti = 105–103 s and pulse repetition time Dt = 102 – 100 s. The PEF treatment time was calculated as tPEF = nti. The electrical conductivity of a sample was measured with a LCR Meter HP 4284A (Hewlett–Packard) at the frequency of 1000 Hz selected as optimal for purposes of removing the polarizing effects on the electrodes and tissue sample. All the output data (current, voltage, electrical conductivity and temperatures) were recorded using a data logger and software developed by Service Electronique UTC, France.

3. Results and discussion The typical drying curves for intact (solid lines) and freeze-thawed (dashed lines) potato tissues at drying air temperatures 30, 50 and 70 C are presented in Fig. 3. Insert shows the drying rate curves in coordinates dx/ dt versus x for the same data. At the beginning of the drying processes, the drying rate passes through the maximum, when the excess surface moisture is removed. Then it decreases with decreasing of x, so the drying process occurs at the falling rate. In all experiments at different temperatures of drying and material damage degrees no constant period of drying rate was observed. Such behaviour is in correspondence with 1 04

0.8

03 -4 -d /dt, 10 s -1

Fig. 2. The construction of PEF-drying chamber.

where r is the measured electrical conductivity value and the subscripts ‘i’ and ‘d’ refer to conductivities of the intact and maximally destroyed tissues, respectively. The conductivity of the maximally destroyed material rd was determined for samples after the PEF treatment in the electric field E = 500 V/cm during the period tPEF of order 1 s (Bazhal, Lebovka, & Vorobiev, 2003; Lebovka et al., 2002). Application of Eq. (1) gives Zr = 0 for an intact tissue and Zr = 1 for a disintegrated material.

2.3. Procedure The initial moisture content was within 83–85%. It was determined by drying samples in an oven at 90 C for 24 h. The samples were in the form of cylinders having diameter d = 40 mm and height h = 10 mm. Drying experiments were done at different temperatures of the drying air in the interval 30–70 C. The preliminary experiments showed that air velocity had a little effect on the drying rate and in all experiments the volumetric flow rate was kept constant and equal to 6 m3 h1. Each experiment was repeated at least three times. In the drying experiments, the dimensionless moisture ratio x versus time t was studied. The value of x was determined as MðtÞ  M e x¼ ; M0  Me

ð1Þ

0.6

70 ˚C

02

50 ˚C

01

30 ˚C 0

0.4

0

0. 5

1

30 ˚C 0.2 50 ˚C 70˚C 0

10000

20000

30000

40000

t,s Fig. 3. The moisture ratio x versus drying time t for intact (solid lines) and freeze-thawed (dashed lines) tissues at different temperatures of drying. Insert shows drying rate curves in coordinates dx/dt versus x for the same data.

N.I. Lebovka et al. / Journal of Food Engineering 78 (2007) 606–613

1

1 0.8

0.8 0.6 Z

Intact

0.4

0.6

0.2 0.4

0

10

-3

10

-2

10

-1

10

0

tPEF, s

Freezethawed 0.2

tPEF=5.10-3s 10-2s 5.10-1s

0

0

5000

10000

15000

20000

25000

30000

t, s Fig. 4. The moisture ratio x versus drying time t for intact, PEF-treated and freeze-thawed treated (dashed line) tissues at 50 C drying temperature. The PEF-pre-treatment was done at room temperature, T = 25 C, electric field strength E = 400 V/cm, pulse duration ti = 103 s, pulse repetition time Dt = 102 s, and different treatment time tPEF, shown at the figure. Insert shows the electrical conductivity disintegration index Zr versus treatment time tPEF for the same PEF treatment conditions.

1

0.8

60

Freezethawed

55

50

Intact

0.6

45

PEF-treated

0.4

T, ˚C

the drying behaviour of potato slices observed in a convective dryer (Akpinar, Midilli, & Bicer, 2003). The absence of constant rate stage can be explained by the shrinkage factor importance for the potato (May & Perre´, 2002). The drying time was affected considerably by the drying temperature and freeze-thawing pre-treatment (Fig. 3). Pre-treatment of potato tissue by pulsed electric fields also allows to enhance the drying. Fig. 4 shows the effect of PEF pre-treatment at electric field strength E = 400 V/cm on the drying kinetics at drying air temperature of 50 C. Here, insert shows the electrical conductivity disintegration index Zr of tissues versus treatment time tPEF for the same PEF treatment conditions. The higher is the damage degree of the PEF-treated tissue, more rapid is the drying process (Fig. 4). The PEF treatment of material releases moisture from the damaged cells and enhances transport processes, which results in increase of the drying rate. However, in any case (even for the totally PEF-damaged tissues with Zr  1) the drying rate observed for the freeze-thawed tissue was never exceeded. This can be explained by the fact that the drying rate depends not only on the quantity of the released water, but also on the structure, density and porosity of material (May & Perre´, 2002; Wang & Brennan, 1995). The textural study of the PEF-damaged and freezethawed potatoes demonstrates that their structures are quite different and less affected by PEF treatment (Lebovka et al., 2004a). Fig. 5 shows the moisture ratio x and temperature evolution in the geometrical centre of a sample T during drying

609

40

35 0.2 30

0

5000

10000

15000

20000

25000

30000

t, s Fig. 5. The moisture ratio x (solid lines) and temperature T (dashed lines) inside the geometrical centre of sample versus drying time t for intact, PEF-treated and freeze-thawed treated tissues at 60 C drying temperature. The PEF-pre-treatment was done at temperature T = 25 C, electric field strength E = 400 V/cm, and time of treatment tPEF = 0.5 s, that corresponds to required for attaining the maximal sample damage (Zr  1).

of the untreated, freeze-thawed and PEF pre-treated tissues at drying air temperature of 60 C. The temperature inside the sample (measured by thermocouple 2, Fig. 2) is continuously growing during drying and reaches the temperature of hot air (60 C) only at the end of the drying process. The observed temperature evolution inside the sample is typical for food products (Luikov, 1968). Two stages of the temperature elevation can be distinguished from Fig. 5. The first stage corresponds to the regime of an intense removing of free water from a biological tissue. The rate of the temperature rise decreases at the end of this stage, which corresponds to the equilibrium between external heating and cooling related to evaporation of the free water. At low contents of the free moisture, the balance between the external heating and cooling processes changes and the second stage of the temperature rise is observed. The highest rate of the temperature rise at this second stage (shown by open circles in Fig. 5) is observed at moisture ratios x equal to xb  0.05–0.07 that correspond to about 0.2–0.3 kg water/kg of dry solid (shown by closed circles in Fig. 5). This moisture level approximately corresponds to the maximum in bound water diffusivities, that widely discussed in the literature (Aguerre & Suarez, 2004; Fish, 1958; Saravacos & Raouzeos, 1984). The second stage of the temperature rise is observed at the point of maximum water diffusivity x  xb, that practically do not depend upon the character of material pretreatment (Fig. 5). In principle, it is possible to apply the PEF treatment during drying and it also allows to enhance the drying process. But electrical contacts between the electrodes and tissue makes worse in the course of drying and the PEF

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treatment become less effective. Fig. 6 compares moisture, temperature and electrical conductivity kinetic for the samples PEF-treated (at E = 300 V/cm) in the course of drying (solid lines) and initially intact (dashed lines). The measured electrical conductivity of a sample decreases rapidly in the course of drying. When PEF treatment starts, the electrical conductivity rises rapidly. PEF induces the tissue damage and the additional free moisture appearing at the sample surface. It results in improvement of contacts between the electrodes and the sample. In this experiment the total time of PEF treatment was rather high, tPEF = 1 s, and the temperature inside a sample increases owing to the ohmic heating effects during the PEF treatment. When PEF treatment stops, both the electrical conductivity and temperature decrease rapidly (Fig. 6). We have used the Fick’s second law solution for estimation of the effective moisture diffusion coefficient Deff in a potato slab (Crank, 1975): ! 1 8 X 1 ð2i þ 1Þ2 p2 Deff t x¼ 2 exp  . ð3Þ p i¼0 ð2i þ 1Þ2 4h2 Though the diffusion theory has some restrictions in description of the experimental drying data (Hamdami, Monteau, & Le Bail, 2004), in our case we used the value of Deff as some empirical parameter that characterises the drying rate. The first five leading terms in the series expansion of Eq. (3) were taken into account in the least square fitting pro-

cedure. Eq. (3) allows to obtain a rather good description of the drying experimental data and the correlation coefficients q lie in the interval of 0.991–0.995. Fig. 7 presents the moisture effective diffusion coefficient Deff for intact, PEF pre-treated and freeze-thawed potato tissues at different drying temperatures. Dashed lines in Fig. 7 shows the fitting of calculated Deff values to the Arrhenius law:   DU =R Deff ¼ D1 exp  ; ð4Þ T þ 273:15 where D1 is the limiting diffusion coefficient at an infinitely high temperature, DU is the activation energy and R is the universal gas constant. The activation energies estimated from this equation are: DU = 21 ± 1 kJ/mol, DU = 20 ± 2 kJ/mol, DU = 27 ± 4 kJ/mol, and the limiting diffusion coefficients are D1 = (12 ± 5) · 106 m2/s, D1 = (13 ± 8) · 106 m2/s, D1 = (5 ± 3) · 104 m2/s for intact, PEF-treated and freezethawed treated potato tissues, respectively. The moisture effective diffusion coefficient Deff is a nonlinear function of the conductivity disintegration index Zr (Fig. 8). We may suppose that a PEF-damaged tissue can be modeled as a mixture of the intact cells with a diffusion coefficient Deff,i and damaged cells with a diffusion coefficient Deff,d. The moisture effective diffusion coefficient Deff in an parallel model of diffusion (Saravacos & Kostaropoulos, 1996) may be approximated as Deff ¼ PDeff;d þ ð1  P ÞDeff;i ;

1

ð5Þ

where P is a fraction of the damaged cells, or a material damage degree.

PEF

0.8 0.6

T, ˚C 20

0.4 0.2 0

70

60

50

40

30

15 0

5000

10000

t, s

15000

20000

3 o

40

2

/

T, ˚C

45

35

Def f , 10 -9 , m2 /s

10 50

Freeze-thawed

PEF-treated 5 Intact

1 30 25

0

5000

10000

15000

0 20000

t, s Fig. 6. The moisture ratio x, temperature inside the geometrical centre of sample T and ratio of conductivities r/ri (ri is initial conductivity) versus drying time t for PEF-treated (solid lines), and initially intact (dashed lines) samples at 50 C drying temperature. The PEF-treatment in the course of the drying starting from the time t = 1000 s. Electric field strength E is 300 V/cm, pulse duration ti is 103s, time of PEF treatment tPEF is 1.0 s, and total duration of pulses application is 900 s.

0.00036

0.00038

0.0004

1/ RT Fig. 7. Arrhenius plots of the effective diffusion coefficient Deff versus 1/RT for intact, PEF pre-treated and freeze-thawed pre-treated potato tissues. Symbols are the experimental data, solid lines are the result of the linear least mean square fitting.

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611

Freeze-thawed Freeze-thawed

10

Deff , 10 -9, m 2 /s

Def f , 10 -9, m 2 /s

10

8

PEF-treated

6

8

PEF-treated 6

Thermally-treated Intact

4

4

0

0

0.2

0.4

0.6

0.8

1

Z Fig. 8. Effective diffusion coefficient Deff versus conductivity disintegration index for PEF pre-treated potato tissues (closed symbols). The open symbol is the data for freeze-thawed pre-treated tissue. The drying temperature is 50 C, the PEF pre-treatment conditions are the same as shown in Fig. 4.

Relation between the damage degree P and experimentally measured conductivity disintegration index Zr can be approximated with the help of the empirical Archie’s equation (Archie, 1942): P ¼ Z 1=m r ;

ð6Þ

where m is the, so-called, Archie’s exponent, and experimentally estimated values of m for different biological tissues fall within the range of 1.8–2.5 (Lebovka et al., 2002). The least square fitting of the experimental data to Eqs. (5) and (6) gives the dashed line in Fig. 8 and obtained value of the Archie’s exponent is m = 1.68 ± 0.04. This result is consistent with our previous estimations of m values for different biological tissues (Lebovka et al., 2002). The moisture effective diffusion coefficient Deff is a parameter sensitive to the pre-treatment procedure, and the highest values of Deff are always observed for freezethawed pre-treatment (Figs. 7 and 8). The PEF pre-treatment allows to decrease the drying temperature and for the totally PEF-damaged tissues with Zr  1 the drying temperature can be decreased approximately on 20 C (Fig. 7). The cells damage is also occurred at thermal treatment. At higher temperatures (T > 50 C), the cellular membranes begin to suffer a noticeable irreversible damage ¨ ste, 1994; Thebud (Andersson, Gekas, Lind, Oliveira, & O & Santarius, 1982). Lebovka, Praporscic, Ghnimi, and Vorobiev (2005) discussed the effect of the thermally induced damage of potatoes treated at different temperatures. Fig. 9 shows the effective diffusion coefficient Deff estimated from the moisture drying experiment for tissues thermally pretreated at different temperatures. The potato

20

40

60

80

T, ˚C Fig. 9. Effective diffusion coefficient Deff versus temperature of thermal pre-treament (symbols). Dashed lines shows the levels of Deff for intact, PEF pre-treated (Zr  1) and freeze-thawed pre-treated tissues. The drying temperature is 50 C.

samples were heated in the potato juice during the time required for attaining the maximum degree of the sample damage (Zr  1): t  4 h at 50 C, t  2 h at 60 C and, t  1 h at 70 C. The data show that the thermally induced damage at high pre-treatment temperature (T = 70 C) has practically no beneficial effect on the drying rate. Thermal pre-treatment at a mild temperature (T = 50 C) markedly increases the effective moisture diffusion coefficient Deff and gives an effect that is comparable with the PEF-pretreated samples. Note that also a noticeable difference between the textural properties of freeze–thawed and thermally treated (at temperature T=65 C during of 2 h) tissues is observed for potato (Lebovka et al., 2004a). We can suppose, that the thermal pre-treatment at different temperatures results in different structure of pretreated tissues that influence the drying kinetics. The potato cells are surrounded by the strong cell walls and include the large content (67%) of starch (Lewis & English, 1991). The tissue thermal modification may be associated with different phenomena induced by heat: changing of inner chemical structure of the cell walls through hydrolitic degradation reactions, starch gelatinization, protein insolubilization, breakdown of the interlamellar layer of cell walls, their swelling, expulsion of trapped air, etc. (Huang & Bourne, 1983; Rao & Lund, 1986). Heating of potato tissue in water at temperatures 60 ± 10 C results not only plasmolysis of cells, but also alters pectins in the primary and secondary walls and causes partial cells separation (Andersson et al., 1994). Restricted diffusional transparence of thermally weaken cells possibly reflects the thermal changes in starch granules, but the physical mechanism of observed differ-

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