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Influence of pulsed electric field energy on the damage degree in alfalfa tissue
Journal of Food Engineering 95 (2009) 558–563
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Journal of Food Engineering journal homepage: www.elsevier.com/locate/jfoodeng
Influence of pulsed electric field energy on the damage degree in alfalfa tissue Tanya K. Gachovska a, Akinbode A. Adedeji b,*, Michael O. Ngadi b a b
Department of Electrical Engineering, 209N Walter Scott Engineering Center University of Nebraska, Lincoln, NE 68588-0511, United States Department of Bioresource Engineering, McGill University, Macdonald Campus 21,111 Lakeshore Road, Ste-Anne-de-Bellevue, Quebec, Canada H9X 3V9
a r t i c l e
i n f o
Article history: Received 24 December 2008 Received in revised form 31 March 2009 Accepted 10 June 2009 Available online 16 June 2009 Keywords: Pulsed electric field Damage degree Energy Capacitance Extraction Alfalfa juice
a b s t r a c t The objectives of this study were to investigate the influence of pulsed electric field (PEF) parameters on the damage degree of alfalfa mash, and to determine the relationship between the maximum damage degree and the energy used. Alfalfa mash was treated with PEF at various electric field strengths of 1.25, 1.90, and 2.50 kV/cm. The capacitance of the discharge capacitor was varied from 0.5 to 1.5 lF in steps of 0.5 lF. The pulse number was increased gradually to the point where the impedance became constant. There was no significant increase in the rate of damage beyond 0.5 kJ applied energy. The rate of change of the damage degree at 0.5 kJ was highest when the capacitance was 1.5 lF for all the voltages. Increase in the electric field strength led to decrease in energy needed to obtain the maximum damage degree. To achieve an efficient result for alfalfa juice extraction, the capacitance of the discharge capacitor should preferably be 1 lF or more. In order to minimize energy consumption for a given damage degree in alfalfa, it is desirable to have the highest energy per pulse and fewer number of pulses. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Alfalfa is recognized as an energy-efficient agronomic crop and is most widely used. Alfalfa has been noted for its high nutritional qualities, especially in terms of its crude protein content (15–20%), vitamins (A, D, E, K, C, B12, Biotin, Folic acid, Choline, Pyridoxine, Niacin, Panthothanic acid Riboflavin, Thiamin), and different kinds of minerals including calcium, phosphorus, copper, potassium, magnesium, iron, manganese, sulfur, sodium and chlorine, cobalt, zinc, iodine, and selenium (Bolton, 1962; Ensminger and Olentine, 1978; Hanson et al., 1988). Young alfalfa leaves use as vegetable has been reported in China (Sturtevant, 1919). Alfalfa sprouts are consumed directly by humans (Juiceing-for-Health, 2008; Takyi et al., 1992). Alfalfa juice is extracted from the entire alfalfa plant to maximize the nutritional content by reducing the fiber content. Alfalfa juice concentrate is used widely as a supplement in some health food products. Therefore alfalfa product can be considered a nutraceutical or functional food. A lot of studies have been done on alfalfa juice extraction using different pressing methods. However, juice has not been extracted completely (Duran and Nunez–Arenas, 1988; Sinha et al., 2000). Approximately 52% of total alfalfa water content was extracted using pressures greater than 4 MPa (Sinha et al., 2000). Pulsed electric field (PEF) is an innovative non-thermal technology that involves application of quick burst of electrical energy to biological material leading to membrane disruption and perme* Corresponding author. Tel.: +1 514 398 4400x0708; fax: +1 514 398 8387. E-mail address: [email protected] (A.A. Adedeji). 0260-8774/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2009.06.015
ability. This technique has been used to enhance efficient drying of foods such as carrot (Gachovska et al., 2008), okra (Adedeji et al., 2008), potato (Lebovka et al., 2007), red bell pepper (AdeOmowaye et al., 2003) and Japanese radish (Raphanus sativus L.) (Bajgai and Hashinaga, 2001). It has been successfully used as a pretreatment to enhance juice extraction from plants. Gachovska et al. (2006) used PEF to increase juice extraction from alfalfa leaves. It was applied to enhance juice yield from apple, carrot, beet and other fruits and vegetables (Bazhal and Vorobiev, 2000; Eshtiaghi and Knorr, 2002). The principles behind PEF treatment are that high power and small duration of the pulses result in instant penetration into the plant’s anatomy. The cell membranes of a plant tissue treated with PEF disintegrate, leading to increased permeability of the cell walls, consequently, increasing the juice yield (Bazhal et al., 2004). The potential of increase extraction of juice from alfalfa is promising. Plant tissue has been considered as an electrical model (circuit) because PEF treatment changes its electrical properties. Different models have been presented to substitute the plant tissue in an electrical circuit (Chaylahyan, 1962; Hayden et al., 1969; Meleshenko, 1965; Stout et al., 1987; Zhang and Willison, 1992). As a whole, the plant tissue has resistive and capacitive properties. One of the models that make a good fit between an equivalent electric circuit and the morphological structure of plant tissue is shown in Fig. 1 (Chaylahyan, 1962; Meleshenko, 1965). The resistor R1 corresponds to the resistance of the protoplasm, while R2, R3 (non-linear resistors) are the resistances of the inter-cellular fluid and the membrane, respectively. For a live plant tissue, R2 R1 and R3 R1. The values of resistors R2 and R3 depend on the ap-
T.K. Gachovska et al. / Journal of Food Engineering 95 (2009) 558–563
559
Nomenclature capacitance of discharge capacitor, lF frequency of the measurement, Hz the distance between the spheres, mm pulse number resistance, ohm (X) damage degree
C f lsph n R Sd
V V0 W X Z0 Z
plied voltage (Chaylahyan, 1962; Meleshenko, 1965). The non linear capacitor C in the circuit presents the capacitance of the membrane. The impedance Z of an intact tissue for this model can be substituted for by series connection of resistance R and reactance X
Z ¼ R þ jX
ð1Þ
where,
R ¼ R2
ðR1 þ R3 ÞðR1 þ R2 þ R3 Þ þ x2 C 2 R23 ðR1 þ R2 ÞR1 ðR1 þ R2 þ R3 Þ2 þ x2 C 2 R23 ðR1 þ R2 Þ2
and
X¼
CR22 R23 2 R3 Þ þ 2 C 2 R23 ðR1
ðR1 þ R2 þ
x
þ R2 Þ2
x = 2pf- angular frequency. When PEF is applied to the plant tissue, the cell membrane breaks and becomes permeable. This leads to the capacitance in the circuit to become zero and to short the resistance of the membrane R3. In this wise, the electrical circuit of the plant tissue becomes a resistive circuit and the impedance will not exceed the resistance of the protoplasm R1, this can be calculated by Eq. (3):
R¼
R1 R2 R1 þ R2
plant tissue. Determination of damage degree of the plants by measuring tissue impedance was reported in other studies (Greenham and Daday, 1957; Wilner and Brach, 1970). The damage degree was also determined as a ratio of the polarization coefficient before and after treatment (Sinyuhin, 1967; Svitalka, 1975). The polarization coefficient in this case is equal to the ratio of the impedance measured for low and high frequency. The method of conductivity as a ratio of impedance before and after treatment was used to determine the damage degree, Sd, was reported by several authors (Armyanov et al., 1998; Klimov, 1970; Savchuk, 1971; Sinyuhin, 1967):
Sd ¼
x
ð2Þ
Obviously, it is evidence that the most important part in the cell structure in its interaction with electrical energy is the cell membrane. It determines the change in the electrical impedance of the plant tissue during PEF treatment. Increase in the permeability of the membrane due to electro-plasmolysis triggers the liquid to pass from the cell to the inter-cellular spaces. This leads to a decrease in the resistance R2 and change in the impedance of the
Fig. 1. Equivalent substitute circuit of plant tissue: R1 – the resistance of the protoplasm; R2 (non-linear resistor) – the resistance of the inter-cellular fluid; R3 (non-linear resistor) the resistance of the membrane; C (non-linear) capacitor – the capacitance of the membrane.
treatment voltage, V break voltage, V energy per pulse, kJ reactance, ohm (X) magnitude of impedance before PEF treatment, ohm (X) magnitude of impedance after PEF treatment, ohm (X)
Z0 Z
ð3Þ
The damage degree of plant tissue (electro-plasmolysis) depends on many factors such as: products components, intensity of the electric field, type of pulse waveform, treatment time; and number of pulses (Rastogi, 2003). In this study, alfalfa mash was treated with exponential decay pulses. The sum of applied energy, W P , was calculated using Eq. (4):
W P ¼ Wn ¼ 0:5CV 2 n
ð4Þ
The objectives of this study were to investigate the influence of pulsed electric field (PEF) parameters on the damage degree of alfalfa mash and amount of juice extracted, and also to determine the relationship between the maximum damage degree and spent energy.
2. Materials and methods Approximately 20 cm long fresh alfalfa stems with leaves at pre-blossoming stage were obtained from the farm of Macdonald Campus of McGill University in the Fall of 2005. The stems were sorted, cleaned and ground with a domestic food processor (Oster, Sunbeam Products Inc., China) for 5 min to obtain a homogeneous mash. The mash was kept in a closed vessel to prevent evaporation prior to use. The mash had an initial moisture content of 81.1% (g/g, wet basis). The conductivity of the mash was measured as 3.5 mS cm. The mash was mixed properly before conducting the experiment. Approximately 40 g of the alfalfa mash was introduced into the treatment chamber (batch system) which was used for high voltage treatment, impedance measurement and juice extraction. The chamber (4.2 cm in diameter and 8.0 cm height) and the juice collector are shown in Fig. 2. The treatment chamber consisted of an insulated cylinder (C) made of plastic (Polyoxymethylene, Derlin). A cylindrical plunger (E) (ground) and a disc base (B) (high voltage) were used as electrodes during PEF treatment and impedance measurement. They formed a rigid structure during pressing. The perforated cylindrical plunger and the base were made of stainless steel. Extracted juice was filtered through a stainless steel sieve (D) placed on top of the perforated plunger. Juice extracted during pressing was collected in a plastic collector (G) placed under the treatment chamber. The volume of the treatment chamber was 110 cm3.
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The samples were treated at the electric field strengths of 1.25 1.9 and 2.5 kV/cm with capacitance of discharge capacitor of 0.5, 1 and 1.5 lF. The voltage was registered with a high voltage probe (P6015A, Tektronix, Beaverton, USA) which was connected to a digital oscilloscope (54621A, Agilent, Malaysia). The pulse number was increased gradually to the point where the impedance of the alfalfa mash became constant.
D
B
C
A
F
E
2.2. Determination of the damage degree
H
I K L
Fig. 2. Schematic diagram of pulsed electric field and pressing treatment chamber. Labels: (A) stainless steel disc base (upper electrode), (B) electrical connection outlet on disc electrode, (C) rubber gasket; (D) plastic cylinder, (E) sieve, (F) stainless steel perforated cylindrical plunger (lower electrode), (G) electrical connection outlet on cylindrical electrode, (H) plastic collector; (I) stainless steel connector; (K) plastic tube; (L) plastic container.
2.1. High pulsed electric field treatment Pulsed electrical field treatment of the alfalfa mash was achieved using a pulsed electric field generator shown in Fig. 3. A variable autotransformer (Powerstat Type 3PN116C 0–140 V, Superior Electric Co., Bristol, CT) was used to supply voltage to the circuit and to regulate the pulse frequency. The pulse frequency was 1 Hz (Eshtiaghi and Knorr, 2002). A high voltage transformer (Model 62159A, Apotex Imaging Inc., Canada) transformed the voltage from the autotransformer. The voltage was rectified by a high voltage diode. Resistor was used to limit initial charge current that passed through the capacitor. The capacitor was used to store energy that quickly discharges through the sample after the discharger breaks. The initial treatment voltage applied to the treatment chamber depended on the distance between the 15 mm diameter stainless steel sphere discharger. The break voltage V0, applied to the treatment chamber for this diameter of the sphere can be calculated by the following Eq. (1) (Armyanov et al., 1998): 0:75
V 0 ¼ 4:85lsph
ð5Þ
The damage degree of a plant tissue is an important index that shows the effectiveness of PEF treatment as it demonstrates the level of electro-plasmolysis in the plant. When an exponential decay PEF generator is used, the damage degree depends on the basic treatment parameters: electric field strength, capacitance of discharge capacitor and pulse number (Lebovka et al., 2002; Armyanov et al., 1998). Electrical impedance of the alfalfa mash was measured to determine the damage degree of the alfalfa plant tissue due to PEF treatment. The impedance was measured at a frequency of 1 kHz, by connecting the disk base (B) and the cylindrical plunger (E) to the LCR meter (LCR-745, Leader Electronics Corp., Japan) as shown in Fig. 2. The measurements were carried out before and after PEF treatment of alfalfa mash. The damage degree was determined using Eq. (3). The measurements after PEF treatments were recorded for every increase in the pulse number until constant impedance was achieved. Temperature of the system remained practically the same at ambient condition before and after the treatment. 2.3. Alfalfa pressing The pressing process was a batch operation. Pressure was applied with the aid of a Universal Testing Machine (Model 4502, Instron, Norwood, MA and Series IX software) through a plunger head of 4.15 cm diameter at a displacement of 10 mm/min to 40 g of alfalfa mash poured into a metallic cylinder (4.25 cm diameter and 5.5 cm height) with perforated base connected to a drain (Fig. 2). All samples, both treated and untreated were pressed at 4 MPa for 2 min and the alfalfa juice extract was collected and weighed. All the experiments were conducted in triplicate. Nonlinear regression of the data was performed on CurveExpert 1.3 (2005). Statistical analysis was carried out using SAS version 8.2. (2001).
2.2
2
Damage degree
G
1.8
1.6
0.5 µF 1 µF
1.4
1.5 µF 1.2
1 0.0
1.0
2.0
3.0
4.0
5.0
6.0
Energy, kJ Fig. 3. Electrical circuit diagram for the pulsed electric field generator used to treat alfalfa mash.
Fig. 4. Influence of the total treatment energy on the damage degree for electric field 1.25 kV/cm.
561
2.2
6.50
2
6.00
1.8
5.50
Energy, kJ
Damage degree
T.K. Gachovska et al. / Journal of Food Engineering 95 (2009) 558–563
0.5 µF
1.6
1 µF 1.5 µF
1.4
5.00
0.5 µF 4.50
1 µF 1.5 µF
1.2
1 0.0
4.00
1.0
2.0
3.0
4.0
5.0
3.50 1.25
6.0
1.50
Energy, k J
1.75
2.00
2.25
2.50
Electric field, kV/cm
Fig. 5. Influence of the total treatment energy on the damage degree of alfalfa mash in an electric field of 1.9 kV/cm.
Fig. 7. Influence of electric field on the total treatment energy for the maximum damage degree.
2.2
Damage degree
2
1.8
0.5 µF
1.6
1 µF 1.5 µF
1.4
1.2
1 0.0
1.0
2.0
3.0
4.0
5.0
6.0
Energy, kJ Fig. 6. Influence of the total treatment energy on the damage degree for electric field 2.5 kV/cm.
3. Results and discussions For a practical application PEF to enhance juice extraction, it is important to know the basic parameters of PEF treatment including – electric field strength, capacitance of the discharge capacitor and pulse number, that will result in the maximum damage degree using minimum energy. The influences of applied energy on the calculated values of damage degree, using Eq. (4) for three different capacitances (0.5, 1 and 1.5 lF) of the discharge capacitor, are presented in Figs. 4–6. From the obtained data, it can be concluded that increase in applied energy was as a result of the increase in the pulse number, which subsequently led to the increase in the
damage degree. However, the rate at which damage degree increases diminished as the applied energy (pulse number) increased. For example at 2.5 kV/cm and 2.5 lF, the rate of energy increase was 61.3% from 0 to 1 kJ, while it was 2.61% when the energy increased from 3 to 4 kJ. There was a decrease observed in the impedance as the energy applied to the system increased. The decrease in impedance of the plant tissue could be explained by cell membrane rupture which resulted in diminished capacitance of the cell. Continuous increase in the pulse number led to further cell damage up until the impedance became constant indicating complete saturation of cells breakdown. Similar results of saturation of damage degree were reported by Armyanov et al. (1998), and Eshtiaghi and Knorr (2002). The pulse threshold that causes this extent of damage degree depends on the PEF parameters and the plant tissue (Arevalo et al., 2004). There was no substantial change in the rate of damage degree beyond the point when applied energy was 0.5 kJ and this was used as the set point to model the data of applied energy and damage degree for all the PEF treatments (Table 1). The relative stability in the rate of change of damage degree after the 0.5 kJ mark could be as a result of substantial saturation of cell membrane breakdown after certain number of pulses have been applied. This was reported by Lebovka et al. (2002) and Eshtiaghi and Knorr (2002). The data were fitted to a couple of models and the exponential model presented in Eq. (6) gave the best fit for the data of applied energy and damage degree shown in Figs. 4–6, as it gave the least standard error and the highest correlation coefficient, R (Table 1). The analysis was carried out on CurveExpert 1.3 (2005). The applied energy = x, kJ and the damage degree = y:
Y ¼ aðb ecx Þ
ð6Þ
Table 1 Calculated values for regression coefficients of the exponential model for the damage degree at 0.5 kJ applied energy. Electric field, kV/cm
Capacitance, lF
a
b
c
Standard error
Correlation coefficient, R
Rate of energy change at 0.5 kJ, %
2.50
1.5 1.0 0.5 1.5 1.0 0.5 1.5 1.0 0.5
1.09 1.08 1.05 1.06 1.03 1.05 1.01 1.05 1.34
1.92 1.94 1.97 1.94 1.98 1.96 2.01 1.98 1.75
0.82 0.79 0.61 0.73 0.71 0.53 0.66 0.48 0.29
0.018 0.047 0.073 0.024 0.058 0.032 0.053 0.046 0.026
0.997 0.994 0.987 0.996 0.991 0.996 0.996 0.993 0.996
54.3 49.7 44.9 50.7 49.7 40.7 47.2 37.9 22.4
1.90
1.25
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T.K. Gachovska et al. / Journal of Food Engineering 95 (2009) 558–563
Table 2 The effect of PEF on alfalfa juice extraction and mash compression for a damage degree (max) of 2.02. Capacitance, lF 0 1.50
1.00
0.50
Electric field, kV/cm 0 2.50 1.90 1.25 2.50 1.90 1.25 2.50 1.90 1.25
Amount of juice extracted per 40 g of Alfalfa, g a
10.10 13.70b 13.92b 13.40b 13.88b 13.69b 13.45b 13.20b 13.47b 13.33b
Max. PEF energy applied, kJ
Alfalfa mash compression, mm
0 3.90a 5.27b 5.81c 4.05a 5.63bc 6.00c 5.00b 5.91c 6.25c
55.34a 51.49b 51.30b 51.62b 51.49b 51.30b 51.62b 51.95b 51.64b 51.66b
Mean with the letter on the same column are not significantly different at P < 0.05. Max-maximum.
For untreated sample, x = 0, and the damage degree Y(0) = a(b1) = 1 because impedance remained the same i.e. Z0/Z = 1 for intact/untreated. The model parameters a, b and c for all treated variables are shown in Table 1. Standard error and correlation coefficient ranged between 0.018–0.073 and 0.987–0.997, respectively. Initially, the damage degree of the plant tissue depended on the energy discharged by the capacitor to the plant tissue. With increase in pulse number, the total applied energy increased and the damage degree also increased. The rate of change of the damage degree then decreased exponentially until it becomes zero. The complete saturation of damage degree was significantly affected by the electric field strength as shown in Figs. 4–6. For 1.25 kV/cm, the final saturation point was at about 6 kJ (Fig. 4), for 1.9 kV/cm, the damage degree complete saturation was attained at about 5 kJ, while for 2.5 kV/cm, the saturation point was about 4.5 kJ. This implies the higher the electric field strength, the quicker the final saturation point for cell damage is attained. Fig. 7 further shows the dependency of damage degree saturation on electric field strength at three different capacitances (0.5, 1 and 1.5 lF). The results of the experiment indicate that an increase in the electric field strength and the capacitance of the system would decrease the total applied energy necessary to achieve maximum damage degree. The minimum applied energy was obtained when alfalfa mash was treated at the highest electrical field strength and capacitance (2.5 kV/cm and 1.5 lF). To have an efficient result for juice extraction using a decay PEF generator, a high value of capacitance, in the range of microfarad, is necessary. Eshtiaghi and Knorr (2002) reported using a bank of capacitors with value of 5 lF for the pretreatment of sugar beet for efficient extraction. The difference between the total applied energy (to obtain maximum damage degree for the lowest electric field strength – 1.25 kV/cm) and capacitance of 0.5 and 1.5 lF was only 7.5%. Meanwhile, for the highest electric field strength 2.5 kV/cm, this difference was 28.2%. This implies that application of the smallest energy obtained the maximum damage degree. Therefore, the highest voltage per cm which corresponds to few pulses should be used for maximum cell permeabilization. Treatment with high pulse energy (high electric field strength) usually forms a detonation wave that expends quickly into plant tissue causing the damage to the cell membrane leading to the expression of juice from the mash. There was no significant variation in the quantities of extracted juice from the alfalfa mash after the attainment of maximum damage degree and a compression force of 4 MPa had been applied for 2 min (Table 2). However, there was significant difference between the control and PEF treated samples. A 31.7% increase in juice extraction was recorded compared to the control. The same level of alfalfa mash compression was obtained for all the samples which could be due to the fact that they were all treated to the same point of cell breakdown saturation. Lack of significant differences between the quantities of extracted alfalfa juice and alfalfa mash com-
pressions of the treated samples indicates that the same damage degree can be obtained by using different treatment parameters. However, the amount of energy (electric field and pulse number combinations) needed to achieve this damage degree differs. 4. Conclusion Increase in pulse number (applied PEF energy) resulted in increase in damage degree. However, the rate of increase in damage degree showed a decreasing trend as the pulse number (the applied energy) increased. Exponential models provided a good fit for the applied energy and the damage degree for the different treatment parameters. The maximum rate of change of damage degree was recorded at the highest capacitance of 1.5 lF for all the electric field strengths when the applied energy reached 0.5 kJ. A decrease in the capacitance led to decrease in the rate of change of damage degree while increase in the electric field led to a decrease in energy needed to obtain the maximum damage degree. For an exponential decay PEF generator, a high value of capacitance in the range of lF (microfarads) was needed to achieve maximum juice extraction. It is suggested that fewer number of pulses and higher energy per pulse be used in order to consume the least energy to obtain maximum damage degree. Maximum damage degree obtained by different treatment parameters produced the same compression and quantity of juice extraction. Acknowledgement The authors will like to acknowledge the funding support of Natural Sciences and Engineering Research Council of Canada (NSERC). References Adedeji, A.A., Gachovska, T.K., Ngadi, M.O., Raghavan, G.S.V., 2008. Effect of pretreatments on drying characteristics of okra. Drying Technology 26 (10), 1251–1256. Ade-Omowaye, B.I.O., Rastogi, N.K., Angersbach, A., Knorr, D., 2003. Combined effects of pulsed electric field pre-treatment and partial osmotic dehydration on air drying behaviour of red bell pepper. Journal of Food Engineering 60 (1), 89– 98. Arevalo, P., Ngadi, M., Bazhal, M., Raghavan, G.S.V., 2004. Impact of pulsed electric field on the dehydration and physical properties of apple and potato slices. Drying Technology 25 (5), 1233–1246. Armyanov, N., Stefanova, S., Stoyanova, T., Gachovska, T., 1998. Investigation of the energy consumption of the tobacco leaves feed to the eclectic spark machinery. Electrotechnic and Electronics, Sofia 1, 51–53. Bajgai, T.R., Hashinaga, F., 2001. High electric field drying of Japanese radish. Drying Technology 19 (9), 2291–2302. Bazhal, M.I., Ngadi, M.O., Raghavan, G.S.V., 2004. Modeling compression of cellular systems exposed to combined pressure and pulsed electric field. Transactions of the ASAE 47 (1), 165–171. Bazhal, M., Vorobiev, E., 2000. Electrical treatment of apple cossettes for intensifying juice pressing. Journal of the Science of Food and Agriculture 80 (11), 1668–1674.
T.K. Gachovska et al. / Journal of Food Engineering 95 (2009) 558–563 Bolton, J.L., 1962. Alfalfa. Botany Cultivation and Utilization. Leonard Hill Books Ltd., London. Chaylahyan, L.M., 1962. Electrical structure of live tissue and mechanism of behavior of neural impulse. Biophysica, USSR 7 (5), 639–652. CurveExpert 1.3, 2005. A comprehensive curve fitting package for windows.
563
Lebovka, N.I., Shynkaryk, N.V., Vorobiev, E., 2007. Pulsed electric field enhanced drying of potato tissue. Journal of Food Engineering 78 (2), 606–613. Meleshenko, C., 1965. Question of nature of change of electrical characteristic plant tissue external conditions have been changed. Biophysica, USSR 10 (5), 507– 513. Rastogi, N.K., 2003. Application of high-intensity pulsed electrical fields in food processing. Food Reviews International 19 (3), 229–251. SAS, Institute Inc., 2001. SAS system release 8.2 Cary 27513 USA. Savchuk, V.N., 1971. An investigation of electric spark as a work organ for treatment of sun flowers before harvesting. PhD thesis. Volgograde, USSR. Sinha, S., Sokhansanj, S., Crerar, W.J., Yang, W., Tabil, L.G., Khoshtaghaza, M.H., et al., 2000. Mechanical dewatering of chopped alfalfa using an experimental piston– cylinder assembly. Canadian Agricultural Engineering 42 (3), 153–156. Sinyuhin, A. M., 1967. Polarization coefficient of the cell as index vitality of the tree plants. Electrical treatment of materials. USSR 2. Stout, Darryl G., Hall, John W., McLaughlin, Neil B., 1987. In vivo plant impedance measurements and characterization of membrane electrical properties: the influence of cold acclimation. Cryobiology 24 (2), 148–162. Sturtevant, E.L., 1919. Sturtevant’s Notes on Edible Plants. Agricultural Experiment Station. J.B. Lyon Co, Albany, New York. Svitalka, P.I., 1975. Investigation and chouse of optimum parameters electric spark method of thin out sugar beet. PhD paper, Kiev, USSR. Takyi, E.E.K., Kido, Y., Rikimaru, T., Kennedy, D.O., 1992. Possible use of alfalfa (Medicago sativa) as supplement in infant nutrition: comparison of weight gained by rats fed on alfalfa and a popular weaning diet. Journal of the Science of Food and Agriculture 59 (1), 109–115. Wilner, J., Brach, E.J., 1970. Comparison of radio telemetry with another electric method for testing winter injury of outdoor plants. Canadian Journal of Plant Science 50, 1–8. Zhang, M.I.N., Willison, J.H.M., 1992. Electrical impedance analysis in plant tissue: the effect of freeze-thaw injury on the electrical properties of potato tuber and carrot root tissue. Canadian Journal of Plant Science 72, 545–553.
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Journal of Food Engineering journal homepage: www.elsevier.com/locate/jfoodeng
Influence of pulsed electric field energy on the damage degree in alfalfa tissue Tanya K. Gachovska a, Akinbode A. Adedeji b,*, Michael O. Ngadi b a b
Department of Electrical Engineering, 209N Walter Scott Engineering Center University of Nebraska, Lincoln, NE 68588-0511, United States Department of Bioresource Engineering, McGill University, Macdonald Campus 21,111 Lakeshore Road, Ste-Anne-de-Bellevue, Quebec, Canada H9X 3V9
a r t i c l e
i n f o
Article history: Received 24 December 2008 Received in revised form 31 March 2009 Accepted 10 June 2009 Available online 16 June 2009 Keywords: Pulsed electric field Damage degree Energy Capacitance Extraction Alfalfa juice
a b s t r a c t The objectives of this study were to investigate the influence of pulsed electric field (PEF) parameters on the damage degree of alfalfa mash, and to determine the relationship between the maximum damage degree and the energy used. Alfalfa mash was treated with PEF at various electric field strengths of 1.25, 1.90, and 2.50 kV/cm. The capacitance of the discharge capacitor was varied from 0.5 to 1.5 lF in steps of 0.5 lF. The pulse number was increased gradually to the point where the impedance became constant. There was no significant increase in the rate of damage beyond 0.5 kJ applied energy. The rate of change of the damage degree at 0.5 kJ was highest when the capacitance was 1.5 lF for all the voltages. Increase in the electric field strength led to decrease in energy needed to obtain the maximum damage degree. To achieve an efficient result for alfalfa juice extraction, the capacitance of the discharge capacitor should preferably be 1 lF or more. In order to minimize energy consumption for a given damage degree in alfalfa, it is desirable to have the highest energy per pulse and fewer number of pulses. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Alfalfa is recognized as an energy-efficient agronomic crop and is most widely used. Alfalfa has been noted for its high nutritional qualities, especially in terms of its crude protein content (15–20%), vitamins (A, D, E, K, C, B12, Biotin, Folic acid, Choline, Pyridoxine, Niacin, Panthothanic acid Riboflavin, Thiamin), and different kinds of minerals including calcium, phosphorus, copper, potassium, magnesium, iron, manganese, sulfur, sodium and chlorine, cobalt, zinc, iodine, and selenium (Bolton, 1962; Ensminger and Olentine, 1978; Hanson et al., 1988). Young alfalfa leaves use as vegetable has been reported in China (Sturtevant, 1919). Alfalfa sprouts are consumed directly by humans (Juiceing-for-Health, 2008; Takyi et al., 1992). Alfalfa juice is extracted from the entire alfalfa plant to maximize the nutritional content by reducing the fiber content. Alfalfa juice concentrate is used widely as a supplement in some health food products. Therefore alfalfa product can be considered a nutraceutical or functional food. A lot of studies have been done on alfalfa juice extraction using different pressing methods. However, juice has not been extracted completely (Duran and Nunez–Arenas, 1988; Sinha et al., 2000). Approximately 52% of total alfalfa water content was extracted using pressures greater than 4 MPa (Sinha et al., 2000). Pulsed electric field (PEF) is an innovative non-thermal technology that involves application of quick burst of electrical energy to biological material leading to membrane disruption and perme* Corresponding author. Tel.: +1 514 398 4400x0708; fax: +1 514 398 8387. E-mail address: [email protected] (A.A. Adedeji). 0260-8774/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2009.06.015
ability. This technique has been used to enhance efficient drying of foods such as carrot (Gachovska et al., 2008), okra (Adedeji et al., 2008), potato (Lebovka et al., 2007), red bell pepper (AdeOmowaye et al., 2003) and Japanese radish (Raphanus sativus L.) (Bajgai and Hashinaga, 2001). It has been successfully used as a pretreatment to enhance juice extraction from plants. Gachovska et al. (2006) used PEF to increase juice extraction from alfalfa leaves. It was applied to enhance juice yield from apple, carrot, beet and other fruits and vegetables (Bazhal and Vorobiev, 2000; Eshtiaghi and Knorr, 2002). The principles behind PEF treatment are that high power and small duration of the pulses result in instant penetration into the plant’s anatomy. The cell membranes of a plant tissue treated with PEF disintegrate, leading to increased permeability of the cell walls, consequently, increasing the juice yield (Bazhal et al., 2004). The potential of increase extraction of juice from alfalfa is promising. Plant tissue has been considered as an electrical model (circuit) because PEF treatment changes its electrical properties. Different models have been presented to substitute the plant tissue in an electrical circuit (Chaylahyan, 1962; Hayden et al., 1969; Meleshenko, 1965; Stout et al., 1987; Zhang and Willison, 1992). As a whole, the plant tissue has resistive and capacitive properties. One of the models that make a good fit between an equivalent electric circuit and the morphological structure of plant tissue is shown in Fig. 1 (Chaylahyan, 1962; Meleshenko, 1965). The resistor R1 corresponds to the resistance of the protoplasm, while R2, R3 (non-linear resistors) are the resistances of the inter-cellular fluid and the membrane, respectively. For a live plant tissue, R2 R1 and R3 R1. The values of resistors R2 and R3 depend on the ap-
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559
Nomenclature capacitance of discharge capacitor, lF frequency of the measurement, Hz the distance between the spheres, mm pulse number resistance, ohm (X) damage degree
C f lsph n R Sd
V V0 W X Z0 Z
plied voltage (Chaylahyan, 1962; Meleshenko, 1965). The non linear capacitor C in the circuit presents the capacitance of the membrane. The impedance Z of an intact tissue for this model can be substituted for by series connection of resistance R and reactance X
Z ¼ R þ jX
ð1Þ
where,
R ¼ R2
ðR1 þ R3 ÞðR1 þ R2 þ R3 Þ þ x2 C 2 R23 ðR1 þ R2 ÞR1 ðR1 þ R2 þ R3 Þ2 þ x2 C 2 R23 ðR1 þ R2 Þ2
and
X¼
CR22 R23 2 R3 Þ þ 2 C 2 R23 ðR1
ðR1 þ R2 þ
x
þ R2 Þ2
x = 2pf- angular frequency. When PEF is applied to the plant tissue, the cell membrane breaks and becomes permeable. This leads to the capacitance in the circuit to become zero and to short the resistance of the membrane R3. In this wise, the electrical circuit of the plant tissue becomes a resistive circuit and the impedance will not exceed the resistance of the protoplasm R1, this can be calculated by Eq. (3):
R¼
R1 R2 R1 þ R2
plant tissue. Determination of damage degree of the plants by measuring tissue impedance was reported in other studies (Greenham and Daday, 1957; Wilner and Brach, 1970). The damage degree was also determined as a ratio of the polarization coefficient before and after treatment (Sinyuhin, 1967; Svitalka, 1975). The polarization coefficient in this case is equal to the ratio of the impedance measured for low and high frequency. The method of conductivity as a ratio of impedance before and after treatment was used to determine the damage degree, Sd, was reported by several authors (Armyanov et al., 1998; Klimov, 1970; Savchuk, 1971; Sinyuhin, 1967):
Sd ¼
x
ð2Þ
Obviously, it is evidence that the most important part in the cell structure in its interaction with electrical energy is the cell membrane. It determines the change in the electrical impedance of the plant tissue during PEF treatment. Increase in the permeability of the membrane due to electro-plasmolysis triggers the liquid to pass from the cell to the inter-cellular spaces. This leads to a decrease in the resistance R2 and change in the impedance of the
Fig. 1. Equivalent substitute circuit of plant tissue: R1 – the resistance of the protoplasm; R2 (non-linear resistor) – the resistance of the inter-cellular fluid; R3 (non-linear resistor) the resistance of the membrane; C (non-linear) capacitor – the capacitance of the membrane.
treatment voltage, V break voltage, V energy per pulse, kJ reactance, ohm (X) magnitude of impedance before PEF treatment, ohm (X) magnitude of impedance after PEF treatment, ohm (X)
Z0 Z
ð3Þ
The damage degree of plant tissue (electro-plasmolysis) depends on many factors such as: products components, intensity of the electric field, type of pulse waveform, treatment time; and number of pulses (Rastogi, 2003). In this study, alfalfa mash was treated with exponential decay pulses. The sum of applied energy, W P , was calculated using Eq. (4):
W P ¼ Wn ¼ 0:5CV 2 n
ð4Þ
The objectives of this study were to investigate the influence of pulsed electric field (PEF) parameters on the damage degree of alfalfa mash and amount of juice extracted, and also to determine the relationship between the maximum damage degree and spent energy.
2. Materials and methods Approximately 20 cm long fresh alfalfa stems with leaves at pre-blossoming stage were obtained from the farm of Macdonald Campus of McGill University in the Fall of 2005. The stems were sorted, cleaned and ground with a domestic food processor (Oster, Sunbeam Products Inc., China) for 5 min to obtain a homogeneous mash. The mash was kept in a closed vessel to prevent evaporation prior to use. The mash had an initial moisture content of 81.1% (g/g, wet basis). The conductivity of the mash was measured as 3.5 mS cm. The mash was mixed properly before conducting the experiment. Approximately 40 g of the alfalfa mash was introduced into the treatment chamber (batch system) which was used for high voltage treatment, impedance measurement and juice extraction. The chamber (4.2 cm in diameter and 8.0 cm height) and the juice collector are shown in Fig. 2. The treatment chamber consisted of an insulated cylinder (C) made of plastic (Polyoxymethylene, Derlin). A cylindrical plunger (E) (ground) and a disc base (B) (high voltage) were used as electrodes during PEF treatment and impedance measurement. They formed a rigid structure during pressing. The perforated cylindrical plunger and the base were made of stainless steel. Extracted juice was filtered through a stainless steel sieve (D) placed on top of the perforated plunger. Juice extracted during pressing was collected in a plastic collector (G) placed under the treatment chamber. The volume of the treatment chamber was 110 cm3.
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The samples were treated at the electric field strengths of 1.25 1.9 and 2.5 kV/cm with capacitance of discharge capacitor of 0.5, 1 and 1.5 lF. The voltage was registered with a high voltage probe (P6015A, Tektronix, Beaverton, USA) which was connected to a digital oscilloscope (54621A, Agilent, Malaysia). The pulse number was increased gradually to the point where the impedance of the alfalfa mash became constant.
D
B
C
A
F
E
2.2. Determination of the damage degree
H
I K L
Fig. 2. Schematic diagram of pulsed electric field and pressing treatment chamber. Labels: (A) stainless steel disc base (upper electrode), (B) electrical connection outlet on disc electrode, (C) rubber gasket; (D) plastic cylinder, (E) sieve, (F) stainless steel perforated cylindrical plunger (lower electrode), (G) electrical connection outlet on cylindrical electrode, (H) plastic collector; (I) stainless steel connector; (K) plastic tube; (L) plastic container.
2.1. High pulsed electric field treatment Pulsed electrical field treatment of the alfalfa mash was achieved using a pulsed electric field generator shown in Fig. 3. A variable autotransformer (Powerstat Type 3PN116C 0–140 V, Superior Electric Co., Bristol, CT) was used to supply voltage to the circuit and to regulate the pulse frequency. The pulse frequency was 1 Hz (Eshtiaghi and Knorr, 2002). A high voltage transformer (Model 62159A, Apotex Imaging Inc., Canada) transformed the voltage from the autotransformer. The voltage was rectified by a high voltage diode. Resistor was used to limit initial charge current that passed through the capacitor. The capacitor was used to store energy that quickly discharges through the sample after the discharger breaks. The initial treatment voltage applied to the treatment chamber depended on the distance between the 15 mm diameter stainless steel sphere discharger. The break voltage V0, applied to the treatment chamber for this diameter of the sphere can be calculated by the following Eq. (1) (Armyanov et al., 1998): 0:75
V 0 ¼ 4:85lsph
ð5Þ
The damage degree of a plant tissue is an important index that shows the effectiveness of PEF treatment as it demonstrates the level of electro-plasmolysis in the plant. When an exponential decay PEF generator is used, the damage degree depends on the basic treatment parameters: electric field strength, capacitance of discharge capacitor and pulse number (Lebovka et al., 2002; Armyanov et al., 1998). Electrical impedance of the alfalfa mash was measured to determine the damage degree of the alfalfa plant tissue due to PEF treatment. The impedance was measured at a frequency of 1 kHz, by connecting the disk base (B) and the cylindrical plunger (E) to the LCR meter (LCR-745, Leader Electronics Corp., Japan) as shown in Fig. 2. The measurements were carried out before and after PEF treatment of alfalfa mash. The damage degree was determined using Eq. (3). The measurements after PEF treatments were recorded for every increase in the pulse number until constant impedance was achieved. Temperature of the system remained practically the same at ambient condition before and after the treatment. 2.3. Alfalfa pressing The pressing process was a batch operation. Pressure was applied with the aid of a Universal Testing Machine (Model 4502, Instron, Norwood, MA and Series IX software) through a plunger head of 4.15 cm diameter at a displacement of 10 mm/min to 40 g of alfalfa mash poured into a metallic cylinder (4.25 cm diameter and 5.5 cm height) with perforated base connected to a drain (Fig. 2). All samples, both treated and untreated were pressed at 4 MPa for 2 min and the alfalfa juice extract was collected and weighed. All the experiments were conducted in triplicate. Nonlinear regression of the data was performed on CurveExpert 1.3 (2005). Statistical analysis was carried out using SAS version 8.2. (2001).
2.2
2
Damage degree
G
1.8
1.6
0.5 µF 1 µF
1.4
1.5 µF 1.2
1 0.0
1.0
2.0
3.0
4.0
5.0
6.0
Energy, kJ Fig. 3. Electrical circuit diagram for the pulsed electric field generator used to treat alfalfa mash.
Fig. 4. Influence of the total treatment energy on the damage degree for electric field 1.25 kV/cm.
561
2.2
6.50
2
6.00
1.8
5.50
Energy, kJ
Damage degree
T.K. Gachovska et al. / Journal of Food Engineering 95 (2009) 558–563
0.5 µF
1.6
1 µF 1.5 µF
1.4
5.00
0.5 µF 4.50
1 µF 1.5 µF
1.2
1 0.0
4.00
1.0
2.0
3.0
4.0
5.0
3.50 1.25
6.0
1.50
Energy, k J
1.75
2.00
2.25
2.50
Electric field, kV/cm
Fig. 5. Influence of the total treatment energy on the damage degree of alfalfa mash in an electric field of 1.9 kV/cm.
Fig. 7. Influence of electric field on the total treatment energy for the maximum damage degree.
2.2
Damage degree
2
1.8
0.5 µF
1.6
1 µF 1.5 µF
1.4
1.2
1 0.0
1.0
2.0
3.0
4.0
5.0
6.0
Energy, kJ Fig. 6. Influence of the total treatment energy on the damage degree for electric field 2.5 kV/cm.
3. Results and discussions For a practical application PEF to enhance juice extraction, it is important to know the basic parameters of PEF treatment including – electric field strength, capacitance of the discharge capacitor and pulse number, that will result in the maximum damage degree using minimum energy. The influences of applied energy on the calculated values of damage degree, using Eq. (4) for three different capacitances (0.5, 1 and 1.5 lF) of the discharge capacitor, are presented in Figs. 4–6. From the obtained data, it can be concluded that increase in applied energy was as a result of the increase in the pulse number, which subsequently led to the increase in the
damage degree. However, the rate at which damage degree increases diminished as the applied energy (pulse number) increased. For example at 2.5 kV/cm and 2.5 lF, the rate of energy increase was 61.3% from 0 to 1 kJ, while it was 2.61% when the energy increased from 3 to 4 kJ. There was a decrease observed in the impedance as the energy applied to the system increased. The decrease in impedance of the plant tissue could be explained by cell membrane rupture which resulted in diminished capacitance of the cell. Continuous increase in the pulse number led to further cell damage up until the impedance became constant indicating complete saturation of cells breakdown. Similar results of saturation of damage degree were reported by Armyanov et al. (1998), and Eshtiaghi and Knorr (2002). The pulse threshold that causes this extent of damage degree depends on the PEF parameters and the plant tissue (Arevalo et al., 2004). There was no substantial change in the rate of damage degree beyond the point when applied energy was 0.5 kJ and this was used as the set point to model the data of applied energy and damage degree for all the PEF treatments (Table 1). The relative stability in the rate of change of damage degree after the 0.5 kJ mark could be as a result of substantial saturation of cell membrane breakdown after certain number of pulses have been applied. This was reported by Lebovka et al. (2002) and Eshtiaghi and Knorr (2002). The data were fitted to a couple of models and the exponential model presented in Eq. (6) gave the best fit for the data of applied energy and damage degree shown in Figs. 4–6, as it gave the least standard error and the highest correlation coefficient, R (Table 1). The analysis was carried out on CurveExpert 1.3 (2005). The applied energy = x, kJ and the damage degree = y:
Y ¼ aðb ecx Þ
ð6Þ
Table 1 Calculated values for regression coefficients of the exponential model for the damage degree at 0.5 kJ applied energy. Electric field, kV/cm
Capacitance, lF
a
b
c
Standard error
Correlation coefficient, R
Rate of energy change at 0.5 kJ, %
2.50
1.5 1.0 0.5 1.5 1.0 0.5 1.5 1.0 0.5
1.09 1.08 1.05 1.06 1.03 1.05 1.01 1.05 1.34
1.92 1.94 1.97 1.94 1.98 1.96 2.01 1.98 1.75
0.82 0.79 0.61 0.73 0.71 0.53 0.66 0.48 0.29
0.018 0.047 0.073 0.024 0.058 0.032 0.053 0.046 0.026
0.997 0.994 0.987 0.996 0.991 0.996 0.996 0.993 0.996
54.3 49.7 44.9 50.7 49.7 40.7 47.2 37.9 22.4
1.90
1.25
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Table 2 The effect of PEF on alfalfa juice extraction and mash compression for a damage degree (max) of 2.02. Capacitance, lF 0 1.50
1.00
0.50
Electric field, kV/cm 0 2.50 1.90 1.25 2.50 1.90 1.25 2.50 1.90 1.25
Amount of juice extracted per 40 g of Alfalfa, g a
10.10 13.70b 13.92b 13.40b 13.88b 13.69b 13.45b 13.20b 13.47b 13.33b
Max. PEF energy applied, kJ
Alfalfa mash compression, mm
0 3.90a 5.27b 5.81c 4.05a 5.63bc 6.00c 5.00b 5.91c 6.25c
55.34a 51.49b 51.30b 51.62b 51.49b 51.30b 51.62b 51.95b 51.64b 51.66b
Mean with the letter on the same column are not significantly different at P < 0.05. Max-maximum.
For untreated sample, x = 0, and the damage degree Y(0) = a(b1) = 1 because impedance remained the same i.e. Z0/Z = 1 for intact/untreated. The model parameters a, b and c for all treated variables are shown in Table 1. Standard error and correlation coefficient ranged between 0.018–0.073 and 0.987–0.997, respectively. Initially, the damage degree of the plant tissue depended on the energy discharged by the capacitor to the plant tissue. With increase in pulse number, the total applied energy increased and the damage degree also increased. The rate of change of the damage degree then decreased exponentially until it becomes zero. The complete saturation of damage degree was significantly affected by the electric field strength as shown in Figs. 4–6. For 1.25 kV/cm, the final saturation point was at about 6 kJ (Fig. 4), for 1.9 kV/cm, the damage degree complete saturation was attained at about 5 kJ, while for 2.5 kV/cm, the saturation point was about 4.5 kJ. This implies the higher the electric field strength, the quicker the final saturation point for cell damage is attained. Fig. 7 further shows the dependency of damage degree saturation on electric field strength at three different capacitances (0.5, 1 and 1.5 lF). The results of the experiment indicate that an increase in the electric field strength and the capacitance of the system would decrease the total applied energy necessary to achieve maximum damage degree. The minimum applied energy was obtained when alfalfa mash was treated at the highest electrical field strength and capacitance (2.5 kV/cm and 1.5 lF). To have an efficient result for juice extraction using a decay PEF generator, a high value of capacitance, in the range of microfarad, is necessary. Eshtiaghi and Knorr (2002) reported using a bank of capacitors with value of 5 lF for the pretreatment of sugar beet for efficient extraction. The difference between the total applied energy (to obtain maximum damage degree for the lowest electric field strength – 1.25 kV/cm) and capacitance of 0.5 and 1.5 lF was only 7.5%. Meanwhile, for the highest electric field strength 2.5 kV/cm, this difference was 28.2%. This implies that application of the smallest energy obtained the maximum damage degree. Therefore, the highest voltage per cm which corresponds to few pulses should be used for maximum cell permeabilization. Treatment with high pulse energy (high electric field strength) usually forms a detonation wave that expends quickly into plant tissue causing the damage to the cell membrane leading to the expression of juice from the mash. There was no significant variation in the quantities of extracted juice from the alfalfa mash after the attainment of maximum damage degree and a compression force of 4 MPa had been applied for 2 min (Table 2). However, there was significant difference between the control and PEF treated samples. A 31.7% increase in juice extraction was recorded compared to the control. The same level of alfalfa mash compression was obtained for all the samples which could be due to the fact that they were all treated to the same point of cell breakdown saturation. Lack of significant differences between the quantities of extracted alfalfa juice and alfalfa mash com-
pressions of the treated samples indicates that the same damage degree can be obtained by using different treatment parameters. However, the amount of energy (electric field and pulse number combinations) needed to achieve this damage degree differs. 4. Conclusion Increase in pulse number (applied PEF energy) resulted in increase in damage degree. However, the rate of increase in damage degree showed a decreasing trend as the pulse number (the applied energy) increased. Exponential models provided a good fit for the applied energy and the damage degree for the different treatment parameters. The maximum rate of change of damage degree was recorded at the highest capacitance of 1.5 lF for all the electric field strengths when the applied energy reached 0.5 kJ. A decrease in the capacitance led to decrease in the rate of change of damage degree while increase in the electric field led to a decrease in energy needed to obtain the maximum damage degree. For an exponential decay PEF generator, a high value of capacitance in the range of lF (microfarads) was needed to achieve maximum juice extraction. It is suggested that fewer number of pulses and higher energy per pulse be used in order to consume the least energy to obtain maximum damage degree. Maximum damage degree obtained by different treatment parameters produced the same compression and quantity of juice extraction. Acknowledgement The authors will like to acknowledge the funding support of Natural Sciences and Engineering Research Council of Canada (NSERC). References Adedeji, A.A., Gachovska, T.K., Ngadi, M.O., Raghavan, G.S.V., 2008. Effect of pretreatments on drying characteristics of okra. Drying Technology 26 (10), 1251–1256. Ade-Omowaye, B.I.O., Rastogi, N.K., Angersbach, A., Knorr, D., 2003. Combined effects of pulsed electric field pre-treatment and partial osmotic dehydration on air drying behaviour of red bell pepper. Journal of Food Engineering 60 (1), 89– 98. Arevalo, P., Ngadi, M., Bazhal, M., Raghavan, G.S.V., 2004. Impact of pulsed electric field on the dehydration and physical properties of apple and potato slices. Drying Technology 25 (5), 1233–1246. Armyanov, N., Stefanova, S., Stoyanova, T., Gachovska, T., 1998. Investigation of the energy consumption of the tobacco leaves feed to the eclectic spark machinery. Electrotechnic and Electronics, Sofia 1, 51–53. Bajgai, T.R., Hashinaga, F., 2001. High electric field drying of Japanese radish. Drying Technology 19 (9), 2291–2302. Bazhal, M.I., Ngadi, M.O., Raghavan, G.S.V., 2004. Modeling compression of cellular systems exposed to combined pressure and pulsed electric field. Transactions of the ASAE 47 (1), 165–171. Bazhal, M., Vorobiev, E., 2000. Electrical treatment of apple cossettes for intensifying juice pressing. Journal of the Science of Food and Agriculture 80 (11), 1668–1674.
T.K. Gachovska et al. / Journal of Food Engineering 95 (2009) 558–563 Bolton, J.L., 1962. Alfalfa. Botany Cultivation and Utilization. Leonard Hill Books Ltd., London. Chaylahyan, L.M., 1962. Electrical structure of live tissue and mechanism of behavior of neural impulse. Biophysica, USSR 7 (5), 639–652. CurveExpert 1.3, 2005. A comprehensive curve fitting package for windows.
563
Lebovka, N.I., Shynkaryk, N.V., Vorobiev, E., 2007. Pulsed electric field enhanced drying of potato tissue. Journal of Food Engineering 78 (2), 606–613. Meleshenko, C., 1965. Question of nature of change of electrical characteristic plant tissue external conditions have been changed. Biophysica, USSR 10 (5), 507– 513. Rastogi, N.K., 2003. Application of high-intensity pulsed electrical fields in food processing. Food Reviews International 19 (3), 229–251. SAS, Institute Inc., 2001. SAS system release 8.2 Cary 27513 USA. Savchuk, V.N., 1971. An investigation of electric spark as a work organ for treatment of sun flowers before harvesting. PhD thesis. Volgograde, USSR. Sinha, S., Sokhansanj, S., Crerar, W.J., Yang, W., Tabil, L.G., Khoshtaghaza, M.H., et al., 2000. Mechanical dewatering of chopped alfalfa using an experimental piston– cylinder assembly. Canadian Agricultural Engineering 42 (3), 153–156. Sinyuhin, A. M., 1967. Polarization coefficient of the cell as index vitality of the tree plants. Electrical treatment of materials. USSR 2. Stout, Darryl G., Hall, John W., McLaughlin, Neil B., 1987. In vivo plant impedance measurements and characterization of membrane electrical properties: the influence of cold acclimation. Cryobiology 24 (2), 148–162. Sturtevant, E.L., 1919. Sturtevant’s Notes on Edible Plants. Agricultural Experiment Station. J.B. Lyon Co, Albany, New York. Svitalka, P.I., 1975. Investigation and chouse of optimum parameters electric spark method of thin out sugar beet. PhD paper, Kiev, USSR. Takyi, E.E.K., Kido, Y., Rikimaru, T., Kennedy, D.O., 1992. Possible use of alfalfa (Medicago sativa) as supplement in infant nutrition: comparison of weight gained by rats fed on alfalfa and a popular weaning diet. Journal of the Science of Food and Agriculture 59 (1), 109–115. Wilner, J., Brach, E.J., 1970. Comparison of radio telemetry with another electric method for testing winter injury of outdoor plants. Canadian Journal of Plant Science 50, 1–8. Zhang, M.I.N., Willison, J.H.M., 1992. Electrical impedance analysis in plant tissue: the effect of freeze-thaw injury on the electrical properties of potato tuber and carrot root tissue. Canadian Journal of Plant Science 72, 545–553.