029_HANDB OF FRUITS_AND PROSESSING_2006_688

92 Part I: Processing Technology

(in Hungarian) Budapest, Hungary, Mez´o´gazdasa´gi Imre, L. 1974. Sza´r´ıta´si ke´ziko¨nyv. (Handbook of
Kiado´, 231–241. Drying.) (in Hungarian) Budapest, Hungary,
Choi, Y. and Okos, M. R. 1986. Effects of temperature M´u´szaki Ko¨nyvkiado´, 39–59.
and composition on the thermal properties of foods.
In: Food Engineering and Process Applications Jowitt, R., et al. 1983. Physical properties of foods.
Vol. 1: Transport Phenomena, Eds. Le Maguer, M. London, Applied Science Publishers, 50–127.
and Jelen, P., England, Elsevier Applied Science
Publishers Ltd., 93–101. Kilpatrick, P. W., Lowe, E. and Van Arsdel, W. B.
Chung, D. S. and Pfost, H. B. 1967. Adsorption and 1955. Tunnel dehydrators for fruit and vegetables.
desorption of water vapor by cereal grains and their Adv. Food Res. 50:385.
products. Trans. A.S.A.E. 10:549.
Constenla, D. T., Lozano, J. E. and Crapiste, G. H. Ko¨rmendy, I. 1985. Heat-and material transports in
1989. Thermophysical properties of clarified apple canning technological procedures II. E´lelmeze´si
juice as a function of concentration and temperature. Ipar. 39(12):463–470.
J. Food Sci. 54(3):663–668.
Crank, J. 1967. The Mathematics of Diffusion. Lazar, M. E. and Farkas, D. F. 1971. The centrifugal
London, Oxford University Press, 55–123. fluidized bed. 2. Drying studies on pieces-form
Crapiste, G. H. and Rotstein, E. 1986. Sorptional foods. J. Food Sci. 36:315.
equilibrium at changing moisture contents. In:
Drying of Solids, Ed. Mujumdar, A. S., Wiley Lewis, M. J. 1987. Physical Properties of Foods and
Eastern Ltd., New Delhi, India, 41–47. Food Processing Systems. Chichester, England,
Ginzburg, A. Sz. 1968. Sza´r´ıta´s az e´lelmiszeriparban. Ellis Horwood, 210–295.
(Drying in Food Industry) Budapest, Hungary,
M´u´szaki Ko¨nyvkiado´, 39–47. Lozano, J. E., Rotstein, E. and Urbicain, M. J. 1983.
Ginzburg, A. Sz. 1969. Application of Infrared Shrinkage, porosity and bulk density of foodstuffs
Radiation in Food Processing. Chemical and at changing moisture contents. J. Food Sci. 51:113.
Process Engineering Series, London, Leonard Hill.
Ginzburg, A. Sz. 1976. E´ lelmiszerek Lozano, J. E., Urbicain, M. J. and Rotstein, E.
sza´r´ıta´selme´lete´nek e´s technika´ja´nak alapjai. 1979. Thermal conductivity of apples as a
(Principles of Drying Theory and Techniques of function of moisture content. J. Food Sci.
Foods.) (in Hungarian) Budapest, Hungary, 44(1):198–199.
Mez´o´gazdasa´gi Kiado´, 28–51.
Gion, B. 1986. Simulation of food drying. (in Maroulis, Z. B., Tsami, E., Marinos-Kouris, D. and
Hungarian) E´lelmeze´si Ipar. 40(3):110–125. Saravacos, G. D. 1988. Application of the GAB
Gion, B. 1988. Simulation of drying technological model to the moisture sorption isotherms for dried
processes on IBM-AT computer for potato and fruits. J. Food Eng. 7:63–78.
Jerusalem artichoke roots. (in Hungarian)
E´lelmeze´si Ipar. 42(6):220–224. Mazza, G. 1984. Sorption isotherms and drying
Gion, B. and Barta, J. 1998. Processing of dried cubes rates of Jerusalem Artichoke. J. Food Sci. 49:
and flour from Jerusalem Artichoke. J. Food Phys. 384.
9:15–22.
Guggenheim, E. A. 1966. Applications of Statistical Menon, A. S. and Mujumdar, A. S. 1987. Drying of
Mechanics. Oxford, Clarendon Press, 186–206. solids: Principles, classification, and selection of
Halsey, G. 1948. Physical adsorption on non-uniform dryers. In: Handbook of Industrial Drying, Ed.
surfaces. J. Chem. Phys. 16:931. Mujumdar, A. S., First edition, New York, NY,
Heldman, D. R. 1975. Food Process Engineering. Marcel Dekker Inc., 3–46.
Westport, CT, The Avi Publishing Company Inc.,
96–103. Mohr, K. 1984. Oualita¨tserhalt der Frocknung durch
Henderson, S. M. 1952. A basic concept of Computersimulation. Lebensmittelindustrie
equilibrium moisture. Agr. Eng. 33:29. 34(4):150–151.
Iglesias, H. A. and Chirife, J. 1976. Prediction of the
effect of temperature on water sorption isotherms of Mohsenin, N. N. 1984. Electromagnetic Radiation
food materials. J. Food Technol. 11:109. Properties of Foods and Agricultural Products. New
York, NY, Gordon and Breach Science Publishers,
25–30.

Nelson, S. O. 1983. Dielectric properties of some fresh
fruits and vegetables at frequencies of 2.45 to
22 GHz. Trans. A.S.A.E. 26:613.

Pfost, H. B., Mauer, S. G., Chung, D. S. and Milliken,
G. A. 1976. Summarizing and reporting equilibrium
moisture data for grains. A.S.A.E. Paper 76–3520.
1976. A.S.A.E Meeting, St. Joseph, MI.

Ratti, C. 1991. Design of dryers for vegetable and fruit
products. Ph.D. Thesis. Universidad Nacional del
Sur, (in Spanish) Bahia Blanca, Argentina.

5 Fruit Drying Principles 93

Ratti, C., Crapiste, G. H. and Rotstein, E. 1989. A new Szabo´, Z. 1987. Drying. In: E´ lelmiszeripari m´u´veletek
water sorption equilibrium expression for solids e´s ge´pek. (Procedures and Machines of Food
foods based on thermodynamics considerations. industry.) (in Hungarian), Eds. Szabo´, Z., Csury, I.
J. Food Sci. 54(3):738–742. and Hidegkuti, Gy. Budapest, Hungary,
Mez´o´gazdasa´gi Kiado´, 491–556.
Ratti, C. and Mujumdar, A. S. 1995. Infrared drying.
In: Handbook of Industrial Drying, Ed. Mujumdar, Thompson, T. L. 1972. Temporary storage of
A. S., Second edition, New York, NY, Marcel high-moisture shelled corn using continuous
Dekker Inc. aeration. Trans. A.S.A.E. 15:333.

Sandu, C. 1986. Infrared radiative drying in food Van Arsdel, W. B., Copley, M. J. and Morgan, A. I.
engineering: A process analysis. Biotechnol. Prog. 1973. Food Dehydration. Second edition, Westport,
2(3):109–119. CT, Avi Publishing Co., 50–139.

Schiffman, R. F. 1987. Microwave and dielectric Wolf, W. and Jung, G. 1985.
drying. In: Handbook of Industrial Drying, Ed. Wasserdampfsorptionsdaten fu¨r die
Mujumdar, A. S., First edition, New York NY, Lebensmitteltrocknung. Zeitschrift fu¨r
Marcel Dekker Inc., 327–356. Lebensmitteltechnologie 36(2):36–38.

Shatadal, P. and Jayas, D. S. 1992. Sorption isotherms Wolf, W., Spiess, W. E. L. and Jung, G. 1985. Sorption
of foods, In: Drying of Solids, Ed. Mujumdar, A. S., Isotherms and Water Activity of Food Materials.
New York, NY, International Science Publisher., New York, NY, Elsevier, 1–5.
433–448.
GENERAL REFERENCES
Singh, R. P. 1992. Heating and cooling processes for
foods. In: Handbook of Food Engineering, Eds. Kudra, T. 2001. Advanced Drying Technologies.
Heldman, D. R. and Lund, D. B., New York, NY, Marcel Dekker, New York, 265–303.
Marcel Dekker Inc., 247–255.
Mujumdar, A. S. 2000. Drying Technology in
Singh, R. P. and Mannapperuma, J. D. 1990. Agriculture and Food Sciences. Science Pub. Inc.,
Developments in food freezing. In: Biotechnology New Hampshire, USA, 313 pp.
and Food Process Engineering, Eds. Schwartzberg,
H. G. and Rao, M. A., New York, NY, Marcel Mujumdar, A. S. 2004 Dehydration of Products of
Dekker Inc., 309–329. Biological Origin. Science Publishers, Inc., New
Hampshire, USA, 541 pp.
Suzuki, K., Kubota, K., Hasegawa, T. and Hosaka, H.
1976. Shrinkage in dehydration of root vegetables. Oetjen, G. W. 2004. Freeze-Drying. John Wiley &
J. Food Sci. 41:1189. Sons, Weinheim, Germany, 407 pp.

Handbook of Fruits and Fruit Processing
Edited by Y. H. Hui

Copyright © 2006 by Blackwell Publishing

6
Non-Thermal Pasteurization of Fruit

Juice Using High Voltage Pulsed
Electric Fields

Zsuzsanna Cserhalmi

Introduction are drastically decreased by irreversible disruption of
Mechanisms of Inactivation by PEF cell membranes.
PEF System
Effect of PEF on Food Preservation MECHANISMS OF INACTIVATION
BY PEF
Effect of PEF on Microorganisms
Effect of PEF on Enzymes The mechanism of inactivation of microorganisms
Effect of PEF on Food Quality exposed to PEFs has not been fully clarified yet.
Application The most commonly accepted theory is based on
Future of PEF the dielectric breakdown of cell membranes result-
References ing in changes in membrane structure and perme-
ability which occur at a critical breakdown voltage
INTRODUCTION (Kinosita and Tsong, 1977a, b; Zimmermann et al.,
1974; Coster and Zimmermann, 1975; Castro et al.,
The pulsed electric field (PEF) process is a new and 1993). It is suggested that an external electric field in-
innovative non-thermal minimal processing technol- duces a transmembrane potential over the cell mem-
ogy that is used as an alternative preservation pro- brane that is larger than the normal potential of the
cess for fruit juices. The aim of this technology is to cell. When the overall membrane potential reaches
inactivate microorganisms and to decrease the activ- a critical value, rupture takes place. According to
ity of enzymes in order to increase the shelf life of Zimmermann (1986), this value is around 0.7–1.1
food products without undesirable heat and chemical V for most cell membranes. As shown in Figure
effects. 6.1, the cell membrane can be regarded as a capac-
itor filled with dielectric materials with a low di-
The theoretical basis of PEF technology is the use electric constant. Accordingly, free charges can be
of an external electric field to destabilize cell mem- accumulated on both sides of membrane surfaces
branes and form one or more pores in them. PEF tech- (Fig. 6.1a). These charges occur by the application
nology applies high voltage pulses (generally 20– of an external electric field and increase the poten-
80 kV/cm) for very short time (␮s to ms), producing tial difference across the membrane, known as the
PEFs between two electrodes. This technique is very transmembrane potential. The accumulated charges
similar to electroporation, used in cell biology and ge- on two sides of the membrane are opposite and at-
netic manipulation of cells. But, in the case of foods, tract each other resulting in membrane compression,
the applied pulses are shorter and much more intense. which leads to reduction of membrane thickness (Fig.
The aim of the application of high voltage pulses to 6.1b). An elastic or viscoelastic restoring force stands
foods is not only to disrupt temporarily the cell mem- opposite to the increased compression. Since electric
branes of microorganisms. However, in this process,
the microorganisms are also killed or their numbers

95

96 Part I: Processing Technology
E
V’m V > V’m

cytoplasm medium

a b cd

Figure 6.1. Schematic diagram of dielectric breakdown of cell membrane (according to Zimmermann, 1986).
(a) cell membrane with potential V’m, (b) membrane compression, (c) reversible pore formation, and (d) irreversible
pore formation.

compression increases more rapidly with decreasing where F is the shape factor, r is the cell radius (␮m),
membrane thickness than the elastic restoring com- and E is the electric field strength (V/␮m). With the
pression forces, the cell membrane ruptures (Fig. same magnitude of external electric field, the induced
6.1c). It occurs if the critical breakdown voltage is transmembrane potential is greater in a larger cell. By
reached by a further increase in the external field this means, the larger cells are more sensitive to PEF
strength (Zimmermann, 1986). than smaller ones. The shape factor is 1.5 for spheri-
cal cells. For non-spherical cells, Zimmermann et al.
The change in membrane permeability can be re- (1974) derived a mathematical equation. The equa-
versible or irreversible, depending on the external tion is based on the assumption that the cell shape
electric field strength. When it is equal to or only consists of a cylinder with two hemispheres at each
slightly exceeds the critical value, the increase in end. In this case, the shape factor is
membrane permeability is reversible and the change
in cell membrane can be recovered within a few sec- F = L(1 − 0.33d),
onds after the treatment. In the food industry, when
the aim of PEF application is to kill microorganisms where L is the length of cylinder and d is the diameter.
in order to extend the shelf life of food products, The shape factor (F) and the critical field strength
the magnitude of the electric field strength has to be
carefully taken into consideration. for four microorganisms are presented in Table 6.1
(Aronsson, 2002).
The PEF-induced transmembrane potential (Vm)
depends on the electric field strength and on the cell The electrical breakdown of biological cell mem-
size (Hu¨lsheger et al., 1983): branes can be explained by mechanisms other than
dielectric breakdown. These were comprehensively
Vm = FrE, summarized, e.g., by Barbosa-Ca´novas et al. (1998a).

Table 6.1. Shape Factor and Critical Electric Field Strength of Microorganisms

Shape Factor Critical Electric
Field Strength (kV/cm)

Microorganism Minimum Maximum Minimum Maximum

Escherichia coli 1.09 1.23 12.2 14.8

Listeria innocua 1.09 1.37 36.6 36.7

Leuconostoc mesenteroides 1.24 1.31 23.0 30.4

Saccharomyces cerevisiae 1.07 1.31 2.2 18.7

6 Non-Thermal Pasteurization of Fruit Juice Using High Voltage PEF 97

According to these authors, the breakdown can occur that are submerged in a refrigerated water bath, set or
for the following possible reasons: (1) threshold controlled at the treatment temperature. The pre- and
transmembrane potential and compression of cell post-treatment temperatures in each pair of chambers
membranes, (2) viscoelastic properties of cell mem- are monitored by thermocouples attached to the exit
branes, (3) fluid mosaic arrangement of lipids and of the chamber pair. The system is able to generate
proteins in cell membranes, (4) structural defects in pulse durations up to 10 ␮s. However, a minimum
cell membranes, and (5) colloid osmotic swelling. pulse duration of 2 ␮s is recommended to obtain a
The basis of these changes was explained by those good waveform. The electric field strength (E), one
who studied the electrical breakdown of biological of the most important factors influencing microbial
membranes based on model systems such as lipo- inactivation, is calculated by
somes, planar bilayers, and phospholipid vesicles.
E = U/d,
PEF SYSTEM
where U is the peak voltage of the applied voltage (V)
A PEF processing system consists of five main com- and d is the distance between the electrodes (cm). The
ponents such as a power source, high voltage genera- maximum output voltage and current are 10 kV and
tor, capacitor bank, switch, and a treatment chamber 50 A, respectively. The maximum pulse frequency
to which the voltage, current, and temperature con- of the system is 5000 Hz, but too high a frequency
trolling, sample handling, and packaging systems can causes unnecessary heating of the sample. Therefore,
be connected (Fig. 6.2). The power source charges the its usage must be avoided. The total treatment time
capacitor bank and a switch is used to discharge the (t) can be calculated from
energy from the capacitor bank across the food that
is held in a treatment chamber. number of pulses received in each chamber (np),
number of chambers (n),
A laboratory scale continuous flow PEF system pulse duration (␶ ) [t = npn␶ ].
(OSU-4B) designed and constructed at the Ohio
State University in the United States is presented in One of the most important parts of a PEF system is
Figure 6.3. This equipment is able to generate both the treatment chamber that consists of two carbon or
bipolar and unipolar square wave pulses. A syringe metal electrodes. Stainless steel is generally applied
pump system consisting of four syringes (syringe A but other metals may be preferable to reduce electro-
for starter, B for the sample, C for the treated sample, chemical attack (Barsotti et al., 1999). Parallel plates,
and D for waste) is integrated with the system and parallel wires, concentric cylinders, and a rod-plate
run, using a total of 60 ml of sample. The sample are the possible electrode configurations discussed
is pumped through six PEF treatment chambers con- by Hofmann (1989). Parallel plates are the simplest
nected in series containing stainless steel electrodes in design and produce the most uniform distribution
with a gap of 0.29 cm (Fig. 6.4). The flow rate of of electric field (Jeyamkondan et al., 1999). Numer-
the system is up to 2 ml/s. After treated at each pair ous types of static and continuous flow treatment
of chambers, the sample is cooled via attached coils chambers are known which are named after the de-
signers. Examples include Sale and Hamilton (1967),

Power High Treatment Sample
source voltage chamber handling
generator
system

Pulse Temperature Packaging
controller control system

unit Figure 6.2. PEF system.

98 Part I: Processing Technology

AB

C D
Aseptic Bags- Outlet Valve
or bottles

Inlet Valve

Fuid Handling Module

PDP PRV

Flow Pressure Pulse Generator Module
PEF Chamber Module

V

Current

Trigger

Pulse Controller
f, dt, τ

Figure 6.3. OSU-4B PEF system. Temperature Control unit

Food inlet Outlet

High
voltage
pulse
generator

Electrodes Insulation

Figure 6.4. Schematic configuration of a continuous PEF treatment chamber.

6 Non-Thermal Pasteurization of Fruit Juice Using High Voltage PEF 99

Charging Discharge switch
resistor

Rs Energy Treatment
storage chamber
Power capacitor
supply
C

Figure 6.5. Simplified circuit for exponential decay pulse generation.

Dunn and Pearlman (1987), Grahl et al. (1992) static Voltage
treatment chamber, and Dunn and Pearlman (1987)
continuous flow treatment chamber. For a compre- Time
hensive summary of the different chambers, see
Zhang et al. (1995) and Barbosa-Ca´novas et al. Figure 6.6. Exponential decay pulse.
(1998b). Also, commercially available PEF systems
for food processing are reviewed by Barsotti et al. without bactericidal effect. A typical exponential de-
(1999). cay pulse can be characterized by a 1–3 ␮s pulse
width and 10–100 kV/cm peak electric field strength.
The treatment chambers are designed to avoid di- The square pulse waveform maintains a peak voltage
electric breakdown of foods, which occurs when the for a longer time than the exponential decay pulses
applied electric field strength exceeds the dielectric (Fig. 6.7). Consequently, it is more energy efficient
strength of the food, as indicated by a spark. and requires less cooling effort (Zhang et al., 1995).
The generation of square pulses is complex and usu-
In the treatment chamber, electric pulses go ally involves a pulse-forming network with an ar-
through the food samples. Depending on the specific ray of capacitors, inductors, and solid-state switching
electronic system, the pulses may be exponential de- devices (Fig. 6.8). The high cost involved prohibits
cay, square wave, oscillatory, or bipolar. The expo- practical application of this system.
nential decay and square wave pulses are the most
commonly applied waveforms in the PEF technol-
ogy. Although, exponential decay pulses are easier
to obtain, their efficiency has failed to produce the
desired effect. In the electrical circuit, a power sup-
ply charges a capacitor bank connected in series with
a charging resistor (Rs) (Fig. 6.5). When a trigger
signal is applied, the charge stored in the capacitor
flows through the food in the treatment chamber. Ex-
ponential decay pulse rises rapidly to a maximum
value and decays slowly to zero (Fig. 6.6). Conse-
quently, the pulses have a long tail with a low electric
field, resulting in excess heat generated in the food

100 Part I: Processing Technology

Voltage where ␴ is the conductivity (S/cm), ␶ is the pulse
length (␮s), and E is the electric field strength
Time (V/␮m).

Figure 6.7. Square wave pulse. When ␶ and E are constants, W depends only on
␴ and, as a result, the application of the same num-
During PEF processing for food pasteurization, the ber of pulses generates higher energy input when ␴
following main operating parameters should be con- is higher (Wouters et al., 2001a). The other problem
sidered: voltage (2–50 kV), electrode gap distance for food systems with high conductivity in relation to
(0.2–7 cm), electric field strength (1–100 kV/cm), PEF is the generation of excessive heat. A high con-
number of pulses (1–120), pulse width (1 ␮s–10 ms), ductivity of foods results in low field strengths and
pulse frequency (1–1000 Hz), shape of pulses, elec- excessive production of heat through the generation
trical characteristics, and in the case of a continuous of current. A low conductivity results in a more ef-
treatment chamber, the flow rate of liquid food. A re- fective PEF treatment (Jayaram et al., 1993; Wouters
quired residence time (Tr) of sample in a continuous et al., 1999). Lowering the conductivity of food in-
treatment chamber is needed to choose the flow rate creases the difference between the conductivity of the
correctly (Tr = volume of one chamber/flow rate). medium and the microbial cytoplasm and weakens
the membrane structure due to an increased flow of
In order to understand the effect of electric fields ionic substances across the membrane (Jayaram et al.,
on foods, it is very important to know the electrical 1993).
properties of a food item, especially the resistivity.
Barsotti et al. (1999) showed that food resistivity, EFFECT OF PEF ON FOOD
the reciprocal of conductivity, ranges from 0.4 to PRESERVATION
100 ·m. The electrical resistivity of a food sam-
ple has an effect on the whole circuit that is related Effect of PEF on Microorganisms
to the efficiency of energy transfer and temperature.
A change in conductivity affects the pulse energy Sale and Hamilton (1967) were the first to report
[W(J/ml)] given by the following formula: the effect of PEF on microorganisms. Escherichia
coli, Staphylococcus aureus, Micrococus lysodeikti-
W = ␴␶ E2, cus, Sarcina lutea, Bacillus subtilis, Bacillus cereus,
Bacillus megatherium, Clostidium welchii, Saccha-
romyces cerevisiae, and Candida utilis were sus-
pended in neutral sodium chloride solution and ex-
posed to a 5–25 kV/cm electric field strength that
was applied as a series of direct current pulses from
2 to 20 ␮s. The authors demonstrated that microbial
inactivation was non-thermal and it happened by
irreversible loss of membrane function. The sensitiv-
ity of microorganisms to this treatment was different

Charging Inductor Discharge
resistor switch

Rs Treatment
chamber
Power
supply

Figure 6.8. Simplified circuit for Energy storage capacitors
square wave pulse generation.