Ohmic Heating of Peaches in the Wide Range
of Frequencies (50 Hz to 1 MHz)
Mykola V. Shynkaryk, Taehyun Ji, Valente B. Alvarez, and Sudhir K. Sastry
shaped instant reversal bipolar pulses, and frequencies varying within 50 Hz to 1 MHz. Thermal damage of tissue was
evaluated from electrical admittivity. It showed that the time for half disruption (τ T ) of tissue was required more than
10 h at temperatures below 40 ◦ C. However, cellular thermal disruption occurred almost instantly (τ T < 1 s) at high
temperatures (> 90 ◦ C). Electrical conductivity σ o and admittivity σ o ∗ of tissue at T o = 0 ◦ C and their temperature
coefficients (m, m∗ ) were calculated. For freeze–thawed tissues, σ and σ ∗ as well as m and m∗ were nearly indifferent
to the frequency. However, for the intact tissue, both σ o , σ o ∗ and m, m∗ were frequency dependent. For freeze–thawed
product, the power factor (P) was approximately equal to 1 and indifferent to the frequency and temperature. On the
other hand, strong frequency dependence was observed for intact tissue with the minimum P approximately equal to
0.68 in the range of tens of kHz. The time required to reach a target temperature tf was evaluated. The tf increased
with frequency up to the middle of the range of tens of kHz and thereafter continuously decreased. Samples exposed to
the low-frequency electric field demonstrated faster electro-thermal damage rates. The textural relaxation data supported
more intense damage kinetics at low-frequency OH. It has been demonstrated that a combination of high-frequency OH
with pasteurization at moderate temperature followed by rapid cooling minimizes texture degradation of peach tissue.
Keywords: electrical conductivity, frequency, ohmic heating, peach, power factor, texture
Practical Application: In this study, we investigated the electric field frequency effect on the rate of OH of peaches.
It was shown that the time required for reaching the target temperature is strongly dependent upon the frequency.
Samples exposed to low-frequency OH demonstrated higher electro-thermal damage rates. It has been shown that the
combination of high-frequency OH with pasteurization at moderate temperature followed by rapid cooling minimizes
texture degradation of peach tissue. Obtained results provide new information on the impact of electric field frequency
on OH, which is useful for OH process design.
Introduction
Application of ohmic heating (OH) to food product thermal
preservation is being intensively discussed. The interest is stimulated by the possibility of reducing treatment time, resulting in
products of superior quality compared to those processed conventionally (Kim and others 1996; Castro and others 2003). According
to recent literatures, plant products are most suitable and most often used for OH processing (Leadley 2008). Generally, fruit and
vegetables exhibit sufficient conductivity to reach the required
temperatures in less than one min at comparatively low electric
field strengths (E < 100 V.cm−1 ) (Palaniappan and Sastry 1991;
Wang and Sastry 1997; Sarang and others 2008).
In OH processing, the product is heated volumetrically by dissipation of electrical current, passing through the food. The rate
of heating is directly proportional to the square of the electric
MS 20100491 Submitted 5/6/2010, Accepted 6/29/2010. Authors Shynkaryk
and Sastry are with Dept. of Food, Agricultural, and Biological Engineering, The Ohio
State Univ., 590 Woody Hayes Drive, Columbus, OH 43210, U.S.A. Authors Ji and
Alvarez are with Dept. of Food Science and Technology, The Ohio State Univ., 2015
Fyffe Rd., Columbus, OH 43210, U.S.A. Direct inquiries to author Shynkaryk
(E-mail: m.shynkaryk@gmail.com).
R
2010 Institute of Food Technologists
doi: 10.1111/j.1750-3841.2010.01778.x
C
Further reproduction without permission is prohibited
field strength and the electrical conductivity. While the electric
field strength during OH can be precisely controlled, the behavior of plant tissue electrical conductivity is very complex, and
depends on multiple factors, such as frequency, temperature, and
the electro-thermal damage kinetics of the tissue cell membranes
(Lebovka and others 2005, 2006, 2007a; Kulshrestha and Sastry
2006).
Different aspects of OH processing are being intensively discussed in the literatures. Usually, to minimize undesirable electrochemical reactions between electrodes and food product, an
increase in the alternating electric field frequency is required
(Amatore and others 1998; Samaranayake and Sastry 2005; Samaranayake and others 2005). Such an increase leads to a tissue electrical impedance decrease (Shaw and Galvin 1949; Ohnishi and
others 2004). On the other hand, the electroporation process of
plant tissue cell membranes was reported to be more pronounced
at low frequencies (Mehrle and others 1990; De Vito and others
2008; Kulshrestha and Sastry 2003). Furthermore, product temperature increase affects both electrical conductivity and electroporation rate, and at relatively high temperatures (> 50 ◦ C), thermal
plasmolysis can be observed (Wang and Sastry 1997; Lebovka and
others 2007b).
There have been some efforts to clarify the impact of electric
field frequency on OH rate of vegetable food. Lima and others
Vol. 75, Nr. 7, 2010 r Journal of Food Science E493
E: Food Engineering &
Physical Properties
Abstract: The ohmic heating (OH) rate of peaches was studied at fixed electric field strength of 60 V.cm−1 , square-
Frequency impact on ohmic heating . . .
E: Food Engineering &
Physical Properties
(1999) studied the electrical conductivity and temperature rise
of turnip under OH at 4 frequencies (4, 10, 25, and 60 Hz). It
was shown that heating rate increased with decreasing frequency.
Imai and others (1995) observed a similar effect during the OH
of Japanese white radish in the range of frequencies 50 Hz to
10 kHz and E = 40 V.cm−1 . In both studies, it was concluded
that the electroporation mechanism can be determinant. Lima
and Sastry (1999) and Kulshrestha and Sastry (2003) stated that
lowering of electric field frequency for the moderate electric field
(MEF) isothermal processing of red beet resulted in higher tissue
damage and improved the mass transfer processes. However, so far,
the electroporation rate during OH at different frequencies was
not visualized, and OH of vegetable tissues at frequencies above
10 kHz was not studied.
This study was focused on the effect of electric field frequency
on the OH rate of soft cellular tissue. Peach was chosen for investigation as a product currently commercially processed by OH.
The kinetics of electro-thermal disintegration was studied at fixed
electric field strength of 60 V.cm−1 , varied frequencies within
50 Hz to 1 MHz and target temperatures of 60 to 90 ◦ C. The degree of tissue damage was evaluated from electrical admittivity data
and validated by textural stress relaxation tests. The result would
be expected to provide information on the impact of electric field
frequency on OH rate, which would be useful for OH process
design.
Material and Methods
Material
Peach tissue was chosen as the object of investigation. Peaches of
good and uniform quality were purchased from the local grocery
store (Kroger, Columbus, Ohio, U.S.A.) and refrigerated at 4 ◦ C
until used. Samples came from the same lot and had approximately
the same size and degree of ripeness. Cylinders of fresh tissue
(diameter d = 0.025 m and height h = 0.008 m) were used in
experiments.
Experimental setup
Figure 1 shows a schematic diagram of the OH treatment experimental setup. Electric field treatment was applied using a 500A
power amplifier (Industrial Test Equipment, Port Washington,
N.Y., U.S.A.) coupled with a Tektronix AFG3252 function generator (Tektronix Inc., Richardson, Tex., U.S.A.). The function
generator allowed generation of various wave forms at frequencies
from 1 mHz to 240 MHz and maximum output level 5 V. The
amplifier could deliver 500 W of power and boost signals in the
range 10 Hz to 1 MHz to a maximum output of about 55 V at
5 V input. When operating the function generator and the amplifier output signals (voltage at the sample) were monitored on a
Tektronix MSO4034 four channels oscilloscope (Tektronix Inc.).
Electric field was applied to the sample in a cylindrical 0.025
m i.d. glass treatment cell via platinized-titanium electrodes connected to the power amplifier with 0.3 m long CL1254 wires
(Belden Inc., Richmond, Ind., U.S.A.). To improve the electrical
contact between the electrodes and the sample, both were previously moistened with fresh peach juice. The temperature at the
centre of the sample was measured with a 2 type-T thermocouples
introduced through the opening of the cell. The current flowing
through the circuit was monitored by a Pearson current transformer, model 150 (Pearson Electronics Inc., Palo Alto, Calif.,
U.S.A.) with a working range of 40 Hz to 20 MHz and sensitivity
of 0.5 V/A. All the output data (current, voltage, and temperature)
were collected using an Agilent 34970A data acquisition switch
unit and HP BenchLink Data Logger software (Agilent Technologies Inc., Palo Alto, Calif., U.S.A.) to program the sampling rate.
Electrical properties measurements
The relationship between electrical properties, temperature, and
frequency of intact and the maximally damaged peach tissues were
studied at temperatures in the range 0.5 to 40 ◦ C and frequencies
in the range 50 Hz to 1 MHz. To obtain maximally damaged
tissue, cylinders of fresh peach were frozen at −18 ◦ C and then
thawed. The sample housed in the treatment cell and enclosed by
the electrodes was placed in the water bath kept at the desired
temperature. When the temperature equilibrated, a measurement
of the sample conductance (G), admittance (Y ), and dissipation
factor (DF) were done with a HP 4285A LCR meter (HewlettPackard, Palo Alto, Calif., U.S.A.). All measurements were taken
with 1V test signal level and medium measuring speed. The tissue
conductivity σ and admittivity σ ∗ were calculated using G and Y
data by:
σ =
G4h
;
πd 2
(1.1)
σ∗ =
Y4h
πd 2
(1.2)
The P was calculated using DF values by:
DF
P = √
1 + DF 2
(2)
The power factor (P) represents the ratio between the amounts
dissipated into heat and absorbed/returned to the source power.
Application of the previously mentioned equation gives a dimensionless number between 0 and 1 with P = 1 for a circuit dissipating all energy into heat.
Figure 1–Schematic diagram of the experimental setup.
E494 Journal of Food Science r Vol. 75, Nr. 7, 2010
Thermal treatment measurements
To highlight the contribution of thermal degradation to the
damage of peach tissue during OH the thermal treatments were
done in the temperature range of 40 to 60 ◦ C. The cell with
freshly prepared peach sample was preheated to the target temperature by OH at 30 V.cm−1 and frequency of 1 MHz in less
Frequency impact on ohmic heating . . .
Z=
(σi∗ − σ ∗ )
(σi∗ − σd∗ )
(3)
where σ ∗ is the sample admittivity at given temperature and the
subscripts “i” and “d” refer to the admittivity of intact and maximally damaged (freeze–thawed) peach tissue. Application of the
previously mentioned equation gives Z = 0 for an intact material
and Z = 1 for a maximally damaged tissue.
OH treatment measurements
The disintegration kinetics of peaches under OH was studied at
a fixed electric field strength of 60 V.cm−1 , square shaped instant
reversal bipolar pulses, and frequencies varying within 50 Hz to
1 MHz. The electric field was applied to the sample settled in
the treatment cell at room temperature (25 ◦ C) and turned off
when temperature inside the tissue reached 90 ◦ C. The peach
tissue damage rate under OH was estimated in real time from Eq.
3 where the tissue electrical admittivity (σ ∗ ) was calculated from
OH voltage-current data and values of σ ∗ i and σ ∗ d at appropriate
temperatures T were calculated by:
∗
∗
∗
σi,d
= σ0,i,d
(T − T0 )
(4)
1 + m i,d
2.03 (Jandel Scientific, San Rafael, Calif., U.S.A.) software was
used.
Results and Discussion
The impact of temperature on peach tissue
Vegetable tissue thermal treatment is known to cause noticeable
damage to cell membranes and to improve mass transfer processes
(Taiwo and others 2001; Tedjo and others 2002; Lebovka and
others 2007c). On the other hand, membrane rupture leads to a
significant rise of the tissue electrical conductivity and may affect
the process of OH (Sarang and others 2007; Lebovka and others
2007a, 2007b). The time of thermal impact, required to reach
a high level of damage, depends on the temperature and differs
by individual differences in fruit and vegetables. To highlight the
contribution of the thermal degradation on damage to peach tissue
during OH, thermal treatments were studied in the temperature
range of 40 to 60 ◦ C. The insert to Figure 2 shows typical curves
of electrical admittivity disintegration index Z against thermal
treatment time at different temperatures T.
It can be seen that even a slight temperature increase noticeably
accelerates the temperature induced damage kinetics. At relatively
high values of Z (0.7 to 0.8), a deceleration of further admittivity
increase is observed and the final Z was smaller than its maximum
level in all cases. The characteristic thermal damage time τ T was
estimated as a thermal treatment time required for attaining onehalf of the maximum Z value (Z = 0.5) and obtained τ T values
were presented in the form of an Arrhenius plot (Figure 2). The
activation energy was calculated by the least square fitting of the
Arrhenius equation to the line slope:
Wp
(5)
τT = τT,∞ exp
R(T + 273.15)
∗
is the electrical admittivity at the reference temperwhere σ o,i,d
ature To , and m∗ i,d is the electrical admittivity temperature coefficient in ◦ C−1 . More detailed explanation of employed method
can be found elsewhere (Lebovka and others 2007a).
Texture stress relaxation test
The presence of the OH induced tissue damage was validated
with a texture stress relaxation test. The OH treatment was started
at 25 ◦ C and stopped when the temperature inside the sample
reached the target value. After treatment, the sample was immediately removed from the treatment cell and immersed in freshly
squeezed peach juice kept at 0 ◦ C in a 100 mL beaker. When
the temperature inside a sample decreased to room temperature,
2 cylindrical samples (0.012 m i.d. and 0.008 m in length) were
prepared with a cork borer for further rheological measurements.
The stress relaxation test was carried out using an Instron 5542
texture analyzer (Instron, Norwood, Mass., U.S.A.). The preloading at 0.1 N was applied to compensate a nonuniformity of 2 ends
of a sample. Measurements were done at room temperature, deformation rate 0.2 mm.min−1 and with the resolution 0.1 s. The
maximum load force was N = 10 N, and force relaxation curves
were recorded during 300 s. All the output data were collected
using Bluehill Materials Testing Software (Instron).
Data analysis
Each experiment was done at least in triplicate, except for the
texture relaxation test, in which 6 replications were done, and
mean and standard deviations of the data were calculated. For the
experimental data least-square fitting, the Table Curve 2D version
Figure 2–Arrhenius plot of the characteristic thermal damage time (τ T )
against inverse temperature (1/T) for peach tissue; symbols are the experimental data, dashed line is the result of their linear least mean square
fitting, the error bars represent the standard deviations; solid line is the
calculated characteristic thermal damage time (τ T ) for sugar beet tissue,
insert shows the disintegration index (Z) compared with time of thermal
treatment (t) for peach tissue.
Vol. 75, Nr. 7, 2010 r Journal of Food Science E495
E: Food Engineering &
Physical Properties
than 60 s and placed into the heating bath, maintained at the desired temperature. Electrical admittivity was measured using OH
setup at frequency 1 kHz and low voltage (1 V.cm−1 ). Data were
collected using the Agilent 34970A data acquisition switch unit
programmed to scan current, voltage, and temperature every 10 s.
Thermally induced damage in the peach tissue was estimated as
changes of admittivity disintegration index Z with the time t. The
admittivity disintegration index Z was calculated as in Lebovka and
others (2002):
Frequency impact on ohmic heating . . .
E: Food Engineering &
Physical Properties
where Wp = 198 ± 19 kJ.mol−1 is the activation energy, τ T ,∞
(approximately equal to 10−29 s) is the limiting time, R = 8.314
J.K−1 .mol−1 is the Universal Gas constant and T is the temperature
in ◦ C.
An extrapolation of measured data to the regions of low (<
40 ◦ C) and high temperatures (> 60 ◦ C) shows that thermal
treatment at temperatures below 40 ◦ C will not significantly affect
membrane integrity even at a long time of exposure as the time
of half disruption is rather high (τ T > 10 h). On the other hand,
a tissue preheating up to 90 ◦ C will result in almost instantaneous
tissue damage (τ T < 1s). The solid line on Figure 2 is an Arrhenius
plot of the characteristic thermal damage time for sugar beet tissue
calculated based on values of Ws = 170 kJ.mol−1 and τ T ,∞ =
10−23 s obtained by Lebovka and others (2007a). It can be seen
that the peach tissue exhibits rather high sensitivity to temperature
with the 1 to 1.5 log gap in Arrhenius plots as compared to sugar
beet.
Effect of frequency and temperature on electrical
properties of the peach tissue
Figure 3 shows the effect of frequency and temperature on
the electrical conductivity (σ ) for intact and maximally damaged
peach tissues. The measurements were done within the range of
frequencies f = 50 Hz to 1MHz and temperatures T of 0.5 to
40 ◦ C to avoid the effects related to thermal degradation of material. A conductivity rise with the frequency was observed for
intact samples at all temperatures. Such behavior is typical for
plant tissues and it is mainly due to the changes of dielectric properties with increasing frequency. According to Schwan (1957),
there are 3 distinct dielectric dispersions at low, radio, and microwave frequencies defined as α-, β-, and γ - dispersions. The
50 Hz to 1 MHz measurements range cover 2 first dispersions,
where the α- dispersion is located in the range up to few kHz
and the β-dispersion is the tens of kHz to tens of MHz range.
The tissue electrical conductivity also changes corresponding to
these 3 dispersions. The most dramatic rise of conductivity was
observed in the β-dispersion range, reflecting the capacitive cell
membrane charging through the cell interior and exterior media.
In contrast to the intact tissue, the conductivity of freeze–thawed
sample demonstrated a minor increase at high frequencies that may
evidence a presence of some undamaged cells.
An increase of the tissue temperature resulted in a conductivity
rise. For both damaged and intact tissues, the typical linear dependence of electrical conductivity (σ ) against the temperature (T)
was observed within the measured range of frequencies. The tissue electrical conductivity σ o and admittivity σ o ∗ at the reference
temperature T o and their temperature coefficients m and m∗ were
estimated employing Eq. 4. The temperature range used for the
intact tissue σ o, σ o ∗ and, m and m∗ values calculation was limited
to 30 ◦ C due to a visible departure from linearity at 40 ◦ C indicating that some thermal softening of membranes is implemented.
Obtained results are presented in Figure 4.
It can be seen that for freeze–thawed tissue, values of σ o ∗ and σ o
as well as m∗ and m perfectly match each other, indicating that there
was no reactance from the cell membranes, and changes slightly
upon the frequency. The situations seem to be more complicated
for the intact tissue. While the σ o ∗ and σ o continuously increased
against the frequency, the m and m∗ had minima at about 30 and 50
kHz, respectively. Also, for intact tissue, a distinction between σ o
and σ o ∗ values was observed, which was evidence of the presence
of reactive load in the circuit. To evaluate the possible impact of
reactive load on the OH process, the P was calculated from Eq.
2. Figure 5 illustrates the response of P-values in a spectrum of
frequencies 50 Hz to 1MHz and temperature range 0.5 to 40 ◦ C
for the intact ( ) and maximally damaged () peach tissues.
At low frequency up to 100 Hz, the value of P was equal to
1 for both samples within the studied temperature range. Further
frequency increase resulted in the decrease of P of intact tissue
with the minimum of about P = 0.68 in the tens of kHz followed by the continuous increase up to the end of the studied
frequency spectra. Also, it can be seen that increase of the sample
temperature leads to the shift of the whole curve to the region
Figure 4–Variation of the tissue electrical conductivity (σ o ) and admittivity
(σ o ∗ ) at the reference temperature (T o ) and their temperature coefficients
Figure 3–Typical electrical conductivity frequency dependence at different
(m, m∗ ) with frequency for intact ( , ) and maximally damaged (, )
temperatures for the intact (solid line) and maximally damaged (dashed
peach tissues, solid symbols correspond to the conductivity and open symline) peach tissues. The error bars represent the standard data deviations bols to the admittivity data, the error bars represent the standard data
of 3 observations.
deviations of 3 observations.
E496 Journal of Food Science r Vol. 75, Nr. 7, 2010
of the higher frequencies. Observed behavior can be attributed
to the possible decrease of the cells membranes charging time
with the temperature increase. The minimum value of P practically did not change significantly up to the temperature of 30 ◦ C
with the sharp increase at 40 ◦ C that can be related with the structural transitions softening inside membranes (Zimmermann 1986).
Membranes structures softening may facilitate ion flow leading to
DF increase. In contrast to the intact product, for the damaged
sample, no noticeable changes of P with the frequency or temperature were observed. Such behavior allows us to conclude that
tissue with disrupted cell membranes loses the ability to absorb and
store electric energy and can be considered as a simple resistance.
In the present study, when the electric field is applied directly
to the sample, the heating rate can be predicted using conductivity
data, and P allows estimating an extra current flowing trough the
circuit. However presence of a low P may considerably complicate
prediction of the heating rate for a solid–liquid system where
components with different P values are heated simultaneously.
Effect of frequency on the OH rate
The temperature increase inside the peach tissue during OH in
the case when thermal exchange with outside media is negligible
(adiabatic conditions) is governed by following equation of energy
balance:
ρC p
dT
= E 2 σ0 (1 + m T)
dt
(6)
where E is the electric field strength, ρ is the density, and Cp is the
specific heat capacity. Integrating Eq. 6 and expressing the time tf
required to obtain a desired temperature Tf we obtain following
expression:
tf =
ρC p
1 + m Tf
ln
m σo E 2
1 + m Ti
(7)
where Ti is the initial temperature and value of σ o and m for the
intact and totally damaged peach tissue at appropriate frequency
can be found from Figure 4.
The adiabatic approximation can be applied when the total time
of OH tf is far below the time of thermal diffusion τ D that can
be estimated as τ D approximately equal to h2 /2d, where h is the
height of the sample in m, and d = k/ρCp is the thermal diffusivity,
where k is the thermal conductivity. Using values of k = 0.581
W.m−1 .K−1 , ρ = 970 kg.m−3 , and Cp = 3820 J.kg−1 .K−1 given
for peach tissue by Whitelock and others (1999) and h = 8∗ 10−3
m; the time of thermal diffusion is τ D = 204 s that is significantly
higher than the total treatment time tf under E = 60 V.cm−1 ,
which was usually less than 50 s.
A comparison between the (analytically) predicted and experimental time tf required to reach a target temperature of
90 ◦ C at E = 60 V.cm−1 at different frequencies for intact and
freeze–thawed tissue is presented in Figure 6. It is clear that the predicted time will follow the σ o and m and as expected, for totally
damaged tissue (dash–doted line) tf remains practically constant
over the studied frequency range. For intact tissue (dashed line) a
continuous decrease of tf with frequency increase was observed,
and at the highest frequency of 1 MHz, it almost reached the line
obtained for freeze–thawed tissue. The experimentally obtained
data for freeze–thawed tissue () were slightly higher compared to
those predicted analytically. As it was discussed earlier, the heat loss
can be excluded as the factor affecting heating rate. However, a
nonuniformity of electrical contact between the sample and electrodes due to the presence of some air bubble and the voltage drop
of about 3% on the generator during the treatment could lead to
such results.
For the intact tissue ( ), a strong difference between predicted
and experimental tf values was observed at frequencies below 50
kHz. On the other hand, the OH rates at frequencies above > 50
kHz were in relatively good agreement with those obtained analytically. Significantly accelerated temperature elevation as compared
with the theoretical predictions was previously stated by Lebovka
and others (2007a) for an ohmic treatment of sugar beet at 60 Hz.
Such behavior was attributed to the sharp increase of the electrical conductivity due to the tissue cells membrane electroporation.
Imai and others (1995) observed that increasing of the electric field
Figure 6–The predicted and experimental time (tf ) required to reach a
Figure 5–P compared with frequency for intact ( ) and maximally damaged target temperature of 90 ◦ C at OH field strength of E = 60 V.cm−1 com() peach tissue at different temperatures, the error bars represent the pared with frequency for intact and freeze–thawed tissue. The error bars
standard data deviations of 3 observations.
represent the standard data deviations of 3 observations.
Vol. 75, Nr. 7, 2010 r Journal of Food Science E497
E: Food Engineering &
Physical Properties
Frequency impact on ohmic heating . . .
Frequency impact on ohmic heating . . .
E: Food Engineering &
Physical Properties
frequency decreases the heating rate of Japanese white radish in
the range of frequencies 50 Hz to 10 kHz. They assumed that the
electroporation mechanism can be suppressed at high frequency.
To estimate the electroporation induced tissue electrical conductivity rise during OH the admittivity disintegration index Z rise
during the treatment at frequencies 100 Hz, 1 kHz, 10 kHz, 100
kHz, and an electric field strength of E = 60 V.cm−1 was evaluated
(Figure 7).
The OH at low frequency f = 100 Hz demonstrated a sharp
increase of the disintegration index (Z), right after the treatment
application, followed by slow increase with temperature rise resulting in relatively high final Z value. The increase of disintegration
index Z with the temperature was expected. Lebovka and others
(2007b) demonstrated that the combination of the vegetable tissue
preheating to mild temperatures with simultaneous application of
an electrical treatment causes a synergetic effect on the damage
rate. Zimmermann (1986) experimentally observed a noticeable
drop of the breakdown transmembrane voltage for a single membrane near the temperature of 50 ◦ C. On the other hand, an increase of OH frequency resulted in a decrease of the disintegration
rate and final Z values. Similar effects were previously observed
during microbial inactivation (Martin-Belloso and others 1997;
Canatella and others 2001) and vegetable tissue electroporation
(Mehrle and others 1990; De Vito and others 2008) by pulsed
electric fields. The observed suppressed disintegration rate at elevated frequencies correlates with the theory of cell membrane
electroporation (Weaver and Chizmadzhev 1996). According to
this theory, a membrane breakdown occurs when the transmembrane potential (Um ) exceeds a threshold value of about 0.2 to 1.0
V. Jeltsch and Zimmermann (1979) obtained following solutions
of the Laplace equation for the transmembrane potential of a single
spherical cell exposed to an external d-c electric field:
Um =
t
3
Ea cos ϑ 1 − exp
2
τ
Effect of frequency and endpoint
temperature on peach texture
To confirm a presence of electrically induced tissue damage,
force relaxation tests were performed. Fresh peaches with approximately the same rigidity were selected to ensure good reproducibility of the textural measurements. Figure 8 shows the
textural relaxation data compared with time for intact,
freeze–thawed, and ohmically preheated to 50 ◦ C peach tissue
at frequencies f = 100 Hz, 1 kHz, 10 kHz, 50 kHz, 100 kHz, 1
MHz, and electric field strength of E = 60 V.cm−1 .
It can be seen that the OH treatment applied at 100 kHz and
1 MHz does not affect cell membrane integrity and the relaxation curves perfectly match those obtained for fresh tissue (dashed
curve). Such a result is in good accordance with the admittivity
disintegration index (Z) obtained earlier at 100 kHz (Figure 7).
The texture examination of samples treated at lower frequencies
revealed a dramatic drop of the textural strength with the frequency decrease. Generally, the texture relaxation data displayed
even higher levels of tissue damage compared to the electrical ad(8) mittivity disintegration index (Z). Underestimated damage degree
Figure 7–Admittivity disintegration index (Z) of peach tissues compared
with temperature for ohmic heat processing at frequencies 100 Hz, 1 kHz,
10 kHz, 100 kHz, and electric field strength of E = 60 V.cm−1 . Error bars
represent data standard deviations of 3 replications. Insert shows the
transmembrane potential (Um ) compared with frequency (f ) for a single
cell in d-c field of E = 60 V.cm−1 .
E498 Journal of Food Science r Vol. 75, Nr. 7, 2010
where a is the radius of the cell, τ the time constant of the charging
process of the membrane, t the pulse duration, and θ is the angle
between the external field direction and the radius-vector r on the
membrane surface.
Using typical values for plant tissue cells a = 100 μm, τ = 30 μs
given by De Vito and others (2008), and an applied electric field
E = 60 V.cm−1 , a drop of the transmembrane potential (Um ) with
increasing frequency can be estimated (see inset to the Figure 7).
It can be seen that Um decreases significantly above about 1 kHz
and already at 100 kHz only about 1% of equilibrium potential is
attained. It allows concluding that tissue damage observed at 100
kHz and elevated temperatures T > 60 ◦ C is mainly due to the
electrically assisted thermal plasmolysis.
Figure 8–Relaxation of the relative force (N/Nmax ) compared with time for
intact (dashed curve), freeze–thawed (dash–dotted curve), and ohmically
preheated to 50 ◦ C peach tissues at frequencies of 100 Hz, 1 kHz, 10 kHz,
100 kHz, and electric field strength E = 60 V.cm−1 . Error bars represent
data standard deviations of 6 replications.
Frequency impact on ohmic heating . . .
activation energy, as compared to the peach tissue (Wp = 198
kJ.mol−1 ), may significantly reduce the lag between the time of
peach and yeast cells thermal damage when lowering pasteurization temperature. Comparing the time τ T required for attaining
one-half of the maximum Z for peaches (Eq. 5) and obtained F
values for yeasts at T = 65 ◦ C it can be found that τ T = 102 s
while F is only 8 s. On the other hand at T = 60 ◦ C τ T = 293 s
for peaches and F = 261 s for yeasts.
Conclusions
OH can be a real alternative to conventional heat processing
techniques, allowing the processor to obtain products with a superior quality. However, selection of the operating conditions,
such the electric field frequency, voltage, end-point temperature,
and the treatment time should be done with caution. An application of the low-frequency electric field may lead to intensive cell
membrane electroporation, resulting in an unpredictable electrical
conductivity rise and affected product texture. Peach processing
at frequencies in the range of tens of kHz allows for significant
suppression of electroporation (especially at f > 50 kHz). However, relatively low tissue electrical conductivity at such frequencies
dramatically increases the time required to reach the desired temperature. Moreover, a nonlinear conductivity rise due to a residual
electroporation and thermal plasmolysis at elevated temperatures,
as well as the low P, significantly complicates process modeling
for multiphase systems. On the other hand, peach preservation at
frequencies above 100 kHz allows elimination of electroporation,
an increased P, and ability to obtain good prediction of the heating rate regardless of the presence of some thermal damage at high
temperatures. It has been shown that the right combination of
processing parameters (200 kHz at E = 60 V.cm−1 to 65 ◦ C for
8 s) allows for minimization of texture degradation of peaches
tissue during the thermal preservation.
Figure 9–Relaxation of the relative force (N/Nmax ) compared with time
for intact (dashed curve), freeze–thawed (dash–dotted curve) and peach
tissues pasteurized ohmically at different temperatures at a frequency of
Substituting values of D and D1 at appropriate temperatures T 200 kHz and electric field strength of E = 60 V.cm−1 . Error bars represent
−1
and T1 a value of Wy = 695 kJ.mol can be obtained. Such high data standard deviations of 6 replications.
Vol. 75, Nr. 7, 2010 r Journal of Food Science E499
E: Food Engineering &
Physical Properties
can be related to the nature of electrical admittivity and rigidity.
The product electrical admittivity strongly depends on the moisture distribution and availability of percolative channels inside the
tissue (Stauffer and Aharony 1992). On the other hand the tissue relaxation response is interrelated with cell walls, membranes,
and turgidity. Recently, Grimi and others (2010) compared disintegration indexes obtained with measurement of electrical and
acoustical properties of apple tissue. The researchers demonstrated
that the disintegration index (Z) evaluated from electrical measurements underestimated the level of tissue damage and the acoustic
method better reflected the real degree of damage. In the present
study, the results show that the OH frequency can influence the
electrical and textural properties in different ways.
From a practical point of view, it is clear that to obtain a product
with minimally affected texture and maximum nutrient retention
after the thermal preservation, a right combination of the electric
field strength, frequency, end-point temperature, and treatment
time needs to be selected. For the peach, as a high acid product
with a pH varying within 3.3 to 4.1 (Leadley 2008), a pasteurization treatment is required to obtain a 6 log reduction of the most
thermally resistant pathogenic or spoilage microorganisms (Gaze
and Betts 1992). Garza and others (1994a) isolated and identified
172 microorganisms from commercial peach puree consisting of
bacteria, molds, and yeasts. They concluded that Saccharomyces cerevisiae is the most important and thermally resistant microorganism
responsible for product spoilage. Later, the researchers identified
the yeast strain 173 as the most thermally resistant with the thermal
death time (TDT) curve LogD = 19.79 to 0.33T for heating in
McIlvaine buffer with pH = 4, where D is the time in minutes
required for 1 logarithmic cycle reduction of yeasts population at
the temperature T in ◦ C (Garza and others 1994b). Estimating
the lethality value F required to obtain a 6 log cycle reduction
at different temperatures, we obtain the F = 261 s at 60 ◦ C, =
8 s at 65 ◦ C, = 0.26 s at 70 ◦ C, = 2.6∗ 10−4 s at 80 ◦ C, and =
2.6∗ 10−7 s for 90 ◦ C. At temperatures higher than 70 ◦ C OH
was stopped immediately when desired temperature was attained.
Holding time in those cases was estimated to be about 1 s. Figure 9 presents the texture examination for peach disks pasteurized
ohmically at different temperatures using an electric field strength
E = 60 V.cm−1 and frequency f = 200 kHz.
It can be seen that exposure of the sample to high temperature
(T = 90 ◦ C) even for a short period of time followed by the
immediate immersion in cold juice at 0 ◦ C, significantly damages
cell membranes and removes cell turgor. A decrease of the pasteurizing temperature from 90 to 65 ◦ C considerably improved tissue
firmness. Such behavior can arise from the very short treatment
time at high temperatures, which can not be controlled well that
result in thermally overtreated peach tissue and accelerated force
relaxation. The temperature decrease to 65 ◦ C increased treatment
time up to 8 s and improved processes control and texture. On the
other hand, further temperature decrease to 60 ◦ C demonstrated
even softer texture than thermal treatment at 70 ◦ C for 1 s. Such
effects can be expected taking into account a high activation energy of yeast cells (Wy ) that can be evaluated from TDT data by
following equation (Karel and Lund 2003):
D
TT1
Wy = 2.303R
(9)
log
T1 − T
D1
Frequency impact on ohmic heating . . .
Acknowledgments
The authors appreciate the financial and research support provided in part by the Ohio Agricultural Research and Development
Center, The Ohio State Univ., and in part by Rudolf Wild and Co.
References to commercial products or trade names are made with
the understanding that no endorsement or discrimination by The
Ohio State Univ. is implied. The authors also thank Dr. Suzanne
A. Kulshrestha for her help in preparation of the manuscript.
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