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Theuse of carbon electrodes in ohmic cooking of meat patties.

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ASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERING
Asia-Pac. J. Chem. Eng. 2007; 2: 474–479
Published online 17 August 2007 in Wiley InterScience
(www.interscience.wiley.com) DOI:10.1002/apj.084
Research Article
The use of carbon electrodes in ohmic cooking of meat
patties
Benjamin Gin and Mohammed Farid*
Department of Chemical and Materials Engineering, The University of Auckland, Private Bag 92019, Auckland, New Zealand
Received 11 December 2006; Revised 23 February 2007; Accepted 23 February 2007
ABSTRACT: Ohmic cooking is the process of cooking by passing an electrical current through food. The electrical
conductivity of the food causes heating and hence a rise in the interior temperature of the food. Previous research
has shown that combined ohmic and plate cooking process improves the cooking time of hamburger patties over
a conventional plate cooking process. However, it has been observed that after ohmic cooking the electrode plates
suffered from corrosion or pitting even when current is passed at high frequency. In this research, carbon plates were
tested to find out if the combined ohmic and plate cooking would result in pitting. Loss of carbon material, believed
to be caused by pitting corrosion, was observed on the plates after ohmic cooking at a power frequency of 50 Hz.
Frequencies of 5 and 100 kHz were tried to eliminate the pitting. It was shown that the higher frequencies resulted in
reduced pitting.  2007 Curtin University of Technology and John Wiley & Sons, Ltd.
KEYWORDS: ohmic cooking; pitting; graphite; carbon
INTRODUCTION
Ohmic heating describes the process of passing an
electrical current through a system in which the material
to be heated is part of the circuit. The resistance of
the material causes heat to be generated, therefore
this process is also known as direct resistance heating,
electrical resistance heating or Joule heating (Alwis and
Fryer, 1990).
The concept of ohmic heating has been applied to
the heating of solid foods. Recently a new method of
cooking meat patties has been patented (Farid, 2002).
This method combines conventional plate and ohmic
heating. A study into the quality of meat patties cooked
using this method (Özkan et al ., 2004) showed that
there were no significant differences in the properties
of the patties cooked using this method compared to
that of conventional cooking. It was also found that by
using combined ohmic and conventional heating, the
time required to cook was reduced to almost half.
Studies into ohmic heating of foods have shown
that electrochemical corrosion damages the electrodes
(Jayasinghe, 2004; Samaranayake and Sastry, 2005).
Pitting corrosion has been observed in electrodes used
for ohmic cooking of meat patties (Jayasinghe, 2004).
Pitting is a form of localised corrosion that produces
*Correspondence to: Mohammed Farid, Department of Chemical
and Materials Engineering, The University of Auckland, Private Bag
92019, Auckland, New Zealand. E-mail: m.farid@auckland.ac.nz
 2007 Curtin University of Technology and John Wiley & Sons, Ltd.
holes or pits (Smith, 1993). A number of different
frequencies were previously tested with stainless steel
plates and various coating materials (Jayasinghe, 2004)
in an effort to minimise the corrosion. The conclusion
was that corrosion could be minimised using high
frequency power. With stainless steel, little pitting was
observed even at frequencies as high as 100 kHz. Such
traces of metal may contaminate the food, especially
with the presence of chromium in the steel. In this
project, the stainless steel plates were replaced with
graphite plates for use as electrodes, which were tested
for corrosion under different frequencies. This study
then uses a qualitative approach to investigate the power
frequency effect on the pitting occurring in graphite
electrode plates.
MATERIALS AND METHODS
A Breville SG600B domestic sandwich plate griller
(Auckland, NZ) was modified to allow for combined
conventional and ohmic heating. The upper and lower
heating plates were connected to an alternating current
(AC) power supply and modified so that electrode plates
of any material could be attached. This was the same
grill used by Jayasinghe (2004) and Özkan et al . (2004).
Two different graphite materials were used for this
study. Graftech International Ltd (Ohio, USA) supplied
one type of graphite material of dimensions 263 mm ×
161 mm by 1.5 mm thick for trials. Also, a food grade
Asia-Pacific Journal of Chemical Engineering
graphite material coded 2020 carbon of dimensions
268 mm × 163 mm by 5 mm thick was obtained from
Carbone Lorraine (Auckland, NZ). Two plates of the
same type of graphite material were then attached to
the grill. The grill was allowed to preheat up to 150 ◦ C.
Sunflower oil was then brushed onto both the top and
bottom surfaces. A k-type thermocouple was then rested
onto the bottom plate. A second k-type thermocouple
was then inserted into a predrilled hole in the side
of the meat patty to record the temperature change
during cooking. The meat patty was then placed onto
the bottom plate and the top plate was closed. Plate and
ohmic cooking was then started.
A simplified schematic of the experimental set-up
with the completed circuit path is shown in Fig. 1.
Figure 2 shows the grill inside a safety cage which
protects the operator from being harmed during the
ohmic cooking. The two thermocouples that are used
can also be seen in Fig. 2. A qualitative technique was
used to compare the pitting found in the plates – the
amount of pitting was compared by visual observation
of the plates after cooking.
The combined ohmic system could be set up in
three different ways for different frequency AC power
Figure 1. Schematic showing the ohmic cooking
experimental set-up. This figure is available in colour
online at www.apjChemEng.com.
USE OF CARBON ELECTRODES IN OHMIC COOKING
supplies. With one set-up, a 50-Hz sinusoidal AC could
be used. The applied voltage used was 50 V. A second
set-up allowed for any frequency between 5 and 20 kHz
to be utilised, but with this, the maximum voltage that
could be used was 30 V. The last set-up allowed for the
use of 100 kHz and a voltage of 50 V.
DISCUSSION OF RESULTS
Voltage/current and temperature
characteristics
During ohmic cooking, the current rose as the patty
thawed, and then decreased as moisture was lost (Özkan
et al ., 2004). Similar maximum currents of close to 14
A were reached using the stainless steel, graphite and
2020 carbon plates. This shows that both the plates are
electrically conductive.
Figure 3 shows plots for the centre temperature
of meat patties during conventional and combined
cooking. The centre temperature of the meat patty
cooked using the combined method increases at a faster
rate and approaches the upper temperature faster than
with conventional grilling. From Fig. 3, it can be seen
that as the current rises, the rate of heating of the
meat patty increases, evidenced by the steeper rise in
temperature. The current then reaches a peak and starts
to drop since the electrical conductivity of the meat
patty decreases as it is cooked.
To cook safely, the centre temperature of the meat
patty must reach 72 ◦ C and remain so for 15 s (Özkan
et al ., 2004). According to this criteria, the cooking
time could be reduced to almost half by using combined
plate and ohmic cooking. In Fig. 3, the cooking time
under conventional plate cooking is 137 s, whereas
under combined cooking, the time has been reduced
to only 74 s.
Figure 2. (a) The modified Breville SG600B inside the safety cage, (b) grill open with two k-type
thermocouples. This figure is available in colour online at www.apjChemEng.com.
 2007 Curtin University of Technology and John Wiley & Sons, Ltd.
Asia-Pac. J. Chem. Eng. 2007; 2: 474–479
DOI: 10.1002/apj
475
476
B. GIN AND M. FARID
Asia-Pacific Journal of Chemical Engineering
Plot showing the centre temperature change during cooking for
conventional and combined ohmic cooking with the current plotted for
the combined ohmic cooking. This figure is available in colour online at
www.apjChemEng.com.
Figure 3.
the area, where the meat patties were cooked at
100 kHz.
The process of ohmic cooking causes removal of
material from the surface of the plates. This is thought to
be pitting. Pitting is localised corrosion through chemical attack. This type of corrosion results in the formation of holes or pits in the material. In the plates
supplied by Graftech Industries Ltd, these pits can be
seen on the surface. In Figs 4 and 5 these pits can be
seen as the darker areas (circled). Figure 4 shows many
more darker areas than Fig. 5. It can be clearly seen
that ohmic cooking using 50 Hz results in more pitting – or removal of material – than 100 kHz. Figure 6
Figure 4. The surface of the Graftech plate after cooking
ten meat patties at 50 Hz. The circled areas shown are
some of the areas where pitting has occurred. This figure is
available in colour online at www.apjChemEng.com.
Graftech plates
Meat patties were cooked with these plates using
power frequencies of 50 Hz and 100 kHz. The surface of these graphite plates has a texture; it is not
smooth as can be seen in Figs 4–6. Figure 4 shows
the surface of the plate after cooking ten meat patties using a power frequency of 50 Hz. The large
hole in the top right corner is a hole for mounting
the electrode to the cooker. Figure 5 shows the surface of the graphite plate after cooking 14 meat patties at 100 kHz. The dark circular area on the left
half of Fig. 6 shows the area, where the meat patties were cooked using 50 Hz. The right half shows
 2007 Curtin University of Technology and John Wiley & Sons, Ltd.
Figure 5. The surface of the Graftech plate after cooking 14
meat patties at 100 kHz. The circled area shown is the area
where some pitting has occurred. This figure is available in
colour online at www.apjChemEng.com.
Asia-Pac. J. Chem. Eng. 2007; 2: 474–479
DOI: 10.1002/apj
Asia-Pacific Journal of Chemical Engineering
highlights this by showing the two affected areas side
by side. The left half of the plate has a much darker
area which outlines the shape of the meat patty than the
right half.
Figure 7(a) shows the surface of an unused Graftech
plate. Figure 7(b) shows that some localised pitting
occurred even at a high frequency of 100 kHz. This
corrosion, or pitting, could be due to effects such as
localised overheating. Non-uniform current distribution
could occur because of the irregularity of the meat patty
surface leading to hot spots. Liu et al . (2006) concluded
that pitting may be due to localised arcing.
Carbon plates
Power frequencies of 50 Hz and 5 kHz were used for
ohmic cooking on these 2020 carbon plates. These
plates had a smooth finish, though there were some
horizontal lines across the plates which must have
resulted during the manufacturing of the plates. Figure 8
The Graftech plate showing side by side the
areas where cooking was conducted at 50 Hz (left) and
100 kHz (right). This figure is available in colour online at
www.apjChemEng.com.
USE OF CARBON ELECTRODES IN OHMIC COOKING
shows the surface of the plate after ten meat patties
have been cooked at 50 Hz. At 50 Hz, the surface of
the plate started to look as shown in Fig. 8 after five or
six cooking cycles. Five meat patties were then cooked
using a power frequency of 5 kHz and Fig. 9 shows
the surface of the plate after cooking. If five more
patties were cooked at 5 kHz, the surface would become
slightly more deteriorated, but would not look as bad as
the surface after cooking ten patties at 50 Hz as shown
in Fig. 8. The circled area of Fig. 9 is not pitting, but
is a patch of cooked meat which could not be removed
from the surface. Figure 10 shows the same plate with
the two areas side by side, where the left half utilised
a frequency of 50 Hz and the right half a frequency of
5 kHz.
The grade 2020 carbon plates obtained from Carbone
Lorraine exhibit deterioration of material from the
Figure 6.
Figure 8. The surface of the 2020 carbon plate after
cooking at 50 Hz. This figure is available in colour online at
www.apjChemEng.com.
Figure 7. (a) Microscopic image of the unused Graftech plate, (b) microscopic image of the Graftech plate
after cooking 14 meat patties at 100 kHz. This figure is available in colour online at www.apjChemEng.com.
 2007 Curtin University of Technology and John Wiley & Sons, Ltd.
Asia-Pac. J. Chem. Eng. 2007; 2: 474–479
DOI: 10.1002/apj
477
478
B. GIN AND M. FARID
Figure 9. The surface of the 2020 carbon plate after
cooking at 5 kHz. This figure is available in colour online
at www.apjChemEng.com.
Figure 10. The 2020 carbon plate showing side by side
the areas where cooking was conducted at 50 Hz (left)
and 5 kHz (right). This figure is available in colour online at
www.apjChemEng.com.
Asia-Pacific Journal of Chemical Engineering
Figure 12. Enlarged view of the 2020 carbon plate showing
the pitting that occurred. This figure is available in colour
online at www.apjChemEng.com.
surface of the plates. Though there is some evidence
of pitting which can be seen in Fig. 11(b) and 12, there
is more a removal of material from the surface which is
seen in Figs 8–10. The shape of where the meat patty
was cooked on can be seen in Fig. 8 – the whiter areas
show the places where material has been removed from
the plate and the darker areas show the places in the
plate where material has not been removed. For the
higher frequency cooking, Fig. 9 evidences much less
removal of material – an outline of the shape of a meat
patty can be seen, but the space occupied by the white
areas is much smaller than that seen on the plate cooked
using 50 Hz.
Figure 11(a) shows a microscopic image of an unused
plate where there are no signs of pitting. The pitting due
to ohmic cooking can be seen on the microscopic level
by the darker areas in Fig. 12. This shows that there is
Figure 11. (a) Microscopic image of the unused 2020 carbon plate, (b) microscopic image of the
2020 carbon plate after cooking five meat patties at 5 kHz. This figure is available in colour online at
www.apjChemEng.com.
 2007 Curtin University of Technology and John Wiley & Sons, Ltd.
Asia-Pac. J. Chem. Eng. 2007; 2: 474–479
DOI: 10.1002/apj
Asia-Pacific Journal of Chemical Engineering
pitting occurring, and Figs 8–10 show that on a macroscopic scale, general removal of the surface material is
experienced as well as pitting.
USE OF CARBON ELECTRODES IN OHMIC COOKING
International and Carbone Lorraine for supplying a
number of graphite plates for testing.
REFERENCES
CONCLUSIONS
The results from this investigation show that utilising
the higher power frequencies results in less pitting – or
loss of material – than by using the readily available
power supplied at 50 Hz. There is definite loss of
material occurring during ohmic cooking and the pitting
that has occurred is thought to be due to localised
overheating or arcing.
Acknowledgements
Auckland Uniservices Ltd for financial support of this
project. We would also like to acknowledge Graftech
 2007 Curtin University of Technology and John Wiley & Sons, Ltd.
Alwis AAPde, Fryer PJ. The use of direct resistance heating in the
food industry. J. Food Eng. 1990; 11: 3–27, DOI:10.1016/02608774(90)90036-8.
Farid M. International Patent, #WO 02/102215 A1, 2002.
Jayasinghe S. Ohmic cooking of hamburger patties, Masters Thesis
from the Department of Chemical and Materials Engineering, The
University of Auckland, 2004.
Liu Z, Jayasinghe S, Gao W, Farid M. Electrical corrosion on ohmic
heating electrodes. In CHEMECA Conference; Auckland: New
Zealand, 2006.
Özkan H, Ho I, Farid M. Combined ohmic and plate heating of
hamburger patties: quality of cooked patties. J. Food Eng. 2004;
63: 141–145, DOI:10.1016/S0260-8774(03)00292-9.
Samaranayake CP, Sastry SK. Electrode and pH effects on electrochemical reactions during ohmic heating. J. Electroanal. Chem.
2005; 577: 125–135, DOI: 10.1016/j.jelechem.2004.11.026.
Smith WF. Foundations of Materials Science and Engineering (2nd
edn). McGraw-Hill: Singapore, 1993.
Asia-Pac. J. Chem. Eng. 2007; 2: 474–479
DOI: 10.1002/apj
479
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