ELECTROCHEMICAL REACTIONS DURING OHMIC HEATING
DISSERTATION
Presented in Partial Fulfillment of the Requirements for
the Degree Doctor of Philosophy in the Graduate
School of The Ohio State University
By
Chaminda Padmal Samaranayake, B.Sc. Honors
∗∗∗∗∗
The Ohio State University
2003
Dissertation Committee:
Professor Sudhir K. Sastry, Adviser
Approved by
Professor Q. Howard Zhang
Professor David B. Min
Professor Russ C. Hille
Adviser
Food Science and Nutrition
Graduate Program
© Copyright by
Chaminda Padmal Samaranayake
2003
ii
ABSTRACT
Electrochemical reactions, chemical reactions at electrode/solution interfaces
induced by current, are undesirable during ohmic heating. These reactions may be
avoidable or suppressible through an understanding of electrochemical behavior of ohmic
heaters. Though numerous studies have dealt with the applications of ohmic heating, little
is known regarding electrochemical aspects.
Electrochemical behavior of four types of electrode materials: titanium, stainless
steel, platinized-titanium, and graphite, was studied at (initial) pH 3.5, 5.0, and 6.5 using
60 Hz sinusoidal alternating current. Concentrations of metal ions and elemental carbon
migrated into the heating media were determined by inductively coupled plasma (ICP) –
mass, and -emission spectrometers. Hydrogen gas accumulation in the headspace of the
ohmic heater, and pH changes of the heating media were also measured. Stainless steel
was found to be the most electrochemically active electrode material, whereas platinizedtitanium exhibited relatively inert electrochemical behavior at all the pH values. The
potential use of platinized-titanium electrodes for ohmic heating operations was further
demonstrated on a pilot scale.
Effects of frequency, pulse width, and delay time of a pulsed ohmic heating
technique on electrochemical reactions were studied, in comparison with conventional
ii
(60 Hz, sine wave) ohmic heating. Analyses of electrode corrosion, hydrogen generation,
and pH measurements suggest that the pulsed ohmic heating is capable of significantly
reducing the electrochemical reactions of titanium, stainless steel, and platinized-titanium
electrodes. The delay time was found to be a critical factor.
Electrochemical and secondary chemical reactions during 60 Hz ohmic heating of
ascorbic acid in citrate-phosphate buffer with stainless steel electrodes were characterized
by a number of analytical methods. Electrode corrosion showed marked effects on the
heating buffer medium forming metal-phosphates and metal-citrate complexes. Effects of
reactions on pH, buffer capacity, and ascorbic acid degradation are discussed.
Free radical generation was investigated by spin trapping with 5,5-dimethyl-1pyrroline N-oxide (DMPO), and employing electron spin resonance (ESR) spectroscopy.
The frequency range of 1 – 8 kHz is recommended to suppress free radical generation
with platinized-titanium electrodes. Ohmic heating operated at 60 Hz (sine wave) and 10
kHz (pulses) indicated the generation of •OH radicals.
iii
Dedicated to all who assisted my education
prior to and during my study at The Ohio State University.
iv
ACKNOWLEDGMENTS
I would like to thank my adviser, Dr. Sudhir Sastry, for intellectual support,
guidance, and financial aid offered to me through out my degree program. I wish to thank
Drs. Howard Zhang, David Min, Russ Hille, and Richard McCreery for kindly serving on
my dissertation and candidacy examination committees.
I gratefully acknowledge the collaboration of Dr. Russ Hille, and Craig Hemann
(Department of Molecular and Cellular Biochemistry) in the free radical study (chapter
5). I am also grateful for the hospitality received from Hille’s lab.
A special thanks goes to Brian Heskitt for making all my experimental setups, and
for the technical assistance provided to me through out this research. I thank Dr. John
Olesik, director of microscopic and chemical analysis research center at OSU, for
providing ICP-MS, ICP-OES, SEM, and SEM-EDX analytical services. I appreciate the
assistance received from Dr. Johnie Brown, the former associate director of campus
chemical instrument center-mass spectrometry laboratory at OSU, with GC-MS and ESIMS analyses.
Finally, I wish to thank Drs. Salengke and Ilkay Sensoy, the former members of our
ohmic heating group, and Pisit Wongsa-ngasri for their wide range of support during my
study at OSU. My appreciation is extended to Karthik Vembu who helped me in cleaning
v
up viruses and restoring the programs in my computer, and also to Rakhith U.C. for his
contribution to some digital images.
vi
VITA
August 26, 1970 …………………..Born – Chilaw, Sri Lanka
1991 – 1995 ……………………….B.Sc. (Chemistry) Special Degree, with
First Class Honors pass
(subsidiary subject: Mathematics)
University of Sri-Jayawardenepura, Sri Lanka
1995 ……………………………….Demonstrator (Inorganic Chemistry),
Department of Chemistry,
University of Sri-Jayawardenepura, Sri Lanka
1996 – 1998 ……………………….Assistant lecturer (Phys. and Environ. Chemistry),
Department of Chemistry,
University of Kelaniya, Sri Lanka
1998 – present …………………….Graduate Research Associate,
The Ohio State University
PUBLICATIONS
Research Publications
1.
Assiray A., Sastry S.K., and Samaranayake C. (2003); Degradation kinetics of
ascorbic acid during ohmic heating with stainless steel electrodes, Journal of applied
electrochemistry, 33 (2), pp. 187-196.
2.
Edirisinghe E.M.R.K.B., Samaranayake C.P., Bamunuarchchi A., Walpola S., and
De Alwis A.A.P. (1997); Nutrient retention in ohmic heating; 7 th International Congress
on Engineer and Food (ICEF – 7), Brighton U.K., SA 43-46.
3.
Samaranayake C.P., De Alwis A.A.P., and Bamunuarchchi A. (1996); Peroxide
formation in ohmic heating of meats; Ceylon Journal of Science (Physical Sciences),
3(1), pp. 30-35.
vii
Published Abstracts
1.
Samaranayake C.P. and Sastry S.K. (2003); Electrochemical corrosion of
platinized-titanium electrodes during ohmic heating; Institute of Food Technologists
annual meeting, Chicago, IL.
2.
Samaranayake C.P. and Sastry S.K. (2002); Electrochemical reactions during
ohmic heating; Institute of Food Technologists annual meeting, Anaheim, CA.
FIELDS OF STUDY
Major Field: Food Science and Technology
viii
TABLE OF CONTENTS
Page
Abstract …………………………………………………………………………………. ii
Dedication …………………………………………………………….………………… iv
Acknowledgments …………………………………………….………………………… v
Vita …………………………………………………………………………………….. vii
List of Tables ……………………………………………………………………………xii
List of Figures ………………………………………………………………...……….. xiv
Chapters:
1.
Introduction ………………………………………………….……………………. 1
1.1 The ohmic heating process ……………………………….…………………….. 1
1.2 Electrochemical reactions ……………………………………………………… 2
1.2.1 Electrode/solution interface ……………………………………………… 2
1.2.2 Electrochemical reactions induced by alternating currents ……………… 3
1.2.3 Electrochemical reactions during ohmic heating ………………………… 4
1.3 Effects of electrochemical reactions on the ohmic heating process …………… 5
1.3.1 Electrode corrosion ………………………………………………………. 6
1.3.2 (Partial) Electrolysis ……………………………………………………… 6
1.3.3 Generation of free radicals ……………………………………………….. 8
1.3.4 Loss of energy ……………………………………………………………. 9
1.4 Research objectives …………………………………………………………….. 9
Symbols...……………………………………………………………….………… 11
ix
References …………………………………………………………………………. 12
2.
Electrochemical behavior of various electrode materials during ohmic
heating at pH 3.5, 5.0, and 6.5…………….……………………………………… 17
Abstract ……………………….…………………………………………………… 17
Introduction ………………….…………………………………………………….. 18
Materials and Methods ………….……………………………………………….… 19
Results and Discussion ………….………………………………………………… 27
Conclusions …..…..……….……………………………………………………….. 37
Symbols …….…..……….…….……….………………………………………...… 38
References …………………….…………………………………………………… 39
3.
Effect of pulsed ohmic heating on electrochemical reactions ………….……… 53
Abstract ………………………………………………………………………….… 53
Introduction …………………………………………………….……………….…. 54
Materials and Methods ……………….………………………………………….… 55
Results and Discussion ………………….………………………………………… 61
Conclusions …..…...……………………………………………………………….. 66
Symbols ……..…..……….…………….………………………………………...… 67
References ………….……………………………………………………………… 68
4.
Electrochemical reactions during 60 Hz ohmic heating of ascorbic acid
in buffer medium with stainless steel electrodes ………………..……………… 89
Abstract ………………………………………………………………………….… 89
Introduction …………………………………………………….……………….…. 90
Materials and Methods ……………….………………………………………….… 91
Results and Discussion ………………….………………………………………… 97
Conclusions …..…...……………………………...………………………………. 105
Symbols ……..….………...………….…….……………………………………... 106
References ………….……...…...………………………………………………… 107
5. Investigation of free radical generation during ohmic heating ……………… 121
Abstract ……………………………………..……………………………………. 121
Introduction …..…………………………….………………….……………….… 122
Materials and Methods ………..……….…………………………………………. 123
Results and Discussion ……………….…….…….……………………………… 127
Conclusions …..…...……………………………………………………………… 129
x
Symbols ……..…..……….…………….…..……………………………………... 130
References ………….……………………..……………………………………… 131
6.
Conclusions …….…………...…………………………………………………… 142
List of References ………………..…………………………………………………... 145
xi
LIST OF TABLES
Table
Page
2.1 Comparison of corrosion rates (in ppb per KJ) of the electrode materials
with respect to the migrations of their major (surface) elements at different pH
values ……………………………………………………………………..……... 41
2.2
pH changes of the heating media observed with stainless steel electrodes
at different pH values ………………………………………………………….… 41
2.3
Pt and Ti concentrations (in parts per trillions) of the ohmically heated
heating medium in the pilot scale study …………………………………….…… 42
2.4
Comparison of estimated metal intakes via consumption of an 8 oz ohmically
heated meal with the published upper-level daily dietary exposure limits for
adult consumers. The estimation is based on unit conversions: 1 ppt =
1 picogram/ g; 8 oz = 227 g; 1 picogram = 10-12 g = 10-6 µg ……………….…… 42
3.1
Frequency-pulse width combinations with required initial electrical
conductivities, and cooling water temperatures for
stainless steel electrodes ………………………………………………….……… 69
3.2
Frequency-pulse width combinations with required initial electrical
conductivities, and cooling water temperatures for
titanium electrodes ……………………………………………………….………. 70
3.3
Frequency-pulse width combinations with required initial electrical
conductivities, and cooling water temperatures for
platinized-titanium electrodes ………………………………………….………… 71
3.4
Frequency-pulse width combinations with required initial electrical
conductivities, and cooling water temperatures for
graphite electrodes ………………………………………………………..……… 72
4.1 Ohmic heating conditions ……………………………………...……………….. 109
xii
4.2
Some indicators of the electrochemical processes at different power
densities and NaCl concentrations ……………………………………………… 110
4.3
Chemical compositions (as weight %) of electrode deposits at different
power densities and NaCl concentrations. The values are means of five
replicates (n=5) with respective standard deviations in parentheses …………… 111
4.4
Minimum migratory metal ion concentration [Mn+] needed to precipitate
some metal-phosphates and metal-hydroxides in the presence of the same
[Na2HPO4] as in the citrate-phosphate buffer system at pH 3.5 …………….….. 112
4.5 The effect of AA-induced Fenton’s reaction on buffer pH, in comparison
with the observed pH changes at different power densities and NaCl
concentrations ……………………………………………………………..……. 113
4.6
The spectral maxima (λmax) and the respective absorptivity coefficients of
1:1 Fe(III)-citrate, and the ohmically heated heating medium at
1.5 Wcm-3 (1.0% NaCl) ………………………………………..……………….. 114
4.7 GC-MS characteristics and % losses of the buffer components ………..………. 114
5.1
Selected ohmic heating conditions to study free radical generation.
See figure 5.3 for typical time-temperature history for all these
ohmic heating conditions ……………………………………………………….. 133
xiii
LIST OF FIGURES
Figure
Page
1.1
The concept of ohmic heating …………………………………………………… 15
1.2
A simplified electrical equivalent circuit of the interface during the application
of AC; Cd: electrical double layer capacitor, Rct: charge-transfer resistance,
Rs: electrolyte resistance ………………………………………………………… 15
1.3
Mechanisms of generating free radicals by electrolysis and electrode corrosion. . 16
2.1 The laboratory scale ohmic heater ………………………………………………. 43
2.2
Schematic diagram of the laboratory scale experimental setup …………………. 44
2.3 Typical time vs. temperature curve for all electrodes at all pH values during
ohmic heating ………………………………………………………………….… 45
2.4 Typical time vs. current curve for all electrodes at all pH values during
ohmic heating ……………………………………………………………….…… 45
2.5
Typical SEM micrograph of titanium electrodes …………...…………………… 46
2.6
Typical SEM micrograph of stainless steel electrodes ………………………….. 47
2.7
Typical SEM micrograph of platinized-titanium electrodes ……………….……. 48
2.8 Typical SEM micrograph of graphite electrodes ……………...………………… 49
2.9
Hydrogen generation with titanium electrodes during ohmic heating …….…….. 50
2.10 Hydrogen generation with stainless steel electrodes during ohmic heating …….. 50
2.11 Identified graphite corrosion products by GC-MS analysis; (a) 2-hydroxy,
propanoic acid (lactic acid) (MW: 90), (b) 2-hydroxy, 4- methyl,
pentanoic acid (MW: 132) ………………………………….…………………… 51
xiv
2.12 Positive ion ESI-MS spectra of the heating medium before and after
ohmic heating ………………………………………………………….………… 52
3.1 The ohmic heater used for both pulsed and conventional ohmic heating
experiments ……………….………………………………..……………………. 73
3.2
Schematic diagram of the experimental setup used for pulsed ohmic heating ….. 74
3.3 Typical time vs. temperature curve for all the ohmic heating experiments …...… 75
3.4
Schematic diagram of the centering of bipolar pulses within the period to
study the effects of frequency and pulse width …….……………………….…… 76
3.5
Typical pulse waveforms at 10 kHz for different pulse widths. The top
and the bottom waves in each diagram represent the current and
the voltage, respectively ……………….………………………………………… 77
3.6
Typical pulse waveforms at 4 kHz for different pulse widths. The top
and the bottom waves in each diagram represent the current and
the voltage, respectively …….…………………………………………………… 78
3.7
Schematic diagram of the centering of bipolar pulses within the period to
demonstrate the effect of delay time ……….……………………………….…… 79
3.8
Typical pulse waveforms for different delay times. The top
and the bottom waves in each diagram represent the current and
the voltage, respectively ….…………………………………………..………….. 80
3.9
The corrosion rates (in ppb per KJ) of stainless steel electrodes with respect to
Fe at different frequencies and pulse widths. Number shown in parenthesis is
the corresponding frequency (kHz) for each pulse width (µs). The corresponding
duty cycle is indicated as a percentage. ‘Con’ represents the corrosion rate for
conventional ohmic heating. Different letters: a, b, c, d, and e denote significant
differences (p ≤ 0.05) of mean (n = 3) corrosion rates ……………………...….. 81
3.10 The corrosion rates (in ppb per KJ) of stainless steel electrodes with respect to
Cr at different frequencies and pulse widths. Number shown in parenthesis is
the corresponding frequency (kHz) for each pulse width (µs). The corresponding
duty cycle is indicated as a percentage. ‘Con’ represents the corrosion rate for
conventional ohmic heating. Different letters: a, b, c, and d denote significant
differences (p ≤ 0.05) of mean (n = 3) corrosion rates ………………….………. 82
xv
3.11 The corrosion rates (in ppb per KJ) of titanium electrodes with respect to
Ti at different frequencies and pulse widths. Number shown in parenthesis is
the corresponding frequency (kHz) for each pulse width (µs). The corresponding
duty cycle is indicated as a percentage. ‘Con’ represents the corrosion rate for
conventional ohmic heating. Different letters: a, b, c, d, and e denote significant
differences (p ≤ 0.05) of mean (n = 3) corrosion rates …………………..……… 83
3.12 The corrosion rates (in ppb per KJ) of platinized-titanium electrodes with respect
to Pt at different frequencies and pulse widths. Number shown in parenthesis is
the corresponding frequency (kHz) for each pulse width (µs). The corresponding
duty cycle is indicated as a percentage. ‘Con’ represents the corrosion rate for
conventional ohmic heating. Different letters: a, b, c, and d denote significant
differences (p ≤ 0.05) of mean (n = 3) corrosion rates ………………….………. 84
3.13 The corrosion rates (in ppb per KJ) of platinized-titanium electrodes with respect
to Ti at different frequencies and pulse widths. The presence of asterisk (*)
indicates corrosion rate either < 0.001 ppb/ KJ or undetectable. Number shown
in parenthesis is the corresponding frequency (kHz) for each pulse width (µs).
The corresponding duty cycle is indicated as a percentage. ‘Con’ represents the
corrosion rate for conventional ohmic heating. Different letters: a, and b denote
significant differences (p ≤ 0.05) of mean (n = 3) corrosion rates ……………… 85
3.14 The corrosion rates (in ppm per KJ) of graphite electrodes with respect to
elemental carbon at different frequencies and pulse widths. The presence of
asterisk (*) indicates an undetectable corrosion rate. Number shown in
parenthesis is the corresponding frequency (kHz) for each pulse width (µs). The
corresponding duty cycle is indicated as a percentage. ‘Con’ represents the
corrosion rate for conventional ohmic heating. Different letters: a, and b denote
significant differences (p ≤ 0.05) of mean (n = 3) corrosion rates ……………… 86
3.15 The corrosion rates (in ppb per KJ) with respect to Fe for different delay times.
‘Con’ represents the corrosion rate for conventional ohmic heating. Different
letters: a, b, and c denote significant differences (p ≤ 0.05) of mean (n = 3)
corrosion rates …………………………………………………………………… 87
3.16 The corrosion rates (in ppb per KJ) with respect to Cr for different delay times.
‘Con’ represents the corrosion rate for conventional ohmic heating. Different
letters: a, b, c, and d denote significant differences (p ≤ 0.05) of mean (n = 3)
corrosion rates …………………………………………………………………… 88
4.1
Buffer capacities (in µmol pH-1ml-1) at 0.25% (w/v) NaCl. [1] and [2]
correspond to the amounts of metal ions migrated at 0.5 Wcm-3 and
0.75 Wcm-3, respectively ………………………………………………………. 115
xvi
4.2
Buffer capacities (in µmol pH-1ml-1) at 0.50% (w/v) NaCl ……………………. 116
4.3
Buffer capacities (in µmol pH-1ml-1) at 1.0% (w/v) NaCl ………….………….. 117
4.4
The buffer solution before being subjected to ohmic heating (a), and
the ohmically heated medium at 1.5 Wcm-3 (1.0% NaCl) (b) …………………. 118
4.5
UV-Visible absorption spectra of 1:1 Fe(III)-citrate(
), and
the ohmically heated medium (
) at 1.5 Wcm-3 (1.0% NaCl) …………...… 119
4.6
Total ion chromatograms of TBDMS derivatized solutions of the unheated
buffer (A), and the ohmically heated medium at 1.5 Wcm-3 (1.0% NaCl) (B).
The CA and P peaks represent citric acid and phosphate, respectively ……..… 120
5.1
Schematic diagram of the pressurized ohmic heater …………….…………….. 134
5.2
Schematic diagram of the experimental setup used for the ohmic heating ……. 135
5.3
Typical time-temperature histories for ohmic and conventional heating ……… 136
5.4
Schematic diagram of the centering of bipolar pulses within the period (T)
at each frequency (f). The positive and negative pulses having the same
pulse width (tp) were equally spaced by adjusting the delay time (td)
as T = 2 (tp + td) ………………………………………………………………… 137
5.5
The ESR spectrum of the DMPO-OH reference. This signal represents
spin concentration of 0.63 µM …………………………………………………. 138
5.6
Typical ESR spectra of ohmic and conventional heating experiments, in
comparison with the ESR spectrum of DMPO-OH reference. The signals
at 60 Hz (sine wave) and 10 kHz correspond to average spin concentrations
of 0.14 and 0.11 µM, respectively …………………………..…………………. 139
5.7
Chemistry of •OH and O2•− trapping by DMPO in the presence and absence
of ethyl alcohol ………………………………………………………………… 140
5.8
Comparison of typical ESR spectra of the ohmic heating experiments
carried out at 60 Hz (sine wave) and 10 kHz in the presence (2%, v/v)
and absence of ethyl alcohol …………………………………………………… 141
xvii
CHAPTER 1
INTRODUCTION
In recent decades, technologies utilizing electrical energy directly into food
processing have attracted renewed interest in the food industry. Some of those are now
being used on a commercial scale for processing of a broad range of food products.
Research in this area will provide the food processor with the opportunity to produce new
and value-added food products with enhanced quality attributes preferred by consumers.
Since heating is one of the traditional and still widely popular treatments applied to food
both in the industry and the home, electroheating technologies have gained increasing
industry interest and attention. Ohmic heating, a well-known electroheating technique,
has extensively developed during the past two decades; and today it is in commercial
scale operation for processing of a number of food products, especially those containing
particulates.
1.1
The ohmic heating process
As shown in figure 1.1, the concept of ohmic heating is quite simple. The passage of
electric current through an electrically conductive food material obeys Ohm’s law (V =
IR); and heat generation due to the electrical resistance of the food, is given by:
1
Pheat = I2R
(1.1)
The design of ohmic heaters is governed by the electrical conductivity of the food. Since
most food materials contain a considerable amount of free water with dissolved ionic
species, the conductivity is high enough for a heating effect to occur.
Applications of ohmic heating in the food industry emerged in the 1930’s as a
pasteurization process for milk (Getchell, 1935; Moses, 1938). Then, the technique has
been applied to blanching of vegetables (Mizrahi et al., 1975), thawing of frozen foods
(Naveh et al., 1983); and recently, pasteurization and sterilization of liquid and
particulate food products that can be packed under aseptic conditions (Parrott, 1992;
Zoltai et al., 1996). In addition, a large number of potential future applications exist for
ohmic heating, including its use in evaporation, dehydration, fermentation, and extraction
(Butz et al., 2002).
1.2
Electrochemical reactions
1.2.1 Electrode/solution interface
To understand the electrochemical behavior of ohmic heaters, it is necessary to
identify the characteristics of the electrode/solution interface. As described in interfacial
electrochemistry, the electrode/solution interface is analogous to a parallel combination
of a resistor and a capacitor (Rubinstein, 1995). A simplified electrical equivalent circuit
of the interface during the application of alternating current (AC) is shown in figure 1.2.
In reality, the so-called electrical double layer capacitor (Cd) can hold only a limited
number of charges. Once it is fully charged or ‘saturated’, it becomes a ‘leaky’ capacitor,
2
and charge-transfer occurs between the plates of the capacitor generating faradaic current,
consequently initiating electrochemical (i.e. faradaic-type) reactions.
However, most electrode/solution interfaces also exhibit a potential range (1 – 2 V
wide at most) where no faradaic reactions can take place. If the potential difference
across the double layer is maintained within its faradaic reaction-free potential window,
electrons from the electrode cannot be transferred to the electrolyte and nor can ions from
the electrolyte react at the electrode. The only phenomenon occurring at the electrode is a
periodic change in charge density on both sides of the interface. Under such
circumstances the current flowing through the interface becomes purely capacitive.
1.2.2 Electrochemical reactions induced by alternating currents
In AC circuits, both current and voltage oscillate as a wave at a certain frequency.
When AC is applied to an electrolytic cell, the double layer capacitors of the
electrode/solution interfaces charge and discharge periodically. If the frequency of the
AC wave is low, the capacitors can be fully charged during the rising part of the wave
turning on electrochemical reactions. Those reactions involve simultaneous cathodic (i.e.
reduction) and anodic (i.e. oxidation) half-reactions; and the overall reactions produce
periodic concentration changes of redox species at the electrode surfaces. The extents of
those chemical changes primarily depend upon the frequency of the applied AC signal
and the chemistry of the electrolytic cell.
Electrochemical phenomena induced by AC were first reported in the early
nineteenth century, and it was a common difficulty encountered in measuring
conductivity of electrolytes. Shaw (1950) has reported that when an alternating current is
3
applied to an electrolytic cell, the cell shows both dissipative and reactive characteristics.
Bentley et al. (1957) observed corrosion of stainless steel, platinum, and gold electrodes
when low-frequency (50 Hz) alternating currents were passed through concentrated acids;
and this corrosive effect was not evident at frequencies greater than a few kHz. They
further encountered distorted waveforms of voltage across the test cells at the frequency
of 50 Hz; and they attempted to correlate this waveform distortion with corrosion. Most
of the previous investigations under AC induced corrosion have been briefly reviewed by
Kulman (1961). He implied that AC induced electrolysis is closely associated with the
corrosion of electrodes. Then, Venkatesh et al. (1979) have reviewed some of the
fundamentals of AC induced electrochemical processes. Their discussion indicates that
when a sinusoidal alternating electric field is applied to an electrolytic cell, a direct
current (DC) or a ‘faradaic rectification current’ is generated at each of the electrode
surface; and this DC component of the current is related to the amplitude of the applied
voltage signal. Another discussion and a review of AC induced anodic and cathodic
reactions, and the effect of frequency have been given by Venkatachalam et al.(1981).
Recently, Lalvani et al. (1994, 1996) and Bosch et al. (1998) carried out some theoretical
studies for predicting AC induced corrosion; and they also reported that the corrosion
behavior strongly depends on the amplitude of applied voltage signal.
1.2.3 Electrochemical reactions during ohmic heating
Typically, ohmic heaters are powered by low-frequency (50 - 60 Hz) AC coming
from the public utility supply, because that mainly minimizes the cost and power supply
complexity. Under such alternating frequencies, a part of the current passing through the
4
electrode/solution interfaces causes electrochemical processes. Although a number of
studies have addressed the basic engineering and heat transfer aspects of ohmic heating
over the years, surprisingly limited attention was paid to electrochemical processes.
The possibility of electrochemical reactions occurring at electrode/solution interfaces
during ohmic heating has been described by Stirling (1987). He demonstrated the
selection of safe maximum current density that minimizes the faradaic current to less than
0.1% of the total current, using a platinized-titanium/ saturated NaCl ohmic cell.
Palaniappan et al. (1991), Uemura et al. (1994), Assiry (1996), Reznick (1996), Wu et al.
(1998), and Assiry et al. (2003) observed apparent electrolysis of the heating medium and
electrode corrosion during ohmic heating. Some of these authors reported that those
electrochemical effects diminish with increasing frequency. A broad discussion of
fundamental electrochemistry related to ohmic heating has been given by Amatore et al.
(1998); and the use of high alternating frequencies was suggested to inhibit adverse
electrochemical effects. Tzedakis et al.(1999) has recently examined the electrochemical
behavior of platinum and platinized-titanium electrodes for ohmic sterilization of some
commercial food products. Their results indicate that, at the frequency of 50 Hz, only
platinized-titanium would be capable of suppressing the electrochemical behavior.
1.3
Effects of electrochemical reactions on the ohmic heating process
Food materials are inherently a complex mixture of several different chemical
compounds. During ohmic heating, various electrochemical reactions can potentially
occur. In addition, some of the products of those electrochemical reactions may initiate a
number of secondary chemical reactions. Although it is not possible to examine the
5
effects of all those electrochemical and chemical processes, the following more
frequently encountered electrochemical reactions and their effects on the ohmic heating
process cannot be overlooked.
1.3.1 Electrode corrosion
In ohmic heating, electrodes are necessary to convey the current to the food material
to be heated. During heating, electrode corrosion occurs mainly via electrodissolution
induced by the low-frequency AC. For metallic electrodes (M), a generalized anodic halfreaction for the electrode corrosion can be written as follows.
M (solid) ⇔ M n+ (aqueous) + n e, (where n = 1,2,3….)
(1.2)
The metal ions (M n+) migrated into the heating medium are basically contaminants, and
may have some toxic potential. However, on the other hand, the electrode corrosion
might represent an opportunity to introduce essential minerals into the processed foods.
Since food systems are generally rich in ligands, the migrated transition metal ions
can form various coordination complexes. Those metal complexes typically have
characteristic colors; and therefore, they may involve alteration of color of the processed
foods. It is also known that, some transition metal ions have catalytic effects for certain
food reactions, such as lipid oxidation. Therefore, the electrode corrosion may have an
impact on flavor quality of the processed food products.
1.3.2 (Partial) Electrolysis
Most food formulations subjected to ohmic heating contain more than 50% water.
During ohmic heating, the low-frequency AC induces electrolysis of the water generating
6
H2 and O2 gases at the electrode/solution interfaces. The corresponding anodic and
cathodic half-reactions, and the overall electrolysis reaction are as follows.
Anodic half-reaction:
2H2O (liquid) ⇔ O2 (g) + 4H+(aqueous) + 4e
(1.3)
Cathodic half-reactions:
2H+(aqueous) + 2e ⇔ H2 (g)
(1.4)
2H2O (liquid) + 2e ⇔ H2 (g) + 2OH−(aqueous)
(1.5)
2H2O(liquid) ⇔ 2H2 (g) + O2 (g)
(1.6)
The overall reaction:
Molecular oxygen generated by electrolysis can oxidize almost all the oxidizable
food components, particularly lipids and vitamins like ascorbic acid (Vitamin C). The
molecular oxygen also involves electrode corrosion, or formation of insulating species on
the electrode surfaces partially passivating the electrodes (Tzedakis et al., 1999). Because
of the high flammability and explosive nature, uncontrolled liberation of hydrogen gas
might pose safety concerns in large-scale continuous ohmic heating practice.
However, the liberation of gas bubbles at the electrode/solution interfaces does not
necessarily indicate the overall electrolysis reaction (equation 1.6). Sometimes, the
anodic half-reaction for electrode corrosion (equation 1.2) may be accompanied with one
of the cathodic half-reactions for electrolysis (equation 1.4 or 1.5) resulting in electrode
corrosion with H2
(g)
overpotential for Cl2
generation. In addition, since some electrode materials show low
(g)
liberation than oxidation of water to O2
(g),
one of the cathodic
half-reactions for electrolysis (equation 1.4 or1.5) may be also coupled with the following
7
anodic half-reaction (equation 1.7), especially when there is a significant amount of
chloride ions in the heating medium, resulting in H2 (g) and Cl2 (g) generation.
2Cl −(aqueous) ⇔ Cl2 (g) + 2e
(1.7)
1.3.3 Generation of free radicals
Electron transfer associated with electrochemical reactions at the electrode/solution
interfaces leads to generation of radical species (Schafer, 2001). However, such radical
generation specifically during ohmic heating has not yet been reported. Konya (1979)
described the formations of hydroxyl (•OH) and hydroperoxyl (•OOH) radicals, and
hydrogen peroxide (H2O2) in oxygen evolution (equation 1.3) during the electrolysis of
water. The cathodic half-reactions of hydrogen generation (equations 1.4 and 1.5) are
also mediated via hydride radicals (H•) (Sawyer, 2003). Some (hypothetical) mechanisms
of generating free radicals by electrolysis and electrode corrosion are illustrated in figure
1.3.
Since electrolysis and corrosion reactions occur in the microenvironments of the
electrodes, the radical species formed under ohmic heating conditions might be H•, and
oxygen-containing free radicals, such as •OH, •OOH, and superoxide anion radicals
(O2•−), as well as the molecules like H2O2 and singlet oxygen (1O2). These reactive
oxygen species can aggressively attack food components, in particular lipids, vitamins,
and amino acids causing oxidative degradation of those nutrients. However, on the other
hand, since the above reactive oxygen species can function as bactericides, the generation
of free radicals might improve the sterilization efficiency of ohmic heating.
8
1.3.4 Loss of energy
In ohmic heating, the current is exclusively for the purpose of heating, and no
electrochemical phenomena are desirable. However, at any time, total current (I
total)
across an electrode/solution interface is given by equation 1.8 (Tzedakis et al., 1999).
I total = I c + I f
(1.8)
The current that involves heat generation by passing through the food material is
obviously the capacitive current (I c). The faradaic current (I f) is associated with
electrode/solution interfaces, and causes electrochemical reactions. Therefore, I f can be
regarded as a ‘stray’ current, and it is essentially a loss of useful electrical energy.
Tzedakis et al. (1999) reported that the ratio of I f / I c could even be 20- 40 %.
1.4
Research objectives
The overall objective of this research was to acquire a better understanding of
electrochemical behavior of ohmic heaters. The following were the specific objectives of
the investigations.
1. To investigate the behavior of some electrodes at different pH values during ohmic
heating;
2. To test the feasibility of using pulse inputs to minimize electrochemical processes;
3. To characterize electrochemical processes during ohmic heating of ascorbic acid;
4. To investigate free radical generation during ohmic heating.
Various analytical techniques were used to characterize and quantify the
electrochemical, and subsequent chemical reactions. The metal ions migrated into the
heating medium were measured by state-of-the-art Inductively Coupled Plasma (ICP)
9
spectrometers. Results from this research contribute to the production of safe and high
quality ohmically processed food products, and to smooth and efficient ohmic heating
practice.
10
SYMBOLS
AC
alternating current
DC
direct current
e
electron
I
current (A)
Pheat
amount of heat liberation (W)
R
electric resistance (Ω)
V
voltage (V)
11
REFERENCES
Amatore C., Berthou M., and Hebert S.(1998); Fundamental principles of
electrochemical ohmic heating of solutions; Journal of Electroanalytical Chemistry; 457,
pp. 191-203.
Assiry A.M (1996); Effect of ohmic heating on the degradation kinetics of ascorbic acid;
Ph.D thesis, The Ohio State University.
Assiry A., Sastry S.K., and Samaranayake C. (2003); Degradation kinetics of ascorbic
acid during ohmic heating with stainless steel electrodes, Journal of applied
electrochemistry, 33 (2), pp. 187-196.
Bently R., and Prentice T.R. (1957); The alternating current electrolysis of concentrated
acids; J. Appl. Chem.; 7 (November), pp. 619-626.
Bosch R.W., and Bogaerts W.F.(1998); A theoretical study of AC-induced corrosion
considering diffusion phenomena; Corrosion Science, 40(2/3), pp.323-336.
Butz P., and Tauscher B. (2002); Emerging technologies: chemical aspects; Food
Research International, 35, pp.279-284.
Getchell B.E.(1935); Electric pasteurization of milk; Agr. Eng., 16(10), pp.408-410.
Konya J. (1979); Notes on the “non-faraday” electrolysis of water; Journal of
electrochemical society: electrochemical science and technology, 126(1), pp. 54-56.
Kulman F.E. (1961); Effects of alternating currents in causing corrosion; Corrosion;
17(3), pp. 34-35.
Lalvani S.B. and Lin X.A. (1994); A theoretical approach for predicting AC-induced
corrosion; Corrosion Science, 36(6), pp. 1039-1046.
Lalvani S.B. and Lin X.A. (1996); A revised model for predicting corrosion of materials
induced by alternating voltages; Corrosion Science, 38(10), pp. 1709-1719.
Mizrahi S., Kopelman I.J., and Perlman. J.(1975); Blanching by electroconductive
heating; J. Food Technology, 10, pp. 281-288.
Moses B.D. (1938); Electric pasteurization of milk; Agr. Eng., 19(12), pp.525-526.
Naveh D., Kopelman I.J., and Mizrahi S. (1983); Electroconductive thawing by liquid
contact; J. Food Technology, 18, pp. 171-176.
12
Palaniappan S., and Sastry S. (1991); Electrical conductivity of selected juices:
Influences of temperature, solid content, applied voltage, and practical size; Journal of
Food Process Engineering, 14, pp. 247-260.
Parrott D.L.(1992); Use of ohmic heating for aseptic processing of food particulates; J.
Food Technology, 12, pp. 68-72.
Reznick D.(1996); ohmic heating of fluid foods; J. Food Technology; 5, pp. 250-251.
Rubinstein I. (1995); Physical electrochemistry: principles, methods, and applications,
Rubinstein I. (Ed.), Chapter 1: Fundamentals of physical electrochemistry; Marcel
Dekker, Inc., New York, pp. 1-4.
Sawyer D.T. (2003); Electrochemical transformations of metals, metal compounds, and
metal complexes: invariably (ligand/ solvent)-centered; Journal of molecular catalysis A:
Chemical, 194, pp. 53-67.
Schafer H.J.(2001); Organic electrochemistry (Fourth edition), Lund H. and Hammerich
O. (Ed.), Chapter 4: Comparison between electrochemical reactions and chemical
oxidations and reductions; Marcel Dekker, Inc., New York, pp. 207-221
Shaw M. and Remick A.E.(1950); Studies on alternating current electrolysis; J. of
Electrochemical Society; 97(10), pp. 324-334.
Stirling R. (1987); ohmic heating - a new process for the food industry; Power
Engineering Journal; 1(6), pp. 365-371.
Tzedakis T., Basseguy R., and Comtat M.(1999); Voltammetric and coulometric
techniques to estimate the electrochemical reaction rate during ohmic sterilization;
Journal of Applied Electrochemistry; 29(7), pp. 821- 828.
Uemura K., Noguchi A., Park S.J., and Kim D.U. (1994); ohmic heating of food
materials- Effect of frequency on the heating rate of fish protein; Developments in Food
Engineering - Proceedings of the 6 th International Congress on Engineering and Food;
Blackie Academic & Professional Press, London, pp. 310-312.
Venkatachalam S. and Mehendale S.G.(1981); Electrodissolution and corrosion of metals
by alternating currents; Journal of Electrochemical Society, India; 30-3, pp. 231-237.
Venkatesh S. and Chin D. (1979); The alternating current electrode processes; Israel
Journal of Chemistry, 18, pp.56-64.
13
Wu H., Kolbe E., Flugstad B., Park J.W., and Yongsawatdigul J.(1998); Electrical
properties of fish mince during multi-frequency ohmic heating; Journal of Food Science;
63(6), pp. 1028-1032.
Zoltai P. and Swearingen P. (1996); Product development considerations for ohmic
processing; J. Food Technology, 5, pp. 263-266.
14
I
Electrodes
~
V
Alternating
current power
supply
R
Electrical analogue
Food
ohmic heating
Figure 1.1: The concept of ohmic heating
Cd
Rs
Rct
Figure 1.2: A simplified electrical equivalent circuit of the interface during the
application of AC; Cd: electrical double layer capacitor, Rct: charge-transfer resistance,
Rs: electrolyte resistance.
15
Electrolysis
Electrode corrosion
Chlorine generation
H+ (or H2O)
Cl•
M2+(aqueous)
Cl2 (g)
H•
H2O
H2O2
H2 (g)
e
+
OH
H +e
OCl
Mn+(aqueous)
−
−
Fenton’s Reaction
•
•
OOH
OH
O2•−
Cl − + H2O
Fe3+
Fe2+
1
−
O2 (g)
(Singlet oxygen)
O2•−
H+
OH
H+
H2O2
3
O2 (g)
(Triplet dioxygen)
Figure 1.3: Mechanisms of generating free radicals by electrolysis and electrode
corrosion.
16
CHAPTER 2
ELECTROCHEMICAL BEHAVIOR OF VARIOUS ELECTRODE MATERIALS
DURING OHMIC HEATING AT pH 3.5, 5.0, AND 6.5
ABSTRACT
Undesirable electrochemical phenomena at electrode/solution interfaces during
ohmic heating can be avoided or effectively inhibited by choosing an appropriate
electrode material. We attempted to understand the electrochemical behavior of four
types of electrode materials: titanium, stainless steel, platinized-titanium, and graphite at
pH 3.5, 5.0, and 6.5. The electrodes were comparatively examined using 60 Hz sinusoidal
alternating current at a RMS voltage of 110 V. Analyses of surface morphologies of the
electrode surfaces, electrode corrosion, hydrogen gas generation, and pH change of the
heating medium were performed. The results highlight the relatively inert
electrochemical behavior of platinized-titanium electrodes at all the pH values. Pilot scale
study at 39.8 kW further demonstrates the potential use of platinized-titanium electrodes
for ohmic heating with commonly available low-frequency alternating currents. The
amounts of migrated Pt and Ti due to electrode corrosion were well below dietary
exposure limits of those elements.
17
INTRODUCTION
Electrodes in ohmic heating can be regarded as a ‘junction’ between a solid-state
conductor (i.e. current feeder) and a liquid-state conductor (i.e. heating medium). They
play a vital role by conveying the current uniformly into the heating medium. Various
materials, so far, have been used as electrodes in different ohmic heating studies and
applications. Those materials include platinized-titanium (Stirling, 1987; Tzedakis et al.,
1999), platinum (Tzedakis et al., 1999), titanium (Assiry, 1996), aluminum (Mizrahi et al,
1975; Uemura et al., 1994), carbon/graphite (Gatchell, 1935; Moses, 1938),
dimensionally stable anode (DSA)-type (Amatore et al., 1998), stainless steel (Assiry,
1996; Wu et al., 1998; Assiry et al., 2003), and rhodium plated stainless steel (Palaniappn
et al., 1991). During ohmic heating at low-frequency (50 - 60 Hz) alternating currents, it
was reported that corrosion of electrodes and apparent (partial) electrolysis were often
encountered with most of those electrodes.
In electrochemistry, it is generally known that both physical and chemical properties
of electrodes (specifically, the electrode surfaces) have a great influence on
electrochemical processes at the electrode/solution interfaces. With some electrodes, a
particular electrochemical reaction may occur slowly or not at all; but with another type
of electrodes, the same reaction may be faster under the same set of conditions. Such
information about electrodes under ohmic heating conditions is, therefore, important to
avoid or inhibit the electrochemical reactions by choosing appropriate electrode
materials.
The anodic and cathodic half-reactions of electrolysis have strong pH dependence
(see equations 2.1 - 2.3).
18
Anodic half-reaction:
2H2O (liquid) ⇔ O2 (g) + 4H+(aqueous) + 4e
(2.1)
Cathodic half-reactions:
2H+(aqueous) + 2e ⇔ H2 (g)
(2.2)
2H2O (liquid) + 2e ⇔ H2 (g) + 2OH−(aqueous)
(2.3)
The overall electrolysis reaction:
2H2O(liquid) ⇔ 2H2 (g) + O2 (g)
(2.4)
In general, pH also affects electrode corrosion. Moreover, food formulations subjected to
ohmic heating have various pH values. Therefore, the objectives of the present study
were to comparatively investigate the behavior of various electrode materials at different
pH, and to seek the extents of reactions in pilot scale during an ohmic heating process.
MATERIALS AND METHODS
Electrodes: Titanium, stainless steel (316), platinized-titanium, and graphite were used
as electrodes. All electrodes were rectangular (7.5 cm × 5.2 cm) with slight curvature
(radius ~ 4.5 cm) (see figure 2.1), and had the same geometric dimensions.
Chemicals: ACS grade sodium chloride (Fisher Scientific, NJ), Citric acid monohydrate
(Aldrich, WI), and sodium bicarbonate (Fisher Scientific, NJ); anhydrous sodium sulfate
(Cooper Natural Resources, Inc., TX); trace metal grade concentrated nitric acid (Fisher
Scientific, PA); AR grade dichloromethane (Mallinckrodt, KY), and concentrated HCl
(Fisher Scientific, NJ); HPLC grade acetonitrile (Mallinckrodt Baker Inc., NJ); and
TBDMS
{N-Methyl-N-[(tert-butyldimethyl)silyl]
19
trifluoroacetamide}
(Regis
Technologies, Inc., IL) were purchased from the suppliers. Demineralized double
distilled water (Resistivity: 3 megohm; pH: 5.5) was obtained from the Reagent
Laboratory Store, at The Ohio State University.
Scanning Electron Microscopic (SEM) Analysis
Surfaces of the electrodes before ohmic heating were examined by a JEOL JSM-820
scanning electron microscope.
Laboratory scale Ohmic heating
Experimental setup:
The extents of electrochemical reactions would be greater with a batch unstirred
ohmic heater compared to those with a flow-through stirred ohmic heater, since the flow
constantly dilutes the amounts of corrosion products in the heating medium, and the
stirring may disperse charge buildup at the electrode/solution interfaces minimizing
charge saturation of the electrical double layers. A batch unstirred ohmic heater was,
therefore, used for these experiments, in order to promote more pronounced
electrochemical behavior, which makes detection easy. The ohmic heater was powered
by typical low-frequency (60 Hz) sinusoidal alternating current. Figures 2.1 and 2.2 show
the ohmic heater and experimental setup, respectively. An external cooling system was
operated by a Haake F3 Fisons thermostatic water bath having inflow and outflow
attached to the ohmic heater. The cooling was required to perform the experiments for a
longer time, and at relatively low temperatures and currents to avoid potential explosion
20
hazards due to excessive hydrogen accumulation (in the headspace) and possible arc
formation.
Heating media:
Experiments were performed with each type of electrode at (initial) pH = 3.5, 5.0,
and 6.5 (at 25 °C) using freshly prepared aqueous heating media. The selected pH values
represent the entire pH range of food formulations subjected to ohmic heating. Those
desired pH values of the heating media were achieved by either citric acid or sodium
bicarbonate. Initial electrical conductivity of the heating media was adjusted by NaCl
(0.1%, w/v), and was kept constant at 2.35 mS cm-1 (at 25 °C) in this comparative study
to achieve prolonged heating exposure while controlling temperature and current at
intended levels. All the above components used for the pH and conductivity adjustments
are common ingredients in food formulations. The heating media were not buffered in
order to determine if pH changes were caused by electrochemical reactions.
Heating procedure:
A volume of 250.00 (± 0.12) ml was subjected to ohmic heating in each experimental
run. Effective geometric surface area of each electrode involved in ohmic heating was
constant at 21.5 cm2 with that volume. The ohmic heater was operated at a RMS voltage
of 110 V, which is the common single-phase public utility supply voltage in the US.
Duration of heating was kept constant for the purpose of comparison, and was limited to
307 seconds by the upper detection limit (250 ppm) of the hydrogen detector, and
pronounced hydrogen generation with some electrode – pH combinations. Since each
electrode material had a different electrical conductivity, and resulted in different
21
effective (capacitive) current, controlling temperature and current during ohmic heating
required different cooling water temperatures and/or input voltages, as described below.
Temperature of the cooling water was varied from 13 - 17 °C for the metallic
electrodes to yield the same time-temperature and time-current histories at all the pH
values during ohmic heating. However, since the graphite electrodes exhibited much
higher heating rates and currents that could not be compensated by external cooling,
those electrodes were examined at a RMS voltage of 97 (± 1) V, instead of the 110 V,
and cooling water temperature of 23 (± 1) °C to obtain the same time-temperature and
time-current histories as those of the metallic electrodes (see figures 2.3 and 2.4). Since
rates of electrochemical reactions depend upon the temperature and amount of current
(Venkatachalam et al., 1981) passing through the ohmic cell, this precise matching of
time-temperature and time-current histories was considered necessary to eliminate
temperature and current as variables. The voltage reduction, however, resulted in a
reduced power input for the same time-temperature and time-current histories. Therefore,
all data used for comparison were normalized on the basis of unit energy input.
The ohmic heating experiments were randomized with respect to the electrode
material at each pH. Three replications were made per material at each pH. The
electrodes were thoroughly rinsed using demineralized double distilled water before each
run. Adherent films formed on the titanium and stainless steel electrode surfaces during
ohmic heating were removed by brushing and cleaning after the three replicates at each
pH.
22
Analysis of electrode corrosion
Concentrations of Fe (from the stainless steel electrodes), Ti (from the titanium
electrodes), Pt (from the platinized-titanium electrodes), and elemental carbon (from the
graphite electrodes) migrated into the heating media were taken as measures of electrode
corrosion. In each experimental run, once the ohmic heating was completed, a 25.00 (±
0.03) ml sample was pipetted out after removing the electrodes, and thoroughly mixing
the fluid. A 25.00 (± 0.03) ml sample of the respective unheated heating medium was
used as a method blank. All the samples were collected into polypropylene sample
bottles, and then stabilized by adding concentrated nitric acid (5%, v/v). Quantitative
analyses of the metal ions were performed by a Perkin-Elmer Sciex ELAN 6100 DRC
inductively coupled plasma - mass spectrometer (ICP-MS) (AOAC, 2000). The elemental
carbon concentrations were determined by a Perkin-Elmer Optima 3000 DV inductively
coupled plasma - optical emission spectrometer (ICP-OES) monitoring the emission
spectra near 193.03 nm.
Hydrogen generation and pH measurements
A series U hydrogen detector (CEA Instruments, Inc., NJ) was used to measure
headspace hydrogen gas generated during ohmic heating. The pH of the medium before
and after the ohmic heating treatment was measured by a Cole-Parmer 59003 Benchtop
pH meter (resolution: 0.01 pH) at 25 °C.
23
Analysis of migrated graphite corrosion products
An aqueous heating medium was prepared using only NaCl, and without adding any
citric acid or sodium bicarbonate. Initial pH of the heating medium happened to be 5.52
at the same initial electrical conductivity. Ohmic heating was performed as previously
described, such that the time-temperature and time-current histories were as same as that
of the metallic electrodes. Analysis of electrode corrosion, and measurements of
hydrogen generation and pH were also carried out as described above. Identification of
the chemical nature of some elemental carbon species migrated into the heating medium
was attempted by using gas chromatography – mass spectrometry (GC-MS), and
electrospray ionization – mass spectrometry (ESI-MS), as described below.
GC-MS analysis:
The pH measurements revealed the migration of soluble acidic organic compounds.
Those compounds were extracted by 2.5 ml of dichloromethane after acidifying 250.00
(± 0.12) ml of the ohmically heated medium to a pH of 1 by adding 1:1 (v/v) HCl. The
extraction involved 1 minute shaking in a separating funnel, and a settling time of about 5
minutes. About 0.5 ml of dichloromethane extract could be collected, and it was
evaporated to dryness under reduced pressure at room temperature. The acidic
compounds were derivatized by a 1:2 mixture of TBDMS/ acetonitrial and incubating at
60 °C for 1 hour, for the GC-MS analysis. Unheated medium of the same volume
subjected to the same extraction and derivatization procedures was used as a method
blank; and was run under the same GC-MS conditions to identify the migrated acidic
organic compounds.
24
Gas Chromatography was carried out splitless through a 95% dimethyl/ 5% diphenyl
polysiloxane column (30 m × 0.32 mm ID; 0.25 µm film) using temperature ramps of 30
°C/ min from 40 to 180 °C and 7 °C/ min from 180 to 350 °C, with He as the carrier at
the flow rate of 2 ml/ min. The injector and GC-MS interface temperatures were at 222
and 260 °C, respectively. Mass spectra were acquired in scan mode within the m/z range:
42 – 839, at a rate of 1.4 scan/ sec. Structural identification of GC peaks was performed
by means of NIST-98 library search database.
ESI-MS analysis:
The migration of polar organic compounds was further detected by using a Bruker
Esquire electrospray ionization mass spectrometer operated in positive ion mode. Heating
medium samples before and after ohmic heating treatment were directly infused into the
electrospray source at 5-10 µl min-1 using a capillary voltage of 3500 V, source
temperature 250 °C, and nitrogen as the drying gas. Data were acquired using Bruker
Dolltonics DataAnalysis 2.0 software, in continuum mode until acceptable averaged data
were obtained.
Pilot scale study of electrode corrosion
An aqueous (tap water) solution having initial pH 3.5, and initial electrical
conductivity 1 Sm-1 was used as the heating medium. The pH and electrical conductivity
were adjusted by citric acid monohydrate (0.03 %, w/v), and sodium sulfate (0.8 %, w/v),
respectively. The pilot scale flow-through ohmic heater contains three cylindrical ohmic
cells. Each cell consisted of two platinized-titanium electrodes; with each electrode
having a surface area of 142.7 cm2.
25
Pre-heated (by steam) heating medium at 60 °C was pumped through the ohmic
heater followed by cooling in a scraped-surface heat exchanger. The heater was operated
at 39.8 kW using 60 Hz sinusoidal alternating current under steady state conditions with
continuous liquid flow of 6.8 liters min-1. The inlet and outlet temperatures to the ohmic
heater were 58 and 138 °C, respectively.
Approximately 1 liter samples were collected both from the feed tank (initially
containing 220 liters) and after running through the heat-hold-cool cycle of the ohmic
processor at steady state. From each of the 1 liter samples, a 25.00 (± 0.03) ml sample
was pipetted out into a polypropylene sample bottle, and then stabilized by adding
concentrated nitric acid (5%, v/v) for the chemical analysis. The above sampling
procedure was carried out two times for each experimental run, and the whole experiment
was triplicated. Pt and Ti concentrations were determined by a high resolution (double
focusing sector based) ThermoFinnigan Element 2 inductively coupled plasma – mass
spectrometer (ICP-MS).
Data analysis
Total energy input, in the laboratory scale studies, was determined by integrating the
power input (Pinput = Vrms Irms) vs. time curve for each experimental run. Electrode
corrosion normalized per unit energy input was defined as ‘corrosion rate’. Descriptive
statistics including means and standard deviations were calculated for the quantitative
measurements. Two-factor analysis of variance was used to determine if the type of
electrode material and pH had significant effects on corrosion rate. Changes in pH of the
heating media observed with some electrodes were analyzed by one-factor analysis of
26
variance to determine the effect of pH. Difference between the Pt and Ti concentrations
in the pilot scale study was evaluated by two-sample paired t-test. Tukey’s specific
comparison test determined which particular means were significantly different.
Significance of differences was defined as p≤ 0.05. SPSS 11.5 for windows (SPSS Inc.,
2002) statistical software package was used for the statistical analyses.
RESULTS AND DISCUSSION
Electrode corrosion
An important consideration in ohmic heating is the amount and the chemical nature
of corrosion products migrated into the food during the application of electrical power.
Table 2.1 shows a comparison of corrosion rates (in ppb/ KJ of energy input) of the
electrode materials with respect to the migrations of their major (surface) elements at
different pH values. Analysis of variance suggests that both the type of electrode material
and pH, as well as their interactions have significant effects on corrosion rate. As can be
seen from table 2.1, the corrosion rates of titanium and platinized-titanium electrodes are
not significantly different at any pH, however these are significantly lower (p ≤ 0.05)
than the corrosion rates of stainless steel and graphite electrodes. On the other hand, the
corrosion rates of stainless steel and graphite electrodes are not significantly different
except at pH 3.5. Each of the electrode material exhibits a higher corrosion rate at pH 3.5
than that of the other pH values.
27
SEM Analysis
Figures 2.5 - 2.8 show typical SEM micrographs (magnification: × 2000;
accelerating voltage: 20 kV) of the four electrodes before the ohmic heating experiments.
Although the apparent geometric surface area of the electrodes involved in the ohmic
heating was kept constant, surface morphologies imply the existence of markedly
different surface areas in the micro scale. Since the capacitance is directly proportional to
the surface area, an electrode having a larger microscopic surface area possesses higher
electrical double layer capacitance per unit apparent geometric surface area. The
electrical double layer of such an electrode is capable of holding more charges before the
double layer capacitor becomes ‘leaky’ inhibiting faradaic-type reactions at the electrode/
solution interface. Moreover, the current flowing through the interface then becomes
mostly capacitive. Therefore, the use of electrodes having large microscopic surface area
is beneficial in terms of inhibiting the faradaic processes at the interfaces, as well as
achieving more capacitive current for the heat generation. Amatore et al. (1998) have
already introduced this concept suggesting the use of electrodes with large surface
roughness. However, the applications of highly porous electrodes may be hindered
because of the possibility of harboring tiny food particles, microorganisms, or
macromolecular food components in the interstitial spaces within the electrodes.
As can be seen from figure 2.5, the titanium electrodes had a smooth surface
indicating virtually no enhancement of surface area in the micro scale, and thereby a poor
double layer capacitance. The SEM micrograph of the stainless steel electrodes (figure
2.6) also indicates no considerable enhancement of the microscopic surface area,
although it appears to have some cracks and valleys on the surfaces. The tiny cracks,
28
valleys, and bumps spread over the surfaces of platinized-titanium electrodes (see figure
2.7) would certainly enhance the microscopic surface area possessing a much higher
double layer capacitance. Clearly, the graphite electrodes had a very large microscopic
surface area because of the high surface roughness (see figure 2.8) possessing the highest
capacitance out of the four electrodes. The highly efficient heat generation attributed by
this huge capacitance could not be controlled by the external cooling system; and
therefore necessitated a lower voltage, instead of the 110 V, in order to obtain the same
time-temperature and time-current histories as that of the metallic electrodes. According
the SEM analysis, variation of the double layer capacitance of the electrodes can be
represented as: titanium < stainless steel << platinized-titanium << graphite.
Titanium electrodes
Titanium is considered to have high corrosion resistance and biocompatibility
characteristics (Assiry, 1996). It is generally the material-of-choice for chloride
environments. Although the SEM micrograph indicates poor double layer capacitance,
the corrosion rates of titanium electrodes at all the pH values were significantly lower (p
≤ 0.05) than those of the stainless steel and graphite electrodes. The possible reason could
be the oxide layer that covers active titanium metal, protecting it against corrosion. Since
titanium exhibits high affinity towards oxygen (James et al., 1976), the protective oxide
layer could be formed by reacting with atmospheric oxygen even before using the metal
as electrodes, and also during the ohmic heating treatments. Tzedakis et al. (1999)
discussed the possibility of forming rutile (TiO2) due to electrochemical processes during
ohmic heating, partially passivating the titanium electrodes.
29
Titanium forms various oxides having different colors, such as TiO2 – anatase
(yellowish), TiO2 – rutile (white), TiO1.9 - oxygen deficit (bluish), and Ti3O5 (violet)
(James et al., 1976). At all the pH values, we observed formations of adherent surface
films with a yellowish-brown color with some blue and violet coloration, which
therefore, imply the electrochemical generation of oxygen during ohmic heating.
However, there was no detectable pH change of the heating medium at any pH. The
hydrogen generation shown in figure 2.9, therefore, probably indicates the occurrence of
electrolysis (equation 2.4) on the passivated titanium electrode surfaces. It can be seen
that the probable electrolysis becomes more pronounced at pH 6.5 than that of the other
pH values. Because of the high penetration ability of hydrogen into titanium, the
hydrogen generation may cause surface embrittlement followed by surface disintegration
enhancing electrode corrosion (James et al., 1976). Under acidic conditions (i.e. pH 3.5),
however, TiO2 can undergo the following cathodic half-reactions (James et al., 1976)
migrating titanium ions into the heating medium resulting in a higher corrosion rate.
TiO2 (solid) + 4H +(aqueous) + 2e ⇔ Ti 2+(aqueous) + 2 H2O (liquid)
(2.5)
TiO2 (solid) + 4H +(aqueous) + e ⇔ Ti 3+(aqueous) + 2 H2O (liquid)
(2.6)
Stainless steel electrodes
Stainless steel is an iron-chromium alloy containing at least 11% chromium. The
grade designated by 316 belongs to austenitic family of stainless steels, and contains
chromium (17%), nickel (10%), and molybdenum (2%) as major alloying elements
(Redmond, 1996). In the food industry, stainless steels are in widespread use as food
contact surfaces. The stainless steel electrodes exhibited pronounced corrosion rates,
30
hydrogen generation, and also pH changes of the heating media at all the pH values. In
addition to the chemical reactivity of stainless steel, the lack of double layer capacitance,
as implied by the SEM analysis, would be responsible for the pronounced
electrochemical behavior. The observed adherent surface films formed on the stainless
steel electrodes during ohmic heating, were transparent, with a light golden color and
some brown rust. The films, however, did not uniformly cover the electrode surfaces, and
showed several cracks. Since there were pH changes in the heating media (see table 2.2),
the hydrogen generation shown in figure 2.10 does not solely represent the overall
electrolysis reaction (equation 2.4). Based on the observations, assignment of
predominant electrochemical reactions is attempted as follows.
At pH 3.5, the following anodic half-reaction (equation 2.7) could couple with the
cathodic half-reaction 2.2 resulting accelerated corrosion and hydrogen generation
compared to those of the other pH values, as given by the overall reaction 2.8.
Anodic half-reaction: M (solid) ⇔ M2+(aqueous) + 2e, (where, M = Fe, Cr, Ni, Mo)
(2.7)
Overall reaction: M (solid) + 2H+(aqueous) ⇔ M 2+(aqueous) + H2 (g)
(2.8)
The resulting pH change (i.e. increase of pH) could be attributed to the loss of H+(aqueous)
ions as the hydrogen gas. Further, under the acidic conditions, the migrated Fe2+(aqueous)
ions might undergo Fenton’s reaction (equation 2.9) liberating OH−(aqueous) ions into the
heating medium (Tomat et al., 1979).
Fe2+(aqueous) + H2O2 ⇔ Fe3+(aqueous) + •OH + OH−(aqueous)
(2.9)
The required H2O2 for the above reaction can be generated by the following cathodic
half-reaction (equation 2.10).
O2 (g) + 2H+(aqueous) + 2e ⇔ H2O2
31
(2.10)
At the other pH values, the significantly high (p ≤ 0.05) pH changes associated with
corrosion and hydrogen generation can be explained by the following overall reaction
(equation 2.11), which is the combination of the anodic half-reaction 2.7 and cathodic
half-reaction 2.3.
M (solid) + 2H2O (liquid) ⇔ M2+(aqueous) + H2 (g) + 2OH−(aqueous)
(2.11)
It may also be possible to have the pH changes together with the generation of hydrogen
and chlorine gases due to the association of the following anodic half-reaction 2.12 and
the cathodic half-reaction 2.3 giving the overall reaction 2.13.
Anodic half-reaction:
2Cl −(aqueous) ⇔ Cl2 (g) + 2e
Overall reaction: 2H2O (liquid) + 2Cl −(aqueous) ⇔ Cl2 (g) + H2 (g) + 2OH−(aqueous)
(2.12)
(2.13)
Platinized-titanium electrodes
Platinization of titanium electrodes has been a popular choice because of the high
cost of pure platinum electrodes for industrial processes (Indira et al., 1968; Iniesta et al.,
1999). On the other hand, platinization is also an effective method of passivating titanium
(James et al., 1976). The platinized- titanium electrodes exhibited significantly lower (p ≤
0.05) corrosion rates compared to those of the stainless steel and graphite electrodes at all
the pH values. Further, there were no signs of hydrogen or any other gas evolution at the
electrode/solution interfaces, and also no detectable pH change of the heating medium at
any pH. The rich double layer capacitance, as indicated by the SEM analysis, would be
the major reason for this superior electrode performance. Tzedakis et al. (1999) have
already demonstrated the superiority of platinized-titanium electrodes over platinum
electrodes for ohmic sterilization of food products with low-frequency alternating
32
currents. Iniesta et al. (1999) reported that the large surface area of platinized-titanium
electrodes also affects adsorption controlled surface processes, such as hydrogen
adsorption-desorption, and the surface oxidation.
Graphite electrodes
Graphite, one of the allotropic forms of carbon, has been used as an electrode
material in electrochemical applications for a long time. Although there are various types
of commercially available graphitic carbons, polycrystalline graphite (PCG) is the
material most often referred to as ‘Graphite’ (McCreery, 1999). In spite of the very rich
double layer capacitance as indicated by SEM analysis, the corrosion rates of graphite
electrodes at all the pH values were significantly greater (p ≤ 0.05) than those of titanium
and platinized-titanium electrodes. However, as in the case of platinized-titanium
electrodes, there were no signs of gas evolution at the electrode/solution interfaces, and
also no detectable pH change of the heating medium at any pH. The above
electrochemical behavior can be explained by means of the chemical structure of
graphite, as follows.
Graphite consists of sp2 hybridized carbon atoms arranged as parallel sheets of
hexagonal rings. Since sp2 hybridized carbon is capable of forming covalent bonds and
has a propensity towards adsorption of a broad range of substances, graphite electrode
surfaces usually contain various functional groups and oxides (McCreery, 1999).
Therefore, the migration of surface functional groups and oxides as organic compounds
was anticipated during ohmic heating. In the analysis of those migratory corrosion
products, the heating medium exhibited about – 0.03 pH units change indicating the
33
migration of acidic organic compounds. The observed corrosion rate was 6.1 ± 0.1 ppb
KJ-1. Figure 2.11 shows the identified organic compounds by GC-MS analysis. The
quasimolecular ion [M+H]+ peaks observed at m/z 91 and m/z 133 in the ESI-MS
spectrum of the heating medium after the ohmic heating treatment (see figure 2.12)
correspond well with the identified polar organic compounds by the GC-MS. In addition,
the peaks observed at m/z 109 and m/z 123 may represent [M+H]+ ions for ortho- or
para-quinones (C6H4O2) and their corresponding methylated counterparts, respectively.
Such types of quinones are reported to be present on most carbon surfaces (McCreery,
1999; Tarasevich et al., 1987). It is clearly seen that the organic compounds migrated into
the heating medium always contain more than one carbon atom per molecule. Therefore,
migration of even a few molecules results in intense corrosion rates if the corrosion is
measured as elemental carbon.
Since sp2 hybridized carbon has a high affinity towards oxygen (McCreery, 1999),
surface oxides and oxygen-containing functional groups could be formed by reacting
with atmospheric oxygen even before using the graphite as electrodes, and also due to the
electrochemical reactions during ohmic heating. Soffer et al. (1972) suggested the
following anodic and cathodic half-reactions related to the electrolysis of water creating
oxides and functional groups on graphite electrode surfaces.
Anodic half-reactions:
C + H2O (liquid) ⇔ C − O + 2H +(aqueous) + 2e
(2.14)
C + H2O (liquid) ⇔ C − OH + H +(aqueous) + e
(2.15)
34
Cathodic half-reaction:
C + H2O (liquid) + e ⇔ C − H + OH−(aqueous)
(2.16)
where ‘C’ represents sp2 hybridized carbon on the graphite surface. During ohmic
heating, once a set of carbon atoms leaves the graphite surface as organic compounds a
new set of sp2 hybridized carbon atoms in the graphite structure is exposed to the heating
medium, and keep undergoing the corrosion process. The migration of compounds into
the heating medium could be due to thermal, electric field, and pH (of the heating
medium) effects. The significantly high (p ≤ 0.05) corrosion rate observed at pH 3.5 may
be due to acid catalyzed hydrolyses of ester and ether linkages on the graphite electrode
surfaces facilitating the migration of functional groups as compounds than at the other pH
values.
The equations 2.14 - 2.16 basically indicate the adsorption of electrolysis products on
the electrodes causing oxidation and reduction of the surfaces, ultimately creating some
functional groups. Soffer et al. (1972) also reported the adsorption of chloride ions on the
graphite electrodes according to the following anodic half-reaction.
C + Cl −(aqueous) ⇔ C− Cl + e
(2.17)
Such adsorption processes as well as very rich double layer capacitance apparently
inhibited the gas evolution at the electrode/solution interfaces. Although there were no
detectable pH changes of the heating media in the presence of citric acid and sodium
bicarbonate, pH change of the electrolyte due to positive and negative charging of the
electrodes is considered to be a unique property of high surface area graphite electrodes
(Soffer et al., 1972). In general, graphite electrodes are also considered to have a wider
faradaic reaction-free potential window compared to that of the metallic electrodes
35
(McCreery, 1999). However, ordinary faradaic processes, such as generation of hydrogen
and oxygen due to the electrolysis of water without adsorption on the surfaces, can also
take place irrespective of the chemical nature of the surface groups (Soffer et al., 1972).
Pilot scale study of electrode corrosion
In the laboratory scale studies, platinized-titanium exhibited the best electrode
performance out of the four electrodes tested. Therefore, it was subjected to further
investigation on a pilot scale for electrode corrosion. Table 2.3 shows Pt and Ti
concentrations of the ohmically heated medium after running through the heat exchanger
when the ohmic heater was at the steady state. The concentrations of Pt and Ti in the
blank (i.e. the heating medium in the feed tank before being subjected to ohmic heating)
were too low for reliable measurements. Since platinized-titanium exhibited the highest
corrosion at pH 3.5 in the laboratory scale studies, the values shown in table 2.3 would be
the ‘worst-case’ concentrations. Using those concentrations, intakes of the metal
contaminants were evaluated with respect to a typical meal of 8 oz (227 g) comparing
with recently published upper-level daily dietary exposure limits for adult consumers (see
table 2.4). It can be seen that the estimated metal intakes via consumption of an
ohmically heated meal of 8 oz are far below the published upper-level daily dietary
exposure limits. Therefore, ohmic heating may be performed in pilot scale without
significant electrode corrosion using platinized-titanium electrodes; and the migrations of
Pt and Ti may result in concentrations that are far below the published dietary exposure
limits.
36
CONCLUSIONS
Using the alternating frequency of 60 Hz, we demonstrated that electrochemical
behavior of an electrode material is unique to the material itself. Although, in general, the
large microscopic surface area can suppress the electrochemical processes, the type and
extent of electrochemical reactions are determined by the chemical nature of the electrode
surface, as well as the pH of the heating medium. All the electrode materials exhibited
intense electrode corrosion at pH 3.5 compared to that of the other pH values. Although
the titanium electrodes were having a relatively high corrosion resistance, apparent
electrolysis was seen at all the pH values during ohmic heating. Stainless steel was found
to be the most electrochemically active electrode material during ohmic heating at all the
pH values. It was proven that, the intense corrosion of graphite electrodes was due to the
migration of surface functional groups and oxides as organic compounds during ohmic
heating; and the pH of the heating medium seemed to facilitate such migrations. Because
of the relatively inert electrochemical behavior, platinized-titanium would be the
electrode material-of-choice for ohmic heating at all the pH values. The potential use of
platinized-titanium electrodes for ohmic heating operations was further demonstrated in
pilot scale at 39.8 kW; and the concentrations of migrated Pt and Ti were far below the
published dietary exposure limits.
37
SYMBOLS
e
electron
Irms
RMS current (A)
MW
molecular weight
m/z
mass to charge ratio
Pinput
power input (W)
ppb
parts per billion
ppm
parts per million
ppt
parts per trillion
RMS
root-mean-square
Vrms
RMS voltage (V)
38
REFERENCES
Amatore C., Berthou M., and Hebert S.(1998); Fundamental principles of
electrochemical ohmic heating of solutions; Journal of Electroanalytical Chemistry; 457,
pp. 191-203.
Assiry A.M (1996); Effect of ohmic heating on the degradation kinetics of ascorbic acid;
PhD thesis, The Ohio State University.
Assiry A., Sastry S.K., and Samaranayake C. (2003); Degradation kinetics of ascorbic
acid during ohmic heating with stainless steel electrodes, Journal of applied
electrochemistry, 33 (2), pp. 187-196.
Getchell B.E.(1935); Electric pasteurization of milk; Agr. Eng., 16(10), pp.408-410.
Indira K.S, Sampath S., and Doss K.S.G.(1968); Recent trends of platinized titanium as
node material in electrochemical industries; Chemical processing & Engineering
(February), pp. 35-37.
Iniesta J., Gonzalez-Garcia J., Fernandez J., Montiel V., and Aldaz A. (1999); On the
Voltammetric behavior of platinized titanium surface with respect to the specific
hydrogen and anion adsorption and charge transfer processes; Journal of materials
chemistry, 9 (12), pp. 3141-3145.
James W.J., and Straumanis M.E.(1976); Encyclopedia of electrochemistry of the
elements, Bard A.J. (Ed.), Vol.(V), Chapter V-7 : Titanium; Marcel Dekker Inc., New
York, pp. 305 – 386.
McCreery R.L.(1999); Interfacial Electrochemistry: Theory, Experiment, and
Applications, Wieckowski A. (Ed.), Chapter 35 : Electrochemical properties of carbon
surfaces; Marcel Dekker Inc., New York, pp. 631 – 646.
Mizrahi S., Kopelman I.J., and Perlman. J.(1975); Blanching by electroconductive
heating; J. Food Technology, 10, pp. 281-288.
Moses B.D. (1938); Electric pasteurization of milk; Agr. Eng., 19(12), pp.525-526.
Official methods of analysis of AOAC International – 17th edition (2000); Vol.1
(Agricultural Chemicals, Contaminants, Drugs), Ch.9: metals and other elements at trace
levels in foods, pp. 46-59.
39
Palaniappan S., and Sastry S. (1991); Electrical conductivity of selected juices:
Influences of temperature, solid content, applied voltage, and practical size; Journal of
Food Process Engineering, 14, pp. 247-260.
Redmond J.D. (1996); Marks’ Standard handbook for mechanical engineers (Tenth
Edition); McGraw – Hill Companies, Inc., pp. 6(32) – 6(33).
Reilly C.; Metal contamination of food, second edition (1991); Elsevier Science
Publishers Ltd., New York; pp. 14-15 & 235-237.
Soffer A., and Folman M. (1972); The electrical double layer of high surface porous
carbon electrodes; Journal of Electroanalytical Chemistry, 38, pp.25 – 43.
Stirling R. (1987); Ohmic heating - a new process for the food industry; Power
Engineering Journal; 1(6), pp. 365-371.
Tarasevich M.R., Bogdanovskaya V.A., and Zagudaeva N.M.(1987); Redox reactions of
quinones on carbon materials; Journal of Electroanalytical Chemistry, 223, pp.161-169.
Tomat R., and Rigo A. (1979); Oxidation of polymethylated benzenes promoted by •OH
radicals; Journal of Applied Electrochemistry, (9), pp. 301-305.
Tzedakis T., Basseguy R., and Comtat M. (1999); Voltammetric and coulometric
techniques to estimate the electrochemical reaction rate during ohmic sterilization;
Journal of Applied Electrochemistry; 29(7), pp. 821- 828.
Uemura K., Noguchi A., Park S.J., and Kim D.U. (1994); ohmic heating of food
materials- Effect of frequency on the heating rate of fish protein; Developments in Food
Engineering - Proceedings of the 6 th International Congress on Engineering and Food;
Blackie Academic & Professional Press, London, pp. 310-312.
Venkatachalam S. and Mehendale S.G.(1981); Electrodissolution and corrosion of metals
by alternating currents; Journal of Electrochemical Society, India; 30-3, pp. 231-237.
Wu H., Kolbe E., Flugstad B., Park J.W., and Yongsawatdigul J.(1998); Electrical
properties of fish mince during multi-frequency ohmic heating; Journal of Food Science;
63(6), pp. 1028-1032.
Ysart G., Miller P., Crews H., Robb P., Baxter M., De L’Argy, Lofthouse S., Sargent C.,
and Harrison N. (1999); Dietary exposure estimates of 30 elements from the UK total diet
study; Food Additives and Contaminants, Vol.16 (9), pp. 391-403.
40
Corrosion rate* (ppb/ KJ)
Electrode type
pH = 3.5
pH = 5.0
pH = 6.5
Titanium(Ti)
0.26 a (0.21)
0.03 a (0.01)
0.05 a (0.03)
Stainless steel (Fe)
14.20 b (1.95)
8.33 c (0.30)
11.43 b,e (1.51)
Platinized-titanium (Pt)
0.25 a (0.10)
0.07 a (0.04)
0.05 a (0.02)
Graphite (C)
26.6 d (2.2)
7.2 c (0.0)
8.4 c,e (1.1)
* Numbers in parentheses represent standard deviations of the means (n=3). Different
superscript letters with the means denote significant differences (p ≤ 0.05).
Table 2.1: Comparison of corrosion rates (in ppb per KJ) of the electrode materials with
respect to the migrations of their major (surface) elements at different pH values.
∆ pH*
pH = 3.5
pH = 5.0
pH = 6.5
+ 0.04 a (0.01)
+ 0.18 b (0.03)
+ 0.10 c (0.01)
* Numbers in parentheses represent standard deviations of the means (n=3). Different
superscript letters with the means denote significant differences (p ≤ 0.05). Positive signs
indicate increase of pH.
Table 2.2: pH changes of the heating media observed with stainless steel electrodes at
different pH values.
41
Element
Concentration* / ppt
Pt
61.6 a (10.3)
Ti
69.2 a (14.6)
* Numbers in parentheses represent standard deviations of the means (n=6). The means
with the same superscript letter are not significantly different (p > 0.05).
Table 2.3: Pt and Ti concentrations (in parts per trillions) of the ohmically heated heating
medium in the pilot scale study.
Element
Estimated intake via 8 oz
meal (µg)
Published upper-level daily
dietary exposure limits (µg/ day)
Pt
0.014
0.3*
Ti
0.016
600**
* Ysart et al. (1999)
** Reilly (1991)
Table 2.4: Comparison of estimated metal intakes via consumption of an 8 oz ohmically
heated meal with the published upper-level daily dietary exposure limits for adult
consumers. The estimation is based on unit conversions: 1 ppt = 1 picogram/ g; 8 oz =
227 g; 1 picogram = 10-12 g = 10-6 µg.
42
2
1
6
5
4
3
1 – Hydrogen gas sensor
2 – Teflon coated thermocouple
3 – Electrodes: slightly curved electrodes (electrode gap: 9 cm)
4 – Cooling water inlet
5 – Cooling water outlet
6 – Removable lid: the electrodes, hydrogen sensor, and thermocouple, are attached. It
was tightly clamped to the cell body during ohmic heating.
Figure 2.1: The laboratory scale ohmic heater.
43
Thermocouple
ohmic heater
Isolation
module
Hydrogen
gas meter
0 – 250 ppm
60 Hz/ 0-110 V ~
Public utility
Supply
V
A
Variac
Data
logger
V = voltage transducer
A = Current transducer
Microcomputer
Figure 2.2: Schematic diagram of the laboratory scale experimental setup.
44
40
0
Temp./ C
50
30
20
10
0
0
100
200
300
time/ seconds
Figure 2.3: Typical time vs. temperature curve for all electrodes at all pH values during
ohmic heating.
2.00
Current/ A
1.50
1.00
0.50
0.00
0
100
200
300
time/ seconds
Figure 2.4: Typical time vs. current curve for all electrodes at all pH values during ohmic
heating.
45
Figure 2.5: Typical SEM micrograph of titanium electrodes.
46
Figure 2.6: Typical SEM micrograph of stainless steel electrodes.
47
Figure 2.7: Typical SEM micrograph of platinized-titanium electrodes.
48
Figure 2.8: Typical SEM micrograph of graphite electrodes.
49
250
pH = 3.5
pH = 5.0
pH = 6.5
200
[H2 ]
p 150
p
m 100
50
0
0
100
200
300
time/ seconds
Figure 2.9: Hydrogen generation with titanium electrodes during ohmic heating.
250
pH = 3.5
200
pH = 5.0
[H2]
p 150
p
m 100
pH = 6.5
50
0
0
100
200
300
time/ seconds
Figure 2.10: Hydrogen generation with stainless steel electrodes during ohmic heating.
50
O
OH
O
OH
OH
OH
(a)
(b)
Figure 2.11: Identified graphite corrosion products by GC-MS analysis; (a) 2-hydroxy,
propanoic acid (lactic acid) (MW: 90), (b) 2-hydroxy, 4- methyl, pentanoic acid (MW:
132).
51
Int.
× 104
0.5
After
91.0
Int.
×104
Before
123.0
149.0
2.5
0.0
200 m/z
100
2.0
75.1
149.0
109.0
1.5
133.0
1.0
0.5
0.0
20
40
60
80
100
120
140
160
180
m/z
Figure 2.12: Positive ion ESI-MS spectra of the heating medium before and after ohmic
heating.
52
CHAPTER 3
EFFECT OF PULSED OHMIC HEATING ON ELECTROCHEMICAL
REACTIONS
ABSTRACT
Minimization of electrochemical reactions during ohmic heating would be desirable.
This study examines a pulsed ohmic heating technique to determine its effect on
electrochemical reactions. Effects of pulse parameters, such as frequency, pulse width,
and delay time were studied, in comparison with conventional ohmic heating (60 Hz, sine
wave) using various electrode materials. Analyses of electrode corrosion, hydrogen gas
generation, and pH change of the heating medium were performed. The results suggest
that pulsed ohmic heating is capable of significantly reducing the electrochemical
reactions of stainless steel, titanium, and platinized-titanium electrodes, in comparison to
conventional 60 Hz ohmic heating. The importance of allowing enough delay time for
discharge of the electrical double layers after each pulse input is emphasized.
53
INTRODUCTION
In ohmic heating, the current is ideally used only for heat generation; and
electrochemical processes at the electrode/ solution interfaces must be avoided or
significantly inhibited. It is already known that the above criteria can be easily achieved
by using high frequency alternating currents allowing only minimal charging of electrical
double layers (Amatore at al., and Wu et al., 1998). However, the potential use of high
frequency generators, especially for industrial scale ohmic heating, is principally limited
by cost considerations. In recent years, power semiconductor devices have developed
significantly; and now their applications are emerging in many disciplines (Baliga, 1998;
Grant, 1996). Insulated Gate Bipolar Transistor (IGBT), a member of a broad power
semiconductor family, is basically a rapid switching device that enables the application of
current and voltage as high frequency short duration pulses. The use of such a device for
ohmic heater circuitry is a relatively inexpensive alternative route of moving into high
frequencies.
Pulse waveforms derived from an IGBT can be independently manipulated by
adjusting various pulse parameters including frequency, pulse width, and delay time (offtime between adjacent pulses); and therefore differ from the waveforms typically
generated by means of high frequency generators. So far such pulse application for the
purpose of ohmic heating has not been reported. Furthermore, the effects of these pulse
parameters on electrochemical reactions during ohmic heating have not yet been
understood. Therefore, the objective of this study was to test the effect of using pulse
inputs derived from an IGBT on the electrochemical reactions during ohmic heating, in
comparison with conventional ohmic heating (60 Hz, sine wave). The results would be
54
expected to provide basic understanding of operating conditions, which would be useful
in future applications of pulsed ohmic heating in the food industry.
MATERIALS AND METHODS
Chemicals: ACS grade sodium chloride (Fisher Scientific, NJ), and citric acid
monohydrate (Aldrich, WI); and trace metal grade concentrated nitric acid (Fisher
Scientific, PA) were purchased from the suppliers. Demineralized double distilled water
(Resistivity: 3 megohm; pH: 5.5) was obtained from the Reagent Laboratory Store, at
The Ohio State University.
Experimental setup
The laboratory scale ohmic heater shown in figure 3.1 was used for both pulsed and
conventional (60 Hz, sine wave) ohmic heating experiments. The ohmic heater was
connected to an IGBT power supply that was capable of delivering bipolar potential
pulses, as shown in figure 3.2. The IGBT power supply had a fixed peak voltage (Vp) of
170 V with switching frequency up to 10 kHz. The same experimental setup attached to a
variac and powered by the public utility supply, instead of the IGBT power supply, was
used for the conventional ohmic heating experiments. Stainless steel (316), titanium,
platinized-titanium, and graphite were used as electrodes in all the experiments, except in
the study on delay-time effects, where only stainless steel was used. All electrodes were
rectangular (7.5 cm × 5.2 cm) with a slight curvature (radius ~ 4.5 cm) (see figure 3.1),
and had the same geometric dimensions. The effective geometric surface area of each
55
electrode involved in ohmic heating was constant at 21.5 cm2 with a volume of 250.00 (±
0.12) ml of heating medium.
An external cooling system was operated by a Haake F3 Fisons thermostatic water
bath having inflow and outflow attached to the ohmic heater. The cooling was required
for precise matching of time-temperature history of each experiment (see figure 3.3).
Since reaction rates depend upon temperature, performing the experiments under equal
temperature conditions was considered necessary to eliminate temperature as a variable.
The cooling also allowed for prolonged heating exposure at relatively low temperatures
minimizing safety risks while increasing the extent of electrochemical reactions, and
facilitating detection.
Heating media
Experiments were performed at an initial pH of 3.5 (at 25 °C) using freshly prepared
aqueous heating media. This particular pH value was specifically chosen, since it was
determined from our previous studies (chapter 2) to be the pH of the worst-case scenario
for all electrode materials with respect to corrosion. The desired pH value of the heating
media was achieved by citric acid. Initial electrical conductivity was adjusted by NaCl,
and was varied for different experiments (described below) to maintain the same timetemperature history. The above components used for the pH and conductivity adjustments
are common ingredients in food formulations. The heating media were not buffered to
determine if pH changes were caused by electrochemical reactions.
56
Experimental procedure
Effects of frequency and pulse width:
To study the effects of frequency and pulse width, two switching frequencies, 10 and
4 kHz, were chosen. These were considered as representing upper and lower frequency
ranges for minimized electrode corrosion, based on the findings of Wu et al. (1998) who
reported drastically reduced corrosion of stainless steel electrodes at frequencies > 5 kHz.
Figure 3.4 shows the centering of bipolar pulses within the period (T) at a given
frequency (f). Both positive and negative pulses of the bipolar pulse inputs had the same
pulse width (tp), and were equally spaced by adjusting the delay time (td) according to the
following relationship.
T = 2 (tp + td)
(3.1)
The values for pulse width were arbitrarily chosen allowing at least 15 µs delay time.
From preliminary experiments, we found that about a 10 – 15 µs delay time was
necessary to prevent hydrogen generation, and to yield symmetric positive and negative
pulses. Since varying the frequencies and pulse widths essentially varies the duty cycle
(θ) and thereby power input (Pinput) (see equations 3.2 and 3.3), the uses of different
initial electrical conductivities and cooling water temperatures were necessitated to
maintain the same time-temperature history. Under these circumstances, all data used for
comparison were normalized on the basis of unit energy input.
θ = 2tp / T
Pinput = VpIpθ
(3.2)
(3.3)
A volume of 250.00 (± 0.12) ml was subjected to ohmic heating in each experimental
run. Tables 3.1 – 3.4 show the frequency-pulse width combinations with required initial
57
electrical conductivities, and cooling water temperatures for various electrode materials
used in this study. Duration of heating was kept constant with all the experiments for the
purpose of comparison (184 seconds). The shapes of corresponding pulse waveforms for
voltage and current are shown in figures 3.5 and 3.6. The conventional ohmic heating
was carried out with all electrodes at a RMS voltage of 110 V, which is the common
single-phase public utility supply voltage in the US.
The ohmic heating experiments of each electrode material were completely
randomized with respect to order of experimentation. The electrodes were thoroughly
rinsed using demineralized double distilled water before each run. All experiments were
triplicated; and adherent films formed on the stainless steel and titanium electrode
surfaces during ohmic heating were removed by brushing and cleaning after each three
replicates. Analyses of electrode corrosion, hydrogen generation, and pH measurements
were performed according to the procedures described below.
Effect of delay time:
Preliminary experiments were conducted at the frequencies and pulse widths
described in tables 3.1 – 3.4 using all electrode materials, in parallel with the above
experiments. Figure 3.7 shows the centering of bipolar pulses within the period (T) at a
given frequency (f) obeying the following relationship.
T = 2tp + td1 + td2
(3.4)
The delay time denoted by td1 was varied from 0 to td1 = td2 (i.e. until pulses were equally
spaced). These experiments indicated hydrogen generation at all the frequencies and
pulse widths, and with all electrode materials, when td1 = 0 µs. Interestingly, unlike in
58
conventional ohmic heating, the gas evolution was seen only at the electrode to which the
neutral wire was attached. This hydrogen generation was more pronounced at 10 kHz
with shorter pulse widths (where heating media contained higher NaCl concentrations).
We further noticed that the hydrogen generation was gradually diminished with
increasing td1, and completely disappeared when td1 > 10 µs.
In order to illustrate the effect of delay time, a heating procedure with analysis of
electrode corrosion was carried out at the frequency of 10 kHz and the pulse width of
25.0 µs using stainless steel electrodes only. Stainless steel was specifically chosen, since
it exhibits pronounced electrochemical behavior compared to that of the other electrode
materials. The selection of frequency, 10 kHz, was simply to illustrate the enhanced
hydrogen generation, especially when td1 = 0 µs. However, shorter pulse widths (less than
25.0 µs) were avoided, because of more pronounced hydrogen generation that raised
safety concerns. The delay time, td1, was varied as 0.0, 5.0, 10.0, 15.0, 20.0, 25.0 µs. The
shapes of corresponding pulse waveforms for voltage and current are shown in figure 3.8.
All these experiments were randomized and triplicated as before. The results were also
compared with those of conventional ohmic heating.
Analysis of electrode corrosion
Concentrations of Fe and Cr (from the stainless steel electrodes); Ti (from the
titanium electrodes); Pt and Ti (from the platinized-titanium electrodes); and elemental
carbon (from the graphite electrodes) migrated into the heating media were taken as
measures of electrode corrosion. In each experimental run, once the ohmic heating was
completed, a 25.00 (± 0.03) ml sample was pipetted out after removing the electrodes,
59
and thoroughly mixing the fluid. A 25.00 (± 0.03) ml sample of the respective unheated
heating medium was used as a method blank. All the samples were collected into
polypropylene sample bottles, and then stabilized by adding concentrated nitric acid (5%,
v/v). Quantitative analyses of the metal ions were performed by a Perkin-Elmer Sciex
ELAN 6100 DRC inductively coupled plasma - mass spectrometer (ICP-MS) (AOAC,
2000). The elemental carbon concentrations were determined by a Perkin-Elmer Optima
3000 DV inductively coupled plasma - optical emission spectrometer (ICP-OES)
monitoring the emission spectra near 193.03 nm.
Hydrogen generation and pH measurements
A series U hydrogen detector (CEA Instruments, Inc., NJ) was used to measure
headspace hydrogen gas generated during ohmic heating. The pH of the medium before
and after the ohmic heating treatment was measured by a Cole-Parmer 59003 Benchtop
pH meter (resolution: 0.01 pH) at 25 °C.
Data analysis
Total energy input in the pulsed ohmic heating experiments was determined by
integrating the power input (equation 3.3) vs. time curve for each experimental run. The
total energy input was also calculated for conventional ohmic heating using its power
input (Pinput = Vrms Irms) vs. time curve. Concentration of metal migration normalized per
unit energy input was defined as ‘corrosion rate’. The corrosion rates were calculated for
each migratory element. Descriptive statistics including means and standard deviations
were calculated for the quantitative measurements. The corrosion rates with respect to
60
each migratory element were individually analyzed using one-factor analysis of variance
to determine if: (1) the frequency-pulse width combinations of pulsed ohmic heating
together with conventional ohmic heating; and (2) the delay times of pulsed ohmic
heating together with conventional ohmic heating; had significant effects on corrosion
rate. Tukey’s specific comparison test determined which particular means were
significantly different. Significance of differences was defined as p≤ 0.05. SPSS 11.5 for
windows (SPSS Inc., 2002) statistical software package was used for the statistical
analyses.
RESULTS AND DISCUSSION
Effects of frequency and pulse width
Stainless steel electrodes:
Figures 3.9 and 3.10 show the corrosion rates with respect to Fe and Cr, major
elements of stainless steel, at different frequencies and pulse widths, in comparison with
those of conventional ohmic heating. It can be seen that the corrosion rates become
enhanced at 4 kHz, compared to those at 10 kHz. It is also seen that significantly (p ≤
0.05) reduced corrosion rates can be achieved for the same duty cycle when a higher
frequency and a shorter pulse width are used. The figures further demonstrate that pulsed
ohmic heating is capable of significantly (p ≤ 0.05) reducing corrosion rates, compared
to conventional ohmic heating. In all pulsed ohmic heating experiments, there were no
signs of hydrogen or any other gas evolution at the electrode/solution interfaces, and also
no detectable pH change of the heating medium at any of the frequencies and pulse
widths. However, with conventional ohmic heating, 6 (± 2) ppm hydrogen gas
61
accumulation in the headspace, and + 0.04 pH change of the heating medium were
observed.
Titanium electrodes:
The corrosion rates of titanium electrodes become enhanced at 10 kHz longer pulse
widths, and at 4 kHz shorter pulse widths (see figure 3.11). Therefore, higher duty cycles
with reduced corrosion rates can be achieved using 4 kHz with longer pulse widths. The
figure further indicates significantly (p ≤ 0.05) reduced corrosion rates of pulsed ohmic
heating, compared with conventional ohmic heating. There were no signs of gas
evolution at the electrode/solution interfaces, and also no detectable pH changes of the
heating media in both pulsed and conventional ohmic heating experiments.
Platinized-titanium electrodes:
Figures 3.12 and 3.13 show the corrosion rates with respect to Pt and Ti at different
frequencies and pulse widths, in comparison with those of conventional ohmic heating.
As can be seen, the corrosion rates become greatly enhanced at 10 kHz with increasing
pulse width, whereas 4 kHz yields greatly reduced corrosion rates with all the pulse
widths. Therefore, higher duty cycles with reduced corrosion rates can be achieved using
lower frequencies and longer pulse widths in pulsed ohmic heating with platinizedtitanium electrodes. It is also seen that pulsed ohmic heating significantly (p ≤ 0.05)
reduces the corrosion rates with respect to Pt, compared to conventional ohmic heating.
However, conventional ohmic heating yields a reduced corrosion rate with respect to Ti,
compared to pulsed ohmic heating at 10 kHz. Both pulsed and conventional ohmic
heating experiments did not indicate any signs of gas evolution at the electrode/solution
interfaces; and also there were no detectable pH changes of the heating media.
62
Graphite electrodes:
Pulsed ohmic heating yields reduced corrosion rates at 10 kHz with shorter pulse
widths, and at 4 kHz with longer pulse widths, in the case of graphite electrodes (see
figure 3.14). However, it is seen that pulsed ohmic heating can enhance the corrosion
rate, compared to conventional ohmic heating. As in the cases of titanium and platinizedtitanium electrodes, we neither observed any signs of gas evolution, nor any detectable
pH changes of the heating media in both pulsed and conventional ohmic heating
experiments.
Effect of delay time
Figures 3.15 and 3.16 represent variations of the corrosion rates (with respect to Fe
and Cr) with delay time, in comparison with the corrosion rates for conventional ohmic
heating. It is evident that delay time has a significant effect on corrosion rate. When there
was no delay time (i.e. td1 = 0.0 µs), we observed 71 (± 25) ppm hydrogen gas
(selectively generated at the electrode to which the neutral wire was attached)
accumulation in the headspace, and a + 0.33 (± 0.02) pH change of the heating medium,
in addition to the significantly (p ≤ 0.05) greater corrosion rates. These observations can
be explained by means of the corresponding pulse waveforms as follows.
The shapes of the corresponding pulse waveforms shown in figure 3.8 indicate
markedly incomplete discharge of the double layers after each positive pulse, when there
was no delay time (see ‘Delay: 0 µs’). As a result, with continued pulsation, there was
likely to be negative charge (e.g. Cl − and citrate ions) accumulation in the vicinity of the
electrode to which the hot wire was attached, and, simultaneously, a positive charge (e.g.
63
H+ and Na+) accumulation at the electrode to which the neutral wire was attached. The
latter phenomenon led to selective hydrogen generation at that particular electrode,
together with a positive pH change (i.e. an increase of pH) due to the loss of H+ ions in
the heating medium (see equation 3.5). Then, in order to maintain electrical neutrality,
the electrode to which the hot wire was attached had to liberate (positive) metal ions into
the heating medium resulting in enhanced corrosion rates (see equation 3.6).
Cathodic half-reaction:
Anodic half-reaction:
2H+(aqueous) + 2e ⇔ H2 (g)
(3.5)
M (solid) ⇔ M2+(aqueous) + 2e, (where, M = Fe, Cr, Ni, Mo) (3.6)
The overall reaction:
M (solid) + 2H+(aqueous) ⇔ M 2+(aqueous) + H2 (g)
(3.7)
With increasing delay time, the corrosion rates were drastically reduced, and became
insignificant after 10 µs (see figures 3.15 and 3.16). We further noticed no signs of gas
evolution at any of the electrode/solution interfaces when td1 > 10 µs. Also, the heating
media did not indicate a detectable pH change when delay time was ≥ 5 µs. These
observations also relate to the shapes of pulse waveforms shown in figure 3.8, which
indicates gradual completion of discharge of the double layers yielding more symmetric
positive and negative pulses, with increasing delay time. It can be seen that the symmetry
of positive and negative pulses remains almost unchanged after the delay time of 10 µs.
The above results demonstrate the importance of allowing enough delay time for
discharge of the double layers after each pulse input. The results further indicate that
pulsed ohmic heating with insufficient delay times can be worse than conventional ohmic
heating. The symmetry of positive and negative pulses of the pulse waveforms may be
used as a reliable indicator to determine the sufficiency of delay time in pulsed ohmic
64
heating. On the other hand, the delay time requirement in pulsed ohmic heating limits the
accomplishment of higher duty cycles, especially at higher frequencies. For instance,
when allowing a delay time of 15 µs, the maximum duty cycle at 10 kHz is 70%,
compared with the 88% maximum duty cycle at 4 kHz.
65
CONCLUSIONS
Electrochemical reactions during ohmic heating with stainless steel, titanium, and
platinized-titanium electrodes can be significantly (p ≤ 0.05) reduced, in some cases, to
undetectable levels by use of IGBT pulse inputs. For stainless steel electrodes, pulsed
ohmic heating at higher frequencies and shorter pulse widths yields the lowest rates of
electrochemical reactions. However, pulsed ohmic heating at lower frequencies and
longer pulse widths is more effective in suppressing the electrochemical reactions of
titanium and platinized-titanium electrodes, while achieving higher duty cycles. In
general, pulsed ohmic heating is not capable of suppressing the electrochemical reactions
of graphite electrodes. Delay time was found to be a critical factor in pulsed ohmic
heating. The sufficiency of a given delay time is dependent on the symmetry of positive
and negative pulses of the pulse waveforms.
66
SYMBOLS
e
electron
f
frequency (Hz)
Ip
peak current (A)
Irms
RMS current (A)
Pinput
power input (W)
ppb
parts per billion
ppm
parts per million
RMS
root-mean-square
T
period (µs)
td, td1, td2 delay times (µs)
tp
pulse width (µs)
Vp
peak voltage (V)
Vrms
RMS voltage (V)
Greek letters
θ
duty cycle
67
REFERENCES
Amatore C., Berthou M., and Hebert S.(1998); Fundamental principles of
electrochemical ohmic heating of solutions; Journal of Electroanalytical Chemistry; 457,
pp. 191-203.
Baliga B. J.; Power semiconductor devices for variable frequency devices; IEEE
Technology update series: Power electronics technology and applications II (selected
conference papers), Lee F.C. ed. (1998), pp. 50-60.
Grant D. (1996); Power semiconductor
Microelectronics Journal, 27, pp. 161-176
devices
–
continuous
development;
Wu H., Kolbe E., Flugstad B., Park J.W., and Yongsawatdigul J.(1998); Electrical
properties of fish mince during multi-frequency ohmic heating; Journal of Food Science;
63(6), pp. 1028-1032.
68
Frequency / Hz
10 000
4 000
60 (sine wave)
Pulse width (µs), or
RMS voltage (V)
Initial electrical
conductivity/
mS cm-1 (at 25 0 C)
Cooling water
temp. / (± 1) 0 C
10.0
3.75
31
15.0
2.97
28
25.0
2.23
26
35.0
1.90
23
30.0
3.34
31
62.5
2.28
27
75.0
2.04
25
100.0
1.76
20
110.0
1.64
19
110 (Vrms)
2.59
29
Table 3.1: Frequency-pulse width combinations with required initial electrical
conductivities, and cooling water temperatures for stainless steel electrodes.
69
Frequency / Hz
10 000
4 000
60 (sine wave)
Pulse width (µs), or
RMS voltage (V)
Initial electrical
conductivity/
mS cm-1 (at 25 0 C)
Cooling water
temp. / (± 1) 0 C
10.0
3.80
30
15.0
2.99
28
20.0
2.56
27
25.0
2.21
25
30.0
3.30
31
50.0
2.58
29
75.0
2.05
24
100.0
1.78
21
110 (Vrms)
2.58
28
Table 3.2: Frequency-pulse width combinations with required initial electrical
conductivities, and cooling water temperatures for titanium electrodes.
70
Frequency / Hz
10 000
4 000
60 (sine wave)
Pulse width (µs), or
RMS voltage (V)
Initial electrical
conductivity/
mS cm-1 (at 25 0 C)
Cooling water
temp. / (± 1) 0 C
10.0
3.76
29
15.0
2.94
27
20.0
2.51
25
25.0
2.17
23
30.0
3.24
28
50.0
2.53
26
75.0
2.02
24
100.0
1.76
21
110 (Vrms)
2.53
26
Table 3.3: Frequency-pulse width combinations with required initial electrical
conductivities, and cooling water temperatures for platinized-titanium electrodes.
71
Frequency / Hz
10 000
4 000
60 (sine wave)
Pulse width (µs), or
RMS voltage (V)
Initial electrical
conductivity/
mS cm-1 (at 25 0 C)
Cooling water
temp. / (± 1) 0 C
10.0
3.04
30
15.0
2.48
28
20.0
2.10
26
25.0
1.86
24
30.0
2.85
31
50.0
2.16
28
75.0
1.71
24
100.0
1.46
21
110 (Vrms)
2.19
29
Table 3.4: Frequency-pulse width combinations with required initial electrical
conductivities, and cooling water temperatures for graphite electrodes.
72
2
1
7
7
6
5
4
3
1 – Hydrogen gas sensor
2 – Teflon coated thermocouple
3 – Electrodes: slightly curved electrodes (electrode gap: 9 cm)
4 – Cooling water inlet
5 – Cooling water outlet
6 – Removable lid: the electrodes, hydrogen sensor, and thermocouple, are attached. It
was tightly clamped to the cell body during ohmic heating
7 – Attachments of differential voltage probe
Figure 3.1: The ohmic heater used for both pulsed and conventional ohmic heating
experiments.
73
Isolation
module
ohmic heater
Oscilloscope
~
Hydrogen
gas meter
0 – 250 ppm
V
Digital
multimeter
A
Data
logger
Pulse generator
60 Hz, 3 phase
Power supply
~
~
~
+
IGBT
Microcomputer
Rectifier
_
V = Differential voltage probe
A = Current monitor
N
IGBT Power supply
Figure 3.2: Schematic diagram of the experimental setup used for pulsed ohmic heating.
74
50
30
0
Temp./ C
40
20
10
0
0
50
100
150
time/ seconds
Figure 3.3: Typical time vs. temperature curve for all the ohmic heating experiments.
75
tp
td/2
td
td/2
tp
T = 1/ f
Figure 3.4: Schematic diagram of the centering of bipolar pulses within the period to
study the effects of frequency and pulse width.
76
10 µs
15 µs
25 µs
20 µs
35 µs
Figure 3.5: Typical pulse waveforms at 10 kHz for different pulse widths. The top and
the bottom waves in each diagram represent the current and the voltage, respectively
77
30 µs
50 µs
62.5 µs
75 µs
100 µs
110 µs
Figure 3.6: Typical pulse waveforms at 4 kHz for different pulse widths. The top and the
bottom waves in each diagram represent the current and the voltage, respectively.
78
tp
td2/2
td2/2
td1
tp
T = 1/ f
Figure 3.7: Schematic diagram of the centering of bipolar pulses within the period to
demonstrate the effect of delay time.
79
Delay: 0 µs
Delay: 5 µs
Delay: 10 µs
Delay: 15 µs
Delay: 20 µs
Delay: 25 µs
Figure 3.8: Typical pulse waveforms for different delay times. The top and the bottom
waves in each diagram represent the current and the voltage, respectively.
80
12.50
e
[Fe] in ppb/ KJ
10.00
d
7.50
cd
5.00
bc
ab
2.50
a
a
bc
ab
ab
0.00
10
15
25
35
30 62.5 75 100 110 Con
(10) (10) (10) (10) (4) (4) (4) (4) (4)
20% 30% 50% 70% 24% 50% 60% 80% 88%
Figure 3.9: The corrosion rates (in ppb per KJ) of stainless steel electrodes with respect to
Fe at different frequencies and pulse widths. Number shown in parenthesis is the
corresponding frequency (kHz) for each pulse width (µs). The corresponding duty cycle
is indicated as a percentage. ‘Con’ represents the corrosion rate for conventional ohmic
heating. Different letters: a, b, c, d, and e denote significant differences (p ≤ 0.05) of
mean (n = 3) corrosion rates.
81
d
1.80
[Cr] in ppb/ KJ
1.50
1.20
c
0.90
bc
0.60
abc
ab
0.30
a
a
abc
ab
a
0.00
10
15
25
35
30 62.5 75 100 110 Con
(10) (10) (10) (10) (4) (4) (4) (4) (4)
20% 30% 50% 70% 24% 50% 60% 80% 88%
Figure 3.10: The corrosion rates (in ppb per KJ) of stainless steel electrodes with respect
to Cr at different frequencies and pulse widths. Number shown in parenthesis is the
corresponding frequency (kHz) for each pulse width (µs). The corresponding duty cycle
is indicated as a percentage. ‘Con’ represents the corrosion rate for conventional ohmic
heating. Different letters: a, b, c, and d denote significant differences (p ≤ 0.05) of mean
(n = 3) corrosion rates.
82
0.20
e
[Ti] in ppb/ KJ
0.15
0.10
bcd
cd
d
abc
0.05
a
ab
abc
a
0.00
10
15
20
25
30
50
75 100 Con
(10) (10) (10) (10) (4) (4) (4) (4)
20% 30% 40% 50% 24% 40% 60% 80%
Figure 3.11: The corrosion rates (in ppb per KJ) of titanium electrodes with respect to Ti
at different frequencies and pulse widths. Number shown in parenthesis is the
corresponding frequency (kHz) for each pulse width (µs). The corresponding duty cycle
is indicated as a percentage. ‘Con’ represents the corrosion rate for conventional ohmic
heating. Different letters: a, b, c, d, and e denote significant differences (p ≤ 0.05) of
mean (n = 3) corrosion rates.
83
0.100
d
[Pt] in ppb/ KJ
0.075
c
0.050
0.025
ab
ab
b
ab
ab
ab
a
0.000
75 100 Con
50
30
25
20
15
10
(10) (10) (10) (10) (4) (4) (4) (4)
20% 30% 40% 50% 24% 40% 60% 80%
Figure 3.12: The corrosion rates (in ppb per KJ) of platinized-titanium electrodes with
respect to Pt at different frequencies and pulse widths. Number shown in parenthesis is
the corresponding frequency (kHz) for each pulse width (µs). The corresponding duty
cycle is indicated as a percentage. ‘Con’ represents the corrosion rate for conventional
ohmic heating. Different letters: a, b, c, and d denote significant differences (p ≤ 0.05) of
mean (n = 3) corrosion rates.
84
0.100
a
[Ti] in ppb/ KJ
0.075
0.050
a
ab
ab
0.025
b
0.000
b
b
b
*
*
*
b
10
15
20
25
30
50
75 100 Con
(10) (10) (10) (10) (4) (4) (4) (4)
20% 30% 40% 50% 24% 40% 60% 80%
Figure 3.13: The corrosion rates (in ppb per KJ) of platinized-titanium electrodes with
respect to Ti at different frequencies and pulse widths. The presence of asterisk (*)
indicates corrosion rate either < 0.001 ppb/ KJ or undetectable. Number shown in
parenthesis is the corresponding frequency (kHz) for each pulse width (µs). The
corresponding duty cycle is indicated as a percentage. ‘Con’ represents the corrosion rate
for conventional ohmic heating. Different letters: a, and b denote significant differences
(p ≤ 0.05) of mean (n = 3) corrosion rates.
85
0.20
ab
[C] in ppm/ KJ
0.15
ab
b
0.10
ab
ab
ab
ab
0.05
a
0.00
a
*
10
15
20
25
30
50
75 100 Con
(10) (10) (10) (10) (4) (4) (4) (4)
20% 30% 40% 50% 24% 40% 60% 80%
Figure 3.14: The corrosion rates (in ppm per KJ) of graphite electrodes with respect to
elemental carbon at different frequencies and pulse widths. The presence of asterisk (*)
indicates an undetectable corrosion rate. Number shown in parenthesis is the
corresponding frequency (kHz) for each pulse width (µs). The corresponding duty cycle
is indicated as a percentage. ‘Con’ represents the corrosion rate for conventional ohmic
heating. Different letters: a, and b denote significant differences (p ≤ 0.05) of mean (n =
3) corrosion rates.
86
a
[Fe] in ppb/ KJ
150.0
100.0
50.0
b
b
c
c
c
c
0.0
0
5
10
15
20
Delay time/ micro-seconds
25
Con
Figure 3.15: The corrosion rates (in ppb per KJ) with respect to Fe for different delay
times. ‘Con’ represents the corrosion rate for conventional ohmic heating. Different
letters: a, b, and c denote significant differences (p ≤ 0.05) of mean (n = 3) corrosion
rates.
87
a
[Cr] in ppb/ KJ
30.0
15.0
b
c
c
c
c
10
15
20
25
d
0.0
0
5
Con
Delay time/ micro-seconds
Figure 3.16: The corrosion rates (in ppb per KJ) with respect to Cr for different delay
times. ‘Con’ represents the corrosion rate for conventional ohmic heating. Different
letters: a, b, c, and d denote significant differences (p ≤ 0.05) of mean (n = 3) corrosion
rates.
88
CHAPTER 4
ELECTROCHEMICAL REACTIONS DURING 60 Hz OHMIC HEATING OF
ASCORBIC ACID IN BUFFER MEDIUM WITH STAINLESS STEEL
ELECTRODES
ABSTRACT
This study was aimed at understanding electrochemical and secondary chemical
reactions during ohmic heating of ascorbic acid in a 3.5 pH buffer medium. Ohmic
heating experiments were performed at different power densities and NaCl concentrations
using 60 Hz sinusoidal alternating current and stainless steel electrodes. A number of
reactions seem to occur in the cell during ohmic heating. Electrode corrosion is shown to
have marked effects on the buffer medium as well as ascorbic acid degradation. The uses
of more inert electrode materials, and/or high frequency alternating currents are
suggested to minimize electrocatalytic effects on ascorbic acid.
89
INTRODUCTION
Ascorbic acid (AA), also known as vitamin C, has been the subject of numerous
investigations in many scientific disciplines, including food science, medicine, and
biochemistry. In recent years, AA has gained a renewed interest as a nutraceutical since it
possesses antioxidant properties providing potential health benefits. AA is considered to
be one of the most heat sensitive nutrients in foods, and its degradation has been reported
to vary with pH, oxygen, enzymes, metal catalysts, initial concentration, and light
(Assiry, 1996). This inherent instability of AA is a major concern in thermal food
processing. Although a number of studies have examined AA degradation under
conventional heat treatments, a little information is available related to ohmic heating.
Assiry (1996) studied degradation kinetics of AA under ohmic heating conditions
with stainless steel electrodes, and compared it with conventional heating. The results
indicate that, at pH 3.5, although kinetics of AA degradation can be described adequately
by a first order model for both conventional and ohmic heating, a number of
electrochemical as well as secondary chemical reactions appear to have some effects on
the kinetic parameters. Edirisinghe et al. (1997) found a greater loss of AA in ohmic
heating with carbon electrodes, compared to that in conventional heating. Electric field
interactions and electrode effects were suggested as being possible explanations for this
enhanced AA loss. In contrast, Lima et al. (1999) found that the electric field had no
significant effects on AA degradation.
Assiry et al. (2003) noted the influence of reactions at electrode/solution interfaces
on degradation of AA in buffer medium during 60 Hz ohmic heating with stainless steel
electrodes. Their study however does not include a detailed delineation of possible
90
reactions, and suggests a follow-up study to characterize and quantify specific reactions.
Therefore, we attempted to fill this gap by studying electrochemical and secondary
chemical reactions revisiting Assiry et al’s (2003) ohmic heating conditions. Our present
study is a comprehensive approach to understanding the reaction kinetics of AA in the
buffer medium during 60 Hz ohmic heating with stainless steel electrodes.
MATERIALS AND METHODS
Chemicals: ACS grade sodium chloride (Fisher Scientific, NJ), Citric acid monohydrate
(Aldrich, WI), L – Ascorbic acid (Fisher Scientific, NJ), special low-carbonate NaOH
(EM Science, NJ); AR grade 1:1 Fe (III)-citrate hydrate, FeCl3, CrCl3, NiCl2 (Aldrich,
WI), Na2HPO4 .12 H2O (J.T. Baker, NJ); trace metal grade concentrated nitric acid
(Fisher Scientific, PA); HPLC grade acetonitrile (Mallinckrodt Baker Inc., NJ); and
TBDMS
{N-Methyl-N-[(tert-butyldimethyl)silyl]
trifluoroacetamide}
(Regis
Technologies, Inc., IL) were purchased from the suppliers. Demineralized double
distilled water (Resistivity: 3 megohm; pH: 5.5) was obtained from the Reagent
Laboratory Store, at The Ohio State University.
Ohmic heating procedure
Experiments were performed using the same batch isothermal stirred ohmic heater
and setup used by Assiry et al. (2003). The same ohmic heating procedure was also
followed without any modification. Table 4.1 shows a summary of the treatments
subjected to investigation. Experiments were completely randomized with respect to
order of experimentation. Two replications were made for each treatment.
91
Analyses of electrode corrosion
Metal ion migration into the heating medium, and deposits formed on the electrode
surfaces were examined as described below.
Quantitative analysis of metal ions:
In each experimental run, once the ohmic heating was completed, a 25.00 (± 0.03) ml
sample of the heating medium (i.e. buffer solution containing 2.26 µmol ml-1 AA at the
beginning) was pipetted out. A 25.00 (± 0.03) ml sample of the respective unheated
buffer solution was used as a method blank. All the samples were collected into
polypropylene sample bottles, and then stabilized by adding concentrated nitric acid (5%,
v/v). Concentrations of Fe, Cr, and Ni, the three major elements of stainless steel,
migrated into the heating medium were determined by a Perkin-Elmer Optima 3000 DV
inductively coupled plasma - optical emission spectrometer (ICP-OES) monitoring the
emission lines at characteristic wavelengths for Fe (234.349, 238.204, 239.562, and
259.939 nm), Cr (205.560, 267.716, 283.563, 284.325, and 357.869 nm), and Ni
(221.648, 231.604, 232.003, and 341.476 nm).
Gravimetric analysis of electrode deposits:
When ohmic heating was completed in each individual experiment, the electrodes
were allowed to dry at room temperature for about 1 hour. Then, the electrode deposits
on both electrodes were carefully scraped off using a piece of stainless steel foil. Weights
of the electrode deposits were measured by a Mettler AE 166 (DeltaRange) analytical
balance.
92
Characterization of electrode deposits:
Chemical compositions of electrode deposits were determined by a JEOL JSM-820
scanning electron microscope equipped with an Oxford energy dispersive x-ray analyzer
(SEM-EDX). The analysis was performed at 15 kV accelerating voltage, 300 µA beam
current, and 100 second time, after mounting each of the deposit samples on carbon tape,
and again coated with carbon. The results were acquired using INCAEnergy+ software.
Five replicate measurements were made for each deposit sample.
Measurements of hydrogen generation
A 250-ppm series U hydrogen detector (CEA Instruments, Inc., NJ) was used to
measure the headspace hydrogen gas. However, since hydrogen generation overloaded
the sensor before the end of each experiment, the time needed to accumulate 250 ppm of
hydrogen gas in the headspace (volume: 60 cm3) was recorded.
Electrical conductivity measurements
Electrical conductivities of the heating medium after ohmic heating and the
corresponding buffer solution before being subjected to ohmic heating, were measured in
each experimental run using a Cole-Parmer 19101-00 digital conductivity meter at 25 °C.
pH and buffer capacity measurements
Measurements of pH were also carried out in each experimental run for both the
heating medium after ohmic heating and the corresponding buffer solution before being
subjected to ohmic heating, using a Cole-Parmer 59003 Benchtop pH meter (Resolution:
93
0.01 pH units) at 25 °C. Since some ohmic heating treatments significantly increase the
buffer pH, the buffer capacities of the citrate-phosphate buffer system at each NaCl
concentration were determined by the following titrimetric method at 25 °C. In order to
test the effect of temperature on buffer capacity, the same titrimetric method was also
carried out at 80 °C after incubating the buffers at 80 °C for 60 minutes using a water
bath.
Determination of buffer capacity:
A 100.00 (± 0.12) ml sample of each buffer solution was titrated with 1.00 M NaOH
solution using a micro-burette. The pH was recorded at each 0.25 ml NaOH addition up
to 2.00 ml using the same pH meter. The procedure was duplicated. Buffer capacity (β)
was calculated according to the following equation.
β = 1.00 Vb × 103 / ∆ pH (100 + Vb )
µmol pH−1 ml−1
(4.1)
where Vb is the accumulated volume (ml) of the titrant.
Measurements of pH and buffer capacity were also performed using the following
procedures, to determine the effects of AA-induced Fenton’s reaction and metal
complexation on the pH and buffer capacity, respectively.
Effect of AA-induced Fenton’s reaction on buffer pH:
The same amount of migrated Fe (found in the corrosion analysis) was artificially
introduced in the form of Fe(III) (i.e. FeCl3) into the corresponding buffer solution
containing 2.26 µmol ml-1 AA. The mixture was incubated at 80 °C for 60 minutes with
gentle magnetic stirring, using the water bath. Since FeCl3 is inherently acidic, its
concentrated stock solution was prepared in each case; and the pH was adjusted to the
same value as that of the buffer, using NaOH just before addition into the (buffer + AA)
94
mixture. A control experiment was also performed by incubating the corresponding
buffer at 80 °C for 60 minutes without adding any AA and Fe. Measurements of pH were
carried out after the incubation period, and cooling to 25 °C. The above procedure was
duplicated.
Effect of metal complexation on buffer capacity:
The same amounts of migrated Fe, Cr, and Ni (found in the corrosion analysis) were
artificially introduced in the forms of Fe(III), Cr(III), and Ni(II) (i.e. FeCl3, CrCl3, and
NiCl2) into the corresponding buffer solution. The (buffer + metal ions) mixture was
incubated at 80 °C for 60 minutes with gentle magnetic stirring, using the same water
bath. Again, since those metal chlorides are inherently acidic, a concentrated stock
solution containing all the three metal ions was prepared in each case; and the pH was
adjusted to the same value as that of the buffer, using NaOH just before addition into that
particular buffer. At the end of the incubation period, buffer capacity was determined at
80 °C titrating with 1.00 M NaOH solution as before. The above procedure was also
duplicated. Buffer capacity (β) was calculated modifying the equation 4.1, as follows.
β = 1.00 Vb × 103 / ∆ pH (100 + Vb + Vm)
µmol pH−1 ml−1
(4.2)
where Vm is the volume of metal ion stock solution added (1.0 ml).
Spectrophotometric analysis of Fe (III) – citrate at 1.5 Wcm-3
UV-Visible absorption spectra of both the ohmically heated medium (at 1.5 Wcm-3
with 1.0% NaCl) and an aqueous solution of 1:1 Fe (III) – citrate, were recorded on a
Shimadzu double beam spectrophotometer (Columbia, MD) having a 1.0 cm quartz cell,
95
and fitted with UV-2401 PC photometric software. The 1:1 Fe (III) – citrate solution, the
reference, was freshly prepared using demineralized double distilled water containing 1.0
% (w/v) NaCl. Its pH was as same as that of the ohmically heated medium (pH 8.1, at 25
°C), and was adjusted by NaOH. Both spectra were recorded at 25 °C using 1.0 %(w/v)
NaCl solution (pH 8.1) as a blank. The same reagent blank was also used for necessary
dilutions of the samples.
GC-MS analysis of the buffer components at 1.5 Wcm-3
A 1.0 ml sample of the heating medium after the ohmic heating treatment at 1.5
Wcm-3 was withdrawn and cooled down to room temperature. A 50 µl aliquot of the 1.0
ml sample was subjected to vacuum drying at room temperature using a Savant speed-vac
concentrator SVC-100H (Farmingdale, NY). The dried aliquot was then derivatized by
adding 100 µl of 1:1 TBDMS/ acetonitrial mixture, followed by incubating at 60 °C for 1
hour. A 50 µl sample of the corresponding unheated buffer solution subjected to the same
vacuum drying and derivatization procedures was used as the reference. 1 µl aliquots
were introduced into a ThermoFinnigan Trace 2000 GC-MS (San Jose, CA) at the
following operating conditions.
Gas chromatography was carried out splitless through a 95% dimethyl/ 5% diphenyl
polysiloxane column (30 m × 0.32 mm ID; 0.25 µm film) using temperature ramps of 30
°C/ min from 40 to 180 °C and 7 °C/ min from 180 to 350 °C, with He as the carrier at
the flow rate of 2 ml/ min. The injector and GC-MS interface temperatures were at 222
and 260 °C, respectively. Mass spectra were acquired in scan mode within the m/z range:
96
42 - 839 at a rate of 1.4 scan/ sec. Identification of the GC peaks corresponding to the
buffer components, citric acid and phosphate, was performed by means of the NIST-98
library search database. The remaining buffer components in the ohmically heated
medium were estimated by selected ion monitoring (SIM mode) at the respective [M-57]+
ions.
Data analysis
The duplicated quantitative measurements were averaged. Group differences were
determined by one-factor analyses of variance together with Tukey’s specific comparison
test. Significance of differences was defined as p≤ 0.05. SPSS 11.5 for windows (SPSS
Inc., 2002) statistical software package was used for the statistical analyses.
RESULTS AND DISCUSSION
Electrode corrosion
The extents of metal migration and electrode deposit formation shown in table 4.2
indicate significantly greater (p ≤ 0.05) electrode corrosion, when ohmic heating is
performed at 1.5 Wcm-3 (1.0% NaCl) as compared to the other treatments. It is also seen
that the electrode corrosion becomes considerably lower at the same power density (i.e.
0.75 Wcm-3) with lower NaCl concentration, and at higher power density with the same
NaCl concentration (i.e. 0.25%). The increase of electrical conductivities implies the
migration of metal ions into the heating media.
Table 4.3 shows the chemical compositions of electrode deposits at different power
densities and NaCl concentrations. The electrode deposits are apparently richer in Cr. It is
97
further evident that the P contents of the deposits are much greater, compared with the
typical P content (< 0.1 wt%) of stainless steels (Davis, 1994). This suggests the
formation of metal-phosphates on the electrode surfaces during ohmic heating treatments.
In addition, formations of metal-oxide rusts (e.g. Fe2O3. xH2O), (soluble) metal-chlorides,
and metal-hydroxides may be suspected due to the possibilities of generating O2
(equation 4.3) and Cl2 (equations 4.4 and 4.5) gases, and the Fenton’s reaction (equation
4.6) (Tomat et al., 1979).
2H2O(liquid) ⇔ 2H2 (g) + O2 (g)
(4.3)
2H+(aqueous) + 2Cl−(aqueous) ⇔ H2 (g) + Cl2 (g)
(4.4)
2H2O (liquid) + 2Cl −(aqueous) ⇔ Cl2 (g) + H2 (g) + 2OH−(aqueous)
(4.5)
Fe2+(aqueous) + H2O2 ⇔ Fe3+(aqueous) + •OH + OH−(aqueous)
(4.6)
where H2O2 can be generated by the following cathodic half-reaction.
O2 (g) + 2H+(aqueous) + 2e ⇔ H2O2
(4.7)
In order to demonstrate the tendencies to form particular metal-phosphate and metalhydroxide compounds, the minimum amounts of migratory metal ions needed for
precipitation were theoretically calculated using their respective solubility products (Ksp)
(see table 4.4). It can be seen that a discharge of just one picomole of Cr (III) ions (per
dm3 of heating medium) from the electrodes is sufficient to form CrPO4
(solid).
The
reaction between Cr (III) and phosphate ions apparently occurred on the electrode
surfaces (i.e. at the electrode/solution interfaces) yielding Cr- and P-rich electrode
deposits. Similarly, the presence of FePO4 (solid) in the electrode deposits is highly likely,
since the required amount of Fe (III) is much less compared to the Fe (II) and Ni (II)
98
requirements for precipitation as phosphates. Among the metal-hydroxides, Fe(OH)3 (solid)
is the one that can be formed with the least amount of migratory Fe (III) ions.
Hydrogen generation
Hydrogen generation seems to be accelerated with increasing power density and
NaCl concentration (see table 4.2). The hydrogen generation could be accompanied with
electrode corrosion (equations 4.8 and 4.9), electrolysis (equation 4.3), and chlorine
generation (equations 4.4 and 4.5).
M (solid) + 2H+(aqueous) ⇔ M 2+(aqueous) + H2 (g)
(4.8)
M (solid) + 2H2O (liquid) ⇔ M2+(aqueous) + H2 (g) + 2OH−(aqueous)
(4.9)
where, M = Fe, Cr, Ni, Mo.
pH and Buffer Capacity
As can be seen in table 4.2, the ohmic heating treatment performed at 1.5 Wcm-3
(1.0% NaCl) resulted a significantly greater (p ≤ 0.05) pH change. The pH of that
particular medium reached 8.1 (at 25 °C), apparently destroying the buffer system. Also,
the pH change became significantly higher (p ≤ 0.05) at the same power density (i.e. 0.75
Wcm-3) with higher NaCl concentration. The observed pH changes simply indicate the
electrochemical nature of the ohmic cell. In fact, there may be various different
electrochemical and secondary chemical reactions capable of affecting the pH. The
extents of those reactions would be different with different ohmic heating conditions.
The electrochemical reactions that either generate OH−(aqueous) ions (e.g. equations
4.5, 4.6, and 4.9), or consume H+(aqueous) ions liberating hydrogen gas (e.g. equations 4.4
99
and 4.8) may have considerable effects in increasing the buffer pH. In particular, AA is
known to function as a free radical catalyst in the presence of transition metal ions, and
generates H2O2 and free radical intermediates exhibiting prooxidant properties (Deutsch
et al.,1994 and1998; Zhao et al., 1995)(equations 4.10, 4.11, and 4.12). This H2O2 readily
participates in Fenton’s reaction (equation 4.6) causing pronounced generation of
OH−(aqueous) ions.
AA + Fe3+(aqueous) ⇔ oxidized AA + Fe2+(aqueous)
AA + O2 (g)
⇔ oxidized AA + O2•–
2O2•– + 2H+(aqueous) ⇔ H2O2 + O2 (g)
(catalyzed by Fe2+(aqueous))
(4.10)
(4.11)
(4.12)
In McIlvaine type citrate-phosphate buffer systems, pH is primarily determined by
the relative proportions of citric acid monohydrate, and Na2HPO4 .12 H2O (Elving et
al.,1956; Dean,1992). Obviously, the formation of metal-phosphates on the electrode
surfaces causes the loss of phosphate in the buffer system. On the other hand, citric acid
is one of the well-known sequestrients frequently used in food formulations to chelate
metal ions that catalyze certain food reactions, such as lipid oxidation. Therefore, some of
the metal ions migrated into the heating media react with the citric acid forming metalcitrate complexes, ultimately causing the loss of free citric acid in the buffer system.
Since the buffer capacity is directly related to the buffer composition, the formations of
metal-phosphates and metal-citrate complexes would alter the original buffer capacity.
Effect of AA-induced Fenton’s reaction on buffer pH:
Table 4.5 demonstrates that the amount of migrated Fe in some of the ohmic heating
treatments is capable of significantly increasing the buffer pH. During ohmic heating, the
reaction shown in equation 4.11 would be facilitated by the electrochemical generation of
100
O2
(g)
and some of the migrated Fe in the form of Fe2+(aqueous) ions enhancing OH−(aqueous)
generation by Fenton’s reaction (equation 4.6). Some negative pH changes observed in
our model system indicate metal-citrate complexation as shown in the following
generalized equation (equation 4.13).
x Mn+(aqueous) + y citric acid ⇔ (M-citrate complex) (aqueous) + z H+(aqueous)
(4.13)
where x, y, and z represent the stoichiometric coefficients. The H+(aqueous) ions released
into the heating media decrease the pH, especially at low concentrations of migrated
metal ions, apparently competing with the AA-induced Fenton’s reaction. During ohmic
heating, however, there are number of other reactions inside the ohmic cell capable of
consuming the H+(aqueous) ions.
Effect of metal complexation on buffer capacity:
As can be seen from the figures 4.1 – 4.3, metal complexation with the buffer
components lowers the original buffer capacity. The extent of buffer capacity loss is
consistent with the amounts of migrated metal ions. The loss of buffer capacity results in
increasing pH during ohmic heating.
Spectrophotometric and GC-MS analyses at 1.5 Wcm-3
The results of the preceding analyses indicate the significantly enhanced
electrochemical nature of the ohmic cell at 1.5 Wcm-3 (1.0% NaCl). Figure 4.4 shows the
corresponding buffer solution, and the ohmically heated medium for this particular
treatment. The intense dark color of the heated medium implies the existence of soluble
metal complexes, particularly metal-citrates. In fact, citric acid, a hydroxy tricarboxylic
acid, is known to form mononuclear (Matzapetakis et al., 1998; Abrahamson et al.,
101
1994),
polynuclear
(Still
et
al.,1980),
as
well
as
mixed
metal-citrate
complexes(Manzurola et al, 1989) having different stoichiometries. The pH, and the
metal-to-citric acid ratio are considered to have a significant influence in formation of
such complexes. Preliminary calculations indicate the presence of 6 mM citric acid
(initially) in the buffer solution. The electrode corrosion yields approximately 10 mM,
Fe; 2 mM, Ni; and 0.7 mM, Cr. Based on the relative concentrations, and the
predominant oxidation states of the metal ions and their complex forming ability with
citric acid (Dean, 1992), the most abundant metal-citrate species in the heated medium is
likely be 1:1 Fe(III)-citrate.
UV-visible absorption spectra of 1:1 Fe(III)-citrate (i.e. the reference), and the heated
medium are shown in figure 4.5. It can be seen that both absorption curves track each
other very closely strongly supporting the above theoretical prediction. Table 4.6 shows
the virtually identical λmax values for the major characteristic absorbance peak in the UV
region. The heated medium, however, exhibits an ill-defined shoulder around 250 nm,
which may represent a compound other than this 1:1 Fe(III)-citrate complex. Above pH
2, 1:1 Fe(III)-citrate is known to exist as an anionic dimer, [Fe2(cit)2(H2O)2]2- , absorbing
UV and blue light (Abrahamson et al., 1994; Hao et al., 2001).
The gas chromatograms (in figure 4.6) show almost complete disappearance of the
buffer components in the heating medium after it was subjected to the ohmic heating
treatment. The estimations with respect to specific [M-57]+ ions (Ohie et al., 2000)
further indicate the tremendous losses of those buffer components during ohmic heating
(see table 4.7). Therefore, the results of GC-MS analysis are in agreement with the
observed pH change, and the formations of metal-phosphates and 1:1 Fe(III)-citrate
102
complex. Although both AA and dehydroascorbic acid (DHAA) are derivatizable by this
TBDMS procedure (Deutsch et al., 1994 and 1998), none of them was detected in the
ohmically heated medium.
Electrocatalytic effects on AA degradation
Assiry et al. (2003) proposed the following regression model (equation 4.14) to
predict the first order rate constant k (min-1) for AA degradation under ohmic heating
conditions.
ln (k) = ln (k0) – (ET/ RT)
(4.14)
where k0 = 3.7735 [NaCl]-13.2 exp ( [NaCl]2 ) (Irms)-12.92 ; and
(ET/ R) = -32718 + 246 Vrms – 1.44 (Vrms)2 + 25823 (Irms /Vrms ) + 26876 [NaCl].
As can be seen, AA degradation heavily depends upon voltage (Vrms), current (Irms), and
NaCl concentration. In the present study, the dependence of these three variables (Vrms
and Irms were combined as power density) on the electrochemical processes, particularly
electrode corrosion, hydrogen generation, and pH changes, was observed throughout.
Therefore, AA degradation during ohmic heating is logically associated with the
electrochemical processes.
The metal ions migrated into the heating media catalyze oxidative degradation of AA
(Assiry, 1996), particularly via the AA-induced Fenton’s reaction. The metal migration
also causes pH changes, which in turn affect AA degradation (Assiry, 1996). The
electrolytic generation of oxygen (equation 4.3) further promotes the oxidative
degradation of AA. It is known that the primary oxidation product of AA, DHAA which
103
is as biologically active as AA, can be irreversibly hydrolyzed and degraded to over 50
different compounds ( Deutsch, 1998; Niemela, 1987).
In the present study, we identified some reactions responsible for the observations
reported by Assiry et al. (2003). These reactions affect the citrate-phosphate buffer
system as well as AA degradation. With such reactions, the differences in degradation
rate between conventional and ohmic heating observed by Assiry et al. (2003) are hardly
surprising. We would like to note that the system we studied is a model only, and is not a
representation of a real ohmic heating system designed for food processing, which may
consist of different electrode materials, such as platinized-titanium. Moreover, real foods
have not shown a greater susceptibility to AA degradation as in this buffer system (Lima
et al., 1999). Still, our study explains the chemistry underlying the kinetics of AA
degradation, and may helpful to understand the electrocatalytic effects on real food
systems. We further note that the unsuitability of citrate-phosphate buffers and stainless
steel electrodes for this type of studies. The uses of more inert electrode materials, such
as platinized-titanium (Tzedakis et al., 1999), and/or high frequency alternating currents
(Amatore et al., 1998) would be viable options.
104
CONCLUSIONS
Some electrochemical and secondary chemical reactions during ohmic heating of AA
in a 3.5 pH citrate-phosphate buffer medium have been successfully identified. Electrode
corrosion forms electrode deposits, also causing metal ion migration into the heating
medium. The electrode deposits are believed to consist mainly of CrPO4
(solid)
and
FePO4(solid) that are formed simply by precipitation of some migrated Cr(III) and Fe(III)
ions on the same electrode surfaces as the insoluble phosphates. The migrated metal ions
are involved in changing the buffer pH, and affect AA degradation. The presence of 1:1
Fe(III)-citrate complex was identified in the heated medium at 1.5 Wcm-3 (1.0% NaCl).
GC-MS analysis indicates complete destruction of the buffer system during this particular
ohmic heating treatment. The results of this study highlight the needs of using more inert
electrode
materials,
and/or
high
frequency
electrocatalytic effects.
105
alternating
currents
to
minimize
SYMBOLS
AA
ascorbic acid
DHAA dehydroascorbic acid
Irms
RMS current (A)
m/z
mass to charge ratio
ppm
parts per million
RMS
root-mean-square
Vrms
RMS voltage (V)
Greek letters
β
buffer capacity in the alkaline direction (µmol pH−1 ml−1)
λmax
spectral maxima (nm)
106
REFERENCES
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spectroscopic studies of complexes of iron (III) with citric acid and other carboxylic
acids; Inorganica Chemica Acta, 226, pp 117-127.
Amatore C., Berthou M., and Hebert S.(1998); Fundamental principles of
electrochemical ohmic heating of solutions; Journal of Electroanalytical Chemistry; 457,
pp. 191-203.
Assiry A.M (1996); Effect of ohmic heating on the degradation kinetics of ascorbic acid;
PhD thesis, The Ohio State University.
Assiry A., Sastry S.K., and Samaranayake C. (2003); Degradation kinetics of ascorbic
acid during ohmic heating with stainless steel electrodes, Journal of applied
electrochemistry, 33 (2), pp. 187-196.
Davis J.R. (Ed.) (1994); ASM Specialty Handbook (Stainless Steel); ASM International,
The Materials Information Society, pp. 3-5.
Dean J.A. (Ed) (1992); Lange’s Handbook of Chemistry, 14th Edition, McGraw-Hill Inc,
pp. 8.6-8.11, 8.83-8.103, and 8.109-8.112.
Deutsch J.C., Santhosh-Kumar C.R., Hassell K.L., and Kolhouse J.F. (1994); Variation in
Ascorbic Acid oxidation routs in H2O2 and cupric ion solution as determined by GC/MS;
Analytical Chemistry,66, pp. 345-350.
Deutsch J.C. (1998); Spontaneous hydrolysis and dehydration of dehydroascorbic acid in
aqueous solution; Analytical Biochemistry, 260, pp. 223-229.
Edirisinghe E.M.R.K.B., Samaranayake C.P., Bamunuarchchi A., Walpola S., and De
Alwis A.A.P. (1997); Nutrient retention in ohmic heating; 7 th International Congress on
Engineer and Food (ICEF – 7), Brighton U.K., SA 43-46.
Elving P.J., Markowitz J.M., and Rosenthal I.(1956); Preparation of buffer systems of
constant ionic strength; Analytical Chemistry, 28(7), pp. 1179-1180.
Hao X., Wei Y., and Zhang S.(2001); Synthesis, crystal structure and magnetic property
of a binuclear iron(III) citrate complex; Transition Metal Chemistry, 26, 384-387.
Harris D.C.(1999); Quantitative Chemical Analysis - fifth edition, W.H. Freeman and
Company, New York, Ap: 12-14.
107
Lima M., Heskitt B.F., Burianek L.L., Nokes S.E., and Sastry S.K.(1999); Ascorbic acid
degradation kinetics during conventional and ohmic heating; Journal of Food Processing
Preservation, 23, pp. 421-434.
Manzurola E., Apelblat A., Markovits G., and Levy O. (1989); Mixed-metal
hydroxycarboxylic acid complexes; J. Chem. Soc., Faraday Trans.1, 85(2), pp.373-379.
Matzapetakis M., Raptopoulou C.P., Tsohos A., Papaefthymiou V., Moon N., and
Salifoglou A.(1998); Synthesis, spectroscopic and structural characterization of the first
mononuclear, water soluble Iron-Citrate complex, (NH4)5Fe(C6H4O7)2 . 2H2O; J. Am.
Chem. Soc., 120, pp. 13266-13267.
Niemela K. (1987); Oxidative and non-oxidative alkali-catalyzed degradation of Lascorbic acid; Journal of Chromatography, 399, pp. 235-243.
Ohie T., Fu X., Iga M., Kimura M., and Yamaguchi S. (2000); Gas chromatography-mass
spectrometry with tert.-butyldimethylsilyl derivatization: use of the simplified sample
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Chromatography B, 746, pp. 63-73.
Still E.R., and Wikberg P. (1980); Solution studies of systems with polynuclear complex
formation. 2. The nickel (II) citrate system; Inorganica Chimica Acta, 46, 153-155.
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radicals; Journal of Applied Electrochemistry, (9), pp. 301-305.
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108
Parameter
NaCl concentration of the buffer
(w/v %)
Value
a
b
Power density (W/ cm3)
Corresponding voltage (Vrms)
(at 60 Hz) / (± 1) V
Corresponding current (Irms)
(at 60 Hz) / (± 1) A
0.25
0.50
1.0
0.5
0.75
0.75
1.5
33.0
40.3
33.9
41.1
3.0
3.7
4.4
7.3
Isothermal temperature / °C
80 ± 2
Duration of heating/ minutes
60
a
McIlvaine type citrate-phosphate buffer system. 0.1313% (w/v) citric acid
monohydrate, and 0.2390% (w/v) Na2HPO4 .12 H2O form the buffer having
pH 3.5 ± 0.25 (at 25 °C) depending on the NaCl concentration.
b
Volume = 200.0 (± 0.80) cm3
Table 4.1: Ohmic heating conditions.
109
NaCl concentration of the
buffer (w/v %)
Power density (W/ cm3)
0.25
0.50
1.0
0.5
0.75
0.75
1.5
39.4 a
27.9 a
58.9 a
586.4 b
3.6 a
1.2 a
2.8 a
36.9 b
8.2 a
5.4 a
11.9 a
117.4 b
Weight of the
electrode deposit * / mg
6.4 a
3.4 a
6.9 a
192.9 b
Approximate time to
accumulate 250 ppm
H2 gas * / min
19 a
12 b
4c
2c
% Electrical conductivity
change * (at 25 °C)
+ 4.3 a
+ 6.4 a
+ 7.1 a
+ 6.1 a
% pH change * (at 25 °C)
+ 1.3 a,b
0a
+ 4.8 b
+ 119.8 c
[Fe] / ppm
Metal
Migration *
[Cr] / ppm
[Ni] / ppm
* Mean values (n=2) in the same row with different superscript letters are significantly
different (p ≤ 0.05).
Table 4.2: Some indicators of the electrochemical processes at different power densities
and NaCl concentrations.
110
NaCl
concentration
of the buffer
(w/v %)
Power density
(W/ cm3)
0.25
0.50
1.0
0.5
0.75
0.75
1.5
Fe
15.19 (2.81)
12.90 (1.96)
18.99 (1.95)
23.26 (8.30)
Cr
21.63 (3.85)
22.71 (3.69)
27.59 (3.58)
30.78 (11.42)
Ni
1.96 (0.29)
1.94 (0.25)
2.61 (0.32)
4.13 (0.85)
Mo
1.70 (0.38)
1.80 (0.12)
1.54 (0.26)
2.62 (1.17)
P
7.95 (0.73)
8.49 (0.50)
7.63 (0.71)
2.36 (0.97)
Si
0.14 (0.07)
0.16 (0.03)
0.14 (0.04)
0.40 (0.10)
Cl
1.34 (0.18)
1.29 (0.40)
1.45 (0.29)
2.15 (0.98)
Na
1.76 (0.36)
2.12 (0.26)
1.70 (0.37)
1.07 (0.23)
O
45.05 (1.99)
48.58 (6.51)
38.70 (4.69)
33.36 (6.59)
Table 4.3: Chemical compositions (as weight %) of electrode deposits at different power
densities and NaCl concentrations. The values are means of five replicates (n=5) with
respective standard deviations in parentheses.
111
Metal-Phosphate
Solubility product
(Ksp)
FePO4 (solid)
1.3 × 10 −22
4.87 × 10 −12
Fe3(PO4)2 (solid)
1 × 10 −36
1.12 × 10 −5
2.4 × 10 −23
9 × 10 −13
Ni3(PO4)2 (solid)
5.0 × 10 −31
8.88 × 10 −4
Fe(OH)3 (solid)
4 × 10 −38
1.26 × 10 −6
Fe(OH)2 (solid)
8.0 × 10 −16
7.94 × 105
Cr(OH)3 (solid)
6.3 × 10 −31
19.95
Cr(OH)2 (solid)
2 × 10 −16
2.0 × 105
d
Metal-Hydroxide
Minimum
[M ] / moles dm-3
(where n = 2,3)
n+
Compound
c
a
b
CrPO4 (solid)
Ni(OH)2 (solid)
2.0 × 10 −15
2.0 × 106
a
[Na2HPO4] = 0.0067 M; and HPO42− ⇔ H+ + PO4 3 −, where Ka = 1.26 × 10 −12 (Dean,
Ka
1992).
b
Ksp at 18 – 25 °C (Dean, 1992).
c
Fe3(PO4)2 .8H2O (Harris, 1999).
d
CrPO4 .4H2O (green).
Table 4.4: Minimum migratory metal ion concentration [Mn+] needed to precipitate some
metal-phosphates and metal-hydroxides in the presence of the same [Na2HPO4] as in the
citrate-phosphate buffer system at pH 3.5.
112
NaCl
concentration of
the buffer
(w/v %)
Power density
(W/ cm3)
Observed % pH change*
in ohmic heating
% pH change*
(Buffer + AA + Fe3+)
0.5
+ 1.3 a,b
- 1.0 a
0.75
0a
- 2.1 a
0.75
+ 4.8 b
+ 1.6 b
0.25
0.50
1.0
1.5
+ 119.8 c
+ 25.4 c
* All the pH measurements were carried out at 25 °C. Mean values (n=2) in the same
column with different superscript letters are significantly different (p ≤ 0.05).
Table 4.5: The effect of AA-induced Fenton’s reaction on buffer pH, in comparison with
the observed pH changes at different power densities and NaCl concentrations.
113
Sample
λmax / nm
1:1 Fe(III)-citrate
221.00
The ohmically heated
heating medium
219.50
Absorptivity coefficient
0 cm
-1
-1
( E 01..025
%( w / v ) ) 73.08 ml g . cm
0 cm
( E 51 .%(
v /v) )
0.35 cm-1
Table 4.6: The spectral maxima (λmax) and the respective absorptivity coefficients of 1:1
Fe(III)-citrate, and the ohmically heated heating medium at 1.5 Wcm-3 (1.0% NaCl).
Retention
time/ min
[M]+ (TBDMS)
(m/z)
[M-57]+
(m/z)
% Loss
Citric acid (CA)
18.8
648
591
99.8
Phosphate (P)
11.2
440
383
99.9
Buffer component
Table 4.7: GC-MS characteristics and % losses of the buffer components.
114
5.00
Buffer capacity
4.00
3.00
Buffer at 25 °C
Buffer at 80 °C
2.00
Buffer + metal ions [1]at 80 °C
1.00
Buffer + metal ions [2]at 80 °C
0.00
3.5
4.5
5.5
6.5
7.5
8.5
pH
Figure 4.1: Buffer capacities (in µmol pH-1ml-1) at 0.25% (w/v) NaCl. [1] and [2]
correspond to the amounts of metal ions migrated at 0.5 Wcm-3 and 0.75 Wcm-3,
respectively.
115
Buffer capacity
5.00
4.00
3.00
Buffer at 25 °C
2.00
Buffer at 80 °C
1.00
Buffer + metal ions at 80 °C
0.00
3.5
4.5
5.5
6.5
7.5
pH
Figure 4.2: Buffer capacities (in µmol pH-1ml-1) at 0.50% (w/v) NaCl.
116
8.5
Buffer capacity
5.00
4.00
3.00
Buffer at 25 °C
2.00
Buffer at 80 °C
1.00
Buffer + metal ions at 80 °C
0.00
3.5
4.5
5.5
6.5
7.5
pH
Figure 4.3: Buffer capacities (in µmol pH-1ml-1) at 1.0% (w/v) NaCl.
117
8.5
Figure 4.4: The buffer solution before being subjected to ohmic heating (a), and the
ohmically heated medium at 1.5 Wcm-3 (1.0% NaCl) (b).
118
A
Figure 4.5: UV-Visible absorption spectra of 1:1 Fe(III)-citrate(
heated medium (
) at 1.5 Wcm-3 (1.0% NaCl).
119
), and the ohmically
100
A
Relative Abundance
75
CA
50
P
25
0
6
8
10
12
14
16
18
20
22
24
26
28
Time (min)
100
B
Relative Abundance
75
50
25
P
CA
0
6
8
10
12
14
16
18
20
22
24
26
28
Time (min )
Figure 4.6: Total ion chromatograms of TBDMS derivatized solutions of the unheated
buffer (A), and the ohmically heated medium at 1.5 Wcm-3 (1.0% NaCl) (B). The CA and
P peaks represent citric acid and phosphate, respectively.
120
CHAPTER 5
INVESTIGATION OF FREE RADICAL GENERATION DURING
OHMIC HEATING
ABSTRACT
Free radical generation in food processing is generally a concern. Research in this
area would aid to avoid or inhibit radical generating events during food processing. There
have been no studies reported about free radical generation during ohmic heating.
However, electrochemical phenomena at the electrode/solution interfaces may be
involved in generating radicals. This study investigates free radical generation during
ohmic heating at different frequencies. Ohmic heating experiments were carried out with
platinized-titanium electrodes using an aqueous heating medium containing 5,5-dimethyl1-pyrroline N-oxide (DMPO) as a spin trapping agent. Electron spin resonance
spectroscopy (ESR) was used for detection of radicals. No radical generation was evident
with pulsed ohmic heating operated at 1, 4, and 8 kHz under these experimental
conditions suggesting an operational frequency range effective in suppressing free radical
generation. However, results indicate generation of •OH radicals with conventional lowfrequency (60 Hz, sine wave) ohmic heating, and also with pulsed ohmic heating
operated at 10 kHz.
121
INTRODUCTION
Free radicals are by definition chemical species containing unpaired electrons. In
biological systems, free radicals are thought to play a major role in many oxidative
processes within cells, and have been implicated in a number of human diseases as well
as aging (Reiter et al., and Wickens, 2001). Generally, free radicals can be generated in
both chemical and biological systems by multiple pathways. In electrochemistry,
heterogeneous electron transfer associated with electrochemical reactions at the
electrode/solution interfaces is known to generate radical species (Schafer, 2001; Sawyer,
2003). This electrochemical generation of radicals has practical importance in the areas
of organic electrosynthesis (Schafer, 2001) and water treatment (Malik et al., 2001; Sun
et al., 1997). Some electrotechnologies used in food processing, such as high voltage arc
discharge (U.S.FDA, 2000) and pulsed electric discharge (Anpilov et al., 2002), are
considered to generate radicals and reactive molecular species, which in turn affect
microbial inactivation. Since there are redox type reactions at the electrode/solution
interfaces, the possibility of generating radicals during ohmic heating cannot be ruled out.
Tzedakis et al.(1999) have already briefly discussed this possibility implying the
formation of hydroxyl (•OH) and superoxide anion (O2•−) radicals during ohmic heating.
However, no conclusive evidence of radical generation during ohmic heating has so far
been reported.
As generally known, free radicals are short-lived and highly reactive. They can
readily react with various food components including lipids, vitamins, and amino
acids/proteins causing damaging effects on these nutrients, as in the cases of biological
systems (Reiter et al., Wickens, and Hawkins et al., 2001). In particular, oxygen122
containing free radicals cause oxidation of these food components, and also consume
antioxidants present in food formulations. Therefore, in addition to the nutritional losses,
the oxidation of foods leads to produce undesirable flavor, toxic, and color compounds,
which make foods less acceptable or unacceptable to consumers (Min et al., 2002).
The objective of this study was to investigate free radical generation during ohmic
heating. More specifically, since electrochemical reactions diminish with increasing
frequency (Wu et al., 1998; Uemura et al., 1994), we studied free radical generation at a
range of frequencies. A comparison was also made with conventional heating. Electron
Spin Resonance (ESR) spectroscopy with a spin trapping technique was employed for the
detection of radicals. The results will provide basic understanding of free radical
generation by ohmic heating.
MATERIALS AND METHODS
Chemicals: ACS grade sodium chloride (Fisher Scientific, NJ), absolute (99.5%) ethyl
alcohol (Aldrich, WI); and 97% 5,5-dimethyl-1-pyrroline N-oxide (DMPO) (Aldrich,
WI) were purchased from the suppliers. Demineralized double distilled water
(Resistivity: 3 megohm; pH: 5.5) was obtained from the Reagent Laboratory Store, at
The Ohio State University.
123
Heating procedure
Heating medium:
An aqueous NaCl (1.06%, w/v) solution containing 3.5 mM DMPO (as spin trapping
agent) was used as the heating medium in all the experiments. Electrical conductivity and
pH of the heating medium were 17.5 mS cm-1 and 5.2, respectively. In order to
distinguish between •OH and O2•− trapping by DMPO (Pritsos et al., 1985; Finkelstein et
al., 1980), a heating medium containing 2% (v/v) ethyl alcohol with the same amounts of
NaCl and DMPO was also used for some selected experiments. The heating media were
freshly prepared using fresh DMPO for each experiment.
Ohmic heating:
A pressurized batch ohmic heater made of a Pyrex glass tube (inside diameter: 2.5
cm), and equipped with platinized-titanium electrodes was used for the ohmic heating
experiments (see figure 5.1). The ohmic heater was attached to either a 60 Hz public
utility supply (through a variac) or an IGBT pulsed-power supply (described in chapter
3), as shown in figure 5.2. Ohmic heating experiments were carried out at 60 Hz (sine
wave), and at 1, 4, 8, and 10 kHz with IGBT pulse inputs. Appropriate pulse widths were
selected to achieve the same time-temperature history with all the ohmic heating
experiments (see table 5.1 and figure 5.3). Bipolar pulses at each frequency were centered
within the period, as shown in figure 5.4. The current (RMS) density was in the range of
3 – 10 kA m-2.
A volume of 30.0 (± 0.40) ml was subjected to ohmic heating for 43 seconds with
gentle magnetic stirring and 150 kPa (initial) air pressure reaching the end-point
temperature of 123 (± 1) °C (sterilizing temperature), in each experimental run. Once the
124
heating was completed, the heating medium was rapidly cooled to room temperature by
immersing the ohmic heater into a water bath containing chilled water (0 - 1 °C). Then, a
3.0 ml sample was withdrawn for ESR analysis after further mixing the heated fluid. A
3.0 ml sample of the unheated medium was used as a method blank. All the samples were
collected into polypropylene sample bottles (avoiding any headspace), and were stored in
ice for up to 4 hours prior to the ESR analysis. It was verified that the intensity of ESR
signal (shown in figure 5.5) remained unchanged during this period.
Conventional heating:
Conventional heating was carried out in a glass vial (volume: 4.0 ml) using an oil
bath maintained at 227 (± 1) °C. In each experimental run, a volume of 3.5 ml was
subjected to the heating, followed by rapid cooling to room temperature using the same
chilled water bath. The same end-point temperature was achieved (without boiling)
within the same duration of heating as in the ohmic heating experiments (see figure 5.3).
The same sampling procedure for the ESR analysis was also followed.
All the above experiments were randomized and triplicated with minimal exposure of
light.
ESR analysis
ESR spectroscopy is the most definitive method of studying free radicals. Spin
trapping with DMPO, a frequently used nitrone spin trap, enables ESR detection of •OH,
O2•−, H•, carbon-centered free radicals, and solvated electrons (esolv) (Hanaoka, 2001;
Pritsos et al., 1985; Sargent et al., 1976). Therefore, we used this analytical method to
acquire precise information of our experimental conditions. ESR spectra of all the
125
samples were collected on a Bruker ESP 300 spectrometer equipped with an ER 035M
NMR gaussmeter and Hewlett-Packard 5352B microwave frequency counter, using a
standard flat cell. The spectrometer was operated at 9.77 GHz microwave frequency, 10
mW microwave power, 100 kHz field modulation, and 5 G modulation amplitude.
Quantification of approximate amounts of free radicals was obtained by
measurement of the ESR spectrum of a relatively stable flavosemiquinone radical
prepared by flavoquinone/ flavodoxin radical-generating system. The spectrum was
double integrated and compared with the double integrals of the ESR spectra of the
samples that exhibited characteristic ESR signals.
DMPO-OH reference:
The generation of DMPO-OH radical adducts either via trapping of •OH radicals, or
as the decay product of DMPO-OOH (unstable) radical adducts formed by trapping of
O2•− radicals is well-known (Shi et al., 1998; Pritsos et al., 1985; Finkelstein et al., 1980).
We obtained DMPO-OH radical adducts by the latter method using xanthine/ xanthine
oxidase as a source of O2•− (Shi et al., 1998) and treating with DMPO. The ESR spectrum
of this particular radical adduct was recorded at the same spectrometer settings as
described above, and was used as the reference. Figure 5.5 shows the characteristic
quartet (1:2:2:1) spectrum of the DMPO-OH radical adducts with hyperfine splittings of
aN = aH = 14.9 G.
126
RESULTS AND DISSCUSSION
As can be seen from figure 5.6, pulsed ohmic heating operated at 1, 4, and 8 kHz did
not indicate any radical generation. However, a DMPO-OH signal appeared, when the
frequency was raised to 10 kHz. The ohmic heating carried out at 60 Hz (sine wave) also
exhibited this characteristic signal. As previously described, trapping of both •OH and
O2•− radicals by DMPO yields DMPO-OH radical adducts. In the presence of ethyl
alcohol, an efficient •OH scavenger, trapping of •OH radicals (i.e. generation of •OH
radicals during ohmic heating) can be verified, because of the formation of DMPOCH(OH)CH3 radical adducts (see figure 5.7), simultaneously inhibiting the characteristic
DMPO-OH signal (Pritsos et al., 1985; Finkelstein et al., 1980). Figure 5.8 shows the
effect of ethyl alcohol on the ESR spectra of the ohmic heating experiments carried out at
60 Hz (sine wave) and 10 kHz. It is clearly seen that ethyl alcohol caused almost
complete disappearance of the characteristic DMPO-OH signal at both frequencies,
strongly implying the generation of
•
OH radicals. The carbon-centered DMPO-
CH(OH)CH3 radical adduct formed in the presence of ethyl alcohol, is known to exhibit
an ESR signal composed of six identical lines. However, such an ESR signal was not
evident, possibly because of the very weak signal intensities. The ESR spectra shown in
figures 5.6 and 5.8 were reproducible at least twice in each case.
The conventional heating procedure did not indicate any free radical generation (see
figure 5.6). The ESR spectrum of the method blank ensured that there were no interfering
signals. Further, the ESR spectra of both the conventional heating and the method blank,
together with the ohmic heating in the presence of ethyl alcohol (which is a competitive
inhibitor) clearly demonstrated that the observed DMPO-OH signals at 60 Hz (sine wave)
127
and 10 kHz were not simply due to artifacts caused by heat or nucleophilic addition of
water to DMPO described by Robert et al., 2002 and Makino et al., 1990. In our previous
laboratory scale studies (chapter 3), we observed enhanced corrosion of platinizedtitanium electrodes during ohmic heating at these frequencies. The migrated Pt and Ti at
the above frequencies could function in a Fenton-like reaction generating •OH radicals.
The •OH radical is highly electrophilic (Hawkins et al. and Reiter et al, 2001), and
therefore can aggressively attack electron-rich molecules (i.e. virtually all food
components) causing their oxidation. The best protection against the •OH is considered to
be the prevention of its formation (Reiter et al, 2001). Therefore, ohmic heating would be
better performed at the frequencies (1 – 8 kHz) where no radical generation was detected.
In this study, although radical generation was detected at 60 Hz (sine wave) and 10 kHz
(pulses), our results may not imply the occurrence of radical generation in pilot scale. Our
pilot scale study of electrode corrosion (chapter 2) indicated extremely low Pt and Ti
migrations (ppt levels at 39.8 kW), which may not allow occurring a Fenton-like reaction
that generate •OH radicals. Moreover, since food systems are inherently complex and
consist of natural antioxidants, such as tocopherols, and other phenolics and
polyphenolics, some amount of electrochemically generated radicals can be tolerated
without undergoing significant changes during long storage periods.
128
CONCLUSIONS
Free radical generation during ohmic heating can be suppressed by using IGBT pulse
inputs. The operational frequency, however, needs to be 1 ≤ f < 10 kHz with platinizedtitanium electrodes. Ohmic heating operated at 60 Hz (sine wave) and 10 kHz (IGBT
pulses) indicated the generation of •OH radicals.
129
SYMBOLS
aN, aH
hyperfine splitting constants (G)
f
frequency (Hz)
ppt
parts per trillion
RMS
root-mean-square
T
period (µs)
tp
pulse width (µs)
td
delay time (µs)
130
REFERENCES
Anpilov A.M., Barkhudarov E.M., Christofi N., Kop’ev V.A., Kossyi I.A., Taktakishvili
M.I., and Zadiraka Y. (2002), Pulsed high voltage electric discharge disinfection of
microbially contaminated liquids; Letters in applied microbiology, 35, pp. 90-94.
Finkelstein E., Rosen G.M., and Rauckman E.J. (1980); Spin trapping of superoxide and
hydroxyl radical: practical aspects; Archives of biochemistry and biophysics, 200(1), pp.
1-16.
Hanaoka K. (2001); Antioxidant effects of reduced water produced by electrolysis of
sodium chloride solutions; Journal of applied electrochemistry, 31, pp. 1307-1313.
Hawkins C.L., and Davies M.J. (2001); Generation and propagation of radical reactions
on proteins; Biochimica et biophysica acta, 1504, pp. 196-219.
Makino K., Hagiwara T., Hagi A., Nishi M., and Murakami A. (1990); Cautionary note
for DMPO spin trapping in the presence of iron ion; Biochemical and biophysical
research communications, 172 (3), pp. 1073-1080.
Malik M.A., Ghaffar A., and Malik S.A.(2001); Water purification by electrical
discharges; Plasma sources science and technology, 10, pp.82-91.
Min D.B., and Boff J.M. (2002); Chemistry and reaction of singlet oxygen in foods;
Comprehensive Reviews in Food Science and Food Safety – Institute of Food
Technologists, 1, pp. 58-72.
Pritsos C.A., Constantinides P.P., Tritton T.R., Heimbrook D.C., and Sartorelli A.C.
(1985); Use of high-performance liquid chromatography to detect hydroxyl and
superoxide radicals generated from mitomycin C; Analytical Biochemistry, 150, pp. 294299.
Reiter R.J., Tan D., Manchester L.C., and Qi W. (2001); Biochemical reactivity of
melatonin with reactive oxygen and nitrogen species; Cell biochemistry and biophysics,
34, pp. 237-256.
Robert R., Barbati S., Ricq N., and Ambrosio M. (2002); Intermediates in wet oxidation
of cellulose: identification of hydroxyl radical and characterization of hydrogen peroxide;
Water research, 36, pp. 4821-4829.
Sargent F.P., and Gardy E.M.(1976); Spin trapping of radicals formed during radiolysis
of aqueous solutions. Direct electron spin resonance observations; Canadian journal of
chemistry, 54, pp. 275-279.
131
Schafer H.J.(2001); Organic electrochemistry (Fourth edition), Lund H. and Hammerich
O. (Ed.), Chapter 4: Comparison between electrochemical reactions and chemical
oxidations and reductions; Marcel Dekker, Inc., New York, pp. 207-221
Shi X., Leonard S.S., Liu K.J., Zang L., Gannett P.M., Rojanasakul Y., Castranova V.,
and Vallyathan V. (1998); Cr(III)-mediated hydroxyl radical generation via Haber-Weiss
cycle; Journal of inorganic biochemistry, 69, pp. 263-268.
Sun B., Sato M., and Clements J.S.(1997); Optical study of active species produced by a
pulsed streamer corona discharge in water; Journal of electrostatics, 39, pp. 189-202.
Tzedakis T., Basseguy R., and Comtat M.(1999); Voltammetric and coulometric
techniques to estimate the electrochemical reaction rate during ohmic sterilization; J. of
Applied Electrochemistry; 29(7), pp. 821- 828.
Uemura K., Noguchi A., Park S.J., and Kim D.U. (1994); ohmic heating of food
materials- Effect of frequency on the heating rate of fish protein; Developments in Food
Engineering - Proceedings of the 6 th International Congress on Engineering and Food;
Blackie Academic & Professional Press, London, pp. 310-312.
U.S. FDA- Center for food safety and applied nutrition (June 2, 2000); Kinetics of
microbial inactivation for alternative food processing technologies – High voltage arc
discharge.
Wickens A.P.(2001); Ageing and the free radical theory; Respiration physiology, 128, pp.
379-391.
Wu H., Kolbe E., Flugstad B., Park J.W., and Yongsawatdigul J.(1998); Electrical
properties of fish mince during multi-frequency ohmic heating; Journal of Food Science;
63(6), pp. 1028-1032.
132
Frequency / Hz
RMS voltage (V) or pulse width (µs)
60 *(sine wave)
110 V (RMS)
1 000
226
4 000
57
8 000
27
22
10 000 *
* Ohmic heating experiments were also carried out with the heating medium containing
2% (v/v) ethyl alcohol. Lowering of electrical conductivity due to the presence of ethyl
alcohol was compensated by using 115 V (RMS) and 25 µs pulse width.
Table 5.1: Selected ohmic heating conditions to study free radical generation. See figure
5.3 for typical time-temperature history for all these ohmic heating conditions.
133
1
6
P
7
3
2
2
4
5
1 – Coated thermocouple
2 – Platinized-titanium electrodes (electrode gap: 6.1 cm; geometric surface area: 4.8 cm2
per electrode)
3 – Rubber gaskets
4 - Magnetic stirring bar
5 – Steel flanges clamped together with bolts
6 - Removable lid: thermocouple and airflow tubing are attached. It was also clamped to
the cell body with bolts during ohmic heating
7 – Inlet airflow
P – Air pressure gauge
Figure 5.1: Schematic diagram of the pressurized ohmic heater.
134
Isolation
module
Pressurized
ohmic heater
Oscilloscope
~
V
Digital
multimeter
A
Data
logger
V = Differential voltage probe
A = Current monitor
Microcomputer
~ 60 Hz Public utility supply/
IGBT Power supply
Figure 5.2: Schematic diagram of the experimental setup used for the ohmic heating.
135
120
0
Temp./ C
100
80
60
40
Ohmic
20
Conventional
0
0
10
20
30
40
time/ seconds
Figure 5.3: Typical time-temperature histories for ohmic and conventional heating.
136
tp
td/2
td
td/2
tp
T = 1/ f
Figure 5.4: Schematic diagram of the centering of bipolar pulses within the period (T) at
each frequency (f). The positive and negative pulses having the same pulse width (tp)
were equally spaced by adjusting the delay time (td) as T = 2 (tp + td).
137
80000
Intensity
40000
0
-40000
-80000
3405
3435
3465
3495
Gauss
3525
3555
Figure 5.5: The ESR spectrum of the DMPO-OH reference. This signal represents spin
concentration of 0.63 µM.
138
Blank
Conventional
1 kHz
4 kHz
8 kHz
10 kHz
60 Hz
(sine wave)
DMPO-OH
3405
3435
3465
3495
3525
3555
Gauss
Figure 5.6: Typical ESR spectra of ohmic and conventional heating experiments, in
comparison with the ESR spectrum of DMPO-OH reference. The signals at 60 Hz (sine
wave) and 10 kHz correspond to average spin concentrations of 0.14 and 0.11 µM,
respectively.
139
•
OH
−
+
OH (H )
OOO
N
CH3CH2OH
•
O
O2•−
DMPO-OOH
•
CH(OH)CH3
(α-hydroxyethyl radical)
N
+
O
−
DMPO
•
OH
CH
N
CH3
O
OH
N
•
•
O
OH
DMPO-CH(OH)CH3
DMPO-OH
Figure 5.7: Chemistry of •OH and O2•− trapping by DMPO in the presence and absence of
ethyl alcohol.
140
10 kHz
With
alcohol
Without
alcohol
With
alcohol
60 Hz (sine wave)
Without
alcohol
3405
3435
3465
3495
3525
3555
Gauss
Figure 5.8: Comparison of typical ESR spectra of the ohmic heating experiments carried
out at 60 Hz (sine wave) and 10 kHz in the presence (2%, v/v) and absence of ethyl
alcohol.
141
CHAPTER 6
CONCLUSIONS
1.
With 60 Hz (sine wave) ohmic heating, corrosion of all the electrode materials
(titanium, stainless steel, platinized-titanium, and graphite) is enhanced at pH 3.5
compared to that at pH 5.0 and 6.5.
2.
Stainless steel was found to be the most electrochemically active electrode material
during ohmic heating.
3.
Corrosion of graphite electrodes yields soluble organic compounds due to the
migration of surface functional groups and oxides during ohmic heating.
4.
Among the materials tested in our study, platinized-titanium can be considered as
the electrode material-of-choice for ohmic heating with commonly available
low-frequency (60 Hz, sine wave) alternating currents.
142
5.
Pulsed ohmic heating significantly inhibits the electrochemical reactions of
stainless steel, titanium, and platinized-titanium electrodes, in comparison to
conventional 60 Hz ohmic heating.
6.
Pulsed ohmic heating at higher frequencies and shorter pulse widths yields the
lowest rates of electrochemical reactions of stainless steel electrodes.
7.
Pulsed ohmic heating at lower frequencies and longer pulse widths is more effective
in suppressing the electrochemical reactions of titanium and platinized-titanium
electrodes, while achieving higher duty cycles.
8.
In general, pulsed ohmic heating is unable to suppress the electrochemical
reactions of graphite electrodes.
9.
Delay time (off-time between adjacent pulses) was found to be a critical factor in
pulsed ohmic heating.
10. Ohmic heating (60 Hz, sine wave) of ascorbic acid in citrate-phosphate buffer with
stainless steel electrodes showed the formations of metal-phosphates and
metal-citrate complexes, also indicating electrocatalytic effects on ascorbic acid
degradation.
143
11. Free radical generation during ohmic heating with platinized-titanium electrodes
can be effectively suppressed by using IGBT pulse inputs at the frequency range of
1 – 8 kHz.
12.
Ohmic heating with platinized-titanium electrodes operated at 60 Hz (sine wave)
and 10 kHz (pulses) indicated the generation of •OH radicals.
144
LIST OF REFERENCES
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