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 Adviser Professor Russ C. Hille Food Science and Nutrition Graduate Program

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© Copyright by

Chaminda Padmal Samaranayake

2003

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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 platinized-

titanium 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

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(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-1-

pyrroline 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.

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Dedicated to all who assisted my education prior to and during my study at The Ohio State University.

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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 ESI-

MS 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

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up viruses and restoring the programs in my computer, and also to Rakhith U.C. for his

contribution to some digital images.

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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.

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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

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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

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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

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Symbols ……..…..……….…………….…..……………………………………... 130 References ………….……………………..……………………………………… 131 6. Conclusions …….…………...…………………………………………………… 142

List of References ………………..…………………………………………………... 145

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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

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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

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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

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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

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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

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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

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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:

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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,

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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

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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

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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

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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 half-

reaction 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

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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)

The overall reaction:

2H2O(liquid) ⇔ 2H2 (g) + O2 (g) (1.6)

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) generation. In addition, since some electrode materials show low

overpotential for Cl2 (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

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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.

9

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)

10

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.

11

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)

12

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.

13

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.

14

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.

15

I Electrodes

Alternating ~ Food V R current power supply

Electrical analogue 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.

16

Figure 1.3: Mechanisms of generating free radicals by electrolysis and electrode corrosion.

Mn+(aqueous)

3O2 (g) (Triplet dioxygen)

Electrolysis

Electrode corrosion

H• H2 (g)

H2O2

H+ + e

•OOH •OH

e

OH −

O2•− H+

1O2 (g) (Singlet oxygen)

Fenton’s ReactionOCl −

Cl − + H2O

H+ H2O2

Fe3+

Cl2 (g)

Cl•

H2O

Chlorine generation H+ (or H2O)

M2+(aqueous)

Fe2+

O2•−

OH −

17

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.

18

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).

19

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] trifluoroacetamide (Regis

20

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

21

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

22

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.

23

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.

24

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.

25

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.

26

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

27

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.

28

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,

29

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.

30

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,

31

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 (2.10)

32

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 (2.12)

Overall reaction: 2H2O (liquid) + 2Cl −(aqueous) ⇔ Cl2 (g) + H2 (g) + 2OH−(aqueous) (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

33

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

34

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)

35

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

36

(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.

37

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.

38

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)

39

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.

40

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.

41

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 = 3.5

pH = 5.0

pH = 6.5

∆ pH*

+ 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.

42

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.

43

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.

1

2

3

4

5

6

44

Figure 2.2: Schematic diagram of the laboratory scale experimental setup.

60 Hz/ 0-110 V ~ Public utility Supply

Isolationmodule

Hydrogen gas meter 0 – 250 ppm

Data logger

Microcomputer

V

AVariac

V = voltage transducer A = Current transducer

Thermocouple

ohmic heater

45

0

10

20

30

40

50

0 100 200 300time/ seconds

Tem

p./ 0 C

Figure 2.3: Typical time vs. temperature curve for all electrodes at all pH values during ohmic heating.

0.00

0.50

1.00

1.50

2.00

0 100 200 300

time/ seconds

Cur

rent

/ A

Figure 2.4: Typical time vs. current curve for all electrodes at all pH values during ohmic heating.

46

Figure 2.5: Typical SEM micrograph of titanium electrodes.

47

Figure 2.6: Typical SEM micrograph of stainless steel electrodes.

48

Figure 2.7: Typical SEM micrograph of platinized-titanium electrodes.

49

Figure 2.8: Typical SEM micrograph of graphite electrodes.

50

0

50

100

150

200

250

0 100 200 300time/ seconds

[H2]ppm

pH = 3.5pH = 5.0pH = 6.5

Figure 2.9: Hydrogen generation with titanium electrodes during ohmic heating.

0

50

100

150

200

250

0 100 200 300time/ seconds

[H2]ppm

pH = 3.5pH = 5.0pH = 6.5

Figure 2.10: Hydrogen generation with stainless steel electrodes during ohmic heating.

51

OH

O

OH

OH

O

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).

52

Figure 2.12: Positive ion ESI-MS spectra of the heating medium before and after ohmic heating.

75.1

91.0

109.0

123.0

133.0

149.0

0.0

0.5

1.0

1.5

2.0

2.5

20 40 60 80 100 120 140 160 180 m/z

Int. × 104

149.0

0.0

0.5

100 200 m/z

After BeforeInt. ×104

53

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.

54

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 (off-

time 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

55

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

56

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 time-

temperature 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.

57

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 (3.2)

Pinput = VpIpθ (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

58

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

59

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,

60

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

61

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

62

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 platinized-

titanium 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.

63

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 platinized-

titanium 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.

64

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: 2H+(aqueous) + 2e ⇔ H2 (g) (3.5)

Anodic half-reaction: 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

65

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.

66

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.

67

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

68

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 devices – continuous development; Microelectronics Journal, 27, pp. 161-176 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.

69

Frequency / Hz

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

10 000

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

4 000

110.0

1.64

19

60 (sine wave)

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.

70

Frequency / Hz

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

10 000

25.0

2.21

25

30.0

3.30

31

50.0

2.58

29

75.0

2.05

24

4 000

100.0

1.78

21

60 (sine wave)

110 (Vrms)

2.58

28

Table 3.2: Frequency-pulse width combinations with required initial electrical conductivities, and cooling water temperatures for titanium electrodes.

71

Frequency / Hz

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

10 000

25.0

2.17

23

30.0

3.24

28

50.0

2.53

26

75.0

2.02

24

4 000

100.0

1.76

21

60 (sine wave)

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.

72

Frequency / Hz

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

10 000

25.0

1.86

24

30.0

2.85

31

50.0

2.16

28

75.0

1.71

24

4 000

100.0

1.46

21

60 (sine wave)

110 (Vrms)

2.19

29

Table 3.4: Frequency-pulse width combinations with required initial electrical conductivities, and cooling water temperatures for graphite electrodes.

73

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.

1

2

3

4

5

6

7 7

74

Figure 3.2: Schematic diagram of the experimental setup used for pulsed ohmic heating.

V = Differential voltage probeA = Current monitor

Isolation module

Hydrogen gas meter 0 – 250 ppm

ohmic heater

~

Oscilloscope

Data logger

Microcomputer

V

A Digital multimeter

Rectifier

+

_

N

~

~

~ IGBT

Pulse generator

IGBT Power supply

60 H

z, 3

pha

se

Pow

er su

pply

75

0

10

20

30

40

50

0 50 100 150time/ seconds

Tem

p./ 0 C

Figure 3.3: Typical time vs. temperature curve for all the ohmic heating experiments.

76

Figure 3.4: Schematic diagram of the centering of bipolar pulses within the period to study the effects of frequency and pulse width.

td td/2 td/2

T = 1/ f

tp

tp

77

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

10 µs 15 µs

25 µs

35 µs

20 µs

78

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.

30 µs

62.5 µs 75 µs

100 µs 110 µs

50 µs

79

Figure 3.7: Schematic diagram of the centering of bipolar pulses within the period to demonstrate the effect of delay time.

td1 td2/2 td2/2

T = 1/ f

tp

tp

80

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.

Delay: 0 µs Delay: 5 µs

Delay: 10 µs Delay: 15 µs

Delay: 20 µs Delay: 25 µs

81

0.00

2.50

5.00

7.50

10.00

12.50

10(10)20%

15(10)30%

25(10)50%

35(10)70%

30(4)

24%

62.5(4)

50%

75(4)

60%

100(4)

80%

110(4)

88%

Con

[Fe]

in p

pb/ K

J

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.

a

ab

aba

d

cd

bc bcab

e

82

0.00

0.30

0.60

0.90

1.20

1.50

1.80

10(10)20%

15(10)30%

25(10)50%

35(10)70%

30(4)

24%

62.5(4)

50%

75(4)

60%

100(4)

80%

110(4)

88%

Con

[Cr]

in p

pb/ K

J

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.

a a

a

ab

c

bc

abc abcab

d

83

0.00

0.05

0.10

0.15

0.20

10(10)20%

15(10)30%

20(10)40%

25(10)50%

30(4)

24%

50(4)

40%

75(4)

60%

100(4)

80%

Con

[Ti]

in p

pb/ K

J

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.

ab

bcdd

a

cd

abc

a abc

e

84

0.000

0.025

0.050

0.075

0.100

10(10)20%

15(10)30%

20(10)40%

25(10)50%

30(4)

24%

50(4)

40%

75(4)

60%

100(4)

80%

Con

[Pt]

in p

pb/ K

J

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.

ab ab

ab

c

bab

ab a

d

85

0.000

0.025

0.050

0.075

0.100

10(10)20%

15(10)30%

20(10)40%

25(10)50%

30(4)

24%

50(4)

40%

75(4)

60%

100(4)

80%

Con

[Ti]

in p

pb/ K

J

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.

ab

ab

a

a

b b b b b

* * *

86

0.00

0.05

0.10

0.15

0.20

10(10)20%

15(10)30%

20(10)40%

25(10)50%

30(4)

24%

50(4)

40%

75(4)

60%

100(4)

80%

Con

[C] i

n pp

m/ K

J

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.

*a a

abab b

ab ab

ab

ab

87

0.0

50.0

100.0

150.0

0 5 10 15 20 25 ConDelay time/ micro-seconds

[Fe]

in p

pb/ K

J

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.

a

cb b

c c c

88

0.0

15.0

30.0

0 5 10 15 20 25 Con

Delay time/ micro-seconds

[Cr]

in p

pb/ K

J

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.

c

a

b dc c c

89

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.

90

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

91

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.

92

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.

93

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:

94

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)

95

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,

96

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:

97

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

98

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 metal-

hydroxide 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)

99

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

100

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) (4.10)

AA + O2 (g) ⇔ oxidized AA + O2•– (catalyzed by Fe2+

(aqueous)) (4.11)

2O2•– + 2H+

(aqueous) ⇔ H2O2 + O2 (g) (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 metal-

citrate 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

101

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.,

102

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

103

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

104

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.

105

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 alternating currents to minimize

electrocatalytic effects.

106

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)

107

REFERENCES

Abrahamson H.B., Rezvani A.B.; and Brushmiller J.G.(1994); Photochemical and 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.

108

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 L-ascorbic 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 preparations and the automated data system to screen for organic acidemias; Journal of 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. 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. Zhao M.J., and Jung L.(1995); Kinetics of the competitive degradation of deoxyribose and other molecules by hydroxyl radicals produced by the Fenton reaction in the presence of ascorbic acid; Free Radical Research,23(3),pp.229-243.

109

Parameter

Value

NaCl concentration of the buffer a (w/v %)

0.25

0.50

1.0

Power density b (W/ cm3)

0.5

0.75

0.75

1.5

Corresponding voltage (Vrms) (at 60 Hz) / (± 1) V

33.0

40.3

33.9

41.1

Corresponding current (Irms) (at 60 Hz) / (± 1) A

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.

110

NaCl concentration of the buffer (w/v %)

0.25

0.50

1.0

Power density (W/ cm3)

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

[Fe] / ppm Metal Migration * [Cr] / ppm [Ni] / ppm

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

4 c

2 c

% 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

0 a

+ 4.8 b

+ 119.8 c

* 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.

111

NaCl concentration of the buffer (w/v %)

0.25

0.50

1.0

Power density (W/ cm3)

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.

112

Compound

Solubility product b

(Ksp)

Minimum [Mn+] / moles dm-3

(where n = 2,3)

FePO4 (solid)

1.3 × 10 −22

4.87 × 10 −12

c Fe3(PO4)2 (solid)

1 × 10 −36

1.12 × 10 −5

d CrPO4 (solid)

2.4 × 10 −23

9 × 10 −13

Metal-Phosphate a

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

Metal-Hydroxide

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, 1992). Ka 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.

113

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

0.25

0.75

0 a

- 2.1 a

0.50

0.75

+ 4.8 b

+ 1.6 b

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.

114

Sample

λmax / nm

Absorptivity coefficient

1:1 Fe(III)-citrate 221.00 ( cmvwE 0.1

)/%(025.0 ) 73.08 ml g-1. cm-1

The ohmically heated heating medium

219.50 ( cmvvE 0.1

)/%(5 ) 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).

Buffer component

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

Table 4.7: GC-MS characteristics and % losses of the buffer components.

115

0.00

1.00

2.00

3.00

4.00

5.00

3.5 4.5 5.5 6.5 7.5 8.5pH

Buf

fer c

apac

ity

Buffer

Buffer

Buffer + metal ions [1]

Buffer + metal ions [2]

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.

at 25 °C

at 80 °C

at 80 °C

at 80 °C

116

0.00

1.00

2.00

3.00

4.00

5.00

3.5 4.5 5.5 6.5 7.5 8.5pH

Buf

fer c

apac

ity

Buffer

Buffer

Buffer + metal ions

Figure 4.2: Buffer capacities (in µmol pH-1ml-1) at 0.50% (w/v) NaCl.

at 25 °C

at 80 °C

at 80 °C

117

0.00

1.00

2.00

3.00

4.00

5.00

3.5 4.5 5.5 6.5 7.5 8.5pH

Buf

fer c

apac

ity

Buffer

Buffer

Buffer + metal ions

Figure 4.3: Buffer capacities (in µmol pH-1ml-1) at 1.0% (w/v) NaCl.

at 25 °C

at 80 °C

at 80 °C

118

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).

119

Figure 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).

A

120

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.

6 8 10 12 14 16 18 20 22 24 26 28

Time (min)

0

25

50 75

100

Rel

ativ

e A

bund

ance

CA

P

6 8 10 12 14 16 18 20 22 24 26 28

Time (min)

0

25 50 75

100

Rel

ativ

e A

bund

ance

CAP

A

B

121

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-dimethyl-

1-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 low-

frequency (60 Hz, sine wave) ohmic heating, and also with pulsed ohmic heating

operated at 10 kHz.

122

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, oxygen-

123

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.

124

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

125

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

126

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.

127

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 DMPO-

CH(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)

128

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 platinized-

titanium 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.

129

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 platinized-

titanium electrodes. Ohmic heating operated at 60 Hz (sine wave) and 10 kHz (IGBT

pulses) indicated the generation of •OH radicals.

130

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)

131

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. 294-299. 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.

132

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.

133

Frequency / Hz

RMS voltage (V) or pulse width (µs)

60 *(sine wave)

110 V (RMS)

1 000

226

4 000

57

8 000

27

10 000 *

22

* 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.

134

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.

P

1

22

3

4

5

76

135

Figure 5.2: Schematic diagram of the experimental setup used for the ohmic heating.

Isolation module

Pressurized ohmic heater

~

Oscilloscope

Data logger

Microcomputer

V

A Digital multimeter

V = Differential voltage probe A = Current monitor

~ 60 Hz Public utility supply/ IGBT Power supply

136

0

20

40

60

80

100

120

0 10 20 30 40time/ seconds

Tem

p./ 0

C

Ohmic

Conventional

Figure 5.3: Typical time-temperature histories for ohmic and conventional heating.

137

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).

td td/2 td/2

T = 1/ f

tp

tp

138

-80000

-40000

0

40000

80000

3405 3435 3465 3495 3525 3555Gauss

Inte

nsity

Figure 5.5: The ESR spectrum of the DMPO-OH reference. This signal represents spin concentration of 0.63 µM.

139

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.

DMPO-OH

60 Hz (sine wave)

10 kHz

8 kHz

4 kHz

1 kHz

Conventional

Blank

140

Figure 5.7: Chemistry of •OH and O2

•− trapping by DMPO in the presence and absence of ethyl alcohol.

N

O

+

−

N

O

OOH•

OO− (H+)

N

O

O H•

N

O

CH CH3

OH

•

•OH

CH3CH2OH

O2•−

•CH(OH)CH3 (α-hydroxyethyl radical)

•OH

DMPO

DMPO-OOH

DMPO-OH DMPO-CH(OH)CH3

141

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.

60 Hz (sine wave)

10 kHz

With alcohol

Without alcohol

Without alcohol

With alcohol

142

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.

143

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.

144

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.

145

LIST OF REFERENCES Aboul-Enein H.Y., Kladna A., Kruk I., and Lichszteld K. (2003); Effect of psoralens on Fenton-like reaction generating reactive oxygen species; Biopolymers, 72(1), pp. 59-68. Abrahamson H.B., Rezvani A.B.; and Brushmiller J.G.(1994); Photochemical and 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. 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. 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. 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. 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.

146

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. Dodge C.J., and Francis A.J. (1997); Biotransformation of binary and ternary citric acid complexes of iron and uranium; Environmental Science & Technology, 31(11), pp. 3062-3067. Donnet J.B. (1968); The chemical reactivity of carbons; Carbon, 6, pp. 161-176. 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. Ershler B. (1947); Investigation of electrode reactions by the method of charging-curves and with the aid of alternating currents; Discussions of the Faraday Society, 1, pp. 269-277. Evans C.A.(1979); Spin trapping; Aldrichimica Acta, 12(2), pp. 23-30. Finkelstein E., Rosen G.M., Rauckman E.J., and Paxton J. (1979); Spin trapping of superoxide; Molecular Pharmacology, 16, pp. 676-685. 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. Floyed T.L. (1995); Instructor’s edition for electric circuits fundamentals and electronics fundamentals: circuits, devices, and applications (Third edition), Chapters 8 and 15; Prentice Hall, Inc., Columbus, Ohio, pp. 317- 320 and 626-661. Getchell B.E.(1935); Electric pasteurization of milk; Agr. Eng., 16(10), pp.408-410.

147

Gilbert T.W., Newman L., and Klotz P. (1968); Mixed-metal hydroxycarboxylic acid complexes, The chromium (iii) inhibition of the solvent extraction of indium (iii); Analytical Chemistry, 40(14), pp. 2123-2130. Golub D., Oren Y., and Soffer A. (1987); The electrochemical double layer of carbon and graphite electrodes, Part IV: Dependence of carbon electrode dimensions and electrical capacity on pH; Journal of Electroanalytical Chemistry, 227, pp. 41-53. Grant D. (1996); Power semiconductor devices – continuous development; Microelectronics Journal, 27, pp. 161-176 Hanaoka K. (2001); Antioxidant effects of reduced water produced by electrolysis of sodium chloride solutions; Journal of applied electrochemistry, 31, pp. 1307-1313. Hanna P.M., Chamulitrat W., Mason R.P. (1992); When are metal ion-dependent hydroxyl and alkoxyl radical adducts of 5,5-dimethyl-1-pyrroline N-oxide artifacts?; Archives of biochemistry and biophysics, 296(2), pp. 640-644. 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. Harbour J.R., Chow V., and Bolton J.R. (1974); An electron spin resonance study of the spin adducts of OH and HO2 radicals with nitrones in the ultraviolet photolysis of aqueous hydrogen peroxide solutions; Can. J. Chem., 52, pp. 3549-3553. Harris D.C.(1999); Quantitative Chemical Analysis - fifth edition, W.H. Freeman and Company, New York, Ap: 12-14. Hawkins C.L., and Davies M.J. (2001); Generation and propagation of radical reactions on proteins; Biochimica et biophysica acta, 1504, pp. 196-219. Horlick G. (1992); Plasma-source mass spectrometry for elemental analysis; Spectroscopy, 7(1), pp. 22-27. Hu C. and Liu K. (1999); Voltammetric investigation of platinum oxides. I. Effects of ageing on their formation/ reduction behavior as well as catalytic activities for methanol oxidation; Electrochemica Acta, 44, pp. 2727-2738. 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.

148

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. Koike M. and Fujii H. (2001); The corrosion resistance of pure titanium in organic acids; Biomaterials, (22), pp. 2931 – 2936. Konya J. (1979); Notes on the “non-faraday” electrolysis of water; Journal of electrochemical society: electrochemical science and technology, 126(1), pp. 54-56. Kremer M.L. (1999); Mechanism of the Fenton reaction. Evidence for a new intermediate; Phys. Chem. Chem. Phys., 1, pp. 3595-3605. 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. 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. Lindeburg M.R. (1998); Engineer-in-training reference manual, Chapter 48: Alternating current circuits; 8th edition, Professional publications, Inc., Belmont, CA, pp. 48-4 – 48-5. 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. 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.

149

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. 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. 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. 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. Niemela K. (1987); Oxidative and non-oxidative alkali-catalyzed degradation of L-ascorbic acid; Journal of Chromatography, 399, pp. 235-243. 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. 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 preparations and the automated data system to screen for organic acidemias; Journal of Chromatography B, 746, pp. 63-73. Pai S., Tsau Y., and Yang T. (2001); pH and buffering capacity problems involved in the determination of ammonia in saline water using the indophenol blue Spectrophotometric method; Analytica Chimica Acta, 434, pp. 209-216. 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.

150

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. 294-299. Rabah M.A., Nassif N., and Azim A.A.A. (1991); Electrochemical wear of graphite anodes during electrolysis of brine; Carbon, 29(2), pp. 165-171. Ramshaw R.S. (1993); Power electronics semiconductor switches, Chapters 1 and 7; Chapman & Hall, Inc., New York, pp. 1-23 and 294-319. 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. 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. Reznick D.(1996); ohmic heating of fluid foods; J. Food Technology; 5, pp. 250-251. 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. 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. 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. 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

151

Scott V.D., Love G., and Reed S.J.B. (1995); Quantitative electron-probe microanalysis (second edition), Chapters 4 and 6; Ellis Horwood Ltd., New York, pp. 61-75 and 93-109. Shaw M. and Remick A.E.(1950); Studies on alternating current electrolysis; J. of Electrochemical Society; 97(10), pp. 324-334. 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. Shriver D.F. and Atkins P.W. (1999); Inorganic Chemistry (third edition), Chapter 6: Oxidation and reduction; W.H. Freeman and Company, New York, pp. 177-210. Soffer A., and Folman M. (1972); The electrical double layer of high surface porous carbon electrodes; Journal of Electroanalytical Chemistry, 38, pp.25 – 43. Speiser C., Baumann T., and Niessner R. (2001); Characterization of municipal solid waste incineration (MSWI) bottom ash by scanning electron microscopy and quantitative energy dispersive X-ray microanalysis (SEM/EDX); Fresenius J. Anal. Chem., 370, pp. 752-759. 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. Stirling R. (1987); ohmic heating - a new process for the food industry; Power Engineering Journal; 1(6), pp. 365-371. Stott F.H.(1984); Some recent experience with scanning electron microscope in corrosion research; Journal of Microscopy, 133 (2), pp. 191-204. 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. Tanner S.D., Baranov V.I., and Volkopf U. (2000); A dynamic reaction cell for inductively coupled plasma mass spectrometry (ICP-DRC-MS), Part III: Optimization and analytical performance; J. Anal. At. Spectrom., 15, pp. 1261-1269. 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. Thomas M.J.K. (1996); Ultraviolet and visible spectroscopy, Ando D.J. (Ed.), Chapter 5: Qualitative analysis and structural relationships; second edition, John Wiley & Sons, Ltd., England, pp. 163-187.

152

Thomson B.A., Iribarne J.V., and Dziedzic P.J. (1982); Liquid ion evaporation/ mass spectrometry/ mass spectrometry for the detection of polar and labile molecules; Analytical Chemistry, 54(13), pp.2219-2224. Tomat R., and Rigo A. (1979); Oxidation of polymethylated benzenes promoted by •OH radicals; Journal of Applied Electrochemistry, (9), pp. 301-305. Tomov T. and Tsoneva I. (2000); Are the stainless steel electrodes inert?; Bioelectrochemistry, 51, pp. 207-209. 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. UK patent applications GB 2067390 A and 2068200 A (1981); Apparatus for heating electrically conductive flowable media. 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. Van den Broeke L.T., Graslund A., Nilsson J.L.G., Wahlberg J.E., Scheynius A., and Karlberg A.T. (1998); Free radicals as potential mediators of metal-allergy: Ni2+- and Co2+ -mediated free radical generation; European Journal of Pharmaceutical Sciences, 6, pp. 279-286. 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. Voyksner R.D. (1992); Elecrospray LC/MS – can it be used to determine lower molecular weight molecules?; Nature, 356, pp. 86-87. Wickens A.P.(2001); Ageing and the free radical theory; Respiration physiology, 128, pp. 379-391.

153

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. Zhao M.J., and Jung L.(1995); Kinetics of the competitive degradation of deoxyribose and other molecules by hydroxyl radicals produced by the Fenton reaction in the presence of ascorbic acid; Free Radical Research,23(3),pp.229-243. Zoltai P. and Swearingen P. (1996); Product development considerations for ohmic processing; J. Food Technology, 5, pp. 263-266.