Embed Size (px)
Citation preview
HYBRID COMBINATION OF EMERGING FOOD PROCESSING TECHNOLOGIES:
MICROWAVE AND PULSED OHMIC HEATING
A DISSERTATION SUBMITTED TO THE GRADUATE DIVISION OF THE UNIVERSITY OF HAWAIʻI AT MĀNOA IN PARTIAL FULFILLMENT OF THE
REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY
IN
MOLECULAR BIOSCIENCES AND BIOENGINEERING
AUGUST 2014
By
Seung Hyun Lee
Dissertation Committee:
Soojin Jun, Chairperson Alvin Huang
Eun Sung Kan Samir K. Khanal
Wei-Wen Winston Su
Keywords: Multiphase foods, thermal uniformity, microwave, ohmic heating,
simultaneous combination, COMSOL simulation
ii
ACKNOWLEDGEMENT
I can distinctly remember the moment when I entered University of Hawaii. At that time,
everything was unfamiliar to me and it was so afraid when I faced to the first academic obstacle.
However, whenever I feel daunted, I have tried to pull myself together and to raise my ability to
a higher academic level. Thanks to kind consideration and heartful encouragement from my
advisor, family, and friends, I could finish my course without any accident. In here, I would like
to show everyone my sincere appreciation.
I would like to thank Dr. Soojin Jun who gave me a great opportunity in my life. I really
appreciate for his unlimited support and mentorship. He has encouraged me in doing my best and
being positive. He also made me focus on only research without financial problem and gave
sincere advice on my academic life and future.
My sincere thanks goes to Dr. Samir Khanal who is my committee member and
introduced me the concept of bioprocessing. His class was all things wonderful and I was
surprised to his passion and solicitude for students. If there was not his support and advice, I
would still wander around a maze.
Dr. Eun Sung Kan who is also my committee member and a lay of sunshine to me
deserves my gratitude. When I had a trouble organizing committee, he gave me a ready consent.
Through his counsel and guidance, my research could advance on right way.
I would like to thank Dr. Winston Su for his academic guidance throughout my UH life.
I am indebted to him for his encouragement and support in my research. The learning from him
was always interesting and enjoyable.
iii
I would also like to express my thanks to Dr. Alvin Huang for his invaluable knowledge
and for serving as one of my earliest academic mentors at UH. His considerable help guided my
academic work on the right track. I cannot fully express in words the depth of my gratitude.
My parents and family are who gave me life, boundless love, and words of
encouragement. I would like to express my heartfelt gratitude and really see them in this moment.
Thanks to their unlimited care, I became a well grown man and have enjoyed my good life. I am
also thankful my friends who always cheer up and help me whenever I have a big problem. Let
me say them “I love you”.
Research group members at Food Processing Laboratory who kept me going through
rough way and gave me inspiration about research are deserved thanks of me.
iv
ABSTRACT
Conventional thermal processing of foods containing particulates significantly rely on
convective and conductive heat transfer and tend to be overly conservative in ensuring microbial
safety, thus compromising quality. Temperature lags inside particles of solid-liquid mixture
foods could lead to the danger of under-processing and therefore risking the food’s safety.
Advanced food processing technologies such as microwave heating and ohmic heating have been
developed in the last few decades as alternatives to conventional processing methods. These
advanced technologies could contribute to shorten processing times, energy savings, and highly
balanced safe food; however, they alone still cannot guarantee food safety without damaging the
food’s quality. Therefore, a new concept to combine microwave and ohmic heating has been
extensively evaluated. This combination technology would optimize each of the individual
technology’s strengths and reduce each of their individual weaknesses.
In this study, a dual cylindrical microwave and ohmic combination continuous flow
heater was designed and fabricated to heat treat solid-liquid mixture foods without under-
processing the solid particulates. The electric field distribution under microwave and ohmic
heating was numerically analyzed; the use of cylindrical microwave cavity was suitable to
maximize the electric strength in the combination heater. Thermal profiles of solid-liquid
mixtures consisted of chicken and potato particles, and sodium chloride solution (0.5, 1.25, and
2.0% concentration) at different solid fraction 10 and 15% were collected and compared. These
profiles were recorded for both individual heating (either microwave or ohmic heating) and
combination heating (microwave and ohmic together) until the exit temperature of either solid
particles for solution reached 80°C.
v
In individual ohmic heating, particle size and slat concentration affected temperature
differences between the particulates and the solution. In individual microwave heating, the
solution temperature lagged behind the particle temperature with salt concentrations up to 1.25%,
regardless of particle size and solid fraction; however, a different tendency was observed in the
food mixtures including 2% salt concentration. The maximum temperature differences between
solid and liquid phases obtained by individual microwave and ohmic heating were 7.1±1.7 and
11.9±2.9°C, respectively. In the combination heating, only small temperature gaps between
solid particles and liquid (maximum difference < 3.08°C) at low salt concentrations (up to 1.25%)
were observed. A 3D block diagram constructed using the controlled ranges of salt concentration,
particle size, and solid fraction estimated by empirical polynomial equations was used to
describe temperature similarities between solid and liquid phases when combination heating was
applied. This unique continuous flow combination heater has the potential to thermally process
multiphase foods with improvements in heat distribution, energy efficiency, and food quality.
Computational modeling for individual microwave and ohmic heating, and the
combination heating in a continuous flow system were established. The modeling of the
continuous flow system was important for future designing of a production-scale unit. Accurate
prediction of temperature distribution in solid-liquid mixture foods was demonstrated using the
computational modeling. Numerical modeling of the combination heating technique was a
challenge due to simultaneous changes in heat and particulate flow pattern in the continuous flow
system. This study employed the COMSOL software which enabled a numerical model to
simulate the complicated simultaneous changes during the combination heating technique. The
temperature profile of the combination heating was simulated during a lethality test of
Escherichia coli K12 inoculated carrot balls. The carrot ball’s exit temperature reached 90°C in
vi
56 seconds, as predicted by computational model. The reliable prediction of particulate
temperature in this complicated system can be attributed to the integration of the two-
dimensional moving mesh method and the arbitrary Lagrangian–Eulerian (ALE) equation in the
COMSOL software.
Results indicated that the combination heating technique has potential to inactivate E.
coli K12 in carrot balls with improved lethal activities. The simulated flow patterns and thermal
profiles of multiphase foods showed the heating mechanism and the movement of particulates
during combination heating. The experimental data were in good agreement with the simulated
heating profile of particulates with a maximum prediction error of 5%.
1
TABLE OF CONTENTS ACKNOWLEDGEMENT .............................................................................................................. ii
ABSTRACT ................................................................................................................................... iv
LIST OF FIGURES ........................................................................................................................ 4
LIST OF TABLES .......................................................................................................................... 6
Chapter 1 ......................................................................................................................................... 7
INTRODUCTION .......................................................................................................................... 7
References ............................................................................................................................................... 13
Chapter 2 ....................................................................................................................................... 15
LITERATURE REVIEW ............................................................................................................. 15
2.1 Introduction ....................................................................................................................................... 15
2.2 Emerging food processing technologies ........................................................................................... 15
2.2.1 Microwave heating ..................................................................................................................... 15
2.2.3 Radio frequency heating ............................................................................................................ 16
2.2.4 Ohmic heating ............................................................................................................................ 17
2.2.5 Pulsed electric field .................................................................................................................... 17
2.2.6 High pressure processing ........................................................................................................... 18
2.3. Current emerging combination technologies for food processing ................................................... 19
2.3.1 Microwave combination technology .......................................................................................... 19
2.3.2 Infrared radiation combination technology ................................................................................ 26
2.3.3 High pressure processing combination technology .................................................................... 31
2.3.4 Radio frequency electric field combination technology ............................................................ 35
2.3.5 Ohmic heating combination technology .................................................................................... 36
2.3.6 Pulsed Electric Field combination technology ........................................................................... 37
2.4 Conclusion ........................................................................................................................................ 42
2.5 References ......................................................................................................................................... 49
Chapter 3 ....................................................................................................................................... 62
Development of a dual cylindrical microwave and ohmic combination heater for processing of particulate foods ............................................................................................................................ 62
3.1 Introduction ....................................................................................................................................... 62
3.2 Theoretical background ..................................................................................................................... 64
3.2.1 Electromagnetic field propagation in microwave heating system ............................................. 64
3.2.2 Determination of wave propagation in a waveguide .................................................................. 65
2
3.2.3 Heat generation for ohmic heating ............................................................................................. 66
3.3 Design parameters ............................................................................................................................. 66
3.3.1 Rectangular waveguide .............................................................................................................. 66
3.3.2 Cylindrical cavity resonator ....................................................................................................... 69
3.3.3 Impedance matching .................................................................................................................. 71
3.3.4 Impedance matching for microwave heating system ................................................................. 71
3.4 Results and discussion ...................................................................................................................... 72
3.4.1 Microwave power launcher ........................................................................................................ 72
3.4.2 Cylindrical microwave cavity resonator .................................................................................... 74
3.4.3 Rectangular waveguide .............................................................................................................. 75
3.4.4 Impedance matching .................................................................................................................. 76
3.4.5 Ohmic heating applicator ........................................................................................................... 79
3.4.6 Overall design of microwave and ohmic combination heater .................................................... 82
3.5 Conclusion ........................................................................................................................................ 83
3.6 References ......................................................................................................................................... 84
Chapter 4 ....................................................................................................................................... 90
Minimization of thermal lags in the simultaneous microwave and ohmic combination heating of particulate foods ............................................................................................................................ 90
4.1 Introduction ....................................................................................................................................... 90
4.2 Materials and methods ...................................................................................................................... 91
4.2.1 Particle and liquid mixtures preparation .................................................................................... 91
4.2.2 The measurement of electrical conductivity of food samples .................................................... 93
4.2.2 Experimental protocol ................................................................................................................ 93
4.3 Results and discussion ...................................................................................................................... 96
4.3.1 Electrical conductivities of particle and solution samples ......................................................... 96
4.3.2 Heating patterns of particle-liquid mixtures under different heating methods .......................... 97
4. 4 Conclusion ..................................................................................................................................... 107
4.5 References ....................................................................................................................................... 108
Chapter 5 ..................................................................................................................................... 110
Computational modeling for heating profile and the validation of thermal lethality of multiphase foods in a dual cylindrical microwave and ohmic combination heater ...................................... 110
5.1 Introduction ..................................................................................................................................... 110
5.2 Materials and methods .................................................................................................................... 112
3
5.2.1 Particle and liquid mixtures preparation .................................................................................. 112
5.2.2 Microorganism cultivation ....................................................................................................... 113
5.2.3 Measurement of electrical conductivity and dielectric properties............................................ 113
5.2.4 Experimental protocol .............................................................................................................. 114
5.2.5 Microbial enumeration ............................................................................................................. 115
5.2.6 Model development.................................................................................................................. 115
5.2.6.1 Model parameters .............................................................................................................. 115
5.2.6.2 Ohmic heating ................................................................................................................... 116
5.2.6.3 Microwave heating ............................................................................................................ 117
5.2.6.4 Heat transfer ...................................................................................................................... 117
5.2.6.5 Solution of incompressible fluid flow including particle .................................................. 118
5.2.6.6 Forces on the particles....................................................................................................... 119
5.2.6.7 Arbitrary Lagrangian–Eulerian (ALE) moving mesh method .......................................... 121
5.2.6.8 Simulation strategy ........................................................................................................... 122
5.2.6.9 The Assumptions for the simulation ................................................................................. 126
5.3 Results and discussion .................................................................................................................... 127
5.3.1 Electrical conductivities and dielectric properties of carrot and base solution ........................ 127
5.3.1 Heating patterns of carrot ball and liquid mixture under different heating modes .................. 128
5.3.2 Inactivation of E.coli K-12 in carrot balls treated under different heating modes ................... 130
5.3.3 Code validation ....................................................................................................................... 133
5.3.4 Hydrodynamic field ................................................................................................................ 134
5.3.5 Electric field and temperature distributions ............................................................................ 136
5.4 Conclusion ...................................................................................................................................... 138
5.5 References ....................................................................................................................................... 140
CHAPTER 6 ............................................................................................................................... 146
FUTURE WORKS...................................................................................................................... 146
APPENDIX A ............................................................................................................................. 149
NUMERICAL MODELING IN COMSOL MULTYPHYSICS ................................................ 149
APPENDIX B ............................................................................................................................. 164
SUMMARY OF THE MOST IMPORTANT SIGNS AND SYMBOLS .................................. 164
4
LIST OF FIGURES
Figure 1. 1 Schematic diagram of the microwave and ohmic combination heater ....................... 11
Figure 2. 1A schematic of the combined microwave and hot air equipment for the peeled longan drying (Varith et al. 2007) ............................................................................................................ 21
Figure 2. 2 Schematic view of far infrared-hot air dryer (Ponkham et al., 2012) ........................ 28
Figure 2. 3 A schematic of the designed pulsed electric field and thermal treatment combination pasteurization system (Bazhal et al., 2006) .................................................................................. 38
Figure 3. 1 Electric and magnetic field lines for TE and TM modes in rectangular waveguide . 67
Figure 3. 2 A cylindrical cavity resonator ................................................................................... 69
Figure 3. 3 A calibration kit (open, short, and matched load) and vector network analyzer ....... 72
Figure 3. 4 The dimension of microwave power launcher: (a) top view, (b) side view .............. 73
Figure 3. 5 The dimension of cylindrical microwave cavity resonator: (a) top view, (b) side view....................................................................................................................................................... 75
Figure 3. 6 Schematic diagram of taper connection between two different waveguides ............ 75
Figure 3. 7The dimension of rectangular waveguide: (a) top view, (b) side view ...................... 76
Figure 3. 8 Electric field distribution in the microwave heating chamber after the adjustment of both position and inserting depth of stubs .................................................................................... 77
Figure 3. 9 The positions and inserting depths of stubs on microwave heating unit: .................. 78
Figure 3. 10 Impedance matching of (a) Chamber 1 and (b) Chamber 2: Smith charts to show the reflection coefficient S11, which means the characteristic impedance (Z0) is 50 Ω ................ 79
Figure 3. 11 The dimension of (a) microwave and ohmic combination applicator, and ............. 80
Figure 3. 12 Electric field distribution in the ohmic heating applicator ...................................... 81
Figure 3. 13 A schematic diagram of the microwave and ohmic combination heater: (a) A front view, (b) A side view, (c) A 3D schematic, and (d) A cross-sectional view of the chamber ....... 82
Figure 4. 1 A schematic diagram of the experimental set-up ....................................................... 95
Figure 4. 2 Electrical conductivities of potato, chicken breast, and the base solutions with different salt concentration ........................................................................................................... 97
Figure 4. 3 Heating patterns of particle-liquid mixtures (0.5% salt concentration) under microwave, ohmic, and combination heating. (a) 10% mass fraction, and (b) 15 % mass fraction..................................................................................................................................................... 101
5
Figure 4. 4 Heating patterns of particle-liquid mixtures (1.25% salt concentration) under microwave, ohmic, and combination heating. (a) 10% mass fraction, and (b) 15% mass fraction...................................................................................................................................................... 102
Figure 4. 5 Heating patterns of particle-liquid mixtures (2.0% salt concentration) under microwave, ohmic, and combination heating. (a) 10% mass fraction, and (b) 15% mass fraction..................................................................................................................................................... 103
Figure 4. 6 3D block diagram constructed at different controllable ranges of key variables for the prediction of (a) ΔTps and (b) ΔTcs falling in the variations less than ±2°C from targeted exit temperature, 80°C. ...................................................................................................................... 106
Figure 5. 1 A schematic diagram of the experimental set-up ..................................................... 114
Figure 5. 2 The schematic diagram of particle-particle and particle-wall collision model ........ 121
Figure 5. 3 A flowchart of simulation procedure ........................................................................ 123
Figure 5. 4 Simulated electric field distributions for (a) microwave and (b) ohmic heating. ..... 123
Figure 5. 5 (a) 3D tetrahedral meshes for estimating the electric field strengths of microwave 125
Figure 5. 6 Electrical conductivities of the base solution with 0.5% NaCl and carrot ............... 127
Figure 5. 7 Dielectric properties of the base solution base solution with 0.5% NaCl and carrot...................................................................................................................................................... 128
Figure 5. 8 Heating patterns of carrot ball-solution mixtures under microwave, ohmic, and combination heating. (a) 1.3 cm diameter and (b) 1.8 cm diameter ........................................... 129
Figure 5. 9 Log reductions of E. coli K-12 in carrot balls after treatment with ohmic heating, microwave, and the combination heating. (a) 1.3 cm diameter and (b) 1.8 cm diameter .......... 133
Figure 5. 10 Velocity distributions of three particles with a carrier medium in the microwave and ohmic combination heater ........................................................................................................... 135
Figure 5. 11 Simulated temperature distributions of solid liquid mixture under different heating methods ....................................................................................................................................... 138
6
LIST OF TABLES
Table 2. 1 Summary of combination technologies applied to various food processing areas ..... 44
Table 3. 1 Standard waveguide dimensions .................................................................................. 67
Table 3. 2 The values of pnm for the TEnm mode in cylindrical cavity resonator .......................... 70
Table 3. 3 Inserting depths of stubs on microwave heating units ................................................. 78
Table 4. 1 Applied microwave power and voltages for microwave (MW), ohmic (OH), and combination heating modes (MW & OH) depending on NaCl concentration. ............................. 95
Table 4. 2 The constants of polynomial equations for the prediction of temperatures of solution, chicken, and potato ..................................................................................................................... 104
Table 5. 1 The material properties and parameters used for the simulation .............................. 116
Table 5. 2 Comparison of the drag coefficients by the code and published data ...................... 134
Table 5. 3 The comparison between the experimental temperature values and the simulated temperature values of carrot balls and solution .......................................................................... 137
7
Chapter 1
INTRODUCTION
Conventional thermal processing of food products has been most commonly considered
as the simple and effective way to preserve food. However, excessive thermal treatment for
processing food products has frequently caused serious deterioration in quality aspects such as
texture, color, flavor and the destruction of bioactive compounds (Choi et al., 2006). Minimally
processed food products that retain their fresh and nutritional qualities have received great
attention from customers in recent years. In order to meet rising demands from customers and to
shorten processing times, food engineers and scientists have enthusiastically endeavored to
explore new technologies to replace conventional cooking methods. As alternatives to
conventional methods, new food processing approaches, using advanced technologies such as
microwave and ohmic heating, have been widely investigated and developed. Advanced
technologies in processing of various food products were able to obtain comparable food quality
in a shorter processing time and with less energy input (Nguyen et al., 2013). Furthermore,
emerging food processing technologies have provided outstanding merits such as a rapid heating
rate, better heating uniformity, and improved inactivation of microbial contaminants (Knorr,
1955; Tang et al., 2002; Jun and Sastry, 2005). Despite of the development of these advanced
food processing technologies, they still have been challenged to achieve an even temperature
distribution in solid-liquid food mixtures (Choi et al., 2011).
Microwave heating has been intensively used to heat up "ready to eat" foods because
microwaves heat foods in a rapid and direct manner (Hossan et al., 2010; Lee and Jun, 2011).
Since microwave heating has an interaction with polar water molecules and charged ions within
8
food, volumetric heating is produced by the induced frictional energy from the realignment of
water molecules and the conductive migration of charged ions in the alternating magnetic field
(Wang et al., 2009). In industrial applications, this volumetric heating was drastically able to
reduce the come up time required to achieve a target process temperature, thereby reducing the
total cumulative thermal treatment and better preserving the thermo-labile constituents (Coronel
et al., 2003). Continuous flow microwave heating in the food industry suggests a great potential
to replace the conventional heat exchangers in an aseptic processing system due to its rapid
heating of food products. Substantial improvements in color, flavor, and nutrient retention of
continuous flow microwave system treated products have been reported (Giese, 1992; Nikdel et
al., 1993). However, problems associated with microwave heating are numerous. Such problems
include: non-uniform heating, localized heat zones because of the variation in dielectric, physical,
and thermal properties of food components (Pitchai et al., 2012).
Ohmic heating, the internal heat generation by applying alternating current through a
food product, has been applied to various food applications such as extraction, bleaching,
cooking for meat products, and pasteurization since it provides a constant heating rate for a
single solid or liquid phase food with a high energy transfer efficiency of approximately 95%
(Shim et al., 2010). In addition, it has potential applications in the food industry for processing of
solid-liquid mixture foods, also termed particulate foods (Palaniappan and Sastry, 2002). The
optimal product condition for ohmic processing of a solid-liquid mixture food is that the solid
particles and the liquid have the same or similar electrical conductivities; however, most
vegetables and meats have lower electrical conductivities than liquids. Particulate foods heat at
rates depending on relative conductivities of the phases and the volume fractions of the
respective phases (Sastry and Palaniappan, 1992). If a particle of low conductivity is surrounded
9
by a high conductivity environment, this particle will thermally lag the fluid and be under-
processed (Sastry, 1992). Regarding concern about food safety, it is important to pre-estimate the
electrical conductivities of solid and liquid phases. By increasing the electrolytic content in the
solids, such low-conductivity particles may be heated at a similar rate as or faster than the
surrounding fluid. Therefore, a pretreatment procedure to increase the electrical conductivity of
solid particles (i.e. salt infusion) could enhance the heating uniformity of multiphase foods;
however, this treatment is relatively time consuming and requires extra energy input (Wang and
Sastry, 1993; Zareifard et al., 2003).
A new concept of hybrid combination technologies for the processing of solid or liquid
phase foods or multiphase foods has been intensively evaluated to minimize the processing
procedures. The combination of existing food processing technologies has a significant impact
on the preservation of food products with enhanced qualities. For instance, liquid pasteurization
using pulsed electric field (PEF) assisted with thermosonication (TS) developed by Walkling-
Ribeiro (2010) did not experience any significant losses of physical properties (pH, Brix°, and
conductivity) of orange juice but provided an extended shelf life, as compared with high
temperature short time (HTST) pasteurization. In addition, to resolve the uneven temperature
distribution issue, microwave heating has been assisted with conventional heating methods, such
as hot air and vacuum, and microwave absorbents (Goksoy et al., 1999; Maskan, 2001).
However, most studies of combination technologies for a variety of food processes have been
aimed at the fundamental understanding of the combination process, or have been conducted in
small batch or a sequential strategy. In addition, the aforementioned combination methods
required specific conditions that substantially relied on the physical and chemical properties of
targeted foods.
10
A microwave and ohmic combination heating technology has been proposed and tested
for heating uniformity of particulate foods. It was expected that combining these two heating
technologies would be beneficial for uniform heating of multiphase foods since it would allow
liquid parts of the product to be heated immediately via current, and solid particles (independent
of their conductivity values) to be heated to their cores rapidly via microwave. Nguyen and
others (2013) have attempted to develop a continuous flow, simultaneous microwave and ohmic
combination heater, coupled with a polytetrafluoroethylene (PTFE) ohmic tube with two
electrodes at opposing ends fed through the center of a rectangular microwave chamber, for
achieving consistent heat distribution in solid-liquid food mixtures (Fig. 1.1). It was validated
that solid particles and liquid were simultaneously heat treated via electromagnetic wave and
electrical current, thus improving thermal treatment of multiphase foods with enhanced thermal
uniformity. However, only limited microwave energy was transmitted to solid-liquid food
mixtures in a PTFE tube due to a scattered field strength in multi-mode microwave cavity.
Moreover, the solid-liquid food mixtures used in the previous study consisted of one kind of
vegetable (carrot) in a liquid, which seemed to be an over-simplification of solid-liquid mixture
foods.
11
Figure 1. 1 Schematic diagram of the microwave and ohmic combination heater
(Nguyen et al., 2013)
Therefore, it was necessary to design and fabricate a better microwave cavity in order to
maximize the electric field strengths form microwave. A cylindrical shaped microwave cavity
was employed in this study because it is known that a highly localized hot spot generated by only
one standing microwave could be easily obtained in this cavity. This new cylindrical, continuous
flow microwave and ohmic combination heater could result in better thermal distribution within
solid-liquid food mixtures and deliver maximum microwave energy to multiphase foods. In
terms of microbial inactivation, the under-processed solids in particulate foods could be a good
shelter for surviving foodborne pathogens. The uniformity in temperature distributions of food
during the thermal processing is crucial, in particular when particulate foods are to be sterilized
or aseptically processed. Accordingly, the effectiveness of the combination heater on microbial
inactivation in multiphase foods should be examined. In addition, computational modeling of
simultaneous microwave and ohmic heating would validate the thermal lethality and the heating
12
profile of multiphase foods treated by the combination heating technology. The model could
provide a practical solution for uniform thermal treatment of multiphase foods by manipulating
transient electrical conductivities and the thermal properties of specific heat and thermal
conductivities.
Thus, this study was intended to accomplish the following specific objectives:
Objective 1: Explore the combination technologies in thermal and non-thermal food processing
area
Objective 2: Design and fabricate a microwave heating unit equipped with a single mode
cylindrical cavity resonator for continuous flow, simultaneous ohmic and microwave
combination heating
Objective 3: Optimize the continuous flow, simultaneous microwave and ohmic heating
combination heater by evaluating its effectiveness on multiphase foods with different electrical
conductivities and dielectric properties at varying voltage and power levels
Objective 4: Validate the computational model for simultaneous microwave and ohmic heating
of multiphase foods, and evaluate the lethal effectiveness on microorganisms of the combination
heater in multiphase foods.
13
References
Castro, S.M., Loey, A.V., Saraiva, J.A., Smout C., Hendrickx, M. (2006). Inactivation of pepper
(Capsicum annuum) pectin methylesterase by combined high-pressure and temperature
treatments. Journal of Food Engineering, 75, 50-58.
Choi, W., Nguyen, L. T., Lee, S. H., and Jun, S. (2011). A Microwave and Ohmic Combination
Heater for Uniform Heating of Liquid–Particle Food Mixtures. Journal of food science, 76(9),
E576-E585.
Choi, Y., Lee, S.M., Chun, J., Lee, H.B., Lee, J. (2006). Influence of heat treatment on the
antioxidant activities and polyphenolic compounds of Shiitake (Lentinus edodes) mushroom.
Food Chemistry, 99, 381-387.
Coronel, P., Truong, V.D., Simunovic, J., Sandeep, K.P., Cartwright, G.D. (2005). Aseptic
processing of sweetpotato purees. Journal of Food Science 70 (9), E531–E536.
Göksoy, E. O., James, C., and Corry, J. E. L. (2000). The effect of short-time microwave
exposures on inoculated pathogens on chicken and the shelf-life of uninoculated chicken meat.
Journal of food engineering, 45(3), 153-160.
Hossan, M. R., Byun, D., and Dutta, P. (2010). Analysis of microwave heating for cylindrical
shaped objects. International Journal of Heat and Mass Transfer, 53(23), 5129-5138.
Jun, S., Sastry, S.K. (2005). Modeling and optimizing of pulsed ohmic heating of foods inside
the flexible package. Journal of Food Processing Engineering, 28, 417–436.
Knorr, D. (1995). Hydrostatic pressure treatment of food: microbiology. In New Methods of
Food Preservation, Blackie Academic & Professional. Bishopbriggs, UK .
Lee, S.H., Jun, S. (2011). Electrophoresis induced breakdown of cellular structures in taro skins
for enhancement of sugar release. Transactions of the ASABE, 54, 1041–1047.
14
Maskan, M. (2001). Drying, shrinkage and rehydration characteristics of kiwifruits during hot air
and microwave drying. Journal of Food Engineering, 48, 177-182.
Nguyen, L.T., Choi, W., Lee, S.H., Jun, S. (2013). Exploring the heating patterns of multiphase
foods in a continuous flow, simultaneous microwave and ohmic combination heater. Journal of
Food Engineering, 116, 65-71.
Shim, J.Y., Lee, S.H., Jun, S. (2010). Modeling of ohmic heating patterns of multiphase food
products using computational fluid dynamics codes. Journal of Food Engineering, 99, 136–141.
Tang, J., Feng, H., Lau, M. (2002). Microwave heating in food processing. In X. Young & J.
Tang (Eds.), Advances in bioprocessing engineering. New Jersey, Scientific Press, USA.
Walkling-Ribeiro, M, Noci, F., Cronin, D.A., Riener, J., Lyng, J.G., Morgan, D.J. (2010). Shelf
life and sensory attributes of a fruit smoothie-type beverage processed with moderate heat and
pulsed electric fields. LWT-Food Science Technology,43, 1067-1073.
Wang, J., Tang, J., Wang, Y., Swanson, B. (2009). Dielectric properties of egg whites and whole
eggs as influenced by thermal treatments. LWT-Food Science and Technology, 42, 1204-1212.
Wang, J., Tang, J., Wang, Y., Swanson, B. (2009). Dielectric properties of egg whites and whole
eggs as influenced by thermal treatments. LWT-Food Science and Technology, 42, 1204-1212.
Zareifard, M. R., Ramaswamy, H. S., Trigui, M., & Marcotte, M. (2003). Ohmic heating
behaviour and electrical conductivity of two-phase food systems. Innovative Food Science &
Emerging Technologies, 4(1), 45-55.
15
Chapter 2
LITERATURE REVIEW
2.1 Introduction
This section provides the basic concepts of emerging food processing technologies and
the effectiveness of combination technologies in thermal and non-thermal food processing areas.
Since the combination technologies have a great potential to enhance overall qualities of food
products and simplify food processing steps without any pretreatment step. Emerging and
developed combination technologies have been evaluated in various types of food processing
such as drying, baking, and pasteurization. Therefore, the various applications of combination
technologies from recently published literatures will be discussed.
2.2 Emerging food processing technologies
2.2.1 Microwave heating
Microwaves(MW), which are a part of electromagnetic spectrum and have a frequency
range between 300 MHz and 300 GHz, have been extensively employed to various food
processing such as drying, tempering, and cooking because MW heat foods in a rapid and direct
manner (Lee and Jun, 2011). Therefore, MW heating can shorten processing time and thoroughly
secure thermo-labile constituents (Coronel et al., 2003). In order to understand the interaction
between electromagnetic field and food, the research for measuring dielectric properties of food
should be conducted (Buffler, 1993). Dielectric properties (ε = ε' jε'') are consisted with two
factors (ε': dielectric constant, real part and ε'': dielectric loss factor, imaginary part). Dielectric
16
constant (ε') indicates the ability to store electric energy. On the other hand, dielectric loss factor
(ε'') is closely associated with the conversion of electric energy to thermal energy. Dielectric
properties of food are mainly affected by frequency, temperature, and moisture contents (Calay
et al., 1994; Tang et al., 2002; Venkatesh and Raghavan, 2004; Sakiyan et al., 2007; Wang et al.,
2009).
2.2.2 Infrared heating
Infrared radiation (IR), which is also a part of electromagnetic spectrum and can be
classified into three regions (Near-, Mid-, and Far- infrared) corresponding to the spectral
wavelengths, has been exploited in various thermal processing owing to its inherent advantages
such as radiation impingement on the surface of food, rapid penetration into food, and enhancing
food quality (Sakai and Hanazawa, 1994; Datta and Ni, 2002). Among three regions, far infrared
and near infrared radiation (FIR and NIR) have been employed for food processing because FIR
energy is directly absorbed by food material via molecular vibration and NIR instantly produces
heat on the surface of food products with approximately 1mm of penetration depth (Sandu, 1986;
Nindo et al., 1995). Nowak and Lewicki (2004) suggested that IR heating could be a practical
alternative to conventional food dehydration method due to high thermal efficiency and ease of
control.
2.2.3 Radio frequency heating
Similar to MW heating, radio frequency (RF) heating, which is classified as dielectric
heating, is rapidly and volumetrically heat up solid phase or semi-solid phase food products
(Wang et al., 2003). The distinct difference between MW and RF is a region of electromagnetic
spectrum and the frequency range from 3 Hz to 300 MHz is dominantly applied for RF heating
17
(Tang, 2005). RF energy can be produced by passing alternating or direct current thorough food
samples located between two electrodes without direct contact because the electromagnetic field
converted from the electrical energy stimulate the migration of ions within food products (Wang
et al., 2006). RF heating can result in uniform heating on the surface of food products, eventually
decreasing thermal lag between the surface and the interior of food products (Rowley, 2001);
however, RF heating rate is substantially depending on dielectric properties of food (Birla et al.,
2008)
2.2.4 Ohmic heating
On the contrary to RF heating, the mechanism of ohmic (OH) heating is that internal heat
dissipation can be generated by applying alternating current through food products with direct
contact to two electrodes. OH heating have made considerable contribution to thermal uniformity
improvement in single phase foods (Nguyen et al., 2013). The energy conversion efficiency
during OH process is remarkably high as compared with other thermal processing methods
(Salengke, 2000; Jun and Sastry, 2005; Shim et al., 2010). In addition, it has been essential to
investigate non-thermal effect (electroporation) of OH on the permeability of cell membrane
with reducing heat generation (Lee and Jun, 2011). The electrophoretic force with electro-
osmosis under high intensity electric field can improve the ionic diffusion throughout the
membrane (Kulshrestha and Sastry, 2003; Sensoy and Sastry, 2004).
2.2.5 Pulsed electric field
Non-thermal processing technologies such as ultra high pressure and pulsed electric field
processing has gained a lot of interests from food industry to satisfy the demand of consumers
who are anxious to have less processed and healthful food products. In pulsed electric field (PEF)
18
processing, short period (1-100 µs) of a high electric field (5-50 kV/cm) is most commonly
applied to liquid phase food products filled between two electrodes. Furthermore, this technology
is well known as short burst of electricity for eliminating pathogens (Jin et al., 2009; Saldana et
al., 2009). Pores in cell membrane can be reversibly or irreversibly damaged or reformulated by
electrophoretic force generated by high intensity PEF (Barbosa-Canovas et al., 1999; Garcia et
al., 2003; Gurtler et al., 2010). PEF may guarantee the quality of food product and efficiently
inactivate microorganisms without the degradation of flavor and nutritional values resulted from
conventional thermal treatment (Ayhan et al., 2001). In addition, Qin and others (1995) reported
that PEF is an energy efficient process compared to conventional pasteurization method,
especially applying in a continuous flow system.
2.2.6 High pressure processing
High pressure processing (HPP), one of the well developed non-thermal techniques, has
been widely applied to inactivate pathogenic microorganisms and enzymatic activity i.e. pectin-
methyl-esterases (PME) in food products during isostatic pressure processing without thermal
pretreatment (Torrecilla et al., 2005). HPP subjects either solid or liquid foods, with or without
packaging to pressure from 100 to 800 MPa (Farkas and Hoover, 2000). Since high pressure
uniformly and instantaneously exerts a force throughout food products regardless of its geometry,
size, and composition, it has a great effect on the inactivation of pathogenic microorganism such
as Salmonella typhimurium and Listeria monocytogenes by five decimal reduction (Torres and
Velazquez, 2005). When high compression is applied to water, fat, and oil, the temperature rise
in water is ranged in only 3 °C per 100 MPa, while the compression heating value for fat and oil
was up to 9 °C per 100 MPa (Rasanayagam et al., 2003). It clearly showed that compression
heating values was significantly depending on the composition of liquid phase food products.
19
2.3. Current emerging combination technologies for food processing
2.3.1 Microwave combination technology
Although food products heated by MW have better retention in color, texture, and flavor
compounds compared with conventionally treated products, MW heating is associated with
numerous problems, such as non-uniform heating, partial over-heating, and limited penetration
(Gentry and Roberts, 2005; Nguyen et al., 2013). Conventional methods i.e., vacuum drying (VC)
and hot air (HA) heating, can preserve the promising quality of perishable agricultural products
without any damage during processing; however, it takes a long time and consumes more energy
with low energy efficiency to complete the processing (Varith et al., 2007a). MW technology
combined with conventional methods has been investigated particularly in drying and baking
processes.
Ren and Chen (1998) evaluated the effectiveness of MW-HA combination drying on
American ginseng roots. The moisture content versus time curves and the drying rates by MW-
HA were obviously influenced by sample size (16 and 24 mm in diameter), while the effect of
MW energy on drying rate was consistently diminished as the drying progressed. The required
time reaching the desired moisture level (10%) in both samples were significantly changed
depending on MW power. However, MW power did not affect the color values in both final
products. The empirical Page's model was established to describe drying trend of American
ginseng roots by MW-HA. MW-HA drying have an ability to substantially reduce drying time of
functional foods with maintaining the quality attributes of final product.
MW-HA drying of kiwifruit was investigated with respect to color change, shrinkage,
and rehydration (Maskan, 2001a, b). When combined MW and HA method finished drying, in
color parameters of kiwifruit, L* and b* values significantly decreased and a* value increased as
20
compared with HA drying. The color of final products dried by individual MW became more
brown than that by HA and MW-HA drying. The different kinetic models for the change in color
parameters (L*, a*, and b*) were in a good agreement with experimental data. Since the dried
products by MW-HA combination shows less shrinkage than that by individual MW drying, the
better rehydration characteristic could be achieved in MW-HA combination method. Thus, MW-
HA drying showed positive potential for preservation of high quality food products.
Sharama and Prasad (2001) studied MW-HA drying of garlic (Allium sativum) cloves.
The drying time increased with increase in HA velocity at a constant MW power. At higher
velocity, heat removal from the product increased due to higher film heat transfer coefficient (h
v0.4 for cross airflow). In addition, the drying rate increased when higher temperature of HA was
applied at constant MW power. The accelerated drying rates may attribute to internal heat
generation and the so-called 'liquid movement' within the material when it was exposed to MW.
The color and flavor strength of dried garlic by MW-HA were better than that by HA. The
empirical Page's model could appropriately address the MW-HA drying data.
The drying kinetics of apple cylinders (Granny smith) under MW-HA dehydration
system was developed (Andres et al., 2004). In this study, vacuum impregnation with an isotonic
solution was supplemented as a pretreatment prior to drying. The drying rate curves of vacuum
impregnated samples were dramatically different from those of fresh samples and the drying
kinetic of impregnated samples were slower than it of fresh samples. However, greater volume
reduction was promoted in impregnated samples than in non-impregnated samples. The slower
drying kinetics of impregnated samples has a close relation to its greater volume reduction
because the coupled mass transfer with deformation-relaxation has influence on the drying
kinetics. In addition, scanning electron microscopy (SEM) images showed the morphological
21
difference between fresh and impregnated samples. In the empirical model, the predicted data
was quite similar to experimental data.
MW-HA drying process for peeled longan that is one of the most important crops in
Thailand was developed and evaluated (Varith et al., 2007b). The designed MW-HA dryer were
composed with microwave oven, electric finned heater (hot air blower), cooling system, and
weighing system (Fig. 2.1).
Figure 2. 1A schematic of the combined microwave and hot air equipment for the peeled longan drying (Varith et al., 2007b)
Hot air generated from an electric finned heater was constantly provided into the bottom of
microwave oven through a stainless steel duct and then was out to the outlet duct. The control
panel was used to regulate MW power and weight loss from the sample was simultaneously
measured. HA temperature did not affect drying characteristic of peeled longan in MW-HA
system, while MW power could effectively be facilitated to shorten drying time. MW-HA was a
superior method to eliminate the major and minor moisture content of peeled longan than the
conventional HA method. In addition, MW-HA process yielded a unique convex-shaped drying
rate period, followed by a falling rate period. By using MW-HA drying process, a uniform
22
browning on dried peeled longan could be obtained. Furthermore, the overall color quality (L*,
a* and b*) index of dried samples by MW-HA was not significantly different from commercial
products.
Vacuum (VC) drying has been considered an alternative to HA drying and proposed to
overcome problems relative to conventional drying process; however, it is known that VC
equipment requires high operation cost and heat transfer in VC may seriously limit drying rate
(Cui et al., 2004). The development of MW-VC combination drying has been investigated to
enhance heat transfer in VC.
MW-VC drying for model fruit gel (pectin gel) was studied (Drouzas et al., 1999). The
MW energy distribution in the chamber was indirectly determined at atmospheric pressure, by
placing samples of the model gel at 5 fixed locations in the cavity and estimating the drying rate.
According to the fixed locations, the highest and lowest drying rates, corresponding to “hot” and
“cold” spots were observed in the oven. MW-VC drying resulted in an acceleration of the drying
rate of model fruit gel. The MW-VC created a significant porous structure (puffing) of the gel
samples, facilitating the transport of the water vapor. High VC pressure at a constant MW power
yields normally better quality. The color of dried gel by MW-VC at high VC pressure was better
(lighter) than the color of the MW-HA dried product at atmospheric pressure. Combined MW-
VC drying of fruit material has a promising potential for high-quality dehydrated products.
Durance and Wang (2002) investigated MW-VC dehydration of tomatoes. MW-VC
dehydration was much faster than conventional HA drying, particularly towards the end of the
drying process. In sequential HA and MW-VC drying process, the substitution of MW-VC at the
final portion of an HA dehydration process could potentially result in the improved overall
23
energy efficiency. The structure of dried tomato by MW-VC was more puffy than it by HA. In
addition, better rehydration rate was observed in the sample dried by MW-VC.
Dehydration characteristics of button mushroom (Agaricus bisporus) under MW-VC
process were examined (Giri and Prasad, 2007). At constant MW power level and product
thickness, the drying rate at higher VC level was slightly raised; however, the impact of VC
pressure on drying time was not substantial rather than it of MW. As drying progressed, the
absorption of MW energy decreased with an increase in the loss of moisture in the sample and
the drying rate became gradually less during later part of drying. Furthermore, dried sample by
MW-VC at low VC pressure and high MW power showed better rehydration rate than dried
samples by HA and MW-VC at high VC pressure. By comparing with SEM images of dried
samples by different methods, the less shrinkage and collapse of cellular structure were observed
in dried sample by MW-VC at low VC pressure. It may be because of the shorter drying time,
lower drying temperature, and the extension of some tissues induced by internal water vapor.
The empirical models for predicting experimental data from MW-VC drying process obviously
indicated that the drying rate and Page's model constants (K- and k- values, respectively) were
highly influenced by MW power and thickness of sample slices. When MW-VC was applied to
dry mint leaves, similar results with dehydration of mushroom were observed (Therdthai and
Zhou, 2009).
The application of MW combined with IR has been considered to be more efficient in
regards to mass and heat transfer; therefore, many researchers have attempted to explore the
synergistic effect of MW-IR combination on the variety of food products.
The effectiveness of MW-IR baking on breads, cookies, cakes, and rice cakes was
evaluated by the number of researchers. (Keskin et al., 2004, 2005, 2007; Sumnu et al., 2005;
24
Sakiyan et al., 2007a,b; Turabi et al., 2008; Ozkoc et al., 2009 a, b). The moisture loss and
firmness of baked food products by MW-IR increased with the increase in MW and IR powers;
however, MW power were more effective on weight loss than IR power level due to high
internal pressure and concentration gradients, which increased the flow of liquid through the
food to the boundary. In the case of color values, the effect of IR power was more dominant than
MW power. Since near-infrared radiation (NIR) generated by halogen lamp has low penetration
depth, NIR energy might accumulate on the surface of products. MW-IR combination baking
method was employed for the baked goods (dough) including different gums such as xanthan,
guar, xanthan-guar blend, and k-carrageenan (Keskin et al., 2005). It was found that xanthan-
guar blend could enhance the quality of bread in MW-IR baking rather than other gums in terms
of specific volume and porosity. However, the undesirable quality of baked breads was found in
the bread formulated with k-carrageenan, which had lowest porosity and specific volume and
highest dielectric properties. Turabi and others (2008) reported that cakes formulated with
xanthan gum had higher specific volume and firmness than the cake including xanthan-guar
blend, resulting in better quality of final samples. In addition, no significant difference between
total color changes (ΔE) of MW-IR baked cakes containing different gums type was found. The
effectiveness of gums on macro and micro-structure in baked bread in MW-IR and conventional
ovens was validated using SEM analysis (Ozkoc et al., 2009). SEM images showed that pore
area fraction of bread formulated with different gums and baked in MW-IR oven was
significantly higher than that of conventionally baked ones. The pores of breads, which was
baked in MW-IR oven and gum was not added, were close to each other that coalescence of the
gas cells to form channels were observed; however, more uniform structure was found in the
samples formulated with xanthan and baked in MW-IR. Larger pores were observed in the
25
sample including carrageenan. Furthermore, the fat replacers (such as purawave, lectigran, malto
dextrin, and simpleasse) were exploited in the cakes baked by MW-IR (Sakiyan et al., 2007a).
For cake samples baked in MW-IR, no significant effect of the fat addition was observed on the
hardness of samples; however, among samples formulated with fat replacer and baked in MW-IR,
the highest hardness value was found in the sample including simplesse that is a protein based fat
placer due to the unique interaction of protein with MW. By the usage of MW-IR baking, the
baking time could be significantly saved and it was possible to obtain the quality of bread
comparable with that of conventionally baked bread.
MW-IR heating was also applied in thawing of frozen potato puree (Seyhun et al., 2009).
During MW thawing process, the dielectric constant (ε') and loss factor (ε'') of potato puree
samples were dramatically changed by the increases in temperature and frequency. Even though
MW-IR combination heating considerably attributed to decrease tempering time and to change
the heating pattern of the potato puree, an increase in either MW or IR power led to uneven
temperature distribution during tempering. A finite difference model were also developed to
predict temperature profile of frozen puree during MW-IR tempering. The predicted data was
well fitted the experimental data with very high correlation coefficient value close to 0.998 and
the model could be facilitated to design MW-IR thawing for other frozen food products.
MW-IR oven which halogen lamps providing near IR were installed at both top and
bottom in the chamber for roasting of hazelnut was investigated (Uysal et al., 2009). The change
in color values and roasting time were dominantly affected by upper halogen lamp power. It was
also observed that the use of upper lamp was effective to prevent the sogginess problem of
microwave heating. The force needed to break the hazelnut kernel decreased by increasing MW
26
and upper halogen power levels. Fatty acid composition of conventionally roasted hazelnut was
not significantly distinct from the sample roasted in MW-IR oven.
2.3.2 Infrared radiation combination technology
Even though IR heating is considered a promising method especially for drying process,
the observed problems in IR drying are scorching heat on uncertain area of surface of food
products and a limited IR penetration depth (Sakai et al., 1993). Case hardening is a troublesome
problem occurred in conventional HA drying process because the surface of food material is
dried first and as drying process progress, the dried surface of food becomes the barrier of heat
transfer (Ratti, 1994). To prevent undesirable phenomenon caused by either IR or conventional
heating methods, the number of studies for the dehydration of food products using integrated IR
and conventional methods have been conducted..
IR assisted with HA drying process for fruit and vegetable has been mainly evaluated and
developed (Afzal et al., 1999; Umesh Hebbar et al., 2004; Praveen Kumar et al., 2006;
Nathakaranakule et al., 2010; Jaturonglumlert and Kiatsiriroat, 2010; Nuthong et al., 2011;
Ponkham et al., 2012; Siriamornpun et al., 2012).
Dehydration of barley was carried out under both IR-HA and individual HA drying
methods (Afzal et al., 1999). It was clearly observed that drying rate and time of barely under
IR-HA system were considerably higher and shorter than in HA drying alone, respectively.
Similar to the MW-HA drying method, slower air velocity led to shorter drying times; hence
relatively low specific energy consumption was needed. However, slower air velocity gave rise
to an increase in temperature of barley, finally causing more quality deterioration than faster air
velocity. Under IR-HA drying, the presence of IR considerably reduced the required total energy
with higher efficiency compared with HA drying alone at all given air temperatures.
27
When whole longan and longan fruit leather were dried by IR-HA system, the
coterminous result with the dehydration of barley was achieved (Jaturonglumlert and Kiatsiriroat,
2010; Nuthong et al., 2011). At the same drying air temperature and velocity, IR power clearly
resulted in an acceleration of drying rate in the dehydration of both samples. The models with
variable parameters (the mass transfer coefficient (hD), the effective diffusion coefficient (Deff),
and drying constant (k)) in combined IR-HA drying of whole longan were developed as a
function of IR power, air temperature, and air velocity (Nuthong et al., 2011). By comparing the
predicted and experimental values, the models were validated and had a fairly good fit for the
empirical data at all conditions. Additionally, the developed models could be used to obtain the
empirical parameters of the Nusselt number (Nu). The scanning electron microscopy (SEM)
images of dried longan by IR-HA obviously showed that higher power of IR created more
porous (puffing) structure in the sample (Nathakaranakule et al., 2010). It was concluded that the
puffing structure led to less shrinkage, hardness, and toughness of dried longan by IR-HA and
provided higher rehydration percentage as compared with dried product by HA.
A IR-HA system was developed for drying of carrot, potato, and pineapple as shown in
Fig. 2.2 (Umesh Hebbar et al., 2004; Ponkham et al., 2012). In the case of carrot and potato, it
was clear that the HA drying method caused a constant drying rate period followed by a "falling
rate period" where includes an unsaturated surface and evaporation occurred below the surface
(Umesh Hebbar et al., 2004; Uretir et al., 1996). In addition, the slight case hardening was
observed in the final product dried by HA method. Since IR-HA mode had a higher dehydration
falling rate closely associated with mass transfer, the drying in combination mode was completed
in a short time. With respect to color and case hardening, the better quality of dried product by
IR-HA was found rather than final products dehydrated by individual IR and HA drying modes.
28
The specific energy consumption for the IR-HA drying was reduced nearly by 63% over HA
drying alone and individual IR heating required much less energy than HA drying alone. An
analogous result with dried potato and carrot by IR-HA drying was produced in the dried
pineapple by IR-HA system (Ponkham et al., 2012). Modeling for predicting moisture diffusion
coefficient, color, shear force ratio, and shrinkage was developed and could describe the
experimental data.
Figure 2. 2 Schematic view of far infrared-hot air dryer (Ponkham et al., 2012)
Kumar and others (2006) investigated the thin layer models for IR-HA drying of onion
slices. The experiments were carried out at three different drying temperatures, slice thickness,
inlet air temperatures, and air velocities. k (drying parameter) values in almost all developed
empirical models (Page, Modified Page, Fick's, and Exponential models) became higher as
drying temperature was raised. It was obvious that drying parameter was significantly influenced
by drying temperature. Slice thickness was closely linked with drying time but did not
significantly affect to the change in k value. This results may be associated with the variation in
penetration depth of IR with the change in slice thickness, influencing heat and mass transfer. On
the contrary to the research conducted by Afzal and others (1999), the slowest air velocity
29
reduced drying rate. It was concluded that the developed Page and Modified Page models
provided a better fit with experimental data over other empirical models.
Chemical properties of dried marigold flower under different dehydration methods
(freeze drying (FD), HA, and IR-HA) were analyzed (Siriamornpuna et al., 2012). In order to
determine carotenoid content in the sample, a standard mixture of lycopene, β-carotene, and
lutein was used for high performance liquid chromatography (HPLC) analysis. The dried sample
under IR-HA has the highest amount of lycopene and the highest content of lutein was found in
the dried sample under FD and FIR-HA. There was a significant difference between the β-
carotene contents of fresh and dried marigold samples. The dried marigold samples by HA and
IR-HA methods showed the highest level of β-carotene. Freeze drying (FD), which is well
known as non-thermal drying method, decreased β-carotene content as compared with the fresh
sample. The results clearly indicated that each drying method could be suitable for different
products depending on the type of chemical compounds considered to be the most important.
The highest antioxidant activities was seen in the sample dried by IR-HA. It could be explained
by the fact that since FIR creates internal heating with molecular vibrations of materials, it may
have the capability to breaking down the covalent complex molecular structures and release
some antioxidant compounds from polymers. The amounts of total flavonoid content (TFC) and
total phenolic content (TPC) in dried material by IR-HA method were higher than fresh, FD, and
HA dried samples. Wanyo and others (2011) also reported that the phenolic compounds in dried
mulberry leaves by IR-HA and individual HA drying methods may differ in terms of its binding
status, depending on specific aspect of their chemical structures.
Dondee and others (2011) tested HA fluidized-bed (HA-FB) dryer assisted with IR
heating to minimize the crack and breakage of dehydrated soybean. In the case of individual IR
30
drying process, the cracking and breakage percentage of soybean was slightly raised with an
increase in IR power and drying time; however, that under IR-HA drying was negligible when
compared with HA-FB at same level of moisture content. The SEM images illustrated that the
membrane and cell wall of dried sample under low IR power level (4 and 6 kW) were intact.
However, at high IR power level (8 kW) the membrane inside the cell wall was extracted outside,
probably due to the stress formation during drying and led to cell wall rupture. Furthermore, the
combined IR-HA-FB method resulted in a minor change on the color of final product.
Banana slice drying process was carried out using integrated IR-VC system (Swasdisevi
et al., 2009). At the same VC level, moisture reduction rate increased with a raise in drying
temperature (depending upon IR power) owing to the increased temperature variation between
the drying product and the surrounding and the increased moisture diffusivity. A decrease in the
absolute pressure of the drying chamber increased the moisture reduction rate. As mentioned
above, the thickness of slices substantially affected to the drying time because the distance over
which the moisture had to travel prior to being removed from the surface of slices increased. The
developed mathematical modeling did not well fit to the experimental data at initial stage
because of the use of the heat transfer coefficient (hc) calculated from the constant drying rate
period. However, as drying progressed, a good agreement between the calculated and
experimental temperature data was observed from the quarter period of drying time.
Uniquely, the combined IR and vibration system was used for high moisture paddy to
determine its drying kinetics (Das et al., 2009). It was clearly observed that the drying rate
period and the moisture diffusivity (Deff) were significantly dependent on the IR intensity and the
distance between sample and IR lamp that was closely associated with the penetration depth of
31
IR . An empirical Page model for to anticipating the moisture ratio in paddy was successfully
developed.
Banana slices were dehydrated by combined IR with low-pressure superheated steam
(LPSS) (Nimmol et al., 2007). Super heated steam drying can provide dried products with less
quality deterioration than HA drying; however, most fruits and vegetables are damaged at the
superheated steam temperature corresponding to the atmospheric pressure. Therefore, LPSS was
developed to prevent damaging products during the drying process. Although LPSSD has a great
potential to produce high-quality dried products, LPSS drying slowly progressed rather than HA
drying process. It was found that the required drying time for banana slice under IR-LPSSD
method was longer than IR-VC at almost all drying conditions. In addition, IR-LPSSD
significantly affected to excessive changes in lightness and redness of dried banana slices.
However, SEM images and X-ray microtomographic images clearly illustrated that dried banana
slices by IR-LPSSD method had larger and more pores compared with those dried by IR-VC.
2.3.3 High pressure processing combination technology
High pressure processing (HPP) has been mainly applied to pasteurize liquid food
products; however, it often could not inactivate bacterial spores (i.e., Bacillus and Salmonella)
which have a strong resistance to high temperature and acidic condition (Krebbers et al., 2003).
Therefore, thermal treatment has been applied to HPP as pretreatment step. The effectiveness of
HPP combined with thermal treatment (TH) on the inactivation of pectin-methyl-esterases (PME)
and the inactivation kinetics in various agricultural products were evaluated by the number of
researchers (Castro et al., 2006; Valdramidis et al., 2007; Sila et al., 2007; Roeck et al., 2009,
2010). Experimental data and kinetic modeling clearly indicated that the higher pressure and
temperature than at least 600 MPa and 50 °C, respectively, and pH values of food products
32
significantly impacted on catalytic activity of PME. In the research done by Castro and others
(2006), it was mentioned that an antagonistic effect of pressure below 300 MPa and temperature
above 54 °C has frequently been encountered for enzyme inactivation/protein denaturation.
Valdrmidis and other (2007) found that the inactivation of PME in carrot required more strong
treatment condition than E. coli inactivation.
The transition of chemical properties in food products containing high content of protein
such as egg white during TH-HPP treatment was also studied (Plancken et al., 2007; Gupta et al.,
2011; Grauwet et al., 2010, 2011). Chemical properties and reaction in protein such as chemical
marker M-2 (4-hydroxy-5-methyl-3(2H)-furanone), ovomucoid, and denaturation were
significantly influenced by increase in pressure and temperature. Furthermore, pH value was the
one of important parameters that could attribute to the quality of final products in TH-HPP
treatment.
Verbeyst and others (2011) investigated the kinetics of anthocyanin degradation in
raspberries treated by TH-HPP treatment. Degradation of anthocyanin was accelerated in high
temperature and pressure treatment. On the degradation kinetics of different anthocyanins in
raspberries, the activation energy at atmospheric pressure was not distinct from it at elevated
pressures. In both models which were a combined Arrhenius-Eyring model and a model
consisting of an exponential equation with staple factors (temperature, pressure, and
temperature-pressure), the stronger effect of temperature than of on the degradation rate
constants was indicated by estimating the parameters corresponding to temperature and pressure.
The optimal condition for TH-HPP processing of strawberries was determined to reduce
the degradation of physical and nutritional qualities after processing and during storage (Terefe
et al., 2009). Temperature rather than pressure had the greatest effect on the color quality of
33
strawberries and an increase in hue angle of the sample following treatment at higher
temperature conditions was caused by pigment degradation. The observed changes in the color
quality of samples were much lower as compared to what was observed in thermally processed
strawberries. During refrigeration condition storage, continuous change in the color quality of
thermally processed strawberries, which was statistically significant, was occurred. Polyphenol
oxidase (PPO) in strawberry was found to be highly resistant to combined high pressure-mild
temperature treatment and was not significantly inactivated in the combination method.
Furthermore, none of individual HPP and thermal treatments had statistically a significant effect
on PPO in strawberry. However, TH-HPP caused significant inactivation of peroxidase (POD) in
the strawberry samples. At longer treatment time, the inactivation effect became dominant
because there would be no further enzyme release. Increase in processing pressure resulted in
increased inactivation regardless of the processing temperature. None of processing parameters
(pressure, temperature, and treatment time) had statistically significant effect on the total
polyphenol content and total anthocyanin content of strawberries.
Similarly, total antioxidant activity, microbial inactivation, and color change of
strawberry juice, tomato, and carrot purees treated by TH-HPP system were studied (Krebbers et
al., 2003; Rodrigo et al.,2007; Patras et al., 2009). TH-HPP sterilization (700 MPa, 30 s, and
80~90 °C) reduced B. stearothermophilus spore in tomato purees with at least 4.5 log microbial
reduction level, reaching a final temperature of 121°C during 30 s. Unlike conventional thermal
processing, a simultaneous adiabatic temperature increase in HPP treatment was evenly
transmitted within the sample, making the effect of HPP treatment independent on size or
geometry. However, HPP processing at ambient or elevated temperature had no effect on the
retention of total lycopene content (Krebbers et al., 2003). Furthermore, the color degradation
34
rate constants in tomato puree and strawberry juice increased with the treatment temperature
rather than pressure level, which indicated that as the temperature increased the rate at which
color degradation occurred also increased (Rodrigo et al., 2007). Antiradical powers and
phenolic contents of tomato and carrot purees processed by TH-HPP treatment at 70°C and
pressure range between 400 and 600 MPa were significantly higher than unprocessed and
thermally processed samples. After processing at 600 MPa, over 93.7 % ascorbic acid in tomato
puree was retained and large significant increase (172 %) in extracted carotenoids occurred as
compared to unprocessed samples. Since carotenoids are the major pigments presenting in both
and carrot and tomato, the consistent increases in hue angle may be a reflection of changes in
total carotenoid contents (Patras et al., 2009).
The transition of physical properties in carrot treated by TH-HPP and TH methods was
determined (Rastogi et al., 2008). Carrot samples were pretreated by each HPP (pressure: 100 -
400 MPa), heat treatment (temperature: 50 - 70 °C), and calcium chloride (CaCl2,
concentration:0.5 - 1.5 %) immersion and then, processed by HPP-TH (700 MPa, 105 °C for 15
min) and TH processing (105 °C for 15 min). Without any pretreatment, the hardness values of
carrot samples treated by HPP-TH and TH alone were 14.08 N and 4.36 N, respectively. Among
three pretreatments, the highest hardness value (33.65 N) was achieved from the sample treated
with 1.0 % of calcium chloride pretreatment and TH-HPP method because the calcium content of
the sample has a positive effect in preserving hardness. Furthermore, when three pretreatments
were combined (200 MPa, 60 °C, and 1.0%), the hardness of treated sample by TH-HPP was 129
N. It was clearly observed that the combined pretreatments could enhance hardness of food
samples with synergistic effect of TH-HPP combination treatment.
35
2.3.4 Radio frequency electric field combination technology
Ukuku and Geveke (2010) developed the combined UV light and RF electric field
(RFEF) system to inactivate Escherichia coli K-12 in apple juice. Apple juice was preheated up
to 25, 30, and 40 °C, and then treated by individual UV, RF treatment and combined UV with
RF treatment. The individual UV and RFEF treatment at 40 °C showed the minimum surviving
population of E. coli K-12 in the juice. A higher bacterial inactivation was expected when the
two treatments were combined; however, the determined number was only approximately 0.6 log
microbial reduction higher than UV treatment alone. Although inactivation of E. coli K-12 in
apple juice was not influenced by the combination system, UV substances determined in the
juice treated by combined treatment was substantially different from individual UV treated
sample. The results suggested that combination treatment would damage the bacterial cells and
lead to more leakage of intracellular UV-substances than individual treatment.
Combined RF with HA treatment was investigated to improve the quality and mold
control of enriched white bread (Liu et al., 2011). Prior to RF-HA treatment, the bread columns
inoculated with mold spores were kept under a sterile hood in order to equilibrate moisture
content in the breads. Additionally, target HA and treatment temperatures controlled by electrical
fan heater and RF power were evaluated to maximize mold lethal condition. Target treatment
temperature of 58 °C and HA temperature which kept bread surface temperature of 58 °C were
determined as the optimal mold lethal condition in this research. Visible mold growth was
observed from the surface of untreated bread loaves stored for 5 weeks at room temperature; on
the other hand, it was found in the sample after combined RF-HA treatment took after four extra
weeks. Moisture migration from bread crumb to the crust was caused by generation of internal
vapor pressure during the RF heating. The consequent moisture loss in the bread crumb and
36
increased moisture at the crust led to a more even distribution of moisture in the treated bread
samples. Combined RF and HA treatment had a little effect on bread water activity during
storage. In addition, the hardness of bready baked under RF-HA system is comparable to the
sample baked by conventional method.
The effectiveness of RF assisted cryogenic (liquid nitrogen, Cryo) freezing on meat was
determined (Anese et al., 2012). To evaluate the effect of RF assisted Cryo freezing on the color
change of frozen meat cubes, the meat samples were frozen by the different freezing procedures
(RF-Cryo, Cryo, and air). However, a significant difference of total color changes between
frozen meat samples using different freezing methods was not observed. On the other hand, drip
loss from RF Cryo frozen meat was much lower than the amount observed during thawing of air
and cryo frozen meat samples. A firmness of thawed meat previously subjected to RF-Cryo
freezing was not considerably different from that of the unfrozen sample. The better cellular
structure observed in the frozen meat by RF-Cryo method could be attributed to depression of
the freezing point by RF pulse, thus producing more nucleation sites. It was likely that the
applied low voltage electric field strength allowed polar water molecules to rotate without
inducing thermal effect.
2.3.5 Ohmic heating combination technology
Combined OH and plate heating system for cooking hamburger patties was developed for
the enhancement of physical properties of the patties (Ozkan et al., 2004; Gin and Farid, 2007).
The domestic plate grill was modified for combination system. The plate was preheated first and
then 50V of alternating current was applied for OH heating. The required cooking time was
determined as 117 s and 163 s for the combined and conventional techniques, respectively. The
elasticity index of the conventionally cooked meat has a slightly higher value than it of cooked
37
meat by OH-Plate heating. This suggested that the meat cooked by the combination system
would be less chewy. Otherwise, the mechanical properties of the meats cooked by individual
plate and OH heating methods were very similar. The application of OH heating for cooking of
hamburger patties did not affect the taste and texture of the meat. From the experiments, it was
concluded that the meat quality was not altered substantially by applying OH heating, which was
evident from the values of the moisture and fat contents (Ozkan et al., 2004).
Another approach for combined OH and plate grill for cooking of meat patties was
conducted with graphite plates for use as electrodes (Gin and Farid, 2007). The grill was also
allowed to preheat up to 150 °C. The center temperature of the meat patty cooked using the
combined method increased with a faster heating rate and approached the upper temperature
faster than with conventional grilling. As a current raised, heating rate of the meat patty
increased, evidencing by the steeper rise in temperature. As similar as the research done by
Ozkan and others (2004), the cooking time could be reduced to almost half by using OH-Plate
cooking. The applied low frequency (50 Hz) resulted in more pitting- or removal of material than
high frequency (100 kHz). This corrosion, or pitting, could be because of localized overheating
and an electrochemical reaction on the surface of electrode. Non-uniform current distribution
could be occurred owing to the irregularity of the meat patty surface leading to hot spots. The
higher frequency for OH heating was more acceptable to prevent occurring the corrosion on the
surfaces of electrodes rather than lower frequency.
2.3.6 Pulsed Electric Field combination technology
Synergistic effect of combined thermal treatment (TH) and pulsed electric field (PEF) on
inactivation of microorganisms in liquid food products has been investigated by the number of
researchers (Li et al., 2005; Bazhal et al., 2006; Amiali et al., 2007; Riener et al., 2008;
38
Walkling-Ribeiro et al., 2010). In these studies, thermal treatment (TH) and pulsed electric field
(PEF) were sequentially applied for pasteurization of liquid food products. Figure 2.3 shows the
schematic of the designed PEF and TH combination pasteurization system (Bazhal et al., 2006).
Prior to PEF treatment, the liquid food product was preheated up to certain temperatures in the
hot water bath. Preheated sample was flowed between two disc electrodes and then high electric
field strength in the range 9 to 15 kV/m with different pulse numbers and high frequency was
applied. The pulse width and frequency were adjusted using an external transistor-transistor logic
(TTL) with a frequency trigger. Increasing the pretreatment temperature of liquid food product
(apple juice and liquid egg yolk) and higher electric field strength had a significant effect on the
inactivation of POD, PPO, and E. coli O157, as well as, showed lower D-values (Amiali et al.,
2007; Riener et al., 2008).
Figure 2. 3 A schematic of the designed pulsed electric field and thermal treatment combination
pasteurization system (Bazhal et al., 2006)
The impact of PEF strength on the relative activity for POD and PPO was considerably
noteworthy and the linear relationship between the increase in field strength and the decrease in
relative activity was found (Riener et al., 2008). TH-PEF system contributed to improve the
microbiological safety, shelf life and overall quality except for the color change in a fruit juice
39
smoothie-type beverage as compared to mild thermal pasteurization (Walkling-Ribeiro et al.,
2010). A first order kinetic model was developed to predict of inactivation of E. coli O157 in
liquid whole egg using TH-PEF combination and it was obviously concluded that the kinetic
constant for the model was influenced by pretreatment temperature, pulse residence time, width,
numbers and intensity (Bazhal et al., 2006). In addition, TH-PEF system was applied to
eliminate Lactobacillus plantarum in salad dressing (Li et al., 2005). Higher than 6 log reduction
in L. plantarum was gained in the sample treated by TH-MEF; however, the decrease in pH
values of the treated samples at different electric field strengths was observed after one week of
storage in refrigeration condition. The transition of pH value of salad dressing
Manothermosonication (MTS), which ultrasound is in conjunction with heat and pressure
treatments, was integrated with PEF for the control of Listeria innocua in a smoothie beverage
(Palgan et al., 2012). When MTS was followed by PEF, the microbial log reduction of L.
innocua counts was higher when compared to that for the sequence of PEF and MTS. The
greater inactivation observed in the sequence of MTS and PEF might be because of greater cell
injury by MTS (as a first hurdle) through cavitation, possibly enhancing susceptibility to the
subsequent PEF treatment. The combined MTS and PEF / UV / high intensity light pulses (HILP)
system was tested for the better retention of shelf life and sensory in orange and carrot juice
(Caminiti et al., 2012). None of the selected combination treatments (MTS+PEF, MTS+UV,
MTS+HILP) induced any change in the pH of the orange and carrot juice blend, while Brix°
values were little distinct depending on combination treatments. All combination treatments
induced ‘well visible’ changes in total color difference (ΔE) of fruit juice and could not
contribute to enhance the sensory attributes of the juice. It was suggested to reduce the intensity
of the MTS treatments for the minimization of the adverse sensory effects on the flavor of the
40
product. The highest inactivation of pectin methyl-esterase (PME) was obtained from the sample
treated under MTS-PEF; however, all combination treatments were efficacious to inactivate
PME activity by approximately 78 %.
The pasteurization of orange juice was carried out under PEF assisted with
thermosonication (TS) which US is integrated with heat treatment (Walkling-Ribeiro et al.,
2009). As coterminous as the results from the research conducted by Caminiti and others (2012),
there was not considerable change in physical and chemical properties (pH, Brix°, and
conductivity) and the shelf life of the juice treated under PEF-TS method was extended as
compared with high temperature short time (HTST) pasteurization. However, significant color
difference (ΔE) between the juice samples treated by PEF-TS combination and HTST
pasteurization was observed due to high processing temperature, finally degrading color
attributes in orange juice. In addition, UV and PEF in a sequential hurdle treatment for the
pasteurization of apple juice was developed to ameliorate shelf life and overall quality (Noci et
al., 2008). The microbial reduction in apple juice treated by sequence of PEF and UV was lower
than that by sequence of UV and PEF. None of the processed juices differed significantly from
fresh juice with respect to pH, conductivity, and Brix°. In addition, the relative antioxidant
capacity was not affected by different treatments.
Combined UV, TH, and PEF treatment for the reduction of Staphylococcus aureus in
apple juice was evaluated (Walking-Ribeiro et al., 2008). Before UV-TH-PEF treatment, the
germicidal effect of UV on S. aureus in apple juice was evaluated. Although inactivation of S.
aureus decreased linearly as UV exposure time increased, only 2.2 log reduction from the
sample exposed to UV for 30 min was obtained because of the insufficient irradiation dosage and
the limited penetration depth of UV. However, when the UV hurdle was assisted with pre-
41
heating at 46 °C and PEF with electric field strength of 40 kV/cm for 100 μs, a bacterial
reduction of 9.5 log was obtained, which was much higher than that produced by HTST
pasteurization. Both PEF treatment time and the filed strength led to significant difference in
bacterial inactivation rather than an increase in UV exposure time. It was also observed that an
increase in electric field strength and total treatment time caused a higher total specific energy
input and generally led to a higher total microbial reduction. Among samples treated by the
combined UV and PEF hurdle approach, no significant differences in color were found; however,
HTST treatment caused a significant changes in color attributes with degradations in L*, a* and
b* values. Therefore, the hurdle-treated apple juice could be considered to be of comparable
color quality to the untreated juice.
Martin and others (2001) developed the combined 18-Tesla pulsed magnetic field (PMF)
with selective non-thermal technologies (HPP, US, and PEF) to validate the inactivation effect
on E. coli ATCC 11775 in ten milliliter of nutrient broth. The inactivation of E. coli in buffers
treated by combined 18-T PMF with other technologies was not more effective than those treated
by individual HPP, US, and PEF. It was concluded that 18-T PMF was not suitable of
inactivation of E. coli when applied after mild HPP, US, and PEF treatments, however, higher
magnetic field might enhance the inactivation of microorganisms.
The sequential treatment of PEF, partial osmotic dehydration and fluidized bed dryer was
employed for dehydration of red bell pepper (Ade-Omowaye et al., 2003). Samples treated by
PEF were immersed in sucrose and sodium chloride solution as a part of partial osmotic
dehydration prior to fluidized bed drying process. An increase in electric field strength resulted
in increasing damaged cell membrane area; however, the area was not extended depending on
42
pulse number of PEF. Additionally, it was found that increasing field strength did not
considerably affect drying rate.
In order to eliminate Salmonella enteritidis in liquid whole egg, combination treatment of
PEF, HPP, and US was employed (Huang et al., 2006). There were total six different hurdle
systems; PEF-US, US-PEF, HPP-US, US-HPP, HHP-PEF, and PEF-HPP. The reduction of
Staphylococcus enteritidis in samples treated by both combinations (PEF-US and US-PEF) was
less than that of additive sum of individual PEF and US treatments. It may because PEF
attributed in an “all or nothing” event, i.e. it may not injure but inactivate the microorganisms
fully. When the sequences of HPP-US and US-HPP were used, no synergistic effect was
observed. This was due to a more resistant sub-population remained after an initial treatment and
cells of S. enteritidis, that were easily killed, were inactivated by initial treatment. In addition,
both treatments that have a change in sequential order (PEF-HPP and HPP-PEF) did not exhibit a
synergistic effect. Among these combinations, HPP-US treatment showed the highest
inactivation in S. enteritidis and linear inactivation rates over increasing exposure time.
Physical properties of carrot, potato, and apple treated by PEF-TH were investigated
(Lebovka et al., 2004a,b). Cell membranes of samples treated by PEF-TH were significantly
damaged and an increase in preheating temperature with keeping constant electric field could
result in obvious softening of tissue.
2.4 Conclusion
The present review has demonstrated the successful applications of combination
technologies for various food processing. The combination technologies in thermal and non-
thermal processing significantly attribute to preserve better quality of food products than
43
conventional methods without serious damages in terms of physical properties such as hardness
and morphological transition and to substantially increase inactivation of enzyme and
microorganisms in pasteurization process. In addition, the combination technology could
simplify food processing procedure with the reduction of processing time and energy
consumption particularly in drying process as compared with conventional method. The
combination technologies applied to various processing for food products were summarized in
Table 2.1.
Although combination technologies provided significant advantages in a variety of food
processing areas, some quality aspects of food products processed with conventional methods
were still better than those with combination methods with respect to color transition and texture.
When advanced technologies were sequentially applied for the processing, the order of sequence
could significantly impact on the final quality of food products depending upon electrical,
physical, and chemical properties of raw food materials. Furthermore, the majority of
combination technologies introduced from previous researches was evaluated on lab scale
systems.
Therefore, it is essential to concretely investigate parameters which can affect the
working abilities of combination technologies such as dielectric properties and chemical
properties of food products. Additionally, in order to practically apply combination technology
in commercial sites for processing of food products, the future research should be conducted in
large scale system and more precise numerical modeling should be needed to predict the
effectiveness of combination technology.
44
Table 2. 1 Summary of combination technologies applied to various food processing areas
Type of Combination Authors Affiliation Type of
Processing Evaluation
Factors Products Comments
MW + CA
Sharma and Prasad (2001)
Indian Institute of Technology, India
Drying
Moisture ratio Drying rate Garlic Experimental
Maskan (2001) University of Gaziantep,
Turkey
Moisture ratio Color transition Kiwifruit Empirical
Model
(2001) Moisture ratio
Drying rate Shrinkage
Kiwifruit
Experimental
Ren and Chen (1998) University of Hong Kong, Hong Kong
Moisture ratio Drying rate
Color transition Ginseng
Andres et al. (2004) Polytechnic University of Valencia, Spain
Moisture ratio Drying rate
SEM analysis Apple cylinder Empirical
Model
Varith et al. (2007) Maejo University, Thailand
Moisture ratio Drying rate
Color transition Longan Experimental
MW + VC
Durance & Wang (2002)
University of British Columbia, Canada
Moisture ratio Drying rate Tomato
Energy consumption
analysis
Giri and Prasad (2007)
Indian Institute of Technology, India
Moisture ratio Drying rate
SEM analysis Mushroom Empirical
Model
Drouzas et al. (1999) National Technical University, Greece
Moisture ratio Drying rate
Color transition Fruit Gel Experimental
MW + VC + CA
Therdthai and Zhou (2009)
Kasetsart University, Thailand
Moisture ratio Drying rate
Color transition SEM analysis
Mint leaves
MW + IR
Uysal et al. (2009) Middle East Technical University, Turkey Roasting
Moisture ratio Drying rate
Chemical and Physical properties
Hazelnut Empirical Model
Sumnu et al. (2005) Middle East Technical University, Turkey
Baking
Moisture ratio Color transition
Physical property Cake
Experimental
45
MW + IR
Ozkoc et al. (2009) The Scientific and
Technological Research Council of Turkey
Baking
Moisture ratio Physicochemical &
Spectrometry properties Bread
Experimental
(2009)
Macro & micro structure of Gum
analysis SEM analysis
Bread
Seyhun et al. (2009) Middle East Technical University, Turkey Thawing
Tempering Dielectric & Physical
properties
Frozen Potato Puree
Sakiyan et al. (2007) Selcuk University, Turkey
Baking
Formulation transition Dielectric & Physical
properties Bread
(2007) Color transition Physical property Cake
Turabi et al. (2008)
Middle East Technical University, Turkey
Formulation transition Color transition
Physical property Rice Cake Empirical
Model
Keskin et al. (2004) Moisture ratio
Color transition Physical property
Bread
Experimental (2005) Color transition Physical property Cookie
(2007) Formulation transition Dielectric property of
Gum Bread
IR + CA
Umesh Hebbar et al. (2004)
Central Food Technological Research
Institute, India Moisture ratio Potato, Carrot
Energy consumption
analysis
Afzal et al. (1999) The United Graduate
School of Agricultural Sciences, Japan
Moisture ratio Drying rate
Physical property Barley
Energy consumption
analysis
Nathakaranakule et al. (2010)
King Mongkut's University of Technology Thonburi,
Thailand
Moisture ratio Drying rate
SEM analysis Longan
Energy consumption
analysis
Jaturonglumlert and Kiatsiriroat (2010)
Chiang Mai University, Thailand
Drying
Drying rate Heat & Mass transfer Longan
Empirical & Numerical
Model
Nuthong et al. (2011) Chiang Mai University, Thailand
Moisture ratio Drying rate
Mass transfer Longan
Empirical & Numerical
Model
46
IR + CA
Wanyo et al. (2011)
Mahasarakham University, Thailand
Drying
Color transition Chemical property
Spectrometry analysis Thai tea Experimental
Ponkham et al. (2012)
Moisture ratio Drying rate
Color transition Pineapple Empirical
Model
Siriamornpun et al. (2012)
Color transition Chemical property
Spectrometry analysis Marigold Experimental
Praveen Kumar et al. (2006)
Central Food Technological Research
Institute, India Moisture ratio Onion Empirical
Model
IR + LPS
Nimmol et al. (2007) King Mongkut's University of Technology Thonburi,
Thailand
Moisture ratio Color transition SEM analysis
Banana Experimental
Leonard et al. (2008) University of Liege, Belgium
X-ray microtomography
analysis Banana Experimental
IR + FB Dondee et al. (2011) Mahasarakham University, Thailand
Moisture ratio Color transition SEM analysis
Soybean Experimental
IR + VB Das et al. (2009) Indian Institute of Technology, India
Moisture ratio Drying rate Paddy Empirical
Model
IR + VC Swasdisevi et al. (2009)
King Mongkut's University of Technology Thonburi,
Thailand Moisture ratio Banana
Empirical & Numerical
Model
HPP + TH
Krebbers et al. (2003)
Agrotechnological Research Insititue,
Netherlands Sterilization
Color transition Chemical property
Spectrometry analysis
Whey protein gel Experimental
Castro et al. (2006)
Katholieke University, Belgium
Sterilization Lethality level Pepper Empirical Model
Sila et al. (2007) Extraction Chemical property Carrot
Experimental
Plancken et al. (2007)
Pasteurization
Physicochemical property Egg
Rodrigo et al. (2007) Color transition Chemical property
Tomato puree Strawberry
juice
Valdramidis et al. (2007)
Katholieke University, Belgium
Sterilization Lethality level Carrot
Empirical & Numerical
Model
47
HPP + TH
Grauwet et al. (2010)
Katholieke University, Belgium
Separation (Ovomucoid) Chemical property Egg
Empirical Model
(indicator)
Roeck et al. (2010) Sterilization Physical & Chemical properties Carrot Empirical
Model
(2011) Separation (Ovomucoid) Chemical property Egg
Empirical Model (TEMP
mapping)
Verbeyst et al. (2011)
Sterilization
Chemical property Isothermal-isobaric
degradation Raspberry
Experimental Rastogi et al. (2008) The Ohio University, USA
Chemical & Physical properties
SEM analysis Carrot
Sampedro et al. (2008)
Instituto de Agroquimica y Tecnologia de Alimentos,
Spain Pasteurization Lethality level Orange juice-
milk
Patras et al. (2009) University College Dublin, Ireland
Color transition Chemical property
Tomato & Carrot purees
Terefe et al. (2009) Innovative Food Center, Australia Sterilization
Color transition Physical & Nutritional
Properties Strawberry Empirical
Model
RF + CA Liu et al. (2011) China Agricultural University, China Sterilization Moisture ratio
Physical property White bread
Experimental
RF + LNR Anese et al. (2012) Universita degli Studi di Udine, Italy Freezing Color transition
SEM analysis Meat
RF + UV Ukuku and Geveke (2010)
U.S. Department of Agriculture, USA Pasteurization Inactivation efficiency Apple juice
OH + HP Ozkan et al. (2004) University of Auckland,
New Zealand
Cooking
Chemical & Physical properties Meat patty
Gin and Farid (2007) Corrosion rate SEM analysis Meat patty
PEF + TH
Lebovka et al. (2004) Universite de Technologie
de Compiegne, France
Physical property Apple
(2004) Physical property Apple, Carrot, Potato
Li et al. (2005) The Ohio State University, USA
Pasteurization
Lethality level Salad dressing
Experimental Bazhal et al. (2006) McGill University, Canada
Liquid whole
egg
48
PEF + TH
Amiali et al. (2007) McGill University, Canada
Pasteurization
Egg yolk
Experimental
Riener et al. (2008) University of College Dublin, Ireland Chemical property Apple juice
PEF + UV Walkling-Ribeiro et al. (2008)
School of Agriculture, Ireland
Color transition Lethality level
Apple juice
PEF + TH (2010) Fruit smoothie type beverage
PEF + TS (2009) Color transition Shelf life test Orange juice
PEF + US HPP + US
Huang et al. (2006) University of Guelph, Canada
Lethality level
Liquid whole egg
PEF + MS Palgan et al. (2012) Institute of Food and Health, Ireland
Smoothie type beverage
PEF + PMF Martin et al. (2001) Washington State University, USA Sterilization E. coli buffer
PEF + POD Ade-Omowaye et al. (2003)
Berlin University of Technology, Germany Drying Moisture ratio
Physical property Red bell pepper
PEF + UV Noci et al. (2008) School of Agriculture, Ireland
Pasteurization
Color transition Chemical property
Lethality level Apple juice
PEF + MS HILP + MS UV + MS
Caminiti et al. (2012) UCD Institute of Food and Health, Ireland
Color transition Chemical property Sensory analysis
Orange and Carrot juices
Note) MW: Microwave heating, CA: Convective Air, VC: Vacuum, IR: Infrared heating, LPS: Low-pressure superheated steam, FB:
Fluidized bed, VB: Vibration treatment, HP: Hot plate heating, TH: Thermal treatment, LNR: Liquid nitrogen reservoir, UV: Ultraviolet
irradiation, TS: Thermosonication, US: Ultrasound sonication , HPCD: High intensity light pulse, MS: Manothermosonication, PMF:
Pulsed magnetic field, POD: Partial osmotic dehydration
49
2.5 References
Ade-Omowaye. B.I.O., Rastogi, N.K., Angersbach, A., Knorr, D. (2003). Combined effects of
pulsed electric field pre-treatment and partial osmotic dehydration on air drying behavior of red
bell pepper. Journal of Food Engineering, 60, 89-98.
Afzal, T,M., Abe, T., Hikida, Y. (1999). Energy and quality aspects during combined FIR-
convection drying of barley. Journal of Food Engineering, 42 177-182.
Amiali, M., Ngadi, M.O, Smith, J.P., Raghavan, G.S.V. (2007). Synergistic effect of
temperature and pulsed electric field on inactivation of Escherichia coli O157:H7 and
Salmonella enteritidis in liquid egg yolk. Journal of Food Engineering, 79, 689-694.
Andres, A., Bilbao, C., Fito, P. (2004). Drying kinetics of apple cylinders under combined hot
air–microwave dehydration. Journal of Food Engineering, 63, 71-78.
Anese, M., Manzocco, L., Panozzo, A., Beraldo, P., Foschia, M., Nicoli, M.C. (2012). Effect of
radiofrequency assisted freezing on meat microstructure and quality. Food Research
International, 46, 50-54.
Ayhan, Z, Yeom, H.W., Zhang, Q.H., Min, D.B. (2001). Flavor, color, and vitamin C retention
of pulsed electric field processed orange juice in different packaging materials. Journal of
Agricultural Food Chemistry, 49, 669-674.
Barbosa-Canovas, G. V., Gongora-Nieto, M., Pothakamury, U., Swanson, B. (1999). Food
processing by PEF. Preservation of Foods with Pulsed Electric Fields (first ed.) (pp. 156−171).
San Diego, California: Academic Press.
Bazhal, M.I. , Ngadi, M.O., Raghavan, G.S.V., Smith, J.P. (2006). Inactivation of Escherichia
coli O157:H7 in liquid whole egg using combined pulsed electric field and thermal treatments.
LWT-Food Science Technology,39, 419-425.
50
Birla, S.L., Wang, S., Tang, J., Tiwari, G. (2008). Characterization of radio frequency heating of
fresh fruits influenced by dielectric properties. Journal of Food Engineering, 84, 270–280.
Buffler,C. R. (1993). Microwave cooking and processing. Van Nostrand Reihold., New York,
USA.
Calay, R.K., Newborough, M., Probert, D., Calay, P.S. (1994). Predictive equations for the
dielectric properties of foods. International Journal of Food Science and Technology, 29, 699-
713.
Caminiti, I.M., Noci, F., Morgan, D.J., Cronin, D.A., Lyng, J.G. (2012). The effect of pulsed
electric fields, ultraviolet light or high intensity light pulses in combination with
manothermosonication on selected physico-chemical and sensory attributes of an orange and
carrot juice blend. Food and Bioproducts Processing, 90, 442-448.
Castro, S.M., Loey, A.V., Saraiva, J.A., Smout C., Hendrickx, M. (2006). Inactivation of pepper
(Capsicum annuum) pectin methylesterase by combined high-pressure and temperature
treatments. Journal of Food Engineering, 75, 50-58.
Choi, Y., Lee, S.M., Chun, J., Lee, H.B., Lee, J. (2006). Influence of heat treatment on the
antioxidant activities and polyphenolic compounds of Shiitake (Lentinus edodes) mushroom.
Food Chemistry, 99, 381-387.
Coronel, P., Simunovic, J., Sandeep, K.P. (2003). Temperature profiles within milk after heating
in a continuous flow tubular microwave system operating at 915 MHz. Journal of Food Science,
68, 1976–1981.
Cui, Z. W., Xu, S. Y., Sun, D. W. (2004). Microwave–vacuum drying kinetics of carrot slices.
Journal of Food Engineering, 65(2), 157-164.
51
Datta, A.K., Ni, H. (2002). Infrared and hot-air-assisted microwave heating of foods for control
of surface moisture. Journal of Food Engineering, 51, 355-364.
Dondee, S., Meeso, N., Soponronnarit, S., Siriamornpun, S. (2011). Reducing cracking and
breakage of soybean grains under combined near-infrared radiation and fluidized-bed drying.
Journal of Food Engineering, 104, 6-13.
Drouzas, A.E., Tsami, E., Saravacos, G.D. (1999). Microwave/vacuum drying of model fruit gels.
Journal of Food Engineering, 81, 117-122.
Durance, T.D., Wang, J.H. (2002). Energy consumption, density, and rehydration rate of vacuum
microwave- and hot-air convection- dehydrated tomatoes. Journal of Food Science, 67, 2212-
2216.
FARKAS, D. F., HOOVER, D. G. (2000). High pressure processing. Journal of Food Science,
65(s8), 47-64.
Garcia, D., Gomez, N., Condon, S., Raso, J., Pagan, R. (2003). Pulsed electric fields cause
sublethal injury in Escherichia coli. Letters in Applied Microbiology, 36, 104-144.
Gentry, T.S., Roberts, J.S. (2005) Design and evaluation of a continuous flow microwave
pasteurization system for apple cider. Lebensmittel Wissenschaft Technology, 38, 227–238.
Gin, B., Farid, M. (2007). The use of carbon electrodes in ohmic cooking of meat patties. Asia-
Pacific Journal of Chemical Engineering, 2, 474-479.
Giri., S.K., Prasad., S. (2007). Drying kinetics and rehydration characteristics of microwave-
vacuum and convective hot-air dried mushrooms. Journal of Food Engineering, 78, 512-521.
52
Grauwet, T., Plancken, I.S., Vervoort, L., Matser, A., Hendrickx, M., Loey, A.V. (2011).
Temperature uniformity mapping in a high pressure high temperature reactor using a temperature
sensitive indicator. Journal of Food Engineering, 105, 36-47.
Gurtler, J. B., Rivera, R. R., Zhang, H. Q., Geveke, D. J. (2010). Selection of surrogate bacteria
in place of E. coli O157:H7 and Salmonella Typhimurium for pulsed electric field treatment of
orange juice. International Journal of Food Microbiology, 139, 1-8.
Huang, E., Mittal, G.S., Griffiths, M.W. (2006). Inactivation of Salmonella enteritidis in liquid
whole egg using combination treatments of pulsed electric field, high pressure and ultrasound.
Biosystems Engineering, 94, 403-413.
Das, I., Das, S.K., Satish Bal. (2009). Drying kinetics of high moisture paddy undergoing
vibration-assisted infrared (IR) drying. Journal of Food Engineering, 95, 166-171.
Jaturonglumlert, S., Kiatsiriroat, T. (2010). Heat and mass transfer in combined convective and
far-infrared drying of fruit leather. Journal of Food Engineering, 100, 254-260.
Jin, T., Zhang, H., Hermawan, N., Dantzer, W. (2009). Effects of pH and temperature on
inactivation of Salmonella typhimurium DT104 in liquid whole egg by pulsed electric fields.
International Journal of Food Science and Technology, 44, 367–372.
Jun, S., Sastry, S.K. (2005). Modeling and optimizing of pulsed ohmic heating of foods inside
the flexible package. Journal of Food Processing Engineering, 28, 417–436.
Keskin, S. O., Sumnu, G., Sahin, S. (2004). Bread baking in halogen lamp–microwave
combination oven. Food Research International,37, 489-495.
Keskin, S. O., Sumnu, G., Sahin, S. (2007). A study on the effects of different gums on dielectric
properties and quality of breads baked in infrared-microwave combination oven. European Food
Research Technology, 224, 329-334.
53
Keskin, S. O., Sumnu, G., Sahin, S. Koksel, H., Sumnu, G. (2005). Halogen lamp–microwave
combination baking of cookies. European Food Research Technology, 220, 546-551.
Krebbers, B., Master, A.M., Hoogerwerf, S.W., Moezelaar, R., Tomassen, M.M., Berg, W.
(2003). Combined high-pressure and thermal treatments for processing of tomato puree:
evaluation of microbial inactivation and quality parameters. Innovative Food Science and
Emerging Technologies, 4, 377-385.
Kulshrestha, S., Sastry, S.K (2003). Frequency and voltage effects on enhanced diffusion during
moderate electric field (MEF) treatment. Innovative Food Science and Emerging Technology, 4.
189-194.
Lebovka, N.I., Praporscic, I., Vorobiew, E. (2004). Combined treatment of apples by pulsed
electric fields and by heating at moderate temperature. Journal of Food Engineering, 65, 211-217.
Lebovka, N.I., Praporscic, I., Vorobiew, E. (2004). Effect of moderate thermal and pulsed
electric field treatments on textural properties of carrots, potatoes and apples. Innovative Food
Science and Emerging Technology, 5, 9-16.
Lee, S.H., Jun, S. (2011). Electrophoresis induced breakdown of cellular structures in taro skins
for enhancement of sugar release. Transactions of the ASABE, 54, 1041–1047.
Li, S.Q., Zhang, H.Q., Jin, T.Z., Turek, E.J., Lau, M.H. (2005). Elimination of Lactobacillus
plantarum and achievement of shelf stable model salad dressing by pilot scale pulsed electric
fields combined with mild heat. Innovative Food Science and Emerging Technology, 6, 125-133.
Liu, Y., Tang, J, Mao, Z., Mah, J., Jiao, S., Wang, S. (2011). Quality and mold control of
enriched white bread by combined radio frequency and hot air treatment. Journal of Food
Engineering, 104, 492-498.
54
Maritin, M.F.S., Harte, F.M., Lelieveld, H., Barbosa-Canovas, G.V., Swanson, B.G. (2001).
Inactivation effect of an 18-T pulsed magnetic field combined with other technologies on
Escherichia coli. Innovative Food Science and Emerging Technologies, 2, 273-277.
Maskan, M. (2001). Drying, shrinkage and rehydration characteristics of kiwifruits during hot air
and microwave drying. Journal of Food Engineering, 48, 177-182.
Maskan, M. (2001). Kinetics of colour change of kiwifruits during hot air and microwave drying.
Journal of Food Engineering, 48, 169-175.
Nathakaranakule, A., Jaiboon, P., Soponronnarit, S. (2010). Far-infrared radiation assisted drying
of longan fruit. Journal of Food Engineering, 100, 662-668.
Nguyen, L.T., Choi, W., Lee, S.H., Jun, S. (2013). Exploring the heating patterns of multiphase
foods in a continuous flow, simultaneous microwave and ohmic combination heater. Journal of
Food Engineering, 116, 65-71.
Nimmol, C., Devahastin, S., Swasdisevi, T., Soponronnarit, S. (2007). Drying of banana slices
using combined low-pressure superheated steam and far-infrared radiation. Journal of Food
Science, 81, 624-633.
Nindo, C.I., Kudo, Y., Bekki, E. (1995). Test model for studying sun drying of rough rice using
far-infrared radiation. Drying Technology: An International Journal, 13, 225-328.
Noci, F., Riener, J., Walkling-Ribeiro, M., Cronin, D.A., Morgan, D.J., Lyng, J.G. (2008).
Ultraviolet irradiation and pulsed electric fields (PEF) in a hurdle strategy for the preservation of
fresh apple juice. Journal of Food Engineering, 82, 141-146.
Nowak, D., Lewicki, P.P. (2004). Infrared drying of apple slices. Innovative Food Science and
Emerging Technologies, 5, 353– 360.
55
Nuthong, P., Achariyaviriya, A., Namsanguan, K., Achariyaviriya, S. (2011). Kinetics and
modeling of whole longan with combined infrared and hot air. Journal of Food Engineering, 102,
233-239.
Ozkan, N., Ho, I., Farid, M. (2004). Combined ohmic and plate heating of hamburger patties:
quality of cooked patties. Journal of Food Engineering, 63, 141-145.
Ozkoc, S.O., Sumnu, G., Sahin, S. (2009). The effects of gums on macro and micro-structure of
breads baked in different ovens. Food Hydrocolloids, 23 2182-2189.
Ozkoc, S.O., Sumnu, G., Sahin, S., Turabi, E. (2009). Investigation of physicochemical
properties of breads baked in microwave and infrared-microwave combination ovens during
storage. European Food Research Technology, 228, 883-893.
Palgan, I., Munoz, A., Noci, F., Whyte, P., Morgan, D.J., Cronin, D.A., Lyng, J.G. (2012).
Effectiveness of combined pulsed electric field (PEF) and manothermosonication (MTS) for the
control of Listeria innocua in a smoothie type beverage. Food Control, 25, 621-625.
Patras, A., Brunton, N., Pieve, S.D., Butler, F., Downey, G. (2009). Effect of thermal and high
pressure processing on antioxidant activity and instrumental colour of tomato and carrot purées.
Innovative Food Science and Emerging Technologies, 10, 16-22.
Plancken, I.V., Loey, A.V., Hendrickx, M. (2007). Kinetic study on the combined effect of high
pressure and temperature on the physico-chemical properties of egg white proteins. Journal of
Food Engineering, 78, 206-216.
Ponkhama, K., Meeso, N., Soponronnaritb, S., Siriamornpunc, S. (2012). Modeling of combined
far-infrared radiation and air drying of a ring shaped-pineapple with/without shrinkage. Food and
Bioproducts Processing, 90, 155-164.
56
Praveen Kumar, D.G., Umesh Hebbar, H., Ramesh, M.N. (2006). Suitability of thin layer models
for infrared–hot air-drying of onion slices. LWT-Food Science Technology,39, 700-705.
Qin, B. L., Chang, F. J., Barbosa-Cánovas, G. V., & Swanson, B. G. (1995). Nonthermal
inactivation of Saccharomyces cerevisiae in apple juice using pulsed electric fields. LWT-Food
Science and Technology, 28(6), 564-568.
Rasanayagam, V., Balasubramaniam, V.M., Ting, E., Sizer, C.E., Bush, C., Anderson, C. (2003).
Compression heating of selected fatty food materials during high-pressure processing. Journal of
Food Science, 68, 254-259.
Rastogi, N.K., Nguyen, L.T., Balasubramaniam, V.M. (2008). Effect of pretreatments on carrot
texture after thermal and pressure-assisted thermal processing. Journal of Food Engineering, 88,
541-547.
Ratti, C. (1994). Shrinkage during drying of foodstuffs. Journal of Food Engineering, 23, 91-105.
Ren, G., Chen., F. (1998). Drying of American ginseng (Panax quinquefoZium) roots by
microwave-hot air combination. Journal of Food Engineering, 38, 433-443.
Riener, J., Noci, F., Cronin, D.A., Morgan, D.J., Lyng, J.G. (2008). Combined effect of
temperature and pulsed electric fields on apple juice peroxidase and polyphenoloxidase
inactivation. Food Chemistry, 109, 402-407.
Rodrigo, D., Loey, A.V., Hendrickx, M. (2007). Combined thermal and high pressure colour
degradation of tomato puree and strawberry juice. Journal of Food Engineering, 79, 553-560.
Roeck, A.D., Duvetter, T., Fraeye, I., Plancken, I.V., Sila, D.N., Loey, A.V., Hendrickx, M.
(2009). Effect of high-pressure/high-temperature processing on chemical pectin conversions in
relation to fruit and vegetable texture. Food Chemistry, 115, 207-213.
57
Roeck, A.D., Mols, J., Duvetter, T., Loey, A.V., Hendrickx, M. (2010). Carrot texture
degradation kinetics and pectin changes during thermal versus high-pressure/high-temperature
processing: A comparative study. Food Chemistryistry, 120, 1104-1112.
Rowley, A. T. (2001). Radio frequency heating. In P. Richardson (Ed.), Thermal technologies in
food processing. Cambridge, Woodhead, USA.
Sakai, N., Fujii, A., Hanzawa, T. (1993). Heat transfer analysis in a food heated by far infrared
radiation. Nippon Shokuhin Kogyo Gakkaishi, 40, 469–77.
Sakai, N., Hanzawa, T. (1994). Applications and advances in far-infrared heating in Japan.
Trends in Food Science and Technology, 5, 357-362.
Sakiyan, O., Sumnu, G., Sahin, S., Meda, V. (2007). The effect of different formulations on
physical properties of cakes baked with microwave and near infrared-microwave combinations.
Journal of Microwave Power and Electromagnetic Energy, 41, 20-26.
Sakiyan, O., Sumnu, G., Sahin, S., Teshmeda, V. (2007). Investigation of dielectric properties of
different cake formulations during microwave and infrared–microwave combination baking.
Journal of Food Science, 72, E205-E213.
Saldaña, G., Puértolas, E., López, N., García, D., Álvarez, I., Raso, J. (2009). Comparing the
PEF resistance and occurrence of sublethal injury on different strains of Escherichia coli,
Salmonella Typhimurium, Listeria monocytogenes and Staphylococcus aureus in media of pH 4
and 7. Innovative Food Science and Emerging Technologies, 10, 160−165.
Salengke, S. (2000). Electrothermal Effects of Ohmic Heating on Biomaterials: Temperature
Monitoring, Heating of Solid–Liquid Mixtures, and Pre-treatment Effects on Drying Rate and
Oil Uptake. PhD Thesis, The Ohio State University, Ohio, USA.
58
Sandu, C. (1986). Infrared radiative drying in food engineering: a process analysis.
Biotechnology Progress, 2,109–119.
Sensoy, I., Sastry, S.K. (2004). Extraction using moderate electric fields. Journal of Food
Science, 69, FEP7–FEP13.
Seyhun, N., Ramaswamy, H., Sumnu, G., Sahin, S., Ahmed, J. (2009). Comparison and
modeling of microwave tempering and infrared assisted microwave tempering of frozen potato
puree. Journal of Food Engineering, 92, 339-344.
Sharama, G. P., Prasad, S. (2001). Drying of garlic (Allium sativum) cloves by microwave-hot
air combination. Journal of Food Engineering, 51, 99-105.
Shim, J.Y., Lee, S.H., Jun, S. (2010). Modeling of ohmic heating patterns of multiphase food
products using computational fluid dynamics codes. Journal of Food Engineering, 99, 136–141.
Sila, D.N., Smout, C., Satara, Y., Truong V., Loey, A.V., Hendrickx, M. (2007). Combined
thermal and high pressure effect on carrot pectinmethylesterase stability and catalytic activity.
Journal of Food Engineering, 78, 755-764.
Siriamornpuna, S., Kaisoona, O., Meeso, N. (2012). Changes in colour, antioxidant activities and
carotenoids (lycopene, b-carotene, lutein) of marigold flower (Tagetes erecta L.) resulting from
different drying processes. Journal of Functional Foods, 4, 757-766.
Sumnu, G., Sahin, S., Sevimli, M. (2005). Microwave, infrared and infrared-microwave
combination baking of cakes. Journal of Food Engineering, 71, 150-155.
Swasdisevi, T., Devahastin, S., Sa-Adchom, P., Soponronnarit, S. (2009). Mathematical
modeling of combined far-infrared and vacuum drying banana slice. Journal of Food
Engineering, 92, 100-106.
59
Tang, J. (2005). Dielectric properties of foods. In: The Microwave Processing of Foods.
Woodhead, Cambridge, USA.
Tang, J., Feng, H., Lau, M. (2002). Microwave heating in food processing. In X. Young & J.
Tang (Eds.), Advances in bioprocessing engineering. New Jersey, Scientific Press, USA.
Terefe, N.S., Matthies, K., Simons, L., Versteeg, C. (2009). Combined high pressure-mild
temperature processing for optimal retention of physical and nutritional quality of strawberries
(Fragaria×ananassa). Innovative Food Science and Emerging Technologies, 10, 297-307.
Therdthai, N., Zhou, W. (2009). Characterization of microwave vacuum drying and hot air
drying of mint leaves (Mentha cordifolia Opiz ex Fresen). Journal of Food Engineering, 91, 482-
489.
Torrecilla, J.S., Otero, L., Sanz, P.D. (2005). Artificial neural networks: a promising tool to
design and optimize high-pressure food processes. Journal of Food Engineering, 69, 299-306.
Torres, J.A., Velazquez, G. (2005). Commercial opportunities and research challenge in the high
pressure processing of foods. Journal of Food Engineering, 67, 95-112.
Turabi, E., Sumnu, G., Sahin, S. (2008). Optimization of baking of rice cakes in infrared-
microwave combination oven by response surface methodology. Food Bioprocess Technology, 1,
64-73.
Ukuku, D.O., Geveke, D.J. (2010). A combined treatment of UV-light and radio frequency
electric field for the inactivation of Escherichia coli K-12 in apple juice. International Journal of
Food Microbiology, 138, 50-55.
Umesh Hebbar, H., Vishwanathan, K.H., Ramesh, M.N. (2004). Development of combined
infrared and hot air dryer for vegetables. Journal of Food Engineering, 65, 557-563.
60
Uysal, N., Sumnu, G., Sahin, S. (2009). Optimization of microwave–infrared roasting of
hazelnut. Journal of Food Engineering, 90, 255-261.
Valdramidis, V.P., Geeraerd, A.H., Poschet, F., Ly-Nguyen, F., Van Opstal, I., Van Loey, A.M.,
Michiels, C.W., Hendrickx, C.W., Van Impe, J.F. (2007). Model based process design of the
combined high pressure and mild heat treatment ensuring safety and quality of a carrot simulant
system. Journal of Food Engineering, 78, 1010-1021.
Varith, J., Dijkanarukkul, P., Achariyaviriya, A., Achariyaviriya, S. (2007). Combined
microwave-hot air drying of peeled longan. Journal of Food Engineering, 81, 459-468.
Varith, J., Noochuay, C., Netsawang, P., Hirunstitporn, B., Janin, S., Krairiksh, M. (2007).
Design of multimode-circular microwave cavity for agri-food processing. Proceedings of Asia-
Pacific Microwave Conference Microwave Conference, APMC, Asia-Pacific, 1-4.
Venkatesh, M.S., Raghavan, G.S.V. (2004). An overview of microwave processing and dielectric
properties of agri-food materials. Biosystems Engineering, 88, 1-18.
Verbeyst, L., Crombruggen, K.V., Plancken, I.V., Hendrickx, M., Loey, A.V. (2011).
Anthocyanin degradation kinetics during thermal and high pressure treatments of raspberries.
Journal of Food Engineering, 105. 513-521.
Walkling-Ribeiro, M, Noci, F., Cronin, D.A., Riener, J., Lyng, J.G., Morgan, D.J. (2008).
Reduction of Staphylococcus aureus and quality changes in apple juice processed by ultraviolet
irradiation, pre-heating and pulsed electric fields. Journal of Food Engineering, 89, 267-273.
Walkling-Ribeiro, M, Noci, F., Cronin, D.A., Riener, J., Lyng, J.G., Morgan, D.J. (2010). Shelf
life and sensory attributes of a fruit smoothie-type beverage processed with moderate heat and
pulsed electric fields. LWT-Food Science Technology,43, 1067-1073.
61
Walkling-Ribeiro, M, Noci, F., Cronin, D.A., Riener, J., Lyng, J.G., Morgan, D.J. (2009). Shelf
life and sensory evaluation of orange juice after exposure to thermosonication and pulsed electric
fields. Food and Bioproducts Processing, 87, 102-107.
Wang, J., Tang, J., Wang, Y., Swanson, B. (2009). Dielectric properties of egg whites and whole
eggs as influenced by thermal treatments. LWT-Food Science and Technology, 42, 1204-1212.
Wang, S., Birla, S.L., Tang, J., Hansen, J.D. (2006). Postharvest treatment to control codling
moth in fresh apples using water assisted radio frequency heating. Postharvest Biology and
Technology, 40, 89–96.
Wang, Y., Wig, T.D., Tang, J., Hallberg, L.M. (2003). Sterilization of foodstuffs using radio
frequency heating. Journal of Food Science, 68, 539-544.
Wanyo, P., Siriamornpuna, S., Meeso, N. (2011). Improvement of quality and antioxidant
properties of dried mulberry leaves with combined far-infrared radiation and air convection in
Thai tea process. Food and Bioproducts Processing, 89, 22-30.
62
Chapter 3
Development of a dual cylindrical microwave and ohmic combination heater for
processing of particulate foods
3.1 Introduction
Microwave technology has been utilized in a wide range of fields including
communication systems, radar systems, and medical systems during the past decades (Pozar,
2005). In particular, the domestic microwave oven has become an indispensible home appliance
owing to simple operation and rapid heating or thawing of food products that are prepared in
advance. However, the microwave power intensity in the domestic microwave oven (the typical
example of multi-mode microwave cavity) is not evenly distributed in the target food material,
finally resulting in non-uniform heating inside food (Barlow and Marder, 2003). Various factors
that affect heating uniformity in food materials during microwave heating, i.e. dielectric
properties, volume, geometry, and shape of the food materials have been investigated
(Anantheswaran and Liu, 1994; Zhu et al., 2007). However, as far as can be determined from
accessible literature, there was a dearth concerning the design and geometric parameters of
microwave resonant cavities for processing food products.
Microwave cavity resonators for food processing can be classified into two common
types: single mode and multimode cavity resonators. The presence of different resonance modes
can consequentially lead to multiple hot spots apparently disconnected in the multimode cavity
(Bradshaw et al., 1998). The multimode cavity is versatile to thermally process various food
materials that have large volume, complicated shape, and different dielectric properties (Das et
al., 2009). In general, several fundamental standing modes can be generated in the cavity;
63
however, it is not guaranteed that all resonance modes will be generated and excited (Chan and
Reader, 2000). The multimode cavity have suffered from inherent problems such as poor energy
efficiency, electric field strengths, and the demand of high power (Whittles et al., 2003). The
practical approaches for the enhancement of thermal uniformity in a multimode cavity have been
explored by a number of researchers. Tang and others (2008) reported that the use of magnetron
with higher frequency (5.8 GHz) could increase the power density and processing efficiency by
four times more than conventional 2.45 GHz magnetron. Another advantageous approach for a
multimode cavity with a fixed dimension is to install multiple microwave feeding ports
(magnetrons or power inputs) in the cavity (Tran, 1992). However, the suggested approaches
require supplementary equipment i.e., a mode stirrer that is closely associated with the heating
patterns of food material and microwave leakage, and tremendous time to achieve critical and
detail design of the multimode cavity.
On the other hand, the single mode cavity resonator was designed and exploited to
support only one standing wave at the source frequency (Sun et al., 2004). The single mode
cavity would have important advantages over multimode cavity due to the improvement of
energy efficiency and the capability for precise control of processing (Asmussen et al., 1987).
When same microwave power inputs are applied in multimode and single mode cavities, the
electric field strength established in single mode cavity is much higher than in multimode cavity
(Metaxas and Meredith, 1983). Thus, single mode cavity has been intensively utilized to heat
treat food products having a limited volume and low dielectric properties due to the existence of
only one hot spot with well-defined electric field pattern (Boldor et al., 2008; Kybartas and
Ibenskis, 2011).
64
Single mode cavity resonators are most commonly built in rectangular and cylindrical
shape. The rectangular cavity resonator can be simply made from a rectangular waveguide by
shorting both ends with metal plate. However, the cylindrical cavity usually has higher quality
factor (Q) which implies lower microwave energy loss in a cavity resonator than rectangular
cavity (Chen et al., 2004). It is able to achieve highly localized heating zone at the central axis of
the cylindrical cavity resonator. Thus, the cylindrical single mode microwave cavity is suitable to
apply for continuous flow thermal processing with constant electric field strength.
In addition, the design of an ohmic heater with pulsed square waveforms at higher
frequencies and the use of chemically inert materials for electrodes have made this technique
more reliable and successful in commercial practices. However, when ohmic heating is applied
for the thermal processing of multiphase foods, the different heating rates of solid and liquid are
frequently observed because of the difference between electrical conductivities of particles and
liquid. The different heating rates can lead to under-processed particulates.
Therefore, the development of microwave and ohmic combination heater for the
processing of multiphase foods has been proposed because solid particle and liquid are
simultaneously heat treated via electric current and electromagnetic wave depending upon their
dielectric properties. The study in this section was aimed to design and fabricate dual cylindrical
microwave and ohmic combination heater for the processing of particulate foods.
3.2 Theoretical background
3.2.1 Electromagnetic field propagation in microwave heating system
The majority of microwave heating system are mainly composed of microwave power
source (magnetron which generates the electromagnetic radiation), the transmission line
65
(waveguides that deliver high electromagnetic power from power source to the cavity and can be
considered as conductor), and the cavity resonator (rectangular or cylindrical cavity resonator)
(Thostenson and Chou, 1999). The electric and magnetic field distributions in microwave heating
system can be numerically analyzed by Maxwell's equations (Geedipalli and others 2008).
Furthermore, the design parameters associated with the transmission line and the cavity resonator
may be easily derived from Maxwell‘s equations (Zhao et al., 2011):
∇ × 𝐸 = 𝑗𝜔𝜇�𝐻, ∇ × 𝐻 = jω𝜀�𝜇�𝐻, ∇ ∙ 𝐸 = 0, ∇ ∙ 𝐻 = 0 (1)
where E is the electric field (V/m), H is the magnetic field (A/m), ω is the angular velocity
(rad/s), μ0 is the free space permeability (1.25664×10-7 H/m) and ε0 is the dielectric constant of
free space ( 12108548 −×. F/m).
3.2.2 Determination of wave propagation in a waveguide
According to the particular geometry of microwave waveguide, only specific patterns of
electric and magnetic fields, which are also known as modes, can exist as propagating wave. The
propagation modes within waveguide can be determined by which field components are
dominant (Mekis et al., 1996). In design procedure of wave guide, the most important step is to
determine the wave propagation in waveguide. Wave propagation in wave guide can be
classified into three modes: transverse electric (TE), transverse magnetic (TM), and transverse
electromagnetic (TEM) modes. Under TE mode, the electric field components are transverse to
the direction of propagation (Ez = 0, no longitudinal electric field); while magnetic field has both
transverse and longitudinal components (Hz ≠ 0). On the other hand, TM mode has a completely
different characterization against TE mode (Ez = 0, Hz ≠ 0). The longitudinal components of
electric and magnetic fields cannot exist in TEM mode (Ez = 0, Hz = 0).
66
3.2.3 Heat generation for ohmic heating
The rate of ohmic heating can be determined by the square of the applied electric field
strength and electrical conductivity of food. In particular, the electrical conductivities of foods
are the linear function of temperature and can be estimated by the following equation (Sarang et
al., 2008);
𝜎 = ��∙ �� (2)
where V is the applied voltage (V), σ is the electrical conductivity (S/m), I is the current (A), L is
the distance between the electrodes (m), and A is the contact area (m2).
Temperature distribution inside food under ohmic heating is governed by;
𝜌𝐶�����
= ∇(k∇T) + 𝑈 (3)
where ρ is the density (kg/m3), Cp is the specific heat (J/kg K), t is the time (s), k is the thermal
conductivity (W/m K), T is the temperature (K) and U is the internal energy source. The internal
energy source of ohmic heating is given by:
𝑈 = 𝜎(𝑇)|𝛻𝑉|� (4)
In addition, the electric field distribution in an ohmic heating applicator can be defined by
Laplace's equation (De Alwis and Fryer, 1990);
∇[𝜎(𝑇)∇𝑉] = 0 (5)
3.3 Design parameters
3.3.1 Rectangular waveguide
Waveguide can be produced in various shape, most commonly rectangular or cylindrical
shape. The rectangular waveguide has been used for the microwave heating system because of
67
efficient microwave energy transmission from microwave power source to the cavity resonator
(Zhu et al., 2007; Rattanadecho et al., 2008; Cha-um et al., 2009). In addition, it was frequently
exploited to design high millimeter wave systems (Deslandes and Wu, 2001). Standard
rectangular waveguides have aspect ratio which is close to 2:1 (width : height) and the size of
waveguide is dependent on the frequency range; however, for specific purpose, the reduced-
height waveguides can be sometimes exploited (Cooper and Carter, 1991). Standard WR430 and
WR340 can be employed in microwave heating system powered by the magnetron at 2.45 GHz.
Their specific sizes and frequency ranges are listed in Table 3.1.
Table 3. 1 Standard waveguide dimensions
Waveguide designation
Width (mm)
Height (mm)
Frequency rage (GHz)
WR430 109.22 54.61 1.70 ~ 2.60 WR340 86.36 43.18 2.20 ~ 3.31
Figure 3. 1 Electric and magnetic field lines for TE and TM modes in rectangular waveguide
TE and TM modes in rectangular or cylindrical waveguide can be determined by cutoff
frequency (fc) which is a frequency below which all lower frequencies are attenuated in
waveguide. Field lines for TE and TM modes in rectangular waveguide are illustrated in Fig. 3.1.
Above cutoff frequency, microwave power can be transferred from the source to the cavity
68
without any attenuation. TE mode is dominantly applied in the rectangular waveguide and is
usually expressed as TEmn mode (m and n are numbers of half wavelength variations of fields in
the horizontal and longitudinal directions of the rectangular waveguide, respectively). The
dominant mode, which has the lowest cutoff frequency and lower order of m and n in the
waveguide, is TE10 mode (Pozar, 2005). Since all transverse field components (Ex, Ey, Hx ,and Hy)
in TEmn mode become zero (no wave propagation in the waveguide) when both m = n = 0
substitute the formula for electric (E) and magnetic (H) fields, TE00 mode in the waveguide does
not exist (Elshafiey, 2011). In order to estimate all transverse field components in TEmn mode,
the propagation constant (β) should be calculated by the following equation:
𝛽 = �𝑘� − ������− ���
��� , 𝑘 = 𝜔√𝜇𝜀 (6)
where k is the wave number of medium, μ is the permeability of the medium (H/m), ε is the
dielectric constant of medium (F/m), a and b are the width and height of rectangular waveguide,
respectively.
A cutoff frequency fc of TEmn mode can be given by:
𝑓� = ���√��
�������
+ ������ (7)
where c is a speed of light (2.998 × 108 m/s).
In TE10 mode, a cutoff frequency can be expressed as ���√��
or ���√��
(εr is the relative dielectric
constant of material filled in waveguide).
In the design of the distributed electric (E) and magnetic (H) field structures, the guide
wavelength (λg) can be used and defined as the distance between two equal phase planes along
the waveguide (Veronis and Fan, 2005). In addition, it is also a function of the lower cutoff
69
wavelength and has a longer wavelength than the wavelength (𝜆 = �� , ) in free space (Sisodia
and Gupta, 2005). The guide wavelength (λg) is determined through:
𝜆����� = ���
(8)
The ratio of the transverse components of the electric (E) and magnetic (H) can be
represented by the wave impedance that is the direct analog of a circuit impedance given by the
voltage to current ratio (Chan and Reader, 2000). The wave impedance for TE mode is estimated
by:
𝑍�� = �
������� ��
, 𝜂 = �������
(9)
where η is the intrinsic impedance of the medium (Ω), and f is the operating frequency (Hz).
3.3.2 Cylindrical cavity resonator
As similar with single mode rectangular cavity resonator, single mode cylindrical cavity
resonator can be built from a cylindrical waveguide by closing both ends with metal plate as
shown in Fig. 3.2. (Puschner and Atlas-Elektronik, 1967).
Figure 3. 2 A cylindrical cavity resonator In the cylindrical cavity resonator, the dominant TE mode is the TE111 (Pozar, 2005). By
integrating cylindrical cavity resonator and rectangular waveguide, the desirable field strength
70
distribution at specific locations inside the cavity can be achieved and it is able to adjust heating
direction (Sun et al., 2004).
The propagation constant (β) of the TEnm mode is can be derived from equation (5) and
the values of p'nm for the TEnm mode are shown in Table 3.2. Furthermore, m, n, and l refer to the
number of variations at the standing wave pattern in the x, y, and z directions inside the cavity
(Pozar, 2005).
𝛽 = �𝑘� − ��′����� (10)
where a is the radius of the cylindrical cavity resonator.
Table 3. 2 The values of pnm for the TEnm mode in cylindrical cavity resonator
n p'n1 p'n2 p'n3 0 3.832 7.016 10.174 1 1.841 5.311 8.536 2 3.054 6.706 9.970
Then, the cutoff frequency (fc) of TEnm mode is:
𝑓� = ��′�����√��
(11)
A resonant frequency is a key factor that determines the dimension of the cavity and
filling materials (Hardy and Whitehead, 1981). It can be calculated using following equation:
𝑓� = ���√��
���′�����
+ ������ (12)
where d is a height of the cylindrical cavity resonator.
71
3.3.3 Impedance matching
During the course of designing of a microwave heater, impedance matching is an
essential procedure to make sure that a maximum microwave power is transmitted and wave
reflection is minimized (Pozar, 2005). In addition, while electromagnetic waves travel down the
waveguide, generating the currents that may cause ohmic losses and gradual attenuation of
waves should be prevented by impedance matching (Chen and Reader, 2000). There are several
types of impedance matching such as matching with lumped elements or single, double, and
series stub matching. Series stubs matching method was applied in this study because it is
practical and convenient from the design aspect for microwave heating system (Berliner and
Bender, 2004). A stub is an open or short circuit transmission line connected either in parallel or
in series with transmission feed line which implies a waveguide at a certain distance from the
load that means the targeted food passing through the central axis of the cylindrical cavity
(Torungrueng and Thimaporn, 2005). In general, the distance between series stubs can be λg/3
and λg /6 or λg /4.
3.3.4 Impedance matching for microwave heating system
Figure 3.3 shows the calibration kit for the impedance matching for microwave heating
system. A microwave dummy probe connected to a vector network analyzer (VNA, N9923A,
Agilent Technologies, Santa Clara, CA) was used to measure the scattering parameter (S11, the
reflection coefficient). Before impedance matching, the probe was calibrated with open and short
circuits, and the matched load which has 50 Ω characteristic impedance (Z0). The S11 value close
to 50 Ω at the center of Smith chart, which stands for minimum wave reflection, was obtained by
tuning locations and lengths of 5 matching stub towers on the top surface of the custom-designed
waveguide.
72
Figure 3. 3 A calibration kit (open, short, and matched load) and vector network analyzer
3.4 Results and discussion
3.4.1 Microwave power launcher
The dimension of microwave power launcher can be determined by microwave power
source operating at certain frequency. US Federal Communication Commission (FCC) allocated
only two frequencies (915 MHz with wavelength of 0.328 m and 2450 MHz with wavelength of
0.122 m) for industrial, scientific, and medical (ISM) application (FCC, 1988). Therefore, the
magnetrons operated at 915 MHz and 2.45 GHz for the purpose of microwave heating have been
commonly used as microwave energy source. Furthermore, the magnetron at 2.45 GHz has been
intensively exploited for small and laboratory scale applications because it is suitable with
respect to energy efficiency and cost (Bosisio et al., 1974). The microwave power launcher
originated from 2.45 GHz can be fabricated using WR430 rectangular waveguide that have
cutoff frequency of 1.37 GHz at TE10 mode, width of 109.2 mm, and height of 54.6 mm. Even
though the standard size of WR430 waveguide is recommended for the simple design of
microwave power launcher operating at the frequency range between 1.7 and 2.6 GHz, the
modified WR430 waveguide usually having the reduced width can be utilized for maximum
73
microwave power transfer to the load (Richard et al., 2002). The modified WR430 waveguide,
which the dimension were 95.3 mm wide, 54.6 mm high, and 159 mm long, was designed for the
microwave power launcher as shown in Fig. 3.4. The width of the modified WR 430 waveguide
was also applied for the microwave heating device to increase microwave power (Amano et al.,
2010). The magnetron operating at 2.45 GHz was adopted for the microwave power source in
this study. According to the width and height of the launcher and a frequency of 2.45 GHz in
TE10 mode, the calculated propagation constant (β), cutoff frequency (fc10), and guide wavelength
(λg) from equations (2), (3), and (4) were 39.52, 1.57 GHz, and 0.159 m, respectively. The length
of the launcher was same with the calculated guide wavelength (λg); while the distance between
the output antenna of magnetron and the right end of the launcher was 148 mm that was the
guide wavelength (λg) of WR430 waveguide corresponding a frequency of 2.45 GHz in TE10
mode. The launcher was built with nickel-coated brass.
Figure 3. 4 The dimension of microwave power launcher: (a) top view, (b) side view
74
3.4.2 Cylindrical microwave cavity resonator
The cylindrical cavity resonator coupled with WR430 rectangular waveguide operating
on TE10 mode was designed to be operated on axis-symmetric TE111 mode (Fig. 3.5). The
calculated cutoff frequency (fc10) in WR430 rectangular waveguide was 1.372 GHz. In order to
match cutoff frequencies in the rectangular waveguide and cylindrical cavity, the radius of
cylindrical cavity was determined using Eq. (7). The diameter of the designed cylindrical cavity
was 127mm that was longer the wavelength of 2.40 GHz (λ=125mm) since the general
frequency resolution of the magnetron operating at 2.45 GHz is ± 0.05GHz. The distance
between the center of the cylindrical cavity and the end of waveguide was 125 mm as same as
the wavelength of 2.40 GHz; while approximately half of guide wavelength of 2.40 GHz (λg
=152 mm) was applied to the length of the rectangular waveguide (80 mm) for efficient coupling
the rectangular waveguide and the cylindrical cavity (Sreekanth, 2003). When the rectangular
waveguide (109.2 × 54.6 × 80 mm3, W × H × L) was considered as the rectangular cavity
resonator operating at 2.45 GHz in TE101 mode, the resonant frequency (fr) could be calculated
from the following equation;
𝑓� = ���√����
�������
+ ������
+ ������ (8)
where d is the length of the rectangular cavity resonator.
For estimating the height of the cylindrical cavity resonator, the determined resonant frequency
(fr = 2.32 GHz) from Eq. (8) was substituted to Eq. (7). The calculated height (82 mm) was
exploited for the design of the cylindrical cavity resonator. The cylindrical cavity resonator was
made from stainless steel.
75
Figure 3. 5 The dimension of cylindrical microwave cavity resonator: (a) top view, (b) side view
3.4.3 Rectangular waveguide
When it is necessary to couple two rectangular waveguides having different cross
sections, a gradual waveguide taper can be used to minimize microwave power loss from the
reflected wave (Cooper and Carter, 1991). In order to deliver high performance at high power
levels, the length of taper should be at least two times longer than the guide wavelength (λg) of
input rectangular waveguide (Fig. 3.6).
Figure 3. 6 Schematic diagram of taper connection between two different waveguides
However, when there is not an enough space to place the taper for the connection of two
waveguides, it is technically feasible to directly couple the cross sections of input and output
waveguides by maintaining the length of input waveguide to 40mm which is almost 1/4
76
wavelength of 2.45 GHz (Kimura et al., 2009). This coupling method could be applied to
microwave power system that requires high energy transfer efficiency from microwave energy to
thermal energy (Cull and Carnahan, 1988). In addition, microwave power could be sequentially
transmitted via a transition from WR430 to WR 284 waveguides (72.1mm in width and 34.0mm
in height) when 2.45 GHz magnetron was adopted for microwave power supply (Atwater et al.,
1997; Wheeler et al., 2002). Although the usable frequency range of WR284 waveguide is
between 2.60 and 3.95 GHz, this waveguide is frequently implemented for 2.45 GHz operation at
average power levels by 6 kW (Kaur et al). In this study, WR284 rectangular waveguide having
length of 296mm (2λg of WR430 waveguide) and built with nickel-coated brass was used for
transmitting high microwave power to the narrow cylindrical cavity as shown in Fig. 3.7.
Figure 3. 7 The dimension of rectangular waveguide: (a) top view, (b) side view
3.4.4 Impedance matching
The complex impedances between the load and the microwave power source should be
matched to minimize the reflected wave and to protect the magnetron. In microwave
communication system, the distance between stubs can be determined depending on the guide
wavelength (λg) as aforementioned in the section; while the positions of stubs on microwave
heating unit are often decided by trial and error, experience (Chan and Reader, 2000). A common
method for the impedance matching is to insert metallic rods or screws into the waveguide with
77
specific depth at certain locations; however, the misarrangement of stubs on the waveguide
disables the entire microwave heating system. Therefore, prior to fabrication of the developed
microwave heating unit, both positions and inserting depths of stubs on the designed power
launcher, rectangular waveguide, and cylindrical cavity resonator were tuned in the
computational simulation using COMSOL Multiphysics software (COMSOL 3.5, COMSOL,
Inc., Palo Alto, CA) in order to save time and laborious work for impedance matching.
Figure 3. 8 Electric field distribution in the microwave heating chamber after the adjustment of
both position and inserting depth of stubs
After tuning both positions and depths of stubs in the simulation, the electric field
distribution in the microwave heating cavity was also simulated as shown in Fig. 3.8.
Electromagnetic field generated from a magnetron was determined using the electromagnetic
waves module with generalized minimal residual methods (GMRES). The electric field strength
simulated in the microwave heating cavity ranged between 2.245 and 5.618 kV/m. The cross-
sectional view of the chamber obviously indicated that the electric field was intensively focused
at the center and the maximum microwave power density was found at the areas adjacent to inlet
and outlet of applicator. Based on the result from the simulation, the positions and inserting
78
depths of stainless steel rods (10mm in diameter, 120mm in height) were applied for fabrication
of the microwave heater (Fig. 3.9).
Figure 3. 9 The positions and inserting depths of stubs on microwave heating unit:
(a) top view, (b) side view
Table 3. 3 Inserting depths of stubs on microwave heating units
Microwave heating units
#1 #2
A 65.00 65.50
B 60.90 61.30
C 67.30 66.60
D 65.15 65.10
E 68.55 68.40
79
The reflection coefficients (S11 values) for the first and second microwave chambers (Fig.
8 (a) and (b)) were measured by filling water in the PTFE tube and recorded while the inserting
depths of 5 matching stubs were adjusted as listed in Table 3.3. During the stub matching process,
the marker indicating S11 value moved close at the center of Smith chart where indicates
minimum wave reflection (Fig. 3.10). Return loss (R.L = 20 log S11 dB), which is a loss of power
delivery from the microwave generator to a load, was calculated using obtained S11 value (Pozar,
2005). The values of S11 and return loss (dB) for first and second chambers after impedance
matching were 0.12/-18.463 dB and 0.045/-27.002 dB, respectively. The results clearly implied
that the magnetrons would be rarely damaged by wave reflection and the maximum microwave
power could be transmitted to the working chambers.
Figure 3. 10 Impedance matching of (a) Chamber 1 and (b) Chamber 2: Smith charts to show
the reflection coefficient S11, which means the characteristic impedance (Z0) is 50 Ω
3.4.5 Ohmic heating applicator
The ohmic and microwave combination applicators were fabricated using two
polytetrafuoreoethylene tubes (Fig. 3.11 (a), 190 mm long, 25.4/38.1mm inner/outer diameter,
Virgin electrical grade Teflon@ PTFE, Santa Fe Springs, CA). Since the PTFE has quite low
dielectric constant (approximately 2.1) and is considered as the transparent material to
microwave such as glass, microwave can penetrate PTFE tube with negligible dielectric heating.
80
Two titanium tubular electrodes (Fig. 3.11 (b), 25 mm long, 25.4/31.8 mm inner/outer diameter,
Tico Titanium Inc., Wixom, MI) were placed at the both ends of PTFE tube. Titanium electrodes
have shown strong resistance to acidic condition and low corrosion rate. In addition, electrolytic
reactions i.e., gas production and dissolution are rarely occurred on the surface of titanium
electrode (Lima et al., 1999). The ohmic power supply based on an integrated gate bipolar
transistor (IGBT, SKYPERTM, SEMIKRON Inc., Hudson, NH) was exploited to generate
alternating current with pulsed square waveform, the maximum frequency of 20 kHz, on/off duty
cycle of 0.2, and a max current of 100 Amps. The pulsed square waveform with high frequency
was known to minimize the undesired electrochemical reactions to occur at the interfaces
between the electrodes and food samples because of no formation of electrons on the electrical
doubly layers. The gap of 190 mm between the electrodes was the required minimum distance to
prevent dielectric heating on the surface of the electrode by microwave.
Figure 3. 11 The dimension of (a) microwave and ohmic combination applicator, and
(b) titanium electrodes
81
The electric field distribution in the ohmic applicator was also simulated using COMSOL
Multiphysics software (COMSOL 3.5, COMSOL, Inc., Palo Alto, CA). A conductive media DC
module with parallel direct sparse solver (PARDISO) was used to simulate the electric field
distribution in the applicator. When COMSOL simulation for electric field distribution in the
ohmic applicator used 200 V as an applied root mean square (RMS) voltage, the estimated
electric field strength was from 675.4 to 3650.2 V/m (Fig. 3.12). Although quite uniform electric
field distribution was observed between ring shaped electrodes, the electric field strength in the
areas adjacent top and bottom of solid particulates was relatively low, probably due to particles'
field disruption (Shim et al., 2010). The electrical resistance (Ω) in those particular areas was
relatively lower than solid particles and the majority of electrical current passed through high
conductive solution (Sastry and Palaniappan, 1992; Salengke and Sastry, 2007; Shim et al.,
2010). Furthermore, the localized field overshoot (approximately 3.6 kV/m) occurred at both
side edges of the electrodes. This phenomenon can result in numerous problems such as the
corrosion on the edges of electrodes in batch system; however, the overall performance of the
heater in a continuous flow mode was not significantly influenced by the localized field
overshoots (Nguyen et al., 2013).
Figure 3. 12 Electric field distribution in the ohmic heating applicator
82
3.4.6 Overall design of microwave and ohmic combination heater
Dual cylindrical microwave heating chambers were designed and fabricated to
concentrate and resonate the electric field strengths from ohmic and microwave power sources
(Fig. 3.13 (a)). The chamber had two ports connected to waveguides that were built with nickel-
coated brass and 5 towers for stub matching of microwave. Two magnetrons (2450 MHz; Model
OM75S, Samsung) which could deliver up to 900 W each were mounted on the end of each
waveguide (Fig. 3.13 (b) and (c)). One polytetrafluoroethylene ohmic tube installed through the
cavity was transparent to microwave, hence yielding negligible dielectric heating. Two titanium
ring-shaped electrodes were placed at both ends of the ohmic heating tube (Fig. 3.13 (d)). The
electrodes were connected to a power supply based on an integrated-gate-bipolar-transistor.
Figure 3. 13 A schematic diagram of the microwave and ohmic combination heater: (a) A front
view, (b) A side view, (c) A 3D schematic, and (d) A cross-sectional view of the chamber
Matching
stub Microwave
cavity
Waveguide
Magnetron
Electrode
PTFE
Adapter
(c) (d)
(a) (b)
83
3.5 Conclusion
Single mode cylindrical microwave cavity for the simultaneous microwave and ohmic
combination heater was successfully designed and fabricated to achieve the maximum
microwave power density at the central axis. All components consisted in the designed
microwave heating unit were suitable to transmit maximum microwave power to the load. The
simulated electric field distribution in the cavity indicated that the single mode microwave
heating would be able to constantly generate highly localized heating zone in the cavity. In
addition, S11 values (reflection coefficient) obtained from the impedance matching could imply
the minimum power loss and wave reflection. The results from this study showed that the
concentrated microwave power at the centerline under single frequency microwave method
would be beneficial for thermal treatment of multiphase foods without an attenuation of
microwave power.
84
3.6 References
Amano, H., Furuya, S., Kuga, M., & Ogura, T. (2010). U.S. Patent Application 13/138,328.
Anantheswaran, R. C., and Liu, L. (1994). Effect of viscosity and salt concentration on
microwave heating of model non-Newtonian liquid foods in a cylindrical container. Journal of
Microwave Power and Electromagnetic Energy, 29, 119-126.
Asmussen, J., Lin, H. H., Manring, B., and Fritz, R. (1987). Single‐mode or controlled
multimode microwave cavity applicators for precision materials processing. Review of Scientific
Instruments, 58, 1477-1486.
Atwater, J. E., Dahl, R. W., Garmon, F. C., Lunsford, T. D., Michalek, W. F., Wheeler, R. R.,
and Sauer, R. L. (1997). Miniature microwave powered steam sterilization chamber. Review of
Scientific Instruments, 68, 3924-3925.
Barlow, S., and Marder, S. R. (2003). Single-mode microwavw synthesis in organic materials
chemistry. Advanced Functional Materials, 13, 517-518.
Berliner, L., & Bender, C. J. (Eds.). (2004). EPR: Instrumental Methods: Instrumental Methods
(No. 21). Springer.
Boldor, D., Balasubramanian, S., Purohit, S., and Rusch, K. A. (2008). Design and
implementation of a continuous microwave heating system for ballast water treatment.
Environmental Science and Technology, 42, 4121-4127.
Bosisio, R. G., Spooner, J., and Granger, J. (1974). Asphalt road maintenance with a mobile
microwave power unit. Journal of Microwave Power, 9(4), 381-386.
Bradshaw, S. M., Wyk, E. J. v., and Swardt, J. B. d. (1998). Microwave heating principles and
the application to the regeneration of granular activated carbon. The Journal of The South
African Institute of Mining and Metallurgy.
85
Chan, T. V. C. T., and Reader, H. C. (2000). Understanding microwave heating cavities. Artech
House Publishers.
Cha-um, W., Rattanadecho, P., and Pakdee, W. (2009). Experimental analysis of microwave
heating of dielectric materials using a rectangular wave guide (MODE: TE10) (Case study:
Water layer and saturated porous medium). Experimental Thermal and Fluid Science, 33, 472-
481.
Chen, L. F., Ong, C. K., Neo, C. P., Varadan, V. V., and Varadan, V. K. (2004). Microwave
electronics: measurement and materials characterization. John Wiley & Sons.
Cooper, R. K., and Carter, R. G. (1991). High power RF transmission. Proceedings of the CERN
Accelerator School: RF Engineering for Particle Accelerators, Oxford, UK, 92-03.
Cull, K. B., and Carnahan, J. W. (1988). Design considerations and preliminary characterizations
of a kilowatt-plus microwave-induced plasma. Applied Spectroscopy, 42, 1061-1065.
Das, S., Mukhopadhyay, A. K., Datta, S., and Basu, D. (2008). Prospects of microwave
processing: An overview. Bulletin of Materials Science, 31, 943-956.
De Alwis, A. A. P., and Fryer, P. J. (1990). A finite-element analysis of heat generation and
transfer during ohmic heating of food. Chemical Engineering Science, 45(6), 1547-1559.
Deslandes, D., and Wu, K. (2001). Integrated microstrip and rectangular waveguide in planar
form. IEEE Microwave and Wireless Components Letters, 11, 68-70.
Elshafiey, T. M. F. (2011, 25-28 Sept. 2011). Simple novel qualitative approach for
electromagnetic wave analysis of rectangular waveguides without usage of Maxwell's equations.
Paper presented at the IEEE Symposium on Wireless Technology and Applications (ISWTA).
86
Geedipalli, S., Datta, A. K., and Rakesh, V. (2008). Heat transfer in a combination microwave-
jet impingement oven. Food and Bioproducts Processing, 86, 53-63.
Hardy, W. N., and Whitehead, L. A. (1981). Splitring resonator for use in magnetic resonance
from 200-2000 MHz. Review of Scientific Instruments, 52, 213-216.
Kaur, J., Saini, M., Singh, R., & Singh, K. Performance Improvements in Microwave Wave
Guide.
Kimura, Y., Kawaguchi, H., Kagami, S., Furukawa, M., and Shindo, H. (2009). A New Method
of Line Plasma Production by Microwave in a Narrowed Rectangular Waveguide. Applied
Physics Express, 2(12), 126002.
Kybartas, D., Ibenskis, E., and Surna, R. (2011). Single mode circular waveguide applicator for
microwave heating of oblong objects in food research. Electronics and Electrical Engineering,
114, 79-82.
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 and
Preservation, 23, 421-443.
Mekis, A., Chen, J. C., Kurland, I., Fan, S., Villeneuve, P. R., and Joannopoulos, J. D. (1996).
High transmission through sharp bends in photonic crystal waveguides. Physical Review Letters,
77, 3787-3790.
Metaxas, A. A., & Meredith, R. J. (1983). Industrial microwave heating (No. 4). IET.
Nguyen, L. T., Choi, W., Lee, S. H., and Jun, S. (2013). Exploring the heating patterns of
multiphase foods in a continuous flow, simultaneous microwave and ohmic combination heater.
Journal of Food Engineering, 116(1), 65-71.
87
Pozar, D. M. (2005). Microwave engineering, 3rd. Danvers, MA: Wiley.
Püschner, H., and Atlas-Elektronik, F. K. (1967). Microwave Heating Technique in Europa.
Journal of Microwave Power, 2(2), 31-44.
Rattanadecho, P., Suwannapum, N., Chatveera, B., Atong, D., and Makul, N. (2008).
Development of compressive strength of cement paste under accelerated curing by using a
continuous microwave thermal processor. Materials Science and Engineering: A, 472, 299-307.
Richard R. Wheeler, James E. Atwater, Akse, J. R., Holtsnider, J. T., and Luna, B. (2002).
Development and testing of a microwave powered regenerable air purification technology
demonstrator. SAE Technical Paper.
Salengke, S., and Sastry, S. K. (2007). Experimental investigation of ohmic heating of solid–
liquid mixtures under worst-case heating scenarios. Journal of food engineering, 83(3), 324-336.
Sarang, S., Sastry, S. K., and Knipe, L. (2008). Electrical conductivity of fruits and meats during
ohmic heating. Journal of Food Engineering, 87, 351-356.
Sastry, S. K., and Palaniappan, S. (1992). Mathematical modeling and experimental studies on
ohmic heating of liquid-particle mixtures in a static heater1. Journal of Food Process Engineering,
15(4), 241-261.
Shim, J., Lee, S. H., and Jun, S. (2010). Modeling of ohmic heating patterns of multiphase food
products using computational fluid dynamics codes. Journal of food engineering, 99(2), 136-141.
Sisodia, M. L., and Gupta, V. L. (2005). Microwave Engineering:(as Per UPTU Syllabus). New
Age International.
Sreekanth, P. V. (2003). Digital Microwave Communication Systems: With Selected Topics in
Mobile Communications. Universities Press.
88
Sun, R., Kempel, L. C., Zhou, S., Zong, L., and Hawley, M. C. (2004). Electromagnetic
modeling of an adaptable multimode microwave applicator for polymer processing. 20th Annual
Review of Progress in Applied Computational Electromagnetics.
Tang, X. W., Jiao, S. J., Gao, Z. Y., and Xu, X. L. (2008). Study of 5.8 GHz magnetron in
microwave deicing. Journal of Electromagnetic Waves and Applications, 22, 1351-1360.
Thostenson, E. T., and Chou, T. W. (1999). Microwave processing: fundamentals and
applications. Composites Part A: Applied Science and Manufacturing, 30, 1055-1071.
Torungrueng, D., and Thimaporn, C. (2004). A generalized ZY Smith chart for solving
nonreciprocal uniform transmission‐line problems. Microwave and Optical Technology Letters,
40(1), 57-61.
Tran, V. N. (1992). An applicator design for processing large quantities of dielectric material.
DEAKIN UNIV VICTORIA (AUSTRALIA).
Veronis, G., and Fan, S. (2005). Bends and splitters in metal-dielectric-metal subwavelength
plasmonic waveguides. Applied Physics Letters, 87.
Wheeler Jr, R. R., Atwater, J. E., Akse, J. R., Holtsnider, J. T., and Luna, B. (2002).
Development and testing of a microwave powered regenerable air purification technology
demonstrator. Development, 1, 2403.
Whittles, D. N., Kingman, S. W., and Reddish, D. J. (2003). Application of numerical modelling
for prediction of the influence of power density on microwave-assisted breakage. International
Journal of Mineral Processing, 68, 71-91.
Zhao, X., Yan, L., and Huang, K. (2011). Review of numerical simulation of microwave heating
process. Advances in Induction and Microwave Heating of Mineral and Organic Materials, 27-48.
89
Zhu, J., Kuznetsov, A. V., and Sandeep, K. P. (2007). Mathematical modeling of continuous
flow microwave heating of liquids (effects of dielectric properties and design parameters).
International Journal of Thermal Sciences, 46, 328-341.
90
Chapter 4
Minimization of thermal lags in the simultaneous microwave and ohmic combination
heating of particulate foods
4.1 Introduction
Thermal processing of multiphase foods is often challenging due to non-uniform heating,
which can give rise to food safety risk as well as losses of nutritional and sensory qualities.
Conventional cooking approach for particulate foods requires relatively long processing time,
high energy consumption, and several procedures to achieve even temperature distribution in
particulate foods because it substantially depends upon convection and conduction for heat
transfer from heating source to particle foods (Mullin, 1995; Nguyen et al., 2013). One of the
feasible solutions to minimize the temperature variation between solid and liquid phases with
rapidity and quality maintenance is an application of advanced volumetric heating methods such
as microwave and ohmic heating.
However, several studies for the thermal behaviors of solid-liquid food mixtures under
ohmic heating showed that the heating rate of solid particles lagged behind liquid having a
higher electrical conductivity, and arbitrary temperature distribution in multiphase foods were
often reported (Sastry and Palaniappan, 1992; Salengke and Sastry, 2007; Sarang et al., 2008;
Shim et al., 2010).
The temperature distribution patterns of different shaped food samples treated by
microwave heating were observed (Ramaswamy et al., 1991; Vilaryannur et al., 1998). In
cylindrical shaped meat sample, hot spots were along the central axis of the cylinder and the
lowest temperature was at the region between the center and the surface during microwave
91
heating; while for slab shaped meat samples, cold spot was located near the geometric center and
hot spots occurred along the corners (Ramaswamy et al., 1991). Furthermore, when the
cylindrical shaped potato was thermally treated by microwaves, the highest temperature values
were measured along central axis and the surface (Vilaryannur et al., 1998). The results from
previous studies accounted for that a higher concentration of microwave energy at the center of
cylindrical shaped food samples caused the pronounced core heating effects and non-uniform
temperature distribution even in single phase food.
A dual cylindrical microwave and ohmic combination heater was designed and fabricated
to deliver maximum microwave energy to particulate foods without leaving solids under-
processed. The simultaneous microwave and ohmic combination heating technique proposes that
food particles are heated by microwave independent of their electrical conductivity and liquid
phase is heated via electric current, which will eventually eliminate the drawbacks of individual
technologies and enhance heating uniformity. The objectives of the study in this were to explore
the heating patterns of multiphase liquid-particle foods using the designed combination heater,
and to optimize the effect of the combination heating by tuning operation parameters such as
voltage and power levels.
4.2 Materials and methods
4.2.1 Particle and liquid mixtures preparation
The particle sizes and the concentrations of sodium chloride and carboxymethyl cellulose
(CMC) solutions in particle-liquid mixtures were imitated to be similar to commercial canned
chunky soup products. Prior to preparing solid-liquid mixture foods as model foods, particle
sizes were determined to be similar to those in commercial soups on the market. For instance, the
92
lengths of particulate cubes in commercial chunky soup products (such as Campbell's soup-
Chunky chicken vegetable) ranged from 0.5 cm to 1.2 cm. The averaged lengths of small and
large particles in those were approximately 0.5 cm and 1 cm. Therefore, these specific sizes were
used to prepare solid particles. Chicken breast and potato purchased from a local market were
used as chunky foods and cut into two fixed size cubes (0.5 and 1 cm in length). A base solution
was prepared with 1.5% (W/V) CMC (Pre-Hydrated CMC 6000, TICGUM, White Marsh, MD)
solution and NaCl solution (Morton Salt, Chicago, IL) with different concentrations (0.5, 1.25,
and 2%, W/V), and then evenly mixed with particle cubes. Base solutions at three different salt
concentrations were exploited to test the capability of the combination heating technology on
minimization of temperature gap between solid particles and solution. In addition, the solution
with 1.5% (W/V) CMC concentration was suitable for the sine pump (MR 125, Waston-Marlow,
Wilmington, MA) (Nguyen et al., 2013). Solid mass fractions in total volume of the mixtures
were fixed at 10 and 15% (W/V). The calculated volume of the microwave and ohmic heating
applicator (Teflon tube) was 86.1 mL. If the particulates passed through the Teflon tube in an
alignment, approximately 3.8 and 6.6 pieces of chicken breast and potato cubes cut into 1 cm,
respectively, at 10% mass fraction could be simultaneously treated by microwave and ohmic
heating. The 10% mass fraction was good enough to investigate heating uniformity of the
combination heating technology in the environment that a particle of low electrical conductivity
was surrounded by a high conductive solution. In the case of 15% mass fraction, approximately
5.8 and 9.9 pieces of chicken breast and potato cubes cut into 1 cm could be heated in the
applicator. However, further increase in the percentage of particulate foods (Mass fraction, Mf)
over 15% frequently caused pipe clogging due to limited spacing of Teflon tube. Therefore, the
93
maximum percentage of particulate foods, which was empirically determined in this
investigation, was 15%.
4.2.2 The measurement of electrical conductivity of food samples
As aforementioned in section 3.2.3, the electrical conductivity of food, which can be
determined by Eq. (1), significantly affects the ohmic heating rate (Sarang et al., 2008);
𝜎 = ��∙ �� (1)
where V is the voltage (V), σ is the electrical conductivity (S/m), I is the current (A), L is the
distance between the electrodes (m), and A is the contact area (m2).
For the measurement of electrical conductivities of samples, solid food samples (potato and
chicken breast) were cut into cylindrical shape of 25.4mm in diameter and 10mm in height. Each
sample was placed in PVC cylinder-shaped ohmic cell (25.4mm in diameter and 70mm in height)
and was sandwiched between two electrodes. A pair of clamps was used to maintain tight contact
between the sample and the electrodes and then a thermocouple (K-type KK-K-30, Omega
Engineering Inc., Stamford, CT) was inserted at the center of sample via a small hole in the
chamber surface. The electrical conductivities of base solutions with different salt concentrations
were measured using the same procedure. A RMS voltage of 30V and frequency of 10 kHz, and
duty cycle of 0.2 were applied for the measurement. All data points of temperature, voltage, and
current were recorded and monitored by a data logger (Agilent 34970A, Agilent Technologies,
Santa Clara, CA) in a real time.
4.2.2 Experimental protocol
Figure 4.1 shows a schematic of the experimental setup. The combination heater was
composed of a feeder tank containing solid-liquid mixtures, a sine pump, a power supply for
94
ohmic heating, cylindrical microwave chambers including microwave power source, and a data
acquisition (DAQ). After mixing solid food cubes with CMC base solution, the mixtures were
kept at room temperature for 30 min to equilibrate temperatures of solid and liquid phases.
Samples in the feeder tank were pumped through the combination heater using a sine pump in
which solid particles could move without any physical damages. Depending upon salt
concentration in the base solution, different power inputs for individual heating mode (MW and
OH) and combination heating mode (MW & OH) were determined by preliminary trials so that
the exit temperature of either solid or liquid phase reached approximately 80 °C (Table. 4.1);
however, the flow rate and the pump speed were fixed at 300 ±8 ml/min and 85 rpm,
respectively. The temperature values of solid and liquid phases were measured before and after
different heating methods using thermocouple sensors (K-type, KK-K-30, Omega Engineering
Inc., Stamford, CT). A total thermal treatment time from the inlet of the system to the outlet was
50.7±1.35 sec. Transient values of voltage, current, and temperature were monitored and
recorded using the data logger (Agilent 39704A, Agilent Technologies, Inc., Santa Clara, CA).
All experiments were conducted in a duplicate.
95
Figure 4. 1 A schematic diagram of the experimental set-up
Table 4. 1 Applied microwave power and voltages for microwave (MW), ohmic (OH), and
combination heating modes (MW & OH) depending on NaCl concentration.
Analysis of variance (ANOVA) and T-test (P <0.05) analyses were performed to
determine the temperature differences between potato, chicken, and CMC solution, depending on
staple variables such as the heating method (OH, MW, and MW & OH), solid particle size (Sp,
cm), particle mass fraction (Fm, %, W/V) and salt concentration (Cs, %, W/V) using the SPSS
software (V. 11, SPSS Inc., Chicago, IL).
NaCl 0.5% 1.25% 2.0% MW 1000 W 800 W 700 W OH 320 V 230 V 180 V
MW & OH
800 W + 240 V
700 W + 185 V
600 W + 140 V
96
4.3 Results and discussion
4.3.1 Electrical conductivities of particle and solution samples
Figure 4. 2 indicates the relationship between electrical conductivities of both solid and
liquid phases and temperature values. It was clearly observed that the electrical conductivities of
solid particles (potato and chicken breast) were much lower than those of base solutions with
different salt concentration. The electrical conductivities of the base solutions with different
NaCl concentrations increased linearly with temperature increases. As temperature increased
from 20 °C to 80 °C, the electrical conductivities of 0.5%, 1.25%, and 2.0% increased from
1.060, 2.350, and 3.596 S/m to 2.138, 4.711, and 7.209 S/m, respectively. It was evidenced that
an increase of salt concentration in base solution significantly affected the electrical
conductivities of base solutions.
The electrical conductivity of potato showed biphasic linearity with temperature values. It
is known that potato is rich in starch. The electrical conductivity of potato substantially increased
after 325 K (52°C). It was reported that the significant changes in the electrical conductivity of
potato treated by ohmic heating occurred in the temperature range between 40 and 50 °C or 75
and 80 °C (Haden et al., 1990). The biphasic correlation found in the figure could be attributed to
starch gelatinization, leading the changes in thermal properties of samples such as heat capacity
and thermal conductivity. The biphasic correlation observed during the measurement could be
attributed to starch gelatinization, eventually leading the changes in thermal properties of potato
such as heat capacity and thermal conductivity. However, the electrical conductivity of chicken
breast constantly increased without any considerable change.
97
Figure 4. 2 Electrical conductivities of potato, chicken breast, and the base solutions with
different salt concentration
4.3.2 Heating patterns of particle-liquid mixtures under different heating methods
Temperature variations between solid particles and liquid in the mixtures with three
different salt concentrations are shown in Figs. 4.2, 4.3, and 4.4.
Ohmic heating: When solid-liquid mixtures were treated by ohmic heating alone, the
magnitudes of temperature differences (ΔTps = Tpotato - Tsolution, ΔTcs = Tchicken - Tsolution) for the
mixtures with the same particle size (Sp) and mass fraction (Mf) increased with an increase in the
salt concentrations (Cs) of as shown in Figs. 4.2 and 4.3. However, there were little significant
temperature variations (ΔTps and ΔTcs) when Cs ranged between 0.5% and 1.25%, and between
1.25% and 2.0% (P > 0.05). An increase of Mf with the same values of Sp and Cs did not make a
significant impact on the magnitudes of ΔTps and ΔTcs (P > 0.05). Furthermore, the temperature
variation between solid particles (ΔTcp = Tchicken - Tpotato) at the fixed values of Sp and Mf did not
substantially change even with an increase in the value of Cs (P > 0.05). It might be because
98
electrical conductivities of chicken and potato were similar at the temperature range from 40 to
100°C (Ye et al., 2004, Sarang et al., 2008; Shim et al., 2010).
On the other hand, the large particle size (Sf = 1cm) led to the change in the magnitude of ΔTps,
ΔTcs, and ΔTcp at low and medium Cs (0.5 and 1.25%). In particular, the values of ΔTps and ΔTcp
at Mf of 10% was significantly affected by Sf (P < 0.05, Figs. 4.2 (a) and 4.3 (a)). It was noted
that larger particle size could develop more heat transfer resistance between liquid and solid
particles, eventually resulting in uneven temperature distribution in multiphase foods (Chen et al.,
2010). A number of studies reported that the ohmic heating pattern of multiphase foods could
vary due to several factors i.e., electrical conductivities of solid and liquid phases, the size, shape,
and constituents of solid particle, and the applied electric field strength (Haden et al., 1990;
Sastry, 1992; Sastry and Palaniappan, 1992; Zareifard et al., 2003; Salengke and Sastry, 2007;
Shim et al., 2010). During ohmic heating, in general, temperature of liquid phase was higher than
solid particles. The maximum temperature variation found between solid and liquid phases was
11.87±2.91°C (ΔTps, Cs = 2%, Sp = 1 cm, Mf = 10%).
Microwave heating: For microwave heating, the magnitude of ΔTps, when Mf of 10% and
identical values of Sp were used, did not substantially change as Cs increased from 0.5% to 1.25%
(P > 0.05, Figs. 4.2 (a) and 4.3 (a)). However, as shown in Figs.4.2 (b) and 4.3 (b), the value of
ΔTps at Mf of 15% became smaller by increasing Cs up to 1.25% and especially the difference in
the values of ΔTps was found in Fig. 4.2 (b) (P < 0.05). In addition, the magnitude of ΔTps at Mf
of 15% decreased with an increase in the value of Sp; however, no statistical difference was
found at Cs of 0.5% (P < 0.05). An increase in Cs by 1.25% did not substantially affect to the
change in the magnitude of ΔTcs at Mf of 10% (P > 0.05, Figs. 4.2 (a) and 4.3 (a)). The value of
ΔTcs at Sp of 0.5 cm and Mf of 15% decreased with an increase in Cs by 1.25% (P < 0.05, Figs.
99
4.2 (b) and 4.3 (b)). A relatively small temperature variation between solid particles (ΔTcs) was
obtained (P > 0.05).
The effect of the increase in Sp on the magnitude of temperature differences between solid
particles and solution was observed at ΔTps with the values of Mf of 15% and Cs of 0.5%. The
experimental data reported by Kent and Kreess-Rogers (1986) also described that particle size
did not substantially affect microwave heating rate as compared to other factors, i.e. density and
composition of solid particle. However, higher Mf under identical conditions of Sp and CS slightly
affected temperature variations between solid particles and liquid. The effect of Mf on those was
observed at ΔTps at Sp of 1 cm and ΔTcs at Sp of 0.5 cm (P < 0.05, Fig. 4.3). It was obvious that
potato and chicken particles thermally led to the base solution at low and medium Cs (0.5% and
1.25%) and a maximum thermal lead was 7.11±1.74°C (ΔTps, Cs= 0.5%, Sp = 0.5 cm, Mf = 15%,
Figs. 4.2 and 4.3); however, the temperature of solution was higher than those of solid particles
at high Cs (2.0%). It might be that the limited microwave power delivered to the solid particle
surrounded by the solution having high electrical conductivity or dielectric properties. This
finding was in an agreement with the study of Koskimiemi and others (2011), reporting that an
increase of salt concentration in liquid phase attenuated microwave penetration into unblanched
vegetable cubes and decreased average temperatures of solid particles.
Combination heating: When microwave and ohmic heating simultaneously treated the solid-
liquid mixture, the magnitudes of ΔTps, ΔTcs, and ΔTcp at low and medium Cs (0.5 and 1.25%)
were small and there were not considerable temperature differences between chicken, potato, and
solution (less than 3.1°C), irrespective of Sp and Mf (P > 0.05). The experimental data could be
used to support the aforementioned hypothesis. For the combination heating, even temperature
distribution of both solid and liquid phases was obtained because microwaves with strong
100
penetration depth heated up solid particles having low electrical conductivities rather than carrier
medium and ohmic current could treat liquid phase with higher salt concentration (Figs. 4.2 and
4.3). However, the temperature variation between solid particles and liquid dramatically
increased when combination heating was applied to heat up particulate solution at high Cs (2%)
as compared with ΔTps and ΔTcs at low and medium Cs (P < 0.05, Fig. 4.4). Although the
combination heating mode could obtain the temperature similarity between solid and liquid
phases as Cs increased by 1.25%, its effectiveness could vary depending upon multiphase foods
of which electrical conductivities and dielectric properties are significantly different. However,
such limitation can be resolved by designing the protocol recipes to manipulate the mixing ratios
of individual microwave and ohmic heating resources at first and second stage heaters. The
optimized hybrid power heating modulation will lead to uniform heating of food mixtures with
various electrical conductivities and dielectric properties.
101
Figure 4. 3 Heating patterns of particle-liquid mixtures (0.5% salt concentration) under
microwave, ohmic, and combination heating. (a) 10% mass fraction, and (b) 15 % mass fraction
(a)
Δ Δ Δ
NOTE: Error bars indicate the 95% confidence intervals (CI) based on
the average of temperature difference between particulates and solution
ΔTps = Tpotato - Tsolution, ΔTcs = Tchicken - Tsolution, ΔTcp = Tchicken - Tpotato
Δ Δ Δ
(b)
102
Figure 4. 4 Heating patterns of particle-liquid mixtures (1.25% salt concentration) under
microwave, ohmic, and combination heating. (a) 10% mass fraction, and (b) 15% mass fraction.
(a)
Δ Δ Δ
NOTE: Error bars indicate the 95% confidence intervals (CI) based on
the average of temperature difference between particulates and solution
ΔTps = Tpotato - Tsolution, ΔTcs = Tchicken - Tsolution, ΔTcp = Tchicken - Tpotato
Δ Δ Δ
(b)
103
Figure 4. 5 Heating patterns of particle-liquid mixtures (2.0% salt concentration) under
microwave, ohmic, and combination heating. (a) 10% mass fraction, and (b) 15% mass fraction
(a)
Δ Δ Δ
NOTE: Error bars indicate the 95% confidence intervals (CI) based on
the average of temperature difference between particulates and solution
ΔTps = Tpotato - Tsolution, ΔTcs = Tchicken - Tsolution, ΔTcp = Tchicken - Tpotato
Δ Δ Δ
(b)
104
The estimation of the control ranges of key variables (i.e. Cs, Sp, and Mf) used in the developed
combination heater was an essential process in this study to achieve the adequate range of
temperature variations between solid and liquid phases. The concept similar to the discriminant
analysis was applied to build up the acceptable ranges of key variables for the optimized
combination heating method (Press and Wilson, 1978; Abdullah et al., 2004). Prior to the
estimation of the workable range of key variables, temperatures of solution, chicken, and potato
have been predicted using the empirical polynomial equations given by:
𝑇𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒��������,�������,��� ������ = ��𝑎� ∙ 𝑀� + 𝑎�� ∙ 𝑆� + �𝑎� ∙ 𝑀� + 𝑎���𝐶�� + 0��𝑏� ∙ 𝑀� + 𝑏�� ∙ 𝑆� +
�𝑏� ∙ 𝑀� + 𝑏���𝐶� + �𝑐� ∙ 𝑀� + 𝑐�� ∙ 𝑆� + 𝑐� ∙ 𝑀� + 𝑐� (1)
The predicted temperature values of solution, chicken, and potato were accurately consistent
with experimental observations with the r2 values close to 1 and the constants of polynomial
equations for each variable were listed in Table 4.2.
Table 4. 2 The constants of polynomial equations for the prediction of temperatures of solution,
chicken, and potato
Solution Chicken Potato a1 -214.30 25.03 -137.65 a2 13.45 -21.85 8.68 a3 168.54 -3.38 51.02 a4 -11.93 7.49 -4.87 b1 529.11 70.38 868.13 b2 -43.60 25.13 -100.25 b3 -443.21 -78.35 -442.08 b4 38.73 -3.03 56.22 c1 -303.62 -7.98 -713.47 c2 28.87 -10.86 90.07 c3 224.24 18.60 383.08 c4 60.13 81.98 30.01
105
For practical application for the processing of food products, the value of Sp was
normalized as a particle diagonal per a tube diameter within the combination heating applicator.
The normalized values of Sp and Mf in the combination heater were determined to range
between 0 and 0.9, and between 0 and 0.25, respectively. The controllable ranges of salt
concentration for the developed heating unit to achieve the desired temperature variations (±2°C,
at targeted exit temperature, 80°C) defined for ΔTps and ΔTcs were between 0 and 1.68%, and
between 0 and 1.71%, respectively. In addition, a 3D block diagram, which could identify and
quantify heating uniformity between solids and carrier medium, were constructed using the
controllable ranges of key variables (Fig. 4.5). The desired temperature variations of ΔTps or
ΔTcs (±2°C at targeted exit temperature, 80°C) can be obtained when the values of key variables
are located within each block unit.
The appropriate ranges of ΔTps and ΔTcs imply that the combination heating technique
leads to improved temperature similarities between solid and liquid phases and enhanced energy
efficiency of the combination heating technology. This finding could explain that the
combination heating has the potential to thermally treat multiphase foods with uniformity of
temperature distribution in various ranges of salt concentration, particle size, and mass fraction.
106
Figure 4. 6 3D block diagram constructed at different controllable ranges of key variables for the
prediction of (a) ΔTps and (b) ΔTcs falling in the variations less than ±2°C from targeted exit
temperature, 80°C.
(a)
(b)
107
4. 4 Conclusion
A continuous flow, simultaneous microwave and ohmic combination heater was
successfully fabricated and tested for thermal uniformity of multiphase foods. It was clearly
observed that temperature differences between solid and liquid phases was substantially high
under individual heating modes; however, there was little temperature lag at low salt
concentrations (up to 1.25%) in a relatively short time (52 sec) when microwave and ohmic
heating were simultaneously applied. Temperature lag of solid particles in multiphase foods was
observed when high salt concentration (2.0%) was applied, which, however, seems to be
unrealistic in food systems. This finding could highlight potential opportunities to thermally
process particulate foods with improved thermal analogy and no need for pretreatment steps. A
next study will include validation of the effectiveness of the combination heating technology in
terms of physicochemical and microbiological achievement, i.e. lethality testing for solid-liquid
mixture foods.
108
4.5 References
Abdullah, M.Z., Guan, L.C., and Azemi Mohd B.M.N. (2001). Stepwise discriminant analysis
for color grading of oil palm using machine vision system. Chemical Engineering Research &
Design, 79, 223-231.
Chen, C., Abdelrahim, K., and Becherich, I. (2010). Sensitivity analysis of continuous ohmic
heating process for multiphase foods. Journal of Food Engineering, 98, 257-265.
Halden, K., de Alwis, A.A.P., and Fryer, P.J. (1990). Changes in the electrical conductivity of
foods during ohmic heating. International Journal of Food Science and Technology 25 (1), 9–25.
Kent, M., and Kress-Rogers, E. (1986). Microwave moisture and density measurements in
particulate solids. Transactions of Instrumentation, Measurement and Control, 8(3), 167–168.
Koskiniemi, C. B., Truong, V. D., Simunovic, J., and McFeeters, R. F. (2011). Improvement of
heating uniformity in packaged acidified vegetables pasteurized with a 915 MHz continuous
microwave system. Journal of Food Engineering, 105, 149-160.
Mullin, J. (1995). Microwave processing. In G. W. Gould (Ed.), New methods of food
preservation. Blackie Academic and Professional. Bishopbriggs, UK.
Nguyen, L. T., Choi, W., Lee, S. H., and Jun, S. (2013). Exploring the heating patterns of
multiphase foods in a continuous flow, simultaneous microwave and ohmic combination heater.
Journal of Food Engineering, 116, 65-71.
Press, S.J., and Wilson, S. (1978). Choosing between logistic regression and discriminant
analysis. Journal of the American Statistical Association, 73, 699-705.
Ramaswamy, H. S., Pillet, T., and Fakhouri, M. (1991). Distribution and equalization of
temperature in a microwave heated food model. ASAE Paper No. 913518. St. Joseph, MI.
109
Salengke, S., and Sastry, S.K. (2007). Experimental investigation of ohmic heating of solid–
liquid mixtures under worst-case heating scenario. Journal of Food Engineering 83 (3), 324–336.
Sarang, S., Sastry, S.K., and Knipe, L. (2008). Electrical conductivity of fruits and meats during
ohmic heating. Journal of Food Engineering 87, 351–356.
Sastry, S., and Palaniappan, S. (1992). Mathematical modeling and experimental studies on
ohmic heating of liquid–particle mixtures in a static heater. Journal of Food Process Engineering
15, 241–261.
Sastry, S.K. (1992). A model for heating of liquid–particle mixtures in a continuous flow ohmic
heater. Journal of Food Process Engineering 15, 263–278.
Shim, J.Y., Lee, S.H., Jun, S. (2010). Modeling of ohmic heating patterns of multiphase food
products using computational fluid dynamics codes. Journal of Food Engineering 99 (2), 136–
141.
Vilayannur, R. S., Puri, V. M., and Anantheswaran, R. C. (1998). Size and shape effect on non-
uniformity of temperature and moisture distributions in microwave heated food materials: Part 1
Wan, D., and Turek, S. (2007). Fictitious boundary and moving mesh methods for the numerical
simulation of rigid particulate flows. Journal of Computational Physics, 222(1), 28-56.
Ye, X., Ruan, R., Chen, P., and Doona, C. (2004). Simulation and verification of ohmic heating
in static heater using MRI temperature mapping. Swiss Society of Food Science and Technology,
37, 49-58.
Zareifard, M.R., Ramaswamy, H.S., Trigui, M., and Marcotte, M. (2003). Ohmic heating
behaviour and electrical conductivity of two-phase food systems. Innovative Food Science and
Emerging Technologies, 4, 4
110
Chapter 5
Computational modeling for heating profile and the validation of thermal lethality of
multiphase foods in a dual cylindrical microwave and ohmic combination heater
5.1 Introduction
Microwave heating has shown the great effectiveness on the destruction of microbial
contaminants and foodborne pathogens such as Bacillus subtilis and Escherichia coli particularly
for the sterilization of either solid or liquid phase foods (Wang et al., 2003; Dev et al., 2008).
When microwave radiation was applied for the sterilization of the packaged inhomogeneous
foods, the product has a longer shelf life owing to the improved lethal activities (Tang et al.,
2008). Furthermore, microwave radiation was implemented to treat ballast water inoculated with
several invasive organisms such as microalgae (Nannochloropsis oculata) and oyster larvae
(Crassosstrea virginica) in continuous flow system and consequently the complete inactivation
of the targeted microorganisms with more than 80% power utilization efficiency was achieved in
the relatively low temperature range from 43 to 55 °C (Boldor et al., 2008). Although a number
of studies reported that microwave radiation could substantially enhance the destruction of
microbial contaminants in various foods, it was extremely hard to obtain the consistent reduction
rate even under the identical experimental condition because of the complex relationship
between the dielectric properties of foods and microwave. Moreover, uneven temperature
distribution inside sphere shaped and homogeneous model food treated in the domestic
microwave oven was validated by the numerical simulation and the experiment (Navarrete et al.,
2012). The cold spots inside food could be a good shelter for foodborne pathogens during
microwave heating.
111
Ohmic heating was employed to disrupt both interiors and exteriors of cell walls and to
improve enzymatic hydrolysis for subsequent ethanol process using wastes from food processing
(Lee and Jun, 2011). In addition, Palaniappan and others (1992) found that the mild
electroconductive heat treatment and the electrical pretreatment could save the thermal energy
required for subsequent inactivation of Escherichia coli at certain temperature. The disruption of
cell membrane and the sublethal injury of food borne pathogen may occur by electroporation that
is the formation of pores in cell membranes in response to electric field (Lima and Sastry, 1999).
Therefore, ohmic heating has a potential to process food products with improved quality in terms
of the retention of nutrients and the prevention of microbial spoilage because of the efficient
inactivation of microorganisms by the electrothermal effect and electroporation. A continuous
pilot scale ohmic heating system was developed for aseptic processing of apricots in syrup and it
was reported that the product maintained the microbiological stability during the storage period
of 52 weeks (Pataro et al., 2011). Furthermore, the sterilization capability of a continuous ohmic
heating system for producing low-acid foods containing particulates was examined and the
desirable inactivation rate more than 5 log reduction of Clostridium sporogenes spores in solid-
liquid mixture foods was obtained with the absence of viable microorganisms at the target
treatment temperature (Kamonpatana et al., 2013). However, the separate pre-heating processing
for apricot pieces and syrup was essential and the multiphase foods having the similarity in
electrical conductivities of both solid particulate and liquid could be processed in the above
mentioned continuous ohmic heating systems.
In order to understand the interaction between the electromagnetic field and the food, the
necessity of the simulation for microwave heating system has been increased and many
researchers have made modeling efforts to investigate the heat transfer mechanism of microwave
112
heating in foods often using the commercial software based on finite element method.
Furthermore, the numerical modeling using computational fluid dynamics (CFD) codes for
ohmic heating process of solid-liquid foods has been developed (Salengke and Sastry, 2007;
Shim et al., 2010). However, the majority of numerical models for multiphase foods processed
by microwave or ohmic heating system have dealt with a simplistic scenario including a single
solid particle surrounded by carrier medium and have been carried out in a static state. The
aforementioned modeling approaches are over-simplified and far from the reality. Therefore, the
simulation of simultaneous microwave and ohmic heating in continuous flow system was
necessary to predict the heating patterns of multiphase foods. If successful, the developed model
will provide the practical solution for uniform heating of multiphase foods by manipulating their
transient electrical conductivities plus thermal properties such as specific heat and thermal
conductivities.
The study in this section was intended to evaluate the effectiveness of the combination
heating on microbial inactivation in multiphase foods and to develop a numerical model using
COMSOL software for flow, movement of particulates, and temperature distributions in
continuous flow multiphase foods when microwave and ohmic combination heating was applied.
5.2 Materials and methods
5.2.1 Particle and liquid mixtures preparation
Carrots as solid particles were cut into ball shaped pieces with different sizes (1.3 and 1.8
cm in diameter) and then immersed in Escherichia coli K-12 suspension having an initial load of
108 CFU/ml. Sodium chloride solution (Morton Salt, Chicago, IL) at 0.5% concentration(W/V)
113
were mixed in 1.5% (W/V) carboxymethylcellulose (CMC PH 6000, TICGUM, White Marsh,
MD) base solution.
5.2.2 Microorganism cultivation
Frozen stock cultures of E. coli K12 obtained from the Food Microbiology Laboratory,
University of Hawaii (Honolulu, Hawaii), were streaked on agar plates and incubated at 37°C for
24 h. The strain was transferred into tryptic soy broth (TSB, BD Diagnostic Systems, Franklin
Lakes, N.J.) and incubated at 37°C for 24 h. The initial concentration of E. coli K12 in the carrot
ball was enumerated by serial dilutions and plate counting methods.
5.2.3 Measurement of electrical conductivity and dielectric properties
The electrical conductivities of carrot and CMC base solution with 0.5% salt
concentration were determined by using the method mentioned in section 4.2.2. Dielectric
properties of food samples were measured using a vector network analyze connected with a high
temperature dielectric probe kit (85070E, Agilent Technologies, Santa Clara, CA) at the
frequency of 2.45 GHz. The tolerant temperature range of the probe is from -40°C to 200°C,
which allows measurements versus frequency and temperature. Before measuring dielectric
properties of carrot and base solution, the probe was sequentially calibrated with air, a PTFE
shorting block, and distilled water at 25°C. During measurement, the probe and food sample
were placed in a custom made test cell to prevent water from evaporating at high temperature.
Circulating solution in the jacket of the test cell was practical to control sample target
temperature and the temperature value of the sample was measured using a thermocouple
inserted into the center of the sample.
114
5.2.4 Experimental protocol
As similar with the experimental protocol in Chapter 4, the base solution filled in the
feeder tank were flowed into the combination heater using a sine pump. Carrot balls were
inserted in the T-shaped tube installed between the combination heater and the sine pump (circle
A in Fig. 5.1). The applied power inputs for individual heating mode (MW and OH), and
combination heating mode (MW & OH) were 1000 W, 380 V, and 800 W & 270 V, respectively.
When the exit temperature of solid particle reached 90°C in 52 sec, the particulates were
collected for lethality testing of the each heating mode. The flow rate and the pump speed were
fixed at 300±8 ml/min and 85 rpm, respectively. In order to record and monitor all data points of
temperature, voltage, and current in real time, a data acquisition unit (DAQ, Agilent 39704A,
Agilent Technologies, Inc., Palo Alto, Cal.) was used during the experiment (Fig. 5.1).
Figure 5. 1 A schematic diagram of the experimental set-up
115
5.2.5 Microbial enumeration
Plate counting argar (PCA) method was employed to determine microbial load of
particulates. A carrot ball immediately collected after each thermal treatment was weighed and
placed in a stomacher bag (Whirl-Pak, Nasco, Fort Atkinson, WI) including 40 mL of 0.1%
sterile peptone water. The carrot ball and peptone water were homogenized using a laboratory
blander (Stomacher 400C, Seward Inc., London, England) at 260 rpm for 2 min. 1 mL taken
from each bag was serially diluted in 9 mL of 0.1% sterile peptone water, and 0.1 mL of the
appropriate diluted sample was spread on agar plates in triplicate. The plates were incubated at
37°C for 24 h, and bacterial numbers were estimated.
5.2.6 Model development
5.2.6.1 Model parameters
The model parameters used in this study were numerically calculated and obtained from
the previous literatures (Chakrabandhu and Singh, 2004; Semmar et al., 2004; Yang and Zhu,
2007; Broniarz-Press and Pralat, 2009; Singh and Heldman, 2001; Nguyen et al., 2013). The
diameter and initial temperature of carrot particle and solution were 1.3 cm and 23 °C,
respectively. Inlet velocity was 0.01 m/s corresponding to the applied flow rate of 300 ml/min in
the experiment. The microwave frequency was set up as 2.45 GHz. For the numerical modeling
of heating patterns of carrot ball and solution under individual heating mode and combination
heating mode, the applied power inputs in the experiments (MW, OH, and MW & OH) were
used. The density (ρ), specific heat (Cp), and thermal conductivity (k) of carrot were 641 kg/m3,
3.864 kJ/kg∙K, and 0.626 W/m∙K, respectively (Singh and Heldman, 2001). The temperature
dependant ρ, Cp, k, and dynamic viscosity (μ, cP) of the base solution, which were
experimentally measured in previous studies (Chakarabandhu and Singh, 2004; Semmar et al.,
116
2004; Yang and Zhu, 2007; Broniarz-Press and Pralat, 2009), were exploited for the numerical
modeling. In addition, thermo-physical properties (such as electrical conductivity (λ, S/m) and
dielectric constant (εr)) of carrot and the base solution were taken from the study done by
Nguyen and others (2013). The summarized material properties and parameters for the
simulation are listed in Table. 5.1.
Table 5. 1 The material properties and parameters used for the simulation
Parameter Type Value Unit
k Carrot 0.626 W/m∙K
Thermal conductivity (W/m·K) Solution Broniarz-Press and Pralat (2009)
Cp Carrot 3.864 kJ/kJ∙K
Specific heat (kJ/kg·K) Solution Semmar et al. (2004)
λ Carrot Nguyen et al. (2013)
Electrical conductivity (S/m) Solution Nguyen et al. (2013)
εr Carrot Nguyen et al. (2013)
Relative permittivity Solution Nguyen et al. (2013)
ρ Carrot 641 kg/m3
Density (kg/m3) Solution Chakrabandhu and Singh (2004)
U0 Solution 0.01 Inlet velocity (m/s)
μ Solution Yang and Zhu (2007) Dynamic viscosity (Pa·s)
D Carrot 0.01 Diameter (m)
To - 23 Initial temperature (°C)
5.2.6.2 Ohmic heating
The electric field distribution in an ohmic applicator can be determined by solving
Laplace's equation (Salengke and Sastry, 2007);
∇ ∙ (λ ∇V) = 0 (1)
where λ is the electrical conductivity (S/m).
117
with boundary conditions:
V|Z=0 = V0 which means a ground state at the electrode,
V|Z=L = VL which means the applied electric potential to the electrode,
𝑛 ∙ (𝜆 𝛻𝑉) = 0 which means the electrical insulation at the applicator tube.
where n is the unit vector perpendicular to the scattering plane of the applicator tube.
5.2.6.3 Microwave heating
The electromagnetic energy distribution inside the cylindrical microwave cavity was
governed by the constraint relations of Maxwell's equations (Zhao et al., 2011);
∇ × 𝐸 = 𝑗𝜔𝜇�𝐻, ∇ × 𝐻 = 𝑗𝜔𝜀�𝜇�𝐻, ∇ ∙ 𝐸 = 0, ∇ ∙ 𝐻 = 0 (2)
5.2.6.4 Heat transfer
The volumetric heat generation from microwave and ohmic heating can be calculated by
the following equations (Alwis and Fryer, 1990; Salvi et al., 2011);
𝑄�� = 2𝜋𝜀�𝜀"𝑓|𝐸|� (3)
𝑄�� = 𝜆(𝑇) ∙ |∇𝑉|� (4)
where ε" is the dielectric loss (J/s), f is the applied frequency (Hz).
The temperature distribution in the base solution due to conduction and convection can
be estimated by solution Fourier's energy balance equation (Salvi et al., 2011);
����
+ 𝑢∇𝑇 = ����
∇�𝑇 + �(�� �� �� �� ��&��)
��� (5)
where u is the velocity (m/s), and T is temperature (K).
with boundary conditions:
−𝑛 ∙ (−𝜆 𝛻𝑉) = 0 which means the thermal insulation at the applicator tube.
118
5.2.6.5 Solution of incompressible fluid flow including particle
Navier-Stokes equation, which are derived from Newton's second law, has been widely
used for computing time dependant incompressible flows (Chen and Lobo, 1995; Reddy et al.,
2013). However, it was complicated to obtain a time-accurate solution for an incompressible
flow because the explicit time derivative was not included in the continuity equation (Kim and
Moin, 1985). Therefore, the incompressibility and density of fluid flow were assumed to be
constant for solving incompressible fluid flow (Tegze and Toth, 2013). The Navier-Stokes and
continuity equations are given by (Huang et al., 2012);
���𝜌 + ∇ ∙ (𝜌𝑢) = 0 (6)
𝜌 � ���𝑢 + (𝑢 ∙ ∇)𝑢� = ∇ ∙ 𝜎 + 𝑉� (7)
where σ is the stress tensor (N/m2), and Vf is the volume force (N/m3).
Maxey and others (1997 and 2001) developed the specific model for the motion of
particle laden flows that the fluid motion equations were exploited to compute the entire region
involving the volume occupied particles. The motion for incompressible flow and particle can be
calculated using the following equation (Eq. 8) included in the force-coupling model because the
movement of particles is able to lead the body forces imposed on the fluid.
𝜌 �����
+ 𝑢 ∙ 𝛻𝑢� = −𝛻𝑝 + 𝜇𝛻�𝑢 + 𝑉� (8)
𝑉� = 𝑉�(𝑥, 𝑡) = ∑ �𝐹(�)(𝑡)∆(�) �𝑥 − 𝑌(�)(𝑡)������ (9)
where p is the pressure (Pa), x is the position vector of solution, Y(n)(t) is the position vector of
particle depending on the time (t), N is the total number of particles, F(n)(t) is the force
119
monopole strength equal to the hydrodynamic drag on the nth particle and ∆(n)(x) is the Gaussian
envelope function.
Instead of Dirac delta function that shows the effect of a point singularity, the Gaussian
function smoothly varying function ∆(x-Y(n)) in a numerical scheme was applied in this study and
can be explained by using the length scale (σn) of the nth particle (Maxey and Patel, 2001);
∆�(𝑥) = (2𝜋𝜎��)���exp �− ��
����� (10)
The length scale of the nth particle is associated with the radius (rn) of the nth particle;
𝜎� = ��√�
(11)
The force monopole strength was estimated by the sum of the external force on the
particle and the inertia of the particle.
𝐹(�)(𝑡) = 𝐹(�)��� + 𝐹(�)��� (12)
where 𝐹(�)��� is the external force on the particle and 𝐹(�)��� is the inertia of the particle.
The external force acting on the particle is only the buoyancy force which is able to be
neglected in the case of the flow with high viscosity.
5.2.6.6 Forces on the particles
In a motion of incompressible flow and particles, the particles move through the medium
by virtue of the pressure gradient force and hydrodynamic viscous drag acts between particles
and the medium (Takeda et al., 1994; Hill et al., 2003). In addition, the drag force represents the
additional force on the particles because of the velocity associated with the medium (Manninen
et al., 1996). The force exerting on the particles is affected by the stress tensor term (σ) in Eq. (7);
𝜎 = −𝑝𝐼 + 𝜇[∇𝑢 + (∇𝑢)�] − ��𝜇(∇ ∙ 𝑢)𝐼 (13)
120
where I is the identity tensor and T in (𝛻𝑢)� is the transpose of a matrix.
The hydrodynamic force on the particle can be determined by integrating the translational
velocity and mass of particles with the stress tensor over the surfaces of particles (Glowinskin et
al., 1999);
𝐹 = ∮ 𝜎�(�) ∙ 𝑛𝑑𝑆 = 𝑚��(�)
��� (14)
where F is the hydrodynamic force, σp(n) is the surface stress tensor of the nth particle, S is the
surface of particle, m is the mass of particle, and u(n) is the velocity of the nth particle.
If two particles are overlapped during the modeling, the movement of particles becomes
over-constrained in the overlap area (Singh et al., 2003). To avoid this situation, the numerical
solution known as collision model, which imposes a particle-particle repulsive force to ensure
that the distance between particles never becomes too close, was applied in the previous studies
(Glowinski et al., 1999; Singh et al., 2000 and 2003; Wan and Turek, 2007; Galdi et al., 2008).
The modified collision model to exert a short-range repulsive forces to the particles was
exploited in this study as given below (Glowinski et al., 1999);
𝐹�(�)(𝑡) = ∑ �𝐹�(�,�)(𝑡)��������
+ 𝐹�(�)(𝑡) (15)
where FR(n)(t) is the total short-ranged repulsive forces exerted on the nth particle, FP(n,m)(t) and
FW(n)(t) are the short-ranged repulsive forces exerted on the nth particle by the mth particle and by
wall, respectively.
To prevent particles from colliding each other or the walls of the microwave and ohmic
applicator, the particle-particle and the particle-wall repulsive forces were applied (Glowinski et
al., 1999; Wan and Turek, 2007; Galdi et al., 2008);
𝐹�(�,�)(𝑡) = �0
���
(𝑌(�)(𝑡) − 𝑌(�)(𝑡))(𝑟� + 𝑟� + 𝛿 − 𝑑(�,�))��
,𝑑(�,�) > 𝑟� + 𝑟� + 𝛿,𝑑(�,�) ≤ 𝑟� + 𝑟� + 𝛿 (16)
121
𝐹�(�)(𝑡) = �0
���
(𝑌(�)(𝑡) − 𝑌(�)(𝑡))(2𝑟� + 𝛿 − 𝑑(�)�)��
,𝑑(�)� > 2𝑟� + 𝛿,𝑑(�)� ≤ 2𝑟� + 𝛿 (17)
where d(n,m)=|Y(n)(t)-Y(n)(t)| is the distance between the centers of mth and nth particles,
d(n)'=|Y(n)(t)-Y(n)'(t)| is the distance between the centers of nth particle and the nth imaginary
particle (Fig. 5.2), δ is the repulsive force range (1.5∆h), ∆h is the local mesh sizes near the
particles, 𝜀� and 𝜀� are small positive stiffness parameters for particle-particle and particle-wall,
and are defined as (∆h)2 and 𝜀�/2, respectively.
Figure 5. 2 The schematic diagram of particle-particle and particle-wall collision model
5.2.6.7 Arbitrary Lagrangian–Eulerian (ALE) moving mesh method
The arbitrary Lagrangian–Eulerian (ALE) moving mesh technique is a finite element
formulation that the computational scheme is transient depending upon time. This moving mesh
method has been frequently employed to describe the motion of particle in fluid flow and to
derive a number of procedures for fluid-structure interaction analysis (Onate et al., 2004; Al
Quddus et al., 2008). In this study, ALE method was used to trace the movement of particles in
fluid flow. With equations (14) to (17), the summations of the forces exerting on the surface of
each particle were calculated by integrating the forces acting on the surfaces of particles and then
it was settled to form the geometric mesh nodes on the surfaces of particles in COMSOL
simulation. The explicit update of computational mesh for the region surrounding particles was
completed until the mesh quality reached 0.6. Based on the previous deformed mesh, the new
122
computational geometry with the update of new data was created. By repeating this procedure,
the large deformation of computational mesh could be described with the transient mode state.
5.2.6.8 Simulation strategy
Numerical modeling for the prediction of heating patterns of carrot ball-liquid mixture
treated under individual heating mode (MW and OH) and combination heating mode (MW &
OH) was conducted by finite element codes using COMSOL Multiphysics software (COMSOL
4.1, COMSOL, Inc., Palo Alto, CA). The numerical modeling was performed with COMSOL
Java Application Programming Interface which has an interaction with Java programming
language (Java Platform JDK (Java Development Kit) 7u21). Since the programming language
was especially designed to execute an object oriented programming with data fields, COMSOL
Multiphysics linked with Java interface has been intensively applied to simulate the complicated
movements of fluid flow occupied with particles and the electric fields of microwave and ohmic
because the.
A numerical modeling for thermal behavior and movement of solid-liquid food mixtures
under continuous individual heating mode and combination mode required tremendous
computational time to simultaneously solve the all partial differential equations coupling ALE
moving mesh technique with heat transfer. Therefore, the numerical modeling was performed
with two different strategies for the efficient management of computational time; 1) the electric
field distributions in microwave and ohmic heating were estimated in 3D simulation and 2) the
aforementioned differential equations were solved in 2D simulation. The procedure flowchart
used for simulating the combination heating under continuous flow including particles is shown
in Fig. 5.3
123
Figure 5. 3 A flowchart of simulation procedure
Above all, the Maxwell’s and ohmic equations were solved to estimate the electric field
strengths inside the applicator filled with two carrot balls and solution on the stationary state (3D
environment). The cross sectional 2D electric field strength data (Fig. 5.4), which were cropped
perpendicular to the longitudinal direction at the cavity in 3D environment, were imported into
2D simulation part to calculate the heat transfers produced by each heating chamber.
Figure 5. 4 Simulated electric field distributions for (a) microwave and (b) ohmic heating.
124
Then, the fluid dynamics were solved to achieve the forces (viscous drag and pressure
forces) exerting on the surfaces of solid particles by simultaneously updating the gradient of
velocity and pressure of carrier medium in 2D simulation. The calculated forces were substituted
in the ordinary differential equations (ODEs) to solve the velocities and displacements of the
solid particles. After the calculation of ODEs, the translations of the solid particles were traced
by using the ALE moving mesh technique. At each time step occurring the mesh deformation
was occurred due to the movements of the particles, the mesh quality and the convergence rates
(0.01) for each physics were checked. If the mesh quality was content with the targeted quality
factor number (0.6), new computational geometry was created based on the previous mesh. The
large displacements of the particles to move through the long tube were traced. Finally, the same
procedures were repeated until the preset time was reached. The partial differential equations to
govern the different physics were solved simultaneously using the transient mode with a linear
solver.
To reduce the computational time and increase the convergence rate, a generalized
minimal residual (GMRES), parallel direct sparse solver (PARDISO), and multi-frontal
massively parallel sparse (MUMPS) solvers were used for electromagnetic waves and general
heat transfer, fluid dynamics, and electric field of ohmic, respectively. The mesh to describe the
movement of the fluid medium with solid particles inside the PTFE tube showed large
deformation with respect to time even though the geometries of the electrodes for ohmic heating
and the cavities for microwave heating were fixed. Therefore, the Identity Pairs technique was
applied to connect the governing physics sharing the interface boundaries such as the wall of the
tube. To solve the Maxwell’s wave equations, the maximum mesh sizes for each sub-domain
were set inner 1/4 of the wavelengths of each sub-domain. To increase the mesh quality and
125
minimize the number of mesh, the domains were discretized with unstructured triangular meshes
using advanced front method (AFM) which included fine meshes defined by a maximum
element size on the surfaces of the particles and large meshes allowed to grow larger away from
the particles. As mentioned above, this simulation was separately conducted in 3D and 2D
environment. The continuous domain was divided into discrete elements. The total numbers of
164,960 tetrahedral and 149,265 triangular mesh elements were generated for 3D and 2D
simulations, respectively (Fig. 5.5). It took 30 hours to accomplish the simulation using a sever
level personal computer (Intel® Xeon® CPU [email protected] (2 Processors), G.SKILL RAM
48GB@1,600MHz).
Figure 5. 5 (a) 3D tetrahedral meshes for estimating the electric field strengths of microwave
and ohmic heating, and (b) 2D triangular meshes for anticipating particle movements,
fluid low and temperature distribution
126
5.2.6.9 The Assumptions for the simulation
The following assumptions were made during the entire simulation process in order to
analyze the heat transfer and the movements of particles and liquid flow;
1. The carrot ball used in this study is considered as single phase material where salt
diffusion from the carrier medium is limited. Thus, its ionic concentration is assumed to
be self-contained and uniform at any location.
2. The incompressibility and density of carrier medium are to be constant for solving
incompressible fluid flow.
3. All components for microwave heating chamber are perfect electrical conductors and
reflect all of the incident energy.
4. The electrical conductivities and dielectric properties of carrot balls and carrier medium
are temperature dependant.
5. Dielectric heating on the Teflon tube is negligible and Teflon tube is perfectly transparent
to microwave penetration.
6. The mass transfer is negligible and can be ignored.
7. The dominant wave propagation mode of the microwave power port in the microwave
heating system is TE10 mode at 2.45 GHz.
8. The tangential components of the electric and magnetic fields are zero at the wall of
cavity (Etangential and Htangential = 0, Campanone et al., 2012; Geedipalli et al., 2008).
9. The electric field strengths from microwave and ohmic heating power sources are to be
constant.
127
5.3 Results and discussion
5.3.1 Electrical conductivities and dielectric properties of carrot and base solution
As similar with the results reported from section 4.3.1, the electrical conductivity of
carrot significantly increased after 57°C. This significant change in the electrical conductivity
could be because of heat induced quality deterioration i.e., starch gelatinization and the
destruction of cell membrane by electroporation (Shim et al., 2010; Lee and Jun, 2011). As
temperature increased, the electrical conductivity of the base solution with 0.5% NaCl
concentration linearly increased. It was obvious that the electrical conductivity of the base
solution having more active ion components was much higher than that of carrot.
Figure 5. 6 Electrical conductivities of the base solution with 0.5% NaCl and carrot
Dielectric properties of the base solution and carrot are shown in Fig. 5.7. Note that
dielectric properties of carrot were calculated from the equation reported by Sipahioglu and
Barringer (2003). The dielectric constant of carrot was slightly lower than that of the base
solution; however, its dielectric loss was higher. Even though the dielectric constants decreased
with an increase in temperature values, the dielectric loss factors consistently increased with
128
temperature increase. Dielectric loss factor (ε'') indicates the conversion efficiency of electrical
energy to thermal energy; thus, during individual microwave heating, carrot ball may thermally
lead to the base solution.
Figure 5. 7 Dielectric properties of the base solution base solution with 0.5% NaCl and carrot.
5.3.1 Heating patterns of carrot ball and liquid mixture under different heating modes
Figure 5.5 shows the average temperature values of carrot ball and base solution after
different heating methods. A significant temperature variation between solid and liquid phases
was clearly observed in the individual heating modes. Although only one carrot ball surrounded
by 0.5% NaCl solution was treated under the individual heating mode (MW and OH), the results
obtained from the experiment were quite similar to the results addressed in section 4.3.3.
129
Figure 5. 8 Heating patterns of carrot ball-solution mixtures under microwave, ohmic, and
combination heating. (a) 1.3 cm diameter and (b) 1.8 cm diameter
During ohmic heating, the average temperature values of solutions with 1.3 and 1.8 cm
diameter carrot balls were about 10.5 and 14.8 °C, respectively, higher than those of the carrot
balls (Fig. 5.5 (a) and (b)). By increasing the diameter of carrot ball, the temperature variation
between solid particulate and solution increased. In ohmic heating of multiphase foods, it was
noted that larger particle size could lead to more heat transfer resistance from liquid phase to
solid particulates, finally resulting in serious non-uniform temperature distribution in multiphase
foods (Chen et al., 2010). Furthermore, an increase in particulate food size caused the worst case
heating of multiphase foods with leaving particle foods under-processed (Salangke and Sastry,
2007).
On the other hands, the opposite trend with ohmic heating was observed in microwave
heating. Carrot balls having different diameter (1.3 and 1.8 cm) in the base solutions had thermal
leads by about 7.9 and 7.5 °C, respectively (Fig. 5.5 (a) and (b)). The temperature difference
between solid and liquid phases was not substantially changed with an increase in diameter of
particulates. The temperature distribution patterns of different shaped and sized food samples
during microwave heating were experimentally investigated and simulated using the
130
computational modeling by a number of researchers (Ramaswamy et al., 1991; Vilayannur et al.,
1998; Campanone and Zarizky, 2005; Hossan et al., 2010). Depending upon the size and shape
of food samples, the cold and hot spots in food sample were observed at different locations;
however, even though the diameter of spherical shaped food increased, a higher concentration of
microwave energy was absorbed in the core of the sample (Rahman, 2007). Coronel and others
(2005) noted that the highest temperature was measured in proximity of the central axis of the
application during continuous flow microwave heating. Therefore, the solid particulate, which
was surrounded by the base solution and might move more closely to the centerline, might
absorb more microwave energy than the base solution (Nguyen et al., 2013).
When the carrot ball and base solution were simultaneously heated by microwave and
ohmic heating methods, the huge temperature difference between those was not found regardless
of diameter and the maximum difference was less than 2.7 °C (Fig. 5.5 (a) and (b)). As similar
with the results in section 4.2.2, the small temperature gap was because carrot ball was treated by
means of microwave penetration and the electrical current heated the base solution faster than
carrot ball. The results from this study supported that the simultaneous microwave and ohmic
combination heating technology could be a promising methods to thermally treat solid-liquid
mixture foods with securing uniform temperature distribution.
5.3.2 Inactivation of E.coli K-12 in carrot balls treated under different heating modes
The microbial log reductions of E. coli K-12 in carrot balls treated under different heating
modes are shown in Fig. 5.6. The inactivation rates of E. coli K12 were different depending upon
heating method.
131
The microbial log reductions in carrot balls with different diameter (1.3 and 1.8 cm) were
3.89±0.13 and 3.23±0.18, respectively. This result clearly showed that the principle mechanisms
of microbial inactivation in ohmic hating were thermal in nature (FDA, 2000). When ohmic
heating was applied to inactivate E. coli in homogeneous liquid food, the D values (decimal
reduction time) achieved from the ohmically and uniformly heated samples were much faster
than those calculated from conventional heating (Pereira et al., 2007); however, the significant
temperature variation between solution and carrot ball might result in low microbial log
reductions because carrot ball did not sufficiently receive the heat treatment with leaving a small
portion of the electrical current to go to less conductive particle. During ohmic heating of solid-
liquid mixture, the worst case particle that heated more sluggish than normal particles and
solution would take longer time to meet the target lethality (Kamonpatana et al., 2013).
Furthermore, the inactivation rate might be affected by the increase in diameter affected. Even
though Escherichia coli strains are less sensitive to heat than other foodborne pathogens such as
Clostridium botulinum, the heat treatment time based on theoretical background of residence
time distribution was not enough to eliminate E. coli K-12 in carrot balls. Another possible factor
to consider this result was the lack of conductive heat transfer from the solution to carrot ball that
could lead to lowest heating rate being in the solution parallel to the inclusion particle (Salengke
and Sastry, 2007).
Microwave heating could give rise to 4.21±0.26 and 4.06±0.45 log reductions in carrot
balls (1.3 and 1.8 cm Φ, respectively). By contrast to the result from ohmic heating, similar
reduction rate was obtained regardless of particle size. However, microwave heating could not
meet the target lethality corresponding to 5 log microbial reduction that is the minimum
requirement for the pasteurization. Although the concentrated microwave energy can be
132
absorbed inside solid particles that have a relatively small portion in solid-liquid mixture foods,
short microwave treatment could not completely eliminate foods pathogens the mixture foods
(Apostolou et al., 2005). Microwave radiation might cause the morphological change in the
cellular membrane of E. coli; however, this effect appeared to temporary because it was
validated using scanning electron microscopy (SEM) analysis that the shape of the cell was
recovered within 10 min after microwave exposure (Shamis et al., 2011). Moreover, when potato
omelet inoculated with Salmonella Enteritidis was pasteurized using microwave heating, there
was not a substantial change in the bacteria population up to 30s treatment at 300W and 450W,
causing microbial reductions lower than 0.5; while longer treatment time than 40s at 800W could
cause 4.8 log reduction (Valero et al., 2014). Hamoud-Agha and others (2013) reported that non-
uniform inactivation in the homogeneous sample could be resulted from the uneven temperature
distribution and the inactivation efficiency was entirely different depending on locations within
the sample.
However, reductions higher than 5 log were achieved for the particulates treated under
the microwave and ohmic combination heating (5.18 ±0.22 and 5.04 ±0.19 in 1.3 and 1.8 cm Φ,
respectively). Simultaneous microwave and ohmic combination heating seems to have a
synergistic effect on inactivation of E. coli K-12 in carrot balls because strong microwave
penetration might devastate the bacteria invaded the core of the particulate and ohmic current
was probably able to treat the entire surface of particulate. A thermally uniform and sufficient
treatment for solid-liquid food mixture under microwave and ohmic combination heating
technology could improve the microbial lethality in solid particles. In addition, the combination
heating technology showed the potential to expand the capability for sterilization process of
multiphase foods.
133
Figure 5. 9 Log reductions of E. coli K-12 in carrot balls after treatment with ohmic heating,
microwave, and the combination heating. (a) 1.3 cm diameter and (b) 1.8 cm diameter
5.3.3 Code validation
To validate the force coupling method code developed in this study, a well-known test in
which a spherical particle was located in the center of a cylindrical tube was simulated. There
were the other conditions that the diameter ratio of the particle and tube was 0.1, the length of the
tube was twice longer than the diameter of the tube and the hydrodynamic flow was fully
developed at the inlet. The particle Reynolds number and drag coefficient of a particle were
defined as follows.
μ
DρU tP
0Re = (18)
22
0
2trπρU
FC DD = (19)
where Rep is the particle Reynolds number, Dt is the diameter of the tube, CD is the drag
coefficient of the particle, FD is the drag force and rt is the radius of tube.
The detailed procedure to get the drag force under the hydrodynamic state combined
with the FCM code followed the method explained by Dance and Maxey (2003). The drag
coefficients obtained from the code developed in this study were compared with the other
134
published data as shown in Table 5.1 and the results showed a good agreement within an error
of 6%.
Table 5. 2 Comparison of the drag coefficients by the code and published data
Case Rep = 10 Rep = 100 Etc.
Present work 4.13 1.05
Published data 1 4.02 1.06 Zhu et al., 2008
Published data 2 4.26 1.11 Clift et al., 1978
5.3.4 Hydrodynamic field
The base solution was prepared using 1.5% of CMC which made the carrier medium high
viscous. The viscosity values at 20 and 90°C were 0.03 and 0.36 Pa·s, respectively. The
COMSOL flow condition to solve incompressible fluid dynamics was laminar fluid condition
(Reynolds number = 8.38) due to the slow inlet velocity (0.01 m/s). The hydrodynamic flow of
the solid-liquid mixture was fully developed in 2 s because the solution was viscous enough to
drive the whole mixture to move in a short time. A carrot ball, which flew into the combination
heater first, was denoted as particle A (shown in Fig. 5.7). In that order, the next carrot ball was
particles B.
135
Figure 5. 10 Velocity distributions of three particles with a carrier medium in the microwave and
ohmic combination heater
When particles passed through the first microwave and ohmic heating chamber at 20s, the
moving path of particles A and B was little toward left-side of central axis because the
hydrodynamic forces of fluid flow affected the particle movements. At 30s, the hydrodynamic
wake observed between Particles influenced the moving paths of Particle A and B. The distance
between Particles A and B was gradually increased from 40 s. As time increased, strong and
widespread hydrodynamic forces were consistently formed between the particles and the right-
side wall of the applicator; however, the movement of particle A were not substantially
influenced by those. On the other hand, the movement of particle B was impacted by the strong
hydrodynamic wake generated by the presence of particle A. The longest distance between
Particles A and B was observed when Particle A arrived at the end of the applicator. The forced
coupling method, which imposes a particle-particle repulsive force to ensure that the distance
between particles never becomes too close, was affordable to determine the orientations of solid
136
particles in continuous flow system.
5.3.5 Electric field and temperature distributions
The electric field strength of microwave (shown in Fig. 5.4) was concentrated at the
center of the applicator; however, the intensity gradually decreased from the center (5.618 kV/m)
to outside (2.245 kV/m). The maximum microwave power intensity was found at the areas
adjacent to inlet and outlet of the applicator. In addition, the intensity inside carrot balls was not
different from the outside intensity. It was obvious that temperature values of particles were
increased faster than those of solution while particles traveled the combination applicator. It
might be because the moving paths particles were not strayed out of the central axis of the
applicator. (Fig. 5.8).
In ohmic heating, the electric field distribution inside the applicator was relatively
uniform (Fig. 5.4). It appears that the field strengths inside particles were higher than solution.
Moreover, it was evidenced that the electric field strengths adjacent to the surfaces of particles
were much lower than the other areas. The localized field overshoots were observed at both side
edges of the electrodes; however, this phenomenon could be negligible because it was occurred
in very small area and the overall performance of continuous flow system could not be
influenced by such field overshoots.
The maximum temperatures of either solid or liquid phase for ohmic, microwave and
combination heating were targeted at 90°C. The significant temperature differences between
particles during individual heating method (MW or OH) were found (Fig. 5.8). The experimental
temperature values and simulated temperature values of carrot balls and the base solution are
given in Table. 5.3.
137
Table 5. 3 The comparison between the experimental temperature values and the simulated temperature values of carrot balls and solution
Heating method
Experiment (°C) Simulation (°C) Carrot ball
(1.3 cm in Φ) Solution Carrot ball (1.3 cm in Φ) Solution
Microwave 90.60 ± 1.97a 82.72 ± 0.93 b 87.20 80.45 Ohmic heating 78.53 ± 0.97 a1 89.01 ± 1.24 b1 82.52 92.12 Combination 88.26 ± 2.37 a2 89.79 ± 2.58 a2 87.83 90.69
Even though the electric field strengths near the boundary surfaces of particles were
much lower than the other areas inside applicator during ohmic heating process, the simulated
data showed that the temperature values of the solution were higher than those of inner particles.
The temperature values observed near the surfaces of the particles were similar to those of the
solution. It should be because of the limited heat conduction from the solution to the core of
particle in the continuous flow system. The simulation for continuous ohmic heating validated
that the difference between electrical conductivities of carrot and base solution could result in
non-uniform temperature distribution in multiphase foods.
By contrast to ohmic heating, the particles thermally led to solution during microwave
heating. Although the electric filed strengths in particles and solution were fairly similar, the
microwave power could be absorbed in solid particles that have substantially lower volume than
solution. In addition, uneven temperature distribution near particles was observed. Since the
dielectric loss (ε") of carrot was higher than that of solution, the more microwave power could be
absorbed in the particles.
In combination heating, the simulated temperature values of particles and solution were
not significantly different and solution temperature was 2.86 °C higher than temperatures of
particles. It was because the sufficient electric current and microwave power could
simultaneously treat particles and solution with improved thermal uniformity. It was clearly
138
apparent that the microwave and ohmic combination heating was efficient for uniform heating of
both liquid and particulates. The overall simulated results were in good agreement with the
experimental results addressed in the section 5.3.1.
Figure 5. 11 Simulated temperature distributions of solid liquid mixture under different heating
methods
5.4 Conclusion
A simultaneous microwave ohmic combination heater was tested to inactivate E. coli K-
12 in carrot balls surrounded by high electrical conductive solution. The log reductions (5.18
±0.22 and 5.04 ±0.19) of E. coli K-12 in carrot balls with 1.3 and 1.8 cm diameter treated under
the combination heating was obtained. A synergistic effect of microwave and ohmic combination
could improve the microbial lethality with the ensured thermal uniformity. It was presumed that
microwave could effectively penetrate into the core of solid particle and the entire surface of
particle could be treated by ohmic current. This finding suggested that a microwave and ohmic
139
combination heater could provide an effective and simplistic pasteurization process of
multiphase foods with improved heating uniformity, nutritional quality, and microbial lethality.
Governing equations for estimating the electric field strengths from microwave and
ohmic heating, ALE moving mesh methods, and forced coupling method were successfully
coupled to predict temperature distribution in multiphase foods and the movements of particles
and fluid flow. The numerical modeling based FEM codes could solve all partial differential
equations associated with heat transfer in the microwave and ohmic combination heating. In
addition, the separate simulation strategy could efficiently manage computational time in 3D and
2D environment. The simulation for the combination heating could highlight uniform
temperature distribution in solid-liquid mixture foods. The predicted temperature values of
carrot balls and solution were closely matched to the experimental data with the maximum
prediction error of 5%. The numerical model for prediction of the detailed heating patterns of
solid-liquid mixture foods under the microwave and ohmic combination heating can be
implemented to design commercial scale combination heater with ensured temperature similarity
between solid particles and liquid.
140
5.5 References
Al Quddus, N., Moussa, W. A., and Bhattacharjee, S. (2008). Motion of a spherical particle in a
cylindrical channel using arbitrary Lagrangian–Eulerian method. Journal of colloid and interface
science, 317(2), 620-630.
Apostolou, I., Papadopoulou, C., Levidiotou, S., and Ioannides, K. (2005). The effect of short-
time microwave exposures on Escherichia coli O157: H7 inoculated onto chicken meat portions
and whole chickens. International journal of food microbiology, 101(1), 105-110.
Boldor, D., Balasubramanian, S., Purohit, S., and Rusch, K. A. (2008). Design and
implementation of a continuous microwave heating system for ballast water treatment.
Environmental science & technology, 42(11), 4121-4127.
Broniarz-Press, L., and Pralat, K. (2009). Thermal conductivity of Newtonian and non-
Newtonian liquids. International Journal of Heat and Mass Transfer, 52(21), 4701-4710.
Campañone, L. A., and Zaritzky, N. E. (2005). Mathematical analysis of microwave heating
process. Journal of Food Engineering, 69(3), 359-368.
Campañone, L. A., Paola, C. A., and Mascheroni, R. H. (2012). Modeling and simulation of
microwave heating of foods under different process schedules. Food and bioprocess technology,
5(2), 738-749.
Chakrabandhu, K., and Singh, R. K. (2005). Rheological properties of coarse food suspensions in
tube flow at high temperatures. Journal of food engineering, 66(1), 117-128.
Chen, C., Abdelrahim, K., and Beckerich, I. (2010). Sensitivity analysis of continuous ohmic
heating process for multiphase foods. Journal of food engineering, 98(2), 257-265.
141
Chen, J. X., and Lobo, N. D. V. (1995). Toward interactive-rate simulation of fluids with moving
obstacles using Navier-Stokes equations. Graphical Models and Image Processing, 57(2), 107-
116.
Coronel, P., Truong, V.D., Simunovic, J., Sandeep, K.P., Cartwright, G.D. (2005). Aseptic
processing of sweetpotato purees. Journal of Food Science 70 (9), E531–E536.
Dance, S. L., and Maxey, M. R. (2003). Incorporation of lubrication effects into the force-
coupling method for particulate two-phase flow. Journal of computational Physics, 189(1), 212-
238.
De Alwis, A. A. P., and Fryer, P. J. (1990). A finite-element analysis of heat generation and
transfer during ohmic heating of food. Chemical Engineering Science, 45(6), 1547-1559.
Dev, S. R. S., Raghavan, G. S. V. and Gariepy, Y. (2008). Dielectric properties of egg
components and microwave heating for in-shell pasteurization of eggs. Journal of Food
Engineering 86 (2), 207-214.
Galdi, G. P., Rannacher, R., Robertson, A. M., and Turek, S. (2008). Hemodynamical flows.
Delhi Book Store.
Geedipalli, S., Datta, A. K., and Rakesh, V. (2008). Heat transfer in a combination microwave–
jet impingement oven. food and bioproducts processing, 86(1), 53-63.
Glowinski, R., Pan, T. W., Hesla, T. I., and Joseph, D. D. (1999). A distributed Lagrange
multiplier/fictitious domain method for particulate flows. International Journal of Multiphase
Flow, 25(5), 755-794.
Hamoud-Agha, M. M., Curet, S., Simonin, H., and Boillereaux, L. (2013). Microwave
inactivation of Escherichia coli K12 CIP 54.117 in a gel medium: Experimental and numerical
study. Journal of Food Engineering, 116(2), 315-323.
142
Hill, R. J., Saville, D. A., and Russel, W. B. (2003). Electrophoresis of spherical polymer-coated
colloidal particles. Journal of colloid and interface science, 258(1), 56-74.
Hossan, M. R., Byun, D., and Dutta, P. (2010). Analysis of microwave heating for cylindrical
shaped objects. International Journal of Heat and Mass Transfer, 53(23), 5129-5138.
Huang, C. C., Gompper, G., and Winkler, R. G. (2012). Hydrodynamic correlations in
multiparticle collision dynamics fluids. Physical Review E, 86(5), 056711.
Kamonpatana, P., Mohamed, H. M., Shynkaryk, M., Heskitt, B., Yousef, A. E., and Sastry, S. K.
(2013). Mathematical modeling and microbiological verification of ohmic heating of a solid–
liquid mixture in a continuous flow ohmic heater system with electric field perpendicular to flow.
Journal of Food Engineering, 118(3), 312-325.
Kim, J., and Moin, P. (1985). Application of a fractional-step method to incompressible Navier-
Stokes equations. Journal of computational physics, 59(2), 308-323.
Lee, S. H., and Jun, S. (2011). Enhancement of sugar release from taro waste using ohmic
heating and microwave heating techniques. Transactions of the ASABE, 54(3), 1041-1047.
Lima, M., and Sastry, S. K. (1999). The effects of ohmic heating frequency on hot-air drying rate
and juice yield. Journal of food engineering, 41(2), 115-119.
Manninen, M., Taivassalo, V., and Kallio, S. (1996). On the mixture model for multiphase flow.
Maxey, M. R., and Patel, B. K. (2001). Localized force representations for particles sedimenting
in stokes flow. International journal of multiphase flow, 27(9), 1603-1626.
Maxey, M. R., Patel, B. K., Chang, E. J., and Wang, L. P. (1997). Simulations of dispersed
turbulent multiphase flow. Fluid Dynamics Research, 20(1-6), 143.
143
Navarrete, A., Mato, R. B., and Cocero, M. J. (2012). A predictive approach in modeling and
simulation of heat and mass transfer during microwave heating. Application to SFME of
essential oil of Lavandin Super. Chemical Engineering Science, 68(1), 192-201.
Nguyen, L. T., Choi, W., Lee, S. H., and Jun, S. (2013). Exploring the heating patterns of
multiphase foods in a continuous flow, simultaneous microwave and ohmic combination heater.
Journal of Food Engineering, 116(1), 65-71.
Oñate, E., Idelsohn, S. R., Del Pin, F., and Aubry, R. (2004). The particle finite element
method—an overview. International Journal of Computational Methods, 1(02), 267-307.
Palaniappan, S., Sastry, S. K., and Richter, E. R. (1992). Effects of electroconductive heat
treatment and electrical pretreatment on thermal death kinetics of selected microorganisms.
Biotechnology and bioengineering, 39(2), 225-232.
Pataro, G., Donsì, G., and Ferrari, G. (2011). Aseptic processing of apricots in syrup by means of
a continuous pilot scale ohmic unit. LWT-Food Science and Technology, 44(6), 1546-1554.
Pereira, R., Martins, J., Mateus, C., Teixeira, J. A., and Vicente, A. A. (2007). Death kinetics of
Escherichia coli in goat milk and Bacillus licheniformis in cloudberry jam treated by ohmic
heating. Chemical Papers, 61(2), 121-126.
Rahman, M. S. (Ed.). (2007). Handbook of food preservation. CRC press.
Ramaswamy, H. S., Pillet, T., and Fakhouri, M. (1991). Distribution and equalization of
temperature in a microwave heated food model. ASAE Paper No. 913518. St. Joseph, MI.
Reddy, N. S., Rajagopal, K., Veena, P. H., and Pravin, V. K. (2013). A Pressure Based Solver for
an Incompressible Laminar Newtonian Fluids. International Journal of Fluids Engineering, 5(1),
21-28.
144
Salengke, S., and Sastry, S. K. (2007). Experimental investigation of ohmic heating of solid–
liquid mixtures under worst-case heating scenarios. Journal of food engineering, 83(3), 324-336.
Salvi, D., Boldor, D., Aita, G. M., and Sabliov, C. M. (2011). COMSOL Multiphysics model for
continuous flow microwave heating of liquids. Journal of food engineering, 104(3), 422-429.
Semmar, N., Tanguier, J. L., and Rigo, M. O. (2004). Analytical expressions of specific heat
capacities for aqueous solutions of CMC and CPE. Thermochimica acta, 419(1), 51-58.
Shamis, Y., Taube, A., Mitik-Dineva, N., Croft, R., Crawford, R. J., and Ivanova, E. P. (2011).
Specific electromagnetic effects of microwave radiation on Escherichia coli. Applied and
environmental microbiology, 77(9), 3017-3022.
Shim, J.Y., Lee, S. H., and Jun, S. (2010). Modeling of ohmic heating patterns of multiphase
food products using computational fluid dynamics codes. Journal of food engineering, 99(2),
136-141.
Singh, P., Joseph, D.D., Hesla, T.I., Glowinski, R., and Pan, T.W. (2000). Direct numerical
simulation of viscoelastic particulate flows. J. Non-Newton. Fluid Mech. 91, 165–188.
Singh, P., Hesla, T. I., and Joseph, D. D. (2003). Distributed Lagrange multiplier method for
particulate flows with collisions. International Journal of Multiphase Flow, 29(3), 495-509.
Singh, R. P., and Heldman, D. R. (2001). Introduction to food engineering. Gulf Professional
Publishing.
Sipahioglu, O., Barringer, S.A. (2003). Dielectric properties of vegetables and fruits as a function
of temperature, ash, and moisture content. Journal of Food Science, 68 (1), 234–239.
Takeda, H., Miyama, S. M., and Sekiya, M. (1994). Numerical simulation of viscous flow by
smoothed particle hydrodynamics. Progress of Theoretical Physics, 92(5), 939-960.
145
Tegze, G., & Tóth, G. I. (2013). A GPU cluster optimized multigrid scheme for computing
unsteady incompressible fluid flow. arXiv preprint arXiv:1309.7128.
Tang, Z., Mikhaylenko, G., Liu, F., Mah, J. H., Pandit, R., Younce, F., and Tang, J. (2008).
Microwave sterilization of sliced beef in gravy in 7-oz trays. Journal of Food Engineering, 89(4),
375-383.
US Food and Drug Administration. (2000). Kinetics of microbial inactivation for alternative food
processing technologies. Government Printing Office, Washington, DC, 4-1.
Valero, A., Cejudo, M., and García-Gimeno, R. M. (2014). Inactivation kinetics for Salmonella
Enteritidis in potato omelet using microwave heating treatments. Food Control, 43, 175-182.
Vilayannur, R. S., Puri, V. M., and Anantheswaran, R. C. (1998). Size and shape effect on non-
uniformity of temperature and moisture distributions in microwave heated food materials: Part 1
Wan, D., and Turek, S. (2007). Fictitious boundary and moving mesh methods for the numerical
simulation of rigid particulate flows. Journal of Computational Physics, 222(1), 28-56.
Wang, Y., Timothy, D. W., Juming, T., and Linnea, M. H. (2003). Dielectric properties of foods
relevant to RF and microwave pasteurization and sterilization. Journal of Food Engineering 57
(3), 257-268.
Yang, X. H., and Zhu, W. L. (2007). Viscosity properties of sodium carboxymethylcellulose
solutions. Cellulose, 14(5), 409-417.
Zhao, X., Yan, L., and Huang, K. (2011). Review of numerical simulation of microwave heating
process. Advances in Induction and Microwave Heating of Mineral and Organic Materials, 27-48.
146
CHAPTER 6
FUTURE WORKS
This study was conducted to explore the potential of ohmic and microwave combination
technology for thermal processing of multiphase foods consisted with solid particle and liquid
having different electrical conductivities. The results reported within this study have indicated
that the combination technology could contribute to minimize thermal lag between solid
particulate and liquid without pretreatment step. A synergistic effect of microwave and ohmic
combination heating technology could improve the microbial lethality in solid-liquid mixture
foods. It was presumed that microwave and strong electrical current could effectively access and
damage bacteria inoculated in core and crust of solid particles, respectively. Furthermore, the
continuous flow microwave and ohmic combination heating model was successfully validated by
the designed experimental setup and model solid-liquid mixture foods.
However, when electrical conductivity and dielectric properties of solid particulates and
liquid were so different (solution with 2% salt concentration), the combination heating was not
capable to achieve thermal similarity between solid particles and liquid and its effectiveness on
thermal uniformity would be questionable. The clear inactivation mechanism and the germicidal
effect of microwave and ohmic combination heating technology should be validated in order to
extend the practical application of combination heating technology into commercial sites for
processing of food products including particulates (i.e. chunky-beef soup and fruit based
desserts). In addition, the modeling work for the heating patterns and the movements of
multiphase foods treated by the combination heating in 3D environment is necessary because the
147
flow regime with moving particle orientation inside the field strength distribution is crucial and
can be more accurately interpreted in 3D coordination.
Therefore, some of key points, which can assist better understanding of the mechanism
and the precise control of microwave and ohmic combination heating technology, should be
considered for future studies as listed in brevity below.
1. To efficiently process multiphase foods without pretreatment step under the combination
heating, it is essential to build a database for temperature dependant electrical and
dielectric properties of food materials that are main ingredients of solid-liquid mixture
foods (such as carrot, potato, beef, and solution at different range of salt concentration).
The predictive equations for those properties can be established using the database
empirically measured and the published data from previous studies. Consequentially, the
equations will be helpful to design the protocol recipes to manipulate the mixing ratios of
individual microwave and ohmic heating resources.
2. Recently, many types of microbial indicators and methods that can be exploited for
microbial validation in various thermal processing have been developed depending on
target foodborne pathogens and process. However, new type of indicator, which both
microwave and electrical current can penetrate and pass through, should be invented for
exploring the germicidal effect of the combination heating on target pathogen
(Escherichia coli or Bacillus strains). The ball shaped bioindicator will be fabricated
using silicone rubber (polydimethylsiloxane, PDMS) mixed with carbon black. In order
to make the indicator having similar electrical conductivities with solid particle foods,
the different concentration of carbon power will be employed. The bioindicator will be
filled with bacterial suspension and then applied in the microbial validation under the
148
combination heating technology. Furthermore, radio frequency identification (RFID)
tags will be used to monitor residence time distribution (RTD) of the fastest or slowest
moving particle in the combination heater.
3. Sterilization is necessary for the complete elimination of foodborne pathogens in food
products. Therefore, key parameters (microwave power supply, the diameter of
applicator, and holding tube) needed for scaling up of current combination heating unit
to industrial sterilization capacities should be taken into consideration. In order to obtain
higher production rates that can guarantee uniform thermal treatment for complex foods,
the magnetron operating at 2.45 GHz should be replaced with that at 915 MHz which has
longer wavelength (32.79 cm) closely associated with penetration depth into solid
particle. A pressure regulator (REG) connected to a compressed air tank has to be
installed in the combination heater to achieve the commercially required temperature
(121 °C). In addition, holding tube providing sufficient heat treatment for the slowest
moving particle will be included in the heater to ensure sterility of solid-liquid mixture
foods.
4. For escalating the scope and accuracy of the computation, the dynamic moving mesh
methods implying the rotating of particulate in the combination heating will be
introduced in new 3D modeling work. A new model will be extended to solve 3D
incompressible Navier-Stoke equations and to determine structure of fluid wake pattern
in the combination heater. The particles concentration and orientation in a continuous
flow system will be interpreted in the new 3D model.
149
APPENDIX A
NUMERICAL MODELING IN COMSOL MULTYPHYSICS
The purpose of Appendix A is to describe the procedures of numerical modeling in
COMSOL Multiphysics 4.1. The electric field distributions in the microwave and ohmic
combination heater were computed in a stationary, frequency-domain electromagnetic and
electric current analyses. In addition, it was illustrated how fluid flow and solid particle could
deform computational structures during the simulation and how to solve for the flow and particle
movement in a continuously deforming geometry using the arbitrary Lagrangian-Eulerian (ALE)
technique. The ALE method can treat the dynamics of the deforming geometry and the moving
boundaries with a moving grid. The geometry of the microwave and ohmic combination heater
was drawn using 3D AutoCAD software.
In this appendix, the procedures for the simulation of electric field distribution in
microwave heating chamber and the simple example of ALE technique were handled. Clarify
that the description style for the procedure of numerical modeling followed the format of
COMSOL Multiphysics users guide.
150
Modeling Instructions for the electric field distribution in microwave heating 1. MODEL WIZARD
a) In the Add physics tree, select Radio Frequency and choose Electromagnetic Waves,
Frequency Domain
b) In the Studies subsection, select Preset Studies and choose Stationary
2. GLOBAL DEFINITIONS
a) A set of parameters for creating geometry is usually defined in Parameters which is a
submenu of Global Definition
b) However, all geometry in this study was drawn using 3D AutoCAD. Therefore, it was not
necessary to define all parameters for creating geometry
3. GEOMETRY
a) In the Model Builder, under Model 1 right-click Geometry 1 and choose Import.
b) Under the Import section, click Browse and navigate ACIS file (*.sat) which is converted
from 3D AutoCAD file
c) Click Import as shown in Fig. 1
151
Figure 1. Geometry of microwave heating chamber
4. MATERIALS
a) In the Model Builder, under Model 1 right-click Materials and choose open Material
Browser
b) In the Material Browser, select Built-In → Air and Water, liquid
c) Right-click and choose Add Material to Model from the menu
d) Under Model 1 → Materials click Air
e) In Geometric Entity Selection, choose Domain and in Selection, select the Domains for
microwave heating chamber (Fig. 2)
f) Under Model 1 → Materials click Water, liquid
g) In Geometric Entity Selection, choose Domain and in Selection, select the Domains for
Teflon tube
152
Figure 2. Material selections in the geometry of microwave heating chamber
5. ELECTROMAGNETIC WAVES, FREQUENCY DOMAIN
a) In Electromagnetic Waves, Frequency setting window, select All domains for microwave
heating chamber and Teflon tube from Domain Selection list
b) Under Equation, choose User defined from the Frequency
c) Type 2.45 [GHz] in the f edit field (Depending on the operating frequency of magnetron, 915
MHz or 2.45 GHz can be typed)
5.1 Wave Equation, Electric 1
a) In the Model Builder, under Model1 right-click Electromagnetic Waves, Frequency and
choose Wave Equation, Electric 1
b) In the Wave Equation, Electric settings window, under Domain Selection list, select All
domains for microwave heating chamber and Teflon tube
5.2 Perfect Electric Conductor 1
153
a) In the Model Builder, under Model1 right-click Electromagnetic Waves, Frequency and
choose Perfect Electric Conductor 1
b) In the Perfect Electric Conductor settings window, under Boundary Selection list, choose
All boundaries except for the boundaries of Teflon tube
5.3 Perfect Electric Conductor 1
a) In the Model Builder, under Model1 right-click Electromagnetic Waves, Frequency and
choose Initial Values 1
b) In the Initial Values settings window, under Domain Selection list, choose All domains for
microwave heating chamber and Teflon tube
Figure 3. The selection of the domain for microwave power source
5.4 Port 1
a) In the Model Builder, under Model1 right-click Electromagnetic Waves, Frequency and
choose the boundary condition Port
154
b) In the Initial Values settings window, under Boundary Selection list, choose the domain
for microwave power source (magnetron) as shown in Fig. 3
c) Under Port Properties section, choose On from the Wave excitation at this port
d) Under Port input power, type 1000 in the Pin edit field (Depending on the power of
magnetron, the number can be various)
e) Under Port Mode Settings, specify E0 vector in Electric field
f) Type E0 vectors as
0 x
cos(pi*(x-0.00[cm])/dg[cm])
[V/m] y
0 z
(dg: Wave guide depth (9.53 cm) is equal to the width of WR430 waveguide)
g) Under Propagation constant, type 2*pi/c_const*sqrt((2.45 [GHz])^2-c_const^2/(4*dg^2)) in
β edit field
(c_const: the speed of light (2.998 × 108 m/s))
6. MESH
a) Under Model 1 right-click Mesh 1 and choose Free Tetrahedral
6.1 Size 1
a) In the Geometric Entity Selection setting window section, choose Domain in Geometric
entity level and select the domains for microwave heating chamber in Selection list
b) In Element Size section, choose General physics from Calibrate for and click Custom
button
c) In Element Size Parameters section, click the Maximum element size check box and type
0.0314 in the associated edit field
(In this study, the mesh element size was limited smaller than π (3.14 cm))
d) Click the Build All button (Fig. 4)
6.2 Size 2
155
a) In the Geometric Entity Selection setting window section, choose Domain in Geometric
entity level and select the domains for Teflon tube in Selection list
b) In Element Size section, choose General physics from Calibrate for and click Custom
button
c) In Element Size Parameters section, click the Maximum element size check box and type
0.0315/sqrt(80) in the associated edit field
d) Click the Build All button
7. STUDY
a) In the Model Builder window, under Study 1 click Step 1: Stationary
b) Right-click Study 1 and choose Compute
8. RESULTS
a) In the Model Builder window, under Results click Electric field
b) Right-click Electric field and choose Slice 1
c) In Plot settings window, under Plane Data, select Plane as zx-planes as shown in Fig. 5
156
Figure 4. The creation of computational mesh for microwave heating chamber
Figure 5. The simulated electric field distribution in microwave heating chamber
157
The example of the arbitrary Lagrangian-Eulerian (ALE) technique After the electric field distributions in microwave and ohmic heating were analyzed in 3D simulation, the estimated electric field strengths were adopted in 2D simulation to solve differential equations for fluid dynamics, forced coupling methods, and the movement of particles in the combination heater. This example demonstrates the modeling particle and fluid flow interaction coupled with heat transfer from microwave and ohmic heating in COMSOL Multiphysics. In addition, it addresses how the movements of particles and fluid flow are predicted under the microwave and ohmic combination heating method by using the arbitrary Lagrangian-Eulerian (ALE) technique. 1. MODEL WIZARD
a) In the Add physics tree, select Radio Frequency → Electromagnetic Waves, Frequency
Domain (for microwave heating), AC/DC → Electric Currents (for ohmic heating), Fluid
Flow → Single-Phase Flow → Creeping Flow (for Stokes flow), Heat Transfer → Heat
Transfer in Solids (for heat transfer in solid particulate foods), and Mathematics → Deformed
Mesh → Moving Mesh (for ALE technique),
b) In the Studies subsection, select Preset Studies and choose Time Dependent
2. GEOMETRY
a) In the Model Builder, under Model 1 right-click Geometry 1 and choose Import.
b) Under the Import section, click Browse and navigate ACIS file (*.sat) for the geometry of
the microwave and ohmic combination heater drawn in two-dimensional environment
c) Click Import as shown in Fig. 6
158
Figure 6. Geometry of the microwave and ohmic combination heater drawn in 2D environment
3. MATERIALS
a) In the Model Builder, under Model 1 right-click Materials and choose open Material
Browser
b) In the Material Browser, select Built-In → Air, Titanium beta-21S, Water, liquid, and
Water, liquid (2)
c) Right-click and choose Add Material to Model from the menu
d)Under Model 1 → Materials click Water, liquid (2)
f) In Material settings window, change Property of Water liquid (2) to Property of Carrot (these
properties were taken from the previous studies) under Material Contents for Water liquid (2)
g) From this step, follow same steps d),e), f), and g) for Materials in Modeling Instruction for
the electric field distribution in microwave heating
4. ELECTROMAGNETIC WAVES, FREQUENCY DOMAIN
a) Follow same steps for Electromagnetic waves, frequency domain in Modeling Instruction for
the electric field distribution in microwave heating
A
A
B
B
159
5. CREEPING FLOW
a) In the Model Builder window, click Model 1 → Creeping Flow
b) Under Creeping Flow settings window, choose the domain for liquid flow from Domain
Selection list
c) Under the Physical Model section, select Incompressible flow from the Compressibility list
5.1 Fluid properties 1 and Initial Values 1
a) Right-click Creeping Flow and choose the Fluid properties and Initial Values 1
b) In Fluid properties and Initial Values 1settings windows, choose the domain for liquid
flow from Domain Selection list
5.2 Inlet 1
a) Right-click Creeping Flow and choose the Inlet 1
b) Under Inlet settings window, choose only the boundary for Inlet from Boundary Selection
list
c) Select velocity from Boundary Conditions
d) Click the check box for Numerical inflow velocity under Velocity
e) Type the velocity corresponding to flow rate in the U0 edit field
5.3 Walls
a) Right-click Creeping Flow and choose the Wall
b) Under Wall settings window, choose the boundaries for the microwave and ohmic
applicator (Teflon tube) from Boundary Selection list
5.4 Outlet 1
a) Right-click Creeping Flow and choose the Outlet 1
b) Select only the boundary for Outlet from Boundary Selection list
6. HEAT TRANSFER IN SOLID
a) In the Model Builder window, click Model 1 → Heat Transfer in Solids, Heat Transfer in
Fluids, and Heat Sources
160
b) Under Heat Transfer in Solids settings window, choose the domains for particulate foods
from Domain Selection list
c) Under Heat Transfer in Fluids settings window, choose the domains for liquid flow from
Domain Selection list
d) Under Heat Sources settings window, choose the domains for heat source (Domains for
microwave and ohmic heating) from Domain Selection list
7. ELECTRIC CURRENTS
a) In Electric Currents setting window, select the domains for the electrodes, the microwave
and ohmic applicator (Teflon tube), liquid flow, and particulate foods from Domain Selection
list
7.1 Current Conservation 1 and Initial Values 1
a) Right-click Electric Currents and choose Current Conservation 1 and Initial Values 1
b) In the Current Conservation 1 and Initial Values 1 settings window, choose the domains
for the electrodes, Teflon tube, liquid flow, and particulate foods from Domain Selection list
c) In Initial Values 1 settings window, type 0 in the V edit field (Electric potential section)
7.2 Electric Insulation 1
a) Right-click Electric Currents and choose Electric Insulation 1
b) In the Electric Insulation 1 settings window, choose only the boundaries for Teflon tube
from Boundary Selection list
7.3 Electric Insulation 1
a) Right-click Electric Currents and choose Electric Insulation 1
b) In the Electric Insulation 1 settings window, choose only the boundaries for Teflon tube
from Boundary Selection list
7.4 Electric Potential 1
a) Right-click Electric Currents and choose Electric Potential 1
161
b) In the Electric Potential 1 settings window, choose only the boundaries for upper electrodes
from Boundary Selection list (Circle A in Fig. 1)
c) Under Electric Potential, type the applied ohmic voltage in V0 edit field (Electric Potential
Section).
7.4 Ground 1
a) Right-click Electric Currents and choose Ground 1
b) In the Electric Potential 1 settings window, choose only the boundaries for bottom
electrodes from Boundary Selection list (Circle B in Fig. 1)
8. MOVING MESH
a) Under Model 1, right-click Moving Mesh and choose Fixed Mesh 1
b) In Fixed Mesh 1 setting windows, choose the domains for the electrodes, Teflon tube, and
the microwave power sources from Domain Selection list
c) Right-click Moving Mesh and choose Prescribed Mesh Displacements
d) In Prescribed Mesh Displacements setting windows, choose the domains for liquid flow
and particulate foods from Domain Selection list
e) Right-click Moving Mesh and choose Prescribed Mesh Velocities
f) In Prescribed Mesh Velocities setting windows, choose only the domains for particulate
foods from Domain Selection list
g) Under Prescribed Mesh Velocity section, click the check boxes for Prescribed x velocity
and Prescribed y velocity, and type velocity equations in Vx and Vy edit fields
9. MESHES
a) Under Model 1, right-click Meshes and choose Deformed Configurations
b) Under Deformed Configurations (1~3) setting windows, choose only the domains for
particulate foods from Domain Selection list (Moving mesh)
c) Under Deformed Configurations (4~13) setting windows, choose the domains for liquid flow,
Teflon tube, the electrodes, and the microwave power sources from Domain Selection list
(Simultaneously Deformed mesh) as shown in Fig. 7
162
10. STUDY
a) In the Model Builder window, under Study 1 click Step 1: Time-dependant
b) Right-click Study 1 and choose Compute
11. RESULTS
a) In the Model Builder window, under Results click Electric fields or Velocity or Pressure or
Temperature or Electric Potentials
b) Right-click velocity and choose Surface as shown in Fig. 8
163
Figure 7. The creation of computational deformed mesh (moving mesh) for the movements of
liquid flow and particulate foods in the ohmic and microwave combination heater
Figure 8. The simulated velocity magnitude of fluid flow and particulate foods in the microwave
and ohmic combination heater
164
APPENDIX B
SUMMARY OF THE MOST IMPORTANT SIGNS AND SYMBOLS
A Area (m2) a Width of rectangular waveguide or
Diameter of cylindrical cavity (m)
b Height of rectangular waveguide (m) C Speed of light (2.998 × 108 m/s)
CD Drag coefficient of the particle Cp Specific heat (J/kgK)
Cs Salt concentration (%) d Length of rectangular waveguide or
Height of cylindrical cavity (m)
Dt Diameter of the tube E Electric field (V/m)
f Frequency (Hz) F Hydrodynamic force (N)
F(n)(t)
The force monopole strength equal to
the hydrodynamic drag on the nth
particle
fc Cutoff frequency (Hz)
FD Drag force Fp(n,m)(t)
The short-ranged repulsive forces
exerted on the nth particle by mth
particle
fr Resonant frequency (Hz) FR(n)(t) The total short-ranged repulsive
forces exerted on the nth particle
FW(n)(t) The short-ranged repulsive forces
exerted on the nth particle by wall H Magnetic field (A/m)
I Electrical current (A) I Identity tensor
k Thermal conductivity (W/m∙K) k Wave number
L Distance between the electrodes (m) m, n
Numbers of half wavelength
variations of fields in the horizontal
and longitudinal directions of the
rectangular waveguide
Mf Mass fraction (%) N The total number of particles
p Pressure (Pa) Pv Microwave power density(W/m2)
Q Quality factor R Electrical resistance (Ω)
Rep Particle Reynolds number rn Radius of the nth particle
165
rt Radius of tube S Siemens (1/R)
S The surface of particle (m2) s11 Reflection coefficient
Sp Particle size (m) T Temperature (K)
T Transpose of a matrix t time (s)
U Internal energy (J) u Velocity (m/s)
u(n) The velocity of the nth particle U0 Inlet velocity (m/s)
V Voltage (V) Vf Volume force (N/m3)
x The position vector of solution x,y,z Coordinates in the rectangular
waveguide and cylindrical cavity
Y(n)(t) The position vector of particle Z0 Characteristic impedance (Ω)
ZTE Wave impedance for TE mode (Ω) β Propagation constant
δ Repulsive force range ε Dielectric constant (F/m)
ε" Dielectric loss (J/s) ε0 Dielectric constant of free space
(8.854 × 10-12 F/m)
εp Stiffness parameter for particle-
particle εr Relative dielectric constant (F/m)
εw Stiffness parameter for particle-wall η Intrinsic impedance (Ω)
λ Electrical conductivity (S/m) λ Wavelength (m)
λg Guide wavelength (m) μ Dynamic viscosity (cP: 10-3 kg/m∙s)
μ Permeability (H/m) μ0 Permeability of free space (1.25664
× 10-7 H/m)
ρ Density (kg/m3) σ Electrical conductivity (S/m)
σ Stress tensor (N/m2) σn Length scale of nth particle
σp(n)
Surface stress tensor of the nth
particle ω Angular velocity (rad/s)
∆ (x) Gaussian envelope function ∆h Mesh size
