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Continuous flow microwave heating: Evaluation of system efficiency and enzyme inactivation kinetics

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Continuous Flow Microwave Heating:
Evaluation of System Efficiency and Enzyme Inactivation Kinetics
by
Man Guang Lin
D epartm ent of Food Science and Agricultural Chemistry
M acdonald Campus, McGill University
Montreal, Quebec
Canada
December, 2004
A thesis submitted to McGill University in partial
fulfillment of the requirements for the degree of M aster of Science
© M an Guang Lin, 2004
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
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ABSTRACT
A continuous flow microwave heating system was set up by using one domestic
microwave oven (1000W nominal output at 2450MHz). Water was run through the coil
centrally located inside the oven cavity for microwave heating.
Microwave absorption
efficiency was evaluated by measuring inlet and outlet temperatures o f coil as a function of
system variables. In order to optimize the coil configuration, the influence of tube diameter
(6.4, 7.9 and 9.7mm); initial temperature (10, 20 and 30 °C); number o f turns (3.5, 4.5 and
5.5); coil diameter (61.5, 88, 102, 121 and 153 mm) and pitch (16, 18, 20, 22 and 24mm)
were evaluated, respectively at different flow rates (240, 270, 300, 330 and 360ml/min).
In
helical systems, Dean number is used as a measure of secondary flow which enhances
mixing of the fluid providing uniform heating even under laminar flow conditions. Results
showed that microwave absorption efficiency was a compromise between coil volume and
Dean number. Therefore, a helical coil (110 mm high) with a coil diameter of 108 mm, tube
diameter o f 8.2 mm, 5.5 turns demonstrated the highest efficiency, fast heating rate, more
uniform heating and less temperature fluctuations. The optimized coil configuration
parameters were used subsequently to set up continuous-flow microwave heating system.
Alkaline phosphatase (ALP) is an indigenous enzyme in raw milk and is used as an
indicator of milk pasteurization because its thermal resistance is slightly greater than that of
nonsporeforming pathogens. Raw milk was subject to conventional batch heating in a wellagitated water bath in the temperature range of 60 to 75 °C for different time intervals. The
associated ALP residual activities were evaluated.
Based on gathered time-temperature
profiles, the effectiveness of come-up and come-down times were taken into account for the
first order rate kinetic data handling. The D-values of ALP varied from 1250 s at 60 °C to
1.7 s at 75 °C with a z-value of 5.2 °C.
Kinetic data obtained were used for subsequent
comparison with those obtained under continuous-flow microwave heating and continuous
flow thermal holding.
A continuous-flow microwave heating system was set up by using two microwave
ovens with 1000 W nominal output at 2450 MHz and three optimized glass coils (two for
microwave heating, one for thermal holding). Raw milk was run through the system for
i
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microwave heating and thermal holding treatments in the pasteurization temperature range
(65-70 °C). The outlet temperatures were adjusted as a function o f inlet temperatures and
flow rates. The average residence time was determined by dividing the sample volume by
the volumetric flow rate. The associated ALP residual activities were evaluated. Based on
time-temperature profiles along the coil length during microwave heating and thermal
holding, the residence times were converted to effective heating times at various
temperatures for first order rate kinetic data handling. D-values o f ALP ranged from 17.6 s
at 65 °C to 1.7 s at 70 °C with a z-value of 4.9 °C under continuous-flow microwave heating
condition, 128.2 s at 65 °C to 13.5 s at 70 °C with a z-value of 5.2 °C under continuous-flow
thermal holding condition.
Results showed that continuous-flow thermal holding was a little more efficient than
conventional batch heating for ALP inactivation.
Moreover, ALP inactivation during
continuous-flow microwave heating was an order of magnitude faster than during
conventional thermal heating thereby confirming the existence of enhanced thermal effects
from microwave.
ii
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RESUME
Un systeme de chauffage micro-ondes a flux continu a ete mis au point en utilisant un
four a micro-ondes domestique (une production nominale de 1000W a 2450MHz). L'eau a
ete circulee a travers la bobine situee au centre a l'interieur de la cavite de four pour le
chauffage a micro-ondes. L'efficacite absorbante a micro-ondes a ete evaluee en mesurant les
temperatures d’entree et de sortie de la bobine comme une fonction des variables du systeme.
Pour optimiser la configuration de la bobine, l'influence du diametre de tube (6.4, 7.9 et
9.7mm); de la temperature initiale (10, 20 et 30°C) ; le nombre de tours (3.5, 4.5 et 5.5) ; le
diametre de la bobine (61.5, 88, 102, 121 et 153 millimetres) et le pitch (16, 18, 20, 22 et
24mm) ont ete evalues, respectivement a differents debits (240, 270, 300, 330 et 360ml/min).
Dans des systemes helicoidaux, le nombre Dean est utilise comme une mesure du flux
secondaire qui augmente le melange du fluide permettant un chauffage uniforme meme dans
les conditions de flux de laminaire. Les resultats ont montre que l'efficacite absorbante a
micro-ondes etait un compromis entre le volume de la bobine et le nombre Dean. Pour cette
raison, une bobine helicoidale (110 millimetres de haut) avec un diametre de bobine de 108
millimetres, un diametre de tube de 8.2 millimetres et 5.5 tours ont montre l'efficacite la plus
elevee, un vitesse de chauffage rapide, un chauffage plus uniforme et une faible fluctuation
de la temperature. Les parametres optimises de la configuration de la bobine ont ete utilises
par la suite pour mettre au point le systeme de chauffage micro-onde a flux continu.
La phosphatase alcaline (ALP) est une enzyme indigene dans le lait cru et est utilisee
comme un indicateur de pasteurisation de lait car sa resistance thermale est legerement plus
grande que celle des pathogenes non sporulants. Un traitement conventionnel a ete applique
au lait cru en le plagant dans un bain-marie bien agite et a des variations de temperature de 60
a 75°C pour differents intervalles de temps. L’activite residuelle de ALP a ete evaluee. Base
sur les profils temps-temperature obtenus, l'efficacite de la montee et de la descente de la
temperature a ete tenue en compte lors du traitement des donnees afin de determiner les
vitesses cinetiques du premier ordre. Les valeurs D de ALP ont varie de 1250 s a 60°C a 1.7 s
a 75°C avec une valeur z de 5.2°C. Des donnees cinetiques obtenues ont ete utilisees pour la
comparaison suivante avec celles obtenues en appliquant un chauffage micro-ondes a flux
continu et un retenu thermique a flux continu.
iii
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Un systeme de chauffage micro-ondes a flux continu a ete mis au point en utilisant
deux fours a micro-ondes avec une production nominale de 1000W a 2450 MHz et trois
bobines en verres optimises (deux pour le chauffage a micro-ondes, une pour retenu
thermique). Le lait cm a ete circule a travers le systeme de chauffage a micro-ondes et le
systeme de retenu thermique aux temperatures de pasteurisation (65-70°C). Les temperatures
de sortie ont ete ajustees comme une fonction des temperatures d'entree et des debits. Le
temps de residence moyen a ete determine en divisant le volume de l’echantillon par le debit
volumetrique. Les activites residuelles de ALP ont ete evaluees. En se basant sur les profils
temps-temperature le long de la bobine pendant le chauffage a micro-ondes et le retenu
thermique, les temps de residence ont ete convertis en temps effectifs du chauffage a
differentes temperatures pour le traitement des donnees des vitesses cinetiques du premier
ordre. Les valeurs D de ALP ont varie de 17.6 s a 65°C a 1.7 s a 70°C dans le cas du
chauffage micro-ondes a flux continu, de 128.2 s a 65°C dans le cas du retenu thermique a
flux continu.
Les resultats ont montre que le retenu thermique a flux continu etait un peu plus
efficace que le chauffage conventionnel pour l’inactivation de l’ALP. De plus, l’inactivation
de l’ALP durant le chauffage micro-ondes a flux continu etait un ordre d'ampleur plus rapide
que pendant le chauffage conventionnel confirmant ainsi l’existence de l’augmentation des
effets thermiques a partir d’une micro-onde.
iv
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ACKNOWLEDGEMENTS
I wish to extend immense gratitude to Dr. Hosahalli S. Ramaswamy, my thesis
supervisor, for his constructive supervision, valuable suggestions, encouragement and
insightful editing throughout the course o f my study.
His profound knowledge, rigorous
scientific approach, professional way of solving problems helped me to improve the ability of
thinking and solving problems, which will become a valuable asset throughout my entire
life. I will never forget his heart moral and financial support.
I would like to take this opportunity to express gratefulness to the professors and staff
at the Department of Food Science and Agricultural Chemistry for the support given to me
during my study at Macdonald Campus of McGill University.
Thanks especially to Drs.
Songmin Zhu, Cuiren Chen and Le Bail, Yanwen shao, Heping Li, Yang Meng, Nikhil
Hiremath and Anuradha Gundurao for their knowledge, time and experience as well as
valuable suggestions. I also wish to thank Mr. Ali. R. Taherian for his assistance in French
translation o f the abstract.
I wish to extend my sincere thanks to the colleagues and friends in the Food
Processing Group: Minli Chi, Neda Maftoonazad, Hong Jin, Baboucarr Jobe, Shafi Zaman,
Jassim Ahemd, Mritunjay Dwivedi and Pedro Alvaraz, who created the friendly environment
to make my work and study efficiently. Without their support and understanding, the pilot
plant work would not have been smoothly finished.
Finally, I would like to express my deep gratitude to my family, especially my wife
Ying Chen and lovely son Zhaoxiang Lin for their love, support and care. Also, my special
appreciation goes to my mother, my mother-in-law, my sisters and brother for their
understanding and support.
v
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TABLE OF CONTENTS
Page
ABSTRACT
i
RESUME
iii
ACKNOWLEDGEMENTS
v
TABLE OF CONTENTS
vi
NOMENCLATURE
ix
LIST OF TABLES
xi
LIST OF FIGURES
xii
CHAPTER 1
INTRODUCTION
1
CHAPTER 2
LITERAURE REVIEW
5
2.1
Thermal processing
5
2.2
History of microwave processing
5
2.3
Principles of microwave heating
7
2.3.1
9
How a magnetron works
2.4
Basic components of microwave oven
9
2.5
Parameters affecting microwave heating
11
2.5.1
Dielectric properties
11
2.5.2
Frequency
12
2.5.3
Power of microwave
12
2.5.4
Mass
12
2.5.5
Moisture content
12
2.5.6
Density
13
2.5.7
Temperature
13
2.5.8
Physical geometries
13
2.5.9
Thermal conductivity
14
2.5.10 Specific heat
14
2.5.11 Secondary flow in curved pipe
14
vi
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2.6
Advantages and limitations of microwave heating
2.7
Microwave application in food processing
2.9
16
2.7.1
Meat and poultry processing
17
2.7.2
Tempering
18
2.7.3
Baking
18
2.7.4
Drying
18
2.7.5 Pasteurization and sterilization
2.8
15
Inactivation/Destruction kinetics
19
21
2.8.1
Thermal destruction
21
2.8.2
Arrhenius equation
22
2.8.3
Thermal Death Time (TDT)
22
2.8.4
Lag correction
26
2.8.5
Lethality concept
26
2.8.6
Microwave kinetics
28
Mechanisms of inactivation/destruction
28
2.9.1
Thermal effects
28
2.9.2
Kinetic effects
29
2.9.3
Chemical effects
29
2.10 Microwave thermal, non-thermal and enhanced effects
29
2.10.1 Evidence with microorganisms
31
2.10.2 Evidence with enzymes
34
2.10.3 Evidence with nutrients
35
2 .11 Summary o f literature review
35
CHAPTER 3
INFLUENCE OF SYSTEM VARIABLES ON THE EFFICIENCY
OF CONTINUOUS FLOW MICROWAVE HEATING
36
3.1 Abstract
36
3.2 Introduction
37
3.3 Materials and Methods
40
3.3.1
Continuous-flow microwave heating system
40
3.3.2
Measurement oftime-temperature profiles
40
3.4 Results and Discussion
3.4.1
44
Influence o f number o f turns
vii
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44
3.5
3.4.2
Influence of coil diameter
46
3.4.3
Influence of tube diameter
54
3.4.4
Influence of initial temperature
60
3.4.5
Influence of pitch
60
Conclusions
CHAPTER 4
63
MICORWAVE PASTEURIZATION OF MILK: EVALUATION
OF PHOSPHATASE INACTIVATION KINETICS
64
4.1
Abstract
64
4.2
Introduction
64
4.3
Materials and Methods
66
4.3.1
Preparation of milk samples
66
4.3.2
Conventional water bath thermal treatment
66
4.3.2.1 Effectiveness o f heating time and kinetic data analysis
66
Continuous-flow microwave and thermal holding treatments
68
4.3.3.1 Effectiveness o f microwave heating time and kinetic data
analysis
71
Estimation of ALP activity and data analysis
72
4.3.3
4.3.4
4.4 Results and Discussion
72
4.4.1
Inactivation kinetics during conventional batch heating
72
4.4.2
Inactivation kinetics during continuous flow microwave heating
76
4.4.3
Inactivation kinetics during continuous flow thermal holding
78
4.4.4
4.5
Kinetic comparison: conventional water bath heating, continuous
flow thermal holding and continuous flow microwave heating
Conclusions
CHAPTER 5
GENERAL CONCLUSIONSAND RECOMMENDATIONS
REFERENCES
81
83
84
86
viii
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NOMENCLATURE
A
Mean residual enzyme activity, unit/ml
Ao
Initial enzyme activity, unit/ml
C
Concentration of any component of interest
d
tube diameter, m
D
Coil diameter, m
E
Microwave absorption efficiency, %
f
Flow rate, mL/min
F
Process lethality, s
j
Complex constant
k, ki, k2
Reaction rate constant, k at Ti, k at T2
n
Order of reaction
P
Output of microwave oven, W
R
Universal gas constant, kJ/mole
S
Frequency factor
t
Time, s
At
Change in time, s
T
Temperature, °C
AT
Change in temperature, °C
V
Fluid average velocity, m/s
z
Temperature sensitivity indicator, °C
cp
Specific heat, kJ/kg °C
dp
Penetration depth of microwave
De
Dean number
D i, D 2
Decimal reduction time at Tj, T2
Ea
Activation energy, kJ/mole
F0
F value at 121 °C, s
Hr
Heating rate, °C/s
Re
Reynolds number
te
Effective heating time, s
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tan 8
Dielectric loss tangent
ko
Free space microwave length
Greek Symbols
*
8
Relative complex permittivity
8
Relative dielectric constant
8
Relative dielectric loss
P
Fluid density, kg/m3
P
Fluid viscosity, Pa.s
Subscripts
i
Initial condition
0
Outlet condition
ref
Reference
Abbreviations
ALP
Alkaline Phosphatase
PME
Pectin Methlylesterase
PPO
Polyphenol Oxidase
AOAC
Associate o f Official Analytical Chemists, USA
CDT
Come-down Time
CUT
Come-up Time
FCC
Federal Communications Commission, USA
HTST
High Temperature Short Time
ISM
Industrial, Scientific and Medical
IT
Initial temperature
PME
Pectin Methyl Esterase
RF
Radio Frequency
TDT
Thermal Death Time
TTI
Time Temperature Indicator
MW
Microwave
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LIST OF TABLES
Table
Page
2.1
Major unit operations in microwave food processing
17
2.2
Types o f radiation with wavelength and quantum energy
30
2.3
Quantum energy required for breakage o f chemical bonds
30
2.4
Published data of studies investigating microwave effects on different samples
3.1
Design o f parameters in continuous-flow microwave heating system
43
3.2
Calculation of microwave absorption efficiency vs. flow rate for different
coil diameter (5.5 turns)
51
Calculation of microwave absorption efficiency vs. Dean number for
different coil diameter (5.5 turns)
54
Calculation of microwave absorption efficiency vs. flow rate for different
tube diameter (5.5 turns)
5
Calculation of microwave absorption efficiency vs. Dean number for
different tube diameter (5.5 turns)
59
3.6
Heating rate and microwave absorption efficiency for different coil height
60
4.1
Thermal kinetic parameters (D- and z- values) of ALP in milk at various
temperatures under conventional isothermal water bath heating condition
73
Thermal kinetic parameters (D- and z- values) of ALP in milk at various
temperatures under continuous-flow microwave heating
76
Thermal kinetic parameters (D- and z- values) of ALP in milk at various
temperatures during continuous-flow isothermal holding
78
3.3
3.4
3.5
4.2
4.3
4.4
32
Thermal kinetic parameters (D- and z- values) of ALP in milk under conventional
isothermal water bath heating, continuous-flow MW heating and continuous-flow
thermal holding conditions
81
xi
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9
LIST IF FIGURES
Figure
Page
2.1
Electromagnetic spectrum
8
2.2
Microwave heating mechanism
8
2.3
Top cross-sectional view of the cathode and anode of the magnetron
10
2.4
Semi-logarithmic survivor curve
23
2.5
Thermal death time curve to obtain z-value
25
3.1
Secondary flow in the form of a pair of vortices rotating in opposite
directions
38
3.2
Schematic diagram of continuous-flow microwave heating system
41
3.3
Coil geometric parameters
42
3.4
MW heating curves for coils with 3.3 turns (a), 4.5 turns (b) and
5.5 turns (c) as a function of flow rate at initial temperature 20°C
45
Flow rate as a function of Dean number for coils with different
number of turns
47
Temperature rise as a function of flow rate for coils with different
number of turns
47
3.7
Heating rate as a function of flow rate for coils with different number of turns
48
3.8
MW absorption efficiency as a function of flow rate for coils with
different number of turns
48
3.9
Temperature rise as a function of flow rate for coils with different coil diameter
50
3.10
Heating rate as a function of flow rate for coils with different coil diameter
50
3.11
MW absorption efficiency as a function of flow rate for coils with
different coil diameter
52
MW absorption efficiency as a function of coil diameter at initial
temperature 20°C and flow rate 5.0ml/s
52
Dean number as a function of flow rate for coils with different diameter
at initial temperature 20 °C
53
3 .5
3.6
3.12
3.13
xii
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3 .14
MW absorption efficiency as a function of Dean Number at initial
temperature 20°C and flow rate 5.0ml/s
53
3.15
Temperature rise as a function of flow rate for coils with different tube diameter
55
3.16
Heating rate as a function flow rate for coils with different tuber diameter
55
3.17
MW absorption efficiency as a function of flow rate for coils with
with different tube diameter
57
3.18
MW absorption efficiency as a function of tube diameter
57
3.19
Dean number as a function of flow rate for coils with different tube diameter
58
3.20
MW absorption efficiency as a function of Dean number for different coil
diameters
58
Exit temperatures as a function of initial temperature for (a) Coil No. 1
and (b) Coil No.2
61
3.22
Dean number as a function of flow rate for (a) Coil No. 1 and (b) Coil No.2
62
4.1
Schematic diagram o f conventional isothermal water bath heating
67
4.2
A schematic diagram of continuous-flow microwave heating and isothermal
holding system set-up
69
Time-temperature profile (dotted line) and corresponding lethal rate profile
(solid line) for milk under isothermal water bath heating condition
74
Survival curves o f ALP in milk during isothermal water bath heating as a
function of uncorrected heating times
74
Survival curves o f ALP in milk during isothermal water bath heating as a
function of corrected heating times
75
Temperature sensitivity curves for ALP in milk during conventional bath
heating treatment showing uncorrected (dotted line) and corrected
(solid line) heating times
75
Time-temperature profiles for milk at inlet of oven 1 (Ti), outlet of
oven 1 ( T 2 ) , outlet of oven 2 ( T 3 ) and outlet of the holding coil ( T 4 )
in continuous-flow microwave heating system at initial temperature
20 °C and flow rate 480ml/min
77
Time-temperature profile of milk during continuous-flow microwave
heating (dotted line) and computed effective times (solid line) at exit
temperature 70 °C
77
3.21
4.3
4.4
4.5
4.6
4 .7
4.8
xiii
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4.9
Survival curves o f ALP in milk during continuous-flow microwave
heating as a function of uncorrected heating times
4.10
Survival curves o f ALP in milk during continuous-flow microwave
heating as a function of corrected heating times
4.11
Temperature sensitivity curves for inactivation rates of ALP in milk
under continuous-flow microwave heating before time corrected
(dotted line) and after time corrected (solid line)
4.12
Survival curves o f ALP in milk during continuous-flow thermal
holding as a function of uncorrected heating times
4.13
Survival curves o f ALP in milk during continuous-flow thermal
holding as a function of corrected heating times
4.14
Temperature sensitivity curves for inactivation rates of ALP under
different conditions
xiv
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CHAPTER
1
INTRODUCTION
Thermal processing has been the most widely used method in food processing for
almost two centuries. Its primary objective is to extend shelf life of foods with assurance o f
safety by destroying food pathogens and by reducing or preventing food deterioration arising
from enzyme activity and microbial spoilage as well as chemical reactions.
The heat
treatment however affects quality attributes o f food products especially thermolabile
nutrients and organoleptic components.
In recent years, food processing has undergone
technological advances which have resulted in marketing minimally processed products.
Several techniques including microwave heating, ohmic heating and other novel heating
techniques, non-thermal processing technologies such as high pressure processing, electrical
pulse treatment, light pulses, ultrafiltration and irradiation, and addition of preservatives or
their combinations have been investigated as alternatives to conventional thermal processing
(Mertens and Knorr, 1992).
Microwave heating has been applied to the food processing since the 1950’s.
Presently, successfully implemented microwave processing operations in food industry
include cooking, blanching, drying, tempering, baking, pasteurization and sterilization
(Decareau and Peterson, 1986; Mudgett, 1989; Giese, 1992).
Microwave heating offers
several distinct advantages over conventional thermal techniques such as (1) improved
retention o f thermolabile constituents in the fluids such as milk and fruit juices due to the
possibility of employing HTST processing conditions (Mudgett, 1986), (2) reduced process
time due to fast heating rate, (3) improved energy efficiency, (4) controllable heating.
Microwave heating provides a better alternative to solve the fouling problem of heat
transfer surface inherent in conventional heat exchangers (plate/tubular). A severe problem
in conventional high-temperature short-time (HTST) processing of liquid food is the fouling
due to the solid deposits of food components exposed to the high temperature inner surface
of heating exchangers. Fouling not only reduces the heat transfer but also causes flavor
changes as fouled material decomposes.
Moreover, the incidence of fouling in heat
1
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exchangers can represent excessive costs in terms of capital invested and the maintenance of
equipment (Boot, 1989).
Currently, there are two ways to remove the fouling in food
industry: chemical cleaning and mechanical cleaning depending on the nature of the fouling.
Compared with conventional HTST processing, the internal volumetric heating associated
with microwaves keeps the tube surfaces relatively cooler than the product.
Hence, the
fouling problem could potentially be completely eliminated.
The dairy industry is the fourth largest sector of the Canadian agri-food economy
after grains, red meats and horticulture. In 2003, dairy farming generated $4.5 billion in total
farm cash receipts. During the same period, sales from Canadian dairy processors amounted
to $11 billion, representing 15.2% of all processing sales in the Canadian food and beverage
sector (CDIC, 2004).
Alkaline phosphatase (ALP) is a naturally occurring enzyme in raw
milk. The negative test for ALP activity is used as an indicator of adequate pasteurization in
the dairy industry because thermal resistance of ALP is slightly greater than that of common
pathogens in milk. A number o f studies have reported successful microwave pasteurization
of milk (Hamid et al., 1969; Jaynes, 1975; Chiu et al., 1984; Merin and Rosenthal, 1984;
Knutson et al., 1988; Kudra et al., 1991). In order to establish a microwave pasteurization
process for milk, one needs to understand the mechanisms of action of microwave process on
milk and ALP.
However, available kinetic parameters relating ALP inactivation under
microwave heating condition are very limited.
The widespread use of helical pipes as heat transfer equipment in food processing and
numerous engineering fields, such as chemical engineering, refrigeration, air conditioning
and nuclear engineering is due to the unique flow patterns resulting from tube curvature and
the advantage of volume compactness (Prabhanjan, 2001).
The flow patterns are
substantially more complex than in straight tubes because curvature induces a centrifugal
force that distorts the cross-section velocity profile compared to that in a straight tube, and is
manifested as what is usually termed a “secondary flow pattern” (Prabhanjan, 2001). The
secondary flow can result in thorough mixing of fluid providing uniform heating even under
laminar flow conditions.
The principle objective of thermal processing optimization is to promote
maximization of product quality, minimization of undesirable changes, minimize cost, and
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maximize profits, efficiency, etc., with most of them interested (Chen, 2001). By combining
helical coil technique with microwave, continuous-flow microwave heating has been
recognized as a promising technique for milk and other liquid foods pasteurization due to fast
heating rate, high quality retention, uniform heating, energy saving opportunities, prevention
o f surface fouling and easy to access to clean-up. Coil configurations influence microwave
absorption efficiency and heating characteristics in continuous-flow microwave heating
system. However, little attention has so far been given for optimizing coil configuration
parameters to improve microwave absorption efficiency and reduce heating time.
Although microwave heating has so many advantages over conventional heating,
industrial microwave processing of food did not develop rapidly due to lack of information
on product safety and quality (Mudgget, 1986). Moreover, there is a controversy about
microwave heating whether the non-thermal effect exits.
In recent years, studies have
focused on different procedures, approaches, experimental designs, techniques, and
biological systems to distinguish thermal and non-thermal effects of microwave heating and
resolve the controversial question (Tajchakavit, 1998; Tong, 2002). Olsen et al. (1966) is
probably among the first who postulated the existence of non-thermal effect.
In recent
years, there have been several studies which suggest that microwaves have additional lethal
effects other than those ascribed by heat. Several theories have been advanced to explain
how electromagnetic fields might kill microorganisms without heat as summarized in a
review by Knorr et al. (1994). On the other hand, several studies have refuted the existence
of non-thermal effects with pure microbial strains (Tiexeira-Pinto et al., 1960; Goldblith and
Wang, 1967; Bluhm and Ordal, 1969; Fujikawa et al., 1992).
In order to introduce continuous-flow microwave heating technique to dairy industry,
special approach is required to evaluate ALP inactivation kinetics under continuous-flow
microwave heating condition.
The objectives of this research were to optimize coil
configuration parameters to improve microwave absorption efficiency and establish kinetic
data of ALP inactivation for continuous-flow microwave heating.
The specific objectives
were to:
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1. Evaluate system variables such as coil diameter, tube diameter, number of turns and
initial temperature to optimize coil configuration parameters to improve microwave
absorption efficiency, heating rate, reduce heating time and temperature fluctuations.
2. Evaluate thermal inactivation kinetics of ALP in milk under conventional batch
heating condition.
3. Evaluate thermal inactivation kinetics of ALP in milk under continuous-flow
microwave heating and continuous-flow thermal holding conditions
4. Compare ALP inactivation kinetics under continuous-flow microwave heating with
conventional heating.
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CHAPTER 2 LITERATURE REVIEW
2.1 Thermal processing
Thermal processing has been used as a preservation method for perishable food
products since the early 1800's. The objective of thermal processing was to extend food shelf
life with assurance o f microbiologic safety.
Depending on the severity o f the heat treatment
and the purpose o f the process, different thermal processes regimes such as pasteurization
and sterilization, can be defined (Lund, 1975).
In addition to inactivation of unhealthy
microorganisms, thermal treatment also leads to desirable results, such as protein coagulation,
textual softening, and formation o f aroma components (Tong, 2002). However, at the same
time, it results in degradation o f quality attributes of food products especially thermolabile
nutrients and organoleptic quality.
Consumers’ demand for high quality, fresh, and
convenience food is the driving force of development and application of new food processing
technologies.
Several techniques including microwave heating, ohmic heating and other
non-thermal processing technologies such as high pressure processing, electrical pulse
treatment, light pulses, ultrafiltration, irradiation, addition of preservatives or their
combinations have been investigated as alternatives to conventional thermal processing
(Mertens and Knorr, 1992).
2.2 History of microwave processing
While Nicholas Appert, the inventor of canning, was still alive and thriving,
Michael Faraday postulated the electromagnetic field in 1832 (Decareau, 1985). In 1873,
less than 10 years after Pasteur had introduced the concept of pasteurization, James Clerk
Maxwell-working from Faraday’s hypothesis predicated mathematically the existence and
behavior of radio waves. Heinrich Hertz, a professor of physics at Karlsruhe, Germany,
verified Maxwell’s theory by experiment in 1885. Jacques Arsene d’Arsonval used Hertz’s
first high-frequency oscillator to experiment on the effects of high-frequency (500,0001,500,000 Hz), low-voltage alternating current on animals. He noted, as had Hertz, that
the only effect was production o f heat. This discovery led to the first high-frequency heat
therapy unit under d ’Aronval’s direction in 1895 at the Hotel Dieu hospital in Paris.
Microwave is the name given to electromagnetic waves arising as radiation from an
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electrical distribution across a broad spectrum of frequencies between 300 MHz and 300
GHz (wavelengths between 1 m and 1 mm). The magnetron, a microwave generator, was
first developed at the University of Birmingham in England about 1940. A prototype was
then brought to the United States and, consequently, the microwave radar was developed.
Radar (coined from the words “radio detection and ranging” by two U.S. Navy officers,
Furth and Tucker) was first used in 1934 for detection of aircraft (Page, 1962).
Microwave techniques and many of the applications of microwaves were developed during
and just prior to World War II when most of the efforts were concentrated on the design
and manufacture of microwave radar, navigation, and communications equipment for
military use (Decareau and Peterson, 1986).
Many peacetime uses of microwaves were developed just after World War II.
In
1946, Dr. Percy Spencer worked at radar-related research project for Raytheon Corporation.
He was testing a new vacuum tube called a magnetron when he discovered that the candy
bar in his pocket had melted. This intrigued Dr. Spencer, so he tried experiments with
other foods near the magnetron and he found that microwave could cause rapid temperature
rise within foods.
Dr. Spencer fashioned a metal box with an opening into which he fed
microwave power. The energy entering the box was unable to escape, thereby creating a
higher density electromagnetic field. When food was placed in the box and microwave
energy fed in, the temperature of the food rose very rapidly, thus microwave oven was
invented (Gallawa, 2003). At the same year, Raytheon Co. developed a microwave oven
and patented “Radarange” as the trademark. In this unit the magnetron and horn feed
assembly for directing energy were located above the cooking zone, essentially beaming
energy directly onto the food being cooked. This unit had a relatively small oven to hold
the food. By June 1947, a second-generation Radarange microwave oven with a larger
oven cavity had been developed and used in a large fine restaurant chain in Boston for
thawing and heating frozen main courses that had been prepared in a central kitchen, thus
heralding one o f the more important applications of radio frequency energy (Decareau,
1985). In 1948, Proctor and Goldblith used this unit to carry out their studies, perhaps the
first of the kind, using microwave energy for cooking and food processing (Decareau,
1985).
Several studies were carried out by Proctor and Goldblith in 1951 including
blanching of vegetables, coffee roasting, and the effect of cooking and baking on vitamin
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retention.
Work on freeze-drying of foods was reported by Jackson et al. in 1957
(Decareau and Peterson, 1986).
Microwave processing began on a commercial scale in the early 1960s when the first
conveyorized microwave system operating at 915 MHz and 25 KW was introduced by
Cryodry Corporation o f San Ramon, California. At about the same time, Litton Industries
developed a conveyorized ovens powered by 2.5 KW, 2450 MHz modular sources. One
system combined microwave power and saturated steam operating at 2450 MHz with a total
power of 130 KW was introduced in 1966 for poultry precooking. By the middle of 1966, a
number o f potato chip companies in both the United States and Europe were using
microwave finish dryers to control chip color (Decareau, 1985; Decareau and Peterson, 1986).
While the popularity o f the home microwave oven as a consumer appliance began in 1970s,
commercial microwave systems for tempering and drying also hit their stride in the middle of
the 1970s (Tajchakavit, 1998).
2.3 Principles of microwave heating
Microwaves cover a broad spectrum of frequencies, usually considered to range from
300 MHz to 300 GHz (Decareau & Peterson, 1986) as illustrated in Figure 2.1. In order to
avoid interference with the radio frequencies used for telecommunication purposes, only
certain frequencies are allocated by the Federal Communications Commission (US FCC) for
industrial, scientific and medical use (ISM).
The most common frequencies used within
North America are 915 MHz and 2450MHz (Decareau, 1985). The penetration depths range
from 8 to 22 cm for 915 MHz and 3 to 8 cm for 2450 MHz depending on moisture content
(Decareau, 1985).
When microwave are intercepted by dielectric materials such as food, they interact
with the dielectric material, giving up energy which results in a temperature increase to the
material (Decareau and Peterson, 1985).
There are two main mechanisms by which
microwaves produce heat in dielectric materials: dipole rotation and ionic polarization
(Figure 2.2.). Water is the most common polar materials in foods. Under normal conditions,
polar molecules are randomly oriented. As an alternating field is applied, the polarity of the
field is varied at the rate of microwave frequency and molecules attempt to align themselves
with the changing field and flip-flop several million times per second.
As a result, heat is
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3x10 m
3x10 m
3x10 m
3x10 m
VLF
Infra Sonics
Audible
10 Hz
100 Hz
10 kHz
3000 m
3 00 m
3 0m
LF
MF
30 KHz
VHF
Radio TV
300 KHz
3 MHz
30 cm
30 MHz
3 cm
1
3 mm
0.3 mm
1
1
UHF
SHF
EHF
Infrared
Microwaves
1
300 MHZ
3 mm
HF
Ultra Sonics
10 Hz
3 GHz
'
Sub millimeter
1
1
30 GHz
300GHz
3000GHz
Figure 2.1 Electromagnetic spectrum (Soruce: Decareau and Peterson, 1986)
W ater M b Lecu le
S odium
C hlorine
Ion
Ion
105
A Ltemating
E lectric
V
f
F ie ld
t
*
Dipolar Interaction
Ionic Interaction
Figure 2.2 Microwave heating mechanism (Source: Tong, 2002)
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generated instantaneously as a result of internal molecule friction. Ionic polarization occurs
when ions in solution move in response to an electric field. Ions carry an electric charge and
are accelerated by the electric field. Kinetic energy is given up by the field to ions, which
collide with other ions, converting kinetic energy into heat.
These interactions are also
affected by the state of the constituents, whether they are bound or free, e.g., bound ions have
much lower microwave absorptivities (Decareau and Peterson, 1986; Ramaswamy and van
de Voort, 1990).
2.3.1
How a magnetron works
As described by Van Zante (1973), the magnetron (Figure 2.3) is the heart of the
microwave oven. In its simplest form the magnetron consists o f the two elements of an
electron tube - a cathode and anode each of which is in circular form with the anode resonant
cavities (anywhere from four to eighty). A magnet (permanent or electric) is placed around
the anode to provide a magnetic field. When the cathode is heated by means of an electrical
filament, it gives off negatively charged electrons which are attracted to the positively
charged anode. The magnetic field of the magnets around the anode cause the electrons to
move in an orbital fashion rather than a straight line as they jump from the cathode to the
anode under an electrical pressure of 4,000 to 6,000 volts. As the electrons approach the
anode, they pass by the resonator cavities of the anode and this cause the electrons to
oscillate at a very high frequency (2450 MHz or 915 MHz). The high-frequency oscillations
o f the electrons in the magnetron are picked up by a small antenna on the top the magnetron
tube. These oscillations are transmitted through a wave guide to a feed box from whence they
distributed into the oven cavity.
2.4 Basic components of microwave oven
The basic components of a microwave oven are well described by (Van Zante, 1973).
In general a microwave oven consists of following components:
(a)
a boxlike cavity (RF cavity) which encloses the food within sealed metallic walls so
that the distributed energy absorbed by the food is received directly from the
microwave source or reflected from the walls
(b)
a door at the front of the cavity with energy-sealing or -trapping devices which
retain the microwaves within the oven
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Resonant cfcvlfciss
Cathode |
.g|0 6 fro»s
Anode
Figure 2.3 Top cross-sectional view of the cathode and anode of the magnetron.
The electron “wind” between the anode and cathode generates the
high-frequency microwaves which are picked up and then transmitted
to the oven by means of the antenna. (Adapted from a drawing courtesy
o f Amana Refrigeration, Inc.)
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(c)
the power supply which draws electrical current from the line and converts it to the
form required by the microwave generator
(d) the microwave generator or power tube, usually spoken of as the “magnetron”,
which is an oscillator capable of converting electrical power into microwaves
(e)
the transmission and coupling section, which transfers the generated
microwaves to the oven and which includes the wave guide and a feed box
(f)
a rotating disc or stirrer, usually located at the bottom of the feed box and at the
top of the oven, which distributes the microwaves into the oven and also
disturbs standing wave patterns that may tend to form
(g)
Energy sealing or trapping structure to prevent leakage of energy from the oven
(h) Operating controls and safety interlocks
2.5 Parameters affecting microwave heating
As summarized by Schiffmann (1986), microwave heating of food materials is
affected by a number of properties of the equipment and materials being heating.
2.5.1
Dielectric properties
Microwave properties of basic interest in food processing were discussed by Mudgett
(1982) in terms o f complex permittivities for biological materials (von Hippel, 1954):
( 2 . 1)
e=s -je
where the real component is the dielectric constant e' and the imaginary component is the
dielectric loss factor s" of the material, j is the complex constant. The dielectric constant is a
measure of a material’s ability to store electric energy, and the loss factor is a measure o f its
ability to dissipate the electrical energy in the form of heat.
Complex permittivity is a
measure of a material’s ability couple electrical energy from a microwave power generator
(magnetron). The ratio of the dielectric loss to the dielectric constant, defined as the loss
tangent:
( 2 .2 )
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is related to the material’s ability to be penetrated by an electrical field and to dissipate
(attenuate) electrical energy as heat. Materials are classified on the basis of loss tangent.
Those that are highly lossy are those that absorb microwave energy efficiently. Materials
that are highly transparent have low loss factors, such as Teflon, glass, kerosene.
2.5.2 Frequency
There are two available frequencies for microwave heating - 915 MHz and 2450
MHz; their wavelengths in are 33 and 12.2 cm, respectively. The commonly used equation
for determining the value of the penetration depth is given by:
dp =
—7[~J\ + tan 2 ^ ) - l ] 1/2
2n:^2s
(2-3)
where X0= free space microwave wavelength (for 2450 MHz, \= 1 2 .2 cm); 8 = the loss
tangent of the material; e = the relative dielectric constant of the material . The smaller the dp,
the more surface and uneven heating.
2.5.3
Power of microwave
Most industrial microwave systems operate at microwave power output ranging from
5 to 100 kw. The speed of microwave heating is usually controlled by varying the power
output. The higher the power output, the faster the heating of a given mass.
2.5.4 Mass
There is a direct relationship between the mass and the amount of microwave power
which must be applied to it to achieve the desired heating. When the mass is small, this
might best be done in a batch oven, whereas a larger throughput would often be better done
in a conveyorized system.
Such conveyorized systems have the added advantage of
providing greater heating uniformity by moving the product through the microwave field.
2.5.5
Moisture content
Water is usually the major influence in how well materials, particularly foods, absorb
microwave energy. Usually, the more water present, the higher the dielectric loss factor and,
hence, the better the heating. In general: (a) the higher the moisture content, the higher the
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dielectric constant; (b) dielectric loss usually increases with increasing moisture content, but
it levels off at a value in the range o f 20-30% and may decrease at still higher moisture
contents; and (c) the dielectric constant of a mixture usually lies between those of its
components (Tinga, 1970).
2.5.6
Density
The density of a product has an effect upon its dielectric constant. The dielectric
constant o f air is 1.0, and air is for all practical purposes, completely transparent at the
industrial heating frequencies.
Thus, air inclusions will reduce a material’s dielectric
constant. Hence, as a material’s density increases, so does its dielectric constant, often in an
almost linear fashion.
Very porous materials, such as bread dough, because of the air
inclusion, and become good insulators when they are baked and their density decreases.
2.5.7
Temperature
The temperature of the material plays a role in microwave heating in several ways:
(a)
The dielectric loss may increase or decrease with temperature, depending upon the
material. Since temperatures and moisture levels change during heating, they may have
a profound effect upon the dielectric constant, dielectric loss factor, and loss tangent,
and it is important to know what functional relationships exist between these
parameters in any materials.
(b)
Freezing has a major effect upon a material’s heating ability because of the vastly
different dielectric properties of ice and water. Whereas water is highly absorptive and
heats well, ice is highly transparent and does not heat well at all. For example, the
relative dielectric constant, loss tangent, loss factor are 78, 0.16 and 12.48 for water at
25 °C, 3.2, 0.0009 and 0.0029 for ice, respectively.
(c)
The initial temperature of food products being heated by microwaves should either be
controlled or known, so the microwave power can be adjusted to obtain uniform final
temperatures. In other words, if the microwave system is set to raise the temperature of
a material from 20 °C to 80 °C, but the initial temperature is only 15 °C, it will raise the
temperature only 75 °C unless the microwave power into the system is increased.
2.5.8
Physical geometries
As summarized by Schiffmann (1986), the size and shape of food materials being
heated play an important role in the distribution of heat within the food product. The closer
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the size is to the wavelength, the higher will be the center temperature. Smaller particulates
require less heat than larger ones.
In addition, the more regular is the shape, the more
uniform will be the heat distribution within the product. Round is better than square, and a
torus is an ideal shape. A higher surface to volume ratio results in more rapid heating (Giese,
1992). Therefore, the heating rate for a sphere will be different from that for a cylinder with
the same volume. The' relationship between load geometry, load orientation and oven cavity
parameters such as cavity size and geometry, however, are yet to be established (Buffler,
1993).
2.5.9
Thermal conductivity
This may have an important effect when heating large materials where the depth of
penetrate is not great enough to heat uniformly to the center, or when the microwave heating
time is long. In cases where the time is short, thermal conductivity will play a secondary role,
and it may be necessary to extend the heating time to achieve its benefit.
2.5.10 Specific heat
The specific heat indicates the amount of energy required to raise the temperature of a
unit mass of the food by a degree. It is this property which can cause a material which has a
relatively low dielectric loss to heat well in a microwave field. For example, the specific heat
of cooking oil is 2.0 KJ/Kg °C, whereas that of water is 4.2 KJ/Kg. °C. Therefore, oil heats
considerably faster than water because of its far lower specific heat.
2.5.11 Secondary flow in curved pipe
When a fluid flows in a helically coiled tube, centrifugal forces cause secondary fluid
motion in the form o f a pair of vortices rotating in opposite directions.
Dean first showed
that the parameter De = Re -\Jd / D now known as the Dean number (Re is the Reynolds
number, d is tube diameter, D is coil diameter), is the unique dynamic similarity parameter
governing fluid motion in helical coiled tubes.
Many workers have shown that secondary
flow increases heat and mass transfer rates in addition to the rate of momentum transfer, the
late resulting in an increased pressure drop (Dravid, 1971).
The heat transfer rates are
usually a few percent to several-fold higher in a helical coil, the amount depending on types
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o f flow regime (laminar or turbulent), fluid properties and helix configuration (Prusa and Yao,
1982).
2.6
Advantages and limitations of microwave heating
The most prominent characteristic of microwave heating is volumetric heating, where
materials can absorb microwave energy directly and internally and convert it to heat. This
characteristic leads to advantages of microwave processing over conventional thermal
processing. Volumetric heating greatly reduces the processing time, improving the retention
of thermolabile nutrients. Fouling is the solid deposits of food components exposed to the
high temperature inner surface due to a series of chemical reactions. For viscous liquid foods,
scraped surface of heat exchangers have been developed to minimize fouling problems by
continuously sweeping the hot surfaces with scraper blades.
Combining with other
techniques, such as helical coil, microwave heating processes an alternate convenience by
which surface fouling could be completely solved since lower temperature gradients from the
surface to the center. Microwaves offer particularly more of an advantage to food with poor
conductivity.
In drying, although there is less amount of water, the available water
selectively absorbs microwave energy and rapidly diffuses water vapor to the surface
(Tajchakavit, 1998).
Since microwave heating occurs only within the food not the surrounding and heat is
produced only where it is needed, so energy consumption is considerably reduced.
An
increased processing rate and advances in designing microwave equipment have also lead to
possible compact microwave equipments (Tajchakavit, 1998). During processing, heating
conditions can be controlled instantaneously by starting up, shutting down system, or
adjusting power level.
Cleaning can also be carried out almost immediately since the
equipment is cool after operation. Moreover, microwave can be combined with other energy
sources and chemicals.
As with all processing techniques, application of microwave energy for food
processing has some limitations.
Decareau (1985) pointed out that the difficulty of
development of microwave processing in food industry was that the communication between
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microwave engineers and food manufacturers had been very bad and besides, the food
industry is reluctant to invest their money to adopt new technology. This is due to a lack of
knowledge and understanding about the interaction of microwaves and food including
information on product safety and quality as well as a non-uniform heating characteristic.
Buffler (1993) also provided several reasons for the failure of microwave food processing
including: (1) only 50% efficiency can be expected compared with gas thus rendering
microwaves advantageous only under some circumstances, (2) unfamiliar heating techniques
and (3) poor perception due to operation safety, although the cost imbalance might not be as
great if all the running costs are considered. Ohlsson (1991) pointed out that microwave
heating is not suited for large foods due to uneven heating characteristic, also, he mentioned
that in microwave pasteurization and sterilization, very high requirement on heating
uniformity must be met in order to fulfill the quality advantages.
To alleviate the problems realized from the limitations, a more in-depth
understanding of the microwave heating principle and a good design o f the microwave
device that should be compatible to the application are essential. A flexible, proven and cost
effective system is also required. Furthermore, the application o f the microwave heating
technique in conjunction with other heating techniques also provides better output in some
cases including microwave finished drying. With the advent of these processes, the problem
can be greatly reduced.
In addition, with the technological advances in microwave
susceptors, problems such as lack of browning and crispness in microwave processed foods
have been improved (Decareau, 1985).
2.7
Microwave applications in food processing
As summarized by Schiffmann (2001), a number of microwave processes have been
developed in food processing industry. The two most successful uses of microwave energy
for food processing, in terms of number of operating systems, are tempering meat, poultry,
fish, fruit, and butter and the precooking bacon.
Each hour, 300 major food companies
temper a total of approximately 1000 tons of bulk frozen food for further processing. At the
same time, processors in the North American and United Kingdom precook about 2,500,000
slices of bacon each hour (Eves, G. 1996). Currently used microwave-based industrial food
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processing operations are summarized in Table 2.1.
Table 2.1 Major unit operations in microwave food processing
Application
Major objective
Products
Cooking
Modify flavor and texture
bacon, chicken, sausage patties
Tempering
Raise temperature below
freezing
Cause expansion, make edible
foams and reduce undercooked
core
Reduce moisture content
meat, fish, poultry, vegetables,
fruit and butter
Bread, sweet dough products,
proofing/baking yeast raised
donuts, cake, doughnut
cookies, biscuits, potato chips,
pasta, snack, crackers, cereal,
nonfat and low-fat nonfried
potato chips
Fresh pasta, ready meals,
bread, milk
Baking
Drying
Pasteurization
and
Sterilization
2.7.1
Inactivate vegetative microbes,
Inactivate microbial spores
Meat and poultry processing
The use o f microwaves in processing meat accounts for some of the earliest
installations of microwaves in the food industry, as well as the applications accounting for
most of the current operational systems (Schiffmann, 2001). The driving forces for adoption
of microwaves to meat processing vary, but rely upon the controlled internal heating which
provides speed, yield, and end product quality not achievable by conventional techniques.
Microwave precooking of bacon causes less physical shrinkage, so the structure has greater
final volume and better control of the final shape and dimensions than when grilled.
Currently no other process matches or exceeds its advantages. As reported by Schiffmann,
there are now well over 30 continuous microwave bacon cookers operating in the United
States, most of large microwave power and product output. However, only 5 microwave
systems were employed for chicken precooking and sausage patties cooking.
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2.7.2 Tempering
Without doubt, this is the single most successful application of microwave heating in
industry. There are at least 400 tempering systems operating in the United Sates alone,
primarily for tempering o f frozen meat, but also for fish, poultry, vegetables and fruit. In the
United Kingdom there are several large systems, up to 200 kW, utilized for tempering of
frozen beef, as well as butter. 915 MHz tempering systems, batch and continuous, are sold
worldwide, including the United States, Europe, United Kingdom, China, Japan, Korea, and
Australia (Schiffmann, 2001).
2.7.3
Baking
The various unit operations involved in baking, especially proofing and baking, lend
themselves to microwave processing because the heat transfer problems encountered by
conventional means are easily overcome by microwave heating (Schiffmann, 2001). The
addition of microwaves to baking processes can increase the speed dramatically - 3, 4, or
more times faster because air is transparent to microwaves, the foam allows the microwaves
to penetrate easily to the center of the product. When properly applied, usually combined
with a conventional heat source, the microwaves seek out the wetter interior, while the
external heat bakes the outer layers of the product.
2.7.4
Drying
Microwave drying employs a completely different mechanism. The internal pressure
gradient generated by the microwave field can effectively pumps water to the surface.
Microwaves and RF can speed up almost any drying process in which the liquid being
evaporated. Always there is the use of additional heat - hot air, ambient or forced circulation;
infrared; or some combination of these.
In fact microwave heating usually represents a
minor part of the total heat energy required for drying, the reason being cost. Microwaves
can be applied to a drying process throughout the drying process at a low level, or at the end
for finish drying, or prior to the hot air drying (Schiffmann, 2001).
The advantages o f microwave drying are listed below:
(a) Efficiency: In most cases the energy couples into the solvent, not the substrate.
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(b) Nondestructive: Drying can be done at low ambient temperatures; no need to maintain
high surface temperatures. This leads to lower thermal profiles.
(c) Reduction of migration: Solvent often mobilized as a vapor thereby not transporting
other materials to the surface.
(d) Leveling effects: Coupling tends toward wetter areas.
(e) Speed: Drying times can be shortened by 50 % and more.
(f) Uniformity o f drying: By a combination of more uniform thermal profiles and
leveling.
(g) Conveyorized systems, less floor space, reduced handling: No need for batch
processing in most cases.
(h) Product improvement in some cases: Eliminates case hardening, internal stresses, etc.
As reported by Schiffmann (2001), numerous microwave systems have been applied in drying
potato chips, pasta drying, finish drying biscuits and crackers, and cereal cooking and drying.
2.7.5 Pasteurization and sterilization
Pasteurization is a thermal process, which provides a partial sterilization of substances by
inactivating pathogenic microorganisms, notably vegetative cells of bacteria, yeast, or molds.
The effectiveness with which such organisms are deactivated is a measure of the adequacy of
pasteurization. Pasteurization is usually carried out at a temperature of 100 °C or lower and the
products have to be refrigerated.
Sterilization processes are designed to inactivate
microorganisms or their spores which would grow and cause spoilage or health hazards under the
normal conditions of storage. Thermal sterilization is usually done at temperatures in excess of
100 °C which means they are usually done under pressure (Schiffmann, 2001).
Microwaves pasteurization has been industrially accomplished for decades, especially in
Europe. The equipment used for pasteurization consists of a long conveyorized tunnel
microwave oven where the product is heated up by microwaves to the pasteurization
temperatures. Microwave pasteurization has been successfully used to process ready meals in
hermetically sealed microwavable trays to increase the shelf life.
These products are
successfully distributed and sold in the refrigerated state in Europe where refrigerated or “ready”
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meals are quite popular and o f good quality (Schiffmann, 2001). Pasteurization of fresh pasta
(i.e., not dried) in the package had commercial success in the United States, especially when
combined with controlled atmospheres. However, the extent of use is low. It is employed to a
greater extent in Europe and Japan (Schiffmann, 2001). The process has also been successfully
used for bread products, soft cheese, fruits and sugar mixture (Buffler, 1993).
A commercial microwave pasteurization for yogurt has been used in Germany employing
two frequencies, 40 kW at 27 MHz and 8 kW at 2450 MHz. The low frequency technique
increased the temperature of the main body of the container of yogurt while being conveyed
through a water bath operating at 60 °C. The high frequency technique heated the top portion of
the products. This process provided extended shelf life (Decareau, 1985). Extended shelf life
and improved quality due to the significant reduction in process time are major advantages of
microwave pasteurization over conventional pasteurization (Tajchakavit, 1998).
Schiffmann (2001) reported that Natick laboratories in the United States developed
systems for sterilizing products such as chicken fricassee and beef stroganoff in pouches, in an
attempt to prepare shelf-stable individual rations for military applications.
laminated polyester and polypropylene/polyester pouches were used.
Polyethylene-
The pouches were first
conventionally preheated to 98 °C, then exposed to microwave heating to raise the temperature
(in a pressure vessel) to 121 °C, followed by a holding period in water at 121 °C, to achieve
sterilization, and finally rapid cool-down with water. The Multitherm Process of Alfa-Laval AB
in Sweden also developed system to process microwaved food in pouches that were immersed in
a fluid medium having a dielectric constant at least one-half that of the product. Adjusting the
temperature of this circulating medium could then control the temperature of the product surface.
The work was dropped in the late 1970s due to lack of a suitable packaging material. However,
such materials are now available.
Microwave heating has not been used for sterilization as successfully as for
pasteurization (Buffler, 1993). The presence of hot and cold spots has been a major concern due
to the ability of C. botulinum to grow readily in a nonacidic anaerobic condition (pH >4.6). The
process has not been commercialized in North America, however, microwave-sterilized pasta
dishes have been successfully marketed in Europe (Tajchakavit, 1998).
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2.8 Inactivation/Destruction kinetics
2.8.1
Thermal destruction
Reaction kinetic parameters for thermal inactivation/destruction of enzymes and
microorganisms as well as quality attributes are required for basically three reasons (Lenz and
Lund, 1980): (1) for establishing a thermal process, (2) for minimizing loss of a quality factor
and (3) for shelf life testing. The kinetic parameters available in the literature as described by
Hayakawa et al. (1981) can be categorized into the following two groups: (1) those based on
unistep or multi-step reaction kinetics employing application of some characteristic formulae and
(2) those based on empirical analysis of time-temperature curves of the inactivation.
has been widely used for kinetic data analysis for thermal processing.
The latter
The mechanism of
inactivation/destruction o f various attributes including enzymes and microorganisms can be
based on the generalized nth-order kinetic model (Labuza, 1980):
dc
— = - k Cn
dt
(2.4)
dc
where — = the time rate o f change of concentration (C); k = the reaction rate constant; n = the
dt
order of reaction.
Generally, for many food components, enzymes and microorganisms, the fist order
kinetics model adequately describes the destruction (Stumbo, 1973). However, according to
Labuza (1982 a, b), nutrient losses due to thermal processing can generally characterized to
follow a zero- or first- order kinetics.
Considering a first-order reaction rate, integrating Equation (2.4) between limits Ci at
time ti and C2 at time t 2 and converting the natural logarithm to base 10 results in:
^ 0 2 = ^ 0 !- -^ 2.303
(t2-ti)
(2.5)
The temperature dependency of the reaction rate constant (k) can be described by many
theoretical and experimental relationships (Adamson, 1986).
Among those, two principle
methods have been proposed.
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2.8.2
Arrhenius equation
One o f the relationships which was empirically introduced by Arrhenius in 1889 is based
on a thermodynamic approach:
Ea
k = S e ' RT
(2.6)
where k = the reaction rate constant at T, S = the frequency factor, Ea = the activation energy for
the reaction, R = the universal gas constant, and T = absolute temperature.
Rearranging
Equation (2.6) for two temperatures Ti and T2 with their corresponding reaction rate constants,
Ki and K2, results in:
log h , =
E a ■( 1
- 1
)
ky
2 .3 0 3 R Tx T2
(2.7)
2.8.3 Thermal Death Time (TDT)
Thermal Death Time (TDT) which is the alternate concept of describing the temperature
dependency of the reaction rates in food that is based on empirical considerations proposed by
Bigelow (Lund, 1975).
The logarithm o f the number of survivors following a heat treatment at a particular
temperature plotted against heating time will give a straight line curve (Figure 2.4). These curves
are commonly called survivor curves. The D-value or decimal reduction time represents the time
required to reduce the number of survivors by 90% in survival curve.
Graphically, this
represents the time range between which the survival cure passes through one logarithmic cycle
(Figure 2.4). The survival curve indicates that:
slope = ___ *2 ~ ?1____
!ogC2 —log Cj
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(2.8)
where C s and C? represent the survivors following heating time ti and t?. respectively. When t?-ti
---1) and Lou fC|VLog (C?) = 1.00. the slope - - J_ . From Equation (2.5), we can know that the
D
D-value is reciprocally related to k as follows:
D =
2.303
(2.9)
k
i .00
log C
r
Time at a constant temperature
Figure 2.4 Semi-logarithmic survivor curve
The logarithmic nature o f the survivor or destruction curve indicates that complete
lesiruction of microorganisms or enzymes is not a theoretical possibility, because a decimal
fraction o f the population should remain even after an infinite number o f D-values. In practice,
.■alculated fractional survivors are treated by a probability approach; for example, a surviving
oonuiation of 10 S/unit would indicate one survivor in 10 s units.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Another term that generally contradicts this logarithmic destruction approach is often
employed: thermal death time (TDT), which is the heating time required to cause
microorganisms, enzymes, or nutrients death or destruction.
Such data are obtained by
subjecting number of survivors to a series of heat treatments at a given temperature and testing
for survivors.
Comparing TDT approach with the decimal reduction approach, it can easily be
recognized that TDT value depends on the initial load of survivors (while D value is not).
Moreover, if TDT is always measured with reference to a standard initial load or load reduction,
it would represent a multiple of the D-value. For examples, if TDT represent the time to reduce
the population from 1012 to 10°, then TDT is a measure of 12 D values. OR
TDT = nD
where n is the number of decimal reductions. The D-value depends strongly on the temperature
employed.
Higher temperatures obviously result in smaller D-values.
The temperature
sensitivity of D-values at various temperatures is normally expressed as a thermal resistance
curve with log D-values plotted against temperature (Figure 2.5). The temperature sensitivity
indicator is defined as a z value, which represents a temperature range that results in a 10-fold
change in D-values, or graphically it represents the temperature range through which the D-value
curve passed through one logarithmic cycle.
r _
Mathematically,
(72 ~ T\)
(2.10)
Pog(A)-log(Z>2)]
where Di and D 2 are D-values at Ti and T2 , respectively. The D-value at any given temperature
can be obtained from a modified form of the above equation using a reference D-value (Do at a
reference temperature, Tr, usually 121 °C for thermal sterilization):
D = D0 i o (7>~r ) / Z
(2.11)
To determine kinetic parameters of inactivation/destruction of many food components,
the food components in appropriate media are subject to heat treatment at different temperatures.
However, there is always a log period associated prior to reaching the temperatures of the
surrounding heating or cooling media. It is, therefore, essential to properly correct the times
24
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associated during the lag periods especially when the heating time is fairly short.
In some
processes, a shorter lag period could be obtained by using steam as the heating medium which
provides a better heat transfer. This, however, does not necessarily eliminate the lag corrections.
log Di -log D2 = 1.00
J3
log D
I
Q
Ml
O
-J
iogD:
Temperature (°C)
Figure 2.5 Thermal death time curve to obtain z-value
The temperature dependency of k and D are, however, at variance when considering the
TDT and Arrhenius concepts. The TDT approach is based on the assumption that the thermal
death time (or D-value) of microorganisms, enzymes, or nutrients follows a semi-logarithmic
relationship and relates proportionally with temperature as follows:
log
(212)
A
where Di and D2 are D-values at Ti and T2, respectively.
Since the D-value is based on a
logarithmic destruction, the completer inactivation/destruction of enzyme activity or of the
microbial population is theoretically not feasible.
An adequacy of heat treatment therefore
employed a probability approach for process calculation.
From Equation (2.7) and (2.12), the relationship between Ea and z can be obtained as
follows:
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z
2 3m R T T
(2 .1 3 )
-----------
E.
It should be noted that both TDT and Arrhenius concepts contradict each other since the
reaction rate constant (k) is reciprocal of the absolute temperature for the Arrhenius concept with
Ea as the slope index o f a semi-logarithmic plot. Whereas the D-value is a direct proportional to
temperature in the TDT concept with z as the slope index of a semi-logarithmic plot. However,
Lund (1975) suggested that the two concepts are reconcilable at small temperature ranges.
According to Ramaswamy et al.(1989), both concepts have been proven to be suitable for
studying inactivation/destruction kinetics; however, they demonstrated that erroneous results
could be associated with conversion of parameters from one concept to another depending on
associated reference temperature and temperature range employed. The authors recommended
the use o f the lower and upper limit of the experimental temperature range instead o f the
approach suggested by Lund (1975) where T is assumed to be proportional to 1/T for small
temperature ranges. The TDT concept is more widely applied in thermal process calculation
(Hallstrom et al., 1988; Ramaswamy et al., 1989).
2.8.4
Lag correction
To determine kinetic parameters of inactivation/destruction of many food components,
the food components in appropriate media are subject to heat treatment at different temperatures.
However, there is always a lag period associated prior to reaching the temperatures o f the
surrounding heating or cooling media. It is, therefore, essential to properly correct the times
associated during the lag periods especially when the heating time is fairly short.
In some
processes, a shorter lag period could be obtained by using steam as the heating medium which
provides a better heat transfer. This, however, does not necessarily eliminate the lag corrections
(Tajchakavit, 1998).
2.8.5
Lethality concept
Lethality (F-value) is a measure of the heat treatment or sterilization processes.
To
compare the relative sterilizing capacities of heat processes, a unit o f lethality needs to be
established. For convenience, this is defined as an equivalent heating of 1 minute at a reference
temperature, which is usually taken to be 121 °C for the sterilization processes. Thus the F value
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wouldrepresent
a certain multiple or fraction of the D-value depending on the type of the
survivor. In arealprocess, the food component passesthrough changes in temperatures and the
lethal effects o f temperatures are integrated over the heating time based on the time-temperature
profile giving a process lethality (F):
t
F = JlO
0
(T-Tref)
z dt
(2.14)
Moreover, a relationship like equation (2.11) also holds good with reference to the F-value:
F = F0 10 <Tref~T)/z
(2.15)
The Fo in this case will be the F-value at the reference temperature (Tref).
A reference (or
phantom) TDT curve is defined as a curve parallel to the real TDT or thermal resistance curve
(i.e., having the same z value) and having a TDT (F value) of 1 min at 121 °C. With a phantom
TDT curve so defined, it will be possible to express the lethal effects of any time-temperature
combination in terms of equivalent minutes at 121 °C or lethality or
F0 = F 1 0 <T'Tref)/Z
(2.16)
Thus, an F-value of 20 min at 115 °C is equivalent to an F0 of 5.02 min, while the same F-value
at 125 °C is equivalent to an F0 of 50.24 min when z = 10 °C. In these situations, it is assumed
that heating to the appropriate temperature and the subsequent cooling are instantaneous. For
real processes where the food passes through a time-temperature profile, it should be possible to
use this concept to integrate the lethal effects through the various time-temperature combinations.
The combined lethality so obtained for a
process is called process lethality and is also
represented by the symbol F0. Furthermore, with reference to the processing situation, the
lethality can be expressed as related to a specific location (normally thermal center) or any other
arbitrarily chosen location or the integrated over the product. From microbiological safety point
of view, the assurance of a minimal lethality at the thermal center is of utmost importance, while
from a quality standpoint it is desirable to minimize the overall destruction throughout the
product (Chen, 2001).
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M ost the early literature relevant to PME inactivation discarded the heating lag correction
by assuming instantaneous increase in temperature (Rouse and Atkins, 1952; 1953; Atkins and
Rouse, 1953; Ulgen and Ozilgen 1991).
There are several reports showing application of the
time correction to the lag periods (Eagerman and Rouse, 1976; Nath and Ranganna, 1977a,b,c;
Versteeg et al., 1980; Ramaswamy and Ranganna, 1981; Marshall et al. 1985; Wicker and
Temelli, 1988; Tajchakavit and Ramaswamy, 1998). Graphical and numerical integration based
on time-temperature history of the sample. The procedure is similar to the one described in
Hayagawa et al. (1981) who suggested an iterative computerized procedure for estimation of
kinetic parameters by correcting isothermal heating times for heating and cooling lags.
2.8.6 Microwave kinetics
As in thermal inactivation/destruction, microwave inactivation/destruction kinetics of
food constituents such as enzymes, microorganisms and quality attributes are required for
establishing microwave processing.
Similar concepts can be applied for determining kinetic
parameters during microwave heating, however, non-isothermal heating conditions are involved
in this case. The food constituent basically experiences changing temperature with time. The
procedure is more complex than with the isothermal procedure. There have been only a few
studies describing kinetics during microwave heating. A numerical integration o f lethality o f
PME in orange juice, and Saccharomyces cerevisiae and Lactobacillus plantarum in apple juice
(Tajchakavit and Ramaswamy, 1998) was used based on time-temperature profiles to predict
thermal inactivation effects during microwave heating.
2.9
Mechanisms of inactivation/destruction
As summarized by Tajchakavit (1998), the mechanisms contributing to microbial death
or enzyme inactivation can be classified into 3 major groups: (1) thermal effects, (2) kinetic
effects and (3) chemical effects.
2.9.1 Thermal effects
At the molecular level, the structural changes of the bound water layer may be affected
by conventional thermal energy (Cleary and Mills, 1970). This also leads to changes in stability
and functionality o f macromolecules and subsequently the biological processes in the
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cells/molecules such as denature of proteins and nucleic acids inducing inactivation/ destruction
of enzymes or microorganisms.
2.9.2
Kinetic effects
The net charges of cells may be influenced in an electromagnetic field of microwaves and
could lead to a rapid oscillation that exceeds the elastic limitation of the cell wall resulting in the
disruption o f the cell membrane (Carroll and Lopez, 1969; khalil and Villota, 1988; Palaniapan
et al., 1990). This may lead to modification of membrane permeability, leakage o f cellular
contents such as release of bound water, loss of cell functionality and, eventually, death o f cells.
The passage of electron current could also induce the alternation of growth and nerve processes
o f cells (Cope, 1976).
In addition, and alignment of micromolecules in the field which is
sometimes referred to as the pearl chain formation may cause the breakdown of lipids and
formation o f free fatty acids (Teixeira-Pinto et al., 1960; Wildervanck et el., 1959; Olsen et al.,
1966; Rosen, 1972).
2.9.3 Chemical effects
The quantum energy levels of microwaves (Table 2.2) are several magnitudes lower than
that required to break chemical bonds (Table 2.3), it is therefore unlikely that microwaves could
break any type of chemical bond in foods (Rosen, 1972; Pomeranz and Meloan, 1987).
However, it is likely that free radicals of oxygen, hydrogen, hydroxyl and hydroperoxyl may be
formed by simultaneous absorption of energy (Olsen et al., 1966; Rosen, 1972). Microbial cells
may also be selectively heated by microwaves depending on their chemical composition and the
surrounding medium (Carroll and Lopez, 1969). In view of food as a whole, it is possible that
microwave energy may be concentrated in micro- or macroscopic layers of food resulting in
unconventional chemical reactions due to higher local temperature than the average temperature
(Lorenz, 1976).
2.10 Microwave thermal, non-thermal and enhanced effects
The applications of microwave heating in food processing have undergone intensively for
more than 40 years (Decareau, 1985). Numerous studies have been carried out on the effects of
microwave heating on food to achieve pasteurization or even sterilization at lower temperature or
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in shorter time and these hastened the controversial issue on non-thermal microwave effects,
which is not attributed to conventional heating.
Olsen (1966) is probably among the first who postulated the non-thermal effects of
microwave heating. Several published studies have demonstrated that inactivation/ destruction
by microwaves cannot be solely explained by conventional thermal effects (Grecz et al., 1964;
Olsen et al., 1966; Culkin and Fung, 1975; Chipley et al., 1980; Dreyfuss and Chipley, 1980;
Khalil and Villota, 1988).
Some early work by Gray in 1968 (Decareau, 1985) lead to the
granting of patents for food sterilization using microwave energy at low temperatures. As
reviewed by Decareau (1985), relevant work between 1944 and 1983 concluded that microbial
Table 2.2 Types of radiation with wavelength and quantum energy
Typical wavelength (cm) Quantum energy (eV)
i 0-i°
1,240,000
Radiation type
Gamma rays
X - rays
10'9
124,000
Ultraviolet light
0.00003
4.1
Visible light
0.00005
2.5
Infrared light
0.01
0.012
Microwaves
10
0.000012
Radio
30,000
0.000000004
Source: Rosen (1972)
Table 2.3 Quantum energy required for breakage of chemical bonds
Bond
Quantum Energy (eV)
H-OH
5.2
h -c h 3
4.5
h -n h c h 3
4.0
h 3c -c h 3
3.8
ph CH2-COOH
2.4
Source: Rosen (1972)
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inactivation/destruction is solely due to heating. Some early work attempted to determine the
microwave effects without heating by introducing dried samples to microwaves and reported no
difference from conventional heating (Delany et al., 1968; Vela and Wu, 1979). However, Vela
and Wu suggested that water may play an important role in enhancing any additional microwave
effects.
Some published data for investigating non-thermal microwave effects are presented in
Table 2.4.
The results have further stimulated this controversial issue since the information has
been provided ambiguously and skeptically.
Doubts have been mainly associated with the
temperature measurement, temperature exposure of the samples, improper comparison of
microwave and conventionally treated samples, and microwave exposure period (Tajchakavit,
1998).
2.10.1 Evidence with microorganisms
Radio frequencies (RF) can induce biological changes in cells such as changes in the
functioning of the nervous system that may ultimately disturb the function of the cell membrane.
Non-thermal effects of RF were demonstrated in the early work of Fleming (1944). Abramov et
al. (1984) studied the effects of RF (250-500 MHz) on the metabolism of yeast cells and found
that acetal production was induced by the RF treatment. Ponne et al. (1996) reported membrane
damage o f a simple cell model such as liposome vesicles at sublethal temperatures exposed to
RF field at frequencies of 27 and 100 MHz, but no such effects were found in more complex
cells like Erwinia carotovara cells. However, contradictory results on the effects of RF have
been reported in other studies (Brown and Morrison, 1954; Carroll and Lopez, 1969).
Numerous studies have investigated the existence of microwave non-thermal effect.
Grecz et al. (1964) reported a greater destruction of Clostridium sporogenes sproes heated in
microwaves than that in equivalent conventional heating.
Olsen et al. (1966) reported that
Fusarium spores can be inactivated at low temperatures using microwaves as compared with
conventional heating. Culkin and Fung (1975) found additional effects of microwaves at 915
MHz on the destruction of E. coli and S. typhimurium in microwave cooked-soup (batch heating)
and reasoned that some of the apparent non-thermal effects may be exerted through molecular
level responses of the biological materials to the generated thermal energy. It was suggested that
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Table 2.4 Published data of studies investigating microwave effects on different samples
Test sample
Frequency
(MHz)
Additional
Effects
Reference
Microorganisms
C. sporogenes
Fusarium solani, Fusarium phaseoli
E. coli, B. stbtils
2450
2450
2450
Yes
Yes
No
Grecz et al. (1964)
Olsen et al. (1966)
Goldblith and Wang (1967)
S. faecalis, S. cerevisiae
S. Cerevisiae, E. coli, B.substilis
E. coli, S. iyphimurium
Active dry yeast, E. coli
S. aureus
S. aureus
E. coli
C. sporogenes
E. coli, S. aureus
Bacillus subtillis
S. cerevisiae, L. plantarum
2450
60
915
2450
2450
2450
2450
2450
2450
2450
2450
No
No
Yes
No
Yes
Yes
No
No
Yes
Yes
Yes
Lechowich et al. (1967)
Carroll and Lopez (1969)
Culkin and Fung (1975)
Vela and Wu (1979)
Dreyfuss and Chipley (1980)
Khalil and villota (1988)
Fujikawa et al. (1992)
Welt et al. (1994)
Odani et al (1995)
Wu and Gao (1996)
Tajchakavit et al (1998)
60
No
Lopez and Baganis (1971)
2800
No
Belkhode et al. (1974a)
2450
2450
2450
2450
2450
2450
Yes
Yes
Yes
Yes
Yes
Yes
Henderson et al. (1975)
Lorenz (1976)
Esaka et al. (1987)
Kermasha et al. (1993a)
Kermasha et al. (1993b)
Enzymes
Peroxdae, Polyphenol oxidase,
Pectin methylesterase, Catalase,
Alpha-amylase
Glucose-6-phosphate
dehydrogenase
Peroxidase
Peroxidase, PPO
Lipoxygenase, Trypsin inhibitor
Lipase
Lipoxygenase
PME
2450
Yes
Tajchakavit et al. (1998)
Tong et al. (2002)
Thiamine
2450
No
Goldblith et al. (1968)
Thiamine
Thiamine
2450
2450
No
No
Van Zante and Johnson (1970)
Welt and Tong (1993)
Alpha-amylase
Nutrients
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the thermal effects were contributed from generated heat and non-thermal effects were apparent
since there were greater decreases in survival in the top region than those in the middle and
bottom regions. Such decreases may arise from the higher microwave intensity in the top region
than in the other regions. Deryfuss ad Chipley (1980) characterized some of the effects of sublethal microwave heating on cells Staphylococcus aureus.
The results indicated that higher
enzymatic activities in microwave treated cells can not be explained solely by thermal effect.
Mudgett (1986) calculated the lethality of if. coli strain in the continuous system and compared it
with experimentally measured values. Experimental microbial lethality was somewhat greater
than that predicted by numerical integration. The author opinioned that it could have resulted
from sensitivity o f the kinetic effects from the selective absorption of microwave energy by the
test organism based on high intracellular conductivity. In addition, Khalil and Villota (1988)
compared cultures of S. aureus heated by microwaves and conventional method at 50 °C and
indicated a greater reduction by microwaves.
In contrast, a number o f studies refuted the existence o f non-thermal effects. Lechowich
et al. (1969) introduced S. faecalis and S. cerevisiae cells to microwave exposure at low
temperatures and showed that no lethal effects of microwaves other than those contributed heat.
Goldblith and Wang (1967) concluded from experiments on heating of aliquots of E. coli and B.
subtilis by microwave and conventional methods, that only thermal effects contributed to the
destruction of the spores since there were no markedly differences in the survival of spores for
the same time-temperature exposure.
Fujikawa et al. (1992) showed that the microwave
destruction profile o f E. coil was similar to that of conventional heating by considering the same
temperature increase during heating for comparison. In addition, Welt et al. (1994) reported no
additional effects other than heat contributed from microwave heating o f C. sporogenes spores.
However, the spores were not exposed to microwave heating continuously since temperatures
were controlled by an on and off mode whereby during the off period temperatures were
maintained by thermal.
Tajchakavit and Ramaswamy (1998) reported that no significant destruction was
observed at low temperatures (< 40 0C) for S. cerevisiae in apple juice in both continuous-flow
and batch heating conditions.
However as temperature increased beyond 50 °C, microwave
heating gave consistently higher destruction rates than conventional heating. This leads to the
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hypothesis that the observed microwave effects are not entirely “non-thermal”, but are possibly
temperature dependent.
Hence, the effect was described as “enhanced thermal effects of
microwave heating” rather than the traditionally used “non-thermal” effects.
2.10.2 Evidence with enzymes
Enzyme inactivation by microwave heating has been reported as a consequence of
increase in temperature. Henderson et al. (1975) studied inactivation of horseradish peroxidase
using 2450 MHz microwave by circulating carbon tetrachloride as a coolant to control the
temperature o f the sample (0.8 ml) at ~ 25 °C. They reported significant enzyme inactivation at
high absorbed power (> 125 W/cm3 for 20 min or > 60 W/cm3 for > 20 min) and reasoned that
protein denaturation may be due to local heat generation within the sample. As reviewed in
Lorenz (1976), the microwave inactivation pattern of peroxidase and PPO in potato was similar
to that obtained for conventional blanching, but at a faster rate. Esaka et al. (1987) investigated
the inactivation of lipoxygenase and trypsin inhibitor in winged bean seeds by microwave
heating and compared the results with those from conventional heating. They concluded that
microwave heating was more effective, however, the effectiveness of come-up time was not
considered in their studies. Kermasha et al. (1993) studied the effect of microwaves on wheat
germ lipase and soybean lipoxygenase and suggested that enhanced effects associated with
microwave heating may be due to the interaction of polar parts of enzymes with the alternating
electromagnetic field o f microwaves.
Tajchakavit and Ramaswamy (1997) reported that
continuous-flow microwave heating is more effective than conventional heating for PME in
orange juice at temperature > 50 °C (no significant inactivation was observed at temperature < 40
°C) and indicated the possibility of exist of enhanced thermal microwave effects.
In their
experiments, effectiveness of come-up and come-down periods (lag) were taken into
consideration based on time-temperature history of test samples. The equivalent timetemperature treatments were accommodated for comparison between microwave and
conventionally treated samples.
However, Lopez and Baganis (1971) studied inactivation of several enzymes including
peroxidase, polyphenol oxidase (PPO), PME, catalase and alpha amylase and found no apparent
effects of RF at 60 MHz.
34
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2.10.3 Evidence with nutrients
Critical reviews on effects o f microwaves on nutritive values of foods have been given by
Lorenz (1976) and Cross and Fung (1982). Some studies have been carried out on thiamine
degradation by microwaves at sublethal or low temperatures. Goldblith et al. (1968) exposed a
certain amount of thiamine solution to microwaves while maintaining the sample temperature
constantly low by circulating cool kerosene through the jacket o f the condenser. They reported
no destruction of thiamine from microwave heating at 33 °C for 15 and 30 min demonstrating no
apparent non-thermal effects. Welt and Tong (1993) reported no significant difference between
thiamin degradation kinetics obtained from microwave and conventional heating. However, in
their experiment, microwave power was only 50% of the fully operated power meaning that
during the heating process, it was an effect of combined microwave and thermal (Tajchakavit,
1998).
2.11
Summary of literature review
Microwave heating can be effectively use for food processing as one of the minimally
processing methods. The continuous-flow microwave heating is a promising technique for milk
pasteurization for dairy industry due to fast heating rate, high quality retention, non-fouling and
energy saving. The coil configuration affects the nature of the secondary flow, which improves
heat transfer in continuous-flow microwave heating system.
However, attention has not been
devoted to optimizing coil configuration parameters to improve system efficiency and
establishing inactivation kinetic data for ALP (indicator o f raw milk pasteurization) under
continuous-flow microwave heating condition. In order to apply this technique to dairy industry
and verify the controversial argument of non-thermal microwave effects, the further experiments
for system efficiency evaluation and ALP inactivation kinetics studies should be carried out.
35
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CHAPTER 3
INFLUENCE OF SYSTEM VARIABLES ON THE EFFICIENCY OF CONTINUOUS
FLOW MICROWAVE HEATING
3.1 ABSTRACT
A domestic microwave oven (1000W nominal output at 2450 MHz) was modified to
permit the continuous flow o f water run through a helical coil centrally located inside the oven
cavity for microwave heating. Microwave absorption efficiency and heating characteristics
were evaluated by measuring inlet and outlet temperatures at various flow rates. The average
residence time o f water was computed by dividing the sample volume by the volumetric flow
rate under the test condition. In order to optimize coil configuration, the influence o f tube
diameter (6.4, 7.9, 9.7mm); initial temperature (10, 20, 30 °C); number of turns (3.5, 4.5, 5.5);
coil diameter (61.5, 88, 102, 121 and 153 mm); pitch (16, 18, 20, 22, 24 mm) were evaluated,
respectively at different flow rates (240, 270, 300, 330, 360ml/min). The influence of Dean
number was also evaluated.
Results from this study showed that (1) higher number of turns resulted in higher
microwave absorption efficiency and temperature change, lower heating rate and lower
temperature fluctuations; (2) larger tube or coil diameter gave larger coil volume and lower
Dean numbers, larger coil volume caused microwave absorption efficiency to increase and
lower Dean numbers caused microwave absorption efficiency to decrease; (3) higher initial
temperatures resulted in higher exit temperatures and higher Dean numbers; (4) faster flow
rates resulted in lower exit temperatures, higher efficiency, but didn’t affect heating rate
significantly; (5) higher Dean number resulted in more uniform heating, intermediate values
yielded higher efficiency; (6) microwave absorption efficiency was a compromise between
coil volume and Dean number. Overall, a helical coil (110 mm high) with a coil diameter of
108 mm, tube diameter of 8.2 mm, 5.5 turns demonstrated the highest efficiency, fast heating
rate, more uniform heating and less temperature fluctuations.
36
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3.2 INTRODUCTION
Curved or helically coiled pipes are usually used as heat exchanger in food processing
and numerous engineering fields, such as chemical engineering, refrigeration, air
conditioning and nuclear engineering due to the unique flow patterns resulting from tube
curvature and the advantage of volume compactness. Many experimental and theoretical
papers have reported on convective heat transfer and temperature profiles in helical coiled
tubes (Yasuo Mori, 1964; Dravid, 1971; Patankar, 1973; Janssen, 1978; Le Bail, 2000 and
Prabhanjan, 2001).
The results showed that the centrifugal force resulted in the secondary
flow which increased the heat and mass transfer as compared with the values obtained from
straight tubes.
Thomson (1876) reported the first observation of the striking effects of curvature on
open-channel flow.
Eustice (1911) observed the trajectories of ink injected into water
flowing in tubes wound around pipes of different diameter. Dean (1927) was one of the first
researchers in this area, using a perturbation technique; he analyzed the secondary flow field
as a deviation from Poiseuille flow. As illustrated in Figure 3.1, Dean indicated that there
exits a secondary flow in the form of a pair of vortices rotating in opposite directions. The
lines in the figure “ represent what may loosely be called the projections of the paths of fluid
elements on the cross-section of the pipe” (Dean, 1927). Then Dean began to develop a
mathematical framework to explain the streamlines observed by Eustice, later Dean (1928)
turned his attention to a mathematical explanation of the reduction in flow rate due to
curvature, earlier observed experimentally by Eustice (1910).
In attempting to explain the
reduction in flow in a curved tube compared to that in a straight tube at the same axial
pressure gradient, Dean introduced the variable K defined as 2N2 r/R where N is the
Reynolds number, r is tube radius, R is coil radius, K is the precursor of the Dean number
(De), which is now expressed as Eqn. 3.1:
(3.1)
Re = d * v * P = 4 * f * p
n
3.14 *d*j u
37
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
(3.2)
Figure 3.1 Secondary flow in the form of a pair of vortices rotating in opposite
directions (Dean, 1927)
38
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Where,
Re=Reynolds number
De= Dean number
D = coil diameter (m)
d = pipe diameter (m)
v = average velocity (m/s)
f = flow rate (mL/min)
p = fluid density (kg/m3)
p = fluid viscosity (Pa.s)
Dean number is used as a measure of secondary flow in helical systems.
Microwave heating has been considered to result in a faster heating rate and more
homogeneous heat dissipation in comparison with conventional heat treatment due to
volumetric heat dissipation which could provide quality advantage for heat sensitive
product (Mudgett, 1986). The internal heat generation during microwave heating is also
expected to keep the tube surface temperature at a relatively lower level thereby reducing the
problems associated with surface fouling (Le Bail, 2000).
Continuous-flow systems are
considered to yield more uniform heat treatment as compared to bulk bath-mode offering
advantages of improved product quality and higher nutrient retention (Burton, 1972). Since
coil configuration influences the level of Dean number and coil volume, hence, coil
configuration influence microwave absorption efficiency and heating characteristics in
continuous-flow microwave heating system.
However little attention has so far been given for optimizing coil configuration
parameters to improve microwave absorption efficiency and reduce heating times which are
the objectives of the study at this section. The optimized coil configuration parameters were
subsequently used to set up continuous-flow microwave heating system for evaluating
39
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
alkaline phosphatase inactivation kinetics during continuous-flow microwave heating and
continuous-flow thermal holding.
3.3
3.3.1
MATERIALS AND METHODS
Continuous-flow microwave heating system
A domestic microwave oven with 1000W nominal output at 2450 MHz (Sunbeam,
Model: SMW 1150, Curtis International Ltd. 315 Attwell Drive, Toronto, Ontario, M9W
5C1) was modified to accommodate the continuous-flow microwave heating of water (Figure
3.2). Glass beakers (made from Pyrex® glass) with different sizes were used to support high
performance Norprene® tubing (Cole-Parmer Instrument Company) to form coils (Figure 3.3)
with different geometric parameters. One stainless steel container (25 liters) was used to
feed water to a calibrated variable-speed metering pump (Masterflex®, Model No: 7524-40,
Cole-Parmer Instrument Company), which circulated water through the Norprene® tubing
coil centrally located inside the MW oven cavity (dimension: 38.4 x 34.9 x 21.9 cm ).
In
order to maintain a stable flow rate, one plastic buffer (Cole-Parmer Instrument Company)
was installed between the metering pump and the microwave oven, the direction of the water
flow in the system was upward in order to have a better control of flow rate.
The water
container and tubing were insulated to prevent heat loss. Inlet and outlet temperatures were
continuously monitored using T-type thin-wire (0.381 mm diameter) copper-constantan
thermocouples (Omega Engineering Inc., Stamford, CT) centrally inserted in the tubings just
outside the microwave oven with their surface parallel to the direction of the fluid flow.
Before installation each thermocouple was welded on a copper fin to permit the measurement
of mean temperature of flowing fluid. The modified thermocouples were calibrated against
an ASTM mercury-in-glass thermometer. The accuracy of the temperature measurements
was ± 0 . 1 °C. Temperatures were computer recorded via a data-logger (Dash-8, Omega
Engineering Inc., Stamford, CT). In order to get better mixing condition and reduce the
temperature gradient across the radius of the tube, two static mixers (8mm diameter, 50 mm
length, Teflon, Omega Engineering Inc.) were installed near each end of coil inside the
microwave oven cavity.
3.3.2
Measurement of time-temperature profiles
Water was run through the system long enough to purge out all water previously
present in the system, then the data-logger and microwave were turned on.
The system was
40
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
MW oven
2450 MHz 1000W
pump
water
static
b u ffe r
static
mixer
m ix e r
static
mixer
ikltH-loupei
Figure 3.2 Schematic diagram of continuous-flow microwave heating system
41
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
coil diameter
4 ----------------------------------------------------------------------------------------------------------- p ,
t
tube diameter
No. of turns
Figure 3.3 Coil geometric parameters
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
run for 10 min after establishing steady-state condition indicated by a constant outlet
temperature. During microwave heating inlet and outlet temperatures were recorded every 2
seconds.
Microwave absorption efficiencies associated different system parameters were
evaluated as detailed in Table 3.1. The water to be run through the microwave was kept in a
controlled temperature environment for achieving the desired initial temperature.
The
average residence time of heated water was determined by dividing the sample volume by the
steady state volumetric flow rate. Microwave absorption efficiency was calculated according
to the equation 3.3:
r
* (T —T\* f
E= C-L -V -°- J l L l .*ioo%
P
(3.3)
where: E= microwave absorption efficiency (%)
f = flow rate (ml/s)
Cp= heat capacity (J/g.K)
P = output of microwave oven (W)
T0= outlet temperature (°C)
Tj = inlet temperature (°C).
Heating rate was calculated according to the equation 3.4: Hr =
(3.4)
At
where: Hr = heating rate (°C/s)
AT = temperature rise (°C)
At = residence time in microwave oven (s)
Table 3.1 Design of parameters in continuous-flow microwave heating system
No. of
turns
Tube
diameter
(mm)
Coil
diameter
(mm)
61.5
Initial
temperature
(°C)
Different
pitch
(mm)
16
270
3.5
6.4
88
10
18
300
4.5
7.9
102
20
20
330
5.5
9.7
121
30
22
Flow rate
(ml/min)
240
370
153
24
43
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Scan
time
(s)
2
3.4 RESULTS AND DISCUSSION
3.4.1
Influence of number of turns
Except the three number of turns (3.5, 4.5, 5.5), the coils had same coil diameter
(120mm), tube diameter (7.9 mm) and pitch (18mm). Each coil was evaluated at 240, 270,
300, 320 and 340 mL/min, respectively with an initial temperature of 20°C.
Time-
temperature profiles for the three coils (Figure 3.4 a, 3.4 b and 3.4 c) were obtained by
pumping water through the continuous-flow microwave heating system at different flow rates
showing typical lag periods prior to achieving steady state. The figures show exit
temperatures as a function of heating time from the time microwave oven was turned on.
Time-temperature profiles observed for the three coils during heating were similar.
The
nonlinearity in time-temperature data during early phase of heating (the lag period) can easily
be explained by the heat sink contributed by the coil and environment within the cavity as
explained by Kudra et al. (1991).
The equilibrium exit temperature and heating time were
dependent on the flow rate and number of turns. Faster flow rates resulted in lower exit
temperatures due to shorter residence times. As illustrated in Figures 3.4 a (3.5 turns), 3.4 b
(4.5 turns) and 3.4 c (5.5 turns), exit temperatures generally stabilized at temperatures of
about 55 °C, 58 °C and 60 °C (indicated by ‘S’ in each figure) after about 1.6, 1.8 and 2.0
minutes for coil N o.l, No.2 and No.3, respectively, at a flow rate about 242 mL/min. The
residence time during microwave heating was determined by dividing the sample volume by
the volumetric flow rate. At the same flow rate, coil with higher number of turns had a
bigger coil volume, which resulted in a longer residence time and a higher exit temperature.
Hence, coils with more number of turns needed a longer time to achieve temperature
equilibrium. Figures 3.4 shows that a faster flow rate resulted in a lower exit temperature
and a lower temperature fluctuation.
The lower exit temperature was due to the shorter
residence time in microwave oven.
Although fluid flow profiles for the experiment conditions were expected to be
essentially laminar, the use of helical coil created a secondary flow, which enhanced mixing of
flowing fluid and provided uniform heating even under laminar conditions. Dean number is
the unique dynamic parameter governing the secondary flow in the curved tube. Tajchakavit
(1998) reported the temperatures rise in the microwave oven was somewhat non-linear with the
temperature registered generally higher than in a linearly increasing profile.
The temperature
44
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
55
50
45
&
40
♦ 242ml/m
+
a
35
X 295ml/m
30
■ 318ml/m
25
20
15
271 ml/m
3.5 turns (a)
(
+ 336ml/m
L
60
55
^ 4 4 * ^ * A 4 ^ A A A 4 A 4 a A A A * A A A A A A a A A A A a a A a 4 4 4 4 a a A A A aA A aaA ^J.
A
■>.;.:>:
■:■;•:■■:■■'■-'-■'■ •--•-•>•!:■:•-• ■'■■:■-:■s
*:v •:■:
i
s
K
i
l
l
50
45
♦ 242ml/m
40
a
271 ml/m
® 296rri/m
35
+ 320rr1/m
30
X 337ml/m
25
4.5 turns (b)
20
15
65
60
55
50
45
40
35
30
r - -
.4 ^ *
. . . f a a a a A A A A A A a a a a A a A A A A A A a aA AAAA A * AAA AAAA& aAA
-
* * * * * * “ «“ ***
»»»»»”” *aa
***
-
♦ 243ml/m
-
a
.t
270ml/m
♦
« 2 9 7 ml/m
&
-s- 319ml/m
25 ♦
20 ’?
15
5.5 turns (c)
X 338ml/m
Time (min)
Figure 3.4 MW heating curves for coils with 3.3 turns (a), 4.5 turns (b) and 5.5
turns (c) as a function of flow rate at initial temperature 20 °C
(S=stabilization time)
45
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
o f fluid at 50 % coil length was used as mean temperature, which was calculated by Eqn. 4.3 as
detailed later. The mean temperature associated density and viscosities were used to calculate
Dean numbers.
As presented in Figure 3.5, higher number of turns resulted in higher Dean
numbers because higher number of turns caused higher exit temperatures, and the associated
Dean numbers increased due to lower fluid viscosity (due to temperature rise). Figure 3.5 also
shows that faster flow rates resulted in higher Dean numbers.
As water runs through the coil in the microwave oven, the temperature increased from
an initial temperature, Tj, at the inlet, to a final temperature, T0, at the outlet. Figure 3.6
shows the temperature rise as a function of flow rates, essentially demonstrating a linear
relationship. More number o f turns and lower flow rates resulted in longer residence time in
the microwave oven, which allowed fluids to absorb more microwave energy and resulted in
a higher temperature rise.
According to Eqn. 3.4, heating rate was calculated by dividing the temperature rise by
the average residence time. Figure 3.7 shows that heating rate as a function of flow rate,
again showing a linear relationship. Higher number of turns resulted in a lower heating rate.
At the same flow rate, lower number of turns resulted in a higher heating rate due to the
smaller coil volume and shorter residence time.
Microwave absorption efficiency was calculated by Eqn. 3.3.
Figure 3.8 shows
higher number o f turns resulted in higher microwave absorption efficiency.
As described
earlier, higher number o f turns resulted in higher temperature rise. At this specific situation,
the higher temperature rise caused a higher microwave absorption efficiency. Lower flow
rates resulted in a higher exit temperature, which caused more heat loss from surrounding.
Thus, microwave absorption efficiency was decreased as flow rate decreased.
3.4.2
Influence of coil diameter
In this section, 5 coil diameters were evaluated. Except for different coil diameters
(153, 121, 102, 88 and 61.5mm), the coils had same parameters (tube diameter: 7.9mm, 5.5
turns, coil height: 88 mm). Each coil was evaluated at flow rates 240, 270, 300, 330 and
350mL/min, respectively with initial temperature 20°C.
Typical evolution o f temperature rise as a function o f mean flow rate for different coil
diameters is presented in Figure 3.9.
As expected, lower flow rate resulted in higher
temperature rise; however, the five coil diameters demonstrated different magnitudes of
46
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
360
Dean n u m b e r
340
320
300
♦ 5.5 tu rn s
280
s 4 .5 ru rn s
A 3.5 tu rn s
260
240
220
3.5
4.0
4.5
5.0
5.5
6.0
Flow rate (ml/s)
Figure 3.5 Flow rate as a function of Dean number for coils
with different number of turns
45
40
O
5.5 tu rn s
4.5 tu rn s
A 3.5 tu rn s
35
30
25
20
3.5
4.0
4.5
5.0
5.5
6.0
Flow rate (ml/s)
Figure 3.6 Temperature rise as a function of flow rate for coils with
different number of turns
47
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
1.8
«
1.7
■■
1.6
1.5
O)
1.4
♦ 3 .5 turns
s 4 .5 turns
1.3
A 5 .5 turns
1.2
3.5
4
4.5
5
5.5
6
Flow rate (ml/s)
Figure 3.7 Heating rate as a function of flow rate for coils with
different number of turns
75
♦ 3.5 turns
& 4.5 turns
— 70 - A 5.5 turns
A--"
>»
c 65
©
o
E
UJ
60
55
4.0
4.5
5.0
5.5
6.0
Flow rate(ml/s)
Figure 3.8 MW absorption efficiency as a function of flow rate
for coils with different number of turns
48
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
temperature rise at the same flow rate. Coil diameter 153mm (the biggest coil volume) and
61.5mm (the smallest coil volume) demonstrated the lowest temperature rise, coil diameter
102mm (the 3rd biggest coil volume) achieved the highest temperature rise, coil
diametersl21mm (the 2nd biggest coil volume) and 88mm (the 4th biggest coil volume)
demonstrated the intermediate temperature rise. The results show that coil diameter was not
the only parameter affecting temperature rise.
Figure 3.10 shows heating rate as a function of flow rate. Under this specific condition,
flow rate had no significant influence on heating rate.
Faster flow rates resulted in lower
temperature rise and shorter residence time, thus, as determined by Eqn. 3.4, flow rate didn’t
affect heating rate significantly.
However, the fluid had less heat loss to microwave
environment at lower exit temperature when fluid was run at faster flow rate. Hence, faster
flow rate slightly increased heating rate. Figure 3.10 shows smaller coil diameter resulted in
faster heating rate due to smaller coil volume, showing the similar tendency as Figures 3.7.
Figure 3.11 shows microwave absorption efficiency as a function of flow rate. As
expected faster flow rate resulted in higher microwave absorption efficiency. However, at the
same flow rate the five coil diameters demonstrated the different magnitude of efficiency. Coil
diameter 102 mm achieved the highest absorption efficiency, coil diameters 88mm and 121mm
demonstrated the intermediate efficiency, coil diameters 153 mm and 61.5mm yielded the
lowest efficiency, showing the relationship between microwave absorption efficiency and coil
diameter was not linear. Results again showed that coil diameter was not the only parameter
determining microwave absorption efficiency. In order to achieve an optimal coil diameter, it
is necessary to explore the relationship between coil diameters and microwave absorption
efficiency. In the experiment, water was run through coils from initial temperature of 20 °C to
exit temperature of 42.8 - 61.3 °C at different flow rates varied from 4.0 to 5.93 mL/s, the flow
rate at intermediate value of 5.0 ml/s is used to evaluate the relationship between microwave
absorption efficiency and coil diameters. Based on Figure 3.11, each coil diameter is allocated
an equation to accommodate the relationship of microwave absorption efficiency vs. flow rate.
The associated data on coil diameter, tube diameter, flow rate, initial temperature and
relationship of microwave absorption efficiency vs. flow rate are presented in Table 3.2.
At
initial temperature 20°C and flow rate 5 ml/s, microwave absorption efficiency as a function of
49
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
45
♦ D=61,5mm
40
is D=88mm
o
i D=102nrn
35
+ D=121nnm
xD=153nm
I
O
25
20
3.5
4.0
4.5
5.0
5.5
6.0
6.5
Flow rate (ml/s)
Figure 3.9 Temperature rise as a function of flow rate for coils with
different coil diameter
2.1
1.9
w> 1.7
O
o
1.5
♦ D=153rrm
D=121mm
A D=88rrm
1.3
x D=102rrm
O 1.1
c
're
o> 0.9
X D=61.5rrm
X
0.7
0.5
3.5
4.0
4.5
5.0
5.5
6.0
6.5
7.0
Flow rate (ml/s)
Figure 3.10 Heating rate as a function of flow rate for coils with
different coil diameter
50
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
coil diameter is presented in Figure 3.12 with a corresponding equation Y = -0.0055X2 +
1.1842X + 2.8245 (where Y is MW absorption efficiency, X is coil diameter).
The equation
indicates when coil diameter was 108 mm, the system demonstrated the highest microwave
absorption efficiency.
Table 3.2 Calculation of microwave absorption efficiency vs. flow rate for different
coil diameter (5.5 turns)
Coil diameter Tube diameter Flow rate Initial temperature
(mm)
(mm)
(ml/s)
CO
Relationship of MW absorption
vs. flow rate (Y=MW efficiency,
X=flow rate)
Efficiency
(%)
153
7.9
5
20
Y = 0.5647X + 52.683
55.5
121
7.9
5
20
Y = 0.660 IX + 61.689
65.0
102
7.9
5
20
Y = 1.3005X + 61.047
67.6
88
7.9
5
20
Y = 0.7143X + 60.324
55.5
61.5
7.9
5
20
Y = 0.525X + 52.283
55.0
It is necessary to explore the relationship of Dean number vs. microwave absorption
efficiency.
Dean numbers were calculated as detailed earlier.
In the experiments of this
section Dean numbers varied from 232 to 471. Figure 3.13 shows Dean number as a function
of flow rate at initial temperature 20°C.
Based on Figure 3.13, each coil is allocated an
equation for relationship o f Dean number vs. flow rate. The associated data are presented in
Table 3.3. Based on Table 3.3, a relationship of microwave absorption efficiency vs. Dean
number at initial temperature 20°C and flow rate 5ml/s is obtained and illustrated in Figure 3.14
with a corresponding equation Y= -0.0019X2+1.2975X-57.42 (where Y is MW absorption
efficiency, X is Dean number).
As indicated in Figure 3.14, the Dean number increased with
decreasing coil diameter, which benefited the increase of microwave absorption efficiency.
However, the lower coil volume caused microwave absorption efficiency to decrease. Coil
diameter 153mm had the highest coil volume, but the lowest Dean number. Coil diameter
61.5mm had the highest Dean number, but the lowest coil volume.
achieve high efficiency.
Both of them didn’t
The intermediate value of Dean number corresponding coil diameter
102 mm yielded higher efficiency.
The equation indicates when Dean number was 341, the
system achieved the highest efficiency.
51
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
70
X
_ 65 s°
+ D=153nm
X D=121mm
>>
O
c 60
0)
o
£
m 55
A D=102rrTTi
# D=88mn
* D=61.5rrm
Hr:;
50 -------4.0
3.5
4.5
5.0
5.5
6.0
5
Flow rate (ml/s)
Figure 3.11 MW absorption efficiency as a function of flow rate for coils with
different coil diameter
At IT=20°C Flow rate=5ml/s
D=102 mm
D= 121mm
D=88 mm
o
c
.2
o
IE
iii
y = -0.0055X2 + 1 ,1842x + 2.8245
(w hen D=108 mm. Efficiency =Max)
j 53mm
D=61.5mm
50
70
90
110
130
150
170
Coil diam eter (mm)
Figure 3.12 MW absorption efficiency as a function of coil diameter
at initial temperature 20°C and flow rate 5.0 ml/s
52
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
500
450
♦ 153mm
^ 400
E
1 350
c
2 300
O
250
121mm
-8 8 m m
200
3.7
4.2
4.7
5.2
5.7
Flow rate (ml/s)
6.2
6.7
Figure 3.13 Dean number as a function of flow rate for coils with
different diameter at initial temperature 20 °C
At IT=20°C Flow rate=5 ml/s
70
68
66
^ 64
>62
O
C 60
d>
O 58
£ ~
y = -0.001 9X2 + 1 ,2975x - 1 57.42
(when De=341, Efficiency = Max)
111 OD
54
52
50
250
D=153mm
D=61.5mm
300
350
400
450
D ean n u m b er
Figure 3.14 MW absorption efficiency as a function of Dean Number
at initial temperature 20°C and flow rate 5.0ml/s
53
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 3.3 Calculation of microwave absorption efficiency vs. Dean number for
different coil diameter (5.5 turns)
Coil diameter
(mm)
Coil
Tube
diameter height
(mm)
3.4.3
Flow rate
(ml/s)
(mm)
Relationship of Dean
Initial
number vs. flow rate
temperature
(Y=Dean number, X=
flow rate)
(°C)
Dean
number
Efficiency
(%)
153
7.9
88
5
20
Y = 38.404X + 76.894
269
55.5
121
7.9
88
5
20
Y = 43.132X + 103.65
319
65.0
102
7.9
88
5
20
Y = 4 8 .2 5 X + 112.32
354
67.6
88
7.9
88
5
20
Y = 50.334X +120.18
372
55.5
61.5
7.9
88
5
20
Y = 57.142X + 138.23
424
55.0
Influence of tube diameter
Under continuous-flow microwave heating condition, tube diameter is one of
parameters determining coil volume and Dean number. It is necessary to explore how tube
diameters influence microwave absorption efficiency and heating rate.
In this section, three
tube diameters (6.4, 7.9 and 9.7mm) were evaluated. Except tube diameters, the three coils had
the same number o f turns (5.5), coil height (112mm) and frame diameter (beaker diameter:
108mm).
Experiments were carried out at flow rate 240, 270, 300, 330 and 370mL/min,
respectively at initial temperature 20°C.
Figure 3.15 shows temperature rise as a function of flow rate. As expected faster
flow rate resulted in lower temperature rise, however, the three tube diameters demonstrated
the different magnitude of temperature rise. Tube diameter 9.7 mm had the largest coil
volume, but yielded the 2nd highest temperature rise, tube diameter 7.9 mm had the 2nd
largest coil volume and achieved the highest temperature rise; tube diameter 6.4 mm had the
smallest coil volume and demonstrated the lowest temperature rise. The results showed that
the relationship between tube diameter and temperature rise was nonlinear.
Figure 3.16 shows heating rate as a function of flow rate indicating a linear
relationship. Smaller tube diameter achieved higher heating rate due to smaller coil volume,
showing the same tendency as previous sections.
At faster flow rates, the lower temperature
54
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
45
40
O
o
♦
d = 6.4m m
A.
d= 9.7m m
I-I
o 30
I25
20
3.5
4.0
4.5
5.5
5.0
6.0
6.5
Flow rate (m l/s)
Figure 3.15 Temperature rise as a function of flow rate for coils with different
tube diameter
2.5
A
W 2.0
o
o
d=6.4mm
d=7.9mm
▲d=9.7mm
©
**
5
a>
c
©
I
0.5
0.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
7.0
Flow rate (ml/s)
Figure 3.16 Heating rate as a function flow rate for coils with different tuber
diameter
55
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
rises prevented the system from achieving faster heating rate. On the other hand, at slower
flow rates the longer residence prevented the system from achieving faster heating rate. Thus,
flow rate didn’t affect heating rate significantly.
As indicated in the Figure 3.17, tube diameter 9.7, 7.9 and 6.4 mm achieved the
second highest, the highest and the lowest microwave absorption efficiency, respectively at
same flow rates. The results show a nonlinear relationship between microwave absorption
efficiency and tube diameter.
In order to optimize tube diameter, it is necessary to explore
the relationship between tube diameter and system efficiency. According to Figure 3.17,
each tube diameter has a corresponding equation to accommodate the relationship of
microwave absorption efficiency vs. flow rate. The associated data on coil diameter, tube
diameter, coil height, flow rate, initial temperature and Eqn. of MW absorption efficiency vs.
flow rate are presented in Table 3.4. Table 3.4 induces a relationship of tube diameter vs.
MW absorption efficiency (Figure 3.18) with a corresponding equation Y = -1.5926 X2 +
26.169X - 35.082 (where Y is MW absorption efficiency, X is tube diameter), which
indicates when tube diameter was 8.2 mm, the system demonstrated the highest efficiency.
Figure 3.19 shows Dean number as a function of flow rate. The calculation of Dean
number was detailed as earlier.
As expected smaller tube diameter and faster flow rate
resulted in higher Dean number.
It is necessary to explore the relationship between Dean
number and MW absorption efficiency under this specific condition. According to Figure
3.19, each tube diameter has a corresponding equation to accommodate the relationship of
Dean number vs. MW absorption efficiency. The associated data such as coil diameter, tube
diameter, coil height, flow rate, initial temperature, relationship o f Dean number vs. flow rate,
Dean number and MW absorption efficiency are presented in Table 3.5.
From table 3.5, a
relationship of Dean number vs. MW absorption efficiency is established as shown in Figure
3.20 with a corresponding equation Y= -0.0036X2 + 2.2595X - 278.45 (where Y is MW
absorption efficiency, X is Dean number), which indicates when Dean number was 314, the
system achieved the highest MW absorption efficiency.
Reynolds number (calculated by Eqn. 3.2) associated flow rates under the experiment
condition was < 2100, indicating the flow condition was laminar.
However, the use of
helical coil created the secondary flow which resulted in the thorough mixing of the fluid and
provided uniform heating.
Hence, the system achieved the highest efficiency of about 73%
56
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
"T
^
o
.2
i|
ill
76
75
74
73
72
71
70
69
68
67
66
65
3.5
♦ d=6.4mm
sb d=7.9mm
& d=9.7mm
4.0
4.5
5.0
5.5
6.0
Flow rate(ml/s)
6.5
7.0
Figure 3.17 MW absorption efficiency as a function of flow rate for coils
with different tube diameter
At IT=20 C, Flow rate=5ml/s
73
_
72
S'
~ 71
>*
o
C 70
.2
o
£
69
LU
d=9.7mm
68
Y = -1.5926X2+ 26.169X - 35.082
(When d=8.2mm, Efficiency=Max)
67
d=6.4mm
66
6.0
6.5
7.0
7.5
8.0
8.5
9.0
9.5
10.0
T ube daim eter (mm)
Figure 3.18 MW absorption efficiency as a function of tube diameter
57
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
440
400 -
& 360
E
c 320
O 280 -
Q
240 ii,
200
-
3.5
d=9.7mm
J_______________ I_______________ 1_____
4.0
4.5
5.0
5.5
6.0
6.5
Flow rate (ml/s)
Figure 3.19 Dean number as a function of flow rate for coils with different tube
diameter
At IT=20°C, flow rate=5ml/s
73
72
71
70
69
y = -0.0036X2 + 2 .2 5 9 5 X - 2 7 8 .4 5
(when X=314, Efficiency=Max)
68
67
66
260
280
300
320
340
360
Dean num ber
Figure 3.20 MW absorption efficiency as a function of Dean number for different
coil diameters (Reynolds number: 900-1500)
58
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
when Dean number was 314 at flow rate 5mL/s and initial temperature 20 °C.
The results
show that the intermediate value of Dean number yielded the highest efficiency.
Table 3.4 Calculation of microwave absorption efficiency vs. flow rate for different
tube diameter (5.5 turns)
Coil
diameter
(mm)
Tube
diameter
(mm)
Coil
Flow
Height rate
(mm) (ml/s)
Initial
temperature
(°C)
Eqn. of MW absorption efficiency Efficiency
vs. flow rate (Y=MW efficiency,
(%)
X=flow rate)
119.5
6.4
112
5
20
Y = 1.301X + 60.655
67.2
121
7.9
112
5
20
Y = 1.9145X + 62.687
72.3
122.5
9.7
112
5
20
Y = 1.6681X + 60.57
68.9
Table 3.5 Calculation of microwave absorption efficiency vs. Dean number for
different tube diameter (5.5 turns)
Coil
diameter
(mm)
119.5
121
122.5
Tube
diameter
(mm)
6.4
7.9
9.7
Coil
Height
(mm)
Flow
Rate
(mL/s)
Initial
Temperature
(°C)
Eqn. of Dean number vs.
flow rate (Y=Dean number,
X=flow rate)
Dean
number
Efficiency
(%)
112
5
20
Y = 44.816X + 122.7
347
67.2
112
5
20
Y = 42.216X+ 104.52
316
72.3
112
5
20
Y =36.344X + 98.145
280
68.9
59
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3.4.4 Influence of initial temperature
In this section, two coils were employed to evaluate the influence of initial
temperatures (10, 20 and 30 °C) on exit temperatures and Dean number. Figures 3.21 (a) and
(b) show that higher initial temperature resulted in higher exit temperature, faster flow rate
resulted in lower exit temperature. Both Figures 3.22 (a) and (b) show that higher initial
temperature and faster flow rate resulted in higher Dean number.
3.4.5
Influence of pitch
As water runs through continuous-flow MW heating system, the curvature of helical
coil induced a centrifugal force which created the secondary flow in the form of a pair of
vortices rotating in opposite directions (Figure 3.1).
In helical coil, flow translation is
favored as pitch increases while rotation is favored as pitch tends to 0 (torus). The influence
of pitch is therefore to alter the relative dominance of translation and rotation. The more
dominant is the translation, the less one can expect symmetry of the vortices (at conditions at
which two vortices exits).
As pitch increases, the velocity at which the two vortices are
symmetrical is greater (in the limit o f infinite pitch, there are never two vortices) (Prabhanjan,
2001).
Pitch influenced flow motions and patterns in the coil, thus pitch affected MW
absorption efficiency and heating rate. Some results from this study (Tables 3.3 and 3.4)
show (coil diameter: 121 mm, tube diameter: 7.9 mm, 5.5 turns) MW absorption efficiency
increased from 65 % at pitch 16mm (Table 3.3) to 72.3% at pitch 20 mm (Table 3.4),
showing higher pitch achieved higher MW absorption efficiency (Table 3.6). However,
when pitch tends to be infinite, the coil become a straight tubing, thus, the system will give
low efficiency due to the disappearance of the secondary flow. This can be inferred that the
optimal pitch should be a compromise between 0 and infinite height. However, due to the
limited space in the MW oven cavity, the influence of pitch could not be investigated
extensively.
Table 3.6 Heating rate and microwave absorption efficiency for different coil height
Tube
diameter
(mm)
Coil
diameter
(mm)
Coil height
(mm)
Number
o f turns
Initial
temperature
(°C)
Flow
rate
(ml/s)
Heating
rate (°C/s)
Efficiency
(%)
7.9
121
88
5.5
20
5
1.29
65
7.9
121
112
5.5
20
5
1.44
72.3
60
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o
o
60
g> 55
« 50
111
3.5
4.0
5.5
4.5
5.0
Flow rate (ml/s)
6.0
6.5
(a)
(Coil N o.l: tube diameter 7.9mm, coil diameter 121mm, coil height 88mm, 5.5 turns)
70
65
60
55
50
45
40
35
30
3.5
4.0
4.5
5.0
Flow ra te (ml/s)
5.5
6.0
6.5
(b)
(Coil No.2: tube diameter 7.9mm, coil diameter 88mm, coil height 88 mm, 5.5 turns )
Figure 3.21 Exit temperatures as a function of initial temperature for (a) Coil No.l
and (b) Coil No.2
61
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500
450
i_ 400
o
XT'
■| 350
C 300
« 250
e
♦
-
---------
♦ IT=10 oC
Q 200
IT=20oC
150
IT=30oC
100
3.5
4.0
4.5
5.0
5.5
6.0
6.5
Flow rate (ml/s) (a)
(Coil N o.l: tube diameter 7.9mm, coil diameter 121mm, coil height 88mm, 5.5 turns)
500
450
JJ 400
E
c
350
c
2
300
Q
250
200
3.5
4.0
4.5
5.0
5.5
6.0
6.5
Flow rate (ml/s) (b)
(Coil No.2: tube diameter 7.9mm, coil diameter 88mm, coil height 88 mm, 5.5 turns)
Figure 3.22 Dean number as a function of flow rate for (a) Coil No. 1 and (b) Coil
No.2
62
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3.5
CONCLUSIONS
A continuous-flow microwave heating system was set up to monitor inlet and outlet
temperatures of water to optimize coil configuration parameters such as coil diameter, tube
diameter, number of turns, initial temperature, flow rate and pitch to improve microwave
absorption efficiency, heating rate, reduce heating time and temperature fluctuations. Under
continuous-flow microwave heating condition, microwave absorption efficiency was found
to be a function o f flow rate, initial temperature and coil configuration. Results from this
study show that
(1) higher number of turns resulted in higher microwave absorption
efficiency and temperature rise, lower heating rate and lower temperature fluctuations; (2)
larger tube or coil diameter gave larger coil volume and lower Dean numbers, larger coil
volume caused microwave absorption efficiency to increase and lower Dean numbers caused
microwave absorption efficiency to decrease; (3) higher initial temperatures resulted in
higher exit temperatures and higher Dean numbers; (4) faster flow rates resulted in lower exit
temperatures, but didn’t affect heating rate significantly; (5) higher Dean number resulted in
more uniform heating, intermediate values yielded higher efficiency; (6) larger coil volume
or slower flow rate resulted in longer time to achieve steady-state condition; (7) microwave
absorption efficiency was a compromise between coil volume and Dean number. Overall, a
helical coil (110 mm high) with a coil diameter of 108 mm, tube diameter of 8.2 mm, 5.5
turns yielded the highest efficiency, fast heating rate, more uniform heating and less
temperature fluctuations.
Constant exit temperatures could be obtained upon achieving steady-state condition.
The system could be used for studying microwave inactivation kinetics by employing
different combinations of initial temperature and flow rate to achieve various residence times
and exit temperatures.
63
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CHAPTER 4
MICROWAVE PASTEURIZATION OF MILK: EVALUATION OF
PHOSHPATASE INACTIVATION KINETICS
4.1 ABSTRACT
Raw milk was subjected to conventional isothermal water bath heating, continuous
flow microwave heating and continuous flow thermal holding in the pasteurization
temperature range (60 - 75°C), and then immediately cooled in an ice-water bath.
associated alkaline phosphatase (ALP) residual activities were evaluated.
The
Base on the
gathered time-temperature profiles, the come-up time (CUT) and come-down time (CDT)
contributions were assessed and included for first order rate kinetic data handling. The timecorrected D-values o f ALP varied from 1250 s at 60°C to 1.7 s at 75°C with a z-value o f
5.2°C under conventional batch heating conditions, 128.2 s at 65°C to 13.5 s at 70°C with a zvalue of 5.2°C under continuous-flow thermal holding condition, 17.6 s at 65 °C to 1.7 s at
70°C with a z-value of 4.9 °C under continuous-flow microwave heating condition. D values
associated ALP inactivation under microwave heating were therefore an order of magnitude
lower than under conventional thermal heating.
The results thus emphasize that ALP
inactivation occurs much faster under microwave heating condition than conventional
heating thereby confirming the existence of enhanced thermal effects from microwave.
Because of the enhanced effects, MW pasteurization would reduce the severity o f the
treatment and hence potentially offer pasteurized milk of higher quality.
4.2 INTRODUCTION
Milk serves as an
excellent
culture
and protective
medium
for certain
microorganisms, particularly bacterial pathogens, whose multiplication depends mainly on
temperature and competing microorganisms and their metabolic products (Heeschen, 1994).
The purpose o f milk pasteurization is to minimize possible health hazards arising from
pathogens and extend temporary shelf life with minimal chemical, physical and organoleptic
64
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changes.
Alkaline phosphatase (ALP) is an indigenous enzyme in raw milk.
Because
thermal resistance o f ALP is recognized to be slightly greater than that of pathogens
commonly present in milk, a negative test for ALP is used as an indicator of adequate
pasteurization. Traditional pasteurization o f milk is routinely carried out under continuous
high-temperature short-time (HTST) heating conditions using heat exchangers (tubular/plate)
followed by a brief period of holding (72-75°C for 15-20 seconds), subsequent cooling in
heat exchangers (< 4°C within 2 hrs), followed by hermetical sealing in sterile packages
under aseptic conditions. A drawback for these continuous HTST pasteurization processes is
the equipment fouling due to food components exposed to the high temperature inner surface
o f heating exchangers.
Microwave heating provides a better alternative to solve the fouling problem o f heat
transfer surface inherent in conventional heat exchanger (plate/tubular). This is mainly due
to the internal volumetric heating associated with microwaves keeping the tube surfaces
relatively cooler than the product. Continuous-flow microwave heating is a promising
technique for milk pasteurization due to fast heating rate, high quality retention, uniform
heating, energy saving opportunities, prevention of surface fouling and easy access to clean­
up. A number of studies have reported successful microwave pasteurization of milk (Jaynes,
1975; Chiu et al., 1984; Decareau, 1984, 1985; Knutson et al., 1988; Kudra et al., 1991).
However, little attention has so far been given for establishing kinetic data of ALP
inactivation for continuous-flow microwave heating.
In order to establish a processing schedule for milk pasteurization, kinetics o f ALP
inactivation are required. The objectives o f this study were, therefore, (1) to evaluate the
inactivation kinetic parameters (D and z-values) of ALP in milk during conventional batch
heating in the pasteurization temperature range of 60-75 °C, (2) to evaluate the inactivation
kinetic parameters of ALP during continuous-flow microwave heating and continuous flow
thermal holding in the temperature range o f 65-70°C, and (3) to compare the inactivation
kinetic parameters of ALP during continuous-flow microwave heating with conventional
heating.
65
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4.3 MATERIALS AND METHODS
4.3.1 Preparation of milk samples
Raw, fresh milk was obtained from a local farm late in the afternoon on the day
preceding experimentation and kept at 4 °C until use. 100 ml of raw milk was heated in
water bath at 100 °C for 20 min then immediately cooled in ice-water bath and stored at 4 °C.
The boiled milk was used as control blank (no residual ALP activity).
4.3.2 Conventional water bath thermal treatment
2 ml aliquots o f raw milk (20 °C) in glass tubes (13mm O.D., 100 mm length) were
subjected to heat treatments in a well-agitated water bath (Model: 1267-62, Cole-Parmer
Instrument Company) at selected temperatures for different time intervals, then cooled
immediately in an ice-water bath (Figure 4.1). Temperatures employed for heat treatment were
60, 65, 70 and 75°C, respectively. All treatments were duplicated. ALP residual activities
were determined as detailed later. The CUT and CDT temperatures profiles were gathered
during the heat treatment using T type thin-wire (0.381 mm diameter) copper-constantan
thermocouples (Omega Engineering Inc., Stamford, CT) attached to a data-logger (Hewlett
Packard 34970A, 20 channel multiplexer).
4.3.2.1 Effectiveness of heating time and kinetic data analyses
The CUT and CDT were taken into account for correcting the kinetic
parameters (D and z-values).
Briefly, the heating times (including the come-up period) were
first considered without any correction, and based on a semi-logarithmic relationship of
residual activity o f ALP with time (first order kinetics), the decimal reduction time (D-value,
or time to reduce 90% of ALP activity) was computed for each temperature (as the negative
reciprocal regression slopes of the linear portion of log residual ALP activity versus time
curve).
The D-values on logarithmic scale were then regressed against the temperature to
obtain the temperature sensitivity indicator, z (as the negative reciprocal regression slope of the
log D versus temperature curve).
Using this computed z value and the gathered time-
temperature profile during both CUT and CDT, the effective heating time (te) during both CUT
and CDT were obtained using the process lethality Eqn (4.1) (Ramaswamy and Ranganna,
66
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data-logger
water bath
milk
Figure 4.1 Schematic diagram of conventional isothermal water bath heating.
67
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
1981; Awuah, etal., 1993; Tajchakavit, 1997):
te= F = j l O (T‘Tref)/z dt
0
(4 1 )
where T= temperature of test liquid (°C); Tref = reference temperature (°C) or exit temperature
o f the second oven in microwave treatment); t = come-up or come down time (s) and z =
temperature sensitivity value (°C). The effective portions of CUT and CDT as calculated were
then added to the isothermal hold-time (heating time) to obtain the corrected heating time.
Using these corrected times, the D values, and subsequently the z value, were recalculated.
The process was repeated several times until the difference between two successive z values
was less than 0.5 % (Ramaswamy and Ranganna, 1981).
4.3.3 Continuous-flow microwave and thermal holding treatments
Tow domestic microwave ovens with nominal output 1000 W at 2450 MHz
(Sunbeam, Model: SMW 1150, Curtis International Ltd. 315 Attwell Drive, Toronto, Ontario,
M9W 5C1) were modified to set up continuous-flow microwave heating system (Figure 4.2).
The microwave ovens had cavity dimensions: 38.4 cm long, 34.9 cm wide and 21.9 cm high.
The two ovens were connected by an insulated short Norprene® tubing (7.9mm ID., ColeParmer Instrument Company). Two helical coil (coil diameter: 105 mm, tube ID.: 7.9 mm;
6 j turns; coil height: 110 mm, coil volume 100 ml) made of Pyrex® glass tubing were
centrally located in the two microwave oven cavities. The test fluid in a insulated stainless
steel container (25 liters) was circulated through coils for microwave heating using a
calibrated variable-speed metering pump (Masterflex®, Model No: 7524-40, Cole-Parmer
Instrument Company).
Inlet, middle and outlet temperatures were continuously gathered
using T-type thin-wire thermocouples (Omega Engineering Inc., Stamford, CT) centrally
inserted in the tubings just outside the microwave ovens with its surface parallel to the
direction of the fluid flow and attached to the data-logger (Hewlett Packard 34970A, 20
channels multiplexer).
Flow rates were measured at the outlet. The average residence time
o f the test liquid in each coil was determined by dividing the coil volume by the steady state
volumetric flow rate o f the liquid running through the system.
Each thermocouple was welded on a copper fin and calibrated against an ASTM
mercury-in-glass thermometer before installation. The accuracy of the temperature measured
68
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
holding coil
T4
MW OVEN
< ---------------•
|g»ve*
:
q-.
"
1
1
*\
I i
▼ i
sampling
sampling
■iSEailC
i jm ix e r
'
' '/s
•
}
t
static
mixer
pump
buffer
* ............ »
data-logger
i " . ......................... i
Figure 4.2
A schematic diagram of continuous-flow microwave heating and
isothermal holding system set-up
69
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
by the modified thermocouple was ± 0.1 °C. In order to maintain a stable flow rate, one
plastic buffer (Cole-Parmer Instrument Company) was installed between metering pump and
the first oven, and the direction o f the fluid flow in the MW system was kept upward in order
to have a better control of flow. Static mixers (8mm diameter, 50 mm length, Teflon, Omega
Engineering Inc.) were installed at the inlet and outlet ports of each glass coil to ensure the
better mixing condition and reduce temperature gradient across the radius of the tube. In the
thermal holding set-up, an insulated short Norprene® tubing (7.9mm id) was used to
introduce the microwave heated fluid exiting from the second oven to a well insulated helical
coil (coil diameter: 106 mm, tube inner diameter: 7.9 mm; 6 \ turns; coil height: 110 mm, coil
volume 100 ml) made from Pyrex® glass tubing. One T-type thin-wire (0.381) thermocouple
(modified as detailed earlier) was installed immediately outside the holding coil and attached
to the data-logger.
Before each run, raw milk was preheated to required initial temperatures (10-30°C) in
a water bath (< 50 °C) for 10-20 min then immediately transferred to a well insulated
stainless steel container (storage tank) for microwave treatment. According to kinetic data
obtained from conventional batch heating in this study (details are given later in Results and
Discussion), the D-value of ALP at 60°C was 20.8 min with a z-value o f 5.18 °C, and the
computed D-value at 50°C was 1985 min, thus the preheating treatment didn’t affect ALP
activity in raw milk. The preheated milk was run through the system long enough to purge
out water used for stabilizing the system or microwave heated milk previously present in the
system and to establish the steady state condition indicated by constant temperatures at the 4
thermocouple locations. The initial temperature and flow rate were pre-adjusted (based on
preliminary runs) to obtain several heating times at each exit temperature.
During
microwave heating, temperatures at 4 thermocouple locations were recorded at every 1
second intervals. After establishing steady state conditions, test sample was withdrawn first
from the exit of the holding coil (downstream) followed by the exit after the second oven at
temperatures between 65 and 70°C. The treated samples (<10 mL) were collected into a pre­
cooled glass conical flask and cooled immediately in ice-water bath. All treatments were
duplicated. Subsequently, ALP residual activities were determined as detailed later. After
completing an experiment, the stainless steel storage container was washed and then water
was run through the system at high flow rate for 20 min to purge out residual milk in the
70
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
system, and then microwave ovens were turned on, the system was sanitized by circulating
hot water in a closed system for 30 min at low flow rates to maintain all exit temperature at >
70°C.
4.33.1 Effectiveness of microwave heating time and kinetic data analyses
Continuous-flow microwave heating used in the study was a fully non-isothermal
heating treatment consisting of a come-up period inside the oven during microwave heating
and a come-down period outside during cooling, with ‘no MW hold’ period. The temperature
o f test samples increased from an initial temperature at the entry port o f MW Oven 1 to a final
temperature at the exit port o f MW Oven 2. It is necessary to determine temperature profile
inside each oven to estimate effective microwave heating time. Tajchakavit and Ramaswamy
(1998) used a Luxtron fiber optic probe to measure temperatures of test samples inside
microwave oven at three locations (0.25, 0.5 and 0.75 tube length from the entrance) along the
coil length for continuous-flow microwave heating system. They reported that temperature rise
in microwave oven was somewhat non-linear with the temperature registered generally higher
than in a linearly increasing profile. The temperatures along the coil length inside microwave
oven can be generalized as follows:
At 25% o f total residence time:
Ti/4= T0 - 0.64*(To - T;)
(4.2)
At 50% o f total residence time:
Tm = T0 - 0.333*(To - Tj)
(4.3)
At 75% o f total residence time:
T3/4 = T0 - 0.097*(To - Ti)
(4.4)
where T;, T 1/4 , T 1/2 , T3/4 and T0 were temperatures at entrance port, one-fourth, middle, threefourth and exit port positions along the coil length, respectively.
Thus, by using above
equations temperature profiles inside each microwave oven could be determined based on the
temperature profiles obtained from thermocouples installed outside microwave ovens.
Again briefly, as described earlier with conventional heating, the D-values at the
exit temperature were first calculated from the regression of log residual ALP activity versus
uncorrected heating time (i.e. using the average residence time), and then the z-value was
obtained as negative reciprocal slope of log D versus temperature. Using the calculated z
value, the heating times were then corrected using eqn. (4.1), and corrected D-values, and
71
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
subsequently the corrected z, were calculated. As before, this step was repeated until 0.5%
convergence o f z-value was obtained (Tajchakavit and Ramaswamy, 1998).
Because there were two microwave ovens, the effective heating time, te = tei + Uz,
where tei,te 2 were the effective heating times for MW Oven 1, MW Oven 2, respectively. It
should be recognized that the mechanism by which microwave heats foods is different from
the conventional, and that this could have an entirely different influence on ALP inactivation.
In the microwave heating system, heating is done inside the microwave oven and cooling
outside.
The calculation of effective portion of come-down time (CDT) for microwave
heating was similar to that for conventional heating. The CDT contribution to the ALP
inactivation was considered relatively small as compared with total effective time (detailed
later) since the sample temperature was almost instantaneously dropped to below lethal levels
in the pre-cooled conical flask surrounded by crushed ice, thus the contribution of CDT for
ALP inactivation was not accommodated.
4.3.4 Estimation of ALP activity and data analysis
After heat treatment, the residual ALP activity in milk was measured essentially
according to AOAC method (979.13) quantitative spectrophotometric procedures. Phenol
calibration curves were run with each batch of unknowns
4.4 RESULTS AND DISCUSSION
4.4.1 Inactivation kinetics during conventional batch heating
Although test samples in the well agitated water bath were only 2 ml, a certain time
lag was inevitable before the sample reached the target temperatures. Likewise a small delay
was accompanied during cooling. Ignoring CUT and CDT contributions to lethality could
result in serious errors in kinetic parameters determination especially at high temperatures
where a short time can induce appreciable inactivation.
Figure 4.3 shows a typical come-up period time-temperature profile for milk subject
to conventional isothermal water bath heating at 70°C. The lethal rate curve was constructed
by plotting the lethal rate for each data point against associated heating time. The effective
time was calculated by integration of lethal rate curve. Based on gathered data points, the
72
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integration was achieved by dividing the area under the lethal rate curve into several
trapezoids and calculating the area of each individual trapeze. As illustrated in Figure 4.3,
the effective portion of come-up period was larger than that of come-down period, where
heating at 70 °C for 110 s yielded an effective time of 32.2 s.
A coupled come-down temperature profile is also illustrated in Figure 4.3 for the
same condition. The temperatures decreased rapidly and reached levels where thermal
inactivation was negligible within about 5-6 s. The effectiveness of CDT was 0.3 s, therefore
much smaller as compared with 32.2 s (effectiveness of CUT), however, it was included in
the computation o f kinetic data under conventional batch heating condition.
Figure 4.4 and 4.5 show typical survival curve of ALP both before and after making
come-up and come-down period corrections.
As expected, the inactivation behavior
indicated characteristic first-order rate kinetics.
The inactivation rates increased with
increasing temperatures. D-values ranged from 1250 s at 60°C to 8.9 s at 75°C before
heating time corrected, 1250 s at 60°C to 1.7 s at 75°C after heating time corrected. The
difference between uncorrected and corrected D-values increased with an increasing
temperature.
As compared with lower temperature (60 and 65°C),
treatment at higher
temperatures (70 and 75 °C) required longer come-up periods and shorter heating time,
hence, the lag periods became more apparent and significant when the temperatures did not
reach the higher target temperatures during shorter heating times.
Figure 4.6 shows the
uncorrected and corrected z-values of ALP in milk during the conventional batch heating
were 6.88 and 5.18 °C, respectively. The results are summarized in Table 4.1.
Table 4.1 Thermal kinetic parameters (D- and z- values) of ALP in milk at various
temperatures under conventional isothermal water bath heating condition
Exit temperature
ofthe second oven
(°C)
60
65
70
75
Z value (°C)
D-values (s)
Uncorrected
1250.0
188.7
28.1
8.9
6.88
CUT + CDT
Corrected
1250.0
182.7
16.6
1.7
5.18
73
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O
Temperature
65
0)
+ -i
2
co 50
Lethality
co
JZ
d>
0
10 20 30 40 50 60 70 80 90 100 110 120 130
H eating tim e (s)
Figure 4.3 Time-temperature profile (dotted line) and corresponding lethal rate profile
(solid line) for milk under isothermal water bath heating condition
R esid en ce time (s)
0
200
400
600
800
1000 1200 1400 1600
0.0
-0.5
60°C
—
- 1.0
g
<
-1.5
O)
-
-
2.0
70°C
75°C
65°C
-2.5
-3.0
Figure 4.4 Survival curves of ALP in milk during isothermal water bath
heating as a function of uncorrected heating times
74
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Effective time (s)
0
200
400
600
800
1000
1200
1400
1600
0.0
-0.5
-
60°C
1.0
-1.5
-
2.0
-2.5
65°C
70°C
-3.0
Figure 4.5 Survival curves of ALP in milk during isothermal water bath
heating as a function of corrected heating times
3.5
3.0
2.5
Q
2.0
z -
6.88 °C
O)
O
1.5
1.0
0.5
0.0
a.
55
60
65
70
75
80
T em perature (°C)
Figure 4.6 Temperature sensitivity curves for ALP in milk during conventional
batch heating treatment showing uncorrected (dotted line) and
corrected (solid line) heating times
75
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4.4.2 Inactivation kinetics during continuous flow microwave heating
Raw milk was subjected to continuous-flow microwave heating with initial
temperatures and flow rates appropriately adjusted to obtain desired exit temperatures. Figure
4.7 shows at initial temperature 10°C and flow rate 480 ml/min, the outlet temperatures of
MW Oven 1 (T 2 ), Oven 2 (T3 ) and holding coil (T4 ) stabilized after 20, 60 and 100 s,
respectively from the time microwave ovens were turned on. The nonlinearity in timetemperature data during early phase of heating (lag period) can easily be explained by the
heat sink contributed by the coil and environment within the cavity as explained by Kudra et
al. (1991). Samples were generally withdrawn only after the stability was achieved.
A
typical time-temperature profile of test samples inside microwave ovens and their
corresponding effective times during continuous-flow microwave heating are illustrated in
Figure 4.8.
As previously described for kinetic data analyses, the effective microwave
heating time, te, at the exit temperature were computed based on eqn (4.1) by taking the
effectiveness o f CUT into account. The temperature profiles of 3 locations (0.25, 0.5 and
0.75 coil length from entrance) inside each microwave oven were determined by eqn (4.2),
(4.3), (4.4) based on temperature profiles gathered from thermocouples outside microwave
ovens. A typical cumulative effective heating time curve is also illustrated in Figure 4.8
giving a come-up period effectiveness of 2.34 s/ 34.14 s for the CUT period.
The D-values
and z-values were corrected for microwave heating time as detailed in the methodology and
the results are summarized in Table 4.2.
Table 4.2 Thermal kinetic parameters (D- and z- values) of ALP in milk at
various temperatures under continuous-flow microwave heating
Exit temperature
o f the second oven
(°C)
65
67
70
Z value (°C)
D-values (s)
Time-uncorrected
91.7
53.8
19.5
7.39
Time-corrected
17.6
6.3
1.7
4.92
76
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75 ]
70
65
60 -
Q.
45
40
20
0
10
20
30
40
50
60
70
80
90
100
Time (s)
• T1
• T2
* 73
x T4
Figure 4.7 Time-temperature profiles for milk at inlet of oven 1 (Ti), outlet
of oven 1 (T2 ), outlet of oven 2 (T3) and outlet of the holding
coil (T4) in continuous-flow microwave heating system at initial
temperature 20 °C and flow rate 480ml/min
80
70
£
60
2.5
♦ O ven 1
* O ven 2
Temperature
j
„
2.0
® 50
f
-
/:
40
Q>
Q_ 30
E
1.0
4)
E
-w
<U
>
+3
0
0.5
N
15
® 20
10
VI
y
0.0
0
10
15
20
25
30
35
R esidence time (s)
Figure 4.8 Time-temperature profile of milk during continuous-flow microwave
heating (dotted line) and computed effective times (solid line)
at exit temperature 70 °C
77
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Initially, the microwave kinetics was carried out from 60 to 75°C. However, it was
observed that when the exit temperature of the second MW Oven, T3 < 63 °C, ALP
inactivation was not apparent because of shorter heating times. On the other hand, ALP was
very sensitive to microwave heating at higher temperatures, ALP was completely inactivated
when T3 was >72°C. Therefore, ALP kinetics were evaluated only at 65, 67 and 70°C.
Figures 4.9 and 4.10 illustrate microwave inactivation kinetics of ALP in milk for
both uncorrected and corrected heating times. The corrected D-values ranged from 17.6 s at
65 °C to 1.7 s at 70 °C with corresponding z-value of 4.9 °C.
Results again indicated
characteristics first-order reaction kinetics during microwave heating.
The rate of
inactivation generally increased with an increase in temperature. The temperature sensitivity
curves o f ALP in milk during continuous-flow microwave heating were 7.39 for heating
times uncorrected and 4.92 s for heating time corrected (Figure 4.11).
4.4.3
Inactivation kinetics during continuous flow thermal holding
As illustrated in Figure 4.2, milk was passed through a glass holding coil after
microwave treatment. There was only a small drop in temperature (< 1 °C) between the inlet
port and the outlet port o f holding coil.
Since thermal holding was a conventional heating
process, the time-temperature profile could be linearly constructed according to inlet and out
temperatures, as well as average residence time. Based on the time-temperature profile, zvalue o f 5.18 °C from conventional batch heating was used for the first calculation o f
effective heating times at various exit temperatures as per Eqn (4.1).
The D-values and z-values were corrected as detailed in the methodology and the
results are presented in Table 4.3. Figure 4.12 and 4.13 show survival curves o f ALP during
isothermal holding before and after heating time corrected, respectively. The corrected Dvalues ranged from 128.2 s at 65 °C to 13.5 s at 70 °C with corresponding z-value of 5.15 °C.
Table 4.3 Thermal kinetic parameters (D- and z- values) of ALP in milk at various
temperatures during continuous-flow isothermal holding (holding coil
volume= 100ml)________________________________________________
D-values (s)
Exit temperature of the
second oven (°C)
Time-uncorrected
Time-corrected
128.2
65
73.0
45.7
67
47.4
70
16.8
13.5
5.18
5.15
Z value (°C)
78
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Redidence time (s)
0
5
10
15
20
25
30
35
40
0.0
65°C
-0.5
O
o
67°C
£
3
-
1.0
-
1-5
(0
8
E
o
H
.
70°C
-2 .0
-2.5
Figure 4.9 Survival curves of ALP in milk during continuous-flow microwave
heating as a function of uncorrected heating times
E ffective tim e (s)
2
3
4
0.0 a
65 C
<
- 0.8
Figure 4.10 Survival curves of ALP in milk during continuous-flow microwave
heating as a function of corrected heating times
79
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2.5 r
2.0
z = 7.39 °C
1.5
O)
5
z = 4.92 °C
1.0
0.5
0.0
64
65
66
67
68
69
70
71
lO/
Temperature ( C)
Figure 4.11 Temperature sensitivity curves for inactivation rates of ALP in milk
under continuous-flow microwave heating before time corrected
(dotted line) and after time corrected (solid line)
Residence time (s)
0
-
2
4
6
8
10
12
14
16
18
65 °C
0.2
-0.4
I
<
67 °C
-0.6
"5
o
-
1.0
-
1.2
75 °C
Figure 4.12 Survival curves of ALP in milk during continuous-flow thermal
holding as a function of uncorrected heating times
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Results indicated inactivation kinetics of ALP during continuous flow thermal holding
followed conventional first order kinetics.
4.4.4
Kinetic comparison: conventional water bath heating, continuous flow thermal
holding and continuous flow microwave heating
The D-values of ALP varied from 1250.0 s at 60 °C to 1.7 s at 75 °C with z-value of
5.18 °C for conventional batch heating, 128.2 s at 65°C to 13.5 s at 70°C with a z-value of
5.15°C for continuous-flow thermal holding, 17.6 s at 65 °C to 1.7s at 70 °C with z-value of
4.92°C for continuous-flow microwave heating. The results are summarized in Table 4.4.
Figure 4.14 shows the comparison of temperature sensitivity o f ALP in milk between two
heating systems.
Since no significant ALP inactivation was observed at lower exit temperatures (< 63
°C with corresponding residence time < 28.2 s when initial temperature = 10 °C) during
continuous flow microwave heating, the belief on existence of non-thermal is not appropriate
in this study. However, at higher temperatures 65-70 °C, ALP inactivation under continuousflow microwave heating occurred about 10 times faster than conventional heating showing
there were additional effects associated with microwave heating which were temperature
dependent. The effect was described as “enhanced thermal effects of microwave heating”
than the traditionally used “non-thermal effects” which did not exist (Tajchakavit and
Ramaswamy, 1998).
Table 4.4 Thermal kinetic parameters (D and z-values) of ALP in milk under
conventional isothermal water bath heating, continuous-flow MW
heating and continuous-flow thermal holding conditions
Temperature
(°C)
60
65
67
70
75
Ratio of D-values
(compared with
MW heating)
z-value (°C)
D-values (second)
Conventional water
bath heating
1250.0
182.7
16.6
1.7
Continuous-flow
thermal holding
Continuous-flow
MW heating
128.2
48.5
13.5
17.6
6.3
1.7
1 0 .4 -9 .8
7.3 - 7 .9
5.18
5.15
4.92
81
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Effective heating tim e (s)
0
5
10
15
20
25
30
0.0
-
0.2
65°C
-0.4
-
1.0
-
1.2
67°C
70°C
Figure 4.13 Survival curves o f ALP in milk during continuous-flow thermal
holding as a function of corrected heating times
3.5
3.0
conventional batch heating
z =5.18 °C
2.5
continuous-flow
thermal holding
O 2.0
o>
z = 5.15 °C
1.5
1.0
continuous-flow
MW heating
0.5
z = 4.92 °C
0.0
55
60
65
70
75
80
Temperature (°C)
Figure 4.14 Temperature sensitivity curves for inactivation rates of ALP under
different conditions
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D-values associated ALP inactivation under continuous-flow thermal holding was 25% lower
than under conventional batch heating, but the nature of ALP inactivation between
continuous flow thermal holding and conventional batch heating was similar. Tajchakavit
and Ramaswamy (1998) reported that continuous-flow thermal holding had higher
inactivation rate (lower D-value) for PME in orange juice as compared with the conventional
bath heating and suggested (1) there may be some residual effect from the microwave heat
treatment causing the inactivation in the isothermal holding tubes to be better than in
conventional heating or (2) it may be due to the inherent variations in the continuous set-up
with respect to radial temperature profiles and residence distribution.
4.5 CONCLUSIONS
The application of isothermal batch heating, continuous flow microwave heating and
thermal holding for the inactivation of ALP in milk were explored and compared in
pasteurization temperature range (60-75°C). Based on gathered time-temperature profiles,
CUT and CDT were suitably corrected for obtaining kinetic parameters. Inactivation kinetics
of ALP during above heating conditions followed typical first-order kinetics. Within the
range of temperatures and samples sizes employed in this study, continuous-flow thermal
holding was a little more efficient than conventional batch heating for ALP inactivation. The
continuous flow microwave heating inactivated ALP in milk by an order of magnitude faster
than conventional thermal heating. Results confirmed the existence of enhanced thermal
effects from microwave.
83
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CHAPTER 5
GENERAL CONCLUSIONS AND RECOMMENDATIONS
GENERAL CONCUSIONS
Continuous-flow microwave is a promising technique for milk pasteurization due to
fast heating rate, high quality retention, uniform heating, energy saving opportunities,
prevention o f surface fouling and easy access to clean-up. Coil configurations influence
microwave absorption efficiency and heating characteristics in continuous-flow microwave
heating system.
However, little attention has so far been given for optimizing coil
configuration parameters to improve microwave absorption efficiency or establishing kinetic
data of alkaline phosphatase (ALP) inactivation for continuous-flow microwave heating,
which are the objectives o f this research.
A microwave oven was modified to accommodate the continuous-flow of water and
to evaluate the influence of system variables on the system efficiency. The optimized coils
were then installed in the system to study microwave inactivation kinetics of ALP in milk for
pasteurization purposes.
Raw milk was subject to isothermal water bath heating (bath
system) and continuous-flow microwave heating and continuous-flow thermal holding in
pasteurization temperature range (60-75 °C). The associated ALP residual activities were
evaluated. Based on gathered time-temperature profiles, effectiveness of come-up and come­
down times were suitably corrected for first order rate kinetic data handling.
The results from system evaluation showed that a helical coil (110 mm high) with a
coil diameter of 108 mm, tube diameter of 8.2 mm, 5.5 turns demonstrated the highest
efficiency, fast heating rate, more uniform heating and less temperature fluctuations. ALP
inactivation kinetic data showed that the D-values o f ALP were 1250, 182.7, 16.6 and 1.7 s
at 60, 65, 70 and 75 °C, respectively with a z-value o f 5.18 °C under isothermal water bath
heating condition; 128.2, 48.5 and 13.5 s at 65, 67 and 70 °C, respectively with a z-value of
5.15 °C under continuous-flow thermal holding condition; 17.6, 6.3 and 1.7 s at 65, 67 and 70
°C, respectively with a z-value of 4.92 °C under continuous-flow microwave heating
condition.
This study showed that continuous-flow thermal holding was a little more
efficient than conventional batch heating for ALP inactivation.
D-values associated ALP
inactivation during continuous-flow microwave heating were an order of magnitude lower
84
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than during conventional heating. The results thereby confirm the existence of enhanced
thermal effects from microwave.
RECOMMENDATIONS FOR FUTURE RESEARCH
The further study could be developed by using artificial neural networks for modeling
continuous-flow microwave heating and optimizing system variables based on the achieved
experiment data.
The scope of the present study could also be broadened by using
established microwave kinetic data for industrial-scale microwave milk pasteurization.
The
research base could be broadened by optimizing system variables for continuous-flow
microwave heating with higher microwave power and heating volumes on a pilot scale to
improve system efficiency and reduce microwave heating time. The current research could
be extended to the study of shelf stability, destruction kinetics of pathogens and quality
attributes such as flavors, colors and other organoleptic qualities for microwave treated milk
in order to demonstrate the commercial feasibility o f the process.
85
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