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A study of microwave -water interactions in bread system

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A STUDY OF MICROWAVE-WATER INTERACTIONS
IN BREAD SYSTEM
BY TAESOO PARK
A dissertation submitted to the Graduate School-New Brunswick
Rutgers, The State University of New Jersey
in partial fulfillment o f the requirements
for the degree of
Doctor o f Philosophy
Graduate program in Food Science
Written under the direction o f
Professor Kit L. Yam
and approved by
New Brunswick, New Jersey
May, 2000
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ABSTRACT OF THE DISSERTATION
A Study of Microwave-Water Interactions in Bread System
by Taesoo Park, Ph.D.
Dissertation Director: Dr. Kit L. Yam
During microwave (MW) heating of bread there is a rapid
loss of moisture, and after microwaving the mechanical strength
of the bread is often increased greatly (sometimes referred
to as "toughening").
This phenomenon is a clear evidence of
some interactions occurring between the microwave and the bread
system, although the specific mechanisms of those interactions
are not understood.
We hypothesized that a large part of those
interactions is related to the water behavior in the bread
system.
To gain a better understanding the interactions between
MW energy and bread system, three specific tasks were performed.
First,
the moisture
loss
of bread
during microwaving was
measured to quantify how MW energy interacted with water in
bread.
Second, a simple proximity sensor method developed in
this laboratory was used to measure the change of mechanical
strength of bread.
was
applied
to
Third,
assess
Inverse Gas Chromatography
both water
sorption properties
ii
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(IGC)
and
thermodynamics of sorption for non-MW heated and MW heated bread
systems.
The MW heating had a significant effect on the moisture
loss
of bread as
shown by
increasing MW energy input.
increased moisture
losses
with
There was a linear relationship
between moisture losses and energy input within the range of
experiment conditions.
The behavior of the moisture loss rate
curves suggested the possibility of internal structural changes
due to the microwaving process.
The mechanical
strengths
of breads were analyzed by measuring the creep compliance values
under constant stress.
The creep compliance value of MW heated
bread decreased significantly with increasing MW heating time.
The creep compliance value of MW heated bread was significantly
lower than those of non-MW heated bread although both had the
same moisture content level, suggesting the collapse of aerated
structure during microwaving by losing water molecules.
Inverse gas chromatography (IGC) was shown to be a rapid
method to obtain data on the water sorption process in bread
systems . MW heated breads absorbed less water than non-MW heated
breads
(i.e.,
they were less hygroscopic).
This result was
confirmed by the less negative free energy and enthalpy values
in the MW heated bread system than those of non-MW heated bread.
This result suggests that for the MW heated bread, the active
iii
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water binding sites might be masked by some structural changes
in starch-gluten matrix, which resulted in decrease of water
sorption properties and an increase in the mechanical strength.
iv
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ACKNOWLEDGEMENTS
I would like to express my appreciation to my advisor.
Dr. Kit Yam, for allowing me to work in his laboratory and on
a project that I was deeply interested in. I will always be
grateful for the time he spent in helping me to develop my
critical thinking skills.
I would like to thank my committee members Dr. George Halek
and Dr. Mukund Karwe for their kind assistance,
during my last year.
especially
I also thank Dr. Tarik Roshdy, my outside
committee member, for his guidance and support throughout my
dissertation research.
I
wish
to
encouragement
Especially,
thank
my
and belief
the support,
parents
that
and
friends
for
their
I could achieve
this
goal.
love and smile of my wife, Mihyun,
and my son, Byunghun, will always be remembered.
v
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TABLE
OF C O N TE N TS
Page
Abstract of T h e s i s ......................................... ii
Acknowledgements and Dedication ............................ v
Table of C o n t e n t s .......................................... vi
List of T a b l e s ............................................. ix
List of F i g u r e s ............................................. x
Chapter I
Introduction ................................................ 1
Chapter II
Literature Review .......................................... 7
A. Microwave Heating ....................................... 7
1.
Microwaves............................................7
2 . Microwave Heating Mechanism..........................7
3.
Microwave - Food Interactions ....................... 9
B. Bread Background ....................................... 16
1 . Bread Structure..................................... 16
1.1. Wheat Starch.................................... 20
1.2. Gluten.......................................... 22
2 . Water in B r e a d ...................................... 24
3 . Effect of Microwave Heating on Br e a d ................25
C. Theory of Water S o r p t i o n ................................29
1.
Water Structure..................................... 29
2.
Interactions of Water with f o o d ................... 3 0
3 . Water Sorption...................................... 33
4 . Thermodynamics of Water Sorption................... 37
D. Inverse Gas Chromatography (IGC) ........................ 41
vi
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1. Background.......................................... 41
2 . Application of IGCM e t h o d ........................... 42
Chapter II
Characterization of Microwave Interaction with Bread System
........................................................... 44
A.
Introduction........................................... 44
B.
Materials and M e t h o d s ................................. 45
1. Materials........................................... 45
2 . M e t h o d s ............................................. 46
2.1. Microwave Oven................................. 46
2.2. Microwave Heating of Bread.................... 46
C . R e s u l t s ................................................ 49
D.
C o n c l u s i o n ............................................. 58
Chapter III
The Compression Properties
of Microwave Heated Bread .... 62
A.
Introduction........................................... 62
B.
Materials and M e t h o d s ................................. 63
1. Materials........................................... 63
2 . Simple Proximity Sensor M e t h o d ..................... 64
C . R e s u l t s ................................................ 67
D.
C o n c l u s i o n ............................................. 71
Chapter IV
Inverse
Gas
Chromatographic
Characterization
of
Microwave
Heated Bread ............................................... 7 5
A.
Introduction........................................... 7 5
B.
Materials and M e t h o d s ..................................7 6
1. Material............................................. 76
2 . M e t h o d ............................................... 7 6
vii
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2.1. IGC Method......................................76
2.1.1. Sample Preparation
76
2.1.2. IGC Setup............................... 77
2.1.3. Experiment.............................. 80
2.1.4. Data Analysis........................... 81
2.2. Static Method for Water Sorption Isotherm..... 81
C . R e s u l t s ................................................. 82
1. System Calibration.................................. 82
2 . Analysis of IGC Chromatogram........................ 86
2.1. Typical IGC Chromatogram....................... 86
2.2. Reproducibility of IGC Chromatogram........... 89
2.3. Sorption Isotherms with the
IGC and Static Method
................................................ 89
2.4. Water Sorption of Microwave Heated Bread....... 90
3 . Thermodynamics of WaterSorption.................... 112
3.1. Free Energy of Sorption....................... 112
3.2. Enthalpy of Sorption.......................... 113
3.3. Entropy of Sorption........................... 121
D. Conclusion............................................. 134
Chapter V
Conclusions............................................... 136
RE FERENCES ................................................ 138
AP PENDICES................................................ 162
V I T A ...................................................... 175
viii
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LIST OF TABLE
Page
Microwave Related Studies ................................. 15
ix
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LIST OF FIGURES
Page
Microwave and its Propagation............................8
1.
2 . Microwave heating mechanisms:
(A) Dipole polarization (B) Ionic polarization........ 11
3 . Scanning electron micrograph of starch and gluten in
b r ead................................................... 19
4.
5.
6.
7.
8.
8.
10.
11.
12.
13.
14.
Kramer
A.VC-80
Effect
Power
Effect
shear-compression cell tracing of b r e a d ......... 28
Microwave o v e n ................................... 48
of absorbed microwave powers on moisture content:
50%, 7 5%, and 100%, Heating time 40 seconds.... 53
of absorbed microwave powers on moisture loss
rate:
Power
Effect
Power
Effect
50%, 75%, and 100%, Heating time 40 seconds
54
of absorbed microwave power on moisture content:
100% and Heating times 10, 20, 30, 40 seconds... 55
of absorbed microwave power on moisture loss
rate:
Power 100% and Heating times 10, 20, 30, 40 seconds... 56
Effect of absorbed microwave power on moisture content:
Powers 50%, 7 5%, and 100%,
Heating times 30,40,60 seconds......................... 57
Effect of absorbed microwave power on moisture lossrate:
Powers 50%, 7 5%, and 100%,
Heating times 30, 40, 60 seconds....................... 60
Relationship between %moisture loss rate and heating
times: Powers 50%, 75%, and 100%,
Heating times 10, 20, 30, 40 seconds................... 61
Schematic diagram for proximity sensor m ethod ......... 66
Creep compliance of non-microwave heated bread:
Moisture contents 18.3%, 24.7%, 31.7%, and 40.9%,
Stress levels of 1.53kPa............................... 68
15. Creep compliance of microwave heated bread:
x
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16.
17 .
18.
19.
20.
Power 100%, Heating times 20, 30, and 40 seconds,
Stress levels of 1.53kPa............................... 69
Creep compliance of non-microwave heated and microwave
heated bread: Power 100%, Heating times 20, and 30
seconds and moisture contents of 31.7% and 24.7%,
Stress levels of 1.53kPa............................... 72
Creep compliance of non-microwave heated and microwave
heated bread: Power 100%, Heating times 20 seconds
and moisture contents of 40.9%,
Stress levels of 1.53kPa............................... 73
Creep compliance of bread as a function of % Moisture
loss.................................................... 74
Schematic Diagram of IGC Column Assembly...............79
Area versus Inject water amount from empty column
at 30°C, flow rate 40 c c / m i n ...........
84
21. Resident time versus Inject water amount from empty
column at 30°C, flow rate 40 cc/min .................... 85
22. Typical IGC chromatograms: empty column and non-MW
heated bread............................................88
23.
IGC chromatogram at 30°C- multiple injection for
non-MW heated b r e a d .................................... 92
24.
IGC chromatogram at 30°C- multiple injection for
MW heated bread: power 100% for 20 seconds.............93
25.
IGC chromatogram at 30°C- multiple injection for
MW heated bread: power 100% for 30 seconds.............94
26. Correlation between IGC and Static method:
non-MW heated bread at 30°C............................ 95
27.
IGC chromatograms: empty column and non-MW heated and
MW heated bread: power 100% for 30 seconds.............96
28. IGC sorption isotherms of non-MW heated bread and
MW heated bread: Power 100% for 30 seconds at
temperature 30°C ....................................... 99
xi
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29.
IGC sorption isotherms of non-MW heated bread and
MW heated bread: power 100% for 30 seconds at
temperature 40°C...................................... 100
30. IGC sorption isotherms of non-MW heated bread and
MW heated bread: power 100% for 30 seconds at
temperature 50° C ...................................... 101
31.
IGC sorption isotherms of non-MW heated bread at temperature
30, 40, and 50°C...................................... 103
32.
IGC sorption isotherms of MW heated bread: power 100%
for 20 seconds at temperature 30, 40, and 50°C ....... 104
33. IGC sorption isotherms of MW heated bread: power 100%
for 30 seconds at temperature 30, 40, and 50°C ....... 105
34. IGC sorption isotherms of MW heated bread: power 100%
for 20 and 30 seconds at temperature 3 0 ° C ............ 106
35.
IGC sorption isotherms of MW heated bread: power 100%
for 2 0 and 30 seconds at temperature 4 0 ° C .............107
36.
IGC sorption isotherms of MW heated bread: power 100%
for 20 and 30 seconds at temperature 50°C.............108
37.
IGC sorption isotherms of non-MW heated and
bread: power 100% for 20 and 30 seconds at
MW heated
temperature 30°C ...................................... 109
38.
IGC sorption isotherms of non-MW heated and
bread: power 100% for 2 0 and 3 0 seconds at
MW heated
temperature 40°C...................................... 110
39.
IGC sorption isotherms of non-MW heated and MW heated
bread: power 100% for 20 and 30 seconds at
temperature 50°C...................................... Ill
40.
»Gs of sorption as a function of temperature for
non-MW heated bread at 30, 40, and 50°C...............115
41.
»Gs of sorption as a function of temperature for MW
heated bread:
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power 100% for 30 seconds at 30, 40, and 50°C.........116
42.
«Gs of
sorption as a function of
non-MW heated and MW heated bread:
wateruptakefor
power 100% for 30 seconds at 3 0 ° C ............. 117
43.
«Gs of sorption as a function of water uptake for
non-MW heated and MW heated bread:
power 100% for 30 seconds at 40°C ............. 118
44.
»Gs of sorption as a function of water uptake for
non-MW heated and MW heated bread:
power 100% for 30 seconds at 50°C............. 119
45.
Plot of Ln (Aw) as a function of absolute
temperature
at 30, 40, and 50°C for non-MW heated b r e a d ...123
46.
Plot of Ln (Aw) as a function of absolute
temperature
at 30, 40, and 50°C for MW heated bread:
power 100% for 20 seconds......................124
47. Plot of Ln (Aw) as a function of absolute temperature
at 30, 40, and 50°C for MW heated bread:
power 100% for 30 seconds......................125
48. »Hs of
sorption as a function of
wateruptakefor
non-MW heated and MW heated bread:
power 100% for 30 seconds at 30°C'............ 126
49.
»Hs of sorption as a function of water uptake for
non-MW heated and MW heated bread:
power 100% for 30 seconds at 40°C ..................... 127
50.
»Hs of sorption as a function of water uptake for
non-MW heated and MW heated bread:
power 100% for 30 seconds at 50°C..................... 128
51.
»Ss of sorption as a function of water uptake for
non-MW heated and MW heated bread:
power 100% for 30 seconds at 30°C..................... 131
52.
»Ss of sorption as a function of water uptake for
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non-MW heated and MW heated bread:
power 100% for 3 0 seconds at 40°C.....................132
53.
»Ss of sorption as a function of water uptake for
non-MW heated and MW heated bread:
power 100% for 3 0 seconds at 50°C.....................133
xiv
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1
Chapter I
INTRODUCTION
The use of microwave (MW) energy for processing and cooking
foods has increased greatly in recent years.
It has been
estimated that more than 92% of the U.S. households own at least
one MW oven (Arsenault, 1994). As microwaves penetrate to the
center of a food they cause violent oscillations of polar
molecules (especially water), which create frictional heat to
cook the food in a relatively short time (Giese, 1992) .
MW
heating offers a rapid and economic method for cooking or
reheating food products of high quality in terms of taste,
texture, and nutritional contents (Shiffmann, 1992).
Consumer often relate texture changes with the quality
of bread.
During microwave (MW) heating of bread there is a
rapid loss of moisture, and after microwaving the mechanical
strength of bread is often increased greatly (referred to as
"toughening") (Higo et a l ., 1981a; Lambert et a l ., 1992; Roger
et al, 1990) .
This phenomenon is a clear evidence of some interactions
occurring between the microwave and the bread system, although
the specific mechanisms of those interactions are not well
understood.
By reason of the cause-and-effeet of nature, there
must be a cause for the MW-induced toughening of MW heated bread.
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2
Higo et a l . seems to be one of the first groups who conducted
extensive research to elucidate the role of MW energy on the
toughness of bread. They conducted a series of studies on topics
such as changes in bread water content
(1983c), differences
of lipid extractability (1982), changes in properties in bread
(1981a) , and changes in starch granules under limited conditions
(1981b). They attributed the toughening of bread heated with
MW to the starch fraction.
of
starch
They found an increase in the degree
gelatinization,
extractability
of
starch,
and
multiple changes in shape and size of starch granules.
Roger et a l . (1990) characterized the deformation of bread
using a Kramer shear-compression cell.
They concluded the
disulfide cross-linking of the proteins could not explain the
toughening of bread heated with MW.
Yamauchi et al.
(1993)
investigated the role of starch and gluten on toughening of
white bread heated with MW.
They reported that the rapid
toughening of MW heated bread was mainly caused by the changes
of starch-gel in bread that further contributed to the gradual
hardening after the initial rapid hardening.
They also found
that the gluten in bread did not contributed to the toughening
of MW heated bread.
Although the MW-induced toughening of bread has been a
subject of investigation for many years, the cause is not well
understood or conclusive.
It is reasonable to speculate that
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3
the
increase
in mechanical
strength
(toughening)
must
be
attributed to some physical changes, chemical changes, or other
unknown changes due to interactions between MW energy and bread
system. We hypothesized that a large part of those interactions
is related to the water characteristics in bread system.
As
known, the microwaves interact strongly with water molecules,
which is a major component of baked bread (approximately 40%,
w.b.).
Therefore,
a
specific
interaction
is
one
between
microwaves and water in bread, which can cause the changes of
bread during MW heating.
In this study the water characteristic is evaluated in
terms of the ability of water sorption of microwaved bread.
The specific water binding sites are primarily responsible for
the ability of water sorption and these binding sites are believe
to be -OH group or polar amino acid chains in starch-protein
matrix of bread.
Water is partly bound in starch-gluten matrix and/or partly
distributed within the intermolecular spaces of the proteins
and the swollen, partially gelatinized starch (Stear,
When
microwaves penetrate
into bread,
1992).
they cause violent
oscillations of the water molecules that have electric dipole
moments (called as dipole polarization) . The friction generated
by the oscillation of the water molecules provides enough energy
to heat the bread in a relatively short time (Mudgett, 1988;
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4
Schiffmann, 1990).
When MW heating is applied to the starch-gluten matrix
of bread,
(1)
it is possible that the frictional heat result in
breaking of inter- or intra-molecular hydrogen bonds by
stretching of the chain; (2) loss of bound water, and molecular
reorientation;
forming
(3) hydrogen bonding between adjacent chains
crystalline
macromolecule
of
areas.
starch
During
moves
mechanically stronger structure.
closer
cooling
stages,
together
to
the
form
a
If the availability of water
binding sites is decreased, the moisture sorption is decreased.
To gain a better understanding the interaction of MW energy
with bread system, this work was aimed at studying the effect
of MW heating on the moisture loss rate, mechanical strength,
and
the
water
sorption
properties
of
bread
system.
The
experimental methods include a MW oven with built-in balance
to observe the extent of moisture loss under various power levels
and
microwaving
times.
The
study
also
explored
the
applicability of methods such as a simple proximity sensor
method and Inverse Gas Chromatography (IGC) to assess both the
mechanical strength and the water sorption properties of bread
system.
An IGC analysis method was developed to provide a simple
determination of water sorption of food.
The IGC method, for
determination of water sorption, has two significant advantages
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5
First,
the method can rapidly produces a sorption isotherm
from a single chromatographic elution peak within hours . Second,
the data enables to study of the thermodynamics of water sorption.
Due to its high sensitivity of the detector system,
IGC has
been employed as a rapid method in water sorption studies of
food
systems
including
protein,
starches,
gliadin,
bakery
products, corn meal extrudate, and freeze-dried coffee (Helen,
1983; Paik, 1984, Tanaka, 1984; Apostolopoulos, 1985; II, 1987,
1991; Lin, 1993).
The specific tasks of this research were:
(1)
Study how MW energy interacts with water in bread
study the effect of MW energy input on change of
moisture content of bread during heating
study the effect of MW energy input on moisture loss
of bread during heating
relate MW energy input to moisture loss of bread
(2)
Study how MW energy affects mechanical properties of bread.
develop a simple proximity sensor method to measure
compression properties of bread
determine creep compliance of non-MW heated and MW
heated bread with same moisture contents
(3)
Characterize water sorption properties of non-MW heated
and MW heated bread using IGC method.
verify
IGC
method
by
measuring
water
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sorption
6
properties of bread
study the effect of temperature on water sorption
of non-MW heated and MW heated bread
determine thermodynamic parameters of water sorption
for non MW heated and MW heated bread system
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7
CHAPTER II
LITERATURE REVIEW
A. Microwave Heating
1. Microwaves
Microwaves are defined as electromagnetic waves in the
frequency (f)range of 300 to 300,000 MHz (Ohlsson, 1989) . Most
MW ovens used by consumers operate at a frequency 2,450 million
cycles per second (2,450 MHz) , and in free space (air is a good
approximation)
the
wavelength
(X)
associated
with
this
frequency (f = c/ X, where c = speed of light = 3 x 108 m/sec
and f = frequency) is 12.24 cm (Decareau, 1985; Robertson, 1992) .
The energy is delivered in sine waves, with the electric and
the magnetic components of the electromagnetic wave oriented
perpendicular to each other along their propagating direction
(Figure 1).
2. Microwave Heating Mechanisms
There are two main mechanisms by which microwaves produce
heat in dielectric material: dipolar polarization and ionic
polarization (Decareau and Peterson,
1986; Giese,
1992).
Dipolar polarization (Figure 2-A) is the heating mechanism
for polar molecules such as water molecules.
In the presence
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00
Z
Y
H = MAGNETIC FIELD
E * ELECTRIC FIELD
DIRECTION OF
PROPAGATION
Figure 1. Microwave and its Propagation
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A Plane Electromagnetic Wave
9
of an electric field,
the polar molecules attempt to align
themselves with the rapid changing,
field.
alternating electrical
The polarity of the field is varied at the rate of the
MW frequency that causes a continuous vibration of the water
molecules within the food material.
The vibration energy is
converted to heat through frictional dissipation.
Ionic
polarization
(Figure
2-B)
occurs
when
ions
in
solution move in response to an electric field. When an electric
field is applied,
the positive charges experience a force in
the direction of the field and the negative charges are repelled
opposite to the field direction.
Energy is generated when
numerous collisions occur, and much heat is dissipated.
The
more concentrated the solution and the greater the frequency
of collisions,
the more energy is dissipated as heat.
3. Microwaves-Food Interactions
Materials may reflect, transmit, or absorb microwaves (or
MW energy)
according to their interaction with the MW field
(Schiffmann, 1990; Ohlsson, 1989). Dielectrics absorb MW energy
and convert it into heat.
Reflecting materials are typically
metals, where the MW energy creates surface current penetrating
just a few microns into the material.
Transparent materials
on the other hand let microwaves pass through with negligible
absorption.
Food materials are examples of absorbing type
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10
materials because the major constituent of food is water.
The intensity of heat per unit time developed in foods
depends on the electrical field strength and frequency,
on the dielectric properties of the food material.
heating mechanisms
are
closely related
and
The MW
to such dielectric
properties of food materials as complex permittivity
(£*),
dielectric constant (£' ) , dielectric loss (£") , and loss tangent
(tan 9) . The complex permittivity is customarily defined by
the following equation (von Hippel, 1954):
£*=£'-
j £"
(1)
tan 0 = £"/£ 1
(2)
where j is imaginary number
The complex permittivity is related to the ability of material
to couple electrical energy from the MW field.
The dielectric
constant is the measure of the speed that the electromagnetic
wave goes through a material.
It reflects the ability of a
material to store electromagnetic energy (Lewis, 1987).
The
dielectric loss factor is an intrinsic property of the food
and is a measure of a material's ability to dissipate electrical
energy.
It indicates the efficiency with which electromagnetic
radiation is converted to heat.
Thus a large loss factor
indicates that the food would heat rapidly.
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(a) frequency » zero
(b) frequency a low
In) frequ en cy a zero
t
(cl frequency ■ low
(d) frequency ■ high
(b) frequency > low
crQ O Q ^ Q
(c) frequency = low
(d| frequency ■ high
W q¥* J
^
Dipole polarization
ionic polarization
Figure 2. Microwave Heating Mechanism
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Dipole polarization and Ionic polarization
12
Materials with high loss factor are termed lossy materials and
are very suitable for MW heating
tangent
(Mudgett,
1989).
(also known as the dissipation factor)
The loss
is the ratio
of the loss factor to the dielectric constant and represents
the energy loss characteristics of the material.
As
the
attenuated,
microwaves
i.e.,
they
pass
lose
through
their
the
food,
energy.
The
they
are
energy
converted to heat at the point where the energy is lost.
is
The
average MW power absorbed, which can be converted into heat
for a unit volume, Pv (W/m3) , is given by (Decareau and Peterson,
1986, Ohlsson et a l ., 1974):
Pv = 2 tc f e'E2 £"
(3)
where Pv = power absorbed in a volume of material
= dielectric constant
(8.854 farad/m),
(W/m3) , E'
E = electrical field
strength (V/m), f = frequency of the microwave system (Hz),
and £“ = dielectric loss factor for the material.
When microwaves strike the surface of a material,
they
arrive with some initial level of power P0 that depends on the
power output of the magnetron,
the uniformity of the field.
the size of oven cavity,
and
As the microwaves penetrate the
material, some power is absorbed and the wavelengths are changed,
depending on the dielectric constant of the material.
The power
not attenuated at some depth from the surface is determined
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13
from the attenuation factor based on Lambert absorption.
The
penetration depth of power is defined as the depth in a material
at which the MW power level is 37% of the surface value (1/e) :
Pz = P0 exp(-a'z)
= P0 exp(-z/£)
(4)
where P2 = power absorbed at depth z from the surface (W) , P0
= incident power in free space
(W) , a' = attenuation factor
(cm'1) , z = depth from the surface of dielectric
(cm) , and
§
= l/2a = penetration depth (cm) .
According to Mudgett (1985 a, b ) , the penetration depth
increases with decreasing moisture content of a product, but
it decreases with increasing frequency. When processing a large
(thick) product of high moisture content at 2,450 MHz, severely
non-uniform temperature profiles may occur.
to limit the product size
(thickness)
It is desirable
to a level consistent
with the penetration depth at the processing frequency.
This
limitation is not so severe for products of low to intermediate
moisture content, because penetration depth of the product is
much longer than that of high moisture products.
Dielectric constant (£’) and dielectric loss factor (£")
of distilled water at 20°C and 2,450 MHz are 77.4 and 9.2
respectively.
The penetration depth of the water is 1.7 cm
based on the above data.
According to equation (4) , this means
at most 33% of incident energy is absorbed at 1.7 cm depth of
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14
water.
In the case of bread,
dielectric constant is 4.0,
dielectric loss factor is 2.0, and penetration depth is 2.0
cm (Hegenbart, 1992).
In the conversion of dielectric loss
(£") to heat,
the
frequency response of the foods dictates the total heating rate.
The dielectric properties
of
foods at MW
frequencies are
decided by their water and salt contents. Water is the major
absorber of microwaves in foods and, consequently, the higher
the moisture content, the better the heating.
Nelson
(1990)
has
studied
the
frequency and moisture
dependence of the complex dielectric constant of shelled, yellow
dent field corn, in the frequency range 50 MHz to 11 GHz.
The
dielectric constant increases with the moisture content and
decreases with frequency.
Bengtsson and Risman
(1971) have
measure the dielectric properties of various foodstuffs, at
2.8 GHz, using a cavity perturbation technique.
that both the amounts of polar component
They reported
(mainly water) and
the addition of charge carriers played an important role on
the changes of dielectric properties.
In Table 1, microwave related studies are presented.
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15
Table 1. Microwave Related Studies
Dielectric properties: Kent, 1982, 1985, 1987; Kress-Roger &
Kent, 1987; Mudgett, 1985a, 1986a, 1986b; Mudgett et a l .,
1970,
1980; Nelson,
1990; Ohlsson et a l ., 1974;
Tong,
1994a.
Modeling of heat and mass transfer: Ayappa et a l ., 1991, 1992;
Bengston et a l ., 1980; Chen et a l ., 1990; Datta,
1990,
1991; De Alwis et a l ., 1990; Lin, 1991; Mudgett,
1985b;
Pangrle et a l ., 1991; Shivhare et a l ., 1991; Wagter, 1984;
Wei and Davis, 1985; Tong, 1993a.
Quality and safety: Buffler, 1991, 1992; Buffoot et a l ., 1989;
Decareau,
1967,
1985;
Forey,
1985;
Gerster,
1989;
Heddleson et a l ., 1991; Keenan, 1983; Kudra et a l ., 1990;
LePage et a l ., 1989; Mudgett et a l ., 1982; Palaniappan
et a l ., 1990; Spite,
1984; Tong, 1994b.
Application in food: Decareau and Peterson, 1986; Edgar, 1986;
Loh et a l ., 1983; Ming & Baghurst,
1991; Mudgett,
1982,
1988; Pei, 1982 ; Radajewski et a l ., 1988; Shiffmann,1992;
Soudaet al., 1989; Tong 1993b, 1993c; Whorton, 1990; Zalie,
1989 .
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16
B. Bread Background
1. Bread Structure
The basic formula for bread is wheat flour, yeast, salt,
and water. Other ingredients that are often found in the formula
are fat, sugar, milk or milk solids, oxidants, various enzyme
preparations,
surfactant,
and
additives
against molds.
Each component in the formula serves a function
in producing the loaf of bread.
countries
on all
continents,
protecting
bread
Wheat is cultivated in most
and while about
30,000 wheat
varieties belonging to 14 species are grown throughout the world,
only about 1000 varieties are of commercial significance.
Most
of the varieties grown for bread production belong to the species
Triticicum aescivxim (Pomeranz,
1980) .
Wheat is unique in several aspects among the cereals flours.
First,
it
properties.
can
form
a
cohesive
dough
with
viscoelastic
The second unique attribute of wheat flour dough
is its ability to retain gas.
the production of light,
This property is essential to
leavened products and is generally
attributed to the gluten protein.
The third unique feature
is the ability of wheat flour dough to set in the oven during
baking.
Setting is the transformation of dough to bread.
This
involves a change in complex viscosity that is sufficient for
the bread to retain its shape when it is taken from the oven.
Baked products are inherently complex systems because they
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17
involve many ingredients.
During the baking of bread doughs,
heat induces physical and chemical changes in the components
of the dough system.
structure
with
These heat-induced changes yield a stable
subjectively
texture characteristics.
of
mixing,
gas
cell
desirable
flavor,
aroma,
and
Extensive studies into the process
formation
during
proofing,
and
crumb
structure of the bread were conducted by Baker (1941) . He noted
that a bread crumb was not stained by iodine unless the crumb
had been cut or ruptured and concluded that the crumb matrix
was composed of impervious gluteneous material.
The starch
granules were completely covered by a protein layer and were
segregated to the interior of the matrix.
noted
that with adequate mixing,
Bechtel et a l . (1978)
the gluten
in dough was
transformed into strands or sheets, which formed a matrix in
which starch and other components were embedded and dispersed
as shown in Figure 3.
Direct examination of bread with SEM (scanning electron
microscope)
provided
information
on
gas
cell
size
and
distribution in the bread crumb and showed the size and shape
of starch granules in the gas cell walls (Khoo et al., 1975;
Varriano-Martson, 1981; Hoseney, 1986).
The general consensus
are that the starch granules retain their integrity in the bread
crumb, but they are deformed and flattened so that their long
axes lie parallel to the surface of the gas cells within the
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18
bread.
Auerman (1977) reported Chat bread-crumb structure depends
on the formation of the cell walls, which formed the crumb
porosity, as spongy framework.
A microscopic examination of
these pore cell walls revealed that they consisted of a mass
of coagulated gluten protein with embedded swollen, partially
gelatinized starch granule.
These starch granules expanded,
but remained parallel and surrounded on all sides by coagulated
proteins.
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19
Scanning
Electron
Micrographs
3
and Gluten
Figure
of Starch
in Bread
(Bechtel
et al. 1978)
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20
1.1. Wheat Starch
It has been known that wheat starch is a natural high
polymer, built up through successive condensation of glucose
units by enzymatic process in plants.
This gives a long linear
chain of about 500 to 200 glucose units.
classified as either linear molecules
(amylopectin).
(1— >4)-linked
Amylose
is
linear
a-D-glycopyranosyl
Starch molecules are
(amylose) or branched
isostactic
units
with
polymers
an
of
average
molecular weight of about 250,000 (typical for corn starch).
Amylopectin consists of several hundred short chains of 13
to 25 glucose units linked with (1—>4) bonds.
These short chains
are joined with each other at their ends with
(1—>6) bonds.
Its molecular weight is very large, ranging from 50 million
to 100 million (Pomeranz, 1991).
Native
molecules.
starches contain large amylose and amylopectin
Each branched amylopectin molecule forms a part
of many crystallites in the periodically spaced crystalline
shell-like layer of the granule and extended through many layers
of crystallites.
On the other hand,
the non-crystalline or
amorphous portion of these molecules are connected or occupy
the space between the crystalline layer. Within the crystalline
region the molecule chains are packed closely and held in place
by hydrogen bonds between glucose molecules.
The crystalline
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21
regions consist of about 30% of the whole granule.
The major
component of the crystalline regions is amylopectin with amylose
very little contributing to the crystallinity (Marousis et al.,
1989; Chinachoti and Steinberg, 1986, 1988).
The amorphous phase of the starch granules is the region
of
the
starch
most
readily
penetrable
molecular-weight, water-soluble solutes.
the amorphous phase undergoes
by
water
and
low
With water uptake,
limited reversible swelling,
which results in a consequent swelling of the entire granule
(French, 1984; Harri et a l ., 1988). Low molecular weight, water
soluble
substances
readily
penetrate
the
granule,
if
the
amorphous phase is adequately swollen by water or other hydrogen
macerial. At room temperature, penetration into the amorphous
phase is limited to solutes less than about 1000 Daltons.
Bechtel et a l . (1978) found using light and transmission
electron microscopy that protein strands provided a matrix
network in a mixed wheat flour dough and that matrix formation
required adequate mixing.
was
gelatinized into
protein strands.
In the baked bread, most of the starch
fibrous
strands
interwoven with
thin
Chabot et a l . (1979) reported that air cells
in bread were about 20 |im thick.
Starch granules were embedded
in a matrix but in most cases were disguised by the protein
covering them. Small vacuoles were covered in the protein layer
covering
the
granules.
During
baking,
starch
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granules
22
gelatinized and were flexible enough to fit around air cells.
Under conditions of limited water availability, a strong bond
between starch and proteins may be formed.
1.2. Gluten
Wheat gluten is known as a natural amorphous polymer.
According to the comprehensive studies of Osborne (1979), the
protein was divided into four classes based on solubility:
albumin, soluble in water; globulins, soluble in salt solutions
(10% NaCl was frequently used) , but insoluble in water; gliadin,
soluble in 70-90% alcohol; and glutenin, insoluble in neutral
aqueous solutions, saline solutions, or alcohol.
Gliadin is part of the gluten protein that is soluble in
7 0% aqueous ethanol.
of
the wheat
components
to
It comprises approximately 3 5% to 40%
flour proteins.
Gliadin imparts
the viscoelastic properties
the viscous
of gluten.
The
viscosity of gluten (dough) is generally attributed to gliadin
component due to its relatively small molecular size and compact
tertially structure.
Most gliadin components consist of single
chains containing intra-polypeptide disulfide bond.
The amino
acid composition of gliadin shows that approximately 35% of
the total amino acid residues consist of glutamic acid.
Almost
all of the glutamic acid of gliadin is present as glutamine.
The high glutamine content promotes hydrogen bonding in the
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23
gluten complex (Hoseney, 1986).
Glutenin is a fraction of the gluten proteins that is
insoluble in 7 0% aqueous ethanol but soluble in dilute acid
or alkali.
It comprises approximately 35% to 45% of wheat
endosperm protein.
the
viscoelastic
Glutenin imparts the elastic component to
properties
of
gluten.
The
amino
acid
composition of glutenin shows a high content of glutamic acid.
Thus,
there are many amide groups that can form intra- and
inter-molecular
hydrogen
bonds.
This
extensive
hydrogen
bonding is considered a very important feature of the physical
(rheological) properties of hydrated glutenin (Pomeranz, 1989).
During dough mixing, fully hydrated and developed gluten
proteins form a web of fibrils with many microscopic vacuoles.
After fermentation,
larger air cells.
this protein lattice structure contains
Many small air cells enmesh minute starch
granules within them.
The veil-like protein coating on the
surface of the starch granules, due mainly to increase in the
size of the air cells, stretches and rolled up into fibrils.
These fibrils, after kneading and proofing, aggregate and form
longer and larger fibrils.
A bread crumb is characterized by
thin walls and large gas cells.
The strands are swollen and
fused, and the veil-like protein and starch form a cohesive
mass
(Hoseney, 1986).
Under the influence of the higher temperature in the oven.
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24
the protein coagulates,
forming a framework and fixing the
porous volume of the bread. The cell walls of the crumb, which
consist of starch and protein, represent a swollen system in
which one part of the water molecules is thermodynamically bound,
and the other part distributed in the intermolecular spaces
of
the
denatured
proteins,
and
the
swollen,
partially
gelatinized starch (Knjaginicev, 1970).
2. Water in Bread
Bread is made up about 40% of water, which is the most
abundant component of bread next to flour.
Water in flour is
hydrogen-bonded to the hydroxyl group and to oxygen in starch.
Most amino acid groups are capable of interacting with water.
Pentosan gums are a minor component (about 1.0%-1.5% of wheat
flour) that binds relatively large amount of water (Bushuk and
Hiynka, 1964) .
Adding water while mixing
the dough assures
adequate
distribution of the components and proper development of the
dough.
Adequate water absorption and mixing are essential for
proper dough development and the production of good bread.
When water levels are too low, dough is stiff and lacks cohesion.
At excessively high water levels, a batter or flour suspension
results and proper dough development cannot take place (Pomeranz,
1986, 1988) .
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25
Water plays an important role in the major changes that
take places during the baking of dough: starch gelatinization,
protein denaturation, yeast and enzyme inactivation, and flavor
and color formation.
Water contents and its distribution also
govern the shelf life of bread, which is influence by incidence
of microbial damage, softness of the crumb, crispness of the
crust,
crumb hardening,
crumbliness,
and many other changes
associated with overall staling and lowered consumer acceptance
Water affects
the texture of crumb and crust of bread by
plasticizing and softening the starch-protein matrix,
which
alters the mechanical strength of the product (Katz and Labuza,
1981).
3. Effect of Microwave Heating on Bread
Microwave heating has been used to refresh staled bakery
products.
The mechanism of reheating of bread with a microwave
oven is very different from that with a conventional oven.
For example, reheating of bread with a conventional oven usually
takes more than 5 minutes.
Heat is transferred from the oven
cavity to the surface of bread by convection,
become dry and crispy.
causing it to
On the other hand, reheating of bread
with a microwave oven takes only a few seconds.
Since most
bread have higher moisture content at the insider than at the
surface, microwaving the bread causes the inside temperature
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26
to be higher. In this case, both the heat transfer and moisture
transfer are in the direction from inside to the surface (Ghiasi
et al., 1984; Lambert et a l ., 1992).
When starch (wheat starch)
is a major component of the
baked products and the moisture content is intermediate to low,
surface crisping results from the formation of a dehydrated
continuous retrograded starch film.
MW heating is known for
its non-uniform heating that normally results in a broad range
of
granular
swelling
that
can
produce
undesirable
phase
differences and separations within a food product (Lorenz, 1973;
Martin & Tsen,
1981).
Goebel et a l . (1984) studied starch granule swelling over
the range of water levels commonly found in starch-based food
system and developed classification of the stages of granule
swelling.
They showed that at each water: starch ratio the range
of stages of swelling and matrix development was larger in MW
heated samples than in convection heated one.
It implied that
when a material such as starch absorbed more water as in gel
formation, heating rate was increased in that location due to
the interaction of MW energy with water.
Zylema et a l . (1985) studied model wheat starch systems
heated
by
MW
heating
and
conduction
heating
method.
No
structures unique to a given heating method were found.
The
distribution of various swollen granules and the range in degree
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27
of swelling within the samples depended on the heating method
and subsequent the heat and water transport.
Roger etal.
(1990), who tested crumb with the Kramer shear shell and the
Instron universal testing machine,
detected not only a peak
compression force but also a distinct shoulder when crumb was
reheated by MW oven
(Figure 4) . The shoulder,
which was
interpreted as showing a resistance to pulling or stretching
(toughness)
of
the crumb,
was
absent when reheating
in a
conventional oven. They reported that the MW-induced toughening
effect is not the result of cross-linking by disulfide bond
formation.
There is a definite time-temperature relationship
involved in the disulfide cross-linking of proteins. The longer
the
gluten
is
cross-linking
temperature
Therefore,
held
occurs.
is short,
at
elevated
With
MW
temperatures,
heating,
the
the
more
at
high
time
so very few cross-links are
formed.
the cross-linking of the proteins cannot explain
the toughening of bread heated or baked in a MW oven.
Higo et al.
(1981 a,b, 1982, 1983 a,b,c) attributed the
toughening of bread heated in a MW oven to the starch fraction.
They found increase in the degree of starch gelatinization,
extractability of starch, and multiple changes in the shape
and size of starch granules.
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28
M
FORCE (KG)
*0
at
to it -
10
t*
0
01
00
00
MMUTES
Figure 4. Kramer shear-compression cell tracing of bread;
control bread (---- ), tough bread (---- )
(Roger et al. 1990)
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29
C. Theory of Water Sorption
Water serves as: (1) a reactant and reaction medium (e.g. ,
Maillard reaction);
(e.g.,
enzyme
conformation;
activity);
and
of macromolecule,
mobility
in
(2) a determinant of protein reactivity
(3)
a
stabilizer
of
biopolymer
(4) a facilitator of the dynamic behavior
i.e.,
relation
the influence of water on molecular
to
the
properties
and
functions
macromolecule (van den Berg and Bruin, 1981; Jeffrey,
of
1982) .
There is also considerable evidence that the physical states
of water are closely related to
chemical,
the structural,
and sensory properties of food.
physical,
The significance
of water in food systems has been studied in several articles
(Karel, 1975a,b; Fuzek, 1980; Katz & Labuza, 1981; Leung, 1987;
Halek et a l ., 1989) .
The scope of this dissertation will
primarily with the interactions of water and water sorption
properties of foods.
1. Water Structure
A water molecule is a nonlinear, polar molecule consisting
of two hydrogen atoms covalently bonded to an oxygen atom. An
unsymmetrical charge distribution is a result of the V-like
form of a water molecule (sp configuration) with the bond angle
of which is 104° near the perfect tetrahedral angle of 109°.
The electronic charge density is concentrated around the oxygen
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30
atoms due to its high electronegativity.
The asymmetry charge
created by this structure makes water highly polar and gives
it a strong propensity for creating hydrogen bonds
Woods,
(Aurand &
1973 ; Fennema, 1985) .
The
directed
nature
of
orbitals
leads
to
the
three-dimensional structure for a collection of water molecules
packed as in water or ice. Within this packing the distance
of
a
hydrogen
bond
between
adjacent
molecules is approximately 0.177 nm.
oxygen
and
hydrogen
Each water molecule can
participate in four hydrogen bonds since there are two positive
poles of hydrogen atoms and a negative pole with two sets of
orbitals at the oxygen atom (Labuza, 1968).
2. Interaction of Water with Foods
Water may exist in food at three states
Rockland & Nishi, 1980) .
(Labuza,
1984;
A certain amount may be present as
free water in the intergranular spaces and within the pores
of the foods.
Such water retains its usual physical properties
and serves as a dispersing agent for the colloidal substances
and as a solvent for the crystallizing compounds. Part of the
water is absorbed on the surface of the macromolecular colloids
(starch,
pectin,
cellulose,
and proteins).
This water
is
closely associated with the absorbing macromolecule by forces
of absorption attributable to van der Waals forces or to hydrogen
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31
bond formation.
Finally,
combination with various
some water is in a bound form in
substances,
that
is,
as water of
hydration.
The interaction of water vapor with solid foods depends
strongly on the vapor pressure, the temperature, and the binding
energies between the foods and vapor.
What is often observed
is that interaction with solids can create an overall binding
energy in the range of 40-60 kJ/mole
(9.56-14.34 kcal/mole)
because two H-bonds are usually formed per molecule of water.
This
binding
energy
non-polar gases,
is much
stronger
than that
seen
for
such as nitrogen or oxygen that are in the
range of 4-8 kJ/mole. This bond strength (20-30 kJ/mole for
each H-bond) is somewhat higher than the hydrogen bond strength
between molecules in liquid water (15-25 kJ/mole) . This becomes
important when considering the mechanism of water adsorption
and attempting to describe water sorption isotherms
(Labuza
and Busk, 1979) .
Many parameters may affect water-protein interactions.
One of the most important factors is the number and nature of
hydration sites.
Depending on polarity and conformation of
these sites, protein strongly either interacts or limits its
contacts with water.
as
dispersibility,
The functional properties of protein such
swelling,
solubilities,
gelation,
water
holding capacity, etc., are related how to water interacts with
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32
the protein. Once the structural integrity of protein is altered,
those useful functional properties are also changes
Sc Anderson,
1968; Berlin et al., 1970; Berlin,
Wolfenden
(Berlin
1981).
(197 8) suggested that water in proteins tend
to associate with the C=0 group rather than the N-H group of
the peptide bond.
It is believed that water binding occurs
at the polar side groups in the low vapor pressure region.
Water,
acting as "mobility catalyzer", may promote breaking
of hydrogen bonds in the metastable structure of proteins, with
subsequent rearrangement of stearic conformations and reforming
of hydrogen bonds to maintain the new and thermodynamically
more stable structures
(Chirgadze & Ovsepyan,
1972).
The conformational alteration of proteins results in the
changes of the nature and availability of hydration sites may
affect the thermodynamics of water binding.
Transition of the
protein molecule from compact, ordered conformation to a random
coil leads to exposure of previously buried active sites that
can interact with water.
Thus, a denatured, unfolded protein
can bind more water than in its native conformation (Hermansson,
1977).
There are many chemical groups in carbohydrates such as
C-H or CH2, oxygen atoms,
hydroxyl,
carboxyl,
sulphate,
methoxyl groups have a potential for water binding
et al., 1977).
and
(Jeffrey
The accessibility of moisture to hydrophilic
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33
sites
plays
an
carbohydrates,
important
which
role
depends
in
water
heavily
on
binding
the
with
molecular
configurations and the polysaccharide structures (Frank et a l .,
1973) .
Owing to abundant hydroxyl groups,
liable
to be
soluble
polysaccharides
may
in water.
take
accessibility that arises
closely arrangements
consisting
of
place
polysaccharides are
However,
due
to
insolubility of
decrease
in
water
from the compact structures with
the
fully extended polymer chains
of perfectly uniform,
linear molecules.
Water
associated with starch exist as bound and free moisture with
the bound water held mainly by hydrogen bonding to the hydroxyl
groups of the carbohydrates (Carrillo, 1988).
3. Water Sorption
Water found in the matrix of a food is generally classified
as either mobile or immobile. Mobile water (called unbound water
or free water) is loosely adsorbed on the surface of the food.
Immobile or bound water is either i) hydrated in the crystalline
structure,
ii) linked by hydrogen bonding, or iii) sorbed or
entrapped within the amorphous structure
(Zografi, 1988).
Sorption is a general term that encompasses both adsorptive
and
absorptive
process.
Adsorption
is
envisioned as
collection of water molecules on the surface of foods.
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the
The
34
vapor molecules remain at the surface of the foods and are not
incorporated into
the
foods.
Absorption is more properly
described as the process of the foods taking up the vapor
molecules and incorporating them into its internal structure.
Both adsorption and absorption may occur and the overall
process is therefore termed water sorption.
The basis for assessing the water sorption with foods has
been through an analysis of the water activity (Aw) in the system
under study (Troller & Christian, 1978) . Activity is defined
as the relationship between the fugacity of the components in
the mixture f and the fugacity of the component in the pure
state f0 .
Aw = £/f0
Fugacity
quantifies
the
(5)
interaction
of
a
gas
phase
component of second phase (gas or liquid) where the partial
vapor pressure of the gaseous component in the system was less
than that of the pure gas.
For cases where the pressures are
low enough to assume ideal gas behavior, fugacity.
Aw = P/Po
(6)
where P is the vapor of water in the system and PQ is the vapor
pressure of the pure water under the temperature and pressure
conditions of the experiments.
Water activity defined on this
base can be equated with equilibrium relative humidity (ERH)
divided by 100:
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35
ERH = 100 (P/P0) = 100 Aw
(7)
Both Aw and ERH are commonly used to describe the free water
of a material.
Numerically ERH equals the relative humidity
generated by the product in a closed system.
Since the proportion of free and bound water varies with
temperature, it has been noted that Aw is temperature dependent
and should be defined at a specific temperature.
systems,
water
activity,
at
constant
In most food
moisture
content,
increases with an increase in temperature, i.e., food products
become
less
hygroscopic
with
an
increase
in
temperature
(Bandyopadhyay et a l ., 1980) . The food industry has long been
recognized
the
usefulness
of
free
water
measurement.
Correlation of water activity in foods with microbial growth
(Labuza,
1984),
storage problem,
textural quality
(Bourne,
1986) have been studied.
The water sorption of a food material is commonly expressed
as a plot of the amount of water adsorbed as a function of the
relative humidity or activity of the vapor surrounding the
material
(Labuza,
1968) . During the water sorption process,
the water molecules attracted by the solid surface remains as
a bound state for a certain period.
This is considered as a
dynamic equilibrium because the number of molecules leaving
the foods per unit time is equal to the number of sorbed molecules,
with the total number of water molecules remaining constant.
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36
The equilibrium may be established between the vapor phase
and the solid phase, i.e., no more gain or loss of water molecules
in the solid phase.
The relation between the concentration
of water vapor above a solid food system and its concentration
sorbed in the food system, at constant temperature, is called
a sorption isotherm.
The sigmoid shape of the water sorption
isotherm has been attributed to the qualitative difference in
the affinity of water for hygroscopic solids.
Several mathematical equations have been reviewed in the
literature
materials
for describing water sorption isotherms of
(Labuza,
1968; Chirife and Iglesias,
et a l ., 1978; van den Berg & Bruin,
food
1978; Boquet
1981).
The isotherm model with the greatest popularity is the
B.E.T isotherm after the work of Brunauer et a l . (193 8) .
The
general equation for the B.E.T isotherm can be written as:
V/Vn = (CAw) / (1-Aw) [l+(C-l)Aw]
where V = moisture content
(8)
(g H 2O/IOO g of dry solid) , Vm =
monolayer value, A« = water activity, and C = a constant.
Generally equation (8) can be rearranged as:
AW/(1-AW)V = 1/C + Aw(C-l)/VmC
(9)
Which suggests that a plot of Aw/ (l-Aw)V vs. A« should yield
a straight line. From the slope, S = (C-l)/Vn,C, and the intercept,
I = l/VmC, the monolayer value can be calculated.
assumptions are not entirely
true
for most
foods,
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Since the
the BET
37
isotherm has been applied only between water activities from
0.1 to 0.5 (Aguerre et al., 1984, 1989).
The weakness of this
equation is that it assumes that the influence of the solid
on the enthalpy and entropy of the adsorbed species extends
only to the first adsorbed layer and that all subsequent layers
have properties identical to that of "bulk" phase.
It ignores
the possibility of structural effect past the monolayer level.
The Guggenheim, Anderson, and de Boer equation recognized
as G.A.B. was developed to account for the strength of solid/gas
interaction to a depth of greater than one molecular diameter
at the solid surface.
It was well suited for correlating the
influence of water activity on the equilibrium moisture content
of many foods at a constant temperature (Guggenheim, 1966; van
den Berg,
1981) .
The equation is normally written in the
following form:
V = VmCgKAw/ (1-KAW) [ (1-KAW)+CgKAw]
where V = moisture content
(10)
(g H20/100g of dry solid) , Vm =
monolayer value, Cg = the Guggenheim constant, K = correction
factor.
4. Thermodynamics of Water Sorption
Temperature is another parameter that affects the amount
of water adsorbed by food at a specific relative pressure.
Due
to
the nature
of
the adsorption process,
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
a change
in
38
temperature results in a shift of the adsorption isotherm.
The temperature-dependence water sorption isotherms have been
widely used to study water-food interactions by calculating
the thermodynamic functions of water sorption (Mazza,
Labuza et
al.,
1985;
Saravacos
et
a l ., 1986).
198 0;
From
the
temperature dependence of the water sorption isotherms it is
possible to derive thermodynamic parameters such the free of
energy
(AGS) , enthalpy
(AHS) , and entropy
(ASs) of sorption
(Everret, 19 50) .
The free energy required for the transference of a water
molecule from the vapor state to the solid is a quantitative
measurement of the affinity of the solid for the gas vapor (Bull,
1944) .
The free energy of sorption
(AGs) is a quantitative
measure of the affinity of the food for water.
It can be
considered as
system
a measure
of
the
accomplish the sorption process.
ability
The
of
value
the
of
AGS at
to
any
temperature can be calculated from the following equation:
AGS = RT In P/P0 = RT In Aw
(11)
where R = the universal constant and T = absolute temperature
(AK) .
From
the
thermodynamic
standpoint,
sorption
is
a
spontaneous process and is therefore accompanied by a decrease
in free energy of the system, i.e., AGS is negative.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
The more
39
negative AGS, the stronger the tendency toward sorption.
When
the system is in thermodynamic equilibrium, AGS = 0.
When vapor is sorbed by a solid (i.e., sorption),
is always given off
(i.e., exothermic process).
heat
Similarly,
if vapor is pumped out of a sorbent (i.e., desorption), heat
is absorbed (i.e., endothermic process). The quantity of heat
released
(or
absorbed)
during
sorption
process
isosteric heat or the enthalpy of sorption (AHS) •
is
called
Therefore,
it is directly related to the energy of interaction between
the sorbate and the sorbent.
A knowledge of the net heat of
adsorption provides an indication of the binding energy of water
molecules also the adsorption mechanism involved.
The
isosteric
enthalpy
of
sorption
(AHS) is
a molar
quantity directly related to the energy of interaction between
sorbed water molecules and sorption sites in the food material,
thus providing information on the exothermic or endothermic
nature of the interaction.
AHS is usually obtained from the
isotherm data measured at least three temperatures using the
Clausius-Clapeyron equation
in the
following modified
form
(Coelho et a l ., 1979):
(din P/dT) m = (AHS/R)
(12)
where P = the partial pressure of water in equilibrium with
moisture uptake m and R = universal constant
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40
The Equation 14 also can be integrated into the following
working expression:
In P = (AHS/R)(1/T)
(13)
According to this equation a plot of In P vs. 1/T yields
a straight line.
The slope of the isosteric plot should be
equal to -AHS/R from which AHS can be calculated.
The change
of enthalpy is a measure of the energy changes occurring upon
sorption of water molecules onto a sorbent.
Thus, the enthalpy
changes can be related to the binding or repulsive forces of
the system, depending on the sign of AHS values . Negative values
denote the existence of binding forces as opposed to positive
values shown by the repulsive forces (Apostolopolous & Gilbert,
1990) .
Once AGS and AHS are known,
the respective entropy of
sorption (ASs) can be calculated using Gibb's equation.
ASs = (AHs-AGs)/T
(14)
The variation of entropy may be related to the spatial
arrangements occurring at the water-sorbent
interface in a
defined state (i.e., temperature and pressure).
Thus ASs can
be used to characterize the degree of order or disorder existing
in the system and are conducive to the interpretation of such
processes
as
crystallization,
(Iglesias et a l ., 1976).
dissolution,
and
swelling
The term describes the changes in
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41
randomness or disorder of the food/water system during sorption
process.
Negative
ASs values
are
associated
with
reduced
randomness as the interaction between water and food proceeded
spontaneously.
D. Inverse Gas Chromatography (IGC)
1. Background
Inverse gas chromatography (IGC) is an analytical method
where the "unknown" sample material is used as the stationary
phase in a gas chromatographic system and analysis is conducted
by monitoring interactions of an interactive marker compound
with the sample material.
Water is an important probe because
of its effect on the properties of and its interaction with
food ingredients (Gray and Guillet,
1972).
A particular useful case is where the stationary phase
is a macromolecule such as a starch or a protein and the mobile
phase contains a water vapor.
The retention volume of the water
vapor phase
the
is related to
sorption properties
of
the
substrates as a function of vapor pressure. The response height
likewise depends on the concentration of water vapor as solute
in the exiting carrier gas (Paik & Gilbert, 1986).
Both the
test temperature and the concentrations of the phase can be
readily varied experimentally,
so that IGC systems can be
considerable use in kinetic as well as thermodynamics studies.
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42
The three basic interactions seen in IGC are,
adsorption,
redistribution,
and desorption.
namely;
In adsorption,
the water vapor is carried into the column and adsorbed onto
the initial layers of the stationary phase.
with the initial layers occurs rapidly.
has been introduced into the system,
The interaction
After the water vapor
subsequent pure carrier
gas causes redistribution of it onto the subsequent portion
of the column.
the
water
The assumption made is that equilibrium between
vapor,
adsorbate
and
subsequent
layers
of
the
adsorbent is attained rapidly during this redistribution of
sorbate through the length of the column.
Finally desorption
occurs and the water vapor is desorbed into the effluent stream.
2. Application of IGC Method
The IGC technique has been widely used in the study of
physicochemical properties of foods and polymers such as phase
transitions, crystallinity, and sorption isotherms, with their
attendant thermodynamic parameters (Smith, 1982; Gilbert, 1989;
Gilbert
and
Roshdy,
1989;
II,
1991;
Avital
et
a l ., 1990;
Demertzis et a l ., 1991; Apostolopoulos and Gilbert, 1984, 1988,
1990; Lin, 1993) .
Excellent reviews of the entire subject of IGC were done
by many workers such as Braun & Guillet (197 6) , Rabek (1980) ,
and Gilbert (1984) . Advantage of this approach is the speed.
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43
simplicity,
methods.
and accuracy over the conventional
gravimetric
Simplicity of operation of GC equipment with the wide
availability can give
further
Apostolopoulos and Gilbert
advantages
over
techniques.
(1988) studied sorption of coffee
solubles using static and several IGC methods, including frontal
and pulse or elution methods.
Ferng & Gilbert (1987), working
with starches, made a critical evaluation of the quantitative
analysis of frontal IGC.
IGC also has been successfully used as a rapid method system
for studying the thermodynamic properties of a solid used for
the stationary phase in relation to a mobile gas phase containing
selected solutes such as water.
of
physical
phenomena
such as
Theoretical interpretation
adsorption was
achieved
by
calculating thermodynamic parameters: a free energy of sorption,
enthalpy, and entropy (Riganakos et al, 1989; Speccio,
1987;
Lin, 1993; Gilbertetal, 1995). Method for determining sorption
by IGC method was well introduced in Wadgaonkar's work (1992) .
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44
Chapter III
CHARACTERIZATION OF MICROWAVE INTERACTION WITH BREAD SYSTEM
A. Introduction
The change of bread with microwave (MW) heating are complex
and many changes, such as an increase in gelatinization and
a
decrease
in
bound
water
and
moisture
content,
occur
simultaneously (Higo et al, 1981 a,b; Higo & Nokuchi,
1987).
Upon exposure to MW energy to refresh bread, it is well known
that the mechanical strength (referred as toughening) of a crumb
increased significantly (Rosenberg and Bohl, 1987).
Reasoning by the cause-and-effeet of nature, there must
be a cause for the MW-induced toughening.
It is reasonable
to speculate that the MW-induced toughening must be attributed
to physical, chemical, and/or other unknown changes.
the cause is not fully understood or conclusive.
However,
In bread,
water is the major constituent and plays a dominant role in
the interaction with microwave energy.
Therefore, there must
be a strong interaction between MW energy and water in bread,
which can cause the toughening of MW heated bread.
Cloke et a l . (1984) viewed the water loss rate relating
to the physicochemical transition and to the developed structure
of model cake during reheating.
Yamauchi et a l . (1993) studied
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45
the changes in the physical properties of MW heated bread.
They reported that the rapid staling rate of MW heated bread
was
caused
by an
increase
in
the
retrogradation
rate
of
starch-gel in bread, which was caused by a decrease in moisture
level with MW heating.
Despite
many
studies
on
water
loss
in
relation
to
physicochemical changes of bakery system during MW heating,
there have been no reports of dynamic studies of the water loss
of MW heated bread.
It is appropriate to consider the water
loss properties of the MW heated bread as an indicator of
MW-water interaction.
Therefore, the objective of our present
work was centered on studying the dynamic water loss of MW heated
bread.
More specifically, water losses were monitored on bread
heated with different levels of MW energy inputs.
B. Materials and Methods
1. Materials
In
this work
loaves
of white pan bread,
sliced
into
approximately 12 mm thick, were purchased from local supermarket
The samples were packaged in Zipper Sandwich bags to minimize
moisture loss during storage and used within 24 hours.
Average
weight of a slice of bread was approximately 10 g and initial
moisture content was 38.32%
(wet basis) or 0.621 g water/g
dry matter . The moisture contents of the samples were determined
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46
by the hot-air oven drying method (AOAC 44b)
2. Methods
2.1. Microwave Oven
To perform MW heating experiments, a MW oven was needed
that would produce a uniform electric field strength around
the testing samples.
To obtain the field uniformity, a CEM
model AVC™-80 (CEM Corp., Indian Trail, NC) MW solid moisture
analyzer was used (Figure 5) . Unlike a home MW oven that cycles
turns on and off to control the power, the CEM AVC™-80 MW oven
delivered constant electric field strength during heating.
The MW oven was capable of controlling variable power
levels (from 0% to 100% = approximately 600 watt) and heating
time at 2450 MHz . It was also equipped with a built-in analytical
balance measuring the weight changes of sample during heating.
The MW oven was connected with a computer that allowed the
recording of weight changes in second-interval using a data
acquisition system.
2.2. Microwave Heating of Bread
A slice of bread was placed on
the balance
positioned at the center of the oven cavity.
levels
and
60
(50, 75, and 100%) and heating times
seconds,
based
on power
levels)
that was
Various power
(10, 20, 30, 40
were
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selected
for
47
manipulating
the different MW energy input.
The built-in
chemical balance measured the dynamic weight changes of bread
mainly
due
experiments
to
the
were
moisture
conducted
losses
during
triplicate
in
heating.
each
All
selected
condition.
The MW energy input is defined as the product of power
level (watt) and heating time (second) per unit weight of bread
samples.
The moisture loss rate was calculated by following
equation:
[weight at ti - weight at t]
Moisture loss Rate
---------------------------------
(g water/sec)
[ti - t] (second)
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(15)
1.
2.
3.
4.
5.
Door Seal
Door Interlocks
40 Character Alphanumeric Display
Numeric Keys
Function Keys
6.
7.
8.
9.
10.
11.
Power Switch
Balance Air Shield
Balance Pan
Key Lock
Tie Down
Floor Stub
Figure 5. Microwave Oven AVC™-80
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
AVC'“-80 FRONT VIEW
49
C. Results
The effects of power levels and MW heating times on the
moisture
content
and the moisture
presented in Figures 6 through 12.
loss
rate
of bread are
In general the moisture
loss rate increased and the moisture content decreased with
increasing power level and microwaving times, i.e., increasing
MW energy input.
Three power levels (50%, 7 5%, and 100%) selected for the
present work are corresponding to 300 watt, 450 watt, and 600
watt, respectively.
The difference in the observed moisture
content among the three replications of all the sets was less
than 2%.
Figure 6 shows the effect of changing the power levels
at constant microwaving
content of bread.
time
(40 seconds)
on the moisture
It is evident that the moisture content
decreased significantly as the power level was increased.
power level was 100%,
When
the moisture content of bread dropped
from 0.621 g water/g d.m.(dry matter) to 0.284 g water/g d.m.
The moisture content of bread decreased to 0.443 g water/g
d.m. at 7 5% power level, and for 50% power, the moisture content
decreased to 0.530 g
water/g d.m.
The effect of power level on the moisture loss rate of
bread is given in Figure 7.
The higher MW power level caused
the higher the moisture loss rate at constant heating time (40
seconds) . When microwaving for 25 seconds at power level 100%,
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50
the moisture loss rate reached at the maximum level (0.01244
g/sec) and the moisture content dropped from 0.621 g water/g
d.m. (initial) to 0.469 g water/g d.m.
The decrease in moisture
loss rate after 2 5 seconds microwaving was observed.
This is
likely due to the changes in the availability of water molecules .
In the beginning of MW heating, when the moisture content of
the bread was high, therefore, its dielectric loss factor was
high, the maximum absorption of microwave power by the water
molecules took place.
As the microwaving was proceeded,
the
availability of water molecules was decreased by the internal
resistance to moisture transfer.
The result of Cloke et a l . (1984) suggested that the changes
of moisture loss rate in reheating of stored cakes was related
to the availability of water molecules due to the change of
structure by pore structure shrinkage, phase transition, and
matrix redistribution.
For power level of 7 5%, the maximum
moisture loss rate was obtained at microwaving time for 40
seconds.
However, at power level of 50%, the moisture loss
rate increase gradually until the microwaving time reached 40
seconds.
Also
studied were
the
effect
of microwaving
time
on
moisture content and moisture loss rate of bread at constant
power level
(100%) .
As shown in Figure 8,
the longer the
microwaving time caused the more moisture content decreased.
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51
When microwaving time was 10 seconds,
the moisture content
of bread dropped from 0.621 g water/g d.m to 0.612 g water/g
d.m.
For microwaving time 40 seconds,
decreased to 0.284 g water/g d.m.
the moisture content
Figure 9 shows that the
moisture loss rate of all bread samples heated with different
microwaving times (10, 20, 30, and 40 seconds) at constant power
level (100%) were followed similar trend.
The effects of the MW energy input on breads were studied
by manipulating the power levels (50, 75, and 100% = 600 watt)
and the microwaving times (30, 40, and 60 seconds) (Figure 10) .
Total MW energy input
(18 kw. second)
was obtained by the
combinations of the power levels and the microwaving times.
The moisture contents of MW heated bread decreased significantly
at high power level (100%) with short time (30 seconds) condition.
In Figure 11, it is also clearly shown that the moisture loss
rate of MW heated bread with high power level and short time
is much higher than that at low power level (50%) and long time
(60 seconds).
The effect of the MW power levels (50, 75, and 100%) and
the MW heating times
(10, 20, 30, and 40 seconds) on the %
moisture loss of bread is shown in Figure 12 . It was observed
that there was a high degree of correlation between the heating
times
and
% moisture
loss
in bread during
heating.
The
relationship between MW heating time and % moisture loss at
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52
power 10 0% was expressed by the following linear regression
equation, and the coefficient of correlation was 0.967.
% moisture loss = 1.3899 x (MW heating time)-15.671
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53
matter)
0.7
water/g
dry
0.6
*
CONTENT
(g
0 .5 *
MOISTURE
0 .4 “
0.2
0
5
10
15
TIME
Figure
6.
20
2 5
3 0
3 5
40
(second)
Effect of absorbed microwave powers on moisture
content: 50%, 75%, and 100% power, heating time 40
seconds.
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54
0.0125
0.0100
0.0075
MOISTURE
LOSS
RATE
(g
water/sec)
0.0150
0.0050
0.0025
0.0000
0
5
10
15
TIME
20
25
30
35
40
(second)
Figure 7. Effect of absorbed microwave powers on moisture loss
rate:
50%,
75%,
and 100% power,
heating
seconds.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
time 40
55
matter)
0.7
water/g
dry
0.6
CONTENT
(g
0 .5
MOISTURE
0.4
0 3 ]
0.2
0
5
10
15
TIME
Figure
8.
Effect
content:
of
20
25
30
3 5
40
(second)
absorbed microwave
power
on moisture
100% power and heating time 10, 20,
40 seconds
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
30,
56
water/sec
0.0150
0.0125
o
0/
9
8 .
0.0050
MOISTURE
,
a.
a»
0.0075
LOSS
RATE
(g
0.0100
0*
S*
0.0025
□
W
O
A
6m
40
30
20
lO
sec.
sec.
sec
sec.
0.0000
0
5
1 0
15
TIME
20
25
3 0
35
40
(second)
Figure 9. Effect of absorbed microwave power on moisture loss
rate:
100% power and heating time 10,
seconds
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
20,
30,
40
matter)
57
0.5
0.4
0.3
0.2
□
100 % f o r
30
sec.
•
75%
fo r
40
sec.
O
50 % f o r
60
sec.
-t
------1--------- 1--------- 1----------- -------- 1
0
10
20
TIME
Figure
10.
Effect
30
■---------1------------------- r
-
MOISTURE
CONTENT
(g
water/
g dry
---
0.6
4 0
50
60
(second)
of absorbed microwave power on moisture
content: 50%, 75%, 100% power and heating time 30,
40, 60 seconds
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58
D. Conclusion
The microwave-bread interaction was characterized by the
manipulation of microwave energy input on white bread slices.
The effect of MW energy input on the moisture content and the
moisture loss rate of white bread were studied using a AVC™-80
microwave oven which could control the microwave power levels
and the microwaving times, and monitor the weight changes during
MW heating.
From the experimental runs carried out,
conclusions can be made:
the following
(1) in general, the moisture losses
were found to be proportional to the MW energy input. The higher
the power level and the longer the heating time, i.e. , the higher
the energy input cause the more moisture losses; (2) the moisture
loss rate increased with an increase in absorbed MW energy.
When microwaving for 2 5 seconds at 100% power
level
, the
moisture loss rate reached at the maximum level (0.01244 g/sec)
and
the
moisture
content
dropped
d.m. (initial) to 0.469 g water/g d.m.
from
0.621
g
water/g
The decrease in moisture
loss rate after 2 5 seconds microwaving was observed.
phenomenon
may
due
to
the
high
energy
input
on
This
the
starch-gluten-water matrix of bread which could affect the
availability of water in bread;
(3) there was a high degree
of correlation between the heating times and % moisture loss
in bread during heating.
The relationship was expressed by
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59
the following linear regression equation, and the coefficient
of correlation was 0.98.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
60
0.0150
□
•
o
0
a
O
0.0125 -
for
75%
for
50% for
100%
30
sec .
45 sec.
60
sec.
U
0
jj
0
S
0 . 0 1 0 0
w
Eh
*
-
0.0075 -
01
oi
0
►3
0.0050 -
H
PS
Eh
S3
01
H
O
s
0.0025 -
0.0000
60
TIME
(second)
Figure 11. Effect of absorbed microwave power on moisture loss
rate: 50%, 75%, 100% power and heating time 30, 40,
60 seconds
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61
50
3.1(21 * 0.3049s
R»2 « 0.(91
7.7375 ♦ 0.(5»7s
0.967 O
15.(71 ♦ 1.3t99s
LOSS
40
MOISTURE
30
20
o \°
10
0
0
10
20
TIME
30
40
50
(second)
Figure 12. Relationship between % moisture loss rate and heating
times: 50%, 75%, 100% power, heating time 10, 20,
30, 40 seconds
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
62
Chapter III
THE COMPRESSION PROPERTIES OF MICROWAVE HEATED BREAD
A. Introduction
Upon exposure to microwave (MW) energy to refresh bread,
it is well known that the mechanical strength
(referred as
"toughening") of crumb is increased significantly (Rosenberg
and Bohl, 1987).
Reasoning by the cause-and-effect of nature,
there must be a cause for the MW-induced toughening of bread.
However,
the cause is not fully understood or conclusive.
Despite the importance of changes in mechanical strength
accompanying the MW heating of bread, there was few published
data exist on the analysis of mechanical properties for MW heated
bread.
One of the difficulties in obtaining data was that the
deformation is occurred within a short time period.
And there
were no universally accepted method and apparatus for measuring
the mechanical properties such as creep compliance of a MW heated
bread.
The American Association of Cereal Chemists
(1961)
used the Baker compressimeter as a standard (AACC 74-10).
Its
disadvantages were that the data obtained was a single force
reading
and was
not
convertible
to measure more
detailed
mechanical properties. The experimental errors associated with
reading the scale of this instrument were also significant.
Another difficulty in using that system was the deformation
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
63
of a bread crumb had to be monitored within a very short time
(less than one minute).
Therefore, we developed a simple and sensitive method for
measuring the creep compliance of bread using a linear proximity
sensor method in this lab.
The proximity sensor measured only
the deformation of a crumb under the constant stress applied
by metal disc.
The creep compliance value was increased with
time and approached a steady state where the compliance remained
constant.
In present work,
the creep compliance-time curve
was determined to evaluate the mechanical properties of the
MW heated bread.
The changes of creep compliances for MW heated
and dried breads were determined as a function of moisture
content to elucidate the effect of MW heating on the change
of mechanical properties of bread.
B. Materials and Methods
1. Materials
A commercial sliced white pan bread was purchased at a
local supermarket. Flat specimens were prepared from this item.
The specimen had a thickness of 12 mm.
Bread was treated with
MW heating under the same conditions as shown in previous study
(Chapter II) .
Dried breads were prepared by exposing them to
the air for different times at 25°C.
of samples
Initial moisture content
(average 39.5%, w.b.) was determined by oven dry
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
64
(AOAC 4 4 b ) .
2. Simple Proximity Sensor Method
The method was consisted of carefully placing a metal disc
of a constant weight on a sample and measuring the displacement
of the disc with time using a linear proximity sensor.
the
determination
of
normal
creep
behavior
of
For
bread,
an
apparatus (Figure 13) was developed in this lab using a simple
proximity sensor with a sensing range of 4.5 to 9.5 mm and a
repeatability of within 0.03 mm.
The method used for measuring normal creep behavior was
as follows: a slice of bread was placed on the adjustable jack
raised so that the slice touched flatly the bottom surface of
the applied load.
The specimens had a diameter 50 mm and
thickness about 12 mm.
The metal disc was 3 5 mm in diameter
and weights (150 g) was used to provide a constant stress level
of 1.53kPa.
The load (metal disk) was released from the clamp.
The movement from its null point was due to the deformation
of the bread.
The proximity sensor set over the load detected
the distance between the sensitive face of a sensor and the
upper surface of the load at real time.
The deformation due
the compression of the load to the crumb of the bread was
monitored every second for approximately 1 minute.
The
proximity sensor was operating on the reluctance principle
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65
responded to the closeness of a ferromagnetic object.
sensor could measure
the distance between a
This
ferromagnetic
surface and the sensitive face of the sensor without physical
contact.
This was interfaced with a Macintosh II computer with
a Strawberry Tree ACM 2-12 interface board.
Computer software
was used for control of the machine during data acquisition
and for subsequent data processing.
of machine voltage versus real
constant stress.
This included conversion
time output
to strain with
The compression property was obtained from
the stress-strain curves of the sample.
In a creep-compliance
test, the strain due to an imposed stress will be recorded as
a function of time.
Stress,
strain,
and creep compliance were defined as
follows:
Force
(16)
Stress
Cross sectional area
Compression of sample
(17)
Strain
Sample thickness
Strain
Creep compliance
(cm2/Dyne* 10s)
(18)
Stress
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Schematic Diagram of Compressimeter
with Proximity Sensor
Proximity
Sensor
Load(steel)
BREAD
tty
Adjustable Jack
Figure 13. Schematic diagram for proxim ity sensor method
Mac-ll
computer
Data Acquisition
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
VO
VO
67
C. Results
Figures 14 through 18 show the experimental results from
the creep compliance test for dried bread and MW heated bread
at 1.53 kPa stress level.
mean of triplicates.
Each of data point represents the
For dried bread, creep compliances were
measure at moisture content of 39.5%
31.7%, 24.1%, and 18.3%.
(w.b., non-MW heated),
These moisture content ranges covered
the loss of water for 3 hours drying in the air at 25°C.
Figure 14 shows that immediately after the initial loading
of metal disc, the creep compliance values are increased within
short time periods in all samples.
Constant creep compliance
value was observed in about 20 seconds after for the fresh bread
with moisture content of 39.5% and after 35 seconds for other
samples. The lower the moisture content caused the lower the
creep compliance.
This indicates that the decrease in creep
compliance (increase in mechanical strength) of non-MW heated
bread was caused by a decrease in moisture content due to
evaporation.
Figure 15 shows the creep compliances of MW heated bread
at power level 100% for 20, 30, and 40 seconds. When microwaving
for 20 seconds, the moisture content dropped from 39.5% to 33.5%,
to 30.2% for 30 seconds, and to 25.90% for 40 seconds.
The
creep compliance decreased significantly for each bread as the
moisture decreased.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
68
in
«
o
H
□
A
40.9%
31.796
O 24 .796
A 18.3%
51
*
«
a
>1
Q
\
ci
<
S
u
w
U
<
H
(W
S
o
u
01
w
w
«
u
50
TIMB
Figure 14
(second)
Creep compliance of non-microwave heated bread:
Moisture contents 18.3%,
Stress level 1.53kPa
Reproduced with
24.7%,
31.7%,
permission of the copyright owner. Further reproduction prohibited without permission.
40.9%,
69
CREEP
COMPLIANCE
(cmA2/Dyne*10A5)
O in itia l
A 20 s e c .
O 30 s e c .
A 40 s e c .
1UlMiilllMiiii*
o
10
30
20
TIME
40
50
(second)
Figure 15. Creep compliance of microwave heated bread:
power,
heating times 20,
30,
40 seconds,
level 1.53kPa
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
100%
stress
70
To
elucidate
the
compliance of bread,
effect
of
MW
heating
on
the
creep
the creep compliance of MW heated bread
and non-MW heated bread with same moisture content levels were
obtained
(Figure
16) .
At a given
time,
there were
large
differences of creep compliances between the non-MW heated bread
and MW heated bread. Independent of moisture content, MW heated
breads had lower creep compliances than non-MW heated bread.
This result suggests that the increased mechanical strength
of MW heated bread may not be only caused by the moisture loss
during MW heating.
It may due to other physicochemical changes
which affect the change of structure.
Figure
17
shows
the differences
of creep
between non-MW heated and MW heated bread after
in Zipper bag without losing moisture.
the
mechanical
significantly,
strengths
indicating
of
the
6
compliances
days storage
The results show that
tested
progress
breads
of
decreased
staling
during
storage periods. The creep compliances for the MW reheated bread
after
6
days storage were lower than
6
-day stored bread.
This
suggests that the staled bread is not fully refreshed by MW
heating.
From Figure 18,
there was a high degree of correlation
between the % moisture loss and the change of creep compliances
for the plots of both MW heated and non-MW heated bread.
shown,
the creep compliances decreased
(in other word,
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
As
the
71
mechanical strength increased) for each bread as the % moisture
loss level increased.
At a given moisture loss, there were
large differences of creep compliance between two bread.
The
MW heated bread was high mechanical strength than was non-MW
heated bread.
This suggests that there may be a physicochemical
changes affecting the structure of MW heated bread crumb and
resulting in the increase of mechanical strength of bread.
D. Conclusions
The changes of mechanical strength for MW heated and non-MW
heated
bread
were
investigated
by
measuring
the
creep
compliances of bread using a simple proximity sensor method
developed in this lab.
The creep compliances of MW heated and
non-MW heated bread were significantly decreased as the decrease
of moisture contents.
Independent of moisture content,
MW
heated breads had lower creep compliances than non-MW heated
bread.
At a given moisture loss, there were large differences
of creep compliance between non-MW heated and MW heated bread.
The MW heated bread showed higher mechanical strength than
non-MW heated bread.
These results suggest that the structural
changes of MW heated bread decreased the availability of water
as plasticizer and resulted in the increase of mechanical
strength of bread.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
72
(cmA2/Dyne*10A5)
A
A
CREEP
COMPLIANCE
5-
O
□
31.7% (Non-MW)
30.2%(MW heated)
24.7% (Non-MW)
25.9%(MW heated)
50
TIME
Figure
16.
(second)
Creep compliance of non-microwave heated and
microwave heated bread: 1 0 0 % power, heating times
20 and 3 0 seconds, moisture contents of 31.7%,
24.7%. stress level 1.53kPa
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
(cmA2/Dyne*10 *5)
73
CREEP
COMPLIANCE
4-
A
□
Non-MW
20 sec.(MW heated)
A 6 day stored (Non-MW)
O 6 day stored(MW heated)
0
10
20
TIME
Figure
17.
30
40
50
(second)
Creep compliance of non-microwave heated and
microwave heated bread: 1 0 0 % power, heating time
2 0 seconds and moisture content of 40.9%, stress
level 1.53kPa
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
74
6
Non-MW boated
% MW heated
y - 4.9533
0.911
y « 4.7854
R*2
0.975
5
4
COMPLIANCE
3
2
CREEP
(cm*2/Dyne*10A5)
□
1
0
0
5
10
MOISTURE
15
LOSS
20
25
30
(%)
Figure 18. Creep compliance of bread as a function of % Moisture
loss
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
75
Chapter IV
INVERSE GAS CHROMATOGRAPHIC CHARACTERIZATION OF
MICROWAVE HEATED BREADS
A. Introduction
The interactions of water with food components play an
essential role in the sensory, physical and chemical properties
of the food.
Water is known to have a significant effect on
the mechanical and textural properties of biopolymers such as
foods (Halek et al, 1989) . As discussed in the literature review,
water activity measurement and water sorption isotherm provide
a simple convenient means of evaluating interactions between
water and food systems.
An
Inverse
Gas
Chromatography
(IGC)
method
has
been
employed as a rapid method to study water sorption of food
systems including protein, starches, gliadin, bakery products,
corn meal extrudate, and freeze-dried coffee (Helen, 1983; Paik,
1984, Tanaka, 1984; Apostolopoulos, 1985; II, 1987, 1991; Lin,
1993).
However,
this method has not been used to study the
water sorption of microwaved bread.
In this study the applicability of using IGC analysis to
study the water sorption of microwaved bread was evaluated.
The sorption data were used to estimate thermodynamic parameters
that could provide insights about the interactions between
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76
microwave and the food system.
B. Materials and Methods
1. Materials
Locally purchased samples of white pan bread were used
as test material.
Fresh bread slices, selected from the middle
part of loaf, were approximately 12 mm thick.
The slices,
packaged in Zipper Sandwich bags to minimize moisture loss
during storage, were used within 24 hours.
The initial moisture
content of each sample was analyzed by the oven dry method (AOAC
44-b).
2. Methods
2.1. IGC Method
2.1.1. Sample Preparation
Samples of microwave heated bread were obtained by heating
the bread slices on a glass tray placed inside an AVC™-80
Microwave oven for 20 or 30 seconds at 100% power (approx. 600
Watt) . All the samples were initially equilibrated for 2 days
in a desiccator with P2 O 5 to ensure complete dryness prior to
experiment. All dried bread slices were gently grinded in mortar
and then sieved through a 100 mesh screen.
Moisture contents
of all samples were less than 2 g H 2 0/100 g dry bread.
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77
2.1.2.
IGC Setup
A
schematic
diagram
of
the
IGC
setup
experimental work is shown in Figure 19.
used
in
this
A Hewlett-Packard
Model 5890A with a Model 5705A thermal conductivity detector
(TCD)
(Hewlett-Packard, Avondale, Pennsylvania) was modified
for the IGC work.
The instrument settings were as follows:
Helium flow rate
= 40 ml/min
Injection port temperature
= 150°C
Column temperature
=30,
Detector (TCD) temperature
The
helium
pressure
was
40, and 50°C
= 140°C
regulated
at
Chromatographic grade (ultra pure) helium was used.
60psi.
The helium
then passed through the pre-column and IGC column to the detector.
The flow rate was measured at the detector outlet using a
digital flowmeter OPTIFLOW 520 (Humonics CO.).
The injection
port was kept at 150°C to allow instant evaporation of the
injected water.
Distilled water was used at all times.
The
temperature of column was regulated by the GC and was controlled
within
±1°C.
The
column
temperature
was
monitored
by
a
thermocouple attached on the surface of the midpoint of the
column.
The IGC column assembly consisted of a pre-column and a
sample column.
The pre-column was a 60" coil of 1/8" o.d.
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78
stainless steel tubing. The sample column was made up of a 1/8"
to 1/4"
Swagelok reducer
(Westchester valve
Briarcliff, NY) with nuts and ferrules.
& Fitting Co,
The sample chamber
inside the sample column was about 15 mm in length and 5 mm
in
diameter
(295mm3 total
volume).
All
the
connections were made of stainless steel.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
column/tube
o\
Personal
Computer
Thermal
Conductivity
Detector
_L
Injection
Port
A/D Converter
I
88888
IGC Column
Pre-Column
Figure 19. Schematic diagram o f IGC Column Assembly
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
He Carrier
Gas Tank
80
2.1.3. Experiment
The sample column was dried at 90°C in a hot-air oven for
an hour, cooled in a desiccator for at least two hours, and
then weighed.
A silanized glass wool (Supelco., Bellefronte,
PA.) was first inserted into the sample column followed by
filling the remaining column with bread sample prepared earlier.
Typical bread samples had weights ranging from 40-45 mg. After
packing of sample column with the bread sample, the silanized
wool was used as a filler at the end of the column.
The sample
column was weighed before and after bread sample was put in.
The
sample
column was
mounted
to
the
IGC assembly,
then
pre-conditioned in the GC by passing the helium for an hour
before experiment.
For
each experimental
run
40
(il distilled
water was
injected into the injection port.
The injected water was
vaporized and carried by helium gas.
With empty sample column,
injected water passed through sample column without sorption.
On the other hand when the sample column contained bread samples,
it became a reactor wherein the water-bread interactions took
place.
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81
2.1.4 Data Analysis
The TCD generated an analog voltage signal which was
proportional to the amount of water vapor in the helium gas
stream.
with
An analog to digital
12
bit
resolution)
(A/D) converter, model DASH -
and the analog
8
input multiplexer,
instrumentation amplifier were used for data collection.
The
analog signal from the TCD detector was collected as a continuous
voltage output.
data.
An NEC-AT computer was used for collecting
The Labtech Notebook software (Laboratory Technologies
Corp.,
Cambridge,
MA) , version 4.1,
was used for the data
acquisition, while for the data analysis the Lotus 1-2-3 program,
version 2.01, by Lotus Devel. Corp. was used.
Further analysis
of data to obtain water sorption was performed using a macro
program named Solo 123 developed by Giannakakos (1990).
2.2. Static Method for Water Sorption Isotherm
Water sorption isotherms also were obtained using the
static
method
described
by
procedures are as follows.
in aluminum pans.
1
Gal
(1981).
The
experiment
-gram bread samples were placed
Each pan was stored inside a 1-pint Mason
jar containing a saturated salt solution.
The salt solutions
and their respective relative equilibrium humidities were LiCl
(11%), CH3COOK (23%), MgCl 2 (32%), K2CO3 (42%), Mg(N0 3 ) 2 (52%),
NaN0 2
(64%),
and NaCl
(75%)
(Rockland,
1960; Young,
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
1967).
82
The jars were then kept in a storage room controlled at 3 0°C.
The pans were weighed every week until no weight change
could be absorbed, meaning the system was in equilibrium in
terms of water sorption.
Each datum reported was the average
of three replicates. The water sorption isotherms ware obtained
by
plotting
moisture
uptake
versus
equilibrium
relative
humidity at 3 0°C.
C. Results
1. System Calibration
It was shown by Ferng (1987) and Gilbert (1995) that the
empty column provides data for calibration of the TCD response
as a function of water concentration.
Since there is no sorption,
the area of resulting detector response was proportional to
the amount of water injected at any retention time.
To obtain area constant (Ka) and time constant (Kc) , the detector
response was analyzed by injecting 5, 10, 15, 20, and 40 |ll
of water in the empty column at 30°C.
was recorded as a voltage output.
The TCD detector response
As shown in Figures 20-21,
there was a linear dependence (R2 = 0.99) between the injection
mass and the residence time or the area of the chromatogram.
The peak height remained the same for the different amounts
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
83
of
injected water and the area under curve
increased from 742 to 5994.41.
(AUC)
linearly-
The area constant and the time
constants were obtained from the slope of Figures 20 and 21,
respectively.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
84
6000
5000
AREA
4000
3000
2000
1000
1.000
2.9946 + 146.71 x
0
5
10
15
20
INJECTED
25
WATER
30
35
40
45
(ul)
Figure 20. Area (time X response voltage) versus Inject water
amount from empty column; at 3 0°C, flow rate 40 cc/
min
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
85
2000
1750
1500
•d
G
o
o
o
n
1250
1000
U
s
H
n
750
500
250
5.2456 + 45.453X
0
5
10
15
20
INJECTED
25
WATER
30
1.000
35
40
45
(ul)
Figure 21. Resident Time versus Inject water amount from empty
column; at 30°C, flow rate 40 cc/ min
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86
2. Analysis of IGC Chromatogram
2.1. Typical IGC Chromatogram
The characterization of the non-MW heated and the MW heated
bread
(power
100%,
heating 20
or
3 0 seconds)
conducted using injection of distilled water.
this
testing gave
some
indications
interacts with these bread samples.
as
systems was
The results of
to how water vapor
A typical IGC sorption
chromatogram of bread system at 30°C is shown in Figure 22.
The amount
of
water passed
through
the
sample
column was
monitored by the TCD.
From tQ to tmaxI the water was seen to strong interaction
with the bread system.
Interaction with the bread caused loss
of water from the carrier gas. This constituted the adsorption
portion of the chromatogram.
During this adsorption phase,
the water vapor, assumed plug flow through the sample column,
was adsorbed on bread system.
If there was no interaction
between water vapor and bread, the chromatogram of bread would
have the identical shape with that of empty column run.
The high initial
difference
response.
most
between
losses of water is seen with the large
empty column
response
and bread
sample
There were increased interactions of water with the
energetic,
"primary'', water
binding
sites
of
bread
(referred to as monolayer) . As these sites were occupied, less
water interacted with the bread sample and more water was passed
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
87
through the sample to
the TCD.
After primary
(monolayer)
sorption was nearly completed, secondary (bilayer) and tertiary
sites were filled as time approached tm^.
These were less
energetic sites and did not have as high an affinity for the
water vapor.
The chromatogram in this region had a much lower
slope because the sorption of water in this region became slow.
It was also possible that as water was sorbed, some swelling
of the sample material might occur, exposing additional sorption
sites.
At time tmax, the maximum detector response represented
as Vmax was obtained.
At this point of the chromatogram,
the
water vapor stream had passed completely through the IGC column.
In empty column, because there was no more water left in the
stream of helium gas,
abruptly.
to
the corresponding TCD response dropped
In the sample column with bread, the area from tmax
the end of
the chromatogram represented the desorption
portion of the chromatogram.
This phase monitored the evolution
of water from the bread sample, representing the adsorbed water
in bread came out to dry helium gas stream then passed through
the TCD.
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88
3.5
Vmax
Empty column
Broad sample
□ O B
3.0
2.5
o
►
2.0
w
01
2
ft
0
1.5
01
M
ft
1.0
0.5
tm a x
to
0.0
1000
20 0 0
TIME
3000
(second)
4000
Figure 22. Typical IGC chromatogram: empty column and non-MW
heated bread
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
89
2.2. Reproducibility of IGC Chromatogram
Three sample columns with bread samples were prepared to
study
the
reproducibility
of
TCD detector
chromatograms were obtained by injecting 40
at 30°C.
(X l
IGC
of water water
After the first run, when the detector response was
zero (complete desorption), a sample column
second sample column prepared earlier.
run,
response.
was replace with
At the end of the second
third sample column with bread were replaced to second
run sample column.
Figures 23-25 show the IGC chromatogram for non-MW heated
and MW heated bread at 30°C
chromatogram
shows
no
(data on Appendices 1-3).
significant
differences
in
the
IGC
TCD
response among the three runs through the whole time range.
This data suggests that the IGC method has a good reproducibility
for generating IGC chromatogram which could be used to analyze
the water sorption characteristics of bread.
2.3. Sorption Isotherms with the IGC and Static Method
The sorption data generated by the IGC method and the static
salt solution method for non-MW heated bread at 3 0°C are plotted
In Figure 26.
The water sorption isotherm obtained from the
IGC method was in fair agreement with that from the static
experiments. It suggests the two method provide different means
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90
of studying the moisture sorption characteristics of food.
While comparing the sorption isotherm it can be observed
that up to a water vapor pressure of about 17 mmHg the isotherm
obtained from IGC method are a little below those from the static
method. Helen (1983) obtained similar results by testing bakery
products with static and IGC method and noted that nearly but
not quite all sorption equilibrium was reached in IGC sorption
isotherm.
Van den berg and Bruin
(1981) suggested that for
a multicomponent food system the equilibrium in water sorption
was some kind of "pseudoequilibrium" state.
2.4. Water Sorption of Microwave Heated Bread
Figure 2 8
shows the IGC chromatogram of empty column,
non-MW heated and MW heated bread
(power 10 0%,
3 0 second).
The IGC chromatogram allows one to visualize the different water
sorption characteristics of bread system.
the chromatogram,
As can be seen from
the detector response values of MW heated
bread were higher than that of non-MW heated bread prior to
W.ax- This is indicating that less water is adsorbed by MW heated
bread than non-MW heated bread.
The lesser the water uptake
the higher the detector response value in IGC chromatogram.
This differences may due to the structural changes occurred
in starch-gluten matrix of MW heated bread.
A graphical illustration of these data is given in Figures
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
91
28-30. These sorption isotherms more clearly demonstrate the
differences of water uptake shown in chromatogram analysis of
MW heated and non-MW heated bread system.
did
not
have
the
typical
sigmoid
characteristics of most food system.
of all
tested bread
All sorption curves
shape,
which
were
the
Generally, water uptake
increased gradually with
water
vapor
pressure until the sorption isotherm started to curve upward
steeply.
As Apostolopoulos
(1985)
have
reported
sorption
isotherms of this type were characteristic of food system, like
freeze-dried coffee, which are high in soluble low molecular
weight polysaccharides.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
92
3.0
1st Run
2nd Run
3rd Run
RESPONSE
(volt)
2.5
2.0
1.5
1.0
0.5
0.0
0
1000
2000
TIME
Figure 23.
4000
30 0 0
(second)
IGC chromatogram at 30 C-multiple
injection for
non-MW heated bread
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
93
3.0
2nd Run
3rd Run
RESPONSE
(volt)
2.5 '
0.5
0.0
0
1000
2000
TIME
3000
4000
(second)
Figure 24. IGC chromatogram at 30°C - multiple injection for
MW heated bread: power 100% for 2 0 seconds
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
94
3.0
1st Run
2nd Run
3rd Run
2.5
•U
o
i-l
2.0
>
w
CO
2
O
cu
CQ
H
0$
1.5
1.0
0.5
0.0
0
1000
2000
TIME
3000
4000
(second)
Figure 25. IGC chromatogram at 30°C-multiple injection for MW
heated bread: power 100% for 3 0 seconds
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
95
20.0
“ I
O
M
0
«
17.5-
6
>»
IGC
A Static-lst
□ Static-2nd
A Static-3rd
15.0 o
12.5 -
10.0
-
bi
2.5 *
0.0
0
5
WATER
10
VAPOR
15
PRESSURE
20
25
(mmHg)
Figure 26. Correlation between IGC and Static method isotherm:
non-MW heated bread at 3 0°C
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
96
Empty column
Bread sample
.5
RESPONSE
(volt)
.0
.5
.0
1.5
.0
.5
0.0
0
1000
2000
TIME
Figure 27. IGC chromatogram:
3000
4000
(second)
empty column and non-MW heated
and MW heated bread: power 100% for 3 0 seconds
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
97
It can be seen that all MW heated breads absorbed less
moisture than did non-MW heated bread at the same water vapor
pressure.
The gradual and slow water uptake of MW heated bread
at the lower water vapor pressure region, suggested that the
number of active sites favoring water sorption on the matrix
of starch-gluten in bread must be relatively less.
(1975)
Suggett
reported that polysaccharides or other carbohydrate
polymers in contact with water lend their free-OH groups and
rings or glycosidic oxygen for interaction with water molecules .
These group were the active sites which when they were present
on the surface of a solid, then attracted water molecules more
strongly than did surrounding components.
Because
of
MW
heating,
the
polymeric
chains
of
starch-protein matrix could be altered and packed tightly. By
forming crystalline regions,
-OH groups could be tied up in
intermolecular cross-linking, making them unavailable for water
sorption.
It should also be noted that due to the difference
of amount of MW energy input,
the MW heated bread with more
energy (microwaving for 3 0 seconds) shows less absorption of
water than MW heated bread for 20 seconds at power 100%.
This result suggests that in the MW heated bread matrix,
less active sites available for food-water interactions than
in the non-MW heated bread.
This may supportive that the
mechanical strength of MW heated bread can be increased by
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
98
decreasing the amount of water molecules that are available
as plasticizer.
The same observations were made at all three
temperatures.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
20.0
17.5
dry
matter)
99
CONTENT
(g
water/100g
15.0
12.5
10.0
7.5
MOISTURE
5.0
30°C
40°C
50°C
2.5
0.0
0
10
20
WATER
30
VAPOR
40
50
PRESSURE
60
70
80
(mmHg)
Figure 28. IGC sorption isotherms of non-MW heated bread and
MW
heated
bread:
power
100%
for
30
temperature 3 0°C
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
seconds
at
20.0
17.5
dry
matter)
100
CONTENT
(g
water/lOOg
15.0
12.5
10.0
7.5
MOISTURE
5.0
30°C
40°C
50°C
2.5
0.0
0
10
20
WATER
30
VAPOR
40
50
PRESSURE
70
60
80
(mmHg)
Figure 29. IGC sorption isotherms of non-MW heated bread and
MW
heated
bread:
power
100%
for
30
temperature 40°C
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
seconds
at
20.0
□
•
30°C
40°C
O 50°C
17.5
dry
matter)
101
CONTENT
(g
water/lOOg
15.0
12.5
10.0
7.5
MOISTURE
5.0
2.5
0.0
0
10
20
WATER
30
VAPOR
40
50
PRESSURE
60
70
80
(mmHg)
Figure 30. IGC sorption isotherms of non-MW heated bread and
MW
heated
bread:
power
100%
for
3 0 seconds
temperature 50°C
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
at
102
Water sorption isotherms at three temperatures (3 0, 40,
and 50°C) are presented in Figures 31-39.
Water sorption data
calculated from the IGC experiments are presented in Appendices
5-7.
Temperature affects the mobility of the water molecules,
and will therefore influence the rate for achieving equilibrium
between the water vapor and sorbed phase. Temperature increase
will results in a downward shift in the slope of the isotherm.
As may be seen the extent of water uptake increases with
increasing temperature.
These isotherms are constructed using
water vapor pressure at each temperature as the water activity.
If the water activity is expressed in terms of water vapor
pressure, a different phenomenon is more readily seen.
These
multiple-temperature data were used to calculate thermodynamics
of water sorption for bread systems.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
20.0
17 .5 '
□ Non-MW heated
• MW—100%/ 30 sec
dry
matter)
103
water/lOOg
15.0 -
12 .5 '
-
MOISTURE
CONTENT
(g
10.0
0.0
*
0
5
WATER
1 0
VAPOR
15
PRESSURE
20
25
(nunHg)
Figure 31. IGC sorption isotherms of non-MW heated bread at
temperature 30, 40, and 50°C
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
20.0
O
•
17.5
Non-MW heated
MW-100%, 30 aec
dry
matter)
104
CONTENT
(g
water/lOOg
15.0
12.5
10.0
7.5
MOISTURE
5.0
2.5
0.0
0
1 0
WATER
2 0
VAPOR
30
PRESSURE
40
50
(mxnHg)
Figure 32. IGC sorption isotherms of MW heated bread: power
100% for 20 seconds at temperature 30, 40, and 50°C
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
20.0
17.5 -
O Non-MW heated
• MW-100*6, 30 sec
dry
matter)
105
12.5 -
10.0
-
MOISTURE
CONTENT
(g
water/lOOg
15.0 '
2.5 '
0.0
0
1 0
2 0
WATER
30
VAPOR
40
50
PRESSURE
60
70
8 0
(mnHg)
Figure 33. IGC sorption isotherms of MW heated bread: power
10 0% for 30 seconds at temperature 30, 40, and 50°C
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
20.0
□
•
17.5
MW-100’s, 20 sec
MW-100\, 30 sec
dry
matter)
106
CONTENT
(g
water/lOOg
15.0
12.5
10.0
7.5
MOISTURE
5.0
0.0
0
5
WATER
1 0
VAPOR
15
PRESSURE
20
25
(mmHg)
Figure 34. IGC sorption isotherms of MW heated bread: power
100% for 20, 3 0 seconds at temperature 30°C
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
20.0
17.5 '
□
•
MW-100%, 20 sec
MW-100%/ 30 sec.
dry
matter)
107
12.5 "
10.0
-
MOISTURE
CONTENT
(g
water/lOOg
15.0 '
2.5 *
o.o O'
0
1 0
WATER
2 0
VAPOR
40
30
PRESSURE
50
(nunHg)
Figure 35. IGC sorption isotherms of MW heated bread: power
100% for 20, 3 0 seconds at temperature 40°C
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
20.0
O
•
MW-10096, 20 sec
MW-100%, 30 sec
17.5 '
dry
matter)
108
12.5 -
10.0
-
MOISTURE
CONTENT
(g
water/lOOg
15.0 '
0.0
0
1 0
20
WATER
30
VAPOR
40
50
PRESSURE
60
70
80
(mmHg)
Figure 36. IGC sorption isotherms of MW heated bread: power
100% for 20, 30 seconds at temperature 50°C
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
20.0
17.5 -
Non-MW heated
MW-100*, 20 sec
MW-100’s, 30 sec.
dry
matter)
109
12.5 -
10.0
-
MOISTURE
CONTENT
(g
water/lOOg
15.0 -
0.0 Q
0
10
WATER
2 0
VAPOR
30
PRESSURE
40
50
(mmHg)
Figure 37. IGC sorption isotherms of non-MW heated and MW heated
bread: power 100% for 20, 3 0 seconds at temperature
3 0°C
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
20.0
Non-MW heated
MW-100%, 20 sec
MW-100%, 30 sec
17.5
dry
matter)
110
CONTENT
(g
water/100g
15.0
12.5
10.0
7.5
MOISTURE
5.0
2.5
0.0
0
5
WATER
10
VAPOR
15
PRESSURE
20
25
(mmHg)
Figure 38 . IGC sorption isotherms of non-MW heated and MW heated
bread: power 100% for 20, 30 seconds at temperature
40°C
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Ill
20.0
□
17.5 '
O
A
Non-MW heated
MW-100%, 20 sec
MW-100\, 30 sec
>»
15.0 -
12.5 -
10.0
-
0.0 Q
0
1 0
2 0
WATER
30
VAPOR
40
50
PRESSURE
60
70
80
(mmHg)
Figure 3 9 . IGC sorption isotherms of non-MW heated and MW heated
bread: power 100% for 20, 3 0 seconds at temperature
5 0°C
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
112
3. Thermodynamics of Water Sorption
The thermodynamic parameters for the three samples (non-MW
heated, MW heated at power 100%,
30 seconds) are plotted in
Figures 42-54.
3.1. Free Energy of Sorption
The free energy of sorption
(AGS) is a key factor in
explaining the water sorption properties of bread because it
is a quantitative measure of the affinity of the food for water.
It can be considered as a measure of the ability of the system
to accomplish the sorption process.
could
provide
a
criterion
The change of free energy
whether
water
sorption
is
a
spontaneous (-AGS) or non-spontaneous process (+AGS) depending
on the sign of AGS values.
From
the
thermodynamic
standpoint,
sorption
is
a
spontaneous process and is therefore accompanied by a decrease
in free energy of the system, i.e., AGS is negative.
The more
negative AGS the stronger the tendency towards sorption.
the system is in a thermodynamic equilibrium AGS =
0
When
.
The AGS values calculated for non-MW heated and MW heated
bread (power 100% for 30 seconds) , at 30°C, 40°C, and 50°C are
presented in Appendix
8
.
In Figures 40-41, the AGS values are
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
113
presented
as
temperatures.
a
function
of
moisture
uptake
at
all
three
Depending on the microwave heat, treatment, the
AGS values for the water sorption of bread, ranged from about
-0.75 kcal/mole to -0.12 kcal/mole.
indicate
that moisture
The negative AGS values
sorption on bread
is a spontaneous
process.
As moisture content increases AGS becomes less negative
(i.e, water sorption is less favorable for samples of higher
moisture content) . This observation explained by the attractive
forces of the primary and secondary binding sites discussed
earlier.
Figures 42-44 show that the AGS values of MW heated samples
had significantly less negative values than non-MW heated bread
at all tested temperatures.
The difference was more apparent
at low moisture content. The less negative AGS values for MW
heated bread indicated that MW heated bread had less water
sorption
than
non-MW
heated
bread.
This
observation
is
suggested that less water binding sites were available on the
matrix of MW heated bread.
3.2. Enthalpy of Sorption
The enthalpy of sorption (AHS) is a molar quantity directly
related
to the energy of
interaction between
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
sorbed water
114
molecules
and
sorption
sites
in
the
food
material,
thus
providing information on the exothermic or endothermic nature
of the interaction.
The change of enthalpy is a measure of
the energy changes occurring upon sorption of water molecules
on to a sorbent.
Thus, the enthalpy changes can be related to the binding
or repulsive forces of the system, depending on the sign of
AHS values.
Negative values denote the existence of binding
forces as opposed to positive values indicated by the repulsive
forces {Apostolopolous & Gilbert,
1990).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
115
0.8
30° C
40° C
50° C
-delta
Gs
(kcal/mole)
0.7 -
0.1
-
0.0
0
.0
2 .5
MOISTURE
5 .0
7.5
CONTENT
(g
10.0
12 .5
water/lOOg
15.0
dry
17. 5
matter)
Figure 40. A G S of sorption as a function of temperature for
non-MW heated bread at 30, 40, and 50°C
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
116
0.6
30° C
40° C
50° C
(kcal/mole)
0.3
-delta
0.4
6s
0.5
0.2
0.1
0.0
0.0
2 .5
MOISTURE
5 .0
7 .5 10.0
CONTENT
(9
12 .5
water/lOOg
15.0
dry
17.5
matter)
Figure 41. AGS of sorption as a function of temperature for
MW
heated
bread:
power
100%
for
3 0 seconds
30,40,and 50°C
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
at
117
0.8
-delta
6s
(kcal/mole)
Non-MW heated
MW-100%, 30 84
0.0
0
.0
2 .5
MOISTURE
5 .0
7.5
CONTENT
(g
10.0
12 . 5
w a t e r / 10Og
15.0
dry
17.5
matter)
Figure 42. AGS of sorption as a function of water uptake for
non-MW heated and MW heated bread: power 100% for
30 seconds at 30°C
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
118
0.8
Non-MW heated
MW-100. 30 sec
0.6
Gs
0.5
0.4
-delta
(kcal/mole)
0.7
0.3
0.2
0.1
0.0
0
.0
2 .5
MOISTURE
5 .0
CONTENT
7.5
(g
10.0
12.5
water/100g
15.0
dry
17.5
matter)
Figure 43. AGS of sorption as a function of water uptake for
non-MW heated and MW heated bread: power 100% for
3 0 seconds at 40°C
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
119
0.8
Non-MW heated
MW-100*, 30 sec
0.6
Gs
0.5
0.4
-delta
(kcal/mole)
0.7
0.3
0.2
0
.0
2 .5
MOISTURE
5 .0
CONTENT
7.5
10.0
12 .5
(g water/lOOg
15.0
dry
17.5
natter)
Figure 44. AGS of sorption as a function of water uptake for
non-MW heated and MW heated bread: power 100% for
3 0 seconds at 50°C
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
120
The AHS values of non-MW heated and MW heated bread
were determined using Equation 13.
By plotting the natural
logarithms of the water vapor pressure of the 30°C, 40°C, and
5 0°C at
fixed water uptake versus
the
inverse of absolute
temperature (K) , straight lines with slopes equal to AHS/R were
obtained (Figures 45-47) . The slopes of these lines were used
to calculate AHS values
subsequently.
The AHS values,
as
calculated for non-MW heated and MW heated bread, are presented
in Appendices 9-10.
The AHS values of water sorption of bread were negative
within the entire water uptake and temperature covered.
negative
sign
attracting
of
the AHS value
forces
in
the
verified
binding
of
the
water
The
existence
of
molecules
on
starch-gluten matrix of bread system.
Figures 48-50 shows that the AHS values of MW heated system
had a less negative value than non-MW heated bread at all tested
temperature of 3 0°C, 40°C and 50°C.
At very low water uptake
region (less than 2.5 g H 2 0/ g 100 bread solid) , the most active
(energetically most favorable) binding sites became occupied
by water as reflected by the high negative AHS values about
-12.3 kcal/mole for non-MW heated bread and -11.5 kcal/mole
for MW heated bread.
This also meant that the binding forces
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
121
involved in the sorption of water with MW heated bread were
weaker than those with the non-MW heated bread.
For MW heated
bread, as water uptakes increased over 2.5 g H 2 0/ g 100 bread
solid,
the AHS values started leveling off at values around
10.7 kcal/mole.
3.3. Entropy of Sorption
The variation of entropy may be related to the spatial
arrangements occurring at the water-sorbent interface in a
defined state (i.e. , temperature and pressure) . Thus the water
sorption of water molecules on starch-gluten matrix could change
the original state of bread-water system to produce entropy
changes
(ASs) .
These entropy changes
(ASs) can be used to
characterize the degree of order of randomness existing in the
bread-water system at different stage of the sorption process
(Iglesias et a l ., 1976) . More structured systems have negative
ASs
values
associating
with
reduced
randomness
as
the
interaction between water and food proceeded spontaneously.
Entropy changes which accompanied water sorption on breads
were calculated using Equation 14.
The ASs value for non-MW
heated and MW heated breads , at 30°C, 40°Cand50°C, are presented
in Appendix 12 . Entropy changes for non-MW heated and MW heated
breads, depending on the temperature and water uptake are given
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
122
in Figures 51-53.
Depending on the type of bread, the temperature, and the
water uptakes, ASs values varied from -32 to -38.9 cal/mole
K.
The difference in the entropy change (ASs) between non-MW
heated
and
MW
heated
bread
was
related
to
the
sorption
characteristics of starch-protein matrix due to MW heating.
For MW heated bread, as the amount of sorbed water increased
up to monolayer value (approximately 2.5 g H2 0/100 g of bread
solid), the ASs value dropped to about -34.1 cal/mole K.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
123
4.25 -
(Aw)
3.75 '
In
3.25 -
2.75 '
2.25
3 .05
3 .10
3 . 15
3.20
TEMPERATURE
3.25
3.30
3 .35
(1/°K)
Figure 45. Plot of Ln (Aw) as a function of absolute temperature
at 30, 40, and 50°C for non-MW heated brea.d
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
124
4.25 -
(Aw)
3.75 ‘
In
3.25 -
2.75 -
2.25
3 .05
3 .10
3 . 15
3 .20
TEMPERATURE
3.25
3.30
3 .35
(1/°K)
Figure 46. Plot of Ln (Aw) as a function of absolute temperature
at 30, 40, and 50°C for MW heated bread: power 100%
for
2 0
seconds
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
125
4.25 -
(Aw)
3.75 -
ln
3.25 '
2 .75 -
2.25
3 .05
3 .10
3 . 15
3.20
TEMPERATURE
3.25
3.30
3 . 35
(1/°K)
Figure 47 . Plot of Ln (Aw) as a function of absolute temperature
at 30, 40, and 50°C for MW heated bread: power 100%
for 3 0 seconds
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
126
13.0
□
•
Non-MW heated
MW-100%, 30 sec.
(kcal/mole)
12.0
Hs
11.5
-delta
12.5 '
11.0
'
-
10.5 '
10.0
0
.0
2 .5
MOISTURE
5 .0
7.5
CONTENT
(g
10.0
12.5
water/100g
15.0
dry
17.5
matter)
Figure 48. AHS of sorption as a function of water uptake for
non-MW heated and MW heated bread: power 100% for
3 0 seconds at 3 0°C
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
127
13.0
D
•
Non-MW heated
MW-100%, 30 sec
(kcal/mole)
11.5
-delta
12 .0
Hs
12.5 -
11.0
"
-
10.5 -
10.0
0 .0
2 .5
MOISTURE
5 .0
CONTENT
7.5
10.0
12.5
(g water/lOOg
15.0
dry
17.5
matter)
Figure 49. AHS of sorption as a function of water uptake for
non-MW heated and MW heated bread: power 100% for
30 seconds at 40°C
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
128
13.0
□
•
Non-MW heated
MW-100%, 30 sec
(kcal/mole)
12.0
Hs
11.5
-delta
12.5 -
11.0
-
-
10.5 '
10.0
0 .0
2 .5
MOISTURE
5 .0
CONTENT
7.5
10.0
12 . 5
(g water/lOOg
15.0
dry
17 . 5
matter)
Figure 50. AHS of sorption as a function of water uptake for
non-MW heated and MW heated bread: power 100% for
3 0 seconds at 50°C
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
129
Figure 51-53 show a distinctive temperature dependence
of
the ASs values,
for bread system.
As
the
temperature
increased the ASs values became less negative, the system less
ordered.
This could be related to the higher kinetic energy
acquired by the water molecules at higher temperature, which
provided them with more mobility and made it easier for them
to bounce off
structural
the active water binding
instability
and
disorder
was
sites.
Thus more
produced
at
the
bread-water interface.
The entropy changes
(ASs) for MW heated bread at 3 0°C
indicated highly water uptake dependent (Figure 51).
At low
water uptake, the ASs value for the MW heated bread was about
-39.3
cal/mole K,
increased.
then dropped gradually as water uptakes
The observed ASs values at low water uptake were
close to those reported by Kuntz and Kauzman (1974) for protein.
Smith
(1982)
for corn starch,
and Helen
(1983)
for bakery
products.
MW heated bread had more negative ASs value than that of
non-MW heated bread.
Water sorption on MW heated bread resulted
in a more ordered system than non-MW heated bread.
The reason
for that may be the structural differences between non-MW heated
and MW heated bread, in the number of active sites, the strength
and the availability of these binding sites for water sorption
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
130
on the starch-protein matrix of bread due to MW heating.
The data indicates that microwave heated bread became more
ordered structure at low moisture uptake.
water molecules
sorbed on
It seems that the
the most active sites produce a
swelling of the polymer matrix containing the amylose molecules .
This swelling is accompanied by an orientation of the structure.
This
orientation
of
water
molecules
is
indicated by
the
increase in entropy of the matrix with increasing moisture
contents.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
131
40
MW-100%, 30 aec
•
Non-MW heated
38 '
(cal/mole
°K)
39 '
□
-delta
Ss
36 -
34 -
33 -
32
0 .0
2 .5
MOISTURE
5 .0
7.5
CONTENT
(g
10.0
12.5
water/lOOg
15.0
dry
17.5
matter)
Figure 51. ASs of sorption as a function of water uptake for
non-MW heated and MW heated bread: power 100% for
3 0 seconds at 3 0°C
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
132
40
(cal/mole
°K)
39 -
□
MW-100%, 30 sec.
®
Non-MW heated
38 -
37 -
-delta
Ss
36 -
35 -
34 -
33 -
32
0 .0
2 .5
MOISTURE
5 .0
CONTENT
7.5
10.0
12.5
(g water/lOOg
15.0
dry
17.5
matter)
Figure 52. ASs of sorption as a function of water uptake for
non-MW heated and MW heated bread: power 100% for
3 0 seconds at 40°C
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
133
40
□
•
38 '
(cal/mole
°K)
39 '
MW-100%, 30 sec.
Non-MW heated
-delta
Ss
36 -
34 -
33 -
32
0 .0
2 .5
MOISTURE
5 .0
7. 5
CONTENT
(g
10.0
12.5
water/100g
15.0
dry
17.5
matter)
Figure 53. ASs of sorption as a function of water uptake for
non-MW heated and MW heated bread: power 100% for
3 0 seconds at 50°C
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
134
D. Conclusion
Inverse gas chromatography was shown to be a reproducible
and a suitable for rapidly generating data on the effect of
MW heating on the water sorption process in bread systems.
The sorption measurements were made at temperatures (30°C, 40cC,
and 50°C) for non-MW heated and MW heated bread samples.
MW heated breads absorbed less water than non-MW heated
breads at all temperatures and all water concentrations, i.e.,
they were less hygroscopic.
This conclusion was confirmed by
the less negative AGS values
in the MW heated bread system
indicating that it had less "chemical potential" for moisture
transfer from surrounding atmosphere.
In the non-MW heated
bread system an active site binding was manifested by the highly
negative AHS value.
In the MW heated bread system, however,
AHS values were less negative value.
It can be speculated that the high energy input by MW
heating caused breaking intermolecular hydrogen bonding
in
starch-gluten matrix. Therefore, in MW heated bread, the active
water binding sites might be masked by structural changes,
possibly taking place in starch-gluten matrix and resulting
in the change of water sorption properties.
Such changes of
MW heated bread would decrease the availability of water binding
sites
on
that
matrix
and
result
in
lower
water
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
sorption
135
properties and giving rise to a mechanical strength of MW heated
bread system.
This result suggests that MW-induced toughening
of bread are due to structural
changes which affect water
sorption properties of microwave heated bread.
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136
Chapter V
CONCLUSIONS
To gain a better understanding the interaction of MW energy
with bread system, this work was aimed at studying the effect
of MW heating on the moisture loss rate, mechanical strength,
and the water sorption properties of bread system.
First, the moisture loss of bread during microwaving was
measured to quantify how MW energy interacted with water in
bread.
From the experimental runs carried out, the following
conclusions were made:
(1 ) the higher the power level and the
longer the heating time, i.e. , the higher the energy input cause
the more moisture losses;
(2 ) the behavior of moisture loss
rate curves suggested the possibility of internal structural
changes due to the microwaving process;
(3) there was a high
degree of correlation between moisture losses and MW energyinput within the range of experimental conditions.
Second, a simple proximity sensor method developed in this
laboratory was used to measure the change of mechanical strength
of bread.
The
creep
compliance value of MW
heated bread
decreased significantly with increasing MW heating time.
The
creep compliance value of MW heated bread was significantly
lower than those of non-MW heated bread although both had the
same moisture content level, suggesting the collapse of aerated
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
137
structure during microwaving by losing water molecules.
The
result also suggests that the structural changes of MW heated
bread decreased the availability of water as plasticizer and
resulted in an increase of mechanical strength of bread.
Third,
Inverse Gas Chromatography
(IGC) was applied to
assess both water sorption properties and thermodynamics of
sorption for non-MW heated and MW heated bread systems.
IGC
was shown to be a rapid method for obtaining data on the water
sorption process in bread systems.
MW heated breads absorbed
less water than non-MW heated breads,
hygroscopic.
i.e.,
they were
less
This result was confirmed by the less negative
free energy of sorption (AGS) and enthalpy (AHS) values in the
MW heated bread system than those of non-MW heated bread.
MW
heated bread had more negative entropy (ASs) value than that
of non-MW heated bread.
The data indicates that MW heated bread
became more ordered structure at low moisture uptake. The result
is consistent with the above suggestion that, in the MW heated
bread,
the
active water binding
sites
might
be masked
by
structural changes in starch-gluten matrix and resulted in the
decrease of water sorption properties in increase the mechanical
strength of MW heated bread system.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
138
REFERENCES
Aguerre,
R.J.,
Suarez,
C.,
and
Violiaz,
P.E.
1984.
Calculation of the variation of the heat of desorption
with moisture content on the basis of the BET theory. J.
Fd. Technol. 19:325.
Aguerre,
R.J.,
type
Suarez,
C.,
multilayer
and Violiaz,
sorption
P.E.
1989. New BET
isotherms.
Part.
I.
Theoretical derivation of the model. Lebensm.-Wiss. u.
Technol. 22:188.
Anderson, K. H., Lorense, M. W. and DeVey, D. E. 1989. Method
of microwave heating of starch-based product.
Apostolopoulos, D.
and Gilbert,
S.G.
1984.
Water
sorption
of coffee solubles by inverse gas chromatography.
In
"Instrumental
analysis
of
food,
Vol.2".
Ed.
Charalombous, G. and Inglette, G., Academic Press,
by
Inc.
New York.
Apostolopoulos,
D. 1985.
Inverse gas chromatography as used
in studying water sorption of coffee solubles. Ph.D.
Thesis. Rutgers University, New Jersey.
Apostolopoulos,
D. and Gilbert,
S. G. 1988. Frontal inverse
gas chromatography as used in studying water sorption
of coffee solubles. J. Food Sci. 53(3): 882-884.
Apostolopoulos,
of
D. and Gilbert,
coffee
solubles
S. G.
by
1990. Water sorption
frontal
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
inverse
gas
139
chromatography: Thermodynamic consideration. J. FoodSci.
55 :475.
Arsenault,
M.
1994.
Industrial
microwave
processing.
Food
Processing. June, 1994. 91-96.
Auerman,, L.J.
1977. Technology in Breadmaking., 1st German
Edition. Leipzig. Page 282-284.
Aurand, L. W. and Woods, A. E. 1973 . Water. In "Food Chemistry, "
1, AVI Publishing Company, Westport, CT.
Ayappa,
K.
G. , Davis,
H.T.,
Davis,
E .A . and
Gordon,
J.
1991. Analysis of microwave heating of materials with
temperature-dependent
properties.
AIChE
J. ,
37(3):
313-322.
Ayappa, K. G., Davis, H. T., Davis, E. A., and Gordon, J. 1992.
2D finite element analysis of microwave heating. AIChE
J. 38(10): 1577 - 1592.
Avital, Y., Mannaheim, C.H., Miltz, J. 1990. Effect of carbon
dioxide atmosphere on staling and water relations in bread.
J. FoodSci.
55 (2) :413-461.
Baker, J.C. 1941. The structure of the gas cell in bread dough.
Cereal Chem. 18, 34-41.
Bandyopadhyay, S.,
adsorption
Weisser,
isotherms
H.,
of
and Loncin,
foods
at
high
M.
1980. Water
temperatures.
Lebensm. w i s s . U. Technol. 13:182.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
140
Bechtel, D. B., Pomeranz, Y. , and DeFrancisco, A. 1978. Bread
making
studied
by
light
and
transmission
electron
microscopy. Cereal Chem. 55. 392-401.
Bengtsson, N. E. and Risman, P.O. 1971. Dielectric properties
of foods at 3 GHz as determined by a cavity perturbation
technique.
J.Microwave Power,6(2):107-123.
Bengtsson, N. E. and Ohlsson, T. 1980. Application of microwave
and high frequency heating in food processing. J. Microwave
Power. 15(1): 45-52.
Berlin, Z., Kliman, P. G. and Pallansch, M. J. 197 0. Changes
in state of water and proteinaceous systems.
J.
Coll.
Interface Sci. 34:488.
Berlin,
E.,
Anderson,
B.
A.
1968.
Water
vapor
sorption
properties of various dried milks and wheys. J. Dairy Sci.
51: 1339.
Berlin, E. 1981. Hydration of milk proteins. In "Water Activity:
Influences on Food Quality," edited by Rockland,
L. B.
and Stewart, G. F., Academic Press, New York, pg.467.
Boquet, R. , Chirife, J . , and Iglesias, H. A. 1978. Equations
for fitting water sorption isotherms: II. Evaluation of
various two-parameter models. J. Food technology. 13: 319.
Bourne, M.C. 1986. Effect of water activity on texture profile
parameters of apple flesh. J. Texture Stud. 17:331.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
141
Braun, J. and Guillet, J. E. 1976. Study of polymers by inverse
gas chromatography.
In
"Advances
in polymer science",
v .21. Springer-verlaag, Berlin.
Brunauer,
S., Emmett,
of gases
P. H. and Teller, E. 1938. Adsorption
in multimolecular
layers.
J.
Am.
Chem.
So c .
60:309.
Buffler, C. R. 1991. Microwave oven safety: non-pathological
issues. ASAE paper 916612. ASAE. St. Joseph. MI.
Buffler, C. R. 1992.
"Microwave Cooking and Processing", Avi
Book, Van Nostrand Reinhold, NY.
Bull, H. B. 1944. Adsorption of Water Vapor by Proteins. J.
Am. Chem. Soc. 66(2) : 1499.
Buffoot, D., Griffin, W. J. and James, S. J. 1989. Microwave
pasteurization of prepared meals. J. Food Engineering.
2: 145-156.
Bull, H. B. and Breese, K. 1968. Protein hydration. 1. Binding
sites. Arch. Biochem. Biophys. 128:488.
Bushuk, W. and Hlynka, I. 1964. Water as a constituent of flour,
dough, and bread. Baker's Dig. 38(6): 43-46.
Carrillo, P.J., Gilbert, S.G., and Daun, H. 1988. Starch/solute
interaction in water sorption as affected by pretreatment.
J. Food Sci. 53;1199.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
142
Chamberlin,
N.
1973.
Microwave
energy
in
the
baking
of
bread. Food Trade Rev. 43(9):8 .
Chabot,
J. F., Hood,
L. F.,
and Liboff, M. 1979.
Effect of
scanning electron microscopy preparation methods on the
ultrastructure of white bread. Cereal Chem. 56: 462-464.
Chen,
P. and Schmidt, S. 1990. An integral model for drying
of
hygroscopic
and
nonhygroscopic
dielectric heating. Drying Technology.
materials
8
with
(5) :907-930 .
Chinachoti, P. and Steinberg, M. P. 1986. Crys tall ini ty of waxy
maize
starch
as
influenced
absorption and desorption,
by
ambient
sucrose
content
temperature
and water
activity. J. Food Sci. 51: 997.
Chinachoti, P. and Steinberg, M. P. 1988. Interaction of sucrose
with gellatin,
egg albumin and gluten in freeze-dried
mixture as shown by water sorption. J. Food sci. 53:932.
Chirgadze, Y. N. and Ovsepyan, A. M. 1972. Hydration mobility
in peptide structures. Biopolymers. 11:2179.
Chirife, J. and Iglesias, H. A. 1978. Equation for fitting in
peptide structure. Biopolymer. 11: 2179.
Coelho, U., Miltz, J., and Gilbert, S. G. 1979. Water binding
on collagen by inverse gas chromatography: thermodynamic
considerations. Macromolecules 12:284.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
143
Datta,
A. K. 1990. Heat and mass transfer in the microwave
processing of food. Chemical Engineering Progress. June,
pp 47-53.
Datta, A. K. 1991. Mathematical modeling of microwave processing
as a tool to study safety. ASAE paper 916614. ASAE. St.
Joseph, MI.
Decareau, R. V. 1967. Application of high frequency energy in
the baking field. Baker's Dig. 41(6):52.
Decareau,
R.
V.
1985.
" Microwave
in
the Food
Processing
Industry". Academic Press, Inc. New York, NY.
Decareau, R. V. and Peterson, R. A. 1986. “Microwave Processing
and Engineering". Ellis Harwood Series in Food Science
and Technology, Weinheim,
De Alwis,
Federal Republic of Germany.
A. A. P, and Fryer,
P. J. 1990. A finite element
analysis of heat generation and transfer during ohmic
heating
of
food.
Chemical
Engineering
Series.
45(6):
1547-1559.
Demertzis, P.G., Riganakos, K.A., and Kontominas, M.G. 1991.
Study
of
water
sorption
behavior
of
pectins
using
a
computerized elution gas chromatographic technique. 54,
421-428 .
Derby,
R. I., Miller,
B.
1975.
B. S., Miller,
Visual
B. F., and Trimbo,
observation
of
wheat
H.
starch
gelatinization in limited water system. Cereal Chem. 52:
702-713 .
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
144
Donnelly, B. J . ( Fruin, J. C., and Scallet, B. L. 1973. Reactions
of oligosaccharide. III. Hygroscopic properties. Cereal
chem. 50:512.
Edgar, R. 1986. The economics of microwave processing in food
industry. Food Technology. 40(6): 106-111.
Everett,
D. H.
1950.
Thermodynamics of adsorption.
Part.l.
General consideration. Trans. Faraday Soc. 46:453.
Fennema, O. R. 1985. Water and Ice. In "Food Chemistry.
by Fennema, Or. R.
11
Ed.
Marcel Dekker, Inc. New York. NY.
Feng, A. 1987.
Ferng,
A.
and Gilbert,
S.G.
1987.
New method
for sorption
isotherms by chromatography. 47th IFT meeting. Las vegas.
Nevada, June, 1987.
Forey,
A.
M.
1985.
pasteurization
Modelling
process.
M.S.
a
continuous
Thesis.
microwave
University
of
Massachusetts, MA.
Frank, F. Reid, D. S., and Sugget, A. 1973. Conformation and
hydration of sugars and related compounds in dilute aqueous
solution. J. Solution
Chem. 2: 99.
French, D. 1984. Physical and chemical organization of starch
granule.
In "Starch:chemistry and technology". 2nd Ed.
Ed. R. L. Whistler, J. N. Bemiller. and E. F. Paschall,
Academic Press, New York, NY.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
145
Fuzek, J. F. 1980. Glass transition temperature of wet fibers:
Its measurement and significance. In "Water in polymer".
S. P. Rowland, (ed. ) . ACS Symposium series 127. Washington,
D. C. American Chemical Society, pp:515-530.
Gal, S. 1981. Recent developments in techniques for obtaining
complete sorption isotherms. In "Water activity influences
on food quality. " Ed. by L.R. Rockland and S.G.F. Steward,
Academic press. New York.
Gerster, H. 1989. Vitamin losses with microwave cooking. Food
Science and Nutrition. 42F: 173-181.
Ghiasi, K., Hoseney, R. C., Zeleznak, K. and Rogers, D. E. 1984.
Effect of waxy barly starch and reheating on firmness of
bread crumb. Cereal Chem. 61(4): 281-285.
Giannakakos, P. N. 1990. Water relations in lysozyme-glucose
system. Ph. D thesis, Rutgers University, New Brunswick,
New Jersey.
Giese, J. 1992. Advances in microwave food processing. Food
Technology, 46(9): 118-123
Gilbert, S. G. 1984. Inverse gas chromatography. In "Advances
in Chromatography". Vol.23. Ed. byGiddings, J.C., Grushka,
E. Marcel Dekker, Inc., New York.
Gilbert,
S. G. 1989. Modified frontal chromatographic method
for water
sorptions
of
biological
macromolecules.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
In
146
"Inverse gas chromatography" Ed. D. R. Llyod. ACS symposium
series 391. Am. Chem.Soc. publisher, Washington,
D. C.
pg. 306-317.
Gilbert, S. G. and Roshdy, T.H. 1989. The use of inverse gas
chromatography in food science research. In "Flavors and
Off-flavors,
Proceeding of the
6
th International Flavor
Conference". Ed. by Charalambous, G., Elservier Sci., Pub.,
Amsterdam, Netherlands.
Goebel,
N.
K. , Grider,
J.,
Davis,
E.
A.
and
Gordon,
J.
1984. The effect of microwave energy and convection
heating
on
wheat
starch
granule
transformations.Food
Microstructure, Vol.3, pp. 73-82.
Gray, D. G., and Guillet,
method
for
the
Macromolecules.
Guggenheim,
E.
J. E. 1972. A gas chromatographic
study
of
sorption
on
polymers.
5(3): 316.
A.
1966.
"Applications
of
statistical
mechanics." Clarendon Press, Oxford.
Halek, G. W. , Paik, S.W., and Chang, K.L.B. 1989. The effect
of moisture content on mechanical properties and texture
profile parameters of corn meal extrudate. J. of Texture
Studies. 20:43.
Hari,
P.K.,
Agarwala,
A.K.,
and
Garg,
S.
1988.
Moisture
accessibility of hydrophilic sites in starch and modified
starch. Starch 40(6):221.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
147
He, H. and Hoseney, R. C. 1992. Effect of the quantity of wheat
flour protein on bread loaf volume. Cereal Che. 69(1);
17-19.
Heddleson, R. A., Doores, S., and Mast, M. G. 1991. Survival
of Salmonella species heated by microwave energy in a
liquid menstruum containing food components. J. of Food
Protection. 54 (8):637-642.
Hegenbart,
S. 1992. Microwave quality:
Coming of Age.
Food
product design. June. Page 29-52.
Helen, H. I. 1983. Moisture sorption of dry bakery products
by inverse gas chromatography for package optimization.
Ph.D. thesis, Rutgers University, New Brunswick, NJ.
Helen, H. I. and Gilbert, S. G. 1985.
Hermansson, A.M. 1977. Functional properties of proteins for
foods-water vapor sorption. J. Fd. Technol. 12:777.
Higo, A. and Noguchi, S. 19 87. Process of bread hardening by
microwave heating. Nippon Shokuhin Gakkaishi,
34, 474.
Higo, A., Ohkubo, M . , and Shimazaki, M. 1981a. Hardening of
food texture induced by microwave irradiation. II. Changes
in properties of starch in bread accompanied by hardening.
J. Home. Econ. Japan. 32: 178.
Higo, A., Ohkubo, M. , and Shimazaki,
food
texture
induced
by
M. 1981b. Hardening of
microwave
irradiation.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
III.
148
Changes in starch granule under water limited conditions.
J. Home. Econ. Japan. 32: 18 5.
Higo, A., Ohkubo, M. , and Shimazaki, M. 1982. Hardening of food
texture induced by microwave irradiation. V. Differences
in lipid extractability of some model systems with regard
to the water content. J. Home. Econ. Japan. 33: 221.
Higo, A., Shimazaki, M . , and Noguchi, S. 1983a. Hardening of
food
texture
induced by microwave
irradiation.
VIII.
Effect of starch-gel formation on hardening of bread. J.
Home. Econ. Japan. 34: 83.
Higo, A., Shimazaki, M . , and Noguchi, S. 1983b. Hardening of
food texture induced by microwave irradiation. IX. Study
of the specific effects of microwaves. J. Home. Econ. Japan
34: 251.
Hoseney, R. C. 1986. Principle of cereal science and technology.
AACC. St. paul. MI.
Iglesias, H.A. andChirife, J. 1984. Technical note:Correlation
of BET monolayer value moisture content in foods with
temperature. J. Fd. Technol. 19:503.
Iglesias,
H.A.,
Chirife,
J. ,
and
Violiaz,
P.
1976.
Thermodynamics of water vapor sorption by sugar beet root.
J. Fd. Technol. 11:91.
II, B.
1987. Water sorption of Glidin. M.S Thesis.
University, New Brunswick, New Jersey.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Rutgers
149
II,
B.
1991.
Water
sorption
of Gliadin.
J.
of
Food
Sci.
1977.
Some
56(2):510-531.
Jeffrey,
G. A.
, Gress,
M.
E.,
and Takagi,
S.
experimental observation on H-O hydrogen bond length in
carbohydrate crystal structure. J. Am. Chem. Soc. 99: 609.
Jeffrey, G. A. 1982. Hydrogen bonding in amino acid acids and
carbohydrates.
Activity.
In Molecular
Structure
and
Biological
Griffin, J. F. And Duax, W. L. (Ed.), p. 135.
Elsevier Science Publishing Co, Inc.
Karel,
M.
1975a.
Water activity and
food preservation.
In
"Principle of food science. Part II. Physical principles
of food preservation". Ed. by Karel, M . , Fennema, O.R.,
and Lund, D.B., Marcel Dekker,
Inc. New York.
Karel, M. 1975b. Physico-chemical modification of the state
of water in foods-A speculative survey. In "Water relations
in foods". Ed. by Duckworth, R.B., Academic press. Inc.,
New York.
Katz, E. E. and Labuza, T. P. 1981. Effect of water activity
on the sensory crispness and mechanical deformation of
snack food products. J. Food Sci. Vol. 46.,pg 403.
Keenan, M. C. 1983. Prediction of thermal inactivation effects
in
microwave
heating.
M.
S.
Thesis.
University
of
Massachusetts at Amherst. MA.
Kent,
M.
1985.
Dielectric properties
of
frozen biological
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
150
material. J. Bioelectricity. 4(2): 349-362.
Kent, M. 1987.
"Electrical and dielectric properties of food
materials". Science and Technology Pub.England.
Kent, M. and Meyer, W. 1982. A density independent microwave
moisture meter for heterogeneous food stuffs. J. of Food
Engineering,
1:31-42.
Khoo, U., Christianson, D. D., and Inglett, G. E. 1975. Scanning
and transmission microscopy of dough and bread. Baker's
Dig. 49(4): 24-26.
Kim, S. K. and D'Appolonia, B. L. 1977. Bread staling studies.
II. Effect of protein content and storage temperature on
the role of starch. Cereal Chem. 54:216.
Knjaginicev,
M.J.
1970.
Russ.
Journal
Mendelejew Institute, Moscow,
of
Chem.
Allunion
10(3):227-286.
Kress-Rogers, E. and Kent. M. 1987. Microwave measurement of
powder moisture and density. J. of Food Engineering. 6:
345-376.
Kudra, T., Raghavan, G. S. V., VandeVoort, F. R. and Ramaswamy,
H. S. 1990. Microwave heating of dairy products. ASAE Paper.
906603. ASAE. St. Joseph. MI.
Kuntz, I. D. and Kauzmann, W. 1974. Hydration of proteins and
polypeptides. Adv. Protein Chem. 28:239.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
151
Labuza, T. P. 1968. Sorption phenomena in foods. Food Technol.
22:264.
Labuza,
T.P.
1984. Moisture sorption:
Practical aspects of
isotherm measurement and use. AACC., St. Paul., Minnesota.
Labuza,
T.P. and Busk, G.C.1979. An analysis
of water binding
in gels. J. Food Sci. 44:1385.
Labuza,
T.P.,
Kaanane,
A.,
and Chen, J.Y.
1985.
Effect of
temperature on the moisture sorption isotherms and water
activity shift of
two dehydrated
foods.
J.
Food Sci.
50 :385.
Lambert, L. L. P., Gordon, J. anddavis, E. A. 1992. Water loss
and structure development in model cake systems heated
by
microwave
and
convection
methods.
Cereal
Chem.
69 (3) :303-309.
LePage,
C. A.,
Gordon,
J.
and Davis, E. A.
1989.
Physical
analysis of isolated gluten model systems heated in an
experimental conventional-microwave oven.
Cereal Chem.
6 6 (1): 33-38.
Leung,
H. K. 1987.
Influence of water activity on chemical
reactivity. In "Water Activity:Theory and application to
food." Ed. by Rockland, L. B. and Beuchat, L. R. , Marcel
Dekker, Inc. New York.
Lewis, M.J. 1987. Microwave and dielectric heating. In "Physical
properties
of
foods
and
food
processing
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
systems".
152
Eliiswood Ltd., Chichester, England.
Lin, Y. E. 1991. Heating characteristics of simulated solid
foods in a microwave oven. Ph.D Thesis. The Pennsylvania
State University, University Park, PA.
Lin,
H.
1993,
Water
sorption
properties
of
corn
meal
extrudates;Thermodynamic approach by the methods of water
sorption isotherms and inverse gas chromatography. Ph.D
Thesis. Rutgers University, New Jersey.
Loh, J. and Breene, W. M. 1983. A laboratory microwave sterilizer
and its possible application toward improving texture of
sterilized vegetables . J. Food Processing and Preservation.
7: 77-92.
Lorenz, K,m Charman,
E., and Dilsaver, W. 1973. Baking with
microwave energy. Food Technol. 27 (12) :28.
Marousis, S. N. , Karathanos, V. T. , and Saravacos, G. D. 1989.
Effect of sugars on the water diffusivity in hydrated
granular starches. J. Food Sci. 54(6): 1496
Martin,
D.
J.,
and
Tsen,
C.
C.
1981.
Baking
high-ratio
white layer cakes with microwave energy. J. Food Sci.
46:1507.
Mazza, G. 1980. Thermodynamic considerations of water sorption
by horseradish roots. Lebensm. wiss. u. Technol. 13:13.
Mingos, M. P. and Baghurst, D.R. 1991. Application of microwave
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
153
dielectric
heating
effects
to
synthetic
problems
in
chemistry. Chem. Soc. Rev. 20. 1-47.
Mudgett,
R.
E.
1985a.
Dielectric
properties
of
food.
In
"Microwaves in the food processing industry", Ed. R. V.
Decareau, Academic Press. Inc., New York, NY.
Mudgett,
R.
E.
1985b.
characteristics.
Modeling
In Microwaves
microwave
in the
heating
food processing
industry", Ed. R. V. Decareau, Academic Press. Inc., New
York, NY.
Mudgett,
R.
E.
1986a.
Microwave
properties
of
heating
characteristics of food. Food Technology. 40 (2) :121-13 5 .
Mudgett,
R.
E.
1986b.
Electric
properties
of
foods.
In
"Engineering properties of foods" . Ed. Rao M. A. and Rizvi
S. S. H., Marcel Dekker, Inc., New York, NY.
Mudgett, R. E. 1988. Electromagnetic energy and food processing.
Journal of Microwave Power and electromagnetic energy.
23 (4) : 225-230 .
Mudgett, R. E. 1989. Microwave food processing. A scientific
status summary by the IFT expert panel on food safety and
nutrition. Food technology. 43(1): 117-126.
Mudgett, R. E., Goldith, S. A., Wang, D. I. C., and Westphal,
W. B. 1977. The prediction of dielectric properties in
solid foods of high moisture content at ultrahigh and
microwave
frequencies.
J.
of
Food
Processing
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
and
154
Preservation. 1: 119-151.
Mudgett, R. E., Goldith, S. A., Wang, D. I. C., and Westphal,
W. B. 1980. Dielectric behavior of semi-solid food at ow,
intermediate,
and high moisture content.
J.
Microwave
Power. 15(1): 27-36.
Mudgett.
R. E. and Schwartzberg,
H. G. 1982. Microwave food
processing, pasteurization, and sterilization
A review.
AIChE symposium Series. 78(218): 1-11.
Nelson, S. O. 1990. Correlating dielectric properties of solids
and particulate samples through mixture relationships.
ASAE paper 906604. ASAE, St. Joseph, MI.
Nelson,
S. O., Lawrence,
K. C . , and Kraszewski,
A. W. 1990.
Sensing moisture content of pecans by RF impedance and
microwave resonator measurement. ASAE Paper . 903554. ASAE.
St. Joseph. MI.
Ohlsson, T, Henriques, M. and Bengtsson, N. E. 1974. Dielectric
properties of model meat emulsions at 900 and 2800 MHz
in relation to their composition. J. FoodSci. 39: 393-414.
Ohlsson,
T.
1989.
Dielectric
properties
and
microwave
processing. 1989 SBMO International Microwave Symposium
Proceedings. Sao Paulo, Brazil.
Osborne, T.B. 1979.
"The proteins of wheat kernel". Carnegie
Institute of Washington, Washington D.C.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
155
Paik, S. W. 1984. The state of water in food components related
to germination of mold
spores.
Ph.D.
thesis,
Rutgers
University, New Brunswick, NJ.
Paik, S. W. and Gilbert, S.G. 1986. Water sorption isotherms
on sucrose and starch by inverse frontal gas chromatography.
J. Chromatography. 351:417.
Palaniappan, S., Sastry, S. K. , and Richer, E. R. 1990. Effect
of
electricity
on
microorganism:
A
review.
J.
Food
Processing and Preservation. 14: 393-414.
Pangrle, B. J., Ayappa, K. G., andCavis, H. T. 1991. Microwave
thawing of cylinders. AIChE J. 37(12): 1789-1800.
Pei, D. C. T. 1982. Microwave baking-New developments. Baker's
D i g . 56(1):8.
Pixton, S.W. and Warburton, S. 1976. The relationship between
moisture content and equilibrium relative humidity of
dried figs. J. Stored Products Res. 12:87.
Pomeranz, Y. 1980. Wheat flour components in breadmaking. In
"Cereal
for food and beverage".
Ed.
by Inglett,
E.D.
Academic Press., New York. Page 201-232.
Pomeranz, Y. 1986. Modern cereal science and technology. VCH
publish, New York. NY.
Pomeranz,
Y.
(ed.) 1988.
"Wheat: chemistry and technology."
2 vols. AACC. St. Paul. MI.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
156
Pomeranz,
Y.
(ed.) 1989.
"Wheat is unique." AAC C . St. Paul.
MI.
Pomeranz, Y. 1991. Functional properties of food components.
2nd Ed. Academic press,
Inc. San Diego, CA.
Rabek, J. F. 1980. Inverse gas chromatography. In " Experimental
method
in
polymer
chemistry-Physical
principles
and
application" Wiley Interscience, New York. NY.
Radajewski, W. , Jolly, P. and
Abawi, G. Y. 1988. Grain drying
in a continuous flow drier supplemented with a microwave
heating system. J. Agricultural Engineering Research, 41:
211-225.
Riganakos,
K.A. , Demertzs,
P.G., and Kontominas, M.G.
198 9.
Gas chromatography study of water sorption by wheat flour.
J. Cereal Science. 9:261.
Robertson, G. L. 1992. Packaging of Microwavable Foods, In "Food
packaging : Principle and Practice", Marcel dekker, Inc.,
NY, pp 409-430.
Rockland,
L. B. and Nishi,
activity on
S. K.
food product
1980.
Influence of water
quality and
stability.
Food
Technol. 34(4):42.
Rockland,
L.
B. and Stewart,
G.
F.
1981.
"Water Activity:
Influences on Food Quality." Academic Press, New York.
Roebuck,
B.D.
and Goldblith,
S.A.
1975.
Dielectric
properties at microwave frequencies of agar gels.
J. Food
Sci., 40: 899-902.
Roger, D. E., Doescher, L. C. and Hoseney, R. C. 1990. Texture
characteristics of reheated bread. Cereal Chem.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
67(2):
157
1 8 8 -1 9 1 .
Roos, Y. H. 1987. Effect of moisture on the thermal behavior
of
strawberries
studied
using
differential
scanning
calorimetry. J. Food Sci. 52:146.
Roos, Y.H. 1993. Water activity and physical state effects on
amorphous food stability. J. Food Process and Preservation.
16:433-447 .
Rosenberg, U. and Bogi, W. 1987. Microwaving thawing, drying,
and
baking
in
the
food
industry.
Food
Technology.
41(6) :92-99.
Saravacos, G. D. Tsiourvas, D. A., and Tsami, E. 1986. Effect
of temperature on the water sorption isotherms of Sultana
raisins.
Schiffimann,
J. Food Sci. 51: 381.
R. F.
1990.
Microwave foods
:
basic
design
consideration. Tappi Journal, March, pp 209-212.
Schiffimann,
R.
F.
1992.
Microwave
food processing:
past,
present, and future. Paper 148, presented at 52nd Annual
Meeting of Inst, of Food Technologists, New Orleans, LA,
June 21-24.
Shivhare,
U.
Modeling
S.,
Raghavan, G.
S., and Bosisio,
of microwave drying of corn through
R. G.
1991.
diffusion
phenomena. ASAE paper 913520. ASAE. St. Joseph, MI.
Smith,
D. S. 1982. The thermodynamic and structural aspects
Reproduced with permission of the copyright owner Further reproduction prohibited without permission.
of
water
sorption
chromatography.
by
Ph.D
starches
thesis.
using
Rutgers
inverse
gas
University.
New
Brunswick, New Jersey.
Souda, K. B., Akyel, C. and Bilgen, E. 1989. Freeze dehydration
of milk using microwave energy. J of Microwave Power and
electromagnetic energy. 24 (4):195-202 .
Speccio, J. J. 1987. The effect of carbon dioxide and nitrogen
on the thermodynamic state of water in collagen.
Ph.D
Thesis. Rutgers University, New Brunswick, NJ.
Spite,
G.
T.
1984.
Microwave-inactivation
of
bacterial
pathogens in various controlled frozen food compositions
and in a commercially available frozen food products. J.
of Food Protection. 47(6): 458-462.
Stear,
C.
A.
1992.
"Handbook
of
breadmaking
technology".
Elsevier Applied Science. New York. NY.
Suggett,
1975.
Tanaka, S. 1984. A study of protein-probe interaction by inverse
gas chromatography. Ph.D. Thesis. Rutgers University, New
Brunswick, NJ.
Tong, C. H. and Lund, D. B. 1993a. Microwave heating of baked
dough
products
with
simultaneous
heat
and
moisture
transfer. J. Food eng. 19(4): 319.
Tong, C. H., Welt, B. A., and Steet, J. A. 1993b. Utilization
with permission of the copyright owner. Further reproduction prohibited without permission.
159
of microwaves in the study of reaction kinetics in liquid
and semisolid media. Biotechnology progress.
9(5): 481.
Tong, C. H., Lentz, R. R., and Lund, D. B. 1993c. A microwave
oven
with
variable
temperature
continuous
controller.
power
and
a
feedback
Biotechnology progress.
9(5):
488 .
Tong, C. H., Lentz, R. R., and Rossen, J. L. 1994a. Dielectric
properties of pea purea at 915 MHz and 2450 MHz as a function
of temperature. J. of Food Sci. 59(1): 121.
Tong, C. H., Welt, B. A., and Rossen, J. L. 1994b. Effect of
microwave
sporogenes
radiation
(PA 3679)
on
inactivation
of
Clostridium
spores. Applied and environmental
microbiology. 60(2): 482.
Troller, J.A. and Christian, J.H.B. 1978. Water activity methods.
In "Water activity”. Ed. by Troller and Christian,
Page
14. Academic Press, New York.
van den Berg, C. and Bruin, S. 1981. Water activity and its
estimation in food systems: theoretical aspects. In "Water
Activity: Influences in Food Quality, " edited by Rockland,
L. B. and Stewart, G. F., Academic Press, New York,
p.
1.
van den Berg, C. 1981. Vapour sorption equilibria and other
water-starch interactions; a physico-chemical approach.
Ph.D.
thesis,
Agricultural
Univ.,
Wageningen,
Netherlands.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
the
160
van
den Berg,
C., and Bruin,
S. 1981.
Influences on food quality"
(Ed.)
In
"Water activity:
Rockland,
L.B.,
and
Stewart, G.F., Academic Press, New York.
Varriano-Martson,
microscopy
E.
in
1981.
cereal
Integrating
science.
light
Foods
and
electron
World.
26(10),
558-561.
von Hippel, 1954. "Dielectrics and Waves" . MIT press, Cambridge,
MA.
Wagter,
C. 1984.
Computer simulation predicting temperature
distribution
generated
by microwave
absorption
multilayered media. J. Microwave Power. 19(2):
in
97-105.
Wei, C. K. and Davis. E. A. 1985. Heat and Mass transfer in
water-laden sandstone: Microwave heating. AIChE u. 31(5):
842-848.
Whorton, C. and Reineccius, G. 1990. Current developments in
microwave flavors. Cereal Food World, pp 553-55 9.
Wolfenden, R. 1978. Interaction of the peptide bond with solvent
water: A vapor phase analysis, biochemistry. 17:210.
Zalie,
J.
P.
1989.
Meeting
the microwave
challenge.
Food
Processing. May. pp 23-24.
Zografi, G. 1988. States of water associated with solids. Drug.
Dev. Ind. Pharm. 14:1905-1919.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
161
Zylema,
B. J.,
Grider,
J. A.,
gordon,
J. and Davis,
E. A.
1985. Model wheat starch system heated by microwave
irradiation and conduction with equalized heating time.
Cereal Chem. 62(6): 447-453
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
162
APPENDICES
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163
Appendix 1. IOC Chromatogram datasnon-MW heated braad
Time
(second)
0
1
5
10
15
20
100
200
400
600
800
1000
1200
1400
1600
1800
2000
2100
2200
2300
2400
2500
2600
2700
2800
2900
3000
3200
3400
3600
3800
4000
4100
4300
4400
4600
4800
5000
5200
5400
5600
run -1
0.0000
0.0391
0.0391
0.0342
0.1440
0.4004
1.4893
1.7993
2.0605
2.1997
2.3145
2.4121
2.5073
2.5830
2.6489
2.6758
2.6448
2.6270
2.5244
2.2339
1.5649
0.7983
0.4541
0.3076
0.2368
0.1807
0.1514
0.1123
0.0879
0.0732
0.0659
0.0537
0.0513
0.0464
0.0345
0.0190
0.0044
0.0044
0.0044
0.0044
0.0044
Response (volt)
run -2
run-3
0.0000
0.0391
0.0391
0.0342
0.1440
0.4004
1.4869
1.7993
2.0521
2.2000
2.3145
2.4087
2.4957
2.5826
2.6347
2.6696
2.6348
2.6696
2.6348
2.6270
2.5130
2.2347
1.5652
0.8000
0.4521
0.3076
0.2368
0.1807
0.1514
0.1123
0.0879
0.0732
0.0659
0.0537
0.0513
0.0464
0.0344
0.0042
0.0042
0.0042
0.0042
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
0.0000
0.0392
0.0391
0.0342
0.1440
0.4004
1.4956
1.8000
2.0521
2.2174
2.3304
2.4347
2.5217
2.6087
2.6782
2.7043
2.6608
2.6270
2.5043
2.2260
1.5478
0.7739
0.4541
0.3076
0.2368
0.1807
0.1514
0.1123
0.0879
0.0732
0.0659
0.0537
0.0513
0.0464
0.0339
0.0192
0.0044
0.0042
0.0042
0.0042
0.0042
164
Appendix 2. IGC Chroxatograx data: MW heated bread
Power 100%, 20 seconds
Time
(second)
0
1
5
10
15
20
100
200
400
600
800
1000
1200
1400
1600
1800
2000
2100
2200
2300
2400
2500
2600
2700
2800
2900
3000
3200
3400
3600
3800
4000
4100
4300
4400
4600
4800
5000
5200
5400
5600
run -1
Response (volt)
run -2
run-3
0.0000
0. 0 0 0 0
0.0000
0.0610
0.0659
0.0635
0.1978
0.3906
1.4429
1.7847
2.0898
2.2803
2.4243
2.5342
2.6127
2.6831
2.7344
2.7539
2.6831
2.5757
2.0532
1.5601
1.0767
0.6006
0.3735
0.2710
0.0610
0.0659
0.0635
0.1980
0.3911
1.5514
1.8847
0.0625
0.0710
0.0748
0.2451
0.4111
1.5536
1.9234
2.1011
2.1211
2.2904
2.4455
2.5898
2.6279
2.7111
2.7399
2.7777
2.7031
2.6024
2.2732
1.5861
1.1955
0.6131
0.3755
0.2758
0.2119
0.1855
0.1455
0.1247
0.1028
0.0880
0.0755
0.0555
0.0348
0.0189
0.0044
0.0044
0.0044
0.0042
0.0042
0.0042
0.0042
2.3111
2.4486
2.5988
2.6311
2.7211
2.7400
2.7784
2.7031
2.6024
2.1846
1.5755
1.1955
0.6231
0.3777
0.2810
0.2213
0.1809
0.1455
0.1247
0.1028
0.0880
0.0755
0.0555
0.0348
0.0189
0.0044
0.0044
0.0044
0.0042
0.0042
0.0042
0.0042
0.2100
0.1709
0.1465
0.1147
0.0928
0.0830
0.0684
0.0435
0.0248
0.0153
0.0049
0.0044
0.0044
0.0044
0.0044
0.0044
0.0044
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
165
Appendix 3. XGC Chromatogram data: MW heated bread
Power 100%, 30 seconds
Time
(second)
0
1
5
10
15
20
100
200
400
600
800
1000
1200
1400
1600
1800
2000
2100
2200
2300
2400
2500
2600
2700
2800
2900
3000
3200
3400
3600
3800
4000
4100
4300
4400
4600
4800
5000
5200
5400
5600
run-1
Response (volt)
run-2
0.0000
0.0360
0.0488
0.1270
0.5054
0.9204
2.0996
2.2681
2.3950
2.4902
2.5610
2.6294
2.6831
2.7319
2.7759
2.8101
2.2974
1.8921
1.4746
1.0425
0.6396
0.3979
0.2710
0.1978
0.1514
0.0000
0.0360
0.0488
0.1270
0.6054
1.0124
2.0876
2.2345
2.3655
2.4603
2.5348
2.5994
2.6531
2.7019
2.7669
2.7999
2.2544
1.8223
1.4012
1.0025
0.5999
0.3255
0.2899
0.1978
0.1514
0.1221
0.1001
0.1221
0.1001
0.0684
0.0537
0.0415
0.0415
0.0415
0.0415
0.0415
0.0415
0.0345
0.0185
0.0044
0.0044
0.0044
0.0044
0.0684
0.0537
0.0415
0.0415
0.0415
0.0415
0.0415
0.0415
0.0348
0.0192
0.0042
0.0042
0.0042
0.0042
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
run-3
0.0000
0.0360
0.0488
0.1270
0.6154
1.0124
2.0955
2.2789
2.4124
2.5124
2.5946
2.6598
2.7111
2.7519
2.7899
2.8231
2.3123
1.9344
1.5258
1.0465
0.7699
0.4544
0.3555
0.2544
0.2014
0.1625
0.1324
0.0684
0.0537
0.0415
0.0415
0.0415
0.0348
0.0189
0.0044
0.0044
0.0044
0.0042
0.0042
0.0042
0.0042
166
Appendix 4. Static Sorption Data: non-KW heated bread
Experiment
run -1
Water Activity
(mmHg)
0.0
0.00
3.5
7.3
13.4
16.6
0.62
0.75
1.23
1.85
2.99
20.4
24.9
7.51
14.44
0.0
0.00
3.5
7.3
0.88
10.2
run -2
13.4
16.6
20.4
1.29
1.29
1.92
3.58
7.71
24.9
15.01
0.0
0.00
3.5
7.3
0.42
10.2
1.27
13.4
16.6
20.4
1.81
2.54
24.9
14.02
10.2
run-3
Moisture Uptake
(g H 2O/100 g d.m)
1.11
7.01
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
167
Appendix 5. Sorption Data : non-aicrovave heated bread
Temperature
Moisture Uptake
(g H2O/100 g d.m)
3 0 *C
1.0
2.5
5.0
7.5
10.0
12.5
15.0
17.5
4 0 *C
1.0
2.5
5.0
7.5
10.0
12.5
15.0
17.5
50 °C
1.0
2.5
5.0
7.5
10.0
12.5
15.0
17.5
R.H
(%)
Water Activity
Vapor Pressure
(nunHg)
0.42
0.51
0.60
0. 64
0.67
0.70
0.72
0.74
13.21
16.07
19.03
20. 34
21.39
22.25
23.04
23.61
0.43
0.51
0.61
0.65
0.68
0.70
0.73
0.75
23.51
28.33
33.47
35.68
37.34
38.78
40.16
41.38
0.44
0.52
0.60
40.70
48.29
55.88
60.59
63.65
66.15
68.64
70.96
0.66
0.69
0.72
0.74
0.77
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
168
Appendix 6. Sorption Data : microwave heated bread
power 100% for 20 seconds
Temperature
Moisture Uptake
(g H 20/100 g d.m)
30 *C
0.47
2.5
5.0
7.5
0.55
0.62
15.02
17.57
19.73
0.66
21.00
0.69
0.72
0.73
0.75
22.09
22.75
23.36
23.90
0.49
0.56
0.62
27.10
30.76
34.41
36.73
38.51
39.72
40.83
41.83
12.5
15. 0
17.5
o
•
o
Water Activity
Vapor Pressure
(nunHg)
1.0
10.0
1.0
2.5
5.0
7.5
10.0
12.5
15.0
17.5
50 *C
R.H
(%)
1.0
2.5
5.0
7.5
10.0
12.5
15.0
17.5
0.66
0.70
0.72
0.74
0.76
0.55
0.61
0.68
0.72
0.74
0.77
0.80
0.81
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
50.79
56.33
62.72
66.15
69.10
71.42
73.73
75.30
169
Appendix 7. Sorption Data : microwave heated bread
power 100% for 30 seconds
Temperature
Moisture Uptake
(g H 2O/100 g d.m)
30 *C
4 0 *C
Water Activity
Vapor Pressure
(nunHg)
1.0
0.47
2.5
0.55
15.02
17.57
5.0
7.5
10.0
12.5
15.0
17.5
0.62
19.73
0.66
21.00
0.69
0.72
0.73
0.75
22.09
22.75
23.36
23.90
0.49
0.56
0.62
27.10
30.76
34.41
36.73
38.51
39.72
40.83
41.83
1.0
2.5
5.0
7.5
10.0
12.5
15.0
17.5
50 *C
R.H
(%)
0.66
0.70
0.72
0.74
0.76
2.5
5.0
0.55
0.61
0.68
7.5
10.0
12.5
15.0
17.5
0.72
0.74
0.77
0.80
0.81
1.0
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
50.79
56.33
62.72
66.15
69.10
71.42
73.73
75.30
170
Appendix 8. Free energy (AG) of sorption for bread
Temperature
Moisture Uptake
(g H2O/100 g d.m)
30 *C
1.0
2.5
5.0
7.5
10.0
o
•
o
12.5
15.0
17.5
-0.529
-0.411
-0.309
-0.269
-0.239
-0.215
-0.194
-0.179
-0.371
-0.310
-0.251
-0.217
-0.195
-0.174
-0.156
-0.141
-0.356
-0.313
-0.247
12.5
-0.532
-0.416
-0.312
-0.273
-0.244
- 0.220
- 0.211
-0.184
-0.166
15.0
17.5
-0.199
-0.181
-0.145
-0.131
1.0
-0.526
-0.417
-0.323
-0.271
-0.316
-0.258
-0.208
-0.181
12.5
-0.239
-0.215
-0.160
-0.143
15.0
17.5
-0.191
-0.170
-0.126
-0.114
1.0
2.5
5.0
7.5
10.0
50 *C
Free energy (AG)
non-MW
MW heated
(kca1/mole)
2.5
5.0
7.5
10.0
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
171
Appendix 9. Calculation of Enthalpy (AH) for bread
Sample
Moisture Uptake
3.1
(g H 2O/100 g d.m)
non-MW heated
0.44
0.52
0.60
0.68
0.69
0.72
0.74
0.77
0.42
0.51
5.0
7.5
0.60
0.64
0.67
0.70
0.72
0.74
0.70
0.73
0.75
12.5
15.0
17.5
MW heated bread
P=100%, 30 secs.
0.43
0.51
0.61
0.65
1.0
2.5
10.0
MW heated bread
P=100%, 20 secs.
Temperature (* K)
3.3
3.2
Water Activity
0.66
1.0
2.5
5.0
7.5
0.47
0.55
0.62
0.49
0.56
0.62
0.55
0.61
0.66
0.66
10.0
0.69
0.70
12.5
15.0
17.5
0.72
0.73
0.75
0.72
0.74
0.76
0.72
0.74
0.77
1.0
2.5
5.0
7.5
0.47
0.55
0.62
0.49
0.56
0.62
0.55
0.61
0.66
0.66
10.0
0.69
0.72
0.73
0.75
0.70
0.72
0.72
0.74
0.77
0.74
0.76
0.80
0.81
12.5
15.0
17.5
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
0.68
0.80
0.81
0.68
172
Appendix 10. Calculation of Enthalpy (AH) for bread
II
I
II
j!
Sample
Moisture Uptake
H
M
(g H 2O/100 g d.m)
non-MW heated
1.0
2.5
5.0
7.5
10.0
12.5
15.0
17.5
MW heated bread
P=100%, 20 secs
1.0
2.5
5.0
7.5
10.0
12.5
15.0
17.5
MW heated bread
P=100%, 30 secs
1.0
2.5
5.0
7.5
10.0
12.5
15.0
17.5
Temperature (•K)
3.2
3.3
Ln(Water Activity)
3.71
3.88
4.02
4.10
4.15
4.19
4.22
4.26
3.15
3.34
3.51
3.57
3.62
3.65
3.69
3.72
2.58
2.77
2.94
3.01
3.06
3.10
3.13
3.16
3.93
4.03
4.14
4.19
4.25
4.26
4.30
4.32
3.30
3.42
3.53
3.60
3.65
3.68
2.71
2.98
3.04
3.09
3.12
3.71
3.73
3.15
3.17
3.44
3.51
3.62
3.67
2.84
2.94
3.04
3.09
3.13
3.17
3.20
3.22
4.03
4.12
4.02
4.24
4.27
4.30
4.33
4.34
3.72
3.74
3.78
3.81
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
2.86
173
Appendix 11. Enthalpy (AH) of sorption for bread
Temperature
Moisture Uptake
(g H 20/100 g d.m)
30 *C
40°C
2.5
5.0
7.5
10.0
12.5
15.0
17.5
•11.843
•11.714
•11.505
•11.379
•11.339
•11.240
11.222
■11.158
•11.174
•10.927
•10.897
•10.844
•10.834
•10.823
•10.842
■10.927
1.0
•11.843
2.5
5.0
7.5
•11.714
■11.505
•11.379
■11. 339
•11.240
•11.175
-10.927
-10.697
-10.784
-10.834
-10.823
1.0
10.0
12.5
15.0
17.5
50 °C
Enthalpy (AH)
non-MW
MW heated
(kcal/mole)
1.0
2.5
5.0
7.5
10.0
12.5
15.0
17.5
•
■
11.222
■11.158
•11.843
•11.714
■11.505
•11.379
■11.339
•11.240
■
11.222
•11.158
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
-10.842
-10.927
•11.175
•10.927
-10.697
-10.084
-10.834
-10.823
•10.842
■10.927
174
Appendix 12. Entropy (AS) of sorption for bread
Temperature
Moisture Uptake
Entropy (AS)
non-MW
MW heated
(g H2O/100 g d.m)
(cal/mole *K)
30 *C
1.0
2.5
5.0
7.5
10.0
12.5
15.0
17.5
-35.13
-35.70
-34.94
-34.90
-34.96
-34.00
-35.14
-35.47
-37.86
-37.63
-37.14
-36.83
-36.95
-36.52
-36.52
-36.35
40 °C
1.0
2.5
5.0
7.5
10.0
-34.00
-34.58
-33.82
-33.74
-33.83
-33.87
-34.01
-34.33
-36.70
-36.42
-35.96
-35.68
-35.81
12.5
15.0
17.5
50 °C
1.0
2.5
5.0
7.5
10.0
12.5
15.0
17.5
-32.96
-32.53
-32.73
-32.73
-32.79
-32.84
-32.97
-33.30
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
-35.38
-35.39
-35.23
-35.68
-35.46
-34.97
-34.66
-34.77
-34.35
-34.35
-34.19
175
VITA
Taesoo Park
1981-1986
B.S. in Food Science and Technology, Korea
University, Korea
1986-1988
M.S. in Food Science and Technology, Korea
University, Korea
1988-1995
Research assistant, Department of Food Science,
Rutgers University
2 0 00
Ph.D. in Food Engineering, Department of Food
Science, Rutgers University
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
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