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Comparison of microwave and conventional heating of a wheat starch-gluten-water model system

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O rder N u m b e r 9 303395
C om parison of microwave and conventional h eating of a w heat
starch-gluten-w ater m odel system
Umbach, Sharon Lynn, Ph.D.
University of Minnesota, 1992
UMI
300 N. ZeebRd.
Ann Arbor, MI 48106
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Comparison of Microwave and Conventional Heating
of a Wheat Starch-Gluten-Water Model System
A Thesis
Submitted to the Faculty of the Graduate School
of the University of Minnesota
By
Sharon Lynn Umbach
In Partial Fulfillment of the Requirements
for the Degree of
Doctor of Philosophy
October, 1992
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U N I V E R S I T Y OF M I N N E S O T A
This is to certify that I have examined this bound copy of a doctoral thesis by
Sharon Lynn Umbach
and have found that it is complete and satisfactory in all respects,
and that any and all revisions required by the final
examining committee have been made.
F ugenia A. Oavi<;
Name o f Faculty Adviser(s)
Signature o f Faculty Adviser(s)
O cto b er 0 6 . 1992
Date
GRADUATE
SCHOOL
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ABSTRACT
The effects of conventional (CV) and microwave (MW) heating on a
model wheat starch, vital wheat gluten system were studied. Three moisture
contents (35%, 50%, and 65%) and up to five ratios of starch:gluten were
studied. MW power was such that heating took place more quickly than
during CV heating conditions. Temperature profiles during heating showed
the importance of moisture content for curve shape and that all samples
reached a high enough tem perature for starch gelatinization and gluten
denaturation to occur. Scanning electron microscopy supported temperature
data in that physical changes for starch had taken place.
Texture was evaluated as the force required to compress the sample.
Differences were found for heating method, moisture content, and sample
composition. Moisture content had the major effect and was inversely
related to force. In general, CV heated samples required more force to
compress than the MW heated samples. The high starch samples at 50%
moisture showed the opposite affect.
Self-diffusion coefficients determined by nuclear magnetic resonance
(NMR) showed that there was a redistribution of water between starch and
gluten after heating for the faster more mobile water in the samples. Little
difference was found between the two heating methods. The attenuation
factor which was based on the dielectric constant and loss showed differences
in the way the sample interacted with electromagnetic energy.
The gluten was fractioned with dilute HC1 to give six groups of protein.
Starch appeared to have an effect on how the gluten denatured. W hen the
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fractions were characterized with SDS-PAGE it was found that each fraction
contained a wide range of proteins with different molecular weights.
Differences were found in the band patterns between the CV- and MW-heated
samples. 13C NMR also was used, which provided information on the
protein and carbohydrate component of the fractions showed differences
between the CV and MW heated samples, especially for the carbohydrate
component.
From this study, it appears that differences between the CV and MW
heated samples are due to the type of denaturation the gluten undergoes,
which may be influenced by the presence of starch and the amount of water
available.
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ACKNOWLEDGEMENTS
I would like to thank all the people who have been a part of my life
during the time I worked to complete this thesis. The people are spread over
two continents in four countries and are far too numerous to mention. Some
of them helped in a scientific manner to allow me to complete the research.
But many more persons provided me w ith opportunities to learn nonsdentific skills, develop as a person, and most importanly to enjoy life! These
skills will be invaluable to me for the rest of my life. The process of obtaining
a Ph.D. is a long tortuous path that without peers, family, and colleagues
around to make it bearable, it would be a very long and lonely journey. I will
remember you all.
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CONTENTS
LIST OF TABLES.....................................................................................................vi
LIST OF FIGURES...................................................................................................vii
Chapter 1 INTRODUCTION...............................................................................1
Chapter 2
REVIEW OF THE LITERATURE................................................... 4
2.1 Heating...................................................................................................4
2.1.1 Conventional Heating..........................................................4
2.1.2 Microwave Heating...............................................................5
2.1.2.1 Dielectric Properties.................................................6
2.1.3 Events During Baking.............................................................8
2.1.4 Patents about Microwave Heated Cereal Products............9
2.2 Cereal Products......................................................................................11
2.2.1 Composition of Dry Ingredients...........................................11
2.2.1.1 Gluten.........................................................................11
2.2.1.2 Starch......................................................................... 13
2.2.1.3 Interaction of Gluten, Starch, and W ater............. 13
2.2.1.4 Role of Gluten in Dough and Bread
Formation............................................................................... 14
2.2.2 Appearance.............................................................................. 15
2.2.3 Texture.....................................................................................18
2.3 Water-Macromolecular Interactions................................................ 20
2.3.1 Proton Nuclear Magnetic Resonance................................. 21
2.3.2 Water-Macromolecular Interactions as Studied by
N M R .................................................................................................. 22
2.3.3 Water Behavior as Examined by Dielectric
Properties.......................................................................................... 24
2.4 Examination of Gluten........................................................................ 25
2.4.1 Fractionation and Electrophoresis of Unheated
Gluten.................................................................................................28
2.4.2 l 3C NMR of Unheated Gluten..............................................34
2.4.3 Effects of Heating....................................................................35
2.5 Objectives of This Study...................................................................... 37
Chapter 3 MATERIALS AND METHODS........................................................39
3.1 The Model Systems............................................................................. 39
3.2 Heating Methods.................................................................................. 40
3.3 Temperature Profiles.......................................................................... 40
3.4 Water Loss After Heating and Cooling.............................................41
3.5 SEM........................................................................................................41
3.6 Texture Measurements.........................................................................42
3.7 Determination of Self-diffusion Coefficients...................................43
3.8 Determination of Dielectric Properties............................................. 44
3.9 Examination of Gluten Proteins......................................................... 45
iv
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3.9.1 Fractionation...........................................................................45
3.9.2 SDS-PAGE...............................................................................47
3.9.3 Carbon-13 NMR Spectroscopy..............................................48
Chapter 4 RESULTS
................................................................................... 50
4.1 Physical Characteristics of the Samples.............................................. 50
4.2 Heat Penetration...................................................................................52
4.3 Moisture Content After Heating........................................................ 60
4.4 SEM......................................................................................................... 61
4.5 Instron Data........................................................................................... 67
4.6 Self-diffusion and Dielectric Property Data....................................... 72
4.6.1 The Three Ingredients Separately.......................................72
4.6.2 The Mixed Samples...............................................................75
4.6.2.1 General Information on the PGSE-NMR
Experim ents.......................................................................... 75
4.6.2.2 Unheated Samples.................................................... 78
4.6.2.3 Heated Samples........................................................ 81
4.7 Qualitative Information from the Fractionation Procedure.......... 87
4.8 SDS-PAGE...............................................................................................93
4.9 13C NMR.................................................................................................103
4.9.1 Spectral Assignments.............................................................. 106
4.9.2 Discussion of l 3C NMR D ata.................................................112
4.10 Selected Amino Acid Profiles........................................................... 116
4.11 Overall Discussion............................................................................... 120
Chapter 5 CONCLUSIONS..........................................
121
LITERATURE.......................................................................................................... 125
Appendix I Raw data of texture experiments.....................................................139
Appendix II Raw data of D value experiments..................................................140
Appendix HI Raw data of dielectric constant (k1) experiments........................ 143
Appendix IV Raw data of dielectric constant (k") experiments........................ 146
Appendix V Representative 13C spectra of unheated samples........................149
Appendix VI Representative 13C spectra of conventional heated
sam ples.............................................................................................................. 152
Appendix VII Representative 13C spectra of microwave heated
sam ples.............................................................................................................. 155
v
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LIST OF TABLES
4.1 Moisture contents of the center of representative heated samples,
on a wet basis, as determined by the vacuum oven m ethod.................... 60
4.2 Average weight of samples after heating, before compression w ith
the Instron....................................................................................................... 70
4.3 Moisture contents, self-diffusion coefficents, and dielectric
properties of the components of the model system...................................72
4.4 Dielectric constant for unheated samples.......................................................79
4.5 Dielectric loss for unheated sam ples............................................................. 80
4.6 Dielectric constant for heated samples.......................................................... 84
4.7 Dielectric loss for heated samples.................................................................... 85
4.8 pH values during HC1 fractionation procedure........................................... 88
4.9 Peaks present in 13C spectra of all samples.....................................................109
4.10 Chemical assignments for peaks of fraction spectra................................... 112
4.11 Percentage of amino acids in select fractions...............................................119
vi
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LIST OF FIGURES
2.1
3.1
3.2
4.1
4.2
4.3
Models of interactions between gluten components............................... 17
30 cm2 Ottawa Texture Measuring System test cell.................................. 42
Flow chart of fractionation procedure......................................................... 46
Conventionally heated samples.................................................................. 51
Microwave heated samples.......................................................................... 51
Representative temperature profiles for CV heating of 35%
moisture content samples with all five starch:gluten ratios....................54
4.4 Representative temperature profiles for CV heating of 50%
moisture content samples with all five starch:gluten ratios...................55
4.5 Representative temperature profiles for CV heating of 65%
moisture content samples with all five starch:gluten ratios...................56
4.6 Representative temperature profiles for MW heating of 35%
moisture content samples with all five starch:gluten ratios.................. 57
4.7 Representative temperature profiles for MW heating of 50%
moisture content samples with all five starch:gluten ratios.................. 58
4.8 Representative temperature profiles for MW heating of 65%
moisture content samples with all five starchrgluten ratios.................. 59
4.9 Representative scanning electron micrographs of unheated
samples........................................................................................................... 63
4.10 Representative scanning electron micrographs of 35% moisture
samples........................................................................................................... 64
4.11 Representative scanning electron micrographs of 50% moisture
samples........................................................................................................... 65
4.12 Representative scanning electron micrographs of 65% moisture
samples........................................................................................................... 66
4.13 Represenative results from Instron compression.................................... 68
4.14 Effect of composition on the force required to compress heated
samples........................................................................................................... 69
4.15 Sorption isotherms for dry ingredients of model system at 23 °C......... 74
4.16 Plot showing the effect of increasing gradient pulse duration on
echo attenuation for the PGSE-NMR experiment.....................................75
v ii
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4.17 Self-diffusion coefficients for unheated starch and vital wheat
gluten mixtures at three different moisture contentsl7 Self­
diffusion coefficients for unheated starch and vital wheat gluten
mixtures at three different moisture contents...........................................78
4.18 Attenuation factors for unheated starch and vital w heat gluten
m ixtures...........................................................................................................81
4.19 Self-diffusion coefficients for (A) conventional and (B) microwave
heated starch and vital wheat gluten mixtures...........................................83
4.20 Attenuation factors for (A) conventionally and (B) microwave
heated starch and vital wheat gluten mixtures...........................................87
4.21 Structures of the amino acids and their three letter codes.......................91
4.22 Electrophoregrams of unheated fractions.................................................. 94
4.23 Electrophoregrams of fractions from 35% moisture samples that
were heated......................................................................................................95
4.24 Electrophoregrams of fractions from 50% moisture samples that
were heated......................................................................................................96
4.25 Electrophoregrams of fractions from 65% moisture samples that
were heated......................................................................................................98
4.26 Sample spectra of solvents used for 13C NMR experiments....................105
4.27 Examples of sample 13C spectra...................................................................108
v i ii
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CHAPTER 1
INTRODUCTION
The use of microwave (MW) ovens has increased tremendously over
the last few decades, and as a result a wide variety of food products are being
heated by this method. Cereal-based products such as breads and rolls
commonly are heated in a MW oven, yet they have a tendency to become
very tough and chewy when cooled. It is not commonly known w hy this
occurs. In order to understand the mechanism by which this happens, it is
important to evaluate the interaction of wheat flour and water in these types
of foods. The structural development of baked products is strongly
influenced by w heat flour proteins even though they comprise only 7 to 12%
of the flour with respect to starch. Thus, water, starch, and gluten interactions
are probably very important to the toughening problem during MW heating.
Water is of considerable importance because it structures itself around
molecules. The manner by which it interacts could be different when a
product is heated by MW radiation, especially since water is essential for the
MW heating of food. Water is a dipolar molecule which tries to align itself in
the alternating electromagnetic waves produced by the microwave oven.
This rapid movement towards alignment causes friction which provides for
the transfer of electric energy to heat energy. This generation of heat along
with other thermal properties that contribute to heat transfer is responsible
for overall heating. The water will have different levels of interaction with
the other molecules present which affects the w ater mobility as well as the
motion of the molecules.
1
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Starch contains carbohydrates which have macromolecular structures
that are strongly influenced by the presence of water during heating, which
results in granule swelling and loss of granule integrity. The two polymers
found in the starch granule are amylose, which is more straight chain, and
amylopectin which has more branch points. It is known that bread type
products stale (undergo retrogradation) upon storage due to the reorientation
and interaction of the starch molecular polymers which result in a tough
texture to the product. Generally this phenomenon occurs over time. The
reason this behavior (if it is the same) is accelerated during MW heating
might be related to the rapid heating and subsequent cooling of the product.
Wheat proteins are required to incorporate air in dough and maintain
air cells during baking. Research has been conducted to determine what
happens to the protein chains when they are mixed during dough formation.
After hydration of the molecules, there is an unfolding of the protein chains
with sulfhydryl and disulfide interchanges taking place. What
transformations occur as proteins are heated are not fully understood. Gluten
is difficult to study due to the complex nature of these proteins. They range
in molecular weight from 11,000 to over a million daltons and therefore
differ in their solubilities. The lower molecular weight components are
loosely termed gliadins and the higher molecular weight components are
termed glutenins.
In this thesis, the effect of conventional and MW heating on starch and
gluten in the presence of water will be evaluated. The appearance will be
characterized by scanning electron microscopy and textural properties will be
evaluated with an Instron. Dielectric properties and self-diffusion coefficients
will be used to evaluate water interactions with the macromolecules. The
2
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protein will be separated by pH and the fractions characterized by molecular
weight differences as shown by sodium dodecyl sulfate polyacrylamide gel
electrophoresis and
nuclear magnetic resonance.
3
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CHAPTER 2
REVIEW OF THE LITERATURE
2.1 Heating
Heating is a common unit operation that is applied to foods in order to
make them appealing, safe, and nutritious. The process causes the sample to
increase in temperature which provides energy for water diffusion and
macromolecular transformations, such as starch gelatinization or protein
denaturation. The dehydration and new molecular structure will affect the
texture of the food, which may be desirable, but m ay also be undesirable.
Conventional heating has been more extensively studied than microwave
heating which is a more recent method of increasing the tem perature of a
food, and is more complex in the heat generation process. Although the basic
mechanisms of both heating methods are understood, the effect each has on
the macromolecules, which affect the textural properties, is not clearly
worked out.
2.1.1
Conventional Heating
During conventional (CV) heating food is placed in a hot oven and the
differential in temperature between the sample and the surrounding air is a
driving force which results in conductive heat transfer from the surface to the
center of the sample. In general, the increase in temperature is linear and the
greater the temperature differential, the faster the food heats. The rate of
heating is inversely proportional to the heat capacity of the material. A food
with a small heat capacity (or specific heat) will require less energy to increase
in temperature by one degree, and thus will heat more rapidly. The thermal
4
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conductivity and density are inherent properties of the sample and play a
large role on how the sample heats. As the tem perature increases the water
gains enough energy that it is able to form steam and move to the surface of
the sample. The basic principles of mass transport prevail, and the liquid
water moves from high to low concentration, which is from the center of the
sample to the outside edge (Wei et al., 1985a). If water is required for
transformations of the macromolecules the flow of water to the surface will
be interrupted altering evaporation rates, unless excess water is present. This
was shown for cakes in which starch gelatinization inhibited the water loss
rate (Gordon et al., 1979). The composition of the sample is also important in
determining how it heats in the oven (Cloke et al., 1984)
2.1.2 Microwave Heating
Microwave (MW) heating is somewhat more complex than CV heating
because conductive-convective heating is taking place as well as
electromagnetic radiation coupling (Datta, 1990). The importance and
complexity of microwave heating has been discussed in various reviews
(Buffler and Stanford, 1991; Engelder and Buffler, 1991; Davis and Gordon,
1990; Lee, 1989; Mudgett, 1986; Decareau, 1985). Microwaves are
electromagnetic waves, which are most often at a frequency of 2450 MHz (in
household microwave ovens). When the waves encounter a dipolar
molecule (most commonly water) it tries to align itself in the alternating
electric field. This movement creates friction which results in a conversion
of the electrical energy to heat energy. As the waves enter the sample at the
surface they may be reflected, transmitted, or absorbed. The extent that each
option will occur is determined by the sample composition. If the waves are
5
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able to penetrate deeper into the sample, more heat will be generated
internally, rather than solely move from the surface inwards. The size and
shape of the sample will also affect where the waves will focus. For
rectangular shaped products there may be excess corner heating, whereas with
spheres waves may concentrate in the geometric center.
The general principles of heat transfer that were occurring during CV
heating also apply during MW heating as the energy is distributed from the
coupling molecules to the rest of the material. Since the heat capacity is a
critical factor in the heating rate, a smaller heat capacity (or specific heat) will
enable one to use less energy to increase the temperature of the sample by one
degree than when the heat capacity is large. A difference in MW heating,
relative to CV heating, is that the air surrounding the sample is most often
not at an elevated temperature. This results in greater heat loss from the
surface. Also with MW heating of foods, the wattage is generally large
enough that the rate of heating is very rapid compared with CV heating.
These differences in events during MW heating can result in water
movement opposite to that of CV heating (i.e., from the surface to the center)
(Wei et al, 1985b). Some experimental work has been done to elucidate the
physico-chemical effects on baked products of MW heating (Lambert et al.,
1992; Baker et al., 1990; Umbach et al, 1990; LePage, 1989).
2.1.2.1 Dielectric Properties
The specific dielectric properties which pertain to MW interactions that
result in heating are given by von Hippel (1954) and have been discussed by
various scientists with regard to food applications (Engelder and Buffler, 1991;
Decareau and Peterson, 1986; Mudgett, 1986,1985). The ability of water to
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facilitate the conversion of MW electrical energy to heat energy is dependent
upon the molecular mobility. If the water is bound to the macromolecules
that are present and is unable to rotate there will be limited heat formation.
On the other hand, if the water is not affected by the surrounding molecules,
then the conversion will be expedited and the rate of temperature increase
will be relatively higher. The mobility of the macromolecules also plays a
role. Water movement may be affected by dissolved salts or other solutes.
These ions will also be affected by the oscillating electromagnetic field and
will move to try and align themselves. The nature of the ions and their
concentration will influence the amount of coupling that will occur.
Collisions of ions as they move converts kinetic energy to heat energy. The
dielectric properties are used as averages of the quantifiable measures of the
ability of a sample to interact with the MW energy.
There are three main dielectric properties: the dielectric constant (k') and
loss (k"), and the attenuation factor (a). These variables are used as indicators
of how a food system will store the electromagnetic radiation (k') (i.e., energy
that is reflected vs. transmitted), which is related to polarity, and how it will
transform or dissipate the electric energy into heat (k”). A loss tangent or
dissipation factor (tan 8) is a ratio of k" to k’, and represents the energy loss
characteristics of the sample.
The attenuation factor is a measure of how the
waves will be affected when they go from the surrounding air into the food
medium. The factors that affect a (cm-1) are demonstrated in the following
equation (von Hippel, 1954), where X is the wavelength (cm):
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The inverse of a is penetration depth which provides information on how
far a wave will penetrate into the sample before being reduced to 36% (1 / e) of
its original intensity.
Equipment has recently become available that allows non-destructive
measurement of dielectric properties of food samples (Anon., 1992). Hewlett
Packard has developed an open-ended coaxial probe which is part of a
microwave transmission line. The probe, which has a MW signal at the
frequency desired coming out the end of it, is placed in contact with the food.
The food will affect the reflected signal and this altered signal can be used to
calculate k' and k".
Despite the advantage of knowing the dielectric properties in order to
predict how a food will heat in the MW oven, food systems have not been
widely characterized by these variables. Part of the reason for this lack of data
is that the dielectric properties are dependent on temperature and the
physico-chemical nature of the sample which are constantly changing during
the heating process. Both the dielectric constant and loss of water decrease as
the tem perature increases at the frequency of household microwave ovens.
Conversely to water, the change in the dielectric loss for ions is directly
proportional to the change in temperature (Mudgett, 1985).
2.1.3 Events During Baking
As mentioned in section 2.1.1 there will be a temperature gradient in the
interior of the sample as it attempts to equilibrate. The extent of the
tem perature gradient determines how fast the food will heat (Varilek and
Walker, 1984). As long as there is excess water present the maximum
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temperature reached by the food is 100°C (Godsalve et al., 1977; Gordon et al.,
1979; Galletti et a l, 1980). The temperature at the surface may be higher than
the center due to dehydration during CV heating.
Traditionally, temperature is monitored during heating with a metal
thermocouple. This method was used by Marston and Wannan (1976) to
show that bread heats more slowly as one moves towards the center from the
surface. The temperature increase allows water to evaporate from the surface.
Heat also supplies energy for gelatinizing starch and denaturing gluten (see
Section 2.2.1). The effect the composition has on the heating rate can be
monitored. The ingredients in a cake can have an effect on the temperature
profile as was found by Gordon et al. (1979) w hen different ratios of flour to
starch were used.
Metal thermocouples cannot be used in a MW oven as the metal may
reflect or arc and cause irregularities in the electromagnetic field. Fiber optic
temperature sensors are used instead (Wickersheim and Sun, 1987). This
method has been used successfully to monitor heating of cereal-based systems
(Lambert et al., 1992; Baker et al., 1990; Umbach et al., 1990; LePage, 1989).
2.1.4 Patents about Microwave Heated Cereal Products
Baked products made from cereal flour that are heated in a MW oven
generally do not have a desirable texture. This problem is of such great
interest to the food industry that a number of patents have been issued
attempting to solve the problem. In 1984, Ottenberg commented on the
unpalatability of MW heated rolls m ade with wheat flour and offered a roll
reformulation as a solution. The most im portant alteration to the traditional
formula is the inclusion of 10 to 20% (on wheat flour basis) of long-grain rice
9
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flour and vital wheat gluten to maintain the starch to protein balance. A year
later, Ottenberg (1985) published another patent where any starches could be
used, as long as granular size was small (< 20 microns). In both of these
patents "a variety of people" found the improved formulation gave a food
that had superior crust texture after MW reheating compared to a traditional
formulation. Ottenberg does not provide any mechanism by which the starch
would affect the texture, or why the "crystal size" of the starch would be
important. (Note: The author uses the word "crystal" for starch, and it can be
assumed that the "granule" is being described.) Anderson et al. (1989), rather
than examining the formulation of the product, examined the whole process
by which the food is heated and cooled. The authors claimed that retarding
the moisture loss during cooling was a sufficient method to retain
palatability. Cochran and others (1989; 1990) also have patented methods of
MW heating baked goods. They claimed in one patent (1989) that the
addition of chemically modified starches would absorb water to improve
palatability. In the other patent (1990), a protein modifier (L-cysteine) was
incorporated in the baked product to break disulfide bonds which was claimed
to improve texture. Huang et al. (1991) were able to patent the idea that the
inclusion of "preferential binding materials such as surfactants . . . hydrogen
bond breakers" would interact with gluten to give a more desirable texture.
They also felt that the inhibition of starch granule swelling was important in
order to maintain desirable texture. This discussion of patents illustrates that
there is no general consensus as to what key ingredient controls texture of
baked products. That only ONE component is responsible for the texture in
baked products seems unlikely. Thus the modification of a system for MW
heating appears to be complex.
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2.2 Cereal Products
2.2.1 Composition of Dry Ingredients
2.2.1.1 Gluten
Gluten is the "proteinaceous material that is recovered after washing out
of flour-water doughs the soluble and occluded substances of flour." (Bloksm a
and Bushuk, 1988) and was first described by Beccari in 1745 (see Shewry and
Tatham, 1989). Despite its singular name and years of study, gluten is a very
heterogeneous mixture of proteins that has been extensively studied, but not
totally understood. In general it is characterized by its solubility, as
popularized by Osborne in 1907 (for more detail, see Section 2.4.1.1). Two
main groups were found: the prolamins, called gliadin, which are alcohol
soluble, and the glutelins, called glutenins, which are acid soluble. This is a
very simplified way to divide gluten based on solubility and Section 2.4.1.1
describes some of the variety of other methods that have been used. The
following paragraph provides a brief summary of gluten composition.
More recent investigations of wheat proteins were reviewed by Shewry et
al. (1986) to examine in more detail the structure of the gliadin and glutenin.
The gliadins are generally smaller with a , (3, and y subfraction designations
whose molecular weights range from 30,000 to 40,000 daltons and the co
fractions whose molecular weight is between 44,000 and 74,000 (Shewry et al.,
1986). All the gliadins contain cysteine except the co fraction. Reduction of
disulfide bonds may result in a change in mobility by sodium dodecyl sulfatepolyacrylamide electrophoresis at low pH. The glutenins are larger molecules
composed of protein chains that are disulfide bonded to each other to give
molecular weights of over a million. Shewry and co-workers (1986) divided
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glutenin into two groups - low and high molecular weight (LMW and
HMW). The LMW group is thought to be aggregates of gliadin, held together
by hydrogen bonds and hydrophilic interactions. This idea is supported by the
amino acid profiles which are similar to gliadin. The HMW group has a
different amino acid profile (more glycine and less proline) than gliadin and
the proteins are stabilized by inter-chain disulfide bonds. This same basic
system of characterizing the gluten was used in an electrophoretic study by
Fullington et al. (1987). These workers divided gluten into four groups based
on molecular weights: A l, >80,000; A 2 ,51,000-80,000; A3,40,000-50,000; A4,
28,000-39,000. Proteins with a molecular weight between 8,000 and 27,000
were classified as albumins and globulins. A recent review (Shewry et ah,
1992) discussed the HMW glutenins in great detail with an emphasis on the
genetics. In 1983 Payne and Lawrence suggested a system of assigning
electrophoretic bands to the glutenin subunits. They compared their
preferred lettering system to the more commonly used numbering system,
which includes up to 22 possible subunits in wheat glutenin, not all of which
will be expressed by one variety.
In order to examine gluten it can be easily isolated from a flour dough
with water because the starch washes out to leave the gluten as a cohesive
mass. Vital wheat gluten (VWG) is one term that can be used to refer to
functional gluten after it has been separated from the water soluble
components of wheat flour and dried. It can be specifically defined as "gluten
that has not been denatured and thus has the ability to form a dough that will
retain gas" (Hoseney, 1986) or gluten that forms a viscoelastic mass upon
complete hydration (Ortalo-Magne and Goodwin, 1991).
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2.2.1.2 Starch
Starch is the largest component of wheat flour and is essential for its
dough and bread-forming properties (Sandstedt, 1961). This carbohydrate is
made up of two main polymers - amylose and amylopectin - which are
composed of the glucose monomer, connected together with an a(l->4) bond.
Amylopectin also has <x(l->6) bonds which give branch points on the polymer
chain. When the wheat kernal has been formed, these polymers will be
found in units called granules which are spherical and range in size from 10100 pm; the exact structure (i.e., location in the granule of amylose and
amylopectin) is not known. Various discussions about starch are available
(Whistler et al., 1984; Zobel, 1988). Upon heating, with adequate water, at
about 60°C the starch granules will swell and the amylose will leach out. In a
baked product such as bread, the starch may undergo structural changes that
will affect the texture, a phenomenon referred to as staling that has been
examined over the years by various researchers (Willhoft, 1973; Russel, 1983;
Avital et a l, 1990; Wilson et al., 1991). Wilson et al. (1991) reported that the
amylopectin portion of the starch is responsible for staling based on
molecular structural changes.
2.2.1.3 Interaction of Gluten, Starch, and Water
In order to make a basic dough, two ingredients are required: flour and
water. The outer layers of the flour particles become hydrated and with
stirring get removed to reveal more dry parts which can become hydrated,
and so on. Once the particles are completely hydrated, subsequent mechanical
action results in the development of the dough. Although wheat flour
contains primarily starch, it is the wheat proteins that are the crucial
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components in the dough that retains carbon dioxide from a leavening agent,
and then upon baking and expansion will retain the shape (Pomeranz, 1983).
The gluten chains are affected by the mixing (or kneading) and expand to
form the structure of the system (Hoseney and Finney, 1974).
The water is not evenly distributed between the dough main components
(starch and gluten) and exists at different levels of hydration in localized
areas. The amino acid side chains provide numerous opportunities for
hydrogen bonding with water (Bushuk and Hlynka, 1964). From a rheological
study, Webb et al. (1970) felt that water was present either free, lightly bound,
or firmly bound in flour, water, salt doughs. Wynne-Jones and Blanshard
(1986) examined starch gels and found that at 23% moisture all water was
bound. There was quite a large difference in mobility between 33 and 38%
moisture in the starch gels. MacRitchie (1976) felt that the water was also not
evenly distributed among the dough components and that m ore water is
associated with the protein than the starch. During baking there is a
redistribution of the water as the starch gelatinizes and takes water from the
denaturing protein (Bushuk and Hlynka, 1964; Bushuk, 1966). The
significance of this water transport phenomena was demonstrated by Eliasson
(1983) with differential scanning calorimetry. She showed that the amount of
gluten and the moisture content affect the temperature of gelatinization.
2.2.1.4 Role of Gluten in Dough and Bread Formation
Numerous studies have been carried out to try and determine which part
(if any one in particular) of the whole gluten is m ost im portant for the
formation of high quality doughs and bread. There is no general consensus
in the literature, although glutenin seems to be most im portant but gliadin
14
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also contributes to structural development, as was reported by Chakraborty
and Khan (1988b). This generally agrees with MacRitchie (1987a) who found
that gliadin decreases dough strength and glutenin increases dough strength
and loaf volume. Bushuk (1985) divided glutenin and found that part of it
was inversely related and part was directly related to loaf volume, with
gliadin having no significant effect. Conversely to that, Hoseney et al. (1969a)
reported that gliadins control loaf volume and glutenin is responsible for the
mixing requirements of the dough.
Over the years, various models have been proposed about the interaction
between gliadin and glutenin. Wall (1979) suggested the gliadin was less
associated with the system and the glutenin, which has m ore surface area
could non-covalently hold the dough together (Fig. 2.1A). Lasztity (1986)
proposed his own model, after reviewing a number of models available in
the literature, in which there was more bonding between the gliadin and
glutenin (Fig. 2.1B). Weegels and coworkers (Weegels and Hamer, 1991;
Weegels et al., 1991) proposed that during dough formation hydrophilic
regions of gliadin and glutenin interact to bring the protein chains together
(Fig. 2.1C). This provides the opportunity for disulfide bonds to form and
provide stability to the structure.
2.2.2 Appearance
One is often interested in the physical appearance of a baked product, such
as bread, that goes beyond shape and color. Scanning electron microscopy
(SEM) is a relatively simple technique, requiring little preparation when used
to examine bread. SEM allows one to see both the air cells (at a lower
magnification) and the starch granules (at a higher magnification) (Taranto,
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1983). Air cells are a common feature of baked cereal products that one may
want to investigate. Different ingredients and heating methods m ay have an
effect on the size, shape, and numbers of air cells. Part of the microscopic
appearance of a bread-like product is the size and shape of the starch granules.
When enough water is present the starch granules will swell and exude
some of their contents which will alter the morphology of the granules. The
further the extent of gelatinization the greater will be the change in granule
shape relative to the ungelatinized granules. Starch is not the only
component in a baked product, one also sees material between the granules.
This matrix before heating would most likely be gluten and after heating
would include some of the molecules that were exuded from the starch
granules.
SEM has been used by many investigators to examine starch granules in
bread and other baked products after heating (Khoo et al., 1975; Hoseney et al.,
1977; Varriano-Marston et al., 1980). Air cell size has been examined in cakes
to relate to bulk density (Attenburrow et al., 1989) and cross-sectional area
(Lambert et al., 1992; Cloke et al., 1984). Wheat starch has been examined with
SEM to determine if MW heating has any effect on the starch granules
compared to those that have been CV heated. Both Goebel and Zylema and
their coworkers (1984 and 1985, respectively) examined starch-water mixtures
by SEM and did not find any unique macrostructures after MW heating.
Goebel et al. (1984) did find more diversity in the granular shape after MW
heating as compared to CV heating. Air cell size and starch granule
morphology was similar between a CV and MW baked bagel (Umbach, 1989).
Huang et al. (1991) in their patent state that a dough after MW heating had
larger starch granules than if it was CV heated.
16
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A
s°
G lu te n in
R e s id u e
"-4s
^W.----:-/. ZOg.Q
_ Q ,.-.—T ’ - r v ^ '
‘ •<—
^
D ough p r o te in s
6
JUJJT /w//'x
* ....
\--w ^
random coil
^A A ,
disulphide bond
hydrogen or hydrophobic bond
aggregated gluten
J g lu ten in
t e e
la eyacatn
lla oiiacsn^
l la c r o c *
1 chomicai aggregation
la DOiymariauon oy S-*S
B DOtymariutton oy
ito iioasa
\m
M
fl laatnm
8Cl(J
'lib
b hemtcaiiuiaaa
D phyaicol aggregation
io asecrete
o
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S-S bond
hydrophobic
site
carbohydrate
Us prot»r»~orotain interaction
IB ofotetn-noio urcaraction
canxfiy<nita-orot#«
Figure 2.1 Models of interactions between gluten components. A) Wall, 1979;
B) Lasztity, 1986; C) Weegels and Hamer, 1991.
17
•OT*
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2.2.3 Texture
From the patents discussed in the previous section, the importance of
texture is evident. The texture that a loaf of bread possess when it is first
removed from a CV oven does not remain constant, but changes as the bread
ages. This phenomenon, characterized by a decrease in softness, is often
referred to as staling. Texture generally relates to the behavior of the food
when it is chewed and chewing can be thought of as essentially compressing
the food between the teeth. Thus, it follows that a mechanical device that is
measuring texture should simulate compression.
Attempts have been made to quantify these textural changes using
various methods (Willhoft, 1973; Voisey et al., 1974; Kamel, 1987; Joensson
and Toemaes, 1987; Baker et al., 1987), while the official AACC method
(number 74-10) employs a Baker compressimeter. For whatever m ethod is
chosen, in order to have consistent results when compressing a sample
between two parallel plates, sample size and shape should be kept constant
(Brinton and Bourne, 1972; Voisey et al., 1974). These factors, as well as rate of
compression and area compressed, were evaluated by Baker et al. (1986a,b).
They found that the most significant factors were crosshead speed, plunger
area, and degree of compression for bread crumb. A compression rate of 100
m m /m in was recommended by Hibberd and Parker (1985) and Baker and co­
workers (1986a). Other variables in bread that have been evaluated for their
effect on texture measurements are location in the loaf where the sample is
taken from (Hibberd and Parker, 1985; Baker et al., 1987) and the amount of
gluten (Lasztity, 1980).
Another method for measuring texture was described by Voisey (1971)
which may be used with a variety of foods. The Ottawa Texture Measuring
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System (OTMS) has a number of different test cells that can be used with an
Instron, and there are inter-changeable parts to allow different textural
characteristics to be measured. The test cell has high sides so when the
plunger comes down and compresses the sample it cannot go out from
underneath the plunger. One option is a wire extrusion insert so that the
sample is sheared on the wires when forced through with the plunger. This
system has been reported to be used for fruits and vegetables (Voisey, 1970)
and meat (Voisey and Larmond, 1977) but not for bread-like products.
Some work has been done to study the effects of MW heating on the
texture of cereal products. Bread that was baked in a CV oven was found to be
firmer than if baked in a MW oven when texture was measured with a
penetrometer (Lorenz et al., 1973). This difference was attributed to a higher
moisture content in the MW heated bread. Sensory panels did not agree with
the penetrometer data. Dahle and Sambucci (1987) found an increase in
extensibility (amount bread is deformed before rupture) and no change in
strength (maximum force before rupture) for MW thawed bread compared to
room tem perature thawing. Rogers et al. (1990) used a Kramer shearcompression cell to examine sliced bread and gluten balls that had been
reheated by one of three methods - steaming, convection oven, and
microwave oven. An advantage of the Kramer shear method is that firmness
and toughness could be differentiated. They were able to support the claim
that toughening found with m ia o w ave heating was due to the gluten.
Some researchers have thought of bread and cakes as being foams and
used this idea in the interpretation of the compression of these foods between
parallel plates. Attenburrow et al. (1989) and Swyngedau et al. (1991) have
reported on stress-strain curves to demonstrate that the compression first
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destroys the integrity of the cell walls and then compresses the cell wall
material.
From the work that has been published, there does not appear to be any
consensus on which is the "best" method to use to measure the texture of a
baked cereal product. The method chosen appears to be dependent on the
parameters needed to be studied and, possibly, availability of equipment.
2.3
Water-Macromolecular Interactions
Water is a simple molecule with a structure that allows it to hydrogen
bond to molecules which have hydroxyl, amino, carbonyl, or sulfhydryl
groups. There are various possible levels of interaction between the water
molecule and the macromolecule. By measuring the moisture content at
different water activities it is possible to hypothesize when the
macromolecules would have a single layer of water surrounding them (a
high level of interaction) and when there w ould be subsequent layers (lower
levels of interaction) (Fennema, 1985). Isotherm data evaluated by the
Brunauer, Emmett, and Teller or Guggenheim, Anderson, and DeBoer
methods provide a theoretical basis to predict the amount of water required to
form a single layer of water on the surface of the macromolecule (Fennema,
1985). This monolayer value is dependent on the nature of the
macromolecule. The monolayer can also be thought of as rather immobile
water. If the moisture content is great enough, there will also be mobile
water. The extent of this mobility is of interest. In order to measure water
mobility, it is desirable to use a method that is specific for water, as well as
being non-destructive to the interaction between the water and the
macromolecule.
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The type of interaction between water and starch or gluten has been of
interest for a number of years (Bachrach and Briggs, 1947; Bushuk and
Winkler, 1957; Volman et al., 1960; Lee, 1970; Chinachoti and Steinberg, 1988).
More detail on these interactions will be given in section 2.2.1.4 where dough
formation is discussed in detail. Interactions of water w ith various proteins
(Lumry, 1973) or with starch (Tanner et al., 1991) have been discussed .
2.3.1 Proton Nuclear Magnetic Resonance
In the 1940's nuclear magnetic resonance (NMR) was beginning to
become popular as a method for examining nuclei in solutions. Bloembergen
et al. (1948) published a rigorous approach to the theory of nuclear magnetic
relaxation. In 1952 Bloch and Purcell jointly won the Nobel prize for physics
for their work on NMR (Bloch, 1953; Purcell, 1953). For NMR experiments
one places the sample (with all its nuclei) in a magnetic field and by carefully
choosing the frequency and duration of the pulses that the sample is subjected
to, specific nuclear behavior can be examined. Proton mobility is often
examined to determine water behavior, but the protons on macromolecules
can also be evaluated. A more complete description of NMR is given by
Harris (1983).
Pulsed gradient-spin echo NMR (PGSE-NMR) is a very specific type of
NMR experiment that can be used to determine self-diffusion coefficients,
m2/s (D). One is therefore measuring mobile protons, usually water, while
maintaining the integrity of the sample. This m ethod was first proposed in
1965 by Stejskal and Tanner. When water interacts with other substances, its
self-diffusion coefficient decreases from that of free water. This method
requires a field gradient to be pulsed on the sample to positionally identify the
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nuclei. When the gradient is removed, the nuclei have the opportunity to
diffuse (or move). The gradient is p u t back on and if the nuclei are not in the
same place they were the first time the gradient was on, the signal will be
attenuated. The greater the attenuation the larger is D of the nuclei (Blum,
1986).
It is important to know that the D value is providing information on the
Brownian (or random) motion of the nuclei of interest and not Fickian
(concentration dependent) motion (Karger et al.,1988). The pulsed gradient
method of tracing the movement of molecules leads to a measurement of
Brownian motion in the absence of concentration gradients, thus leading to a
non-invasive determination of the true self-diffusion coefficient.
Water has
a D of 2.3 x 10'9 m2/ s at 25°C (Callaghan et al., 1980).
2.3.2 Water-Macromolecular Interactions as Studied by NMR
For a number of years NMR has been used to examine starch and water
interactions. A review of NMR applications for food has been written (Belton
and Colquhoun, 1989). Collison and McDonald (1960) were able to
demonstrate the increased mobility of the macromolecular side chains of
starch after gelatinization by examining the line width of the proton signal.
This technique was used to more fully characterize water behavior during
gelatinization (Jaska, 1971). In 1968 Sterling and Masuzawa also examined
starch with proton NMR and demonstrated the increasing interaction
between starch and water when starch gelatinizes. Boundness of water in
doughs and starch has been examined with NMR (Toledo et al., 1968;
Shanbhag et al., 1970).
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NMR can be used to measure relaxation times of water in order to
examine its mobility or level of interaction with the macromolecules. Tait et
al. (1972) examined other aspects of proton and deuteron signals to examine
mobility and layers of water on starch. Various researchers have shown that
gelatinization results in changes in relaxation times, increased starch
polymer mobility, and the presence of different forms of water (Chinachoti et
al., 1991; Hennig and Lechert, 1974; Lelievre and Mitchell, 1975; Hennig and
Scholz, 1976; Leung et al., 1976). Wheat flour-water doughs were found to
contain two populations of water - mobile and immobile - based on the
proton transverse relaxation time (Leung et al., 1979). Three populations of
water were found for wheat flour doughs investigated by d'Avignon et al.
(1990). When wheat starch gels were MW heated and it was found, from fH
transverse relaxation measurements, that there was a movement of water
from white powder regions to gel regions from an initial moisture content of
50% w /w (Wynne-Jones and Blanshard, 1986). Two populations of water
were found for com starch when
(Lang and Steinberg, 1983) and for cereal
starches when *H and 170 (Richardson et al., 1986,1987a, 1987b, 1987c) nuclei
were examined.
Gluten has not been examined by NMR to as great an extent as starch. A
discussion of various ideas of water protein interactions was presented by
Bryant and Halle (1982). A more detailed study of the relaxation of 170 nuclei
to examine protein hydration has been done (Halle et al., 1981) where it was
found that mobility decreases with increasing protein concentration.
Relaxation times for dry gluten were measured and it was found that there
was m ore than one population of proton nuclei (Belton et al., 1988). The
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lipids present in the gluten affected the transverse relaxation and the
disulfide bonded regions are critical for spin lattice relaxation.
Little research has been published that shows the ability to determine D
values for food systems and to use the information to characterize molecular
level events. Lechert (1981) investigated the effect of moisture content and
temperature on potato starch and found that D is directly proportional to both
parameters. The association between water and protein in cheese was
examined by Callaghan et al. (1983a). A study of wheat starch pastes showed
that there was a lack of correlation between Theological measurements and D
values (Callaghan et al., 1983b). A nonlinear relationship between moisture
content and self-diffusion coefficients was demonstrated for water in potato
starch (Lechert et al., 1980) and wheat endosperm (Callaghan and Lelievre,
1979). The non-linear effect of moisture content, from 24 to 54%, on the D of
gluten was demonstrated by Back et al. (1991).
2.3.3 Water Behavior as Examined by Dielectric Properties
The effect of temperature on the dielectric properties of meat and
vegetables has been clearly demonstrated (Bengtsson and Risman, 1971). For
potato starch-water mixtures of greater than 40% moisture measured at 3.0
GHz, an increase in moisture content resulted in an increase of k' and no
change in k"; gelatinization caused an increase in both properties (Roebuck et
al., 1972). The effect of temperature on the dielectric properties of sunflower
oil, mashed poatoes and mashed bananas was demonstarted by Akyel et al.
(1983). After heating to 55°C and cooling, k’ and k" for the bananas decreased.
A coaxial probe was used to measure the dielectric constant and dielectric loss
for various fruits and vegetables (Tran et al., 1984) over a range of frequencies.
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The experimental results correlated with the theoretical Maxwell equation
predictions. The values at 2,400 MHz were between 57.6 and 68 for k' and 12.3
and 19.5 for k".
Little work has been published with measurements made at 2450 MHz.
Using the cavity perturbation technique, various ingredients for a cake were
measured (Brand, 1987). The dry ingredients all had low dielectric properties,
whereas the water was much higher. When the ingredients were combined
in a batter, there was an interactive, not additive, effect on k1and k". If the
moisture content was greater than 20%, Fleischmann (1991) found that the k'
and k" for whey proteins increased with increasing moistue content, and k"
reached a maximum at about 60% moisture. Miller et al. (1991) demonstrated
the effect chemically modifying starch has on the dielectric properties.
2.4 Examination of Gluten
One way to understand a large group of complex proteins, such as gluten,
is to break them down into smaller groups. This can be achieved through the
use of solvents of differing characteristics to obtain numerous sub-fractions.
There are a wide variety of procedures available in the literature and each one
results in fractions of different composition. Peptide chains with similar
composition or physical characteristics will all be soluble in one solution.
The fractions that result from a procedure are by no means absolute and thus
the method used m ust always be clearly defined. Once these fractions are
obtained, it is desirable to characterize them to understand the composition of
the proteins.
Wheat proteins have been extensively studied using sodium-dodecyl
sulfate polyacrylamide gel electrophoresis (SDS-PAGE) as is evident by the
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number of gels illustrated in published material on gluten. This method is
specific enough that it is often used for identification of cultivars by
examining the gliadins or the glutenins (Ng et a l, 1988).
Poly-acrylamide gel electrophoresis (PAGE) is a method of separating
charged particles based on either their molecular size or charge. The
procedure first published by Laemmli (1970) has been widely adopted for these
experiments. A field for the molecules to travel in is produced with
acrylamide monomer which is polymerized to produce a network with spaces
adequate for the mobility of the proteins of interest. The proteins are p u t on
the gel and a charge is applied which causes the proteins to move at a rate that
is generally proportional to their size. Smaller molecules will be less retarded
by the gel and will migrate further. If sodium dodecyl sulfate (SDS) is added
to the sample buffer, the detergent will coat the protein and neutralize the
charges so the movement of the molecules will only depend on size (Weber
and Osborn, 1969). After the current has been turned off, the protein is
stained with a dye (often coomassie blue) in order to see the bands of protein.
Molecules of known molecular weight are also put in a lane of the gel and a
standard curve can be generated of the distance traveled vs. log molecular
weight (Robyt and White, 1987).
A more recent method that has been used to examine wheat proteins is
carbon-13 nuclear magnetic resonance (13C NMR). The following discussion
of the methodology was taken from Breitmaier and Voelter (1990).
Solution state 13C NMR is when the molecules are hydrated sufficiently
so that the carbon atoms have some mobility. The 13C nuclei is naturally
abundant, but at a lower concentration than JH, and thus more scans are
often required to achieve the desired signal to noise ratio that will result in
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well resolved spectra. The basic NMR theory is the same as for protons
mentioned in section 2.3.1. The 13C spectra is the Fourier Transform of the
free induction decay (FID) that results from the relaxation of the carbon atoms
after they have experienced the radio frequency pulse of a sufficient length to
perturb the nuclei 90°. The mobile carbon atoms in the sample will produce a
signal which results in a peak in the spectra. Each carbon nucleus has a
different environment surrounding it, which results in a slightly different
frequency (chemical shift), with which it relaxes.
Some deuterium atoms may be added to the sample, either in the form of
deuterium oxide or as part of the solvent molecule, so that the spectrometer
can lock on the deuterium signal. All the chemical shifts of the carbon atoms
will then be given relative to the deuterium signal. The chemical shifts cover
the range of 0 to 200 ppm. For example, the chemical shift for the carbonyl
carbon is about 175 ppm and for the alkane carbons is about 40 ppm. The
signal is often proton decoupled. This removes the influences of multiplet
lines of a single to make the spectra clearer, which occurs by having only one
peak per carbon atom and is attributed to the nuclear Overhauser effect.
Individual amino acids will have unique spectral features when compared to
each other. By comparing amino acid spectra to a protein spectrum, the
amino acid composition of the protein can be determined. Difficulties in
identification of peaks arise when signals overlap. The greater the
concentration of the protein in solution, the larger will be the signal. If the
concentration becomes too large, one risks having the mobility of the carbon
atoms in the peptide chains inhibited, thus possibly eliminating those carbon
signals.
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2.4.1 Fractionation and Electrophoresis of Unheated Gluten
Osborne has been called the father of gluten fractionation. A procedure
was described by Wrigley and Bietz (1988) where four solvents were used to
solubilize the wheat proteins: water for the albumins, 0.5 M sodium chloride
for the globulins, 70% ethanol for the prolamines (gliadins), and 0.05 M acetic
acid for the glutelins (glutenins). When subsequent fractionation studies
have used the same basic methodology, the same protein fraction names were
retained for fractions that were assumed to be similar in composition to those
of Osborne.
Blish (1945) provided information on Osborne's work with a thorough
discussion of the work he and other researchers did up to 1945. An emphasis
is given on the effects of heat and solvents on the gluten proteins. Another
excellent review of work done on wheat proteins from 1745 (by Beccari) to
1976 is given by Kasarda et al. (1976) and provides a m odem perspective on
gluten studies. This section will discuss some of the key work covered in that
review, as well as significant work done since the chapter was written in
order to summarize the different solubility methods used in this field.
A type of Osborne separation was reported by Jones et al. (1959) in which
gluten was dispersed in 0.01 N acetic acid and m ade to 70% ethanol final
solution. When the pH was increased to 6.5, half of the proteins precipitated
and these were thought to be the glutenins. This method combined with
other fractionation techniques was utilized in early electrophoresis work.
This same technique was used by Bietz and Wall (1972). The authors used gel
filtration to separate the proteins that were in an SDS mercaptoethanol
solution. PAGE that was subsequently done was able to differentiate
subfractions of the gliadins and glutenin components. Bietz et al. (1973)
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provided a good review of the methods used to separate the gluten proteins
and some methods to examine the composition of the fractions. Gel filtration
can be used to further subfractionate the glutenin and gliadin fractions (Bietz
and Wall, 1973). Bietz et al. (1975) published a clearly outlined modified
Osborne fractionation procedure for use in genetic studies of wheat.
Chen and Bushuk (1970) published a detailed modified Osborne
procedure for flour in which centrifugation played a critical role. 0.5 M NaCl
solution and water extracts were used initially to isolate the water and salt
insoluble fraction. They obtained 70% ethanol soluble and insoluble fraction,
with the latter fraction further extracted with 0.05 M acetic acid to obtain an
acid insoluble fraction using centrifugation. Differences were found between
the protein contents of the fractions between various wheats with a large
portion of the hard red spring wheat (HRSW) protein falling into the residue
category. Chen and Bushuk (1970) called the acid soluble fraction the
glutenin, which is contrary to Shogren et al. (1969) who called the ad d
insoluble fraction (pH 4.7) the glutenin. A few years later Bushuk and
another co-worker (Orth and Bushuk, 1973) found that the Osborne method
resulted in a very impure glutenin fraction based on PAGE.
In an attempt to improve on the previous solubility methods, Meredith
and Wren (1966) reported their own unique solution which contained 0.1 M
acetic add, 3 M urea, and 0.01 M cetyltrimethyl ammonium bromide
(CH3 (CH 2 )i 5 N(CH 2 )3 +Br‘). The solvent was given the acronym AUC. Urea
will aid the solubilization by dissociating the protein chains. Urea in such a
low concentration does not drastidy effect protein structure. A comparison
was m ade between two similar solvents: AUC (Meredith and Wren, 1966)
and AUB with Brij 35 substituted for cetyltrimethyl ammonium bromate.
29
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Similar PAGE gels were produced with Brij 35 being preferred for its lack of
charge. The overall recommended procedure which involved solubilizing
the proteins in AUC, 70% ethanol, then precipitating the glutenins by
increasing the pH to 6.4, required about six days. 0.01 M acetic acid was used to
disperse the glutenin.
Bottomley et al. (1982) found a sodium dodecyl sulfate (SDS)-Tris solution
superior to AUC and CUC (cetyltrimethylammonium bromide, urea, and
sodium citrate) in the amount of protein extracted and suitability for gel
filtration when the extracted proteins were compared by SDS-PAGE.
McMaster and Bushuk (1983) reported a rather complex procedure involving
AUC, ethanol, centrifugation, and dialysis to separate glutenin and gliadin in
order to examine the carbohydrate associated with each fraction. They
implied that different monosaccharides may influence protein solubility in
ethanol.
Kobrehel and Bushuk (1977) tried a novel approach using sodium stearate
to solubilize glutenins. Ham auzu et al. (1979) also solubilized glutenin in
sodium stearate and added tris-hydrochloric acid to prevent the rapid (less
than five hour) precipitation of the protein. They felt that the hydrophobic
interactions of the peptide chains were important and the aggregation of the
glutenin was disrupted by the sodium stearate but not to the extent of a
chemical reduction of the disulfide links, based on SDS-PAGE and gel
filtration work. This last finding was also published by Wasik et al. (1979)
using a sodium stearate method. Graveland et al. (1979) used 1.5% SDS
solution, ethanol, and centrifugation to separate wheat flours and reported
that there were six glycoprotein fractions; three were SDS-insoluble.
Subsequent research showed that there was only one SDS-insoluble
30
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gylcoprotein and the previous fractions only contained sugars that were not
bound to the proteins (Graveland et al., 1982).
Considerable work has been done using only acids to separate the gluten
fractions. A multicomponent study was undertaken using decreasing lactic
acid concentrations (and therefore increasing pH's) to separate the various
fractions (Shogren et al., 1969). The band separation was not very clear from
starch gel electrophoresis, but differences in fraction composition were
observed. This work was carried out further by the same group of authors
(Hoseney et al., 1969b) who also included an ultracentrifuge in the separation
method. Chung et al. (1979) found that a greater amount of protein could be
extracted with 0.05 N than 0.005 N acetic acid. A lengthy procedure using
primarily 0.05 M acetic acid and centrifugation was used by Preston and
Tipples (1980) to separate gluten into an add soluble and insoluble fractions.
Rather than fully reconstitute the fractions, they were added to flour in small
amounts and their influence was manifested in that way. Unpublished work
that was referred to indicated that the acid soluble portion was a mixture of
gliadin and glutenin-like components.
Three years later, Jones et al. (1983) were successful in separating gluten
fractions with lactic add (0.005 N) and ultracentrifugation. Unpublished data
mentioned that there was no loss of functionality. Finney (1985) summarized
the article by Jones et al. (1983) and went on to further explain the significance
in baking of each fraction.
MacRitchie (1978) found a decrease in loaf volume with a fractionation
method using 0.1 M acetic a d d and centrifugation. He found the acid soluble
fraction to be composed of gliadins and glutenins. The next year MacRitchie
(1979) showed the effect of pH on gluten solubility in acetic add. MacRitchie
31
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(1985) found that when the gluten was in contact with acetic a d d there was a
loss in functionality. This conclusion may be questionable because the baking
test to determine functionality did not work due to the presence of the acetate
ion which depressed yeast activity. Dilute HC1 (0.05 M) appeared to have less
affect than 0.1 M acetic add; the two acids were not compared at the same
concentration. The study also found that up to 87% protein could be
solubilized, with an HCI concentration of 4.5 x 1(H M and the aid of a
homogenizer (spedfically an Ultraturrax), indicating that both gliadin and
glutenin could be ad d soluble. Baking tests indicated that the shearing action
of the Ultraturrax did not alter the vitality of the proteins. It was not
indicated whether the gluten proteins were specifically affected by the acid.
The lower the pH, the more deleterious effect it had on the peptide chains.
Also, it was found that loaf volume was related to the pH of the dough.
Whether this was due to the extraction method or protein chain interactions
under the specific conditions used was not discussed. The molarity of the HCI
used (between 4 x lO-4 and 45 x 1(H) had a direct relationship to the percent
soluble protein. Lactic acid, which is sometimes used, was not examined.
Letting the protein remain in the acid for up to 48 hours did not result in
deterioration of the peptide chain.
A successive HCI fractionation of gluten was used by MacRitchie (1987b)
to examine functionality of the protein group components. But, SDS-PAGE
showed that the nine fractions overlapped in the big groups of glutenin,
gliadin, and residual proteins (MacRitchie et al., 1991). Eliasson and Lundh
(1989) also found the proteins to be of a variety of molecular weights in the
fractions that resulted from this HCI extraction procedure.
32
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Chakraborty and Khan (1988a) conducted an extensive comparison of
three separation methods. One method was that of Hoseney et al. (1969a,b)
(which used the method outlined in Shogren et a l, 1969) in which lactic acid
was used as the solvent for separation. The second method was of Orth and
Bushuk (1972), which was a modification of Osborne taken from Chen and
Bushuk (1970). The third method came from MacRitchie (1978) and used
acetic acid as the solvent for the proteins. Flow charts, for easy comparisons,
of the details of the isolation procedures are given in the article by
Chakraborty and Khan (1988a). The sensitivity of the MacRitchie method to
the type of mixer used was shown. The SDS-PAGE results showed that the
Chen and Bushuk (1970) method resulted in different separation compared to
the other two.
One test of a fractionation method is to determine if the functionality of
the fractions has been effected by the treatment. Therefore, a comparison of
functionalities in reconstitution studies were perfomed (Chakraborty and
Khan, 1988b). It was found that 70% ethanol is a strong enough denaturant to
alter functionality of gliadins in a negative manner. Thus, Chakraborty and
Khan (1988b) concluded that the Hoseney et al. (1969a,b) method was the best
of the three. The Chen and Bushuk method resulted in denaturation of the
proteins, and the MacRitchie method did not result in an equally effective
separation of gliadin and glutenin.
From the above discussion it is evident that there are a wide variety of
methods for fractioning gluten. Each solvent will interact with the proteins
differently and thus the fractions will be slightly different. When choosing a
procedure, one must consider how the fractions will be characterized. Many
solvents do not interfere with electrophoresis. If one is interested in doing
33
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13c NMR (see next section), it is not desirable to have a solvent that is going
to interfere w ith the spectra. One m ust also think about whether each
fraction should be subjected to a different solvent or if it is preferable to have
one solvent. The fractionation procedure of MacRitchie (1987b) that uses
dilute HCI is a method that satisfies this criteria because it uses only one
solvent with no carbon atoms.
2.4.2 13C NMR of Unheated Gluten
Solution state 13C NMR has been used to a limited extent to examine
gluten. In 1981 Baianu published the first *3C spectra of gliadins using a
solvent of deuterated acetic acid with 5% propanol. The advantage of 13C in
its resolution capabilities over *H is demonstrated. They assigned peaks to
amino acids. The next year Baianu and co-workers (1982) expanded their
NMR work to look at the effects different concentration and heating on the
13C spectra of gliadin in 10 mM acetic acid. At 32% (w /v) there was a
broadening of the peaks, and this was not observed at 15.8% (w /v). A higher
temperature (up to 60°C) did not appear to have that much effect on the
spectra. More recently, Belton et al. (1987) looked at hydrated gluten by 13C
NMR and reported a spectra that was similar to that of Baianu et al. (1982). It
should be noted that in the Belton study, the lyophilized gluten was hydrated
to about 60% moisture, and was thus a "plastic mass".
2.4.3 Effects of Heating
After the gluten has been heated, the protein m ay separate differently
than it did before heating. A different kind of fractionation procedure may be
34
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necessary because the proteins are denatured which may affect their
conformation and chemical solubility.
One of the earliest studies to look at the effects of temperature (70-90°C)
and moisture content (10-60%) on gluten was done by Pence et al. (1953).
After subjection to the treatments, changes in solubility of the gluten in dilute
acetic a d d were evaluated. As moisture content or tem perature of heating
was increased, the rate of denaturation increased. Muller and Bale (Muller,
1969; Bale and Muller, 1970) evaluated gluten as a polymer, such as rubber.
They found that gluten denatures over a range of temperatures starting at
50°C and extending for 40 more degrees by finding that there was an increase
in the number of cross links with heating. The exact nature of these
interactions was not discussed. Hansen et al. (1975) further investigated heat
(108-174°C) and moisture (18-33%) effects on gluten through an examination
of changes in solubility in urea, electrophoresis, and gel filtration. As the
temperature was increased, there was a decrease in solubility followed by an
increase when heating was to 174°C. The chromatographic separation
depended upon the treatment conditions and was altered after heating
relative to the unheated sample. The authors felt that the conditions
experienced during baking, where the internal temperature does not exceed
100°C, would result in changes to the protein that would be less severe than
those reported in the article.
Jeanjean et al. (1980) used 0.5 M NaCl or 60% ethanol or one of three 0.025
M borate buffers with sodium stearate. Solubility decreased in 60% ethanol
and salt, but increased in 1% mercaptoethanol/0.025 M borate buffer after
heating. This change in solubility was thought to be due to hydrophobic
interactions. Gluten, at 60% moisture, was heated to 90°C by Booth et al.
35
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(1980) and the changes in the protein examined by measuring changes in
extractability of the gluten components in CUC. It was found that glutenin
aggregated (with disulfide bonds being involved) resulting in diminished
solubility, which was confirmed with starch gel electrophoresis. This work
was extended to investigate further the involvement of disulfide bonding
when gluten was heated (Schofield et al., 1983). Heating up to 75°C resulted
in complete loss of functionality of the gluten and a decreased extractability in
SDS-Tris buffer, which was due to polymerization of the glutenins, as found
by gel filtration and propanol solubility. These methods showed a
polymerization of the gliadin proteins when gluten was heated to 100°C.
After measuring free sulfhydryl groups of gluten that had been heated to
75°C, the authors concluded that the changes caused by heating were due to
sulfhydryl-disulfide interchange reactions of the glutenin proteins, not a loss
of sulfhydryl groups. A subsequent report (Schofield et al., 1984) mentioned
that at greater than 70°C gliadins appeared to polymerize. LeGrys et al. (1981)
reported that when VWG, at 65% moisture, was heated above 55°C there were
irreversible changes in the storage and loss moduli as measured with
dynamic rheometry. Rogers et al. (1990) used a simple extraction in 1% SDS
and up to 1.5% P-mercaptoethanol of gluten that had been reheated by
convection or microwave methods. The authors found that MW reheating
formed fewer disulfide cross-links between the protein chains. Weegels and
Hamer (1991) found that when gluten at greater than 20% moisture was
heated to 80°C, solubility in SDS and the level of free sulfhydryl groups
decreased.
Baianu et al. (1982) examined gliadins in great detail and gluten to a lesser
extent by 13C NMR. These investigators saw a difference in the spectra when
36
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the wheat protein sample (32% w /v) was heated to 60°C and spectra recorded.
More recently, Belton et al. (1987) investigated the effects of heating on gluten
by 13C solution state NMR. Gluten was hydrated to about 60% moisture and a
spectra was taken before and after heating to 80°C, and little change was
found, but the authors felt they were only monitoring a small portion of the
gluten. Gluten, gliadin, and glutenin (as separated by lactic add) were freezedried and hydrated with water for examination by 13C NMR (Ablett et a l.,
1988) at 30°C and after heating to 80°C. In the unheated form, all three
protein systems had very similar spectra, with heating having no effect.
2.5 Objectives of This Study
There is a great deal of interest in using a MW oven to heat cereal-based
foods. Little work has been done to examine these systems at the molecular
level to understand the structural changes that have occurred. This research
was undertaken to investigate the effects of MW heating on cereal-based
systems and compare the results to those systems when CV heated. To
eliminate complications and interactions, a model system containing the
prim ary ingredients of a cereal product will be used. The experimental design
used throughout the study will be based on five ratios of wheat starch:vital
w heat gluten at three different moisture contents. Temperature profiles will
be obtained during heating to determine the final tem perature to ascertain
that protein denaturation and starch gelatinization are occurring. The texture
of the samples after heating will be measured. Overall appearance and starch
granule morphology will be assessed with SEM. An investigation of the
components at the molecular level will be carried out both before and after
heating to assess the effect of the heating method. Water will be examined
37
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with PGSE-NMR to obtain self-diffusion coefficients. Dielectric properties
will be determined. The gluten will be separated into fractions based on pH
with HC1. Each of the fractions, after freeze drying, will be characterized by
SDS-PAGE and 13C NMR.
38
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CHAPTER 3
MATERIALS AND METHODS
3.1 The Model Systems
All moisture contents will be given as a percentage on a wet basis (i.e.,
(g w ater/g solids + water) multiplied by 100). The two dry ingredients in the
samples were vital wheat gluten (VWG) (Whet-pro 80, Ogilvie Mills,
Minneapolis, MN) [proximate analysis: 7.58% moisture, 76.5% protein, 0.15%
fat, 0.41% ash] and wheat starch (Aytex-P, General Mills Chemicals, Inc.,
Minneapolis, MN) [proximate analysis: 10.8% moisture, 0.45% protein, 0.15%
fat, <0.02% ash]. These two ingredients were used throughout the study in
five ratios (100:0,80:20,50:50, 20:80, and 0:100) of starch:gluten. Each of these
combinations was m ade to three different moisture contents 35%, 50%, and
65% (not including the water present in the sample). In general, to prepare all
the samples, the dry powders were weighed, mixed, the appropriate amount
of water added, and the ingredients thoroughly combined. Specifically, for
the unheated samples, a 2-g ball was made by weighing the water with the dry
ingredient mixture. For the heated samples, an 80-g ball was made, with the
water measured volumetrically. Two samples could not be made, out of the
fifteen, because of the hydration properties of the gluten. At 35% moisture, a
homogeneous mixture of the 80 g sample of 20:80 and 0:100 starch:gluten
compositions was not possible. It was possible with some effort to make a 2 g
ball of these compositions for some of the measurements.
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3.2 Heating Methods
For all experiments conducted in Minnesota, conventional (CV) and
microwave (MW) heating was carried out in an environmental oven as
described by Hung (1980). No air flow was used. Samples were p u t in a
beaker with a diameter of 5.4 cm and height 5.2 cm. Conventional heating
was done at 190°C for 20 min with the beaker sitting on a mesh platform
suspended in the center of the oven cavity. Microwave heating was done at
400 W (2450 MHz) for 90 s with the beaker sitting on a teflon platform.
The exception to the above heat treatment was when the samples were
prepared for the self-diffusion coefficient measurements, which were done in
New Zealand. At Massey University samples were placed in a 250 ml beaker
(diameter 6 cm and height 9.5 cm) and heated in a household CV oven at
190°C for 25 min or in a household MW oven (700 W Panasonic), equipped
with a turntable, at full power for 45 s.
3.3 Temperature Profiles
Temperature profiles of samples prepared in Minnesota were obtained.
During CV heating, type T thermocouples were connected to a recording
device (Omega Engineering, Inc., OM400 Series Multichannel Data Logger,
Stamford, CT) and the temperature in four positions in the sample were
recorded each minute. An apparatus was used to try and provide consistency
in the locations monitored, but the curvature of the wire and the expansion
of the sample prevented precisely repeatable positions from being measured.
During MW heating, four fiber optic temperature probes were used (Luxtron
Model 750 Fluoroptic Temperature System, Mountain View, CA) which were
interfaced to a computer to record the temperature in four locations in the
40
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sample every 10 s. Again, due to difficulty in maintaining probes in an exact
position, reproducible locations of temperature measurement were not
always possible.
3.4 Water Loss After Heating and Cooling
All samples were weighed before heating and after cooling for 30 min
to determine the amount of water loss and how much water rem ained in the
samples when the experiments were conducted. As well, the moisture
content for eight of the samples (35% - 100:0,50:50; 50% - 100:0, 50:50, 0:100;
65% - 100:0, 50:50, 0:100) was determined using the vacuum oven method
(AACC, 1983) using a four place analytical balance. The Al pans and lids were
oven dried and kept in a desiccator prior to use. Calculations were made on a
wet basis.
3.5 SEM
Specimens were cut, with a razor blade, from the same locations as
where the D values and dielectric properties were measured. Representative
samples of the 3x3x5 factorial design were examined (35% 100:0, 50:50; 50%
100:0,50:50,0:100; 65% 100:0,50:50,0:100) before and after heating by both
methods. The pieces were affixed to Al stubs with Ag paint and left to dry
over CaCC>3 in a desiccator. The samples were coated with A u /P d in a
vacuum evaporator and then viewed on a Philips SEM (model PSEM 500) at
12 kV. Micrographs were taken with a Polaroid camera.
41
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3.6 Texture Measurements
The Instron Universal Testing Instrument (Model 1122, Canton, Mass.)
with a 500 kg load cell was used to measure the force required to compress
both the MW and CV heated samples (all thirteen compositions). The Ottawa
Texture Measuring System test cell (Voisey et al., 1972) that was 30 cm2 was
used to hold the sample so it could not move sideways, only down through
the wire grid of the insert, as shown in Fig. 3.1. The nine wires of the insert
had a diameter of 0.236 cm and were 0.333 cm apart. In order to be consistent,
considering there were different physical characteristics of the samples, the
piston on the shaft that was connected to the load cell, stopped 16 mm from
the top of the rungs for all samples. The cross-head speed was 200mm/min
and the paper moved at 100 m m /m in. The maximum peak height, in kg,
was determined for each sample and the data statistically analyzed.
Figure 3.1 30 cm2 Ottawa Texture Measuring System test cell
42
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The maximum force that could be measured without damaging the wire grid
was 200 kg. Due to the pattern in the variance of the data, a transformation
was necessary. The natural logarithm of the individual values was taken for
the Analysis of Variance (ANOVA) (p=0.05) that was done on the Statistical
Analysis System (SAS, 1987). The contrasts that are reported are on the actual
data.
3.7 Determination of Self-diffusion Coefficients
The complete 3x3x5 factorial study was done. Immediately after
mixing, D values were obtained for the unheated samples. It was not possible
to obtain data for one of the heated samples. At 65% moisture, the 100:0
sample could be easily made up, but it was too sticky after heating to insert
into the NMR tube for measuring.
Self-diffusion coefficients (D), m 2/s , were determined at 30°C using a
JEOL FX60 pulse Fourier transform spectrometer w ith modifications described
elsewhere (Callaghan et al.r 1980) using the method of Stejskal and Tanner
(1965). 4 mm (inner diameter) NMR tubes were used for obtaining D values.
For single component diffusion the ratio of the echo signal with and without
the application of the pulse gradient (i.e., the echo attenuation) was written
according to the Stejskal-Tanner relation (Stejskal and Tanner, 1965) as
= exp(-kD)
(1)
where k= y282G2(A-8/3), y is the nuclear gyromagnetic ratio, 8 is the gradient
pulse duration, G is the gradient amplitude, and A is the time separation of
the two gradient pulses. In the experiments reported here, G was 4.72 T /m , A
was 5 ms, and 8 was varied between 0.1 and 2 ms. Where two components
are present, equation (1) may be modified to read
43
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with An being the normalized amplitude of the component with diffusion
coefficient Di, and (1-An) being the normalized amplitude of the component
with diffusion coefficient D2 . Fits using this two component relation were
carried out using a program based on a least squares method. The D values
were obtained with a calibration accuracy of ±0.5%. All D values was analyzed
by ANOVA (p < 0.05) for unbalanced data using SAS (1987).
3.8 Determination of Dielectric Properties
A reflectance method was used to determine the dielectric properties with
a Hewlett Packard Network Analyzer (Model 8752A) and a Hewlett Packard
Dielectric Probe (Model 85070A). Microwaves at 2450 MHz were emitted from
the end of the probe into the sample (at 22°C) and reflected back. These
reflected waves were interpreted by the Network Analyzer to give data that
was required to simultaneously calculate k' and k". The attenuation factor, a,
could then be calculated with the following equation
a
(3)
where X = 12.245 cm and tan28 is equal to the ratio of k" to k’ (von Hippel
1954). The data for k', k", and a were each analyzed separately with SAS
(1987) for unbalanced data.
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3.9
Examination of Gluten Proteins
3.9.1 Fractionation
The method of MacRitchie (1987b) was used to separate both unheated
and heated (MW and CV) gluten. The following heated samples were chosen
because they contain either all gluten (0:100) or enough gluten with sufficient
starch (50:50) to examine the possible influence of the carbohydrate: 35%
50:50; 50% 50:50, 0:100; 65% 50:50, 0:100. A schematic of the fractionation
procedure is shown in Fig. 3.2. The experiment was always started with a
sample that contained 20 g of gluten in 400 ml 0.6 mM HC1 (pH = 3.6). For the
samples that contained less than 20 g of gluten (i.e., 65%, 0:100), more than
one sample was required, with a portion of the second sample used to obtain
20 g. If a sample contained more than 20 g of gluten, only a portion was used
that contained about 20 g. For the 50:50 samples, there was a starch layer that
appeared in the first centrifuge tubes, and this was scraped off and discarded.
The MacRitchie method used nine extractions, but preliminary work showed
that there was not much protein in the fractions greater than five, so the
procedure was stopped there. 50 ml 0.6 mM HC1 was added to the final
residue. The pH was recorded for all supernatants and the final residue
(hereafter termed the "residue") after the addition of HC1. The solutions were
adjusted to a final pH of 5.8 with 0.1 M NaOH (pH = 11.6). These solutions
were p u t into plastic bottles, frozen in the blast freezer at -20°C for 1 1 /2 to 2
hours and freeze dried until all moisture was removed. The powder that
resulted was stored at room temperature for the 13C NMR and electrophoresis
experiments. The fractionation was duplicated for all 11 samples, and if any
large discrepancy occurred between the initial pH's of the two replicates, the
experiment was done a third time.
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20 g gluten + 400 m l 0,6 mM HC1
Homogenize 2 m in
\ /
Centrifuge 20 min, 6000 rpm
^
If starch layer present,
discard
\ l
supernatant 1
residue 1
pH to 5.8 w ith 0.1 MNaOH,
lyophilice
200 m l 0.6 mM HC1
Homogenize 2 m in
\1 /
Centrifuge 20 min, 6000 rpm
I
supernatant 2
residue 2
pH to 5.8 w ith 0.1 MNaOH,
lyophilice
200 m l 0.6 mM HC1
Homogenize 2 m in
\l/
Centrifuge 20 min, 6000 rpm
l /
supernatant 3
residue 3
pH to 5,6 w ith 0.1 M NaOH,
lyophilice
100 ml 0.6 mM HC1
Homogenize 2 m in
\l/
Centrifuge 20 m in, 6000 rpm
supernatant 4
residue 4
pH to5.8w ith0.1 M NaOH,
lyophilice
100 m l 0.6 m M H Q
Homogenize 2 m in
\l/
Centrifuge 20 min, 6000 rpm
l/
supernatant 5
pH to 5.8 w ith 0.1 M N aO H ,
lyophilice
-1
final residue
pH to 5.8 w ith 0.1 M N aOH,
lyophilice
Figure 3.2 Flow chart of fractionation procedure
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3.9.2 SDS-PAGE
SDS-PAGE was used to examine each of the 60 products of the
fractionation that were also examined by 13C NMR. A combined procedure of
Ng et al. (1988) and MacRitchie et al. (1991) was used. The sample buffer was
made up of 0.0177 M Tris-HCl (adjusted to pH 6.8 with 0.1 M NaOH), 1% w /v
SDS, 20% w /v sucrose (to increase density of the solution), and 0.006% w /v
Pyronin Y (a tracking dye). 250 pi sample buffer was added to 1 mg of the
freeze dried gluten, left at room temperature, and vortexed several times over
the course of the day. The samples were put in a refrigerator at 4°C overnight,
vortexed the next morning and centrifuged at 10,000 rpm for 10 min; 15 pi
was loaded into each lane of the gel. In order to have a 5% v /v (3mercaptoethanol solution, 12.5 pi of p-mercaptoethanol was added to the
remaining 235 pi of sample buffer in the microcentrifuge tubes. The samples
were then treated in the same manner as indicated above (Burnouf and Bietz,
1989).
The BioRad Protean II Slab Cell system for electrophoresis (BioRad
Laboratories, Richmond, CA) was used with a stacking and separating gel
with a thickness of 1 mm. The buffers used for the gels were 1.0 M Tris-base,
adjusted with concentrated HC1 to pH 6.8 for the stacking gel and to pH 8.8 for
the separating gel. The composition of the stacking gel was 2.9% w /v
acrylamide and 0.043% w /v bis-acrylamide. The composition of the
separating gel was 17.6% w /v acrylamide and 0.083% w /v bis-acrylamide. A
comb with 15 lanes was used. A BioRad power supply unit was used (BioRad
Laboratories, Model 1000/500).
The electrode buffer of 0.025 M Tris-base, adjusted to pH 8.3 with
concentrated HC1,1.44% w /v glycine, and 0.1% w /v SDS was used when the
47
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gels were run at constant current in a walk-in cooler (7.2°C) with the
following conditions (for two gels): one hour at 40 mA, one hour at 50 mA,
and two hours at 60 mA, or until the tracking dye reached the end of the gel.
When the separation was complete, the gels were removed from between the
glass plates and put in the stain which was prepared following the method of
MacRitchie et al. (1991). The gels were left in the dye solution a total of 24-36
hours; the solution was changed once. As the background of the gels picked
up quite a lot of color, the gels were put in a destain (10% v / v of 100% w /v
TCA, 33% v /v methanol, and 57% v /v water) for about four hours.
Nine standards were used at a concentration of each of 0.1 mg in 200 pi
of the sample buffer, with only 10 pi of the standard solution loaded in a lane.
The standards, from Sigma Chemicals (CITY) were (with molecular weight in
brackets): lysozyme (14,300), (3-lactoglobulin (18,400), trypsinogen (24,000),
pepsin (34,700), egg albumin (45,000), bovine albumin (66,000), phosphorylase
b (97,400), P-galactosidase (116,000), and rabbit myosin (205,000). The three
highest molecular weight standards did not migrate far and were not always
included in the standard mixture.
3.9.3 Carbon-13 NMR Spectroscopy
For each of the six lyophilized products of the fractionation procedure,
that resulted from each of the five compositions after MW or CV heating (i.e.,
a total of 60 samples), a 13C spectra was obtained. For comparison, spectra
were also obtained in the same manner for the fractions of the unheated
gluten. Two solvents were required to dissolve all of the freeze dried
fractions. The first three fractions of dry protein was dissolved in solvent A
which was composed of 0.6 mM HC1,5% w /v sodium dodecyl sulfate (SDS),
48
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and 20% deuterium oxide (D2 O); pH = 4.3. The last three fractions were not
soluble in this solvent, but were soluble in solvent B which was composed of
0.0178 M Tris-HCl buffer (adjusted to pH 6.8 with 0.1 M NaOH), 1% w /v SDS,
20% D 2 O, and 10% (3-mercaptoethanol; pH = 6.8. A ratio of 0.06 g of protein to
1 ml of solvent was desired, but not always possible. For some supernatants,
there was less that 0.03 g of protein and a minimum of 0.5 ml of sample was
required for the spectrometer. The solvent was added to the weighed protein
and vortexed until completely mixed. The protein mixture was put in 5 mm
NMR tubes and run soon after mixing. An NT-300 WB Nicolet
Technologies, Inc. (General Electric, Fremont, CA) spectrometer (in the
Biochemistry department) was used for the measurements at 22°C. The
signal was locked on deuterium and all chemical shifts were m easured
relative to deuterium. The signal was also proton decoupled. A single radio­
frequency pulse of 13 psec, which is about a 90° pulse, was used with a 1.5 sec
delay between each of the 10,500 acquisitions, which took about 5 hours, that
was necessary for each sample in order to obtain an adequate signal to noise
ratio. Line broadening of 3 Hz was used before the Fourier Transform of the
free induction decay. All spectra were plotted relative to the height of an SDS
peak which had a chemical shift of about 25.58 ppm in solvent A and 26.00
ppm in solvent B.
49
•v r r r r*?.
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CHAPTER 4
RESULTS
4.1 Physical Characteristics of the Samples
The samples used in this study were mixtures of starch, vital wheat
gluten, and water. While the samples contained the typical ingredients of a
cereal product, their appearance was atypical, and depended on the
composition. Figs 4.1 and 4.2 show what the samples typically looked like
after heating in Minnesota. Although an attem pt was made to maintain the
same conditions, the samples prepared in New Zealand were not exactly the
same. The crumbly mixture of the high starch samples at 35% moisture
became slightly more cohesive, although remained somewhat crumbly after
heating. At 50% moisture after MW heating the center was an opaque gel,
and the outside was a white powder; the CV heated sample had a white
powder center and opaque gel outside, as was found by Goebel et al. (1984).
Measurements that were taken in subsequent experiments were of the "gel"
region. This same appearance was found by other investigators (Goebel et al.,
1984; Zylema et al., 1985; and Wynne-Jones and Blanshard, 1986) when starch
and water were MW and CV heated. The 50:50-35% moisture sample
exhibited a layering of the dough. It is interesting that the MW heated
samples resulted in a single air cell with a shell of dough around, and during
CV heating numerous air cells resulted and the whole mass of dough
expanded. As the proportion of gluten increased, the sample expanded
during heating, and in some instances collapsed upon cooling. Some of the
samples had large holes in the middle, others had holes more evenly
distributed throughout. For some of the compositions, the samples were
different and others were the same after being heating by both methods.
50
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Figure 4.1 Conventionally heated samples.
Figure 4.2 Microwave heated samples.
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4.2 Heat Penetration
Temperature as a function of time was recorded for all samples during
both methods of heating. Each of the graphs in Figs 4.3-4.8 is a representative
set of temperature profiles for each treatment and composition. Each figure
has three or four plots per graph for different probes located in the same
sample during one heating period. At 35% moisture content (Fig. 4.3) the CV
heated sample heated more slowly than samples containing more moisture
(Figs. 4.4 and 4.5). During MW heating all samples had similarly shaped
curves indicating that the temperature increased in the same manner
regardless of moisture content (Figs. 4.6-.8). At the lowest moisture content
for both heating methods there are similar (more uniform) temperature
profiles throughout the sample as shown by the agreement between probe
temperatures (Figs. 4.3 and 4.6). Heating times (20 min for CV and 90 sec for
MW) were such that all probes in all samples pass through starch
gelatinization and gluten denaturation temperature (60-70°C). CV and MW
heated samples reached 100°C to allow starch gelatinization and protein
denaturation to occur throughout the entire sample, since probes only
measure temperature at specific locations and cannot map the temperature
profile of the entire sample. Even though the conditions were chosen for
heating to maintain consistent experimental protocol the pattern of heating
was not identical for all the samples.
At 50 and 65% moisture content for the CV heated samples, the higher
gluten containing samples heated more slowly than when there was more
starch. The opposite phenomenon was observed for MW heated samples at
50 and 65% moisture content. This is surprising considering that the 0:100
sample at all three moisture contents had a lower dielectric constant (see
52
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Section 4.6.2.2) than the 100:0 sample. MW heated samples had more
variability in temperature between probes within a sample as the moisture
content increased.
In Figs. 4.4 and 4.5 for conventionally heated samples, it may be noted that
some of the temperatures are above 110°C. This was due to the sample rising
in the beaker during heating which shifted the thermocouples causing them
to come out of the sample and measure the surrounding air.
53
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100
60 -
0
5
10
15
20
0
5
10
15
20
100
U
o
0u)
3
3u>
3
QJ
JX
g01
100
0
5
10
15
20
time (min)
Figure 4.3 Representative temperature profiles for CV heating of 35%
moisture content samples with all five starchrgluten ratios. A) 100:0, B) 80:20,
C) 50:50.
54
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100
■D
100
B
100
uo
0
5
10
15
20
0
5
10
15
20
100
100
0
5
10
15
time (min)
20
Figure 4.4 Representative temperature profiles for CV heating of 50%
moisture content samples with all five starchtgluten ratios. A) 100:0, B) 80:20,
C) 50:50, D) 20:80, E) 0:100.
55
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100
10
15
20
100
0
ll
<8
I*
a>
Cu
£u
<
5
10
15
20
5
10
15
20
100
3
0
■C
5
10
15
20
100
0
time (min)
0
5
10
15
20
time (min)
Figure 4.5 Representative temperature profiles for CV heating of 65%
moisture content samples with all five starch:gluten ratios. A) 100:0, B) 80:20,
C) 50:50, D) 20:80, E) 0:100.
56
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100
0
30
0
30
60
90
30
60
90
60
90
100
U
o
' —■.
<U
3
100
0
time (sec)
Figure 4.6 Representative temperature profiles for MW heating of 35%
moisture content samples with all five starch:gluten ratios. A) 100:0, B) 80:20,
C) 50:50.
57
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100
100
B
100
u
o
<TJ
0J
cx,
£0)
60 -
0
30
60
90
0
30
60
90
100
• 60
C
100
time (sec)
0
30
60
90
time (sec)
Figure 4.7 Representative temperature profiles for MW heating of 50%
moisture content samples with all five starchrgluten ratios. A) 100:0, B) 80:20,
C) 50:50, D) 20:80, E) 0:100.
58
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100
100
0
30
60
90
100
3
flj
i_
a
a.
£o
0
30
60
90
0
30
60
90
100
0
30
60
90
100
time (sec)
0
30
60
90
time (sec)
Figure 4.8 Representative tem perature profiles for MW heating of 65%
moisture content samples with all five starch:gluten ratios. A) 100:0, B) 80:20,
C) 50:50, D) 20:80, E) 0:100.
59
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4.3 Moisture Content After Heating
Table 4.1 gives the moisture contents, as determined by the vacuum oven
method, on a wet basis of the center part of representative samples. This is
the area which was measured for the self-diffusion coefficients and dielectric
properties. Nearly all samples were within ±3% of the original target
moisture, and all samples were within ±6%. Some of the samples appeared to
gain m oisture in this area possibly due to macromolecular transitions that
required water. Wherever gelatinization occurred in the all starch samples, it
required a great deal of water which left some regions fo the sample looking
dry and white (see Figs. 4.1 and 4.2). This water distribution resulted in
moisture contents that were higher than expected.
Table 4.1. Moisture contents of the center of representative heated samples,
on a wet basis, as determined by the vacuum oven method.
M oisture Content After
Heating
C onventional
Microwave
Heated
Heated
M oisture
Content Before
Heating
Composition
(S:G)
35%
100:0
37.61
37.67
50:50
40.87
33.66
100:0
52.95
55.17
50:50
52.69
48.63
0:100
52.02
47.49
100:0
60.76
67.88
50:50
61.85
64.39
0:100
66.40
64.17
50%
65%
60
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4.4 SEM
Figures 4.9 to 4.12 are representative scanning electron micrographs of the
samples that were examined. The discussion that follows includes
observations of the samples that were not always photographed.
Fig. 4.9 shows scanning electron micrographs of the unheated samples.
The starch granules are very distinct in the 100:0 (Fig. 4.9 A, C, F) and 50:50
(Fig. 4.9 B, D, G) samples, with the latter containing smooth regions of gluten
interspersed between the regions of starch. The samples m ade only from
VWG (Fig. 4.9 E, H) have very little contrast and exhibit a smooth surface.
The micrographs of the CV heated samples at 35% moisture content (Fig.
4.10 A, B) and MW heated (Fig. 4.10 C, D) samples show a loss of granule
integrity. With the gluten present there was development of holes in the
sample during heating. The change in the morphology of the starch granules
relative to the unheated starch (Fig. 4.9 A, B) is evident. The granules are no
longer smooth and have become swollen and misshapen. For the 100:0, 50%
moisture content sample, the gel region (not the dry white region) was
examined by SEM (Fig. 4.11 A, D). There was enough moisture in the sample
to allow gelatinization to occur as seen with the loss of shape by nearly all the
granules, and the formation of a continuous mass. Once gluten was added
(50:50, Fig. 4.11 B, E) there were smooth regions to interrupt the granules that
were not gelatinized by either heating method. The all gluten samples (Fig.
4.11 C, F) were very similar for both CV and MW - a very smooth topography
interrupted by numerous holes.
At 65% moisture there were more distinct granules in the CV heated
sample than the MW heated one (Fig. 4.12 A, B), although the swelling was
61
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greater than the lower moisture content samples. Gluten did not seem to
affect starch swelling (Fig. 4.12 C, D). The CV heated sample had more distinct
granules than the MW heated one. The two heating methods resulted in
similar appearing sample for 0:100 (Fig. 4.12 E, F).
SEM has shown that starch definitely did swell extensively (thus
gelatinize) during both CV and MW heating at all three m oisture contents.
One can assume that if the temperature and moisture conditions allowed the
starch to transform, the gluten was also able to denature (i.e., undergo the
normal transitions that occur during heating).
62
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Figure 4.9
Representative scanning electron micrographs of unheated samples.
A) 100:0, 35% moisture
B) 50:50, 35% moisture
C) 100:0, 50% moisture
D) 50:50, 50% moisture
E) 0:100, 50% moisture
F) 100:0, 65% moisture
G) 50:50, 65% moisture
H) 0:100, 65% moisture
63
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Figure 4.10
Representative scanning electron micrographs of
35% moisture samples.
A) 100:0, conventionally heated
B) 50:50, conventionally heated
C) 100:0, microwave heated
D) 50:50, microwave heated
64
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Figure 4.11
Representative scanning electron micrographs of
50% moisture samples.
A) 100:0, conventionally heated
B) 50:50, conventionally heated
C) 0:100, conventionally heated
D) 100:0, microwave heated
E) 50:50, microwave heated
F) 0:100, microwave heated
65
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Figure 4.12
Representative scanning electron micrographs of
65% moisture samples.
A) 100:0, conventionally heated
B) 50:50, conventionally heated
C) 0:100, conventionally heated
D) 100:0, microwave heated
E) 50:50, microwave heated
F) 0:100, microwave heated
66
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4.5 Instron Data
The Instron data was recorded as the force (kg) to compress the sample. A
representative curve is shown in Fig. 4.13A. Average values of force required
to compress samples after CV and MW heating are shown in Fig. 4.14.
Although the target weight was 80 g, the actual weight of the samples
before they were put in the oven was about 79 g due to errors in measuring
the water and losses in transfer from the mixing vessel to the baking vessel.
Table 4.2 gives the average weight of the samples that were compressed. Most
samples lost less than 5 g to a maximum of 10 g. There was some moisture
loss during heating which will decrease the after heating weight. As well, not
all of the sample can be removed from the beaker and put in the test cell for
the compression. In general, the CV heated samples lost more than those
that were MW heated. When the weights in Table 4.2 are compared to the
moisture contents of Table 4.1, it can be concluded that most of the moisture
loss was from the top surface of the sample, not from the center.
When the force required for compression of the sample was statistically
analyzed, the values were transformed by taking the natural log, to reduce the
variability in the residuals to satisfy the assumption for the ANOVA test.
From the ANOVA table, for both type III and type IV sums of squares, which
account for unbalanced data and missing cells respectively, all main effects
and interactions were significant (P<0.05). The mean square of the error was
0.182 for the transformed values, which converts to 1.2 kg for the actual force.
The three-way interaction is shown in Fig. 4.14 which is composed of three
plots, one for each moisture content.
67
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0.3
0.2
(0
0>
at
Ut
0.1
0.0 9-
0
20
10
30
strain
Figure 4.13 Represenative results from Instron compression. A) Curve of
force versus time for a sample compressed with the OTMS test cell. B)
The values of the curve in (A) plotted as stress versus strain.
68
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100:0 80:20 50:50 20:80 0:100
200
150
100
50
100:0 80:20 5050 20:80 0:100
Composition (S:G)
Figure 4.14 Effect of composition on the force required to compress heated
samples. Cross-hatched = conventionally heated; solid = microwave
heated. A) 35%, B) 50%, and C) 65% moisture.
69
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Table 4.2. Average weight of samples after heating, before compression with
the Instron (n = 4 to 10).
Weight (g) of sample
Conventional
Microwave
Heated
Heated
M oisture
Content
Composition
(S:G)
35%
100:0
71.2
77.2
80:20
73.6
70.7
50:50
74.8
73.9
100:0
68.3
75.9
80:20
70.7
75.4
50.50
74.1
76.9
20:80
74.1
74.5
0:100
74.1
74.6
100:0
68.6
75.3
80:20
70.1
75.0
50:50
69.0
75.2
20:80
73.5
75.9
0:100
73.2
76.6
50%
65%
When one compares these force values to the weight of the sample that
was compressed, it is evident that a larger sample does not require a greater
force as one might suppose. An inverse relationship between moisture
content and force is clearly evident; as the moisture content increases the
force required for compression decreases. The influence of the heating
method on the force is not straight forward, but rather dependant on the
composition with the effect being variable. At 35% moisture (Fig. 4.14A) for
70
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the CV heated samples, the force could only be quantified for the 80:20 and
50:50 samples because the force was greater than the limits of the apparatus
(200 kg). At the lower moisture contents (35 and 50%) composition has a
greater effect on force; that is as the amount of gluten increases in the
samples, generally the force required for compression increases. This is
consistent with the experiments reported by Lelievre et al. (1987) for bread of
up to 16% protein that also included various wheat starches. All
compositions by either heating method had very similar force requirements
for compression at 65% moisture (Fig. 4.14C). The only sample in which MW
heating resulted in a tougher product than CV heating was the all starch
sample at 50% moisture.
Another more classical way of examining the texture of foods has been
used. Attenburrow et al. (1989) suggested a cake could be thought of as being
a sponge based on the work of Ashby and Gibson (1983) where compression
causes the walls to be bent elastically and then collapse. This behavior can be
observed in curves of stress vs. strain. Stress is defined as the force divided by
the initial cross-sectional area and stress is defined as (the am ount of sample
compressed divided by the initial thickness) times 100 (Attenburrow et al.,
1989). If the force data for some of the samples is plotted in this manner (Fig.
4.13B) the phenomena can be observed to a limited extent. There appears to
be a change in the curve, for the higher gluten samples that contained air
cells, when the cells were collapsed.
Compression of the samples has shown that moisture content has the
largest effect on texture, and at lower moisture contents (35% and 50%) an
increase in the proportion of gluten in the sample results in greater force for
compression. Heating method is more important for the high starch samples
71
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at 35 and 50% moisture. At 50% moisture, the CV heated all starch sample is
softer than the MW heated sample. For the 65% moisture sample, the water
dominates the force and all samples are very soft. The interactions between
the components that translates into texture are not straight forward (an
interrelationship between composition and heating method) and starch and
gluten behave differently under the conditions examined.
4.6 Self-diffusion and Dielectric Property Data
4.6.1 The Three Ingredients Separately
Each of the dry ingredients of the model system was examined separately
to investigate how the water interacted with the starch and gluten. This data
was compared to the results of the same experiments on water (Table 4.3).
Table 4.3. Moisture contents, self-diffusion coefficents, and dielectric
properties of the components of the model system.
Moisture Content
W heat
Starch
W heat
G luten
W ater
10.79%
7.58%
100%
Self-Diffusion Coefficient (m2/s) not available 2.6 x 10-12
3.0 x 10-9
Dielectric Constant
2.23
2.46
76.92
Dielectric Loss
0.23
0.13
9.38
Attenuation Factor
0.0390
0.0212
0.274
The D for the proton nuclei in the VWG powder was about three orders of
magnitude lower than water (Table 4.3), indicating a very limited motion of
the water molecules. Although the starch had a higher moisture content
72
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than the VWG, a proton NMR signal could not be obtained, preventing a
determination of a value of D. If the effect of transverse relaxation is
neglected, the magnitude of a signal should be proportional to the number of
contributing nuclei. One simplistic explanation for the lack of starch proton
NMR signal might be that the VWG contained a greater number of
nuclei
than the starch. The percentage of protons, as a total of all atoms present in
the sample on a molecular weight basis, was calculated. For the calculation of
the protons in the VWG, an amino acid profile (Table 4.9) was used so that
the percent of each amino acid that was proton, the percent of the total
sample that each amino acid comprised, and the protons on the starch (16.9%)
could be used. The starch macromolecular composition was taken to be
repeating glucose units and the entire sample was assumed to be starch and
water. The results of these calculations were that 6.9% of VWG and 6.7% of
starch was proton, which is not a difference that would be great enough to
cause the lack of of signal with the starch.
A more likely hypothesis is that the water molecules in the "dry" starch
are more closely associated with the rigid polysaccharide structure and
therefore their protons have very short transverse relaxation time, making
them "invisible" in the present experiment (i.e., below the monolayer).
Possibly the converse is the case for the VWG (i.e., the water is present at a
level above the monolayer). From the isotherms (Fig. 4.15) the BET equations
gave a monolayer value of 0.046 and 0.070 g w ater/g solid for VWG and
starch, respectively which correspond to aw's of about 0.08 and 0.10. These
data agree with previous studies (Bushuk and Winkler 1957; Volman et al.
1960; and French 1984). The monolayer values, on a dry basis, are not quite
half the moisture contents for both the VWG (0.082) and starch (0.120), which
73
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could be interpreted to mean that the water would be in about the same
"layer" on the macromolecules. What must be considered is the distribution
of this water. Possibly the water that interacts with the protein is located in
specific regions, where it is present in many layers thus there would be water
nuclei that are quite mobile and could give a signal. Bryant and Shirley (1980)
found that water at a protein surface does exhibit some motion and is not
rigidly held. For the carbohydrate it is possible that the w ater is more evenly
distributed and thus is too immobile to give a signal. Although, according to
French (1984), for starch the water will not be evenly distributed at the
monolayer and not all the glucose molecules would have a water molecule
(0.66 molecules water/glucose unit). Volman et al. (1960) thought this
amount of water would be shared between the glucose molecules and fit well
into the amylose helix, where it would likely be quite immobile.
2
0.3
X
S ta rc h
♦
Vital W h e a t G luten
♦
"o
X
(0
D)
u
O
0.2
X
*5
5
■
D)
0.1 ‘
X
X
.
x
^
*
•
•
•
0.0
0.0
1 1. .
0.2
. 1
0.4
w a te r
»...... 1 _1
0 .6
.
1
0 .8
1.0
a c tiv ity
Figure 4.15 Sorption isotherms for dry ingredients of model system at 23 °C.
74
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The dielectric properties of the starch were comparable to those of gluten
(Table 4.3) and both dry powders had values much smaller than water. As
expected, these values indicate that for both dry powder samples there was
very little interaction of the water with the electromagnetic radiation or
dissipation of the electrical energy into heat energy. The small am ount of
mobility observed with the D value for the water in the gluten was not
enough to allow movement of the dipole to any great extent.
4.6.2 The Mixed Samples
4.6.2.1 General Information on the PGSE-NMR Experiments
For all the mixtures, except for the 100:0-35% moisture sample after
heating, two values for self-diffusion were evident from the plot of log(echo
attenuation) vs. 82(A - 8/3) as shown in Fig. 4.16.
O
LU
LU
£
-2
-3
0
1
2
8 2( A - S / 3 )
4
5
6
(msec3 )
Figure 4.16 Plot showing the effect of increasing gradient pulse duration on
echo attenuation for the PGSE-NMR experiment.
75
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The signal, as shown in Fig. 4.16 was composed of a faster (diagonal line),
more free population of water and a slower (horizontal line), more inhibited
population. In all cases, the signals for the two populations appeared to have
the same chemical shift, except for the 0:100-35% moisture sample, when the
two signals could be resolved. The NMR spectrum of this sample was
examined with and without the field gradient being on and then compared
with a pure water peak. These results showed that the peak with the same
chemical shift as the water was most attenuated when the field gradient was
put on which indicates that the population of water most similar to pure
water is attenuated most during the experiments. The other signal is a slower
moving component that has a D value of about an order of magnitude (or
less) slower than the fast component of that sample, indicating it is
interacting more closely with the macromolecules so the frequency of the
NMR signal is shifted away from that of water.
It was found that an alteration of the length of time between the field
gradient pulses resulted in no considerable change in the less bound value of
D. However, the values of D for the more bound water did change, so it
cannot be well quantified. The reliability of this slow component value was
minimal, but general comments can be made. For the unheated samples, the
contribution of the slow component to the signal was between 2% and 25%.
For each composition, the proportion of the slow component diminished
with increasing moisture content. At the higher moisture contents, m ost of
the signal came from the more mobile water, and the amount of water
interacting with the macromolecules was a very small portion of the total
water in the sample. For the heated samples, the slow component
contributed between 4% and 30% of the total signal. The moisture content
76
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did not have as great an effect on the proportion of the signal between the fast
and slow component after heating as it did in the unheated samples. The
50% and 65% moisture content samples were quite similar in the proportion
of the slow component and the 80:20 CV heated was the same for all three
moisture levels.
In the above discussion, it m ust be remembered that these "fast" and
"slow" components are relative terms and are for a mixed population. The
Brownian motion of the slow diffusing group of w ater molecules is more
inhibited by macromolecular interactions than the fast diffusing group, with
the latter being inhibited to some extent because the D values are less than
that for free water. The number of "layers" of water that are contributing to
each component cannot be determined. The discussions of the NMR data
that follow will be only on the faster more mobile component behavior.
The one anomaly of the samples was 100:0-35% which only had one
population after heating. This could mean that all of the protons were in one
environment producing one signal. Another explanation for the behavior is
that the exchange between the populations was faster than the delay between
the gradient pulses, thereby preventing both populations from being
resolved.
The statistical analysis indicated that the three main effects (composition
[starch:gluten ratio], moisture content, and treatment [unheated, CV or MW
heated]) and all the interactions were significant (P<0.05) for D and the three
dielectric properties.
77
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4.6.2.2 Unheated Samples
The D value of the dough varied significantly (P < 0.05) with the amount
of water (Fig. 4.17); the more water that was present, the higher proportion of
more mobile protons, and the greater the diffusion coefficient measured.
When the smallest amount of gluten was added to the starch (80:20 sample),
the value of D decreased, with the effect being more dramatic at the highest
moisture content. On further addition of gluten, D was relatively constant.
25
0
100:0
I______i______I_____
80:20
50:50
I______i______L
20:80
0:100
wheat starchrwheat gluten
Figure 4.17 Self-diffusion coefficients for unheated starch and vital wheat
gluten mixtures at three different moisture contents: 35% (■), 50% (x), and
65% (o).
This suggests that the water-gluten interaction was "saturated" at low gluten
contents and the subsequent D value of the water as gluten was increased was
dominated by this interaction. Further evidence for this hypothesis is given
by the fact that the D value for water in the 50:50 starch sample at the highest
moisture content was very close to that of free water. The proton mobility of
the hydrated starch and VWG was contrary to the mobility of the dry
78
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powders. That is, in the dry starch the water was very tightly held, and when
more water was added, it did not interact with the macromolecules and
remained quite mobile. In gluten dry powder water had some mobility, and
when more water was added it interacted with the protein to result in a D that
was less than starch with the same amount of water. However it is important
to emphasize that in the dry powders the mobility of the water, which is
bound to the macromolecules, reflects the motion of the polymer rather than
the degree of binding or interaction. In the case of the protein, it may well be
that the water more strongly binds to the polymer but that the polymer is
more rotationally mobile than that of the starch. At higher water contents
where the water is in exchange between "free" and "bound" states, the protein
will, therefore, exert a greater attenuation of the translational diffusion of the
water because of this stronger attraction. The dominance of gluten on the
signal and the decrease in mobility in the presence of the macromolecules
was determined by ESR experiments using the probe TEMPO in water with
starch (Pearce et al., 1985) or gluten (Pearce et al., 1988).
When the dielectric properties were measured, it was found that moisture
content had a greater effect on the dielectric constant (Table 4.4) than on the
dielectric loss (Table 4.5).
Table 4.4 Dielectric constant for unheated samples (standard deviation
= 5.45; p < 0.05, n = 5-16).
_______________Starch:gluten ratio_______________
Moisture Content
35%
50%
65%
100:0
24.40
41.37
55.15
80:20
22.71
33.08
54.27
50:50
20:80
0:100
22.82
33.27
40.38
21.78
31.39
38.56
18.61
30.73
38.10
79
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Table 4.5 Dielectric loss for unheated samples (standard deviation =
1.47; p < 0.05, n = 5-16).
Starch :gluten ratio
Moisture Content
100:0
80:20
50:50
20:80
0:100
35%
50%
65%
7.08
8.95
9.46
7.05
7.77
9.74
7.18
8.75
8.21
6.99
8.76
8.11
5.61
8.81
8.23
As moisture content increased, the sample had increased ability to rotate
and couple with the electromagnetic energy but the k' values did not
approach the k' of water. As the amount of protein increased in the system
there was a decrease in k', and the water appeared to interact more with the
macromolecules, especially at the highest moisture content. For these
compositions, the system appears not to change over the three moisture
contents; as more water was added there was no change in the ability of the
sample to transform the electrical energy to heat energy as indicated by the k"
behavior which is quite similar to water. The attenuation factor (Fig. 4.18)
followed the same pattern as k" but the values were all greater than the value
for water.
Starch did not appear to absorb as much water as gluten because when
equivalent amounts of starch or gluten were mixed with water at the higher
moisture contents, the former was liquid and the latter was not. Thus, for the
unheated mixtures, the physical appearance may provide an indication of the
molecular behavior, but not necessarily.
80
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100:0
80:20
5 0:50
20:80
0:100
wheat starchrwheat gluten
Figure 4.18 Attenuation factors for unheated starch and vital wheat gluten
mixtures. 35% (■), 50% (x), and 65% (o).
4.6.2.3 Heated Samples
The decrease in D with heating (from the unheated samples) is evident in
Fig. 19. The effect of gluten on the signal that was evident in the raw dough
data was minimized during heating due to the redistribution of water from
the gluten to the starch or because the free water became bound to the
macromolecules. Possibly the starch transformations during heating
(gelatinization), which would destroy the rigid structure, allowed greater
interaction between the starch macromolecules and the water. As expected,
the denaturation of the gluten did not appreciably change water behavior as is
evident by the 50% gluten samples (Fennema 1977). For most of the heated
samples moisture content had a significant effect on these results as it did for
the unheated samples. The largest difference between the self-diffusion
coefficients for the MW and CV heated samples was the 80:20-65% moisture
sample. The slight moisture content differences between the CV and MW
81
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heated samples reported in Table 4.1 were not enough to result in any
differences in self-diffusion.
At 50% moisture for the all-starch sample, there was a large difference
between the dough and the CV heated sample. This is contrary to the ESR
investigations reported by Pearce et al. (1985) for starch (67% moisture) where
there was no difference in the motion of the spin probe (TEMPO) after
heating.
For the all-gluten sample at 50% moisture, there was a significant
difference between the unheated and heated samples that was not evident
when the moisture content was 65%. When gluten was examined by ESR at
about 50% moisture, it was found that after heating there was an increased
partitioning of the probe into a hydrophobic region of the sample (Pearce et
al., 1988). The motion of water measured by a spin probe is indirect and the
self-diffusion measurement is direct, which could explain the difference in
the results between the two methods.
It has been reported, based on water adsorption studies, that in a dough
more of the water is associated with the starch than the protein (Bushuk and
Hlynka, 1964) thus there would be more hydrogen atoms associated with
starch-interacting water than with protein-interacting water. This appears
contrary to w hat was found in this study, where the gluten water appeared to
dominate the signal. During heating, the water redistributes itself when the
protein denatures and the starch gelatinizes absorbing water given up by the
protein (Bushuk and Hlynka 1964). When the samples were heated, there
was no composition effect indicating that there m ay be an even distribution
of water between the macromolecules. The self-diffusion data reported here
support this idea.
82
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25
20
15
10
5
o :_
100:0
80:20
50:50
20:80
0:100
Pi
Q
100:0
80:20
50:50
20:80
0:100
wheat starch:wheat gluten
Figure 4.19 Self-diffusion coefficients for (A) conventional and (B)
microwave heated starch and vital wheat gluten mixtures. 35% (■), 50% (x),
and 65% (o).
Heating generally caused a decrease in the k’ (Table 4.6) and k" (Table 4.7)
relative to the unheated samples. A redistribution of the water may have
occurred in the samples during CV heating as all three moisture contents did
not have very different k' and k" values. The changes in the sample that
occurred during CV heating, changed the ability of the water to couple with
83
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the microwaves and transform the electrical energy to heat. MW heating did
not cause the same type of changes in the system as CV heating, and the
moisture content had more of an effect on k' and k". Composition played a
large role in the MW heated samples, especially at 50% moisture. The water
appeared to interact more strongly with the denatured gluten than the
gelatinized starch as the high protein samples had lower k' and k" values
than the high starch samples at 65% moisture.
If one examines the moisture content values after heating in Table 4.1
there is some explanation of the k' and k" phenomena observed. At 35%
moisture, for 100:0 the two samples have the same moisture content, and
similar k' and k". When gluten was added, (50:50) the CV sample had a
greater moisture content and higher k' and k". Moisture is not the sole
determinant of the dielectric properties when gluten is present as the heated
values are less than the unheated.
Table 4.6 Dielectric constant for heated samples (standard deviation =
5.45; p < 0.05, n = 5-16).
Moisture Content
100:0
Conventional Heated
35%
16.54
50%
29.43
65%
41.55
Microwave Heated
35%
17.66
50%
29.15
65%
49.13
* not available
Starch:gluten ratio
80:20
50:50
20:80
0:100
24.03
20.52
29.09
19.70
32.85
35.22
n/a*
31.66
31.78
n /a
21.06
28.84
5.77
22.30
43.12
8.76
1.01
36.61
n /a
2.14
27.70
n /a
20.41
24.30
84
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Table 4.7 Dielectric loss for heated samples (standard deviation = 1.47; p
< 0.05, n = 5-16).
M oisture Content
100:0
Conventional Heated
5.21
35%
50%
10.09
65%
11.48
Microwave Heated
35%
50%
65%
* not available
6.49
7.55
9.95
Starch:gluten ratio
80:20
50:50
20:80
0:100
7.59
6.42
7.84
5.57
8.20
8.34
n/a*
7.53
5.88
n /a
5.03
6.05
1.65
5.84
9.32
2.71
0.24
7.85
n /a
0.18
6.22
n /a
5.36
5.48
At 50% the macromolecular transitions are very important for the dielectric
properties because moisture content cannot be used to explain them. For the
samples with a before heating moisture of 65%, moisture content after
heating does appear to provide an indication of the dielectric properties.
The attenuation factor after heating (Fig. 4.20) is similar to the values
before heating for all 65% samples and the 100:0, 35% and 50% samples.
Although not exactly the same, the pattern of the attenuation factor plots
follows more closely k' than k". It is very clear that MW heating has a much
different effect on the transformations in the sample that determine how the
electromagnetic waves are attenuated. The interaction between composition
and moisture content is clearly evident for the MW heated samples.
The effects of heating, which allowed macromolecular transformations,
were evident by the decrease in D value which indicated an increased binding
85
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and redistribution of the water between the starch and gluten, with the starch
having a greater influence. There was not m uch difference between the CV
and MW heated samples in terms of their D values. For all five ratios of
composition, as moisture content increased there was generally an increase in
D (the mobility of the water) and k' (polarity of the system) but k" (a measure
of the ability of the system to dissipate energy) was not affected as much by
moisture content. The k' and k" values of the heated samples were
dependent on the ratio of starch to gluten, w ith the exact effect dependent on
moisture content. The attenuation factor indicated that the two heating
methods did cause different changes in the system in the way in which the
sample interacted with the electromagnetic energy.
86
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0.5
0.4
0.3
0.2
E
u
0.1
L.
O
4->
0.0 L_
a
100:0
a
o
80:20
50:50
20:80
0:100
80:20
50:50
20:80
0:100
0.5
a
0.4
0.3
0.2
0.1
0.0 L_
100:0
wheat starchrwheat gluten
Figure 4.20 Attenuation factors for (A) conventionally and (B) microwave
heated starch and vital wheat gluten mixtures. 35% (■), 50% (x), and 65% (o)
4.7 Qualitative Information from the Fractionation Procedure
Besides providing a means of separating the w heat proteins into smaller
more homogeneous units, additional information about the effects of heating
and sample composition was obtained by measuring the pH 's of the
supernatants (Table 4.8)
87
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Table 4.8 pH values during HC1 fractionation procedure.
M oisture Starch: M ethod
Content G luten of
Ratio Heating 1
2
3
4
5
residue
Unheated
5.3
5.1
4.6
4.2*
3.8*
3.7*
CV
5.3
5.0
4.8
4.6
4.1
4.2
MW
5.3
5.0
4.6
4.4
4.2
4.2
CV
5.3
5.1
4.6
4.4
4.1
4.1
MW
5.4
5.3
4.9
4.7
4.3*
4.3*
CV
5.3
4.7
4.0
3.9
3.7
3.7
MW
5.3
4.8
4.0
3.8
3.7
3.6*
CV
5.4
5.2
4.9
4.7
4.2
4.2
MW
5.4
5.2
4.9
4.7
4.3*
4.2*
CV
5.3
5.0
4.4
4.2
3.9
4.0
MW
5.4
5.2
4.6*
4.1*
3.8
3.8*
35%
50%
50%
65%
65%
50:50
50:50
0:100
50:50
0:100
Initial pH of Each Supernatant3
a = average of duplicate experiments; generally the agreement was within 0.1
pH unit, to a maximum of 0.3.
* = some protein precipitated as the pH was increased to 5.8 with 0.1 M NaOH.
From Table 4.8 it is evident that as the extraction procedure progressed, the
buffering capacity of the proteins diminished and the pH of the supernatants
after centrifugation decreased. The dissolved proteins were able to raise the
3.6 pH of 0.6 mM HC1 to a maximum of 5.4. When fewer proteins are
dissolved the pH approaches that of the solvent. These data for VWG are in
agreement with the behavior of gluten by Eliasson and Lundh (1989) and of
88
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Yecora Rojo (a Californian "good quality") flour that was observed by
j
MacRitchie (1991). Both the pH trend of the supernatants and the solubility of
!
the protein in five fractions are the same; this behavior is unique relative to
j
the other five flours that MacRitchie (1991) examined, but was also found by
Eliasson and Lundh (1989). SI had a pH of 5.3 or 5.4 for all 11 samples. The
j
i
other four supernatants and the residue had pH's that differed depending on
j
i
heating method and composition. The 0:100, 65% MW heated sample most
closely resembled the unheated VWG in the pH values and precipitation
behavior. All three 50:50 samples when compared within one heating
method had quite similar pH values. Moisture content does appear to have
an effect at the higher hydration level because for the 0:100 samples, within a
heating method, the 65% moisture fractions have higher pH's than the 50%
fractions.
The presence of starch in the sample appears to have an effect on how the
protein separates. At 50 and 65% moisture, within a heating method, all
supernatants (except SI) had higher pH's if starch was added to the sample
than when it was not. This shows that there is a fairly strong interaction
between starch and gluten that affects the buffering capacity and therefore the
solubility of the protein. Or, the denaturation is affected by the presence of
starch. The starch results in a protein with a higher buffering capacity which
would relate to the charge on the protein. The electrostatic interactions of the
amino a d d of the protein will affect the charge behavior (or pKa). (See Fig.
4.21 for structures of the amino adds and their three letter codes.) The amino
acid that can be ionized are (in decreasing concentration in VWG): glu, asp,
arg, tyr, his, and lys. Glutamic acid composes 30% of VWG and the other
ionizable amino acid are present at less than 5%, so glu is going to be the
89
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!
dominating factor in the charge (and thus the solubility) of the protein.
There is a possibility that all the starch was not removed after the first
centrifugation and that its' presence as it is carried through the extraction
procedure is affecting the behavior of the protein. This topic will be discussed
further in section 4.9.
Heating method appears to have an effect on the pH values. At 35%
moisture there does not seem to be much difference in the behavior of the
proteins between MW and CV samples. When the moisture is increased to
50%, the gluten that has been CV heated has slightly lower pH values than
the MW sample for all supernatants. It is also interesting that the MW
heated S5 and residue had some proteins that precipitated at about pH 5.5 and
5.0, respectively. Although the supernatant pH values are higher for the MW
heated samples than the unheated sample, the precipitation is similar to the
unheated behavior. For the all gluten sample at 50% moisture, there is very
little difference between the CV and MW heated samples, except the
precipitation of the MW heated residue at about pH 5.2. At 65% moisture for
the 50:50 sample, unlike the 50% sample, there is hardly any difference in the
pH values between the CV and MW heated samples, except, the precipitation
of MW heated S5 and residue at pH 5.5 and 4.7, respectively.
90
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Figure 4.21 Structures of the amino adds and their three letter codes
X
I
I
I
I
I
o
O
X
I 0—
0*
r.
91
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The pH differences are a little greater between the CV and MW heated
samples with a composition of 65%, 0:100. The precipitation behavior of the
MW heated sample is unique to all the samples studied because S3 had some
protein that precipitated, as did S4 (pi about 5.0), and the residue (pi about 4.8).
Only the MW heated samples (except 35% moisture) precipitated any protein
which is the behavior exhibited by the unheated gluten.
Another aspect of the fractionation procedure that can be examined is the
physical appearance of the residues after each centrifugation. W hen starch
was present in the sample, it was less dense than the gluten and could be
found on top of the protein in the centrifuge tube. This is contrary to the
results of Mauritzen and Stewart (1965) and Graveland (1980) who found the
starch at the bottom of the centrifuge tube. The reason for this could be due to
the different methods used, with different solvents, which would affect the
hydration of the starch and gluten altering their density. For nearly all the
samples the gluten sediments were compact and beige in color. The
exceptions were all the samples at 65% moisture and the unheated gluten.
After the second centrifugation, the all subsequent residues were not dense
(pudding-like) and could hold a lot of liquid.
The dry powders that resulted after freeze drying of the first fraction was a
fluffy white powder and the fractions became progressively more beige with
successive extractions. SI and residue had the most protein. Other
supernatants contained only 0.06 g or less of protein. According to
MacRitchie (1987a) the early fractions contain mostly gliadins and the latter
fractions the glutenins. However, in another publication (MacRitchie, 1991) it
was shown by electrophoresis that each fraction contains a m ixture of gliadins
and glutenins. This was also found by Eliasson and Lundh (1989).
92
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The residue proteins appeared to have the greatest buffering capacity and
required the largest amount of base to increase the pH to 5.8, which could
only be because there was a large amount of protein in a small volume of
HC1, relative to the supernatants. As the pH was increased some of the
supernatants either became slightly cloudy or white as the conformation of
the proteins changed in the solution to refract light differently. This
phenomena was also found by Bernardin et al. (1967) for a-gliadin. Ewart
(1980) found that the intrinsic viscosity decreased slightly at a higher pH, but
was not dependent on pH in the range 4.1 to about 7 (i.e., with or without
acetic add). Thus, the size of the molecule in solution appears to not effect
the way the solution refracts light. Kasarda et al. (1968) found that a-gliadin
decreased in helix content and ellipticity (as measured with circular
dichroism) when the pH was increased from 3 to 5. This change in shape
could be because the carboxylate ions become protonated in this range
resulting in an unfolding of the polypeptide.
4.8 SDS-PAGE
From the electrophoretic gel of the fractioned unheated gluten (Fig. 4.22)
the dramatic effect of p-mercaptoethanol is evident by the greater number of
bands in the six lanes on the right side of the gel. There are not m any bands
of protein when the disulfide bonds are not broken, which agrees with the
work of MacRitchie (1991). This indicates that the proteins were m ade up of
large proteins held together with disulfide bonds, and when they were broken
the individual units could travel in the electric field and separate.
There was
not a lot of difference in the molecular weight of the proteins between the
fractions.
93
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Figure 4.22 Electrophoregrams of unheated fractions. Lane 7-standards.
Lanes 1-6 - no P-ME, lanes 9-14 - p-ME. Fractions were loaded in lanes as
follows: Sl-1 & 9; S2-2 & 10; S3-3 & 11; S4-4 & 12; S5-5 & 13; R-6 & 14.
94
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14,300
18,400
24,000
34,700
45,000
66,000
97,400
116,000
205,000
90*60
Figure 4.23 Electrophoregrams of fractions from 35% moisture samples that
were heated. CV (lanes 2,4,7,9,11, 13) and MW (lanes 3,5,8,10,12,14).
Lanes 1 & 6-standards. Fractions were loaded in lanes as follows: Sl-2 &
3; S2-4 & 5; S3-7 & 8; S4-9 & 10; S5-11 & 12; R-13 & 14.
95
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Figure 4.24 Electrophoregrams of fractions from 50% moisture samples that
were heated. A) 50:50 and B) 0:100 starch:gluten ratio.
_
—
—
—
14,300
18,400
24,000
34,700
— 45,000
—
66,000
— 97,400
— 116,000
— 205,000
A) Lanes 5 & 12 standards. CV - lanes 1,3,6,8,10,13. MW - lanes 2,4,7,9,11,14.
Fractions were loaded in lanes as follows: Sl-1 & 2; S2-3 & 4; S3-6 & 7;
S4-8 & 9; S5-10 & 11; R-13 & 14.
96
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B) Lane 5 standard. CV - lanes 1,3/6,8,10,12. MW - lanes 2,4,7,9,11,13.
Fractions were loaded in lanes as follows: Sl-1 & 2; S2-3 & 4; S3-6 & 7;
S4-8 & 9; S5-10 & 11; R-12 & 13.
97
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Figure 4.25 Electrophoregrams of fractions from 65% moisture samples that
were heated. A) 50:50 and B) 0:100 starch:gluten ratio.
14,300
18,400
24,000
34,700
45,000
97,400
116,000
205,000
A) Lanes 3 & 8 standards. CV - lanes 1,4,6,9,11,13. MW - lanes 2,5,7,10,12,14.
Fractions were loaded in lanes as follows: Sl-1 & 2; S2-4 & 5; S3-6 & 7;
S4-9 & 10; S5-11 & 12; R-13 & 14.
98
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14,300
18,400
24,000
34,700
45.000
66.000
97,400
116,000
205,000
B) Lanes 5 & 12 standards. CV - lanes 1,3,6,8,10,13. MW - lanes 2,4,7,9,11,14.
Fractions were loaded in lanes as follows: Sl-1 & 2; S2-3 & 4; S3-6 & 7;
S4-8 & 9; S5-10 & 11; R-13 & 14.
99
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Supernatants 1 and 2 have a very prominent band at 45,000 which is more
faint for S3 and gone in the last three fractions. Based on the categorization of
Fullington et al. (1987) this band would be composed mostly of LMW
glutenins, with possibly some a , p, and y gliadins. S2 and S3 have some low
molecular weight components (about 18-20,000) which could be albumins or
globulins. There are a variety of other bands that appear in different fractions
in the 66-97,000 range. This indicates that proteins of the similar molecular
weight will separate into different fractions. The fifth supernatant and
residue lanes have very few bands, even though the same amount of protein
was loaded as all the other lanes. Despite the addition of P-mercaptoethanol,
the proteins were not broken down to be able to separate. Eliasson and Lundh
(1990) found SI and S2 to be similar by SE-HPLC, and this was also found by
this electrophoresis work. The authors also found S3, 4, and 5 to be similar
and that was not observed with PAGE. S3 has bands at about 18,400 and about
97,000 that are not present in S4 or 5.
The addition of P-mercaptoethanol to the sample buffer caused the two
LMW bands in S2,3, 4, and 5 to become one band. Also, the very large
glutenin molecules that did not move from the origin in the absence of pmercaptoethanol were broken into units with a molecular weight > 97,000.
Overall there was more information about the protein composition of a
fraction to be obtained when P-mercaptoethanol was added to the sample and
no information lost by omitting the gels with no P-ME; only those
electrophoregrams with p-ME will be shown. The P-ME did have an effect on
the standards and cause the bands to have an upside down "V" or arrow
shape.
100
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After either CV or MW heating of the 35% moisture sample (Fig. 4.23)
there was a change in the distribution of the gluten among the fractions from
the unheated samples (Fig. 4.22). The effect of heating gluten on its
electrophoresis patterns has been demonstrated for other extraction
procedures (Booth et al., 1980). The residue shows the greatest effect of
heating with many more bands compared with the gel from the unheated
sample. The presence or absence of bands in the lanes shows that each
fraction is composed of the total range of molecular weights and there are
differences between where the proteins were soluble. SI and 2 are very
similar (with SI having more LMW proteins at about 14,000) as are S3, 4, and
5 with the latter having an extra band at 66,000. The residue has different
bands than the supernatants between 40,000 and 70,000.
The gels were loaded such that the supernatant from the CV and MW
heated samples were side by side (CV on the left, MW on the right) to
facilitate comparison of the effect the heating method had on protein
solubilization. At 35% moisture, with starch present, no difference in the way
the proteins fractioned was evident. Denaturation did occur based on the
temperature profiles and the difference in the PAGE results from the
unheated gel. But the heating method did not alter the way the proteins were
denatured as evaluated by SDS-PAGE.
When the moisture was increased to 50% (Fig. 4.24A), the gel shows much
of the same characteristics of the 35% sample; the bands look similar and the
supernatants within a heating method can be grouped the same. An
exception from the grouping is S5 which has a slightly different band pattern
than S2 and 3 in the six bands at about 66,000. Unlike the 35% moisture
samples, at 50% the additional water is resulting in some differences between
101
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the MW and CV heated samples, except the residue. The HMW proteins are
absent or very faint from all the CV supernatant lanes where there are four
bands present in the MW lanes. There is another band at the same distance as
the point in the arrow of the 66,000 standard band that is absent from the CV
heated lanes. At 50% moisture, the MW heating had a different effect on how
the protein denatured, relative to CV heating, so that the proteins were
soluble at different pH's.
When VWG was heated in the absence of starch at 50% moisture (Fig.
4.24B), the separation of the proteins was very different than when starch was
present. S4 and 5 have fewer bands, and the pattern of the bands in the
residue is different. The lanes do not look like the unheated (Fig. 4.22) so
again, the heating affected the solubility of the gluten in HC1. The various
components of gluten are spread throughout all the fractions. Like the 50:50
sample, this 0:100 sample had gluten that denatured differently depending on
the heating method. The HMW bands (about 97,000) are absent from the
lanes of the CV sample of all five supernatants. S2 of the CV heated does
have much more prominent bands between 45,000 and 66,000 than the MW
heated sample. The CV heated S4 and 5 sample has prominent bands at
45,000 which are absent from the MW heated lanes, but the latter does have
some slightly larger molecular weight proteins. The CV S4, S5, and MW S5
lane all have a large protein at about 116,000. The CV residue has more bands
between 45,000 and 66,000 than the MW heated.
At 65% moisture for the 50:50 composition (Fig. 4.25A) the bands are more
clearly defined than the same composition at 50% moisture. The differences
in bands present and the dye intensity shows that each fraction has some
differences in its composition, but all contain part of the total gluten mixture.
102
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The SI fractions are predominantly composed of proteins with a molecular
weight of about 45,000 which is mostly LMW glutenins, with possibly some a,
(}, and y gliadins and some HMW glutenins. With this high m oisture sample
of equal amounts of starch and VWG, the CV and MW heated lanes appear
almost identical. It appears the proteins separated in the same m anner and
thus were denatured the same. There is some difference in the intensity of
the bands from S5 between heating methods. After CV heating, the bands are
much lighter and appear almost absent at < 45,000 and some bands between
66,000 and 97,000. The band pattern is different between the gels of 50 and
65% moisture, indicating that the moisture content does have an effect on
how the protein denatures.
When no starch was present at 65% moisture (Fig. 4.25B), there was greater
difference between the supernatants than when starch was present. There are
also differences between the CV and MW heated samples that did not occur
when starch was present. SI is very similar whether from MW or CV
heating. S2 is very similar also, but the sample from MW heating appears to
have two extra bands at the point of the 66,000 arrow and at 97,000. S3 after
MW heating has two extra bands side by side at the point of the 66,000 arrow
and the bands that are of next smallest molecular weight are m ore prominent
than after CV heating. The MW S4 and S5 fractions have m any more bands
than the CV of all molecular weights from 45,000 to 97,000. The residue is
very similar after both heating methods.
4.9 13C NMR
The solvents used for dissolving the proteins for the 13C NMR
experiments needed to be investigated for solubilizing properties. It was
103
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initially hoped that the HC1 used to separate the proteins could be used for the
NMR experiments since it contained no carbon atoms. But the proteins were
not soluble in HC1 and formed a gel. The addition of SDS to the HC1 (solvent
A) prevented gel formation and dissolved the first three fractions. W ith the
latter fractions, a gel was formed with this solvent, as well as many other
typical gluten solvents and dimethyl sulfoxide. The sample buffer used for
electrophoresis did not cause the proteins to form a gel and was adapted for
this purpose (solvent B). The solvents were run alone to identify their peaks
(Fig. 4.26) In solvent A, the only carbons present were part of the SDS
[SC>4 (CH2 )iiC H 3 ] and the spectra was similar to that of Creamer and
Richardson (1984). In solvent B the Tris-HCl [C(CH20H)3NH3C1] and p-ME
(SHCH2 CH 2 OH) also appear in the spectra between 10 and 70 ppm. It is
possible that the binding of the detergent to the protein may alter the mobility
of the carbon atoms when compared to protein mobility without SDS. The
detergent was necessary for protein solubilization and was present at a
concentration about half that required for complete saturation of the protein
(Helenius and Simons, 1975), permiting carbon atoms to rem ain mobile.
For the most part the lyophilized sample was soluble in the solvents and
resulted in a clear or opaque solution that varied in color from clear to white
and sometimes had a brown hue. The color did not appear to affect the
spectra. In a few instances the samples had other colors. Fractions 4 and 5 of
CV heated 50:50, 35% had a red hue just after vortexing, but turned beige after
a few seconds of rest. This phenomena also occurred at 50% moisture for CV
50:50 (S5), 0:100 (S4, S5), and MW 0:100 (S4, S5, R). Another unusual color
behavior was exhibited by some of the CV heated 0:100 samples: 50% and 65%
S4 and S5 were dark green after the NMR acquisition.
104
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A
Jl UL
50
100
150
PPM
\
JJjl
:
150
|
10 0
i
i
i
i
.
|
50
;
JLj «
i
I
PPM
Figure 4.26 Sample spectra of solvents used for 13C NMR experiments. A)
solvent A and B) solvent B.
105
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4.9.1 Spectral Assignments
To determine if there was any kind of interaction that would alter the
chemical shifts from the published values (Breitmaier and Voelter, 1990)
amino ad d s were examined in the solvents. (See Fig. 4.21 for structures of the
amino acids.) Many of the amino acids were not soluble in either solvent, but
cys, gly, and ser were examined in solvent A; pro was examined in both
solvents. These data indicated that the solvents had no influence on the
chemical shifts. Thus, literature values could be used to aid in identifying the
carbon atoms responsible for the peaks. From the amino acid profiles
obtained for the gluten, the amino acids that w ould be present in greatest
amount with the greatest chance of producing the spectra were known.
For the samples, the solvent peaks were of course largest, but by plotting
the spectra on an expanded scale the sample peaks were enlarged.
Quantitation of peak size, which would provide information on the amount
of substance that was contributing to the signal, was not possible since there
was not always a constant initial concentration. An attempt was made to
always have the same initial concentration, but the nature of the
fractionation and the amount of dry powder required for a run, did not
always make this possible.
Table 4.9 on the following pages provides information on the presence or
absence of non-solvent peaks from the six spectra (one from each fraction) for
each treatm ent and composition. Sample spectra are provided in Appendices
V to VII. In all samples there were three peaks that were present in every
sample and they were not included in the table. The carboxylic acid side chain
of glu appeared at 177.5 ppm and was fairly large and broad indicating that
106
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there were a num ber of different environments that the group was
experiencing. This is expected, as glu is the m ost abundant amino a d d in the
protein. Near the glu peak another was found at about 173.6 ppm, for the
other carboxylic ad d s in the protein. The peak was broader and smaller than
the glu peak. Probably these groups were relatively more immobilized and
are spread out in more diverse environments. The next peak which appeared
in all the samples had a chemical shift at about 129.3 ppm and was assigned to
the ring side chain of phe, with possibly some influence of tyr. There was
another rather rare peak that appeared at 42.9 ppm only in the spectra of
supernatants 1 and 2 of the unheated samples. It is likely that this peak
represents the Ca of gly, Cp of leu, an d /o r Cg of arg.
At this point the rest of the peaks in Table 4.9 will be assigned as best is
possible, then the differences between samples will be discussed. Exact
assignment is somewhat difficult; the assumption was m ade that some
chemical shifts have occurred for the protein relative to the pure amino acid
and carbon atoms within ±2.0 ppm were chosen. Other investigators have
published spectral assignments that do not completely concur (Baianu et al.,
1982; Belton et al., 1987). The amino acid assignments are taken from table
5.25 of Breitmaier and Voelter (1990) and the carbohydrate assignments are
based on those of Mora-Gutierrez and Baianu (1991) for corn syrup solids.
Between 70 and 80 ppm the peaks were too numerous to try and identify each
one.
107
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vA
'W #
~—
i—
i—
|—
i—
i—
i—
i—
|—
i—
i—
i—
|—
i—
i—
i—
150
100
50
PPM
150
100
SO
PPM
Figure 4.27 Examples of sample 13C spectra. A) CV heated 35% moisture SI
and B) 50% 0:100 MW S2.
108
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Table 4.9 Peaks present in 13C spectra from all samples. The residue is
indicated by "R" in the Frac. no. column. A '*+" indicates the peak was
present; a
indicates the peak was not present in the spectra. There were
three peaks, not included in this table, that appeared in all spectra at: 129.3,
173.6, and 177.5 ppm. See text for details of all spectral assignments.
Sample
Unheated
Frac.
no.
1
2
3
4
5
R
35% ,50:50
1
CV
2
27.8
31.9
47.9
53.1
61.5
+
+
+
+
+
91.9
95.7
99.4
+
+
+
+
-
-
-
70-80
+
+
+
+
+
-
-
+
+
+
+
-
-
-
-
+
-
+
+
-
-
-
-
-
+
-
+
+
-
-
-
-
-
+
-
-
+
-
-
-
-
-
+
+
+
+
+ »
+
+
+
+
+
+
+
+
+
+
+
+
+
3
-
+
+
+
+ »
+
+
+
+
4
-
-
+
+
+
+
+
+
+
5
-
-
+
+
+
+
+
+
+
-
-
+
+
+ »
+*
-
-
+ a
+
+
+
+
+ »
+
+
+
+
+
+
+
+
+
+
+
+
+
R
35% ,50:50
1
MW
2
3
+
+
+
+
+ »
+
+
+
+
4
-
-
+
+
+
+
+
+
+
5
-
-
+
+
+
+
+
+
+
-
-
+
+
+ »
+*
-
-
+a
R
" » " = the peak was much larger than the peaks at 53.1 and 47.9.
= the peaks in the 70-80 range were fewer than usual.
"a" = the peak had a chemical shift of 101.2 ppm.
109
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Frac.
no.
26.8
31.9
47.9
53.1
61.5
70-80
91.9
95.7
99.4
50% ,5 0:50
1
-
-
-
+
+ »
+
+
+
+
CV
2
+
-
+
+
+
-
+
+
-
+
S a m p le
3
+
+
+
+
+
+
+
+
+
+
4
-
-
+
+
+
+
-
5
-
-
+
+
+
+
+
+
+
-
+a
R
50% ,50:50
1
MW
2
3
-
-
+
+
+ »
+*
-
+
+
+
+
+ »
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
4
-
-
+
+
+
+
+
+
+
5
-
-
+
+
+
+
+
+
+
R
50% ,0:100
1
CV
2
3
-
-
-
+
+
+*
-
-
+a
+
+
+
+
+ »
+
+
+
+
+
+
+
+
+
+*
-
-
-
+
+
+
+
+
-
-
-
-
4
-
-
+
+
+
-
-
-
-
5
-
-
-
-
+
-
-
-
-
R
-
-
+
+
-
-
-
-
-
+
+
+
+
+
+
+
+
+
50% ,0:100
1
MW
2
+
-
+
+
+
-
-
-
-
3
-
-
-
+
+
-
-
-
-
4
-
-
-
+
+
-
-
-
-
5
-
-
-
+
-
-
-
-
-
-
+
+
-
-
R
-
-
-
-
" » " = the peak was much larger than the peaks at 53.1 and 47.9.
= the peaks in the 70-80 range were fewer than usual.
"a” = the peak had a chemical shift of 101.2 ppm.
110
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Frac.
no.
26.8
31.9
47.9
53.1
61.5
70-80
91.9
95.7
99.4
65% ,50:50
1
+
+
CV
2
44-
4 -»
44444-
44-
44-
44-
44-
-
-
-
-
-
-
-
-
-
-
-
-
4-*
4444-
-
-
4-
+
4-
4*
4-
4444-
4-»
444-
4-*
-
-
4*
4*
4-a
4-
4-
4-
4-
-
-
-
-
-
-
-
-
-
-
-
-
Sample
+
+
3
+
+
4
-
-
5
-
-
4~
4444-
R
-
-
-
+
+
+
4*
444444-
4444-
4444-
+
65% ,50:50
1
MW
2
3
+
+
4
-
-
5
-
-
R
-
-
65% ,0:100
1
+
+
CV
2
+
+
3
+
+
4
-
-
4"
44-
4+
+
44-
4 -»
44 -»
4444 -»
444-
+
4-
Not enough sample to obtain a spectra.
5
R
+
-
-
4-
4-
4-
-
-
-
-
65% ,0:100
1
+
+
2
+
4-
4 -»
44444-
+
MW
444444-
4444-
4444-
4444-
-
-
-
-
-
-
+
+
4
-
-
5
-
-
44444-
R
-
-
-
3
+
444-
" » " = the peak was much larger than the peaks at 53.1 and 47.9.
= the peaks in the 70-80 range were fewer than usual.
"a" = the peak had a chemical shift of 101.2 ppm.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 4.10 Chemical assignments for peaks of fraction spectra.
Chemical Shift (ppm) Chemical Assignment
26.8
31.9
47.9
53.1
60.5
70-80
91.9
95.7
99.4
Cp -arg, -cys, -glu, -his, -lys; Cy-ile, -leu
Cp -val; Cpy -met; Cs -lys
Cs-pro
Ca -ala, -arg, -leu
Ca -ile, -thr, pro, -val; Cp -ser
C2 ,3,4 of oligosaccharides, amylopectin,
pyranose or furanose forms of fructose
a-reducing end of oligosaccharides
P-redudng end of oligosaccharide
Ci amylopectin
4.9.2 Discussion of 13C NMR Data
In the unheated gluten sample the small amount of residual starch is
evident in the spectra (Table 4.9) of the first fraction. All the carbohydrate
must be soluble at pH 5.3 (or bound to proteins that are soluble at that pH)
because it is not evident in the spectra of the subsequent fractions. This result
from a mass of hydrated gluten is contrary to Belton et al. (1987) who did not
find any mobile starch carbons with solution 13C NMR spectrometry . A
reason for this discrepancy may be that there was too much protein signal that
masked the carbon signal, whereas with the fractioning procedure used in
this study the starch became concentrated in the first fraction. There was no
heat applied during the fractionation procedure that would have gelatinized
the starch to produce mobile carbohydrate carbon atoms, so they must be
inherently mobile in the starch granule. For some reason there is very little
112
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mobility in the protein of the residue so not many peaks are evident and the
few that are there are not very large.
At 35% moisture the CV and MW heated samples are very similar, and
show some differences from the unheated spectras. With regard to the
protein, the peak at 61.5 ppm is much larger than its neighbors at 53.1 and 47.9
ppm in SI and S3, and in the other heated fractions (and the unheated
fractions) the three peaks were similar in size. There are five different amino
acids that could be contributing to the signal at 61.5 ppm and it cannot be
definitively said which one has increased in mobility. But, it can be said that
denaturation upon heating, even at low moisture (35%), resulted in some
changes in protein folding that resulted in increased mobility. In the residue,
this peak was quite small, indicating a loss of mobility of that side chain.
Another interesting observation is that carbohydrates are present in all of the
fractions. It appears that there is an interaction between the gluten and the
starch during heating that causes the starch to stay with the protein through
all the pH's of the fractionation procedure. As shown in Table 4.8 this
interaction affected the solubility which resulted in a pH change from the
unheated gluten. The interaction is not so strong as to inhibit the mobility of
the carbon atoms, because they are able to give sharp spectral peaks. In the
residue, the group of peaks between 70 and 80 ppm was reduced in number
relative to the other fractions (see spectra in Appendices V to VII). There was
not the usual group of three peaks between 90 and 100 ppm for carbohydrates
but rather only a single peak slightly upfield at 101.2 ppm. This peak
represents another compound which is possibly the C-l at the non-redudng
end of amylopectin (Mora-Gutierrez and Baianu, 1991).
113
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Contrary to 35% moisture, at 50% for the same composition, there were
some differences between the CV and MW heated samples in both the
protein and carbohydrate peaks. Moisture content is having an effect on the
separation of the components to the greatest extent when the sample was CV
heated. Some peaks were different between the protein part of the spectras for
each heating method in fractions SI and the residue. The carbohydrate
behaved quite different if it was CV or MW heated. There were a num ber of
peaks that did not appear in the CV heated spectra. Thus, the kind of
interaction that was observed at 35% moisture between the protein and the
starch did not occur with CV heating at 50% moisture.
At 50% moisture for the 0:100 samples, the CV and MW heated spectras
are more similar than when the starch was there. There are a few peaks that
are different between the protein portion of the spectra. In the carbohydrate
portion of the spectra, the two heating methods resulted in almost identical
products which were very similar to the unheated sample. The starch that is
inherently present in the VWG is mostly soluble in the first fraction.
For the 50:50 sample at 65% moisture, there was some difference in the
carbohydrate component of the spectra, but little difference in the protein
component, between the CV and MW heated treatments. After CV heating,
all the carbohydrate that could be resolved solubilized in the first two
supernatant fractions, with just a small amount evident in the residue. For
the MW heated sample, carbohydrate was found in all the supernatant
fractions. Another small difference in the protein between the two heating
methods was the greater mobility, or more carbons contributing to the signal
at 61.5 ppm in S3 after MW heating than after CV heating. This higher
moisture content appears to result in different interactions between the starch
114
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and protein after CV heating than were found at 50% moisture. Like SI for
the 35 and 50% samples, the peak at 61.5 ppm was much larger than its two
nearest protein peaks at 53.1 and 47.9 ppm.
For the 0:100, 65% moisture sample there was little difference between the
protein component of the spectras after each heating method. However it
was surprising that the small amount of inherent starch did interact with the
protein and was carried through supernatants 1 to 4 after MW heating only.
There was no difference in the chemical shifts between the MW and CV
heated sample, which indicates that the carbohydrate interacted with the
protein in a region that did not contain mobile carbons that were contributing
to the spectras. Or, because SDS breaks non-covalent bonds it could have
interrupted the protein-carbohydrate interaction when the powder was
solubilized in solvent A or B, thus breaking the interaction and chemical shift
effects could not be observed. One can assume from the scanning electron
micrographs that most of the starch did gelatinize at both moisture contents,
since a few intact granules could be seen. For some reason the starch at the
highest moisture content interacted strongly with the protein and was carried
through the fractions. It is not likely that the starch was charged so that it
could be soluble by itself at the different pH's. Nor would there be
concentration effects on solubility because the amount of starch present was
much greater in the 50:50 (relative to the 0:100) 65% sample in which all the
starch was solubilized in the first two fractions.
There are a couple of hypotheses why the starch was present in four of the
65% 0:100 fractions. Possibly at 50% moisture the water was all absorbed by
the gluten and there was little left for the starch to fully gelatinize and interact
with the protein, but at 65% the event did occur. Another reason could be
115
>
TVy~,irm.,rv,.VIB.,^1
.. .
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
that at 65% moisture there was enough water not absorbed by the gluten that
it could be free in the granule to interact with the microwaves. If the
microwaves were able to focus within the starch granule and cause enough
heat to be generated there to promote gelatinization, the granules may have
expanded and resulted in carbohydrate molecules that interacted with the
protein and were carried through the fractions. At 50% moisture, the
granules may have received their energy conductively from the gluten and
the macromolecules in the starch would not interact with the gluten in the
same way.
In some of the residues, after freeze drying there were distinct white and
beige powders. For the most part they were mixed together as they composed
the total residue. To check the composition of the two entities from the CV
50:50, 50% sample, the white and beige powders were examined separately by
13C NMR. The spectra of the white powder did not contain any of the usual
protein peaks at about 130 or 170 ppm, but did contain the carbohydrate peaks
between 70-80 and 101.2 ppm. The spectra of the beige powder only contained
the protein peaks and no carbohydrate peaks. In this sample, carbohydrate
was soluble in SI and to a more limited extent in the other fractions. But
there was also some starch that was never soluble and was carried through all
the procedure to end in the final residue.
4.10 Selected Amino A dd Profiles
To try and understand some of the different behavior that was observed
with the fractionation, SDS-PAGE, and 13C NMR data, amino acid profiles
were obtained for S5 and the residue of the four 65% heated samples and the
unheated gluten, (with the exception of MW 0:100 S5 for which there was not
116
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enough lyophilized protein for the analysis). These data are given in Table
4.11 which also gives the percent amino acid in the unfractioned unheated
VWG, for comparison. About half of the amino acids are within 0.5% of the
values for gluten found by McMaster and Bushuk (1983) and half are not.
The notable differences are for glu and pro; the VWG has much lower
amount of glu and more pro. The differences are most likely because two
different varieties of wheat from unknown growing conditions are being
compared.
For the most part, the percentage of each amino acid in the fractions is
close to the unfractioned value. Three exceptions are cys and thr which are
generally lower and tyr which is higher in the latter sample than the
fractions. It can be inferred that proteins that contain cys and thr m ust be
soluble in the earlier fractions and those that contain thr are not soluble.
When one examines all the columns, CV, 0:100, S5 has the highest
percentages of glu, phe, and pro which is countered by the sample having the
lowest levels of most of the other amino adds (except cys and ile). Contrarily,
the unheated residue has the highest level of many amino acids (ala, arg, asp,
gly, his, lys, met, thr, and val) and the lowest level of glu and phe.
The amino ad d profiles were only done once so statistical analysis is not
feasible. A difference of 0.5% is being taken as an arbitrary value, for the
purposes of this discussion, as being large enough that it may have some
influence on the protein behavior.
To determine if a particular amino acid had a preference for where it
solubilized a comparison of S5 and residue of one treatment and composition
can be made. In the heated samples there is greater difference between S5 and
residue than there is for the unheated samples. For the sake of clarity on the
117
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table, only the differences between the two samples from the same
fractionation procedure are shown. A comparison of S5 and the residue with
four amino adds finds the same trends for the unheated and heated samples,
which agree with the results of MacRitchie (1987a). There is more glu and pro
and less asp and lys in S5 than the residue. Examination of S5 and residue of
the 50:50 CV, 50:50 MW, or 0:100 CV show a more variation in the
distribution of all the amino acid in the protein. This indicates that the
denaturation at the highest moisture content affects the distribution of the
amino a d d in these two fractions, which implies different protein solubility.
One also wants to compare the same fractions after CV and MW heating to
determine if the carbohydrate binding is correlated to the presence of an
amino acid. This can be done by comparing the residues of the 50:50 samples
with those of the 0:100. The two heating methods have almost no effect on
the amino acid profiles (except for glu) when the carbohydrate is present, but
when there is no carbohydrate, the heating method does influence the amino
acid profiles.
The 13C NMR peak at 47.9 ppm could best be attributed to pro. Because
this peak is present in S5 of CV 50:50, 65% and not the residue, one would
expect that there would be more pro in S5. This is the case based on the
amino a d d profiles of Table 4.11. However, if the same comparison is made
for the MW heated sample, the pro peak is present in both samples, yet the
amino acid profiles indicate that S5 has much more pro than the residue.
That is, the two residues have the same amount of pro, but the carbon atoms
contributing to the signal are much more mobile after MW heating than CV.
Thus, one can conclude that the amount of amino acid in the sample will not
always indicate the presence of the carbon atoms in the spectra.
118
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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 4.11 Percentage of amino acids in select fractions. The heated samples
had been made up to 65% moisture before heating. The "<" and ">" signs
between two columns indicates that those two values are different by 0.5%.
sample
percentage of each amino acid in the fractions
CV- M W CV- M W CVtotal Unhtd Unhtd
50:50
VW G
ala
arg
asp
cys
glu
co
s*y
his
ile
leu
lys
met
phe
pro
ser
thr
tyr
val
50:50
50:50
50:50
CV-
MW-
0:100
0:100
0:100
S -5
residue
S -5
S-5
residue
residue
S -5
residue
residue
3 .92
4.01
< 4 .8 5
3.28
2.92
4.13
4 .2 0
2.48
3.88
< 4 .4 2
2.61
2.71
< 3 .2 0
2.21
2.18
2 .67
2 .5 7
1.70
2.61
2.88
3.19
3.66
< 4 .7 3
2.98
2.76
3.85
3 .8 7
2.38
3.55
< 4 .2 3
1.28
1.14
1.28
1.30
0.95
0.98
1.13
1.35
1.32
3 4.72 < 35.69
3 1.58
> 3 0 .8 3
38.77
3 1.80
> 29.15
< 7 .2 8
0.71
3 3 .3 8
31.29 > 2 8 .1 7
5.95
7.46
7.71
6.33
> 5 .7 4
7.00
7 .16
4.43
6.47
1.96
1.72
1.97
1.52
1.41
1.63
1.81
1.26
1.75
1.90
3.54
3.69
4.02
4.10
3.81
3.80
4.14
3.54
3.95
3 .9 5
7.04
7.47
7.70
6.81
6.67
7.58
7 .6 6
6.51
7.61
7.80
1.40
1.69
< 2 .4 3
1.34
1.15
1.84
1.89
0.94
1.57
< 2 .1 4
1.37
1.44
1.50
1.19
1.06
1.11
1.26
0.58
1.58
1.49
4 .1 9
3.96
3.98
4.71
5.06
4.09
3 .9 7
5.52
4.05
4 .08
15.17 < 16.54
12.63
1 2.68
17.99
13.02
> 12.23
5.60
5.64
5.59
5.29
< 4 .6 6
5.75
5 .6 6
4.55
5.68
5.50
2.76
3.63
3.99
3.17
3.03
3.77
3 .7 6
2.38
3.49
3.72
2.60
2.45
2.17
2.25
2.11
1.73
1.81
1.77
2.31
2.22
< 5 .6 9
4.10
3.90
5 .28
5 .4 0
3 .7 7
5.15
5.43
14.88
4 .5 9
12.48 > 10.86
5.14
4.11 Overall Discussion
From the fractionation, the MW heated samples at 50 or 65% exhibited
precipitation behavior that resembled unheated gluten, whereas the CV
heated samples did not precipitate. These data indicated the gluten interacted
w ith the starch. What affect this interaction has on the protein solubility
remains to be investigated. The fractionation is a sequential process and the
pH of the solution that the protein was soluble in does not predict the
composition of those proteins. The difference in the behavior of the
carbohydrate solubility indicates that starch may also be affected differently by
the heating method in the presence of gluten. This would explain why the
patents to modify cereal products have influenced both the starch and gluten.
As the only sample that exhibited a firmer texture in the MW heated products
were the 100:0 and 80:20 at 50% moisture, it leads one to suspect starch as
being an important component influencing texture. Further research on this
80:20 sample would be of interest to investigate how the starch is affect by
both heating methods.
120
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CHAPTER 5
CONCLUSIONS
A model system of wheat starch and vital wheat gluten at three moisture
contents was examined to determine how the samples were affected by CV
and MW heating. Temperature profiles showed that the temperature was
high enough to allow starch to gelatinize and gluten to denature. The rate of
heating overall was much faster with the MW conditions used than with the
CV conditions. MW heating rate was not dependent on moisture content as
was CV heating. At 50 and 65% moisture, high starch samples heated at a
faster rate in the CV oven than high gluten samples; the samples behaved in
the opposite manner in the MW oven. SEM illustrated the starch
gelatinization that occurred when the samples were heated. Because the
samples were heated in a beaker, there was little surface area for moisture
loss, so the center of the sample remained close to its before heating moisture
content.
Texture is an important aspect of a food which can be related to the force
required to compress a sample. There is a great deal of interest about the
effects of MW heating on food texture. When the model system samples
were extruded between the wires of an OTMS, test cell heating method,
moisture content, and sample composition were all found to be significant
factors in the force that was required. Moisture content appeared to be the
predominant factor. There was an inverse relationship between the force and
the amount of water in the sample, with all samples at 65% moisture having
about the same texture. There was enough water that it dominated over the
effects of the macromolecules , and prevented them from forming a firm
texture, resulting instead in a soft product. At 35 and 50% moisture, as the
121
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amount of gluten in the sample increased, so did the force. The CV heated
samples required more force for compression than the MW heated samples,
except the 100:0 and 80:20 samples at 50% moisture. An 80:20 starch:gluten
ratio sample with a moisture content of 50% is the sample most like a typical
bread dough.
The self-diffusion coefficient measurements found that there were two
main populations of water in a cereal type product. The effects of heating
were evident by the decrease in D value for all compositions, which indicated
an increased binding, and redistribution of the water between the starch and
gluten. The energy from the increased temperature and the macromolecular
transformations allowed the water to move within the sample. Another
interesting conclusion is that there was not much difference between the CV
and MW heated samples in terms of their D values. The types of transitions
that occurred during the heating process that caused the differences in water
binding were not affected by the rate of heating. For all five ratios of
composition, as moisture content increased there was generally an increase in
the k' (polarity of the system) but k" (a measure of the ability of the system to
dissipate energy) was not affected as much by moisture content. The k' and k"
values of the heated samples was dependent on the ratio of starch to gluten,
with the exact effect dependent on moisture content, k' and k" are measuring
two different aspects of the system, that include both water and
macromolecular motion, k" had reached a threshold in the unheated
samples at 35% moisture and the changes that occurred during heating were
complex (an interrelation between composition and heating method) in their
influence on k". The attenuation factor indicated that the two heating
methods did cause different changes in the system in the w ay in which the
122
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sample interacted with the electromagnetic energy. This would be due to
differences in macromolecular motion as well as water behavior.
A group of experiments was performed that examined the gluten in
50:50 and 0:100 samples. First the gluten was fractionated with dilute HC1.
This procedure allowed the proteins to be separated into six groups with
similar solubilities. The pH measurements of each fraction dem onstrated the
effect starch can have on the denaturation of the gluten. When the p H was
increased with the addition of NaOH, some of the proteins in the latter
fractions that had been MW heated (except 35% moisture) precipitated,
behavior that was also demonstrated by the unheated gluten. CV heated
gluten did not precipitate. This is demonstrating that the proteins, w ith water
available, that were MW heated were not altered to as great an extent as the
proteins that were CV heated. SDS-PAGE was useful in characterizing the
protein in the fractions. Each fraction contained a variety of proteins w ith a
wide range of molecular weights. Differences were observed with
electrophoresis between the CV and MW heated samples that was not
necessarily observed with the pH measurements. The proteins that were
soluble at each pH were different after each heating method. Possibly the
conformations of the chains were affected in a way that changed the
solubility. 13C NMR was also used to characterize the same fractions. This
method had the advantage of also being able to provide information on the
carbohydrate component, but was not as sensitive to the effects of
denaturation as PAGE. Differences were observed between the CV and MW
heated samples, especially in the carbohydrate fractionation behavior. Some
kind of interaction between the protein and the carbohydrate occurred with
123
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heating that allowed the carbohydrate to be present in a fraction a n d /o r
affected which fraction a protein was soluble in.
It appears from this research that the difference in texture between the
MW and CV heated samples is due to an interaction between the starch and
gluten, although the protein does denature differently if it is MW or CV
heated. The amount of water that is present is also important. At 35%
moisture there is not enough water to facilitate the changes that occur at 50%
moisture. If the moisture is increased to 65% there is so much water it
appears to be the dominant factor. More research needs to be conducted to
understand the composition of the carbohydrate in VWG, how it interacts
with the gluten, and how starch and gluten interact when heated. Further
characterization of the protein in the fractions would provide information on
the composition and conformation of the proteins in all the fractions to try
and understand what causes the proteins to be soluble at each pH.
124
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jn.f
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Appendix V Representative *C spectra of unheated samples
Supernatant 1
Supernatant 2
i — i— |— i— i— i— i— |— i i
150
10 0
i
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50
i i
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PPM
149
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Supernatant 3
Supernatant 4
150
100
150
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Supernatant 5
Residue
150
100
151
■wff-r
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Appendix VI Representative 13C spectra of conventional heated samples
Supernatant 1
f
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Supernatant 2
—
|—
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i—
i—
i—
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152
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Supernatant 3
Supernatant 4
i — i— |— i— i— i— i— |— :— i— i— i— |— i— i— i
150
100
50
=>P^
153
Reproduced w ith permission of the copyright owner. Further reproduction prohibited without permission.
Supernatant 5
Residue
150
100
50
154
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Appendix VII Representative 13C spectra of microwave heated samples
Supernatant 1
Supernatant 2
15 0
10 0
155
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Supernatant 3
Supernatant 4
iw
hL a .
r*Jt
1--- 1---1--- 1--- 1--- 1--- 1--- 1--- 1--- 1--- 1--- 1 1 I I I
150
100
50
156
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Supernatant 5
Residue
150
100
157
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
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