close

Вход

Забыли?

вход по аккаунту

?

The effect of microwave energy on the structure and function of food hydrocolloids

код для вставкиСкачать
INFORMATION TO USERS
This manuscript has been reproduced from the microfilm master. UMI
films the text directly from the original or copy submitted. Thus, some
thesis and dissertation copies are in typewriter face, while others may
be from any type of computer printer.
The quality of this reproduction is dependent upon the quality of the
copy submitted. Broken or indistinct print, colored or poor quality
illustrations and photographs, print bleedthrough, substandard margins,
and improper alignment can adversely affect reproduction.
In the unlikely event that the author did not send UMI a complete
manuscript and there are missing pages, these will be noted. Also, if
unauthorized copyright material had to be removed, a note will indicate
the deletion.
Oversize materials (e.g., maps, drawings, charts) are reproduced by
sectioning the original, beginning at the upper left-hand corner and
continuing from left to right in equal sections with small overlaps. Each
original is also photographed in one exposure and is included in
reduced form at the back of the book.
Photographs included in the original manuscript have been reproduced
xerographically in this copy. Higher quality 6" x 9" black and white
photographic prints are available for any photographs or illustrations
appearing in this copy for an additional charge. Contact UMI directly
to order.
University Microfilms International
A Bell & Howell Information Company
300 North Zeeb Road, Ann Arbor, Ml 48106-1346 USA
31 3/76 1-4 700
800/521-0600
R eproduced with permission of the copyright owner. Further reproduction prohibited without permission.
R e p ro d u c e d w ith p erm is sio n o f th e co p yrig h t o w n e r. F u rth e r re p ro d u ctio n pro hib ited w ith o u t p erm is sio n .
O rder N u m b er 9211204
The effect of microwave energy on the structure and function of
food hydrocolloids
Prakash, Anuradha, Ph.D.
The Ohio State University, 1991
UMI
300 N. Zeeb Rd.
Ann Arbor, MI 48106
R eproduced with permission of the copyright owner. Further reproduction prohibited without permission.
R e p ro d u c e d w ith p erm is sio n o f th e co p yrig h t o w n e r. F u rth e r re p ro d u ctio n pro hib ited w ith o u t p erm is sio n .
THE EFFECT OF MICROWAVE ENERGY ON THE STRUCTURE
AND FUNCTION OF FOOD HYDROCOLLOIDS
DISSERTATION
Presented in Partial Fulfillment of the Requirements for
the Degree Doctor-of Philosophy in the Graduate
School of The Ohio State University
By
Anuradha Prakash, M.S.
The Ohio State University
1991
Dissertation Committee:
pproved h
P.M.T. Hansen
M.E. Mangino
G.W. Chism
J. Lindamood
Food Science and Nutrition
R eproduced with permission of the copyright owner. Further reproduction prohibited without permission.
To My Parents
ii
R eproduced with permission of the copyright owner. Further reproduction prohibited without permission.
ACKNOWLEDGEMENTS
I would like to express my sincere gratitude to Dr. P.M.T. Hansen for his
guidance throughout this study. His instruction and enthusiasm have made
my graduate career a rewarding experience.
I am grateful to Dr. M.E. Mangino for his counsel and assistance at every
moment of need; to Dr. G.W. Chism for his suggestions and faith in my
ablities to conduct this study; and to Dr. Lindamood for his guidance during
the course of this research. Special thanks are due to Dr. S. Sastry for his
expert advice; to Dr. J. Mitchell for helping me with electron microscopy; to
Ann for preparing my samples even at short notice; and to Brian Heskitt for
his assistance with the Luxtron measurements.
I gratefully acknowledge Miles Laboratories, especially Dr. Fogg-Johnson,
for funding this research. I wish to offer my sincere thanks to Dr. S.O.Nelson
and his staff members at USDA, Athens, GA for use of their facilities to obtain
the dielectric measurements essential to my research and for providing me
with an understanding of what they mean.
Many thanks are due to Mary Renoll for procuring much needed library
material, and to Tracey and Judy for their assistance in fulfilling all the
paperwork that are requisites for attending Ohio State. I would like to
specially thank Karen for being a wonderful friend and counsel.
iii
R eproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Most importantly, I wish to express my love and gratitude to my best friend,
Jim and the rest of my family- Kittu, Puchika, Sniffles, Regus and Kasha for
their unquestioning love and understanding.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
VITA
October 2 9 ,1 9 6 6 ................................................ Born - Bombay, India
1987.......................................................................B.S., Nirmala Niketan,
Bombay, India
1989
M.S. in Food Science and
Nutrition, The Ohio State
University, Columbus, Ohio
1991- Present...................................................... Graduate Research Associate,
Department of Food Science
and Nutrition, The Ohio
State University
Columbus, Ohio
PUBLICATIONS
Prakash A., Joseph M. and M.E. Mangino. 1990. The effects of added proteins
on the functionality of gum arabic in soft drink emulsion systems. Food
Hydrocolloids, 4(3): 177-184.
Nelson S.O., Prakash A. and K.C. Lawrence. 1991. Moisture and temperature
dependence of the permittivities of some hydrocolloids at 2.45 GHz. J. Micro.
Power Elec. Energy, 26(3): 178-185.
FIELD OF STUDY
Major Field:
Food Science and Technology
V
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
TABLE OF CONTENTS
DEDICATION......................................................................................................... ii
ACKNOWLEDGEMENTS....................................................................................... iii
VITA.........................................................................................................................v
TABLE OF CONTENTS.........................................................................................vi
LIST OF TABLES................................................................................................... viii
LIST OF FIGURES................................................................................................. ix
LIST OF PLATES................................................................................................... x
INTRODUCTION.................................................................................................... 1
WHAT ARE MICROWAVES........................................................................1
MICROWAVE FOOD PRODUCTS............................................................. 2
OBJECTIVES................................................................................................ 3
REFERENCES............................................................................................ 6
CHAPTER
PAGE
I. MICROWAVE HEATING OF FOOD
THE MICROWAVE OVEN............................................................................ 7
THE MECHANISM OF HEATING BY MICROWAVE ENERGY................8
MICROWAVE INTERACTIONS WITH FOODS..........................................13
PROCESSING IN THE FOOD INDUSTRY.................................................. 16
SAFETY........................................................................................................ 19
REFERENCES.............................................................................................20
II. DIELECTRIC PROPERTIES OF FOOD HYDROCOLLOIDS.................... 22
INTRODUCTION..........................................................................................22
DIELECTRIC PROPERTIES.........................................................................23
DEPENDENCY OF DIELECTRIC PROPERTIES......................................26
DIELECTRIC PROPERTIES OF LOW MOISTURE FOODS.................... 28
MATERIALS................................................................................................. 29
METHODS................................................................................................... 32
RESULTS...................................................................................................... 37
DISCUSSION................................................................................................ 60
REFERENCES....................................................
64
III. THE EFFECT OF MICROWAVE IRRADIATION ON THE
STRUCTURE OF CARRAGEENAN-MILK GELS......................................67
INTRODUCTION..........................................................................................67
OBJECTIVES................................................................................................ 72
MATERIALS................................................................................................. 72
METHODS................................................................................................... 73
GEL PREPARATION......................................................................... 73
vi
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
HEAT TREATMENT......................................................................... 73
PREPARATION FOR MICROSCOPY.............................................74
GEL STRENGTH...............................................................................74
RESULTS......................................................................................................75
DISCUSSION................................................................................................85
REFERENCES............................................................................................ 89
IV. THE EFFECT OF MICROWAVE ENERGY ON POTATO
STARCH.......................................................................................................91
INTRODUCTION......................................................................................... 91
GELATINIZATION............................................................................ 92
A COMPARISON OF THE EFFECTS OF MICROWAVE
COOKING VERSUS CONVENTIONAL COOKING ON
STARCH FUNCTIONALITY............................................................ 93
OBJECTIVES................................................................................................95
I. PROTOCOL TO DETERMINE THE PARTICLE SIZE
DISTRIBUTION OF POTATO STARCH GRANULES...............................96
METHOD.......................................................................................... 96
RESULTS.......................................................................................... 99
DISCUSSION.....................................................................................104
II. A COMPARISON OF THE GELATINIZATION PATTERN OF
POTATO STARCH BY CONVENTIONAL AND MICROWAVE
HEATING......................................................................................................105
METHOD.......................................................................................... 105
RESULTS...........................................................................................105
DISCUSSION.....................................................................................124
III. THE EFFECT OF MICROWAVE HEATING ON DRY
STARCH USING TIME, TEMPERATURE AND WATER
ACTIVITY AS PARAMETERS.....................................................................126
METHOD.......................................................................................... 126
RESULTS...........................................................................................126
DISCUSSION.....................................................................................132
REFERENCES............................................................................................ 142
SUMMARY AND CONCLUSIONS........................................................................143
BIBLIOGRAPHY...................................................................................................... 149
vii
R eproduced with permission of the copyright owner. Further reproduction prohibited without permission.
LIST OF TABLES
TABLE
PAGE
1.
Charge, density data and equilibrium moisture
contents of hydrocolloids conditioned for microwave
permittivity measurements over saturated salt solutions
at 25°C........................................................................................... 31
2.
Values for dielectric constants and loss factors at various
moisture and temperature levels (DC-Dielectric constant;
LF- Loss Factor; SK- carrageenan; PS- potato starch;
LBG- gum arabic; GA- gum arabic; and CMC
carboxymethylcellulose)................................................................. 39
3.
Values for dielectric constants and loss factors predicted
by the model equations using moisture content, charge
and temperature as parameters..................................................... 44
4.
Correlation coefficients, r2, obtained from the plots of the
observed vs. calculated values for the dielectric constants
(DC) and loss factors (LF) for each hydrocolloid....................... 57
5.
Comparison of the calculated and counted number
of potato starch granules in 14 samples......................................100
6.
Cummulative particle analysis table for potato starch
granules counted manually from 14 samples...........................102
7.
Cummulative particle analysis table for potato
starch granules calculated from 14 samples............................ 103
8.
Maximum temperature achieved by potato starch
samples heated for varying times by microwave irradiation.... 133
9.
Estimated water activity at which maximum temperature
was recorded for potato starch samples heated for
varying amounts of time in a microwave oven.........................139
R eproduced with permission of the copyright owner. Further reproduction prohibited without permission.
LIST OF FIGURES
FIGURE
PAGE
1.
Basic components of a microwave oven....................................... 9
2.
(a) Random dipole orientation in an unchanged dielectric
capacitor (b) Dipole ordering by a constant electric field,
E applied (Tinga and Nelson, 1973)............................................. 10
3.
Hydrogen bonding of water molecules in a tetrahedral
configuration (Dashed lines represent hydrogen bonds)
12
4.
Generalized physicochemical model for the
dielectric behavior of foods. (Mudgett, 1985)........................... 14
5.
Illustration of intuitive graphical technique to
correct for drying of sample during permittivity
measurements at high temperatures......................................... 36
6.
Moisture dependence of the dielectric properties
of powdered hydrocolloids.........................
51
7.
Goodness of fit between observed and calculated
values for dielectric properties....................................................58
8.
Effect of moisture(%) and stoichiometric charge-to-mass
ratio(moles/kg) on dielectric properties at 25°C ....................... 62
9.
Instron profile plots for carrageenan-milk gels.............................84
10.
Diagrammatic view of the arrangement of potato
starch granules at the apex of the water droplet as
observed under a polarizing light microscope............................ 97
11.
Time-temperature plots for regular starch heated
in a microwave oven................................................................. 127
12.
Plots of water activity versus maximum temperature
achieved for potato starch samples heated in a microwave
oven............................................................................................134
ix
R eproduced with permission of the copyright owner. Further reproduction prohibited without permission.
LIST OF PLATES
PLATES
PAGE
I.
Photomicrograph of untreated 0.6% carrageenan-milk gel
II.
Photomicrograph of 0.6% carrageenan-milk gel
heated for 10 min at 70°C in a water bath.................................77
III.
Photomicrograph of 0.6% carrageenan-milk gel
heated for 20 min at 70°C in a water bath.................................78
IV.
Photomicrograph of 0.6% carrageenan-milk gel
heated for 30 min at 70°C in a water bath.................................79
V.
Photomicrograph of 0.6% carrageenan-milk gel
heated for 10 min at 70°C in a microwave oven......................... 80
VI.
Photomicrograph of 0.6% carrageenan-milk gel
heated for 20 min at 70°C in a microwave oven......................... 81
VII.
Photomicrograph of 0.6% carrageenan-milk gel
heated for 30 min at 70°C in a microwave oven......................... 82
VIII.
Photomicrograph of 0.6% carrageenan-milk gel
prepared using microwave heat and subjected
to no additional heat treatment..................................................83
IX.
Arrangement of potato starch granules at the apex
of a water droplet as viewed under a polarizing
light microscope subjected to no heat treatment.....................106
X.
Potato starch granules heated to 63°C on a.. Kofler stage..... 107
XI.
Potato starch granules heated to 64°C on a Kofler stage..... 108
XII.
Potato starch granules heated to 65°C on a Kofler stage..... 109
XIII.
Potato starch granules heated to 67°C on a Kofler stage..... 110
XIV.
Potato starch granules heated to 68°C on a Kofler stage..... 111
X
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
76
XV.
Potato starch granules heated to 70°C on a Kofler stage....... 112
XVI.
Potato starch granulesheated to 71 °C on a Kofler stage....... 113
XVII.
Potato starch granules heated to 72°C on a Kofler stage....... 114
XVIII.
Potato starch granules heated to 73°C on a Kofler stage....... 115
XIX.
Arrangement of potato starch granules at the apex
of a water droplet as viewed under a polarizing
light microscope subjected to no heat treatment.....................116
XX.
Potato starch granules heated for 15 seconds
in the microwave oven..................................................................117
XXI.
Potato starch granules heated for 15 + 15
seconds in the microwave oven................................................118
XXII.
Potato starch granules heated for 15 + 15 +15
seconds in the microwave oven................................................119
XXIII. Potato starch granules heated for 15 + 15 + 15 +15
seconds in the microwave oven................................................120
XXIV. Potato starch granules heated for 15 + 15 +15 +15 +15
seconds in the microwave oven................................................121
XXV.
Potato starch granules heated for 15 + 15 + 15 +15 +15
+15 seconds in the microwave oven........................................ 122
XXVI. Potato starch granules heated for 15 + 15 + 15 +15 +15
+15+15 seconds in the microwave oven..................................... 123
xi
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
INTRODUCTION
This dissertation is concerned with determining the effects of microwave
energy on the structural and functional properties of food hydrocolloids. The
rapid popularization of microwave appliances for food preparation has lead
to the generation of a new product line - microwaveable foods. Of concern
to food scientists is the development and production of quality
microwaveable foods and this research has been directed toward exploring
the effects of microwave irradiation on aspects of food texture.
WHAT ARE MICROWAVES
Microwaves are electromagnetic waves in the frequency range 300 to
300,000 MHz (megahertz) that corresponds to 1m to 1 mm in wavelength.
At these wavelengths, the components of conventional electronic circuits
tend to behave like individual antenna, dissipating their electrical signals as
radiation (Fuller, 1979). The high frequency of microwaves allow them to be
used for communication purposes. Microwave radiation can penetrate fog
and clouds, travel in straight lines and give distinct shadows and reflections
so that they can be used for distance and direction measurement and in
radar systems. Microwaves are vital for communication purposes because
they can pass through the ionosphere which reflects low frequency
radiation. For this reason the use of microwave frequencies is controlled by
l
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
governmental regulations. In the USA, the Federal Communications
Commission regulates the use of microwave frequencies for industrial,
scientific, medical, communication and navigational purposes (Vasavada,
1990). Microwave energy is absorbed by water and certain other materials
so that they can be used for drying, heating, curing of glues, resins, and
plastics, vulcanization of rubber, sterilizing, sintering ceramics, and hightemperature superconducting materials, synthesis of organic compounds,
acid digestion or decomposition for trace-element analysis and treatment of
tissue in pathology laboratories (Kok and Boon, 1990). Microwave ovens for
home use are operated at 2450 MHz in batch mode whereas, industrial
ovens use 915 MHz in batch and continuous modes.
MICROWAVE FOOD PRODUCTS
Numerous advances have been made in the production of microwavable
foods (Whorton and Reineccius, 1989) which include product types such as
entrees, soups, snacks, vegetables, breakfast foods and bakery goods.
However, most of these products have come into the market only in the last
few years. Whorton and Reineccius offer an explanation as to the sudden
invasion of microwavable products on the market when microwave ovens
have been in kitchens since the sixties. The authors reasoned that the
percentage of households owning a microwave oven reached the 10%
saturation level sometime between 1973 and 1982. Only then did the food
companies acknowledge the significance of the growing new market and
deem it worthy of research dollars (Schiffman, 1989). At the same time, the
technology of microwave ovens was advancing, and the price of consumer
units was decreasing. The combined effect led to a 50% increase in
microwave oven ownership in the six year period between 1982 and 1988
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
and spurred food companies into a frantic effort to develop microwavable
products. In 1988, microwave foods made up more than 10% of all new food
products introduced in the market. This percentage is expected to increase,
indicating a growing demand for microwavable foods.
However, even as consumers demand increased quality and convenience
with microwave products, the flavor and textural qualities have not been
comparable to conventionally prepared food products. Problems inherent to
microwave cooking such as uneven heating, hot and cold spots, lack of
browning and crisping, and sogginess have proved to be major deterrents to
consumer acceptance of microwave foods. So far, there has been a
reluctance by the food industry to focus on fundamental research in the area
of microwave-food technology. There exists among food scientists a basic
lack of understanding of electromagnetic heating and the interactions of
microwaves with food materials.
As the market for microwavable food products keeps growing, it is becoming
increasingly important for the food industry to invest in research that will help
understand how food materials behave under the influence of microwave
irradiation. Dielectric properties of food ingredients, water binding and
moisture migration, flavor development and retention, and physicochemical
properties of foods are areas that merit more investigation.
OBJECTIVES
This research project has been concerned with measuring the effect of
microwave energy on the structural and functional properties of common
food hydrocolloids (food gums). Hydrocolloids are polymeric substances
R eproduced with permission of the copyright owner. Further reproduction prohibited without permission.
with a strong affinity for water. They include a range of polysaccharides and
certain proteins which have functional value as thickening, stabilizing and
gelling agents. Hydrocolloid stabilizers attain their functional properties
through strong interactions between the surfaces of their macromolecules
with solvents and other components of the food system. Thus, preservation
of their large molecular size is important for retention and maximization of
the functional properties of these colloids. Because hydrocolloids possess a
very large molecular size, they are exceptionally vulnerable to the effect of
heat and mechanical energy. This research is designed to provide an
understanding of the effects of microwave heating on the structural integrity
of simple food systems and to accumulate data on the dielectric properties of
functional components which are related to the physical-chemical stability of
manufactured foods. Changes in the molecular conformation, and the
determination of the extent of polymer interaction and/or degradation during
microwave processing will help dilineate the benefits and constraints of this
under-utilized technology. With these factors in mind, the objectives of this
study are:
a. To obtain dielectric data for functional food components used in the
construction of food systems;
b. To relate the dielectric properties of food components to their heating
characteristics in the microwave oven;
c. To determine by rheological means and electron microscopy if microwave
energy alters the strength and structure of food gels prepared from
hydrocolloids;
d. To study the gelatinization pattern of individual potato starch grains
exposed to microwave irradiation as compared to conventional
heating; and
R eproduced with permission of the copyright owner. Further reproduction prohibited without permission.
e. To determine the heating rates of dry starch in a microwave oven as a
function of water activity.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
REFERENCES
Baden Fuller A.J. 1979. Introduction. In " Microwaves - An Introduction to
Microwave Theory and Techniques”, 2nd edition, Pergamon Press,
New York.
Fennema O.R. 1976. Water and ice. In "Principles of Food Science Part 1:
Food Chemistry", O.R. Fennema (Ed.). Marcel Dekker, Inc., New York.
I.R.P.A. 1988. Guidelines on limits of exposure to radiofrequency
electromagnetic fields in the frequency range from 100 KHz to 300
GHz. Health Physics, 54:115-123.
Kok L.P. and M.E. Boon. 1990. Microwaves for microscopy. J. of
Microscopy, 158(3): 291-322.
Schiffman R.F. 1989. Food product safety problems due to microwave
heating. Paper presented at MW Foods '89, 2nd International
Conference on Formulating Food for the Microwave Oven, March 1415. The Packaging group Inc., Milltown, NJ.
VasavadaP.C. 1990. Microwave Processing for the dairy industry. Food
Australia 42 (12): 562-564.
Whorton B.C. and G.A. Reineccius. 1989. Flavor development in microwave
vs. conventionally baked cake. In " Thermal Generation of Aromas,"
T.H. Parliament, R.J. McGorrin and C.T. Ho (Eds.). ACS Symposium
Series 409, American Chemical Society, Washington D.C.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
CHAPTER I
MICROWAVE HEATING OF FOOD
THE MICROWAVE OVEN
Figure 1 is a diagram of a simple microwave oven (Schiffmann, 1989).
Electrical energy is drawn from a wall socket through the power cord at 110
to 220 volts, 60 hertz, to the power supply (6). Here the electrical energy is
converted to DC and stepped up to thousands of volts, which excite the
magnetron (3) to produce microwaves at 915 or 2450 MHz depending upon
the microwave. The microwaves travel down the waveguide (4) which
delivers them to the cavity (1). The food that has to be heated is placed
inside the cavity. An optional mode stirrer (5) serves to distribute the
microwaves uniformly throughout the microwave oven so as to prevent hot
and cold spots during heating. Once inside the oven, waves can be reflected
from the oven sides and floor, transmitted through containers made of glass,
paper and plastic, or absorbed by a medium such as food.
MICROWAVE HEATING VERSUS CONVENTIONAL HEATING
Microwaves heat food material by interacting with regions of positive and
negative charges on water molecules(electrical dipoles) that rotate the
molecules in the electrical field by forces of attraction and repulsion between
oppositely charged regions of the field and the dipoles. This causes
molecular friction which generates heat. Positive and negative ions of
dissolved salts in foods also interact with the electrical field by migrating
7
R eproduced with permission of the copyright owner. Further reproduction prohibited without permission.
toward oppositely charged regions of the electrical field and disrupt
hydrogen bonds with water to generate additional heat. In contrast,
conventional cooking utilizes the heated air of a conventional oven to
increase the surface temperature of the food, driving off the moisture and
resulting in browning and crisping. Heat penetration through the food
occurs by conductive and convective heating depending on the composition
of the food (Brain and Zallie, 1990).
THE MECHANISM OF HEATING BY MICROWAVE ENERGY
The heating of foods by microwave energy is accomplished both by the
absorption of microwave energy by the water molecules, and the release of
this energy as heat, and by the heat conductivity losses, due to ionic
components of the food (Ohlsson, 1989). Thus, both the water content and
the dissolved ion content (often salt) are dominating factors in the microwave
heating of food.
The water molecule is a dipole - it has a positive and a negative end. When
the dipole is subjected to a microwave field that rapidly changes directions,
the dipole attempts to align itself with the changing electrical field (Fig. 2).
To do this, the water molecule needs to overcome inertia and intermolecular
forces in the water. The electrical field thus provides energy for the water
molecule to rotate into alignment. The energy is then lost to the random
thermal motion of the water, when the water molecule realigns in response
to the changing electrical field. This energy is equivalent to the temperature
increase.
R eproduced with permission of the copyright owner. Further reproduction prohibited without permission.
.1..
mm
'
I " t " l
VI
- o
Figurel. Basic components of a microwave oven
R eproduced with permission of the copyright owner. Further reproduction prohibited without permission.
10
(a )
(b )
Figure 2. (a) Random dipole orientation in an unchanged dielectric
capacitor (b) Dipole ordering by a constant electric field, E applied. (Tinga
and Nelson, 1973)
R eproduced with permission of the copyright owner. Further reproduction prohibited without permission.
In addition to the individual molecules, each water molecule can hydrogen
bond with a maximum of four other water molecules (Fennema, 1976). The
resulting tetrahedral arrangement is shown in Figure 3. In the attempts to
align with the electrical field, the hydrogen bonds between the water
molecules are disrupted, utilizing energy from the field. When a new
hydrogen bond is being formed, energy is distributed to the random thermal
movement, increasing the temperature (Walker, 1987). At higher
temperatures, hydrogen bonds become sparse and the thermal movements
are more intense, therefore the energy required for overcoming
intermolecular bonds is less. The dielectric heating of water molecules thus
decreases with increasing temperature.
Hydrated ions, such as sodium and chloride from table salt, try to move in
the direction of the electrical field. The ions are surrounded by water
molecules. In their movement, the ions will transfer, randomly, energy from
this movement to the water molecules. At higher temperatures, the ions are
more mobile and not so tightly bound to the water molecules and can move
more freely and, thus, dissipate more energy. The conductive heating due to
dissolved ions increases with increasing temperature. Microwave
penetration depths within any given product are determined by its electrical
and physical properties and can vary significantly with chemical composition
and temperature of the product, and the processing frequency (Mudgett,
1989).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
A-V"
/
—
Figure 3. Hydrogen bonding of water molecules in a tetrahedral
configuration (Dashed lines represent hydrogen bonds).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
The electrical and physical properties of foods which determine microwave
penetration depth, conventional heat transfer, and overall heating rate are
the dielectric constant and loss factor, heat capacity (specific heat), and
density. The dielectric constant of a product reflects its ability to store
electrical energy, and the loss factor its ability to disssipate electrical energy
as heat. The greater the polarization of the material, the greater will be the
dielectric constant, and the quantity of energy that can be stored in the
material (Tinga and Nelson, 1973). As frequency increases, the total
polarization decreases because of inertia and other effects and hence
dielectric constant decreases with increasing frequency. As a result, the
energy imparted to to the material by the applied field is not used completely
to orient the dipoles, some energy is lost to to the material by increasing the
random thermal motion that exists in every substance. Therefore as
dielectric constant decreases, the dielectric loss factor increases.
MICROWAVE INTERACTIONS WITH FOODS
Figure 4 depicts a generalized physicochemical model for microwave-food
interactions as proposed by Mudgett (1974). According to the author,
microwaves interact with the chemical constituents of intermediate and high
R eproduced with permission of the copyright owner. Further reproduction prohibited without permission.
14
i-----1
z
o
o
Irro to lio n o f
u
H indered
<
u.
QC
3
</>
1
B o u n d W oler
B o u n d S o ils
Free S u rfa ce Charge
------------ -----------------------------------------------------------------------------------------------------
Z
o
<No
Free W o le r
^ solub^
C xclu d e d Volum e
I
.
Free
S a lts
D is s o lv e d Sugars
|c o H P O ^
@@
B ound S a ils
pH E ffe c ts
____ I
Figure 4. Generalized physicochemical model for the dielectric behavior of
foods. (Mudgett, 1985)
R eproduced with permission of the copyright owner. Further reproduction prohibited without permission.
moisture foods by several mechanisms. The authors explanation of these
mechanisms based on the above model is quoted below:
Proteins, lipids, and regions near colloidal surfaces are
relatively inert and do not interact significantly with
microwaves. Proteins are partly soluble and partly
insoluble, with ionizable surface regions that may tightly
bind water or salts to give rise to zeta potentials and
double layer effects associated with free surface charge
(Mudgett and Westphal, 1989). Lipids are also
hydrophobic except for ionizable carboxyl groups of fatty
acid residues and do not interact appreciably with
microwaves in the presence of aqueous ions, because
these are much more interactive and seldomly absorb
microwave energy. There are some direct interactions
with colloidal food constituents at frequencies below the
microwave region, but the bulk aqueous regions of
foods are the major sites for microwave interaction at
intermediate and high moistures. However, there are
probably indirect interactions between hydrocolloids
and water also affecting microwave behavior at
microwave frequencies in ways that are not yet clear.
The principle mechanisms of interaction in multilayer
and capillary regions of moist foods are: (1) rotation of
water molecules, i.e., dipoles with centers of positive
charge on hydrogen atoms and negative charge on
oxygen atoms, and (2) conductive migration of ions, i.e.,
dissolved salts, in an electrical field that reverses its
direction billions of times each second, to disipate
energy. The rotation of tightly bound water dipoles in
monolayer regions is sterically hindered in rotation, as
seen in Fig. 12.1. Tightly bound water also represents a
small fraction of the total water molecules in
intermediate and high moisture products based on
typical bound water levels in foods ranging from 5-10
grams of bound water/100 grams of dry solids (Karel,
1975). Some of the free water molecules are also
bound by the dissolved salts in hydration sheaths. At
lower moisture contents, the salts become more
concentrated and precipitate as their concentrations
exceed saturation levels, thus limiting ionic
conductivities.
Microwaves also interact with alcohols and alcoholic
moieties of dissolved sugars and polysaccharides by
dipole rotation. Alcohols, such as ethanol, are pure
polar solvents whose molecules are hydrogen bonded
in the liquid state and which are freely miscible with
R eproduced with permission of the copyright owner. Further reproduction prohibited without permission.
water. Sugars and noncellulosic polysaccharides are
more or less soluble in water to an extent that may
depend on the number of free aldehydic and hydroxyl
groups in solution available for hydrogen bonding. Low
alcohol or sugar levels in aqueous mixtures do not
modify the interactions of microwaves with water and
dissolved ions to any great extent, but at higher
concentrations do alter the frequency response of water
in such mixtures. For example, frequencies at which
microwaves show maximum interaction in ethanol-water
mixtures are those of pure water and ethanol (Buck,
1965). This is attributed to hydroxyl-water interactions
that stabilize hydrogen bonding of free water. Similar
effects have been shown for high concentrations of
sugars and starch in water, but appear to be negligible
at low concentrations (Roebuck et al., 1972).
Surprisingly, although microwaves do not appear to
interact with lipids and colloidal solids in intermediate or
high moisture foods, they interact strongluy with these
constituents in the absence of moisture as evidenced by
calorimetric measurements of olive oil and bone-dry
food solids and also by ignition and/or charring of food
solids by microwaves following moisture removal.
Calorimetric measurements of olive oil and bone-dry
food solidsshow levels of energy absorption that cannot
be due to free water and ion activity. While the
mechanistic basis for energy absorption in these
materials is not calesr, microwave interactions are
known to result from rotational modes, rather than
vibrational, electronic, or nuclear modes of interaction
characteristic at lower frequencies, and are related to
permanent and induced dipole moments (Pomeranz
and Melaon, 1987). Perhaps the major difficulty in
accounting for these interactions in a quantitative way is
that it is not yet possible to estimate net dipole moments
in complex food mixtures.
PROCESSING IN THE FOOD INDUSTRY
The commercial installations for industrial microwave processing of food
products started on both sides of the Atlantic, in the sixties (Ohlsson, 1989).
Microwaves are often combined with conventional heating sources for food
processing. Microwaves are utilized for internal heating whereas
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
17
conventional sources are used for browning and crisping, to obtain lower
surface microbial counts, or to accelerate drying.
According to Decareau (1985), there are six major classes of possible
microwave processing: tempering, dehydration, blanching, cooking,
pasteurization and sterilization.
Tempering is a term used for partial thawing of frozen foods to an ice content
level at which the material is still firm but well suited for further processing,
such as slicing, grinding, etc (Ohlsson, 1989).
Dehydration: The first large industrial use of microwaves was in the finishdrying of potato chips. This application solved problems of excessive
browning during the finish drying. However, the method was abandoned
after some years because of unexpected differences in drying rates and final
moisture contents. Meanwhile, the handling of the potatoes during storage
was improved, and high quality chips could be produced by vacuum
dehydration. Microwave drying of pasta is a successful microwave process
that continues today (Decareau, 1985). The process consists of three steps:
hot-air pre-drying, hot-air and microwave drying which utilizes microwaves
to drive the moisture in the interior to the previously dried outer surface, and
equalization. In Europe there are a number of installations for finish drying
and expansion of cereal products and powders. Hamid and Boulanger
(1969) reported on the use of microwaves in controlling insect infestation
and moisture content in grain. Decareau (1985) listed the advantages of
microwave drying compared to hot-air drying of grains as increased
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
germination capability of the grain, low temperature, uniform drying,
decreased fuel usage and increased speed.
Blanching using microwave equipment is costly compared to conventional
steam blanchers (Knutson et al., 1987). However, enzyme inactivation by
microwave heating may offer the advantage that heat-sensitive nutrients and
flavor coumpounds are preserved.
Microwaves have several applications for cooking and precooking of foods
especially poultry and bacon. Precooking of these foods is done before the
food is frozen for later use as convenience and institutional foods (Knutson
et al., 1987). Microwave cooking of bacon is combined with steam and/or
hot air. Processes such as microwave rendering of fat, cooking of bakery
products, precooking of meat emulsions have been shown to have
advantages over conventional processes.
According to Ohlsson (1989), pasteurization of packaged foods is done in an
increasing number of installations in Europe and Japan. In Europe,
products such as sliced rye bread are pasteurized to prevent mold growth
and in Japan, cakes and special products are pasteurized for the same
purpose. Microwave pasteurization of prepared ready-to-serve foods to be
distributed chilled is being introduced in European markets. The Swedish
Multitherm system utilizes water as a surrounding medium during microwave
heating to avoid non-uniform heating, whereas in more conventional
systems, microwave pasteurization is preceded by preheating by traditional
means and hot filling, which is combined with a small temperature increase
in the subsequent microwave pasteurization.
R eproduced with permission of the copyright owner. Further reproduction prohibited without permission.
19
According to Knutson et al, 1987, sterilization of food in plastic pouches is
the industrial microwave process with the greatest potential. In 1977,
Kenyon et al, working at the U.S. Army Natick Laboratories, investigated the
feasibility of microwave sterilization of military rations. They reported that
microwave sterilization was a feasible process. In a later article (Ayoub et al,
1974), they reported optimization of time/temperature processing
parameters, packaging and product quality.
SAFETY
The human body is an excellent absorber of microwaves. When tissue is
exposed to microwave irradiation, it will absorb the microwaves and
generate heat inside the body. If the rate of heating is sufficiently high, the
physiological cooling mechanisms of the body will be unable to maintain
normal temperatures leading to heat stress or burns. Safety guidelines for
microwave emission from home and industrial equipment were established
after World War II. In 1953 it was recommended that exposure of human
tissue to microwave energy should be limited to a maximum of 100W/m2
(ten times less than bright sunlight). The American National Standards
Institute (A.N.S.I.) adopted a standard along this line in 1966. Stricter
standards were established in 1982. In 1988 the International Radiation
Protection Agency standard was issued (I.R.P.A., 1988) which is similar to
the 1988 A.N.S.I. standard. (Kok and Boon, 1990). Microwaves are non­
ionizing rays and do not lead to radioactive contamination of microwaved
materials. However, precautions should be taken against some common
hazards of microwave oven use in the home such as undisclosed hot spots,
R eproduced with permission of the copyright owner. Further reproduction prohibited without permission.
20
generation of steam, boilover, volcanic eruption from superheating, frying in
oil, and fires (Schiffman, 1989a).
REFERENCES
Ayoub J.A., D. Berkowitz, E.M. Kenyon and C.K. Wadsworth. 1974.
Continuous microwave sterilization of meat in flexible pouches. J. of
Food Sci., 39: 309-313.
Brain S.M. and J.P. Zallie. 1990. Role and function of starches in
microwaveable food formulation. Food Aust. 42(11): 523.
BuckD.E. 1965. The dielectric spectra of ethanol-water mixtures in the
microwave region. Ph.D. thesis, Massachusetts Institute of
Technology, Cambridge, MA.
Decareau R.V. 1985. "Microwaves in the Food Processing Industry."
Academic Press, New York.
Hamid M.A.K. and R.J. Boulanger. 1969. A new method for the control of
moisture and insect infestations of grain by microwave power. J. of
Microwave Power, 4:11-18.
Fennema O.R. 1976. Water and ice. In "Principles of Food Science Part 1:
Food Chemistry", O.R. Fennema (Ed.). Marcel Dekker, Inc., New York.
I.R.P.A. 1988. Guidelines on limits of exposure to radiofrequency
electromagnetic fields in the frequency range from 100 KHz to 300
GHz. Health Physics, 54:115-123.
Karel M. 1975. Physicochemical modification of the state of water in foods.
In "Water Relations in Foods," R.B. Duckworth (Ed.), Academic Press,
New York.
Kenyon E.M., D.E. Westcott, P. LaCasse, and J.W. Gould. 1971. A system
for continuous thermal processing of food pouches using microwave
energy. J. of Food Sci., 36: 289-293.
Knutson K.M., E.H. Marth and M.K. Wagner. 1987. Microwave heating of
food. LBT, 20:101-110.
Kok L.P. and M.E. Boon. 1990. Microwaves for microscopy. J. of
Microscopy,158(3): 291-322.
R eproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Mudgett R.E. 1974. A physical chemical basis for prediction of dielectric
properties in foods at ultra-high and microwave frequencies. Ph.D.
Thesis, Massachusetts Institute of technology, Cambridge, MA.
Mudgett R.E. 1985. Dielectric properties of food. In "Microwaves in the
Food Processing Industry," by R.V. Decareau. Academic Press, Inc.,
New York.
Mudgett R.E. 1989. Microwave food processing. Food Tech. 43(1): 117126.
Mudgett R.E. and W.B. Westphal. 1989. Dielectric behavior of an aqueous
cation exchanger. J. Micr. Pwr. E. E. 24:33-37.
Ohlsson T. 1989. Dielectric properties and microwave processing. In"
Food Properties and Computer-Aided Engineering of Food
Processing Systems," R.P.Singh and A.G.Medina (Eds.). Kluver
Academic Publishers, pp. 73-92.
Pomeranz Y. and C.E. Melaon. 1987. Food Analysis: Theory and Practice,
2nd ed. Van Reinhold Nostrand Co., New York.
Roebuck B.D., S.A. Goldblith and W.B. Westphal. 1972. Dielectric
properties of carbohydrate-water mixtures at microwave frequencies.
J. Food Sci. 37:199-204.
Schiffman R.F. 1989. Food product safety problems due to microwave
heating. Paper presented at MW Foods '89, 2nd International
Conference on Formulating Food for the Microwave Oven, March 1415. The Packaging group Inc., Milltown, NJ.
Tinga W.R. and S.O. Nelson. 1973. Dielectric properties of materials for
microwave processing-tabulated. J. of Microwave Power, 8(1): 23-28.
Walker J. 1987. The secret of a microwave oven's rapid cooking action is
disclosed. Scientific American. Febr 98-102.
R eproduced with permission of the copyright owner. Further reproduction prohibited without permission.
CHAPTER II
DIELECTRIC PROPERTIES OF FOOD HYDROCOLLOIDS
INTRODUCTION
In microwave heating, the dielectric properties of foods are important for the
interpretation of the influence of microwave energy on the temperature
distribution in the food material (Ohlsson, 1989). Knowledge of the dielectric
properties of food materials is essential for proper understanding of the
heating pattern during microwave heating of foods, both in the cooking of
foods and in reheating of precooked foods from frozen or refrigerated
conditions (Bengtsson and Risman, 1971). Such knowledge is also
important for the development and formulation of foods for microwave
cooking and for the design of microwave equipment used to process and
cook foods.
Data are now available on the dielectric properties of diverse foods,
including meats, fruits and vegetables, grains, fats and carbohydrates
(Bengtsson and Risman, 1971, Tinga and Nelson, 1973). However,
dielectric data are lacking on individual functional food ingredients, such as
hydrocolloids. Hydrocolloids include a range of polysaccharides and certain
proteins which have functional value as stabilizing, thickening and gelling
22
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
23
agents. They are used in many processed foods for control of stability and
textural attributes.
The mechanism of microwave heating differs significantly from that of
conventional heating. Microwave heating could affect the structural and
functional properties of hydrocolloids, which, in turn, would alter the quality
attributes of the food product.
DIELECTRIC PROPERTIES
Dielectrics are a class of substances that are capable of supporting electrical
fields. These substances can be thought of as insulating materials or poor
conductors. They contain few available ions that are capable of transporting
electrical energy. However, they can support electrical fields by the
polarization of their molecules.
This polarization occurs in four different ways:
1) electronic polarization
2) atomic polarization
3) dipole orientation polarization
4) interfacial (or space charge) polarization
Electronic polarization is caused by the slight displacement of the electrons
in individual atoms or molecules due to irradiation at frequencies in the
optical region (e.g., visible light). The molecule or atom experiences
displacement due to the difference in mass of the negative electrons and the
positive nuclei.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Atomic polarization occurs when the asymmetric charge distribution of a
molecule is distorted by an applied electrical field. Most molecules have
asymmetric charge distributions because the electrons tend to localize in the
area of the chemical bond.
Both, electronic and atomic polarizations, are common to all dielectrics. The
resulting displacements will cause an induced dipole moment in the
material. A dipole moment is created when a dielectric molecule is placed in
an electric field. The molecule tends to align itself in a manner such that its
positive center is toward the negative electrode and its negative center is
toward the positive electrode.
The third type of polarization is caused by the dipolar orientation of the
atoms of a molecule. The structure of a polar molecule is such that it has a
permanent dipole moment. When it is placed between two capacitor plates,
the charges on the dielectric are displaced so that the positive charges are
displaced toward the negative electrode and the negative charges are
displaced toward the positive electrode.
The fourth type of polarization is different from the others in that it is not
caused by the displacements of molecules, electrons, or charges. Interfacial
migration is caused by migration of charge carriers in the dielectric due to
electrical conduction (all substances conduct some electricity). The charge
carriers are restricted by the components of the dielectric, and space
charges build up which can cause polarization in nonhomogeneous
dielectrics.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
The electromagnetic field effects of basic interest in the microwave region
are those related to orientation and ionic polarization (Mudgett, 1985). They
lead to instantaneous heat generation within the product due to "molecular
friction," primarily by the disruption of weak hydrogen bonds associated with
the dipole rotation of free water molecules and electrophoretic migration of
dissociated salts, that is, dissolved ions in an electrical field of rapidly
changing polarity (Mudgett, 1985).
Biological materials, in general, may be viewed as nonideal capacitors.
They have the ability to store and dissipate electerical energy from an
electromagnetic field through a set of electrical properties expressed in
complex notation and characterized as "dielectric permittivity," a complex
electrical property with a real component, dielectric constant, and an
imaginary component, dielectric loss. Such materials do not interact with the
magnetic component of the field because of their low magnetic permeability.
The dielectric properties of a material can be described by a complex
dielectric constant e = e' - je" (Tinga and Nelson, 1973). The real part, e',
directly influences the amount of energy that can be stored in a material in
the form of electric fields, whereas, the imaginary part, e", loss factor, is a
direct measure of how much energy a material can dissipate in the form of
heat. The ratio e'Ve' is called the loss tangent.
When a dielectric material first experiences an electric field, the time
required for the material to completely align itself with the field is defined as
the relaxation time. If a dielectric is placed within an alternating electric field,
complete polarization will occur as long as the frequency is lower than the
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
time required for relaxation to occur. As the frequency increases, the
material is no longer able to completely reverse directions before the electric
field itself reverses. Each dipole is a physical entity having mass and inertia
and may be subject to other restraints depending upon the type of material.
In other words, the relaxation time for one of the polarization processes is
exceeded. The applied field forces the molecules away from an equilibrium
position and the energy taken up by the dipoles is retained as stored
potential energy while the field remains. If the field is removed, the dipoles
relax to a new equilibrium position, and energy is released as random
kinetic energy which is returned to the system in the form of heat.
The greater the polarization of the material, the greater will be the dielectric
constant, and the quantity of energy that can be stored in the material. As
frequency increases, it seems reasonable to expect the total polarization to
decrease because of inertia and other effects and hence e' decreases with
increasing frequency. As a result the energy imparted to the material by the
applied field is not used completely to orient the dipoles, some energy is lost
to the material by increasing the random thermal motion that exists in every
substance. Therefore as e' decreases, e" should increase.
DEPENDENCY OF DIELECTRIC PROPERTIES
Frequency
As the frequency increases, the relaxation times begin to be exceeded and
the amount of polarization decreases. The dielectric constant, which is
related to the amount of polarization also decreases. The amount of
decrease is dependent upon the relationship between the various types of
dipole moments.
R eproduced with permission of the copyright owner. Further reproduction prohibited without permission.
27
Moisture content
As the moisture content of a dielectric substance increases, the dielectric
constant will also increase. This is caused by the additional water
molecules because they are dipolar and have a dielectric constant of
approximately 80, which is considerably larger than that of most food
materials. Because of the large difference, a small change in moisture
content will produce a substantial change in the capacitance of a circuit.
This is the basis of capacitive-type moisture meters currently in use.
Density
For most dielectric materials, an increase in the bulk density of a substance
will produce an increase in the dielectric constant due to the increase in the
amount of dielectric material for a constant volume.
Temperature
At high temperatures, the thermal agitation of the molecules tends to
interfere with polarization which causes a decrease in the dielectric
constant. As the temperature drops, the dielectric constant tends to increase
due to diminishing thermal excitation. Temperature might also have an
effect on the viscosity of the fluid. At low viscosities, the dielectric constant to
decreases due to the additional resistive friction produced by the lower
viscosities on the dipole moments.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
DIELECTRIC PROPERTIES OF LOW MOISTURE FOODS
Cited below is an explanation of the dielectric behavior of low moisture
foods as presented by Mudgett (1985).
The possibility of synergistic loss behavior is seen for
food products with high levels of dissolved sugars (e.g.,
syrups or high alcohol content). Water in foods may be
in free or bound states depending on the product's
moisture content; these states are classified by sorption
isotherm measurements at constant water activities or
equilibrium relative humidities as monolayer, multilayer,
and capillary condensation regions (Labuza, 1968,
1980; Labuza et al., 1968). For example, surface
regions of dissolved proteins bind water at various
energy levels in a surface monolayer, as irrotational or
hindered rotational forms that depend on the number of
hydrogen bonds between the molecule of bound water
and reactive surface sites. Water is also bound in
hydration sheaths or layers by the counterions of
dissolved salts due to coulumbic interactions and by
dissolved carbohydrates through hydrogen bonding
between partially charged regions of water molecules
and sterically available hydroxyl substituents. Free salt
concentrations may be limited by saturation effects in the
aqueous regions (e.g., sparingly soluble salts) or by
binding of salts by dissolved proteins in surface regions
subject to ionization by acid-base equilibria described
by the Henderson-Hasselbach equations (Lehninger,
1970). These effects may result in salts binding
particularly for polyvalent ions or in free surface charge
depending on pH levels reflected by hydrogen and
hydroxyl ion concentrations. Aqueous regions are
generally seen to be electroneutral with counterion
distributions resulting from surface charge effects
described by Stern-Gouy double-layer concepts and
expressed electrokinetically as zeta potentials (Sennett
and Olivier, 1965). These are seen to have little effect
on dielectric behavior at microwave frequencies per se,
since dipole and ionic losses in aqueous and surface
regions appear to be determined primarily by free-water
and ion concentrations based on free-water relaxation in
terms of dipole rotation and ionic conductivity in terms of
free salt concentrations. However, at frequencies on the
order of 106 to 108 Hz, free surface charge effects may
result in relaxation of dissolved proteins to give twisting,
bending, and rotation of molecular segments in some
R eproduced with permission of the copyright owner. Further reproduction prohibited without permission.
native or denatured conformational states depending
upon the ionic environment (Hasted, 1961).
de Loor and Meijboom (1966) and Kent and Jason (1975) have suggested
that free-water relaxation and ionic conductivity are the major determinants
of food dielectric behavior at microwave frequencies. Swami and Mudgett
(1981) confirmed this by comparing a model prediction based on this
hypothesis with experimental measurements for a variety of semisolid food
products (Nelson, 1973; To et al., 1974; Ohlsson and Bengtsson, 1975).
Mudgett et al. (1977), developed model predictions from a simplified linear
model for homogeneous two-phase mixtures of aqueous ions and colloidal
solids,
km* = ks*vs + kc*vc
(1)
where:
km*
Complex permittivity of mixture, nondimensional
ks*
Complex permittivity of suspended phase, nondimensional
kc*
Complex permittivity of continuous phase, nondimensional
vs
Volume fraction of suspended phase in mixture, nondimensional
vc
Volume fraction of continuous phase in mixture, nondimensional
based on measurement of moisture contents for several meats, fruits, and
vegetables, and conductivity measurements of fluids extracted from these
products following denaturation of tissue membranes by microwave heating.
MATERIALS
The hydrocolloids used were potato starch (SX 920, (Matheson, Coleman
and Bell), gum arabic (Spray dried arabic CS), locust bean gum
(Germantown Manufacturing Company), carboxymethylcellulose (7HF,
R eproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Aqualon Company) and carrageenan (Seakem CM 514, FMC Corporation),
all in the powdered form. Locust bean gum is a neutral polysaccharide,
potato starch is slightly charged containing .06 to .1% esterified phosphorus
(Waters, 1961). Carrageenan is a strongly acidic polysaccharide due to the
presence of sulphate half esters, whereas gum arabic and CMC are
moderately acidic polysaccharides containing carboxyl groups.
The charge
for carrageenan was expressed as the number of moles of NaS0 3 ‘ per
kilogram of carrageenan calculated from the ester sulfate content as
measured by the method of Hansen and Whitney (1960), modified for atomic
absorption. The charge on CMC was calculated by the method of Zadow
and Hill (1975) from the sodium carboxylate content as determined by the
ASTM method (1988). The charge on gum arabic was calculated from
reported values of the amount of uronic acid anhydride present in the gum,
assuming that all acidity arises from the uronic acid groups. Throughout this
document, charge refers to the stoichiometric charge-to-mass ratio and not
to the effective charge on the polymer. The values for the charge on each
hydrocolloid in the anhydrous state are presented in Table 1.
The samples were conditioned to different moisture levels to determine the
influence of moisture content on the dielectric properties at 2.45 GHz
(gigahertz). The dielectric properties of the samples at different moisture
contents were determined at temperatures from 20 to 100°C at intervals of
ten degrees.
R eproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Reproduced
with
permission
of the copyright ow ner.
Table 1. Density data and equilibrium moisture contents of hydrocolloids conditioned for microwave
permittivity measurements over saturated salt solutions at 25°C
Further reproduction
Hydrocolloid
prohibited
without p erm is sio n .
Potato Starch
Stoichiometric
charge-to-mass
ratio
kg/mole
0.03
Density
Bulk
Solid
density
density
cyCm3
g/cm3
0.80
1.472
Locust bean gum
0.00
0.70
1.479
9.7
11.9
14.1
19.9
Gum arabic
0.82
0.60
1.014
10.6
12.0
14.5
22.7
Carrageenan
2.60
0.75
1.662
10.7
11.9
15.6
25.0
Carboxymethyl
cellulose
3.51
0.60
1.557
11.0
13.1
18.9
32.2
Equilibrium moisture content, %
Potassium Sodium
Sodium Ammonium
acetate
iodide
bromide
sulfate
22.5% RH 38.2% RH 58.5% RH 80.9% RH
11.1
9.6
13.6
19.2
32
METHODS
The water activities of the hydrocolloids were adjusted by placing them in
open jars in stainless steel chambers containing saturated salt
solutionsmaintaining relative humidities of 20, 40, 60 and 80% for two weeks
(Greenspan, 1977). The chambers were equipped with small fans to
provide air circulation. The temperature was maintained at 25°C. The
moisture contents of the samples were determined gravimetrically after
drying triplicate samples in a forced-air oven for one hr at 130°C. Samples
and moisture dishes were cooled in a dessicator before being reweighed.
The reported moisture contents are on a wet basis (Table 1). The dried
samples were saved and their dielectric properties were determined to
obtain additional low moisture level values.
The short-circuited waveguide principle was used to determine the dielectric
properties of the hydrocolloids. This method was first reported by Roberts
and von Hippel (1946). A sample of unknown permittivity is placed against a
short-circuit termination at the end of the waveguide. If the frequency and
the length of the sample are known, the only measurements required for the
determination of e'rand e"r are the standing wave ratios in the waveguide
with the sample in place and with the sample removed, and the positions of
the voltage standing-wave minima (nodes) under these two conditions. The
shift of the node position due to the presence of the sample depends mainly
upon £'r, the relative dielectric constant, and the change in the standingwave ratio depends mainly upon e"r, the relative dielectric loss factor.
Measurements at 2.4 GHz were taken using a 15 cm long short circuit
specimen container connected to a Rohde and Schwarz Type LMC Non-
R eproduced with permission of the copyright owner. Further reproduction prohibited without permission.
33
Slotted Line fed by a Type SLRC Power Signal Generator, and the UBK
indicator was connected to the non-slotted line detector output. These
systems employ precision coaxial components with a characteristic
impedance of 50 ohms. The inner conductor is 9.12 mm in diameter, and
the outer conductor has an inside diameter of 21.00 mm.
A sample-height gage machined from Kel-F to fit the sample holder was
used to measure the sample length. The gage was provided with a scale
calibrated to read sample height directly when the height gage was inserted
directly into the sample holder and placed in contact with the sample
surface. This gage also served to provide a plane top surface for the
granular sample perpendicular to the axis of the coaxial sample holder. The
current standing waves observations are employed with the LMC non­
slotted line. Measurements of the location and width of a standing-wave
mode (voltage standing-wave minimum or current standing-wave maximum)
are made at 3-db, or other known power level, down from the peak, with the
empty short-circuit sample holder connected to the line and then, also, with
the sample in place against the short-circuit termination. The shift of node
location and the change in the node width due to the presence of the sample
are used, along with the sample length and frequency (or guide
wavelength), to perform the calculation of dielectric properties.
For measurements at room temperature (24°C), a Rohde and Schwarz 15cm deep Short-Circuit Specimen Container was used as the sample holder.
For measurements at temperatures from 20 -100 °C, a Rohde and Schwarz
50-mm temperature controlled sample holder (21-mm short-circuited coaxial
air line) was used. The temperature was adjusted to within +/- 0.1 °C by a
R eproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Haake FK Constant Temperature Circulator which pumped a water and
coolant solution through a water jacket around the sample holder. A
thermometer was used to determine the temperature of the sample holder.
Since the size of the sample in the sample holder was very small, the
difference in temperature between the sample and the sample holder was
negligible.
The location of the air node and the node width measurement was taken for
the empty sample holder at the beginning of a series of measurements and
used in all calculations for that series. The need to take air node readings
for each sample measurement was not justified because the frequency of
the power oscillator was very stable and the temperature in the laboratory
was relatively constant and because the air node readings were verified at
the end of each measurement series.
The dielectric properties of particulate materials are highly dependent on
bulk density, i.e., the density of the air-particle mixture. Bulk densities were
determined from sample weight before introduction into the sample holder
and the sample volume calculated from the cross-sectional area of the
sample holder and sample length from the height gage.
The powdered samples were carefully introduced into the sample holder
using a glass funnel and a camel's hair brush to avoid losing any material.
The sample holder was tapped lightly on its base to settle the sample and
the height was measured by inserting the height gage into the sample
holder till its base reached the surface of the sample. The height gage was
removed, the sample holder was connected to the Nonslotted line, and the
R eproduced with permission of the copyright owner. Further reproduction prohibited without permission.
sample node reading was obtained. The room-temperature sample holder
was used to determine the density dependence behavior of the dielectric
properties by measuring the sample nodes for each sample at three different
densities. The different bulk densities were obtained by settling the sample
in the sample holder between measurements.
For measurement of the dielectric properties in the temperature-controlled
sample holder, the air-node readings of the empty sample holder were taken
over the range from 20 to 100°C and found to remain constant. The sample
was placed in the sample holder and sample node readings taken at ten
degree intervals from 20 to 100°C. Although the sample holder remained
closed for each series of temperature measurements, some drying of the
sample could be expected especially at the high temperatures. The
moisture content of the sample before it was introduced into the sample
holder was measured. After the sample node reading at 100°C was made,
the sample was allowed to cool in the sample holder to room temperature,
the sample was removed and used to make another moisture content
determination. Based on the initial and final moisture contents, an intuitive
graphical technique was used to determine the moisture content at each
temperature level (Figure 5).
Because of the difficulty in achieving exactly the same sample bulk density
for each of the measurements on a given hydrocolloid at different moisture
levels, the permittivities were corrected to a common bulk density with the
relationship described previously (Nelson, 1984)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
e2 = ((e11/3-1)p2 + 1 )3
(2 )36
Pi
where e1 and e2the complex permittivities of the air-particle mixture at bulk
densities pi and p2 . respectively.
IS
•
14
13
12
20
40
60
80
100
TEMPERATURE. *C
Figure 5. Illustration of intuitive graphical technique to correct for drying of
sample during permittivity measurements at high temperatures
The validity of this relationship for pulverized materials has been
demonstrated for both the real part of the complex permittivity (dielectric
constant), e\ (Nelson, 1983) and for the complex permittivity as well (Nelson
and You, 1990). It is based on the linear relationship of the cube root of the
R eproduced with permission of the copyright owner. Further reproduction prohibited without permission.
complex permittivity with bulk density, which is consistent with the Landau
and Lifshitz, Looyenga dielectric mixture equation (Nelson and You, 1990)
Once the values for e' and e" were obtained for a common bulk density by
complex algebraic computation from equation (2) for each moisture content
and each temperature level, these values for each of the colloids were
plotted as functions of moisture content, and a second-order polynomial
regression was performed to fit curves to the data.
The calculated values for charge on the hydrocolloids were adjusted to
account for the moisture content using the relationship:
Adjusted charge = Base charge (100-Moisture content!
(3)
100
where, base charge is the charge on the anhydrous material.
Values for dielectric constant and loss factor of all the hydrocolloids were
subjected to stepwise multiple regression analysis and empirical models for
dielectric constant and loss factor were derived.
RESULTS
The equilibrium moisture contents of the five hydrocolloid materials
conditioned over saturated salt solutions are listed in Table 1 in the order of
their affinity for moisture absorption. Also shown in Table 1 are the bulk
densities to which dielectric properties of the powdered samples were
adjusted and the densities of the solid materials determined by air-
R eproduced with permission of the copyright owner. Further reproduction prohibited without permission.
comparison pycnometer measurements at moisture contents of about 11 or
12%. The measured dielectric constants and loss factors for the samples at
the various moisture and temperature levels are presented in Table 2.
These values have been presented as functions of moisture content at
temperatures from 20 to 100 °C in Fig 6. The dielectric constant and the loss
factor increased with moisture content, although the behavior of the loss
factor was less regular. Dielectric constants of all the materials at 20°C were
around 2 when very dry (about 1% moisture) and increased to around 3 in
the 20-25% moisture range. Loss factors at 20°C for the very dry materials
were all about 0.1, and their rate of increase with moisture content was
different for all the hydrocolloids. Loss characteristics of the five powdered
hydrocolloids did not differ greatly at corresponding moisture contents, but
the locust bean gum and carrageenan tended to have somewhat greater
loss factors at high temperatures and high moisture contents. Except for the
locust bean gum and potato starch, the dielectric constant and loss factor
exhibited very little temperature dependence when very dry. The variation in
the permittivity values with temperature increased as moisture content
increased for all of the materials. However, the increase in temperature
dependence with increasing moisture content was much smaller for potato
R eproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 2. Values for dielectric constants and loss factors at various moisture
and temperature levels (DC-Dielectric constant; LF- Loss Factor; SKcarrageenan; PS- potato starch; LBG- gum arabic; GA- gum arabic; and
CMC- carboxymethylcellulose)
SAMPLE
SK
SK
SK
SK
SK
SK
SK
SK
SK
SK
SK
SK
SK
SK
SK
SK
SK
SK
SK
SK
SK
SK
SK
SK
SK
SK
SK
SK
SK
SK
SK
SK
SK
SK
SK
SK
SK
SK
SK
SK
SK
SK
SK
SK
SK
REL. HUM.
80
60
40
20
0
80
60
40
20
0
80
60
40
20
0
80
60
40
20
0
80
60
40
20
0
80
60
40
20
0
80
60
40
20
0
80
60
40
20
0
80
60
40
20
0
TEMP
100
100
100
100
100
90
90
90
90
90
80
80
80
80
80
70
70
70
70
70
60
60
60
60
60
50
50
50
50
50
40
40
40
40
40
30
30
30
30
30
20
20
20
20
20
MOIST
D.C.
L.F.
22.19
14.61
11.33
9.99
1.68
23.50
15.13
11.60
10.27
1.68
23.96
15.33
11.75
10.44
1.68
24.28
15.45
11.84
10.52
1.68
24.50
15.52
11.89
10.58
1.68
24.67
15.56
11.91
10.61
1.68
24.80
15.58
11.93
10.63
1.68
24.90
15.60
11.94
10.64
1.68
24.97
15.60
11.94
10.65
1.68
5.1379
3.4853
2.7847
2.7811
2.1815
5.0933
3.4741
2.7771
2.7552
2.1687
5.0098
3.3950
2.7684
2.7152
2.1544
4.9651
3.2057
2.7174
2.6775
2.1359
4.7166
3.1154
2.6447
2.6005
2.1231
4.5221
2.9942
2.5581
2.5441
2.1074
4.2592
2.9016
2.4920
2.4855
2.0946
4.0536
2.8116
2.4390
2.4400
2.0832
3.9323
2.7501
2.4054
2.4096
2.0761
1.7302
0.6546
0.2302
0.5005
0.0729
1.6926
0.6470
0.2259
0.4492
0.0700
1.4200
0.5613
0.2280
0.3729
0.0657
1.1777
0.4151
0.2073
0.2391
0.0627
1.0092
0.3660
0.1855
0.1998
0.0599
0.9109
0.3119
0.1615
0.1715
0.0626
0.8203
0.2829
0.1440
0.1540
0.0569
0.7641
0.2663
0.1310
0.1464
0.0540
0.7487
0.2512
0.1266
0.1431
0.0469
R eproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 2 (continued)
SAMPLE
PS
PS
PS
PS
PS
PS
PS
PS
PS
PS
PS
PS
PS
PS
PS
PS
PS
PS
PS
PS
PS
PS
PS
PS
PS
PS
PS
PS
PS
PS
PS
PS
PS
PS
PS
PS
PS
PS
PS
PS
PS
PS
PS
PS
PS
REL. HUM.
80
60
40
20
0
80
60
40
20
0
80
60
40
20
0
80
60
40
20
0
80
60
40
20
0
80
60
40
20
0
80
60
40
20
0
80
60
40
20
0
80
60
40
20
0
TEMP
100
100
100
100
100
90
90
90
90
90
80
80
80
80
80
70
70
70
70
70
60
60
60
60
60
50
50
50
50
50
40
40
40
40
40
30
30
30
30
30
20
20
20
20
20
MOIST
D.C.
L.F.
18.38
12.47
10.41
8.93
0.66
18.75
12.98
10.72
9.20
0.66
18.93
13.20
10.88
9.35
0.66
19.03
13.34
10.97
9.45
0.66
19.09
13.44
11.03
9.50
0.66
19.13
13.51
11.06
9.53
0.66
19.15
13.56
11.07
9.55
0.66
19.15
13.60
11.08
9.55
0.66
19.15
13.64
11.08
9.55
0.66
5.1449
3.8642
3.5699
3.3809
2.4057
5.0839
3.7703
3.5438
3.3106
2.3939
4.8321
3.6849
3.4495
3.2027
2.3767
4.6468
3.5989
3.3101
3.0938
2.3476
4.3692
3.4367
3.1646
2.9606
2.3049
4.0974
3.2526
3.0332
2.8719
2.2667
3.8384
3.0339
2.9099
2.7393
2.1821
3.6362
2.9426
2.8146
2.6396
2.1520
3.5059
2.8735
2.7865
2.5695
2.1365
1.0344
0.8616
0.5648
0.4886
0.1959
1.0469
0.8571
0.5618
0.4871
0.1996
1.0280
0.8505
0.5437
0.4618
0.2014
1.0993
0.6996
0.5276
0.4311
0.1852
1.0468
0.6918
0.4935
0.3915
0.1617
0.9409
0.6292
0.4454
0.3552
0.1427
0.8318
0.5960
0.4022
0.3154
0.1156
0.7280
0.5453
0.3601
0.2736
0.1029
0.7018
0.4600
0.3460
0.2506
0.0947
R eproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 2 (continued)
SAMPLE
LBG
LBG
LBG
LBG
LBG
LBG
LBG
LBG
LBG
LBG
LBG
LBG
LBG
LBG
LBG
LBG
LBG
LBG
LBG
LBG
LBG
LBG
LBG
LBG
LBG
LBG
LBG
LBG
LBG
LBG
LBG
LBG
LBG
LBG
LBG
LBG
LBG
LBG
LBG
LBG
LBG
LBG
LBG
LBG
REL. HUM.
80
60
40
20
0
80
60
40
0
80
60
40
20
0
80
60
40
20
0
80
60
40
20
0
80
60
40
20
0
80
60
40
20
0
80
60
40
20
0
80
60
40
20
0
TEMP
100
100
100
100
100
90
90
90
90
80
80
80
80
80
70
70
70
70
70
60
60
60
60
60
50
50
50
50
50
40
40
40
40
40
30
30
30
30
30
20
20
20
20
20
MOIST
D.C.
L. F.
17.98
13.28
10.38
8.66
0.92
18.62
13.54
11.07
0.92
19.00
13.82
11.41
9.34
0.92
19.23
13.93
11.62
9.50
0.92
19.40
14.00
11.74
9.61
0.92
19.51
14.04
11.82
9.66
0.92
19.60
14.06
11.86
9.69
0.92
19.69
14.07
11.89
9.70
0.92
19.74
14.08
11.89
9.72
0.92
4.9914
3.5707
3.3962
3.2958
2.7374
4.9503
3.5590
3.3836
2.6854
4.8697
3.5259
3.3033
3.2330
2.6129
4.6787
3.4340
3.1469
3.1352
2.5707
4.3330
3.2526
3.0204
3.0138
2.4823
3.9989
3.0939
2.8520
2.8449
2.4497
3.7257
2.9205
2.7292
2.7242
2.4048
3.4407
2.7696
2.6023
2.5617
2.2547
3.3290
2.7105
2.5309
2.5538
2.1056
1.3210
0.6919
0.6630
0.6665
0.3707
1.3077
0.6812
0.6601
0.3425
1.3483
0.6574
0.6566
0.6537
0.3169
1.2941
0.5957
0.5799
0.5175
0.2849
1.2045
0.5522
0.5116
0.4683
0.2348
1.1695
0.5090
0.4330
0.3852
0.2195
1.0126
0.4530
0.3799
0.3280
0.2095
0.8903
0.3927
0.3031
0.3138
0.1433
0.8768
0.3679
0.2881
0.2457
0.0685
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 2 (continued)
SAMPLE
GA
GA
GA
GA
GA
GA
GA
GA
GA
GA
GA
GA
GA
GA
GA
GA
GA
GA
GA
GA
GA
GA
GA
GA
GA
GA
GA
GA
GA
GA
GA
GA
GA
GA
GA
GA
GA
GA
GA
GA
GA
GA
GA
GA
REL. HUM.
80
60
40
20
0
80
60
40
20
0
80
60
40
20
0
80
60
20
0
80
60
40
20
0
80
60
40
20
0
80
60
40
20
0
80
60
40
20
0
80
60
40
20
0
TEMP
100
100
100
100
100
90
90
90
90
90
80
80
80
80
80
70
70
70
70
60
60
60
60
60
50
50
50
50
50
40
40
40
40
40
30
30
30
30
30
20
20
20
20
20
MOIST
D.C.
L. F.
20.03
11.79
11.07
9.17
1.13
21.20
12.90
11.45
9.76
1.13
21.68
13.40
11.69
10.02
1.13
21.99
13.72
10.18
1.13
22.20
13.97
11.85
10.31
1.13
22.38
14.13
11.90
10.40
1.13
22.50
14.28
11.95
10.48
1.13
22.61
14.40
11.98
10.55
1.13
22.72
14.52
12.00
10.62
1.13
4.6588
3.1928
2.7820
2.5428
2.0382
4.6083
3.1743
2.7614
2.5221
2.0236
4.4509
3.1291
2.6991
2.4719
1.9972
4.1697
2.6164
2.4363
1.9735
3.8728
2.8058
2.4889
2.3653
1.9550
3.6182
2.6477
2.3675
2.2224
1.9195
3.3180
2.4811
2.2828
2.1704
1.8932
3.0668
2.3776
2.2172
2.1136
1.8801
2.9491
2.3128
2.1770
2.0980
1.8696
1.0669
0.5710
0.4280
0.3483
0.1126
1.0383
0.5649
0.4236
0.3322
0.1112
0.9569
0.5356
0.3721
0.3232
0.1018
0.9024
0.3164
0.3351
0.0898
0.8136
0.4354
0.2701
0.3430
0.0844
0.7511
0.3796
0.2242
0.2334
0.0579
0.6896
0.3005
0.1897
0.1978
0.0631
0.6207
0.2465
0.1624
0.1623
0.0617
0.5898
0.2251
0.1527
0.1392
0.0591
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 2 (continued)
SAMPLE
CMC
CMC
CMC
CMC
CMC
CMC
CMC
CMC
CMC
CMC
CMC
CMC
CMC
CMC
CMC
CMC
CMC
CMC
CMC
CMC
CMC
CMC
CMC
CMC
CMC
CMC
CMC
CMC
CMC
CMC
CMC
CMC
CMC
CMC
CMC
CMC
CMC
CMC
CMC
CMC
CMC
CMC
CMC
CMC
CMC
REL HUM.
80
60
40
20
0
80
60
40
20
0
80
60
40
20
0
80
60
40
20
0
80
60
40
20
0
80
60
40
20
0
80
60
40
20
0
80
60
40
20
0
80
60
40
20
0
TEMP
100
100
100
100
100
90
90
90
90
90
80
80
80
80
80
70
70
70
70
70
60
60
60
60
60
50
50
50
50
50
40
40
40
40
40
30
30
30
30
30
20
20
20
20
20
MOIST
D.C.
L. F.
28.74
17.77
11.41
10.33
1.34
30.52
18.36
12.21
10.63
1.34
31.26
18.57
12.60
10.78
1.34
31.70
18.70
12.81
10.88
1.34
31.99
18.78
12.96
10.94
1.34
32.10
18.83
13.04
10.97
1.34
32.18
18.87
13.10
10.98
1.34
32.20
18.88
13.14
10.99
1.34
32.20
18.90
13.14
11.00
1.34
5.1934
3.0297
2.6077
2.3209
1.9905
5.1252
2.9894
2.5878
2.2941
1.9759
5.0703
2.9314
2.5580
2.2255
1.9748
4.9414
2.8741
2.4651
2.1667
1.9612
4.7647
2.7981
2.3572
2.1228
1.9486
4.5367
2.7022
2.2924
2.0799
1.9120
4.1603
2.5561
2.2364
2.0371
1.8994
3.9245
2.4284
2.1748
2.0113
1.8900
3.8035
2.3736
2.1430
1.9910
1.8827
2.2038
0.7467
0.2853
0.3667
0.0440
2.1446
0.7382
0.2852
0.3505
0.0419
2.0700
0.7020
0.2740
0.2772
0.0408
1.7039
0.6268
0.3079
0.2018
0.0387
1.4760
0.4985
0.2535
0.1491
0.0408
1.2465
0.4054
0.1850
0.1062
0.0293
0.9735
0.3252
0.1386
0.0890
0.0293
0.8601
0.2471
0.1176
0.0814
0.0293
0.8327
0.2236
0.1077
0.0761
0.0272
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 3. Values for dielectric constants and loss factors predicted by the
model equations using moisture content, charge and temperature as
parameters
R eproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Reproduced
with
permission
of the copyright ow ner.
Further reproduction
prohibited
without p erm is sio n .
Sample
SK
SK
SK
SK
SK
SK
SK
SK
SK
SK
SK
SK
SK
SK
SK
SK
SK
SK
SK
SK
SK
SK
SK
SK
SK
SK
SK
SK
SK
SK
SK
SK
SK
SK
SK
SK
SK
Charge
Adg. chrg
2.60
2.02
2.60
2.22
2.60
2.31
2.60
2.34
2.60
2.56
2.60
1.99
2.60
2.21
2.60
2.30
2.60
2.33
2.60
2.56
2.60
1.98
2.60
2.20
2.60
2.29
2.60
2.33
2.60
2.56
2.60
1.97
2.60
2.20
2.60
2.29
2.60
2.33
2.60
2.56
2.60
1.96
2.60
2.20
2.60
2.29
2.60
2.32
2.60
2.56
2.60
1.96
2.60
2.20
2.60
2.29
•
2.60
2.32
2.60
2.56
2.60
1.96
2.60
2.19
2.60
2.29
2.60
2.32
2.60
2.56
2.60
1.95
2.60
2.19
Temp.
100.00
100.00
100.00
100.00
100.00
90.00
90.00
90.00
90.00
90.00
80.00
80.00
80.00
80.00
80.00
70.00
70.00
70.00
70.00
70.00
60.00
60.00
60.00
60.00
60.00
50.00
50.00
50.00
50.00
50.00
40.00
40.00
40.00
40.00
40.00
30.00
30.00
Moisture
22.19
14.61
11.33
9.99
1.68
23.50
15.13
11.60
10.27
1.68
23.96
15.33
11.75
10.44
1.68
24.28
15.45
11.84
10.52
1.68
24.50
15.52
11.89
10.58
1.68
24.67
15.56
11.91
10.61
1.68
24.80
15.58
11.93
10.63
1.68
24.90
15.60
DC (calc)
4.457
3.249
2.823
2.665
1.907
4.486
3.198
2.768
2.624
1.922
4.352
3.098
2.696
2.566
1.937
4.188
2.985
2.615
2.497
1.952
4.002
2.865
2.529
2.425
1.967
3.805
2.740
2.440
2.350
1.982
3.600
2.613
2.350
2.274
1.997
3.388
2.486
LF (calc)
1.231
0.664
0.458
0.380
-0.011
1.240
0.633
0.423
0.351
-0.018
1.173
0.579
0.380
0.315
-0.026
1.093
0.518
0.333
0.273
-0.033
1.002
0.455
0.284
0.229
-0.040
0.907
0.389
0.233
0.184
-0.048
0.807
0.322
0.182
0.139
•0.055
0.705
0.255
(Mr-Or
iCD(DU>0)<00)U)U>0^^,
?TtcoNOi ti na3( \J Nr -c
*) U) in ro c\ JO0 foomu) «DWi tNi n( Dmi
tnou)
0)(DNOjN(Nj(DO>CMOnU)0(Oa)^<0
0'g>OU)ID
o
o
o
o
o
o
o
o
o
o
o
o
6 c o w c ) 0 ) d ( D w 6 o ) 6
t— t—i —
r* i - ^
o
0)pj6 oid o)p )T -'o)
i— i — r i —i — r -
o
MV,/wU WW^ Uv«/
O O O O O O O O O O O O O O O O O O O O
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0jo o o p p p o o o p o
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0* 0 o o ‘ 0 0 0 0 0 0 0 0 0 0 0 0 0
50.00
50.00
50.00
50.00
50.00
r^dr-’ i t i o ^ o V ’ tooidc6
c\j r - r - r»
t- r - t -
3.902
3.206
2.948
2.800
2.155
n o ^ c o i - N t o n i n w t r-lOr-NC\JNO)C?m(I>t
OCOOCD-iJ-fOr^TftDlOtO
NMnu)Q)(O(D(0ino)(O
r ; O) r ; o q O CO r ;
T -T -U ) tD P )'r;T ;ltU ) C \jq r; W
co co cJ c\i
co cocsi c\i
cvicvioicoc^cvicvicvj^l’ coco cocJ^cococooi^cooco c\i
O NC M O N C D C D N O lO Tf
tOOi-h-lOOi-CVJi-i-O
19.13
13.51
11.06
9.53
0.66
0
( O W—O .N C
( O W O O O ^ ff ) W C O S ( O O O C N J ^ O ) M n O ^
W
O7*NO^NNO(OM^cNoPt )‘(C? O
^N
( O CM nr - O T- c no ) Ot \ j( sOtU«)DO
T fT - c n( y o
COg
<0o (CO
^ T - ( J ) < 0 ^ 5f i I O <0 T - O O O M M D I O i - r - N ( O U ) « - r - N U ) T r r - 0
0 0 0 0* 0 0 0 0 1 - ^ 0 0 0 0
d o d o
r d d d d * - ' d d d d d d d d d
0.868
0.562
0.448
0.383
0.099
46
( OC) C) N( MC\ J ( MOJ OOOOOO) O) 0 ) O>0 ) ( OCO( DOO<ONNNNN<O0 ( O( O<O
n
o 8 o
3 3 3 8 o
oj r-' c\i cvi cvi oi o ' o' o
o
d o
S
3 8 3 q
d o
o
0
0
8 3 3 8 q
0
0
0
0
0
8 8 8 3 8 8 3 8 8
0
0
0
0
0
0
0
0
6
oooooooococoocococoocococococoococoococoococococococo
< 0 ( 0 < 0 < 0 C 0 ( 0 < 0 p p p p p p p p p p p p p p p p p p p p p p p p p p
c \ i c \ i c \ i c \ i c \ i c \ i c \ i c \ i d d d d d d d d d o d o d o o d o d o o o d b d o
Table 3 (continued)
0.02
0.03
0.03
0.03
0.03
& 8 K 8 2 &
cvj
T" t -
003
0.03
0.03
0-03
003
»“
^^^^^^^^(oa)(oa)(0(0(oco(0(0(oo)(0(0(0(oco(0(oa)(oa)(0(0(0(0(oa)(oa)
C/ ) CO( / ) COC/ ) CO( / ) C/ ) Q. Cl . CL Q. Q. Q. Q. Q. Q. CL a . Q. CL Q. Q. OL Q. Q. Q. Q. a . Q. Q. CL Q. Q. Q. Q. CL Q.
R eproduced with permission of the copyright owner. Further reproduction prohibited without permission.
47
to in o
d
o
d
cm to co
( O O N ,rrtP)0<Dc»3rocMooj©iou)
0
(\](DCOOOWOfOO)0^0)OMbOWOS
^ © O V W O f f l r f f l O W O I O O Q f f l N O
d
d
d
d
d
d
d
d
d
COt DCOCOb«-C\ JWt DCOO)O)COCOC\ JTt -
U5 ( b l / ) O l / ) ^ t O r a ^ T - 0 1 0 r - 0 ) N
t OT * * “ C O « D t A T - O N ( O I O r * O U ) I O
0 0 ^ 0 0 0 0 ^ 0 0S0 0
d o d d d d d d d d d d
©
'it(0(QN(S(M^(0r'(MOMC0(0(0C0,<tNT-N0)OCn0>C0U}<
' t(00)(0r>O^^0)nQ)^
r-N(OO^CM^Cy|T-^P)i-T-»-C)U)inO(OT-CO(0(3)WO(0(0(*)tOO)r-'t^O){OvO^
ddcMCMCMdcMdcMCNidcMCMCMCM^cocococM^cocodcMTfrcoddcM^cococMCMTrcod
N O C O N r ; i n O ) N ( I ) » - ; r ) c q ( q i O r ^ ( I ) { O T j ; r ^ W i n S t O T - #OJ^; «£| C) qr - cr >i n( \ I O) T- r - Tj ; T-
lO^NintDinominiDin^coincDcocoraiDcuoi^Ncocvjocvjr-Tfwnnoioojoo^
t*iqoiniq^(qouj(Or;(qqioa)0)C\jnioo)(qioqoo)0(q^no)OJ(7)(qiqq'i;qN
0)«r-oido)Wi-0)oo>t*)T-o)ONc)0{oocowr-o>ooio»-o)ooJoi-oidoi^V
o
o
o
o
o
o
o
o
o
o
o
o
o
o
o
o
0 0*000000
o o o o o o o o
o o o o o o o p
6 6 0 6 0 0 0
0
CO CO CM CM CM CM CM
cm
d
d
d
d
d
d
d
o
p
0
o
p
0
o
p
0
o
p
0
o
p
0
o
o
0
0
0 0
0 0
0 0
0 0
0 0
0 0
0
o
o
o
o
o
o
o
o
o
o
o
o
o
o
o
o
o) O
o) C
oO co
o C
oO C
oO o
d d d d d o d d
CO rs .t's .r'-.h -r^to to to
000000)0)0) O
0 8 8 8 8 0 8 8 8 0C90
d
o
p
0
o
d
d
g
d
__
C
_O—
«
w O
wO
wO
OO
O ^O~ O O O
O ______
O O O ____
0
0
0
0
0^ O
0 O
0 _O
0
0^ ~
0 0 0
“ 0 0
* ^)0“C
^« 0“^C^0Ow0OwO
—
—
—
—
—
—
0 O0-------0 0 0 O0 O0 O
0
d
d
o’ d
d
d
d
d
d
d
d
o
d
d
d
d
d
d
d
d
d
d
d
o
d
d
c o c o c n c o c o o o o o 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0
cococococococjcococo
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 p p p p p p p p p o
Table 3 (continued)
o* 0 0 0 0 0 *
d
o
d
o
o
d
d
d
d
d
o
d
d
d
d
d
d
d
d
d
o
d
d
d
d
d
d
d
d
o
d
o
< /> w c o c/)c/x/> c/> c />c/>c/>c/)C 0c/)c/> c0<2££££££££££££££££££££££
Q . Q . Q . Q . a . Q . Q .Q .0 L Q .Q .Q .Q .Q .Q .9 5 5 5 3 3 3 5 f j3 5 5 3 3 9 9 9 5 3 5 9 5 5
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
'u o jssjw jad jnoLjijM pajjqjijojd u o jp n p o jd a j j a q p n j
jau M o jqBuAdoo aq j jo uojssjiujad ipjM paonpcuday
> > > > > > > > > > > > > > > > 0 0 0 0 00(3 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O
O D O C o b a c D b o ' o b o b o b o b o b o b o b o b o o o ’0 0
0 0 0 0
0 0
0 0
0
0
0
0
0
0
0
0
0
0
0
0
3
0
0
0
0
0
0
0
0
0
0
rorororororoiorororofoiororororooooooooooooooooooooooo
0
0
0
0
0
0
0
0
0
2 C D > | n I > J O ) O J > 4n|
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
•vim oo-vj-^j-viooooooooo
- * o i - * ^ c o r o o ) o o o o o o o o
0
5
00
00
0 0
0 0
0 0
0 0
0
0 0
0 0
0 0
0 0
0 0
0 0
0
- ^ o o o o o o o o c D t D o i o o o o o o o o r o r o r o r o r o o o c o w o ^ A ^ - ^ ^ c n c n c n o i t n o o
o o p p o p p p o p p p p o p p p p p p p p p p p p p p p p p p p p p o p p
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
ro
iS co
-*
o -» co
o CT>
-k ro
-k
ro
-*(0 _*[\)-»-i(C )-i-*ooiD
kj
S I4
co O)
1
M
0 I 0 ■ 0«, ,■ 0■ ■ 0K, 0■ » 0»,|J 0 0 0W 0W 0W 0W 0W 0W 0 0W 0w
O O O O O O O O O O O O O O O O O
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
■f^( DOCO- *ACDOCD—
* 4^ < D O < £ > —
* ^ < 0 0 0
0 v i o (D > i b o > i 0 'nJ bo b b) (O b bo * 0 (ti l o b) bo 0 01 10 a>
l o ro
cn O
~ O
~ CO VI vj <‘D W r o r O 0 5 ^ W O « D s 4( D N U ) O ) O ) O M O ) ( V ) A - ‘ W “*
A forococo^rocococoA cocococo^roroio ro co ro ro ro ro cororoiococo fO focococororo
^ ^ t o ^ G J a > ^ o K > A i g U o i o o o ;> i ^ b i b > < » i k t _ f c b > < » t o u i ^ * :‘s i < 0 - * x 4 - * a > p ro < o - ‘ <D
NiMUiyisjro(\)-‘ C o r o c n r o r o c D ( a y A ( o a ) o i o w M O 0 c o o f o - ‘ W O ' J r * w v i m © ^
oo o)(fl^ (o -*o ii\)(D ro iv 3 o i w o ) 8
w i \ ) f o w < o i \ ) a ) - ‘ O i C f l - * ^ ' ^ v n o - * 0) r o ^ ^ o o
-^ o o o o -^ o o o o -^ o o o o -^ o o o o o o p p o p p o o p p p o p o o p p
^
b >^j S0K a^ W
b £b)* Io
wj b
to>co
1 s^ t tO4 'wg N^ j b]
o coj -iw
* -b‘ o
) - io5» ^
oiN
- k rio
o^
to o
J cik
n b)
0 ^
^ wco
wa
- *0
N
o ia
t n^c
K 5^ R ^
o f O ( o w c n '4
j k O 0 O < o o ) W > i o ) < j ) i o o ^ i \ ) O ) < n c » C B ^ o - * r o o o ) - * c n o > i \ ) N r v ) O i o t o ,> i { o o i A ( o
Table 3 (continued)
OOOOOOOOOOOOOOC^ Omr or or or or or or or or or or or onmr or a r o&CDr or o
49
O^niOWCNKOtOCOOXOntON^tOOOtOtOlOMOOOOOr-O^SNOHOJOON
l 0 ( D C D ( \ l 0 J W r - ' f ( \ J { 5 U ? U ) O r - ^ f T - ^ ( 0 ^ l 0 U ) U ) T “ O U ) O) ^ N O O t - Q ^ T t N N O T i n ^ n o q io ^ w o rt v to n o c & ^ w o j O N p jw c io tD w c v jT -q N c o v o o S K ^ c )
dd dd*-^d d d d d d d d d d d d o ’d d d d d d d d d d d ^ d d d d ^ d d d
NOCMOONtOt-NWWt-IOtOf-t^^WOOtOinTrW^tOCMCOtOONi-r-ntSTtin
coinoi-mcoioor-OMnt-f-otn^wi-dionoi-fflow^T-inoiStDinwcocow
WOtD^CNJr^qCOT-r-qCONr-^OINtDr-tDCqiDinr-TfNtn^T-rOCOin^N^COlOTf
d c o d d ^ c o c \ i c M C M ^ c o ’ c \ i d c M c o c \ i ( \ i d c \ i c o ’ c\ i dcMC\ i coc\ j c\ i cuc\ i t r>cocMC\ j *- - : i nroc\ i cM
Or-CJ(Of-OT-W^t-Or-{M-<f
T- w T- r- r- N t- i - t- CM*-
$
o
r-
COT
r > COWCMOCJ CO,t N T - C O i t ( \ J l O » - C O
to- ? 3
IT
) t-N iooior-N S T frtrtinnojto
O
g ilolO
C
M
0 CM1
O
w t- t- O
r- f O 0
1
CM £
P) - T* -
O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O
00000000000000000000000000000000000000
NNNMOUXOtOtDIOIOIOIOIO^^^^^COncOOCOCMCMCMCMCMOOOOOOOOO)
d d d d d d d d d d d d d d d d d d d d d d d d d d d d d d d d d d d d d d
_
S t-CM'M’ t-t*OCMCO*-'»*
s
OCMP)r-nOCMC*)r-0 0 )r-lO(Ot
S
o CM CO t T- C
M
r*
h* CO Tfr
*- CM
fr ^ N r ^ c q t q r ^ N r ^ c q i n c q T - ^ - ; ^ ; ^ ( OOt *
N N N C O (D N N N p (p N N N C C )
o o o o o d d d d d o o o o d d d d d d d d d d d d d d d c M C M c o c o c o c M C M c o c o
C M C M C M C M C M C M C M C M C M < M C M C M C M C M C M C M C M C M C M C M C M C M C M C M C M C M C M C V tC M *- t- t- t- r -r - r - r - » -
Table 3 (continued)
cqocoooooocqocqcqeqcqcqcqcqcqcqccrcqcqcqcqcqcqcqcqcquotoinioininininin
d d d d d d d d d o d d d d d o ’ d d d d d d d d d d o d d c o c o c o c o co cocococo
<<<<<<<<<<<<<<<<<<<<<<<<<<<<<yyy9y99yy
ooooooooooooooooooooooooooooo^gggggggg
t
R eproduced with permission of the copyright owner. Further reproduction prohibited without permission.
50
Ow
OC
i n o o < o h - * - c o r * - * - m h - o o ^ w0, ) (
wO_
w O( 00) ( DO( OI OI ON( ONCOONSOO( 0
vN
t v©m
l f t T - » - ( O N O i - n ( M t * ) t O O t D MmOrN
» -r ^ f s n o i O Q J W Q I ^ O ^ g i i n O i n O g i
W r O t N j ^ r O O O n r O q O l W O O q
ONNOCMO«DU>r)C\IOtOinC\Jr-OOvCM
d d d d ^ - ’ d d d d * —d d d d d d d o d
0 0 00^-0000^-0000
n o to a (\ n oi n (o oo a
W U# X o
»-OCO<OCpC\JCOlOr^C*)CM<DCO
T - < o o o ) p j o 3 r w ^ o t D f j
oj
T - N O N O r - Q r ^
cocOT
- cN
O rr--* r- r^ f_c o. .< o
v r9oW
c o©
co
r rC
s' -5c o
o rNs- O
o m
- o5
5TN
p o) )O
u )T^ r * - o
1 p)
N W C N i t n c o c q T - q ^ n t q o J O J p j c v j q t D N r o ^ opj cno ii D
o W
W r r; f f l q ^ T - q q K p q q o i
’ T j - ' c v i c J c v i ^ c o c v i c ' j ^ ’-
8
^incocvicJ^incocicNiT^^rcNicvicvi^TroioJoir-'TfCMcvicvj
s a_ ia
qm
s«s^s sfQ
^<
5
s s 8 ^rrtgm
a»_
s
m
—
>w
<
—
. • CO CM O
C
O *- r - *-
o
o
o
O
o
o
d
tO
o
o
o
C
o
o
d
Ot
o
p
d
O
r - CO CM O
CO
CO
o o o
o p o
d d d
COON
o o o
o o p
d d d
NNNN
CO
(J5 r
IDN CO | § Q
o
Tf Tf COO r
M; V
CO CM CMCO CO COcvi CM CO
S
o
o
d
tO
o
o
o
(0
o
o
d
(0
o
p
o
(0
o
o
d
’t Q l T t O O ^ O T
°
' J cn
cvi co co d
co
o
o
d
o
p
d
U ) U ) l 0
o
p
d
l0
o
p
d
N © co 3 ©
*- co
a) co C
V
J Si - o » c
CO
oC
cM\ i o i i - o r t
S• co
CM CO CO O
o
p
d
C M C O C O O t- ’ c m c o c o * CO t*CO »— »—
CO * - * - * -
o
p
d
o
p
d
U ) U ) ^ 5f
o
o
d
^
o
o
o
o
o
d
o
p
o
o
o
d
o
o
d
o
o
d
o
o
d
o
o
d
o
o
d
o
p
d
o
p
d
■M- ^ t C O C O C O C O C O C M C M C M C M C M
c o i n i o c M c o c o m t n cm
COCD O) U? O
n c o o r - M - p c o p rr - ^ CO © o
CVJCMCOCOCOCMCMCO co
C
O
C
O
c
v
i
C
M
C
O
C
O
C
O
COCO CMCMCO
52$8!882 3
o
o
d
com i n i n
m;©
© o
cm co
r- M
-
cocvi cvi co co co
Table 3 (continued)
5 5 © 5 5 5 5 5 5 5 5 5 u ? © i n w 5
iniOtf>inu)u>©u>in©tb©i/)©tni/)io«/)io
d d d d d d d d d d d d d d d d d d d d d d d d d d c o d d d c o d c o c o c o c o
555555552255552555555552555552555555
o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o
R eproduced with permission o f the copyright owner. Further reproduction prohibited without permission.
100’ C
80
60
40
in
20
o
ui
Q
0
5
10
20
15
30
25
MOISTURE CONTENT. 7.
2.0
80
in
in
3
60
1.0
40
20
0.5
0.0
0
5
10
15
MOISTURE CONTENT.
20
25
30
%
Figure 6. Moisture dependence of the dielectric properties of powdered
hydrocolloids (a) carrageenan
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
6
5
80
40
O 3
Id
_i
Id
Q
2
1
0
5
15
10
25
20
MOISTURE CONTENT. 7.
100'C
80
K
o
o
60
u.
40
i—
<
<
i/i/>
_l
20
o
0.5
0.0
0
5
15
10
20
25
MOISTURE CONTENT. 7.
I
Figure 6. Moisture dependence of the dielectric properties of powdered
hydrocolloids (b) gum arabic
R eproduced with permission of the copyright owner. Further reproduction prohibited without permission.
53
6
100’ C
5
80
t-
z
<
60
2 4
40
t-
o
o
20
3
LJ
Q
2
1
0
5
10
15
20
25
MOISTURE CONTENT. %
100*C
80
60
40
a:
20
O
i—
o
<
u.
0.5
0.0
0
5
10
15
MOISTURE CONTENT.
20
25
%
Figure 6. Moisture dependence of the dielectric properties of powdered
hydrocolloids (c) locust bean gum
R eproduced with permission of the copyright owner. Further reproduction prohibited without permission.
6
100“C
80
5
t-
60
z
<
i-
40
2 4
o
o
o
20
cc
o 3
u
ui
2
0
5
15
10
25
20
MOISTURE CONTENT. J5
100*C
80
1 60
1.0
oc
40
Ov
o
20
(/)
t/t
0.5
0.0
0
5
10
IS
20
25
MOISTURE CONTENT. 7.
Figure 6. Moisture dependence of the dielectric properties of powdered
hydrocolloids (d) potato starch
R eproduced with permission o f the copyright owner. Further reproduction prohibited without permission.
55
6
80
5
60
tz
<
40
i—
20
o
ut 3
UJ
2
0
5
35
30
25
20
15
10
MOISTURE CONTENT. 7.
2.5
100°C
80
2.0
60
DC
O
►
—
o
<
u.
40
20
0.5
0.0
0
5
10
15
20
25
30
35
MOISTURE CONTENT. 7.
Figure 6. Moisture dependence of the dielectric properties of powdered
hydrocolloids (e) carboxymethylcellulose
R eproduced with permission of the copyright owner. Further reproduction prohibited without permission.
56
starch and locust bean gum than it was for the gum arabic, carrageenan and
carboxymethylcellulose.
The following models were derived by subjecting the values for dielectric
constant and loss factor of all the hydrocolloids to stepwise multiple
regression:
e'r = 2.0789 - 0.00125 (CxT) + 0.0010 (TxM) - 0.0168 (MxC) + 0.00227M2
(4)
r2 = .901
e"r = 0.1291 - 0.00386 (CxM) + 0.000436 (TxM) + 0.00100 M2 - 0.1460 C1/3)
(5)
r2 = .914
where:
e'r: Dielectric constant
e'V: Loss Factor
C: Stoichiometric charge in moles of
charge/kg
T: Temperature in °C
M: Moisture in percent, wet basis
The predicted values for the dielectric properties obtained using the model
equations (4) and (5) are shown in Table 3. The regression was repeated
five times, each time omitting data from a different hydrocolloid, generating
five different equation pairs (one pair for the dielectric constant and loss
factor of each hydrocolloid). The equations were used to predict the values
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
of the dielectric constant and loss factor for the hydrocolloid that was omitted.
The predicted values were plotted against the observed values of dielectric
constant and loss factor for each of the hydrocolloids and the value of the
correlation coefficient, r2, was recorded (Table 4).
Table 4. Correlation coefficients, r2, obtained from the plots of the observed
vs. calculated values for the dielectric constants (DC) and loss factors (LF)
for each hydrocolloid.
Graph of observed DC vs.
calculated DC for:
Potato Starch
.99
Graph of observed LF vs.
calculated LF for:
Potato Starch
.98
Locust bean gum
.95
Locust bean gum
.94
Gum arabic
.98
Gum arabic
.99
Carrageenan
.99
Carrageenan
.95
Carboxymethylcellulose
.99
Carboxymethylcellulose
.96
r*
|2
The five different equation pairs varied only marginally from the model
equations shown above indicating that the empirical models can be applied
to obtain the dielectric properties of hydrocolloids that have moisture content
and charge within the specified experimental limits at temperatures ranging
from 20 to 100°C. The values for dielectric constant and loss factor
predicted by equations (4) and (5) were plotted against the measured values
to observe the goodness of fit (Figure 7).
R eproduced with permission of the copyright owner. Further reproduction prohibited without permission.
6
y = 0.2905 + 0.9023X R = 0.95
5
ED
DC
calculated
£>□
4
3
2
1
1
2
3
4
5
6
DC observed
Figure 7. Goodness of fit between observed and calculated values for
dielectric properties (a) dielectric constant
R eproduced with permission of the copyright owner. Further reproduction prohibited without permission.
59
2
y = 0.0405 + 0.9158x
R = 0.96
LF
calculated
1
0
1
0
1
2
LF observed
Figure 7. Goodness of fit between observed and calculated values for
dielectric properties (b) loss factor
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
60
The r2 values indicate that the predicted values are in good agreement with
the calculated values for the dielectric properties.
DISCUSSION
For both e> and e'V, moisture is the most important determinant. The
contribution by moisture and temperature are both positive. In contrast, the
dielectric constant and loss factor decrease with increasing charge on the
hydrocolloids. Three dimensional plots of the effect of moisture and charge
on e'r and e'V at 25°C are shown in Figure 8. The effect of charge on the
dielectric values might reflect an effect of charge on the availably of water in
these water limiting systems. Water associated with the highly hydrophillic
charged groups may not be free to interact with the microwaves. As the
charge on the hydrocolloids increases, the amount of moisture bound to the
charged groups increases thereby lowering the dielectric constant and loss
factor of the hydrocolloid. In the absence of water, the effect of charge
disappears as in the samples with extremely low moisture content.
Mudgett (1985) has offered a physicochemical basis for the prediction of
dielectric behavior in liquid systems, which may be related to chemical
composition in terms of proximate analysis, for three major mechanisms that
have been observed to modify the relative contribution of water to total
dielectric activity.
The effect of dissolved salts is to depress the dielectric constant and elevate
the dielectric loss with respect to levels observed for water, and these have
been predicted by the Hasted-Debye models for aqueous ionic solutions
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
61
DC
40
30
Charge
20
10
Moisture
Figure 8. Effect of moisture(%) and stoichiometric charge-to-mass
ratio(moles/kg) on dielectric properties at 25°C (a) dielectric constant
R eproduced with permission of the copyright owner. Further reproduction prohibited without permission.
0 o
Moisture
Figure 8. Effect of moisture(%) and stoichiometric charge-to-mass
ratio(moles/kg) on dielectric properties at 25°C (b)loss factor
R eproduced with permission o f the copyright owner. Further reproduction prohibited without permission.
(Mudgett et al., 1974a). These effects are generally related to soluble ash
and protein constituents. The effect of insoluble and immiscible constituents
is to depress both the dielectric constant and loss in aqueous mixtures as
seen in oil-water emulsions, and this effect is related to lipid, protein, and
carbohydrate constituents in 'colloidal' suspensions with water. Such
behavior has been predicted by the Fricke model expressed interms of
complex permittivity (Mudgett et al., 1974b). The effect of interactive
constituents such as alcohols or sugars is to shift the critical wavelength of
aqueous mixtures to wavelengths intermediate to those of their pure
components, based on hydrogen bonding between hydroxyl groups of these
components and water molecules as seen in measurements by Buck (1965)
and Roebuck et al (1972). The effect has been predicted for alcohol-water
mixtures by an empirical combination of the Maxwell model for
noninteractive mixtures and the Debye model for pure polar liquids,
designated as the Maxwell-Debye model for interactive mixtures (Mudgett et
al., 1974b).
All the models mentioned above require the dielectric properties of the
individual components. Measuring the dielectric properties of food
ingredients is the first step towards synthesizing such models. Dielectric
data is necessary to calculate energy absorption within a product and
volumetric power absorption. It is necessary to calculate internal heat
generation, differential heating within a product and to develop conceptual
models for microwave heating effects. Microwave heating characteristics of
foods are related not only to their dielectric properties but also to electrical
transmission properties peculiar to dielectric heating processes and to
thermal and transport properties that affect heat and mass transfer in both
R eproduced with permission of the copyright owner. Further reproduction prohibited without permission.
64
conventional and dielectric heating processes (Mudgett, 1985). Thus
predicting the behavior of a food product in a microwave oven is complex
and cannot be explained or predicted by knowledge of its dielectric
properties alone. However, it is this knowledge that form the basis of current
research that is devoted to relating dielectric properties to microwave
heating characteristics of food.
REFERENCES
Annual Book of ASTM Standards. 1988. R.Storer (Ed.). Section 6.
Philadelphia, PA.
Bengtsson N.E. and P.O. Risman. 1971. Dielectric properties of foods at
3GHz as determined by a cavity perturbation technique. II.
Measurements of food materials. J. Microwave Power 6:107.
BuckD.E. 1965. The dielectric spectra of ethanol-water mixtures in the
microwave region. Ph.D. Thesis, Massachusetts Institute of
Technology, Cambridge, Massachusetts.
de Loor G.P. and F.W. Meijboom. 1966. The dielectric constant of foods and
other materials with high water contents at microwave frequencies. J.
Food Tech. 1:313-322.
Greenspan L. 1977. Humidity fixed points of binary saturated aqueous
solutions. J. Res. Nat. Bur. Stand. (U.S.) 81A (Phys. and Chem.).
1:89-96.
Hansen P.M.T. and R. McL. Whitney. 1960. A qualitative test for
carrageenan ester sulfate in milk products. J. Dairy Sci. 43(2): 175186.
Hasted J.B. 1961. The dielectric properties of water. Prog. Dielectr. 3:102149.
Kent M. and A.C. Jason. 1975. In "Water Relations in Foods," ed. R.B.
Duckworth, Academic Press, New York.
Labuza T.P., S.R. Tannenbaum and M. Karel. 1968. Water content and
stability of low and intermediate moisture foods. Food Technol. 24:
35-36, 38-40, 42.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
LabuzaT.P. 1968. Sorption phenomenon in foods. FoodTechnol. 22:15- 65
24.
Labuza T.P. 1980. The effect of water activity on reaction kinetics of food
deterioration. FoodTechnol. 34:36.
Lehninger A.L. 1970. "Biochemistry," Worth, New York.
Mudgett R.E., A.C. Smith, D.I.C. Wang, and S.A. Goldblith. 1974a.
Prediction of dielectric properties in nonfat milk at frequencies and
temperatures of interest in microwave processing. J. Food Sci. 39:
52.
Mudgett R.E., D.I.C. Wang, and S.A. Goldblith. 1974b. Prediction of
dielectric properties in oil-water and alcohol-water mixtures at 3000
MHz, 25C based on pure component properties. J. Food Sci. 39:
632.
Mudgett R.E., A.C. Smith, D.I.C. Wang and S.A. Goldblith. 1974a. Prediction
of dielectric properties in nonfat milk at frequencies and temperatures
of interest in microwave processing. J. Food Sci. 39(1): 52-54.
Mudgett R.E., D.I.C. Wang and S.A. Goldblith. 1974b. Prediction of
dielectric properties in oil-water and alcohol-water mixtures at 3000
MHz, 25C based on pure component properties. J. Food Sci. 39 (3):
632-635.
Mudgett R.E., S.A. Goldblith, D.I.C. Wang and W.B. Westphal. 1977.
Prediction of dielectric properties in solid food of high moisture
content at ultrahigh and microwave frequencies. J. Food Process.
Preserv. 1:119-151.
Mudgett R.E. 1985. Dielectric properties of food. In "Microwaves in the
Food Processing Industry," by R.V. Decareau. Academic Press, Inc.,
New York.
Nelson S.O. 1973. Electrical properties of agricultural products- a critical
review. Trans. Am. Soc. Agri. Eng. 16(2): 384-400.
Nelson S.O. 1973a. Electrical properties of agricultural products- A critical
review. Trans. ASAE. 16(2): 384-400.
Nelson S.O. 1973b. Microwave dielectric properties of grain and seed.
Trans. ASAE. 16(5): 902-905.
Nelson S.O. 1983. Observations on the density dependence of the
dielectric properties of particulate materials. J. Microwave Power.
18(2): 143-152.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Nelson S.O. 1984. Density dependence of the dielectric properties of wheat66
and whole-wheat flour. J. Microwave Power 19(1): 55-64.
Nelson S.O. and T.S.You. 1989. Relationships between microwave
permittivities of solid and pulverized plastics. J. Phys. D: Appl. Phys.
23: 346-353.
Ohlsson T. and N.E. Bengtsson. 1975. Dielectric food data for microwave
sterilization processing. J. Microwave Power 10(1): 93-108.
Ohlsson T. 1989. In " Food Properties and Computer-Aided Engineering of
Food Processing Systems," (Ed.) Singh R.P. and Medina A.G., p. 73.
Kluver Academic Publishers.
Roberts S. and A.von Hippel. 1946. J. Appl. Phys. 17:610-616.
Roebuck B.D., S.A. Goldblith and W.B. Westphal. 1972. Dielectric
properties of carbohydrate-water mixtures at microwave frequencies.
J. Food Sci. 37:199-204.
Sennett P. and J.P. Olivier. 1965. Colloidal dispersions, electrokinetic
effects and the concept of zeta potential. Ind. Eng. Chem. 57:33-50.
Swami S. and R.E. Mudgett. 1981. Effect of moisture and salt contents on
the dieletric behavior of liquid and semi-solid foods. Paper presented
at Symp. Int. Microwave Power Inst., 16th, 1981, Toronto, Canada.
Swami S. and R.E. Mudgett. 1981. Effect of moisture-salt contents on the
dielectric behavior of liquid and semi-solid foods. Proc. 16th Intl.
Microwave Power Inst. Symposium, Toronto, Canada, June 9-12
(abstract).
Tinga W.R. and S.O. Nelson. 1973. Dielectric properties of materials for
microwave processing - tabulated. J. of Microwave Power. 8(1): 23.
To E.C.H., R.E. Mudgett, D.I.C. Wang, S.A. Goldblith and R.V. Decareau.
1974. Dielectric properties of food materials. J. Microwave Power 9
(4): 303-316.
Waters J.R. 1961. Tappi, 44 (7): 185A.
Zadow J.G. and R.D.Hill. 1975. J. Dairy Res. 42:267-275.
R eproduced with permission of the copyright owner. Further reproduction prohibited without permission.
CHAPTER III
THE EFFECT OF MICROWAVE IRRADIATION ON THE STRUCTURE OF
CARRAGEENAN-MILK GELS
INTRODUCTION
Microwave energy offers a rapid and economical means to process foods.
The increasing consumer demand for convenience and time savings in food
preparation has lead to a home microwave oven saturation of more than
80% (Mudgett, 1989). Microwaves heat food material by interacting with
regions of positive and negative charges on water molecules(electrical
dipoles) that rotate the molecules in the electrical field by forces of attraction
and repulsion between oppositely charged regions of the field and the
dipoles. This causes molecular friction which generates heat. Positive and
negative ions of dissolved salts in foods also interact with the electrical field
by migrating toward oppositely charged regions of the electrical field and
disrupt hydrogen bonds with water to generate additional heat. In contrast,
conventional cooking utilizes the heated air of a conventional oven to
increase the surface temperature of the food, driving off the moisture and
resulting in browning and crisping. Heat penetration through the food
occurs by conductive and convective heating depending on the composition
of the food (Brain and Zallie, 1990).
67
R eproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Microwave heating characteristics of food products vary considerably with
the processing frequency, temperature, and chemical composition and are
determined to a large extent by the physical and electrical properties of the
food material.
Today, microwaveable foods cover a broad spectrum of product types
including entrees, soups, vegetables, breakfast items, and bakery goods
(Whorton and Reineccius, 1990). Many processed foods, whether intended
for the microwave oven or the conventional oven, include the use of
hydrocolloids as stabilizing, gelling and thickening agents.
Carrageenan belongs to the class of naturally sulphated marine colloids of
present commercial importance (Moirano, 1977). Carrageenan has the
unique ability to interact with milk proteins, particularly micellar casein,
leading to the development of numerous products that depend upon milk
reactivity to stabilize milk systems. Applications of carrageenan in milk
systems include frozen desserts such as ice cream and ice milk, pasteurized
milk products such as chocolate and fruit flavored milk, eggnog, filled milk,
cooked flans, custards, puddings, pie fillings, whipped cream, milk shakes,
and yoghurt.
The mechanism of forming proteinaceous gelling systems by interacting
proteins and polysacchaarides has been reviewed by Stainsby (1980). He
reports on the occurence of three kinds of interactions. The simplest is the
electrostatic interaction between positively charged proteins and negatively
charged polysaccharides. The second interaction is the formation of a gel
between two macromolecules bearing the same overall charge, which
R eproduced with permission of the copyright owner. Further reproduction prohibited without permission.
depends upon a highly unequal distribution of the polar residues, thus
providing a region of the protein capable of electrostatic interaction.
Snoeren and coworkers (1975), and Snoeren (1976), have demonstrated by
electrostatic affinity chromatography, a specific interaction in which K-casein
is retained by K-carrageenan gel while other caseinates were not with the
exception of two minor caseins. Furthermore, they determined that K-casein
was the only casein component capable of displacing the cationic dye,
methylene blue, from K-carrageenan at pH 6.7, providing convincing
evidence for the electrostatic nature of the interaction. The third method of
interaction requires highly selective linking where polysaccharides are
bound to proteins through covalent bonds. Lin (1977) has discussed yet
another mechanism, involving the stabilization of calcium sensitive proteins
by sulphated polysaccharides (Hansen, 1982). He demonstrated that
carrageenan interacted with the casein fraction both at the pH of normal milk
(pH 6.7) and at the isoelectric point of casein (pH 4.6). The acid whey and
the ultracentrifugal serum from these systems were both void of
carrageenan. Thus, it appeared that carrageenan does not exist in milk as a
freely dispersed colloid. Consequently, any functional property ascribed to a
viscosity increase of the solvent phase could be discounted for the
carrageenan-milk system. In 1970, Lin and Hansen reported that sulphated
polysaccharides with alternating a-1,3 and p-1,4 glycosidic linkages could
promote micelle formation in casein. They suggested steric effects because
the stabilizing ability of carrageenan was enhanced by sulphates in the C-4
or C-2 positions, whereas an antagonistic effect was observed with sulphate
in the C-6 position. Accordingly, the matrices formed during gelation of
carrageenans appear to be important for the formation of stable interaction
products between carrageenan and calcium sensitive caseins. Chakraborty
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
70
and Hansen (1971) revealed by electron microscopy, the existence of an
ultrastructure where the hydrocolloid regions appeared to match the ionaggregated, double-helix junction zones. On that basis, they proposed that
as-casein could be entrapped by interconnecting strands of K-carrageenan
and that the a s-casein aggregates were distributed as discrete particles in a
three dimensional network of carrageenan, possibly in a double helix
configuration.
Lin and Hansen (1970) related the stabilization capacity of sulphated
polysaccharides to the following factors: (a) molecular weight, (b) presence
of sulphate groups, (c) location of the sulphate, and (d) primary structure.
These factors are also related to their gel forming properties (Lin, 1977).
Hansen (1982) reported that carrageenan also stabilized dilute skimmilk
systems (0.3% milk solids) against coagulation by rennin in the presence of
Ca(ll). Examination of the stabilized paracasein particles by electron
microscopy showed that the system resembled casein micelles in untreated
skimmilk except for the smaller micelle size. He surmised that the effect may
be one of calcium blocking the sites on the protein involved in calcium
precipitation reaction. His finding agreed with the observations of Skura and
Nakai (1981), that stabilization of asi-casein by K-carrageenan was not a
function of any sequestering action, but was possibly related to the
establishment of calcium salt bridges between the polysaccharide and the
protein.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
71
Effect of heat
A hydrolytic effect of heat sterilization on carrageenan was demonstrated in
a synthetic milk salt system by Badui et al. (1978). The heat induced
changes included increased elctrophoretic mobility on cellulose acetate and
agarose, reduced viscosity, increased reducing power and reduced
molecular weight by sedimentation equilibrium centrifugation. The apparent
average molecular weight decreased by 42% after 20 minutes of heat
treatment at 122C. The hydrolytic process was a first-order random
degradation with a velocity constant at 122C approximately twice as large as
has been reported for heat degradation of carrageenan in aqueous solution
at pH 7.0. The increased hydrolysis in the milk salt environment was
attributed to a drop in pH which occurred during heating of that system which
was greater than that for normal milk. The finding that carrageenan is
subject to significant depolymerization when heated in a milk salt system
has a great impact when assessing the functional properties of this polymer
in milk products. For example, Snoeren et al., 1975, have reported a
minimum molecular weight of 100,000 for carrageenan to control creaming
in evaporated milk. Also, Lin and Hansen (1970), have demonstrated the
importance of polymer size with respect to stabilization of alpha-casein with
degraded carrageenan, and Lin (1972) has shown that this stabilization
requires the molecular weight of the carrageenan to be limited to a range of
100,000 to 300,000 daltons. In 1985, Desai and Hansen demonstrated a
high degree of thermal stability for carrageenan that had been heated in
cacodylate buffer, pH 7.0 containing 0.25M sodium chloride. They attributed
this stability to the altered conformation of the polymer in a solution
containing elevated levels of inorganic salts and the absence of oxygen.
This finding concurred with the results of an earlier study by Masson et al.
R eproduced with permission of the copyright owner. Further reproduction prohibited without permission.
(1955), in which he proved that the presence of dissolved oxygen has a
considerable effect on the heat degradation of carrageenan at high
temperatures. Desai and Hansen (1985) concluded that the thermal
degradation of carrageenan was minimized in systems where the electrolyte
concentration was high and where the oxygen had been effectively
removed. Hence, they suggested that it would be beneficial to
manufacturers of concentrated milk products to add the carrageenan in the
late stages of evaporation. This suggestion is also applicable in the
extraction and manufacture of carrageenan where minimum degradation of
the polymer is desired.
OBJECTIVES
The present investigation was carried out to determine the differences in
structure of milk gels prepared with carrageenan when heated in the
microwave oven versus conventional heat, by scanning electron microscopy
(SEM). Microwave heating causes the oscillation of dipolar materials in a
rapidly changing electromagnetic field. The vibration of the charged groups
in the gel structure could affect the electrostatic bonds and cause the gel
structure to be altered. Also, microwave heating could result in degradation
of the polymer strands, affecting overall functionality. The objectives of this
study have been: (a) to observe, by SEM, the effect of heating
carrageenan-milk gels by conventional means and by microwave energy,
and (b) to measure the changes in gel strength using the Instron Universal
Testing Machine.
MATERIALS
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
73
Carrageenan (Type 335 Gelcarin GP 379 ) was obtained from Marine
Colloids.
METHODS
Gel preparation
A 1.2% carrageenan solution was prepared by dispersing the polymer in
deionized water. The solution was heated to 180F and cooled, undisturbed,
overnight in an ice bath. The next day, the carrageenan solution was mixed
with an equal amount of 18% skim milk solution to produce a 0 .6 % milk gel.
The mixture was heated in water bath to 160F with occasional stirring. The
sample was homogenized while hot using a valve homogenizer. The
homogenized solution was poured into 250 ml pyrex beakers upto the 150
ml mark and cooled in an ice bath. The beakers were covered with
aluminum foil and allowed to gel overnight in the refrigerator.
Heat treatment
The next day, the beakers were removed from the refrigerator and placed at
room temperature for 2-3 hours. For conventional heating, the samples
were placed in a water bath at 75°C for periods of 10, 20 and 30 minutes
after they had reached equilibrium temperature. The samples for microwave
treatment, were placed in the center of a Kenmore microwave oven one at a
time. The temperature probe attached to the microwave oven was placed
into the gel and heating was performed using the hold warm control at 75°C.
The samples were heated for 10, 20 and 30 minutes. When the samples
were removed from the microwave oven, the temperature was measured. In
all cases, the measured temperature was between 72 and 75°C.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Immediately after the heat treatments, the samples were cooled in an ice
bath and left overnight in the refrigerator.
A separate batch of gels was prepared in which microwave heat was used
instead of conventional heat in the gel preparation step. The carrageenan
was stirred into the water and the solution was heated at 70°C for three
minutes in the microwave oven. The solution was then placed on a stirrer
and the 18% milk solution was added to it. The mixture was placed into the
microwave oven and heated for 5 minutes at 70°C. After the heating, the
solution was allowed to stir for 5 minutes, then homogenized and allowed to
gel overnight in the refrigerator. The next day, the gel strength was
measured and sections of the gel were cut to prepare them for electron
microscopy.
Preparation for microscopy
Sections of the gel samples measuring approximately 3mm square were cut
out from each sample. They were dehydrated in a series of ethyl alcohol
solutions, starting at a 50% alcohol solution up to 100% alcohol. The
samples were immersed in each solution for a minimum of 30 minutes and
were left overnight in the 70% and 95% solutions. After immersion in 100%
alcohol, the samples were critical point dried using carbon dioxide. After the
drying, the samples were attached to metal stubs using colloidal siver paste
and then coated with gold in a sputter coater. The gels were observed in a
JEOL JSM 820 scanning electron microscope at 20 KeV at a magnification
of 1500X.
Gel strength
R eproduced with permission of the copyright owner. Further reproduction prohibited without permission.
75
The gel strength was measured using the Instron Universal Testing
Machine, Model 1000, with a 10 lb weigh beam, in the compression mode.
The Instron was calibrated as per manufacturer's instructions (Instron
Operating Manual No. 10-1000-1 (B)). The crosshead speed was 50mm/min
and the chart speed was 100 mm/min. The probe used belonged to a
Brookfield viscositimeter, it was an RVT probe and consisted of a plate 3.5
cm in diameter.
RESULTS
Photomicrogaphs of the carrageenan-milk gels are displayed in Plates 1- 8 .
The gel prepared using conventional heat and no additional heat treatment
appears to have a slightly uneven surface with a fine network of
carrageenan fibrils and distinct casein micelles. When the gel was heated
for 10 minutes in a water bath, the resultant gel appears to be clotted and the
casein micelles seem to aggregate, however the carrageenan strands are
still visible. After heating the gel for 20 minutes, the aggregation is more
pronounced and not as many carrageenan fibrils are visible. But after 30
minutes of heating, the gel begins to resemble the original sample; the
surface of the gel is more even, individual casein micelles are visible and
the carrageenan strands can be easily seen.
The gel that had been subjected to microwave energy for ten minutes
appears to be nonhomogeneous. Aggregation of the gel is evident and few
carrageenan strands are visible. Microwave treatment for an additional 10
minutes makes the aggregation more pronounced. After 30 minutes of
R eproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Plate I. Photomicrograph of untreated 0.6% carrageenan-milk gel
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
I
77
Plate II. Photomicrograph of 0.6% carrageenan-milk gel heated for 10 min at
70°C in a water bath
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
I
78
Plate III. Photomicrograph of 0.6% carrageenan-milk gel heated for 20 min
at 70°C in a water bath
R eproduced with permission of the copyright owner. Further reproduction prohibited without permission.
79
Plate IV. Photomicrograph of 0.6% carrageenan-milk gel heated for 30 min
at 70°C in a water bath
H e a t 36 ffiin
: '
3-0
V'
■
■ v
- w
’’
v
'
. ' 4-
■
‘
'V
.v
0y'07
20KU
- ;' X l - 5 0 0 .
lBlr'm WD21
R eproduced with permission of the copyright owner. Further reproduction prohibited without permission.
80
Plate V. Photomicrograph of 0.6% carrageenan-milk gel heated for 10 min
at 70°C in a microwave oven
R eproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Plate VI. Photomicrograph of 0.6% carrageenan-milk gel heated for 20
at 70°C in a microwave oven
R eproduced with permission of the copyright owner. Further reproduction prohibited without permission
82
Plate VII. Photomicrograph of 0.6% carrageenan-milk gel heated for 30 min
at 70°C in a microwave oven
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
83
Plate VIII. Photomicrograph of 0.6% carrageenan-milk gel prepared using
microwave heat and subjected to no additional heat treatment
R eproduced with permission of the copyright owner. Further reproduction prohibited without permission.
84
No Heat
Heat 10 min
Heat 20 min
Heat 30 min
I
I
I
I
I
1
MW 10 min
}
MW 20 min
MW 30 mjn
Figure 9. Instron profile plots for carrageenan-milk gels
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
85
microwave heating, the surface of the gel is highly irregular and a networks
of carrageenan fibres can be seen with casein micelles embedded in them.
When the carrageenan-milk gels were prepared using microwave heating,
there was a vast difference in the appearance of these gels and the ones
prepared conventionally. The microwave prepared gels were aggregated
and had an extremely irregular surface. Fine networks of carrageenan
strands were easily visible on the surface of the aggregated clumps.
Instron profile plots of the carrageenan-milk gels are shown in Figure 9. As
the untreated gel was compressed, the force experienced by the gel was
recorded as a smooth linear curve until the gel was broken. The texture
profiles of the gels heated in a water bath were not as smooth and a break in
the curve was recorded. Also, the force required to break the gels was
higher than that for the untreated gel. The gels that had been microwaved
for 10 and 20 minutes had a smooth compression curve until break point.
After break point, the gel offered some resistance to compression before
returning to zero, unlike the water bath heated gels that went back to zero
immediately after the first break. The gel that had been microwaved for 30
minutes required more force to break than any of the other gels. Also, the
force recorded returned to zero immediately after breaking the gel.
DISCUSSION
Carrageenan gels are thermoreversible. When the gels were heated in the
waterbath set at 70C for 10, 20 and 30 minutes, they melted. After the heat
treatment, the gels were placed in an icebath and allowed to regel. From the
photomicrographs it appears that when the gels were heated for 10 and 20
R eproduced with permission of the copyright owner. Further reproduction prohibited without permission.
minutes, only partial melting of the gel occurred. Thus when these gels were
allowed to reform they appear to be aggregated versions of the original gel.
When the gel was heated for 30 minutes, the gel melted more thoroughly
and reformed to resemble the original gel.
The Instron profiles show that the force required to break the microwaved
and conventionally heated gels is greater than that required to break the
untreated sample. The gels therefore appear to be strengthened when they
are heat treated. The compression curve of the conventionally heated
samples is not smooth, indicating that they may be more brittle than the
untreated or the microwaved gels. Microwave heating results in gels that
are more resistant to compression but not as brittle as the conventionally
heated samples. Microwaving for longer periods of time seems to increase
the gel strength.
Microwave heating of the prepared gels resulted in a structure different from
the conventionally heated gels. Microwave heating appears to affect the
aggregation of the milk proteins more than the carrageenan itself. This is
evident from the differences in structure between the gels prepared by
conventional heat and those prepared by microwave heat. It was suggested
that the aggregated appearance of the microwaved gel might be a result of
lack of stirring of the microwaved gel. However, gels prepared
conventionally without stirring were similar to the ones prepared with stirring.
It can be concluded from this study that there is a significant difference in the
structure of carrageenan-milk gels when heated by microwave irradiation as
compared to conventional heating. In 1984, Goebel et al., examined the
effects of microwave energy and convection heating on wheat starch
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
granule transformations. Wheat starch-water dispersions of varying
compositions were prepared and heated by microwave irradiation as well as
by convection. The dispersion resulted in gelled and nongelled regions that
were evaluated microscopically and macroscopically. They found that in all
samples, weight loss and shrinkage were higher in the convectively heated
samples. Various degrees of gelling were observed, visually and by SEM,
within each sample; the different degrees were determined by the
starch:water ratios, whereas, their locations were determined by the mode of
heating. The gelled areas were the inner regions of the microwaved
samples and the outer regions of the convection samples. In general, the
convection heated samples had fewer differences in the degree of gelation
compared to the microwaved samples.
In 1985, Zylema et al., performed a study on similar wheat starch-water
systems to compare the effects of microwave heating and conduction
heating (with equalized heating times) on the structure of the wheat starch
gels. The authors concluded from their study that the distribution of variously
swollen granules and the range in degree of swelling within the samples
depended on the heating method and subsequent heat transfer. They did
not find any structures unique to either heating method, but attributed the
differences between microwave and conductive heating to differences in
heat and mass transfer within the samples.
Nisizawa (1991) performed a study to evaluate the effects of Cobalt 60
gamma rays on the crosslinks of agar-agar hydrogels. Although gamma
rays are ionizing waves and are high energy compared to microwaves, this
study does examine the effects of electromagnetic radiation on
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
polysaccharide gels. The authors found that gamma irradiation caused
some destruction of the agar-agar molecule resulting in a decrease in the
number of crosslinks reflected in the depression of the melting point of the
gel. They related an increase in the melting point to the recovery of the
crosslinks in elapsed time after irradiation.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
REFERENCES
Badui S., N. Desai and P.M.T. Hansen. 1978. Heat degradation of
carrageenan in a milk salt system. J. Agri. Food Chem. 26:675-679.
Brain S.M. and J.P. Zallie. 1990. Role and function of starches in
microwaveable food formulation. Food Aust. 42(11): 523.
Chakraborty B.K. and P.M.T. Hansen. 1971. Electron microscopy of proteinhydrocolloid interaction systems (abstract). J. Dairy Sci. 54: 754.
Desai N. and P.M.T. Hansen. 1985. Heat stability of carrageenan.
Proceedings of the 3rd International Conference, Wrexham, Clwyd,
Wales, July 1985. In "Gums and Stabilizers for the Food Industry 3",
G.O. Phillips, D.J. Wedlock and P.A. Williams (Eds.).
Goebel N.K., J. Grider, E.A. Davis and J. Gordon. 1984. The effects of
microwave energy and convection heating on wheat starch granule
transformations. Food Microstructure, 3:73-82.
Hansen P.M.T. 1982. Hydrocolloid-protein interactions: Relationship to
stabilization of fluid milk products. A review. Prog. Fd. Nutr. Sci., 6:
127-138.
LinC.F. 1972. Dissertation Abstract Int., 32(7): 3996-B.
Lin C.F. 1977. Interaction of sulphated polysaccharides with proteins
(Review). In "Food Colloids," H.D. Graham (Ed.), AVI Publishinh Co.,
Westport, Conn. pp. 320-346.
Lin C.F. and P.M.T. Hansen. 1970. Stabilization of casein micelles by
carrageenan. Macromolecules, 3: 269-274.
Masson C.R., D. Santry and G.W. Caines. 1955. The degradation of
carrageenan II. Influence of further variables. Can. J. Chem. 33:
1088-1096.
MoiranoA.L. 1977. Sulfated seaweed polysaccharides. In " Food
Colloids", Horace D. Graham (Ed.), Chapter 8, The AVI Publishing
Company, Inc., Westport, CT.
MudgettR.E. 1989. Microwave food processing. Food Tech. Jan 1989, pp.
117-126.
Nisizawa M. 1991. Studies on irradiation of Agar-Agar in the solid state: On
the changes of thermal property of agar-agar hydrogel produced by
irradiation. J. Appl. Poly. Sci. 42:2713-2716.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
90
Skura B.J. and S. Nakai. 1981. Stabilization of alpha s1 -casein by kcarrageenan in the presence of calcium. Can. Inst. Food Sci.
Technol. J. 14:59-63.
Snoeren T.H.M. 1976. Kappa carrageenan. Monograph V, 174.
Nederlands Instituut voor Zuivelonderzoek, Ede. Published by H.
Veeman & Zonen B.V. Wageningen.
Snoeren T.H.M., T.A.J. Payens, J. Jeunink and P. Both. 1975. Electrostatic
interaction between k-carrageenan and k-casein. Milchwissenschaft.
30: 393-396.
Stainsby G. 1980. Proteinaceous gelling systems and their complexes with
polysaccharides. Food Chem. 6:3-14.
Zylema B.J., J.A. Grider, J. Gordon and E.A. Davis. 1985. Model wheat
starch systems heated bu microwave irradiation and conduction with
equalized heating times. Cereal Chem. 62(6): 447-453.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
CHAPTER III
THE EFFECT OF MICROWAVE ENERGY ON POTATO STARCH
INTRODUCTION
Starch is a polysaccharide, consisting of long chains of glucose molecules.
Twenty to thirty percent of the starch molecules consist of the linear amylose
molecules, the remaining 70-80 percent of very large and branched
amylopectin molecules. Starch is extracted in granular form from the cells of
certain plants. Starch granules are insoluble in water below 50°C. The
ability of starch to produce a viscous paste when heated in water is its most
important practical property. The hydrocolloidal properties of starch make it
suitable as a thickener and binder.
The commercial sources of starch are the seeds of grains (maize, wheat,
sorghum, rice), tubers (potato), roots (tapioca, sweet potato, arrowroot), the
pith of the sago palm and bananas.
The properties of the starch vary with the plant source from which it is
derived. All starches occur in nature as minute granules, each having its
inherent characteristics, size and shape. The starch industry uses a
combination of grinding and wet purification techniques to manufacture
starch with a purity of about 98-99.5%. In the manufacturing process, starch
91
R eproduced with permission of the copyright owner. Further reproduction prohibited without permission.
is separated from the other constituents of the milled raw material such as
fibers, proteins, sugars and minerals.
GELATINIZATION
Native starches are insoluble in water below their gelatinization
temperature. When starch granules are heated in water to progressively
higher temperatures, a point is reached where the granule starts to swell
irreversibly. As the temperature of the aqueous starch mixture rises, more
hydration occurs and the granules continue to expand to a greatly swollen
reticulated network, still held together by persistant micelles which have not
been disrupted. The viscosity increases to a maximum that corresponds to
the largest hydrated swollen volume of the granules. The temperature at
which the birefringent cross disappears is the gelatinization temperature of
that granule. Gelatinization temperatures for raw starches are listed as
ranges covering the temperatures at which loss of birefringence is first
noticed and less than 10% remains. As the heating and the agitation of the
mixture continues, the swollen starch granules begin to rupture and
collapse, yielding a viscous colloidal dispersion of swollen granule
fragments, hydrated starch aggregates and dissolved molecules. Potato
starch undergoes a very rapid and exceptionally high swelling at relatively
low temperatures, indicating internal bonding. This is partly due to the
presence of ionizable esterified phosphate groups, which assist swelling by
reason of mutual electrical repulsion. The rapid single-stage swelling at
relatively low temperature is a typical starch polyelectrolyte property.
R eproduced with permission of the copyright owner. Further reproduction prohibited without permission.
A COMPARISON OF THE EFFECTS OF MICROWAVE COOKING VERSUS
CONVENTIONAL COOKING ON STARCH FUNCTIONALITY
In conventional cooking, the hot air of the oven causes the surface of a food
product to reach high temperatures, followed by dehydration and browning
(Zallie, 1988). The major component in baked foods is starch with
intermediate to low moisture contents. In such products, the external heating
results in the formation of a dehydrated continuous concentrated
retrograded starch film on the surface that leads to crisping. The amyloseamylopectin ratio of the starch present can affect the strength of the film
formed and, thus, the degree of crisping. Higher percentages of amylose
will impart greater film strength and more crispiness and crunchiness. In
high moisture starch systems such as gravies and sauces, conventional heat
will sometimes cause the dehydration of the surface resulting in skinning
which is the formation of a dilute starch film or skin.
Conventional cooking occurs from the exterior of the product toward the
center, and starch gelatinization also proceeds in the same fashion.
Conductive and convective heat transfer are responsible for cooking the
interior of a food product. Starch gelatinization in convection heated foods is
usually quite uniform with a narrow range of degree of granular swelling.
A number of factors such as shape of the product, nonhomogeneity, and
resonance effects can cause nonuniform temperature distributions in a
microwave heated food. This can result in a broad range of granular
swelling producing undesirable phase differences and separations within
the product. These effects are small in high moisture foods, however, in low
to intermediate moisture foods, dramatically different patterns of starch
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
94
transformations can occur which are responsible for poor textures such as
toughening and cracking.
A major drawback of microwave cooking is the lack of surface crisping.
Microwave energy causes water within the product to be continuously
converted to steam which while migrating out causes evaporative cooling
and condenses at the surface preventing the dehydration and subsequent
retrogradation necessary for crisping.
Turpin (1989), suggested that conduction heating has very different
thermodynamic effects even though the final objective of heating by
conventional and microwave means may be the same. With conduction
heating, energy is added to the food molecules in the form of heat. With
microwave heating, energy is added in the form of electromagnetic radiation,
at a frequency of 2450 MHz and converts to heat at the target. Huang et al.
(1990), observed the tissue characteristics and starch granule variations of
potatoes after microwave and conductive heating by scanning electron
microscopy. Whole potatoes were heated in a microwave oven for 0.5,1
and 2 minutes and in boiling water for 5,10 and 20 minutes. They found that
both heat treatments caused swelling and partial disruption of the starch
granules, however the swelling patterns were different. The starch granules
from the microwave heated potatoes were observed to be more dense and
compact, while the starch granules from the conventionally heated samples
were more reticulated. The authors theorize that conduction heating could
hydrate more starch causing disruption of the starch granules compared to
the microwaved samples which appeared to be less hydrated. They also
found that in conductive heating potatoes were heated from the outside to
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
the inside, whereas the microwave treated potatoes heated fairly uniformly.
A study by Chen et al. (1971), demonstrates a temperature gradient from the
potato cores to the periphery with microwave heating and the opposite
gradient with boiling water. Collins and McCarty (1969) reported that heat
penetration into potato tubers occured from the outside in for both types of
heat treatments, however it was much more rapid in the microwave heated
tubers. Their study was performed to compare the rates of enzyme
inactivation and heat penetration by microwave energy and boiling water.
They concluded that microwave energy inactivated the enzymes polyphenol
oxidase and peroxidase more rapidly than boiling water. Collison and
Chilton (1974), used a dye staining method to determine the proportion of
starch granules that were damaged as a function of initial water content
during microwave heating and baking in a forced-air-convection oven. Their
findings indicate that the proportion of granules damaged by heating was
more dependent upon the amount of water in the sample than on the
method of heating, although, the granules were damaged much more
rapidly by microwave heating.
OBJECTIVES
Starch plays a major role in influencing the overall quality of many of today's
foods (Zallie, 1988). Depending upon the type of food, starch can be used to
provide different textures, thicken, gel, opacify, retain moisture, inhibit
moisture, extend shelf life, form soft coatings and crisp coatings, control
expansion, stabilize emulsions or to provide oil resistant films. As the
popularity of microwave cooking and processing continues to increase, it is
important to study the effect that microwave energy has on starch properties
R eproduced with permission of the copyright owner. Further reproduction prohibited without permission.
96
and how they can be best utilized for optimal performance in the microwave
oven.
The objectives of this study are:
I. To develop a protocol for the examination of starch granules under a
polarizing light microscope
II. To compare the pattern of gelatinization of starch by conventional and
microwave heating
III. To study the effect of microwave heating on dry starch using time,
temperature and water activity as parameters.
I. PROTOCOL TO DETERMINE THE PARTICLE SIZE DISTRIBUTION OF
POTATO STARCH GRANULES
METHOD
Potato starch was obtained from National Starch and Chemical Corporation,
Bridgewater, New Jersey. Suspensions of potato starch granules were prepared
by dispersing 400 mg of starch in 100 ml of water. The slurry was stirred
continuously on a magnetic stirrer at a controlled speed of 400 rpm using a 1.5 inch
stirbar in a 150 ml beaker. A 1.0 pi sample was withdrawn and placed as a drop on
a coverslip. The coverslip was carefully inverted over the well of a culture
microslide which was installed on the stage of a polarizing light microscope. The
sample was observed using 10x or 25x objectives. Microphotographs were
obtained following 1-2 minutes during which time all the granules migrated to the
tip of the drop forming a circle of particles at the air-liquid interface arranged
according to size as shown graphically in Figure 10.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
97
Na = 40 particles
Nb = 21 -22 particles
Dg = 56 mm
particle diameters
b J®
-* OCb
Nc =10 “ 11 particles
Nd > 2 particles
a = 4 mm
b = 6 mm
c = 8 mm
d = 1 0 mm
• grand circle diameter, (Dg)
Figure 10. Diagrammatic view of the arrangement of potato starch granules at the
apex of the water droplet as observed under a polarizing light microscope
R eproduced with permission of the copyright owner. Further reproduction prohibited without permission.
98
The size of the particles along one of the diameters of the circle were measured
and particle size distribution and number were estimated arithmetically using the
following relationship:
Na = 7c*(Dg-a)/a
Nb = rc*(Dg-2a-b)/b
Nc = jt*(Dg-2a-2b-c)/c
Nd = 7i*(Dg-2a-2b-2c-d)/d
etc.
where:
Na = Number of particles that have a diameter a
Nb = Number of particles that have a diameter b
Nc = Number of particles that have a diameter c
Nd = Number of particles that have a diameter d
Dg = Diameter of the grand circle
Diameters a,b,c and d can be expressed as a range in which case the
mean value of the range is used for calculations.
The circumference of a circle is rcDg, where Dg is the grand diameter of the
circle. The maximum number of granules of diameter 'a' that can fit on that
circumference is rcDg/a. However, the number of granules (Na) of diameter
'a' that can lie just inside the circumference will be 7c(Dg-a)/a (since the
particles lie just within the circumference, diameter 'a' of the particle is
subtracted from Dg, the diameter of the circle). If we assume the circle to be
made up of several concentric circles, the circumference of the next
concentric circle is (Dg-2a). The number of particles of diameter ’b' that can
lie inside this circle will be rc(Dg-2a-b)/b. Similarly, the number of particles of
diameter 'c' that can lie within the next concentric circle will be Nc = 7i*(Dg2a-2b-c)/c. Similarly, the number of particles in each concentric circle can
be calculated to give the particle size analysis of the starch granules present
in the droplet of water.
R eproduced with permission of the copyright owner. Further reproduction prohibited without permission.
To test the validity of this approach, the number of particles present in each
of 14 droplets were counted manually and compared to the number of
particles determined arithmetically. A t-test was conducted and no
significant difference was found.
Particle size analyses were calculated from the estimated number of
particles in different class size intervals, using the following expressions for
diameter of average surface, (A), and diameter of average volume, ( D):
(«
01
RESULTS
Microscopic observation of hanging drops of starch suspensions of defined
volume and starch concentration revealed that the granules quickly
migrated to the tip of the drop under the influence of gravity producing
concentric arrangement of particles distributed according to size (Fig. 9).
Apparently, the granules are located at the water-air interface with minimum
evidence of overlapping. Thus, all of the particles contained in a sample
aliquot can be observed in a single field. This particular feature has
suggested a rapid approach for estimation of particle size and number in
different class size intervals which facilitates particle size analysis without
instrumental means. There are some difficulties in obtaining the same
number of particles in each pi aliquot possibly caused by the rapid
sedimentation of granules during sampling. However, if the syringe is
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
maintained in good condition and the drop carefully applied, reproducible
results can be anticipated. It may be noted that the use of a microscope
objective with a long working distance is mandatory for satisfactory focus.
Table 5 shows the number of potato starch granules in 14 droplets
calculated using the method outlined above and counted manually from
photographs of the droplets.
Table 5. A comparison of the calculated and counted number of potato
starch granules in 14 droplets
Sample
2
3
4
5
6
7
9
13
23
24
8
10
11
12
Calculated Total
270
242
510
373
279
262
435
397
275
271
358
175
307
394
Counted Total
264
296
492
380
376
220
364
348
260
196
312
184
256
308
Comparison between calculated and actual counts showed no statistical
difference. The average number of particles in a 1-p.l drop was 312 ± 21
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
101
( std. error). Particle size was calculated and expressed by the diameters of
average surface area and average volume ( A and D). The number of starch
granules present in each class size for all fourteen samples were counted
and incorporated into a single particle analysis table as shown in Table 6.
R eproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Reproduced
with
permission
of the copyright ow ner.
Table 6. Cummulative particle analysis table for potato starch granules counted
manually from 14 samples
P a rtic le
A n a lysis
Further reproduction
prohibited
Class size
(mm)
0.00 0.00
0.00 0.00
1. 00 3 . 0 0
2. 00 4 . 0 0
3.00 4.00
3.00 5.00
4.00 6.00
5 . 0 0 7. 00
6 . 0 0 8. 00
7 . 0 0 9. 00
Sum
without p erm is sio n .
Mag.
mm.
p .m .
d
0. 00
0. 00
2.00
3.00
3.50
4.00
5.00
6. 00
7.00
8. 00
1 28
db1
db2
2. 65 3 . 0 5
2 0 . 7 23. 8
Counted
n
nd
ndd
0
0
2660
608
236
440
188
52
8
16
4208
0
0
0
0
532010640
1824 5472
8 2 6 2891
1760 7040
940 4700
312 1872
56
392
128 1024
1116634031
db3
3.57
27. 9
db4
A
4.19
2.84
3 2 . 7 2 2. 2
nddd
ndddd
0
0
0
0
21280
42560
16416 49248
10119
35415
281 60 1 1 2 6 4 0
23500 117500
11232
67392
2744
19208
8192
65536
* * * * * *
509499
D
3.07
2 4. 0
ncum
0. 0
0
0. 0
0
2. 0 63
3. 0 7 8
3. 5 83
4. 0 9 4
5. 0 98
6. 0 9 9
7. 0 1 00
8. 0 1 00
ndcum
0. 0
0. 0
2. 0
3. 0
3. 5
4. 0
5. 0
6. 0
7. 0
8. 0
nddcum
0
0
48
64
71
87
96
98
99
1 00
0. 0
0. 0
2. 0
3. 0
3. 5
4. 0
5. 0
6. 0
7. 0
8. 0
ndddcum
0
0
31
47
56
77
90
96
97
1 00
0. 0
0
0. 0
0
2. 0
17
3. 0 31
3.5 39
62
4. 0
5. 0 8 2
6. 0 91
7. 0 9 3
8. 0 1 0 0
nddddcum
0. 0
0. 0
2. 0
3. 0
3. 5
4. 0
5. 0
6. 0
7. 0
8. 0
0
0
8
18
25
47
70
83
87
100
Reproduced
with
permission
of the copyright ow ner.
Table 7. Cummulative particle analysis table for potato starch granules
calculated from 14 samples
P a rtic le
A n a lysis
Further reproduction
prohibited
without p erm is sio n .
Class size
(mm)
0. 0 0 0. 0 0
0.00 0.00
1. 00 3 . 0 0
2.00 4.00
3.00 4.00
3.00 5.00
4.00 6.00
5.00 7.00
6.00 8.00
7.00 9.00
Sum
Mag.
mm.
nm.
d
0.00
0.00
2.00
3.00
3.50
4.00
5.00
6.00
7.00
8.00
128
db2
db1
2.90 3.54
2 2. 6 2 7 . 6
C a lc u la te d
n
nd
ndd
nddd
0
0
2543
921
151
365
201
1 69
1 08
58
4516
0
0
0
0
0
0
508610172 20344
2763 8289 24867
529 1850 6474
1460 5 84 0 23360
1005 5 02 5 25125
1014 6084 3 6504
756 5292 37044
464 3712 29696
1307746264*
db3
4. 40
3 4. 4
db4
A
5.27
3. 2 0
41. 2 25.0
D
3.56
2 7. 8
ndddd
0
0
40688
74601
22659
93440
125625
219024
259308
237568
ncum
0. 0
0
0. 0
0
2. 0 5 6
3. 0 7 7
3. 5 8 0
4. 0 88
5. 0 93
6. 0 96
7. 0 99
8. 0 1 0 0
ndcum
0. 0
0. 0
2.0
3.0
3. 5
4.0
5.0
6. 0
7. 0
8. 0
nddcum
0
0
39
60
64
75
83
91
96
1 00
0. 0
0. 0
2. 0
3. 0
3. 5
4.0
5. 0
6. 0
7. 0
8. 0
ndddcum
0
0
22
40
44
57
67
81
92
1 00
0. 0
0
0
0. 0
2. 0
10
3. 0
22
3. 5 2 5
4. 0 3 7
5. 0 4 9
6. 0 6 7
7. 0 8 5
8. 0 1 0 0
nddddcum
0. 0
0. 0
2. 0
3. 0
3. 5
4. 0
5. 0
6. 0
7. 0
8. 0
0
0
4
11
13
22
33
54
78
100
I
104
Similarly, Table 7 shows the cummulative particle size analysis for the
fourteen samples when the number of particles was calculated using the
method outlined above. A comparison of Tables 6 and 7 shows no
significant difference between the mean values of A and D for the calculated
and counted samples. The density of the starch calculated by using the
diameter of particles of average volume and the average number of particles
in a droplet was determined to be 1.39 -1.48. The literature value for the
density of starch is 1.53 -1.55. Barring experimental errors and other
uncertainties, the lower values for the microscopic method may indicate
some initial swelling of the starch in water.
DISCUSSION
Particle size is an important characteristic of starch powders that affects
various physical properties such as solubility, dispersibility and rate of
gelatinization.
There are several methods of measuring particle size of powders such as
sieving, light and electron microscopy, sedimentation, light scattering,
permeability and various electronic approaches including the use of coulter
counter techniques. Most of these methods entail measurements of particle
diameters. Sophisticated equipment is available to perform automatic
particle size analysis, however they can be expensive.
The developed protocol for the microscopic examination of starch in a
hanging drop permits the simultaneous observation of all granules present
in the sample aliquot. The spontaneous arrangement of particles in
concentric circles facilitates a simplified procedure for measurement of size
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
105
and number for particle size analysis. The slide preparations are convenient
for rapid particle size measurement in quality control.
II. A COMPARISON OF THE GELATINIZATION PATTERN OF POTATO
STARCH BY CONVENTIONAL AND MICROWAVE HEATING
METHOD
The method described above to obtain a particle size distribution of potato
starch was used to observe the gelatinization of the starch granule. The
prepared slides were heated on a Kofler plate and gelatinization was
observed. The samples were heated upto 74C and photographs were taken
at regular intervals to capture the gelatinization process. Other slides
containing the starch granules were heated in a microwave oven for 30
second time periods.
RESULTS
Photographs of potato starch (Plates IX-XXVI) particles exposed to heating
on a Kofler stage or by microwave energy revealed slight differences in the
pattern of gelatinization. In conventional heating, the larger particles
gelatinized first with concomitant loss of birefringence. The smaller particles
were more persistent, although some small particles gelatinized along with
the larger ones. The small particles had a tendency to swell without loss of
birefringence, only then did they hydrate sufficiently to lose color unlike the
larger granules that did not exhibit pronounced swelling before losing
birefringence. Although loss of birefringence was observed in some
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
124
particles at 63C, complete gelatinization occurred only around 74C. The
starch granules heated by microwave energy appeared to gelatinize more
randomly with big and small particles gelatinizing at the same time. In both
types of heating the gelatinization commenced at the hilum and proceeded
in two ways: (a) the particle region containing the hilum gelatinized first
followed by the rest of the particles; (b) the hilum portion gelatinized first and
gelatinzation proceeded along the longitudinal axis of the particle toward the
periphery.
DISCUSSION
No significant differences in the gelatinization pattern of potato starch
granules were observed when heated by microwave irradiation or by Kofler
plate heating.
Schoch and Maywald, (1967) define gelatinization as loss of the
interference cross visible within the granule under polarized light, and the
gelatinization temperature as the point at which this transition occurs.
According to the authors, the hilum of the granule is the point at which
gelatinization usually commences which is located at the center of the
birefringence cross. In the initial stage of gelatinization, the hilum appears to
be darkened. As the temperature is increased, the granules lose their
polarization crossed and undergo a continuous swelling. The authors
observed that while each granule undergoes gelatinization quite sharply,
not all the granules gelatinize at the same time, but rather over a range of 810 degrees. They attributed the range of gelatinization temperatures to
differences in internal bonding forces within individual granules.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Plate IX. Arrangement of potato starch granules at the apex of a water
droplet as viewed under a polarizing light microscope subjected to no heat
treatment
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Plate X. Potato starch granules heated to 63°C on a Kofler stage
r
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission
Plate XI. Potato starch granules heated to 64°C on a Kofler stage
■r
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
109
Plate XII. Potato starch granules heated to 65°C on a Kofler stage
■n
R eproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Plate XIII. Potato starch granules heated to 67°C on a Kofler stage
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Plate XIV. Potato starch granules heated to 68°C on a Kofler stage
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Plate XV. Potato starch granules heated to 70°C on a Kofler stage
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Plate XVI. Potato starch granules heated to 71 °C on a Kofler stage
R eproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Plate XVII. Potato starch granules heated to 72°C on a Kofler stage
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
115
Plate XVIII. Potato starch granules heated to 73°C on a Kofler stage
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
116
Plate XIX. Arrangement of potato starch granules at the apex of a water
droplet as viewed under a polarizing light microscope subjected to no heat
treatment
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Plate XX. Potato starch granules heated for 15 seconds In the microwave
oven
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Plate XXI. Potato starch granules heated for 15 + 15 seconds in the
microwave oven
R eproduced with permission of the copyright owner. Further reproduction prohibited without permission.
119
Plate XXII. Potato starch granules heated for 15 + 15 +15 seconds in the
microwave oven
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
120
Plate XXIII. Potato starch granules heated for 15 + 15 + 15 +15 seconds in
the microwave oven
H W IU W C iv ©
u v o u
' 1i /t ' O '
'>
’
"T /»
- * 0 A,' ■
(f . ' v j . ,
'+
R eproduced with permission of the copyright owner. Further reproduction prohibited without permission.
121
Plate XXIV. Potato starch granules heated for 15 + 15 +15 +15 +15 seconds
in the microwave oven
'
/
/
/
^
^
*
-
■ >
'V
• '/* .V
it
'
*
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
122
Plate XXV. Potato starch granules heated for 15 + 15 + 15 +15 +15 +15
seconds in the microwave oven
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Plate XXVI. Potato starch granules heated for 15 + 15 + 15 +15 +15 +15 +15
seconds in the microwave oven
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
124
particles at 63C, complete gelatinization occurred only around 74C. The
starch granules heated by microwave energy appeared to gelatinize more
randomly with big and small particles gelatinizing at the same time. In both
types of heating the gelatinization commenced at the hilum and proceeded
in two ways: (a) the particle region containing the hilum gelatinized first
followed by the rest of the particles; (b) the hilum portion gelatinized first and
gelatinzation proceeded along the longitudinal axis of the particle toward the
periphery.
DISCUSSION
No significant differences in the gelatinization pattern of potato starch
granules were observed when heated by microwave irradiation or by Kofler
plate heating.
Schoch and Maywald, (1967) define gelatinization as loss of the
interference cross visible within the granule under polarized light, and the
gelatinization temperature as the point at which this transition occurs.
According to the authors, the hilum of the granule is the point at which
gelatinization usually commences which is located at the center of the
birefringence cross. In the initial stage of gelatinization, the hilum appears to
be darkened. As the temperature is increased, the granules lose their
polarization crossed and undergo a continuous swelling. The authors
observed that while each granule undergoes gelatinization quite sharply,
not all the granules gelatinize at the same time, but rather over a range of 810 degrees. They attributed the range of gelatinization temperatures to
differences in internal bonding forces within individual granules.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Goebel et al., (1984), performed a study on the effect of microwave heat and
convective heat on wheat starch granule transformations. The authors
described six stages in the swelling patterns of small and large granules and
the development of an amorphous matrix. In the first three stages, the
authors observed the small granules, approximately 2 to 10 Mm in diameter,
to progressively clump and then swell at the edges to form a swollen,
dimpled granule. The large granules, approximately, 10 to 40 Mm in
diameter, were observed to swell radially to disc shapes and then fold.
Birefringence was retained in the first three stages and development of a
matrix was not detected. Small and large granules could be identified until
stage 4 but began to flow together by stage 5. Matrix development was first
observed in stage 4 and attained a fibrous state in stage 6. Some granule
structure was retained even until stage 6. This range of microstructural
characteristics was seen to decrease as the starch:water ratio was
increased. The authors summarized that for the convection heated samples
fewer stages of starch granule swelling and matrix development were
observed, however, the gelled areas were at similar stages for both modes
of heating.
Starch is an important ingredient in many microwaveable food items.
However, the task of developing an acceptable starch based product is not
an easy one. Goebel et al., (1984) suggest that the reasons for this may be
related to fast heating rates, difference in heat and mass transfer
mechanisms, or specific interactions of the components of foods with
microwave radiation. Very often, the microscopic structure of of foods can be
related to their physical properties. This study has shown that conventional
heating causes large particles to gelatinize before the smaller ones,
R eproduced with permission of the copyright owner. Further reproduction prohibited without permission.
I
126
whereas, in the samples that were subjected to microwave heating, there
was no difference between small and large particles. The process of
gelatinization was the same for both methods of heating, although it occured
much faster in the samples that were microwaved.
III. THE EFFECT OF MICROWAVE HEATING ON DRY STARCH USING
TIME, TEMPERATURE AND WATER ACTIVITY AS PARAMETERS
METHOD
The equilibrium moisture content of potato starch was adjusted to different
levels in dessicators by equilibrating the samples with saturated salt
solutions of different Aw levels ranging from 0.09 to 0.94. The temperature
was recorded using a Luxtron temperature sensing instrument. Samples
weighing about 1 gram were placed in glass vials covered by rubber
stoppers. The luxtron probes were inserted through the vents in the
stoppers into the starch sample. The samples were then subjected to
microwave energy for 30, 60,120,180 and 240 seconds. The heating was
conducted at 50% power corresponding to a 12 second on and 8 second off
cycle, to allow for even heating of the entire sample. Regular starch and
freeze dried starch samples were also heated in a similar fashion. Timetemperature graphs were plotted for each sample. The heated samples
were observed under a polarizing light microscope to observe the changes
in the structure of the starch granules as a result of the heating.
RESULTS
Time-temperature plots for regular starch heated for 0.5,1, 2, 3 and 4
minutes are shown in Figure 11. The temperature increased during the on
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
127
50 -
40-
30
0
10
20
30
40
Time (sec)
Figure 11. Time-temperature plots for regular starch heated in a microwave
oven (a) 0.5 min
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
128
60 -
50 -
40 -
30
0
20
40
60
80
Figure 11. Time-temperature plots for regular starch heated in a microwave
oven (b)1 min
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
129
60
50
40
30
0
20
40
60
80
100
120
Time (sec)
Figure 11. Time-temperature plots for regular starch heated in a microwave
oven (c) 2 min
R eproduced with permission of the copyright owner. Further reproduction prohibited without permission.
130
100
Q.
-
80 -
60 -
40
0
100
200
Time (sec)
Figure 11. Time-temperature plots for regular starch heated in a microwave
oven (d) 3 min
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
I
iI
131
120
100
0
100
200
300
Time (sec)
Figure 11. Time-temperature plots for regular starch heated in a microwave
oven (e) 4 min
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
132
cycles and remained steady during the off cycles. At high temperatures, the
temperature dropped during the off cycles. Similar plots were obtained for
samples of different water activities, the only difference was in the maximum
temperature attained by the samples. Table 8 shows the maximum
temperature achieved by the samples for different heating times. When the
water activity was plotted against maximum temperature for all the samples,
the graphs shown in Figure 11 were obtained.
DISCUSSION
It is apparent from Figure 11 that the final temperature reached by
microwaving potato starch for specific times is not a linear function of
equilibrium moisture content (Aw), but apparently reaches a maximum at a
water activity level of 0.65. Beyond this value the final temperature drops
toward 100C.
Dry starch at very low water activity undergoes a rapid temperature increase
inspite of its low moisture content. After four minutes of heating the final
maximum temperature achieved by the freeze dried starch was 110C.
Although this temperature is much higher than the gelatinization
temperature for potato starch (63C), only slight gelatinization was observed
suggesting that there was insufficient moisture for gelatinization to occur.
The high temperature attained by starch containing almost no moisture
seems to indicate that potato starch exhibits dielectric behavior.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 8. Maximum temperature achieved by potato starch samples heated
for varying times by microwave irradiation
Aw
0.194
0.194
0.194
0.194
0.194
0.220
0.220
0.220
0.220
0.220
0.264
0.264
0.264
0.264
0.264
0.352
0.352
0.352
0.352
0.352
0.419
0.419
0.419
0.419
0.419
0.896
0.896
0.896
0.896
0.896
0.941
0.941
0.941
OM
0.941
Seconds
30
60
120
180
240
30
60
120
180
240
30
60
120
180
240
30
60
120
180
240
30
60
120
180
240
30
60
120
180
240
30
60
120
180
240
Maximum Temperature
49.60
49.22
50.01
77.94
110.51
42.24
44.77
68.82
91.08
105.94
40.90
49.07
63.36
100.47
124.66
51.26
67.75
82.11
110.66
129.96
46.90
63.35
79.41
113.65
129.70
49.29
61.07
78.07
98.47
99.86
42.82
50.31
69.66
91.34
100.82
R eproduced with permission o f the copyright owner. Further reproduction prohibited without permission.
134
52
48
40
0.2
0 .4
0.6
0.8
1.0
AW
Figure 11. Plots of water activity versus maximum temperature achieved for
potato starch samples heated in a microwave oven (a) 0.5 min
R eproduced with permission of the copyright owner. Further reproduction prohibited without permission.
i
135
80-
70-
60-
50-
40
0.2
0.4
0.6
0.8
1.0
Aw
Figure 11. Plots of water activity versus maximum temperature achieved for
potato starch samples heated in a microwave oven (b) 1 min
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
136
90
80
70
60
50
0.2
0 .4
0.6
0.8
1.0
Aw
Figure 11. Plots of water activity versus maximum temperature achieved for
potato starch samples heated in a microwave oven (c) 2 min
R eproduced with permission of the copyright owner. Further reproduction prohibited without permission.
137
130
120 -
110 -
100-
90-
80-
70
0.2
0.4
0.6
0.8
1.0
Aw
Figure 11. Plots of water activity versus maximum temperature achieved for
potato starch samples heated in a microwave oven (d) 3 min
R eproduced with permission of the copyright owner. Further reproduction prohibited without permission.
138
140
130-
o
o
©
L.
3
a
a
a.
E
120 -
110 -
©
100 -
0.2
0.4
0.6
0.8
1.0
Aw
Figure 11. Plots of water activity versus maximum temperature achieved for
potato starch samples heated in a microwave oven (e) 4 min
i
R eproduced with permission of the copyright owner. Further reproduction prohibited without permission.
At the water activity level of 0.4, the final temperature recorded was 130C
without any evidence of charring. However, at higher water activity levels,
the final temperature began to drop toward 100C. This phenomenon seems
to indicate that when the water activity of potato starch is 0.8 or more, there is
sufficient free water for evaporation and evaporative cooling stabilizes the
temperature close to 100C. Even at the higher water activity levels, only
slight gelatinization of the potato starch granules was observed.
From the five plots (Figure 11) of water activity versus temperature, the level
of water activity at which maximum temperature was recorded was
calculated as the point at which the slope of the second degree polynomial
curve was in effect zero. These values are recorded in Table 9.
Table 9. Estimated water activity at which maximum temperature was
recorded for potato starch samples heated for varying amounts of time in a
microwave oven
Time (sec)
240
180
120
"6(3
3b
d(T)/d(Aw)
226-416AW
277-491Aw
212-367Aw
i54-422Aw
79-122AW
Est. Aw
0.54
0.56
0.58
0.60
0.65
It appears from this table that for 30 seconds of heating, the highest
temperature is obtained at a water activity level of 0.65. As the heating time
increases, the level of water activity at which the maximum temperature is
obtained drops, until it reaches 0.54 at 4 minutes of heating.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
140
According to Zallie (1988), the greater the starch:water ratio, the faster the
mixture will heat in a microwave oven. However, gelatinization will not occur
below a critical moisture level which is usually around 30% for most
starches. Below this moisture content, some of the water would exist
adsorbed or bound to the starch. The tightly bound water cannot respond to
the electric field generated at microwave frequencies, this results in a
depression of the dielectric constant. Gelatinized starch does not bind as
much water as raw starch, therefore more water is available to respond to
the alternating electrical field resulting in the elevation of the dielectric
constant.
Roebuck and Goldblith (1972) measured the dielectric properties of
carbohydrate-water mixtures at near microwave frequencies over a wide
range of moisture levels. For the low moisture granular potato starch-water
mixtures (ranging from nearly zero to 37% water by weight), they found that
the dielectric constant was very low and independent of frequency. They
also observed the loss factor to be very small (less than 0.6) and
independent of frequency. The dielectric properties increased sharply at
30% moisture content. The authors suggest that at microwave frequencies,
the dielectric constant of a mixture decreases as the water content
decreases because less water is available for dielectric polarization
(carbohydrates do not show appreciable dipole polarization at microwave
frequencies). Thus, the binding of water by a carbohydrate material plays an
important role in the dielectric properties of a carbohydrate-water mixture.
Also, the water that is bound to the starch exhibits dielectric properties
similar to the starch itself resulting in further depression of the dielectric
constant. As regards to the loss factor, the authors explain that the loss factor
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
of the mixture is dependent on the relaxation of the water molecules since
carbohydrates show dipole relaxation at much lower frequencies. As the
carbohydrate:water ratio increases, the -OH groups of the carbohydrate tend
to stabilize the hydrogen bonds and the loss factor of the mixture becomes
higher than the loss factor of water. At the same time, some of the water that
is bound to the starch is not available to repond to the alternating field.
Therefore, at low moisture contents, the tightly bound water tends to depress
the loss factor considerably. The authors concluded that the dielectric
constant of a carbohydrate-water mixture of any ratio will decrease as the
temperature increases from 0°C. The decrease will be more significant for
higher levels of water. The loss factor is dependent on the availability of
water to the alternating field, provided that ionic conductivity is small. Bound
water will shift the dispersion curve to lower frequencies, hence the loss
factor will increase over the loss factor of water. In samples with high
moisture contents, the loss factor would decrease as the temperature was
increased.
R eproduced with permission of the copyright owner. Further reproduction prohibited without permission.
142
REFERENCES
Chen S.C., J.L. Collins, I.E. McCarty and M.R. Johnston. Blanching of white
potatoes by microwave energy followed by boiling water. J. Food Sci.
36: 742-743.
Collins J.L. and I.E. Mccarty. 1969. Comparison of microwave energy with
boiling water for blanching whole potatoes. Food Technol. 23 (3):
63-66.
Collison R and W.G. Chilton. 1974. Starch gelation as a function of water
content. J. Food Technol. 9:309-315.
Goebel N.K., J. Grider, E.A. Davis and J. Gordon. 1984. The effects of
microwave energy and convection heating on wheat starch granule
transformations. Food Microstructure, 3:73-82.
Huang J., W.M. Hess, D.J. Weber, A.E. Purcell and C.S. Huber. 1990.
Scanning electron microscopy: Tissue characteristics and starch
granule variations of potatoes after microwave and conductive
heating. Food Structure, 9:113-122.
Roebuck B.D. and S.A. Goldblith. Dielectric properties of carbohydratewater mixtures at microwave frequencies. J. Food Sci., 37:199-204.
Schoch T.J. and C.E. Maywald. 1967. Industrial microscopy of starches. In
"Starch: Chemistry and Technology", Vol. 1, Ch. 12 (R.L.Whistler and
E.F. Paschall, Eds.). Academic Press, New York. pp. 637 - 647.
Turpin C.H. 1989. Variable microwave power. Microwave World, 10:8-11.
Zallie Z.P. 1988. The role and function of starches in microwaveable food
formulation. Presented at MW Foods' 88, March 8,1988, Chicago,
Illinois.
R eproduced with permission of the copyright owner. Further reproduction prohibited without permission.
SUMMARY AND CONCLUSIONS
Food scientists involved with formulating food products for use in the
microwave oven have recognized the importance of understanding the
heating mechanism of microwave energy and its interactions with food. Until
recently, development of a microwaveable food product consisted of
adapting a conventional recipe for the microwave oven by a trial and error
process. However, food scientists have realized the advantages of
formulating food products for the microwave oven based on knowledge of
interaction of microwaves with food constituents and the dielectric properties
of food ingredients. Food hydrocolloids are used in many processed foods
as stabilizing, gelling and emulsifying agents. They make a significant
contribution to the overall texture and hence quality of the product. For this
reason, it is important not only to determine how microwaves affect the
functionality of hydrocolloids, but also to make certain that the heating
characteristics of hydrocolloids are compatible with other food constituents.
This research study was designed to contribute to the above goals.
There is no doubt that the topic of this dissertation encompasses much more
than was done in the course of this study. However, this project has made a
significant contribution to the study of microwave interactions with food
ingredients. Food formulators now have data on the dielectric properties of
143
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
various hydrocolloids at different moisture, temperature and charge levels.
Using equations for complex food systems and data on the dielectric
properties of other food ingredients, the dielectric properties of food products
can be calculated and be used in predicting how these foods will heat in a
microwave oven. The study on potato starch revealed no differences in the
gelatinization pattern of starch granules by conventional or microwave
heating. However, there was a considerable difference in the microstructure
of carrageenan-milk gels when they were heated by microwave irradiation
as compared to conventional heating. These results prove the fact that
although the final effect of microwave irradiation is the generation of heat,
there may or may not be additional effects on the structure and function of
food ingredients. Either way, these effects need to be investigated and
documented.
Starch is an important ingredient in many microwaveable foods. However,
starch containing microwaveable food products, especially bakery items, are
still short of achieving the same quality as conventionally prepared food
products. Therefore, the first objective in this study was to observe the
differences in the gelatinization pattern of microwaved and conventionally
heated starch granules. This led to the development of a protocol to
determine the particle size distribution of potato starch granules in a given
sample. A drop of starch slurry was placed on a coverslip which was
inverted over a cultured microslide. Within seconds, the starch granules in
the droplet gathered at the apex of the drop in an almost perfect circular
shape with the largest granules in the center and the smallest towards the
edge. Few simple arithmetic equations and measurement of the diameter of
the circle permitted the calculation of the particle size distribution of the
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
145
potato starch granules within the droplet. A comparison of the actual
number of particles in a droplet and the number of particles estimated
utilizing this method showed no statistical difference. Particle size analysis
was performed on both; the actual and calculated counts, and the results
revealed good agreement between both counts.
This protocol to observe
the particle size distribution of potato starch granules is simple, fast and
does not require the use of sophisticated equipment. Another advantage of
this procedure is that it permits the simultaneous observation of all granules
present in the sample aliquot.
Slides of potato starch granules prepared in the same manner described
above were heated on a Kofler plate and in the microwave oven and the
gelatinization was observed under a polarizing light microscope. Although
no difference in the process of gelatinization was observed, it was found that
in the conventionally heated samples, the smaller particles tend to gelatinize
last. In both types of heating, gelatinization commenced at the hilum and
proceeded in two ways; (a) the particle regio containing the hilum
gelatinized first followed by the rest of the particles; (b) the hilum portion
gelatinized first and gelatinization proceeded along the longitudinal axis of
the particle toward the periphery.
Time-temperature profiles were obtained for dry starch powders equilibrated
to a range of relative humudities and heated in the microwave oven. It was
found that the maximum temperature achieved was not a linear function of
equilibrium moisture content, but reached a maximum at a water activity
level of 0.65. It appeared that as the water activity of the samples was
increased, the maximum temperature reached by the samples also
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
increased, upto a water activity of 0.65. Beyond this Aw, the final
temperature dropped down toward 100C. This phenomenon indicated that
at high water activities there was enough moisture for evaporation, and
evaporative cooling stabilized the temperature close to 100C. No
gelatinization of the strach granules was observed at the low water activity
levels and only slight gelatinization was observed at the higher water activity
levels. Dry starch with very little moisture, also achieved a maximum
temperature of 110C, indicating dielectric heating of the dry starch. The next
step, then, was to determine the dielectric properties of starch and relate
them to behavior of starch under the influence of microwave irradiation.
Although, dielectric data has been tabulated for various food ingredients and
products, information on the dielectric properties of food hydrocolloids is not
yet available. Hydrocolloids have important functional properties such as
thickening, gelling and emulsifying agents in many manufactured foods.
The properties of these polymers contribute significantly to the final
appearance and texture of the product. The dielectric properties of various
hydrocolloids were determined over a Aw range of 0 to 0.8 from
temperatures from 20 to 100C. It was found that the the dielectric constant
and loss factor of all the polymers increased with moisture content and
temperature. The dielectric constants of all the materials were around 2
when very dry and increased to about 3 at 20-25% moisture. Loss factors at
20C were around 0.1 when very dry and the rate of increase with moisture
content was different for all the hydrocolloids. The temperature dependence
of the dielectric constant and loss factor was greater for carrageenan,
carboxymethylcellulose and gum arabic than it was for locust bean gum and
potato starch at all temperatures. It was observed that locust bean gum and
R eproduced with permission o f the copyright owner. Further reproduction prohibited without permission.
147
potato starch are neutral or only slightly charged compared to the other three
polysaccharides that carry considerably more charge. The values or charge
on each polymer were adjusted to account for the moisture content. The
values for the dielectric constant and loss factor were subjected to multiple
regression based on moisture content, charge on each polymer and
temperature of each measurement. Model equations were derived that
could predict the dielectric constant and loss factor for each hydrocolloid if
moisture content, charge and temperature were known. The values
predicted by the model equations were not significantly different from the
measured values indicating that these models could be used for
hydrocolloids within the moisture, temperature and charge range specified
earlier.
Moisture was found to be the most important factor in determing the
dielectric properties of the hydrocolloids. The contributions by moisture and
temperature were positive. However, charge seemed to have a negative
effect on the dielectric properties. It was reasoned that the effect of charge
was related to the availability of water in these low moisture systems. Water
associated with the highly hydrophillic charged groups was not free to
interact with the microwaves. As the charge on the hydrocolloids was
increased, the amount of moisture bound to the charged groups also
increased thereby lowering the dielectric constant and loss factor of the
hydrocolloid. In the absence of moisture, the effect the effect of charge
dissappeared as in the samples with extremely low moisture content.
To determine the effect of microwave energy on the functional properties of
hydrocolloids, a model system consisting of milk gels prepared with
R eproduced with permission of the copyright owner. Further reproduction prohibited without permission.
carrageenan, was used to measure the changes in gel strength and observe
the changes in structure of the gels. Milk gels prepared with carrageenan as
the gelling agent were heated conventionally as well as by microwave
energy. There was a significant difference in the microstructure of the gels
heated by the two different methods as observed by scanning electron
microscopy. When the gels were heated in the water bath, there was some
aggregation of the gel structure, but as the time of heating was increased,
the appearance of the gel approached that of the one subjected to no heat
treatment. In other words, it appeared that heating the gel in a water bath for
long periods of time resulted in a thorough melting of the gel, which upon
cooling reformed to resemble the control. In contrast, the gels heated in the
microwave oven showed more aggregation which seemed to increase as
the time of heating was increased. Although, a carrageenan network was
observed in the gels heated for 30 minutes in the microwave, the milk
proteins seemed to have aggregated together to form large clumps. Such
an effect was also evident in the gels that were prepared using microwave
heating instead of conventional heat. Based on these results, it was
concluded that microwave irradiation affects the functional and structural
properties of carrageenan in a manner different from conventional heating.
R eproduced with permission of the copyright owner. Further reproduction prohibited without permission.
BIBLIOGRAPHY
Annual Book of ASTM Standards. 1988. R.Storer (Ed.). Section 6.
Philadelphia, PA.
Ayoub J.A., D. Berkowitz, E.M. Kenyon and C.K. Wadsworth. 1974.
Continuous microwave sterilization of meat in flexible pouches. J. of
Food Sci., 39:309-313.
Baden Fuller A.J. 1979. Introduction. In " Microwaves - An Introduction to
Microwave Theory and Techniques", 2nd edition, Pergamon Press,
New York.
Badui S., N. Desai and P.M.T. Hansen. 1978. Heat degradation of
carrageenan in a milk salt system. J. Agri. Food Chem. 26:675-679.
Bengtsson N.E. and P.O. Risman. 1971. Dielectric properties of foods at
3GHz as determined by a cavity perturbation technique. II.
Measurements of food materials. J. Microwave Power 6:107.
Brain S.M. and J.P. Zallie. 1990. Role and function of starches in
microwaveable food formulation. Food Aust. 42(11): 523.
Buck D.E. 1965. The dielectric spectra of ethanol-water mixtures in the
microwave region. Ph.D. Thesis, Massachusetts Institute of
Technology, Cambridge, Massachusetts.
Chakraborty B.K. and P.M.T. Hansen. 1971. Electron microscopy of proteinhydrocolloid interaction systems (abstract). J. Dairy Sci. 54: 754.
Chen S.C., J.L. Collins, I.E. McCarty and M.R. Johnston. Blanching of white
potatoes by microwave energy followed by boiling water. J. Food Sci.
36: 742-743.
Collins J.L. and I.E. Mccarty. 1969. Comparison of microwave energy with
boiling water for blanching whole potatoes. Food Technol. 23 (3):
63-66.
Collison R and W.G. Chilton. 1974. Starch gelation as a function of water
content. J. Food Technol. 9:309-315.
149
R eproduced with permission of the copyright owner. Further reproduction prohibited without permission.
150
de Loor G.P. and F.W. Meijboom. i966. The dielectric constant of foods and
other materials with high water contents at microwave frequencies. J.
Food Tech. 1:313-322.
Decareau R.V. 1985. "Microwaves in the Food Processing Industry."
Academic Press, New York.
Desai N. and P.M.T. Hansen. 1985. Heat stability of carrageenan.
Proceedings of the 3rd International Conference, Wrexham, Clwyd,
Wales, July 1985. In "Gums and Stabilizers for the Food Industry 3",
G.O. Phillips, D.J. Wedlock and P.A. Williams (Eds.).
FennemaO.R. 1976. Water and ice. In "Principles of Food Science Part 1:
Food Chemistry", O.R. Fennema (Ed.). Marcel Dekker, Inc., New York.
Goebel N.K., J. Grider, E.A. Davis and J. Gordon. 1984. The effects of
microwave energy and convection heating on wheat starch granule
transformations. Food Microstructure, 3:73-82.
Greenspan L. 1977. Humidity fixed points of binary saturated aqueous
solutions. J. Res. Nat. Bur. Stand. (U.S.) 81A (Phys. and Chem.).
1:89-96.
Hamid M.A.K. and R.J. Boulanger. 1969. A new method for the control of
moisture and insect infestations of grain by microwave power. J. of
Microwave Power, 4:11-18.
Hansen P.M.T. 1982. Hydrocolloid-protein interactions: Relationship to
stabilization of fluid milk products. A review. Prog. Fd. Nutr. Sci., 6:
127-138.
Hansen P.M.T. and R. McL. Whitney. 1960. A qualitative test for
carrageenan ester sulfate in milk products. J. Dairy Sci. 43(2): 175186.
Hasted J.B. 1961. The dielectric properties of water. Prog. Dielectr. 3:102149.
Huang J., W.M. Hess, D.J. Weber, A.E. Purcell and C.S. Huber. 1990.
Scanning electron microscopy: Tissue characteristics and starch
granule variations of potatoes after microwave and conductive
heating. Food Structure, 9:113-122.
I.R.P.A. 1988. Guidelines on limits of exposure to radiofrequency
electromagnetic fields in the frequency range from 100 KHz to 300
GHz. Health Physics, 54:115-123.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
151
Karel M. 1975. Physicochemical modification of the state of water in foods.
In "Water Relations in Foods," R.B. Duckworth (Ed.), Academic Press,
New York.
Kent M. and A.C. Jason. 1975. In "Water Relations in Foods," ed. R.B.
Duckworth, Academic Press, New York.
Kenyon E.M., D.E. Westcott, P. LaCasse, and J.W. Gould. 1971. A system
for continuous thermal processing of food pouches using microwave
energy. J. of Food Sci., 36: 289-293.
Knutson K.M., E.H. Marth and M.K. Wagner. 1987. Microwave heating of
food. LBT, 20:101-110.
Kok L.P. and M.E. Boon. 1990. Microwaves for microscopy. J. of
Microscopy, 158(3): 291-322.
LabuzaT.P. 1968. Sorption phenomenon in foods. Food Technol. 22:1524.
Labuza T.P. 1980. The effect of water activity on reaction kinetics of food
deterioration. Food Technol. 34:36.
Labuza T.P., S.R. Tannenbaum and M. Karel. 1968. Water content and
stability of low and intermediate moisture foods. Food Technol. 24:
35-36, 38-40, 42.
Lehninger A.L. 1970. "Biochemistry," Worth, New York.
Lin C.F. 1972. Dissertation Abstract Int., 32(7): 3996-B.
Lin C.F. 1977. Interaction of sulphated polysaccharides with proteins
(Review). In "Food Colloids," H.D. Graham (Ed.), AVI Publishinh Co.,
Westport, Conn. pp. 320-346.
Lin C.F. and P.M.T. Hansen. 1970. Stabilization of casein micelles by
carrageenan. Macromolecules, 3: 269-274.
Masson C.R., D. Santry and G.W. Caines. 1955. The degradation of
carrageenan II. Influence of further variables. Can. J. Chem. 33:
1088-1096.
MoiranoA.L. 1977. Sulfated seaweed polysaccharides. In " Food
Colloids", Horace D. Graham (Ed.), Chapter 8, The AVI Publishing
Company, Inc., Westport, CT.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
152
Mudgett R.E. 1974. A physical chemical basis for prediction of dielectric
properties in foods at ultra-high and microwave frequencies. Ph.D.
Thesis, Massachusetts Institute of technology, Cambridge, MA.
Mudgett R.E. 1985. Dielectric properties of food. In "Microwaves in the
Food Processing Industry," by R.V. Decareau. Academic Press, Inc.,
New York.
Mudgett R.E. 1989. Microwave food processing. Food Tech. 43(1): 11T126.
Mudgett R.E. and W.B. Westphal. 1989. Dielectric behavior of an aqueous
cation exchanger. J. Micr. Pwr. E. E. 24:33-37.
Mudgett R.E., A.C. Smith, D.I.C. Wang and S.A. Goldblith. 1974a. Prediction
of dielectric properties in nonfat milk at frequencies and temperatures
of interest in microwave processing. J. Food Sci. 39 (1): 52-54.
Mudgett R.E., D.I.C. Wang and S.A. Goldblith. 1974b. Prediction of
dielectric properties in oil-water and alcohol-water mixtures at 3000
MHz, 25C based on pure component properties. J. Food Sci. 39 (3):
632-635.
Mudgett R.E., S.A. Goldblith, D.I.C. Wang and W.B. Westphal. 1977.
Prediction of dielectric properties in solid food of high moisture
content at ultrahigh and microwave frequencies. J. Food Process.
Preserv. 1:119-151.
Nelson S.O. 1973. Electrical properties of agricultural products- a critical
review. Trans. Am. Soc. Agri. Eng. 16(2): 384-400.
Nelson S.O. 1973a. Electrical properties of agricultural products- A critical
review. Trans. ASAE. 16(2): 384-400.
Nelson S.O. 1973b. Microwave dielectric properties of grain and seed.
Trans. ASAE. 16(5): 902-905.
Nelson S.O. 1983. Observations on the density dependence of the
dielectric properties of particulate materials. J. Microwave Power.
18(2): 143-152.
Nelson S.O. 1984. Density dependence of the dielectric properties of wheat
and whole-wheat flour. J. Microwave Power 19(1): 55-64.
Nelson S.O. and T.S.You. 1989. Relationships between microwave
permittivities of solid and pulverized plastics. J. Phys. D: Appl. Phys.
23: 346-353.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
153
Nisizawa M. 1991. Studies on irradiation of Agar-Agar in the solid state: On
the changes of thermal property of agar-agar hydrogel produced by
irradiation. J. Appl. Poly. Sci. 42: 2713-2716.
Ohlsson T. 1989. Dielectric properties and microwave processing. In "
Food Properties and Computer-Aided Engineering of Food
Processing Systems," R.P.Singh and A.G.Medina (Eds.). Kluver
Academic Publishers, pp. 73-92.
Ohlsson T. and N.E. Bengtsson. 1975. Dielectric food data for microwave
sterilization processing. J. Microwave Power 10(1): 93-108.
Pomeranz Y. and C.E. Melaon. 1987. Food Analysis: Theory and Practice,
2nd ed. Van Reinhold Nostrand Co., New York.
Roberts S. and A.von Hippel. 1946. J. Appl. Phys. 17: 610-616.
Roebuck B.D. and S.A. Goldblith. Dielectric properties of carbohydratewater mixtures at microwave frequencies. J. Food Sci., 37:199-204.
Roebuck B.D., S.A. Goldblith and W.B. Westphal. 1972. Dielectric
properties of carbohydrate-water mixtures at microwave frequencies.
J. Food Sci. 37:199-204.
Schiffman R.F. 1989. Food product safety problems due to microwave
heating. Paper presented at MW Foods ’89, 2nd International
Conference on Formulating Food for the Microwave Oven, March 1415. The Packaging group Inc., Milltown, NJ.
Schoch T.J. and C.E. Maywald. 1967. Industrial microscopy of starches. In
"Starch: Chemistry and Technology", Vol. 1, Ch. 12 (R.L.Whistler and
E.F. Paschall, Eds.). Academic Press, New York. pp. 637 - 647.
Sennett P. and J.P. Olivier. 1965. Colloidal dispersions, electrokinetic
effects and the concept of zeta potential. Ind. Eng. Chem. 57:33-50.
Skura B.J. and S. Nakai. 1981. Stabilization of alpha s1-casein by kcarrageenan in the presence of calcium. Can. Inst. Food Sci.
Technol. J. 14:59-63.
Snoeren T.H.M. 1976. Kappa carrageenan. Monograph V, 174.
Nederlands Instituut voor Zuivelonderzoek, Ede. Published by H.
Veeman & Zonen B.V. Wageningen.
Snoeren T.H.M., T.A.J. Payens, J. Jeunink and P. Both. 1975. Electrostatic
interaction between k-carrageenan and k-casein. Milchwissenschaft.
30: 393-396.
R eproduced with permission of the copyright owner. Further reproduction prohibited without permission.
154
Stainsby G. 1980. Proteinaceous gelling systems and their complexes with
polysaccharides. FoodChem. 6:3-14.
Swami S. and R.E. Mudgett. 1981. Effect of moisture and salt contents on
the dieletric behavior of liquid and semi-solid foods. Paper presented
at Symp. Int. Microwave Power Inst., 16th, 1981, Toronto, Canada.
Tinga W.R. and S.O. Nelson. 1973. Dielectric properties of materials for
microwave processing-tabulated. J. of Microwave Power, 8(1): 23-28.
To E.C.H., R.E. Mudgett, D.I.C. Wang, S.A. Goldblith and R.V. Decareau.
1974. Dielectric properties of food materials. J. Microwave Power 9
(4): 303-316.
Turpin C.H. 1989. Variable microwave power. Microwave World, 10:8-11.
VasavadaP.C. 1990. Microwave Processing for the dairy industry. Food
Australia 42 (12): 562-564.
Walker J. 1987. The secret of a microwave oven's rapid cooking action is
disclosed. Scientific American. Febr 98-102.
Waters J.R. 1961. Tappi, 44 (7): 185A.
Whorton B.C. and G.A. Reineccius. 1989. Flavor development in microwave
vs. conventionally baked cake. In " Thermal Generation of Aromas,"
T.H. Parliament, R.J. McGorrin and C.T. Ho (Eds.). ACS Symposium
Series 409, American Chemical Society, Washington D.C.
Zadow J.G. and R.D.Hill. 1975. J. Dairy Res. 42:267-275.
Zallie Z.P. 1988. The role and function of starches in microwaveable food
formulation. Presented at MW Foods' 88, March 8, 1988, Chicago, Illinois.
Zylema B.J., J.A. Grider, J. Gordon and E.A. Davis. 1985. Model wheat
starch systems heated bu microwave irradiation and conduction with
equalized heating times. Cereal Chem. 62(6): 447-453.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Документ
Категория
Без категории
Просмотров
0
Размер файла
5 494 Кб
Теги
sdewsdweddes
1/--страниц
Пожаловаться на содержимое документа