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Temperature distribution and measurement for Grape Puffs(TM) in the microwave vacuum drying system

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ABSTRACT
TEMPERATURE DISTRIBUTION AND MEASUREMENT
FOR GRAPE PUFFS™ IN THE MICROWAVE VACUUM
DRYING SYSTEM
A combination microwave/vacuum drying system (MTVAC) is a unique
dehydration technology to produce puffed dried fruit products. The heating
mechanism o f the microwave process differs from conventional dehydration
processes. Therefore, temperature measurement and control are critical for
product quality in the MTVAC process. The objectives o f this thesis were to
determine the appropriateness o f temperature measurement and evaluate
temperature rise characteristics in relation to the power levels, and the dielectric
properties of grape as a function o f moisture content and temperature. Thompson
seedless grapes and w ater were treated in a batch type o f the MTVAC system. The
results o f water experiments indicated the infrared temperature detector had certain
limitations and the lower power level such as 500 W had faster temperature rising
curve than the other power levels. The grape experiments indicated moisture
content is a major factor that affected the dielectric constant o f grapes.
Chung-Chueh Cheng
August 1996
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TEMPERATURE DISTRIBUTION AND MEASUREMENT
FOR GRAPE PUFFS™ IN THE MICROWAVE VACUUM
DRYING SYSTEM
by
Chung-Chueh Cheng
A thesis
submitted in partial
fulfillment o f the requirements for the degree of
Master o f Science in Agriculture
in the School o f Agricultural Sciences and Technology
California State University, Fresno
August 1996
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UMI Number: 1382037
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APPROVED
For the Department of Enology, Food Science, and Nutrition:
Dennis A. Ferris (Chairman)
Carter D. Clary
.tthew
/
en
Enology, Food Science, and Nutrition
Viticulture Enology Research Center
Industrial Technology
For the Graduate Committee:
Dean, Division of Graduate Studies
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AUTHORIZATION FOR REPRODUCTION
OF MASTER S THESIS
I grant authorization for the reproduction of this thesis in part or
in its entirety without further authorization from me, on the
condition that the person or agency requesting reproduction
absorbs the cost and provides proper acknowledgment of
authorship.
Permission to reproduce this thesis in part or in its entirety must
be obtained from me.
Signature o f thesis writer;.
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ACKNOWLEDGMENTS
This thesis is dedicated to my parents, Mr. and Mrs. Y. T. Cheng, who
supported the costs of my course work at California State University, Fresno, and
encouraged me to complete this research project.
I acknowledge Dr. Matthew M. Yen, Dr. Carter D. Clary, and Dr. Dennis
A. Ferris for their guidance as members of my thesis committee. I extend my
gratitude to California Agricultural Technology Institute (CATI) for research
funding. M y appreciation goes to Kathy W harton and Amel Sanchez for their
assistance in the experiments.
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TABLE OF CONTENTS
Page
LIST OF TA B L ES........................................................................................................ vii
LIST OF F I G U R E S .................................................................................................viii
IN T R O D U C T IO N .................................................................................................
1
Grapes Dehydration P ro c e sse s.........................................................................
1
G o a l s ...................................................................................................................
9
D elim itations...............................................................................................................10
LITERATURE R E V IE W ............................................................................................. 11
I n tr o d u c tio n ...............................................................................................................I I
Temperature Distribution in Microwave System s.................................................. 18
Temperature Measurement in Microwave S y s t e m s ............................................25
Analysis o f Energy Efficiency for Grapes Dehydration Processes . . .
30
Energy Aspect o f W a te r.............................................................................................35
O B J E C T IV E S .............................................................................................................. 37
Water E x p e rim e n ts ...................................................................................................37
Grape E x p e rim e n ts ...................................................................................................38
MATERIALS AND M ETHODS................................................................................ 39
Description o f the Laboratory Batch Microwave Vacuum Un i t . . . .
39
P r o c e d u r e s .............................................................................................................. 43
Data H a n d l i n g .........................................................................................................48
RESULTS AND D IS C U S SIO N ................................................................................ 51
Water E x p e rim e n ts ...................................................................................................51
Grape E x p e rim e n ts ...................................................................................................67
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vi
Page
SUMMARY AND CONCLUSIONS.............................................................
Summary .
78
78
C o n c lu s io n s .............................................................................................................. 80
RECOMMENDATIONS
FOR FUTURE W O R K .................................................82
R E F E R E N C E S .............................................................................................................. 83
A P P E N D IX .................................................................................................................... 88
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U ST OF TABLES
Table
Page
1.
Microwave food processing applications............................................
2.
Specific energy consumption in heated-air drying of
grapes at two different air temperatures, 50 and 60 C
. . .
8
32
3.
Specific energy consumption in combined microwave
with convective drying at a 500-W power level with
two different air temperatures and v e lo c itie s ..................................... 33
4.
Specific energy consumption in microwave vacuum system
at five different treatment t e m p e r a t u r e s ........................................... 34
5.
Summary of microwave energy treatment
for water e x p e r i m e n t s ......................................................................... 44
6.
Summary of microwave energy treatment for grape
experiments
...................................................................
47
7.
Summary o f water column heights for three different weights
of water sa m p le s ......................................................................................56
8.
Effect o f sample weight on temperature rise rate
at 1500 ana 1000 W in a transparent glass b o w l ............................... 58
9.
Summary coupling coefficients and thermal efficiency
with water e x p e rim e n ts..........................................................................62
10. Dielectric properties and penetration depth of grapes,
calculated by Equations 1,2, and 3, dehydrated
by the MTVAC system with 908 g of sample weight
at 1500 W ..................................................................................................68
11. Dielectric properties and penetration depth of grapes,
calculated by Equations 1,2, and 3, dehydrated
by the MIVAC system with 454 g of sample weight
at 1500 W ..................................................................................................69
12. Dielectric properties and penetration depth of grapes,
calculated by Equations 1,2, and 3, dehydrated
by the MTVAC system with 454 g of sample weight
at 1000 W ..................................................................................................69
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LIST OF FIGURES
Figure
Page
1.
Typical counteiflow raisin drying tu n n e l...........................................
2.
W ater and salt content of microwave absoprtion for
a num ber of f o o d s ................................................................................ 14
3.
Microwave absorption for a number of foods in
freezing and ambient tem peratures........................................................16
4.
Dielectric properties of grapes as a function of
moisture content at 25TT, 2,450 M H z ..................................................20
5.
Dielectric properties of water as a function of temperature
at 2,450 M H z ............................................................................................22
6.
Temperature distribution for 25-mm-thick slab of ham and
for mashed p e a s ......................................................................................23
7.
Focusing effect as a function of penetration d e p t h ...............................26
8.
Transmission absorption and reflection of infrared energy
9.
Schematic diagram of the laboratory microwave vacuum
dehydration s y s t e m ................................................................................40
10.
Photograph of laboratory microwave vacuum drying
system
............................................................................................41
11.
Temperature difference between the fiberoptic and
the infrared detectors during the MIVAC process
.
. . . .
.
4
28
52
12.
Effect of emissivity on temperature measurements
at 908 g of water and 3000 W ............................................................. 53
13.
Effect o f emissivity on temperature measurements
at 908 g of water and 1500 W ............................................................. 53
14.
Temperature rise curve of 908 g of water at various power levels
in a transparent glass b o w l ....................................................................55
15.
Temperature rise curve of 908 g of water at various power levels
in a black plastic b o w l ..........................................................................55
16.
Typical layout of three different water weights
formed into three water column h e i g h t s ............................................57
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ix
Page
Figure
17. Nonlinearity of the infrared temperature measurement
of 908 g of water in a transparent glass b o w l ...................................... 59
18. Nonlinearity o f the infrared temperature measurement
o f 908 g of water in a black plastic b o w l ............................................ 59
19. Effect of sample weight on temperature rise rate
at 1500 W m a transparent glass b o w l ..................................................60
20. Effect of sample weight on temperature rise rate
at 750 W in a transparent glass b o w l ..................................................60
21. Coupling coefficients as a function of power level
and sample m a s s ......................................................................................63
22. Thermal efficiency as a function of power level
and sample m a s s ......................................................................................65
23.Thermal efficiency as a function of container’s material
. . .
66
24. The dielectric constant and moisture content of grapes
as a function of dehydration time with 908 g of grape
at 1500 W ..................................................................................................70
25. The dielectric constant and moisture content of grapes
as a function of dehydration time with 454 g of grape
at 1500 W ..................................................................................................71
26. The dielectric constant and moisture content of grapes
as a function of dehydration time with 454 g of grape
at 1000 W ..................................................................................................72
27. The loss factor and moisture content of grapes as a function
of dehydration time with 908 g of grape at 1500 W .
.
.
.
74
28. The loss factor and moisture content of grapes as a function of
dehydration time with 454 g of grape at 1500 W ............................... 75
29. The loss factor and moisture content of grapes as a function of
dehydration time with 454 g of grape at 1000 W ............................... 76
30. Photograph of burning grapes at fixed power
during the MTVAC p ro c e s s ....................................................................77
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INTRODUCTION
The removal of moisture from a food is known as dehydration (Henderson
and Perry, 1976). Dehydration extends the shelf life of food and ensures that the
product remains stable and safe for consumption (Clary, 1994). Preservation of
grapes in the form of raisins requires dehydration and is an important market for
the grape industry. W orldwide raisin production in 1992 was over 1 million tons
and was valued at $120 million (Food and Agricultural Organization [FAO],
1992). Thompson seedless and similar varieties account for the m ajority of global
raisin production (Tulasidas et al., 1993).
Grapes Dehydration Processes
There are several processes used to dehydrate grapes. In California, the sun
drying process is the most common practice. Advancements in technology use
heated air dehydration, which reduces the risk o f crop loss due to inclem ent
weather during the sun drying season (Clary, 1994). Use of heated air dehydration
has resulted in the production of new types of dried grapes that cannot be
produced by the sun drying process. Owing to the extended time requirements for
sun drying and heated air dehydration, research to reduce drying time for raisin
products is currently being conducted by several authors. Using microwave
technology rather than a heated air process, reduces the processing time required
to dehydrate grapes (Tulasidas et al., 1993). Additionally, a combination of
microwave and vacuum system has demonstrated good potential for the
dehydration of grapes. However, this latter system allows grapes to maintain most
of their fresh characteristics, such as color, flavor, and shape (Petrucci and Clary,
1989; Clary, 1994).
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Sun Drying Dehydration
The production o f sun-dried raisins requires a large amount of labor
because grape bunches are cut individually and laid on paper trays between the
vineyard rows. The tray raisins are turned over after 2 wk and are allowed to dry
until their final moisture content is about 14%. Finally, the paper trays are rolled
into packets, picked up, and dumped into wooden bins for delivery to the
processing plant (Bolin et al., 1975; Clary, 1994).
This process has several disadvantages, such as long drying times,
contamination of the product, poor quality, and product losses due to adverse
weather conditions (Raouzeos and Saravacos, 1986; Tulasidas et al., 1993). When
Thompson seedless grapes are dried in the sun, grapes in which polyphenol
oxidase activity has occurred typically change in color from light green to dark
reddish-brown (Singleton etal., 1985).
Heated Air Dehydration
Heated air is used for the dehydration of about 30% of all dried fruits
produced in the United States (Nury et al., 1973). Heated air dehydration of fruit
offers an advantage over sun drying in the presence of inclement weather. The air
is usually heated by natural gas flames. The advantages of using heated air to
dehydrate fruit are (1) increasing the vapor pressure of water in the fruit by
warming it to about 65-70°C; (2) carrying the water vapor away from the fruit;
(3) reducing drying time; and (4) promoting production of a new type of dried fruit
that cannot be produced by sun drying, particularly when color preservation is
required and sulfur dioxide is used (Nury et al., 1973; Petrucci et al., 1993; Clary,
1994). Today, 15% of the California’s grape crop is dried in gas-fired, forced-air
dehydrators for production of golden seedless and dipped raisins (Nury et al.,
1973; Clary, 1994).
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There are several types of air convection processors used for drying food
products, including kiln, cabinet, continuous belt, spray, and crossflow tunnel
dryers (Potter, 1968). Grapes are high in sugar and moisture content, so they are
difficult to transport on belt-type dryers. The most common method for
dehydrating of grapes employs a tunnel dryer (Clary, 1994). The tunnel dryer
consists of a long cabinet that is filled with dehydrator carts (Fig. 1). Each
dehydrator is loaded with stacks of wood trays that are covered with grapes
(Potter, 1968; Clary, 1994). Grapes should be dried in a counterflow tunnel
dehydrator to attain an acceptably low final moisture content (Clary, 1994).
However, prunes and other large fruits can be processed in a concurrent-flow
tunnel dehydrator for drying. As shown in Fig. 1, the w et cars move from right to
left and the drying air moves across the trays from left to right. This is the
principle upon which a counterflow tunnel dehydrator operates. The significance
of counterflow dryer is the heated air. First, heated air contacts the cars containing
nearly dry grapes. Then, as the drying process continues, there is a cooling effect,
wherein moisture is absorbed by the heated air as the fruit continues moving
through the tunnel (Potter, 1968; Clary, 1994).
The effect of air temperature and humidity. Heating air temperature and
humidity are important factors in controlling raisin quality and are critical for
saving time and energy. Air temperature should not exceed 65°C for drying
grapes; if the air temperature rises above 70°C during dehydration, grapes show
burning (Nichols and Christie, 1930; Ponting and McBean, 1970; Eissen and
Muhlbauer, 1984; Clary, 1994). Eissen and Muhlbauer (1984) and Nichols and
Christie (1930) suggested the level of relative humidity in the exhaust air of a
counterflow tunnel should be between 35 and 40%.
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4
AIRFLOW
FAN
BURNER Q A I R
IN
M
FRUIT
EXIT
mtm
AIR Lmmm
RETURNI
AIR
EXHAUST
SIDEVIEW
END VIEW
Fig. 1. Typical counterflow raisin drying tunnel. From Clary (1994)
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The effect o f pretreatment on the drying time and the quality o f grapes.
Pretreating grapes includes dipping them in hot water, sodium hydroxide, fatty
acid, or fatty acid ester to accelerate drying (Ponting and M cBean, 1970; Bolin et
al., 1975; Raouzeos and Saravacos, 1986; Clary, 1994). Dipping grapes in a hot
water bath causes some epidermal cells to rupture and hairline cracks to develop in
the skin. Moisture can easily escape from the tissue through these cracks during
the drying process, thereby reducing the overall drying time (Clary, 1994). During
the m ost commonly used commercial process, the soda-dip process, the grapes are
dipped into boiling water or hot water (82°C) containing 0.25% sodium hydroxide
before dehydration (Bolin et al., 1975; Clary, 1994). Concentration o f sodium
hydroxide and w ater temperature depend on the source and maturity of the grapes.
Clary (1994) reported that 6 to 8 g of sodium hydroxide per liter of water produces
the best results.
In Australia, fatty acid or ester has been used as a pretreating agent for
grapes (Gmcarevic, 1963; Ponting and McBean, 1970; Clary, 1994). Raouzeos
and Saravacos (1986) found that the best dipping solution and conditions were
obtained by pretreating the grapes in a hot solution (82°C) of 0.5% sodium
hydroxide and 2% ethyl oleate for 30 sec. This method resulted in a short drying
time and good quality raisins. The researchers also observed that dipping the
grapes in a higher temperature (100°C) solution resulted in a poor quality of
raisins. Ponting and McBean (1970) found that drying rate increases and drying
time decreases with increasing fatty acid or ester concentrations. However, there
is a limitation when using a high concentration of fatty acid or ester as a
pretreating agent. Because a high concentration of fatty acid or ester does not
accelerate drying, the process is more expensive. Using a high concentration of
fatty acid or ester also produces a noticeable flavor in the raisins. Stafford et al.
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(1974) dipped grapes in a 2% aqueous emulsion of fatty ester or acid, which
provided maximum residues of fatty derivatives of about 140 ppm and offered the
best compromise between drying rate and cost. Bolin et al. (1975) reported that
using 2% of methyl or ethyl oleate accelerated dehydrating rate and did not affect
flavor, even after periods of storage.
Sodium hydroxide dipped raisins that are not treated with ethyl or methyl
oleate require 2 hr longer to dry than do methyl- or ethyl-treated grapes. Oleatetreated raisins are free flowing and are not as sticky as soda-dipped raisins (Bolin
et al., 1975). Additionally, grapes were dipped in a hot sodium hydroxide
solution, which caused small physical fractures in the skins and resulted in a loss
of juice and difficulty in handling (Bolin et al., 1975; Clary, 1994).
Golden seedless raisins. Thompson seedless grapes are treated with sulfur
dioxide to produce golden seedless raisins (Bolin et al., 1975; Clary, 1994). Sulfur
dioxide is highly effective in controlling browning and in preserving original
colors (Petrucci and Clary, 1989; Sapers, 1993). Two to 4 kg of sulfur dioxide in
an airtight enclosure is applied per ton of grapes. Raisins need to contain about
2000 ppm o f sulfur in order to preserve color for 1 yr or longer (Clary, 1994).
Unfortunately, the use of sulfur compounds on food products has been
associated with severe allergy-like reactions in asthmatics. A small portion of the
population in the United States are likely to be sulfite-sensitive (Petrucci and
Clary, 1989; Sapers, 1993). In 1986, the Food and Drug Administration (FDA)
required that sulfited foods be labeled when detectable amounts of sulfite residues
remained in the finished product (Sapers, 1993).
Alternatives of sulfite. Sulfite has multiple functions in food processing,
such as inhibiting enzymatic and nonenzymatic browning and controlling the
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growth of microorganisms (Sapers, 1993). Current alternatives for sulfite are not
equivalent to sulfite in effectiveness, cost, or functionality. Among these, the best
alternative to sulfite is ascorbic acid (Sapers, 1993).
Microwave Process
Research for an efficient drying technique for raisin production is ongoing
because using heated air or the sun to dehydrate grapes is a time-consuming
process. In comparison, microwave processing may be a way to save drying time
for the raisin industry. The microwave method offers several advantages over
conventional heating (Mudgett, 1982; Giese, 1992). These advantages are
(1) rapid and uniform heating for food products; (2) reduction of surface
temperature; and (3) precise control of processing factors, such as faster start-up
and shut-down (Mudgett, 1982; Decareau, 1985; Giese, 1992; Clary, 1994). The
microwave process is currently being used for several food processes including
tempering, blanching, pasteurization, sterilization, cooking, and dehydration
(Table 1) (Decareau, 1986; Clary, 1994).
Tulasidas et al. (1 9 9 3 ,1995a) reported on combining conventional air
heating and microwave heating for dehydration of grapes. Raisins were lighter in
color and processing time was reduced by 60%, as compared to conventional
heating, during this process. Tulasidas et al. (1995a) mentioned that microwave
drying had lower specific energy consumption than conventional drying.
Microwave Vacuum Dehydration
Microwave vacuum dehydration was first used for concentration of orange
juice in France (Decareau, 1985). Adoption of MIVAC* technology to fruits
* This technology is patented by McDonnell Aircraft Company and CSU, Fresno.
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8
T a b le t. Microwave food processing applications. Adapted from
Decareau (1986) and Clary (1994)
Application
Frequency
(MHz)
Products
Baking
915
Bread, donut proofing
Cooking
915
Bacon, sausage, potatoes
Drying
915 or 2,450
Pasta, snack foods, onions
Freezing Drying
915 or 2,450
Meat, vegetables, fruits
Vacuum Drying
915 or 2,450
Fruit juices, seeds, grains
Pasteurization
2,450
Milk, yogurt
Sterilization
2,450
Sliced bread, prepared meals
Tempering
915
Meat, fish, berries
Blanching
2,450
Com, fruits, potatoes
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offers the food processing industry an opportunity to create new and unique puffed
dried fruit products (Petrucci and Clary, 1989). The vacuum system reduces the
temperature necessary to dry the fruit and minimizes oxidation of the product.
Microwave power supplies energy to uniformly vaporize water (moisture) from the
product. This system allows fruit to maintain most of its fresh fruit characteristics
such as color, flavor, and shape, without using chemical treatments for
preservation (Petrucci and Clary, 1989; Clary, 1994).
In 1978, the MTVAC technology was first applied to dehydrate grapes for
raisin production (Petrucci and Clary, 1989). Then this technology was adapted to
grapes by M cKinney et al. (1983) for production of Grape Puffs™. They used a
* rv #
zoned microwave vacuum dehydration system to produce Grape Puffs
. This
process was patented by McKinney and W ear (1987) and described by Petrucci
and Clary (1989).
Goals
The goal of this thesis was to evaluate temperature rise characteristics in
relation to the power levels during the MTVAC process. To measure accurate
temperature rise rate, the appropriateness of the temperature detector is the most
important parameter.
Heating water and grape samples during the MTVAC process are described.
Experimental methods and results are reported. Water experiments served as a
benchmark to evaluate the accuracy of the infrared temperature detector. The rate
of temperature rise can be related to the power levels during the MTVAC process.
Additionally, the temperature rise curve resulting from the water experiments may
be used to gauge the coupling coefficients and thermal efficiency for the MIVAC
process.
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10
Grape experiments measured final temperature and moisture content during
different conditions of the MIVAC dehydration. The results o f the experiments
were used to calculate the dielectric properties of grapes and penetration depth of
microwave energy into the grapes. An understanding o f the dielectric constant and
the loss factor of grapes during the M IVAC process in relation to the moisture
content and temperature changes is essential for better process controls. On the
other hand, the computed penetration depth served to determine the optimum size
of grape to be used for uniform heating during the MTVAC process.
Delimitations
This study employed only a laboratory microwave vacuum drying system.
Consequently, the study’s results cannot be generalized for the continuous
microwave vacuum drying system. Experiments were scheduled according to a
stringent M IVAC operational procedure. Therefore, only a limited set of data
could be analyzed.
The fiberoptic temperature detector was a demonstration unit from a
photoptical company. Only one test run was conducted for the water experiments.
W ater experiments using the vacuum system induced violent evaporation, and no
meaningful data could be obtained. Therefore, water experiments were performed
at atmospheric pressure.
The results obtained from the grape experiments are applicable only to
Thompson seedless grapes, using fixed microwave power levels and an
atmospheric pressure of about 2.7 KPa (20 Torr).
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LITERATURE REVIEW
Introduction
The microwave process has been applied to the food industry since the
1950s (Schiffmann, 1986) and offers distinct advantages over other, more
conventional heating methods (Stuchly and Hamid, 1972; Mudgett, 1982).
However, this process is not fully understood, which limits the optimization
procedure, particularly with regard to uniformity of temperature distribution in the
treated materials (Stuchly and Hamid, 1972). hi order to investigate this problem,
it is necessary first to understand the principle of microwave heating.
The History of the Microwaves
During W orld W ar n, British scientists were working desperately to
produce a source of microwave energy to power radar for the military because o f
the threat of German invasion (Reynolds, 1995). In February, 1940, Professors
Randall and Boot invented an electronic tube that they named a “cavity
magnetron” (Reynolds, 1995). The cavity magnetron generated the large amounts
of pulsed microwave energy required to make radar equipment operational. It was
more powerful and m ore accurate than anything previously designed. Tizard, a
preeminent British scientist, hoped to bring the magnetron to the United States in
order to take advantage of America’s vast production potential (Reynolds, 1995).
During the development of microwave technology for radar, in 1943-1944,
W.C. Brown and P. Derby created the first continuous wave output microwave
generator, which produced 100 W at 3,000 MHz (U.S. Patent 2,463,524) (Thuery,
1991; Clary, 1994). In 1945, Percy Spencer was intrigued by his observation that
microwave energy could generate heat in various food products (Reynolds, 1995).
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12
Not long after Spencer’s discovery, the first microwave oven designed was
marketed by Raytheon in 1947, and patented in 1951 by Percy Spencer (Cockbum,
1958; Decareau and Peterson, 1986; Clary, 1994; Reynolds, 1995).
The early microwave heating units showed potential for cooking. However,
the range of their applications remained limited to laboratories and the food
service industry because o f the high cost of generating microwave pow er and
because of technical problems encountered in controlling temperature (Clary,
1994). It was not until the 1960s that a practical continuous wave system was
devised (Decareau, 1985). By 1970, using microwaves as heat sources was
demonstrated by the marketing success of microwave ovens. Today, U.S.
microwave oven sales consist of more than 10 million units a year, and 92% o f all
U. S. homes have at least one (Baum, 1992; Reynolds, 1995).
Principles of Microwave Heating
Microwave heating is achieved by energy transfer to a dipole molecule
(Ohlsson, 1983). The m ost common dipole molecule is water. Both w ater and salt
content o f foods affect the microwave absorption (Ohlsson, 1983). Defrosting
foods by microwave is faster than by conventional heating (Ohlsson, 1983).
Microwave basics. Microwaves are electromagnetic waves of radiant
energy and have wavelengths between radio and infrared waves on the
electromagnetic spectrum (300 MHz to 300 GHz) (Giese, 1992; Clary, 1994).
Microwaves radiate from a source and can be transmitted, reflected, and refracted.
Microwaves are generated by a magnetron tube, which consists of a cavity, an
anode, and an antenna that emits high-frequency radiant energy signals. The
signal has centers of positive and negative charges that change direction billions of
times per second (Decareau, 1985; Thuery, 1991; Giese, 1992; Clary, 1994).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
13
W hen microwaves are directed toward a food, they are absorbed and
produce heat in the food. There are two major contributors to microwave
absorption and heating in foods. First, polar molecules, primarily water, attempt
to rotate with the microwave electric field. Second, ions, primarily sodium and
chloride, tend to move linearly back and forth within the microwave electric field
(Clary, 1994; Buffler and Stanford, 1995). The motion of these particles in turn
transfers them, as heat, to the other molecules in the food (Buffler and Stanford,
1995). Both the water and salt content of food affects the food’s absorption of
microwaves (Ohlsson, 1983). The water and salt content of microwave absorption
for a number of foods is illustrated in Fig. 2. The foods containing a high amount
of water have high microwave energy absorption at ambient temperature (20°C).
With increasing temperature, water absorbs less microwave energy. On the other
hand, salty foods such as gravy absorb more microwave energy at higher
temperatures (Ohlsson, 1983).
One of the most dramatic features of microwave heating is the rapidity with
which it defrosts frozen foods. However, microwave defrosting still presents
certain difficulties because of the large differences between the microwave
absorption rates and thermal properties o f frozen foods and defrosted foods
(Ohlsson, 1983). In frozen food, m ost of the water present is ice, which restricts
the movement of water molecules and, thus, allows very litde microwave energy to
be absorbed (Ohlsson, 1983; Clary, 1994). However, in defrosted food, the water
present is not frozen. A percentage of the food ingredients is liquid, forming a
concentrated solution comprised of such solutes as salt and sugar, etc., which
allows free movement of water molecules. Thus, the absorption of microwave
energy is much greater in defrosted foods than in frozen foods (Ohlsson, 1983).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
COOKING
OIL
W ater
Content
Salt
Content
WATER
MEAT
GRAVY
0%
100%
70%
70%
0%
0%
1%
4%
Fig. 2. W ater and salt content of microwave absorption
for a number of foods. From Ohlsson (1983)
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15
M icrowave absorption for a number o f foods under freezing and ambient
temperature conditions is shown in Fig. 3. Fig. 3 reiterates the effect indicated in
Fig. 2, that foods with higher salt content, such as gravy, have higher microwave
energy absorption, both at freezing and ambient temperatures (Ohlsson, 1983).
Heat transfer phenomena. Heat transfer is defined as the transmission of
energy from one region to another as a result of the temperature difference
between them. According to the second law o f thermodynamics, the heat moves
from the.region of higher temperature to the region of lower temperature. There
are three modes of heat transmission: conduction, radiation, and convection. In
microwave heating, heat transfer occurs as a simultaneous combination of all three
modes (Stuchly and Hamid, 1972).
Moisture transport phenomena. It is also necessary to investigate the
process of moisture transfer during microwave heating because most foods contain
water. For example, grapes contain an average of 80% water (moisture). The
removal of moisture from the product can be divided into two periods: the constant
and the falling drying-rate periods (Stuchly and Hamid, 1972). During the
constant drying-rate period, the surface of the product remains saturated with
liquid water and dries in a manner comparable to an open surface of water. The
drying rate depends on the ambient conditions and the total water surface area
(Stuchly and Hamid, 1972). The constant drying-rate period continues until the
moisture disappears from the surface. The moisture content at which the drying
rate ceases to be linearly constant is called the critical moisture content.
The falling drying-rate period starts when critical moisture content is
reached and involves two processes: the movement of moisture within the product
to the surface and the removal of the moisture from the surface (Stuchly and
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
COOKING
OIL
Water
Content
Salt
Content
WATER
MEAT
GRAVY
0%
100%
70%
70%
0%
0%
1%
4%
Fig. 3. Microwave absorption for a number of foods in freezing and
ambient temperatures. From Ohlsson (1983)
Reproduced with permission o f the copyright owner. Further reproduction prohibited without permission.
Hamid, 1972). In the falling drying-rate period, the saturated moisture surface
area decreases and the moisture movement within the product cannot provide
enough moisture, so its drying rate is lower than the constant drying rate. The
movement of moisture within the product to the surface becomes the important
factor when the product surface area reaches the unsaturated state.
Microwave frequencies. In the United States, the Federal Communications
Commission’s (FCC’s) approved microwave frequencies are 915 and 2,450 MHz
for microwave h a tin g because other frequencies are close to radio wave
frequencies and may overlap the radar range (Mudgett, 1986; Geise, 1992; Clary,
1994). In microwave heating, the penetration depth at 915 MHz (33 cm) is more
than twice penetration depth, at 2,450 MHz (12.2 cm) (Mudgett, 1982,
Schiffmann, 1986). Up to six additional frequencies, ranging from 433.05 to
246,000 MHz, have been allocated for microwave heating outside the United
States (Thuery, 1991).
Microwave safety considerations. The safety of microwaves remains a
m ajor issue as microwave processing continues to gain acceptance by the food
industry (Mudgett, 1986; Geise, 1992; Clary, 1994). Although athermal effects of
microwaves have been suggested, the sole result of microwave interactions with
foods appears to be their heating effect (Mertens and Knorr, 1992). In 1968,
Public Law 90-602 established emission standards for home and industrial
microwave ovens of 1 tnW/cm2 prior to sale and 5 mW/cm for the duration of
their usage lifetime (Anonymous, 1970; Clary, 1994). Besides, every home
microwave oven door needs to have two interlocks and a monitor that stops
microwave generation when the door is opened (Decareau, 1992). The
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18
measurement o f microwave emission, is made 5 cm from the surface o f the oven
(Clary, 1994).
Temperature Distribution in Microwave Systems
Heating with microwave energy may result in unacceptable food quality
because of the rubbery or soggy textures produced. However, the primary reason
for unacceptable food quality in microwave-heated foods is uneven temperature
distribution (Vilayannur and Puri, 1995). Several factors influence the uniformity
o f temperature distribution, including microwave energy absorption, dielectric
properties, shape and size o f foods, and penetration of foods by microwave energy
(Ohlsson and Bengtsson, 1976; Ohlsson, 1983; Tulasidas et al., 1995b; Vilayannur
and Puri, 1995).
Heating Mechanisms of
Microwaves
Temperature distribution for microwave heating and conventional cooking
are different. During conventional cooking, the highest temperature is on the
surface of the product and the lowest in its center (Ohlsson, 1983). During
microwave heating, energy is absorbed by the foods. The highest temperature may
be anywhere in the food, which causes a hot-and-cold spot phenomenon (Ohlsson,
1983; Buffler and Stanford, 1995).
Dielectric Properties
In order to better understand microwave heating behavior, it is essential that
one has the knowledge o f their dielectric properties (Tulasidas et al., 1995b).
Dielectric properties play a critical role in how microwave energy is distributed in
food products (Buffler and Stanford, 1995). Dielectric properties o f materials are
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19
defined in terms of dielectric constant1 (e1) and loss factor (£"). The dielectric
constant is a measure of a material’s ability to store electrical energy, and the loss
factor is a measure of its ability to dissipate electrical energy (Mudgett, 1986;
Nelson and Kraszewski, 1990; Tulasidas et al., 1995b). The power dissipated
inside a m aterial is proportional to £" and the ratio, £"/£*, is referred to as the loss
tangent. This tangent is an indicator of the material’s ability to generate heat
(Buffler, 1993; Mudgett, 1986; Tulasidas et al., 1995b). Two factors affect the
dielectric properties of food material: the change effects, due to ionic species
concentration, and the dipole, due to moisture (Tulasidas et al., 1995b).
Dielectric properties o f grapes at 2,450 MHz were studied by Tulasidas et
al. (1995b). The researchers observed that for fresh grapes at the ambient
temperature, the values of £' and £" were 69.8 and 17.5, respectively, and for
raisins containing 15 % moisture, these parameters dropped to 7.5 and 2.2,
respectively (Fig. 4). The values of £' and £" are dimensionless and usually range
between 0 and 100. This phenomenon occurs as a result of the reduction of
moisture content in the fruit. Both the dielectric constant and the loss factor of
grapes are decreased by decreasing moisture content during microwave heating.
During m icrowave heating, the dielectric properties are considerably influenced by
higher temperatures when grapes contain a low moisture content (Tulasidas et al.,
1995b).
The dielectric properties of water were predicted by the Debye equation for
pure polar solvents as functions of frequency and temperature (Appendix)
(Mudgett, 1982). Decareau (1985) also described the dielectric properties of water
1 Dielectric constant is an electrical property of materials. The term “dielectric
constant” has been used widely by the electric industry. It is actually not a
constant, but is a function of frequency, temperature, and moisture content.
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20
♦ Dielectric Constant
0
20
40
60
Moisture content, % (wet basis)
80
Fig. 4. Dielectric properties of grapes as a function of moisture content
at 25 C, 2,450 MHz. Data from Tulasidas et al. (1995b)
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21
at 2,450 MHz. For pure water at 20°C, the values of e' and e" are 80.4 and 11,
respectively. Increasing water temperature decreases both e' and e" (Fig. 5).
Penetration Depth o f Microwave
Energy
One of the transmission properties of microwaves that is o f basic interest is
the penetration depth of microwave power into foods. This determines the relative
ability o f a product to attenuate absorbed energy and to couple energy from the
microwave power (Mudgett, 1986).
Over the past 10 yr, researchers have found that the penetration depth of
microwave energy into foods is determined by the dielectric constant and loss
factor. The penetration depth (dp) is commonly defined as the distance from the
surface of a product at which the transmitted power drops to 36.8% of its value at
the surface (Stuchly and Hamid, 1972). The value of 36.8% is derived from a
mathematical construct, the Napierian base e = 2.718 (Stuchly and Hamid, 1972;
Buffler and Stanford, 1995). The exact penetration depth value is extremely
complicated. An approximate equation that explains the penetration depth for
virtually all foods is given by:
dp = X0 J e ' / l i z e ”
Here Xo is the free space microwave wavelength. The M IVAC system uses
2,450 M Hz of microwave frequency so Xo is equal to 12.2 cm (Buffler and
Stanford, 1995).
The differences in penetration depth affect the ability of microwaves to
transfer uniform heat into foods. Fig. 6 illustrates the temperature distribution for
a 25-mm-thick slab of ham and for mashed peas. In the ham, the energy is
absorbed close to the surface because of a very low penetration depth (4 mm at
40°C). However, in the mashed peas, the microwave energy can penetrate into the
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90 ^ ►
♦
80 -
•
♦
♦
70 |
C/5
U
♦
♦
•
60 -
♦
•
o
8 !
50 o
'S I 4 0 £ -3
"3
♦ Dielectric C o n s a n t
■ Loss F a c t o r
30 *
40
60
T e m p e r a t u r e (°C)
■
■
■
■
I
20
-- ■
0 0
--
i
20 1I
10 -
■
S
80
100
Fig. 5. Dielectric properties of water as a function of temperature
at 2,450 MHz. Data from Decareau (1985)
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23
O
60
:
ftas
£ 50
&
&
I 40-
Ham
30 - -
O
O
oCN
Thickness (nrr}
Fig. 6. Temperature distribution for 25-mm-thick slab of ham and for
mashed peas. From Ohlsson (1983)
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24
center of the food and provide a uniform temperature distribution due to higher
penetration depth (12 m m at 40°C). In the ham, the central parts have to be heated
by heat conduction from the hotter surface areas. Heat conduction is m uch slower
than microwave heating, so the risks for overheating of the surfaces are evident.
The remedy for overheating ham on the surface is to heat it slowly, using lower
microwave energy (Ohlsson, 1983).
Size and Shape
The effect of shape and size on temperature distribution was studied by
Ohlsson and Risman (1978), who found that when irregular-shaped foods are
heated with microwaves, non-uniform temperature distribution with hot and cold
spots are formed. These nonuniformities have been attributed to the scattering and
concentration effect. The authors also observed the heating concentration in the
central pan> of cylinders, with diameters in the range of 18 to 35 mm, and of
spheres, in the range o f 20 to 60 mm.
Product shape influences microwave heating patterns and can cause surface,
edge, and center heating effects. This effect is also called “field concentration
effect” (Ohlsson, 1990). It is known that foods with a slab geometry are difficult
to heat, particularly at the comers and edges of the product, during microwave
heating. Since m icrowave energy can be thought of basically as impinging upon
the food from all directions, the product comers are more susceptible to
microwave heating because they are exposed to microwave energy coming from
numerous directions. On the other hand, edges absorb less energy than comers,
and centers tend to be the coldest areas of foods undergoing the microwave
process (Buffler and Stanford, 1995).
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25
The field concentration effect for the spherical and cylindrical geometry of
food depends on penetration depth. If penetration is greater than the dimension of
the food, most of the microwave energy will travel through the product without
being attenuated. If the penetration depth is less than the dimension of the food,
most of the energy w ill be absorbed within one penetration depth from the food
surface. This causes intense heating at the surface, but leaves the cold spot in the
center (Fig. 7a) (Buffler and Stanford, 1995). For intermediate penetration depths,
the microwave energy is not completely dissipated in the outside layer.
Microwave energy is concentrated at the center of food before being attenuated, so
concentrated or focused heating at the center will occur (Fig. 7b) (Buffler and
Stanford, 1995).
The influence o f food container size on temperature distribution was
studied by Vilayannur and Puri (1995). They found that in heating the same
volume of sample, the food contained in the larger dish had a lower dimension
ratio, resulting in higher bulk and center temperatures than in the food contained in
the smaller dish subjected to microwave power of the same mass-to-power ratio.
Tem perature Measurement in Microwave Systems
Temperature control is critical for product quality in microwave systems.
An understanding o f the temperature profile during the process is essential for
better process controls, so temperature measurement is the primary quantification
tool. Temperature distribution in microwave heating varies with the products
being dried, m icrowave power, product load, moisture content, and dielectric
properties, among other factors (Stuchly and Hamid, 1972; Yen and Clary, 1995).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Diameter = 3 indies
a) dp = 1 inch
b) dp = 3 inches
Fig. 7. Focusing effect as a function o f penetration depth.
Adapted from Buffler and Stanford (1995)
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27
Problems with Conventional
Electrical Temperature
Measurement Techniques
Measurement techniques used to determine time-temperature profiles for
products within a microwave field, specifically those using conventional
thermometry systems such as thermistors or thermocouples, are not accurate, since
they interact with the magnetic component of the field and may cause arching at its
surface (Berek and Wickersheim, 1988). Temperature profiles can also be
measured by glass thermometers containing fluid with low thermal expansion
coefficients. However, this detector is bulky and slow in its thermal response
(Bengtsson and Lycke, 1969).
Infrared Temperature Detector
An infrared (IR) detector is capable of reconstructing a thermal image on
the surface of the product (Berek and Wichersheim, 1988). However, the IR
detector has certain limitations. Only the surface temperature is measured, and the
correlation with internal temperature may be difficult to determine. The IR
detector also requires that each object of emissivity be known in order to obtain
absolute temperature readings (Bengtsson and Lycke, 1969). For temperature
measurement using the IR detector, it is important to note the significance of
emissivity.
Principles of infrared temperature detector. Every object has a temperature.
When the object’s temperature equals that of its surroundings, the amount of
thermal radiation absorbed by the object equals the amount emitted by the object.
However, not all the radiation that falls on an object is absorbed. There are three
modes by which the radiant energy striking an object may be dissipated:
absorption (a), transmission (t), and reflection (r) (Fig. 8).
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Emitted
Energy
Reflection (r)
Absorption (a)
*
Transmission (t)
Fig. 8. Transmission absorption and reflection o f infrared energy.
From Holman (1981)
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Therefore,
a+t+r=1
Theoretically, if all the energy falling on an object were absorbed (no
transmission or reflection), absorptivity would be equal to 1. Such an object is
referred to as a “blackbody.” At the equilibrium temperature, all the energy
absorbed would also be emitted. Thus, a = e = 1. This is known as K irchoff s law
(Holman, 1981).
Practically speaking, the emissivity of real objects is always less than 1.
Typical values of some materials at room temperature (25°C) are shown below
(Holman, 1981):
Material
Total Emissivity
Aluminum
0.05
Carbon
0.81
Gold
0.02
Water
0.96
Wood
0.89
As may be inferred, bright and shiny objects have low emissivity values
whereas dull, opaque surfaces have high emissivity values. Further, the emissivity
o f a material m ay vary with temperature and wavelength. W hen using the IR
detector for temperature measurement, the emissivity of the object must be known
and accounted for.
Fiberoptic Temperature Detector
For making on-line temperature measurements during microwave heating,
Fluoroptic Thermometry using a fiberoptic detector is the best technique for
obtaining accurate temperature readings during the microwave process. This
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30
technique is used widely during food and package development testing. The
fiberoptic detector was developed by Luxtron over the last 10 yr and is used
extensively for measuring temperatures in electrically hostile environments, such
as microwave or magnetic fields (Wichersheim and Sun, 1987; Berek and
Wickersheim, 1988). The sensor is phosphor attached to the tip o f an optical fiber.
The fiberoptic detector is neither electrically nor thermally conductive, and its
materials have been selected for minimal dielectric heating. The detector measures
temperatures ranging from -200°C (-328 F) to +440 C (+824 F) (Wichersheim and
Sun, 1987; Berek and Wickersheim, 1988).
Principles of fiberoptic temperature detector. A fiberoptic temperature
detector is made of mixed phosphor with a transparent binder and is formed into a
small disc that is attached to the end of a silica fiber (Berek and Wickersheim,
1988). The fiber can transmit light efficiently to any measuring temperature
location. Phosphor can be made to emit light when excited by radiation of higher
energy level or shorter wavelength (Berek and Wickersheim, 1988). The
particular phosphor used in fiberoptic detectors is magnesium fluogermanate
activated with tetravalent magnese which has a decay time when measuring
temperatures, from about 5 ms at 200°C to 0.5 ms at 450°C (Wichersheim and
Sun, 1987; Berek and Wickersheim, 1988).
Analysis of Energy Efficiency for Grapes
Dehydration Processes
Microwave processes have been established in various food companies and
are generally successful in processing low- and intermediate-moisture food
products (Stuchly and Hamid, 1972; Mudgett, 1986). However, microwave
processing of high-moisture food products has been less successful because of
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31
evaporative cooling effects which reduce the food’s surface temperature (Mudgett,
1982). In this regard, using a combination of microwave and convective heating
methods has proven to be more effective than microwave heating alone (Mudgett,
1982). Moreover, microwave usage offers an increasing economic advantage in
that the cost of more traditional energy sources, such as oil and natural gas,
continue to rise (Mudgett, 1982). On the other hand, because of continuing design
and manufacturing improvements, the cost of microwave processing is decreasing
(Mudgett, 1985).
Tulasidas et al. (1 9 9 3 ,1995a) dehydrated Thompson seedless grapes in a
modified microwave oven, using a combined microwave/conventional heated-air
technique, according to three drying parameters: inlet air temperature, microwave
power density, and air velocity. It was found that the combined microwave/
conventional heated-air technique consumed less energy than did conventional
heating alone.
Clary (1994) used microwave heating in a vacuum environment to
dehydrate Thompson seedless grapes, under an atmospheric pressure of about 2.7
KPa, in the MTVAC system. The dehydrated grapes contained less than 5%
moisture content. The final product also maintained its original color, shape, and
puffed character. Using the MIVAC system was found to have a lower rate of
specific energy consumption than that of conventional heating.
Specific Energy Consumption for
Grape Dehydration Processes
Evaluation of the specific energy consumption required to dehydrate grapes
is an important way to analyze processing costs. The following discussion is
based on the use o f a laboratory scale device to dehydrate grape samples. The
specific energy consumption requirements provide important information for
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32
process design. Specific energy consumption is defined as the total energy used to
dehydrate grapes to final dried products and is expressed as MJ/Kg for fresh grape
samples.
Specific energy consumption in heated-air drying. Using the heated-air
drying method, Tulasidas et al. (1995a) studied the specific energy consumption
required to dehydrate pretreated grape samples at two different air temperatures:
50°C and 60°C. The researchers found that drying at an air temperature o f 60°C
resulted in a reduction of both time and energy, with an 11% lower specific energy
consumption rate than when air temperature was 50°C (Table 2).
Table 2. Specific energy consumption in heated-air drying of grapes at
two different air temperatures, 50 and 60 C. Adapted from Tulasidas et al.
(1995a)
A ir temperature
(°C)
50
60
Air velocity
(m/s)
Dehydration
time (hr)
2.00
2.00
23.7
16.8
Specific energy
consumption
(MJ/Kg)
66.9
60.1
Specific energy consumption in combined microwave with heated-air
drying. Tulasidas et al. (1995a) also used a combination convection and
microwave procedure to dry grapes in a modified microwave oven, then analyzed
this dehydration process for specific energy consumption. Grape samples were
heated under a 500-W power level at two different air temperatures and velocities.
Air temperatures were 50 and 60°C, and air velocities were 1 and 2 m/s. It is
assumed that the most microwave systems are only 50% efficient in converting
line power to microwave power (Decareau and Peterson, 1986; Buffler, 1993).
Therefore, for electrical usage calculations, it is assumed that actual power
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33
consumed is twice the real microwave power usage (Buffler, 1993). Specific
energy consumption data for the combined microwave/convective drying process
at two different air temperatures and velocities are listed in Table 3.
Table 3. Specific energy consumption in combined microwave with
convective drying at a 500-W power level with two different air
temperatures ana velocities. Adapted from Tulasidas et al. (1995a)
Air temperature
T O
50
60
50
60
Air velocity
(m/s)
1.00
1.00
2.00
2.00
Dehydration
time (hr)
Specific energy
consumption
(MJ/Kg)
3.3
3.2
5.6
3.9
5.3
6.1
16.2
14.1
Two results, in particular, should be noted. First, a 1 m/s air velocity
resulted in a shorter drying time and was more energy efficient, with an almost
65% lower specific energy consumption rate than that produced by the 2 m/s air
velocity. Second, at the same air velocity (1 m/s), the 50°C air temperature was
more energy efficient than was 60°C. This result contradicted the results obtained
by the researchers for heated-air drying, which indicated the more efficient
temperature was 60°C. In comparing the specific energy consumption rate for
convective drying with that for combined microwave/convective drying, the
combination dehydration process was more energy efficient. For example, the
specific energy consumption for convective drying at 60°C air temperature was
60.1 MJ/Kg, whereas for microwave/convective drying at 60°C air temperature
and 2 m/s air velocity, it was 14.1 MJ/Kg. In other words, specific energy
consumption for convective drying was more than four times that for the combined
microwave/convective drying. This phenomenon can be explained by the shorter
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34
drying time required by the combined microwave/convective process as compared
with convective drying alone.
Specific energy consumption in the microwave vacuum system. Clary
(1994) dehydrated grapes, using a combined microwave and vacuum (MIVAC)
system. This system produced a new, unique puffed dried fruit, Grape Puffs™.
Clary dehydrated grape samples at five temperatures, ranging from 54 to 77°C, and
at 2.7 KPa (20 Torr) of vessel pressure. Processing at each temperature level was
analyzed for specific energy consumption (Table 4). For electrical usage, it is
assumed that actual power consumed is about 167% of the real microwave power
usage for the M IVAC system (Clary, 1996). In comparison to the drying methods
previously discussed, the MIVAC system required the shortest drying time; it also
had the lowest specific energy consumption. However, specific energy
consumption o f the MIVAC system did not include vacuum pump energy
consumption.
Table 4. Specific energy consumption in microwave vacuum system at
five different treatment temperatures. Adapted from Clary (1994)
A ir temperature
(°C)
54
60
66
71
77
Dehydration time
(hr)
1.72
1.35
1.51
1.17
1.10
Specific energy
consumption
(MJ/Kg)
4.70
4.72
5.25
5.03
5.28
Based on the above observations, it is clear that microwave technology is an
effective new energy source for use in food processing. Microwave energy can
penetrate foods evenly and distribute temperature more uniformly throughout food
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35
products during processing. Additionally, when a combination of microwave and
vacuum systems is used to dehydrate grapes, lower temperature heating is required
(Clary, 1994). This lowered heating allows the final dried grape product to
maintain most of its original fresh characteristics such as color, flavor, and shape
without using chemical treatments for preservation (Clary, 1994). In contrast,
during heating-air drying, grapes are exposed to heat and atmospheric oxygen,
which can change the grapes’ flavor, color, shape, and nutritional value (Petrucci
and Clary, 1989; Clary, 1994). Studies have demonstrated that microwave energy
is the most successful method of reducing processing time. Microwave systems
have also exhibited an economic advantage, such as energy of coupling
coefficients, over conventional technology (Smith, 1984).
Energy Aspect of Water
There are two thermal properties that determine how a food product heats
after microwave energy has been deposited in it (Schiffmann, 1986; Giese, 1992;
Buffler and Stanford, 1995): specific heat and thermal conductivity.
Specific Heat
The specific heat o f a food is the amount of energy required to raise the
temperature by 1° (Giese, 1992). Some examples of specific heat are shown below
(Ohlsson, 1983):
Food
Specific heating
Water
4.2 KJ/Kg, °C
Vegetable
3.6 KJ/Kg, °C
Beef
3.2 KJ/Kg, °C
Bacon
2.1 KJ/Kg, °C
Cooking oil
2.0 KJ/Kg, °C
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36
As may be inferred, foods containing more water have higher specific heat
levels, whereas foods with more fat content have lower levels of specific heat.
Specific heat can be used to measure the amount of energy required to perform a
given heating task (Giese, 1992). During microwave heating, the lower the
specific heat of foods, the more rapid is their rising temperature rate. In
microwave heating, it is possible to calculate coupling coefficients for the
microwave power level if the temperature rising rate and mass weight of the food
sample are known. The equation is : Coupling coefficients = (Sample mass x
Specific heat x Temperature rising rate)/ Microwave power level (Yen and Clary,
1995).
Thermal Conductivity
Thermal conductivity can greatly affect the heating pattern during heating
(Schiffmann, 1986), for example, when large samples are heated and the
penetration capacity of microwave energy is insufficient to heat the samples
uniformly to their centers. On the other hand, when heating time is short, thermal
conductivity plays a secondary role (Schiffmann, 1986).
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OBJECTIVES
Water Experiments
Fresh grapes contain 75 to 80% water. Therefore, it is essential to first
understand the effect of microwave heating with water. Heating water samples at
a fixed pressure of about 100 KPa (760 Toir) and fixed microwave power levels
was used to determine the temperature rise profiles from data collected from the JR
detector, the fiberoptic detector, and a thermocouple. Analyzing temperature rise
profiles during the water experiments is essential to find the limitations of each
temperature detector and energy efficiency of each fixed microwave power level.
Temperature in the MTVAC system is measured by the IR detector, which is
critical because proper temperature control offers good quality o f product during
the dehydration process.
M ost microwave systems have only a 50 % efficiency in converting line
power to microwave power (Decareau and Peterson, 1986; Buffler, 1993).
Therefore, it is essential to understand the energy efficiency of microwave heating.
Results of water experiments were used as a starting point to determine the fixed
power level of coupling coefficients and thermal efficiency based on the principle
of energy conservation or the first law of thermodynamics (Yen and Clary, 1995).
The specific objectives of water experiments were as follows:
1. To compare the effectiveness of measuring temperature with the IR, and
the fiberoptic detector;
2. To define the effect of emissivity on the IR detector temperature
measurement in the microwave heating;
3. To benchmark the microwave heating with water;
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
38
4. To define the effect of dimension ratios of water during the MIVAC
process;
5. To evaluate the coupling coefficients and thermal efficiency for each
fixed microwave power level; and
6. To determine the MTVAC energy consumption.
Grape Experiments
When the grapes were dehydrated at a vessel pressure of 2.7 KPa (20 Torr)
and fixed microwave power levels, a “burning band” around the central portion of
the berry was often observed. In order to reduce burning of grapes during the
MIVAC system, knowledge of their dielectric properties was essential. An
understanding of the penetration depth of microwave energy into the grapes in
relation to the dielectric properties of grapes is important to choose the optimum
size for the MIVAC dehydration.
The specific objectives of grapes experiments were as follows:
1. To calculate the dielectric properties of grapes and the penetration depth
o f microwave energy into the grapes;
2. To determine the relationship between moisture content and the
dielectric constant of grapes during the MIVAC process;
3. To determine the relationship between moisture content and the loss
factor of grapes during the MTVAC process; and
4. To determine the optimum size of grapes for uniform heating during the
MTVAC process.
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MATERIALS AND METHODS
Description of the Laboratory Batch Microwave
Vacuum Unit
The laboratory unit was assembled by McDonnel Douglas Corporation and
delivered to California State University, Fresno for the dehydration experiments.
The unit consisted of a microwave power supply and magnetron, waveguide,
windows, microwave control, vacuum vessel equipped with a turntable, vacuum
pump and vacuum control, and system controls and instrumentation (Figs. 9 and
10) (Clary, 1994).
The power system (Model GL103) included a three-phase, 480-V ac service
rated at 14.4 MJ (4 KW h). It supplied medium ripple microwave power at 2,450
MHz and a maximum output of 10.8 MJ (3 KW h). The power transformers used
three-phase, wye-connected and delta-connected secondaries to produce a
continuous medium ripple waveform. These secondaries were independently
rectified and the dc outputs combined in series to give a 12-phase output ripple
waveform. This design produced a peak-peak ripple of about 5% with a minimum
of filter components (Gerling, 1991; Clary, 1994).
The output of the microwave energy source was regulated by modulating
the current in the electromagnet surrounding the magnetron and controlling the
level of the magnetic field in the magnetron interaction space (Clary, 1994). If the
field was amply high, no electrons would pass the interaction space and no
microwave energy source would be produced (Clary, 1994). A reference signal
from the magnetron was used to control the magnetic coil. The purpose of this
function was to achieve a smooth microwave energy source output without
waveform distortion at output levels from zero to full power. The level of
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
40
VACUUM VESSEL
System Controls
MFRARED
DETECTOR
Ttm pcnturt
Time
P r tttu n
MW Power
VESSEL
DOOR
Vacuum
Pump
Microwave Power
Supply
Fig. 9. Schematic diagram o f the laboratory microwave vacuum
dehydration system. From Clary (1994)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Fig. 10. Photograph o f laboratory microwave vacuum drying system.
From Petrucci and Clary (1989)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
42
microwave power applied to the product was shown by the forward power meter
on the system instruments as power in watts.
The waveguide assembly included a water-cooled circulator (ferrite
isolator) to absorb reflected power, forward and reflected power detectors, and a
waveguide tubing that guided the microwave power to the vacuum vessel. The
waveguide conducted the microwave power into the vacuum chamber and the
microwave energy through a microwave Teflon window. The purpose in this was
to prevent the chamber from depressurizing through the waveguide (Clary, 1994).
The stainless steel chamber was a 90-cm-diameter cylinder, 120 cm long.
One end o f the chamber was fitted with a flange and o-ring for a door. The other
end was adapted for the waveguide and vacuum port (Clary, 1994). Two plexiglas
windows were fitted with screen to prevent microwave emission. The windows
were mounted on opposite sides of the chamber to provide light and a view into
the vessel. Other ports on the chamber provided access to sensors and
instrumentation. An 80-cm-diameter turntable was supported by a vertical shaft
through the bottom of the chamber. This shaft was mounted to a gear motor which
operated at 5 to 10 rpm (Clary, 1994). The purpose of the turntable was to
improve temperature distribution during processing.
The vacuum pump was connected to the vacuum port o f the chamber
through two motorized valves plumbed in parallel. A Busch high-speed vane
vacuum pump (Model R5S 100-132) was used in this system (Clary, 1994). This
pump can remove water vapor and other gases from the chamber and maintain the
chamber pressure. A Schaevitz pressure transducer (Model P3061-15) was used to
measure the absolute pressure in the chamber.
The system controls included the Gerling microwave system and interlocks,
an infrared temperature detector, a timer, vacuum control, and a pressure sensor.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
43
The system was designed to control desired product temperature and time basis.
Chamber pressure was controlled by manually setting the motorized valves to
maintain the desired pressure.
Temperature in the MIVAC system was measured by a Mikron infrared
temperature detector (Model H-Ll(XXX)-O30OFAUOOO) with an operating range of
-18 to 150°C (Clary, 1994).
Procedures
W ater Experiments
The treatment variables were microwave power level, time of exposure,
type of bowl, and amount of water. W ater experiments were conducted using both
a transparent glass bowl and a black plastic bowl. The dimensions and shape of
the two bowls were the same; each had a diameter of 27 cm and was 10 cm high.
The maximum capacity of each bowl was 2.5 L. W ater samples were weighed
under five conditions for fixed microwave application experiments. The first
condition used five bowls of 1362-g water samples. Each bowl was heated at
fixed power levels of 3000,1500,1000, 750, and 500 W , respectively. The
second condition involved five 908-g water samples, each dehydrated at fixed
power levels of 3000,1500,1000,750, and 500 W , respectively. The third
condition used four 405-g water samples, heated at fixed power levels of 1500,
100,750, and 500 W , respectively. The fourth condition involved three 908-g
water samples, dehydrated at fixed power levels of 3000,1500, and 750, and
500 W, respectively. The final condition only used only one 1362-g water sample,
dehydrated at a whole power level of 3000 W (Table 5).
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Table 5. Summary of microwave energy treatment for
water experiments.
T e s t#
Microwave
power (W)
Sample weight
(g)
Containers
1
3000
1362
G
2
1500
1362
G
3
1000
1362
G
4
750
1362
G
5
500
1362
G
6
3000
908
G
7
1500
908
G
8
1000
908
G
9
750
908
G
10
500
908
G
11
1500
454
G
12
1000
454
G
13
750
454
G
14
500
454
G
15
3000
908
P
16
1500
908
P
17
750
908
P
18*
3000
1362
G
♦involved the fiberoptic detector.
G is transparent glass bowl.
P is black plastic bowl.
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45
Conditions 1 ,2 ,3 , and 5 used a transparent glass bowl for holding the water
sample, whereas condition 4 used a black plastic bowl. Additionally, condition 5
involved the use o f a fluorescent fiberoptic detector to measure temperature during
the MIVAC heating. The fiberoptic detector was placed in the bottom of the bowl,
in order to determine the temperature differences between the water on the surface
and at the bottom o f the bowl.
An atmospheric pressure of about 100 KPa (760 Torr) was used in all water
experiments. W ater sample conditions were monitored visually. Time and
temperature were recorded during the process. Experiments were stopped when
IR temperature detector readings ceased to increase. The measurement of the
sample temperature was measured with the IR detector, the fiberoptic detector, and
a thermocouple. The temperature change registered by the IR detector during the
process was used to calculate the thermal efficiency and the coupling coefficients.
The final sample weight of each test was determined by a digital balance
immediately after the samples were processed. The sample weight loss was used
to calculate the therm al efficiency. The thermocouple measured the initial and
final water tem perature during the heating process in the M IVAC system for water
experiments. T hese data established an ideal straight linear temperature curve.
The curve was a reference by which to compare and calibrate the IR detector
temperature m easurem ent
Grape Experiments
Fresh Thompson seedless grapes purchased from a local market, were used
for processing in the MIVAC system. The grapes were prepared by removing their
capstems. The sam ple of single grapes was washed, weighed, and placed on the
turntable of the batch unit under three different conditions. The first condition
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
employed seven batches of grape samples. Each batch weighed 908 g and was
dehydrated by the MTVAC system at a power level of 1500 W for 5 ,1 0 ,1 5 ,2 0 ,
25, 30, and 35 min. The second condition employed five 454-g grape samples,
dehydrated at a power level o f 1000 W for periods of 5 ,1 0 ,1 5 , 20, and 25 min.
The third condition used one 454-g grape sample, dehydrated at 1500 W for
periods of 5 ,1 0 ,1 5 , and 20 m in (Table 6). Before each test, a sample of single
grapes was collected randomly for determination of initial moisture content by the
vacuum method at 70°C (Boland, 1984). All the experiments were repeated three
times.
A cham ber pressure of 2.7 KPa (20 Torr) was used in all grape
experiments, based on minimizing product temperature and maintaining the
stability of the microwave field in the chamber (Clary, 1994). Free water boils at
22.2°C at this pressure (Goff and Gratch, 1951).
Grape sample conditions were monitored visually. Tim e, pressure,
temperature, and microwave power levels were recorded during the process. At
the conclusion o f each experiment, the grape sample was weighed and its
condition noted. Three grape berries from each batch were randomly selected for
determination o f final moisture content and were dried in a vacuum oven at 70°C
for 6 hr (Boland, 1984). The moisture content and temperature o f each grape test
was used to calculate the dielectric properties of grapes and the penetration depth
of microwave energy into the grapes.
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47
Table 6. Summary of microwave energy treatment for
grape experiments
Microwave
Sample
Process
power
weight
time
(W)
(g)
(min)
1
1500
908
5
2
1500
908
10
3
1500
908
15
4
1500
908
20
5
1500
908
25
6
1500
908
30
7
1500
908
35
8
1000
454
5
9
1000
454
10
10
1000
454
15
11
1000
454
20
12
1000
454
25
13
1500
454
5
14
1500
454
10
15
1500
454
15
16
1500
454
20
T e st#
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48
Data Handling
Equation for Calculating Coupling
Coefficients
Coupling coefficients were calculated using temperature rise profiles from
the w ater experiments for each fixed microwave power level. Equation 1 is based
on the principle of energy conservation:
The rate of product temperature change, or, the rate of internal energy
change = microwave energy absorbed by the product
This relationship can be mathematically expressed as:
MC (dT/dt) = Kw
(1)
Where,
M = mass of the the product, in g
C = specific heat of the product, in J/g, °C
dT/dt = rate of temperature change, or, slope of the rising temperature line,
K = a coupling factor between the forward microwave energy and the
product, dimensionless
w = microwave power in W (watt or J/sec) (Yen and Clary, 1995).
Equation for Calculating Thermal
Efficiency
Using temperature-rise profiles and water sample weight differences from
the w ater experiments, thermal efficiency was calculated. Equation 2 is based on
the latent heat of vaporization of water:
The product of temperature and mass change
= amount of microwave energy requires
This relationship can be mathematically expressed as:
{[W i-W f] (2256 J/g) + (TfTj) C W*} = Qw
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(2)
49
W here,
Wi = initial mass of the product to be d rie d , in g
W f = final mass of the product during drying, in g
Tj = initial temperature of the product before drying, in °C
T f = final temperature of the product during drying, in °C
C = specific heat of the product, in J/g, °C
Q = a thermal factor between the forward microwave energy and the
product, dimensionless
w = microwave energy, in W (watt or J/sec) (Decareau, 1985).
Predictive Models for Dielectric
Properties of Grapes
Scientists and engineers have used dielectric properties to explain the
ability of microwaves to heat materials. However, the measurements of the
dielectric constant and loss factor involve using specialized instruments, together
with some tedious and time-consuming procedures. Tulasidas et al. (1995b)
developed predictive models for determination of dielectric properties of grapes.
These predictive models were used in the grape experiments. The models could be
used to compute the values of the dielectric properties of grapes at any given
moisture content and temperature.
The predictive models are shown in Equations (3) and (4):
e' = -31.35+172.17M-+0.62T-57.63M2-0.74MT-0.003T2
(3)
e" = -8.70+78.95M+0.11T-50.34M2-0.35MT-0.00002T2
(4)
W here,
T = temperature °C, 25< T < 80 and
M = moisture content on wet basis, %, 15 < M < 80 (Tulasidas et
al.,1995b).
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50
Equation for Calculating
Penetration Depth
Since microwaves penetrate foods from all directions, the size and shape of
food significantly influences microwave heating patterns (Giese, 1992).
Therefore, analyzing microwave energy penetration depth into foods provides a
tool for uniform heating. Buffler and Stanford (1995) developed a simple equation
for calculating penetration depth value that depends on the dielectric properties of
the product and frequency. This equation would be appropriate for all products.
The equation is shown in Equation (5):
dp = Xo V e ' /2 7t e"
Where,
Xo = free space microwave wavelength, in cm
e' = dielectric constant, dimensionless
e" = loss factor, dimensionless (Buffler and Stanford, 1995).
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(5)
RESULTS AND DISCUSSION
Water Experiments
The Correlation Between the IR
Detector and the Fiberoptic
Detector
The relationship of the temperature rise curve between the IR detector and
the fiberoptic detector is shown in Fig. 11. During the initial heating, the IR and
the fluorescent fiberoptic detectors had a 2.7°C temperature difference. When
power was turned on, during the first 30 sec, the temperature readings of the IR
detector were higher than those obtained by the fluorescent fiberoptic detector.
However, as tim e passed, the IR detector readings never again surpassed those of
the fiberoptic detector. This can be attributed to the effect o f the penetration depth
of the water sample. According to Ohlsson (1983), when water temperature
increases, the penetration depth of water also increases. A t the beginning, the
surface temperature of the water sample was higher than the bottom temperature.
As time passed, microwave energy penetrated deeper into the water sample,
resulting in the higher temperature measurement by the fiberoptic detector.
The Effect o f Emissivitv on the IR
Detector M easurement
If conditions (e.g., same water sample, water mass, and microwave power)
are the same in different experiments, the IR detector’s temperature readings must
be identical. Figs. 12 and 13 show that when one black bowl and one transparent
glass bowl were heated in the MTVAC system under the same conditions, the black
plastic bowl registered higher temperature readings than did the transparent glass
bowl, because the IR detector was set at an emissivity of 0.99. A t this level,
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52
110
100
90
£
2
2
a.
E
«
H
IR Detector
Fiberoptic Detector
40
0
1
2
4
3
Time (min)
5
6
7
Fig. 11. Temperature difference between the fiberoptic and the infrared
detectors during the MTVAC process.
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53
80
70
60
1
I
50
I
|
40
3000W-Glass
30 00 W -Plastic
30
20
0
3
Time (min)
2
4
5
Fig. 12. Effect o f emissivity on temperature measurements
at 908 g o f water and 3000 W.
70
60
/“N
O
V
50
o
45
g.
E 40
u
H
1500W -Glass
1500 W-Plastic
20
0
2
6
4
Time (min)
8
Fig. 13. Effect o f emissivity on temperature measurements
at 908 g o f water and 1500W.
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10
54
the IR detector can absorb all energy only from the blackbody objects. In order to
obtain an absolute temperature, the IR detector requires each object’s emissivity
(Bengtsson and Lycke, 1969). Holman (1981) reported that lower emissivity
levels cause the IR detector to indicate a lower temperature than the true surface
temperature. Because the emissivity of a transparent glass object is approximately
0.94, which is lower than the level set for the experiment, the black bowl had a
higher temperature reading than the transparent one.
Measurements of W ater
Temperature Rising Curve
The heating pattern of the temperature measured by the IR detector during
MIVAC process was studied. Figs. 14 and 15 illustrate two basic principles
underlying temperature measurement by the IR detector in the MIVAC system.
First, when the highest microwave power was used to heat the water sample, the
IR detector obtained the highest temperature readings. Second, when microwave
power was reduced, the highest temperature reading obtained by the IR detector
also decreased. For example, the highest temperature measured at 3000 W was
71°C. On the other hand, when microwave power was reduced to 1500 W, the
o
reading dropped to 61 C (Fig. 14). Fig. 15 indicates that the highest temperature
obtained at 3000 W was 73°C. Reducing microwave power also decreased the
temperature readings.
The Effect of Size on the IR
Detector Measurement During
MIVAC Heating
Theoretically, the rate of temperature rise depends on the mass of sample,
type of material, and microwave power. If twice the amount of the sample mass is
used, heated under the same microwave power, the temperature rise will decrease
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
55
60
W
50
Q.
40
3000W
0
1
2
3
4
5
750W
1500W
6 7 8
Time (min)
9
10 11 12 13
Fig. 14. Temperature rise curve o f 908 g o f water at various power levels
in a transparent glass bowl.
75
70
65
O' 60
©
|
55
I
50
I 45
E
o
H 40
3000W
30
1500W
750W
25
0
5
Time (min)
15
Fig. 15. Temperature rise curve o f 908 g o f water at various power levels
in a black plastic bowl.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
56
by half o f its original rate. On the other hand, if the microwave power used on the
same sample mass is doubled, the rate of temperature rise also doubles.
W ater experiments showed how different masses of water sample inside a
same container have different column heights (Fig 16). The water column heights
at three different water sample weights are shown in Table 7.
Table 7. Summary of water column heights for three different weights of
w ater samples
Sample weight (g)
Water column height (cm)
1362
4.7
908
3.5
454
2.5
Different w ater column heights may affect the rate of temperature rise. The
data in Table 8 indicate that when the sample mass is increased to 908 g of water
sample, the temperature rise rate decreases by half as compared to that of a 454-g
sample. However, this is not true when the sample mass is increased further to
1362 g; the temperature rise rate is no longer inversely proportional to the sample
mass. The ratio o f temperature rise rate of a 1362-g water sample was lower than
those for 454- and 908-g water samples.
Ramaswamy and Pillet-Will (1992) found that heating at a lower
dimensional rate resulted in higher bulk and center temperatures than those
obtained when heating at a higher rate, using the same levels of microwave power
and m ass of product
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57
Diameter = 27 cm
H
H = 10 cm
h = 2.5 cm
(a). 454 g of water in the transparent glass bowl
Diameter = 27 cm
H = 10 cm
h = 3.5 cm
(b). 908 g of water in the transparent glass bowl
Diameter = 27 cm
~T~
H = 10 cm
h = 4.7 cm
(c). 1362 g of water in the transparent glass bowl
Fig. 16. Typical layout o f three different water weights formed into three
water column heights.
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Table 8. Effect of sample weight on temperature rise rate
at 1500 and 1000 W in a transparent glass bowl
Temperature rise rate, °C/min
Pow er level (W)
Sample mass (g) ->
1362
908
454
1500
3.33
5.52
11.04
1000
1.99
3.51
7.02
The Correlation between the IR
Detector and the Fiberoptic
Detector
The relationship between temperature rise rate for the IR detector and
thermocouple are shown in Figs. 17 and 18. In comparison with the thermocouple,
the IR detector had certain limitations. First, when heated water is used in the
MTVAC system, the IR detector did not measure temperatures higher than 74°C.
Second, when the IR detector reached its maximum temperature (74°C), it
remained flat or even decreased slightly.
These limitations can be attributed to moisture evaporation, since
evaporation produces a cooling effect (Ohlsson, 1983). Berek and W ickersheim
(1988) discovered that the surface temperature of 1362 g of roast pork could not
get higher than 60°C due to moisture evaporation. Figs. 14 and 15 indicate the
o
same effect, in that surface temperatures never rose above 74 C.
Those limitations also may be caused by steam. W hen moisture vaporizes
into steam during MIVAC heating, it obstructs the lens of the IR detector.
Consequently, the IR detector was unable to provide accurate temperature
readings.
Figs. 19 and 20 indicate that when heated inside the same type of container,
the lesser m ass of water had a higher temperature ascending rate than did the
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1500W
750W
T (linear)
3000W
1000W
500W
29
39
49
59
69
Temperature (°C)
79
89
99
Fig. 17. Nonlinearity of the infrared temperature measurement
o f 980 g of water in a transparent glass bowl.
106
U
3000W
1500W
750W
T (linear)
2s
2u
o.
E
£
26
36
46
56
66
76
Temperature ( C)
86
96
Fig. 18. Nonlinearity of the infrared temperature measurement
of 980 g of water in a black plastic bowl.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
106
60
65
-
60 -55
-
P 50 ■■
<5
-
908 g
35 -454 g
30 --
20
0
2
4
3
5
7
6
8
9
10
12
II
13
T im e (m in)
Fig. 19. Effect o f sample weight on temperature rise rate
at 1500 W in a transparent glass bowl.
65 --
60 --
45
-
40 --
35 --
0
2
4
6
8
10
12
14
T im e (m in)
Fig. 20. Effect o f sample weight on temperature rise rate
at 750 W in a transparent glass bowl.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
16
61
greater mass. This phenomenon can be explained by the surface area effect. For
example, 4S4 g of water has a smaller surface area. A smaller surface evaporates
less moisture, which in turn produces a more rapidly ascending temperature rate.
Energy Efficiency of W ater
Experiments
The coupling coefficients and thermal efficiency of water at different fixed
powers o f heating are shown in Table 9. For 454 g of water at 1500 W, the
percentages of coupling coefficients and thermal efficiency were 23.3 and 33.17,
respectively. For 454 g of water at 500 W, these parameters increased to 55.76
and 32.5, respectively. In this case, a 500-W power level resulted in higher
thermal efficiency and coupling coefficients than did a 1500-W power level. This
finding is noteworthy, in that it would be expected that higher power levels would
produce higher thermal efficiency and coupling coefficients.
Effect of power level and sample weight on coupling coefficients of water.
The combined effect of heating power level and sample mass on the coupling
coefficients of water is illustrated in Fig. 21. Coupling coefficients were
influenced by water mass and power level in two ways. First, 1362 g of water
heated at a high power level (3000 W) had higher coupling coefficients than did
the other water samples. In comparison, heating water at low power levels (750
and 500 W) produced opposite results. For example, 454 g of water heated at low
power levels (750 and 500 W) demonstrated higher coupling coefficients than did
the other water sample at those power levels. Second, at intermediate power levels
(1500 and 1000 W), the same coupling coefficients were shown for 454 and 908 g
o f water, whereas the 1362-g water sample had lower coupling coefficients at
those levels.
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62
Table 9. Summary coupling coefficients and thermal efficiency with water
experiments
Pow er level
Sample
Material o f
Thermal
Coupling
(W)
mass (g)
containers
efficiency (% )
coefficients (%)
3000
1362
TG bowl
30.53
30.10
1500
1362
TG bowl
27.29
21.10
1000
1362
TG bowl
30.93
18.90
750
1362
TG bowl
39.76
28.20
500
1362
TG bowl
39.25
25.50
3000
908
TG bowl
32.27
29.00
3000
908
BP bowl
17.31
18.40
1500
908
TG bowl
31.56
23.30
1500
908
BP bowl
17.31
30.00
1000
908
TG bowl
46.60
22.20
750
908
TG bowl
40.08
23.80
750
908
BP bowl
20.08
23.40
500
908
TG bowl
44.20
26.60
1500
454
TG bowl
33.17
23.30
1000
454
TG bowl
39.00
22.20
750
454
TG bowl
42.30
32.70
500
454
TG bowl
55.76
32.50
TG bowl: Transparent glass bowl.
BP bowl: Black plastic bowl.
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63
2u
c
1326 g-G
908 g-G
'5a
£
8u
bo
.5
D.
3
O
P 9 0 8 g-P
U
0.05 -
3000W
1500W
1000W
750W
500W
Fig. 21. Coupling coefficients as a function of power level and
sample mass.
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64
Effect of power level and sample weight on thermal efficiency of w ater.
The combined effect o f heating power level and water mass on thermal efficiency
is shown in Fig. 22. W hen the heating power level and container used w ere the
same, therm al efficiency decreased as water mass increased, with one exception at
the 1000-W power level. For example, when 454 g of water was heated at a
500-W pow er level, the sample’s thermal efficiency was 55.76%; the efficiency of
908 g o f w ater heated at the same power level was 42.30%. This can be attributed
to an evaporative cooling effect on the sample’s surface (Giese, 1992), i.e., the
greater the w ater mass, the larger is its surface volume, even when placed in the
same type container as a smaller mass. Therefore, optimum product size, having a
surface area suitable for microwave heating, is an important factor in energy
conservation.
Effect of container’s material on thermal efficiency of water. The way in
which w ater’s thermal efficiency is affected by the type of container used to hold
the sample is illustrated in Fig. 23. Given the same experimental factors, such as
type of sample, sample mass, and microwave heating power level, the thermal
efficiency o f several samples could be expected to be identical. However, in this
case, the container type was also found to affect thermal efficiency. Samples
contained in the transparent glass bowl registered a higher thermal efficiency than
those contained in the black plastic bowl. Moreover, at the 3000- and 750-W
heating pow er levels, the sample in the transparent glass bowl had almost twice the
thermal efficiency of those in the black plastic bowl. This phenomenon can be
explained by the varying thermal conductivity of different materials. Lunardini
(1981) reported that plastic has a conductivity rate of 0.363 W/m °C, whereas the
rate for glass is 0.761 W/m °C. Therefore, all other factors being equal, the
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
65
0.6
0.5 --
3000W
1500W
1000W
750W
500W
Fig. 22. Thermal efficiency as a function of power level and sample mass.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
66
0 3 5 -■ BP Bowl
c
025 -□TXj BowI
M 0.15
3000W
1500W
750W
Fig. 23. Thermal efficiency as a function of container’s material.
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67
thermal efficiency of the water samples contained in the transparent glass bowl
(higher conductivity) is higher than that of samples in the black plastic bowl
(lower conductivity).
Grape Experiments
General Observations
The dielectric properties of each grape sample were computed by Equations
3 and 4, with the known value of temperature and moisture content. Then the
value for penetration depth of each sample was calculated by Equation 5, using the
counting values of dielectric properties.
Tables 10,11, and 12 summarize the experimental data for three different
conditions: dehydration time (min), vacuum oven-determined moisture content
(%), the IR detector-determined temperature reading (°C), computed dielectric
constant, computed loss factor, and calculated penetration depth.
Dielectric Constant of Grapes
During the MTVAC Process
The dielectric constant and moisture content of grapes as functions of
dehydration time are shown in Figs. 24,25, and 26. The average dielectric
constant was 70.6 for grapes at a moisture content of 80%. The data from this
experiment demonstrate the relationships between the dielectric constant and
moisture content with dehydration time. When the curve of the moisture contents
descends, the curve of the dielectric constant also decreases (Figs. 24,25, and 26).
From this, it is clear that the dielectric constant is more closely related to moisture
content and that moisture content has a greater influence on the dielectric constant.
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68
Table 10. Dielectric properties and penetration depth of grapes,
calculated by Equations 1 ,2 , and 3, dehydrated by the Ml VAC
system with 908 g of sample weight at 1500 W
Dehydration
dme(min)
Moisture
content (%)
Temperature
(°Q
Dielectric
constant
Loss factor
Penetration
depth (cm)
0
78.00
23.8
68.90
18.37
0.877
5
77.45
43.3
69.45
15.27
1.059
10
73.75
44.0
6754
15.63
1.021
15
67.38
38.0
63.10
16.86
0.915
20
64.81
45.0
62.35
16.06
0.954
25
47.49
45.0
4950
14.91
0.916
30
43.39
45.0
45.95
14.19
0.927
35
24.87
73.0
39.73
950
1.289
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 11. Dielectric properties and penetration depth of grapes,
calculated by Equations 1 ,2 , and 3, dehydrated by the MlVAC
system with 454 g of sample weight at 1500 W
Dehydration
time(min)
Moisture
content (%)
Temperature
<°Q
Dielectric
constant
Loss factor
Penetration
depth (cm)
0
83.61
18.22
72.33
18.79
0.878
5
80.50
33.33
70.71
16.51
0.959
10
76.57
33.33
68.47
16.97
0.990
15
50.95
37.77
50.58
15.87
0.870
20
33.14
51.66
38.73
11.63
1.040
Table 12. Dielectric properties and penetration depth of grapes,
calculated by Equations 1 ,2 , and 3, dehydrated by the MlVAC
system with 4 5 4 g of sample weight at 1000 W
Dehydration
time(min)
Moisture
content (%)
Temperature
<°Q
Dielectric
constant
Loss factor
Penetration
depth (cm)
0
83.17
20.0
71.05
18.81
0.870
5
83.10
39.4
67.72
14.87
1.074
10
75.56
36.1
64.12
16.61
0.936
15
65.29
35J5
57.56
17.15
0.859
20
45.17
41.6
41.36
14.65
0.852
25
30.99
555
28.91
10.95
0.953
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80
70
60 -c
3cn
C
oQ 50 -£
3
W
C/5
Dielectric constant
'o 40 -s
Moisture content
30 --
0
5
10
15
20
25
Dehydration time (min)
30
35
Fig. 24. The dielectric constant and moisture content o f grapes as a
function of dehydration tune with 908 g o f grape at 1500 W.
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71
Dielectric constant
Moisture content
20
0
5
10
15
Dehydration time (min)
20
Fig 25. The dielectric constant and moisture content o f grapes as a
function o f dehydration time with 454 g o f grape at 1500 W.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Moisture content
£ £ 60
Dielectric constant
CO
C
O
u
u
r
o
a
0
5
15
10
Dehydration time (min)
20
25
Fig 26. The dielectric constant and moisture content o f grapes as a
fimction o f dehydration time with 454 g o f grape at 1000 W.
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73
Loss Factor of Grapes During the
M lVAC Process
The loss factor and moisture content of grapes as functions o f dehydration
time are shown in Figs. 27,28, and 29. The average loss factor value for fresh
grapes was 18.6. The data from this experiment did not show any relationship
between the loss factor and moisture content with dehydration time. Figs. 27,28,
and 29 illustrate that as the moisture content curve descends, the loss factor curve
continues to fluctuate around a constant level before it eventually descends. This
observation indicates there is no relationship between the loss factor and moisture
content. It may be more closely related to ionic molecules, such as sugar and salt
content. In other words, the sugar content concentrations become more inversely
proportional to the grapes’ native moisture content. Thus, the lower the moisture
content of grapes becomes, the higher is their concentration of sugar content. It is
the sugar concentration that generates the ionic conductivity during microwave
heating.
Effect of Penetration of
Microwave Energy on Grapes
It is known that the spherical and cylindrical shapes of the products have a
focusing effect. The focusing effect is more dependent on the penetration depth.
This experiment’s calculation indicates that the penetration depth energy in grapes
is about 0.95 to 1.05 cm. Usually grapes are cylindrically shaped. As such, there
are tw o dimensions of x (major axis) and y (minor axis). Based on the calculation
of penetration depth, it is known that microwave energy always passes through the
y-axis easily. However, microwave energy can only reach the center of the x-axis.
Because of this phenomenon, the core of the grapes absorbs double the amount of
the microwave energy from x- and y-axes. This effect can be evidenced by the
“burning band” around the central portion of the berry (Fig. 30).
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74
Moisture Content
U*
O
J 8 40
a §
J .2 30
•w
Loss Factor
0
5
10
15
20
25
30
35
Dehydration time (min)
Fig. 27. The loss factor and moisture content o f grapes as a function of
dehydration time with 908 g o f grape at 1500 W.
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75
90
80
Moisture Content
70
Co"
o \
<3
k«
O
o
42
in
in
O
_)
60
C
C
o
u
u
u.
3
tn
‘o
50
40
30
20
Loss Factor
10
0
0
5
15
10
Dehydration time (min)
20
Fig. 28. The loss factor and moisture content of grapes as a function of
dehydration time with 454 g of grape at 1500 W.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
76
80
70
Co' 60 -- Moisture Content
^ -g „
ha qj 50
2 s
*5 g 40
8 I
-> -a 30
20
10
Loss Factor
0
0
5
10
15
Dehydration time (min)
20
25
Fig. 29. The loss factor and moisture content o f grapes as a function o f
dehydration time with 454 g o f grape at 1000 W.
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77
mg/mmfflM
/'F p v ^ ? ; l;0 % k^
Fig. 30. Photograph o f burning grapes at fixed power
during the MlVAC process.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
SUMMARY AND CONCLUSIONS
Summary
The raisin grape crop in California is traditionally dried in the sun or by
heated-air systems. Such methods expose grapes to excessive heat and
atmospheric conditions that can change their flavor, color, shape, and nutritional
value. The MlVAC system, a combination of microwave and vacuum drying
processes, allows grapes to be dehydrated at lower temperatures, thereby
preserving many of their original characteristics in the final product. Dehydration
using the MlVAC system produces a new dried grape product that is puffed and
crisp-textured.
Dehydration of grapes using the MTVAC system required lower specific
energy consumption and processing time than did the other two dehydration
processes. Additionally, drying grapes using a combined microwave and
conventional heated-air method in a modified microwave oven also required lower
specific energy consumption than did conventional processes. The lower specific
energy consumption means that the energy coupling coefficients were higher. The
conventional heating processes demonstrated lower energy coupling; however,
their cost is lower because they use natural gas and the price of natural gas is
lower than that of electricity. On the other hand, microwave systems need to use
electricity to provide power.
W ater Experiments
In the MTVAC system, temperature control and measurement were major
determinants of product quality. Temperature was measured by an infrared (IR)
detector. Using an IR detector, measurements were made quickly and easily.
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However, the IR detector measures only the product’s surface temperature, which
makes it difficult to determine a correlation with internal temperature. The
limitations of the IR detector are that (1) only surface temperature distribution can
be detected; (2) it requires a considerable amount of effort to calibrate due to the
strong dependence of emissivity on the surface condition of materials as well as
moisture content; and (3) when moisture vaporizes into steam, the detector may be
obstructed and unable to provide accurate readings.
To obtain accurate temperature measurements using the IR detector, the
following three factors need to be considered, since they influence the IR
detector’s measurement accuracy. (1) The emissivity of the product must be
known. (2) Vaporization occurs during the dehydration of food products, and the
resultant moisture may obstruct the IR detector. This causes a cooling effect due
to an absence of airflow in the water tests, so the chamber is saturated with
moisture vapor. (3) The dimensional ratio of the product appeared to be an
important factor in microwave heating because appropriately sized products absorb
more energy and reduce the cooling effect.
Thermal efficiency and coupling coefficients of water experiments. The
water experiments conducted in this study demonstrated that the lower power
levels had higher thermal efficiency and coupling coefficients. This was an
unexpected result, since it might be assumed that higher power levels would
produce higher thermal efficiency and coupling coefficients. W ater contained in a
transparent glass bowl had a higher thermal efficiency than did water contained in
black plastic bowl during microwave processing, all other conditions being equal.
This phenomenon can be attributed to the greater thermal conductivity of glass and
the lower conductivity of plastic materials.
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80
Grape Experiments
Both moisture and temperature were found to affect dielectric properties of
grapes during the M lVAC process. M oisture was found to have a m ajor impact on
the dielectric constant; when the moisture of grapes decreased, their dielectric
constant also decreased. On the other hand, the loss factor is m ore closely related
to ionic molecules, such as sugar content, because when the m oisture content of
grapes decreased, the loss factor of grapes continued to fluctuate around a constant
level.
Buffler and Stanford (1995) reported that the approximate optimum
diameter for uniform heating would be equal to twice the value o f the penetration
depth in the product. From calculations m ade in this study, penetration depth of
grapes averaged 0.90 to 1.1 cm. To reduce a focusing effect on the grapes during
fixed microwave energy heating, the grapes should be properly sorted according to
the roundness of shape. Ideally, the size o f the grapes should be about twice the
penetration depth o f the microwaves. Therefore, the ideal size o f grapes is
between 1.8 and 2.2 cm.
Conclusions
Based on this study’s findings, the following specific conclusions are made:
1. The IR detector has severe limitations in measuring the temperature of a
product. On the other hand, the fiberoptic detector demonstrates potential to
accurately measure the internal temperature of a product
2. The IR detector measurements are affected by the emissivity of the
product
3. The IR detector had different temperature rise profiles during the water
tests under microwave heating. This phenomenon may have been caused by the
product’s physical properties, such as size.
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81
4. The 500-W microwave power level produced a higher coupling
coefficient (32.50) and thermal efficiency (55.76) than did other microwave power
levels.
5. Moisture was found to be a major influential factor affecting the
dielectric properties of grapes during the fixed microwave power-level dehydration
process.
6. The loss factor would be more related to ionic concentration such as
sugar concentration.
7. The penetration depth of microwave energy into grapes was in the range
of 0.9 to 1.1 cm.
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RECOMMENDATIONS FOR FUTURE W ORK
The present study generated and analyzed temperature rise profiles for the
water experiments and the penetration depth of microwave energy into products
for the grape experiments. An examination of those findings points out the need
for future research to attain better temperature rise profiles during the MlVAC
processes.
Based on this study’s results, the following recommendations for future
research are made:
1. Combination of the fiberoptic detector and the IR detector. The
fiberoptic detector is not electrically or thermally conductive during microwave
heating. A combination using both the IR and fiberoptic detectors appears to be
the best way to determine accurate temperature distribution during the MTVAC
process, because the fiberoptic detector is only capable of measuring at a discrete
number of points.
2. Physical size and shape of the grapes. To reduce a focusing effect, or
burning, on the grapes during fixed microwave energy heating with 2.7 KPa of
pressure, the grapes should be sorted properly, according to the round shape of the
grapes. Additionally, the ideal size diameter for grapes is between 1.8 and 2.2 cm.
3. Dielectric properties of grapes. An understanding of the dielectric
properties of grapes during the process is essential in order to produce better
process controls. There is a need to examine the theoretical aspect of dielectric
properties of food products in relation to their constituents, such as sugar content,
salt content, and so on.
4. Variable power levels for analysis. Similar studies should be extended
to include MTVAC treatments with variable power levels.
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REFERENCES
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REFERENCES
Anonymous. 1970. Performance standards for microwave ovens. Federal
Register 35(194): 15642-15643.
Baum, H. M. 1992. Opening remarks. Campbell Microwave Institute Seminar,
Camden, N.J. January 15.
Bengtsson, E. N and Lycke, E. 1969. Experiment with a heat cam era for
recording temperature distribution in foods during microwave heating.
J. M icrowave Power 4(2): 48-54.
Berek, H. E. and Wickersheim, K. A. 1988. Measuring temperatures in
microwaveable packages. J. Packaging Technology 2(4): 15-21.
Boland, F. E. 1984. Fruits and fruit products. In “AOAC Official Methods of
Analysis,” ed. W. Horwitz, pp. 413-418. AO AC, W ashington, D.C.
Bolin, H.R., Petrucci, V., and Fuller, G. 1975. Characteristics of mechanically
harvested raisins produced by dehydration and by field drying. J. Food
Sci. 40:1036.
Buffler, R. C. 1993. “Microwave Cooking and Processing.” AVI, New York.
Buffler, R. C. and Stanford, A. M. 1995. Effect of dielectric and thermal
rties on the microwave heating o f foods. M icrowave World 12(4):
Clary, C. D. 1994. Application of microwave vacuum and liquid media
dehydration for the production of dried grapes. Ph.D. Dissertation, Michigan
State Univ.
Clary, C. D. 1996. Viticulture and Enology Research Center, California State
Univ., Fresno. Personal Communication.
Cockbum, R. 1958. Microwave in science and technology. The Engineer 205:
802-804.
Decareau, R. V. 1985. “Microwave in the Food Processing Industry.” Academic
Press, New York.
Decareau, R. V. 1986. Microwave food processing throughout the world. Food
Technol. 40(6): 99-106.
Decareau, R. V. 1992. “Microwave Foods: New Product D evelopm ent” Food
and Nutrition Press, Trumbull, Conn.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
85
Decareau, R. V. and Peterson, R. A. 1986. “Microwave Processing and
Engineering.” Ellis Horwood Ltd., Chichester, England.
Eissen, W. and Muhlbauer, W. 1984. Development of low-cost solar grape
dryers. UNESCO Working Meeting on Solar Drying, Perpignan, Prance.
FAO. 1992. “FAO Year Book of Production, 1992.” FAO, Rome.
Gerling, J. E. 1991. “Installation, Operating, and Maintenance Manual. MlVAC
Pow er Generator.” Gerling Laboratories, Modesto, Calif.
Giese, J. 1992. Advances in microwave food processing. Food Technol. 46(9):
118-123.
Goff, J. and Gratch, S. 1951. Handbook of American Society of Heating and
Ventilating Engineers 43:77-78.
Gmcarevic, M. 1963. Effect of various dipping treatments on the drying rate of
grapes for raisins. Am. J. Enol. Vitic. 14:230-234.
Henderson, S. M. and Peny, R. L. 1976. “Agricultural Process Engineering.”
AVI, Westport, Conn.
Holman, J. P. 1981. “Heat Transfer.” McGraw-Hill, New York.
Lunardini, V. J. 1981. “Heat Transfer in Cold Climates.” Van Nostrand
Reinhold, New York.
McKinney, H. F. and Wear, F. C. 1987. Zoned microwave drying apparatus and
process. U.S. Patent 4,640,020. February 4.
McKinney, H. F., Wear, F. C., Sandy, H. L., Petrucci, V. E., and Clary, C. D.
1983. Process of making hollow dried grape. U.S. Patent 4,418,083.
November 23.
Mertens, B. and Knorr, D. 1992. Developments of nonthermal processes for food
preservation. Food Technol. 46(5): 124-133.
Mudgett, R. E. 1982. Electrical properties of foods in microwave processing.
F o o d Technol. 36(2): 109-115.
Mudgett, R. E. 1985. Directions in microwave food processing. In “Radio
Frequency/Radiation and Plasm a Proxessing,” ea. P. N. Cheremisinoff, O. G.
Farah, and R. P. Oullette. Technomic Publishing Co., Lancaster, PA.
Mudgett, R. E 1986. Microwave properties and heating characteristics of foods.
Food Technol. 40(2): 84-93.
Nelson, S. O. and Kraszewski, A. W. 1990. Dielectric properties of materials and
measurement techniques. Drying Technology 8:1123-1142.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Nichols, P. F. and Christie, A. W. 1930. Dehydration of grapes. Calif. Agric.
Exp. Sta. Bull. 500, October.
Nury, F. S., Brekke, J. E., and Bolin, H. R. 1973. Fruits. In “Food Dehydration,”
ed. W. B. Van Arsdel, M. J. Copley, and A. I. Morgan, pp. 158-198. AVI,
Westport, Conn.
Ohlsson, T. 1983. Fundamentals of microwave cooking. Microwave W orld 4(3):
4-9.
Ohlsson, T. 1990. Temperature distribution in microwave heating influence of
oven and food related factors. In “Engineering and Food,” vol. n , pp 231240. ed. by Elsevier Appled Science, New York.
Ohlsson, T. and Bengtsson, N. 1976. Microwave heating profile in foods. A
comparison between heating experiments and computer simulation. A
research note. Microwave Energy Application Newsletter 4(6): 3-8.
Ohlsson, T. and Risman, P. O. 1978. Temperature distribution of microwave
heating spheres and cylinders. J. Microwave Power 13(4): 303-310.
Petrucci, V. E., and Clary, C. D. 1989. Microwave vacuum drying of food
products. EPRI Report CU_6247. Electric Power Research Institute, Inc.,
4312 Hillview Avenue, Palo Alto, Calif. 94304: EPRI.
Petrucci, V. E., Clary, C. D., and Conrad, P. W. 1993. Development o f a dried
foods technology laboratory for demonstration of microwave vacuum drying.
EPRI Report CU-6247. Electric Power Research Institute, Inc., 4312
Hillview Avenue, Palo Alto, Calif. 94304: EPRI.
Ponting, J. D. and McBean, D. M. 1970. Temperature and dipping effects on
drying times of grapes, prunes and other waxy fruits. Food Technol. 24(12):
85-88.
Potter, N. 1968. “Food Science.” AVI, Westport, Conn.
Ramaswamy,H. S. and Pillet-Will, T. 1992. Temperature distribution in
microwave-heated food models. J. Food Quality 15: 435-448.
Raouzeos, G. S. and Saravacos, G. D. 1986. Solar drying o f raisins. Drying
Technology 4(4): 633-649.
Reynolds, L. 1995. The history o f the microwave oven. Microwave World 16(1):
11-15.
Sapers, G. M. 1993. Browning of foods: Control by sulfites, antioxidants, and
other means. Food Technol. 47(10): 75-84.
Schiffmann, R. F. 1986. Food product development for microwave processing.
Food Technol. 40(6): 94-98.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
87
Singleton, V. L., Trousdale, E., and Zaya, J. 1985. One reason sun-dried raisins
brown so much. Am. J. Enol. Vine. 36(2): 111-113.
Smith, R. D. 1984. Microwave power in industry. EPRI Report EM-3645.
Electric Power Research Institute, Inc., 4312 Hillview Avenue, Palo Alto, CA
94304: EPRI.
Stafford, A. E., Fuller, G., Bolin, H. R., and Mackey, B. 1974. Analysis of fatty
acid esters in processed raisins by gas chromatography. Agric. Food Chem.
22:478-485.
Stuchly, S. S. and Hamid, M. K. 1972. Physical parameters in microwave heating
process. J. Microwave Power 7(2): 117-137.
Thueiy, J. 1991. “Microwave: Industrial, Scientific, and Medical Applications.”
Artech House, Boston.
Tulasidas, T. N., Raghavan, G. S. V., and Akyel, C. 1995a. Quality and energy
aspects in microwave drying o f raisins. ASAE meeting Presentation. Paper
No. 95-3181. Chicago, 111., June 18-23.
Tulasidas, T. N., Raghavan, G. S. V., and Norris, E. R. 1993. Microwave and
convective drying of grapes. Transactions of the ASAE 36(6): 1861-1865.
Tulasidas, T. N., Raghavan, G. S. V., Voort, F., and Girard, R. 1995b. Dielectric
properties of grapes and sugar solutions at 2.45 Ghz. J. Microwave Power
ana Electromagnetic Energy 30(2): 117-123.
Vilayannur, R. S. and Puri, V. M. 1995. Size effect on uniformity of temperature
and moisture distributions during microwave heating of food materials.
ASAE meeting Presentation. Paper No. 95-6778. Chicago, 111., June 18-23.
Wickersheim, K. A. and Sun, H. M. 1987. Fiberoptic thermometry and its
applications. J. Microwave Pow er 22(2): 85-94.
Winkler, A. J., Cook, J. A., Kliewer, J. A., and Lider, L. A. 1974. “General
Viticulture.” Univ. of California Press, Berkeley.
Yen, M. and Clary, C. D. 1995. W hy is the grape puff™ puffy? An analysis of
MIVAC temperature curve. Cafif. Agric. Teen. Inst. Res. Bull. #951101.
California State Univ., Fresno.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
APPENDIX
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89
Dp.hye Model For Polar Liquid
The Debye model for polar liquid was used to predict the dielectric
properties of free water as functions of frequency and temperature (Mudgett,
1982):
k 'w =
{ k j- k o /l+ C X s /X .) 2 } +
ko
k"w= {(ks-ko) (kJk)}/{ l+ (kjk?)
Where,
k'w = dielectric constant o f water
k"w = loss factor of water
ks = static dielectric constant of water
ko = optical dielectric constant of water
= critical wavelength of water, cm and
k = process wavelength, cm.
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