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Properties and qualities of microwave-vacuum dehydrated Russet potatoes

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PROPERTIES AND QUALITIES OF MICROWAVE
VACUUM DEHYDRATED RUSSET POTATOES
By
Dewi Setiady, Ph.D
A dissertation submitted in partial fulfillment of the requirements for the degree of
Doctor of Philosophy In Food Science
Washington State University
Department of Food Science and Human Nutrition
May, 2008
UMI Number: 3333940
Copyright 2008 by
Setiady, Dewi
All rights reserved.
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To the Faculty of Washington State University:
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Dewi Setiady find it satisfactory and recommend that it be accepted.
Co-chair
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Co-chair
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11
ACKNOWLEDGEMENTS
I would like to recognize many people for assisting me during my doctoral work. I am
very grateful for meeting and having exceptional people who enriched my life and knowledge. I
would specially like to show gratitude to my advisor, Dr. Barbara Rasco, and my co-advisor, Dr.
Carter Clary, for their never end encouragement, guidance, protection, and support throughout
the course of my study and research at Washington State University.
I also would like to thank my exceptional doctoral committee, Dr. Barry Swanson and
Dr. Juming Tang, for their support, advice, and guidance. I thank Dr. Stephanie Clark, Frank
Younce, Dr. Joseph Power, Dr. Mengshi Lin, Dr. Carolyn Ross, Dr. Valerie Lynch-Holm, and
Dr. Marvin Pitts for help, academic advice, support, and encouragement. I extend many thanks to
my colleagues and friends, Jodi Anderson, Marsha Appel, Carolee Armfield, Dr. Elly
Soeryapranata, Dr. Subba Rao, Dr. Shantanu Argawal, Carol Padiernos, Beverly Ueti, Mary
Guenther, and Dean Guenther for their support, helps, encouragement, and friendship. I am also
very grateful for getting an opportunity to work part-time as a food scientist in R&D for Chukar
Cherry Co., and wish to thank Pamela Montgomery (CEO), J.T. Montgomery (CFO), and
Katherine Young (Production Manager), for their support, encouragement, and business
direction.
I extend my gratitude for a USDA National Need Graduate Fellowship, and WSU
Agriculture Research Center for financial support, which was essential to my research. I thank
Balcom and Moe, Inc., Pasco, WA, for providing fresh and consistent quality of fresh potatoes.
Finally, I would like to thank my family, especially my mother (deceased), father,
brothers, and my aunties and uncles for their support, encouragement, and patience.
PROPERTIES AND QUALITIES OF MICROWAVE-VACUUM
DEHYDRATED RUSSET POTATOES
Abstract
By Dewi Setiady, Ph.D
Washington State University
May, 2008
Co-chairs: Dr. Barbara Rasco and Dr. Carter Clary
Microwave-vacuum drying (MVD) is dehydration method that can provide rapid heat
transfer at reduced drying temperatures compared to atmospheric hot air drying. Several dryer
configurations are reported; in this study, a MIVAC® configuration was used. This MVD
modulates the application of microwave power based upon surface product temperature at any
time during the drying processing. This research evaluated properties and quality of MVD
dehydrated Russet potatoes {Solarium tuberosum) and compared these with heated air dried
(HAD) and freeze-dried (FD) potatoes. Color, texture, microscopic structure, porosity,
rehydration and sensory properties were analyzed in this research. MIVAC® drying behavior was
also determined by evaluating the moisture distribution and the color change after drying.
Drying at a moderate (60°C) temperature at longer times produced more desirable dried
potatoes compared to higher temperature drying for a shorter time due to scorching. MIVAC®
was equipped with a turntable and drying using a rotating turntable produced more even
microwave distribution and reduced the occurrence of scorching. The moisture distribution for
potatoes on the turntable was determined by divided the turntable into three ring regions; an
outer, middle, and center regions. The outer and center rings had a lower moisture distribution
iv
compared to the middle ring. Operating MIVAC with the turntable static showed that hot spots
occurred most often at the middle ring.
Texture Profile Analysis (TPA) and SEM images indicated that the dried MVD potatoes
were crispy, HAD potatoes were hard and brittle due to case hardening, and FD potatoes were
soft and spongy. The color retention of MVD potatoes was higher compared to FD and HAD
potatoes. The rehydration properties of MVD potatoes were similar to HAD, while FD potatoes
had higher rehydration properties. However, FD potatoes lost structural integrity following
rehydration. Sensory analysis indicated that the panelists preferred MVD potatoes among the
drying potatoes tested.
v
TABLE OF CONTENTS
ACKNOWLEDGEMENT
iii
ABSTRACT
iv
TABLE OF CONTENTS
vi
LIST OF TABLES
xi
LIST OF FIGURES
xiii
CHAPTER 1. INTRODUCTION
1
Objectives
3
Hypothesis
4
References
5
CHAPTER 2. LITERATURE REVIEW: OVERVIEW OF MICROWAVE VACUUM
DRYING
6
1. Introduction
6
2. The Concepts of Microwave Vacuum Drying
8
3. Different Configurations of Microwave Vacuum Drying Systems
3.1 Types of Microwave Power Application
15
15
3.1. a. Constant Mode MVD
3.1. b. Pulsed Mode MVD
3.1. c. Incremental Mode MVD
3.1. d. Combination Pulsed and Incremental Mode MVD
3.1. e. Continuous Temperature Control Systems
15
22
30
34
35
3.1. f. Combined Processing between MVD and Other Drying Methods
39
3.2. Sample Holder
52
3.3. Continuous MVD Processing
3.4. Pressure
56
58
vi
3.5. Temperature
59
5. Conclusions and Future Research
60
References
62
CHAPTER 3. DETERMINATION OF HEATING UNIFORMITY DURING MIVAC®
HEATING
69
Introduction
69
Materials and Methods
70
Results and Discussion
71
Conclusions
73
References
73
CHAPTER 4. OPTIMIZING DRYING CONDITIONS FOR MICROWAVE-VACUUM
(MIVAC®) DRYING OF RUSSET POTATOES (SOLANUM TUBEROSUM)
74
Abstract
74
Introduction
75
Materials and Methods
78
Drying Preparation
MIVAC® Drying and Experimental Design
Color
Analytical Methods
Statistical Analysis
Results and Discussion
78
79
79
79
80
80
MIVAC® Drying and Experimental Design
80
Color
87
Conclusions
90
References
91
vn
CHAPTER 5. DISTRIBUTION OF MOISTURE AND MICROWAVE ENERGY FOR
RUSSET POTATOES (SOLANUM TUBEROSUM) DURING MICROWAVE-VACUUM
DRYING UNDER CONTINUOUS TEMPERATURE CONTROL
94
Abstract
94
Introduction
95
Materials and Methods
98
Preparation of Potatoes for Dehydration
Moisture Distribution and Weight Loss during Dehydration
Operation of the Microwave Vacuum MIVAC® Drying System
Color Determination
Analytical Method for Moisture
Statistical Analysis
Results and Discussion
98
98
98
99
100
100
100
Moisture Distribution
Spectrophotometric Color Measurement
Product Color Based Upon Scanner Measurements
Specific Energy (Es) Calculations
100
109
116
118
Conclusions
119
References
119
CHAPTER 6. POROSITY AND DEHYDRATION PROPERTIES OF DEHYDRATED
RUSSET POTATOES USING MICROWAVE VACUUM, HEATED AIR OR FREEZE
DEHYDRATION
121
Abstract
121
Introduction
122
Mathematical Modeling
124
Materials and Methods
126
Drying Preparation
Analytical Methods
126
126
vm
Moisture Analysis
Apparent density Analysis
Color Analysis
Textural Analysis
Scanning Electron Microscopy Examination
Statistical Analysis
Results and Discussion
126
126
127
127
127
127
128
Properties of Dehydrated Potatoes
Porosity Properties of Dried Potatoes
Microscopic Structure of Dried Potatoes
128
129
132
Conclusions
133
References
135
CHAPTER 7. DIFFERENT SCANNING ELECTRON MICROSCOPY (SEM) FIXATION
METHODS FOR REHYDRATED POTATOES
137
Introduction
137
Materials and Methods
137
Preparation of Dehydrated Potatoes
Sample Preparation before Fixation
Fixation Methods
1. Standard (Std)
2. Microwave Osmium (MO) Fixation
3. Microwave (MW) Fixation
137
138
138
138
139
140
Results and Discussion
140
Conclusions
140
References
141
CHAPTER 8. REHYDRATION AND SENSORY PROPERTIES OF DEHYDRATED RUSSET
POTATOES USING MICROWAVE VACUUM, HEATED AIR OR FREEZE DEHYDRATION
145
Abstract
145
Introduction
146
Materials and Methods
148
IX
Drying Preparation
148
Determination of Rehydration Properties
148
Sensory Evaluation of Cooked Potatoes
Analytical Methods
Color Analysis
Texture Analysis
Scanning Electron Microscopy Examination
Statistical Analysis
149
149
149
149
150
150
Results and Discussion
The Rehydration Properties of Potatoes
Sensory Evaluation and Properties of Cooked Potatoes
150
150
154
Conclusions
160
References
160
Chapter 9. Conclusions and Recommendations
162
Conclusions
162
Recommendations
164
x
LIST OF TABLES
Table 2-1. MVD Profile (% Moisture, Product Temperature (°C), and Drying Time) for Peanuts
at 375 Torr at Various Microwave Power Levels (Data from Pominski and Vinnett,
1989)
16
Table 2-2. The HPLC Ginsenoside Profile of North American Ginseng Dried by 3 Different
Methods (Data from Popovich et al, 2005)
21
Table 2-3. Drying Time, Average Drying Rate, and Density of HAD, MVD, FD Dehydrated
Carrots (Data from Lin et al, 1998)
31
Table 2-4. Carotenoid Content (mean values in mg/kg d.m.; n = 3; s < 10%, *s < 20%) and
Water Content (mean values in wet basis (wb); n = 3; s < 10%, s** < 1%) for Carrot
Slices Before and After Conventional (HAD) and Microwave Vacuum (MVD) Dying
Processes (Data from Mayer-Miebach et al, 2005)
33
Table 2-5. Summary of Tomato Dehydration Experiments (n =5) Representing Different
HAD/MVD Drying Combinations (Data from Durance and Wang, 2002)
41
Table 2-6. List of Recent Research Conducted on Microwave Drying Applications by Author in
Chronological Order
46
Table 3-1. The Effects of Different Peak Picker Settings on MIVAC® System Operation
71
Table 4-1. Specific Energy (£,) and Moisture Content (MC) for MIVAC® Potatoes Dried at
Various Drying Temperatures and Times (n=2)
81
Table 5-1. % Weight Loss for MIVAC® Dynamic and Static Drying at A Set Point of 60°C...109
Table 6-1. CIE-Lab Values1 for Blanched, MVD, HAD, and FD Dried Potatoes (n - 80)
129
1
Table 6-2. Moisture Content (kg kgdb" and kg kgWb"') (n = 15) and Different True Density (kgL"1)
Values Based on Krokida (pp (K)), modifications from Krokida (pp (MK)), and
mathematically calculated (pp(MC)) as Equation 10
130
Table 6-3. Apparent Density (ps (kg L"1)), True Density (pp (kg L"1)), and Porosity (s) Values of
MVD, HAD, and FD Potatoes
132
Table 8-1. Texture Profile Analysis (TPA) factors' for Rehydrated Potatoes Dried Using MVD,
HAD, andFDat 10 or 20 min (n = 15)
153
Table 8-2. Preference and Acceptance Sensory Test Results of Control, MVD, FD, and HAD
Potatoes (n =30)
155
XI
Table 8-3. Different Testing Results (from control) of MVD, HAD, and FD Potatoes (n
=30)
155
Table 8-4. Texture Attribute Sensory Test Results of MVD, HAD, and FD Potatoes (n =
30)
,
155
Table 8-5. Texture Profile Analysis (TPA) Factors of Cooked Control, MVD, HAD, and FD
Potatoes
158
Table 8-6. CIE-Lab Values of Uncooked and Cooked Control, MVD, HAD, and FD
Potatoes
158
Table 8-7. Differential CIE-Lab1 Values Between Uncooked and Cooked Control, MVD, HAD,
and FD Potatoes
158
xn
LIST OF FIGURES
Figure 2-1. Experimental Microwave-Vacuum Drying Apparatus (Giri and Prasad, 2007)
52
Figure 2-2. Schematic Diagram of a Pilot-Scale MIVAC Dryer (Clary et al, 2005)
53
Figure 2-3. Schematic Diagram of Microwave-Vacuum-Rotary-Drum Dryer (Kaensup, 2002)..54
Figure 2-4. Schematic of an Experimental Drying Apparatus, (a) STJ-1 Vegetable Direct Fired
Dryer, (b) WZD4S-01 Microwave Vacuum Dryer (Yanyang et al, 2004)
55
Figure 2-5. Schematic Diagram of a Vacuum Microwave Dryer (Hu et al, 2006)
55
Figure 2-6. Schematic Diagram of a Continuous Processing of Microwave Vacuum Dryer (Clary,
1994)
56
Figure 4-1. Specific Energy (Es) and Moisture Content (MC) for blanched Russet potato slices
dried by MIVAC® at lW-h/g
80
Figure 4-2. Forward Power (FP) and Product Temperature (PT) for Blanched Russet Potato
Slices Dried by MIVAC® at a Set Point of 50, 60, or 70°C for 150 min
82
Figure 4-3. Net Power (NP) and Weight (WT) for Blanched Russet Potato Slices Dried by
MIVAC® at Set Point of 50, 60, or 70°C for 150 min
83
Figure 4-4. Reflected Power during Drying of Russet Potato Slices by MIVAC® at Set Points of
50, 60, or 70°C for 150 min
84
Figure 4-5. Chamber Pressure during Drying of Potato Slices by MIVAC® at Set Points of 50,
60, or 70°C for 150 min
85
Figure 4-6. Tristimulus Color Values for MIVAC® Dried Russet Potato Slices at Dryer Set
Points of 50, 60, and 70°C: (a) CIE-Lab L Values, (b) CIE-Lab a* Values, (c) CIE-Lab
b* Values, and (d) Total Color Differential (AE) Values
86
Figure 4-7. Correlation Between % Moisture Content (wb) and CIE-Lab L and a* Values for
MIVAC® Dried Potato Slices
89
Figure 4-8. Correlation Between % Moisture Content (wb) and Total Color Change (AE) Values
for MIVAC® Dried Potato Slices
90
Figure 5-1. % Weight Loss Values of Potatoes Slices Dried in MIVAC® at Various Drying Time
under Dynamic (D) or Static (S) Conditions at Different Location within the Drying
Chamber
104
xm
Figure 5-2. Schematic of the Plate Arrangement for Moisture Distribution Determination and
Visual Observation for Scorching After 45 min Drying at 60°C Using Static Turntable for
Three Separate Experiments; a, 1st test run; b, 2nd test run; c, 3 rd test run. The "starting
point" is the location at the entrance to the drying chamber
105
Figure 5-3. Correlation Between Moisture Content (db) and % Weight Loss Value for MIVAC®
Dried Potatoes Slices
106
Figure 5-4. CIE-Lab Differential L (AL)Values of Potatoes Slices Dried in MIVAC® at Various
Drying Times under Dynamic (D) or Static (S) Conditions at Different Locations Within
the Drying Chamber (See Figure 5-2)
107
Figure 5-5. CIE-Lab Differential a* (Aa*) Values of Potatoes Slices Dried in MIVAC® at
Various Drying Times under Dynamic (D) or Static (S) Conditions at Different Location
Within the Drying Chamber (See Figure 5-2)
107
Figure 5-6. CIE-Lab Differential b* (Ab*) Values of Potatoes Slices Dried in MIVAC® at
Various Drying Times under Dynamic (D) or Static (S) Conditions at Different Location
Within the Drying Chamber (See Figure 5-2)
108
Figure 5-7. Color Difference (AE) Values of Potatoes Slices Dried in MIVAC® at Various
Drying Times under Dynamic (D) or Static (S) Conditions at Different Location Within
the Drying Chamber (See Figure 5-2)
108
Figure 5-8. Correlation Between %Weight Loss and CIE-Lab Differential a* (Aa*) Values for
MIVAC® Dried Potato Slices
110
Figure 5-9. Correlation Between % Weight Loss and CIE-Lab Differential b* (Ab*) Values for
MIVAC® Dried Potato Slices
Ill
Figure 5-10. Correlation Between % Weight Loss and Color Difference (AE) Values for
MIVAC® Dried Potato Slices
112
Figure 5-11. Mean Red Differential (AR) Values of Potatoes Slices Dried in MIVAC® at Various
Drying Times under Dynamic (D) or Static (S) Conditions at Different Location Within
the Drying Chamber (See Figure 5-2)
113
Figure 5-12. Standard Deviation Red Differential (A Std R) Values of Potatoes Slices Dried in
MIVAC at Various Drying Times under Dynamic (D) or Static (S) Conditions at
Different Location Within the Drying Chamber (See Figure 5-2)
113
Figure 5-13. Mean Green Differential (AG) Values of Potatoes Slices Dried in MIVAC® at
Various Drying Times under Dynamic (D) or Static (S) Conditions at Different Location
Within the Drying Chamber (See Figure 5-2)
114
xiv
Figure 5-14. Standard Deviation Green Differential (A Std G) Values of Potatoes Slices Dried in
MIVAC at Various Drying Times under Dynamic (D) or Static (S) Conditions at
Different Location Within the Chamber (See Figure 5-2)
114
Figure 5-15. Mean Blue Differential (AB) Values of Potatoes Slices Dried in MIVAC® at
Various Drying Times under Dynamic (D) or Static (S) Condition at Different Location
Within the Drying Chamber (See Figure 5-2)
115
Figure 5-16. Standard Deviation Blue Differential (A Std B) Values of Potatoes Slices Dried in
MIVAC® at Various Drying Times under Dynamic (D) or Static (S) Condition at
Different Location Within the Chamber (See Figure 5-2)
115
Figure 5-17. Correlation Between Mean Red Values and CIE-Lab (Aa*) Values for MIVAC®
Dried Potato Slices
117
Figure 5-18. Correlation Between Moisture Content (kg/kgdb) and Specific Energy (W-h/g)
Values for MIVAC® Dried Potatoes Slices
118
Figure 6-1. Puncture Test Results (Force & Work) for MVD, HAD, and FD Dried Potatoes (n =
20)
128
Figure 6-2. True Density (kg L"1) Derived from Krokida (pp (K)) and Modifications to Krokida
(pp (MK)) at Various Moisture Contents (kgkgdb"') for (Krokida, 2001)
130
Figure 6-3. True Density (kg L"1) Based on Mathematical Calculation at Various Moisture
Contents (kg kgdb"') for Dehydrated Potatoes
131
Figure 6-4. SEM (70X) Images of Fresh and Dried MVD, HAD, and FD Potatoes
134
Figure 6-5. Cross sectional SEM (70X) Images of Fresh and Dried MVD, HAD, and FD Potatoes
134
Figure 7-1. SEM (70X) Images of Rehydrated Potatoes Made from MVD Potatoes Fixed Using
Different Methods. Std: Standard Method; MO: Microwave Osmium Method; MW:
Microwave Method
142
Figure 7-2. SEM (7OX) Images from Cross Sectional Cuts of Rehydrated Potatoes Made from
MVD Potatoes Fixed Using Different Methods. Std: Standard Method; MO: Microwave
Osmium Method; MW: Microwave Method
142
Figure 7-3. SEM (70X) Images of Rehydrated Potatoes Made from HAD Potatoes Fixed Using
Different Methods. Std: Standard Method; MO: Microwave Osmium Method; MW:
Microwave Method
143
xv
Figure 7-4. SEM (70X) Images from Cross Sectional Cuts of Rehydrated Potatoes Made from
HAD Potatoes Fixed Using Different Methods. Std: Standard Method; MO: Microwave
Osmium Method; MW: Microwave Method
143
Figure 7-5. SEM (70X) Images of Rehydrated Potatoes Made from FD Potatoes Fixed Using
Different Methods. Std: Standard Method; MO: Microwave Osmium Method; MW:
Microwave Method
144
Figure 7-6. SEM (7OX) Images from Cross Sectional Cuts of Rehydrated Potatoes Made from
FD Potatoes Fixed Using Different Methods. Std: Standard Method; MO: Microwave
Osmium Method; MW: Microwave Method
144
Figure 8-1. Rehydration Ratio of Dried Potatoes Dried Using MVD, HAD, and FD at Various
Rehydration Time (n = 3)
151
Figure 8-2. SEM (70X) Images of Rehydrated Potatoes Following MVD, HAD, and FD
154
Figure 8-3. Cross Sectional SEM (70X) Images of Rehydrated Potatoes Following MVD, HAD,
andFD
154
Figure 8-4. SEM (70X) Images of Control, MVD, HAD, and FD Potatoes
159
Figure 8-5. Cross Sectional SEM (7 OX) Images of Control, MVD, HAD, and FD Potatoes.... 159
xvi
CHAPTER 1. INTRODUCTION
Drying and dehydration practices are among the oldest forms of food preservation in
human history. In a modern life, a wide range of dehydrated foods provide convenience to hikers
and travelers, or simply new types of convenient foods for household storage and emergency
preparedness. Dehydrated foods require less space, no energy for storage (e.g. no refrigeration
requirement), and are light weight. The characteristics of high quality dried foods include a fresh
aroma, color, appearance, and flavor. However, there are still many technological challenges
facing the industry in terms of providing high quality, nutritious dehydrated food products in a
cost effective manner. Sun drying was the first drying method used in early times and is still
widely used. Sun-dried tomatoes, raisins, meat jerky, dried aquatic foods, and spices are still
commercially produced using sun drying. However, sun drying depends upon the weather.
Today, drying using heated circulating or static air provides greater process control.
Heated air drying (HAD) is probably the most common commercial drying method. HAD is the
most economical of the mechanical dehydration methods and is often the easiest dehydration
process to use, but HAD produces dried products with sensory qualities that are least like the
fresh food.
Freeze-drying (FD) is a dehydration method that produces better quality products than
HAD. Freeze dried (FD) products tend to be more nutritious, retain better color, and maintain a
greater degree of fresh like flavor after rehydration compared to heated air dried (HAD) foods.
The disadvantages of FD are the time and cost of operation due to the equipment and energy
required. Depending on the products, the FD process can take 20-30 hours, and HAD can take 624 hours, making FD a costly dehydration process.
1
Microwave-vacuum drying (MVD) is a new technology that provides an alternative to
FD. MVD processing can dry fresh fruits and vegetables in a more cost effective and time
efficient manner and still maintain a high quality product. MVD uses microwave energy as a
heating source and applies vacuum during the drying process. MVD allows for more efficient
energy transfer and moisture removal compared to the HAD method which relies on heat
conduction, because of the process of volumetric heating.
In MVD, heat is produced as microwave radiation induces molecular rotation within
polar molecules such as water in the food. A high frequency alternating electric field applied to a
food product causes polar molecules to rotate, and as these molecules attempt to realign
themselves in the electrical field, this rotation causes friction and the friction generates heat. The
ability of a microwave field to penetrate (volumetric heating) to the core of the food decreases
the processing time by up to 90% (Decareau and Peterson, 1986). Utilizing vacuum in
conjunction with microwave heating reduces the boiling point of water in the food and permits
moisture to vaporize at a lower temperature than under atmospheric conditions. This combined
process can improve product quality compared to food dried under atmospheric conditions. Since
the temperature of drying is lower, color and flavor can be maintained, and because the major
mode of heat transfer is not via conduction; case hardening, a major impediment to mass transfer
in atmospheric drying methods, can be limited. Applying a vacuum also reduces the amount of
oxygen available during dehydration limiting the production of free radical products, reducing
lipid oxidation in the food and resulting in less loss in flavor and color compared to similar
products prepared using other dehydration methods. Petrucci and Clary (1989) indicate that
vacuum drying also influences the internal pressure of the product and maintains the structural
integrity of the dried product to a greater degree than is possible with atmospheric drying. This
2
preserves shape and improved rehydration properties because less of the cellular structure has
collapsed.
Krokida et al. (2001) indicate that dehydrated products lose much of their original
structural integrity following rehydration due to the structural damage to tissue had has occurred
during dehydration and results in a greater degree of hysteresis during rehydration. MVD can
maintain the structural integrity of the dried products and contributes to better rehydration
properties, which are important quality factors for dried products that are added to prepared
mixes and add-water-only soups.
Objectives
A MIVAC® dryer, described by Clary et al. (2005), with modifications reported by
Setiady et al. (2007), is used in this research. MIVAC® is a distinct configuration of MVD and
will be explained further in a later chapter. This dissertation extensively examined the properties
and qualities of Russet potatoes {Solarium tuberosum) as a model food treated by MIVAC®. In
addition to some limited research with apples, the primary objectives of this project were to
characterize and compare the properties and quality of potatoes dehydrated using microwave
vacuum drying (MVD), HAD, and FD potatoes. This research had four specific objectives:
1. Study the effect of the drying rate on the color of MVD dried products.
2. Determine the variation in moisture content under different MVD processing protocols.
3. Determine the rehydration properties, color, texture, porosity, and cell structure of
potatoes dried using MVD, FD, and HAD.
4. Compare the sensory attributes of reconstituted and cooked MVD, FD and HAD potatoes
with cooked fresh potatoes.
3
Hypothesis
The general hypothesis of this research is that MVD drying will yield dried potatoes with
improved porosity, rehydration and sensory properties compared to those that have been dried
using HAD or FD. This hypothesis can be divided into four parts:
1. MVD drying profiles will vary depending on the drying temperature. There will be an
optimal drying temperature for preserving product color. Use of an intermediate product
temperature and a longer process time will preserve the color of potatoes pieces to a
greater extent compared to a higher temperature shorter time process which will result in
some scorching.
2. Various MVD drying conditions including time and use of a rotating turntable either in a
static (no rotation) or dynamic (rotating) mode of operation will yield products with
different moisture distributions.
a) Microwave energy distribution patterns will vary for each drying process.
b) Employing a rotating turntable (dynamic mode of operation) will improve the
uniformity of drying among potatoes pieces compared to processes employing a
static turntable at the same temperature or applied power levels.
3. Microwave treatment may create products with different porosity compared to those
prepared using an HAD and FD methods. The MVD treated potatoes will be more puffed
and maintain the original cell structure to a greater extent compared to heated air-dried
and freeze dried potatoes.
4. The MVD potatoes will have improved product characteristics compared to HAD or FD
products including:
a) rehydration properties,
4
b) color,
c) textural attributes and,
d) microscopic structure.
5. MVD processing will yield products with superior sensory attributes compared to HAD
and FD methods.
References
Clary, C. D., Wang, S. and Petrucci, V. E. 2005. Fixed and incremental of microwave power
application on drying grapes under gacuum. Journal of Food Science. 70(5): 344-349.
Decareau, R. V. and Peterson, R. A. 1986. Microwave Processing and Engineering. Ellis
Horwood Ltd. Chichester, England.
Krokida, M. K., and Maroulis, Z. B. 2001. Structural properties of dehydrated products during
rehydration. InternationalJournal of Food Science and Technology. 36:529-538.
Petrucci, V.E. and Clary, CD. 1989. Microwave Vacuum Drying of Food Products. EPRI Report
CU-6247. Electric Power Research Institute: Palo Alto, CA.
Setiady, D., Clary, C , Younce, F. and Rasco, B.A. 2007. Optimizing drying conditions for
microwave-vacuum (MIVAC®) drying of Russet potatoes (Solanum tuberosum). Drying
Technology. 25(9): 1483-1489.
5
CHAPTER 2. LITERATURE REVIEW
OVERVIEW OF MICROWAVE VACUUM DRYING
1. Introduction
Drying and dehydration often yield products with reduced susceptibility to deterioration
or microbial growth because much of the water that supports enzymatic or chemical reactions
and microbial growth has been removed. Drying refers to the use of natural conditions, such as
sun or air, to remove water from a food through evaporation. It is often difficult to dry products
naturally, since the weather conditions must be appropriate. It is also critical to protect the
product from contamination by dust and airborne microbial contamination which is usually
difficult to do in an open environment. Dehydration refers to applying heat under controlled
conditions to remove water from a food by evaporation or sublimation.
The terms of drying and dehydration are used interchangeably even though they have
different meanings. In the era of industrialization, starting in the late 18th century, dehydration
became more favored compared to drying due to the ability to control dehydration conditions,
not dependent upon weather, resulted in a shorter drying time, and in general, better product
quality, and more hygienic conditions. The first and still most common of food dehydration
methods is heated air drying (HAD). HAD tends to be a relatively cost effective method with
lower initial investment and maintenance costs than other dehydration methods. However, HAD
processes require 6 to 24 h drying time depending on the system configuration and the material
to be dried. This is a process time that is longer than microwave or fluidized bed drying.
Jayaraman (1990) indicated that the length of drying of HAD causes severe shrinkage and loss of
cell integrity, particularly for fruit and vegetable products, inhibiting of their full and rapid
rehydration capability.
6
Microwave heating has been widely adopted for domestic food preparation and a variety
of industrial purposes. Tempering, cooking, baking, pasteurization, sterilization, blanching, and
dehydration are some of the uses of commercial microwave processing. Microwave technologies
offer rapid heating that are time and cost efficient and can produce a high quality end product.
The difference between microwave heating and other types of heating is that microwaves do not
transfer heat primarily via conductive nor convective heating from the surface; but rather the heat
is generated by kinetic energy created by molecular motion within the food. This process can
produce a more uniform heating pattern within the food and reduce the processing time by up to
90% compared to conductive heating (Decareau and Peterson, 1986).
Microwave drying offers a means of a more rapid heating of a product than HAD along
with a potentially higher retention of product quality. Many researchers have incorporated
microwave heating with other drying technologies, such as:
- Convective heated air drying (Prabhanjan et al, 1995; McMinn, 2006)
- Spouted bed (Feng et al, 1998 and 1999; Jumah and Raghavan, 2001; Nindo et al, 2003)
- Fluidized bed (Kaensup et al, 1998; Kaensup and Wongwises, 2004; Goksu, 2005;
Stanislawski, 2005)
- Freeze drying (Ang et al, 1977; Arsem and Ma, 1990; Barrett et al, 1997; Chen et al, 1993;
Cohen and Yang, 1995; Ma and Peltre, 1975; Sochanske, 1990; Souda et al, 1989; Wang and
Shi, 1999; Wang and Chen, 2003; Wang et al, 2005; Wu, 2004)
- Vacuum drying (Anthony, 1983; Attiyate, 1979; Bohm et al, 2002; Chen, and Chiu, 1999;
Clary et al, 2005, 2006; Cui et al, 2003, 2004, 2004, 2005, and 2006; Drouzas and Schubert,
et al, 1999; Durance and Wang, 2002; Erie and Schubert, 2001; Farel et al, 2005; Giri and
Prasad, 2007; Gunasekaran, 1999; Hu, et al, 2006; Kaensup et al, 2002; Kim et al, 2000,
7
2000, Kim and Bhowmik, 1994, 1995, 1997; Kim et al, 1997; Kiranoudis et al, 1997; Krulis
et al, 2005. Leiker et al, 2004a; Lian et al, 1997; McMinn, 2004, 2006; Mayer-Miebach et
al, 2005; Moon, 1997; Mousa and Farid, 2002; Mui et al, 2002; Prabhanjan et al, 1995;
Pominski and Vinnett, 1989; Popovich et al, 2005; Regier et al, 2005; Rodriguez et al, 2005;
Sham et al, 2001; Sunjka et al, 2004; Tilton and Vardell, 1982; Velupillai et al, 1989;
Yanyang et al, 2004; Yen and Clary, 1998; Yongsawatdigul and Gunasekaran, 1996a, 1996b;
Yousif et al, 1999, 2000; Zhang et al, 2007).
This review will focus on the development of microwave vacuum drying (MVD)
technologies. The combination of microwave and vacuum offers several benefits compared to
other combinations of dehydration treatments with details explained in later sections. The
objective of this review is to provide information about the past and current research leading to
the development of and improvement in microwave vacuum drying technologies with the
purpose of providing researchers and engineers the possibility of creating a unique and novel
MVD system.
2. The Concepts of Microwave Vacuum Drying
The two main elements of MVD are microwave heating and vacuum. As mentioned
earlier, microwave provides the opportunity for more rapid heating and greater heating
uniformity. Microwave is a part of the electromagnetic spectrum, occupying a ray from 300 MHz
to 300 GHz. The Federal Communications Commission (FCC) regulates the frequencies use for
domestic and industrial electromagnetic heating in the USA and restricts the use of microwave
heating to 915 and 2450 MHz (Hui et al, 2008). A microwave with 2450 MHz, has a penetration
depth up to 10 cm, while the penetration of 915 MHz, used in industrial microwave applications
can be as great as 30 cm. Thus microwaves can penetrate deep into the food, depending on the
8
composition of the product. Microwave can be absorbed, transmitted and reflected. In food, the
absorbed microwave energy generates heat by two mechanisms: ionic polarization and dipole
rotation (Buffler, 1992). Ionic polarization is influenced by the type and concentration of ionic
compounds in the food or solution. Here, the ions move as a result of the electric field. The rapid
movement of ions throughout the material causes collisions between molecules and generates
kinetic energy which is then converted into heat. Ionic polarization is important in foods that
have a high salt concentration. The second mechanism, dipole rotation, involves polar molecules,
such as water. Generally dipole rotation is a more important heating mechanism in foods
compared to ionic polarization since the predominant component in most foods is water.
Applying microwave causes the polar molecules to rotate with alternating electromagnetic field
attempting to orient themselves in it. This molecular rotation creates friction, produces kinetic
energy, and heat. As food products undergo microwave dehydration, it is possible that the
relative role of ionic polarization and dipole rotation could change during drying. Dipole rotation
is probably the most important mechanism during the initial stages of drying due to high
moisture content. However, as the product moisture content decreases during drying, ionic
polarization could play a more important role. This presumes that whatever moisture still present
in the food is mobile allowing for migration of ions throughout the food.
Vacuum reduces the boiling point of water in the food and allows water to vaporize at a
lower temperature than under atmospheric conditions. Therefore, applying vacuum to a
microwave system can improve the efficiency of drying by reducing case hardening and thereby
promoting mass transfer from the surface of the product during dehydration. This results in a
lower temperature for drying and preserves volatile and heat sensitive aromatic and flavor
components. Drying under a vacuum also decreases the amount of oxygen available during
9
drying. Therefore, it limits the production of free radical products, decreasing lipid oxidation in
the food and enhances flavor and color of the dehydrated products. Applying a vacuum also
assists in maintaining the structural integrity of the dried product and preserving shape. Cell
collapse in plant tissue occurs during HAD to a great extent, and this is one of the primary causes
of case hardening. Drying under vacuum helps reduce shrinkage (Wu, 2007). Rehydration
properties are also generally improved when drying occurs under a vacuum since less of the
cellular structure, particularly at the surface of the product, is retained permitting moisture
penetration into the interior of the piece.
The combination of microwave and vacuum has a synergistic effect. Not only is
dehydration more rapid, but product quality is also improved. Drying can occur at a lower
temperature than HAD reducing problems with overheating, leading to a greater retention of
color, flavor, texture, and nutritional value.
Since the development of microwave for RADAR, its heating characteristics were
recognized and its application to dehydration was investigated in the early 1950's. The first
systematic studies combining microwave heating with vacuum drying (MVD) were introduced in
the early 1970's. Attiyate (1979) reported the first industrialized MVD in France in 1978 for
drying orange juice concentrate. Micro-Ondes Internationales (IMI) in France developed and
built the first industrialized MVD for Pampryl S.A. (France) after successfully developing a pilot
scale dryer in 1975. This first industrialized MVD used 2450 MHz and consisted of eight 6000
W magnetrons placed on top of a vacuum drying tunnel. Microwave energy was introduced into
the tunnel, and then the beam was transferred into the tunnel through waveguides. The level and
method of application of microwave power during drying (e.g. continuous or pulsed) was unclear
in these initial dryers. However, it was clear that MVD was operated at 8 to 12 torr (1.07 to 1.60
10
kPa). The advantages of this MVD system were higher quality retention, such as typical orange
flavor, aroma and color, and higher processing efficiency. The processing time was significantly
shorter and involved a lower temperature treatment. The pioneers of this technology envisioned
one of the most promising applications to be the production of high quality dehydrated delicate
fruits with an expectation that higher retention of volatile compounds would result. The capital
investment of this MVD system was 40% less than freeze drying (FD) in the 1970's and the
operating cost was 3 to 4 times less than FD.
In 1978, a group of scientists from McDonnell Aircraft Company (now Boeing
Company) completed the assembly of a MVD system called MIVAC® and held an open house
for a public demonstration in Tifton, Georgia, US. While Attiyate (1979) reported on the MVD
system to dry concentrated orange juice, investigators with MIVAC were developing
applications for drying grain based materials, such as rice, sorghum, corn, soybean and also
peanuts. MIVAC® used 2450 MHz with the maximum power of 6000 W. Microwave energy was
transferred by a waveguide into the interior of the vacuum vessel through a Teflon window.
Several scientists and engineers adopted this system and developed processes for different
applications (Tilton and Vardell (1982), Anthony (1983), Petrucci and Clary (1989) and Clary et
al. (199'4, 2005, 2006, and 2007)).
In 1982, Tilton and Vardell reported the use of MIVAC® to eliminate or reduce the
infestation of stored-product insects in grain. This MIVAC® was equipped with a 91.4 cm dia
drying chamber, a 2450 MHz microwave generator, and a turntable that rotated at 6.5 RPM.
They found out that the growth of the Lesser Grain Borer (Rhyzopertha dominica (F.)) or Rice
Weevil (Sitophilus oryzae (L.)) in vacuum without microwave was inefficient for controlling
these insects. The use of microwave with vacuum required a relatively high level of microwave
11
energy (0.38 PDU or Power Density Unit) with a vacuum of 35 torr (4.67 kPa) could completely
control the infestation of the Lesser Grain Borer or Rice Weevil in wheat and rye. Microwave
only treatment could not eliminate these pests completely. MIVAC® drying with higher
microwave power at a short time was the most effective treatment compared to drying at a lower
power for a longer time. The growth of the Angoumois grain moth (Sitotroga cerealella
(Olivier)) was completely eliminated in rye and corn, and was reduced by 96.8% in wheat by a
treatment at 0.25 PDU microwave energy and a partial vacuum of 35 torr (4.67 kPa) for 10 min
which was the highest microwave power tested for the shorter time treatments. Treating the
Lesser Grain Borer using the same treatment was 100% effective in corn, 99.4% effective in rye,
and 95.6% effective in wheat. Using the same treatment also eliminated the growth of rice
weevil in rye and reduced it by 99.2% in wheat. The Maze Weevil (Sitophilus zeamais
Motschulsky) in corn was completely controlled by treatment at 0.25 PDU at 35 torr for 10 min.
Cost of operation were not addressed.
Anthony (1983) used MIVAC® to dry cotton seed and determined the properties of its oil.
Cotton seeds were dried at 50.7, 20.0, 6.7 kPa at 1.22, 0.61, 0.43 W/cm3 on a rotating turntable
inside the dryer. Applying microwave energy at lower power levels for a longer drying time was
more effective than higher levels of microwave energy and shorter drying times for maintaining
oil quality. Drying using microwave at reduced pressure reduced the drying time and also
resulted in decreased lipid hydrolysis and, as a result, lower free fatty acid content. Other oil
properties were also improved or remained unchanged, while cottonseed germination was
controlled to zero.
Petrucci and Clary (1989) and Clary et al (1994, 2005, 2006, 2007) are among the most
recent scientists developing MIVAC® drying process. Their papers covered the use of MIVAC®
12
to dry grapes, as well as other fruits and vegetables (unpublished). Two types of MIVAC units
were used, one for batch and another for continuous processing. They also modified the
application of microwave power from a constant power level to temperature control that offered
more promising process control for dehydration. The batch MIVAC dryer consisted of a 3000 W,
2450 MHz microwave power supply, wave guide, microwave window, microwave control, a 90
cm in diameter and 120 cm long vacuum vessel, and a vacuum pump (Figure 2-2, pg 53). A turn
table was connected to load cell for monitoring the change of product weight during drying. An
infrared temperature sensor (model H-L10000 infrared detector, Mikron, Oakland, N.J., U.S.A.)
was also installed to monitor product temperature with an electrical reference signal (0 to 1 V)
fed to a microwave power control system. The microwave power supply included an
electromagnet around the magnetron to control power levels at any continuous wave level from 0
to 3000 W. A reference signal from the infrared detector controlled the microwave power to
maintain the set process temperature. Their work will be discussed further in the later chapter.
By the 1990's, many scientists worked on MVD drying of fruits, vegetables, herbs,
shrimp, hides, and wood. Komanowsky (2000) developed a method to dry hides using MVD.
Uniform exposure to the microwave energy could be attained by tumbling the hides inside a
drum. The pressure was maintained about 36 torr (4.8 kPa) to prevent the denaturation of
collagen within the hides. MVD was a faster processing method that resulted in reduced bacterial
growth. Even though MVD was a more expensive process compared to conventional drying, it
was more cost-effective since the fresh split could be recovered and sold as edible-grade
collagen.
Lieker et al. (2004a) describe a process to dry beech (hardwood) using MVD. The MVD
consisted of a cylindrical stainless steel vessel (200 L in volume, 1 m in length, and 0.5 m dia), a
13
turntable, two 3000 W 2450 MHz magnetrons, a mode stirrer for field distribution, and a liquid
seal pump to lower pressure to 33 mbar (3.3 kPa). Load cells were installed to monitor weight.
The infrared temperature detector was installed to monitor surface product temperature, while
four channel fiber optic temperature sensors were used to monitor internal product temperature.
The sample size was 0.3 m in length, 0.15 m in width, and 30 or 50 mm in thickness. Lieker
reported that the center of the wood sample had lower moisture content compared to the surface
of the sample. Drying sample with higher initial moisture resulted in higher energy efficiency
compared to lower initial moisture.
In addition to using MVD for drying food materials and wood, it has been used in the
pharmaceutical sector to dry powders as explained by Farel et al. (2005). A MVD system was
equipped with 650 W and 2450 MHz microwave oven (Brother, Hi-speed cooker, Model No.
MF 3200 dl3) and a glass dessicator placed inside the microwave cavity to which a vacuum
pump was attached. The vacuum pressure was maintained at 61-81 kPa by an actuator valve
using a pressure gauge. The drying rate profile for all powder-solvent systems showed an initial
warm-up period, a constant-rate stage and two falling-rate periods. The combination of both
solvent and powder systems affected the drying kinetics. Materials wetted with acetone, ethanol,
or methanol had a shorter drying time up to 89.8% compared to materials wetted with water. The
temperature profile for all powder-solvent systems was a short warm-up period, a constant rate
period, and a decreasing temperature period. However, the solvents in the mixtures determined
the temperature ranging from highest to lowest in the order of: water, ethanol, methanol, and
then acetone. Reducing the drying pressure increased the drying rates and decreased drying
times.
14
3. Different Configurations of Microwave Vacuum Drying Systems
The basic components of MVD system includes a microwave power supply, vacuum
chamber, wave guide, sample holder, vacuum pump, condenser, and control system. Some
systems use a temperature or power control unit, temperature measuring device, or air flow to
agitate the material during drying. Many configurations of MVD systems have been investigated
based on the strategy used for microwave power application, the pressure or vacuum condition,
configuration of the sample holder, and the temperature controller system. All of these features
influence the drying process and product quality.
3.1 Types of Microwave Power Application
Microwave power is the most variable factor contributing to MVD performance.
Microwave power units can be categorized into several application systems: constant or fixed
mode, pulsed application systems (timer on - off), incremental or staged application (multiple
fixed powered magnetrons), a combination of pulsed and incremental, or continuous temperature
control system. Combination processing methods using MVD with other methods of drying will
also be discussed in this section.
3.1. a. Constant Mode MVD
A MVD with a constant, fixed, or continuous mode involves applying microwave power
at a specific fixed power level during the entire drying process. At the beginning of MVD
development, most scientists used MVD in a constant mode configuration. Even though it is not
clearly presented in their work, Tilton and Vardell (1982) and Anthony (1983) indicate that
MIVAC® was operated in a constant mode.
Clary (1994) began work with MIVAC® operated in a fixed or constant power mode, and
then examined incremental modes as a means of decreasing problems with localized hot spots
15
during drying. Clary et al. (2007) modified MIVAC into a semi-automated continuous
temperature control system.
Pominski and Vinnett (1989) and Velupillai et al. (1989) worked on MVD applying a
constant microwave power during the processing. Pominski and Vinnett (1989) used MVD that
was manufactured by Aeroglide Corp. (Raleigh, NC) to dry peanuts for the production of peanut
flour. The size of the drying chamber was 91.44 cm in diameter by 91.44 cm in length. A 2450
MHz microwave power was installed and could produce up to 2500 watts. A turntable was used
as a product holder. Four power levels: 1400, 1118, 813, and 560 W for drying 22.7 kg peanuts
at 385 torr (51 kPa) were tested. Increasing microwave power resulted in lower product moisture
content, higher product end temperature, and shorter drying times (Table 2-1). Higher
temperature drying of peanuts significantly reduced the raw peanut flavor and increased the
solubility of hexane-extracted protein compared to lower temperature drying and compared to
flour from HAD peanuts.
Table 2-1. MVD Profile (% Moisture, Product Temperature (°C), and Drying Time) for Peanuts
at 375 Torr at Various Microwave Power Levels (Data from Pominski and Vinnett, 1989)
Power
Temperature
Time
%
(min)
Moisture
(Watts)
(°C)
560
68.3
180
9.9
813
80
120
9.5
1118
75
9.7
89
1400
60
7.8
96
Velupillai et al. (1989) dried parboiled rice using a constant mode MVD. The size of the
MVD batch drying chamber was 0.914 m in length and 0.914 m in dia. Microwave power and
vacuum pressure was 2500 W and 1.3 kPa, respectively. In their study, the sample was placed
inside a rotating drum in the drying chamber. Parboiled rice was dried using 600, 1200, or 1800
W microwave power at either 53.3 kPa (low vacuum level) or 6.67 kPa (high vacuum level) to
16
the final moisture content of 14,16, or 18 % (wb). They found that increasing microwave power
and vacuum level decreased the whole kernel yield. The drying rate was increased as the
microwave power and vacuum level was increased. The microwave and vacuum treatments had
no affect on the color of the final products; however, increasing microwave power and vacuum
levels decreased the amount of gelatinized kernels. They concluded that it was feasible to dry
parboiled rice using MVD since the quality of MVD rice was comparable to the conventional
method parboiled rice.
Lin et al. (1999) evaluated the physical and sensory properties of shrimp dried using a
dryer described in his earlier work (1998) in a constant mode. Shrimp were dried at a microwave
power of 2000 or 4000 W and at 100 or 200 mmHg (13.3 or 26.6 kPa), respectively. The drying
rate of MVD was faster than HAD or FD. Decreasing microwave power and/or increasing
vacuum pressure increased the drying time. Shrimp dried at higher microwave power (4 kW) and
lower vacuum pressure of 100 mmHg (13.3 kPa) had less shrinkage, higher rehydration
potential, and higher water retention ability compared to HAD shrimp. Decreasing the
microwave power and/or increasing the vacuum pressure increased shrinkage and decreased
rehydration potential and water retention ability but also led to a tougher texture. MVD shrimp
dried at 4 kW at 100 mmHg (13.3 kPa) had similar sensory rating for texture, color,
aroma/flavor, and overall acceptability as FD shrimp.
Chen and Chiu (1999) investigated the recovery of volatile compounds in dehydrated
onions dried using MVD in a constant mode. A Whirlpool commercial microwave oven was
modified and vacuum pump was installed to make a custom MVD. Onion slices (100 g) were
dried using 600 W microwave power at 100 torr (13.3 kPa) for 1 hr. The levels of the flavor
compounds 2-methyl-2-pentenal and 1-propenyl propyl disulfate in onion decreased rapidly
17
during the first 20 min of the drying process, and only 30 to 73% of these compounds were
retained after drying. The authors indicated that retention of these compounds followed the first
order reaction with the rate constant of 0.13 and 0.31 per min, respectively.
Kim et al. (2000a) investigated the retention of alkamides in dried botanicals used in
nutritional supplements and functional foods. MVD presumably in a constant mode, HAD, and
FD was used to dry Echinacea purpurea leaves and roots. These investigators used a 2450-MHz
MVD (ENWAVE Corporation, Port Coquitlam, BC). E. purpurea leaves were dried using MVD
at 1500 W microwave power at full vacuum of 50 mmHg (6.67 kPa) for 17 min. E. purpurea
roots were dried using MVD at 1000 W microwave power at 50 mmHg (6.67 kPa) (treatment
referred to as: microwave vacuum drying (MVD)) for 25 min or at 200 mmHg (26.6 kPa)
(treatment referred to as: microwave partial vacuum drying (MPVD)) for 25 min. Air flow rates
for MVD and MPVD were 5 and 20 L/min, respectively. The temperatures for products (3840°C) dried at lower pressure (50 mmHg) at the end of the processing were lower than for
products dried at higher pressure (200 mmHg) (48-50°C) as determined using an infrared
thermometer (Cole Parmer, Vernon Hills, IL) (Kim et al. 2000a). FD was found to be the best
drying method for retaining total alkamides. MVD was a better method for drying roots than
HAD at 70°C, although HAD at 50°C was a better method for drying leaves of E. purpurea than
MVD as indicated by the retention of the alkamide components.
In the same year (2000), Kim et al. investigated the retention of caffeic acid derivatives
in dried MVD, HAD, or FD Echinacea purpurea. Echinacea purpurea flowers dried using MVD
at 1000 W microwave power at 50 mmHg (6.67 kPa) for 47 min (Kim et al. 2000b). Microwave
power was measured by an IMPI 2 liter test (Buffler, 1993). The MVD system was equipped
with a high density polyethylene drying basket with a cylinder size of 0.26 m radius and 0.23 m
18
height. The rotation speed of the drying basket was 6 rpm. The caffeic acids were significantly
affected by the drying method. MVD flowers retained the highest levels of chicoric acid and
caftaric acid similar to FD flowers, even though there was significant loss of chicoric acid in
MVD flowers when stored at high moisture content. Flowers dried using HAD at 25°C only
retained 50% of total acids, and those dried by HAD at 70°C retained an even lower amount.
Flowers dried using HAD at 40°C retained more than 85% of caftaric acid and 82-84% chicoric
acid. However, it took 55 h for HAD compared to 47 min for MVD. MVD produced dried
flowers containing similar contents of chicoric and caftaric acids as FD flowers. MVD flowers
retained a greater amount of the natural color and were similar in color to FD flowers. No
enzymatic browning was observed for MVD flowers but browning was significant in the HAD
flowers.
Kaensup et al. (2002) presented data on drying chilies in a microwave vacuum-rotary
drum dryer in a constant microwave mode. This MVD consisted of a 0.33 m in dia and 0.4 m in
length cylindrical stainless steel vacuum chamber, a 2450 MHz magnetron with 800 W
maximum power output, stainless steel wave-guide, and a vacuum pump (Figure 2-3, pg 54). The
rotating drum was made of polypropylene with the diameter of 0.3m and length of 0.3 m. An
electronic inverter was installed to control the speed of the drum. Red chilies were dried at 800
W microwave power, one of 3 vacuum levels of 60, 160, and 260 mmHg (8.00, 21.3, 34.5 kPa),
and one of 3 drum rotation speeds (10, 20, and 25 rpm). Reducing the chamber pressure reduced
the drying time. The rotating drum sample holder reduced the dead region (improved microwave
distribution) affect that occurs in a stationary chamber. The ideal rotation speed was 20 rpm, and
that produced the shortest drying time. The variation between rotation speed and vacuum
pressure had no influence on the specific energy consumption for high moisture content, while
19
for products with lower moisture content, a low rotating speed and low vacuum pressure
consumed less energy.
Clary et al. (2005) used an MVD configuration (MIVAC®) in a constant or "fixed mode"
fashion and reported three stages of temperature profiles for grape dehydration. In a constant
mode, the first drying stage consisted of increasing temperature from ambient. The second
drying stage showed a balance of heating and cooling at a constant temperature plateau, and the
final stage, the temperature increase rapidly as the cooling effect of moisture loss decreased. The
results of applying a constant level of microwave energy during dehydration caused the grapes to
overheat and scorch before reaching the desired moisture content. Because of this problem, these
investigators developed a continuous temperature controlled MVD system for dehydrating this
and other products.
Krulis et al. (2005) investigated the influence of energy input and initial moisture on
physical properties of MVD strawberries. They used a Microwave Vac650 pilot-plant station
(Puschner MikrowellenEnergietechnik, Schwanewede, Germany). The vacuum chamber was
made from a stainless steel with a cylindrical shape of 0.5 m internal dia and 1 m length.
Strawberries (70 g) were placed on a turntable attached to a load cell. A liquid seal rotary pump
was employed to maintain 4 kPa of vacuum. The microwave heating unit consisted of two 2450
MHz magnetrons with a nominal power of 3000 W that could be tuned continuously between
10% and 100% producing microwave power from 300 W to 6000 W. However, Krulis made no
mention how the microwave energy was applied, however it was surmised that it was in a
constant mode. Horn applicators were used for field distribution inside the cavity. A diode based
system was used to measure the power absorbed by the product. A systems controller recorded
power values, pressure, and load cell readings every second. Drying materials with low initial
20
moisture content at high microwave power produced a low density product with a porous
structure exhibiting optimal puffing effects. The drawbacks of this dehydration method were low
moisture evaporation rate and high energy utilization. The best drying efficiency was obtained
when drying raw materials at high initial moisture content at high microwave power input.
Popovich et al. (2005) investigated the retention of ginsenosides in ginseng root dried
using FD, HAD, and MVD techniques. Ginseng roots were dried in MVD by placing them in a
polyethylene basket inside a drying chamber. Their method involved the application of 2450
MHz, 2 kW MVD dryer (ENWAVE Corp, Vancouver, BC, Canada). A perforated drying basket
was made from high-density polyethylene cylinder (0.26 m radius, 0.31m length). The drum
rotated horizontally at a rate of 15 rpm. Ginseng roots were dried at 28.4 mmHg (3.8 kPa)
vacuum at 2000 W microwave power for 13 min. Ginseng roots dried using MVD retained
similar total ginsenoside levels as FD, while HAD had the lowest levels (Table 2-2, pg 53). From
electrospray mass spectrometry (ESI-MS) analysis, 12 ginsenoiside compounds were detected in
FD ginseng roots, but only 10 compounds were detected in the roots dried by MVD or HAD. FD
and MVD resulted in ginseng with improve extraction efficiency and higher retention of total
ginsenoside than HAD.
Table 2-2. The HPLC Ginsenoside Profile of North American Ginseng Dried by 3 Different
Methods (Data from Popovich et al., 2005)
Ginsenosides
HAD
MVD
FD
Rgl (mg/g)
19±0.7
22±0.8
28±0.9
Re (mg/g)
29±0.1
24±0.1
45±0.1
Rbl (mg/g)
Rd (mg/g)
69±0.1
5±0.0
83±0.1
13±0.0
62±0.1
9±0.1
Re (mg/g)1
2.5
3.2
1
124.5
145.2
145
Total (mg/g)
Re amounts were calculated by subtracting the total ginsenosides to the other 4 ginsenosides.
21
Giri and Prasad (2007) investigated the drying kinetics and rehydration characteristics of
mushroom using MVD in a constant mode and HAD. The mushrooms were dried under several
microwave power levels (115,200 or 285 W) at several vacuum pressures (6.5, 15, or 23.5 kPa)
to 6-6.5 % moisture content (wb) (Figure 2-1, pg 52). MVD processing decreased the drying
time to 70-90% compared to HAD and the products had better rehydration characteristics. They
developed a model for the drying rate using an exponential and Page's model for thin layer
drying. Regression analysis indicated that the drying rate constant was mainly affected by the
microwave power, followed by sample thickness, while system pressure had little effect.
However, the system pressure significantly influenced the rehydration ratio. More vacuum
created a more porous dehydrated product that was easier to rehydrate than the other treatments
examined. Empirical models were developed to predict the drying rate constant and rehydration
ratio based upon the parameters of MVD processing.
3.1. b. Pulsed Mode MVD
A pulsed mode applies microwave energy at a certain level of power, but it switches the
power on and off during the entire process to reduce overheating. Most consumer microwave
ovens use a constant or fixed mode operating continuously at the 100% power level, while a
pulsed microwave oven operates at less than 100% output by turning the microwave power on
and off based on a time sequence. A pulsed mode MVD can be operated in a similar manner with
the power controlled with a timer or manually.
Gunasekaran (1990) examined the benefits of applying microwave power (without
vacuum) in constant and pulsed modes of MVD to dry high moisture corn. This was not the
application of first pulsed mode MVD. The author indicated that drying in a constant mode
resulted in a higher drying rate, but it also required a higher total magnetron power-on times.
22
Longer power-on times in the pulsed mode increased the drying rate; however, longer power-off
times did not significantly affect the drying rate.
In 1996, Yongsawatdigul and Gunasekaran investigated MVD in a constant mode and
pulsed mode to dry cranberries. For this experiment, a laboratory scale microwave oven (Zwag,
Model Labotron 500) with two 250 W magnetrons were used and modified for MVD. The unit
consisted of a thick-walled glass bell jar that served as a vacuum chamber positioned on the
middle of a turntable in the unit. The vacuum chamber was maintained at 0-13.3 kPa absolute.
For a constant mode, fresh cranberries (100 g) were dried at 200 or 500 W microwave power and
at 5.33 or 10.67 kPa. In a pulsed mode, fresh cranberries were dried at 250 W at 5.33 or 10.67
kPa for 30 or 60 s power on and 60, 90, or 150 s power off. The temperature was measured by a
thermocouple digital thermometer (Omega, HH99T2) and sample weight measured by an
electronic balance (Mettler, PM 4600). A higher product temperature was obtained using a
constant MVD mode, compared to a pulse mode configuration. The total drying times for MVD
operated in the constant mode were shorter than in the pulse mode; however, drying in the
constant mode required more energy. In the pulse mode, a longer 'power on' cycle led to faster
drying and was more energy efficient than MVD in the constant mode. Shorter power-on time
and longer power-off time settings was more favorable because it permitted moisture diffusion to
occur during the power off time within the cranberries and that accelerated mass transfer. At the
beginning of the drying period, the sample moisture content was high and the cranberries
absorbed more energy. However, as the moisture content decreased, the energy absorption was
also decreased. Constant application during MVD processing was inefficient and degraded the
product quality. The optimum drying condition for MVD to dry cranberries was power on for 30
s and power off for 150 s at 250 W.
23
Yongsawatdigul and Gunasekaran (1996b) described the quality of dried MVD
cranberries. Higher process pressure reduced the redness of the cranberries dried by both
constant and pulse mode MVD. Cranberries dried for longer power-on time had smaller overall
color change (AE) and retained more red color than those dried using a shorter power-on time.
Drying in a constant mode increased the redness of the final product compared to drying in the
pulse mode. However, since drying in the constant mode required less time, this decreased the
oxidation of anthocyanins. Analysis indicated that commercially dried cranberries required a
similar force to cut as HAD cranberries dried in their laboratory. Less force was required to cut
cranberries dried by either constant or pulse mode MVD. This indicated that MVD cranberries
were softer than those dehydrated in a commercial or laboratory HAD. Sensory testing indicated
that cranberries dried using the constant mode MVD, were much harder or tougher than those
dried by pulse mode MVD.
In 1999, Gunasekaran published a review paper emphasizing how critical the proper
choice of the pulsing ratio is during MVD. A longer power off time compared to power on time
was more energy efficient and produces higher quality product, especially for heat sensitive
products like fruits.
Moon et al. (1997) investigated the inactivation of enzymes in a solution using MVD in
pulsed mode compared to HAD. At the beginning of drying, MVD was shown to be a less
effective drying treatment for inactivation of enzymes compared to HAD due to the pause time
(power off cycle). However, MVD proved to be an effective process for inactivating enzymes in
less time compared to HAD.
Kiranoudis et al. (1997) examined the drying kinetics of apple, pear, and kiwi using
MVD. A triple-mode domestic microwave SHARP IEC 705 oven with three levels of microwave
24
power (425, 595, and 850 kW) was used. A glass vessel to hold the sample under vacuum was
positioned in the oven. Vacuum was achieved by a vacuum pump and controlled by a pressure
controller and maintained at 20, 40, and 67 mbar (2, 4, and 6.7 kPa, respectively) within the
chamber. Based on empirical correlations, microwave power was a significant process
parameter, while vacuum pressure only slightly affected the drying constant. In this study, MVD
was shown to provide rapid moisture removal during processing, mainly during the last period of
drying.
In 1999, Drouzas et al. investigated the distribution of the electromagnetic field of MVD
using model fruit gels at 5 different selected locations in the dryer. MVD was used in a vacuum
range of 30-50 mbar (3-5 kPa) and microwave power of 640-710 W. Fruit gels were placed in 5
Petri dishes that were placed on the turntable. The MVD operated in pulse mode of 10 s power
on and 30 s power off. The fruit pectin gel (38.4% moisture content) was dried in MVD to less
than 3% moisture in 4 min while it required more than 8 hrs to dry these same gels using HAD at
60°C to a 10% moisture content. Microwave vacuum drying produced a very porous puffed
structure of the fruit gels and this assisted the transport of the water vapor from the sample
during drying. The drying rate constant increased as microwave power output increased and
decreased as absolute pressure increased in MVD. The authors reported there was uneven
distribution of microwave energy in the oven so the drying material should be placed in specific
location to avoid scorching.
Mousa and Farid (2002) dried banana slices using a domestic microwave oven (SANYO)
with interior dimensions of 330 X 350 X 240 mm, 650W power and 2450 MHz frequency. A
glass vessel was used as a sample holder and was placed in the oven. The vacuum system
consisted of a vacuum pump, a pressure regulator and an air drying unit to purge the water vapor
25
from the system as it was removed from the product. Silica gel was placed inside microwave
cavity to remove as much water vapor as possible from the air before it reached the vacuum
pump. Product temperature was measured using conventional K-type thermocouples that were
connected through a hole in the bottom of the microwave oven. These thermocouples were then
connected through a data logger to a PC. The microwave was set at 28% power which was power
on for 7 s, followed by a power off cycle for 18 s. Pressure was maintained at 30 kPa, the
product temperature during drying was below 50°C. Product temperature increased rapidly to the
evaporating temperature at the beginning of drying, and then increased very slowly after
reaching the evaporation temperature. Product temperature increased sharply after the removal of
the majority of the free water due to the increase in sensible heat and the decrease in latent heat
of vaporization. As moisture content decreased, microwave absorption declined causing lower
thermal efficiency. During the initial drying process, microwave absorption was nearly 100%
efficient and the maximum power could be applied. As moisture content decreased, vacuum
became more important due to the improved mass transfer. This finding was similar to that of
Yongsawatdigul (1996) for drying cranberries. Decreasing the power input was necessary at the
last stage of drying to limit wasting energy and damage to the microwave generating equipment
but most importantly, the damage to product being dried.
Bohm et al. (2002) investigated the application of pulsed mode in MVD for drying
parsley. Vacuum conditions were 40 mbar (4 kPa) during the power on time, but were reduced to
10 mbar (1 kPa) during the power off time. MVD parsley had higher moisture contents (8.1 and
10.8%) compared to parsley dried using HAD (4.2%). However, MVD parsley had a higher
retention of green color, less color degradation after 8 weeks storage, and higher essential oil
26
content compared to HAD parsley. Product dried by MVD processing also had fewer off-flavor
substances.
Sunjka et al. (2004) investigated the advantages and disadvantages of MVD and
microwave convective both in a pulse mode for drying cranberries. The equipment used for
microwave-convective and MVD consisted of an air blower, air heaters, a magnetron, a system
for monitoring and controlling reflected MW power, wave guides, a vacuum pump, pressure and
MW power measuring instruments, a MW chamber with balance, temperature measuring devices
such as thermocouples including optical fiber sensing, and a data analysis system. About 125 g
osmotic frozen cranberries with 57% moisture content were subjected to microwave-convective
drying with 1.00 or 1.25 W/g of sample. The microwave was set at the modes of 30 s on/30 s off
or 30 s on/60 s off. Three 2000 W heaters were used to heat air and a blower was installed to
continuously circulate air across the sample during drying. Air temperature was maintained at
62±2°C with a velocity of ldhO.l m/s.
Osmotic frozen cranberries with 55% moisture content were treated with MVD with 1.00
or 1.25 W/g of sample. The microwave modes were 30 s on/30 s off or 30 s on/45 s off. A
vacuum pump was used to maintain a vacuum at 3.4 kPa. A glass jar held on a balance with a
plastic bag serving as a vacuum chamber. An optical fiber thermometer was used to measure
temperature. Cranberries dried at longer pause times (45 or 60 s) had three to four times lower
overall color differences (AE) and lighter (L*) than those dried at shorter pause times (30 s).
These findings did not confirm those of Yongsawatdigul (1996) that indicated that a longer
power on period reduced the lightness (darker) and increased the redness of the MVD
cranberries.
27
Higher power levels resulted in darker cranberries and increased AE. MVD cranberries
were redder (higher a* value) and more yellow (higher b* value). MVD cranberries had softer
and chewier based on Young's modulus and these finding were similar as Yongsawatdigul
(1996). MVD had a slightly lower average temperature processing than microwave-convective
processing. Sensory panelists indicated that MVD cranberries were tougher and had a
caramelized or burnt taste. Microwave dehydration also yielded a non-uniform product,
especially for MVD samples. Producing burnt cranberries was consistent with this type of MVD
process treatment since drying in this primitive glass jar dryer was static with no product
agitation or movement during drying which could result in localized heating and scorching.
Cui et al. published five papers over the past several years on MVD, two with a pulsed
MVD section and others with a combined processing method. Cui et al. (2004) examined
theoretical models for the drying kinetics of MVD carrot slices during the last stage of drying.
The MVD was operated in the pulsed mode under 100% full power (without pulsation), 80%
power, and 50% power. The description on how long the microwave power was on and off was
unclear. However, drying at 50% power resulted in a higher drying rate. From the model and
experimental data reported in this experiment; the drying rate was constant until the moisture
content (db) was about 2. The drying rate had little deviation when the moisture content ranged
between 2 to 1. As the moisture content decreased below 1, the drying rate experienced a sharp
decline.
Cui et al. (2005) reported on three temperature phases of MVD for processing dried
carrots in a pulsed mode. The first phase was a warming phase in which the product temperature
was raised linearly with time without any moisture removal. Moisture removal was not achieved
until the water saturation temperature of the carrots at a certain vacuum was reached. The second
28
phase of drying was at a constant temperature phase in which most of moisture evaporated out of
sample with little resistance to mass transfer observed. The last phase of drying occurred when
the drying rate slowed and the temperature of the sample increased quickly. This phase was also
called the heating up period.
McMinn (2004) predicted the moisture transfer parameters for microwave and MVD
drying of lactose powder using the Bi-G (Bi for Biot number G for lag phase) drying correlation.
The microwave dryer was operated under pulsed application with 5 min power on and 15=1=1 min
power off. Lactose samples were dried under convective, microwave, combined convective microwave, and combined vacuum-microwave methods. The drying profiles were separated into
four drying phases: initial preheating, constant rate, and two falling rate stages, a different drying
profile than that observed by Cui et al. (2005) who found that three drying phases were present
for MVD drying. The falling rate stages were presented by an exponential function. Microwaveonly drying had a higher drying rate compared to HAD processing. However, the microwave
convective drying had a higher drying rate compared to microwave-only drying. Air temperature
and air velocity had no significant influence on microwave convective drying although it did for
HAD. The essential factor of MVD was chamber pressure, since decreasing the chamber
pressure reduced the drying time.
McMinn (2006) investigated the thin-layer modeling of microwave drying using a 2450
MHz 650 W microwave oven. In this study, the MVD system consisted of a microwave oven, a
glass dessicator located inside the microwave cavity, and a vacuum pump. The pressure chamber
was maintained at s 0-101 kPa (absolute) by an actuator valve, and air was released through a
vent. The Page, Logarithmic, Chavez-Mendez et al. and Midilli et al. models were used in this
study to describe the drying characteristics for dehydrating lactose powder. However, the Midilli
29
et al. model presented the best demonstration of drying kinetic of lactose powder. The drying
rate was dependent on pressure and the supply of external heating/cooling.
3.1. c. Incremental Mode MVD
Incremental application of microwave power is accomplished using multiple microwave
power supplies that are operated independently to apply power at the desired level. The
microwave power is applied at constant mode and then reduced as the dehydration process
proceeds by turning off one or more of the power supplies to avoid overheating.
Lin et al. (1998) investigated the characterization of MVD in an incremental mode using
carrot slices as a model food. The dryer (DRI Dehydration Research Inc., Vancouver, BC) was
equipped with four 2450 MHz 1000 W magnetrons and a cylindrical microwave cavity with a
0.35 m in radius and 0.5 m in length. A computer program controlled the microwave power by
turning on one to four of the magnetrons at any one time. Microwave power was measured using
the IMPI 2-liter test. An air flow valve was installed to control chamber pressure by bleeding
small amounts of air into the drying chamber. One Kg of blanched carrot slices was dried by
MVD to a final moisture content of 10% with 3 kW microwave power for 19 min, then 1 kW for
4 min, then 0.5 kW for 10 min at a constant vacuum of 100 mmHg (13.3 kPa). MVD was a rapid
drying process and had a higher average drying rate compared to HAD or FD (Table 2-3). Mass
transfer was rapid in MVD because of a high vapor pressure differential between the exterior and
interior of carrot during dehydration. The synergic effect of microwave and vacuum expanded
and puffed this product. HAD carrots had the highest density, while FD carrots had the lowest
density.
The rehydration ratio and rate of MVD carrots at 25°C and 100°C were higher than for
HAD. However FD carrots had the highest rehydration ratio and rehydration rate due to their
30
porous and open structure. Rehydration time was reduced when samples were rehydrated at a
higher water temperature. It required less force to puncture MVD carrots compared to carrots
dried by HAD. Rehydrated FD carrots required the least puncture force. This indicated that less
case hardening took place during MVD compared to HAD.
Table 2-3. Drying Time, Average Drying Rate, and Density of HAD, MVD, FD Dehydrated
Carrots (Data from Lin et al, 1998)
Final MC
Treatments
HAD
MVD
FD
(%)
10
10
10
Drying
Time
8h
33 min
72 h
Average Drying
Rate (kg water/kg
dry matter x h)
1.31
19.1
0.013
Density
(s/mL)
1.13
0.55
0.17
HAD carrots were darker, less red and less yellow than the FD and MVD. FD carrots had
the lightest color with a slightly lower yellow hue compared to the MVD carrots. There were
minor color differences in the reconstituted carrots between the MVD, HAD, and FD treatments
compared to when the products were in the dry state.
HAD processing reduced the content of both a- and P-carotene and MVD carrots retained
more a- carotene than HAD. There was no significant change in a- and (3- carotene content in FD
carrots. MVD carrots retained twice the vitamin C compared to HAD carrots, while no
significant loss of vitamin C occurred in FD carrots compared to the untreated controls. MVD
carrots rated significantly higher for overall acceptability and texture compared to HAD and
received sensory ratings as high as FD carrots for color and aroma/flavor. FD carrots were
somewhat preferred for their appearance by the sensory panel and this might have been due to
the greater retention of an intact cell structure. Rehydrated FD carrots received the highest
ratings for appearance, while rehydrated MVD carrots was preferred for the aroma/flavor.
31
Yousif et ah (1999) investigated the effect of various drying methods on the retention of
flavor volatiles and texture an appearance sweet basil (Ocimum basilicum L.) dried using MVD.
A drying drum containing 600 g fresh basil was positioned inside a 26 in (60 cm) dia x 20 in (51
cm) long MVD chamber with 4 kW maximum powers (DRI Dehydration Research, Vancouver,
BC, Canada). The drum was rotated at 11 rpm. Drying occurred at 27 in. of Hg (91.4 kPa) in
incremental mode at 3.2 kW for 12 min, then at 1 kW for 6 min and last at 0.5 kW for 5 min.
Microwave power was measured by the IMPI 2-liter test (Buffler, 1993). The product
temperature was maintained at 45°C with an ambient air flow rate of 3 L/min. The water activity
and moisture content of the dried basil was 0.37% and 7.1%, respectively. HAD basil contained
the same amount of linalool and methylchaicol (estragole) as fresh basil, while MVD basil
retained about 2.5 times more linalool and 1.5 times more for methylchavicol than HAD. Rapid
heating during MVD decreased the drying time and reduced the diffusion of volatile compounds
out of the tissue leaving dried product with higher retention of volatile compounds. MVD dried
basil had lower a* value and were more green following HAD or in comparison with the
commercial dried basil.
MVD basil also had higher rehydration rate than HAD basil at both 30 and 100°C
rehydration temperatures. The authors reported HAD caused a severe shrinkage in the cuticle
layer of the leaf. The internal epidermis and mesophylic cells of the palisade layer appeared to be
collapsed. MVD processing reduced the degree of shrinkage in the cuticle layer, internal
epidermis and palisade layers compared to HAD. MVD basil was more puffed than HAD. MVD
processing was more rapid (0.4 h) compared to HAD for 11.5 h.
Yousif et ah (2000) also investigated the headspace volatiles and physical characteristics
of FD, MVD, and HAD oregano (Lippia berlandieri Schauer). MVD samples were dried by the
32
method of Yousif et al. (1999). MVD oregano retained similar thymol content as fresh or FD
oregano and 1.3 times more than in HAD. HAD oregano were darker, less green, and had lower
rehydration rates than MVD or FD oregano. Based on electron micrographs, MVD oregano
structure looked similar to FD oregano.
Clary et al. (2005) investigated the application of microwave power in the incremental
mode after a series of unsuccessful experiments applying microwave power in the constant mode
for drying grapes. The specific energy required to dry grapes in constant mode MVD was 0.97 to
1.01 W-h/g fresh grapes. In the incremental mode, the total specific energy to dry grapes to a
moisture content of 3.5% was 0.92 W-h/g of fresh grapes. Drying in the incremental stage
required less energy than at constant mode and enabled the grapes to dry to the desired moisture
content. However, they indicated that applying microwave power based on product temperature
might present a better option for process control and result in improved drying performance.
Table 2-4. Carotenoid Content (mean values in mg/kg d.m.; n = 3; s < 10%, *s < 20%) and
Water Content (mean values in wet basis (wb); n = 3; s < 10%, s** < 1%) for Carrot Slices
Before and After Conventional (HAD) and Microwave Vacuum (MVD) Dying Processes (Data
from Mayer-Miebach et al, 2005)
Water
Content
(% w.b.)
Total
Carotenoids
(mg/kg dry
basis (d.m.))
All-transLycopene
(mg/kg d.m.)
All-trans-BCarotene
(mg/kg d.m.)
Raw
carrots
go>*
1107
467
420
Thawed
carrots
92**
978
449
411
Material
HAD
MVD
Drying
Temp
(°C)
Microwave
Power (W)
Drying
Time
(h)
50
-
8
9
997
460
403
60
.
6
9
1079
502
451
70
-
3.5
8
965
473
372*
90
-
2
8
882
495
330
-
400
1.5
8
1174
539*
370*
600 (75min),
240
1.5
8
790
450
288
33
Mayer-Miebach et al. (2005) investigated the thermal stability of frozen carrots, thawed
and then dried by HAD and MVD in a constant and increment mode. Frozen and defrosted
carrots (180 g) were dried using MVD at a pressure of 50 mbar (5 kPa) with either at a constant
400 W power mode or an incremental power of 600 W for 75 min followed by 240 W. Carrots
dried using HAD at 50-90°C retained all-mms-lycopene to some extent after drying. Drying at
mild < 70°C temperatures for up to 8 h was preferred due to the lower (20% degradation) of the
(3-carotene that occurred during drying at 90°C (Table 2-4). All-trans lycopene and all-trans- Pcarotene content remained stable during drying in a constant power MVD method. The authors
concluded that MVD could be used as an alternative drying method since the MVD drying time
was faster (1.5 h) compared to HAD. However, drying using an incremental power MVD
method still resulted in a significant loses of carotenoids.
Regier et al. (2005) investigated the affect of MVD, HAD, or FD on P-carotene retention
in high lycopene carrots. Frozen and defrosted carrots were dried in a similar manner as reported
by Mayer-Miebach et al. (2005). Carrots dried using HAD at 70°C or below retained most of the
P -carotene and lycopene. Drying above 70°C reduced the P-carotene content, while the lycopene
content remained unchanged due to the thermal stability of lycopene up to 90°C. FD carrots
retained the same P-carotenoid content as HAD carrots following dehydration, however, MVD
processing resulted in a greater retention of carotenoids. MVD also required a shorter drying
times (2 hr) compared to 8-10 hr using HAD.
3.1. d. Combination Pulsed and Incremental Mode MVD
A combination of pulsed and incremental heating often offers synergistic effects. This
combination controls the microwave application by lowering the input power without changing
the rate of cycling of the microwave power. Microwave dehydration using this combination
34
drying can also be employed by increasing the period of time in which the power is off. This will
reduce the amount of total power input and can result in less scorching and higher product
quality.
Erie and Schubert (2001) investigated MVD for treating infused apples and strawberries.
The samples were positioned on a tray on the trolley that moved back and forth. Microwave
energy was applied when the tray came near the horn antenna (similar to the pulse system).
Pressure was maintained at 5 kPa and microwave power applied in two incremental stages, first
390 W and then 195W. MVD treatments for fruits that had an osmotic pre-treatment required a
different dehydration process than non-infused fruits. The vitamin C retention of MVD or
osmotic-MVD apples and strawberries was about 60%. Osmotic pretreated MVD dehydrated
products had a higher volume, 20-60% of the original volume for apples, and 20-50% for
strawberries following dehydration, significantly higher than the non-infused controls. SEM
images indicated that the osmotic-MVD apples had a greater cell volume and retained the cell
shape compared to apples that had not been infused prior to dehydration.
3.1. e. Continuous Temperature Control Systems
Most of the previous work has not used a reference for process control. In the
applications that use temperature for control, the method of microwave power applications has
been applied using staged or constant application. A continuous temperature control system of
MVD modulates the application of microwave power during drying process to maintain the
product temperature at a set point. This method can be operated by manually turning the
microwave power on and off based on temperature or modulate microwave power depending on
the system. The benefit of employing this system is the drying process operates under controlled
conditions that can limit overheating and maintain higher product quality.
35
Kim and Bhowmik (1994) were the first scientists to publish on continuous temperature
control of MVD systems, even though product temperature was maintained manually by turning
the microwave power on and off. Both yogurt and concentrated yogurt were dried and compared
to MVD, Spray Drying (SD), and FD yogurts. They used a laboratory scale MVD (Laboratory
Microwave Vacuum Dryer, Model Labotron 500) supplemented with the fiber-optic temperature
probes (Luxton Fluoroptic Thermometer, Model 2000B) to measure temperature of the product
during drying. The sample was placed on a rectangular Teflon dish and dried at 35 to 50°C at a
microwave power of 250 W at 1 kPa. The conventional yogurt had a higher level of equilibrium
moisture content than the concentrated yogurt because of the structural changes that occur to the
protein fraction of the material during the preconcentration steps prior to dehydration. MVD
concentrated yogurt powder had lower level of equilibrium moisture content compared to SD
and FD yogurt. When concentrated yogurt was used as a raw material for SD and FD, there was
no significant difference in the equilibrium moisture content compared to MVD concentrated
yogurt.
In 1995, Kim and Bhowmik investigated the use of MVD for predicting the effective
moisture diffusivity of plain yogurt using the dehydration system described above. Kim and
Bhowmik observed that the slopes of the drying curve provided the basis for the development of
a useful method to predict the effective moisture diffusivity during yogurt drying and that using
calculations based upon the slopes for the drying curves were easier to evaluate compared to
their regular regime method. Therefore, they suggested using MVD to obtain the isothermal
drying data required to estimate diffusivity values for other foods as well.
In 1997, Kim and Bhowmik investigated the thermophysical properties of dehydrated
plain yogurt as a function of moisture content. The changes in specific heat and product
36
shrinkage were linear with changes in moisture content, while the changes in bulk density were
nonlinear. Thermal conductivity changed linearly with temperature and logarithmically with
moisture content. Heat and mass transfer could be determined from this study based upon a
linear specific heat and shrinkage equation, a nonlinear bulk density equation, and a two-variable
thermal conductivity equation.
Kim et al. (1997) investigated the survival of lactic acid bacteria during MVD of plain
yogurt following dehydration. There was a negative linear relationship between water activity
and temperature as reflected in D values (Decimal reduction time) for Streptococcus
thermophilus and Lactobacillus bulgaricus dried at lower temperature ranges of 35-45°C. D
values at higher temperature ranges of 50 to 60°C were not linear but exponentially correlated.
Lactic acid bacteria were very sensitive to high temperature, especially above 45°C. Drying at
50°C in MVD led to a low survival ratio of lactic acid bacteria and produced no advantage
compared to drying at lower temperatures. Therefore, low drying temperature was preferable for
higher survival ratios of lactic acid bacteria. Evaporation at the beginning of drying occurred
rapidly compared to the later period and making it difficult to control product temperature.
Rodriguez et al. (2005) investigated the drying kinetics and quality mushrooms pieces
dried by MVD in a continuous temperature control system. The temperature was controlled by
turning on-off microwave power manually. Their MVD system consisted of a 2450 MHz
magnetron with a maximum microwave power of 300W, waveguide, monomode cavity, water
load to absorb the transmitted energy from the sample, a condenser and a vacuum pump. The
system was equipped with application software to control, monitor, and record pressure,
temperature, mass weight, and microwave power during the drying process. The sample holder
was made from Teflon with small perforations in it to allow water vapor to escape during drying.
37
Three fiber-optie probes were installed to measure temperature at the internal points within the
product, the surface of the product, and in the chamber air. The sample holder was positioned
above the scale to measure the changes in weight, and the weight was recorded electronically.
An electronic valve linked to a pressure sensor (Leybold, model PIRANIPG-3) was used to
regulate chamber vacuum. The total energy input (incident, reflected, and transmitted) was
measured by three diodes (National Electronics, CIM-D'OR and Carlo Gavazzi model EDM 35).
These indicated that there was a difference between the surface and internal product temperature.
Diffusivity coefficients were affected by product temperature. By controlling the dehydration
process by monitoring the internal temperature, it was possible to increase the drying rate and
reduce the final moisture content in a more efficient manner. MVD mushrooms dried at a
moderate power level and temperature had similar quality as FD mushrooms. SEM
micrographics supported this finding.
Clary et al. (2006) described the production of sweet dessert wines from grapes dried
using MVD in a continuous temperature control system. Ice wine is a late-harvest sweet dessert
wine made from grapes frozen in the vineyard. However, the production of this ice wine is
dependent upon the weather. MVD grapes were used as an alternative to making a sweet dessert
wine and the qualities of this wine were compared to sweet dessert wine made from late-harvest
frozen grapes or from fresh grapes frozen by mechanical refrigeration. The three wines had
consistent flavor and aroma profiles as in sweet dessert wines with less fruity flavor in MVD
grapes wine.
Clary et al. (2007) explained further the operation of MVD called MIVAC under a semiautomated continuous control temperature system. As explained in section 2, the reference signal
read the surface temperature of the sample during drying and gave an indication to the
38
electromagnet around the magnetron to modulate control of the microwave power output. The
automatic system operated as follows. The power output was applied according to the
temperature and microwave output set points at the beginning of drying. As the surface
temperature of the product approached the set temperature point, the microwave power output
was reduced to maintain the surface temperature at the temperature set point. They indicated that
MVD with this automatic system had the ability to control the microwave power output and had
better product quality than constant or incremental mode. This system also tested the capability
to maintain the temperature set point and decrease temperature run away which lead to lower
specific energy required. Drying at lower temperature (71°C) caused more efficient processing.
This finding was similar as Rodriguez et al. (2005).
3.1. f. Combined Processing between MVD and Other Drying Methods
This section will review the combination of MVD at using the various microwave modes
and other drying methods. The purpose of this combination is to reduce energy, be more cost
efficient, reduce the occurrence of hot spots, and to decrease drying time.
Sham et al. (2001) investigated the texture of dehydrated apple chips using a combination
HAD and MVD. Apple slices were dried using HAD at 70°C for about 30 min to a moisture
content of-50% (db). Samples were then dried in MVD drum (EnWave Corp., Port Coquitlam,
B.C.) at 2 kW. The high-density polyethylene drum was rotated at a rate of 5 rpm. A water-ring
vacuum pump, model LEMC 60 (Sihi Pumps Ltd., Guelph, Ont., Canada) was used to maintain
vacuum. The pre-dried apple slices were MVD at vacuum 7, 14, 21, 24, and 28 inches of Hg
(23.7, 47.4, 71.1, and 94.8 kPa, respectively) with a constant microwave power of 1.5 kW to
obtain - 5 % db moisture content apple chips. Drying times varied from approximately 10 min at
39
7 in of Hg, to 4 min at 28 in of Hg. MVD apples lost more of their original cellular structure and
increased cell wall rigidity preventing the collapse of structures during drying.
Use of a high vacuum during dehydration increased the crispiness of dried apples. The
slope of the force/deformation curve indicated a high correlation between densities and force due
to the puffing. The densities of dried apples were presented in order of FD < MVD < HAD with
the density of the HAD being significantly higher than MVD. FD left numerous voids within the
tissue structure causing it to be very porous like a sponge, but not crispy. MVD apple chips had a
significantly higher peak slope compared to HAD apple chips in the force/deformation curves
indicating crispness. HAD apple chips showed severe tissue shrinkage, collapsed cell structure,
and almost no open space between the cells. Apples dried using MVD at 7 mm Hg vacuum had
a similar structure to HAD with the exception of small open spaces within the cells. This may
have been a result of the shorter drying time plus a lower drying temperature (less than 60°C),
and an increase in internal water vapor during the MVD processing relative to HAD this
sentence is too long. At a higher pressure (MVD at 2 8-mm Hg), apples exhibited less shrinkage
and a honeycomb network structure, and puffed cells that retained crispness. The cell walls of
FD chips were moderately smoother and thinner than those in MVD chips causing the chips to be
less crisp with a spongy texture.
Durance and Wang (2002) investigated the energy consumption, product density, and
rehydration rate of MVD, HAD and the combination of HAD and MVD of dried tomatoes. They
used a pilot-scale batch 20kW, 2450 MHz microwave vacuum dryer with a 15 in. (38 cm) dia
perforated polyethylene drum placed inside the drying chamber. A PLC control system was
installed to manage power level adjustments from 4 to 20 kW, at multiple process intervals,
vacuum, air flow and air flow rate, and basket rotation. During this drying experiment, the drum
40
was rotated on its horizontal axis at 6 rpm, vacuum was maintained at 6.65kPa, microwave
power was maintained at 16 kW, and dry air (lOL/min) was flushed through the chamber to
increase water vapor removal. The combination HAD and MVD processing treatments based on
percent water removed was 95% HAD/5%MVD, 85%HAD/15%MVD, 70%HAD/30%MVD.
The MVD drying rate described above was 18 times faster than HAD (Table 2-5). The
combination of HAD and MVD had a lower falling rate than MVD alone. However, this
combination would allow drying at a specific time between 0.8 and 14.75 hr. The MVD only
treatment used the least energy, however the lowest energy cost was 70%HAD/30%MVD. MVD
produced a dehydrated tomato with a puffed structure that had a faster rehydration rate compared
to HAD. Tomatoes dried with MVD or a combination of MVD and HAD rehydrated more
quickly and more completely than HAD alone.
Table 2-5. Summary of Tomato Dehydration Experiments (n =5) B^presenting Different
HAD/MVD Drying Combinations (Data from Durance anc 1 Wang, 2002)
HAD Processing
100%/
95%/
70%/
0%/
85%/
MVD Processing
0%»
5% J
30%'
100%!
15%'
13.77
13.77
13.77
13.77
Initial weight (kg)
13.77
2
0.924
N/A
1.624
2.752
4.736
Final HAD weight (kg)
N/A
0.926
0.931
0.923
0.915
Final MVD weight (kg)
14.75
12
6
N/A
HAD processing time (h)
9
N/A
0.05
0.12
0.23
0.81
MVD processing time (h)
14.75
9.12
Total processing time (h)
12.05
6.23
0.81
29900
N/A
HAD kJ per kg water
25000
20900
17300
N/A
8600
MVD kJ per kg water
9500
9100
8900
8600
29900
24200
19100
14800
Total kJ per kg water
N/A
0.27
0.23
0.19
0.17
HAD energy cost/kg water ($)3
4
N/A
0.20
0.19
0.19
0.18
MVD energy cost/kg water ($)
0.27
0.17
0.18
Total energy cost/kg water ($)
0.23
0.19
U.,
3
4
\i\7T\
Final HAD weight was also initial weight of MVD drying.
Natural gas costs estimated at $9/GJ.
Electricity costs estimated at $21/GJ.
Mui et al. (2002) investigated how a combination of MVD and HAD affected the flavor
and texture of banana chips. Banana slices were dried using HAD to remove 60, 70, 80, or 90%
41
of the initial moisture followed by MVD treatment to reduce the product moisture content to 3%.
Banana slices were also dried using only HAD, MVD, or FD. The samples were dried using
MVD by placing them on a plastic tray at 6.5 kPa and 1.5kW microwave power. An infrared
thermometer (model 39650-04, Cole Palmer Instruments Co., Chicago, IL) was installed to
measure product temperature at 1 min intervals. FD banana chips retained the highest levels of
the total volatile compounds including esters and acetates, while MVD banana chips had the
least retention of total volatile compounds. Banana chips dried using a combination of 90%HAD/
10%MVD had the next highest retention of total volatile compounds compared to FD. The
crispiness of the banana chips increased with longer MVD processing times in the combination
HAD/MVD processing treatment. However, all HAD/MVD combinations produced crispier
chips than banana slices dried using only HAD or only FD. Banana chips dried using only MVD
were hard, chewy, and not crisp. The temperature of banana slices dried by MVD reached 7075°C. The HAD dehydration was set at 55°C which was below the starch gelatinization
temperature. Therefore, MVD chips contained gelatinized starch and the HAD chips did not. The
combination of 90%HAD/10%MVD provided advantages over other treatments in terms of
flavor retention, textural qualities and color qualities. FD banana chips were considered to be
undesirable as a snack product since they were not crisp.
Cui et al. (2003) dried sliced garlic using a combination of MVD and HAD. MVD
operated in a combination mode of incremental and pulsed microwave application. Sliced garlic
(120 g) was MVD to about 10% moisture at a 100% power level for 7 min, followed by 50%
power level for 8 min, and finally 18% power level for 20 min, and then HAD (45°C) to about
5% moisture (about 1 h total drying time). The 50% power level was achieved by pulsing the
power input using a power-on time of 10 s and power-off time of 10 s; and the 18% power level
42
was attained by pulsing power-on time of 4 s and power-off time of 18 s. For the control
treatments, garlic slices were dried using FD or HAD to about 5% moisture content with a total
drying time of about 24 h or 6 h, respectively.
The combination of MVD and HAD yielded garlic which was similar in quality to FD
garlic and of significantly higher quality than HAD garlic based for flavor or pungency, color,
texture and rehydration ratio. The MVD treatment resulted in a higher rate of evaporation and a
higher drying rate during the initial drying stages compared to HAD. MVD maintained a lower
product temperature during dehydration. Adding HAD at 45°C after MVD provided an
alternative way to circumvent hot-spots and scorching that has been reported to occur in the last
stage of microwave drying when the moisture content is less than 10%. The authors indicated
that the microwave power applied to the product required a reduction as the moisture content of
the products decreased.
Cui et al. (2004) examined the retention of carotenoids in carrots and chlorophyll in
Chinese chive leaves dried using a combination MVD and HAD or conventional vacuum drying
(CVD). Control treatments in this study were only HAD, only FD or only MVD. In comparison,
samples were dried by MVD to 20% moisture, and then dried by HAD at 45-50°C or (CVD) at
55-60°C or MVD at a reduced power level to a final moisture content of 6%. Products treated by
any of the combinations or with only MVD showed a similar retention of carotenoids in the
dehydrated carrots and chlorophyll for chive leaves as the control FD product. Carrots or chive
dried by HAD had a significantly lower retention of these compounds. MVD or a combined
treatment of MVD with HAD or CVD required no blanching pretreatment step since the drying
temperature was high enough to inhibit the activity of enzymes that are responsible for the color
43
degradation. The reduced oxygen level in MVD also led to reduced oxidation of sensitive
components.
Cui et al. (2006) investigated the combination of MVD and CVD as a method for drying
lingzhi mushroom (Ganoderma lucidum) extract. The concentrated sample was dried by MVD to
10% moisture content, and then dried by CVD at 55-60°C to 6% moisture content. The
combination of MVD and CVD led to retention of almost the same amount of water-soluble
polysaccharides than CVD treatment alone. The total drying time of MVD and CVD treatments
was less than either FD or CVD. Based on this research, the combination of MVD and CVD was
the best method to dry sticky and heat-sensitive materials.
Yanyang et al. (2004) investigated the combination HAD and MVD on dry wild cabbage
with MVD configuration shown in Figure 2-4 (pg 55). This drying combination shortened the
drying time and resulted in higher preservation of chlorophyll and ascorbic acid content in the
dehydrated product compared to HAD. MVD effectively pasteurized the final products. MVD
processing produced bubbles inside the cabbage from the creation of hot spots during processing
and caramelization of sugars occurred in the region of these hot spots adversely affecting product
quality and dehydration efficiency. MVD processing decreased brightness of the final product as
determined by tristimulus color measurements.
Hu et al. (2006) investigated the used of MVD in a constant mode, HAD, and
combination of HAD and MVD for drying edamame beans. They used a MVD dryer (WZD4S-1,
Nanjing, China) with a 4.2 kW microwave power supply at 2450 MHz. The microwave power
was varied from 700 to 4200 W. The size of the microwave cavity was 1.05 x 1.08 x 0.8 m with
six plates in the cavity rotating at 1 rpm (Figure 2-5, pg 55). The drying rate of HAD was fast
during the beginning period, but decreased sharply as drying continued. During the last phase of
44
drying, the air temperature increased and caused damage to the final product. In the MVD
process, the drying rate increased as microwave power and vacuum increased and mass loads
decreased. MVD drying time was shorter compared to HAD. The combination of HAD-MVD
had similar product quality and drying time to only MVD, and this combination had better
product quality and drying time compare to only HAD. The combination processing was more
efficient in terms of mass load, power consumption, and equipment investment compared to only
MVD or HAD, except that HAD processing had lower equipment investment costs. The
optimum drying process for edamame was a combination of HAD-MVD and could be obtained
by HAD processing at 70°C for 20 min and then MVD at 9.33 W/g microwave power output at 95 kPa vacuum for 15 min.
Zhang et al. (2007) investigated the use of MVD for drying predried HAD savory crisp
bighead carp (Hypophthalmichthys nobilis) slices. A microwave oven (Model 7001, Wuxi Qigao
Precision Machining Electronic Instrument Co. Ltd., Wuxi, China) was used and modified to
include a glass jar as a sample holder that was placed inside the microwave oven. A vacuum tube
was installed to draw a vacuum inside the glass jar. A water circulating vacuum pump was used
to maintain vacuum and an air vent valve was installed to regulate vacuum. They indicated that
applying higher microwave power increased the expansion ratio and crispness of the fish slices
and also improved the sensory qualities. MVD in high vacuum enhanced the puffiness and
crispness of the finished product and increased energy efficiency of microwave heating. High
vacuum could prevent scorching of the dried fish muscle. The optimum drying conditions for
bighead carp slices was a pre-drying step using HAD at 50°C for 3.5 h, followed by MVD with
686 ± 3.5 W power for 12 s at 0.095 MPa, paused for 1 min, and then heating in MVD for
another 10 s. A summary of recent research on microwave drying is presented in Table 2-6.
45
2450
grains
Tilton(1982)
yogurt
fruit
Kim (1997)
Kiranoudis
(1997)
Yongsawatdigul
(1996)
cranberries
yogurt
Kim (1994)
2450
2450
2450
2450
2450
rice
grapes
2450
peanuts
Clary (1994)
Pominski
(1989)
Velupillai
(1989)
2450
2450
orange juice
concentrate
Attiyate(1979)
Anthony (1983) cotton seed
F (MHz)
Product
Author
0.425, 0.595,
0.85 kW
0.25 kW
0.25, 0.5 kW
0.25 kW
pulsed
temperature
constant,
pulsed
temperature
2
2,4, 6.7
5.3, 10.7
1
2.7
constant,
incremental,
temperature,
continuous
3kW
53.3,6.7
constant
0.6, 1.2, 1.8
kW
51.00
50.7, 20.0,
6.7
constant
constant*
1.22,0.61,
0.43 W/cm3
4.67
1.07-1.6
Pressure
(kPa)
1.4, 1.1,0.8,
0.56 kW
constant*
constant*
Microwave
Application
0.38 PDU
6kW
Power Used
glass vessel
Teflon dish
glass vessel
Teflon dish
turntable
and
conveyer
belt
drum
turntable
turntable
turntable
Sample
Holder
industrial,
conveyer
belt*
10
Time
(min)
35, 50C
35, 50C
Additional
Information
Results
MVD: rapid moisture removal,
mainly at the last period of dryin
Low drying temperature MVD:
higher survival of lactic acid
bacteria.
MVD: lower MC than SD or FD
Pulsed with power on for 30 s an
power off 150 s at 0.25 kW was
optimum condition, & produced
softer products. Longer power or
smaller color difference and
redder.
Continuous processing: more
energy efficient and retain fresh
quality of the products.
Increase MW & vacuum: increas
drying rate, reduce gelatinized
kernel, & no affect on color.
Increase MW: decrease MC and
raw peanut flavor, increase produi
temperature.
MVD: reduced free fatty acids &
controlled germination to zero.
Combination microwave and
vacuum: more effective to
eliminate grain insects.
MVD: higher quality retention <&
more efficient processing.
Table 2-6. List of Recent Research Conducted on Microwave Drying Applications by Author in Chronological Order
2450
2450
2450
2450
2450*
2450
2450
soluble food
paste
carrot
onion
fruit gel
shrimp
sweet basil
Echinacea
purpurea
Echinacea
purpurea
animal hide
Lian(1997)
Lin (1998)
Chen (1999)
Drouzas(1999)
Lin (1999)
Yousif(799P)
Kim (2000)
Kim (2000)
Komanowsky
(2000)
915
2450
F
(MHz)
Product
Author
constant
2,4kW
2 of 75 kW
1.0 kW
1.5, 1.0 kW
constant*
constant
constant
3.2 kW for 12
min, 1 kW for
6 min, 0.5 kW
for 5 min
incremental
pulsed
constant*
0.64-0.70 kW
0.6 kW
6.7
4.8
6.7 or 26.6
91
13.3,26.6
3-5
13.3
13.3
3 kW for 19
min, 1 kW for
4 min, 0.5 kW
for 10 min
incremental
Pressure
(kPa)
2.5
Microwave
Application
continuous
6kW
Power Used
drum
drying
basket
drying
basket
drum
turntable
conveyer
belt
Sample
Holder
47
MVD: faster drying and reducec
bacterial growth.
MVD: similar as FD in chicoric <!
caftaric acid, & color.
High pressure increase
temperature. FD was better, but
MVD had higher alkamidefor th<
roots, but not the leaves than HAI
17 for
leaves, 25
for roots
4kW & 13.3 kPa: less shrinkage,
higher rehydration potential &
water retention than HAD, &
similar sensory rating to FD.
MVD: uneven microwave energj
distribution.
MVD: retained 30% 2-methyl2pentenal and 73% 1-propenyl
propyl disulfate.
MVD: shorter drying time than
HAD and FD. MVD had higher
rehydration ratio than HAD, but
FD had the highest. FD: the
highest for appearance, MVD:
preffered for aroma/flavor.
Decrease MC: decrease loss facto
and volumetric heating.
Results
23
Additional
Information
MVD: retained 2.5 more linalool
and 1.5 more methylchavicol thai
HAD or fresh; more rapid, more
puffed.
60
33
Time
(min)
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49
si
u
cci
fcL.
2450
2450
2450
Ganoderma
lucidum
extract
grapes for
wine
edamame
lactose
powder
Cui (2006)
Clary (2006)
Hu (2006)
McMinn
(2006)
2450
2450
2450
mushrooms
carrots
ginseng root
2450
Rodriguez
(2005)
Popovich
(2005)
Regier (2005)
frozen
carrots
strawberries.
Krulis (2005)
MayerMiebach
(2005)
Product
Author
F
(MHz
)
0.09 kW
0.7 kW
3kW
0.6 kW for
75 min, 0.24
for 15 min
0.06,0.12,
0.18,0.24
kW
0.4 kW for
30 min, 0.3
kW for 20
min, 0.2 kW
for 10 min
2kW
pulsed
increment
temperature
incremental
and pulsed
temperature
and pulsed
incremental
constant
incremental
constant
0.345, 0.5,
0.83,1.115,
1.225 kW
0.6 kW for
75 min, 0.24
for 15 min
Microwave
Application
Power Used
30, 50,
80
-95.0
4.0
2.50
40
5
3.8
5
4.0
Pressur
e(kPa)
glass
vessel
turntable
6 rotating
plates in
rotary
wheels
(Fig. 5)
turntable
Teflon
box
basket
turntable
Sample
Holder
105
90
13
90
Time
(min)
HAD predried
for 20-40 min
HAD finished
(55-60C) for 3
h
Additional
Information
MVD: Midilli model and
dependent on pressure.
HAD-MVD: less time and
greater product quality than
HAD.
MVD: consistent flavor and
aroma, but less fruity flavor.
MVD-CVD: best to dry stick)
and heat-sensitive materials.
MVD incremental: loss
carotenoids. MVD constant:
all carotenoids and trans Lycopene remained stable.
MVD: similar as FD
improvement of extraction
efficiency & higher retention
of total ginsenoside than
HAD.
MVD: greater retention of
carotenoids and shorter drying
times compared to FD and
HAD.
MVD at moderate power and
temperature: similar quality as
FD.
Low initial MC at high MW
power produced a low density
with a porous structure.
Results
l__l
Ul
2450
2450
mushroom
bighead
carp fish
Giri (2007)
Zang (2007)
2450
grapes
Clary (2007)
F
(MHz)
Product
Author
constant
constant
0.115,0.15,
0.2, 0.25,
0.285 kW
0.686 W
temperature
Microwave
Application
3kW
Power Used
0, 30, 60,
90
6.5, 10,
15,20,
23.5
2.7
Pressure
(kPa)
glass
vessel
glass
vessel
turntable
Sample
Holder
Time
(min)
HAD
predried
50C for 3.5
h
54, 60, 66,
71,77C
Additional
Information
High vacuum: prevent
scorching, increased puffiness
and crispiness.
Results
This system: capable to
maintain the product
temperature & less energy
used.
Dehydration rate was affected
by MW power & sample
thickness, while vacuum
increased porosity.
3.2 Sample Holder
The challenge that faces MVD technology or any technology associated with microwave
is the uneven microwave distribution that can cause hot and cold spots within the product during
dehydration. Scientists have been working to develop methods to distribute microwave energy
more evenly to the samples during dehydration. Sample holders have been constructed to move
the sample inside the vacuum chamber during dehydration as a means of increasing the
uniformity of microwave energy exposure. Operational methods for the sample holders are also
an important function in microwave distribution. This section describes different types of
methods for sample holders that have been developed and used.
Pressure regulating
¥al¥e
Vacuum gauge
Vacuum pump
Balance
Condenser
Figure 2-1. Experimental Micro wave-Vacuum Drying Apparatus (Giri and Prasad, 2007)
Figure 2-1 is the simple configuration for a sample holder. A domestic microwave oven
can be used for this type of experiment. A desiccator or glass jar can be used as a sample holder
that also serves as a vacuum chamber and is placed inside the microwave oven. Other scientists,
including Yongsawatdigul and Gunasekaran (1996a), Kiranoudis et al. (1997), Mousa and Farid
(2002), Sunjka et al. (2004), McMinn (2006), Zhang et al. (2007) employed this method for their
experiments. Drying occurred in a static mode and involved no sample movement using holders
of this configuration. Therefore, microwave energy distribution might occur unevenly and the
52
occurrence of hot and cold spots tends to be higher compared to other sample holder
configurations. Sunjka et al. (2004) reported that the sensory panel identified a burnt taste in
MVD cranberries dried using such a static method, while the texture was soft and chewy as
predicted from Young's modulus determinations.
FIBEROPTIC TEMPERATURE AND PRESSURE DETECTION
PHOTONETICS
DETECTOR
INFRARED TEMPERATURE DETECTION
PRESSURE DETECTION
SYSTEM
CONTROLS
V A C U U M VESSEL
TURNTABLE FOR PRODUCT
NETWORK
SERVER
MICROWAVE
P O W E R SUPPLY
VACUUM
SYSTEM
PROGRAMMABLE
LOGIC
CONTROLLER
INTERFACE
LAB
DELL PC
OPERATIONS
DELL PC
POWER
TRANSDUCERS
DIRECTOR
DELL PC
TURNTABLE DRIVE
A N D SCALE
D E L L HOST C O M P U T E R l
WONDERWARE
INTOUCH
DIRECTOR
DELL PC
Figure 2-2. Schematic Diagram of a Pilot-Scale MIVAC Dryer (Clary et al, 2005)
Figure 2-2 is an example of a MVD using a rotating turntable as a sample holder (Clary
et al, 2005). Drying with this MVD configuration can be conducted with the turntable either not
moving (static) or rotating (dynamic). Drying using the rotating turntable will be more desirable
because microwave energy can be distributed more evenly and minimized the occurrence of
scorching. Tilton and Vardell (1982), Pominski and Vinnett (1989), Drouzas and Schubert
(1996), Lieker et al. (2004a), Krulis et al (2005) also reported using a turntable as their sample
holder.
53
Figure 2-3. Schematic Diagram of Micro wave-Vacuum-Rotary-Drum Dryer (Kaensup, 2002)
Figure 2-3 is an example of a drum type sample holder for MVD that was used by
Kaensup et al. (2002). Kaensup indicated that this configuration evenly dispersed microwave
energy as the drum rotated within the microwave chamber. Velupillai (1989) also used a rotating
drum as a sample holder inside the drying chamber. Komanowsky (2000) developed a method to
dry animal hides and increase the uniformity of exposure to microwave by tumbling the hides
inside a mixer. This system was similar to the drum dying method described by Kaensup et al.
(2002). Popovich et al. (2005) developed a different system by placing ginseng root into a
perforated cylindrical high-density polyethylene drying basket (0.26-m radius and 0.31-m length)
that was rotated horizontally in a dryer at 15 rpm. Other scientists who reported using drum
drying as a sample holder included Durance and Wang (2002), Yousif et al. (1999), and Kim et
al. (2000).
54
Mapstrm
Cabinet Air Dryer
•J
b)
Figure 2-4. Schematic of an Experimental Drying Apparatus, (a) STJ-1 Vegetable Direct Fired
Dryer, (b) WZD4S-01 Microwave Vacuum Dryer (Yanyang et al, 2004)
Figures 2-4 and 2-5 show other configurations of sample holders. Yanyang et al. (2004)
installed a rotary plate with a vertical movement as a sample holder, while Hu et al. (2006)
attached six sample holders to a rotating wheel that moved like a carousel.
Sight
•mmkw sragnetron
wtefcg
wheel
7\
MWawiy
/
^l^L
sample
holder
gorstmi panel
f\
x-mmm.
.4
pump / '
Figure 2-5. Schematic Diagram of a Vacuum Microwave Dryer (Hu et al, 2006)
55
3.3 Continuous MVD Processing
PRODUCT
INFEED
MICROWAVE POWER SUPPLIES
ZONE I - 12 KW (i 2-150 MHz or JO KW n< 'J15 MHz
I
ZONE 2 - G KW i.i 2450 MHz
7 0 N F 1 - PRODUCT RFSTAND FQUAl I7ATION
CONDENSER AND
VACUUM PUMP
PRODUCT
OUTFEED
Figure 2-6. Schematic Diagram of a Continuous Processing of Microwave Vacuum Dryer (Clary,
1994)
Continuous MVD processing offers many advantages, such as limiting hot and cold
spots, providing for more even microwave distribution, increased mass production, and more
efficient operation. As described in an earlier section, Attiyate (1979) reported the first
industrialized MVD system to dry orange juice concentrate. However, there was no explanation
about the configuration or the system of the operation.
Clary (1994) described the application of industrialized MVD in a continuous processing
based on the continuous temperature control system to dry grapes (Figure 2-6). The product
entered the microwave zone I through the product infeed that was equipped with airlock to
maintain vacuum of the drying chamber. The drying chamber was a cylinder vessel of 11.3 m
long and 2.5 m dia with removable shaved heads. The vacuum pump was installed and had a
capability of removing 8.2 kg/h of air and 36 kg/h of water vapor. The pressure of the chamber
was maintained at 20 Torr (2.7 kPa). The products were distributed on the conveyer system (6 m
long and 0.6 m wide) and dried in zone I to intermediate moisture (20-50% moisture) before
56
entering zone II. At zone II, the products traveled in the opposite direction and were dried to 7 %
moisture. The products left zone II and entered zone III, a radiant heating zone, for moisture
equalization and more moisture loss. The radiant heating zone was equipped with two steam
heated infrared panels. Zone III was also equipped with a water cooled panel, and there was no
microwave energy introduced in Zone III. The microwave in zone I and II had the capability to
produce maximum energy of 12 and 6 kW depending upon the desired product temperature. At
each end of drying zone, an IR temperature sensing device was installed to monitor the product
temperature.
Drying in continuous processing required less specific energy and the feeding rate of
fresh grapes was 13.6 kg/h. The final moisture content of dried grapes was between 3 to 4.5%
and the drying time was 90 min. The product temperatures at the end of each zone were 26.6 and
48.9 to 54.4°C, respectively. Grapes dried in continuous MVD processing were of high quality
compared to fresh grapes, retaining a natural fresh color, maintaining an ovoid shape and fresh
flavor, and having crunchy and crispy texture.
Lian et al. (1997) investigated how coupled heat and moisture transfer occurred within a
6000 W commercial MVD operated in a continuous mode. In this study, a 65% solids soluble
food paste was applied to the dryer at a rate of 4.2 kg/h. The conveyer belt speed was 2.65 mm/s.
Three magnetrons were used at 3000 W power was operated with approximately 20% of the
microwave power being reflected. The pressure of vacuum chamber was maintained at 25 mbar
(2.5 kPa). The temperature and moisture distribution in cross section at various locations along
the belt was recorded at different times. As the paste was supplied into the system, the
temperature increased rapidly due to the fast volumetric heating of the product within the
microwave cavity. As the product moved away from the feed nozzle, the increasing temperature
57
rate decreased, reaching a stable value at the end of MVD. The drying rate became significantly
slower as the product approached 10% moisture content. The moisture content of the product
was the most important factor for microwave power dissipation. A decrease in moisture content
was followed by a decrease in loss factor and volumetric heating due to microwave power
dissipation.
3.4 Pressure
Operating under a vacuum allows MVD drying to reduce the boiling point of water
within the product being dehydrated and also reduces the oxygen available for deleterious
reactions such as lipid and colorant oxidation. The boiling point of water at atmospheric pressure
(1 atm = 101.325 kPa) is 100°C, while the boiling point of water at 50 kPa pressure is 84°C and
at 3 kPa, 22°C.
Yongsawatdigul and Gunasekaran (1996a) examined two levels of vacuum pressure: 5.3
and 10.67 kPa in MVD. Using a vacuum pressure of 10.67 kPa resulted in an increased product
temperature compared to treatments at 5.33 kPa. Water evaporated faster at a higher pressure and
this was responsible for maintaining a lower temperature during processing. Kiranoudis et al.
(1997) examined three chamber pressures of 20, 40, and 67 mbar (2, 4, 6.7 kPa), respectively.
They indicated that the vacuum pressure only slightly affected the drying constant. Moon et al.
(1997) investigated MVD at different vacuum conditions at 160, 260, and 360 mmHg (21.3,
34.5, 48.0 kPa), respectively. They indicated that operating MVD at 160 mmHg retained more
ascorbic acid compared to 360 mmHg. Kaensup et al. (2002) examined pressures of 20, 160, 260
mmHg (8, 21.3, 34.5 kPa). They indicated that the drying time was reduced at lower pressures.
These authors reported that level of pressure had no influence on the specific energy
consumption, except at low moisture contents, less vacuum consumed less energy. Rodriguez et
58
al. (2005) indicated that reducing pressure increased the drying rate and produced a lower
moisture dried product. Giri and Prasad (2007) investigated MVD at 6.5,15, and 23.5 kPa and
found lower pressure significantly influenced the rehydration ratio by creating a more porous
dehydrated product, but it had little effect on the dehydration rate.
3.5 Temperature
Temperature is an important factor in food processing since it is used as a processing
parameter to control spoilage, sterilization, contamination, and product quality. In the
dehydration industry, temperature has more of an effect on product quality than on microbial
safety, because low moisture content and water activity tend to limit the microbial growth.
However, high temperature during dehydration can lead to nutritional depletion, textural and
color change, and loss of sensory quality. Since conventional methods of temperature detection
including thermometers and thermocouples are on compatible with a MVD environment, the
most common devices used to measure temperature during the MVD drying are infrared and
fiber optic sensors. While infrared offers convenience, it can only measure the surface product
temperature. Fiberoptic offers greater accuracy for temperature measurement since these sensors
can measure both the internal and the surface product temperature. However, it is difficult to
place fiberoptic sensors on a moving product.
Yen and Clary (1998) investigated the use of fiber optic and infrared sensors to measure
temperature in the MVD (MIVAC) system. At the beginning MIVAC processing using either 1.5
kW or 3 kW forward power, the temperature reading differences between the infrared and fiber
optic sensors was about 5°F (2.78°C). The difference between the methods decreased almost to
zero toward the end of drying. The infrared reading was more consistent than fiber optic readings
for products of a small size, low moisture and low weight. Rodriguez et al. (2005) indicated that
59
there was a difference between the surface and internal product temperature when using these
types of sensors and the monitoring the internal temperature could increase the drying rate and
reduce the final moisture content in a more efficient manner.
Based on the work of Yen and Clary (1998) and Rodriguez et al. (2005) infrared can be
used to measure product temperature because infrared measurements offer a simple and
convenient method, particularly in MVD where there is sample movement which could displace
or break fiber optic probes and where the product size is small, making it difficult to insert a
fiber optic probe. However, setting and reading the temperature from infrared sensors has to be
corrected since infrared is only able to read the surface temperature, while the internal
temperature might be slightly higher than surface temperature as the processing progress.
4. Conclusions and Future Research
Several types of microwave vacuum dehydrations systems have been developed over the
past thirty years for a wide variety of foods and natural products ranging from herbs and
botanicals, to fruits and vegetables, to polysaccharide and protein suspensions or pastes. These
drying systems involve a wide range of configurations and employ both static and dynamic
product handling systems. It is unclear whether any of these configurations have been
commercialized with the exception of the continuous MVD processing unit described by Attiyate
(1979), Clary (1994) and Lian (1997).
Microwave based drying methods can provide rapid heat transfer, however application
needs to be cautiously applied to avoid non-uniform heating throughout the product and
localized hot spots. Applying microwave energy under conditions of constant power is the
simplest microwave drying system; however, overheating can be a major problem and can lead
to scorching. Compared to HAD methods, a MVD system using constant microwave power will
60
often lead to better quality products. MVD products often are of high quality, and though with
different properties, can be used in similar high quality applications as FD materials. By applying
power incrementally, or pulsing, it is possible to reduce the risk of non-uniform heating and
scorching. A pulsed MVD system can also be more energy efficient, especially in the final stages
of the drying process when vacuum provides much of the driving force for mass transfer. Product
produced in a pulsed MVD system is consistently of higher quality than product from a constant
power system. The product has a more puffed texture and higher rehydration rate. Incorporating
a continuous temperature monitoring and control system into the dryer operation can control
scorching and increase the efficiency of operation.
A combination of pulsed or incremental MVD dehydration with other drying systems has
also been used effectively. HAD can be used during the initial stages of dehydration, followed by
MVD, taking advantage of volumetric heating and vacuum the later drying stages when it is most
needed.
The combination of continuous temperature control and pulsed microwave application
might offer synergistic effects, even though there is no research on this subject. Vacuum offers
continuous mass transfer and maintains the structural collapse during power-off in a pulsed
system. However, the product temperature in a pulsed system can be higher than the desired
temperature as the product dries and moisture content decreases. Therefore, future work of
combining continuous temperature control and pulsed system may address this issue.
Other factors that should be considered when designing a MVD system include vacuum
pressure and configuration and operation of the sample holding system. Drying under vacuum
keeps the product temperature lower during drying and may help with the retention of heat labile
compounds. Drying at a higher vacuum, such as 50 kPA had a greater effect than a lower
61
vacuum such as 3kPA. Having a sample holder that can result in greater uniformity of
microwave energy during dehydration is also important. This sample holder should turn or have
the ability to reorient the product in the microwave field during drying. Drying with the sample
in a static condition is inefficient and leads to scorching from the concentration of microwave
energy in a single location (hot spots). A dynamic system would aid in the distribution of
microwave energy and reduce the occurrence of hot spots. Turn tables are probably the simplest
configuration and seem to be effective for increasing heating uniformity during microwave
drying. In a continuous process, incorporating a belt that moves the product through the
microwave field might serve as an effective and simple method. Drying product inside a rotating
drum also eliminates hot spots and increases the evenness of moisture distribution.
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eggplants. Journal of Food Engineering. 83: 422-429.
Yanyang, X., Min, Z., Mujumdar, A. S., Zhou, L.-Q. and Sun J.-C. 2004. Studies on hot air and
microwave vacuum drying of wild cabbage. Drying Technology. 22(9): 2201-2209.
Yen, M. and Clary, C. D. 1998. Fiberoptics vs. Infrared - An assessment of the temperature
measurement in the MIVAC System. Bulletin #980802. California Agricultural
Technology Institute, California State University, Fresno, CA, 1998.
Yongsawatdigul, J. and Gunasekaran, S. 1996a. Microwave-vacuum drying of cranberries: Part
I. Energy use and efficiency. Journal of Food Processing and Preservation. 20: 121-143.
Yongsawatdigul, J. and Gunasekaran, S. 1996b. Microwave-vacuum drying of cranberries: Part
II. Quality evaluation. Journal of Food Processing and Preservation. 20: 121-143.
Yousif, A. N., Seaman, C. H., Durance, T. D. and Girard, B. 1999. Flavor volatiles and physical
properties of vacuum-microwave- and air-dried sweet basil (Ocimum basilicum L.).
Journal of Agricultural and Food Chemistry. 47: 4777-4781.
Yousif, A. N., Durance, T. D., Seaman, C. H. and Girard, B. 2000. Headspace volatiles and
physical characteristics of vacuum-microwave, air, and freeze-dried oregano (Lippia
berlandieri Schauer). Journal of Food Science. 65(6): 926-930.
Zhang, J., Zhang, M., Shan, L. and Fang, Z. 2007. Microwave-vacuum heating parameter for
processing savory crisp bighead carp (Hypophthalmichthys nohilis) slices. Journal of
Food Engineering. 79: 885-891.
68
CHAPTER 3. DETERMINATION OF HEATING UNIFORMITY DURING
MIVAC® HEATING
Introduction
MIVAC® is a microwave vacuum dryer (MVD) that is operated using a semi-automated
continuous temperature control system designed to control the level of microwave power applied
during dehydration. The system measures temperature during drying using an infrared (IR)
detector. As the turntable rotates during the drying process, it transports the product being dried
into the view of an infrared camera. This provides information to the control to maintain a preset
temperature by modulating the microwave power applied. A temperature control device called a
"Peak Picker" averages the output from the infrared temperature detector. The purpose of this
controller is to modulate input power. Through output averaging, it is possible to provide a
smoother signal to the Proportional Integral Differential (PID) controller which responds to
temperature changes by controlling the electric current to an electromagnetic coil around the
magnetron. If the temperature of the product approaches or exceeds a preset temperature, the
microwave power output is decreased.
There is no published data describing the use of this type of controller for microwave
drying. However, this control system appears to have the optimum utility during later stages of
drying when moisture content is low and the product is sensitive to excessive heating. During
this stage of drying, the temperature of the product has reached the preset point and the
microwave power is automatically reduced to relatively low levels.
In this study, infused apples dried to intermediate (25-40%) moisture content were
examined. These dehydrated apples are commercially important and can be further processed to
a low moisture content (3-5%) and used as ingredients in ready-to-eat cereal and snack items.
69
These dried apples need to have a low enough moisture and water activity so that moisture
migration within the packaged cereal or snack does not lead to a loss in flavor or textural
qualities. The in an interest within the industry for a drying method that can produce crunchy
and puffy dried apples (3-5% moisture) from the infused partially dried 25-40% apples that are
made commercially as an intermediate product for fillings and sauces. A MVD method, such as
MIVAC®, may have the capability to produce this type of low moisture content product without
scorching and produces a more uniform puffed texture compared to heated air drying (HAD)
since microwave can remove moisture by volumetric heating, and vacuum may be able to
maintain the cell structure from collapsing during drying.
When drying the infused partially-dried apple, temperature control becomes the most
important factor if the microwave power application is to be properly regulated. Therefore, if the
peak picker is set too low, the system will become too sensitive to the hot spots and shut the
microwave system off prematurely lengthening the drying process. If the set point is too high,
the response rate would be too slow and will lead to product scorching. The hypothesis of this
study is that the response rate of the peak picker will affect the drying characteristics of the
system. The objective is to determine the optimum setting of the peak picker control device to
increase the effectiveness of MIVAC® drying. This study will not only be useful for the
development of processes for dehydrating intermediate moisture product, but it can also be
applied to processes for dehydrating fresh product.
Materials and Methods
Infused pre-dried apples with 25-40% moisture content from Tree Top Inc. (Selah, WA)
were used in this study. Fresh apples were peeled, cored, and cut into 2 in (50.8 mm) cubes, and
treated with a solution of 200 ppm sodium metabisulfite. The apple cubes were infused with hot
70
apple juice concentrate and dried in a tray HAD drier at 170°F (82.2°C) to a moisture content of
25-40% (wb). The pre-dried apples were stored in a plastic bag inside a cardboard carton and
kept in a refrigerated room (about 4°C) until further used. One kg of pre-dried apples were dried
in MIVAC® for 60 or 90 min at 160 or 180°F (71.1 or 82.2°C, respectively) at starting forward
power of 2 kW using different peak picker regimes (Table 1). Apples were placed on a turn table
rotating at 5 RPM. A vacuum was drawn to 5 kPa before the microwave power was applied.
After the drying treatment, dried apples were left in the MIVAC® chamber under vacuum for 8
hours to equilibrate.
The total specific energy, Es (W-h/g fresh product) from each drying process was
calculated based on Clary et al. (2005). Moisture content of apples dried at 160°F (71.1°C) was
determined using AOAC method 44-15A (AOAC, 2000). Four replicates were taken for each
treatment.
Results and Discussion
Table 3-1. The Effects of Different Pea c Picker Settings on MIVAC System Operation
Peak
Moisture
Picker
Time
Temp
Content Es(W(min)
(%FS/s)2
Product Description
(°Q
h/g)1
(%)
No scorching, good color retention,
71.1
1
0.21
crunchy but chewy after third bite.
60
6.5±0.9
60
71.1
2
0.40
Scorching (about 15%), crunchy.
4.7±1.0
90
71.1
1
8.7±1.6
0.38
Scorching (about 10%), crunchy.
90
82.2
N/A
Scorching (about 35%), crunchy
0.1
0.47
82.2
N/A
90
5
0.69
Scorching (about 90%), crunchy
IT,
.
. .
Es, total specific energy in W-h/g of predried apples
%FS/s, % full scale per second; full scale is 148.9°C
2
Table 1 shows the effects of peak picker response time settings on the performance of the
MIVAC® system. Drying at 71.1 °F for 60 min at a peak picker setting of l%FS/s resulted in
dried products with better sensory characteristics and no scorching compared to the other
treatments. The chewiness indicated that a higher moisture content remained entrapped in the
71
center of the product. This problem might be caused by case hardening that occurred during
HAD drying and refrigerated storage, and which then hindered the mass transfer during the
MIVAC® treatment. Increasing the peak picker setting from 1 to 2%FS/s for the same
temperature and time conditions produced scorching and increased the amount of specific energy
which might have caused the overheating observed. This indicates that increasing the peak picker
setting led to a less uniform application of microwave power. Increasing the drying time for the
same peak picker setting of l%FS/s produced scorching, but less scorching than when the setting
was at 2%FS/s for the same temperature and time of operation. The moisture content was higher
and more variable (higher standard deviation) for a peak picker setting of l%FS/s and a drying
time of 90 min compared to the moisture content of apples dried at the same temperature and
peak picker setting for 60 min.
More extreme drying conditions involving a longer (90 min) drying time at higher
temperature (82.2°C) at a peak picker setting of 0.1 or 5%FS/s were also carried out. The lower
setting resulted in reduced specific energy with 35 % of the apples being scorched. Ninety
percent of the apples were scorched at a peak picker setting of 5%FS/s under the same time and
temperature conditions. This indicates that choosing a precise setting for the peak picker will be
necessary to facilitate microwave vacuum operation if a more uniform heating and an efficient
drying process.
Mousa and Farid (2002) explained the effectiveness of vacuum during the last stage of
drying. The efficiency of microwave vacuum drying of banana slices was nearly 100% during
the initial stages of drying. This high efficiency was due to the ease of moisture removal during
the initial drying stage for banana. Vacuum becomes more beneficial as drying continues and as
the moisture content decreases, because vacuum facilitates mass transfer. At the later stages of
72
drying, microwave efficiency decreases to a level that may make it impractical, therefore there
may be a point in the drying process where adding microwave energy is no longer necessary and
where moisture transfer may result from the application of vacuum alone. Further research
should be conducted into the effect of increasing the holding time under vacuum following MVD
to more than 8 h within the MIVAC® chamber to see whether it may be possible to improve
product texture (increase crunchiness and decrease chewiness) of dried apples made from HAD
pre-dried apples by incorporating this final vacuum step. However, the additional 8 h in vacuum
following MVD might not be an economical as it may be too time consuming.
Conclusion
Based on these experiments, MIVAC® should be operated under peak picker setting of
l%FS/s because this resulted in more uniform heating and more energy efficient (lower specific
energy), and produced a minimal amount of scorching compared to using a higher setting. Too
low of a setting use less energy output but might not be sufficient to produce the desired product
final moisture content.
Acknowledgement
This research was supported by a USDA National Need Graduate Fellowship, Tree Top
Inc., and Agriculture Research Center, Washington State University.
References
AOAC. 2000. Official Methods of Analysis, 17th ed. Washington, DC: Association of Official
Analytical Chemist, 37.1.10. 2(37): 4.
Clary, CD., Wang, S. and Petrucci, V.E. 2005. Fixed and incremental of microwave power
application on drying grapes under vacuum. Journal of Food Science. 70(5): 344-349.
Mousa, N. and Farid, M. 2002. Microwave vacuum drying of banana slices. Drying Technology.
20(10): 2055-2066.
73
CHAPTER 4. OPTIMIZING DRYING CONDITIONS FOR MICROWAVEVACUUM (MIVAC®) DRYING OF RUSSET POTATOES {SOLANUM
TUBEROSUM)
Dewi Setiady, Carter Clary, Frank Younce, and Barbara A. Rasco
Authors Setiady, Younce, and Rasco are with theDept. of Food Science and Human Nutrition,
Washington State Univ., Pullman, WA 99164-6376. Author Clary is with theDept. of
Horticulture and Landscape Architecture, Washington State Univ., Pullman, WA 99164-6414.
Direct inquiries to author Clary at cclary@wsu.edu
ABSTRACT
MIVAC® combines microwave heating with vacuum drying. Microwave power is
modulated based upon product temperature and this feature can limit overheating compare to
other microwave vacuum methods. Blanched potatoes were dried at 50, 60 and 70°C for 0 to 150
min. Potatoes dried at 70°C had a lower moisture content in less time compared to potatoes dried
at 50 and 60°C, but the color of the dehydrated potatoes was adversely affected from
overheating. Drying at 60°C for 150 min resulted in dried potatoes with acceptable color. Drying
at 50°C resulted in the production of dehydrated potatoes of acceptable color, however it
required more time.
74
INTRODUCTION
Freeze-drying is a dehydration process that has the ability to preserve the fresh
characteristics of food products including color, nutritional value, and appearance. However,
there are several disadvantages with this method including the time required for dehydration, a
high initial capital investment, limited freezer capacity, and the high cost of operation and
maintenance. This is due to extended drying time that can take more than 20 h. Even with the
demand for high quality product, it is often not economically feasible to capitalize additional
freeze-drying capacity.
Microwave-vacuum drying is an alternative to freeze drying and perhaps heated airdrying and has the potential to produce dehydrated fruits and vegetables more quickly at a lower
cost compared to freeze-drying. Microwave provides more rapid heat transfer than freeze-drying
or air-drying. The ability of a microwave field to generate heat throughout the food decreases the
drying time by up to 90% (Decareau et al, 1986). Utilizing vacuum in conjunction with
microwave permits moisture to vaporize at a lower temperature than under atmospheric
conditions. This improves product quality compared to food dried under atmospheric and freeze
drying conditions. Drying under a vacuum also reduces oxygen exposure, limiting oxidation and
maintaining flavor and color of the dehydrated product. Microwave vacuum drying maintains the
structural integrity of the dried product preserving the original shape of the dried product
compared to air-dried (Petrucci et al, 1989).
Many different methods of microwave vacuum drying have been investigated. The most
important factor that provides a significant effect on drying characteristics and product quality is
the way the microwave power is applied. Faster drying rates will increase the efficiency of the
drying; however microwave field uniformity can be a problem resulting in localized overheating
75
and scorching. Microwave energy can be applied in a constant or fixed mode, pulsed application,
incremental stages, a combination of pulsed and incremental stages or a power modulation
system. A constant or fixed mode means the microwave power is applied at a specific fixed
power level during the entire drying process (Clary et ah, 2005; Durance et ah, 2002; Hu et ah,
2006; Kaensup et ah, 2002; Kim et ah, 2000, Kim et ah, 1995; Popovich et ah, 2005, and
Rodriguez et ah, 20005). A pulsed system applies microwave energy at a certain level of power,
and switches the power on and off during the process to minimize overheating (Bohm et ah,
2002; Cui et ah, 2003; Giri et ah, 2007; Kiranoudis et ah, 1997; Lian et ah, 1997; Moon et ah,
1997; Mousa et ah, 2002, and Yongsawatdigul et ah, 1996). Most consumer microwave ovens
use a constant or fixed mode operating continuously at the 100% power level or a pulsed
microwave at less than 100% output which is controlled by turning the microwave power on and
off using a timer. Incremental application of microwave power is accomplished using multiple
microwave power supplies that are operated independently to apply power at the desired level. It
is advantageous to reduce the level of microwave power applied as drying proceeds when
operating in a constant mode to avoid overheating (Clary et ah, 2005; Drouzas et ah, 1999;
Krulis et ah, 2005, Lin et ah, 1998; Sunjka et ah, 2004, and Yousif et ah, 1999).
Drying configurations that use a combination of pulsed and incremental heating have
been investigated (Cui et ah, 2004; Erie et ah, 2001, and Mayer-Miebach et ah, 2005). This
method controls microwave power input by lowering the amount of power without changing the
cycling of microwave power on and off or increasing the cycling off time of the microwave
power, which will reduce the amount of total power input into the system. Kim et ah (1997) and
Clary et ah (2007) investigated modulating microwave power to control the drying to avoid
76
overheating. Kim controlled the product temperature by turning the microwave power on and off
manually to dry yogurt at 35°C.
MF/AC® is one type of microwave-vacuum drying described by Clary et al. (2007, 2006,
and 2005). In 2005, Clary investigated the application of microwave power in fixed and
incremental stages for drying grapes. In a fixed application of microwave power, Clary noticed
three drying phases: a first phase of temperature increase, a second phase when the temperature
reached a plateau in a balance of heating and cooling, and a third phase of temperature increase
as the product moisture content diminished. When constant power was applied, the grapes
overheated and scorched before reaching the desired moisture content. However, when the
application of microwave power was reduced using an incremental method in which the power
was modulated to lower levels during the constant rate period of the dehydration curve, the
grapes dried to the desired moisture content with a lower amount of energy than at a constant
microwave power application. Clary et al. (2005) also indicated a positive correlation between
microwave power and product temperature, and that modulating microwave power based on the
temperature of grapes could control and maintain product temperature while at the same time
reduce overheating and maintain product quality.
Clary et al. (2006) reported the production of sweet dessert wines from grapes dried in
MIVAC®. They modulated microwave power as the product temperature changed. Wine made
from MIVAC® dried grapes had consistent flavor and aroma profiles, similar to what one would
expect from wines made from grapes frozen in the vineyard, however the wines made from the
dehydrated grapes had a less fruity flavor. Clary et al. (2007) compared MIVAC® dried puffed
grapes to sun dried grapes and found that MIVAC® dried grapes preserved more of the fresh
grape qualities. The output of the microwave power supply produced continuous output that was
77
modulated based on product temperature. This was accomplished using an electromagnet
surrounding the magnetron. For example, to decrease power from 3 kW to a continuous output of
2 kW, the current to the electromagnet was increased. This provided discrete control with
continuous wave output ranging from 0 to 3 kW. An infrared sensor was used to monitor surface
product temperature and control microwave power.
Yen et al. (1998) compared fiberoptic and infrared sensors for temperature measurements
in a MIVAC® system and observed about a 3°C difference between the IR and fiberoptic reading
at the beginning MIVAC® processing with no difference in readings at the end of the drying
period. Temperature measurements using an infrared sensor may be the most appropriate and
efficient method for temperature control since a turntable during drying process and it is difficult
to position fiber optic probes on a rotating table.
The objectives of this research were to determine the optimum drying temperatures and
times for MIVAC® dehydration using blanched Russet potato slices as a model food, and to
examine the behavior of the system using product temperature to modulate microwave power
application.
MATERIALS AND METHODS
Drying Preparation
Russet potatoes (Solarium tuberosum) were peeled, washed and sliced to 3.175 mm
before blanching in boiling water at about 98°C for 2 min. The blanched potato slices were
cooled in ice water for 2 min and drained on paper towels to remove excess water. Each drying
treatment required approximately 1 Kg of sliced and blanched potatoes.
78
MIVAC Drying and Experimental Design
The sliced blanched potatoes were placed on the turntable of MIVAC® and rotated at 5
rpm. A vacuum was drawn to 5 kPa before the microwave power was applied. Treatment
temperatures ranged from the set point temperatures of 50, 60, or 70°C and controlled by setting
a temperature limit on the infrared detector circuit. Drying times were 60, 90, 120, or 150 min
(Table 1). Forward power was set at 1 W/g blanched potato. Each treatment was replicated 2
times.
Forward power, reflected power, product temperature, product weight, chamber pressure,
and processing time were recorded every six seconds and averaged every 5 min using a computer
system and software (Microsoft Vision Basic, Redmond, WA). The total specific energy, Es (Wh/g fresh product) was calculated based on Clary (5).
Color
The color of the fresh and dried potatoes was measured by using a Minolta CM-2002
spectrophotometer (Minolta Camera Co., LTD, Chuo-Ku, Osaka, Japan) with an 11 mm
aperture. Ten measurements were taken for each treatment. The CIE-Lab L*(lightness),
a*(redness (+)/greenness (-)), and b*(yellowness(+)/blueness(-)) values were recorded. Total
color difference (AE) was also calculated, as follows:
AE = ^CI^ + Ca *^ + i& %
Analytical Methods
Moisture content of fresh and dried potatoes was determined using AOAC method 4415A (AOAC, 2000). Five replicates were taken for each treatment.
79
Statistical Analysis
Data were analyzed using SAS 9.1 (System for Windows 2002-2003, Cary, NC)
including analysis of variance and Fisher's least significant difference (LSD) procedure.
Significance was determined at p<0.05.
RESULTS AND DISCUSSION
MIVAC® Drying Curves
The drying curves show the effect the drying treatments at different temperatures and
times on the final moisture of the potatoes (Figure 4-1). There was a significant difference
among temperature and time treatments (p < 0.05). Drying at 70°C decreased drying time
compared to drying at a set point of 50 and 60°C to reach the target moisture content of 8-9%.
-*-Esat50°C
s— Es at 60°C
•&- Es at 70°C
* - MC at 50°C
- * - MC at 60°C
o - MC at 70°C
30
60
120
90
150
Drying Time (min)
Figure 4-1. Specific Energy (Es) and Moisture Content (MC) for blanched Russet potato slices
dried by MIVAC® at 1 W-h/g
80
Potatoes dried at a set point of 70°C for 120 min or more had significantly lower moisture
content than that observed for the other treatments, however drying at a set point of 70°C for 120
min led to scorching. No evidence of scorching was observed at a set point of 70°C for 90 min
but the final product had a higher moisture content. Drying at a set point of 60°C for 150 min
produced dried potatoes with similar moisture content comparable to the 70°C treatment at 120
min but with no scorching. Drying at the 50°C treatment yielded product with a less uniform
moisture distribution in the dried potatoes and a longer drying time. Drying at a set point of 60°C
fori50 min was the optimum treatment employed in this study.
Table 4-1. Specific Energy (Es) and Moisture Content (MC) for MIVAC® Potatoes Dried at
Various Drying Temperatures and Times (n=2)
MIVAC® Set Point Temperature
50°C
Time
60°C
70°C
(min.)
Es(W-h/g)
MC (% wb)
Es (W-h/g)
MC (% wb)
Es (W-h/g)
MC (% wb)
60
0.74±0.06
32.3±7.90
0.84±0.01
30.3±3.70
0.84±0.01
25.6±4.76
90
0.86±0.00
18.0±1.79
0.94±0.00
15.9±1.25
1.06±0.04
10.5±0.78
120
0.83±0.03
12.9±0.86
0.98±0.00
10.5±0.41
1.13±0.04
8.05±0.52
150
0.90±0.01
10.8±1.62
1.04±0.07
8.57±0.99
1.31±0.02
7.06±0.63
Drying at a higher temperature increased the specific energy (Es) at any given time,
except for drying at set points of 60 and 70°C for 60 min (Figure 4-1, Table 4-1). The Es and
moisture content (wb) of potatoes dried at a set point of 60°C for 60 min was 0.84 W-h/g and
30.3%, respectively, while the Es and moisture content of potatoes dried at a set point of 70°C
for 60 min was 0.84 W-h/g and 25.6%, respectively (Table 4-1). This indicated that drying at a
set point temperature of 70°C during the first 60 min was more efficient than other treatments.
81
70
1400
1200
o
S>
0
.1000
|
800
3
(0
—S--
0)
— A - -FPat70°C
b
— X - -PTat50°C
FP at 60°C
i_
o
Q.
re
— e - - FP at 50°C
600 4
+••
u
o
u- 400
+ 20
A A A A Ai
200
—*- -PTat60°C
T3
3
8
- 9 -
-PTat70°C
a.
10
^-o o o o o
—I—
— i —
30
60
90
120
150
Drying Time (min)
Figure 4-2. Forward Power (FP) and Product Temperature (PT) for Blanched Russet Potato
Slices Dried by MIVAC® at a Set Point of 50, 60, or 70°C for 150 min
The results of modulating microwave power based on the surface temperature of the
potatoes for 150 min are summarized on Figure 4-2. A two phase two drying was observed for
potato slices. This is different from the three phases observed for grapes by Clary et al. (2007).
Drying at a higher temperature required longer time before the system automatically reduced the
microwave power (Figure 4-2). After this reduction, the amount of forward power applied for the
higher dryer set points temperature (70°C) was greater than at lower temperatures. Applying
forward power based on an equivalent sample weight increased the product temperature at the
same rate until a temperature plateau was reached. The surface temperature reported in Figure 42 was below the set point since all of these values were based on 6 sec interval readings and
82
averaged every 5 min. These results suggest that MIVAC using product surface temperatures to
control microwave power input should be appropriate for dehydrating heat sensitive materials.
1200
1200
H©-NPat50°C
a - NP at 60°C
-&- NP at 70°C
-*-WTat50°C
-*-WTat60°C
«-WTat70°C
nnnnnpnnnnnrl:
•fr-«~^fr-»-3~»-0 0 0 0 &>
— i —
30
60
90
120
150
Drying Time (min)
Figure 4-3. Net Power (NP) and Weight (WT) for Blanched Russet Potato Slices Dried by
MIVAC® at Set Point of 50, 60, or 70°C for 150 min
The net power is the amount of microwave power absorbed by the product and is
calculated by subtracting the forward power from the reflected power. Small differences in
weight loss were observed among the drying treatments at set points of 50, 60 and 70°C (Figure
4-3). The net power was reduced after 40, 50, and 55 min of drying at set point temperatures of
50, 60 and 70°C, respectively, and the weight loss was about 51.2, 60.0, and 64.5%, respectively.
83
70
-o-50°C
-s-60°C
-A-70°C
35
30
60
90
120
150
Drying Time (min)
Figure 4-4. Reflected Power during Drying of Russet Potato Slices by MIVAC at Set Points of
50,60, or 70°C for 150 min
Drying at a set point of 50°C was a more energy efficient compared to drying at 60 or
70°C because of the lower reflected power (Figure 4-4). Reflected power seems to be
proportionate to forward power but did not amount to more than 5% of the forward power.
Drying at a set point of 70°C was less efficient with the reflected power increasing 1.5 times
from the beginning to the end of the process. The temperature control system reduced microwave
power immediately when the product reached the set point temperature and at this point there
was a decrease in reflected power. The chamber pressure was set to 5 kPa at the beginning of the
drying without any further adjustment during processing and decreased as dehydration proceeded
(Figure 4-5). The decrease in chamber pressure demonstrated a similar trend, and as the
application of microwave forward power decreased, the moisture content decreased.
84
re
Q.
XL
-o-50°C
-s-60°C
3
(A
-^70°C
</>
0
30
60
90
Drying Time (min)
120
150
Figure 4-5. Chamber Pressure during Drying of Potato Slices by MIVAC at Set Points of 50,
60, or 70°C for 150 min
85
00
90
Drying Time (min)
Drying Time (min)
60
120
150
150
-e-60°C
-*-70°C
-e-50°C
(b)
30
30
90
90
Drying Time (min)
60
Drying Time (min)
60
120
120
150
150
Figure 4-6. Tristimulus Color Values for MIVAC® Dried Russet Potato Slices at Dryer Set Points of 50, 60, and 70°C: (a) CIE-Lab L
Values, (b) CIE-Lab a* Values, (c) CIE-Lab b* Values, and (d) Total Color Differential (AE) Values
30
-*-70°C
-s-60°C
-9— 50°C
(c)
Color
Color provided an indication of the effect of drying temperature and drying rate on the
quality of the dried potatoes. The lightness of the dried potatoes was measured and reported as a
CIE-Lab L values with higher L values indicating a lighter color. The combination of drying
times and temperature treatments significantly affected the lightness of the dried potatoes.
Drying at a set point of 70°C produced significantly darker potatoes compared to drying at set
points of 50 and 60°C at any process time (Figure 4-6a). Drying for 60 min yielded whiter
potatoes with higher average L values (67.4, 68.0, 66.3 for drying at a set point of 50, 60, 70°C,
respectively) compared to the fresh-blanched potatoes control (56.8). The lightness of potatoes
dried at 150 min was not significantly different than the lightness of fresh-blanched potatoes,
except the potatoes dried at a set point of 50°C. Therefore, the lightness was decreased when the
potatoes dried longer than 60 min and exhibited the lightness of fresh-blanched potatoes. The
lightness of potatoes dried at a set point of 50°C, 60°C or 70°C for 120 or 150 min, or at 70°C
for 90 min was not significantly different from the lightness of fresh-blanched potatoes.
However, visual observation indicated that drying at 70°C for 120 min or more resulted in
scorching of some of the potato slices, while drying at other temperatures produced no scorched
potatoes. Because of the non-uniformity in color across a dehydrated potato slice, particularly at
the higher drying temperatures, visual assessment of color may offer a better alternative for color
assessment than spectrophotometric measurement using a hand-held tristimulus colorimeter. To
obtain a more representative assessment of color, 10 measurements were taken on each slice to
compensate in part for the small area over which measurements were taken. Regardless, an
overall color measurement for individual slices was not possible using a tristimulus colorimeter
and could not differentiate the scorched from the non-scorched regions nor differentiate a minor
87
color changes across a large piece of potato. There is a need to develop a better method to
analyze an overall color of a large sample such as the sample of sliced potatoes used in this
experiment.
The CIE-Lab a* value indicated increased redness in the dehydration treatments with
MIVAC® dried products having significantly higher a* value compared to the fresh-blanched
potatoes (Figure 4-6b). Higher drying temperature increased the redness of the dried potatoes.
Drying at a set point of 70°C at any given of drying time resulted in significantly higher increase
in redness of the dried product compared to drying at set points of 50 and 60°C, while there was
no significant differences between drying at a set point of 50 or 60°C. Dried potatoes at 60 min
had a higher a* value compared those dried for 90 and 120 min, except if scorching occurred.
Potatoes dried at a set point of 70°C for 120 min had visible scorching with drying at 70°C for
150 min producing a greater number of scorched potatoes slices and an increase in a* value.
The CIE-Lab b* values of MIVAC® dried potatoes exhibited more yellow color than
fresh-blanched potatoes (Figure 4-6c). There was a significant increase b* value or yellowness
during the first 60 min of drying and then a decrease for longer drying time. Potatoes dried at a
set point of 50°C retained a greater yellow color than potatoes dried at a set point of 60 or 70°C,
while there was no significant difference in b* value between potatoes dried at a set points of 60
and 70°C.
Color difference (AE) indicates how the drying treatments affected the apparent visual
color of the dried potatoes (Figure 4-6d). Drying for 60 min at any given set point temperature
produced the highest total color difference (18.1 for 50°C, 19.6 for 60°C, and 16.5 for 70°C)
compared to the fresh blanched potatoes. Longer drying times showed fewer differences in color
between treatments and between dried potatoes and the fresh-blanched potatoes. Drying at 60°C
88
for 120 min ((AE = 7.45) or 150 min ((AE = 7.77) resulted in dried potatoes with the smallest
color difference from the control.
ou
y = 0.466x+53.608
1^ = 09197*
70
60
O en
50
—*"
•L
•
w
• b*
.Q
.5 40
y = 0.3128x+9.2884
R2 = 0.8781
5 30
20
H—-»
10
0
0
i
i
i
i
i
1
5
10
15
20
25
30
35
% Moisture Content (wb)
Figure 4-7. Correlation Between % Moisture Content (wb) and CIE-Lab L and a* Values for
MIVAC® Dried Potato Slices
The correlation between CIE Lab L or b* values to moisture content of the dried potatoes
is shown in Figure 4-7, while Figure 4-8 shows the correlation between total color difference
(AE) and moisture content. There were positive interactions between the lightness (r2 = 0.92) and
yellowness (r2 = 0.878) and total color difference (r2 = 0.901) with moisture content. Drying
potatoes below 13% moisture content resulted in dehydrated products with a color profile similar
to fresh-blanched potatoes. MIVAC offers an alternative to conventional hot air drying methods
and produces product of acceptable quality.
89
OK
y = 0.4603x + 4.4008
R2 = 0.9009
20 -
LU
<
^—"
4•
15 -
*^n
10 5n U
0
i
i
i
i
i
i
5
10
15
20
25
30
35
% Moisture Content (wb)
Figure 4-8. Correlation Between % Moisture Content (wb) and Total Color Change (AE) Values
for MIVAC® Dried Potato Slices
CONCLUSION
Controlling application of microwave power in MIVAC drying to maintain product
temperature at a targeted set point provides better control of drying conditions and minimizes
overheating compared to other methods of controlling the microwave power. A dryer set point
temperature of 60°C for 150 min with lW/g of fresh blanch potatoes was the most favorable
condition evaluated and this yielding potatoes that dried uniformly, retained the best color and
did not scorch.
ACKNOWLEDGEMENT
The support of a USDA National Needs Graduate Fellowship, the Washington Potato
Commission and the Agriculture Research Center, Washington State University is gratefully
acknowledged.
90
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93
CHAPTER 5. DISTRIBUTION OF MOISTURE AND MICROWAVE
ENERGY FOR RUSSET POTATOES (SOLANUM TUBEROSUM) DURING
MICROWAVE-VACUUM DRYING UNDER CONTINUOUS
TEMPERATURE CONTROL
Dewi Setiady, Carter D. Clary, Juming Tang, Barbara A. Rasco
Authors Setiady and Rasco are with the Dept. of Food Science and Human Nutrition,
Washington State Univ., Pullman, WA 99164-6376. Author Clary is with the Dept. of
Horticulture, Washington State Univ., Pullman, WA 99164-6376. Author Tang is with the Dept.
of Biological System Engineering 6120, Washington State Univ., Pullman, WA 99164-6120.
Direct inquiries to author Clary.
ABSTRACT
Uneven microwave distribution can cause hot spots, leading to scorching and poor
product quality. MIVAC®is a type of microwave-vacuum drying (MVD) that can be operated to
modulate microwave power by maintaining a specific product temperature. In addition, using a
rotating turn table can more evenly distribute microwave power across the product surface
during drying. The objective of this research was to determine the moisture and microwave
power distribution during MIVAC® drying of potato slices with and without the use of a rotating
turntable at a dryer set point temperature of 60°C. The moisture distribution patterns were
different for static (no rotating turntable) and dynamic (rotating turntable) drying conditions. The
moisture distribution was more variable for MVD potatoes made under static conditions. Using
the rotating turn table resulted in dehydrated potatoes with more uniform moisture content,
which is particularly important for dehydrating products to a low final moisture content without
94
scorching and less case hardening. The moisture distribution for product dried using the rotating
turntable was modeled by dividing the turntable into three regions of concentric rings.
Dehydration of potatoes in the outer and center rings was more efficient than in the middle ring,
and potatoes dried in the outer ring had more uniform moisture content.
INTRODUCTION
Producing consistent and uniform dried products is the goal of every drying process.
However, uneven moisture distribution within individual pieces of product in the same batch and
between different pieces in the same batch is a problem with most dehydration systems and
results in added cost, time and quality loss. To asses the impact of non-uniform drying, Cronin &
Kearney (1998) investigated and validated a drying model based on a Monte Carlo model to
predict the final moisture content and moisture distribution for a tray heated air dryer (HAD).
A number of alternative dehydrating methods have been evaluated for HAD. One
alternative system is microwave vacuum drying (MVD). Because microwave has the ability to
heat to the core of the sample via volumetric heating rather than just heating the surface of the
food, it can provide more rapid heat transfer and moisture removal during dehydration.
Microwave can decrease processing time by up to 90% compared to conventional heat transfer
(Decareau, 1986). Employing a vacuum as part of the dehydration process decreases the boiling
point of the water within the food and permits moisture to vaporize at a lower temperature than
under atmospheric conditions. The combination of microwave and vacuum often results in more
efficient heat transfer leading to improved product quality compared to products that are dried in
lengthy heated air drying process. A primary disadvantage of MVD is the uneven distribution of
microwave power that can cause uneven heating within the drying chamber, producing hot and
cold spots. Therefore, being able to predict the moisture and energy distribution within a MVD
95
drying chamber is important for process optimization. Drouzas et al. (1999) determined the
electromagnetic field distribution within a MVD drying chamber operated in a pulsed mode at
five random locations by using fruit gels as a model food. These authors concluded that the
energy distribution was not uniform and that by identifying the energy distribution pattern,
drying would be optimized.
There has been little research regarding microwave energy or moisture distribution in
MVD systems. Cheng et al. (2006) determined the microwave field distribution as temperature
changes in the microwave chamber by using nine water-filled Pyrex beakers. They indicated that
the hot spots occurred at the locations closest to the waveguide or microwave entrance, while the
corners of the chamber had less intense energy. These researchers suggested a number of options
to improve microwave energy uniformity including: mode stirrers and turntables, using several
magnetrons and cavities, upgrading the microwave chamber, or employing a continuous
processing unit.
Yang and Gunasekaran (2004) predicted the temperature distribution within microwave
heating units using the Maxwell's equation and Lambert's law. Maxwell's equation provided a
more accurate prediction of temperature distribution compared to models based upon Lambert's
law. Applying microwaves in a pulsed mode produced a more uniform temperature distribution
and was supported by experimental measurements. More uniform heating may be advantageous
even though pulsed mode operation would require longer processing time compared to operation
in a constant power mode. Gunasekaran and Yang (2007) described the temperature distribution
for microwave heated 2% agar gel in Pyrex glass in a unit operated in both a constant or pulsed
mode. A sample glass was placed at the center of the turntables. Heating with constant
microwave power resulted in a temperature distribution that was uneven with high temperatures
96
occurring at the center of the unit. A more uniform temperature distribution was observed by
heating the gel samples with a pulse mode.
Microwave drying units come in different configurations. MVD units are configured
according to how the microwave power is applied and how vacuum is used during the
dehydration process. One specific configuration of MVD, a MIVAC® dehydrator, utilizes a
semi-automated continuous temperature control (Clary et al. (2005, 2006, and 2007); Setiady et
al. (2007)). Clary et al. (2005) described the operation of MVD unit in constant and incremental
modes for drying grapes. In a constant mode, microwave energy is applied at a fixed power
during the process, while in the incremental mode, microwave power is reduced in stages.
Drying using an incremental mode is more energy efficient and produced higher quality dried
grapes without scorching compared to drying under a constant mode. Clary et al. (2007)
modified MVD for temperature control by developing a controller for consistent, microwave
power application during drying that could maintain a uniform product temperature. Drying
grapes under this MVD system provided greater energy efficiency and improved product quality
compared to drying in either a constant or incremental MVD mode without temperature control.
Setiady et al. (2007) dried Russet potatoes using MVD in a system similar to the one
described by Clary et al. (2007) at different dryer set point temperatures (50, 60, and 70°C). They
found that an intermediate 60°C temperature was an optimal drying temperature of those
evaluated. Potatoes dried to intermediate moisture content (35-20%) at a relatively low drying
temperature (50°C) had a high variation in moisture content than when a higher drying
temperature was used. However, drying at 70°C produced burnt potatoes with the localization,
intensity, and degree of scorching being unpredictable. Therefore, the objective of this research
was to determine how the moisture distribution within a dehydrated product, here potatoes, is
97
affected by different levels of microwave energy under MVD using a MIVAC configuration at
60°C, and how the moisture distribution varies under static and dynamic operating conditions.
MATERIALS AND METHODS
Preparation of Potatoes for Dehydration
Russet potatoes (Solarium tuberosum) were used for this experiment. Potatoes were
peeled, washed, and cut into a cylinder of dia 3.5 cm and then to 3 mm slices before blanching in
boiling water at about 98°C for 2 min. The blanched potatoes were cooled in ice water for 2 min
and drained on paper towels to remove excess water.
Moisture Distribution and Weight Loss during Dehydration
Petri plates were numbered and weighed (n=53) (mp). Blanched potatoes slices (3 -4
slices, ca. 10-17 g) were transferred to each of the numbered Petri plates and the weight recorded
(m,). The plates were arranged at specific locations on the turntable (dia 0.75 m) of the MIVAC
dryer (Figure 3). The plate numbered one was facing the door handle of the vacuum vessel. After
drying, each Petri plate was weighed and recorded (m/). Weight Loss (WL) was calculated as
follows:
\i.-mf
Operation of the Microwave Vacuum MIVAC® Drying System
Microwave vacuum MIVAC® system was described by Clary et al. (2005) with
modifications described by Setiady et al. (2007) using an initial forward power of 750 W. The
level of microwave energy applied was determined by controlling surface temperature using an
infrared sensor. The potatoes were dried at a set point of 60°C for 15, 30, 45, 60, or 90 minutes
on a moving 5RPM turn table and for 15, 30, or 45 on a static turntable.
98
Color Determination
The color of the fresh and dried potatoes was measured by using two methods; a
spectrophotometer and an optical scanner. The spectrometer used was a Minolta CM-2002
(Minolta Camera Co., LTD, Chuo-Ku, Osaka, Japan) with an 11 mm measurement aperture, with
CIE-Lab L*, a*, and b* values recorded. L*, a*, and b* values indicate lightness,
redness/greenness, and yellowness/blueness respectively. Differential in lightness, redness,
yellowness, and overall color (AL, Aa*, Ab*, and AE, respectively) were also calculated, as
follows:
A L = Lfr e s h — Ldried
Aa
= a fresh
_
& dried
Ab* = b* f r e s h - b* dri ed
AE = / i l ^ + ia*^ + i i %
Product color using the scanner was obtained by taking a picture of the sample using an
hp PSC 750 scanner (Hewlett-Packard Co., San Diego, CA). Samples were placed directly upon
the scanner surface. To exclude stray light, the scanner surface was covered with a box lined
with heavy black fabric. Pictures were analyzed using National Instruments Lab View 6.1
(National Instruments Corp., Austin, TX) and RGB (red, green, and blue, respectively) values
were obtained. The mean and standard deviation for RGB values were determined. The RGB
values of 1-50 were eliminated to adjust for the black background being used. Gray cards (18%)
(Delta 1/CPM Inc., Dallas, TX) were used to calibrate the scanner. The differences in RGB
before and after drying (AR, AG, and AB) were also calculated, as follow:
A R = Rfresh - Rdried
A G = Gfr e s h ~ Gdried
A B = Bfresh - Bdried
99
Potatoes were also visually observed for the degree of scorching with valued of 1, 2, or 3
assigned for slight scorching, scorched, and severely scorched, respectively.
Analytical Method for Moisture
Moisture content of fresh and dried potatoes was determined using AOAC method 4415A(AOAC,2000).
Statistical Analysis
Variance and Fisher's least significant difference (LSD) procedure were calculated (SAS
9.1 (System for Windows 2002-2003, Cary, NC). Significance was set at p<0.05. All
determinations were made using three replications for each treatment and values from replicates
were averaged.
RESULTS AND DISCUSSION
Moisture Distribution
Setiady et al. (2007) dried potatoes using MIVAC® a set temperature of 50, 60, and 70°C
at various drying time up to 150 min. They suggested that drying potatoes at 60°C for 150 min
using a rotating turntable would be the most favorable drying condition leading to most uniform
final product moisture. However, the drying time of potatoes in this study for dynamic turntable
required less time because of fewer potato slices. Drying using a static turntable produced severe
scorching after 45 min, so drying using this treatment was terminated at this time.
The moisture distribution during MIVAC® drying was determined based upon the weight
loss for samples at 53 different points within the dryer (Figure 5-2). The average moisture
MIVAC® drying using a rotating turntable was not significantly different compared to static
drying. Longer drying time using rotating or static turntable significantly increased weight loss;
however, there was no significant difference in weight loss for drying more than 45 min with a
100
rotating turntable. The location of the plates within the dryer was a significant influence on how
much weight was lost.
The turntable was divided into 3 concentric rings, this way it was possible to model
drying after 15 and 20 min drying periods using weight loss patterns (Figure 5-2). The first ring
location included the outer plates starting from number 1 to 22. The second ring location was the
middle plates (number 23 to 49) and the third ring location was at the center (number 50-53)
were product remained relatively stationary during the drying process. Table 5-1 shows the
moisture distribution between these three rings at various drying times for static and rotating
turntable treatments. For drying with the rotating turntable for 15 and 30 min, potatoes in the first
ring had a higher average weight loss (48.2% for 15 min and 75.4% for 30 min) compared to
those in the second ring (38.9% for 15 min and 66.9% for 30 min). Potatoes at the center had
similar average weight loss (48.0% for 15 min and 75.3% for 30 min) as potatoes dried in the
first ring. Once potatoes had dried to a moisture content of 20% or less (45 min or longer). The
moisture distribution was more even and product location within the dryer had less of an impact.
Processing potatoes under static conditions resulted in no clear moisture pattern and was
highly variable (high standard deviation among standard deviation in drier and between
treatments) (Figure 5-1). Drying using a static turntable (Table 5-1) was most variable following
15 min of drying compared to other drying times. The average weight loss for potatoes in the
third ring was between that of product in the first and second rings (15 min). Potatoes dried for
15 min using the static turntable regardless of their location within the dryer had an average
weight loss of 42.6+11.0%; while potatoes dried using the rotating turntable for the same time
has an average moisture loss of 43.4+6.1%. Longer processing time decreased the variability in
moisture content; however drying under static conditions was still more variable. Potatoes dried
101
for 45 min with the static system had 83.0+3.4% weight loss, compared to potatoes dried under
rotating turntable for the same time (83.4+1.9%).
Using a rotating turntable decreased variability in the moisture distribution even though
the main purpose for it was to create more even energy distribution reducing the tendency for hot
or cold spots. Potatoes dried using the rotating turntable were slightly drier than those using the
static turntable at the same drying time. Specific locations within the dryer tended to exhibit
greater moisture losses without scorching (number 8) under both drying conditions, followed by
22, 52, 11, and 9 also exhibiting greater moisture losses. Product at location (number 27) had the
lowest average weight loss followed by 40, 47, 35 and 43. Product at location number 27 was
slightly burnt in one of the 45 min drying treatment under static condition, and product at
location number 40 was moderately or severely burnt in two of the 45 minutes static drying
treatments. Thus product receiving a greater amount of microwave power (hot spots) might not
necessary have higher drying rates (Drouzas et al., 1999).
The static turntable treatment resulted in a greater chance of scorched product (4, 0, or 2
out of 53 locations were burnt after 30 min drying and 13, 6, or 15 out of 53 locations within 45
min) without achieving the desired moisture content of 7-8% (wb). Drying with a rotating
turntable reduced both the chance and the intensity of scorching (6, 4, and 5 out of 53 locations
had slightly burnt potatoes after 90 minutes drying). The visual observations for scorching under
the static treatment (45 min drying time) provided a useful means for determining the location of
hot spots in the MF/AC® dryer (Figure 5-2). The locations of hot spots in the static treatment
were not consistent between runs (n=3), with the location of scorched product and intensity of
scorching between locations and experimental runs varying. These results support the findings of
Heng et al. (2007) who indicated that microwave distribution in the drying chamber was a
102
moving standing wave and not a pure standing wave or a pure traveling wave, the effect of which
could result in the creation of hot spots. The location of hot spots within the dryer (static, 45 min)
tended to be at number 38 (moderate to severe scorching in all experiments). Potatoes at
locations 37, 40, and 52 had moderate to severe scorching, and product at locations number 23,
41 and 46 had mild to moderate scorching in at least two of the experiment. Severe scorching
was observed (number 1, 3, 4 17, 31, and 48) in one of the experiments. Therefore, from visual
observation, the hot spots could be predicted to be at locations 23, 37, 38, 40, 41, 46, and 52.
103
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Table 5-1. % Weight Loss for MIVAC® Dynamic and Static Drying at a set point of 60°C
Time (min)
Ring
Plate
Turntable Location Number*
15
30
45
60
90
Rotating
First
1 -22
83.9±2.0
85.9±1.4
48.2±4.3
75.4±4.6
84.6±1.7
23-49
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38.9±3.7
66.9±5.8
82.9±1.8 85.2±1.4 86.1±1.2
Third
50-53
48.0±4.1
75.3±4.0
83.8±1.9 85.3±1.6 86.0±1.0
All
1-53
43.5±6.1
71.1±6.7
83.4±1.9 85.0±1.6 86.0±1.3
Stationary First
1-22
72.6±9.7
82.8±3.0
46.2±11.3
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23-49
69.6±10.2 83.4±3.5
39.6±9.9
Third
50-53
72.9±8.9
81.5±4.7
42.8±11.1
83.0±3.4
All
1-53
42.6±10.1
71.1±9.9
Figure 5-1. % Weight Loss Values of Potatoes Slices Dried in MIVAC at Various Drying Time under Dynamic (D) or Static (S)
Conditions at Different Location within the Drying Chamber
W
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Figure 5-2. Schematic of the Plate Arrangement for Moisture Distribution Determination and Visual Observation for Scorching After
45 min Drying at 60°C Using Static Turntable for Three Separate Experiments; a, 1st test run; b, 2nd test run; c, 3 rd test run. The
"starting point" is the location at the entrance to the drying chamber
Evaluating moisture distribution (Figure 5-1) and visual observation (Figure 5-2) of the
product moisture content using the static turntable (45 min) was possible to determine the cold
spot. The product at locations with the least average weight loss and no scorching were identified
as cold spots (number 33, 34,47, and 51) with weight loss of 79.2, 79.0, 78.7, and 76.2%,
respectively. The hot and cold spots were located in the second and third ring of the MIVAC
turntable; therefore, placing the sample only at the first ring could be used to circumvent
scorching, although this would substantially reduce dryer throughput and reduce overall
efficiency of operation. This experiment indicated that even though the hot and cold spot
locations could be predicted to some extent, the actual distribution microwave energy itself was
unpredictable and varied between different experimental runs. Figure 5-3 shows the correlation
between weight loss and moisture content (db) of MIVAC® dried potatoes under static and
dynamic conditions (r2 = 0.98) indication that this correlation could be used to predict moisture
content.
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40
50
60
70
80
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Figure 5-3. Correlation Between Moisture Content (db) and % Weight Loss Value for MIVAC'
Dried Potatoes Slices
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Figure 5-5. CIE-Lab Differential a* (Aa*) Values of Potatoes Slices Dried in MIVAC at Various Drying Times under Dynamic (D)
or Static (S) Conditions at Different Location Within the Drying Chamber (See Figure 5-2)
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Figure 5-4. CIE-Lab Differential L (AL)Values of Potatoes Slices Dried in MIVAC at Various Drying Times under Dynamic (D) or
Static (S) Conditions at Different Locations Within the Drying Chamber (See Figure 5-2)
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Figure 5-7. Color Difference (AE) Values of Potatoes Slices Dried in MIVAC at Various Drying Times under Dynamic (D) or Static
(S) Conditions at Different Location Within the Drying Chamber (See Figure 5-2)
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Figure 5-6. CIE-Lab Differential b* (Ab*) Values of Potatoes Slices Dried in MIVAC at Various Drying Times under Dynamic (D)
or Static (S) Conditions at Different Location Within the Drying Chamber (See Figure 5-2)
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Spectrophotometric Color Measurement
Color measurement using a tristimulus colorimeter were taken and color difference
between fresh and dried product for lightness (AL), redness (Aa*), yellowness (Ab*), and overall
difference (AE) determined as shown in Figure 5-4, 5-5, 5-6 and 5-7. The method used by
Setiady et al. (2007) describing the changes of color values during MIVAC® potatoes drying at a
set point of 50, 60, or 70°C for various drying times was used. The lightness of potatoes dried for
a short period (15 min drying time) was lighter than the fresh-blanched potatoes (Figure 5-4).
However, as the moisture of the potatoes decreased, the lightness of the potatoes was reduced.
There was no significant difference in lightness for different drying time of 15, 30, and 45 min
for the rotating or static turntable. The lightness of potatoes dried under dynamic conditions was
not significantly different compared to the ones dried under static conditions. The lightness of
dried products was no significantly affected by the plate location. By comparing Figure 5-2, 5-3,
and 5-4, there was no correlation between the lightness value of dried potatoes compared to
weight loss or visual observations for scorching.
The redness values of dried potatoes significantly increased at longer drying time for
dynamic or static drying conditions; however there was no significant difference in redness for
drying times of 45 min or longer when the rotating turntable was used (Figure 5-5). Redness of
the dehydrated potatoes varied with the location of the product within the dryer for both static
and dynamic drying conditions. Setiady et al. (2007) indicated that a* values could be used to
predict scorching in dried potatoes. For a static turntable, products at location number 38 had the
highest Aa* value followed by products at location numbers 52, 8, 37, 10, and 41; product at
location number 43, 27, 33, 47,4, 51 had the lowest change in redness. The locations 23, 37, 38,
40, 41, 46, and 52 were likely hot spots, and location numbers 33, 34, 47, and 51, the cold spots.
109
Visual predictions for 4 of the 7 hot spots correlated with Aa* value, while 3 out of 4 cold spots
predicted based upon weight loss and visual observation correlated with Aa* value. Location
numbers 8 and 10 where there was no visually apparent scorching had high Aa* values and were
also locations where the greatest product weight loss occurred, indicating that efficient
dehydration could take place without scorching the product. Figure 5-8 shows the correlation
between weight loss and Aa* value (r2 = 0.89).
*
2
V = 0.0005X2 - 0.0311x + 0.7335
FT = 0.8915
20
40
60
80
100
% Weight Loss
Figure 5-8. Correlation Between %Weight Loss and CIE-Lab Differential a* (Aa*) Values for
MIVAC® Dried Potato Slices
The yellowness values of potatoes dried using MIVAC® were significantly higher for the
longer drying times under both static and dynamic conditions; however drying for 45 min or
longer with the rotating turntable produced no significant difference in yellowness (Figure 5-6).
There was no significant difference in lightness between the drying methods. The location of
product significantly affected the yellowness of dried potatoes, with the yellowness values for
potatoes dried for 15 and 30 min having a similar pattern as weight loss (Figure 5-2). The
yellowness of potatoes dried for 45 min or longer had similar pattern as the redness with location
number 38 and 52 (45 min, static) having the highest peak values. Figure 5-9 shows the
110
correlation between Ab* value and weight loss (r2 = 0.96) indicating that the potatoes with the
lowest moisture had higher yellowness values as would be expected, since this reflects inclusion
or retention of moisture.
16
14
12
10
V = 0.0013X2 + 0.0164X + 0.9591
R2 = 0.9633
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20
60
40
80
100
% Weight Loss
Figure 5-9. Correlation Between % Weight Loss and CIE-Lab Differential b* (Ab*) Values for
MIVAC® Dried Potato Slices
Somewhat unexpectantly, potatoes dried under dynamic conditions for 15, 30, and 45
min had, in general, higher overall color differences compared to potatoes dried under static
conditions for the same drying time (Figure 5-7) although the range for color difference values
was small (around 10). Increasing the drying time using either drying method tended to lead to
higher color differences. The location of product on the turntable had a significant influence on
the overall color difference with patterns similar to what was observed for yellowness. The
correlation between weight loss and color difference was r2 = 0.92 (Figure 5-10). From Figure 58, 5-9, and 5-10, yellowness tended to be more highly correlated to the weight loss than either
redness or overall color difference.
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80
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Figure 5-10. Correlation Between % Weight Loss and Color Difference (AE) Values for
MIVAC® Dried Potato Slices
From this color measurement, it was clear that product at location number 38 containing
severely burnt potatoes had the highest Aa*, Ab*, and AE values. Burnt potatoes had the greatest
overall color difference, redness, and yellowness. Even though potatoes at location numbers 8
and 10 exhibited the greatest weight loss, potatoes dehydrated at this location had not visible
scorching. However, the average Aa*, Ab*, and AE of potatoes at location number 8 and 10 were
higher than for potatoes dried at other locations, except number 38.
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Figure 5-12. Standard Deviation Red Differential (A Std R) Values of Potatoes Slices Dried in MIVAC at Various Drying Times
under Dynamic (D) or Static (S) Conditions at Different Location Within the Drying Chamber (See Figure 5-2)
<
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Figure 5-11. Mean Red Differential (AR) Values of Potatoes Slices Dried in MIVAC at Various Drying Times under Dynamic (D) or
Static (S) Conditions at Different Location Within the Drying Chamber (See Figure 5-2)
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Figure 5-14. Standard Deviation Green Differential (A Std G) Values of Potatoes Slices Dried in MIVAC at Various Drying Times
under Dynamic (D) or Static (S) Conditions at Different Location Within the Chamber (See Figure 5-2)
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Figure 5-13. Mean Green Differential (AG) Values of Potatoes Slices Dried in MIVAC at Various Drying Times under Dynamic (D)
or Static (S) Conditions at Different Location Within the Drying Chamber (See Figure 5-2)
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under Dynamic (D) or Static (S) Condition at Different Location Within the Chamber (See Figure 5-2)
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Figure 5-15. Mean Blue Differential (AB) Values of Potatoes Slices Dried in MIVAC at Various Drying Times under Dynamic (D)
or Static (S) Condition at Different Location Within the Drying Chamber (See Figure 5-2)
-35
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Product Color Based Upon Scanner Measurements
To obtain a more reliable prediction of visual color, an analysis using scanned images
was performed. Potatoes were scanned using a commercial color scanner against a black
background. The scanner images were analyzed by measuring the amount of red, green, blue
(RGB) pixels and mean values recorded. The RGB pixels from 0 to 50 were eliminated to correct
for the black background. The RGB pixel of 0 is black and the RGB pixel of 255 is white. The
mean value was the average of all pixels from 51 to 255 and was used as an indication of the
overall change in RGB, while the standard deviation was used as an indication of the change
spread RGB. Scans of 18% gray cards were also evaluated to determine the light distribution of
the scanner. The RGB mean values of the gray card were 62.4+6.81, 63.2+7.17, and 63.6+7.32,
respectively. These results indicated an even lighting distribution during the picture scanning and
this eliminated the lighting problem that occurred during the photography.
Figure 5-11 and Figure 5-12 show the difference in mean and standard deviation of R
(red color) between fresh-blanched and dried potatoes. Comparing the mean and standard
deviation of R with weight loss, tristimulus redness values, or visual observation could not be
correlated to a moisture distribution pattern, hot or cold spot prediction, or color changes. Drying
at 15 min increased the red color of dried potatoes compared to the fresh-blanched potatoes.
Drying for longer than 15 min decreased the red color of dried potatoes compared to the freshblanched potatoes, but it increased the red color deviation. Product at location number 38 and 8
did not have the highest mean or standard deviation in R values. Figure 5-17 showed the
correlation between CIE-Lab Aa* values with scanner mean R values (r2 = 0.60).
The difference in mean and standard deviation G (green color) of potatoes dried for 15
min showed little changes (Figure 5-13 and 5-14). The longer drying time led to a decrease in
116
green color. There was no correlation between green color to the yellowness, redness, weight
loss or visual observation. Figure 5-15 shows small increases in blue color between freshblanched and dried (15 min) potatoes, while little deviation of the blue pixel color occurred
during 15 min drying (Figure 5-16). Increasing the drying time decreased the blue value with an
increased deviation from fresh-blanched potatoes observed. Again there was no correlation
between blue and other parameters in this study.
-29
*
-3G-J-
- -
-
J
Aa*
Figure 5-17. Correlation Between Mean Red Values and CIE-Lab (Aa*) Values for MIVAC®
Dried Potato Slices
This scanner application was less successful in predicting changes in product color than
tristimulus colorimetry. CIE-Lab measures the color of the product based on the color-opponent
space theory, which is modeled upon the information of human eyes and obtained about color
from cones and rods in an antagonistic system of light to dark, red to green, or yellow to blue.
RGB measures color based on the additive color model which is based upon how light emanates
from its source. RGB does not measure the exact values for color components within a sample
but rather its measure the relative color values within the sample. Spectrophotometric analysis
measures color based in a manner more similar to how our eyes identifies color. However, to
117
obtain an accurate overall color values for a food product sample, many more measurements are
required to ensure that the assessment is representative. With the scanner, only one image need
to be taken, but since the images reflect absorbance on a color intensity (pixel) scale, they may
not correlate well with human perception of color or correlate as closely as measurement that
provide a closer reflection of human perception of color, the purpose behind the CIE-Lab system.
Specific Energy (Es) Calculations
The correlation between moisture content (db) and specific energy (Es) of potatoes dried
at a set point of 60°C under both static and dynamic conditions was negatively correlated (r2 =
0.94) (Figure 5-18). The Es for drying for 15, 30, and 45 min using either a static or rotating
turntable were similar. The Es requirements increased significantly for longer drying times. The
moisture contents of potatoes dried for 15, 30, and 45 min were similar for both drying methods.
Drying longer than 45 min had a higher Es, even though there was no significant reduction in
moisture content of the potatoes dried longer than 45 min.
y = -0.1585Ln(x) + 0.4454
FT = 0.9397
Moisture Content (kg/kgdb)
Figure 5-18. Correlation Between Moisture Content (kg/kgdb) and Specific Energy (W-h/g)
Values for MIVAC® Dried Potatoes Slices
118
CONCLUSION
The moisture distribution patterns were different for MIVAC® drying when a stationary
or rotating turntable were used, with there being greater variability in the moisture content for
Russet potato slices for the static turntable. The moisture distribution for product dried using the
rotating turntable was modeled by dividing the dryer into three regions composed of concentric
rings. Dehydration of products in the outer and center ring was more efficient than in the middle
ring, and potatoes dried in the outer ring had more uniform moisture content. For the stationary
drying configuration, scorched products tended to be concentrated in the middle ring. The
location of hot and cold spots in the dryer could be predicted but it is not the same for each
drying treatments due to non-uniform microwave energy distribution. Dehydrated potatoes with
more uniform moisture content could be produced using a rotating turn table, and this was
particularly important for producing products with a low final moisture content. A significant
number of potatoes dried using the stationary turntable became scorched (up to 12% after 30 min
and up to 30% after 45 min). Products dried using the dynamic system had less of a tendency to
scorch (up to 12% after 90 min).
ACKNOWLEDGEMENT
This research was supported by a USDA National Need Graduate Fellowship, Balcom &
Moe Inc., Pasco, WA, and WSU Agriculture Research Center and the WSU Department of Food
Science. We also would like to thank Dr. Marvin Pitts and Frank Younce, the manager of WSU
Food Processing Pilot Plant, for support and advice.
REFERENCES:
AOAC. 2000. Official Methods of Analysis, 17th ed. Washington, DC: Association of Official
Analytical Chemist, 37.1.10. 2(37): 4.
119
Cheng, W. M., Raghavan, G. S. V., Ngadi, M. and Wang, N. 2006. Microwave power control
strategies on the drying process I. Development and evaluation of new microwave drying
system. Journal ofFoodEngineering. 76: 188-194.
Clary, C. D., Wang, S. and Petrucci, V. E. 2005. Fixed and incremental levels of microwave
power application on drying grapes under vacuum. Journal of Food Science. 70(4): 344349.
Clary, C. D., Gamache, A., Cliff, M., Fellman, J. and Edwards, C. 2006. Flavor and aroma
attributes of Riesling wines produced by freeze concentration and microwave vacuum
dehydration. Journal of Food Processing and Preservation. 30: 393-406.
Clary, C. D., Mejia-Mesa, E., Wang, S. and Petrucci, V. E. 2007. Improving grape quality
using microwave vacuum drying associated with temperature control. Journal of
Food Science. 72(1): E23-E28.
Cronin, K. and Kearney, S. 1998. Monte Carlo modeling of a vegetable tray dryer. Journal of
Food Engineering. 35: 233-250.
Decareau, R.V. and Peterson, R. A. 1986. Microwave Processing and Engineering. Ellis
Horwood Ltd. Chichester, England.
Drouzas, A. E., Tsami, E. and Saravacos, G. D. 1999. Microwave/vacuum drying of model fruit
gels. Journal ofFood Engineering. 39: 117-122.
Gunasekaran, S. and Yang, H. W. 2007. Effect of experimental parameters on temperature
distribution during continuous and pulsed microwave heating. Journal of Food
Engineering. 78: 1452-1456.
Heng, S., Zhu, H., Feng, H. and Xu, L. 2007. Thermoelectromagentic coupling in microwave
freeze-drying. Journal of Food Process Engineering. 30: 131-149.
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, CA
94304.
Setiady, D., Clary, C , Younce, F. and Rasco, B. A. 2007. Optimizing drying conditions for
microwave-vacuum (MIVAC®) drying of Russet potatoes {Solanum tuberosum). Drying
Technology. 25(9): 1483-1489.
Yang, H. W. and Gunasekaran, S. 2004. Comparison of temperature distribution in model food
cylinders based on Maxwell's equations and Lambert's law during pulsed microwave
heating. Journal of Food Engineering. 64: 445-453.
120
CHAPTER 6. POROSITY AND DEHYDRATION PROPERTIES OF
DEHYDRATED RUSSET POTATOES USING MICROWAVE VACUUM,
HEATED AIR OR FREEZE DEHYDRATION
Dewi Setiady, Carter Clary, Juming Tang, Frank Younce, Barry A. Swanson, Barbara A.
Rasco
Authors Setiady, Younce, Swanson, and Rasco are with the Dept. of Food Science and Human
Nutrition, Washington State Univ., Pullman, WA 99164-6376. Author Clary is with the Dept. of
Horticulture, Washington State Univ., Pullman, WA 99164-6376. Author Tang is with the Dept.
of Biological System Engineering 6120, Washington State Univ., Pullman, WA 99164-6120.
Direct inquiries to author Clary.
ABSTRACT
Microwave vacuum drying (MVD) has been studied as an alternative drying method for
making puffed dried products. Here, the porosity, color, texture, and microscopic analysis of
potatoes dried using MVD, heated air (HAD) and freeze dried (FD) were determined. The
porosity was calculated from a ratio of apparent density and true density. A mathematical model
for calculating true density from moisture content and true density was derived. Potatoes dried
by MVD had twice the porosity compared to HAD, while FD had the highest porosity. Puncture
tests and SEM images supported the results from sensory and texture measurements and
indicated that MVD potatoes were crispy, HAD potatoes were hard and brittle, and FD potatoes
were spongy and soft.
121
INTRODUCTION
Heated air drying (HAD) is commonly used to dry food products because it is
economical, requires a relatively low capital investment, and can be relatively energy efficient.
However, severe case hardening can occur decreasing the overall quality of HAD treated foods.
With freeze drying (FD), it is possible to eliminate case hardening, since water removal occurs
through direct sublimation or to water vapor. FD results in little or no shrinkage as opposed to
HAD. However, FD requires a long processing time leading to an expensive operation and also
involves a high capital investment. Microwave vacuum drying (MVD) has been investigated as
an alternative drying method that can produce puffed dried products (Lin et al, 1998; Drouzas et
al, 1999; Yousif et al, 1999; Sham et al, 2001, Durance et al, 2002; Clary et al, 2007) at
potentially lower cost than FD.
MVD can produce puffed products with a porous structure (Drouzas et al, 1999). The
porous structure of MVD products leads to better rehydration characteristics, such as higher
water retention (Lin et al, 1998), faster rehydration time (Durance et al, 2002; Giri et al, 2007)
and more complete rehydration (Giri, 2007). Krulis et al. (2005) and Kim and Bhowmik (1997)
indicated that drying low initial moisture content products at a high microwave power produced
dehydrated products with a low density that were very porous having optimal puffing. Kim and
Bhowmik (1997) also observed that the shrinkage in dried yogurt increased linearly in
relationship to the moisture content, while bulk density changed nonlinearly relative to the
moisture content.
MVD products have a lower apparent density than HAD products, often half as much as
HAD with FD having the lowest apparent density (Lin et al, 1998; Sham et al, 2001; Durance
et al, 2002). Sham et al (2001) observed puffing in drying apple chips dehydrated with MVD at
122
foil vacuum of 28 mmHg (3.8 kPa) through the formation of a honeycomb network structure of
dried apple chips showing closely interconnected cells that had less shrinkage than HAD. MVD
also required less heat compared to HAD as indicated by less shrinkage and stretching of cell
layers in the epidermis and palisade layers of sweet basil (Yousif et ah, 1999).
Krokida et ah (2001) measured the porosity of dehydrated and rehydrated potato, apple,
banana, and carrot produced by different types of dehydration methods including: HAD, FD,
vacuum drying and microwave drying. The porosity (s) was calculated from the apparent density
(pb) and true density (pp) as:
6 = 1-^-
(1)
Apparent density is the measurement of mass solid, water, and air pores in a specific
volume material (volume that was occupied by solid, water, and air pores). The definition of bulk
density is often mistaken for apparent density. Bulk density is the mass of materials for a known
volume container. Bulk density was used originally to measure the density of grains. Therefore,
bulk density has a different meaning compared to apparent density with the bulk density
measures the density of bulk materials and apparent density measures the density of particles.
True density measures the density of solid and water content of a material within a particle
volume plus that of air pores.
Krokida and Maroulis (2001) indicated that the different drying methods had no influence
on the true density of the products; only the moisture content regardless of whether they were
dried or rehydrated influenced the true density. These researchers also provided a true density
graph of potato, apple, banana, and carrot. From these graphs, a mathematical model was drawn
to predict the true density of potatoes based on the moisture content (wb) of a different potato
products.
123
Mathematical Modeling
True density (pp) is mass of solid (ms) plus mass of water content (mw) divided by the
volume of the solid (vs) plus the volume of water (vw) or
mw+ms
(1)
v„, + v„
vs + vw = vp, with vp for true volume, equation 1 can be rewritten using the density of
water (ps) and density solid (ps) as:
v..
v.
V
V
P
(2)
P
f
\
(3)
Pp=Pw — + P,
Moisture content (wb) (MCwb) is the mass of moisture or water (mw) divided by the total
mass, which is the mass of solid (ms), water (mw), and air (ma). The mass of air is set to zero. The
mass of water and the total mass can be substituted for the density of water for the mass of water
or true density for the mass of total.
MC wb
m
mP
p v
(4)
PPVP
Substituting vw/vp in equation 3 with modifications as per equation 4, will lead to
r
\
PP=MCwbPp+Ps
(5)
V
pp=MCwbpp+Ps-psMCwb^
(6)
P.
P„=Ps+MCwbpwbr
p
\-£^
V
•(V)
pwJ
124
ps=pp-MCwbpp
Ps=Pt
(8)
\-MC
wb
(9)
Ps
(
PP =
\-MC
wb
P,w J
I
v
(10)
>
P
t
Pw j
A test can be performed to examine this mathematical model. When MCwb of a product is
0 kg kgwb"1, pp = ps, and when MCwb of a product is 1 kg kgwb"' (MCwb of water), pp = pw = 1 kg
The objectives of this research was to determine the porosity properties of MVD, HAD,
and FD treated Russet potato slices, as well as their dehydrated properties, such as color, texture,
and microscopic structure. True densities of dried potatoes were determined from equation 10.
True density of potato solid pswas taken from Krokida and Maroulis (2001). The true densities
were calculated and compared to those reported by Krokida and Maroulis (2001) for true density.
An MVD method with continuous temperature control described by Clary et al (2005 and 2007)
and modified (Setiady et al, 2007) was used in this experiment. This type of MVD has the
ability to control the drying process by modifying the application of microwave power according
to IR measurements of surface product temperature. Drying in this experiment was performed
under a moderate set point temperature (60°C) which indicated that drying using moderate
temperature produced more desirable dried potatoes than drying at either a higher or lower
temperature (Setiady et al, 2007).
125
MATERIALS AND METHODS
Drying Preparation
Russet potatoes (Solarium tuberosum) were peeled, washed, and cut into cylinders (3.5
cm dia.) and sliced to 3mm before blanching in 98°C tap water for 2 min. The blanched potato
slices were cooled in ice water for 2 minutes and then drained on paper towels to remove excess
water. The potatoes were dried using:
•
MVD: (MIVAC® ) as described by Clary et al. (2005) with modifications (Setiady et al,
2007) at a dryer set point of 60°C for 150 min with 1 W/g fresh potatoes at an initial
forward power, using a rotating turntable at 5 RPM, and a vacuum of 2.27 kPa,
•
HAD: 3 kW in a model UOP 8 (Armfield Technical Education Co. Ltd., Ringwood,
Hampshire, England) at 60°C for 360 min with 1.8 m/s air velocity,
•
FD: in model Freeze Mobile 24 and Unitop 600L (The Virtis Co., Gardiner, NY) at 0.013
kPa with chamber temperature of 22°C and condenser temperature of-55°C.
Analytical Methods
Moisture Analysis
Moisture content of fresh and dried potatoes was determined using AOAC method 4415A(AOAC,2000).
Apparent Density Analysis
Total volume of dried potatoes was determined according to Krokida et al. (2001) by
measuring the volume displacement of the potatoes inrc-heptanewith at an accuracy of 0.5 ml.
The apparent density of the potatoes was determined from the total mass of the potatoes divided
by the total volume.
126
Color Analysis
Instrumental color of the fresh and dried potatoes was measured by using a Minolta CM2002 spectrophotometer (Minolta Camera Co., LTD, Chuo-Ku, Osaka, Japan) with an 11 mm
measurement aperture. The CIE-Lab L*, a*, and b* values were recorded. L*, a*, and b* values
indicate lightness, redness(+)/(-)greenness, and yellowness(+)/(-)blueness respectively. Total
color differential (AE) was also calculated, as follow:
AE = V i l ^ + ia *l + tb *^
Texture Analysis
Dried potatoes (N = 20) for each treatment were analyzed by a puncture test using the
TA-XT2 Texture Analyzer. A slice of dried potatoes was positioned on the middle of a heavy
duty form plat (model HDP/90) with an insert plat with a 9.5 mm dia hole) and punctured until
they fractured using a 5 mm dia round tip cylindrical probe. Force and work were calculated
directly from the force-time curves. Force was determined from the height of the peak and work
was from the area under the force-time curves.
Scanning Electron Microscopy Examination
Dried potatoes were cut and mounted on aluminum stubs and gold coated. All samples
from different drying methods were examined and photographed in a SEM Hitachi S-570
(Hitachi Ltd., Tokyo, Japan) using an accelerating voltage of 20 KV. Micrographs were taken at
a magnification of 70X for longitudinal sections and 200X for transverse sections.
Statistical Analysis
Data were analyzed with a computer software package, SAS 9.1 (System for Windows
2002-2003, Cary, NC), using analysis of variance and Fisher's least significant difference (LSD)
127
procedure. Significance was determined at P<0.05. All determinations were made at least in
duplicate and all were averaged.
RESULTS AND DISCUSSION
Properties of Dehydrated Potatoes
20
DMVD
15
nHAD
• FD
10
5
0
Force (N)
Work (J)
Figure 6-1. Puncture Test Results (Force & Work) for MVD, HAD, and FD Dried Potatoes (n =
20)
The puncture test provided the force required to puncture the potatoes; FD potatoes
required significantly less force to puncture than HAD, but statistically, it was not significantly
lower than the force required to puncture MVD potatoes (Figure 6-1). The force to puncture
HAD potatoes was slightly higher than for MVD. This indicated that HAD produced harder and
more brittle dried potatoes probably due to case hardening caused by structural collapse during
drying, while MVD potatoes had less case hardening. FD potatoes were crumbled, fragile,
spongy, and more easily punctured. Statistically, there was no significant difference among the
amount of work required to puncture MVD, HAD, and FD potatoes, but FD potatoes had a
tendency to require less work to break. Visual observation indicated that FD potatoes had a
structure like a dried sponge that could break easily.
128
HAD processing led to significantly darker potatoes compared to the other treatments
evaluated. FD potatoes had the lightest color (Table 6-1). MVD potatoes were slightly lighter
than the fresh-blanched potatoes. Dehydration significantly increased the redness of dried
potatoes compared to the fresh-blanched control, including FD processing. HAD potatoes were
significantly redder compared to MVD potatoes due to longer drying time, even though HAD
and MVD potatoes dried at the same (60°C) temperature. The yellowness of the dehydrated
potatoes increased significantly during drying. Comparing the effect of drying treatment to the
fresh-blanched on color, FD potatoes had the highest total color differential compared to MVD
and HAD potatoes due to increase lightness. The visual color of FD potatoes were paler
compared to other dried potatoes or to blanched potatoes. From spectrophotometric results and
visual observation, MVD had the highest color retention, while FD lost some of the potato's
original color properties.
Table 6-1. CIE-Lab Values1 for Blanched, MVD, HAD, and FD Dried Potatoes (n = 80)
Sample
L*1
a*1
b*1
AE
b
a
a
Blanched
58.6±3.2
-2.40±0.29
-0.87±1.0
0
C
c
C
MVD
60.3±3.3
-1.07±0.14
6.76±1.6
7.93
a
d
b
HAD
56.5±2.5
-1.00±0.15
4.86±1.4
6.26
d
b
d
FD
81.7±2.5
-1.19±0.20
9.73±1.3
25.5
0
1
Means in a column with different superscripts are significantly different (/>< 0.05).
L*, lightness; a*, redness(+)/(-)greenness; b*, yellowness(+)/(-)blueness; AE, total color differential.
Porosity Properties of Dried Potatoes
From Krokida and Maroulis (2001) the true density graph for potato (Figure 6-2), the
density of solid (ps) from the mathematical model was determined as 1.6 kg L"1. Unfortunately,
Krokida and Maroulis included no y-axis intercept equation that could be used to identify the
true density of the potato based on moisture content (db). Figure 6-3 was redrawn from the graph
of Krokida for true density (pp (K)) and a y-axis intercept equation determined. The trend line of
the true density and moisture content was highly correlated with r2 = 0.996. However, using the
129
y-axis intercept equation to calculate the true density of MVD (1.90 kg L"1), HAD (1.94 kg L"1),
and FD (1.97 kg L"1.) potatoes would exceed the value of for the solid density of the potato itself
of 1.6 kg L"1. (Table 6-2) and from equation 10, however this equation could not be used to
obtain a reasonable prediction of density. A modification of the Krokida trend line was
calculated by assigning 1.6 kg L"'as the density of solid and new y-axis intercept equation (pp
(MK)) was obtained. This modified trend line was also highly correlated with moisture (r =
0.995). The true densities of MVD, HAD, and FD were then calculated using this modification
and were 1.58, 1.59, 1.60 kgL"1, respectively.
1.7
1
5
4
3
2
6
y = 0.0006x - 0.0128x + 0.1084x - 0.4673x + 1.0934x - 1.4033x +
? nn?s <n in
1
/--\
1.6 \
\
uexi 1.5
^
* 1.4
t/>
1
•
R2 = 0.9963 (ppK)
i
3
2
y = -0.005x + 0.0707x - 0.3275x + 1.607 (ppMK)
R2 = 0.9952 (ppMK)
H
PP (K)
•
pp (MK)
- 'Poly-(pp
(MK))
Poly, (pp
(K»
a
at 1.3
-a
a>
s
s»
•
1 •>
^^>>
1.1
i
i
i
2
3
4
i
i
5
-1
Moisture Content (kg kgdb )
Figure 6-2. True Density (kg L" ) Derived from Krokida (pp (K)) and Modifications to Krokida
(pp (MK)) at Various Moisture Contents (kg kgdb"') for (Krokida, 2001)
Table 6-2. Moisture Content (kg kgdb"1 and kg kgwb_1) (n = 15) and Different True Density (kg L"
') Values Based on Krokida (pp (K)), modifications from Krokida (pp (MK)), and mathematically
calculated (pp(MC)) as Equation 10
Sample
MVD
HAD
FD
:
1
MC(kgkg db - )
0.77±0.007c
0.049±0.004b
0.025±0.006a
PP(K)
MC(kgkg wb -')
0.072±0.006c
0.046±0.003b
0.025±0.006a
1
(kg L" )
1.90
1.94
1.97
PP(MK)
(kg L- )
1.58
1.59
1.60
Means in a column with different superscripts are significantly different (p< 0.05).
130
1
PP(MC)
(kg L"1)
1.53
1.56
1.58
The true density obtained from mathematical calculations according to equation 10, were
plotted against moisture content (wb) with the solid density (ps) set at 1.6 kg L"1 and the density
of water (pw) as 1 kg L"1 (Figure 6-3). The true densities of MVD, HAD, and FD dehydrated
potatoes were 1.53,1.56, 1.58 kg L"1, respectively. These true density values were lower
compared to the true density obtained from y-axis intercept equation (equation 10). Krokida and
Maroulis reported the true density of dried FD foods to be 1.6 kg L"1; therefore further studies
need to be conducted to confirm the solid density of potatoes and other food materials and to
validate this true density mathematical models. The moisture content of MVD dried potatoes was
significantly higher than HAD or FD dried potatoes, and the moisture content of FD was the
lowest.
1.7
y = 0.2754X2 - 0.8673x+ 1.5954
- P 1.6
.3P
Rl = 0.9998
1.5
1.4
s 1.3
T3
2 1.2
s-
H 1.1
0.2
0.6
0.4
0.8
1.2
Moisture Content (kg kgwb" )
Figure 6-3. True Density (kg L" ) Based on Mathematical Calculation at Various Moisture
Contents (kg kgdt,"1) for Dehydrated Potatoes
The apparent density of MVD potatoes was significantly lower compared to the apparent
density of HAD potatoes (Table 6-3). However, FD potatoes had the lowest density due to the
porous structure. This finding was similar as previous finding by Lin et al. (1998), Sham et al.
(2001), and Durance et al. (2002). The porosity of MVD potatoes was twice than the porosity of
131
HAD, while the porosity of FD was higher than MVD. Krokida and Maroulis (2001) indicated
that drying methods had no influence on true density, but moisture content influenced the true
density of the products. If the moisture contents from different drying methods were significantly
different, then measuring the porosity directly would probably provide a better overall indication
of the product structure.
Table 6-3. Apparent Density (ps (kg L"1)), True Density (pp (kg L"1)), and Porosity (s) Values of
MVD, HAD, and 1FD Potatoes
Sample
MVD
HAD
FD
0
Ps(kgU 1 )
0.93±0.11b
1.53±0.11c
0.52±0.04a
PoCkgL"1)
1.53
1.56
1.58
s
0.39
0.02
0.67
Means in a column with different superscripts are significantly different (p< 0.05).
Microscopic Structure of Dried Potatoes
Figure 6-4 shows SEM images of dried potatoes from a planar cut, while Figure 6-5
shows SEM images of dried potatoes from a cross sectional cut. The SEM image of MVD
potatoes exhibit signs of cell expansion. Clary et al. (2007) indicated the puffed character of
MVD grapes were distinct from that of sun dried raisins. These SEM images demonstrate the
puffed character of MVD potatoes. The SEM image of MVD potatoes from Figure 6-5 provide
some indication of how the puffed structure of puffmess might be developing.
During drying, MVD potatoes experienced cell walls collapse similar likes HAD
potatoes. However, because of the vacuum, cell collapse could be minimized and if cell
expansion rate exceeded the rate of cell collapsing rate, air pockets could be incorporated within
the cells. Hydrated starch could form enough of a continuous matrix to aid in the retention of air
within the potato, similar to what is observed during extrusion. The SEM image of HAD
potatoes showed the dense structure from cell wall collapse. HAD potatoes cell walls were
compactly stacked on top of each other yielding a brittle structure. The SEM image of FD
132
potatoes indicated the rapture and fragility of cell walls from this treatment. The cross sectional
images of FD potatoes showed the tearing of cell walls from their base. This image of FD
potatoes confirmed the analysis of puncture test. However, these SEM images shows the
different porosity of the dried potatoes with FD potatoes were the most porous and had minimal
shrinkage, while HAD potatoes had the maximum shrinkage and the least porous.
CONCLUSION
Based on Krokida and Maroulis (2001) worked mathematical model was derived to
measure true density based on moisture content and a known solid density. MVD potatoes
significantly had the highest moisture from all the drying treatments, while FD potatoes had the
least moisture. MVD potatoes had the smallest true density, while FD potatoes had the highest
true density. However, the porosity of MVD potatoes was twice than HAD potatoes, while FD
potatoes had the highest porosity. It required more force to puncture MVD potatoes compared to
HAD and FD potatoes. HAD potatoes needs more work to puncture, while FD potatoes required
the least work and force to puncture. From the puncture and SEM images, it indicated that MVD
produced crispy potatoes, HAD produced hard and brittle potatoes, while FD produced spongy
and soft potatoes. MVD potatoes retained more color following drying process compared to
HAD and FD, while FD potatoes loss some color properties.
133
-1^
Figure 6-4. SEM (70X) Images of Fresh and Dried MVD,
HAD, and FD Potatoes
Figure 6-5. Cross sectional SEM (70X) Images of Fresh and
Dried MVD, HAD, and FD Potatoes
ACKNOWLEDGEMENT
This research was supported by a USDA National Need Graduate Fellowship, Balcom and
Moe, Inc., Pasco, WA, and WSU Agriculture Research Center. We also would like to thank
Joseph Powers and Valerie Lynch-Holm, specialist at WSU Franceschi Microscopy and Imaging
Center, for support and advice.
References:
AOAC. 2000. Official Methods of Analysis, 17th ed. Washington, DC: Association of Official
Analytical Chemist, 37.1.10. 2(37): 4.
Clary, C. D., Wang, S. and Petrucci, V. E. 2005. Fixed and incremental of microwave power
application on drying grapes under vacuum. Journal of Food Science. 70(5): 344-349.
Clary, C. D., Mejia-Mesa, E., Wang, S. and Petrucci, V. E. 2007. Improving grape quality
using microwave vacuum drying associated with temperature control. Journal of
Food Science. 72(1): E23-E28.
Drouzas, A. E., Tsami, E. and Saravacos, G. D. 1999. Microwave/vacuum drying of model fruit
gels. Journal of Food Engineering. 39: 117-122.
Durance, T. D. and Wang, J. H. 2002. Energy consumption, density, and rehydration rate of
vacuum microwave- and hot-air convection-dehydrated tomatoes. Journal of Food
Science. 67(6): 2212-2216.
Giri, S. K. and Prasad, S. 2007. Drying kinetics and rehydration characteristics of microwavevacuum and convective hot-air dried mushrooms. Journal of Food Engineering. 78: 512521.
Kim, S. S. and Bhowmik, S. R. 1997. Thermophysical properties of plain yogurt as functions
of'moisture content. Journal ofFood Engineering. 32: 109-124.
Krokida, M. K. and Maroulis, Z. B. 2001. Structural properties of dehydrated
products during rehydration. International Journal of Food Science and
Technology. 36:529-538.
Krulis, M., Kuhnert, S., Leiker, M. and Rohm, H. 2005. Influence of energy input and initial
moisture on physical properties of microwave-vacuum dried strawberries. European
Food Research Technology. 221: 803-808.
Lin, T. M., Durance, T. D. and Seaman, C. H. 1998. Characterization of vacuum microwave,
air and freeze dried carrot slices. Food Research International. 31(2): 111-117.
135
Setiady, D., Clary, C , Younce, F. and Rasco, B.A. 2007. Optimizing drying conditions for
microwave-vacuum (MIVAC ) drying of Russet potatoes (Solarium tuberosum).
Drying Technology. 25(9):1483-1489.
Sham, P. W. Y., Seaman, C. H. and Durance, T. D. 2001. Texture of vacuum microwave
dehydrated apple chips as affected by calcium pretreatment, vacuum level, and
apple variety. Journal of Food Science. 66(9): 1341-1347.
Yousif, A. N., Seaman, C. H., Durance, T. D. and Girard, B. 1999. Flavor volatiles and physical
properties of vacuum-microwave- and air-dried sweet basil (Ocimum basilicum L.).
Journal of Agricultural and Food Chemistry. 47: 4777-4781.
136
CHAPTER 7. DIFFERENT SCANNING ELECTRON MICROSCOPY
(SEM) FIXATION METHODS FOR REHYDRATED POTATOES
Introduction
Scanning Electron Microscopy (SEM) has been widely used to observe the microscopic
structure of different specimens ranging from the cells of plants, animals, and microorganisms.
Before a sample can be observed under conventional SEM, it needs to be prepared in a way that
can maintain as much of the original tissue structure as possible. Different steps of sample
preparation may be required including: chemical fixation, cryofixation, dehydration, embedding,
staining, etc. As the technology improved and the advancement of microwave heating and
dehydration protocols for SEM fixation was developed, new fixation methods which are safety
and faster have become possible.
The objective of this study was to determine the effectiveness of different fixation
methods: standard, microwave osmium, and microwave, on the preparation of rehydrated
potatoes for SEM examination. The rehydrated potatoes were made from potatoes dried using
microwave vacuum drying (MVD), heated air drying (HAD), and freeze drying (FD). Sample
preparation for rehydrated potatoes involved similar steps as fresh potatoes, including sectioning,
fixation, dehydration, and gold coating. In this experiment, microwave heating was used to assist
with the fixation and dehydration steps. Using microwaves, the sample preparation time was
shortened by as much as twenty-fold.
Materials and Methods
Preparation of Dehydrated Potatoes
Russet potatoes (Solanum tuberosum) were prepared and dried according to Chapter 6
under Materials and Methods of Drying Preparation.
137
Sample Preparation before Fixation
Five g dried MVD, HAD, and FD potato were placed in a beaker and reconstituted by
adding 100 ml boiling water. The beaker was placed in a 70°C water bath for 2 min. The excess
water was removed and then the rehydrated potato slices were patted with a paper towel to
remove excess moisture. The potatoes pieces were placed into glass Petri plate filled with ca 10
ml 2%glutaraldehyde, 2% paraformaldehyde in 50mM phosphate buffer. Each potato slice was
cut perpendicular to make small cubes.
Fixation Methods
1. Standard (Std)
The potato cubes from each treatment were placed into a 25 ml scintillation vial
overnight at 4°C that had been labeled and filled with calO ml 2%glutaraldehyde, 2%
paraformaldehyde in 50mM phosphate buffer. After fixation, the buffer was removed from vial
and the samples were washed by adding ca 10 ml 50 mM phosphate buffer to remove the fixative
for 5 min. This washing step was repeated one additional time. The samples were post-fixed in
2% osmium tetroxide in 50mM phosphate buffer for 1 hour. Then the samples were dehydrated
in a series of ethanol solutions containing 30% (IX), 50% (IX), 60 % (IX), 70% (2X), 80%
(2X), 90% (2X), and 100% (2X) for 15 min for each solution for the number of replications
listed. The samples were then washed with 100% ethanol, and dried further in a critical point
dryer (Samdri-PVT-3B, Tousimis Research Co., Rockville, MD). The sample pieces were then
mounted on aluminum stubs and gold coated using sputter coater (Technics Hummer V
(Anatech), San Jose, CA).
138
2. Microwave Osmium (MO) Fixation
A microwave fixation device, Pelco 3450 Laboratory Microwave Processor (Ted Pella
Inc., Redding, CA), was used. Two-500 ml beakers were filled with 400 ml distilled water and
placed in the microwave processor. The pump tubes were placed in one beaker and air hose was
placed in the other beaker. The air pump was turned on and the microwave temperature set to
32°C. A neon bulb based array sensor was used to check for hot spot before samples were placed
into the processor for fixation.
The potato cubes from each treatment were placed in 25 ml scintillation vials that were
labeled, and fixed with 2%glutaraldehyde, 2% paraformaldehyde in 50mM phosphate buffer. A
temperature probe was placed into a vial containing distilled water only (a dummy vial) and held
on ice until ready to use. The sample vials and dummy vial were placed into a container filled
with ice. Then, the sample container was placed into the microwave processor and operated for
2.5 min at 100% power. The sample container was removed from microwave processor and the
buffer was replaced with 50 mM phosphate buffer as a washing step for 5 min to remove the
fixative. The washing step was repeated one more time before second fixation (post-fixation
step) was conducted. The samples were post-fixed in 2% osmium tetroxide in phosphate buffer
for 1 hour at room temperature in a fume hood. The samples were dehydrated in a series of
ethanol solutions containing 30%, 50%, 60% (2X), 70% (2X), 80% (2X), 90% (2X), and 100%
(2X) using the microwave processor for 40 sec at 100% power. Note: it was important to keep
the samples cold by placing the melted ice water with ice around the sample container. The
samples were washed with 100% ethanol, and then dried further in a critical point dryer as
described above. The sample pieces were then mounted on aluminum stubs and gold coated.
139
3. Microwave (MW) Fixation
The microwave fixation method was similar to the microwave osmium (MO) method
described above except that the post-fixation step using 2% osmium tetroxide in phosphate
buffer was omitted. After the second washing following the fixation step, the samples were
directly dehydrated in a serial ethanol solution.
All samples from different fixation methods were examined and photographed in a SEM
Hitachi S-570 (Hitachi Ltd., Tokyo, Japan) using an accelerating voltage of 20 KV. Micrographs
were taken at a magnification of 70X for longitudinal sections and 200X for transverse.
Results and Discussion
Figures 7-1 through 7-6 showed the SEM images of rehydrated potatoes from different
drying methods and using different fixation methods. Different fixation methods seemed to have
a minor effect on the preservation of textural structure of the original samples. Each of these
fixation methods could be used for rehydrated potatoes dried using different drying methods. The
standard methods were more time consuming, and labor intensive than microwave and
microwave osmium methods, and more hazardous than microwave methods. Osmium tetroxide
used for post-fixation is a toxic chemical and can be fatal at low doses. Omitting the osmium
tetroxide step saved an hour of sample preparation time. Despite the increased sample
throughput using a microwave processor, microwave assisted fixation methods would not be the
best methods to use if the sample is very heat sensitive.
Conclusion
Microwave methods can reduce the time and labor involved in preparing biological
samples for SEM evaluation. These methods can also be safer if toxic chemicals such as osmium
140
tetroxide can be omitted. It does no appear that at least for rehydrated potatoes such as those here
that the microwave fixation method results in any loss in physical structure or sample integrity.
Acknowledgement
This research was supported by a USDA National Need Graduate Fellowship and WSU
Agriculture Research Center. We also would like to thank Valerie Lynch-Holm for support and
advice.
References:
Clary, C. D., Wang, S. and Petrucci, V. E. 2005. Fixed and incremental of microwave power
application on drying grapes under vacuum. Journal of Food Science. 70(5): 344-349.
Setiady, D., Clary, C , Younce, F. and Rasco, B.A. 2007. Optimizing drying conditions for
microwave-vacuum (MIVAC®) drying of Russet potatoes (Solanum tuberosum).
Drying Technology. 25(9): 1483-1489.
141
Figure 7-1. SEM (70X) Images of Rehydrated Potatoes Made from MVD Potatoes Fixed Using
Different Methods. Std: Standard Method; MO: Microwave Osmium Method; MW: Microwave
Method
Figure 7-2. SEM (70X) Images from Cross Sectional Cuts of Rehydrated Potatoes Made from
MVD Potatoes Fixed Using Different Methods. Std: Standard Method; MO: Microwave Osmium
Method; MW: Microwave Method
142
Figure 7-3. SEM (70X) Images of Rehydrated Potatoes Made from HAD Potatoes Fixed Using
Different Methods. Std: Standard Method; MO: Microwave Osmium Method; MW: Microwave
Method
Figure 7-4. SEM (70X) Images from Cross Sectional Cuts of Rehydrated Potatoes Made from
HAD Potatoes Fixed Using Different Methods. Std: Standard Method; MO: Microwave Osmium
Method; MW: Microwave Method
143
•/H•'% ^%,X>'
\ , •
?
*' • "
.*"
;;
Figure 7-5. SEM (70X) Images of Rehydrated Potatoes Made from FD Potatoes Fixed Using
Different Methods. Std: Standard Method; MO: Microwave Osmium Method; MW: Microwave
Method
<) MW • i
6
r
ftT
Figure 7-6. SEM (70X) Images from Cross Sectional Cuts of Rehydrated Potatoes Made from
FD Potatoes Fixed Using Different Methods. Std: Standard Method; MO: Microwave Osmium
Method; MW: Microwave Method
144
CHAPTER 8. REHYDRATION AND SENSORY PROPERTIES OF
DEHYDRATED RUSSET POTATOES USING MICROWAVE VACUUM,
HEATED AIR OR FREEZE DEHYDRATION
Dewi Setiady, Carter Clary, Barbara A. Rasco
Authors Setiady, Younce, and Rasco are with the Dept. of Food Science and Human Nutrition,
Washington State Univ., Pullman, WA 99164-6376. Author Clary is with the Dept. of
Horticulture, Washington State Univ., Pullman, WA 99164-6376. Direct inquiries to author
Clary.
ABSTRACT
The quality of the rehydrated, and cooked Russet potatoes dehydrated by microwavevacuum drying (MVD) in a continuous temperature control system, heated air drying (HAD), or
freeze drying (FD) were compared by measuring instrumental color, texture, rehydration
properties, and sensory quality. In addition, SEM images were taken to evaluate the degree of
cell damage resulting from the 3 drying treatments. Both MVD and HAD had similar rehydration
properties. FD potatoes tended to loose textural integrity following rehydration. The MVD
potatoes remained intact following rehydration and retained suitable textural and color properties
as shown by consumer sensory tests, while the FD potatoes lost textural and color properties.
Sensory panelists preferred the MVD, FD and HAD in that order. From this study, it appears that
MVD can provide consumers with a dehydrated product that has characteristics similar or better
than dehydrated products prepared by HAD or FD.
145
INTRODUCTION
In the last decade, there has been a change in potato consumption away from fried to
other types of preparation due to health concerns with fried products. These market trends were
anticipated by the potato industry by offering consumers different variety of processed potatoes,
such as frozen, refrigerated/ready-to-heat and dehydrated products. Dried potatoes are an
important convenience food in both retail and food service markets with instant flaked mashed
potatoes being used in large volumes and sliced or diced dehydrated products used as an
ingredient in soups and stews and cooked potato products like boxed scalloped potatoes.
However, there is still a need to improve the technology for drying potatoes that can provide a
better quality product economically. Heated air drying (HAD) is the most common type of potato
dehydration. However, the relatively long processing time, high loss of the nutrients, browning,
poor textural and sensory of the rehydrated products limit the utility of heated air dried potatoes.
Freeze-drying (FD) is a costly dehydration process, even though it can produce higher quality
dried products.
In the past 3 decades, research has been conducted in microwave vacuum drying (MVD)
as an option to FD and HAD technology. There are different configurations of MVD, depending
on the application of microwave power and vacuum system used. The application of microwave
power contributes to the qualities of the drying process and the final dried products. Microwave
can be applied in MVD in a constant, pulsed, or incremental mode, a combination of pulse and
incremental modes, with or without continuous temperature control.
A continuous temperature control system with a MVD provides a valuable feature by
modulating the application of microwave power based upon the surface product temperature.
Monitoring product temperature limits the degree of over heating and occurrence of hot spots
146
during the drying process, preserving product quality and integrity, especially for heat-sensitive
products. Kim and Bhowmik (1994, 1995 and 1997) dried both yogurt and concentrated yogurt
using MVD with a continuous temperature control system by manually turning the microwave
power on and off. The survival of heat labile lactic acid bacteria was higher after MVD at a low
drying temperature (below 45°C) compared to higher drying temperature. Rodriguez et al. (2005)
also investigated MVD to dry mushroom pieces in a manually continuous temperature control
system and found that this configuration resulted in more efficient drying with a higher drying
rate and a drier product. In their system, drying mushrooms at a moderate temperature and power
level produced similar quality as FD mushrooms.
Clary et al. (2006) dried grapes for the production of sweet dessert wines using MVD
with a semi-automated continuous temperature control system. Sweet dessert wines are usually
made from late-harvest frozen grapes, however in this study, three different types of sweet
dessert wines were compared, those made from; MVD dried grapes, late-harvest grapes, and
fresh grapes frozen by mechanical refrigeration. For the microwave drying, the input power was
applied depending upon the set point at the beginning of drying. As the surface temperature
approached the set point temperature, the applied microwave power was reduced to maintain the
surface temperature at the set point. These three wines were similar in flavor and aroma profiles;
however, wine made from MVD grapes had less of a fruity flavor.
Clary et al. (2007) described the operational of automatic continuous control temperature
system MVD to make puffed dried grapes. MVD with this automatic system had the ability to
control the microwave power output and produced better product quality than the same dryer
operated in either a constant or incremental mode. Using the automated system, the drying
process was better defined and more energy efficiency. Drying at a moderate (71°C) temperature
147
was an optimal condition and produced grapes with a greater degree of puffing compared to
drying at higher (77°C) temperature which burned more than 20% of the grapes. This finding
was in agreement with Rodriguez et al. (2005). MVD may provide nutritional advantages for
grape dehydration. Grapes dried using this type of MVD at moderate temperature retained more
vitamins compared to sun-dried raisins.
Setiady et al. (2007) dried Russet potatoes using MVD in a continuous temperature
control MVD system modeled upon that of Clary et al. (2006 and 2007). Drying at a moderate
dryer set point temperature (60°C) was more efficient and produced better color retention
compared to a low temperature (50°C) or a high temperature (70°C). The objective of this study
was to determine the rehydration and sensory properties, including color, texture, and changes in
microscopic structure, of Russet potatoes dehydrated by MVD operated in a automatic
continuous temperature control mode and compare these materials to those produced by HAD or
FD.
MATERIALS AND METHODS
Drying Preparation
Russet potatoes (Solarium tuberosum) were prepared and dried according to Chapter 6
under Materials and Methods of Drying Preparation.
Determination of Rehydration Properties
Five g dried MVD, HAD, or FD treated potatoes were reconstituted in 100 ml 98°C water
and then held at 70°C water bath for 1, 3, 5, 10, 15, 20, 25 or 30 min, then drained through a
sieve and dried on paper towel. The samples were weighed and rehydration ratio was calculated
as:
Rehydration ratio (RR) = Mass of rehvdrated potatoes (g)
Mass of dried potatoes (g)
148
Sensory Evaluation of Cooked Potatoes
Dried potatoes (200 g) were cooked in 1 L boiling 7% salt solution for 15min for HAD
and MVD; and for 2 min with an 8 min post cook holding period for FD potatoes. Sliced,
blanched potatoes (500 g), previously described were used as a control, and were cooked in 0.7 L
boiling 5% salt solution for 15 min. After cooking, half of the cooking water was decanted off,
and the covered pans containing the cooked potatoes placed in a 70°C water bath during the
period over which the sensory panel was conducted.
Thirty panelists were asked to evaluate the potatoes in a difference from control test and
for texture attribute (0 to 10 point hedonic scales). Forced acceptability and rank preference were
also conducted along with solicitation for additional comments.
Analytical Methods
Color Analysis
Instrumental color of the dried and cooked potatoes was measured by using a Minolta
CM-2002 spectrophotometer (Minolta Camera Co., LTD, Chuo-Ku, Osaka, Japan) with an 11
mm measurement aperture. The CIE-Lab L*, a*, and b* values were recorded. L*, a*, and b*
values indicate lightness, redness(+)/(-)greenness, and yellowness(+)/(-)blueness respectively.
Total color differential (AE), difference in lightness (AL), redness (Aa*), and yellowness (*b)
were also calculated, as follow:
A.L — -Luncooked — i^cooked
Aa
—
a uncooked — & cooked
Ab* = b*uncooked - booked
AE = V C ^ + Ca *^ + C& *^
Texture Analysis
Samples (N=5) for each treatment were analyzed by Texture Profile Analysis (TPA)
using the TA-XT2 Texture Analyzer (Stable Micro System, Haslemeres, England) within 10
149
minutes after rehydration or 2 hr after cooking. On a fiat metal plate, two sliced of rehydrated
potatoes were positioned on top each other and compressed twice to 30%. TPA parameters, such
as hardness, springiness, cohesiveness, gumminess, chewiness and resilient were attained
directly or by calculation using the equations of Bourne (1968) and Peleg (1976) from the forcetime curve of the TPA.
Scanning Electron Microscopy Examination
For rehydrated and cooked potatoes, each sample was cut perpendicular to make small
cubes. The potatoes pieces were placed into glass Petri plate filled with ca 10 ml 2%
glutaraldehyde, 2% paraformaldehyde in 50 raM phosphate buffer. A microwave fixation device,
Pelco 3450 Laboratory Microwave Processor (Ted Pella Inc., Redding, CA), was used as
previously described in Chapter 7).
Statistical Analysis
Data were analyzed with a computer software package, SAS 9.1 (System for Windows
2002-2003, Cary, NC), using analysis of variance and Fisher's least significant difference (LSD)
procedure. Significance was determined at P<0.05. All determinations were made at least in
duplicate and all were averaged.
RESULTS AND DISCUSSION
The Rehydration Properties of Potatoes
Figure 8-1 shows the rehydration properties of MVD, HAD, and FD potatoes at 60°C.
The optimum rehydration time for FD potatoes was 10 min which was less half than that of
MVD and HAD of 25 min. The optimum rehydration ratio (RR) of MVD at 25 min was 3.93.
The RR of MVD at 30 min decreased to 3.81. The optimum RR of HAD potatoes were 3.82 at
25 min and 3.89 at 30 min. The water holding capacity of the FD potatoes was almost twice that
150
for HAD and MVD potatoes, while MVD and HAD had similar water holding capacity. These
results suggest that MVD and HAD had similar rehydration properties.
Tk-MVD
-+-HAD
-*-FD
5
10
15
20
25
30
Time (min)
Figure 8-1. Rehydration Ratio of Dried Potatoes Dried Using MVD, HAD, and FD at Various
Rehydration Time (n = 3)
Table 8-1 gives texture profile analysis (TPA) results for rehydrated MVD, HAD, and FD
potatoes. TPA for both MVD and HAD potatoes was performed for 10 min and 25 min
(optimum) rehydration times. Increasing the rehydration time of MVD potatoes to the optimum
rehydration time from 10 min significantly increased hardness, gumminess, and chewiness, while
it significantly decreased springiness. Increasing the rehydration time of HAD potatoes had the
same effect, except that there was also a significant increase in resilience. At 10 min rehydration
time, MVD potatoes were significantly harder, less springy, cohesive and resilient compared to
HAD potatoes. At the optimum rehydration time, MVD potatoes were significantly softer, less
springy, gummy, chewy, and resilient compared to HAD potatoes. At the optimum rehydration
time, FD potatoes were significantly softer, less gummy, chewy, cohesive, and resilient
compared to MVD and HAD potatoes. FD potatoes tended to loose textural integrity, fragile and
151
easily tear apart following rehydration. Rehydrated MVD and HAD potatoes remained intact
following rehydration.
Figure 8-2 shows SEM images of MVD, HAD, and FD potatoes rehydrated at their
respective optimal times. The uneven surface appearance of MVD rehydrated potatoes occurred
due to the puffed structure of the dried potatoes. HAD potatoes appeared to have a dense form
from surface observation. Figure 8-3 shows cross sectional SEM images of the rehydrated
potatoes at their optimal rehydration times. These cross sectional SEM images indicated that
MVD potatoes might have a structure that retain more water and have a higher volume increase
compared to HAD potatoes, even though MVD potatoes had similar rehydration properties as
HAD potatoes. These images confirm a denser structure of rehydrated HAD from SEM images
as observed in Figure 8-3. HAD potatoes were not fully rehydrated and might not have the
ability to fully rehydrate due to case hardening. Longer rehydration time than 25 min (optimum)
for MVD and HAD resulted in a loss of structural integrity similar to that observed for FD. The
SEM images showed that FD potatoes readily absorbed water but also had a fragile structure.
From the cross sectional images, the cell walls were not intact and this could contribute to their
fragile nature.
152
HAD, and FD at 10 or 20 min (n = 15)
Ch
Rs
a
0.584±0.065a
38.0±9.7
47.7±9.7bB
0.543±0.050aB
0.714±0.084c
33.5±15.3a
59.8±10.3cC
0.667±0.047bC
0.224±0.055A
19.2±5.7A
" Means in a column with different superscripts are significantly different comparing for different rehydration time on MVD and HAD dried potatoes (p <
0.05).
A c
" Means in a column with different superscripts are significantly different for each optimum rehydration properties among MVD (25 min), HAD (25 min) and
FD (lOmin) dried potatoes (P < 0.05).
1
Hd, hardness; Sp, springiness; Co, cohesiveness; Gu, gumminess; Ch, chewiness; Rs, resilience.
a c
Table 8-1. Texture Profile Analysis (TPA) factors1 for Rehydrated Potatoes Dried Using MVP,
Gu
Hd
Sp
Co
Sample
Time
b
b
a
MVD
10
60.6±11.5
0.810±0.071
0.769±0.049
46.9±10.8a
25
82.3±14.4cB
0.773±0.066aA
0.749±0.036aB
61.7±11.8bB
HAD
10
45.5±18.6a
0.891±0.073c
0.817±0.065b
37.6±16.6a
88.6±13.4cB
25
0.824±0.064bcB
0.819±0.036bB
72.8±12.3cC
FD
10
0.805±0.064A
46.7±9.5A
0.511±0.081A
23.9±6.5A
Figure 8-2. SEM (70X) Images of Rehydrated Potatoes Following MVD, HAD, and FD
Figure 8-3. Cross Sectional SEM (70X) Images of Rehydrated Potatoes Following MVD, HAD,
andFD
Sensory Evaluation and Properties of Cooked Potatoes
The results from the preference and acceptability test showed that there were no
significant differences among the three types of dehydrated potatoes after cooking and the
control (blanched cooked) in terms of preference; however, there was a tendency for the
panelists preferred the control over all of the dehydrated products (Table 8-2). For the
dehydrated potatoes, cooked MVD potatoes received a slightly higher preference rating, while
HAD received the lowest preference score. Five out of 30 panelists (0.83) found the control and
MVD potatoes to be unacceptable, while only 10 out of 30 panelists (0.67) found the FD and
HAD products to be unacceptable.
154
Table 8-2. Preference and Acceptance Sensory Test Results of Control, MVD, FD, and HAD
Potatoes (n =30)
Test
Control
MVD
FD
HAD
2.5±1.0a
2.5±1.1
2.2±1.3
Preference
2.7±1.2
25/30
25/30
20/30
20/20
Acceptability
a c
" Means in a column with different superscripts are significantly different (p < 0.05).
a
a
a
Sensory difference testing for overall difference, appearance, aroma and flavor of the
cooked rehydrated potatoes were compared to the control cooked fresh potatoes (Table 8-3).
HAD potatoes were perceived as having the greatest overall difference from the control, while
MVD potatoes were the least different. HAD potatoes were described as being more different
from the control cooked potatoes in appearance and aroma compared to MVD and FD, while FD
potatoes were perceived as having the least difference from control in appearance, flavor, and
aroma.
Table 8-3. Different Testing Results (from control) of MVD, HAD, and FD Potatoes (n =30)
Test
MVD
HAD
FD
a
a
Overall difference
4.3±2.6
5.3±2.4
4.5±2.3a
Appearance
2.7±2.0ab
3.4±2.9b
2.3±2.5a
Fresh potatoes flavor
4.4±2.9a
3.5±2.2a
4.3±2.7a
ab
3.0±2.2
Fresh potatoes aroma
2.6±2.7a
3.5±2.6b
ac
Means in a column with different superscripts are significantly different (p < 0.05).
Panelists indicated that the cooked control (made from fresh blanched potatoes) and the
cooked dehydrated potatoes had similar mealiness (Table 8-4). FD had similar chewiness as
control with the chewiness of the MVD being higher and the HAD being the highest. The control
potatoes had the greatest hardness and the FD potatoes were the softest. MVD potatoes were
harder than FD and significantly softer than HAD and the control. HAD potatoes were
significantly softer than the control.
Table 8-4. Texture Attribute Sensory Test Results of MVD, HAD, and FD Potatoes (n = 30)
MVD
Test
Control
HAD
FD
Mealiness
3.2±1.7a
4.1±2.4a
3.3±2.5a
4.0+2.63
Chewiness
2.1±1.7ab
2.8+1.9b
4.4+1.9C
1.8±1.5a
C
a
b
Hardness
3.9±2.3
1.6±1.4
1.2±1.7a
2.8±2.0
'Means in a column with different superscripts ar e significantly different (p < 0.05).
155
Texture Profile Analysis (TPA) of cooked potatoes indicated that blanched potatoes had a
different texture profile than the cooked potatoes (Table 8-5). Fresh cooked potatoes (control)
were significantly firmer and less cohesive compared to the rehydrated cooked ones. These TPA
results for hardness and chewiness showed similar trends for textural attributes as the sensory
evaluation. The gumminess of the cooked potatoes was similar. FD potatoes were significantly
springier, while MVD potatoes were the least springy. HAD and control potatoes had similar
springiness. HAD and FD potatoes were more cohesive, while control potatoes were the least
cohesive. HAD potatoes were the most resilient, while control and FD were the least resilient.
Table 8-6 shows the Lab color values for uncooked (fresh blanched or dried) and cooked
potatoes. Table 8-7 showed the difference in lightness, redness, yellowness and total color
differences between cooked and uncooked MVD, HAD, and FD potatoes and were calculated
based upon the results from Table 8-6. Cooked fresh blanched potatoes (control) were
significantly less red and yellow compared to MVD, HAD, and FD, and significantly darker than
MVD and HAD. FD potatoes darkened during cooking and was the darkest among the cooked
potatoes tested. The color of HAD and MVD were similar following cooking.
SEM images from Figure 8-4 and 8-5 indicated that cooked fresh potatoes (control) had a
fuller and denser structure indicative of fully cooked starch. Cooking increased the rehydration
capability of the dried potatoes, except for FD. FD were only cooked in boiling water for 2 min
and held 8 min compared to 15 min cooking for both MVD and HAD. However, FD potatoes
had a fragile, soft structure which tore easily. SEM images showed signs of this tearing of the
cell walls. MVD potatoes seemed to retain more water than HAD potatoes and showed fewer
signs of structural damage. The cross sectional HAD image showed that the cell walls were more
156
deformed and this may have been the reason that cooked HAD had a tendency to be harder than
MVD or FD.
Uncooked
56.3±3.2aC
49.6±5.6aB
47.3±3.3aA
71.8±4.7bD
Cooked
55.0±3.2aB
57.6±2.5bC
57.5±2.7bC
52.3±3.3aA
Uncooked
-3.16±0.19bA
-1.49±0.17bC
-1.58±0.17bB
-1.34±0.19bU
a *i
Cooked
-3.32±0.17aA
-3.14±0.13aB
-2.88±0.16aC
-2.95±0.24aC
Uncooked
-2.09±1.3aA
9.85±2.0bC
8.37±2.2bB
12.3±1.8bD
b* I
Sample
Control
MVD
HAD
FD
'L*, lightness; a*,
AL*1
Aa*1
Ab*1
AE
-0.17
-1.23
1.69
2.09
-1.65
8.02
-8.53
11.8
-1.30
-7.88
12.9
10.2
-19.4
-1.60
-12.2
23.0
redness-greenness; b*, yellowness-blueness; AE, color differential.
Table 8-7. Differential CIE-Lab1 Values Between Uncooked and Cooked Control, MVD, HAD, and FD Potatoes
A D
Means in a row with different superscripts are significantly different (p < 0.05).
" Means in a column with different superscripts are significantly different {p < 0.05).
'L*, lightness; a*, redness-greenness; b*, yellowness-blueness.
Sample
Control
MVD
HAD
FD
L*1
Table 8-6. CIE-Lab Values of Uncooked and Cooked Control, MVD, HAD, and FD Potatoes
Cooked
-0.405±1.5bA
1.32±1.5aC
0.496±1.6aB
0.168±1.6aAB
of Cooked Control, MVD, HAD, and FD Potatoes
Co1
Gu1
Ch1
Rs1
0.679±0.065d
162±31.6b
127±25.5b
0.520±0.056d
0.102±0.027a
6.10±2.6a
3.87±1.7a
0.0418±0.021a
a
3.64±l.l
0.185±0.033b
6.06±1.7a
0.0757±0.022b
a
a
5.70±2.4
8.92±3.3
0.102±0.040c
0.223±0.060°
2.16±0.54a
0.236±0.034c
2.89±0.63a
0.0409±0.009a
" Means in a column with different superscripts are significantly different (p < 0.05).
'Hd, hardness; Sp, springiness; Co, cohesiveness; Gu, gumminess; Ch, chewiness; Rs, resilience
a c
Table 8-5. Texture Profile Analysis (TPA) Factors
Sample
HdCN)1
SP1
d
Blanched
237±28.2
0.785±0.043d
Control
60.8±21.7C
0.638±0.068b
MVD
32.6±5.6b
0.597±0.065a
b
HAD
39.5±7.1
0.631±0.063ab
FD
12.5±3.0a
0.742±0.048c
'^•'i^..A
2^
'-
-
Figure 8-4. SEM (70X) Images of Control, MVD, HAD, and
FD Potatoes
0.43 mm
- .*"' v v.
MVD iWffi3M$tm*&>\
0.43 k m
vV.
•
" *v/ 4v,
./"•
0.43 mm
"*?*> s >
Figure 8-5. Cross Sectional SEM (70X) Images of Control,
MVD, HAD, and FD Potatoes
V
v
CONCLUSION
The MVD potatoes remained relatively intact following rehydration and cooking and
retained suitable textural, sensory and color properties. FD potatoes were soft and tended to fall
apart during rehydration and darkened the most during cooking. Dried FD potatoes were
significantly lighter than HAD or MVD. HAD potatoes browned to a greater extent during
dehydration compared to FD and MVD potatoes and cooked HAD were chewier compared to FD
and MVD. FD potatoes had faster rehydration time and higher (almost twice) the water holding
capacity compared to HAD and MVD, while MVD and HAD had similar rehydration properties.
At the optimum rehydration time, FD potatoes were significantly softer, less cohesive, gummy,
chewy, and resilient compared to HAD and MVD dried potatoes.
Panelists exhibited a slight preference for the control, then the MVD, FD and HAD, in
that order. TPA results for chewiness and hardness of cooked potatoes were consistent with
sensory results. The MVD cooked texture and color profiles were similar as HAD. From this
study, it appears that MVD can provide consumers with a dehydrated product that has sensory
characteristics similar to and possibly better than dehydrated products prepared by HAD or FD.
ACKNOWLEDGEMENT
This research was supported by a USDA National Need Graduate Fellowship, Balcom and
Moe, Inc., Pasco, WA, and WSU Agriculture Research Center. We also would like to thank
Valerie Lynch-Holm, the specialist at WSU Franceschi Microscopy and Imaging Center, for
support and advice.
References:
Clary, C. D., Gamache, A., Cliff, M., Fellman, J. and Edwards, C. 2006. Flavor and aroma
attributes of Riesling wines produced by freeze concentration and microwave vacuum
dehydration. Journal of Food Processing and Preservation. 30: 393-406.
160
Clary, C. D., Mejia-Mesa, E., Wang, S. and Petrucci, V. E. 2007. Improving grape quality
using microwave vacuum drying associated with temperature control. Journal of
Food Science. 72(1): E23-E28.
Kim, S. S. and Bhowmik, S. R. 1994. Moisture sorption isotherms of concentrated yogurt and
microwave vacuum dried yogurt powder. Journal of Food Engineering. 21: 157-175.
Kim, S. S. and Bhowmik, S. R. 1995. Effective moisture diffusivity of plain yogurt undergoing
microwave vacuum drying. Journal of Food Engineering. 24:137-148.
Kim, S. S., Shin, S. G., Chang, K. S., Kim, S. Y., Noh, B. S. and Bhowmik, S. R. 1997. Survival
of lactic acid bacteria during microwave vacuum-drying of plain yoghurt. LebensmittelWissenschaft+Technologie. Food Science & Technology. 30(6): 573-577.
Kim, S. S. and Bhowmik, S. R. 1997. Thermophysical properties of plain yogurt as functions
of moisture content. Journal of Food Engineering. 32: 109-124.
Rodriguez, R., Lombrana, J. I., Kamel, M. and Elvira, C. D. 2005. Kinetic and quality study of
mushroom drying under microwave and vacuum. Drying Technology. 23: 2197-2213.
Setiady, D., Clary, C , Younce, F. and Rasco, B. A. Optimizing drying conditions for
microwave-vacuum (MIVAC ) drying of Russet potatoes (Solanum tuberosum). Drying
Technology. 25(9):1483-1489.
161
Chapter 9. Conclusions and Recommendations
This study investigated the moisture and microwave distribution during MVD
(Microwave Vacuum) drying using potato slices as a model food and also examined the quality
of the dehydrated, rehydrated, and cooked potatoes. The quality properties examined included
moisture, color, texture, porosity, rehydration properties, sensory, and microscopy structure.
Moisture distribution in the MVD drying chamber was identified by placing potato slices into
Petri plates that was numbered according to the location of the turntable.
Conclusions
•
MIVAC® operated with a continuous temperature control system resulted in a greater
degree of process control and minimized overheating.
•
Drying potatoes in a MIVAC® dryer at a moderate set point drying temperature (60°C)
for a longer drying times produced dried potatoes which were more uniform, retained
better color, with no scorching compared to MIVAC® drying at higher temperature.
•
Using a rotating turntable significantly reduced the moisture variability in the final dried
product throughout a single batch and decreased the occurrence of scorching.
•
The turntable could be divided into 3 moisture distribution pattern regions; concentric
rings in which the outer and center rings had a more uniform moisture distribution and
more uniform drying.
•
Using a static turntable, hot spots were common and seemed to concentrate in the middle
regions of the turntable.
•
Dried MVD potatoes were crispy, dried HAD potatoes were hard and brittle, while FD
potatoes were soft and spongy.
•
Dried MVD potatoes had the greatest color retention, while dried FD potatoes lost color.
162
Mathematical equation for predicting true density (pp) based on moisture content (wb)
(MCWb) of the products using a known solid density (ps) and water density (pw) of 1 kgL"
'.as:
P>=
~f
l~MCwb
V
N
Pw J
MVD had a greater porosity compared to HAD, while FD potatoes had the highest
porosity. SEM images confirmed this observation for porosity.
MVD potatoes had similar rehydration properties (time and water absorbed) as HAD
potatoes, while FD potatoes had the highest rehydration rate. SEM images indicated that
MVD retained more of the original cellular structure following rehydration and had a
slightly higher water retention compared to HAD. FD potatoes lost structural integrity
following rehydration and had a tendency to fall apart.
Sensory analysis indicated there may have been a tendency for the panelists to prefer the
cooked MVD potatoes over the HAD or FD potatoes. Panelists indicated that cooked
HAD potatoes were chewier and harder compared to MVD and FD. MVD potatoes were
perceived chewier and harder compared to FD.
Cooked MVD potatoes had similar color profile to HAD potatoes, while cooked FD
potatoes were darker and had the greatest overall color difference.
163
Recommendations
From the literature review, there is another MVD configuration that has not been
examined completely, a combination of pulsed drying with continuous temperature control.
During power off time in a pulsed MVD system, vacuum aids with water transport and helps to
maintain the structural integrity of the product, while applying constant microwave at the same
or different power levels will induce rapid heating and although water mass transport may be
high, there is also a greater risk of collapse of the cellular structure. By monitoring product
temperature and modulating microwave power in a pulsed system, it may be possible to
eliminate scorching that can occur during the last drying period. Another benefit of a pulsed
system is that drying may be more efficient, since during the power off period, heat and mass
transfer still occurs. However, drying in a pulsed system is usually longer than under constant
microwave application.
Moisture and microwave distribution in a combination of pulsed and continuous
temperature control system will probably be different than the current continuous temperature
control system; therefore, assessing the moisture and microwave distribution will help to
determine the relative effectiveness of drying in a system with this configuration and set of
operating conditions.
From previous experiments, drying denser fruits or vegetables, such as potatoes, carrots,
pineapple, apple, etc., using MVD resulted in less puffing and more shrinkage compared to
drying less dense fruits and vegetables. The effect of physical properties on MVD of different
types of fruits and vegetables should be examined further. Shrinkage, porosity, color, texture,
and microscopic analysis should be evaluated as a part of the study of these physical properties.
Sensory properties of different dried fruits or vegetables that can be consumed in a dried or
164
reconstituted form without heat should be evaluated, since some problems surrounding the
sensory analysis of cooked potatoes in this experiment, such as keeping the product temperature
and maintaining the moisture and texture during storage after cooking could be reduced.
Potatoes are a good source of antioxidants and are rich in vitamin C. However, drying
might decrease the retention of these beneficial nutrients. Further studies should be conducted to
evaluate the retention of antioxidants and vitamin C in dried MVD, HAD, or FD potatoes will be
essential to determine whether some drying treatments would lead to greater retention of
antioxidants than others. MVD potatoes seemed to have superior sensory and color qualities
compared to HAD and FD products at least for some applications. MVD seemed to retain higher
flavor compounds compared to FD, therefore the values of volatile compounds that are
responsible for MVD potato flavor and color should be identified and compared to the ones in
HAD and FD.
Mathematical models for predicting porosity and density used in this research seemed to
be effective to measure true density of potatoes based on moisture content (wb). However,
further studied needs to be conducted to validate this mathematical model for true density with
experimental values using different variety dried and rehydrated fruits and vegetables. There is
also a need to identify the solid density of different food materials at a MC of 0 kg/kgwb. Solid
density can be evaluated by drying dried MVD, FD, or HAD sample to remove the moisture and
then volume can be measured by using a pycnometer method to measure true volume.
165
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