close

Вход

Забыли?

вход по аккаунту

?

Development of a model system to evaluate ascorbic acid destruction at various time/microwave power levels

код для вставкиСкачать
INFORMATION TO USERS
This manuscript has been reproduced from the microfilm master. UMI films
the text directly from the original or copy submitted. Thus, some thesis and
dissertation copies are in typewriter face, while others may be from any type of
computer printer.
The quality of th is reproduction is d ep en d en t upon th e quality of the
copy subm itted. Broken or indistinct print, colored or poor quality illustrations
and photographs, print bleedthrough, substandard margins, and improper
alignment can adversely affect reproduction.
In the unlikely event that the author did not send UMI a complete manuscript
and there are missing pages, these will be noted.
Also, if unauthorized
copyright material had to be removed, a note will indicate the deletion.
Oversize materials (e.g., maps, drawings, charts) are reproduced by
sectioning the original, beginning at the upper left-hand comer and continuing
from left to right in equal sections with small overlaps.
Photographs included in the original manuscript have been reproduced
xerographically in this copy.
Higher quality 6” x 9” black and white
photographic prints are available for any photographs or illustrations appearing
in this copy for an additional charge. Contact UMI directly to order.
ProQuest Information and Learning
300 North Zeeb Road, Ann Arbor, Ml 48106-1346 USA
800-521-0600
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
DEVELOPMENT OF A MODEL SYSTEM TO EVALUATE ASCORBIC ACID
DESTRUCTION AT VARIOUS TIME/MICROWAVE POWER LEVELS
BY
SHAHNAZ BEGUM
B.Sc.(Hons)., N. W. F. P. Agricultural University o f Peshawar, 1976
M.Sc. (Hons)., N.W.F.P. Agricultural University at Peshawar, 1987
Masters. Ex. Ed., University o f Illinois at Urbana-Champaign, 1993
THESIS
Submitted in partial fulfillment o f the requirements
for the degree o f Doctor o f Philosophy in Food Science and Human Nutrition
in the Graduate College o f the
University o f Illinois at Urbana-Champaign, 2001
Urbana, Illinois
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
UMI Number: 3017021
<8)
UMI
UMI Microform 3017021
Copyright 2001 by Bell & Howell Information and Learning Company.
All rights reserved. This microform edition is protected against
unauthorized copying under Title 17, United States Code.
Bell & Howell Information and Learning Company
300 North Zeeb Road
P.O. Box 1346
Ann Arbor, Ml 48106-1346
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
© Copyright by Shahnaz Begum, 2001
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
UNIVERSITY OF ILLINOIS AT URBANA-CHAMPAIGN
THE GRADUATE COLLEGE
JANUARY 12, 2001
(date)
W E H EREBY RECOMM END T H A T T H E T H ESIS BY
_____________ SHAHNAZ BEGUM___________________________
fm t Tt [
Fn
Development of A Model System to Evaluate Ascorbic Acid
Destruction at Various Time/Power Levels
BE ACCEPTED IN PARTIAL F U L F IL L M E N T OF T H E R EQ UIREM ENTS FOR
th e
d e g r e e o f _______ DQCTQB-QE
EHILQS.QP.HY__________________
Director of T hesis Research
Head of D epartm ent
Committ€e on Final Examinafcioi
Chairperson
t Required for doctor’s degree but not for m aster’s.
0 -3 1 7
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
ACKNOWLEDGEMENTS
I bow to my God for granting me the strength, determination, patience and
perseverance to simply achieve the goal that was set few years ago. This wouldn’t have
done so well, if his (Allah Karim) blessings were not with me.
I would like to pay a very special thank Dr. Susan Brewer, my advisor and
professor, for her continued kindness, support encouragement and friendship throughout
the past years o f my graduate study. I admire her for being so informal and ready to share
knowledge with students beside regular graduate work. I wouldn’t be able to achieve this
goal if she wouldn’t be my advisor. I never needed to make an appointment or felt any
hesitation to discuss the problem arose at any time.
The author would also like to thank my committee members Drs. Karl
Weingartner, Graciela Padua and Robert Reber for serving on my committee, taking the
time to review my work and helping me complete my dissertation. Author would like to
pay a very special thanks to Dr. Karl Weingartner, for his being a good friend and
providing an excellent insight and advice along the way.
The author would like to thank Dr. Mobesa Kushad, and Dr. Dianna Lu (Plant
Sciences) for providing raw material (broccoli) for this project.
To Charlette Prestat, the author expresses her sincere gratitude for her assistance
during statistical analysis for this study.
Thank you, is also extended to her graduate fellows, without their help, her
graduate school experience wouldn’t have been as meaningful.
iii
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
The author expresses her sincere gratitude to her friends Dr. Waheeda Mani
Tehseen at EPA (Federal Office, Virginia) her husband Major Tehseen Farooq Chohan
(Rtd.) and Dr. Singh at EPA (Washington, DC) for their valuable moral support.
Special thanks is extended to her brother Ashfaq Ahmad and his wife Karen
Ashfaq for their financial support to finish her work and raise a family o f five.
Finally, the author would like to express her sincere gratefulness to her children,
Mudassar Bashir, Sadia Bashir, Ziad Bashir and Haider Bashir. Their support,
encouragement, love and great patience have provided her the strength and determination
to attain this goal and strive to the others. Without their being supportive and caring, this
project wouldn’t have been completed.
The author is indebted to her husband, Dr. Muhammad Bashir, who provided her
opportunity to join University o f Illinois and achieve this goal. If he had not invited her to
join him in USA during his doctoral studies, their dream for her to earn a doctor’s degree
would never have come true.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
DEDICATION
This thesis is dedicated to my parents, deceased Ghulam Ahmad and Siad Begum who
are looking down on me today. They had wished for me to earn the higher degree from
foreign school.
They sparked my interest in and appreciation for higher education in science, instilled in
me the meaning and value o f hard work, patience, determination and obedience to All
Mighty God, and taught me to believe in my own abilities and continuous faith in God.
To my children, Mudassar Bashir, Sadia Bashir, Ziad Bashir and Haider Bashir who
astonished me with their patience, love and support. They have my life meaningful.
To my husband Dr. Muhammad Bashir who provided me the greatest opportunity of my
life to study at one the significant school in the world: University o f Illinois at UrbanaChampaign, USA.
To my brothers Ishtiaq Ahmad, Ashfaq Ahmad, Shaukat Ali and Liaqat Ali and sisters
Nadeema Fuad and Farrah Naz with love and appreciation for their love and support.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
TABLE OF CONTENTS
IN TR O D U C T IO N
CHAPTER 1. REVIEW OF LITERATURE
Conventional Heating vs. Microwave Heating
Ascorbic Acid in Vegetables
Factors Affecting Ascorbic Acid in Vegetables
Effects of Processing on Ascorbic Acid in Vegetables
Ascorbic Acid Oxidase in Vegetables
Peroxidase in Vegetables
Interaction o f Peroxidase and Ascorbic Acid in Vegetables
Inhibitors of Peroxidase
Color of Vegetables
Factors Affecting Color of Vegetables
1
7
7
9
13
16
17
18
19
19
19
20
CAHPTER 2. MICROWAVE AND CONVENTIONAL BLANCHING
EFFECTS ON CHEMICAL SENSORY AND COLOR
CHARACTERISTICS OF FROZEN BROCCOLI
Introduction
Objective of the Study
Materials and Methods
Results and Discussion
Conclusion
23
23
24
24
28
30
CHAPTER 3. MICROWAVE BLANCHING EFFECTS ON COLOR,
CHEMICAL AND SENSORY CHARACTERISTICS OF
FROZEN ASPARAGUS
Introduction
Objective of the Study
Materials and Methods
Results and Discussion
Implications
38
38
39
39
42
45
CHAPTER 4. DEVELOPMENT OF A MODEL SYSTEM TO EVALUATE
ASCORBIC ACID PRESERVATION AT VARIOUS TIME
AND MICROWAVE POWER LEVELS
Objective of the Studies
Objective of the Study
Materials and Methods
Experimental Design
Analyses
51
51
52
52
53
53
vi
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Statistical Analyses
Results and Discussion
55
57
CHAPTER 5. OVERALL CONCLUSION
70
CHAPTER 6. SUMMARY
71
CHAPTER 7. IMPLICATIONS AND FUTURE DIRECTIONS
74
REFERENCES
76
157
APPENDIX
APPENDIX A.
A .I.
L-ascorbic acid and L-dehydroascorbic acid
A. 2.
Relationship o f various chemical forms o f ascorbic
acid
A. 3.
Indirect oxidation o f ascorbic acid in the presence of
Peroxidase
A. 4.
Direct action o f ascorbic acid oxidase in the oxidation
of ascorbic acid
A. 5.
Titration curve o f ascorbic acid
A. 6.
Degradation o f ascorbic acid
A. 7.
Oxidation o f ascorbic acid by Fe(III)
A. 8.
Loss o f ascorbic acid in cooking varies with the
method o f preparation.
A. 9.
Chlorophyll structure
A. 10.
Pathways for chlorophyll degradation
158
158
APPENDIX B.
Major components o f microwave oven
Mechanism of microwave heating
Food .parameters
Oven parameters
168
168
169
175
176
APPENDIX C.
The general peroxidatic reaction o f peroxidase
Effect of pH on peroxidase activity
Effect of temperature on peroxidase activity
181
181
184
184
VITA
192
159
160
161
162
163
164
165
166
167
vii
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
LIST OF TABLES
Table I. 1.
Vitamin content and distribution in cooked broccoli
Table 1.2.
Reduced ascorbic acid, dehydroascorbic acid and total ascorbic
acid o f some representative foods (mg/100 g)
Table 1. 1.
Ascorbic acid content o f the edible portion o f fruits and vegetables
values are given as (m g/100 g)
Table 2. 1
Effect o f blanching method on reduced ascorbic acid (RAA)
content o f broccoli
Table 2. 2.
Reduced ascorbic acid retention o f broccoli after blanching and
after 4 weeks frozen storage and cooking
Table 2. 3.
Instrumental lightness (L* value) o f freshly blanched and cooked
broccoli florets and stems
Table 2.4.
Instrumental green color (-a* value) o f freshly blanched and
cooked broccoli florets and stems
Table 2. 5.
Instrumental yellow color (b* value) o f freshly blanched and
cooked broccoli florets and stems
Table 2.6.
Effect o f blanching treatment on sensory characteristics o f
broccoli cooked after 4 weeks in frozen storage
Table 3. 1.
Reduced ascorbic acid content and retention in fresh, blanched
and frozen asparagus
Table 3. 2.
Sensory characteristics o f frozen and cooked asparagus
Table 3. 3.
Instrumental color o f blanched and frozen asparagus stems
and florets
Table 3.4.
Instrumental color o f blanched and frozen asparagus stems
and florets
Table 3.5.
Instrumental color o f blanched and frozen asparagus stems
and florets
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 4. 1.
Effects various time and microwave power levels on moisture
content o f broccoli
Table 4. 2.
Effects various time and microwave power levels on reduced
ascorbic acid content (RAA) (mg/100 g) in broccoli
Table 4. 3.
Effects various time and microwave power levels on reduced
ascorbic acid percent retention in broccoli
Table 4.4.
Effects various time and microwave power levels on
peroxidase activity (units/min) in broccoli
Table 4. 5.
Effects various time and microwave power levels on lightness
(L* value) o f broccoli
Table 4. 6.
Effects various time and microwave power levels on greenness
(a* value) o f broccoli
Table 4. 7.
Effects various time and microwave power levels on
yellowness(b* value) o f broccoli
Table 4. 8.
Effects various time and microwave power levels on hue
angle o f broccoli
Table 4. 9.
Effects various time and microwave power levels on chroma
o f broccoli
Table 4. 10.
Effects various time and microwave power levels on moisture
content o f green bean
Table 4.11.
Effects various time and microwave power levels on reduced
ascorbic acid content (RAA) (mg/100 g) in green bean
Table 4. 12.
Effects various time and microwave power levels on reduced
ascorbic acid percent retention in green bean
Table 4. 13.
Effects various time and microwave power levels on
peroxidase activity (units/min) in green bean
Table 4. 14.
Effects various time and microwave power levels on lightness
(L* value) o f green bean
Table 4. 15.
Effects various time and microwave power levels on greenness
(a* value) o f green bean
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 4. 16.
Effects various time and microwave power levels on
yellowness(b* value) o f green bean
Table 4. 17.
Effects various time and microwave power levels on hue
angle o f green bean
Effects various time and microwave power levels on chroma
o f green bean
Table 4. 18.
Table 4. 19.
Effects various time and microwave power levels on moisture
content o f asparagus
Table 4.20.
Effects various time and microwave power levels on reduced
ascorbic acid content (RAA) (mg 100 g) in asparagus
Table 4.21.
Effects various time and microwave power levels on reduced
ascorbic acid percent retention in asparagus
Table 4. 22.
Effects various time and microwave power levels on
peroxidase activity (units/min) in asparagus
Table 4. 23.
Effects various time and microwave power levels on lightness
(L* value) o f asparagus
Table 4. 24.
Effects various time and microwave power levels on greenness
(a* value) o f asparagus
Table 4. 25.
Effects various time and microwave power levels on
yellowness(b* value) o f asparagus
Table 4. 26.
Effects various time and microwave power levels on hue
angle o f asparagus
Table 4. 27.
Effects various time and microwave power levels on chroma
o f asparagus
Table 4. 28.
Regression equation for broccoli, green bean and asparagus
processed at various time and microwave power levels
Table 4.29.
Prediction models for broccoli, green bean and asparagus
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
LIST OF FIGURES
Figure 2. 1.
Chroma o f freshly blanched and frozen broccoli florest
37
Figure 2. 2.
Chroma o f freshly blached and frozen broccoli stems
37
Figure 4.1.
Experimental design of broccoli processed at various time
and microwave power levels
114
Effect o f microwave power heating at various time and power
levels on moisture content o f broccoli
115
Effect o f microwave heating at various time and power levels
on reduced ascorbic acid (RAA) (mg/100 g) o f broccoli
116
Reduced ascorbic acid retention (%) in broccoli processed at
various time and microwave power levels
117
Peroxidase activity (units/min) in broccoli processed at various
time and microwave power levels
118
Effect o f microwave power heating at various time and power
levels on moisture content o f green bean
119
Effect o f microwave heating at various time and power levels
on reduced ascorbic acid (RAA) (m g/100 g) o f green bean
120
Reduced ascorbic acid retention (%) in green bean processed at
various time and microwave power levels
121
Peroxidase activity (units/min) in green bean processed at various
time and microwave power levels
122
Effect o f microwave power heating at various time and power
levels on moisture content o f asparagus
123
Effect o f microwave heating at various time and power levels
on reduced ascorbic acid (RAA) (mg/100 g) o f asparagusi
124
Reduced ascorbic acid retention (%) in asparagus processed at
various time and microwave power levels
125
Peroxidase activity (units/min) in asparagus processed at various
time and microwave power levels
126
Figure 4. 2.
Figure 4. 3.
Figure 4.4.
Figure 4. 5.
Figure 4. 6.
Figure 4. 7.
Figure 4. 8.
Figure 4. 9.
Figure 4. 10
Figure 4. 11
Figure4. 12.
Figure 4. 13
XI
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 4. 14
Figure 4.15
Figure 4. 16
Figure 4. 17
Figure 4. 18
Figure 4. 19
Figure 4. 20
Figure 4. 21
Figure 4. 22
Figure 4. 23
Figure 4. 24
Reduced ascorbic acid (RAA) and peroxidase activity (PO) in
broccoli processed at various times and 100% microwave
power level
127
Reduced ascorbic acid (RAA) and peroxidase activity (PO) in
broccoli processed at various times and 70% microwave
power level
128
Reduced ascorbic acid (RAA) and peroxidase activity (PO) in
broccoli processed at various times and 55% microwave
power level
129
Reduced ascorbic acid (RAA) and peroxidase activity (PO) in
broccoli processed at various times and 30% microwave
power level
130
Reduced ascorbic acid (RAA) and peroxidase activity (PO) in
green bean processed at various times and 100% microwave
power level
131
Reduced ascorbic acid (RAA) and peroxidase activity (PO) in
green beani processed at various times and 70% microwave
power level
132
Reduced ascorbic acid (RAA) and peroxidase activity (PO) in
green bean processed at various times and 55% microwave
power level
133
Reduced ascorbic acid (RAA) and peroxidase activity (PO) in
green bean processed at various times and 30% microwave
power level
134
Reduced ascorbic acid (RAA) and peroxidase activity (PO) in
asparagus processed at various times and 100% microwave
power level
135
Reduced ascorbic acid (RAA) and peroxidase activity (PO) in
asparagus processed at various times and 70% microwave
power level
136
Reduced ascorbic acid (RAA) and peroxidase activity (PO) in
asparagus processed at various times and 55% microwave
power level
137
Xll
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 4.25
Reduced ascorbic acid (RAA) and peroxidase activity (PO) in
asparagus processed at various times and 30% microwave
power level
Figure 4. 26. Reduced ascorbic acid (RAA) (mg/100 g) actual and predicted
data o f broccoli processed at various microwave power level
vs. time
Figure 4.27.
Figure 4.28.
Figure 4.29.
Figure 4. 30.
Figure 4. 31.
Figure 4. 32.
Figure 4. 33.
Figure 4. 34.
Figure 4.35.
138
139
Reduced ascorbic acid (RAA) (m g/100 g) actual and predicted
data o f broccoli processed at various times vs. microwave
power levels
140
Reduced ascorbic acid (RAA) retention (%) actual and predicted
data o f broccoli processed at various microwave power level
vs. time
141
Reduced ascorbic acid (RAA) retention (%) actual and predicted
data o f broccoli processed at various times vs. microwave
power levels
142
Peroxidase activity (units/min) actual and predicted data o f
broccoli processed at various microwave power level vs time
143
Peroxidase activity (units/min) actual and predicted data o f
broccoli processed at various times vs. microwave power
levels
144
Reduced ascorbic acid (RAA) (mg/100 g) actual and predicted
data o f green bean processed at various microwave power level
Vs. time
145
Reduced ascorbic acid (RAA) (m g/100 g) actual and predicted
data o f green bean processed at various times vs. microwave
power levels
146
Reduced ascorbic acid (RAA) retention (%) actual and predicted
data o f green bean processed at various microwave power level
vs. time
147
Reduced ascorbic acid (RAA) retention (%) actual and predicted
data o f green bean processed at various times vs. microwave
power levels
148
X lll
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 4.36.
Figure 4. 37.
Figure 4. 38.
Figure 4. 39.
Figure 4.40.
Figure 4. 41.
Figure 4.42.
Figure 4.43.
Peroxidase activity (units/min) actual and predicted data o f
green bean processed at various microwave power level vs time
149
Peroxidase activity (units/min) actual and predicted data of
green bean processed at various times vs. microwave power
levels
150
Reduced ascorbic acid (RAA) (m g/100 g) actual and predicted
data o f asparagus processed at various microwave power level
Vs. time
151
Reduced ascorbic acid (RAA) (mg/100 g) actual and predicted
data o f asparagus processed at various times vs. microwave
power levels
152
Reduced ascorbic acid (RAA) retention (%) actual and predicted
Data o f asparagus processed at various microwave power level
vs. time
153
Reduced ascorbic acid (RAA) retention (%) actual and predicted
Data o f asparagus processed at various times vs. microwave
power levels
154
Peroxidase activity (units/min) actual and predicted data o f
asparagus processed at variousmicrowave power level vs. time
155
Peroxidase activity (units/min) actual and predicted data o f
asparagus processed at various times vs. microwave power
levels
156
xiv
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
INTRODUCTION
Foods o f all types are subject to decomposition and quality deterioration over
time. Though living tissues do have some natural resistance to microorganisms, they
become susceptible to degradation after plants are harvested and after animals are
slaughtered. Enzyme-catalyzed chemical changes also cause deterioration in the quality
o f plant and animal tissues used as food.
The respiration process continues in vegetable tissues after harvest. Changes take
place gradually but escalate when the cells die. They cause loss o f food quality,
nutritional value, deterioration o f color and texture and loss o f flavor. In brief, chemical,
physical, biochemical and microbial changes cause spoilage o f fruits and vegetables.
These changes can be prevented easily by using fresh food as soon as possible. However,
this seems virtually impossible, because o f the seasonal overabundance o f food at
harvest. Some crops are harvested once a year. Preservation will help maintain supply of
these foods throughout the year. Other factors, which affect the food supply, include
natural disasters, insect and disease infestations, and wars. Preservation provides for a
more varied diet than the seasonal native food supply can provide. It allows the holding
of foods that can be used as ingredients and transport of foods from one geographic
location to another. Therefore, efforts have been made to preserve surplus foods.
Preservation o f foods includes techniques in which the food is treated to retard
decay and spoilage. Several methods are used commercially and in the home for this
purpose. Freezing or cold storage is one o f the most common methods used for
preservation. Most vegetables to be frozen require a prefreezing, brief heat treatment
known as blanching. Blanching reduces the number o f microorganisms, removes some
1
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
air from the tissues, makes them more compact, and enhances their color (Penfield and
Campbell, 1990). However, its most important function is to inactivate enzymes that
would otherwise cause deterioration in flavor, texture, color and nutrients during storage
(Penfield and Campbell, 1990). Brewer et al. (1995) reported that in addition to
inactivating destructive enzymes, blanching prior to processing reduces microbial load,
improves color, aids in packaging fill, and assists in the development o f a desirable
texture. Heat application to foods to be preserved can be accomplished by several
methods. Newman et al (1967) recommended that for the best quality in the finished
product, vegetables should be frozen as quickly as possible, after harvesting. Fennema et
al. (1973) states that although microbial growth ceases during frozen storage o f food at 7°C or below, chemical changes may occur; many o f these changes can be satisfactorily
controlled by prefreezing blanching of vegetables and storing at -18°C or below. William
et al. (1986) defined the term “blanching” as the heat treatment operations in the
processing o f foods for frozen storage, which prevent deteriorative changes that occur in
foods not so treated. The blanching process involves exposing plant tissue to heat, usually
steam or hot water, for a prescribed time at a specified temperature, usually 70-105°C
(Barret and Theerakulkait, 1995; Williams et al., 1986; Glasscock et al., 1983). Blanching
o f vegetables prior to freezing has some advantages such as the stabilization o f texture,
color, flavor and nutritional quality (William et al., 1986). They also reported that
blanching does have some disadvantages, such as loss in texture, flavor and nutritional
quality; and the wilting o f leafy vegetables which assists in packaging; formation o f a
cooked taste; some loss o f soluble solids (especially in water blanch); and adverse
environmental impact because o f need for large amounts o f water and energy (William et
2
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
al., 1986). Shamaila et al. (1996) reported that the quality o f carrot flavor and texture was
affected by initial water blanching; in addition low concentration o f important flavor
compounds resulted in lower intensity rating o f sensory attributes. Brewer et al. (1995)
reported that preprocessing heat treatment o f vegetables such as broccoli and asparagus
improves color and texture o f vegetables. Blanching by progressive immersion in boiling
water ensured an almost uniform texture along the length o f asparagus spears (SanchezPineda-Infantas et al., 1994). However, Drake and Carmicheal (1986) stated that
blanching could leach large amounts o f nutrients and solids that are never recovered.
Vegetables are good source o f ascorbic acid (vitamin C). Which functions in
collagen formation, dentin formation, tyrosine metabolism, synthesis o f neuro­
transmitters, and aids in utilization o f iron, calcium and folacin. Due to its solubility in
water, water blanching leads to the losses o f ascorbic acid (Fennema, 1985). Leaching
*
during blanching or cooking can be almost entirely eliminated by using microwave
cookers (Proctor and Goldblith, 1948). Typical vitamin losses from broccoli during
cooking and the distribution o f vitamins between the solid and liquid portions are shown
in Table 1 .1 (Fennema, 1985). If both liquid and solid portions are considered, there is no
measurable loss of B vitamins and a 10-15% loss o f vitamin C; if the liquid is discarded,
as is the usual practice, substantial losses occur for both boiling and microwave cooking
(Fennema, 1985). The indicated losses during microwave cooking are untypically high
and could have been minimized by using less cooking water (Fennema, 1985).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 1 .1. Vitamin content and distribution in cooked broccoli
Vitamin Content f%i*
Solid portion
Liquid portion
Cooking method
C
Bt
b2
C
Microwave
64
76
71
23
31
31
Pressure (steam)
72
90
94
6
8
8
Boiling
60
75
69
25
33
33
Bi
b2
Expressed as a percentage retained o f broccoli’s original vitamin content.
Source: From Thomas et al. (1949).
Ascorbic acid is highly sensitive to changes in pH. Under anaerobic conditions
the rate o f ascorbic acid oxidation reaches a maximum at pH 4, declines to a minimum at
pH 2, and then increases again with increasing acidity (Huelin et al. 1971). In aqueous
solutions below pH 7.6, ascorbic acid is not oxidized on exposure to air unless traces of
copper or other materials that catalyze oxidation reaction are present (Freed, 1966). In the
presence o f air and a suitable catalyst, ascorbic acid is oxidized to dehydroascorbic acid
(Feed, 1966). Below pH 4.0, dehydroascorbic acid is fairly stable, however, above pH 4.0
dehydroascorbic acid is oxidized to diketogulonic acid (Freed, 1966). He also stated that
the relatively labile nature of dehydroascorbic acid suggests that once ascorbic acid in a
food has been oxidized to this compound the value o f the product as a source o f vitamin
C has impaired. Unless this is considered, measurement o f both ascorbic acid and
dehydroascorbic acid, it may give misleading biological value for foods that are not to be
consumed immediately (Freed, 1966).
4
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Tables o f food composition continue to record only the reduced ascorbic acid
content o f fruits and vegetables, although the body utilizes both reduced and
dehydroascrobic acid (Guthrie, 1989). It is transported to and enters the cell more easily
in this oxidized form and is apparently reduced again within the cell before it can be used
(Guthrie, 1989). Table I. 2 gives reduced ascorbic acid, dehydroascorbic acid and total
ascorbic acid values for some representative foods.
Table 1.2. Reduced ascorbic acid, dehydroascorbic acid and total ascorbic acid
o f some representative foods (mg/lOOg)
Dehydroascorbic
Total
Foods
Reduced
Ascorbic Acid
Acid
Ascorbic Acid
Asparagus
7.9
26.9
34.8
Broccoli
48.2
9.8
58.0
Brussels sprouts
60.9
4.4
65.3
Cabbage, raw
54.4
22.3
76.7
Cantaloupe
15.5
18.2
33.7
Green pepper
41.0
4.8
45.8
Strawberries
53.8
12.9
66.7
Sweet potatoes
18.8
8.1
26.9
Tomato juice
15.2
2.3
17.5
mg = milligrams, g = grams.
Source: Davey et al. (1956).
Preserving and storing food in the home has been coming back into its own the past few
years (Mead and Chioffi, 1982). Climbing costs o f foods, their doubtful quality and
5
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
consumer’s awareness o f chemical food additives effect on human health, are the reasons
that many families are turning back to the basics o f gardening and storing the foods they
grow through home preservation (Mead and Chioffi, 1982). Once the garden produce
begins, unless it is preserved in some manner, it begins to spoil soon after it is harvested.
This spoilage is caused by microorganisms, physical damage, water loss or by chemical
changes such as those caused by enzymes. The same forces also destroy the vitamins.
Growers and shippers apply a complicated technology to harvested vegetables to control
all the factors o f decay such as different gases and chemicals. Almost all o f these same
controls can be applied in the kitchen to prolong the shelf-life o f vegetables and protect
their flavor and nutrient potency.
M ost o f the vegetables can be home-frozen using different preserving methods
such as drying, salting, canning and freezing. Freezing is often preferred method over
canning because it is easier and quicker (Klugar, 1979). Quality o f some frozen
vegetables is better than the same variety canned (Klugar, 1979). Frozen vegetables,
prepared for table look and taste more like fresh produce than vegetables preserved in any
other way (Klugar, 1979).
Blanching in boiling water or steam for a short period of time is must for almost
all vegetables to be frozen. It slows or stops the action of enzymes that can cause loss of
flavor, color and texture.
Microwave ovens are common household appliances that can be used for
blanching o f vegetables. Microwave ovens can blanch vegetables perfectly, however, the
time and power levels are the important factors for different vegetables regarding the
usage o f this device.
6
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
CHAPTER 1. REVIEW OF LITERATURE
Conventional Heating vs. Microwave Heating
Microwave ovens are considered important pieces o f equipment in home kitchens
as convenience and timesaving devices. Microwave ovens are common household
appliances (Giese, 1992); an estimated 92% o f homes have at least one and 40% have
two microwave ovens (Baum, 1992). Microwave processing offers several distinct
advantages when compared to conventional heating methods (Decareau, 198S; Giese,
1992). These advantages include speed o f operation, energy savings, precise process
control, and faster start-up and shutdown times (Decareau, 1985). Since microwaves
penetrate into a food rather than heating the surrounding air or water only, heating occurs
more rapidly (Giese, 1992). This accelerated heating provides for a higher quality product
in terms o f taste, texture, and nutritional content, as well as increased product weight.
Bennion (1980) and Bowers (1992) reported cooking time reduction as major advantages
o f microwave cooking in comparison to conventional cooking.
During conventional heating, the food is heated by conduction (Potter, 1995); the
heat source in conventional heating methods, causes food molecules to react largely from
the surface inward, so that successive layers heat in turn. Potter (1995) also states that
conventional heating develops a temperature gradient, which bums and dehydrates the
outer surface o f the food before the center temperature rises to an acceptable level.
Microwaves penetrate foods about 1.25 - 2.5 cm deep depending on the density o f the
food (Brewer, 1992); conduction does occur toward the center o f food if the microwaves
are unable to reach due to the large size o f the piece. Furthermore, surface browning does
not occur during microwave cooking, because the energy from the microwaves is being
7
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
transformed into heat within the food, and heat then is conducted to the surface
(McWilliams, 1997). Surface heat is quickly lost to the surrounding air because the
microwaves do not warm the chamber o f the microwave oven itself so dehydration and
browning reactions are absent or minimal.
Most heating can be accomplished in one quarter of time, or less, that
conventional heating requires (Schiffinann, 1986). Chapman et al. (1960) reported that
the time for cooking fresh broccoli stems to optimum tenderness electronically was about
6 min. compared to 13 min by conventional boiling in a small amount of water. In
general, microwave-blanched vegetables are less acceptable in terms o f texture and flavor
and are tougher than those that are steam or water blanched (Drake et al. 1981; Stone and
Young, 1985; Schrumpf and Charley, 1975). Drake et al. (1981) reported that color and
ascorbic acid retention were better in microwave-blanched broccoli. Campbell et al.
(1958) and Chapman et al. (1960) also reported higher ascorbic acid retention in fresh
and frozen broccoli cooked or blanched by microwave than conventionally heat-treated
broccoli. Microwave blanched spinach had significantly more ascorbic acid than that
boiled conventionally in a small amount o f water (Eheart and Gott, 1964). No significant
differences in quality attributes were found in peas, broccoli, or potatoes blanched with
microwave vs. conventional methods when the amount o f water used was constant.
Carotene content and eating quality were unaffected by blanching method. Bowman et al
(1979) cooked 13 fresh and 9 frozen vegetables by three different methods, using
microwave, pressure saucepan or boiling water, all at an altitude o f 5000 feet above sea
level. Each method was satisfactory; no single method was consistently best. Mean
sensory scores were highest for color o f frozen Brussels sprouts and lima beans cooked
8
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
by microwave oven; o f zucchini, frozen broccoli, and onion by boiling; and o f frozen
peas in the pressure saucepan. They reported superior flavor for red McClure potatoes,
frozen Brussels sprouts, and Burbank potatoes cooked by microwave; for frozen spinach,
onions, zucchini, and frozen broccoli by boiling; and for frozen lima beans cooked using
the pressure saucepan. The authors concluded that microwave cooking was an additional,
satisfactory method for cooking vegetables (Referred to APP. B, page 168 for more
information on microwave ovens and mechanism o f microwave heating).
Ascorbic Acid in Vegetables
Perhaps the most talked about vitamin, and the one most widely used as a
supplement, is the one familiarly known as (ascorbic acid), the antiscorbutic (scurvypreventive) dietary essential (Guthrie, 1989). The saga o f the discovery o f this vitamin is
one o f the longest and most interesting in nutritional history; descriptions o f illnesses that
were unmistakably scurvy were reported in a papyrus from 1500 BC found at Thebes and
in the writings o f Hippocrates in 400 BC: scurvy was a major factor shaping the course o f
history, ravaging armies and navies and causing the death o f many explorers and
homesteaders (Guthrie, 1989). Probably as early as 1700, it was observed that a lack o f
fresh fruits and vegetables resulted in scurvy and that this disease could be prevented, and
cured, by proper d iet Since 1906, it has been known that scurvy is a vitamin-deficiency
disease (Freed, 1966; Guthrie, 1989). Ascorbic acid has the distinction o f being the first
micronutrient whose deficiency was recongnized as a cause o f disease (Freed, 1966).
Ascorbic acid is found in two isomeric forms, D-ascorbic acid and L-ascorbic
acid; the D-isomer has about 10% o f the biological activity o f the L-isomer and is added
to foods for nonvitamin purposes (Fennema, 1985) (Fig. A.1, Appendix A, page 158).
9
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Ascorbic acid, present in foods as reduced ascorbic acid, is susceptible to
oxidation, causing it to change on exposure to air to dehydroascorbic acid, which has two
fewer hydrogen atoms (Guthrie, 1989). It is transported to and enters the cell more easily
in this oxidized form and is apparently reduced again within the cell before it can be used
(Guthrie, 1989). Any further oxidation o f dehydroascorbic acid is irreversible, having
produced a biologically inactive form, diketogulonic acid, with no vitamin value
(Guthrie, 1989) (Fig. A. 2, Appendix A, page 159).
Dehydroascorbic acid has considerable antiscorbutic activity because it is readily
reduced to ascorbic acid in the animal body (Freed, 1966). The exact biological activity
has not been satisfactorily determined, but it is probably about 75 or 80% o f that o f
ascorbic acid (Mills, et al. 1949; Gould and Schwachman, 1943). However, the relatively
labile nature o f dehydroascorbic acid suggests that once ascorbic acid in a food has been
oxidized to this compound the value o f the product as a source o f ascorbic acid, has been
impared (Freed, 1966). He further stated that unless this is considered, measurement o f
both ascorbic acid and dehyroascorbic acid may give misleading biological values for
foods that are not to be consumed immediately.
Ascorbic acid activity o f foods and biological materials is associated with their Lascorbic acid content (Freed, 1966). Ascorbic acid is found in all living tissues. Excellent
sources o f ascorbic acid are citrus fruits and their juices, fresh fruits and green leafy
vegetables. An eight fluid ounce serving o f either grapefruit juice or orange juice would
supply the entire Reference Daily Intake (RDI) amount for ascorbic acid (Rouseff and
Nagy, 1994). Many consumers consider orange juice to be the best source o f ascorbic
acid in their diet (Shaw and Moshonas, 1991). h i addition, cauliflower, tomatoes,
10
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
cabbage, broccoli and Brussels sprouts are considered good sources o f ascorbic acid.
Broccoli and asparagus contain 141 mg/100 g and 49 mg/100 g ascorbic acid respectively
(USDA, 1981). The ascorbic acid content in some commonly consumed fruits and
vegetables are shown in Table 1.1.
Table 1.1. Ascorbic acid content of the edible portion o f fruits and vegetables values
are given as (mg/100 g).
Vegetables
Vitamin C contents
Vitamin C contents
Fruits
(m g)
(mg)
Apricots
11
Asparagus
49
Banana
10
Broccoli
141
Blackberries
30
Brussels sprouts
96
Grapefruit
41
Cabbage
33
Kiwifruit
74
Cauliflower
72
Lemon
31
Kale
53
Orange
70
Kohlrabi
89
Mangos
57
Beans
22
Pineapple
24
Peas
16
8
Tomato
22
Apple
Source: USDA Handbook No. 8.1981.
In fruits and vegetables, ascorbic acid is the most labile nutrient because it is
easily oxidized, water soluble, pH, light and heat sensitive, and affected by naturally
occurring enzyme systems (Erdman and Klein, 1982). According to Heimann (1980) the
11
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
loss o f ascorbic acid occurs during harvesting, transportation, and household and
industrial processing o f vegetables through enzymatic oxidation. Ascorbic acid plays a
significant role in oxidation-reduction reactions (deMan, 1990).
Ascorbic acid functions as reducing agent, an antioxidant, and a metal
sequestering agent in specific a food substrates (Borenstein, 1987). Its antioxidant effect
is due to its ability to scavenge oxygen and to protect double bonds. It also decreases the
oxidation state o f many metals, thereby lowering the catalytic activity o f these metals
(Borenstein, 1987).
Some o f the principal biochemical functions o f ascorbic acid in humans are to
maintain a reducing environment and to destroy toxic free radicals resulting from
metabolic products o f oxygen (Rouseff and Nagy, 1994). Enzyme prolyl hydroxylase
converts proline to hydoxyproline in collagen synthesis, defective hydroxylation is one o f
the biochemical lesions in scurvy. Ascorbic acid due to its reducing property, maintains
prolyl hydroxylase in its active form, probably by keeping the enzyme’s iron atom in the
reduced state (Rouseff and Nagy, 1994). Ascorbic acid provides reducing environment to
another enzyme in collagen synthesis lysyl hydroxylase. Mirvish et al. (1975) suggest
that ascorbic acid prevents the reaction ot nitrites with amines and amides to form potent
carcinogenic nitrosamines within the digestive tract. Ascorbic acid prevents oxidation o f
specific chemicals to their active carcinogenic forms (Pipkin et al. 1969). The protective
association between ascorbic acid rich foods and cancers o f the esophagus, stomach and
cervix has been noted (Acherman et al. 1978; Kolonel et al. 1981; Wassertheil-Smoller et
al. 1981).
12
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Factors Affecting Ascorbic Acid in Vegetables
Ascorbic acid is highly sensitive to various types o f degradation; factors that can
influence the nature o f the degradative mechanism include, salt and sugar concentration,
pH, oxygen, enzyme, metal catalysts, initial concentration o f ascorbic acid, and ratio o f
ascorbic acid to dehydroascorbic acid (Fennema, 1985).
The loss o f ascorbic acid in vegetables is catalyzed by a series o f oxidases
containing metals as their coenzymes, such as copper (ascorbic acid oxidase, phenolase)
or iron (peroxidase). Sumner and Somers (1953) reported that ascorbic acid oxidase
attacks ascorbic acid directly while the peroxidases react with ascorbic acid indirectly.
Peroxidase and H 2O2 act upon phenols to form quinones, which subsequently oxidize
ascorbic acid; ascorbic acid oxidase oxidizes ascorbic acid probably by introducing two
OH groups at the double bond (Tauber, 1937) (Fig. A.3., Appendix A, page, 160) and
Fig. A. 4., Appendix A, page, 161). Wang et al. (1995) reported that L-ascorbate has high
ascorbic acid activity and high stability towards oxygen during processing. Maria et al.
(1995) found higher degradation rates o f ascorbic acid when stored for more than 90 days
at -18°C than for that stored less than 90 days. A substantial loss o f ascorbic acid occurs
during rapid chilling of the cooked vegetables; a further linear loss is caused by chilled
storage at 3° C for 3 days (William et al. 1995).
Ascorbic acid contains two enolic groups in its structure; therefore, when it is
treated with a suitable oxidizing agent, two atoms o f hydrogen are lost to produce
dehydroascorbic acid; on treatment with reducing agents such as hydrogen sulfide, one
molecule o f dehydroascorbic acid takes on two hydrogen atoms to form ascorbic acid
(Freed, 1966). Ascorbic acid is stable to but can be destroyed by alkali (Guthrie, 1989;
13
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Potter, 1995). The pH-rate profile for uncatalyzed oxidative degradation is an S-shaped
curve (Fig. A.5., Appendix A, page, 162) that increases continuously through the pH
range corresponding to the pKi o f ascorbic acid and then tends to flatten out above pH
6.0. This seems to indicate that ascorbic acid is the monoanion primary form that
participates in oxidation (Fennema, 1985); normally it reacts as a monobasic acid with
pKa 4.25 in water, however, a second ionization constant, pK 2 11.79, also has been
reported (Sebrell and Harris, 1967).
Under anaerobic conditions the rate o f ascorbic acid oxidation reaches a
maximum at pH 4.0, declines to a minimum at pH 2.0, and then increases again with
increasing acidity (Huelin, 1971). Sebrell and Harris (1967) stated that the rate o f aerobic
oxidation is pH-dependent, exhibiting a maximum at pH 5.0, corresponding to the
reaction with 1 equivalent o f base, and at pH 11.5, corresponding to the reaction with 2
equivalents o f base. The nature o f oxidation products also shows a dependence on pH;
oxalic acid is formed during autoxidation o f ascorbic acid in alkaline solutions but not at
pH 4.0 (Sebrell and Harris, 1967).
Dry crystals o f ascorbic acid are stable on exposure to air and daylight at ordinary
room temperature for long periods o f time. In aqueous solutions below pH 7.6, ascorbic
acid is not oxidized on exposure to air unless traces o f copper or other materials that
catalyze the reaction are present (Freed, 1966). In the presence o f air and a suitable
catalyst (copper, iron and manganese), ascorbic acid is readily oxidized to
dehydroascorbic acid, which is fairly stable below pH 4.0. However, above pH 4.0,
dehydroascorbic acid readily undergoes an irreversible rearrangement to a biologically
inactive material (Freed, 1966). Metal-catalyzed degradation of ascorbic acid occurs
14
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
through formation o f a ternary complex o f ascorbate monoanion, O2 , and a metal ion
(Fig. A.6., Appendix A, page, 163) (Tauber, 1937).
In the presence o f oxygen, ascorbic acid is degraded primarily via its monoanion
to dehydroascorbic acid. The exact pathway and overall rate, is a function o f the
concentration o f metal catalysts (Mn+) (Fig. A.7., Appendix A, page, 164) in the system.
The rate o f formation of dehydroascorbic acid is first order with respect to monoanion of
ascorbic acid, O2 and Mn+ (Huelin, 1971; Khan and Martell, 1967; Kurata and Sakurai,
1967; Kurata et al. 1967). However, when the metal catalysts are Cu*- or Fe+++, the
specific rate constants are several orders o f magnitude greater for spontaneous oxidation,
and thus even a few parts per million o f these metals may cause significant losses o f
vitamin C in food products (Fennema, 1985).
Labuza (1972) reported higher ascorbic acid losses in foods during sun drying due
to exposure to the sunlight. Direct sun-exposure of vegetables, as is often practiced in the
tropics, results in marginal retention o f ascorbic acid (Maeda and Salunkhe, 1981).
Ascorbic acid is also sensitive to UV, X, and gamma radiation (Ritter, 1976). Irradiation
of alkaline solutions o f ascorbic acid has been found to produce an unidentified
compound; the photochemical oxidation o f ascorbic acid can proceed under either
aerobic or anaerobic conditions; x-rays and gamma-irradiation are more effective than
ultraviolet light, but all three produce dehydroascorbic acid and H 2 O2 ; metal ions
especially iron, and oxygen accelerate these reactions (Sebrell and Harris, 1967). Higher
salt levels are destructive to ascorbic acid, but commonly used levels are much less
harmful to ascorbic acid. Thurman and Vahlteich (1929) found higher losses o f ascorbic
acid in cucumbers pickled in 15 to 20% salt (NaCl) than in fresh pickled cucumbers.
15
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Ritter (1976) reported that both ascorbic acid and its sodium salt are very stable in the
absence o f moisture, but in the presence o f moisture and air or other oxidizing agents, the
vitamin can be very labile.
Effect of Processing on Ascorbic Acid in Vegetables
In fruits and vegetables, ascorbic acid is the most labile nutrient since it is easily
oxidized, water soluble, pH*, light*, and heat sensitive, and degraded by a naturally
occurring enzyme system (ascorbic acid oxidase) (Erdman and Klein, 1982; Brewer et al.
1994). Any method o f processing that involves the use o f heat is likely to result in
ascorbic acid content reduction; however, processing in the absence o f oxygen, results in
lower losses o f ascorbic acid (Guthrie, 1989). When frozen and canned foods are picked
at the peak o f maturity and are processed immediately under optimal conditions, the
resulting product may have a higher vitamin C value than the fresh product, for which the
period between harvesting and consumption may be long and characterized by poor
temperature and humidity control. Howard et al (1999) also reported that when
vegetables are processed, by commercial canning or freezing, postharvest handling is
minimized to less than 12 h, so nutrient loss prior to processing is minimal. Therefore,
when “supermarket fresh” vegetables are used to conduct studies on effects o f further
cold storage and thermal processing, results can be confounded by other factors (Howard,
et al., 1999). Blanching o f vegetables before freezing, canning and dehydration is
necessary to destroy enzymes that otherwise would catalyze the destruction o f ascorbic
acid, however, blanching can result in vitamin losses ranging from 16 to 58% (Lund,
1982; Guthrie, 1989). Many o f the best sources o f ascorbic acid are most commonly
consumed raw. In cooked foods, most loss occurs in the early stages o f the cooking
16
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
process; broccoli heads lose 40% o f their ascorbic acid value in the first 10 minutes o f
cooking by leaching or dissolving into the cooking water; cabbage losses more by heat
destruction than by leaching. Dannison and Kirk (1978) reported that dissolved and
gaseous oxygen is a primary factor in the storage stability o f ascorbic acid in dehydrated
food systems. The rate for spontaneous oxidation o f ascorbic acid in aqueous model
systems was proportional to the concentration o f molecular oxygen (Khan and Martell
1967). Maria et al. (1995) found higher degradation rate o f ascorbic acid in ground
asparagus when stored for more than 90 days at -18° C. William et al. (1995) reported
that a substantial loss of ascorbic acid occurred during rapid chilling of the cooked
vegetables such as potatoes, pumpkins, carrots, silverbeet, broccoli and peas; chilled
storage below (3°C) for 3 days caused a further linear loss o f ascorbic acid. Brewer et al.
(1995, 1997) reported minimal losses o f ascorbic acid in frozen asparagus and broccoli at
(-18°C) during 6 weeks frozen storage.
Losses during broccoli preparation by microwave heating; cooking in pressure
cookers, steaming, and boiling were 15%, 20%, 30% and 55%, respectively (Guthrie,
1989) (Fig. A.8., Appendix A, page, 165). Drake et al. (1981) reported that microwave
blanched (high power level) asparagus contained approximately half the ascorbic acid as
boiling water and steam blanched asparagus while Begum and Brewer (1997) found
higher ascorbic acid content in microwave blanched asparagus and broccoli (Brewer et
al., 1995) than in the boiling water and steam blanched samples.
Ascorbic Acid Oxidase in Vegetables
Sumner and Somer (1985) reported that some ascorbic acid oxidizing enzymes act
indirectly, forming quinones and other products o f oxidation, which then act on ascorbic
17
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
acid. These include peroxidase and H2O 2 (Fig. A.3., Appendix A, page, 160), which act
on phenols to form quinones that oxidize ascorbic acid, however, ascorbic acid oxidase,
which is a copper containing enzyme, acts directly (Fig. A. 4., Appendix A, page 161)
upon ascorbic acid. In the presence of gaseous oxygen at pH 5.6, ascorbic acid oxidase
causes L-ascorbic acid to become oxidized to dehydroascorbic acid (Tauber et al. 1935;
Fennema, 1985). Dehydroascorbic acid is readily hydrated in positions 2 and 3 in
aqueous solutions. The enzyme is inactivated in the course o f oxidation and hydrogen
peroxide is not formed (Dowson, 1953).
Peroxidase in Vegetables
Many o f the quality changes in vegetables during frozen storage can be attributed
to enzyme systems including peroxidase, ascorbic acid oxidase, lipoxygenase and
polyphenol oxidase; off-flavor develops rapidly in unblanched vegetables (William et al.
1986); Katsaboxakis and Papanicolaou, 1984; Bottcher, 1975). Peroxidase is widely
distributed in animal and plant matter. Peroxidase and ascorbic acid oxidase belong to the
oxidoreductase group o f enzymes, and are involved in ascorbic acid destruction in foods
(Richardson and Finley, 1985). Peroxidase from horseradish is the best studied o f the
peroxidases; it is a glycoprotein with a hemin prosthetic group and bound Ca2+ (Bowers,
1992; Hiemann, 1980). Peroxidase frees oxygen from H 2 O2 and from certain other
peroxides, and transfers it directly to organic substrates (AH 2) such as mono- and
diphenols, and aromatic amines (Referred to Appendix C, page 181, for details about
general peroxidase reaction).
18
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Interaction o f Peroxidase and Ascorbic Acid
Peroxidase and peroxides oxidize ascorbic acid with great rapidity. In peroxidase
solutions, substances are present which form quinones with peroxide-peroxidase: the
quinones oxidize the ascorbic acid (Fig. A. 3., Appendix A, page, 160), and they, in turn
are reoxidized by peroxide-peroxidase. The oxidation o f ascorbic acid and reduction o f
quinones proceeds until all the peroxide is reduced, resulting in the decomposition o f the
physiologically toxic peroxide (Tauber, 1937). This reaction using vanillin as the
substrate at 20°C shows two peaks, one at pH 5.8 and one at pH 9.0; at pH 9.0 there are
maximum activity and no colored compound is formed during the reaction.
Inhibitors o f Peroxidase
Peroxidase is inactivated by excess hydrogen peroxide, however, activity returns
if the excess is removed with catalase. Hydrocyanic acid, hydrogen sulfide, sodium azide,
nitric oxide, hydroxylamine, sodium dithionite and thiourea inhibit peroxidase (Sumner
and Somer, 1953). Sumner and Myrback (1951) reported that peroxidase activity is
reversibly inhibited by cyanide and sulfide at a concentration o f 10"5-10'6 M;
hydroxylamine, azide and fluoride inhibit at somewhat higher concentrations (about 10~3
M).
Color o f Vegetables
Consumer decisions are greatly influenced by the bright colors o f foods.
Consumer purchases are based primarily on external characteristics such as visual
appearance and texture (Brewer et al., 1995). An important component o f visual
appearance is color (Gnanasekharan et al. 1992). These colors are imparted to foods by a
variety o f pigments, including chlorophylls, carotenoids, anthocyanins and anthoxanthins.
19
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
McWilliams (1997) stated that all chlorophyll-related compounds have the same
basic tetrapyrrole structure, with connecting methyne bridges (Fig. A. 9., Appendix A,
page, 166). Chlorophyll molecules occur in two different forms, the difference being the
functional group (designated as R in the structure) attached to one o f the pyrrole rings
(McWilliams, 1997). A variety o f chlorophylls have been described, including
chlorophyll a, b, c, and d. In foods, chlorophylls a and b are the major pigments occurring
in an approximate ratio o f 3:1 in plants. In leaves, these pigments are located in
chloroplasts (Fennema, 1985). Chlorophyll a, the more abundant form in nature is a
magnesium-chelated tetrapyrrole structure with methyl substitutions at the 1,3, 5, and 8
positions, vinyl at the 2, ethyl at the 4, propionate esterified with phytyl alcohot at 7, keto
at the 9, and carbomethoxy at the 10 position (Clydesdale and Francis, 1976; Krinsky,
1979; McWilliams, 1997). The phytol group is a 20-carbon alcohol with an isoprenoid
structure. The empirical formula o f chlorophyll is C5sH720sN4Mg (Richardson and
Finley, 1985). The total resonance o f the molecule and the presence o f this methyl group
at position 3 results in a blue-green color (Krinsky, 1979). The R group in chlorophyll b
is an aldehyde group. This difference in functional groups results in chlorophyll b having
a yellowish-green color. The ratio o f chlorophyll a to chlorophyll b varies, depending on
the specific plant; it even varies within the plant In broccoli, chlorophyll a is definitely
the dominant form in the blue-green florets, while chlorophyll b is the more dominant
form in the yellow-green stalks (Richardson and Finley, 1985; McWilliams, 1997).
Factors Affecting C olor o f Vegetables
In general, chlorophyll degradation involves the loss o f phytol to form
chlorophyllide or other related products (depending on where phytol is lost in the
20
Reproduced with permission of the copyright owner. Further reproduction prohibited w ithout permission.
degradation pathway): the loss ofM g2+ to form pheophytin; the loss o f Mg2* and phytol
to form pheophorbide and the loss of Mg2* and the carbomethoxy group to form
pyropheophytin a (Clydesdale and Francis, 1976). Chlorophyllase is an enzyme that
catalyzes the removal o f the phytol group in broccoli and other green vegetables to form
chlorophyllide. The phytol group is responsible for the hydrophobicity o f chlorophyll;
removal o f it renders the remaining molecule soluble in water. The action o f
chlorophyllase prior to the cooking o f fresh green vegetables is responsible for the
slightly green appearance o f the cooking water, as some chlorophyllide is dissolved in the
cooking medium (Richardson and Finley, 1985).
Plant pigments are sensitive to changes, which occur during aging, processing and
cooking. Chlorophyll undergoes pronounced changes in color during heating (Fig. A. 10.,
Appendix A, page, 167). On initial heating o f green vegetables, a bright-green color
appears as the air is expelled. On removal o f Mg2* and formation o f pheophytin, the color
o f chlorophyll, changes to an unattractive olive-green (Gold and Weckel, 1959). This
convesion occurs in the presence of dilute acids (Eskin, 1979); Mg2* is easily lost and
replaced by two protons during heat processing or during storage in mild acid
(Richardson and Finley, 1985). Clydesdale (1966) found that HTST (High-Temperature
Short-Time) treatment o f spinach puree resulted in better chlorophyll retention due to the
elevation in pH during HTST treatment (Lin et al. 1971). Retention o f chlorophyll is
favored when vegetables are added to boiling water. The breakdown o f chlorophyll to
pheophytin begins between 5 and 7 minutes after heating begins. I f vegetables are added
to boiling water, heating is more rapid so the actual time required to tenderize the
vegetable is reduced, and color retention is improved (Clydesdale, 1966).
21
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Loss o f green color is the major limiting factor in shelf life o f broccoli (Shewfelt
et al. 1984). When plant cells die during aging, processing or cooking, the proteins bound
to the chlorophyll denature and the magnesium bound in the pigment may be released
causing conversion to pheophytin. This pigment conversion is favored by acid pH (Potter,
1995). These color changes can be forestalled by the addition o f sodium bicarbonate or
other alkali to the cooking or canning water; however, this practice is not looked upon
favorably nor used commercially because higher pH softens cellulose adversely affecting
vegetable texture and enhances vitamin C and thiamin destruction at cooking
temperatures (Charley, 1970; Potter, 1955). Exposure o f chlorophyll to plant acids
extracted during cooking for increasing periods o f time, increases the loss o f chlorophylls
(Sweeney and Martin, 1961). A microwave cooking result in better chlorophyll retention
than conventional cooking but is dependent on the amount o f water used in boiling
(Gordon and Noble, 1959; Sweeney and Martin, 1961). Brewer et al. (1995) found that
microwave blanching darkened broccoli florets and stems; Begum and Brewer (1997)
found asparagus to be greener after microwave blanching and less green after frozen
storage than boiling water blanched asparagus.
22
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
CHAPTER 2. MICROWAVE AND CONVENTIONAL BLANCHING EFFECTS
ON CHEMICAL, SENSORY, AND COLOR CHARACTERISTICS
OF FROZEN BROCCOLI
Introduction
Consumer purchases are based primarily on external characteristics such as visual
appearance and texture. An important component o f visual appearance is color
(Gnanasekharan et al. 1992). Loss o f green color is a major limiting factor in shelf life of
broccoli (Shewfelt et al. 1984). Color can be stabilized by the blanching process (Klein
1992). With increasing awareness o f nutritional quality and attention to dietary
guidelines, consumers are interested in vegetables processed to maintain acceptable
external quality characteristics that ensure nutrient retention (Brewer et al. 1994).
Quality has been defined as “the composite" o f those characteristics that differentiate
individual units o f the product and have significance in determining the degree o f
acceptability by the buyer (Shewfelt 1990).
Blanching o f vegetables before processing reduces microbial load, improves
color, aids in package fill, and assists in the development o f a desirable texture.
Blanching can leach large amounts o f nutrients and solids that are never recovered
(Drake and Carmichael 1986). Broccoli spears blanched in water lose 8-9% solids,
compared with approximately 2% solid loss when blanched with steam (Drake and
Carmichael 1986). Blanching may result in some loss in texture, flavor, and nutritional
quality due to the heating process, formation o f a cooked taste, some loss o f soluble
23
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
solids; and adverse environmental impact because o f the need for large amounts o f water
and energy (Klein 1992).
Peroxidase, the most heat-stable enzyme in plants, is used as an indicator o f
blanching process adequacy (Brewer et al. 1994). Peroxidase combines with endogenous
hydrogen peroxide to produce an activated complex that reacts with a wide range o f food
constituents including ascorbic acid, carotenoids and fatty acids. Some o f these reactions
cause undesirable changes in food materials, including off-flavor, aroma, and color, as
well as loss of some nutrients (Hemeda and Klein 1990).
In vegetables, ascorbic acid is one o f the most heat labile nutrients; it is water
soluble, pH-, light- and beat-sensitive and readily oxidizable by the naturally occurring
enzyme system, ascorbic acid oxidase (Brewer et al. 1994).
Objective o f the Study
The objective of this study was to compare the effects of blanching methods on
the ascorbic acid and moisture content, peroxidase activity, color, and sensory
characteristics of broccoli.
M aterials and Methods
Sample Preparation
Broccoli (cv. Empress) was purchased from a local supplier in late June
(approximately 48 days of age). Three harvests were conducted. Broccoli was
refrigerated (10°C) within 1 h o f harvest, blanched and assayed within 15 h o f harvest.
Broccoli was manually sorted, rinsed with tap water, and florets were cut into 3.7 cm
long pieces with a diameter o f approximately 2.5 cm immediately before blanching.
24
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Broccoli (225 g) was blanched by four methods in covered containers:
conventional boiling water (1900 ml water, 4 min) (BW), steam (300 ml water, 4 min)
(ST), microwave heated in 1 L Pyrex containers (60 ml water, 4 min, 700 W) (MW) in an
Am ana Radarange (model #R5458P) or microwave heated (4 min) in 1 L boilable bags
(Seal-A-Meal™ laminated polyester/polyethylene bags; O2 transmission rate = 0.7671.749 cc/625 cm2/24 h, Dazey Products Co., Industrial Airport, KS) (MWB). Blanching
times and proportions of vegetable/water were based on average times for boiling water
and steam blanching recommendations (USDA 1978). Immediately after blanching, BW,
ST, and MW broccoli were cooled in ice water for 5 min, drained well and packed in 1 L
bags. MWB broccoli was cooled in the bag. The air was pressed out o f the bags, then
the bags were heat-sealed, frozen, and stored at -18C for 4 weeks.
After 4 weeks frozen storage, 20 g broccoli were removed from each bag for
peroxidase assay, and the remainder o f the broccoli (approx. 190 g) was cooked prior to
quality analysis. Broccoli was placed in 125 ml water in 1 L Pyrex covered casserole
dishes and heated (5 min) on 100% power in a 700 W Amana Radarange (model
#R5458P).
Analyses
Moisture content, reduced ascorbic acid content, peroxidase activity and
instrumental color o f raw unblanched, blanched (all treatments), and frozen cooked
broccoli were determined. Sensory analyses were conducted on frozen cooked broccoli.
Moisture Content
Moisture content o f broccoli (5 g) was determined by drying (55C for 12 h, 60C
thereafter) to a constant weight (Wu et a l 1992).
25
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Reduced Ascorbic Acid
Reduced ascorbic acid (RAA) was determined using the titrimetric assay
described by Pelletier (1985). Ground broccoli (25 g) was extracted with 100 ml 6%
metaphosphoric acid for 2 min using a Tekmar tissue homogenizer (Tekmar Corp.,
Cincinnati, OH), made up to 250 ml with 6% metaphosphoric acid and filtered (Whatman
#2). Duplicate aliquots (10 ml) o f filtrate were titrated with 2,6dichlorophenolindophenol.
True retention was calculated as percent o f total RAA in raw unblanched sample
retained in the cooked sample.
True retention = (RAA/e cookedl x (total weight cooked) x 100
(RAA/g raw) x (225 g)
Peroxidase Activity
Peroxidase activity was determined spectrophotometrically as described by
Hemeda and Klein (1990). Vegetable (20 g) was homogenized (1 min) with cold
deionized water (50 ml), made up to 100 ml and filtered (Whatman #42). Substrate (2.9
ml o f 0.3% H 2 O2 , 1% guaiacol in 0.5 M sodium phosphate buffer, pH 6.5) and vegetable
filtraiu (0.1 ml) were combined in a cuvette. Absorbance (470 nm) was read at 1-min
intervals for 2 min. Enzyme activity is expressed as change in absorbance/min; 1 unit of
activity is defined as a change in absorbance o f 0.001/min.
Instrumental Color
Approximately 50 g broccoli stems or florets were chopped into 5 mm pieces and
placed into the sample cup (5 cm diameter) o f a Hunter spectrocolorimeter (LabScan 600,
Hunter and Associates, Reston, VA). Spectral reflectance (400-700 nm) was determined
26
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
using illuminant A, and CIE Lab values were calculated (CIE 1978). Determinations
were made in triplicate. Hue angle was calculated as tan'1 (b/a). Total color was
calculated as La2/b. Chroma was calculated as (a2 + b2)1/2 (Gnanasekharan et al. 1992).
Sensory Evaluation
An 8-member sensory panel was trained during two 1-h sessions to use a 9-point
scale to evaluate color (1 = very pale or dull green, 9 = very bright green), appearance (1
= not at all plump, 9 = very plump), grassy aroma, sweetness, raw starchy flavor, broccoli
flavor, overcooked-canned flavor, aftertaste (astringent, metallic) (1 = none at all, 9 =
very intense), tenderness crispness and wetness (1 = not at all, 9 = very). Judges were
trained with commercially available fresh and frozen broccoli. Judges also scored
samples for degree of liking (1 = dislike extremely, 9 = like extremely). Samples
(approximately 20 g), coded with 3-digit numbers, were presented to judges immediately
after cooking in random order in 50 ml glass beakers covered with watch glasses.
Distilled water (22C) was presented between samples for cleansing the palate.
Evaluations were made in individual sensory booths under controlled environmental
conditions (22C, positive air pressure, incandescent lighting).
Statistical Analyses
Three complete replications were conducted; triplicate determinations were made
from each replicate for chemical and physical characteristics. Broccoli was harvested on
three consecutive days; each harvest was considered a replication. Each batch was
subdivided into fifteen aliquots; three aliquots were subjected to each blanching
treatment. One aliquot was analyzed immediately after blanching, the remaining two
were frozen for four weeks; one frozen aliquot was used for sensory evaluation, the other
27
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
for chemical and color analyses. Data were subjected to analysis o f variance (ANOVA)
treating blanching treatment as the main effect. Means that were significantly different
(p<0.05) were separated using Fisher’s least significant difference (LSD) test. Sensory
data were subjected to a one-way ANOVA for repeated measures and means were
separated using LSD. Data are expressed as least squares means ± standard deviations.
Results and Discussion
Enzyme activity in fresh unblanched samples ranged between 389 and 829 units.
After blanching peroxidase activity was < 3 units (data not shown). In frozen unblanched
broccoli peroxidase activity ranged between 99 and 167 units while frozen blanched
samples had < 7 units. Some slight peroxidase regeneration occurred in blanched
samples while it decreased in unblanched samples during freezing storage. These results
agree with those o f Wang and DiMarco (1972) that during high-temperature short-time
(HTST) treatments o f vegetables, regeneration occurs within hours or days o f heat
treatment and may even occur after several months o f freezer storage. Lu and Whitaker
(1974) noted that even when this enzyme is inactivated during heat treatment, peroxidase
activity often regenerates during storage.
Blanching had no significant effect (p>0.05) on moisture content o f broccoli (data
not shown). These findings are consistent with those o f Newsome (1980) who reported
that small losses o f soluble solids occur by leaching into the water used for blanching
occur.
Ascorbic acid content o f fresh, blanched, and frozen broccoli is shown in (Table
2.1). These data indicate that fresh broccoli contained significantly more reduced
ascorbic acid (RAA) than did blanched samples. However, no treatment differences
28
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
existed in average RAA content among freshly blanched (unfrozen, uncooked) samples.
After 4 weeks o f frozen storage, the lowest RAA content occurred in unblanched samples
followed by BW-, ST-, and MWB-blanched samples (69.09,75.91, and 74.32 mg/100 g,
respectively). The highest RAA content (77.02 mg/100 g) occurred in the MW-blanched
samples. True retention followed the same trend as absolute RAA content (Table 2.2).
These results agree with those o f Newsome (1980) who reported that the water-soluble
vitamins, especially vitamin C, are more susceptible to losses due to blanching.
Blanching with steam, hot air, or microwave ovens does not require immersion in water
and, hence, substantially reduces leaching o f vitamins. Losses of vitamin C can occur
even at freezer temperatures.
Before storage, ST-blanched broccoli had the lowest L* value and unblanched
broccoli had the highest L* value for both stems and florets (Table 2. 3). After frozen
storage no significant lightness differences existed due to blanching treatments. BWblanched broccoli had greener (a* value) stems and ST-blanched broccoli had greener
follicles (Table 2.4).
Unblanched broccoli was less green than all blanched treatments. Before storage,
unblanched florets and stems were less yellow (b* value) than blanched samples (Table
2.5).
Sensory evaluation data revealed that after four weeks frozen storage, broccoli
appearance, texture, flavor, and general acceptability were significantly different due to
blanching treatment (Table 2.6). After four weeks in frozen storage, BW-blanched
broccoli has the highest scores for visual appearance and color; control (unblanched)
samples had the lowest scores for these characteristics. No difference existed between
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
ST ' and MW-blanched samples for these characteristics. Control (unblanched) samples
had the highest texture scores while BW- and MWB-blanched samples had the lowest.
BW- and MW-blanched broccoli had (he highest flavor and off-flavor intensity scores
while control (unblanched) samples had the lowest. General acceptability scores were
highest for BW-, ST- and MW-blanched broccoli and lowest for control (unblanched)
samples. Green color intensity (-a* value) o f both stems and florets increased (p<0.05)
due to blanching; BW- and ST-blanching had the greatest effects and MWB had the least
immediately after blanching and after frozen storage and cooking. Blanching had a
significant effect on yellow color intensity (b* value, Table 2. 5); immediately after
blanching, stems and florets were more yellow than unblanched samples. These
differences remained after frozen storage for florets but not for stems. All blanching
treatments increased chroma (color saturation) o f broccoli florets and stems (Figures 2.1
and 2 .2) compared to unblanched broccoli.
Chroma improvements o f BW-blanched florets, and BW-, ST-, and MWBblanched stems were maintained during frozen storage; chroma o f all other treatments
decreased during storage compared with freshly blanched broccoli.
CONCLUSION
This study indicates that microwave-blanched broccoli retained the greatest
amount of reduced ascorbic acid, had appearance, visual color and texture scores
equivalent to ST-blanched broccoli, and flavor and general acceptability scores
equivalent to BW-blanched broccoli; chroma o f MW-blanched broccoli was as good or
better than ST-blanched broccoli immediately after blanching and after four weeks in
frozen storage.
30
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 2 .1. Effect o f blanching method on reduced ascorbic acid (RAA) content o f
broccoli.
RAA, mg/100 g
Treatment
After Blanching
After Cooking
Control
140.44 ± 0.5 \u
57.96 ± 0.82e
BW
112.26 ± 0 .4 2 d
69.09 ±0.05d
ST
111.79 ±0.83
75.91 ±0.44b
MW
111.83 ± 0.83
77.02 ±0.24*
111.37 ±0.48
74.32 ± 0.35°
MWB
Control = unblanched; BW = boiling water blanched, 4 min; ST = steam
blanched, 4 min; MW = microwave blanched in covered 1 L glass container,
4 min; MWB = microwave blanched in a 1 L boilable bag, 4 min.
2n = 9
4bcdeMeans in a column with different superscript letters are significantly
(p<0.05) different.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 2.2. Reduced ascorbic acid retention of broccoli after blanching and after 4
_______ weeks frozen storage and cooking._____________________________
Percent Retention
Treatment
After Blanching
After Cooking
Control
100.00 ±024**
39.19 ± 0.17d
BW
80.52 ±0.76b
49.59 ± 0.52c
ST
81.92 ±0.76b
54.67 ± 0.91b
MW
84.64 ±0.30b
58.22 ± 0.39“
MWB
82.37 ±0.03b
54.85 ± 0.35b
‘Control = unblanched; BW = boiling water blanched, 4 min; ST = steam blanched,
4 min; MW = microwave blanched in covered 1 L glass container, 4 min; MWB =
microwave blanched in a 1 L boilable bag, 4 min.
2n = 9
“'““‘Means in a column with different superscript letters are significantly (p<0.05)
different
32
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 2. 3. Instrumental lightness (I* value) o f freshly blanched and cooked frozen,
broccoli florets and stems.
Blanching Treatment
Treatments
Control
BW
ST
MW
MWB
After
Blanching
52.00±0.33u
46.19 ±3.29b
41.46 ±3.17c
43.15 ± 2.24bc
43.99 ±3.22**
After
Cooking
42.71 ±1.47
44.52± 5.14
43.96±4.51
47.92± 3.26
41.78 ±3.32
57.74 ±0.68a
52.46 ± 2.21b
49.81 ± 1.80c
48.67 ± 3.66c
52.16 ± 3.63b
Florets
Stems
After
Blanching
After
Cooking
47.51 ± 0.76
52.03 ± 0.97
51.22 ± 1.00
49.82 ± 8.79
49.88 ±3.35
ln = 9
abcMeans in a row with different superscript letters are significantly (p<0.05) different.
33
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 2 .4. Instrumental green color (-a* value) o f freshly blanched and cooked frozen
__________ broccoli florets and stems.________________________________________________________
_____________________________ Blanching Treatment____________________________
Treatments
Control
BW
ST
MW
MWB
Florets
After
Blanching
-6.47± 0.181>C
-10.67 ± 0.40a
-9.60 ± 0 .6 0 *
-9.58 ± 1.06*
-8.58±0.52b
After
Cooking
-6.39 ± 1.20c
-9.92±1.47a
-8.55 ± 0 .4 7 *
-8.98 ± 1.58*
-7.73±0.67b
-6.37± 0.57d
-11.61 ± 0.88a
-11.45 ± 0 .4 8 *
-1 0 .4 2 ± 0 .8 lbc
-9.07±1.38c
Stems
After
Blanching
After
Cooking
-7.40 ± 0.29c
-10.71 ±0.64*
-10.60 ± 1.28tt
-9.10±2.61b
-9.25±1.09b
ln = 9
#bcdMeans in a row with different superscript letters are significantly (p<0.05) different.
34
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 2. 5. Instrumental yellow color (b* value) o f freshly blanched and cooked frozen
broccoli florets.
Blanching Treatment
Treatment
BW
MWB
ST
MW
Control
Florets
After
Blanching
29.12 ± 0.85l,b
33.07 ± 0.37*
31.75 ± 0 .7 4 ab
32.89 ± 1 .6 2 *
33.16 ±1.71a
After
Cooking
22.79 ± 2.96b
30.06 ± 1.33a
29.62 ± 3 .7 9 a
29.96 ± 2.02a
27.79 ± l ^ 8”
30.98 ± 0.63b
33.90 ± 0.7 4 a
32.84 ± 0 .3 6 a
32.62 ± 1.30ab
32.78 ±0.83a
Stems
After
Blanching
After
30.23 ± 2.52
32.19 ± 1 .6 0
30.99 ±2.37
32.06 ±1.64
Cooking
32.82 ± 0.25
‘n = 9
abMeans in a row with different superscript letters are significantly (p<0.05) different.
35
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 2 .6 . Effect o f blanching treatment on sensory characteristics o f broccoli cooked
_________ after 4 weeks in frozen storage._____________________________________
________________________ Blanching Treatment_________________
Characteristic
BW
MW
Control
ST
MWB
Appearance2
2.46 ± 1.103,d 3.96 ±0.72*
3.55 ± 0.50b 3.50 ± 0.80b 3.00 ±0.87°
Color4
1.82±0.91d
4.50 ± 0.60a
3.82 ± 0.80b
3.68 ± 0.78*
3.18 ± 1.00c
Texture5
3.86 ± 0.71“
2.46 ± 1.01c
3.00± 0.93b
3.05 ± 0.72b
2.41 ±0.91c
Flavor6
2.14 ± 1.13d
3.32 ± O ^ 81*
3.23 ± 0.87bc
3.55 ± 0.83“
3.18 ±0.96°
Off-flavor7
2.14± 1.16d
4.50 ± 0.91“
4.2 7 ± 0 .8 8 bc 4.32 ± 0.89abc 3.96 ± 0.95°
General
1.96±0.89c
3.52± 0.85a
3.61 ± 0.64“ 3.15 ±0.81b
3.55 ± 0.79a
Acceptability8
Control = no blanching treatment; BW = boiling water blanched, 4 min; ST = steam
blanched, 4 min; MW = microwave blanched in 1 L covered glass container, 4 min, 700
watts; MWB = microwave blanched in 1 L boilable bags, 4 min, 700 watts.
2n = 24.
3Scale: 1 = very wrinkled, sloughed or broken; 5 = intact.
4Color: 1 = very off-color; 5 = very bright natural.
5Texture: 1 = mushy; 5 = very firm,
fla v o r: 1 = very unnatural; 5 = very fresh.
7Off-flavor: 1 = definite off-flavor, 5 = no ofF-flavor.
8General Acceptability: 1 = very poor; 5 = very good.
abcdMeans in a row with the same superscript letters are not significantly (p<0.05)
different.
36
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 2 .1 . Chroma o f freshly blanched and frozen broccoli florets.
35n 30
n
After blanching
Fresh
BW
ST
Treatments
MW
■ After freezing
MWB
Fresh = untreated; BW = boiling water; ST = steam; MW = Microwave;
MWB = Microwave blanching in bag.
Figure 2. 2. Chroma o f frehly blanched and frozen broccoli stems.
After Blanching
Fresh
BW
ST
MW
■ After Freezing
MWB
Treatments
Fresh = untreated; BW = boiling water; ST = steam; MW = Microwave;
MWB = Microwave blanching in bag.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
CHAPTER 3 MICROWAVE BLANCHING EFFECTS ON COLOR, CHEMICAL
AND SENSORY CHARACTERISTICS OF FROZEN ASPARAGUS
Introduction
The blanching process involves exposing plant tissue to some form o f heat,
usually steam or hot water, for a prescribed time at a specified temperature, usually 7010SC (Barret and Theerakulkait 1995; Williams et al. 1986; Glasscock et al. 1983).
Many o f the quality changes in vegetables in frozen storage are attributed to
enzyme systems including peroxidase, ascorbic acid oxidase, lipoxygenase and
polyphenol oxidase (Williams et al. 1986; Katsaboxakis and Papanicolaou 1984; Bottcher
1975). Off-flavor develops rapidly in unblanched vegetables (Williams et al. 1986).
Ascorbic acid oxidase is known to destroy the active form o f ascorbic acid during
vegetable storage, fresh or frozen (Brewer et al. 1994; Brewer et al. 1995; Klein 1992).
Unless some type o f enzyme inactivation step is employed, ascorbic acid is lost dining
frozen storage (Fennema 1975,1977). Destruction o f this enzyme occurs during
blanching, preserving ascorbic acid content o f frozen vegetables. Lane et al. (1985)
reported no difference (p>0.05) in overall ascorbic acid retention o f vegetables (mustard
greens, green beans, purple hull peas and squash) blanched by microwave (62%), steam
(65%) or boiling water (61%). Mabesa and Baldwin (1979) reported that peas cooked
with added water in microwave ovens contained significantly less ascorbic acid than did
those cooked without water, losses were attributed to leaching out o f ascorbic acid into
the cooking liquid. However, Drake et al. (1981) reported that microwave-blanched
38
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
asparagus contained significantly less ascorbic acid than did asparagus blanched in steam
or boiling water.
O bjective o f the Study
The objective o f this study was to compare the effects o f microwave, boiling
water and steam blanching on chemical, physical and sensory characteristics o f asparagus
immediately after blanching, frozen (-18F) for 1 month and cooking.
M aterials and Methods
Sample Preparation
Asparagus (cv. Sweet purple) was purchased from a local, commercial grower in
late May, 1995. Three pickings (replications) o f asparagus were made in the early
morning; asparagus was transported on ice to the University o f Illinois Food Science
Laboratories. Asparagus was sorted, trimmed to 5 cm, washed, blanched and assayed
within 6 h o f harvest.
Asparagus (225 g) was blanched by four different methods: Conventional boiling
water (BW: 1900 mL water, 4 min), steam (ST: 300 mL water, 4 min), microwave
heated in 1 L Pyrex containers (MW: 60 mL water, 4 min, 700 W) in an Amana
Radarange (model #R5458P), or microwave heated in 1 L boilable bags (MWB) using
laminated polyester/polyethylene bags (Seal-a Meal™ bags; O 2 transmission rate =
0.767-1.749 cc/625 cm/24 h, Dazey Products Co., Industrial AirporLKS). Blanching
times and proportions o f vegetables/water were based on average times for boiling water
and steam blanching recommendation (USDA, 1978). Blanched samples were cooled in
ice water immediately after blanching, drained, bagged and weighed. Air was pressed
out o f bags, bags were heat-sealed and stored at -18C for 4 weeks. After 4 weeks frozen
39
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
storage, asparagus was removed from freezer, 20g asparagus were taken for peroxidase
analysis and the remainder o f was cooked for sensory evaluation then analyzed for
moisture and ascorbic acid content, and instrumental color evaluation.
Analyses
Peroxidase activity, instrumental color, moisture content and ascorbic acid
content o f raw unblanched, blanched (all treatments), and frozen cooked asparagus were
determined. Sensory analyses were conducted on cooked frozen asparagus.
Moisture Content
Moisture content was determined by drying (55C for 12h, 60C thereafter)
asparagus (5 g) to a constant weight (AOAC, 1990).
Peroxidase Activity
Peroxidase activity was determined spectrophotometrically as described by
Hemeda and Klein (1990). Asparagus (20 g) was homogenized with cold deionized
water, made up to 100 mL and filtered (Whatman #42). Substrate (2.9 mL o f 0.3%
H 2 0 2 ,1% guaiacol in 0.5 M sodium phosphate buffer, pH 6.5) and asparagus filtrate (0.1
mL) were combined in a cuvette. Absorbance (470 nm) was read at 1-min intervals for 3
min. Enzyme activity is expressed as change in absorbance/min; 1 unit o f activity is
described as a change in absorbance o f 0.001/min.
Instrumental Color
Asparagus stems (50 g) and tips (50 g) were cut into 5mm pieces and placed into
separate sample cups (5 cm diameter) o f a Hunter Spectrocolorimeter (Labscan 6000,
Hunter and Associates, Reston, VA). Spectral reflectance was determined over the 400700 nm range using illuminant A, and CIE Lab values were calculated (CIE 1978).
40
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Reduced Ascorbic Acid
Reduced ascorbic acid (RAA) was determined using the titrimetric assay
described by Pelletier (1985). Asparagus (25 g) was extracted with 100 mL 6%
metaphosphoric acid for 2 min in a homogenizer, made up to 500 ml with extraction
solution and filtered (Whatman #2). Triplicate aliquots (10 mL) o f filtrate were titrated
with 2,6-dichlorophenolindophenol. RAA true retention calculated as percent o f total
RAA in raw unblanched sample retained in the cooked vegetable.
True retention = (RAA/g cooked) x (cooked wt) (RAA/g raw) x (225 g) x 100
Sensory Evaluation
A 5-point scale (1 = very low, poor, or unnatural; 5 = very high, good or natural)
was used to evaluate appearance, color, texture, natural flavor, off-flavor and general
acceptability of asparagus (frozen, cooked). Commercial frozen and canned asparagus
were used as standards for these quality characteristics to train an 8-member sensory
panel (Poste 1991). Coded with 3-digit numbers, samples (30 g) were presented to
judges in random order immediately after cooking in the microwave (200 g + 125 mL
water, 4 min, 700 W ). Distilled water (22C) was presented to rinse the palate.
Evaluations were made in the Sensory Analysis Center o f the Department o f Food
Science and Human Nutrition, University o f Illinois, under controlled environmental
conditions. Sensory evaluation was conducted only on cooked products after 1 month
frozen storage.
Statistical Analyses
Asparagus was picked in three batches (same day); each batch was considered a
replication. Each batch was divided into fifteen portions corresponding to 5 treatments x
41
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
3 portions/treatment (1 portion for immediate post-blanching analyses; 2 portions for
post-freezing analyses). One frozen portion was used for chemical and physical analyses
and the other was used for sensory evaluation. Three complete replications were
conducted; determination were made in triplicate for each replicate for chemical (RAA,
peroxidase, moisture content) and physical (instrumental color) characteristics. Data
were subjected to analysis o f variance (ANOVA). Significantly different (p<0.05) means
were separated using Fisher's least significant difference (LSD). One-way ANOVA for
repeated measure was conducted for sensory data and means for significantly different
treatments were separated using LSD.
Results and Discussion
Moisture content was highest (94.51%) in fresh, unblanched asparagus (data not
shown). The lowest moisture occurred in MWB-blanched followed by MW-blanched
asparagus. Drake et al. (1981) reported that MW-blanched asparagus had a significantly
lower drip loss than either BW- or ST-blanched asparagus which would seem to
contradict our findings, however, Drake et al. (1981) blanched for 3-4 min in a 1500 watt
oven while we blanched for 4 min in a 700 watt oven. After freezing, no significant
difference in moisture content existed between treatments in the present study.
Peroxidase activity in fresh, unblanched asparagus ranged between 107 and 180
units; after all blanching treatments, enzyme activity ranged from 1 to 7 units (data not
shown). Frozen, unblanched asparagus had peroxidase activity between 98 and 114 units
while frozen, blanched asparagus had 2 units. Essentially no peroxidase regeneration
occurred. The F-value at 82C for asparagus peroxidase has been reported to be between
2 and 27 min to reduce activity to <1% (Schwimmer 1944).
42
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Reduced ascorbic acid (RAA) content of fresh/unblanched, fresh/blanched, and
frozen asparagus, is shown in Table 3 .1 . Drake et al. (1981) reported that MW-blanched
asparagus had only about half the ascorbic acid (mg/100 g) as BW- and ST-blanched
asparagus, however they exposed their MW-blanched samples to about twice as much
microwave energy as we did. In the present study, RAA content was 18.35 mg/100 g in
frozen, unblanched samples; after freezing, MW-blanched asparagus had the highest
RAA content, followed by MWB- and ST-blanched samples. During frozen storage,
significant RAA loss occurred in unblanched asparagus; blanching reduced these losses
but the degree o f reduction depended on the blanching procedure. Brewer et al. (1995)
reported similar RAA losses during frozen storage o f unblanched broccoli.
True retention o f RAA immediately after blanching was highest for MWB- and
MW-blanched samples (90.97 and 88.00%, respectively) (Table 3.1). After frozen
storage, the unblanched samples retained less than 40% of the original RAA; both types
o f microwave blanching resulted in over 80% RAA retention. RAA retention was
optimized when exposure to water during the blanching treatment was minimized.
Brewer et al. (1995) reported lower RAA retention, but similar trends, in frozen,
blanched broccoli; poorer RAA retention in broccoli may have been related to increased
cut surface area exposed to water and heat as broccoli florets were trimmed to similar
sizes.
After frozen storage, unblanched asparagus had the highest appearance score,
while MW-blanched and unblanched asparagus had the highest scolor scores (Table 3.2).
Brewer et al. (1995) reported that, after frozen storage, BW-blanched broccoli had the
highest appearance and color scores followed by ST- and MW-blanching; unblanched
43
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
broccoli had the lowest appearance and color scores. Brewer et al. (1994) reported no
difference in either visual color or appearance scores o f frozen green beans due to
blanching treatment. In the present study no significant texture, flavor or general
acceptability differences in asparagus existed due to blanching treatment. Flavor scores
were generally low (<2.00) for all blanching treatments. General acceptability was >3.25
for all treatments indicating that appearance and color must have had a greater impact on
acceptability than did flavor. Brewer et al. (1994,1995) found that blanching treatment
had significant effects on flavor, off-flavor, tenderness and crispness o f green beans and
broccoli that had been in frozen storage. General acceptability was highest for BW-, STand MW-blanched broccoli and lowest for unblanched, frozen broccoli which paralleled
flavor scores (Brewer et al. 1995).
Instrumental color characteristics o f blanched and frozen asparagus are shown in
(Table 3. 3). Freshly blanched samples were lighter (higher L* values) than samples after
frozen storage. Blanching had no effect on lightness o f asparagus stems or tips; after
frozen storage unblanched asparagus stems were significantly darker than blanched
stems. Unblanched, MW- and MWB-blanched tips were darker than BW-blanched tips.
Brewer et al. (1995) reported that blanching darkened broccoli florets and stems; these
effects were lost after frozen storage.
Table 3 .4 shows that Asparagus was greener (more negative a* value) after
blanching and less green after frozen storage. Immediately after blanching, stems o f
blanched asparagus were greener than fresh (unblanched) samples; BW-, ST- and MWblanched tips were greener than unblanched and MWB-blanched tips. After frozen
storage, blanching treatment differences were lost and both stems and tips were less green
44
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
than immediately after blanching. Similar blanching and frozen storage effects have been
reported for broccoli (Brewer et al. 1995). Drake et al. (1981) reported that MWblanched asparagus and green beans were less green than their BW- and ST-blanched
counterparts. Brewer et al. (1994) reported that BW-, ST-, MW- and MWB-blanched
green beans were greener (higher hue angle) than unblanched beans; this trend was still
evident after frozen storage. All samples were more yellow (higher b* value) after frozen
storage than immediately after blanching. Immediately after blanching, BW-, ST- and
MW-blanched asparagus tips were more yellow than fresh and MWB-blanched tips.
These differences were lost after frozen storage (Table 3. 5). No differences in
yellowness of stems existed either immediately after blanching or after frozen storage.
Brewer et al. (1995) reported similar blanching and freezing effects on yellowness o f
broccoli florets and stems. Similar color changes have been reported in blanched green
beans during frozen storage (Katsaboxakis and Papanicolaou 1984).
Implications
After frozen storage, MW- and MWB-blanched asparagus retained more RAA
than other treatments. Visual color was better in MW-blanched asparagus but other
sensory characteristics were unaffected. Instrumental lightness (L*), greeness (-a*) and
yellowness (b*) changed inconsistently with blanch treatment. From the standpoint o f
nutrient retention, visual color and overall acceptability, MW-blanching for 4 min at 700
watts was the best blanching method for asparagus quality preservation.
45
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 3.1. Reduced ascorbic acid content and retention in fresh, blanched and frozen
_________ asparagus.____________________________________________________
Reduced Ascorbic Acid Content, mg/100 g
Treatment
Unblanched
Blanched
48.75 ± 0.20“
Frozen,
18.35 ± 0.85f
BW1
ST1
MW1
MWB1
44.33 ± 0.37d 45.94 ± 0.31“
47.30 ± 0.56b
46.78 ± 0.92b
40.74 ± 1.1 7“ 42.31 ± 0.27c
44.30 ± 0.35d
43.42 ± 0.54“
cooked
Reduced Ascorbic Acid Retention, mg/100 g
Treatment
Unblanched
BW1
ST1
MW1
MWB1
Blanched
100.0 ±0.102“ 72.90 ± 2.96d 83.91 ± 1.05“
88.00 ± 3.67b
90.97 ± 2.49b
Frozen,
39.65 ± 1.67f
83.85 ± 1.55“
87.35 ±2.20“
61.25 ± 2.40“ 71.76 ± 2.39d
cooked
1. BW = boiling water; ST = steam; MW = microwave blanched; MWB = microwave
blanched in the bag.
2. Means ± standard deviation.
“‘ fMeans in the same column with different superscript letters are different (p < 0.05).
46
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 3.2. Sensory characteristics of frozen/cooked asparagus
Characteristics
Treatment
Appearance
Color
Texture
Flavor
General
acceptability
Unblanched
4.38 ± 0.85a
4.48 ± 0.45a
2.65 ±2.18
1.52 ± 0.58
3.44 ±1.42
BW2
3.58 ± 0.99b
4.04 ± 0.77b
2.63 ± 1.78
1.56 ± 0.58
3.46 ±1.11
ST3
3.48 ± 1.18b
3.73 ± 0.98c
2.56 ±1.80
1.67 ± 0.4 6
3.48 ±1.04
MW4
3.50 ± 1.04b
4.21 ±0.67ab
3.19 ±1.73
1.48 ±0.58
3.31 ±1.14
MWB5
3.46 ± 0.79b
3.77 ± 0.78bc
3.00 ± 1.48
1.67 ± 0.4 6
3.27 ±1.07
1. Sensory scale: 1 = very low, poor or unnatural; 5 = very high, good or natural.
2. BW = Boiling water blanched, 4 min.
3. ST = Steam blanched, 4 min.
4. MW = Microwave blanched, 4 min.
5. MWB = Microwave blanched in Seal-a-Meal bag, 4 min.
a,b,cMeans in same column with same superscript are not different (p > 0.05).
47
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 3. 3. Instrumental color (L*) of blanched and frozen asparagus stems and tips.
Time
Treatment
After blanching
After frozen storage
L* value
Stems
Unblanched
36.65 ± 1.78
28.33 ±2.59a
BW1
34.98 ± 3.57
30.21 ±0.71ab
ST2
38.63 ±3.15
31.43 ±1.91*b
MW3
37.71 ± 2.52
32.30 ±2.91*
MWB4
36.82 ± 1.73
32.04 ±1.89ab
Unblanched
28.14 ±6.71
26.27 ± 1.35
BW1
33.76 ±1.35
30.04 ±2.09
ST2
34.54 ±1.35
27.78 ±0.82
MW3
33.43 ± 1.88
26.71 ±2.06
MWB4
30.29 ± 1.44
26.55 ±0.77
Tips
1. Boiling water blanched, 4 min.
2. Steam blanched, 4 min.
3. Microwave blanched, 4 min.
4. Microwave blanched in Seal-a-Meal bag, 4 min.
a,b,c. Means in the same column with same superscript are not significantly different
(p > 0.05).
48
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 3.4. Instrumental color (a*) of blanched and frozen asparagus stems and tips.
Time
Treatment
After blanching
Stems
After frozen storage
a* value
Unblanched
-5.04 ± 1.12b
-1.66 ±0.73
BW1
-7.68 ±0.64*
-3.19 ± 0.34
ST2
-8.56 ±0.21*
-2.90 ± 0.42
MW3
-8.1 ±2.52*
-3.14 ± 0.25
MWB4
-7.62 ± 0.23*
-3.14 ± 0.87
Unblanched
-2.54 ± 0.42b
-0.22 ± 0.67
BW1
-6.00 ± 0.47*
-1.34 ± 0.47
ST2
-4.64 ± 2 .19*”
-0.16 ±0.25
MW3
-4.92±0.96*b
-0.80 ± 0.85
MWB4
-2.57 ± 1.12b
-0.14 ± 1.08
Tips
1. Boiling water blanched, 4 min.
2. Steam blanched, 4 min.
3. Microwave blanched, 4 min.
4. Microwave blanched in Seal-a-Meal bag, 4 min.
a,b,c. Means in the same column with same superscript are not significantly different
(p > 0.05).
49
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 3.5. Instrumental color (b*) of blanched and frozen asparagus stems and tips.
Time
Treatment
After frozen storage
After blanching
b* value
Stems
Unblanched
8.97 ± 2.59
27.23 ± 2.62
BW
14.44 ±3.48
27.61 ± 3.16
ST
12.68 ±2.41
25.58 ± 1.07
MW
13.35 ±2.55
27.54 ± 0.73
MWB
15.63 ± 0.33
26.09 ± 1.83
Unblanched
18.21 ± 1.18b
6.74 ± 0.29
BW
22.77 ± 1.2 0 “
8.28 ±1.51
ST
22.92 ± 2.3 2 “
7.54 ±3.51
MW
22.41 ± 2 .3 6 “
8.09 ± 1.60
MWB
19.18 ± 1.19b
7.63 ±2.78
Tips
1. Boiling water blanched, 4 min.
2. Steam blanched, 4 min.
3. Microwave blanched, 4 min.
4. Microwave blanched in Seal-a-Meal bag, 4 min.
a,b,c. Means in the same column with same superscript are not significantly different
(p > 0.05).
50
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
CHAPTER 4. DEVELOPMENT OF A MODEL SYSTEM TO EVALUATE
ASCORBIC ACID PRESERVATION AT VARIOUS
TIME/MICROWAVE POWER LEVELS
Objectives of the Studies
A variety of times and temperatures can be used to blanch vegetables. However,
the optimum time and/or temperature is that combination which will maximize/optimize
peroxidase inactivation and minimize RAA losses. To test each vegetable for this optimal
outcome, would be time consuming and costly. Therefore, this project aimed to develop a
mathematical model, which can be applied to a variety o f vegetables for blanching
situations.
A comparison o f boiling water blanching, steam blanching, microwave blanching
and microwave blanching of green vegetables at 100°C and high power level was done.
Physical (color) analysis o f blanched samples from all methods was done visually and by
using a HunterLab Spectrocolorimeter (CIE, 1978). Sensory evaluation o f appearance,
color, texture, flavor, off-flavor and general acceptability o f samples from all blanching
treatments was conducted. Chemical analysis for reduced ascorbic acid determination
using the titrimetric assay described by Pelletier (1985) was done. Moisture content
determination after blanching and after frozen storage was accomplished to a constant
weight (AOAC, 1980). Peroxidase activity for all blanching methods after blanching and
after frozen storage was determined spectrophotometrically as described by Hemeda and
Klein (1990).
51
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
These comparisons and determinations were conducted under identical conditions
(similar time interval, natural pH and boiling temperature or 100% power level using 700
W microwave oven). However, the influences o f different microwave power levels and
various time conditions on chemical, physical and sensory attributes o f green vegetables
were not been studied.
Microwave ovens have become an essential device o f kitchen appliances.
Housewives use it for most o f their cooking and preservation tasks. No research data are
available to answer the questions raised by housewives and gardeners pertaining to
microwave blanching using various times and power levels.
Objective of the study
The objectives of this study were to develop a model system for evaluating
ascorbic acid preservation and peroxidase inactivation using various times/microwave
power levels combinations, and to optimize this method for evaluating other vegetables.
Broccoli used as a “model system” and was subjected to a variety o f time/power level
combinations. This method was then tested using asparagus and green beans to approach
a single conclusion about the applicability o f this standardized method. This was done to
evaluate the ability o f the model to produce comparable (predict) outcomes in other
products relative to actual observed values.
M aterials and M ethods
Broccoli (cv. Brigadiare) was obtained from the Horticulture Department,
University o f Illinois south farms in late June and early July (45 to 55 days o f age).
Samples were refrigerated (10°C) immediately (within 30 min.) after harvest and
analyses were initiated within lh and completed within 8h o f harvest.
52
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Broccoli was manually sorted, rinsed, and stems immediately below the florets were cut
into 0.8cm size piece in order to obtain a uniform sample. Samples were randomly
allocated to treatments in 300g portions. Broccoli was weighed, transferred to glass bowls
(500 ml Pyrex) and covered.
All samples were heated for 1 ,2 ,3 , and 4 minutes in a covered glass bowls (500
ml Pyrex) without added water in the microwave oven (700W, model NN-6370
Panasonic). Broccoli was analyzed for moisture content, color, peroxidase activity and
reduced ascorbic acid content immediately before and after microwave heating at four
power levels (100%, 70%, 55% and 30%).
Experim ental Design
A 4 (power levels) by 5 (times) factorial design was used: Samples were heated in
the microwave oven (700 W) using four different microwave power levels: 30%, 55%,
70%, and 100% for four time periods: 0 ,1 ,2 ,3 or 4 min. The experimental design is
shown in Figure 4 .1 .
Analyses
Instrumental color, moisture content, reduced ascorbic acid content and
peroxidase activity of raw and all microwave heating treatments were determined in
triplicate.
Instrumental Color Determination
Instrumental color was determined using a HunterLab spectrocolorimeter (Hunter
and Assoc. Reston VA). Approximately 25-30 gram sample was placed in sample cup
(2.5-cm diameter). Spectral reflectance (over the 400-700 nm range) was determined
every 10 nm using illuminant A, and CIE Lab values were calculated (CIE, 1978).
53
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Moisture Content
A five-gram sample in an aluminum-weighing pan was dried at SS°C for 12 h,
60°C thereafter to a constant weight (Wu et al. 1992).
Reduced Ascorbic Acid
Ascorbic acid was determined using the AO AC (1990) method. Sample (25 g)
and 150 mL o f 6% metaphosphoric acid extraction solution was homogenized for 3 min
in a Waring Blender at high speed. The mixture was diluted to 250 mL with 6%
metaphosphoric acid extraction solution, and filtered through Whatman 2V filter paper.
Filtrate (5 mL) was titrated against 2-6, dichloroindophenol (DCIP) solution until a slight
pink color persisted for 15 sec. One ml o f a solution o f standard ascorbic acid (Sigma
Chemicals, St. Louis, Mo) was titrated to produce the standard curve. Ascorbic acid was
calculated as m g /100 gram using the formula:
mg reduced ascorbic acid/lOOg sample = (v) x (T> x fovl x 100
s w x Av
Where:
v = volume (mL) o f DCIP used for sample titration
T = amount o f reduced ascorbic acid (mg) equivalent to 1.0 ml DCEP
sw = sample weight (g)
ov = volume o f the initial extract (ml)
Av = volume o f sample actually titrated (ml)
Peroxidase Assay
Chemicals used for this assay were purchased from Sigma Chemicals, St. Louis,
Mo. Reagents, hydrogen peroxide (3%) 0.5 mL was diluted with distilled water to 50 ml
54
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
to produce a 0.3% H 2 O 2 solution. Solution was prepared the day o f use. 105 m l ethanol
(95%) was added to 95 mL of water and volume was made to 200 mL to produce a 50%
EtOH solution. Guaiacol (1.0 ml) was diluted to 100 ml with 50% EtOH solution.
Sodium phosphate (0.05 M) buffer (pH) were prepared from a stock solutions (0.2M) of
monobasic sodium phosphate (27.8 g) in water (1L); (solution A) and dibasic sodium
phosphate (53.65g) in water (1L); (solution B). Solution A (68.5 ml monobasic sodium
phosphate) and solution B (31.5 ml dibasic sodium phosphate) was added to water (100
mL) then volume was made to 200 ml and labeled as “solution C”. Solution C (25ml
buffer) was added to 50 ml o f water and volume was made to 100 ml.
Substrate Preparation: Substrate was prepared by mixing guaiacol (1.0%) in EtoH (10 ml)
with sodium phosphate buffer (100 mL) and with hydrogen peroxide (0.3%).
Peroxidase activity was determined using a Beckman Coulter DU 640 spectrophotometer
(Beckman Coulter™, Fullerton, CA) as described by Hameda and Klein (1990). Sample
(20 g) was homogenized in Waring blender with 50 ml cold deionized water for 2 min. at
high speed. Volume was made up to 100 mL with deionized water and filtered through
Whatman #42 filter paper. Filtrate (0.1 ml) was added to a cuvette containing 2.9 ml
substrate and absorbance was read at one min intervals for 5 minutes at 470 nm. A
change o f 0.001 per minute in optical density (absorbance) equals one unit o f peroxidase
activity.
Statistical Analysis
Three complete replications o f this experiment were conducted. Data were
analyzed as a 5 (times) by 4 (power levels), factorial design using the General Linear
Model (SAS, 1993). Main effects and interactions were considered significant at p <0.05
55
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
and means were separated using probability o f difference. Pearson’s Correlation was
performed to examine the correlations between different factors (Aref, 1995). Multiple
regression analysis using the General Linear Model (SAS, 1993) was conducted to
develop prediction equations for time and power level with respect to peroxidase
inactivation and ascorbic acid retention optimization using time and power level.
Ascorbic acid and peroxidase activity are the dependent variables and time and
power levels were the independent variables; the General Linear Model relates a
dependent variable (Y: RAA or peroxidase) to independent variables (X: time or power
level). The full model prediction equations for the models were:
Y = p0 + P!(X0 + p2(X2) + p3(X i)2 + p4(X2)2 + p5(XiX2)
Where:
Y = peroxidase activity or ascorbic acid retention
X[ = time
X2 -pow er level
po = intercept
ps= regression coefficients
The terms in the full model were evaluated for their contribution as far as
explaining the variation. Those terms which explained insignificant amounts o f variation
were dropped from the model (for that dependent variable) and the analysis was rerun in
order to reduce the model to those terms, which did explain a significant amount o f
observed variation.
56
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Experiment 1. Broccoli (cv. Brigadiare)
At each temperature (power), the thermal destruction curves for ascorbic acid
(RAA) (% retention) and for peroxidase activity (% inactivation) were determined using
a range o f exposure times. The “powers” are fixed in most home microwave ovens (30,
SS, 70 and 100%), such that those powers are what are available for common usage.
Experiment 2. Green beans (cv. Hilea) and asparagus (cv. Sweet purple)
The same experiments were repeated: (samples heated at 4 powers for 5
times. RAA (mg/100 g) and its retention and peroxidase inactivation were
determined.
Thermal destruction curves were plotted. Experimental (observed) values were
compared to those predicted by the regression equations established during experiment 1.
Results and Discussion
Broccoli (cv. Brigadaire)
Fresh broccoli contained 92% moisture (Table 4 .1 ), which is essentially the same
as that reported by (USDA, 1991). Various time and microwave power level interactions
were significant (Fig. 4.2). High (100%) and medium high (70%) power levels
significantly reduced moisture content to about 77-80% compared to medium (55%) and
medium low (30%) power levels which maintained the original moisture content (92%).
Longer times (4 and 3 min) produced lower moisture content than did the shorter times.
This might be due greater evaporative losses during heating process.
Fresh broccoli had 85 mg/g reduced ascorbic acid (Table 4 .2 ), which is slightly
lower than the 93.37 mg/g RAA in raw broccoli reported by the (USDA, 1991). There is
57
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
an inverse relationship with the amount o f moisture content in broccoli, as the moisture
content decreased the RAA content increased in broccoli. Results indicated that the 4 min
processing time produced the highest ascorbic acid content followed by 3 and 2 min (Fig.
4. 3). The 1 min treatment had the lowest RAA. Higher power levels (100% and 70%)
produced broccoli with more RAA at all times than did lower power levels. These results
contradict those o f Drake et al. (1981) who reported that microwave heating causes
significant losses o f reduced ascorbic acid. The higher amount o f reduced ascorbic acid
content present in samples treated with high and medium high power levels may be
attributed to greater water losses through evaporation from the broccoli during heating
process, which effectively concentrated the remaining ascorbic acid in the sample. These
samples were heated without water, so leaching o f soluble contents did not occur during
processing. Microwave heating did not significantly affect the ascorbic acid content o f 13 •
o f 16 vegetables when compared to boiling (Bowman et al. 1975). Microwave heating o f
peas can result in greater retention o f ascorbic acid than cooking conventionally in water
(Campbell, 1990). Peas cooked in the microwave without added water retained more
ascorbic acid than did those with water added (Mabesa and Baldwin, 1979). Hudson et al.
(1985) compared the effects o f steaming, microwave heating and boiling o f frozen and
fresh broccoli on ascorbic acid, thiamine and riboflavin content. They found that
steaming was least detrimental and boiling most detrimental to vitamin content, although
microwave heating and steaming retained equal amounts o f ascorbic acid and thiamine in
frozen samples.
Reduced ascorbic acid retention (percent o f original adjusted for weight) in
broccoli is shown in (Table 4 .3 ) and (Figure 4.3). RAA retention significantly decreased
58
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
during microwave heating at all treatments. However, the pattern was similar as in
reduced ascorbic acid content (mg/100) after microwave heating. Samples subjected to
high power levels and longer times retained greater amount o f reduced ascorbic acid (Fig.
4.4). There was a significant interaction between power levels and time. Increased
reduced ascorbic acid retention was found with the higher levels o f time at all power
levels.
Peroxidase activity o f untreated and treated broccoli is shown in (Table 4.4). Peroxidase
activity in unheated broccoli ranged from 3.74-4.31 units/5 min or 0.75 to 0.86 units/min.
All microwave heating treatments had a significant effect on peroxidase activity. The
lowest peroxidase activity occurred in broccoli heated at high power (100%) for 2 ,3 or 4
min followed by that heated at medium high and medium power levels (70% and 55%)
for 4 or 3 min and 4 min (Fig. 4. 5). Broccoli heated at medium low power level (30%)
had more peroxidase activity than that heated at higher power levels. These results are in
agreement with those of Williams et al. 1986) who reported that 40 and 60% o f the
peroxidase activity remained in a green pea homogenate and in whole peas heated to
60°C, while only 5% o f the original activity remained in peas heated to 70°C. In the
present broccoli study, samples exposed to shorter vs. longer times (1 and 2 min vs. 3 and
4 min) showed similar trends. High, medium high and medium power level treated
broccoli had 5 ,6 and 7% remaining peroxidase activity, respectively, while 25-30% o f
the peroxidase activity remained in broccoli heated at the low power level (30%).
Peroxidase, the most heat-resistant enzyme in plants, is used as an indicator o f blanching
process adequacy (Brewer et al. 1994; Baardseth and Slinde, 1980,1981). Peroxidase
combines with endogenous hydrogen peroxide to produce an activated complex that
59
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
reacts with a wide range of food constituents including reduced ascorbic acid, carotenoids
and fatty acids (Brewer et al. 1995). Some o f these reactions cause undesirable changes in
food materials including off-flavor, aroma and color, as well as loss o f some nutrients
(Hemeda and Klein, 1990), therefore residual activity, no matter how low, is significant
in terms o f product quality.
Results o f instrumental color determinations o f fresh and microwave broccoli are
shown in (Tables 4.5., 4.6., and 4. 7). Fresh broccoli (untreated) was lighter (higher L*
value) than treated broccoli. Broccoli darkened significantly at high and medium high
(100% and 70%) power levels at all times. Broccoli heated at medium and medium low
power levels (55% and 30%) was as lighter or slightly lighter than untreated broccoli.
However, no significant differences were found between treatments and untreated
broccoli lightness. Initial heating intensifies the green colors by expelling gases from
intercellular spaces, so that the way in which light is refracted from the cell surface
changes (Bower, 1992). Microwave heating at various time/power levels had no
significant effect on green color (a* value) o f broccoli (Table 4. 6), even though the -a*
value range was wide (-3.37 to -10.08). Microwave heating at various time/power level
treatments had no significant effect on yellowness (b* value) o f broccoli (Table 4.7).
Hue angle values o f broccoli subjected to various time/power level treatments are
shown in Table 8. Hue or “color” is expressed as hue angle (arc tan'1 a/b) in degrees o f
departure away from the -a* axis (Clydesdale, 1984). Hue is the characteristic associated
with the conventional perceived color name; an angle o f 90° represents a yellow hue.
Objects with higher hue angles are greener, while those with lower hue angles are more
orange-red (Gnanasekharan et al.1992). There were no significant differences among the
60
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
untreated broccoli and 1,3 or min/medium low power level treatments for hue angles.
Broccoli heated for 1,3 or 4 m in at medium power level had the same hue angle values.
High time/power level (4 min/100%) and short time and medium power level (1
min/55%) treatment produce broccoli with the similar hue angles. No significant
interactions were found among 1-3 min/high power level, 1-2,2-3 or 3-4 min/medium
high and 2-3 min/medium power level treatments (Table 4. 8).
Chromaticity (brightness or saturation) of fresh and processed broccoli is shown
in (Table 4.9). Chroma (how much color there appears to be) is two-dimensional and is
specified by pairs o f numbers such as dominant wavelength and purity, or “a” and “b”
values (CIE, 1978). It is that part o f a color specification, which does not involve
luminance and the visually perceived qualities o f hue and saturation taken together (CIE,
1978). Microwave heating treatments had no significant effect on chromaticity o f
broccoli.
Green Beans: (cv. Hilea)
Moisture content o f food materials may be determined as an indicator o f the
quality of food (Penfield and Campbell, 1990). Fresh green beans had 92% moisture
(Table 4. 10). After microwave heating, significant evaporative losses occurred at high
and medium high power levels for 1 through 4min treatments (Fig. 4. 6). However, these
losses were not significant at medium and medium low power levels over the same time
period. These results agree with those o f microwave heating o f broccoli.
Raw green beans had 18mg/100 g reduced ascorbic acid (RAA) (Table 4. 11),
which is twice that reported by the USDA (1991) o f 9.6mg/100 g. These differences
might have been due to pH, variety and other natural factors such as environment.
61
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Microwave heating had significant effects on RAA content o f green beans. These effects
were highly significant at high and medium high power levels for 1 to 4 min with these
treatments having more RAA than the untreated samples (Fig. 4. 7). These increments in
RAA might be attributed to evaporative losses during the heating process. This
hypothesis is supported by the finding that reduced ascorbic acid (RAA) contents in
samples heated at medium and medium low power levels for 1 to 4 min were comparable
with those o f raw samples. These results are in agreement our findings with broccoli
heated at medium and medium low power levels for 1 to 4 min.
Reduced ascorbic retention (%) in green beans was found greater at higher power
levels and longer times than the lower power levels and shorter times (Table 4. 12). There
was a significant interaction between power levels and time. All treatments had a
significant effect on ascorbic acid retention (Fig. 4. 8). These results are in agreement with those found in the broccoli experiment.
Raw untreated green beans had 2.72units/5 min or 0.54 units/min peroxidase
activity (Table 4. 13). After microwave heating (all treatments), peroxidase activity was
decreased significantly. Peroxidase activity remained in broccoli treated at high and
medium high power levels for 3 or 4 m in was 30-32% and 52-50% o f the original
peroxidase activity in raw green bean. While medium and medium low power levels did
not affect the peroxidase activity significantly 67-76% and 169-170% at 4 or 3 min.
However, no significant differences were found between the treatments. Peroxidase is
considered to be the most heat-resistant enzyme. The present study demonstrated the heat
stability o f this enzyme, since complete inactivation o f peroxidase was not achieved even
at the highest microwave power level for 4 min (Fig. 4. 9).
62
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Results o f color determinations (L*, a*, b*) o f green beans are shown in (Tables
4.14., 4 .1 5 and 4.16). No significant differences were found in lightness (L* value)
(Table 4.1 4 ) between raw and treated green beans. However, microwave heating had
significant effects on green color (a* value) o f beans (Table 4.15). Beans that were
subjected to medium and medium low power levels for 1 to 4 min were greener than
beans heated at high and medium high power levels.
Green beans heated at high and medium high power levels (100%) for 1 ,2 ,3 or 4
min and to (70%) for 2 ,3 or 4 min were equally yellow (b* value) (Table 4.16). 1-4
min/high power level, 1 min/ medium power level, 1 or 3 min/medium power level and 1,
3 or 4 min/medium low power level treatments retained the original yellowness. No
structured patterns were found between time and power levels.
Effects o f microwave heating (various time/power levels) on hue angle o f green
beans are shown in (Table 4.17). After microwave heating green beans, the hue angle o f
all samples decreased indicating the expulsion o f air from the intracellular spaces and
compactness o f green pigment. Longer time and higher power level treatments had the
hue angle equal to longer time and medium power level and shorter time and medium
low power levels.
Data for chroma o f green beans are shown in (Table 4. 18). Chroma o f green
beans increased slightly due to microwave heating. However, there were no significant
differences between the treatments.
Asparagus: (cv. Sweet purple)
Raw untreated asparagus had 94% moisture (Table 4.19). After microwave
heating, significant interactions occurred between power and times. While 100% for 3
63
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
min and 70% for 4 min treatments did not differ, asparagus subjected to high power
(100%) level for 4 or 3 min had the lowest moisture content (Fig. 4.10). Medium and
medium low power levels at 3 or 4 min had significant effects on moisture content o f
asparagus.
Reduced ascorbic acid content o f asparagus is shown in (Table 4 .20). Raw
untreated asparagus had 46mg/100 g RAA. Significant interactions were found at higher
power levels for all times compared to lower power levels for 1 min and 4 min. After
microwave heating, RAA content increased significantly at high and medium high power
levels in samples heated for 1 to 4 min (Fig 4.11). Similar trends occurred in ascorbic
acid content in broccoli and green beans. Asparagus heated at high and medium high
power levels for 4 min had the highest ascorbic acid content (80-83 m g/100 g) followed
by samples heated at the same power level for 3 min (72-75 mg/100 g). This may be due
to greater evaporative losses at higher powers and longer times, which changes the ratio
o f solid components to solvent.
Ascorbic acid retention data are given (Table 4.21). Ascorbic acid retention had a
similar pattern as ascorbic acid content (m g /100 g), and percent ascorbic acid retention
in broccoli and green beans. A significant interaction was found between time and power
levels (p<0.05) (Fig. 4.12). Low and medium low power levels reduced ascorbic acid to
~40% o f its original value in 1 min, however, no further changes occurred. High and
medium high power levels reduced ascorbic acid content to ~60% o f original, however,
retention increased at 2 ,3 and 4 min, probably due to water volatilization. These results
were consistent with those o f broccoli and green beans experiments.
64
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Raw asparagus had 2.66 units/5 min or 0.53-units/min peroxidase activity (Table
4.22). Microwave heating significantly reduced peroxidase activity. Similar levels o f
inactivation occurred at 100% and 70% power at all times, at 55% for 2 ,3 or 4 min. and
at 30% for 4 min. However, complete enzyme inactivation did not occur for any o f the
treatments (Fig. 4.13). There was still 26%-34% o f the original peroxidase activity
remaining after maximum microwave heating.
Instrumental color data are shown in (Tables 4. 23,4. 24 and 4.25). Untreated
asparagus was lighter than the microwave treated samples (Table 4.23). Asparagus
subjected to 100% power for 1,2 or 3 min, and to 70% for 2 ,3 or 4 min was significantly
darker than the raw samples. No significant interaction occurred between 100% power
and all times, while these samples were greener than the untreated asparagus (Table 4.
24). Asparagus treated at all power levels for 2 ,3 or 4 min was equally green. Raw
asparagus was more yellow than microwave treated samples (Table 4.25). However, the
difference between raw and asparagus treated at high or medium high power level for 1-4
min and 2 or 3 min were not significant. Similarly, asparagus subjected at medium power
level for 4 min and at medium low power level for 1-3 min had the same yellowness as
had the untreated asparagus.
Hue angle for untreated and treated asparagus is shown in (Table 4.26). Raw,
untreated asparagus had a higher hue angle indicating a greater distance from the origin.
Hue angle decreased significantly after microwave heating. This may be due to the
expulsion o f gases during heating from the intercellular spaces. Hue angle o f asparagus
subjected to 100%, 70% and 55% did not differ over the 4 min treatment period,
however, hue angle o f asparagus subjected to 30% power for 1 min did not differ from
65
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
that o f fresh asparagus while treatments o f medium high power level (70%) and medium
low power level (30%) for 4 min produced asparagus with similar hue angle.
Chroma o f raw and treated asparagus is shown in (Table 4.27). Microwave
heating at various power levels and times had no effect on asparagus chroma. These
results agree with our findings for broccoli and green beans.
Thermal curves for RAA retention and Peroxidase destruction
Broccoli
At each temperature (power), the thermal curves for ascorbic acid (RAA) and (%
retained) and for peroxidase activity (% destroyed) were plotted. Calculations for RAA
(%) retention and peroxidase activity (%) destroyed were made from the thermal curves
(intersection point of two curves) were made.
Reduced ascorbic acid and peroxidase activity as percent o f original in raw
broccoli remained/destroyed were calculated. Broccoli heated at high power level for <1
min retained 116% RAA, (Fig 4.14). While broccoli subjected to medium high power
level for >1 min retained 112% RAA (Fig. 4. 15). Peroxidase activity destroyed in
broccoli heated at high power level for <1 min was 69% (Fig 4.14), while medium high
power for >1 min resulted in 75% destruction o f peroxidase activity. 98% and 81%
retention occurred in broccoli heated at medium and medium low power levels for >1 min
and >2 min (Figs. 4.16 and 4. 17). More than 60% peroxidase activity was left in
broccoli heated at medium power level for >1 min and >50% peroxidase activity
occurred in broccoli heated at medium low power level for >2 min. In general the thermal
66
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
curves and data in tables (broccoli) demonstrated that the highest retention/destruction of
RAA/peroxidase activity occurred at longer time and higher power levels.
In green beans, high and medium high power levels for <1 min produced >122%
RAA retention (Figs. 4. 18 and 4.19). Peroxidase activity destruction at same treatments
was 64% and >60% respectively. Thermal curves for green beans heated at medium and
medium low power levels did not meet at any point (Figs. 4 .2 0 and 4.21). However, the
generalization the thermal curves and data tables demonstrated for RAA retention and
peroxidase activity destruction in green bean is similar to that o f broccoli experiment.
High and medium high power levels for <1/2 min resulted in >47% and >35%
peroxidase activity destruction and 111-113% RAA retention in asparagus (Figs. 4.22
and 4.23). Medium and medium low power levels for <1 min produced 103% and
>102% RAA retention and >54% and 67% peroxidase activity destruction respectively
(Figs. 4 .2 4 and 4. 25). However, asparagus subjected to longer times and higher power
level treatments showed higher RAA retention and higher peroxidase destruction.
Data for regression equations and models developed for broccoli, green beans and
asparagus are shown in Table 28 and Table 29. Data were plotted using predicted values
and actual values vs. time (Fig. 4.26) and power levels (Fig. 4.27). Both expected and
actual values demonstrated similar trends in variation. There was a significant interaction
between time and ascorbic acid content. Which demonstrated an increasing variability as
the time increased. Figure 4.28 and Figure 4.2 9 revealed that there were significant
differences between observed and expected values for ascorbic acid retention, while the
broccoli model and predicted values for RAA retention did not show any variability.
Actual and expected peroxidase activity data using the broccoli model vs. time
67
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
and vs. power are given in (Fig. 4 .3 0 and 4.31). The variation between actual and
expected data due to time was not significant, however, broccoli model and predicted
values for peroxidase activity were comparable. Similar trends were observed when these
values were plotted against power levels (100%)(Fig. 4.31).
Green beans
Data for RAA (mg/100 g) vs. time and vs. power in green beans are shown in
(Figs. 4 .3 2 and 4. 33). Actual values and expected values with green beans appear at the
lower end o f the figures. When the broccoli model was applied to the same data, it
demonstrated similar trends at different time periods, as did the actual and predicted
green bean model values. However, with the broccoli model, both control variables (time
and power) demonstrated a highly significant interaction. The predicted outcomes with
broccoli model were greater than the actual and expected results with the green bean
model.
Green bean RAA retention (%) results were not significant and no model could be
developed. Green bean observed and expected values applying the broccoli model were
plotted vs. time (Fig. 4. 34) and vs. power (Fig. 4. 35). The broccoli model predicted
much lower outcomes than the real results o f RAA retention in green beans at longer
times and high and medium high power levels.
The broccoli model for peroxidase might be effective at high and medium high
power levels at all times (Figs. 4 .3 6 and 4.37), since these levels o f independent
variables (time and power) had greater effects on peroxidase inactivation.
68
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Asparagus
The observed and expected data for RAA (mg/100 g) for asparagus were
comparable (Figs. 4.3 8 and 4.39). However, predicted results using the broccoli model
were higher than the observed values and than predicted values using the asparagus
model. These results are consistent with the green bean RAA results with broccoli model.
The observed and predicted data for RAA retention (%) in asparagus were similar
(Figs. 4. 40 and 4. 41). The broccoli model predicted data varied more than the
observed and expected (using the asparagus model) data o f asparagus RAA retention.
Also, it predicted lower outcomes than the observed and expected data and demonstrated
increasing variability as time and power level increased. These results are consistent with
those observed for the green bean RAA retention using broccoli model.
Peroxidase inactivation data are presented in (Figs. 4 .4 2 and 4.43). High and
medium
high power levels at all time periods using the broccoli model predicted greater
peroxidase inactivation than that actually occurred. The broccoli model predicted similar
outcomes for peroxidase inactivation in asparagus as it did for green bean peroxidase
inactivation.
69
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
CHAPTER 5. OVERALL CONCLUSION
This study indicates that microwave-blanched broccoli retained the greatest
amount of reduced ascorbic acid, had appearance, visual color and texture scores
equivalent to ST-blanched broccoli, and flavor and general acceptability scores
equivalent to BW-blanched broccoli; chroma o f MW-blanched broccoli was as good or
better than ST-blanched broccoli immediately after blanching and after four weeks in
frozen storage.
In overall conclusion, these results indicated that the broccoli model could be
used to predict reduced ascorbic acid RAA content (mg/100 g) and peroxidase
inactivation in green beans and asparagus. However, it was not effective to predict RAA
percent retention in green beans. This might have been due to low initial ascorbic acid
content in green beans to begin with. The low initial ascorbic acid content in green bean
might be attributed to factors such as composition, variety, environmental conditions
(cultivation methods, time and amount and type o f fertilizer), transportation and time
lapsed between actual harvests and processing time.
70
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
CHAPTER 6. SUMMARY
Effects o f microwave and microwave in bag (MW and MW-B) and conventional
boiling water and steam (BW and ST) blanching on physical (color), chemical (ascorbic
acid retention and peroxidase inactivation) and sensory characteristics were compared
using broccoli and asparagus. Vegetables were blanched for 4 min (all treatments),
cooled, packed and storage at -18°C for 4 weeks. Physical and chemical analyses were
conducted before, immediately after blanching and after 4 week frozen storage. Sensory
evaluations were only done on cooked vegetables after 4 week frozen storage.
Results revealed that blanching treatments had a significant effect on reduced
ascorbic acid contents (RAA) (mg/100 g) of broccoli (Brewer et al. 1995). However,
there were no significant differences between the treatments. Similar trends were found
for percent (RAA) retention after 4 week frozen storage. Microwave and boiling water
blanched (MW and BW) broccoli, which was cooked after 4 week frozen storage, had
significantly different RAA retention. Peroxidase activity in raw broccoli was
829units/min. After blanching peroxidase activity was <3units/min. However, some
regeneration o f peroxidase activity occurred in blanched samples and it was increased
from <3units/min. to <7units/min. (Brewer et al. 1995). Blanched broccoli was darker,
BW-blanched broccoli was greener than other treatments. Sensory evaluation showed
that MW-blanched broccoli had higher texture, flavor and general acceptability scores
(Brewer et al. 1995).
Blanching treatments had a significant effect on reduced ascorbic acid content
mg/100 g in asparagus (Begum and Brewer, 1997). There were significant differences
between BW-, ST- and MW- and MW-B-blanching treatments for reduced ascorbic acid
71
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
(RAA) content. However, after 4 week frozen storage, percent RAA retention in
asparagus was significantly different (all treatments).
Peroxidase activity in fresh, untreated asparagus was 108 units/min. After
blanching (all treatments) peroxidase activity was 1 to 7 units/min (Begum and Brewer,
1997) and was 2units/min after 4 weeks o f frozen storage.
After blanching, BW- and MWB-blanched asparagus was darker, and greener.
However, after 4 week frozen storage, asparagus (all treatments) was equally darker and
ST-blanched asparagus was greener than all other treatments (Begum and Brewer, 1997).
Sensory evaluations indicated that there were no significant differences between all
treatments for asparagus appearance. ST-blanched asparagus had the lowest color score,
while all treatments had similar texture, flavor and general acceptability scores.
A model was developed to evaluate ascorbic acid preservation in broccoli at
various time/microwave power levels. The implication o f this model was tested to predict
the RAA retention in other vegetables using green beans and asparagus.
Broccoli was used as model system and was heated using S time periods (0 ,1 ,2 ,
3 and 4 min) and 4 microwave power levels (100%, 70%, 55% and 30%). Processed
broccoli was analyzed for moisture content, reduced ascorbic acid, peroxidase activity
instrumental color. Predictive model was developed using regression analysis. Results
indicate significant interaction between time and power levels. RAA content increased
and peroxidase activity decreased as the time and power levels increased. The model
developed for this study was used to evaluate RAA retention in broccoli, which indicated
the actual and predicted values for RAA retention using this model were similar. The
experiment was repeated using two other vegetables, green bean and asparagus. Results
72
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
indicated similar patterns o f time and power level interactions for RAA retention and
peroxidase destruction in these vegetables. The model developed using broccoli, was
effective for RAA prediction in asparagus and green beans. In conclusion, ascorbic acid
retention and peroxidase destruction in all three vegetables was higher at longer times
and higher power levels. Model developed to evaluate RAA preservation in broccoli was
applicable to predict RAA preservation in green beans and asparagus. These findings are
consistent with those o f HTST treatments, which are effective to destroy microorganisms
and enzymes and preserve nutrient quality o f the product.
73
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
CHAPTER 7.
IMPLICATIONS AND FUTURE DIRECTIONS
Implications
Since the introduction o f the microwave oven into the consumer market, its
popularity has increased for home cooking. Microwave ovens are heavily used in
institutional kitchens, home and microwave processing at industrial level (Bennion, 1980;
Giese, 1992). An estimated 92% o f homes have at least one, and 40% have two ovens
(Baum, 1992). Microwave processing can offer several distinct advantages when
compared to conventional heating methods, including speed o f operation, energy savings,
precise process control and faster start-up and shut-down times (Decareau, 1985; Giese,
1992). However, it is not certain whether microwave cooking has more positive or
negative effects on food quality. Giese (1992) stated that since microwaves penetrate into
a food, heating occurs more rapidly; this accelerated heating provides for higher quality
products in terms of taste, texture, and nutritional content. In addition, today’s consumer
wants low-fat, low-salt, low-cholesterol, high-fiber, low-sugar, and preservative-free
products that are convenient to prepare and that taste good (Harlfinger, 1992).
Microwave processing provides the means to attain these processes.
The model system developed to predict ascorbic acid (RAA) retention at various
time/microwave power levels demonstrated that it might be applicable to predict the
RAA retention in other high initial ascorbic acid content vegetables such as Brussels
sprouts, artichoke, kale, kohlrabi and cauliflower. However, this model should not be
expected to be applicable with vegetables that contain low initial ascorbic acid content
such as peas and com.
74
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Future Directions
The present work was done using only vegetables, both green vegetables. These
results might not be applicable to other than green vegetables. It is, therefore,
recommended that further research should be conducted using vegetables such as carrots,
peas and tomatoes etc. Addition o f water should be considered in future microwave
processing o f vegetables at various time/power levels to evaluate the contribution o f
leaching and oxidation of RAA doing microwave processing separately from the
time/power levels effects. Microwave blanching of high-RAA content, light-colored
fruits would also be o f interest with respect to preservation o f RAA and destruction of
polyphenol oxidase which causes browning o f unblanched, frozen apples, peaches etc.
Also, studies on microwave processing (various time/power levels) using meat (beef,
pork, chicken) and fish with altered pH, with or without additives and packaging should
be carried out. Work needs to done to estimate the parameters such as lethality o f heat
process (power levels) and nutrient/quality factor retention as well as D values for
peroxidase and for reduced ascorbic acid in finished products.
75
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
REFERENCES
Ackerman, L. V., Weinstein, L. B., and Kaplan, H. S. 1978. Cancer o f the esophagus.
In “Cancer in China,” ed. H. S. Kaplan and P. J. Tsuchitaui. Pp. 111-136. Liss,
New York.
AO AC. 1990. Official Methods of Analysis, 17th Ed., Assoc, o f Official Analytical
Chemists, Washington, DC.
Aref, S. 1995. SAS Recipes. P.31. Stipes Publishing L.L.C. Champaign, IL.
Aurand, L. W., and Woods, A. E. 1979. Enzymes. Food Chemistry. Avi. Publ.co.
Westport. Conn. Pp: 187-89.
Baardseth, P. and Slindle, E. 1980. Heat inactivation and pH optima o f
peroxidase and catalase in carrot, swede and Brussels sprouts. Food Chem.
5(2): 169-176.
Baardseth, P. and Slindle, E. 1981. Peroxidase and catalase activity in carrot.
Food Chem. 7,147-154.
Banion, M. 1980. The Science of Food. J. Wiley & sons. N.Y. pp:229-230.
Banwart, G. J. 1981. Control o f Microorganisms by Destruction. Basic Food
Microbiology. Abridged TextBook Ed. Avi Pub. Co. Westport. Conn. Pp: 419420.
Barret, D.M. and Theerakulkait, C. 1995. Quality indicators in blanched,
frozen, stored vegetables. Food Technol. 49(1), 62-64.
Barth, M.M., Perry, A.K., Schmidt, S J . and Klein, B.P. 1990. Misting effects
on ascorbic acid retention in broccoli during cabinet display. J. Food Sci. 55,
1187-1188.
Baum, H. M. 1992. Opening remarks. Campbell Microwave Inst. Seminar, Camden, New
Jersey. Jan. 15.
Begum, S. and Brewer, M. S. 1997. Microwave Blanching Effects on Color, Chemical
and Sensory Characteristics o f Frozen Asparagus. J. Food Qual. 20. Pp: 471-481.
Bhamidipati, S. and R. K. Singh. 1996. Model system for aseptic processing o f
particulate foods using Peroxidase. J. Food Sci. 61 (1): 171-175.
Borenstein, B. 1987. The role o f ascorbic acid in foods. J. Food Technol. 41(11): 98-99.
76
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Bottcher, H. 1975. Enzyme activity and quality o f frozen vegetables. 1. Remaining
residual activity o f peroxidase. Nahrung 19. Pp: 173-175.
Bowers, J. 1992. Food Theory and Applications. 2nd ed. MacMillan Publ. Co. New
York. Pp: 653-654.
Bowman, F., Berg, E. P., Chung, A. L., Gunther, M. W., Trump, D. C. and Lorenz, K.
1975. Vegetables cooked by microwaves vs. conventional methods. Retention
of ascorbic acid and chlorophyll. Microwave Energy Appl. Newsletter 8(3):3-8.
Bowman, F., E. Page, E. E. Remmenga., and D. Trump. 1971. Microwave vs.
Conventional cooking of vegetable at high altitude. American Dietetic
Association Journal 58,427.
Brewer, M. S. 1992. Food Chemistry for Consumers. Hand Book. Pp: 7-8.
Brewer, M. S., Barbara P. Klein, Bharati K. Rastogi and Aiko K. Perry.
1994. Microwave Blanching Effects on Chemical, Sensory and Color
Characteristics of Frozen Green Beans. J. Food Qual. 17 (3). Food and
Nutrition Press, Inc. pp: 245-259.
Brewer, M. S., S. Begum., and Ava, Bozeman. 1995. Microwave and Conventional
Blanching Effects on Chemical, Sensory, and Color Characteristics o f Frozen
Broccoli. J. Food Qual. 18 ( 6 ). Pp: 479-493.
Campbell, C. L., T. Y. Lin, and B. E. Proctor. 1958. Microwave vs. Conventional
Cooking 1. Reduced and total ascorbic acid in vegetables. American Dietetic
Association Journal 34. Pp: 365-366.
Chapman, V. J., J. 0 . Putz., G. L. Gilpin., J. P. Sweeney., and J. N. Eisen. 1960.
Electronis cooking o f fresh and frozen broccoli. Journal o f Home Economics 52.
Pp: 161-163.
CIE (Comission Internationale de I, Eclairage). 1978. Recommendations on uniform
Color spaces— color equations, psychometric color terms. Supplement No. 2 to
CIE Publ. No. 15 (E-l.3.1) 1971/ (TC-1-3), CIE, Paris.
Charley, H. 1970. Food Science. John Wiley and sons. N.Y. pp:462-463.
Clydesdale, F. M.1984. Color measurement. In “Food Analysis: Principles and
Techniques. Vol. 1. Physical Characterization,” Gruenwedel, D. W. and
Whitaker. J. (Eds.). Marcel and Dekker, Inc. New York.
Davey, B.L., Dodds, M. L., FisherJLH., Schuck, C. and Shih, D.C. 1956.
utilization o f ascorbic acid in fruits and vegetables 1. Plan o f study and ascorbic
77
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
acid content o f 24 foods. J. Amer. Diet. Assoc. 32:1064.
Decareau, R. V. 1985. “Microwave in the Food Processing Industry.” Academic Press.
New York.
Decareau, R. V. 1992. “Microwave Foods: New Product Development:” Food and
Nutrition Press, Trumbell, Conn.
deMan, J. M. 1990. Vitamins. Principles o f Food Chemistry. 2nd ed. An Avi Book. USA.
Pp: 347-348.
DeRitter, E. 1976. Stability characteristics o f vitamins in processed doods. J. Food
Technol 30 (1). Pp: 48-54.
DRAKE, S.R., SPAYD, S.E. and THOMPSON, J.B. 1981. The influence o f blanch and
freezing methods on the quality o f selected vegetables. J. Food Qual. 4,271-278.
Drake, S.R., and Carmicheal, D. M. 1986. Frozen vegetable quality as influenced by high
Temperature short time steam blanching. J. Food Sci. 51 (5). Pp: 1378-1382.
Dunford, H. B., and Araiso, T. 1979. Horseradish peroxidase. XXXVI. On the difference
Between peroxidase and metmyoglobin. Biochem. Biophys. Res. Commun.
89. Pp: 764-768.
Dunford, H. B. 1982. Peroxidase. Adv. Inorg. Biochem. 4. Pp: 41-68.
Eheart, M. S. and C. Gott. 1964. Conventional and Microwave cooking o f vegetables.
American Dietetic Association. Journal. 44. Pp: 116-119.
Erdman, J. W. Jr. and Klein, B. P. 1982. Harvesting, processing and cooking influences
on vitamin C in Foods. In Ascorbic acid: Chemistry, Metabolism, and Uses. Pp:
499-532. P. A. Seib and B. M. Tolbert (eds.) Adv. in Chem. ser. 200, American
Chemical Society, Washington, DC.
Fennema, O. R., W. D. Powrie, and E. h. Martin. 1973. Low-temperature Preservation o f
Foods and Living Matter. New York: Marcel and Dekker.
Fennema, O. 1975. Effects o f freeze preservation on nutrients. In "Nutritional
Evaluation o f Food Processing," 2nd ed. R.S. Harris and E. Karmas, Avi Pub.
Co., Westport, Conn. p. 244.
Fennema, O. 1977. Loss of vitamins in fresh and frozen foods. J. Food Qual. 1,3235.
Fennema, O. R. 1985. Food Chemistry. Vitamins and Minerals. 2nd ed. Marcel and
Dekker Inc. New York. Pp: 484-485.
78
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Freed, M. 1966. Methods o f Vitamin Assay. 3rd. ed. The Assoc. Vit. Chem., Inc. pp:
287-340
Garden, J. and I. Nobel. 1964. “Waterless” vs boiling water cooking vegetables. Amer.
Dietet. Assoc. J. 44. Pp: 378-383.
Giese, J. 1992. Advances in Microwave Food Processing. J. Food. Tech. 46 (9). Pp: 118123.
Gilpin, G. L., J. P. Sweeney, V. J. Chapman, and N. J. Eisen. 1959. Effects o f cooking
methods on broccoli; II. Palatability. American Dietetic Association: J. 35.
Pp: 359-360.
Glasscock, S.J., Axelson, J.M., Palmer, J.K., Phillips, J.A. and Taper, L.J. 1983.
Microwave blanching o f vegetables for frozen storage. Home Econ.
Res. J. 11, 149-158.
Gnanasekharan, V., Shewfelt, R.L. and Chinnan, M.S. 1992. Detection o f color changes
in green vegetables. J. Food Sci. 57(1), 149-154.
Gould, B.S. and Schwachman, H. 1943. “ A new method for the bioassay o f antiscorbutic
substances”. J. Biol. Chem. 151: 439-440.
Guthrie, A. H. 1989. Introductory Nutrition. 7th ed. Pp: 383-384. Mosby College Publ.
S t Louis, Missouri.
Harlfinger, L. 1992. Microwave Sterilization. Food Tech. Pp:57-61.
Hemeda, H. M. and Klein, B. P. 1990. Effects o f naturally occurring antioxidants on
peroxidase activity o f vegetables. J. Food Sci. 55. Pp: 184-187.
Howard, L. A., A. D. Wong, A. Perry, and B.P Klein. 1999. p-Carotene and Ascorbic
Acid Retention in Fresh and Processed Vegetables. Pp: 19-23.
Hudson, D. E., Dalai, A. A. and Lachance, P. A. 1985. Retention of vitamins in fresh and
frozen broccoli prepared by different cooking methods. J. Food Qual.8:45-50.
Huelin, F. E., I. M. Coggiola, G. S. Sidhu, B. H. Kennett. 1971. The anearobic
Decomposition o f ascorbic acid in the pH range o f foods and in more acid
Solutions. J. Sci. Food Agr. 22. Pp: 540-542.
IFT. 1989. Microwave food processing. A Scientific Status Summary by the IFT Expert
Panel on Food Safety and Nutrition. Food Technol. 43 (1). Pp: 117-126.
79
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Karel, M., Fennema, O. R. and Lund, D. B. 1975. Priciples o f Food Science.
Part II. Physical principles o f food preservation. Marcel Dekker Inc. New York.
Pp:31-92.
Katsaboxakis, K.Z. and Papanicolaou, D.N. 1984. The consequences o f varying degrees
o f blanching on the quality o f frozen green beans. Pp. 684-690. Proceedings o f
the Seminar o f the European Cooperation in Scientific and Technical Research,
London, 684-690.
Khan, M. M. T., and Martell, A. E. 1967. Metal ion and metal chelate catalyzed oxidation
o f ascorbic acid by molecular oxygen. I. Cupric and ferric chelate catalyzed
oxidation J. Amer. Chem. Soc. 89. pp: 7104-7111.
Klien, B.P. 1992. Fruits and vegetables. In Food Theory and Applications, (J.
Bowers, ed.), pp. 687-766. Macmillan Publishing Co., New York, NY.
Klugar, M. 1979. Preserving summer's bounty. Freezing fruits and vegetables. M. Evans
and Company Inc. New York. Pp: 189-195.
Kurata, T. and Sakurai, Y. 1967. Degradation o f L-ascorbic acid and mechanism o f
Nonenzymatic browning reaction. Part II. Nonoxidative degradation o f
L-ascorbic acid including the formation o f 3-deoxy-L-pentosone. Agr. Biol.
Chem. 31. Pp: 170-176.
Lane, R.H., Boschung, M.D. and Abdel-Ghany, M. 1985. Ascorbic acid retention o f
selected vegetables blanched by microwave and conventional methods. J. Food
Quality 8,139-144.
Labuza, T. P. 1972. Nutrient losses during drying and storage o f dehydrated foods. Crit.
Rev. Food Technol. 3. Pp: 217-240.
Lee, C.Y., Massey, JR., L.M. and Van Buren, J.P. 1982. Effects o f postharvest
handling and processing on vitamin contents o f peas. J. Food Sci. 47,961-964.
Lopez, A., Bocklet, M.F. and Wood, C.B. 1958. Catalase and peroxidase activity
in raw and blanched southern peas. Food Res. 24,548-551.
Lopez, P., and J. Burgos. 1995. Peroxidase stability and reactivation after heat treatment
and Manothermosonication. J. Food Sci. 60 (3). Pp: 451-455.
Lu, A.T. and Whitaker, J.R. 1974. Some factors affecting rates o f heat inactivation
and reactivation o f horseradish peroxidase. J. Food Sci. 39,1173-1175.
MABESA, L.B. and BALDWIN, R.E. 1979. Ascorbic acid retention in peas cooked by
microwaves. J. Food Sci. 44,932-933.
80
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Maria, J. E., Rosaura, F., and Frigola, A. 1995. Ascorbic acid stability in ground
asparagus. J. Food Sci. pp: 1282-1285.
McWilliams, M. 1997. Food: Experimental Perspectives. 3rd. ed. Prentice Hall, Inc.
pp: 102-104.
Mead, G. and Chioffi, N. 1982. Keeping the harvest: Home storage o f fruits and
vegetables. Garden Way Publ. Vermont. Pp: 1-12.
Mills, M.B., Damron, D.M. and Roe, J.H. 1949. “Ascorbic acid, dehydroascorbic acid,
Diketogulonic acid.” Anal. Chem. 21: 707-709.
Mirvish. S. S., Cardesa, A., Wallcave, L., and Shibik, P. 1975. Induction o f mouse lung
adenomas by amines or ureas plus nitrite and N-nitroso compounds. Effect o f
ascrobate, gallic acid, thiocyanate and caffeine. J. Natl. Cncer Inst. 55:633-636.
Mudgett, R. E. 1982. Electrical Properties o f foods in microwave processing. Food Tech.
36(2). Pp: 109-110.
Mudgett, R. E. 1986. Microwave Properties and Heating Characteristics o f Foods. J.
Food Technol. 40 (6 ). Pp: 84-93.
Newman, H. J., W. C. Dietrich, and D. G. Guadagni. 1967-68.Delay in freezing harvested
peas result in detectable off-flavor. Quick Frozen Foods Dec. 1967. Pp: 101.
sm f Jan. 1968. Pp: 64.
Newsome, R.L. 1980. Food safety and nutrition. A Scientific Status Summary, pp.
1-2. Institute o f Food Technologists, Chicago.
Owusu-Ansah, Y. J., and M. Marianchuk. 1991. Microwave inactivation o f Myrosinase
in Canola seeds: A pilot plant study. J. Food Sci. 56 (5). Pp: 1372-1374.
Pelletier, O. 1985. Vitamin C: L-ascorbic and dehydro-L-ascrobic acids. In
Methods o f Vitamin Assay, 4th ed. (J. Augustin, B.P. Kelin and D. Becker, etds.)
pp. 303-348, John Wiley & Sons, New York.
Penfield, M. P. and Campbell, M. A. 1990. Experimental food science. 3 rd ed Fruits and
vegetables. Pp: 294-330.
Pipkin, G. E., Schlegel, J. U., Nishimura, R., and Shultz, G. N. 1969. Inhibitory effect o f
L-ascorbate on tumor formation in urinary bladders implanted with 3hydroxyanthranilic acid. Proc. Soc. Exp.Biol. Med. 131:522-524.
Potter, N. N. 1978. Constituents o f Foods: Properties and significance. Food Science. 3rd
ed. Avi pub. Co. W estport Conn. Pp: 56-57.
81
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Potter, N. N. 1995. Irradiation/Microwave and Ohmic Processing o f Foods. Food
Science. 5th ed. Avi pub. Co. W estport Conn. Pp: 245-263.
Post, L.M., Mackie, O.A., Butler, G. and Larmond, E. 1991. Laboratory
Methods for Sensory Evaluation o f Foods, Publication 1864/E, Agriculture
Canada Research Branch, Ottawa, Canada. Pp: 16-20.
Proctor, B. E., and S. A. Goldblith. 1948. Radar energy for rapid food cooking and
blanching, and its effect on vitamin content. Food Tech. 2. Pp: 95-104.
Rouseff, R. L. and Steven, N. 1994. Health and nutritional benefits o f citrus fruit
components. J. Food Qual. 48(11): 125-132.
Sanchez-Pineda-Infantas, M. T., G. Cano-Munoz, and J. R. Herminda-Bun. 1994.
Blanching, Freezing and Frozen Storage Influence on Texture o f White
Asparagus. J. Food Sci. pp: 821-823.
SAS Institute, Inc. 1993. SAS User’s Guide. SAS Institute Inc., Cary, NC.
Schiffinan, R. F. 1986. Food Product Development for Microwave Processing. J. Food
Tech. 40 ( 6 ). Pp:94-105.
Schiffinan, R. F. 1990. Problems in Standardizing Microwave Oven Performance. J.
Microwave World 11 (3). Pp: 20-23.
Schiffinann, R. F. 1993. Microwave Power: What Does it Mean? J. Microwave World.
Pp: 25-27.
Schrumpf, E. and Charley, H. 1975. Texture o f broccoli and carrots cooked by
Microwave energy. J. Food Sci. 40. Pp: 1025-1027.
Schwimmer, S. 1944. Regeneration of heat-inactivated peroxidase. J. Biol. Chem. 15,
487-489.
Shamaila, M., Durance, T., and Girard, B. 1996. Water Blanching Effects on Headspace
Volatiles and Sensory attributes o f Carrots. J. Food Sci. pp: 1191-1194.
Shewfelt, R.L. 1990. Quality o f fruits and vegetables. Food Technol. 44(6), 99-114.
Shewfelt, R.L., Heaton, E.K. and Betat, K.M. 1984. Nondestructive color
measurement of fresh broccoli. J. Food Sci. 49,1612-1613.
Singh, R.P. and Heldman, D.R. 1993. Introduction to Food Engineering. 2nd ed.
Acad. Press. Pp: 212-215.
82
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Spiro, T. G., Strong, J. D., and Stein, P. 1979. Porphyrin core expansion and doming in
heme proteins. New evidence from resonance Raman spectra o f six-coordinate
high-spin iron (HI) heme. J. Am. Chem. Soc. 101. Pp: 2648-2650.
Stone, M. B. and Young, C. M. 1985. Effects o f cultivars, blanching techniques, and
cooking methods on quality o f frozen green beans as measured by physical and
sensory attributes. J. Food Qual. 7. pp: 255-257.
Sumner, J. B. and Somers, F. G. 1953. Chemistry and Methods o f Enzymes. Academic
Press Inc. New York. Pp: 220-227.
Swami, S. and Mudgett, R. E. 1981. Effect o f Moisture and Salt contents on the
Dielectric Behavior o f Liquid and Semisolid Foods. Proc. IMPI Symp. 16. Pp:
48-51.
Tauber, H. 1937. Enzume Chemistry. John Wiley & Sons, Inc. pp: 178-184.
Thomas, M.H., S. Benner, A. Eaton, and V. Craig. (1949). Effect o f electronic cooking
on nutritive value o f foods. J. Amer. Diet. Ass. 25: 39-45.
USDA. 1978. Home freeing o f fruits and vegetables. Home and Garden Bulletin No.
10. U.S. Government Printing Office, Washington, D.C.
USDA. 1981. Nutritive Value o f the Edible Part o f Food. Handbook No. 8 . U. S. Govt.
Printing Office.
USDA. 1984. Composition of foods: Vegetables and vegetable products. Agriculture
Handbook No. 8-11, U.S. Dept, o f Agriculture, Washington, D.C.
USDA. 1991. Composition o f foods: Vegetables and vegetable products. Agriculture
Handbook No. 11, U.S. Department o f Agriculture, Washington, D. C.
von Hippel, R. A. 1954. “Dielectrics and Waves.” MIT Press, Cambridge, MA.
Wang, X. Y., Seib, P. A., and Ra, K. S. 1995. L-ascorbic acid and its
phosphorylated derivatives in selected foods: vitamin C fortification and
antioxidant properties. Food Sci. pp: 1295-1300.
Wang, S.S. and Dimarco, G.R. 1972. Isolation and characterization o f the native,
thermally inactivated horseradish peroxidase isoenzymes. J. Food Sci. 37,792795.
Wassertheil-Smoller, S., Romney, S. L., Wylie-Rosett, J., Shagle, S., Miller, G., Lucido,
D., Duttagupta, C., and Palan, P. R. 1981. Dietary vitamin C and uterine cervical
83
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
dysplasia. Am. J. Epidem. 114:714-724.
Williams, D.C., Lim, M.H. and Chen, A.O. 1986. Blanching o f vegetables for
freezing—which indicator enzyme to choose. J. Food Technol. 40: 130-140.
Williams, G. P., Helena, R., and Miller, J. C. B. 1995. Ascorbic acid and 5Methytetrahydrofolate Losses in Vegetables with Cook/Chill or Cook/Hot-hold
Foodservice Systems. Pp: 541-546.
Wu, Y., Perry, A. K. and Klein, B. P. 1992. Vitamin C and Beta-carotene in fresh and
Frozen green beans and broccoli in a simulated system, J. Food Qual. 14. Pp:
87-96.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 4.1. Effects o f various time and microwave power levels on moisture content of
broccoli.
Time,
min
100%
Power levels
70%
55%
30%
0
92.326
(0.24)
92.2b
(0.40)
92.45b*
(0 . 1 1 )
92.4“
(0.08)
1
90.12°
(0 .2 0 )
90.36°
(0 . 1 1 )
92.18ab
(0.41)
92.364*
(0.47)
2
88.63d
( 0 .2 1 )
88.50d
(0.69)
92.05b
(0.03)
92. U *
( 0 .2 0 )
3
81.97**
(0.54)
85.22°*
(0.52)
91.89b
( 0 .0 1 )
92.01ab
(0.50)
4
76.88h*
(1.38)
79.83®*
(0.45)
91.32b
(0.78)
91.95ab
(1.67)
'Comparison was made between this mean and the other means.
abcdMeans with in the row/column with same superscript letter are not significantly
different (pX).05).
Power level: 100% = high; 70% = medium high; 55% = medium low; 30% = low.
Q: Values given in parentheses are standard deviations o f the means in the cells.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 4.2. Effects of various time and microwave power levels on reduced ascorbic acid
content (RAA)(mg/100 g) in broccoli.
100%
Power levels
70%
55%
30%
0
84.79'“
(0.91)
85.02*
(0.13)
84.46*
(1.32)
84.68*
(1.03)
1
99.821*
(0.15)
92.83s’
(1.16)
82.12k*
(1.40)
79.891*
( 1 .0 2 )
2
111.46**
( 0 .8 6 )
98.63f
(0.62)
84.06**
(0-71)
80.82w
(0.43)
3
136.60b*
(1.07)
127.91d*
(0.72)
85.98hi’
(0.52)
81.87k
(0.26)
4
144.81“’
(0.35)
134.77*’
(1.35)
86.271*’
(0.64)
Time,
min
.
84.05*
(1-09)
'Comparison was made between this mean and the other means.
abcdMeans with in the row/column with same superscript letter are not significantly
different (p>0.05).
Power level: 100% = high; 70% = medium high; 55% = medium low; 30% = low.
Q: Values given in parentheses are standard deviations o f the means in the cells.
86
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 4. 3. Effects of various time and microwave power levels on reduced ascorbic acid
percent retention in broccoli.
Time,
min
0
100%
100.00*’
(0.00)
Power levels
70%
100.00*
(0.00)
55%
30%
100.00*
(0.00)
100.00*
(0.00)
1
59.97**
(1.07)
55.85s*
(0.50)
48.63h*
(0.67)
41.22'
(0.39)
2
66.76e*
(1.47)
59.05f
(0.61)
49.63h
(0.88)
41.97'
(0.58)
3
82.38°*
(1.02)
75.34d*
(1.61)
50.37h
(1.46)
42.44'
(0.69)
4
84.79b*
(3.59)
80.76°
(3.83)
50.84h
_.(1,05) .
.
__
43.07'*
0^ 1
’Comparison was made between this mean and the other means.
#bcdMeans with in the row/column with same superscript letter are not significantly
different (p>0.05).
Power level: 100% = high; 70% = medium high; 55% = medium low; 30% = low.
O'. Values given in parentheses are standard deviations o f the means in the cells.
87
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 4.4. Effects of various time and microwave power levels on peroxidase activity
(units/min) in broccoli.
Time,
min
0
100%
Power levels
70%
55%
30%
4.3 1*’
(0.23)
4.04*
(0.05)
4.14*
(0.05)
3.74at>*
(0.54)
1
0.97de
(0.54)
1.23d
(0.03)
1.81“*
(0.24)
3.23b
(0.93)
2
0.24e*
(0.13)
0.48*
(0.13)
0.64d*
(0.30)
2.07c*
(0.47)
3
0.23*
(0.14)
0.27*
(0.05)
0.35*
(0.25)
1.28d*
(0.34)
4
0.23*
(0.11)
0.25*
(0.05)
0.29*
(0.20)
1.03d
(0.67)
‘Comparison was made between this mean and the other means.
abcdMeans with in the row/column with same superscript letter are not significantly
different (p>0.05).
Power level: 100% = high; 70% = medium high; 55% = medium low; 30% = low.
0: Values given in parentheses are standard deviations o f the means in the cells.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 4. 5. Effects of various time and microwave power levels on lightness (L* value)
of broccoli.
Time,
min
100%
Power levels
70%
55%
30%
0
45.45s6
(6.48)
45.69#b
(6.25)
45.06s6
(6 .8 6 )
44.95 s6
(7.17)
1
41.24s6
(5.88)
42.16sb
(8.69)
45.42s15
(13.23)
56.67s’
(7.56)
2
39.10b
(5.42)
40.50#b
(5.74)
44.82sb
(13.70)
51.17s6
(1.79)
3
40.126*
(7.89)
40.90s15
(6.48)
45.62s6
(14.82)
51.14sb
(2.67)
4
40.411*
(5.30)
40.66ab
(6.35)
45.05s6
(12.57)
49.37sb
(6.46)
'Comparison was made between this mean and the other means.
abcdMeans with in the row/column with same superscript letter are not significantly
different (p>0.05).
Power level: 100% = high; 70% = medium high; 55% = medium low; 30% = low.
Q: Values given in parentheses are standard deviations o f the means in the cells.
89
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 4.6. Effects of various time and microwave power levels on greenness (a* value)
of broccoli.
Time,
min
100%
Power levels
70%
55%
30%
0
-5.62a
(2.97)
-5.41s
(2.75)
-5.79s
(2.31)
-5.74s
(2.91)
1
-10.08a*
(6.62)
-8.81s
(5.10)
-6.99s
(4.36)
-3.37s
(0.46)
2
-8.95“
(4.88)
-9.22s
(5.35)
-8.37s
(4.55)
-6.84s
(3.22)
3
-9.01s
(5.59)
-8.15s
(5.45)
-7.96s
(4.82)
-7.17s
(2.31)
4
-8.08s
(5.01)
-7.67s
(4.82)
-7.57s
(4.86)
-6.46s
(2.63)
'Comparison was made between this mean and the other means.
abcdMeans with in the row/column with same superscript letter are not significantly
different (p>0.05).
Power level: 100% = high; 70% = medium high; 55% = medium low; 30% = low.
Q: Values given in parentheses are standard deviations o f the means in the cells.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 4. 7. Effects of various time and microwave power levels on yellowness (b* value)
of broccoli.
Time,
min
100%
Power levels
70%
55%
30%
0
26.66a
(15.17)
25.46“
(13.63)
26.44“
(15.17)
26.32*
(15.69)
1
25.15“
(13.23)
24.14“
(13.09)
24.26“
(13.80)
28.58*
(1.70)
2
23.62“
(12.28)
24.42“
(12.35)
24.61“
(14.18)
32.36*
(1.70)
3
24.42“
(14.06)
23.56“
(12.98)
24.94“
(14.37)
31.17“’
( 1 .1 0 )
4
24.58“
(11.94)
24.12“
( 1 1 .8 8 )
24.28“
(13.71)
29.41“
.......... ( 1 .8 8 )
'Comparison was made between this mean and the other means.
abcdMeans with in the row/column with same superscript letter are not significantly
different (p>0.05).
Power level: 100% = high; 70% = medium high; 55% = medium low; 30% = low.
Q: Values given in parentheses are standard deviations o f the means in the cells.
91
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 4. 8. Effects o f various time and microwave power levels on hue angle of broccoli.
Power levels
Time,
min
0
1
100%
70%
55%
30%
-4.41c
(0.43)
-5.44a*
(0.08)
92.45“**
(0 . 1 1 )
92 4abc
(0.08)
2 j jjktano*
_2 32iu»nn*
(0.48)
(0.71)
-3.66d*
(0.13)
-4.45bc
(0.26)
2
-2.60fghijkl*
(0.23)
-2.60fghijldm*
(0.23)
-2.67efghil*
(0.37)
-1.77“
(0.28)
3
-2.60fghijlc*
(0.23)
-3.25defg*
(0.19)
-3 I 2 defgh*
(0.15)
-4.67bc*
(0.50)
4
-3 12defghi*
(0.15)
-3.27def*
(0 . 1 0 )
-3.3de*
(0.07)
-5.14b*
(0.08)
’Comparison was made between this mean and the other means.
abcdMeans with in the row/column with same superscript letter are not significantly
different (p>0.05).
Power level: 100% = high; 70% = medium high; 55% = medium low; 30% = low.
Q: Values given in parentheses are standard deviations o f the means in the cells.
92
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 4.9. Effects o f various time and microwave power levels on chroma of broccoli.
100%
Power levels
70%
55%
30%
0
27.24*
(15.46)
26.02*
(13.90)
27.08*
(15.31)
26.94*
(15.95)
1
27.11*
(14.74)
25.69*
(14.04)
25.25*
(14.46)
28.77*
(1.64)
2
25.26*
(13.21)
26.11“
(13.44)
25.99*
(14.88)
33.14*'
(2.33)
3
26.03*
(15.12)
24.94*
(14.04)
26.18*
(15.15)
32.01*
(1.58)
4
26.08*
(12.64)
25.32*
(12.77)
25.44*
(13.86)
30.07“
(2.52)
Time,
min
'Comparison was made between this mean and the other means.
abcdMeans with in the row/column with same superscript letter are not significantly
different (p>0.05).
Power level: 100% = high; 70% = medium high; 55% = medium low; 30% = low.
Q: Values given in parentheses are standard deviations o f the means in the cells.
93
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 4.10. Effects of various time and microwave power levels on reduced ascorbic
acid percent retention in green beans.
Power levels
Time,
min
100%
70%
55%
30%
0
92.61*
(0.06)
92.53*
(0.36)
91.27*
(0.16)
90.53**
(0.08)
1
90.10*
(0.13)
90.27ef*
(0 .0 0 )
92.11*
(0.03)
92.35*
(0.08)
2
87.998*
(0.77)
89.59f
(0.60)
92.02“
(0 .0 1 )
92.35*
(0.04)
3
85.72h
(0.83)
86.78h*
(0.64)
91.47*bcd*
(0.17)
91.81abc*
(0.57)
4
78.78J*
(1.03)
83.18'*
(1.60)
90.87bcde*
(0.57)
91.81“b*
(0.08)
'Comparison was made between this mean and the other means.
abcdMeans with in the row/column with same superscript letter are not significantly
different (p>0.05).
Power level: 100% = high; 70% = medium high; 55% = medium low; 30% = low.
Q: Values given in parentheses are standard deviations o f the means in the cells.
94
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 4.11. Effects o f various time and microwave power levels on reduced ascorbic
acid (RAA) (mg/100 g) in green beans.
Time,
min
0
100%
Power levels
70%
55%
30%
17.58“
(0.75)
18.38“'
(0.83)
17.20ta
(1.34)
17.67“
(0.71)
1
25.08®*
(0.21)
24.48®
(0.84)
17.25“
(0.37)
14.65*
(0.88)
2
30.66**
(0.64)
28.69**
(0.68)
17.42“
(0.38)
15.05'*
(0.20)
3
47.78c*
(0.33)
46.08d*
(0.06)
17.62“
(0.28)
15.38j*
(0.25)
4
55.71**
(0.60)
52.27b*
(0.63)
18.25*
(1.24)
16.17*
(0.06)
'Comparison was made between this mean and the other means.
abcdMeans with in the row/column with same superscript letter are not significantly
different (p>0.05).
Power level: 100% = high; 70% = medium high; 55% = medium low; 30% = low.
Q: Values given in parentheses are standard deviations o f the means in the cells.
95
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 4.12. Effects of various time and microwave power levels on reduced ascorbic
acid percent retention in green beans.
Time,
min
100%
0
1 0 0 .0 0
**
(0 .0 0 )
Power levels
70%
1 0 0 .0 0
*
(0 .0 0 )
55%
30%
1 0 0 .0 0
1 0 0 .0 0
*
(0 .0 0 )
*
(0 .0 0 )
1
39.03"*
(0.06)
38.63"
(0.35)
26.501
(0.45)
22.41m
(0.51)
2
47.10**
(1.38)
45.09s*
(0.17)
27.12*
(0.27)
23
3
75.00d*
(0.14)
72.70e*
(0.64)
27.38ij*
(0.35)
23.661*
(0.35)
4
85.92b*
(0.97)
82.05c*
(0.16)
28.19**
(0.25)
24.70k*
(0.61)
i 4 to*
(0.35)
'Comparison was made between this mean and the other means.
abcdMeans with in the row/column with same superscript letter are not significantly
different (p>0.05).
Power level: 100% = high; 70% = medium high; 55% = medium low; 30% = low.
Q: Values given in parentheses are standard deviations o f the means in the cells.
96
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 4.13. Effects of various time and microwave power levels on peroxidase activity
___________ (units/min) in green beans.________________________________________
Power levels
Time,
100%
55%
70%
30%
min
2.41a
2.72a"
2.47*
2.304
0
(0.31)
(0.37)
(0 . 1 0 )
( 0 .0 1 )
1
0.24b
(0.03)
0.42b
(0 . 1 1 )
1.19b*
(1.25)
0.78b
( 0 .0 2 )
2
0.17b
(0 .0 0 )
0.29b
(0.13)
1.07b
(1.27)
0.49b
(0 . 1 1 )
3
0.l7b
(0 .0 1 )
0.27b
(0.14)
0.92b
(1.05)
0.41b
(0.08)
4
0 .l 6 b
(0 .0 2 )
0.28b
(0.15)
0.9 l b
(1.06)
0.36b
(0.07)
'Comparison was made between this mean and the other means.
abcdMeans with in the row/column with same superscript letter are not significantly
different (p>0.05).
Power level: 100% = high; 70% = medium high; 55% = medium low; 30% = low.
Q: Values given in parentheses are standard deviations o f the means in the cells.
97
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 4.14. Effects of various time and microwave power levels on lightness (L* value)
__________ o f green beans.___________________________________________ _____
Power levels
Time,
100%
70%
55%
30%
min
0
44.58“
(4.87)
45.93“
(3.14)
46.58a
(2.22)
46.65a*
(3.30)
1
43.99,b
(1.87)
41.57*
(1.17)
43.07*
(2.32)
45.22*
(4.33)
2
40.40b*
(3.11)
42.22ab
(1.44)
44.18*
(2.62)
44.61*
(4.00)
3
41.67ab
(1.85)
41.22*
(1.86)
4 0.87*
(0.34)
43.51*
(3.54)
4
42.23ab
(4.48)
42.55ab
(1.53)
41.38*
(2.76)
43.59*
(4.36)
’Comparison was made between this mean and the other means
abcdMeans with in the row/column with same superscript letter are not significantly
different (p>0.05).
Power level: 100% = high; 70% = medium high; 55% = medium low; 30% = low.
Q: Values given in parentheses are standard deviations o f the means in the cells.
98
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 4.15. Effects of various time and microwave power levels on greenness (a* value)
___________o f green beans.__________________________________________________
Power levels
Time,
100%
70%
55%
30%
min
-6.44p
-6.45p
0
-6.57p
-6.28p
(0.06)
(0.01)
(0.10)
(0.48)
1
-12.38a*
(0.47)
2
-11.75ab*
(0.90)
3
-10.57bcdefgh*
(0.62)
4
-8.74jldmn*
(1.11)
-11 27^**
(0.12)
-11.46abc*
(1.73)
_g
ggcdefghij*
(0.97)
_g y ^jldmno*
(1.51)
-10.62bcdefg*
(0.11)
_7 49ra“0P*
(0.24)
-10.80bcdef*
(1.24)
-11.39abcd*
(0.61)
9.61fghijkl*
(0.16)
-10 93bc(k*8t>‘*
(0.23)
-8.95ijldm’
(0.55)
_9 ggcdefghijk*
(0.45)
‘Comparison was made between this mean and the other means.
abcdMeans with in the row/column with same superscript letter are not significantly
different (p>0.05).
Power level: 100% = high; 70% = medium high; 55% = medium low; 30% = low.
Q: Values given in parentheses are standard deviations of the means in the cells.
99
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 4.16. Effects o f various time and microwave power levels on yellowness
___________ (b* value) o f green beans.__________________________________
Power levels
Time,
100%
70%
55%
min
26.98fletgm
0
26.57*“
26.39*“
(0.32)
(0.15)
( 0 .8 8 )
1
2
3
4
28 9 3 ^ ^ ^
(0-52)
26.09'
(3.86)
28 9 3 ^ ^ ^
(0.78)
30.34abcdefg*
( 1 .6 6 )
(0.47)
29 95»bcdef8»>*
(0-71)
31.42a*
(1.70)
31.06^*
(1.39)
29 32abcdefghi
(o '13)
3127**
(0 .0 0 )
30%
26.35'“
(0.52)
27.62bcdefghi
(2.55)
3051abcd*
(2.62)
29 yjabcdefghi*
29 j^abcdefghi
(1 3 9 )
(2 -8 6 )
30 4 iabcde(*
29
(2.44)
(3-63)
'Comparison was made between this mean and the other means.
abcdMeans with in the row/column with same superscript letter are not significantly
different (p>0.05).
Power level: 100% = high; 70% = medium high; 55% = medium low; 30% = low.
Q: Values given in parentheses are standard deviations o f the means in the cells.
100
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
55
abcdefshi
Table 4.17. Effects of various time and microwave power levels on hue angle o f green
beans.
Power levels
Time,
100%
55%
30%
70%
min
0
-4.28*
-4.28*
-4.28*
-4.47**
(0.19)
(0.19)
(0.19)
(0.30)
1
-2.53e
(0.15)
-2.53*
(0.15)
-2.79“*
(0.04)
-3.60b
(0 .2 1 )
2
-2.63*
(0.04)
-2.53*
(0.25)
-2.80“*
(0.04)
-2.60*
(0.16)
3
-2.82de*
(0.24)
-3.35**
(0.26)
-3.33*
(0.30)
-2.58*
(0.43)
4
-3.55b
(0.14)
-3.62b*
(0.08)
-3.34*
(0.16)
-3.07*“’
(on)
’Comparison was made between this mean and the other means.
abcdMeans with in the row/column with same superscript letter are not significantly
different (p>0.05).
Power level: 100% = high; 70% = medium high; 55% = medium low; 30% = low.
Q: Values given in parentheses are standard deviations o f the means in the cells.
101
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 4.18. Effects of various time and microwave power levels on chroma of green
beans.
Power levels
Time,
100%
30%
70%
55%
min
27.34b
27.90b
27.76b
27.17b
0
(0.32)
(0.14)
(0.76)
(0.87)
1
31.58*”
(0.38)
28.40b
(3.54)
30.81*
(0.63)
28.63b*
(2.40)
2
32.55*
( 1 .2 2 )
32.57*
(0.18)
31.71*
(1.24)
32.59*
(2.24)
3
33.16**
(1.42)
32.64*
( 1 .0 2 )
31.93*
(0.55)
31.33*
(2.77)
32.48“
(0.40)
31.96*
( 2 -1 2 )
31.28*
(3.58)
4
3 0 3 9 *1.
(0.45)
.
.
...
‘Comparison was made between this mean and the other means.
abcdMeans with in the row/column with same superscript letter are not significantly
different (p>0.05).
Power level: 100% = high; 70% = medium high; 55% = medium low; 30% = low.
Q: Values given in parentheses are standard deviations o f the means in the cells.
102
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 4.19. Effects of various time and microwave power levels on moisture content of
__________ asparagus._____________________________________________________
Power levels
Time,
70%
55%
30%
100%
min
93.46s6'
93.67“
93.35s6
93.72“*
0
(0.33)
(0.42)
(0.25)
(0.50)
1
91.50hij
(0.03)
91.91fghij*
(0.04)
93.05bc*
( 0 .0 2 )
93.26s6
(0.07)
2
90.68k*
(0.43)
91.90fghi*
(0 . 1 0 )
92.86bcd
(0.16)
92.97bcd*
(0.17)
3
88.25m
(0.14)
89.301’
( 0 .2 1 )
91.87“"*
( 0 .2 1 )
92.89cde*
(0.43)
4
86.27“*
(0.58)
88.28m*
(0.61)
91.01efgh*
(0.03)
91.44defg*
(0.45)
'Comparison was made between this mean and the other means.
“'’' ‘‘Means with in the row/column with same superscript letter are not significantly
different (p>0.05).
Power level: 100% = high; 70% = medium high; 55% = medium low; 30% = low.
Q: Values given in parentheses are standard deviations o f the means in the cells.
103
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 4.20. Effects of various time and microwave power levels on reduced ascorbic
__________ acid (RAA) (mg/100 g) in asparagus._____________________________
Power levels
Time,
100%
70%
55%
30%
min
0
46.94m
(1.08)
47.19”
(1.38)
46.79m
(0.62)
46.55m
(0.91)
1
58.88**
(0.22)
56.72h*
(0.57)
47.40““
(0.30)
46.89m
(1.22)
2
64.57e*
(0.99)
62.84**
(0.49)
48.71ijk*
(0.23)
46.98m
(0.18)
3
75 .16c*
(0.45)
72.53d*
(1.39)
49.37ij*
(0.57)
47.78jkm*
(0.52)
4
82.95a*
(0.23)
80.15b*
(0.99)
49.53'*
(0.44)
48.71ijkl*
(0.18)
’Comparison was made between this mean and the other means.
abcdMeans with in the row/column with same superscript letter are not significantly
different (p>0.05).
Power level: 100% = high; 70% = medium high; 55% = medium low; 30% = low.
Q: Values given in parentheses are standard deviations o f the means in the cells.
104
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 4.21. Effects of various time and microwave power levels on reduced ascorbic
__________ acid percent retention in asparagus.______________________________
Power levels
Time,
100%
70%
55%
30%
min
0
1 0 0 .0 0 **
1 0 0 .0 0 *
1 0 0 .0 0 *
1 0 0 .0 0 *
( 0 .0 0 )
( 0 .0 0 )
(0 .0 0 )
(0 .0 0 )
1
58.1 l h*
(0.26)
56.13**
(0.72)
46.57nmop*
(0.50)
46.40nop
(0.59)
2
63.02**
(1.27)
61.47s*
(1.82)
48.06iU*
(0.83)
3
74.65d*
(0.35)
72.06'*
(0.37)
48.22ik*
(0.92)
47.34jUmn*
(0.52)
4
81.89b*
(1.03)
78.97°*
(0.54)
48.88j*
( 0 .2 2 )
47
46 7ilmn°*
(0.28)
.g7 jicta*
(1.50)
'Comparison was made between this mean and the other means.
abcdMeans with in the row/column with same superscript letter are not significantly
different (p>0.05).
Power level: 100% = high; 70% = medium high; 55% = medium low; 30% = low.
0 : Values given in parentheses are standard deviations o f the means in the cells.
105
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 4.22. Effects of various time and microwave power levels on peroxidase activity
__________ (units/min) in green beans._______________________________________
Power levels
Time,
70%
30%
100%
55%
min
0
2.60c
(0.03)
2.64aBcf
(0.00)
2.66ab"
(0.00)
2.66a*
(0.01)
1
0.16s
(0.00)
0.16s
(0.01)
0.26**
(0.01)
0.69d*
(0.02)
2
0.14s
(0.00)
0.14s
(0.00)
0.19s
(0.00)
0.35e*
(0.09)
3
0.14s
(0.01)
0.14s
(0.01)
0.18s
(0.00)
0.24f
(0.01)
4
0.14s
(0.02)
0.14s
(0.01)
0.18s
(0.01)
0.15s
(0.05)
'Comparison was made between this mean and the other means.
lbcdMeans with in the row/column with same superscript letter are not significantly
different (pX).05).
Power level: 100% = high; 70% = medium high; 55% = medium low; 30% = low.
Q: Values given in parentheses are standard deviations o f the means in the cells.
106
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 4.23. Effects of various time and microwave power levels on lightness (L* value)
__________ of asparagus.____________________________________________________
Power levels
Time,
100%
55%
70%
30%
min
37.40*
36.59“
0
37.07“
38.13*'
(2.52)
(2.63)
(2.83)
( 2 .8 6 )
1
31.25c‘
(0.24)
36.30*
(4.11)
35.70“b*
(1.63)
36.52*
(0.78)
2
30.74°
(0.45)
32.12bc
( 1 .0 0 )
37.10*
(0.04)
36.50*
(2.28)
3
29.74°
(0.33)
32.58bc
( 1 .0 0 )
35.54*b
(4.29)
32.70b°
(0.41)
4
34.69abc
(0.75)
30.53'
( 1 .0 0 )
34.59*b°
(0.23)
35.55“b
(2.92)
.
‘Comparison was made between this mean and the other means.
*bcdMeans with in the row/column with same superscript letter are not significantly
different (p>0.05).
Power level: 100% = high; 70% = medium high; 55% = medium low; 30% = low.
Q: Values given in parentheses are standard deviations o f the means in the cells.
107
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 4.24. Effects o f various time and microwave power levels on greenness (a* value)
__________ of asparagus.___________________________________________________
Power levels
Time,
100%
70%
55%
30%
min
-2.96c
-3.23°
-3.66c*
-3.20c
0
(1.34)
( 1 .0 2 )
(0.55)
(0.93)
1
-6.39*”
(0.30)
-5.25b*
(1.03)
-4.76b
(0.13)
-3.36°
(0.57)
2
-6.22th
(1.13)
-6.71*
(0.64)
-6.76*
(0.35)
-5.64*b
(0.24)
3
-5.50ab
(0.56)
-6.19“b
(0 .2 1 )
-4.89*b
(0.46)
-6.80**
(0.62)
4
-5.45*b
(0.23)
-4.94bc
(0.14)
-4.94bc
(0.08)
-4.34bc
(1.32)
’Comparison was made between this mean and the other means.
abcdMeans with in the row/column with same superscript letter are not significantly
different (p>0.05).
Power level: 100% = high; 70% = medium high; 55% = medium low; 30% = low.
Q: Values given in parentheses are standard deviations o f the means in the cells.
108
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 4. 25. Effects of various time and microwave power levels on yellowness of
___________asparagus._________________________________________________
Power levels
Time,
70%
55%
100%
30%
min
28.20“”*'
28.57“°
0
28.96“'
28.59*“'
(1.78)
(2 .2 0 )
(2 . 1 2 )
(2.45)
1
25.03bcde
(0.23)
21.53*
(4.51)
24.53°“*
(1.97)
28.20ab°
(1.69)
2
25.76sbcd
(1.45)
26.17#bcd
(1.18)
24.55°“*
(1.74)
25.54“”°“
(1.14)
3
25.07bcde*
(0 .2 1 )
26.42,bcd
(0.67)
24.46“*
(1.26)
26.67*”°“
( 1 .1 0 )
22.76*
(0.91)
24.96b°“*
(0.08)
24.46“*
(0.08)
4
28.42ab
_____ Q : 6 6 ) .........
_
‘Comparison was made between this mean and the other means.
abcdMeans with in the row/column with same superscript letter are not significantly
different (p>0.05).
Power level: 100% = high; 70% = medium high; 55% = medium low; 30% = low.
Q: Values given in parentheses are standard deviations o f the means in the cells.
109
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 4. 26. Effects of various time and microwave power levels on hue angle of
___________asparagus._______________________________________________
Power levels
Time,
100%
70%
55%
30%
min
-10.41*'
-9.54*
- 8 .2 2 b*
-9.57*
0
(0.74)
(0.40)
(0.39)
(0.28)
1
-3.77**
(0.28)
2
-4.30fghi
(0 .0 1 )
3
4
-4.41efghi
(0.38)
-5.22def*
(0.05)
-8.13b
(0.71)
-3.74*
(0.33)
-3.58*
(0.14)
-5.29“*’
(0.98)
-5.29de
(0.98)
-4.26®“
(0.07)
- 5 .1 ^
(0.16)
-3.93®*
(0.06)
-5.12defgh*
(0.16)
-6.50c*
(0.25)
-4.48defghi*
(0.50)
-6 .0 2 cd*
(0.44)
'Comparison was made between this mean and the other means.
abcdMeans with in the row/column with same superscript letter are not significantly
different (p>0.05).
Power level: 100% = high; 70% = medium high; 55% = medium low; 30% = low.
Q: Values given in parentheses are standard deviations o f the means in the cells.
110
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 4.27. Effects o f various time and microwave power levels on chroma of
__________ asparagus.______________________________________________
Power levels
Time,
100%
70%
55%
30%
min
0
26.12ab
28.39“
28.80“
28.77“
(6.49)
( 1 .8 8 )
(2.25)
(2.54)
25.65th
2 1 .6 6
(0.55)
b
(3.92)
24.99““
(1.92)
28.40“
(1.75)
2
26.48““
(1.64)
27.01““
(1.30)
25.47*“
(1.77)
26.16*“
(0.08)
3
25.67ab
(0.32)
27.13““
(0.71)
24.95*“
(1-15)
27.48*“
(1.15)
4
28.94“*
(3.63)
23.29“*
(0.92)
25.45““
(0.63)
24.86*“
..( 1 ,4 1 )
1
_
‘Comparison was made between this mean and the other means
““^M eans with in the row/column with same superscript letter are not significantly
different (p>0.05).
Power level: 100% = high; 70% = medium high; 55% = medium low; 30% = low.
Q: Values given in parentheses are standard deviations o f the means in the cells.
Ill
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 4.28. Regression equations for broccoli, green bean and asparagus processed at
_________ various time/microwave power level.______________________________
Regression equations
Dependent
Po
Pi
P2
P3
p4
-
Ps
R2
CV
0 .8 6
0.69
variables
RAA (mg/100 g)
83.03
9.03
Broccoli
0.26 -
RAA retention
97.51
27.81
0.87
2.94
0.004
0.27
0 .8 8
6 .0
0.36
0.05
0.0002
•
“
0.93
2.59
5.61
(%)
“PO” (units/min) 4.24
Green bean
RAA (mg/100 g)
17.12
5.80
0.16
-
-
-
0.63
RAA retention
•
•
•
•
•
•
•
2.26
0 .2 0
0 .0 2
-
•
”
0.59
4.71
-
0.81
0.125
-
0 .8 6
6.32
0.005
-
(%)
“PO” (units/min)
Asparagus
RAA (mg/100 g)
48.06
5.42
0.15
-
RAA retention
97.34
1 .2 0
0 .0 1
0.26
2.58
0.82
0.04
0.09
(%)
“PO” (units/min)
0 .0 0 0 2
-
1. RAA = reduced ascorbic acid.
2. “PO” = Peroxidase
3. min = minutes
4. - = no values were available.
5. Green bean RAA retention (%) model was insignificant (p>0.05).
112
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 4.29. Prediction models for broccoli, green beans and asparagus.
Dependent variables
Model
Broccoli
RAA (mg/100 g)
E(Y) = 83.03 - 9.03 (XO + 0.26 (XtX2)
RAA retention (%)
E(Y) = 97.51 - 27.81 (XO - 0.87 (X2) + 2.98 (XO2 + 0.004 ((X2)2
+ 0.27 (XtX2)
PO (units/min)
E(Y) = 4.24 - 0.36 (X 0 - 0.05 (X2) + 0.0002 (X2)2
Green Bean
RAA (mg/100 g)
E(Y) = 17.12 - 5.80 (XO + 0.16 (XiX2)
RAA retention (%)
E(Y) = insignificant
PO (units/min)
E(Y) = 2.26 - 0.20 (Xi) - 0.02 (X2)
Asparagus
RAA (mg/100 g)
E(Y) = 48.06 - 5.42 (X 0 + 0.15 (XtX2)
RAA retention (%)
E(Y) = 97.34 -1 3 .5 3 (Xi) - 1.20 (X2) + 0.01 (X2)2 + 0.26 (X LX2)
PO (units/min)
E(Y) = 2.58 - 0.82 (X 0 - 0.04 (X2) + 0.09 (XO2 + 0.0002 (X2)2 +
0.005(X!X2)
1. E(Y) = expected/predicted values for RAA (mg/100 g), RAA (%) retention, peroxidase
activity (units/min).
2. RAA = reduced ascorbic acid.
3. mg = milligrams.
4. g = grams.
5. PO = peroxidase activity.
6 . min = minutes.
113
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 4.1. Experimental design for broccoli heated at various time and
microwave power levels.
Broccoli
Within 1 h of
harvest
Within 4 h of
harvest
Stored at 10°C
Size of pieces
Length = 0.8 cm
Sorted,
rinsed and
cut
time
levels:
0, 1 ,2 ,3 ,
and 4 min
5
4 power
levels:
1 0 0 %,
70%,
55% and
30%
Heated using
microwave oven
and assayed
Assays:
Moisture
content
Reduced
ascorbic acid
(RAA)
Percent RAA
retention
Peroxidase
activity
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 4.2. Effect o f microwave heating at various time and power
levels on moisture content of broccoli.
■ power = 100%
■pow er = 70%
□ power = 55%
□ power = 30%
0
1
2
3
4
Ti me (min)
Reproduced with permission o f the copyright owner. Further reproduction prohibited without permission.
Figure 4. 3. Effect o f microwave heating at various time and power levels
on reduced ascorbic acid (RAA) (mg/100 g) of broccoli.
150
140
_ 130
3 120
110
100
power = 100%"
power = 70%"
power = 55%"
power = 30%"
0
1
2
3
4
Time (min)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 4.4. Reduced ascorbic acid retention (%) in broccoli processed at various
time and microwave power levels.
110 I
100 p
2
90 -
§
80 ■
s
70 -
©
=
power = 100%
power = 70%
power = 55%
power = 30%
60 50 40 0
1
2
3
4
Time (min)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 4. 5. Peroxidase activity (units/min) in broccoli processed at various
times and microwave power levels.
4.5
e
g 3.5
power =100%"
power = 70%"
power = 55%"
power = 30%"
1 2.5
£
>
2
oto
O
Q.
0.5
0
1
2
3
4
Time (min)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 4. 6. Effect o f microwave heating at various time and power levels on
moisture content of green beans.
95
■ power = 100%
■power = 70%
□ power = 55%
□pow er - 30%
0
1
2
time (min)
3
4
•
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 4. 7. Reduced ascorbic acid content (RAA) (mg/100 g) in green bean
processed at various time and microwave power levels.
50 o>
e
o
T*
g 30 -
0
100% power
70% power
55% power
30% power
1
2
3
4
Time (min)
120
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 4. 8. Reduced ascorbic acid retention (%) in green bean processed at
various time and microwave power levels.
120
100
c
o
**
e
0)
S
80-
“ ♦ "p o w e r = 100%
power = 70%
• " p o w e r = 55%
power = 30%
6040 -
20
-
0
1
2
3
4
Time (min)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 4. 9. Peroxidase activity (units/min) in green bean processed at
various times and microwave power levels.
~ 2.5 S
i
power = 100%"
power = 70%"
power = 55%"
power = 30%"
I> Zn 1
O
°* 0.5 -
0
1
2
Time (min)
3
4
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 4.10. Effect of microwave heating at various time and power levels
on moisture content of asparagus.
96
■Power = 100%
■power = 70%
□ power = 55%
□ power = 30%
0
1
2
3
4
Time (min)
123
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 4.11. Reduced ascorbic acid content (RAA) (mg/100 g) in asparagus
processed at various times and microwave power levels.
8580 » 75 power = 100%
power = 70%
power = 55%
’power = 30%
| 65-
5 60
2 55 ‘
50-
Time (min)
124
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 4.12. Reduced ascorbic acid retention (%) in asparagus processed at
various time and microwave power levels.
120
100
c
o
e
12
80 -
power = 100%
power = 70%
power = 55%
power - 30%
60 40 20
-
0
1
2
3
4
Time (min)
125
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 4.13. Peroxidase activity (units/min) in asparagus processed at various
times and microwave power levels.
2.5 i
power = 100%
power = 70%
power = 55%
power = 30%
o
0.5 O
CL
-0.5
Time (min)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 4.14. Reduced ascorbic acid (RAA) and peroxidase activity (PO) in
broccoli processed at various tunes and 100% microwave power
level.
4.5
150
140
3.5
o> 130
o
o
c 120
O)
£ 110
•RAA
2.5
•PO
100
0.5
Q.
1
2
Time (min)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 4. IS. Reduced ascorbic acid (RAA) and peroxidase activity (PO) in
broccoli processed at variou times and 70% microwave power
levels.
140
4.5
E
130
3.5 e
2
01
o 120
e
£
2.5 5£
o
«o
1.5 0in
■rao
0.5 1
110
100
0
1
S.
2
Time (min)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 4.16. Reduced ascorbic acid (RAA) and peroxidase activity (PO)in
broccoli heated at various times and 55% microwave power level.
87
4.5 c
I
86
3.5
85
2.5 *>
a0
a
e
1.5 M
84
a
E 83
82
■RAA
■PO
IB
TJ
81
0.5 1
£
80
0
1
2
Time (min)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 4.17. Reduced ascorbic acid (RAA) and peroxidase activity (PO) in
broccoli processed at various times and 30% microwave power
levels.
87
4.5 e
I
3.5 c
85
3
83
2.5 5>
0
81
E
IB
79
-■ -R A A
-» -P O
9
M
IB
■o
77
0.5 1
£
75
0
1
2
Time (min)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 4.18. Reduced ascorbic acid (RAA) and peroxidase activity (PO) in
green bean heated at various time and 100% microwave power
level.
60
c
50
2.5
I
c
e 40
o
1.5 >
Za
•«
(0
30
20
0.5
10
■RAA
■PO
"Q
1
0
Q.
0
0
1
2
3
4
Time (min)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 4.19. Reduced ascorbic acid (RAA) and peroxidase activity (PO) in
green bean heated at various times and 70% microwave
power level.
60
50
2.5
o 40
o
*
>
30
3
a
e10
20
a
10
0
0.5
Is
0
S.
Time (min)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 4.20. Reduced ascorbic acid (RAA) and peroxidase activity (PO) in
green bean heated at various times and 55% microwave power
level.
e
E
-- 2.5 8
e
3,
" 2
o> 14 ..
12-■
10
8
6
>
-- 1.5 <
5
- -
u
IB
•
M
- -
- -
ID
4 --
2
-RAA
■PO
-- 0.5
- -
i
£
0
1
2
Time (min)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 4.21. Reduced ascorbic acid (RAA) and peroxidase activity (PO) in green
bean heated at various times and 30% microwave power level.
2.5
E
O)
&
O
e
o>
E
1.5
•RAA
*
>
■PO
CO
0.5
0
1
2
3
4
Time (min)
1
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 4.22. Reduced ascorbic acid (RAA) and peroxidase activity (PO) in
asparagus heated at various times and 100% microwave power level.
e
1
-- 2.5 £
e
2 2,
*
>
70 -eo 60 i 5 0 ir
E. 40 -J 30 -“ 2 0 -10
- -
0IS
®
tf>
IS
-- 0.5
- -
79
1o
0.
0
1
2
3
4
Time (mln)
135
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 4.23. Reduced ascorbic acid (RAA) and peroxidase activity (PO) in
asparagus heated at various times and 70% microwave power
level.
-2 .5
~ 70 --
a
-2
o 60 -o
i
5 0 ih
-- 1.5
E. 40 -
RAA
PO
5 30 -*
20
10
--
-- 0.5
- -
Time (min)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 4.24. Reduced ascorbic acid (RAA) and peroxidase activity (PO) in
asparagus heated at various times and 55% microwave power
level.
3
c
E
2.5
£
49.5
o
o
o 48.5
0)
£ 47.5
*
1.5 >
10
e«
n
TJ
46.5
0.5 n
2
45.5
0
1
CL
2
Time (min)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 4.25. Reduced ascorbic acid (RAA) and peroxidase activity (PO)
in asparagus heated at various times and 30% microwave power
level.
48.5
2.5
e
o>
o
o
47.5
a
E
a
1.5 >
*(0
•RAA
■PO
S
10
46.5
0.5
45.5
0
1
2
3
■a
S
0
a.
4
Time (min)
Reproduced with permission o f the copyright owner. Further reproduction prohibited without permission.
Figure 4.26. Reduced ascorbic acid (RAA) (mg/100 g) actual and predicted
data o f broccoli processed at various microwave power levels vs.
time.
160 T
140 -a
e 120 o
"3» 100-E
80
t
a
X
>*
n
*
1
T
I
1
----------------A A
“A------------ * _
4 --------- tr
♦ Observed Y (Hat)
a Predicted Y (Hat)
60 -■
40 -■
20 -0 -1
2
3
Time (min)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 4.27. Reduced ascorbic acid (RAA) (mg/100 g) actual and predicted
data of broccoli processed at various times vs. microwave power
levels.
160
A
♦
140
♦
♦
-A-
120
A
'5) 100
E
80
4-
t
T
t
♦ Observed Y (Hat)
a Predicted Y (Hat)
60
a
I
>
40
20
0
50
100
150
Pow er (levels)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 4.28. Reduced ascorbic acid (RAA) retention (%) actual and predicted
data of broccoli processed at various microwave power levels vs.
time.
120
c 100
f
£
C
0so- 60
|
1
♦
*
♦
♦
♦
♦
A
A
A
A
1
A
♦
w
I
40
z
A
?
20
*
0
I
♦
♦
.
A
A
1
I
-20
♦ Observed Y (Hat)
a Predicted Y (Hat)
1
2
3
J
Time (min)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 4.29. Reduced ascorbic acid (RAA) retention (%) actual and predicted
data of broccoli processed at various times vs. microwave power
levels.
120
100
80
♦
♦
W
60
A
4
40
A
5
t
A
20
A
A
A
A
A
A.
0
(I
A
♦ Observed Y (Hat)
a Predicted Y (Hat)
•
50
A
l
100
1!
-20
Power (levels)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 4.30. Peroxidase activity (units/min) actual and predicted data of
broccoli processed at various microwave power levels vs. time.
5
e 4
E
3 3
c
3,
5>
55
S
b
%
X
♦
A
♦
2
A
♦
A
i
*
1
A'
2
i
0
-1
*
|
1
0.
I
1
1
♦ Observed Y (Hat)
a Predicted Y (Hat)
I
j
A
-2
A
A
-3
Time (min)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 4.31. Peroxidase activity (units/min) actual and predicted data of
broccoli processed at various times vs. microwave power levels.
c .
l l
E
►
♦
c
3
^
>
ts
o
<0
o
W
(0
3
A
2 ■---------- i ------- ----------------------------------A
♦
♦ Observed V (Hat)
H.
A
t
1
a Predicted Y (Hat)
I
IA--------------1
*----------------------------------------------- i-A----0►
50
a
100
1! >0
A
£ -2 >
A
A
A
-3 Power (levels)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 4.32. Reduced ascorbic acid (RAA) (mg/100 g) actual and predicted
data of green bean processed at various microwave power levels
vs. time.
90
80
m
4
A
A
A' '
A
1
A
♦
♦
♦
1
♦
2
3
70
A
o> 60
E 50
▲
A
40
A
A
A
A
A
X
30
20
10
I
•
'♦
♦
I
♦ Observed Y (Hat)
a Predicted Y (Hat)
*
0
Time (min)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 4.33. Reduced ascorbic acid (RAA) (mg/100 g) actual and predicted
data of green bean processed at various times vs. microwave
power levels.
90
80
o>
e
70
T
60
'Si
E, 50
♦ Observed Y (Hat)
a Predicted Y (Hat)
40
30
I
>•
20
10
T
0
50
100
150
Pow er (levels)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 4. 34. Reduced ascorbic acid (RAA) retention (%) actual and predicted
data of green bean processed at various microwave power levels
vs. time.
120
100
a?
c
o
1£
to
>■
t
80
$
60
A
♦ Observed Y (Hat)
a Predicted Y (Hat)
$
40
t
20
t
\
t
t
A
A
1
A
▲
t
1
2
3
*
0
I
J
-2 0
time (min)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 4.35. Reduced ascorbic acid (RAA) retention (%) actual and predicted
data o f green bean processed at various time vs. microwave power
levels.
120
100
c
o
♦5
e
2
S
80
♦
♦
•
♦
|
♦
60
A
-
40
n
*
1
20
X
A
0
I
A
♦ Observed Y (Hat)
a Predicted Y (Hat)
1
A
“A
A
A
A
' 1A
50
#
A
i
4
100
1!
-20
Power levels
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 4.36. Peroxidase activity (units/min) actual and predicted data of green
bean processed at various microwave power levels vs. time.
E
isc
i ji
5
■>
*
10
t
O
a.
♦
I—
m
I
A
-v
♦ Observed Y (Hat)
a Predicted Y (Hat)
-t
1
T
S
-2
-3
Time (min)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 4.37. Peroxidase activity (units/min) actual and predicted data of green
bean processed at various times vs. microwave power levels.
c
£
A -
I
3 .
1
1
A
♦
1
O
» 1
•
f ,-A----t A------------ A
-----------------------T-----------------------
N
•
1
1
50
A
100
♦ Observed Y (Hat)
a Predicted Y (Hat)
1! iO
A
A
A
A
i)
■
Y (Hat) "PO" activity
i
A
l , i ---------- A-----------------------------------------------------------
Power (levels)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 4.38. Reduced ascorbic acid (RAA) (mg/100 g) actual and predicted data
of asparagus processed at various microwave power levels vs. time.
120
A
S 100
A
’
3) 80
E
1
i
_A_
A
♦ Observed Y (Hat)
a Predicted Y (Hat)
A
T
i
A
4
60
(O 40
I
>
20
2
3
Time (min)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 4. 39. Reduced ascorbic (RAA) (mg/100 g) actual and predicted data of
asparagus processed at various times vs. microwave power levels.
120
3 100
*3)
E
80
A
60
a
40
>
20
♦ Observed Y (Hat)
a Predicted Y (Hat)
A
Z
50
100
150
Power (levels)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 4.40. Reduced ascorbic acid (RAA) retention (%) actual and predicted
data of asparagus processed at various microwave power levels vs.
time.
120
g
c
100
r
T
*
r
t
!
1
2
3
“ 40
TS
X
j: 20
A
♦ Observed Y (Hat)
a Predicted Y (Hat)
-A -
0
0
4
5
Time (min)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 4.41. Reduced ascorbic acid (RAA) retention (%) actual and predicted
data of asparagus processed at various times vs. microwave power
levels.
120
2 100
e
o
80
s
1
60
40
♦ I
t
♦ Observed Y (Hat)
a Predicted Y (Hat)
▲
♦
20
0
50
100
150
Pow er (levels)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 4.42. Peroxidase activity (units/min) actual and predicted data of
asparagus processed at various microwave power levels vs. time.
5
c
4
I
3
c
3
o>
*
>
*
m
t
O
(L
t
C*
a
X
k
►
2
A
A
♦
♦
0
I
1
. .r
♦ Observed Y (Hat)
a Predicted Y (Hat)
A
A
1
-1
A
.
2
2
f
2
S
A
j
A
-2
A
A
-3
Time (min)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 4.43. Peroxidase activity (units/min) actual and predicted data of
asparagus processed at various times vs. microwave power levels.
c
E
1
c
3_
5
>
TS
N
s
0
o.
1
IS
x
4
3
2
♦ Observed Y (Hat)
4 Predicted Y (Hat)
1
0
-1
-2
50
4
4
100
T
1! iO
4
4
Power (levels)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
APPENDIX
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
APPENDIX A.
Figure A. 1. L-ascorbic acid and L-dehydroascorbic acid.
o
ii
>
H
C-O H
■c=o
C-O H
•c=o
$
HOCH
I
CH20H
L-oscorbic acid
H
HOCH
c h 2o h
L-dehydroascorbic acid
Source: Fennema (1985).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure A. 2. Relationship of various chemical forms of ascorbic acid.
Ascorbic acid
Reduction
Oxidation
Dehyd roascorbic acid
Oxidation
Diketogiilonic acid
^Biologically inactive form.
Source: Guthrie (1989).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure A. 3. Indirect oxidation o f ascorbic acid in the presence o f
peroxidase.
•4 -
O
•H
(p e ro x id a s e )
(H
=■ H
2O
4 -
2O 2)
O
Quinone-forming phenol
Quinone
C O -
A—O H I
HO— A—OH I
A— O H
K—
HO— A —0 H
I
H —A— i
1.
■ 2H O —H
A
4----------- -—
H O — A — H
>
h
, Quinone
H O — A — H
]
2
Reduced form of
ucorbio a d d
o
—
A — :H
I
'O H
'B
1
H O — C—
C — H
H aa
Oxidised forii of
ascorbic a did
h
Quinone-fonning
phenol
Source: Tauber 91937).
160
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure A. 4. Direct action o f ascorbic acid oxidase in the oxidation of
ascorbic acid.
co-
CO------H O -C -O H
C—OH
iUn
h- A
HO— i —OH
------
2HO—H
+
HO— C—H
h o 4 - h 2
Reduced form of
ascorbic acid
—
—
h
-»
Ascorbic acrd oxidue
jjq
4
—
i
q
jj
+.Ha
H O -C -H 2
Oxidized form of
ascorbic acid
Source: Tauber (1937).
161
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure A. 5. Titration curve of ascorbic acid.
1.0
x
2 4.25
0
3
4
' - .5
6’
7
Source: S tryer(1995).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure A. 6. Degradation of ascorbic acid.
’V-£>3-A't
■aueifip
a ..
pathway
wnaotaiycatf
••r«ai«
o
o
NJ°
°
~Q o
- S :
(OKC)
(WA1
\ i
Bold lines: Major forms with ascorbic acid activity.
Source: Fennema (1985).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure A. 7. Oxidation of ascorbic acid by Fe(IH).
,OH
OH
,OH
.OH
OH
31
33
OH
OH
OH
OH
OH
OH
F ea+
+ Fe
+ Fe
36
35
34
Source: Richardson and Finley (1985).
164
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure A. 8 . Loss o f ascorbic acid in cooking varies with the method o f
preparation.
• • •
• •••
••••
M icrow ave
15%
Steam ing
30%
J& .
Pressure cooking
20 %
B biiing
55%
Source: Guthrie (198S).
1
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure A. 9. Chlorophyll structure.
I
CHn
II
h
A
3 <
I
H
■R
I
I
/V
NoU: In chlorophyll a, R — CHi; in chlorophyll b, R ~ c ' ' '
|
\
I
>
— C H a— G H a
^c- nn / n- c
JJ— C
M g*
H3 \ /
:c
. H
H
i
I
1
"c'
H
!
-
;
J j— H
II
•
y h tj p h y ti n ferntfjw hm TnagTi—him
digi ia raplacad byjhydrogan OB).
\
cr
i
i
c— c=o
(C H aJa t
..
I .
1
I
C O O C H - -------------------------r --------------------- r------------------------------C = 0
^
! H
; H
H
O — C l ^ ’— C H — C — 7 c ^ ) 3 — C — ( C H a ) g 4 - C — ( C H a ) 3 — C — C H g
_____________C H g ________ __C H 3_______ j
chlorophyll
CH3_
C H 3__
phytyl group j
Source: McWilliams (1997).
166
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure A. 10. Pathways for chlorophyll degradation (MG: methyl group).
Chkupphyfl
g f e
Add
Add V heat
heat
Chlorophyllase
Pheophytln
Chlorophyilide
MG
Pheophorbide
ejrbomethoxyl
Phytol pyroptjeophytln
Source: Bowers (1992).
167
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
APPENDIX B.
M ajor components of microwave oven
Microwave Oven
Singh and Heldman (1993) described that a microwave oven consists the
following major components (Fig. B. 1, page 177).
Pow er supply: Power supply draws electrical power from the line and
converts it to the high voltage required by the magnetron. The magnetron usually requires
several thousand volts o f direct current.
M agnetron o r pow er tube: The magnetron is an oscillator capable o f converting the
power supplied into microwave energy. The magnetron emits high-frequency radiant
energy, The polarity of the emitted radiation changes between negative and positive at
high frequencies (e.g., 24.5 billion times per second for a magnetron operating at a
frequency o f 2450 MHz., the most common frequency used for domestic ovens).
Wave-guide o r transm ission section: The wave-guide propagates, radiates, or transfers
the generated energy from the magnetron to the oven cavity. In a domestic oven, the
wave-guide is a few centimeters long; whereas in industrial units it can be a few meters
long. The energy loss in the wave-guide is usually quite small.
Stirrer: The stirrer is usually a fan-shaped distributor, which rotates and scatters the
transmitted energy throughout the oven. The stirrer disturbs the standing wave patterns,
thus allowing better energy distribution in the oven cavity. This is particularly important
when heating nonhomogenous materials like foods.
Oven cavity o r oven: The oven cavity encloses the food to be heated within the metallic
walls. The distributed energy from the stirrer is reflected by the walls and intercepted by
168
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
the food from many directions with more or less uniform energy density. The energy
impinging on the food is absorbed and converted into heat. The size o f the oven cavity is
influenced by the wavelength. The length o f the cavity wall should be greater than onehalf the wavelength and should be any multiple o f a half-wave in the direction o f the
wave propagation. The wavelength o f 2450 MHz frequency was earlier calculated to be
12.2 cm; therefore, the oven cavity wall must be greater than 6.1 cm. The oven cavity
door includes safety controls and seals to retain the microwave energy within the oven
during the heating process.
M echanism of microwave heating
Microwave energy provides another heat source that can be used for food
applications. It is similar to the types o f energy used for radio and television transmission,
and used for radar signal direction (Potter, 1995; Giese, 1992). Electromagnetic radiation
is absorbed, transmitted or reflected. When an object absorbs these energy waves, the
microwave energy can be converted to heat by the absorbing medium. Microwave energy
use in industry began during “World War IT , following intensive research on radar
mechanisms (Potter, 1978).
High frequency electromagnetic microwaves provide energy as they reverse
direction at 915 MHz and 2450 MHz (1MHz = 1,000,000 cycles/sec) compared to the
common electric current which reverses phase at 60 cycles/sec. The frequencies o f 915
(for industrial usage) and 2450 MHz (for home usage) have been approved for food
application in the United States.
169
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Microwaves are defined as electromagnetic waves o f radiant energy; the
wavelength and frequency are different from those o f light and radio waves (Giese,
1992). The microwave wavelength range is 25 million to 0.75 billion nm (lnm =
10'3 mm) or 0.025 - 0.75 m. Microwaves frequencies range from 20,000 to 400 megahertz
(MHz). The microwave frequencies, approved and used in food applications are 2450
MHz (domestic use) and 915 MHz (industrial use) (Potter, 1995; Schiffmann, 1992). At
915 MHz, microwaves penetrate food materials up to 30 cm, at 2450 MHz penetration is
about 10 cm
(IFT, 1989). Microwaves travel in straight lines; they are reflected by
metals, pass through air and many types o f glass, paper and plastic materials, however,
they are absorbed by some food constituents including water (Potter, 1995).
Water is responsible for heat transfer in foods during most cooking processes
because o f its polar character. Water molecules attempt to align with the alternating
positive and negative field o f the microwaves on passage through foods (Fig. B. 2, page
178) and (Fig. B. 3, page 179).The resultant friction generates heat within the material
(Banwart, 1981).
Microwaves are not a form o f heat but energy; they are converted to heat as they
interact with a material during the energy-transfer mechanism (Schiffinan, 1986). During
this process, the microwaves lose electromagnetic energy, which is known as “dielectric
loss” or “loss factor”. The terms “loss factor” and “loss tangent” indicate the amount o f
microwave energy lost during passage through (absorption by) a material under
controlled conditions (Banwart, 1981).
170
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table B. 1. Dielectric constants, loss factors and loss tangents for raw potato at two
____ microwave frequencies.__________________________________________________
Temperature
Dielectric constant
Loss factor
Loss tangent
(°C)
915 MHz 2450 MHz
915 MHz 2450 MHz
915 MHz 2450 MHz
0
71
67
16
21
0.23
0.30
20
65
64
19
15
0.29
0.23
40
60
59
24
13
0.41
0.23
60
54
54
31
14
0.58
0.27
80
49
49
35
17
0.80
0.34
100
45
45
48
18
1.06
0.41
Source: Mudgett (1982).
Mudgett (1982) discussed the basic electric properties shown in (Table B. 1) o f
foods and microwaves o f scientific interest in terms o f complex permittivities for
biological materials (von Hippel, 1954):
Complex permittivity (Mudgett, 1986):
K* = K ’ - j K ”
(1)
Where:
K*:
“the relative complex permittivity’”
K ’:
“the relative dielectric constant’” and
K” :
“the relative dielectric loss”
j:
:constant” .
Where the real component is the “dielectric constant” and the imaginary
component, the “dielectric loss” or “loss factor” o f the material. They defined the terms:
171
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Dielectric constant: “a measure o f a material’s ability to store electrical energy”.
Loss factor: “a measure o f material’s ability to dissipate electrical energy”.
Loss tangent: “the ratio o f loss factor to loss constant”.
LOSS tan @(giVen temperature) = K /K
(2)
Loss tangent is related to the material’s ability to be penetrated by an electrical
field and to dissipate electrical energy as heat (Mudgett, 1982). The material’s complex
permittivity is also related to its ability to couple electrical energy from a microwave
power generator (magnetron or Klystron), in terms o f loading effects generally reflected
by a characteristic product impedance (Mudgett, 1986). Potter (1978) described the
relationship between food load and heating time to a given temperature:
2
kg o f water
takes twice the time to boil as one kg o f water. Schiffinan (1993) explained this as, the
volume exerts the greatest effect on microwave coupling, when the food is a “reasonable”
microwave absorber; a “reasonable” microwave absorber absorbs 70% o f released
energy.
In general, the more water present, the higher the dielectric loss factor and, hence
the more efficient the heating (Schiffinann, 1986). Swami and Mudgett (1981) explained
the significance of the dielectric properties o f foods and other biological materials, at
microwave frequencies, which are determined by their moisture, solids and salt contents,
and are predicted as functions o f frequency and temperature (Fig. B. 4, page 180).
172
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Schiffrnann (1993) stated that the novice tends to focus on the oven’s power.
What is the significance o f microwave power? There are four basic and different
types o f oven powers:
Pt = PI + Pw + Pr or
Pt = Pm + Pw + Pr
Where:
(1) Pt = the total power launched by the magnetron into the oven cavity.
(2) Pr = the power which is reflected back to the tubes.
(3) Pw = the power absorbed by the oven structure itself and
(4) PI = the power absorbed by the material to be heated.
(5) Pm = the measured output power; it may be quite different from Pr which varies by
as much as 15%.
Pm < Pt and is the value that is actually measured by the user on a single oven using a
technique such as “IEC705” .
(6 ) DEC 705 = A standard procedure which includes a method for determining the output
power o f microwave oven. It uses a water load o f 1,000 ± 5 grams in a 190 mm diameter
one liter crystallizing dish (Particularly made for IEC705 procedures). Water at 10 ± 2°C
(T) is added to the room temperature dish and then immediately heated in the microwave
oven for the time ... in seconds (t) required to raise the temperature 10 ± 2°C (T).
173
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Power is calculated from the following equation:
P (Watts) = 4187 x Mw(T 2-Ti) + 0.88 Me (T 2 -T0)/t (sec)
Where:
P = power in watts
4187 and 0.88 = conversion factors
To = ambient temperature in °C
Ti = the initial temperature o f H20 in°C
T2 = the final H20 in °C
t = time in seconds
Mw= the actual mass of water
Me = the mass of the container
Power is not the only factor affecting oven performance. Other factors, which
play a significant role in microwave heating, can be categorized as “food parameters” and
“oven parameters” . Food parameters include nature o f the food, dielectric properties,
shape and size of food and packaging material o f food (Schiffrnann, 1990). Oven
parameters include output wattage, output frequency, presence or absence o f turntable
and materials of manufacture or the turntable (glass vs. metal), age and condition o f
magnetron (Schiffrnann, 1990).
Food param eters
Water, proteins, and carbohydrates are among the polar molecules that line up in a
microwave electric field; heating also results from the movement o f the electrically
charged ions within food (Giese, 1992). The dielectric properties o f food are dependent
on the content of moisture and salts; microwaves exhibit lower penetration in foods with
174
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
higher moisture and salt content (Kent, 1987). Since microwaves penetrate foods from all
sides, shape has a considerable influence on microwave heating patterns. Decreau (1992)
concluded that the sphere is the ideal shape for microwave heating, with cylinders being
the next best shape. At a frequency o f 2450 MHz, heating can be concentrated in spheres
between 20 and 60 mm; for cylinders, maximum center heating occurs at diameters o f 20
to 35 mm (Decareau, 1992). If the size o f food is not ideal for the wavelength and for the
depth of penetration, heating will not be uniform; if the size o f the food is ideal for the
wavelength, the temperature in the center will be very high (Schiffrnann, 1986).
Oven parameters
Oven parameters include power and speed, frequency, and on-time o f the
magnetron. Most microwave systems operate at power outputs ranging from 5 to 100 kW
with some extending beyond these limits (Schiffrnann, 1986); the higher the power
output, the faster the heating for a given mass. The most attractive feature o f microwave
heating is the speed, which is usually controlled by varying the power output. Cooking,
baking, and other food processes are complex physicochemical activities requiring the
input of heat to initiate and accelerate reactions for better outcomes; these reactions must
occur in their proper sequence and be given the proper amount o f time to occur
(Schiffrnann, 1986). The power to heat the food (Pv) is a result o f the interaction o f the
microwave-generated electric field (E) at a given frequency (f) as influenced by the
dielectric loss factor (E”) o f the food. The equation for the development o f power to heat
implies the intimate relationship between the oven and the food (Schiffrnann, 1990):
175
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Pv = KfE2E”
Where:
K = a constant
f = frequency
E = microwave generated electric field
E” = dielectric loss factor
Pv = the power to heat the food (resulted from the interaction o f E and E” at a given f).
As the dielectric properties o f food change either through change in the food’s
temperature or by changing the ratio of food component (w ater: fat), power to heat
changes, although the oven’s output does not change substantially (Schiffrnann, 1990).
The starting temperature o f food affects both dielectric properties and final temperature
achieved. The starting temperature o f water has an impact on loss factor; a starting
temperature at 10°C will give a significantly higher wattage than one starting at 20°C
(Schiffrnann, 1990).
176
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure B. 1. Major components of microwave oven.
wave guide
Magnetron
Oven cavity
Source: Singh and Heldman (1993).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure B. 2. Mechanism o f microwave heating.
•
•
* +
•
•
0
•
•
•
Source: Brewer (1992).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure B. 3. Movement of a dipole in an electric field.
+ive
-ive
A
-ive
AAA
+ive
Source: Decareau and Peterson (1986).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure B. 4. Relationship between dielectric properties o f foods and penetration
depth.
Dit k e l r i c P r o p e r t i e s l o o d \ I ; i p
<
I)
10
20
20
( . ’I'll Mli/1
-10
20
00
70
SO
00
O u l u liii ( 'onsLinl V
Source: Schiffrnann (1993).
180
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
APPENDIX C.
The general peroxidatic reaction of peroxidase:
The general peroxidatic reaction catalyzed by peroxidase which occurs at pH 4.6 at 25° C,
is:
ROOH + AH2 -> H20 + ROH + A
(Bowers, 1992)
Where:
R = H+, CH 3 , or C 2 H 5 , and
AH 2 = Hydrogen donor in the reduced form
A = Hydrogen donor in oxidized form
The molecular weight of crystalline peroxidase, calculated from the hemin
formed, is 44,000 (Theorell, 1940). Welinder (1979) reported that the primary amino acid
sequence o f isoenzyme C (mb), the dominant isoenzyme in horseradish root, is a single
polypeptide chain o f 33,890 daltons with an attached single Fe protoporphyrin IX
residue (5S0.S daltons) and eight chains of carbohydrate (18% o f total weight). The total
weight o f horseradish peroxidase C is about 42,000; the eight carbohydrate chains are
attached via N-linkages to asparagine residues located at positions 13, 57,158,198,214,
250, and 268 which are all located on the surface o f the protein. One tryptophan residue,
important in peroxidase activity, is located at position 117. Histidine residues are located
at positions 40,42, and 170. Spiro et al (1979) reported that H is-170, closest to the
ferriprotoporphyrin IX and in a hydrophobic region o f the amino acid sequence, probably
forms the fifth coordinate position o f the ferric ion. Four o f the coordination bonds o f the
ferric ion are to the nitrogens of the pyrrole rings (Fig. C. 1, page, 187); the sixth
181
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
coordination position o f the ferric ion is thought to be vacant in peroxidase. Theoreil
(1940) purified and separated horseradish and turnip peroxidase into two enzymes,
peroxidase 1 and peroxidase 2: when precipitated in cold picric acid (at about pH 4.5),
and subjected to electrophoresis (at pH 7.5), peroxidase 1 migrated to the cathode, while
peroxidase 2 went to the anode.
Concentrated peroxidase solutions are deep brown and show characteristic
absorption bands at 645,583, and 498 pm (Sumner and Somers, 1953).Theorell (1940)
stated that peroxidase has bands at 640, 583, 548, and 498 pm; bands at 583 and 548 pm
belong to peroxidase 1 and bands at 640 and 498 pm belong to peroxidase 2.
A general overall summary o f the mechanism of peroxidase is shown in (Fig. C.
2, page, 188).
The resting peroxidase, in the Fe (IH) State, is brown with a molar extinction
coefficient at 403 nm of 1.08.105NT1 cm'1; the sixth coordination bond o f the ferric ion
does not appear to be filled with a water molecule (Spiro et al. 1979). Per-Fe (HI) and
H 2 O2 combine to give the enzyme-substrate complex with kt =9.106 m '1 sec'1 at pH 4.6
and 25°C; the complex undergoes oxidation o f the ferric ion to form per-Fe (IV)=0
(compound I) with k3 , a first-order rate constant, being faster than ki (Richardson and
Finley, 1985).
Dunford and Araiso (1979) and Dunford (1982) stated that the formation o f
compound I appears to involve the side chains o f Asp-43 and Arg-38; and proposed a
mechanism involving these two groups with the prosthetic group Fe (HI) o f the enzyme
as shown in (Fig. C. 3, page, 189).
182
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
The conversion o f per-Fe (IV)=0 (compound I) to per-Fe (TV)-OH (compound II)
occurs with a rate constant ki 40-100 times faster than kt in the presence o f a donor; if a
second molecule of H 2O 2 serves as the donor, k 7 is 4.0/sec; this involves the protonation
o f compound I by removing a proton from the donor making the pale red intermediate
(Richardson and Finley, 1985). The conversion o f per-Fe (IV)-OH (compound II) to the
resting enzyme per-Fe (HI) involves a second molecule o f donor. If the donor is a
molecule o f IfcC^.k* is 0.02/sec; kj is considered to be the rate-limiting step for most
donors.
In addition to peroxidative activity, peroxidase also has catalytic, oxidative and
hydroxylation activities, depending on the system (Richardson and Finley, 1985). Plant
peroxidase, in the presence o f hydrogen peroxide, catalyzes the oxidation o f an indefinite
number o f phenols and aromatic amines; other substances, such as tryptophane, bilirubin,
and iodides are also oxidized. These oxidation products may in turn oxidize other
substances. In this way peroxidases plus peroxide can indirectly oxidize a variety of
substances (Sumner and Somers, 1953).
Owusu-ansah and Marianchuk (1991) concluded that microwave heating might be
successfully used to completely inactivate myrocinase (a peroxidase) in whole or flaked
canola seeds. The most important microwave process parameters in this study were
exposure time and power, which the authors stated, was moisture-dependent. Lopez and
Burgos (1995) compared the effect of heat and manothermosonication on inactivation o f
peroxidase and reported that the higher concentrations o f peroxidase were less sensitive
to manothermosonication. These results agree with those o f Putman (1953), who reported
that protein stability against heat and other physical denaturing agents is higher in
183
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
concentrated than in dilute solutions. Lopez and Burgos (1995) found that reactivation
after both treatments was more rapid at room temperature than in cold temperatures.
These findings are in agreement with Aurand and Woods (1979) who reported that the
peroxidase activity in peas could occur on standing which shows that, in some instances,
inactivation is reversible. Brewer et al (1995) also found reactivation o f peroxidase in
frozen broccoli.
Effect of pH on peroxidase activity
Horseradish peroxidase has an optimum pH at 4.5 to 6.5 when guaiacol is the
substrate; with 0-cresol as the substrate it is at 3.5 to 5, and with pyrogallol the optimum
cannot be determined, since the polyphenols formed are autoxydizable (Tauber, 1937).
In alkaline media, from pH 8 to nearly 10, the activity decreases but recovers
immediately upon neutralization (Tauber, 1937). Above pH 9, peroxidase combines with
hydroxyl ions to form a covalent complex, which is inactive; at pH 10 practically all the
enzyme is destroyed. In acid media at pH 4.2 to 3.8 a precipitate is formed and a decrease
in activity occurs. Between pH 3.6 and 3.2 the precipitate redissolves and the enzyme is
completely inactivated; on adjustment to about pH 7 and standing overnight the activity
o f peroxidase is completely restored (Tauber, 1937).
Effect of temperature on peroxidase activity
In general, enzyme activity is temperature dependent; each enzyme has an
optimum temperature, which usually lies between 30° and 50°C, however, a few enzymes
have optimum activities above 50°C. Heat inactivation o f enzymes is used in food
preservation methods such as blanching o f vegetables, high frequency heating,
pasteurization, sterilization and drying by heating (Heimann, 1980). Most heating
184
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
processes in food manufacturing are designed not only to destroy microorganisms but
also to inactivate deteriorative food enzymes and prolong the storage stability o f foods
(Potter, 1978). Peroxidase, in particular, is stable under many o f the conditions
encountered in food processing. It is considered the most heat-stable enzyme found in
food materials (Baardseth and Slinde, 1980; Gardner et al. 1969; Scott, 1975). Williams
et al. (1986) examined the relative stabilities o f four enzymes (peroxidase, lipoxygenase,
catalase and lipase) in English green pea homogenate incubated at 60° C (Fig. C. 4, page,
190). He reported about 30% o f the activity o f peroxidase is lost within 6 minutes, 50%
within 40 minutes, and the remaining 50% activity was quite stable. In whole peas, 40%
o f the peroxidase activity was lost in 5 minutes at 60° C, however, the other 60% activity
was relatively stable. While at 70° C, 50% o f the activity was lost in 5 minutes with about
40% o f the activity being relatively stable (Williams et al. 1986) (Table C. 1 and Fig. C.
5, page, 191).
Table C. 1. Temperature Stability o f Some Enzymes in Green Peas
ti /2 (min)
Enzyme
Homogenate
Whole peas
(60°C)
(60°C)
(70°C)
Peroxidase
40
—a
5.0
Catalase
1.1
6.5
3.6
Lipoxygenase
0.5
8.0
4.0
Lipase
1.0
5.8
2.0
a40% activity lost by 5 min but still 60% activity left after 50 min.
Source: Williams et al. 1986.
185
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Peroxidase is less sensitive to high temperatures than many other enzymes; even
after boiling a crude preparation, some o f the activity may return on standing; this
regeneration o f activity has been associated with the presence o f cytochrome c in the
tissues (Sumner and Somers, 19S3). They reported that highly purified horseradish
peroxidase is scarcely affected by boiling. They found no considerable loss o f activity
when a solution o f purified peroxidase was kept in an ice chest for ten years.
Due to its heat stability, peroxidase is often used as an indicator o f the
effectiveness o f blanching (Bowers, 1992). There are some problems in the use of
peroxidase as the “universal” indicator o f adequate blanching for vegetables and fruits
(Williams, et al. 1986). Since peroxidase is generally the most heat stable enzyme in
vegetables, heating to complete destruction of peroxidase is more than adequate to
destroy the enzymes directly responsible for quality loss (Bottcher, 1975). The quality o f
the blanched and frozen product is better if there is some peroxidase activity left at the
end o f the blanching (Campbell, 1940; Delincee and Schaefer, 1975; Winter, 1969).
Bottcher (1975) concluded that the complete absence o f peroxidase activity indicated
overblanching; Bottcher (1975) recommended that for the best quality products,
blanching to the some predetermined percent of peroxidase activity is sufficient: peas, 26.3%, depending on variety; green beans, 0-7.3%; cauliflower, 2.9-8.2%; and Brussels
sprouts, 7.5-11.5%.
186
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure C. 1. Structure o f feriprotoporphyrin IX (ferric protohemin).
CH=CH2
HOOCCH
dHsCHjCOOH
Source: marcel and Dekker (1972).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure C. 2. A general overall summary o f the mechanism o f peroxidase.
Hf > 2 k
Per-F e(H I)——»— - — » Per-Fe (III) *H202
Peroxidase (brown)
ES corcplex
H20 +
h20
AH*
Per-Fe(IV)-OH
,
V » 7
Canpound II
(pale red)
AHL
y
*
Per-Fe(IV) =0
Conpound I
(green)
Source: Richardson and finley (1985).
188
Reproduced with permission o f the copyright owner. Further reproduction prohibited without permission.
Figure C. 3. Involvement o f the side chains o f peroxidase (Asp-43 and
Arg-38) in the formation o f compound I.
Aso-43
V 'f e
fc d llP
Arg-38
Asp-43
|
Arg-38 Asg-43
f '8
Fe(III)
A
Fe(Hi)
Arg-38 Asn-43
Arg-38
/H
o'
Fe(IV) H
'%
Source: Richardson and fmley(1985).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure C. 4. Rate o f inactivation o f some enzymes in English Peas
incubated at 60° C.
100
80
Peroxidase
o» 40
Lipoxygenase
20
Lipase
X3Qi
50
20
30
40
Time of Incubation at 60*C (minutes)
Source: William (1986).
190
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure C. 5. Time of incubation for enzymes in minutes.
30
100
V
-X
>k
>
o
<
oi
40
o
20
30
40
!
Time of Incubation (minutes)
Source: William (1986).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
VITA
Shahnaz Begum was bom in Peshawar, Pakistan on February 1,1947. She
received her elementary education in Alamgarh Girls Elementary School and graduated
from Government Girls High School, University Town, Peshawar, Pakistan in 1966.
She received her Bachelor degree in Food Science and Technology from
N.W.F.P. Agricultural University, in 1975. The author served at Pakistan Tobacco Board
(Ministry o f Commerce, Federal Government) as research assistant from 1977 to 1987.
She received her Masters in Food Science and Technology from N.W.F.P. Agricultural
University, in 1987. While the author served in Fruits and Vegetable Development Board
(Provincial Government) as Food Technologist from 1987 to 1991.
The author received her Masters in Agricultural Extension Education from
University o f Illinois in 1993. While she served as Laboratory Technician from 1991
to 1992 and at the Urbana School District she served as bilingual teaching assistant from
1989 to 1999.
In January, 1994, Mrs. Begum enrolled at the University o f Illinois as a doctoral
candidate in Department o f Food Science and Human Nutrition.
Mrs. Begum is married to Professor Dr. Muhammad Bashir o f Gujrat, Pakistan
former graduate o f University of Illinois at Urbana-Champaign, IL, USA. The author has
three sons Mudassar Bashir, Ziad Bashir and Haider Bashir and one daughter Sadia
Bashir.
192
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Документ
Категория
Без категории
Просмотров
0
Размер файла
5 971 Кб
Теги
sdewsdweddes
1/--страниц
Пожаловаться на содержимое документа