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Quality evaluation of vegetables processed by microwave sterilization/pasteurization

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QUALITY EVALUATION OF VEGETABLES PROCESSED BY
MICROWAVE STERILIZATION/PASTEURIZATION
By
JING PENG
A dissertation submitted in partial fulfillment of
the requirements for the degree of
DOCTOR OF PHILOSOPHY
WASHINGTON STATE UNIVERSITY
Department of Biological Systems Engineering
May 2014
UMI Number: 3628868
All rights reserved
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a note will indicate the deletion.
UMI 3628868
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To the Faculty of Washington State University:
The members of the Committee appointed to examine the dissertation of JING PENG
find it satisfactory and recommend that it be accepted.
______________________________
Juming Tang, Ph.D, Chair
______________________________
Diane M. Barrett, Ph.D
______________________________
Shyam S. Sablani, Ph.D
______________________________
Joseph R. Powers, Ph.D
ii
ACKNOWLEDGEMENT
Having read so many theses, it is finally my turn to write my own and express my gratitude to
the people who supported me all the way throughout my Ph.D study here in Pullman. It feels so
good!
First of all, I would like to express my deepest gratitude to my primary advisor, Dr. Juming
Tang, who is a role model in my professional development, gives me support and guidance, and
keeps delivering positive energy to me whenever I need it.
I also greatly thank my committee members Drs. Diane M. Barrett (from UC Davis), Shyam S.
Sablani and Joseph R. Powers for their continuous and valuable advice and suggestions during
my whole research study. I am so lucky to have these professionals and experts in their own
areas of expertise on my committee. Thank you for all of your efforts to maintain multiplecampus communication and make meetings happen (in person or through conference calls). I
really appreciate that!
I would also like to thank Drs. Boon Chew and John Fellman for providing advices and
suggestions for my research and study. My great thanks as well go to Dr. Frank Liu, Dr.
Zhongwei Tang, Dr. Jae-Hyung Mah, Stewart Bohnet, Huimin Lin, Frank Younce, Peter Gray,
Feng Li, Vince Himsl, Jonathan Lomber and Galina Mikhaylenko, for their technical support
during my research. I would like to extend my gratitude to my previous and current Food
Engineering Club colleagues who have been with me these years, Shunshan Jiao,
SumeetDhawan, Ofero Abagon Caparino, Fermin Jr. Pangilinan Resurreccion, Roopesh
Syamaladevi, BaluNayak, Yang Jiao, Wenjia Zhang, Donglei Luan, Ellen Rose Bornhorst,
iii
Rossana Villa Rojas, Kanishka Bhunia, Deepali Jain, Poonam Bajaj, Hongchao Zhang, Rajat
Tyagi and Ravi Tadapaneni.
I am also grateful for support and advice from my other friends Keke, Iris, Qianqian, Hong,
Liang, Jeff, Angie, Susan, Daisy and those who showed up in my life here in Pullman and who
make it more beautiful.
Last but not least, I would like to thank my dearest parents and my young brother Xuanqi, who
support me unconditionally and have been endless sources of encouragement and love for me. I
would like to delicate this thesis to them, and to myself.
iv
QUALITY EVALUATION OF VEGETABLES PROCESSED BY
MICROWAVE STERILIZATION/PASTEURIZATION
ABSTRACT
by Jing Peng, Ph.D.
Washington State University
May, 2014
Chair: Juming Tang
Microwave (MW) heating overcomes the disadvantages of slow conductive/convective heat
transfer inherent in conventional thermal processes, and therefore has the potential to produce
safe and high quality vegetable products. This research was conducted to evaluate the quality
attributes of pre-packaged diced carrots after MW pasteurization and of diced tomatoes after
MW sterilization, in comparison with those subjected to conventional thermal processing. A
systematic study of developing MW sterilization or pasteurization processes for tomato and
carrot products, and evaluating their influence on product quality is presented in this thesis. A
MW-assisted sterilization thermal process (MAST) achieving a target F value of no less than 6
min was developed for processing diced tomatoes packaged in 8-oz pouches, which can deliver a
5D thermal treatment to Bacillus coagulans ATCC 8038 spores. For diced carrots, MW assisted
pasteurization processes (MAP) with F90°C = 3 min and F90°C = 10 min were developed to achieve
at least 6 D reductions of NP C. botulinum type E spores. Thermal resistance of the target
v
bacterium (B. coagulans spores) in tomatoes was characterized, and kinetics of texture
degradation of carrots were investigated for developing thermal processes for these products.
Tomato/carrot dices with added salts (NaCl/CaCl2) at commonly used commercial levels were
processed, and their dielectric properties were determined and used for computer simulation of
heating patterns and cold spot locations in sample pouches. The quality related attributes of
processed tomatoes (drained weight, soluble solids, color, texture, ascorbic acid, and lycopene
content) and carrots (color, texture, pectin methylesterase activity, and carotenoids) were
assessed. The results of quality evaluation of the processed products showed that the impact of
MW processing on the quality of vegetables depends on the characteristics of the vegetables and
the specific quality parameters tested.
vi
Table of Contents
Chapter 1. Introduction ................................................................................................................................. 1
1. Research background and problem statements ..................................................................................... 1
2. Objectives ............................................................................................................................................. 5
3. Dissertation outline ............................................................................................................................... 6
References ................................................................................................................................................. 9
Chapter 2. Literature Review-Thermal Pasteurization of Vegetables......................................................... 14
1. Pathogens of concern and process design for thermal pasteurization ................................................. 14
1.1. Regulations and standards of pasteurization in the U.S. .............................................................. 15
1.2. Regulations and standards of pasteurization in Europe ............................................................... 18
2. Effect of thermal pasteurization on vegetable quality......................................................................... 19
2.1. Color ............................................................................................................................................ 19
2.2. Texture ......................................................................................................................................... 20
2.3. Carotenoids .................................................................................................................................. 22
2.4. Phenolics and antioxidant activity ............................................................................................... 24
2.5. Vitamins ....................................................................................................................................... 25
2.6. Other components ........................................................................................................................ 26
3. Enzyme, storage and shelf-life of pasteurized vegetables .................................................................. 27
References ............................................................................................................................................... 30
Chapter 3. Thermal Inactivation Kinetics of Bacillus coagulans Spores in Tomato Juice ......................... 55
1. Introduction ......................................................................................................................................... 56
2. Materials and Methods ........................................................................................................................ 57
2.1. Microorganisms ........................................................................................................................... 57
2.2. Preparation of B. coagulans spores .............................................................................................. 58
2.3. Preparation of tomato juice .......................................................................................................... 58
2.4. Evaluation of cold-storage time on the viability of B. coagulans in sterile distilled water
and its thermal resistance in tomato juice ........................................................................................... 59
2.5. Preparation and pre-conditioning of a mixture of spore suspension and tomato juice ................ 59
2.6. Evaluation of heat resistance of B. coagulans spores using oil bath............................................ 60
2.7. Evaluation of heat resistance of B. coagulans spores using a capillary tube setup ...................... 61
3. Results and Discussion ....................................................................................................................... 62
vii
3.1. Effect of cold-storage time on the viability of B. coagulans in sterile distilled water and its
thermal resistance in tomato juice ....................................................................................................... 62
3.2. Effect of pH on the thermal resistance of B. coagulans ATCC 8038 spores ............................... 64
3.3. Effect of pre-conditioning on the thermal resistance of B. coagulans ATCC 8038 spores ......... 64
3.4. Thermal resistance of B. coagulans ATCC 8038 spores in tomato juice using a
conventional oil bath ........................................................................................................................... 65
3.5. Thermal resistance of B. coagulans 185A spores at pH 4.3 using oil bath .................................. 66
3.6. Thermal resistance of B. coagulans 8038 spores at pH 4.4 (OSU experiments) ......................... 67
4. Conclusions ......................................................................................................................................... 67
References ............................................................................................................................................... 69
Chapter 4. Kinetics of Carrot Texture Degradation under Pasteurization Conditions ................................ 81
1. Introduction ......................................................................................................................................... 82
2. Materials and Methods ........................................................................................................................ 85
2.1. Sample preparation ...................................................................................................................... 85
2.2. Determination of isotonic concentration of carrot tissue ............................................................. 85
2.3. Thermal treatment ........................................................................................................................ 86
2.4. Texture measurement ................................................................................................................... 87
2.5. Kinetic analysis ............................................................................................................................ 87
3. Results and Discussion ....................................................................................................................... 89
3.1. Determination of the isotonic solution for the carrot tissue and its effect on carrot texture ........ 89
3.2. Effects of preheating and calcium treatment on carrot texture .................................................... 90
3.3. Quality versus microbial/enzyme inactivation ............................................................................. 94
4. Conclusions ......................................................................................................................................... 95
References ............................................................................................................................................... 97
Chapter 5. Dielectric Properties of Tomatoes Assisting in the Development of Microwave
Pasteurization and Sterilization Processes ................................................................................................ 110
1. Introduction ....................................................................................................................................... 111
2. Materials and methods ...................................................................................................................... 114
2.1. Sample preparation .................................................................................................................... 114
2.2. Moisture content, pH and total soluble solids ............................................................................ 114
2.3. Determination of dielectric properties ....................................................................................... 115
2.4. Measurement of ionic conductivity............................................................................................ 115
viii
2.5. Determination of power penetration depth ................................................................................ 116
3. Results and discussion ...................................................................................................................... 117
3.1. Physicochemical properties of pericarp, locular and placental tissues of raw tomatoes ............ 117
3.2. Dielectric properties of pericarp, locular and placental tissues of raw tomatoes ....................... 118
3.3. Effect of NaCl ............................................................................................................................ 120
3.4. Effect of CaCl2 ........................................................................................................................... 121
3.5. Effect of ionic conductivity on dielectric loss factor ................................................................. 122
3.6. Penetration depth ....................................................................................................................... 123
4. Conclusions ....................................................................................................................................... 124
References ............................................................................................................................................. 126
Chapter 6. Developing Microwave Sterilization/Pasteurization Processes for Pre-packaged diced
Tomatoes/Carrots ...................................................................................................................................... 143
1. Introduction ....................................................................................................................................... 143
2. Methods and materials ...................................................................................................................... 147
2.1. Preparation of sample pouches .................................................................................................. 147
2.2. MW heating system ................................................................................................................... 148
2.3. Measurement of dielectric properties of tomato/carrot samples ................................................ 149
2.4. Determination of the heating pattern and cold spot in sample pouches ..................................... 150
2.5. Heat penetration test................................................................................................................... 151
2.6. Incubation test for diced tomatoes ............................................................................................. 154
3. Results and discussions ..................................................................................................................... 155
3.1. Dielectric properties of tomato/carrot samples .......................................................................... 155
3.2. Heating pattern and cold spot in sample pouches ...................................................................... 157
3.3. Heat penetration results .............................................................................................................. 158
3.4. Incubation results for diced tomatoes ........................................................................................ 159
4. Conclusions ....................................................................................................................................... 159
References ............................................................................................................................................. 161
Chapter 7. Quality Evaluation of Vegetable Products Thermally Processed with Microwave and
Conventional Methods .............................................................................................................................. 176
1. Introduction ....................................................................................................................................... 178
2. Materials and Methods ...................................................................................................................... 181
2.1. Sample preparation .................................................................................................................... 181
ix
2.2. Thermal treatments .................................................................................................................... 182
2.3. Color of carrot and tomato dices ................................................................................................ 183
2.4. Texture of carrot and tomato dices............................................................................................. 184
2.5. Pectin methylesterase (PME) activity of carrots ........................................................................ 185
2.6. Carotene analysis of carrots ....................................................................................................... 186
2.7. Determination of pH, soluble solids, drained weight of tomatoes ............................................. 187
2.8. Ascorbic acid content of tomatoes ............................................................................................. 187
2.9. Lycopene content of tomatoes ................................................................................................... 188
2.10. Statistical analysis .................................................................................................................... 189
3. Results and Discussion ..................................................................................................................... 189
3.1. Color .......................................................................................................................................... 189
3.2. Texture ....................................................................................................................................... 191
3.3. PME activity of carrots .............................................................................................................. 192
3.4. Carotenoids of carrots ................................................................................................................ 193
3.5. pH, soluble solids, drained weight of diced tomatoes ................................................................ 194
3.6. Ascorbic acid content of tomatoes ............................................................................................. 195
3.7. Lycopene content of tomatoes ................................................................................................... 195
4. Conclusions ....................................................................................................................................... 196
References ............................................................................................................................................. 198
Chapter 8. Conclusions and Recommendations ........................................................................................ 212
1. Major conclusions ............................................................................................................................. 212
2. Contributions to knowledge .............................................................................................................. 214
3. Recommendations for future research .............................................................................................. 215
x
List of Tables
Chapter 2
Table 1. Commonly accepted growth boundaries of pathogenic microorganisms……….. …….37
Table 2. Summary table of regulations of thermal pasteurization of foods (milk, seafood,
egg and juice products) in the United States…………………………………………………..…38
Table 3. Summary table of Standards of prepackaged chilled foods (pasteurized foods) in
Europe……………………………………………………………………………………………41
Table 4: Lethal rates for L. monocytogenes and necessary process to achieve 6-log
reduction of L. monocytogenes………………………………………………...............................42
Table 5. Lethal rates for C. botulinum type B1and necessary process to achieve 6-log
reduction of C. botulinum type B……………………………………………...............................43
Table 6. Effects of thermal pasteurization on the quality of vegetables…………………………45
Table 7. Major enzymes related to the quality of raw and processed vegetables………………..44
Table 8. Scientific publications of pasteurized vegetables related to storage and enzyme
study………………………………………………………………………………………….…..51
Chapter 3
Table 1. The effect of 4˚Cstorage on the viability of vegetative cells and spores of
B. coagulans……………………………………………………………………………..........….72
Table 2.Comparison of D- and z-values of B. coagulansATCC 8038 and 185A spores
in commercial tomato juice………………………………………………………………………73
Table 3. The effect of heating method on the thermal resistance of spores of B. coagulans
ATCC 8038………………………………………………………………………………………74
Chapter 4
xi
Table 1. Coefficients of determination (r2) from kinetic order (n) models for carrot
texture degradation at four temperatures……………………………………………………….101
Chapter 5
Table 1. Moisture content, pH and soluble solids content of pericarp, locular and
placental tissues in raw tomatoes……………………………………………………………….130
Table 2. Dielectric properties of tomato pericarp, locular and placental tissues with 0.2
g/100g of NaCl and 0.055 g/100g of CaCl2 at 915 and 2450 MHz…………………………….131
Table 3. Microwave penetration depth into tomato pericarp, locular and placental tissues
at 915M Hz……………………………………………………………………………………..132
Chapter 6
Table 1. Processing conditions of carrot/tomato products for equivalent MW and HW
processes with regard to microbial safety………………………………………………………163
Table 2. Microbial assay of raw whole tomatoes, diced tomatoes before and
after process…………………………………………………….……………………………....164
Chapter 7
Table 1. Processing conditions for carrot and tomato products for equivalent MW and
HW processes with equivalent process severity..………………………………………………203
Table 2. CIE L*, a*, b* values, total color differences (∆E), and hue angle of carrot and
tomato dices under different treatments………………………………………………...............204
Table 3. pH, drained weight, color, soluble solids of tomato samples before and
after processing…………………………………………………………………………………205
xii
List of Figures
Chapter 2
Figure 1. Vegetable color wheel…………………………………………………………………54
Chapter 3
Figure 1. Schematic of kinetics treatment chamber, with spore suspensions within sample
in capillaries……………………………………………………………………………………...76
Figure 2. D100°C-values of B. coagulans ATCC 8038 spores exposed to different pH levels
in tomato juice..………………………………………………………..........................................77
Figure 3. Effect of pre-conditioning time on D100°C-value of B. coagulans ATCC 8038
spores in commercial tomato juice at pH 4.3…………………………………………….………78
Figure 4. Thermal survivor curves for B. coagulans ATCC 8038 spores at different
temperatures in commercial tomato juice at pH 4.0….………………………………………….79
Figure 5. Thermal survivor curves for B. coagulans ATCC 8038 spores heated at
different temperatures in tomato juice adjusted to pH 4.4 ………………………………………80
Chapter 4
Figure 1. Percent change in weight of excised carrot pericarp discs in 25 ml of aqueous
solution at different mannitol concentrations…………………………………………………..102
Figure 2. Thermal degradation of texture of carrot dices in isotonic solution or distilled
water at different temperatures………………………………………………………..………..103
Figure 3. Effect of preheating (60°C for 20 min) on the thermal texture degradation of
carrots at different temperatures…………………………………………………….………….104
Figure 4. Plot of 1/C vs. time at different temperatures (n=2)………………………………….105
Figure 5. The final texture value (F∞/F0) of pretreated carrot dices as a function of
xiii
Temperature in different solutions……………………...………………………………………106
Figure 6. Reaction rate k of preheated carrot dices as a function of temperature in
different solutions………………………………………………………………………………107
Figure 7. Arrhenius plot of texture degradation rates of carrots immersed in different
solutions with pretreatment……………………………………………………………………..108
Figure 8. TDT curves of target bacteria, enzymes vs. carrot texture…………………...............109
Chapter 5
Figure 1. Illustration of different anatomical structures of tomato fruits....................................134
Figure 2. Schematic diagram of pressure-proof test cells used for dielectric properties
measurement (from Wang et al., 2003)………………………………………………………...135
Figure 3. Dielectric properties of raw tomato locular tissue as a function of temperature
and frequency…………………….…………………………………………………..................136
Figure 4. Dielectric properties of raw pericarp (♦), locular (□) and placental (△) tissues
at 915 (A) and 2450 (B) MHz…………………………………………………………………..137
Figure 5. Dielectric properties of tomato pericarp (♦), locular (□) and placental (△) tissues
with 0.2 g/100g of NaCl at 915 (A) and 2450 MHz (B)……………………………………….138
Figure 6. Dielectric loss factor of tomato pericarp (♦), locular (□) and placental (△) tissues
at 915 (A) and 2450 (B) MHz…………………………………………………………………..139
Figure 7. Ionic conductivity of tomato locular tissue as a function of temperature……………140
Figure 8. Measured ε" and calculated εσ" of tomato locular tissue (A raw sample; B with
NaCl; C with NaCl & CaCl2) as a function of frequency and temperature…………………….141
Chapter 6
Figure 1a. Front view diagram of four sections in the MATS system at WSU………...............165
xiv
Figure 1b. Front view of MAP system at WSU………………………………………………...165
Figure 2. Flowchart of determination of heating pattern in diced tomato pouches……………166
Figure 3. Preparation of sample pouch with Ellab sensor……………………………...............167
Figure 4. Illustration of computer simulation…………………………………………………..167
Figure 5. Dielectric constant and loss factor of tomato puree added with salts (A) and
carrot puree (B) as a function of temperature and frequency…………….…………………….168
Figure 6. Dielectric properties of processed products with temperatures at 915 MHz…………169
Figure 7. Simulation results of temperature profiles at the cold spot in sample pouches
with a small tomato piece or a big piece for temperature measurement……………………….170
Figure 8. Heating pattern and cold spot location in tomato sample pouch……………………..171
Figure 9. Dielectric properties of tomato sample compared with those of five whey
protein model foods……………………………………………………………………...……..172
Figure 10. Heating pattern and cold spot in carrot sample pouch……………………...............173
Figure 11. Example of temperature-time profile at the cold spot in the diced tomato pouch
under MW (A) and HW (B) processing………………………………………………………...174
Figure 12. Example of temperature-time profiles at the cold spot in the diced carrot pouch
under MW (A) and HW (B) processing………………………………………………………...175
Chapter 7
Figure 1. Color parameter b*/a* of diced carrots (A) and a*/b* tomatoes (B) under different
treatments……………………………………………………………………………………….206
Figure 2. Texture of carrot dices (A) and tomato dices (B) by MW and HW processing
under different conditions…………………..…………………………………………………..207
Figure 3. Residual PME activity of carrot dices by MW and HW processing under
xv
different conditions…………………….…………………………………………………….....208
Figure 4. Total carotenoids content, α- and β-carotene contents of diced carrots under
different treatments……………………………………………………………………………..209
Figure 5. Ascorbic acid content of tomato samples under different conditions………………..210
Figure 6. Lycopene content of tomato samples under different condition……………………..211
xvi
Chapter 1. Introduction
1.
Research background and problem statements
Vegetables provide essential vitamins, minerals and dietary fiber for our bodies, and form an
important part of a healthy diet. Abundant phytochemicals commonly found in vegetables, such
as flavonoids, phenols, and carotenoids, bring us health benefits that prevent nutritional
deficiencies and reduce the risk for various cancers and diseases (Van Duyn, 1999; Scheerens,
2001). The increased public awareness of a healthy balanced diet results in increasing vegetable
consumption.
The highly perishable nature of vegetables requires efficient and appropriate preservation
technologies to prolong shelf life while maintaining nutritional value and sensory quality.
Possible preservation methods include cold storage, heating, freezing, drying/dehydration,
chemical preservation, preservation with sugar/acids, concentration, irradiation, or combinations
of those different means of methods. Among these preservation methods, thermal processing is
one of the most effective means. It is widely used to produce a number of vegetable products
such as juice, puree, sauces, soups, jams, slices, dices and chunks. One of the recent trends in
fruit and vegetable processing is using enhanced delivery of thermal energy (e.g. UHT,
microwave, ohmic) combined with new packaging materials and technologies (e.g. aseptic,
modified atmosphere packaging) to provide consumers with increased choices (Dauthy, 1995).
Compared to conventional heating, microwave (MW) heating provides a relatively short heating
time due to its ability to generate volumetric heating within food materials, and thus has the
potential to produce high quality self-stable food products.
1
Application of MW heating in foods has drawn increased attention over the past decades. Many
researchers have reported its application in vegetable processing and its effect on vegetable
quality, for example in carrot juice (Rayman and Baysal, 2011), carrot pieces (Lemmens et al.,
2009), Brussels sprouts (Vian et al., 2007; Olivera et al., 2008), potato (Alvarez and Canet, 2001;
Barba at el., 2008), peas and spinach (Hunter and Fletch, 2002), tomato (Begum and Brewer,
2001), Swiss chard and green beans (Villnaueva et al., 2000), asparagus (Sun et al., 2007), and
sweet potato purees (Steed et al., 2008). However, in most of these publications, the research was
carried out in a 2450 MHz domestic MW oven or a simply modified MW oven with specially
installed temperature sensors. Different heat treatment levels were achieved by adjusting input
power or heating time, normally several minutes for blanching or pasteurization. Limited
information is available on quality changes of vegetables processed by pilot-scale MW
sterilization/pasteurization systems. Sun et al. (2007) studied color, texture, rutin content and
antioxidant activity of asparagus sterilized by a MW-circulated water combination heating
system. Steed et al. (2008) investigated color, phenolic content, anthocyanins, antioxidant
activities and rheological properties of sweet potato purees by a continuous flow MW-assisted
processing. Koskiniemi et al. (2013) evaluated the quality of acidified vegetables (broccoli, red
bell pepper, and sweet-potato) pasteurized by a continuous MW processing, and observed good
retention of color and texture of acidified vegetable pieces after MW pasteurization. However,
no information is available on quality changes in tomatoes or carrots processed by MW
sterilization or pasteurization.
Tomatoes and carrots are two of the most commonly consumed vegetables in the United States.
Americans consume three-fourths of tomatoes in processed form, most of which are thermally
2
processed; and one-fourth of carrots in processed form, largely canned and frozen (Lucier and
Lin, 2007; Lucier and Glaser, 2009). For processed vegetables, texture is one of the primary
marketable characteristics. The changes in texture of vegetables during processing result from
the chemical composition and amount of cell wall and the middle lamella, which are closely
related to enzymatic and non-enzymatic changes in pectin (a cell wall polysaccharide) (Bourne,
1989; Vu et al., 2004). Enzymatic degradation of cell wall pectin is catalyzed principally by
pectin methylesterase (PME), polygalacturonase (PG) and pectate lyase (PL). PME catalyzes the
de-esterification of pectin, yielding carboxylated pectin with release of methanol. The
demethoxylated pectin can (1) crosslink with divalent cation (primarily Ca2+, naturally present in
the tissue or added during processing) between the free carboxylic acid groups on the
polygalacturonic acid backbone of the pectin to form cross-links between pectin chains (a
firming effect) and (2) be a substrate for PG depolymerization (a softening effect). In addition,
PL can be expressed in plant tissues and depolymerize pectins (Sila et al., 2008). Pectin can also
be depolymerized in a non-enzymatic way by β-elimination, a chemical reaction that takes place
at higher pH levels (>4.5) and at temperatures higher than 80ºC (Keijbets and Pilnik, 1974; Sila
et al., 2008)). Since the β-elimination reaction only occurs in pectins with methyl ester groups,
reducing the extent of pectin methylesterification by activating PME in a low-temperature blanch
could reduce the extent of this reaction (Krall and McFeeters, 1998). Anthon et al. (2005)
observed a reduction of about 2/3 texture in diced tomatoes after 1 min heating at 100ºC with
very little additional change over the next 4 min, and Greve et al. (1994ab) found that the
firmness of carrot tissue was lost rapidly in the first 6 min and then more slowly over the next 15
min heating in boiling water. Studies shows that the early, rapid phase of texture loss in both
tomatoes and carrots under high temperature heating is due to the turgor loss resulted from the
3
membrane disruption, while the 2nd slower, prolonged phase of softening occurred in carrots is
mainly contributed to the breakdown of pectins through β-elimination reactions (Grant et al.,
1973; Greve et al., 1994ab; Anthon and Barrett, 2005). Because the pHs of the two vegetables
are different (3.9-4.4 for the tomato while 5.2-5.8 for the carrot) and β-elimination takes place
only at high pH (usually pH >4.5), the β-elimination in carrots under thermal processing is much
more noticeable than in tomatoes. Therefore, tomatoes and carrots were chosen as the typical
vegetables for MW processing in the current study. Although the texture degradation of carrots
during thermal processing has previously been investigated in several studies, most of the
authors immersed the carrot samples in either distilled water or high calcium solutions (usually
0.5% CaCl2), and none of them evaluated the kinetic models with different reaction order nor
selected the best-fit one to estimate the related kinetic parameters (Huang et al., 1983; Bourne,
1989; Vu et al., 2004; Smout et al. 2005).
Successful exploration of MW application to foods relies on a thorough understanding of the
interaction between microwaves and food materials, and on the ability to predict and provide a
desired heating pattern in foods for specific applications (Tang et al., 2002). A good
understanding of the dielectric properties of food materials is essential for developing MW
processing. Information on dielectric properties of tomato or carrots in solutions with salt and
calcium added at commercial product levels is limited and therefore was investigated in this
study. In addition, a sterilization process is designed to kill all microorganisms while minimizing
quality deterioration. Information on thermal resistance of target bacteria in food materials is
needed for developing and validating thermal processes. In the current study, Bacillus coagulans
spores and non-proteolytic Clostridium botulinum type E spores are considered as the target
4
bacteria for MW sterilization of tomatoes and MW pasteurization of carrots respectively,
according to their acidity (tomato products 3.9-4.4; carrot products 5.2-5.8; FDA, 2001) and
desired final products. Gaze and Brown (1990) studied the thermal resistance of NP C. botulinum
type E spores in carrot from 75–90ºC and reported a z-value of 9.84ºC. For the B. coagulans
spores, although their resistance in tomatoes was studied at acid pHs, information about the
resistance of the spores at different acidic pH levels between 4.0 and 4.5 (the commonly
controlled pH range for canned tomato process) is still limited, especially under high
temperatures (York et al., 1975; Pirone et al., 1989; Mallidis et al., 1990; Rodrigo, 1990;
Sandoval et al., 1992; Palop et al., 1999).
2. Objectives
The overall objective of this dissertation was to develop MW sterilization/pasteurization
processes for pre-packaged vegetables, and evaluate their quality in comparison with those
processed by conventional heating. Two vegetables, tomatoes and carrots were chosen because
of their popularity, differences in pH and the fact that β-elimination doesn’t proceed to a great
extent in tomatoes at their pH under high-temperature processing.
The specific aims of this research were to (1) conduct thermal kinetic studies of target
microorganism in tomatoes; (2) conduct kinetic studies of carrot texture degradation under
pasteurization conditions; (3) determine dielectric properties of tomatoes and carrots; (4) develop
MW and conventional thermal processes for pre-packaged diced tomato and carrot products; (5)
evaluate quality of MW and conventional heating processed products.
5
3. Dissertation outline
This dissertation contains seven chapters, as follows:
Chapter 1: Introduction. This chapter generally introduces the background of this research work,
states the current problems, and outlines the objectives and structure of the dissertation.
Chapter 2: Literature review. This chapter is a review of fundament concepts, general methods,
data and applications of thermal pasteurization of vegetables. In detail, it includes pathogens of
concern and process design for thermal pasteurization; effect of thermal pasteurization on
vegetable quality; and enzyme, storage and shelf-life of pasteurized vegetables.
Chapter 3: Thermal inactivation kinetics of Bacillus coagulans spores in tomato juice. This study
was conducted to characterize the thermal resistance of three strains of Bacillus coagulans
(ATCC 8038, 7050 and 185A) spores and vegetative cells in tomato juice, and choose the one
with the highest thermal resistance as the target microorganism for thermal processing of tomato
products. Thermal inactivation kinetics of the target bacteria in tomato juice between 95°C and
115°C were determined. The effects of environmental factors, including cold-storage time, pH,
and pre-conditioning on the thermal resistance of these bacterial spores were also investigated.
Chapter 4: Kinetics of carrot texture degradation under pasteurization conditions. This chapter
describes the texture degradation of carrot dices in different solutions (distilled water, 0.1% and
1.4% CaCl2 solutions) under temperatures ranging from 80 to 110ºC. The effects of preheating
before high temperature treatments on carrot texture were studied and kinetic parameters were
estimated. Data obtained in this chapter were used to recommend processing conditions for
carrot products that could control food pathogens and inactivate enzymes.
Chapter 5: Dielectric properties of tomatoes assisting in the development of MW pasteurization
and sterilization processes. This chapter provides information on the dielectric properties of
6
tomatoes over a frequency range of 300–3000 MHz for temperatures between 22–120°C. Three
tomato tissues, the pericarp tissue (including the skin), the locular tissue (including the seeds)
and the placental tissue were studied separately. The effects of temperature, frequency and salts
(0.2g/100g of NaCl and 0.055g/100g of CaCl2) on the dielectric properties of three tissues were
investigated, and their dielectric loss mechanisms were discussed in this chapter.
Chapter 6: Developing MW sterilization/pasteurization processes for pre-packaged diced
tomatoes/carrots. In this chapter, a MW assisted thermal sterilization (MATS) process was
developed for processing diced tomatoes packaged in 8-oz pouches using a semi-continuous,
915MHz single-mode MW system; while a MW assisted thermal pasteurization (MAP) process
was developed for diced carrots using a 14-kW single-mode MW system. A 3-D computer
simulation model that considered temperature dependent dielectric properties of food materials
provided information about heating patterns and the cold spot location in the sample pouches.
The simulation results were validated with a chemical marker based computer-vision method.
Heat penetration tests were conducted to obtain temperature-time data for identifying the cold
spot in diced tomatoes packaged in pouches, from which a MATS process was designed to
achieve a 5D reduction in Bacillus coagulans ATCC 8038 spores (F105°C = 6.0 min). For diced
carrots, two MAP processes were developed: a 6D reduction in non-proteolytic Clostridium
botulinum type E spores (F90°C=3 min); and a F90°C=10 min process. Incubation test of the
processed tomato products was conducted to validate the MATS processes.
Chapter 7: Quality evaluation of pilot-scale MW/conventional thermal processed vegetable
products. This chapter presents the results of quality evaluation of diced tomato after MW
sterilization and of diced carrots after MW pasteurization, in comparison with conventional
thermal processing. Quality attributes of the processed products by MW/HW heating were
7
evaluated and compared. For diced tomatoes, quality related parameters included drained weight,
soluble solids, color, texture, ascorbic acid and lycopene content; while for diced carrots, quality
attributes of color, texture, pectin methylesterase activity and carotenoids were evaluated.
Chapter 8: Conclusions and recommendations. The main findings and contributions to
knowledge of this research were summarized in this chapter; recommendations for future work
were also given in this chapter.
Chapters 3, 4 and 5 have been published, and their formats followed the requirements of the
target journals. The articles published and manuscripts prepared from this research are listed
below:
1. Peng, J., Tang, J.,Barrett, D.M., Sablani, S.S., and Powers, J. 2014. Kinetics of carrot texture
degradation under pasteurization conditions. J Food Eng. 122, 84‒91.
2. Peng, J., Tang, J., Jiao, Y., Bohnet, S.G., and Barrett, D.M. 2013. Dielectric properties of
tomatoes assisting in the development of microwave pasteurization and sterilization processes.
LWT-Food Sci & Tech. 54, 367‒376.
3. Peng, J., Mah, J.H., Somavat, R., Mohamed, H., Sastry, S., and Tang, J. 2012. Thermal
inactivation kinetics of Bacillus coagulans spores in tomato juice. JFood Prot. 75, 1236‒1242.
4. Peng, J., Tang, J.,Barrett, D.M., Sablani, S.S., and Powers, J. A review of thermal
pasteurization of vegetables. In preparation.
5. Peng, J., Tang, J.,Barrett, D.M., Sablani, S.S., and Powers, J. Thermal pasteurization and
quality evaluation of diced carrots pre-packaged in 8-oz pouches. In preparation.
8
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13
Chapter 2. Literature Review-Thermal Pasteurization of Vegetables
1.
Pathogens of concern and process design for thermal pasteurization
Known as important components for a balanced and healthy diet, vegetables provide essential
vitamins, minerals and dietary fiber for our bodies. A range of phytochemicals commonly found
in vegetables, such as flavonoids, phenols, and carotenoids, also prevent nutritional deficiencies
and reduce the risk for various types of cancer, heart disease, diabetes, diverticulosis, stroke,
hypertension, birth defects, cataracts, and obesity (Scheerens, 2001 and Van Duyn, 1999).
However, vegetables are highly perishable, they need efficient and appropriate preservation
technology to prolong their shelf life while maintaining nutritional value and sensory quality.
With the increased public awareness of healthy diet and the needs for ready-to-eat foods, thermal
pasteurization has regained our attention as an effective vegetable preservation method to
provide safe convenient foods with high nutrients and good sensory quality.
The word “pasteurization” was originally named after the French scientist Louis Pasteur who
invented the process of heating food, such as wine at 55°C for several minutes to kill diseasecausing micro-organisms (pathogens). Thus, traditional pasteurization refers to a heat treatment
of food (usually below 100°C) to destroy all organisms dangerous to health, or a heat treatment
which destroys part but not all microorganism that cause food spoilage or that interfere with a
desirable fermentation (Downing, 1996). Unlike sterilization, pasteurization process doesn’t kill
all the micro-organisms in the foods, only destroying the vegetative pathogenic bacteria and
lowering the level of spoilage organisms that will grow under refrigerated storage.
14
1.1. Regulations and standards of pasteurization in the U.S.
In recent years, the development of emerging technologies which can satisfy the goals of
pasteurization, calls for a broadening of the definition of pasteurization. Therefore, the National
Advisory Committee on Microbiological Criteria for Foods (NACMCF) has determined the
requisite scientific parameters for establishing equivalent alternative methods of pasteurization,
and defined it as below (NACMCF, 2006):
“Any process, treatment, or combination thereof, that is applied to food to reduce the most
resistant microorganism(s) of public health significance to a level that is not likely to present a
public health risk under normal conditions of distribution and storage”.
This definition allows application of a broad range of technologies (one or in combination) to
different food-pathogens for pasteurization such as ohmic heating, microwave heating, steam and
hot water heating, pulsed electric field, chemical treatments, filtration, infrared, and high voltage
arc discharge. In addition to the processing methods, three major points are made in this
definition for developing a pasteurization process:
1. Determining the most resistant microorganism of public health concern for the food;
2. Assess the level of inactivation of target microorganism needed and validate it, to make sure
“not likely to present a public health risk”;
3. Consider the distribution and storage conditions, normally refrigerated.
According to the recommendations of NACMCF, more factors need to be considered for
establishing a successful pasteurization process, such as the impact of food matrix on pathogen
survival, developing specific Hazard Analysis Critical Control Point (HACCP) system and Good
15
Manufacturing Practices (GMPs) for the process, etc. In the current study, we will focus on the
three major points that need to be addressed for process design.
First of all, determine the most resistant microorganism of public health concern for the process.
Table 1 lists the primary pathogens of concern and their general growth conditions (ECEF,
2006). Considering the normal distribution and storage temperatures for pasteurized foods
(<5°C) with some temperature margin, pathogens with a minimum growth temperature lower
than 7.2°C should be considered as potential pathogens and be included in the hazard analysis to
identify the target bacteria for the process. With this in mind, L. monocytogens, B. cereus, nonproteolytic C. botulinum, E. coli O 157:H7, Salmonella, Staphylococcus aureus, V.
parahaemolyticus and Y. enterocolitica should to be taken into account for the hazard analysis.
There is no simple guideline for the target bacteria for a pasteurization process, because the
presence of bacteria in the foods depends on the food characteristics and compositions, the
resistance of bacteria may also vary under different processing technology. However, regulations
and standards related to target bacteria and processing requirements of certain foods by
pasteurization are given by the government in the U.S. for milk, seafood, egg and juice products
(Table 2).
The current processing of milk is governed by the FDA Milk Pasteurized Ordinance, and is
based on two fundamental principles: 1) every particle must be heated to a specified minimum
temperature for a specified time and 2) equipment is properly designed and operated. The first
federal standard for milk pasteurization was established in 1924, requiring a 61.7°C, 30 min
process targeting Mycobacterium tuberculosis (Meanwell, 1927). In 1956, Coxiella burnetii was
16
recognized as the most resistant bacteria of concern, and the current minimum pasteurization
time and temperature combinations (63°C for 30 min or 72°C for 15 s) were established. Later,
Enright (1961) demonstrated a more rigorous pasteurization treatment was needed for three milk
products including cream, chocolate milk and ice cream mix (Table 2).
The pasteurization of seafood in the US is governed by FDA “Fish and fisheries products
hazards and control guidance” (FDA, 2011b).
FDA considers a 6D process for target C.
botulinum (type E and non-proteolytic types B and F) to be generally suitable for pasteurized
seafood products. For the processing requirements, a minimum cumulative total lethality of F90°C
= 10 min is adequate for pasteurized fish and fishery products (Table 2). With regard to blue
crabmeat, the National Blue Crab Industry Pasteurization and Alternative Thermal Processing
Standards requires a process of F85°C=31 min, which exceeds a 12-log reduction of C. botulinum
type E spores. Some products like Dungeness crabmeat contain certain substances (e.g.
lysozyme) that may enable the pathogen to more easily recover after heat damage. In this case, a
longer intense heating is needed (F90°C = 57 min for Dungeness crabmeat, FDA, 2011b).
Under FSIS regulations and FDA criterion (2009 Egg Safety Action Plan, 21 CFR Parts 16 and
118), egg and egg products must be free of viable Salmonella. Thus, Salmonella is the target
microorganism for pasteurized eggs and egg products. A process of 8.75-log reduction of
Salmonella is required for liquid eggs, and 5-log reduction is required for whole eggs. Specific
processing conditions for the eggs and egg products are listed in Table 2.
17
The pasteurization of juice is regulated by the FDA Juice HACCP Hazards and Controls
Guidance. A process with minimum 5-log reduction of most resistant microorganism of public
health significance under HACCP plan is required. The target bacteria are dependent on the juice
product and process, including E. coli O157: H7, Salmonella, Cryptosporidium parvum or C.
botulinum. For acidic juices (pH ≤ 4.6), E.coli O157:H7, Salmonella, and Cryptosporidium
parvum may occur and cause serious foodborne illness outbreaks; while for low-acid juices such
as carrot juice, C. botulinum may be present and produce toxin, and therefore becomes the
pathogen hazard of concern.
In summary, the definition of pasteurization in the US is broad. Regulations and standards with
regard to the pathogens of concern and processing conditions for pasteurization are specific,
depending on the particular product, process conditions and packaging systems. There is no "one
size fits all" approach to achieving microbiological safety.
1.2.
Regulations and standards of pasteurization in Europe
The European Chilled Food Federation provides guidance for producing chilled foods in Europe
(ECFF, 2006). ECFF defines chilled food as “foods that for reasons of safety and/or quality rely
on storage at refrigeration temperatures throughout their entire shelf-life.” According to ECFF
recommendations, the common practice for heat-treated chilled food is to aim for a 6 log
reduction of either (Table 3):
1)
L. monocytogenes (this treatment will control other vegetative pathogens)
2)
Cold growing C. botulinum (this treatment will not control other spore-forming
pathogens such as B. cereus)
18
L. monocytogenes is the most heat-resistant vegetative pathogen while Type B C. botulinum is
the most heat resistant form of non-proteolytic C. botulinum. Tables 4 and 5 give the commonly
accepted lethal rates and temperature-time combinations to achieve 6-log reduction of L.
monocytogenes and type B C. botulinum. It is generally accepted that a mild pasteurization of
low acid food (F70°C=2.0 min) achieving 6 log reduction of L. monocytogenes is suitable for a
shelf life of maximally 10 days at 5°C. A severe pasteurization process of F90°C=10.0 min aiming
at a 6D process inactivation of non-proteolytic C. botulinum allows product a shelf life up to 6
weeks at 5°C (ECFF, 2006; CSIRO Food and Nutritional Sciences, 2010).
2. Effect of thermal pasteurization on vegetable quality
2.1. Color
Color plays a vital role in the consumer acceptance of a vegetable product, and is one of the most
important characteristics of vegetables. This visual appeal mainly comes from pigments such as
chlorophylls, anthocyanins, and carotenoids (lycopene), which provide health and nutrient
benefits (Figure 1). The visual color of vegetables can be numerically expressed by color
models. The CIE model is the most commonly used and its three color values L*(lightness), a*
(redness) and b* (yellowness) can be used individually or in combination in the form of hue,
chroma, or total color difference value.
Published papers related to color changes by thermal pasteurization are listed in Table 6. Most
authors found that the color degradation of vegetables by thermal pasteurization depends mainly
on the heat intensity, duration, media, compounds responsible for the color, and storage time.
19
Lau et al. (2000) found that the surface color changes of asparagus followed a first order
Arrhenius reaction kinetics at temperatures from 70 to 98°C. Loong and Goh (2004) also
reported first order kinetics of color degradation of a mixed vegetable juice (butterhead lettuce,
celery, parsley, apple concentrate and kalamasi lime) between 80 and 100°C. Koskiniemi et al.
(2013) pasteurized three vegetables (broccoli, red bell pepper and sweet potato), and found that
the green color of broccoli floret changed the most while the sweet potato color was stable over
the course of processing. Among all these work, three included the studies of color changes
during the storage period. The discoloration of pressurized vegetable and vegetable products can
occur due to enzymatic browning. Both pectin methylesterase and peroxidase existed in mild
thermal pasteurized carrots (P70°C= 2 min), but neither was present in severe thermal pasteurized
carrots (P90°C= 10 min). The color values of most vegetables decreased in the storage period from
36 to 120 days (Koo et al., 2008; Koskiniemi et al., 2013; Rejano et al., 1997). Only in one study
by Koskiniemi et al. (2013), did the color of broccoli florets not change during an extended
storage or even during thermal processing. The reason was that addition of acid during
equilibration process had a detrimental effect on the broccoli color, converting chlorophyll
(green) into pheophytin (olive green).
2.2. Texture
The texture of processed vegetables is another primary marketable characteristic for customers.
Mechanisms that contribute to the texture loss during heating of vegetables generally include
turgor loss due to the breakdown of cellular membranes, cell wall degradation and disassembly
resulting from enzymatic and non-enzymatic transformations in pectin structure and composition
(Anthon et al., 2005; Greve et al., 1994ab; Sila et al., 2008). For mild pasteurization in which
20
processing temperature is lower than 80°C, vegetable tissue softening due to pectin
depolymerization by non-enzymatic degradation via
β-elimination is negligible due to the
relatively high temperature required for this reaction to take place.
Texture characteristics of vegetables can be evaluated by sensory and instrumental methods.
Sensory evaluation offers the opportunity to obtain a complete analysis of the textural properties
of a food as perceived by the human sense, while instrumental measurements are more
convenient, less expensive, and tend to provide consistent values when used by different people
(Bourne, 1982; Abbott, 2004). Evaluations of vegetable texture in the manuscripts listed in Table
1 were all assessed by instrumental methods, although the specific equipment and method are
product-dependent. Most authors applied force-compression tests by a texture analyzer, only one
study by Koo et al. (2008) used a rheometer to measure the texture of soybean sprouts.
Most authors observed a decreased texture of the processed vegetables in comparison to the raw
materials. Lau et al. (2000) reported a first order reaction for the softening of green asparagus
spears in a temperature range from 70-98°C. In regard to the storage effect on the vegetable
texture, Koskiniemi et al. (2013) pasteurized sweet potato, red bell pepper and broccoli by
continuous microwave (3.5 Kw) for 4 min after which the surface temperatures of vegetable
packs upon exit of the MW cavity achieved 75-80°C, and the vegetables held in insulating molds
for 30 min. They found that the largest texture degradation in the three vegetables occurred over
a 60-day storage period at 30°C. For example, both the fracture peak force and total work of
sweet potato decreased by 85% from post-process to the end of the 60-day storage. One possible
reason they gave in the paper was the addition of NaCl and citric acid resulting in the tissue
21
softening during the storage time. Besides the compounds in the media, residual enzyme in the
processed products may also degrade their texture through enzymatic reaction in the storage.
2.3. Carotenoids
Carotenoids are one of the predominant organic pigments present in vegetables, and include αand β-carotenes (yellow/orange), lycopene (red/orange), xanthophyll (yellow), lutein and
zeaxanthin (green/yellow). They are also one of the important bioactive compounds in carrots,
and can act as antioxidants to reduce the risk of developing degenerative diseases. For example,
α- and β-carotenes are known as vitamin A precursors, responsible for the orange color such as
carrot, sweet potato. Lycopene is considered to be a potential antioxidant and cancer-preventing
agent, responsible for the red color in tomatoes. Lycopene, α- and β-carotenes may undergo
isomerization, oxidation and other chemical changes during thermal processing and storage due
to their highly unsaturated structure (Rodriguez-Amaya and Kimura, 2004; Shi et al., 2003).
The effect of thermal pasteurization on the carotenoids in vegetables depends on the heat
intensity and the properties of the products. Total carotenoids found in vegetables are relative
stable to mild pasteurization. Vervoort et al. (2012) heated carrot pieces from mild pasteurization
(P70°C = 2 min) to severe pasteurization (P90°C = 10 min) and found no considerable differences
occurred in total carotenoid content after processing, or their individual α- or β-carotene
concentration. The authors attributed the stability of carotenes to the protective food matrix,
which preserves them from degradation during pasteurization. They concluded that the applied
pasteurization conditions were not severe enough to cause a notable isomerization and/or
oxidation of the carotenoids in carrots. Similar results were obtained by Lemmens et al. (2013),
also for β-carotene content in carrots. However, Odriozola-Serrano et al. (2009) observed an
22
increase of total carotenoid content, lycopene and β-carotene after pasteurization of tomato juice
at 90°C for 30 s or 60 s. One explanation given was that the homogenization and heat treatment
condition disrupted cell membranes and protein-carotenoid complexes, increasing the
extractability of the carotenoids. Rayman and Baysal (2011) reported a decrease in total
carotenoid content of carrot juice after pasteurization at 100°C for 10 min.
With regard to the changes of carotenoids in vegetables during storage, Odriozola-Serrano et al.
(2008, 2009) reported a decreasing trend of total carotenoid content and lycopene content in
pasteurized tomato juice during 56 days or 91 days storage at 4°C. The authors explained that the
decrease of lycopene content throughout the storage may due to the oxygen availability in the
headspace of the container. Rayman and Baysal (2011) reported an increase in the carotenoid
content in carrot juice after 3 months of storage at 4°C, and attributed it to possible isomerization
of β-carotene.
Thermal pasteurization may also influence the bio-accessibility of carotenoids in vegetables,
depending on the thermal intensity and various other factors. In the paper mentioned above,
Lemmens et al. (2013) also investigated the bio-accessibility of β-carotene in carrot, and found
mild thermal treatments of carrot could enhance the β-carotene bio-accessibility compared to raw
carrots, but the differences were only significant on a less strict significance level. Higher βcarotene bio-accessibility is normally associated with intense thermal processes. Similar results
were obtained by Knockaert et al. (2012), who reported an increased β-carotene bio-accessibility
in carrot puree after thermal pasteurization.
23
2.4. Phenolics and antioxidant activity
Phenolics are important phytochemicals as bioactive compounds in vegetables. Most researchers
have reported phenolics in relation to their antioxidant activity. Effects of thermal pasteurization
on the total phenolics in vegetables were associated with the properties of the food material,
package and storage conditions. Odriozola-Serrano et al. (2008) didn’t find significant changes
in the total phenolic content between pasteurized and fresh tomato juice (90°C for 30/60 s), and
noticed good maintenance of the phenolic compounds during storage which might be due to the
inactivation of the enzymes responsible for its degradation. The same authors later reported no
significant changes in the total phenolic content and individual phenolic concentration between
tomato juices just after thermal pasteurization and fresh ones, but a decrease in later storage
period (Odriozola-Serrano et al., 2009). Same processing conditions (90°C for 30/60 s) and same
storage conditions (up to 56 days at 4°C) were applied in the latter study. The authors
hypothesized that the degradation of phenolic compounds during storage was associated to the
residual activity of peroxidase, but no enzyme activity test was carried out to support this
assumption. Rayman and Baysal (2011) found a decrease in the total phenolic contents of carrot
juice after pasteurization (100°C for 10 min) and a continued decrease during storage up to 4
months at 4°C.
Quercetin is one of the numerous flavonoids commonly found in vegetables. Roldán et al. (2008)
evaluated the total quercetin content in pasteurized onion by-products and their frozen products,
and found a lower value in all the pasteurized products compared to their corresponding frozen
products, but higher than the sterilized ones. The differences in the quercetin content of onion
24
by-products among different treatments were associated to the onion cultivar and the specific
product.
Pasteurization caused an almost 50% loss in the antioxidant activity in carrot juice, and storage
further increased this loss, which related to the decrease in phenolic contents (Rayman and
Baysal, 2011).The antioxidant activity of fresh and pasteurized tomato juice, measured using the
DPPH stable radical assay, didn’t show significant difference from each other (OdriozolaSerrano). However, the antioxidant capacity of tomato juice subjected to heat treatment
decreased with storage time, with a 46.52% reduction of the initial value at 56 days storage for
mild pasteurized juice (90°C for 30 s) and 35.7% for high pasteurized juice (90°C for 60 s). The
authors studied the correlation among different bioactive compounds (lycopene, vitamin C and
phenolic) and antioxidant capacity of tomato juice subjected to different treatments, and
concluded the decrease of antioxidant capacity during storage could be attributed to the losses of
vitamin C and lycopene.
2.5. Vitamins
It is known that vegetables are great sources of various essential vitamins. Vitamin C (ascorbic
acid) is one of the numerous vitamins vegetables provide us. However, vitamin C is readily
changed or brokendown in the presence of oxygen and light, and high temperature will
accelerate this degradation process. Due to its thermolability, vitamin C in vegetables is used as
an indicator for the loss of other vitamins and thermolabile nutrients during thermal processing
(Torregrosa et al., 2006). Thermal degradation of vitamin C in foods has been widely studied,
and is generally reported as a first-order kinetic. Thermal pasteurization of gazpacho (a cold
25
vegetable soup) at 90°C for 1 min reduced the vitamin C level to 79.2% of its initial value (ElezMartínez and Martín-Belloso, 2007). Torregrosa et al. (2006) reported an 83% retention of ascorbic
acid in pasteurized orange-carrot juice (98°C for 21 s). Pasteurizing soybean sprout at 70°C for 2
min reduced its vitamin C from 7.19 mg/100 g to 1.26 mg/100g, and to 1.17 mg/100 g after
processing at 90°C for 10 min (Koo et al., 2008).
Most authors investigated the changes of vitamin C in pasteurized vegetables followed by a
storage study, and reported a decrease of vitamin C during the storage period. The decreasing
level depended on the storage conditions, such as temperature, oxygen content, light and
packages. Odriozola-Serrano et al. (2008) found the contents of vitamin C in pasteurized tomato
juice could be described by a first-kinetic model. For the effect of storage temperature,
Torregrosa et al. (2006) found that the ascorbic acid degradation rate in the orange-carrot juice
stored at 2°C was less than those in the juice stored at 10°C, while Koo et al (2008) reported the
decreased level of ascorbic acid in cook-chilled packaged sprouts stored at 3°C was much like
those stored at 10°C.
One study by Barba (2012) evaluated the three types of vitamin E (α, Ƴ, and δ-tocopherol) and
vitamin D in a pasteurized vegetable beverage (Table 6). The authors noticed a decrease in each
vitamin E and also vitamin D. No storage study was conducted in that study.
2.6. Other components
Other nutrients or relevant quality aspects, such as sugars, dry matter content, fatty acid,
isothiocyanates and furfural in certain processed vegetables have been reported. Vervoort et al.
26
(2012) pasteurized carrots pieces at two levels (F70°C= 2 min and P90°C = 10 min) and analyzed
the dry matter content, sugar profile (glucose, fructose and sucrose), furfural and 5hydroxymethylfurfural in the processed products. Pasteurization caused a significant reduction in
dry matter content and all sugar concentrations, and an increasing intensity did not cause any
further significant changes in those values. No furfural was detected in any of the pasteurized
carrots.
3. Enzyme, storage and shelf-life of pasteurized vegetables
Thermal pasteurization of vegetables aims to inactivate pathogens and endogenous enzymes to
provide safe and high quality products, with a relatively short shelf-life under expected storage
and use conditions. The presence of residual endogenous enzymes in processed vegetable
products may cause quality loss during storage. Therefore together with microbial growth,
enzyme activity can considerably shorten the shelf life of the final products. The principal
enzyme responsible for a specific quality loss is product-dependent. Major enzymes related to
the quality of vegetables are listed in Table 7. When evaluating the quality of pasteurized
vegetable products, most authors conducted a storage study to see the quality changes with
storage time/temperature. Table 8 summarizes the published papers on pasteurized vegetables
related to storage and enzyme study. Few studies investigated the enzyme activity after
processing or during storage. We can also notice that storage temperatures and time in these
work varied from product to product. For most of the pasteurized vegetables, a storage
temperature of 3-5°C was used. For some pickled, high acidic vegetables, the storage
temperature was much higher (23-30°C). The storage time varied from 21 days to 5 months,
depending on the property of products and storage temperature. With regard to the shelf-life of
27
pasteurized foods, there does not appear to be a universally accepted standard for all the
products. Torregrose et al. (2006) calculated the shelf-life of pasteurized orange-carrot juice as
the time taken for the ascorbic acid concentration to reduce to 50% (Table 7). Most authors
didn’t provide reasons for the selected storage conditions for their products.
The shelf life is defined as “the period of time for which a product remains safe and meets its
quality specifications under expected storage and use conditions” (ECFF, 2006). Based on ECFF
(2006), the manufacturer is responsible for determining the shelf life and must take into account
microbiological safety and stability, physical characteristics and organoleptic quality.
Microbiological safety and stability should always be priority for determining the shelf life when
the acceptable shelf life for either physical condition or organoleptic quality exceeds that for
microbial safety. The product shelf life are influenced by a number of factors, including raw
material quality, product formulation (pH, aw), hygiene during manufacturing, scheduled heat or
other preservation treatments, cooling methods applied to products, type of package, storage
temperature and relevant hurdles (CAC, 1999). When determining the shelf life of the products,
Codex Alimentarius Commission (CAC) suggests taking into consideration the potential for
temperature abuse which may occur during manufacture, storage, distribution, sale, and handling
by the consumer. For example, fluid milk is most often held at marginal refrigeration
temperatures of 6.1-7.2°C (43-45°F) instead of the ideal holding temperatures (≤ 3.3°C) to
determine the potential shelf-life (Murphy, 2009). The authors believed these marginal
refrigeration temperatures allow defects and sanitation deficiencies to become more evident.
Therefore, for a food producer determining the safe shelf-life for pasteurized vegetables, the
following information needs to be collected based on ECFF recommendations:
28
1)
Review relevant scientific information containing the characteristics of pathogens;
2)
Use predictive modeling programs (e.g. ComBase, USFA Pathogen Modeling Program or
Growth Predictor) to estimate the growth of pathogens under the storage conditions;
3)
Conduct challenge test with the relevant pathogens where predictive modeling on its own
does not give sufficient confidence to set a safe shelf life;
4)
Collect historical data for similar products;
5)
Conduct storage trials, either by storing products at predetermined temperatures during
specific time periods considering actual chill chain performance under HACCP or testing the
product at minimum three time points for the relevant indicator and spoilage microorganisms as
well as pathogens identified by HACCP.
The information above focuses on a safe shelf life. Desired quality should also be considered
when determining the shelf-life of pasteurized products. This information also provides insights
for researchers to conduct storage study for pasteurized products.
29
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Enright, J.B., Sadler, W.W., and Thomas, R.C. 1957. Thermal inactivation of Coxiella Burnetti
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36
Table 1. Commonly accepted growth boundaries of pathogenic microorganisms (ECEF 2006)
Microorganism
Min temp
Min
(°C)
pH
Min aw
Aerobic/
anaerobic
L. monocytogens
-0.4
4.3
0.92
Facultative
B. cereus
4
4.5
0.93
Facultative
Campylobacter jejuni
32
4.9
0.99
Microaerophilic
C. botulinum Mesophilic/proteolytic
10-12
4.6
0.93
Anaerobic
C. botulinum Pasyschrotrophic/non-
3.3
5.0
0.97
Anaerobic
proteolytic
C. perfringens
(5% NaCl)
12
5.5-
0.935
Anaerobic
5.8
E. coli
7-8
4.4
0.95
Facultative
E. coli O157:H7
6.5
4.5
0.95
Facultative
Salmonella
6
4.0
0.94
Facultative
Staphylococcus aureus
5.2
4.5
0.86
Facultative
V. cholera
10
5.0
0.97
Facultative
V. parahaemolyticus
5
4.8
0.94
Facultative
Y. enterocolitica
-1.3
4.2
0.96
Facultative
37
Table 2. Summary table of regulations of thermal pasteurization of foods (milk, seafood, egg and
juice products) in the United States.
Related regulations in the U.S.
Category
I: The pasteurization of milk is governed by
FDA Milk Pasteurized Ordinance. The target
Milk
microorganism original was Mycobacterium
tuberculosisis (1924), now is Coxiella burnetii
(since 1956). A process to eliminated 100, 000
guinea pig infectious doses is needed.
Examples
Target
Processing requirements
Bacteria
Milk, 1956C. burnetii
63°C (145°F) for 30 min for batch
present
process
Cream;
C. burnetii
66°C (150°F) for 30 min for batch
Chocolate milk
process; 75°C(166°F) for 15 s for
HTST
Ice cream mix
C. burnetii
69°C(155°F) for 30 min for batch
process, 80°C (175°C) for 25 s for
HTST
Related regulations in the U.S.
Category II: For pasteurization of seafood is governed by FDA
(Fish and fisheries products hazards and control
Seafood
guidance). FDA considers a 6D process for target
C. botulinum (type E and non-proteolytic types B
and F) to be generally suitable for pasteurized
seafood products.
Examples
Target
Processing requirements
bacteria
Fish and fishery C. botulinum
F90°C = 10 min, z value is 7°C for
products
type E and non- temperatures less than 90°C, 10°C
generally (e.g., proteolytic
for temperatures above 90°C.
surimi-based
types B and F
products, soups
or sauces)
Blue crabmeat
C. botulinum
F85°C = 31 min, Z value is 9°C.
type E and nonproteolytic
types B and F
Dungeness
C. botulinum
F90°C=57 min, Z value is 8.6°C.
crabmeat
type E and nonproteolytic
types B and F
38
References
FDA, 2011a
References
Enright et al.,
1957
Enright, 1961
Enright, 1961
References
FDA, 2011b
References
FDA, 2011b
FDA, 2011b
FDA, 2011b
Related regulations in the U.S.
References
The current processing of egg products is governed
CFR, 2012ab;
Category III:
by FSIS regulations and FDA criterion (2009 egg
FDA,
Egg products
safety action plan, 21 CFR Parts 16 and 118). The
2009;USDA,
target microorganism is Salmonella for pasteurized
1969
eggs and products.Aprocess of 8.75-log reduction of
Salmonella is required for liquid eggs, and 5-log
reduction is required for whole eggs.
Examples
Products
Processing requirements References
(Min temp & Min
holding time)
Liquid eggs
Albumen (w/o use of
56.7°C (134°F) for 3.5 min CFR, 2012a
chemicals)
or 55.6 (132°F) for 6.2
min
Whole egg
60°C (140°F) for 3.5 min
CFR, 2012a
Whole egg blends (<2% 61.1°C (142°F) for 3.5 min CFR, 2012a
added nonegg
or 60°C (140°F) for 6.2
ingredients); sugar
min
whole egg (2-12% sugar
added); and plain yolk
Fortified whole egg and 62.2°C (144°F) for 3.5 min CFR, 2012a
blends (24-38% egg
or 61.1°C (142°F) for 6.2
solids, 2-12% added
min
nonegg ingredients)
Salt whole egg (≥2% salt 63.3°C (146°F) for 3.5 min CFR, 2012a
added); sugar yolk (≥ 2% or 62.2°C (144°F) for 6.2
sugar added); and salt
min
yolk (2-12% salt added)
Dried egg
Spray-dried albumen
54.4°C (130°F) for 7 days CFR, 2012b
whites
Pan-dried albumen
51.7°C (125°F) for 5 days CFR, 2012b
Related regulations in the U.S.
References
The pasteurization of juice is governed by FDA
CFR, 2011;
Category IV:
(2011. 21 CFR 120.24). A process of 5-log
FDA, 2004
Juice1
reduction of most resistant microorganism of public
health significance under HACCP plan is required.
The target bacteria are dependent on the juice
product and process, including E. coli O157: H7,
Salmonella, Cryptosporidium parvum or C.
botulinum.
Examples
Target bacteria
References
Acidic juice
E.coli O157:H7, Salmonella, and Cryptosporidium FDA, 2004
(pH≤ 4.6)
parvum
Low-acid juices C. botulinum
FDA, 2004
(pH > 4.6)
39
1
Juice is defined by FDA as the aqueous liquid expressed or extracted from one or more fruits or
vegetables, or concentrates of such liquids or purees.
40
Table 3. Standards of prepackaged chilled foods (pasteurized foods) in Europe (CSIRO, 2010;
ECFF, 2006).
Products
Target bacteria
Processing
Shelf-life
requirements
L. monocytogens
6D reduction.
≤ 10 days at 5°C
Common practice of
Heat-treated chilled
F70°C=2.0 min is
foods1
considered suitable.
Non-proteolytic C.
6D reduction,
Up to 6 weeks at
botulinum
common practice of
5°C
F90°C =10.0 min is
universally accepted
1
Chilled food: foods that for reasons of safety and/or quality rely on storage at refrigeration
temperatures throughout their entire shelf life.
41
Table 4: Lethal rates for L. monocytogenes1 and necessary process to achieve 6-log reduction of
L. monocytogenes (ECFF, 2006).
Temperature (°C)
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
1
Time (mins, secs)
43’29”
31’44”
23’16”
17’06”
12’40”
9’18”
6’49”
5’01”
3’42”
2’43”
2’00”
1’28”
1’05”
0’48”
0’35”
0’26”
0’19”
0’14”
0’10”
0’06”
0’05”
0’04”
0’03”
0’02”
0’02”
0’01”
Lethal Rate
0.046
0.063
0.086
0.117
0.158
0.215
0.293
0.398
0.541
0.736
1.000
1.359
1.848
2.512
3.415
4.642
6.310
8.577
11.659
15.849
21.544
29.286
39.810
54.116
73.564
100.000
L. monocytogens is the most heat-resistant vegetative pathogen. Values have been extrapolated
assuming a linear z-value of 7.5°C and as a reference 70°C.
42
Table 5. Lethal rates for C. botulinum type B1and necessary process to achieve 6-log reduction of
C. botulinum type B (ECFF, 2006).
Temperature (°C)
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
1
Time (mins)
270.3
192.3
138.9
100.0
71.9
51.8
37.0
27.0
19.2
13.9
10.0
7.9
6.3
5.0
4.0
3.2
2.5
2.0
1.6
1.3
1.0
Lethal rate
0.037
0.052
0.072
0.100
0.139
0.193
0.270
0.370
0.520
0.720
1.000
1.260
1.600
2.000
2.510
3.160
3.980
5.010
6.310
7.940
10.000
Type B is the most heat resistant form of non-proteolytic C. botulinum. Values have been
extrapolated assuming a linear z-value of 7°C below 90°C and 10°C above 90°C (reference is
90°C).
43
Table 7. Major enzymes related to the quality of raw and processed vegetables.
Category
Texturerelated
enzymes
Enzymes
Pectin
methylesterase
(PME)
Polygalacturon
ase (PG)
Peroxidase
(POG)
Colorrelated
enzymes
Polyphenol
oxidase (PPO)
POD
Anthocyanase
Chlorophyllase
Alliinase
Lypoxygenase
(LOX)
Off-flavorrelated
enzymes
LOX
Hydroperoxida
se lyase (HPL)
Cystine lyase
Main effects
1. Catalyzes the de-esterification of pectin
to create binding sites for divalent cations
on the polygalacturonic acid backbone of
the pectin to form cross-links between
pectin chains (a firming effect); 2.
demethoxylated pectin can also be a
substrate for PG depolymerization (a
softening effect); 3. Causes cloud loss in
juices
Catalyses the cleavage of polygalacturonic
acid, resulting in pectin depolymerisation
(softening effect)
Involved in the oxidative cross-linking of
cell wall polysaccharides
References
Anthon and
Barrett,
2002; Terefe
et al., 2012;
Van
Buggenhout,
2009
Acts on phenols in the presence of oxygen,
catalyses browning
Catalyses the oxidation of phenolics in the
presence of hydrogen peroxide resulting in
browning
Catalyses the hydrolysis of anthocyanins
Catalyses the degradation of chlorophyll,
causes the loss of green color
Hydrolyses the non-protein amino acid,
involved in the discolouratioin of
processed garlic products
Causes the co-oxidation of carotenoids in
the presence of free fatty acids, affects the
color intensity of foods
Terefe et al.,
2012;
Catalyses the oxidation of polyunsaturated
fatty acids, produces volatile off-flavor
compounds
One of the key enzyme in the “LOX
pathway” for producing volative
compounds, the high concentration level of
which results in off-flavor
Cleaves cystine producing ammonia,
responsible for off flavor and off aroma in
broccoli and cauliflower
Terefe et al.,
2012;
44
Table 6. Effects of thermal pasteurization on the quality of vegetables
Quality
parameters
Color
Color
Color
45
Color
Color
Color
Commodity/
products
Carrot pieces
Main focus related to the specific
quality parameter
Compare high pressure and thermal
treatments on an equivalent basis, and
characterize their overall impact on
carrot quality attributes respectively.
Broccoli (florets Evaluate the use of continuous MW
and stems)
processing for pasteurization of
acidified vegetable packages, and the
changes in color and texture of the
products as affected by MW
pasteurization.
Red bell pepper
Evaluate the use of continuous MW
processing for pasteurization of
acidified vegetable packages, and the
changes in color and texture of the
products as affected by MW
pasteurization.
Sweatpotato
Evaluate the use of continuous MW
processing for pasteurization of
acidified vegetable packages, and the
changes in color and texture of the
products as affected by MW
pasteurization.
Pickled garlic
Evaluate the effects of blanching,
preservation treatment and storage time
on the quality of packaged blanched
garlic.
Soybean sprouts Apply soybean sprouts to sous vide and
cook-chill processing systems, and to
evaluate the quality and microbial safty
of the products during storage.
Thermal
technology
Steam
Continuous
microwave
Continuous
microwave
Continuous
microwave
Hot water
bath
Heat (no
information
on the heating
media)
Processing conditions
Source
P70°C10= 2min or P90°C10=
10min
Vervoort et
al., 2012
3.5 KW for 4 min, then
held in insulating molds for
30 min. Surface
temperatures of vegetable
packs upon exit of the MW
cavity were 75-80°C.
3.5 KW for 4 min, then
held in insulating molds for
30 min. Surface
temperatures of vegetable
packs upon exit of the MW
cavity were 75-80°C.
3.5 KW for 4 min, then
held in insulating molds for
30 min. Surface
temperatures of vegetable
packs upon exit of the MW
cavity were 75-80°C.
90°C for 8 min
Koskiniemi
et al., 2013
P90°C= 10 min or P70°C= 2
min
Koo et al.,
2008
Koskiniemi
et al., 2013
Koskiniemi
et al., 2013
Rejano et al.,
1997
Table 6 (cont.)
Quality
Commodity/
parameters
products
Color
Asparagus
Color
Texture
Broccoli
Texture
Asparagus
Texture
Red bell pepper
46
Texture
Vegetable juice
(made from
butterhead
lettuce, celery,
parsley, apple
concentrate and
kalamasi lime)
Carrot pieces
Main focus related to the specific quality
parameter
Develop kinetic model to describe the
textural and color changes of green
asparagus spears during short time
cooking and thermal pasteurization.
Determine the kinetics of green and total
color degradation of acidified vegetable
juice between 80-100°C.
Thermal
technology
Water bath
Processing conditions
Source
70-98°C for different time
intervals
Lau et al.,
2000
Oil batch
80, 90 and 100°C for 0, 20,
40, and 60 s
Loong and
Goh, 2004
Compare high pressure and thermal
treatments on an equivalent basis, and
characterize their overall impact on
carrot quality attributes respectively.
Evaluate the use of continuous MW
processing for pasteurization of acidified
vegetable packages, and the changes in
color and texture of the products as
affected by MW pasteurization.
Investigate the effect of microwave
pasteurization on the heating uniformity
and textural quality of pickled asparagus
in glass bottles in comparison with the
hot water pasteurization method.
Evaluate the use of continuous MW
processing for pasteurization of acidified
vegetable packages, and the changes in
color and texture of the products as
affected by MW pasteurization.
Steam
P70°C10= 2min or P90°C10=
10min
Vervoort et
al., 2012
Continuous
Microwave
3.5 KW for 4 min, then held in Koskiniemi
insulating molds for 30 min.
et al., 2013
Surface temperatures of
vegetable packs upon exit of
the MW cavity were 75-80°C.
80°C for 10 s
Lau and
Tang, 2002
Microwave
or hot
water
Continuous
microwave
3.5 KW for 4 min, then held in Koskiniemi
insulating molds for 30 min.
et al., 2013
Surface temperatures of
vegetable packs upon exit of
the MW cavity were 75-80°C.
Table 6 (cont.)
Commodity/
products
Asparagus
Texture
Pickled garlic
Texture
Carrot pieces
Texture
Soybean
sprouts
Texture
Sweatpotato
Carotenoids
Carrot pieces
Carotenoids
Carrot juice
47
Quality
parameters
Texture
Main focus related to the specific
quality parameter
Develop kinetic model to describe the
textural and color changes of green
asparagus spears during short time
cooking and thermal pasteurization.
Evaluate the effects of blanching,
preservation treatment and storage time
on the quality of packaged blanched
garlic.
Investigate the effect of in pack thermal
preservation processes in a retort
system on particular carrot quality
aspects (texture).
Apply soybean sprouts to sous vide and
cook-chill processing systems, and to
evaluate the quality and microbial safty
of the products during storage.
Evaluate the use of continuous MW
processing for pasteurization of
acidified vegetable packages, and the
changes in color and texture of the
products as affected by MW
pasteurization.
Thermal
technology
Water bath
Compare high pressure and thermal
treatments on an equivalent basis, and
characterize their overall impact on
carrot quality attributes respectively.
Study the effect of electroplasmolysis
and microwave application on the yield
and quality of carrot juice during
production and storage.
Steam
Processing conditions
Source
70-98°C for different
time intervals
Lau et al.,
2000
Hot water bath
90°C for 8 min
Rejano et
al., 1997
Steam
Actual P70°C=1.85 min;
or P90°C=9.67 min
Lemmens
et al., 2013
Heat (no
information on
the heating
media)
Continuous
microwave
P90°C= 10 min or P70°C=
2 min
Koo et al.,
2008
3.5 KW for 4 min, then
held in insulating molds
for 30 min. Surface
temperatures of
vegetable packs upon
exit of the MW cavity
were 75-80°C.
P70°C10= 2min or P90°C10=
10min
Koskiniemi
et al., 2013
MW: flow rats 90-287
mL/min at 540, 720 and
900 W using a MW
oven; traditional heat:
100°C for 10 min
Rayman
and Baysal,
2011
MW heating or
traditional heat
Vervoort et
al., 2012
Table 6 (cont.)
Commodity/
products
Tomato juice
β-carotene
Carrot puree
β-carotene
Carrot pieces
Lycopene
Tomato juice
Phenolics
Carrot juice
Phenolics
Tomato juice
Phenolics
Onion byproducts
(juice, paste
and bagasse)
48
Quality
parameters
Carotenoids
Main focus related to the specific quality
parameter
Evaluate and compare the effects of high
intensity pulsed electric fields
processing and heat pasteurization on
the quality of tomato juices.
Investigate the effect of different
processing techniques of carrot puree on
β-carotene concentration, isomerisation
and bioaccessibility (in vitro).
Investigate the effect of in pack thermal
preservation processes in a retort system
on particular carrot quality aspects ( βcarotene bio-accessibility).
Evaluate and compare the effects of high
intensity pulsed electric fields
processing and heat pasteurization on
the quality of tomato juices.
Study the effect of electroplasmolysis
and microwave application on the yield
and quality of carrot juice during
production and storage.
Thermal
technology
Heat
exchanger coil
in hot water
bath
Steel tubes in
water bath
Evaluate and compare the effects of high
intensity pulsed electric fields
processing and heat pasteurization on
the quality of tomato juices.
Evaluate onion by-products stabilized by
different treatment to show their
bioactive, antioxidant, and antibrowning
properties for the potential to be food
ingredient.
Heat
exchanger coil
in hot water
bath
Steam
Processing conditions
Source
90°C for 30 or 60 s
OdriozolaSerrano et
al., 2009
P90°C=10min
Knockaert et
al., 2012
Steam
Actual P70°C=1.85 min;
or P90°C=9.67 min
Lemmens et
al., 2013
Heat
exchanger coil
in hot water
bath
MW heating
or traditional
heat
90C for 30 or 60 s
OdriozolaSerrano et
al., 2008
MW: flow rats 90-287
mL/min at 540, 720
and 900 W using a
MW oven;
Traditional heat:
100°C for 10 min
90C for 30 or 60 s
Rayman and
Baysal,
2011
100°C for 11-17 min
Roldán et
al., 2008
OdriozolaSerrano et
al., 2008
Table 6 (cont.)
Quality
Commodity/
parameters
products
Phenolics
Tomato juice
49
Vitamin C
Tomato juice
Vitamin C
Gazpacho (a cold
vegetable soup)
Vitamin C
Orange-carrot
juice
Vitamin C
Soybean sprouts
Vitamin D
Vegetable juice
(mainly made
from tomato,
green pepper,
green celery,
cucumber, onion,
carrot, lemon)
Main focus related to the specific
quality parameter
Evaluate and compare the effects of
high intensity pulsed electric fields
processing and heat pasteurization on
the quality of tomato juices.
Evaluate and compare the effects of
high intensity pulsed electric fields
processing and heat pasteurization on
the quality of tomato juices.
Study the effects of high intensity
pulsed electric field on vitamin C and
antioxidant capacity of gazpacho, and
compared with thermal pasteurization.
Study the degradation of ascorbic acid
in orange-carrot juice treated by PEF or
thermal pasteurization to establish its
shelf life.
Apply soybean sprouts to sous vide and
cook-chill processing systems, and to
evaluate the quality and microbial safty
of the products during storage.
Evaluate the impact of high-pressure
processing on vitamin E, vitamin D and
fatty acid profiles in vegetable
beverages, and in comparison with
traditional pasteurization.
Thermal technology
Heat exchanger coil
in hot water bath
Processing
conditions
90°C for 30 or 60 s
Source
OdriozolaSerrano et
al., 2009
Heat exchanger coil
in hot water bath
90C for 30 or 60 s
OdriozolaSerrano et
al., 2008
Tubular heatexchanger in hot
water bath
90°C for 1 min
Plate heat exchanger
98°C for 21 s
ElezMartínez
and MartínBelloso,
2007
Torregrosa
et al., 2006
Heat (no information
on the heating
media)
P90°C= 10 min or
P70°C= 2 min
Koo et al. ,
2008
Plate heat exchanger
90°C for 15 s
Barba et al.,
2012
Table 6 (cont.)
Quality
Commodity/
parameters
products
Vitamin E Vegetable juice
(mainly made
from tomato,
green pepper,
green celery,
cucumber, onion,
carrot, lemon)
Antioxida Gazpacho (a cold
nt activity vegetable soup)
Carrot juice
Antioxida
nt activity
Onion byproducts
(juice, paste and
bagasse)
Tomato juice
50
Antioxida
nt capacity
Antioxida
nt capacity
Quercetin
Onion byproducts
(juice, paste and
bagasse)
Main focus related to the specific quality
parameter
Evaluate the impact of high-pressure
processing on vitamin E, vitamin D and
fatty acid profiles in vegetable beverages,
and in comparison with traditional
pasteurization.
Thermal
technology
Plate heat
exchanger
Processing
conditions
90°C for 15 s
Study the effects of high intensity pulsed
electric field on vitamin C and antioxidant
capacity of gazpacho, and compared with
thermal pasteurization.
Study the effect of electroplasmolysis and
microwave application on the yield and
quality of carrot juice during production
and storage
Tubular heatexchanger in
hot water bath
90°C for 1 min
Elez-Martínez
and MartínBelloso, 2007
MW heating
or traditional
heat
Rayman and
Baysal, 2011
Evaluate onion by-products stabilized by
different treatment to show their bioactive,
antioxidant, and antibrowning properties for
the potential to be food ingredient.
Evaluate and compare the effects of high
intensity pulsed electric fields processing
and heat pasteurization on the quality of
tomato juices.
Evaluate onion by-products stabilized by
different treatment to show their bioactive,
antioxidant, and antibrowning properties for
the potential to be food ingredient.
Steam
MW: flow rats 90287 mL/min at 540,
720 and 900 W
using a MW oven;
Traditional heat:
100°C for 10 min
100°C for 11-17
min
90°C for 30 or 60 s
OdriozolaSerrano et al.,
2008
100°C for 11-17
min
Roldán et al.,
2008
Heat
exchanger coil
in hot water
bath
Steam
Source
Barba et al.,
2012
Roldán et al.,
2008
Table 8. Scientific publications of pasteurized vegetables related to storage and enzyme study.
Vegetables
(products)
Pasteurization
conditions
Storage
conditions
(temp & time)
Quality related
parameters
evaluated with
storage time
Color, texture,
and ascorbic acid,
Enzyme
studied
N/A
Microbial
test
P90°C= 10 min or
P70°C= 2 min
3°C for 36 d;
10°C for 24 d
Carrot juice
MW: flow rats 90287 mL/min at 540,
720 and 900 W
using a MW oven;
traditional heat:
100°C for 10 min
4°C for 4 mo
Total pectin
contents,
carotenoids,
phenolics,
antioxidant
activities, titrable
acidity values
PME
Orangecarrot juice
98°C for 21 s
2°C for 70 days;
10°C for 59 d
Ascorbic acid
N/A
N/A
Tomato juice
90°C for 30 or 60 s
4°C for 91 d
Lycopene,
vitamin C, total
phenolics, and
antioxidant
capacity
N/A
N/A
Tomato juice
90°C for 30 or 60 s
4 ± 1 °C for 56 d
Carotenoids,
phenolics, color,
pH and soluble
solids content
N/A
N/A
51
Soybean
sprouts
Reference
Aerobic,
Koo et al.,
anaerobic and 2008
psychrophilic
bacterial
counts
N/A
Rayman
and
Baysal,
2011
Torregrosa
et al.,
2006
OdriozolaSerrano et
al., 2008
OdriozolaSerrano et
al., 2009
Table 8 (Continued). Scientific publications of pasteurized vegetables related to storage and enzyme study.
Vegetables
(products)
Pasteurization
conditions
52
Storage
conditions
(temp &
time)
30°C for 60 d
Quality related
Enzyme
parameters
studied
evaluated with
storage time
Color, texture,
N/A
microbial stability
Acidified
vegetables
(broccoli, red
bell pepper,
and
sweetpotato)
Continuous MW
heating (3.5 KW) for 4
min, then held in
insulating molds for 30
min. Surface
temperatures of
vegetable packs upon
exit of the MW cavity
were 75-80°C.
70°C for 15 min
Broccoli
lactic acid
bacteria drink
Jalapeño
Preheated at 50°C for
pepper rings
60 min, then held at
75°C for 5 min.
Pickled garlic 90°C for 8 min
5°C For 21 d
Yellow
“banana”
pepper
23°C for 124
d
Volatile
compounds, and
organic acid
Texture, pectic
substances,
methoxyl content
Color, texture,
Chemical
characteristics
(eg. pH, Acidity,
sugar, ethanol)
Texture, fresh
weight, ascorbic
acid, quercetin,
Luteolin, and
capsaicin content
74°C for 10 min
23°C For 5
mo
27 ± 2 °C for
4 mo
Microbial test
Reference
Koskiniem
i et al.,
2013
N/A
Visual signs of
spoilage (turbidity,
mold growth, gas
production),
metabolic products
indicative of
bacterial and yeast
growth (lactic acid
and ethanol)
Lactic acid bacteria
N/A
N/A
Howard et
al., 1997
N/A
N/A
Ohba et
al., 2002
Rejano et
al., 1997
N/A
Lee and
Howard,
1999
Table 8 (Continued). Scientific publications of pasteurized vegetables related to storage and enzyme study.
Vegetables
(products)
Carrot pieces
Pickled asparagus
Pasteurization
conditions
P70°C10= 2min or
P90°C10= 10min
88°C for 10 s
Storage
conditions
(temp &
time)
N/A
Quality related
parameters
evaluated with
storage time
N/A
N/A
N/A
Enzyme
studied
PME and
POD
POD
Microbial
test
N/A
N/A
Reference
Vervoort et al.,
2012
Lau and Tang,
2002
53
Figure 1. Vegetable color wheel (Anonymous, 2013)
54
Chapter 3. Thermal Inactivation Kinetics of Bacillus coagulans Spores in
Tomato Juice
Abstracts: The thermal characteristics of three strains of Bacillus coagulans (ATCC 8038, 7050
and 185A) spores/vegetative cells in tomato juice were evaluated. B. coagulans 8038 was chosen
as the target microorganism for thermal processing of tomato products due to its spores having
the highest thermal resistance among the three strains. The thermal inactivation kinetics of B.
coagulans 8038 spores in tomato juice between 95°C and 115°C were determined independently
in two different laboratories using two different heating setups. The results obtained from both
laboratories were in general agreement, with z-values of 8.3°C and 8.7°C, respectively. The zvalue of B. coagulans 185A spores in tomato juice (pH 4.3) was found to be 10.2°C. The
influence of environmental factors, including cold-storage time, pH, and pre-conditioning upon
the thermal resistance of these bacterial spores were discussed. Results obtained showed that a
storage temperature of 4°C was appropriate for maintaining the viability and thermal resistance
of B. coagulans 8038 spores. Acidifying the pH of tomato juice decreased the thermal resistance
of these spores. A1-h exposure at room temperature was considered optimal for pre-conditioning
B. coagulans ATCC 8038 spores in tomato juice.
Keywords: Thermal inactivation, kinetics, Bacillus coagulans, tomato juice
55
1. Introduction
Bacillus coagulans, a facultative anaerobic spore-forming bacterium, is acid tolerant and grows
well in foods at pH 4.0 to 4.5 at ambient temperature. This is the single organism most frequently
isolated from spoiled canned vegetables acidified to pH 4.0 to 4.5, and has been considered the
primary cause of economically important spoilage in thermally processed tomatoes and tomato
products (8, 20). It results in a type of spoilage commonly referred to as flat sour in tomato based
products (14, 18, 20, 24). Although B. coagulans is a non pathogenic microorganism, it may
cause a food safety hazard due to its ability to increase the pH of acidic foods, processed with a
reduced treatment, to a level that can allow germination of surviving Clostridium botulinum (1,
2).
Thermal processing is the most common and effective method for inactivation of
microorganisms and extending the shelf-life of tomato juice. Most published data related to the
inactivation of B. coagulans spores in food media are based on studies performed with moderate
heat only (18, 24), or combining heat treatment with other technologies, such as high pressure (5,
12, 22). For high temperature heating, Palop et al. (13, 14) studied heat resistance in food
medium (pH 4 and 7) between 105°C and 130°C, using a strain isolated from canned asparagus,
and Mallis et al. (8) obtained the z-value of B. coagulans spores in tomato serum (pH 4.24) at
temperatures in a narrow range (95-105°C) . Several studies also reported on inactivation of B.
coagulans spores by hydrodynamic cavitation (10), supercritical CO2 micro-bubble (4), and
high-pressure with nisin (3).
The equilibrium pH of tomatoes varies from 4.0 to 4.7, depending on the variety and ripeness
(21). The practice of acidification of canned tomato juice to a pH lower than 4.5 before treatment
56
prevents the outgrowth of spores surviving heat treatment, particularly spores of Clostridium
botulinum (11). Although the heat resistance of B. coagulans spores in tomatoes has been studied
at acid pH (8, 14, 15, 17, 18, 24), information about heat resistance at different acidic pH levels
between 4.0 and 4.5 (the commonly controlled pH range for canned tomato products) is still
limited, especially under high temperatures. Since the thermal resistance of B. coagulans spores
is strain-dependent, three strains of this microorganism (ATCC 8038, 7050 and 185A) were
investigated in this study, although the first two strains have been used more frequently in
previously published work (10, 16, 17, 23, 24). The objective of this study is to characterize the
thermal resistance of B. coagulans spores in tomato juice product at pH between 4.0 and 4.4
using different heating setups in two different laboratories. The influence of some environmental
factors, including cold-storage, pH, and pre-conditioning upon the thermal resistance of this
microorganism was also investigated. The current study provides theoretical support for
developing and validating thermal pasteurization processes of tomato products.
2. Materials and Methods
2.1. Microorganisms
Bacillus coagulans strains ATCC 7050 and 8038 were purchased from the American Type
Culture Collection (Manasses, VA, USA). Bacillus coagulans strain 185A was obtained from Dr.
V.M. Balasubramaniam at The Ohio State University (6). These strains were grown aerobically
in Nutrient Broth (NB, Difco Laboratories Inc., Detroit, MI, USA) for 48 h at 37°C, and then resuspended in NB containing 20% glycerol. The stock culture was divided into sterile cryogenic
vials (Fisher Scientific, Pittsburgh, PA, USA) and then stored in a freezer (-20°C) until further
use.
57
2.2. Preparation of B. coagulans spores
To induce sporulation of vegetative cells of B. coagulans, the procedures described by Palop et
al. (14) and modified by Milly et al. (10) were employed. Briefly, vegetative cells of B.
coagulans were grown in NB aerobically for 48 h at 37°C and transferred into NB at least 3
times before spore preparation. Spores of B. coagulans were prepared by distributing 1 mL of
actively growing vegetative cells (48 h, 37°C) onto a plate containing Nutrient Agar (NA, Difco;
Becton, Dickinson and Co., Sparks, MD, USA) fortified with 500 mg/L of dextrose (Bacto
Dextrose, Difco) and 3 mg/L manganese sulfate (Fisher Scientific, Pittsburgh, PA, USA). The
inoculated plates were incubated at 50°C for 7 days, where more than 90% sporulation was
obtained as verified by observing the refractive spores under phase-contrast microscopy. Spores
were harvested by flooding plates with 5 ml of cold sterile deionized water, and dislodging
spores from the agar surface with a sterile disposable inoculation loop. After harvesting, the
spores were washed 3 times by centrifugation at 14,000 × g at 4°C for 10 min, resuspended in
sterile deionized water and stored at 4°C until used.
2.3. Preparation of tomato juice
Two forms of tomato juice were used: commercial tomato juice (Campbell Soup Co., Camden,
NJ, USA) was used in the Washington State University (WSU) study involving oil bath heating;
fresh Roma tomatoes bought from a local grocery store (Kroger Inc., Columbus, OH) were used
in The Ohio State University (OSU) study. Tomatoes of bright red color (with an ‘a’ value of 20)
were cut into quarters and then blended to prepare the tomato juice media.
58
2.4. Evaluation of cold-storage time on the viability of B. coagulans in sterile distilled water
and its thermal resistance in tomato juice
Cultured B. coagulans spores (ATCC 8038 and ATCC 7050) were suspended in sterile distilled
water and stored at 4°C. The viable numbers of vegetative cells and spores were counted after
10, 23 and 31 days of storage to study the effect of refrigerated storage on the viability of this
microorganism. For enumeration, the spore suspensions were heat-shocked at 80°C for 15 min,
cooled in a crushed ice water bath and checked microscopically to ensure the absence of
vegetative cells. The spore count was obtained by preparation of 10-fold serial dilutions in sterile
0.1% peptone water. One hundred µL of each dilution was spread-plated onto NA and incubated
for 7 days (ATCC strains) or 2 days (185A) at 37°C. The spore numbers were calculated from
three replicates. The vegetative cells in the spore suspension were counted by the same
procedures but without the heat-shocking step.
To further investigate the response of this microorganism to cold storage, the thermal resistance
of prepared B. coagulansATCC8038 spores in tomato juice (pH 4.0) was measured at 100°C
after 0, 10 and 28 days of cold storage. The thermal resistance of B. coagulans spores in tomato
juice was determined following the procedures described subsequently.
2.5. Preparation and pre-conditioning of a mixture of spore suspension and tomato juice
The pH of tomato juice (Campbell Soup Co., Camden, NJ, USA), initially ranging from 4.0 to
4.1, was adjusted to different values by adding 1M sodium citrate or citric acid to evaluate the
influence of pH of the heating medium on microbial heat resistance. To pre-condition spores in
tomato juice (adjusted to pH 4.3),capillary tubes with a mixture of 50 µL tomato juice inoculated
59
with the spore suspension were placed at 4°C or room temperature for time periods of 1, 2, 3,
and 4h. The D100-values of pre-conditioned mixtures were determined following the procedures
described below.
2.6. Evaluation of heat resistance of B. coagulans spores using oil bath
Thermal resistance of test microorganisms was determined by thermal death time (TDT) tests
and reflected by D- and z-values. D-value is defined as the time required at a certain temperature
for 1-log reduction of the target microorganisms and z-value is the change in temperature
required for a 10-fold reduction of D-values. Since the thermal destruction of B. coagulans
generally follows a first-order reaction based on most published data (14, 18, 20), D values of
B.coagulans spores were obtained by taking the negative reciprocal of the slope from linear
regression of the survivor curves. The z-value was estimated by plotting the log10 D-values
versus heating temperatures and taking the negative reciprocal of the slope from linear
regression. Fifty µL of tomato juice inoculated with spore suspension (initial spore concentration
was 108 CFU/mL) was injected into a glass capillary tube with an inner diameter of 1.8 mm and
an outer diameter of 3 mm (Corning Inc., Corning, NY, USA) using a pipette, and the open ends
of the tubes were heat sealed. The tubes were immersed completely in a circulating oil bath
(Thermo Electron Corporation, Waltham, MA, USA) and heated between 95°C and 115°C for
different time intervals. The come-up times (the time for sample to reach within 0.5°C of the
target temperature) was around 5 sec. After heating, the tubes were removed from the oil bath,
cooled immediately in a crushed ice water bath, and washed in 70% ethyl alcohol. Both tube
ends were cut aseptically and the suspension was flushed out with 3 mL of sterile 0.1% peptone
water. The treated samples were then 10-fold serially diluted in sterile 0.1% peptone water and
60
spread-plated onto NA medium. Based on our preliminary test results, ATCC strains were
incubated for 7 days and the 185A strain for 2 days at 37°C, and then colonies were manually
counted, as described previously.
2.7. Evaluation of heat resistance of B. coagulans spores using a capillary tube setup
This part of the study was performed in the Department of Food, Agricultural and Biological
Engineering at Ohio State University (OSU) using B. coagulans ATCC 8038 spores prepared in
the same way as described previously. Tomato juice (inherent pH ranging from 4.1 to 4.3) was
adjusted to a standard value of 4.4 using sodium citrate to eliminate the varying acidity from
affecting the thermal resistance of the organism. Tomato juice samples inoculated with B.
coagulans spores were heated in conventional capillary cells (19) and treated at temperatures
ranging from 95 to 110°C for different time intervals. The come-up times were 158, 170, 180 and
192 sec when heating from room temperature to 95, 100, 105, and 110 °C, respectively. All zerotime samples were allowed to reach the process temperature, and then immediately cooled to
provide initial count data. Two sample-containing capillary cells mounted on each capillary tube
holder were used for each holding time. All tests were replicated three times.
For system design, capillary tubes used at OSU containing 37 μl of tomato juice inoculated with
B. coagulans spores were plugged at both ends with nonconductive capillary tube sealant. The
capillary tubes were placed inside an external ohmic heating device to enable heating under
pressurized conditions. The samples in capillaries had insulating gel plugs at the end to prevent
their heating ohmically; thus all capillaries heated by heat transfer from the external, ohmically
heated medium. To hold capillary cells in place, they were mounted on cell holders (two cells per
61
holder) attached to the treatment chamber (Fig. 1). Temperatures were measured in selected,
thermocouple containing capillary cells. The system also facilitated rapid post-treatment cooling
through pulling of the treated samples into the cooling section with the help of an attached
thread. A detailed description of the setup and procedures is provided by Somavat et al. (19).
To enumerate spore survival, treated capillary cells were washed with cold 1400 ppm
hypochlorite solution and rinsed with cold sterile water. The capillary washing protocol
developed by Somavat et al. (19) was followed to eliminate any residual hypochlorite from
affecting the final plate count. The clean capillary cells were then crushed inside sterile
polypropylene tubes containing 0.1% peptone water using sterile glass rods. A heat shock of
80°C for 15 min was given to inactivate all vegetative cells. Tenfold serial dilutions in peptone
water were prepared and spread-plated onto TSA plates. Inoculated plates were incubated for 48
hours at 37°C and colonies enumerated.
3. Results and Discussion
3.1. Effect of cold-storage time on the viability of B. coagulans in sterile distilled water and
its thermal resistance in tomato juice
To investigate whether storage at 4°C influences the viability of B. coagulans, viable numbers of
vegetative cells and spores in sterile distilled water were counted over a period of one month.
Table 1 shows that there were few changes in the viable numbers of both vegetative cells and
spores of B. coagulans ATCC 8038 during 4°C storage. In contrast, viable vegetative cells of
ATCC 7050 experienced a one-log reduction after 10 days of cold-storage and no viable
vegetative cells could be detected after 23 days of storage (detection limit: ≤ 5 Log CFU/mL).
62
Since spores produced by ATCC 8038 strain (ca. 108 CFU/ml) were much more numerous than
those obtained from ATCC 7050 strain (below the detection limit, ≤ 5 Log CFU/mL) when grown
under the same conditions, and also due to the stable viability of ATCC 8038 stored in sterile
distilled water at 4°C, B. coagulans ATCC 8038 was chosen for further study.
In addition, since the sporulation temperature of 50°C was relatively high and the sporulation
time of 7 days was relatively long, maintaining the moisture content of sporulation medium was
taken into consideration. The viability of spores produced in a moist chamber was compared
with those cultured without a moist chamber, along with the viability of their vegetative cells
(Table 1). It was found that maintaining the moisture content of sporulation medium decreased
the viability of both spores and vegetative cells, and the reduction of viability was more evident
with spores than with vegetative cells. Thus, B. coagulans was grown on sporulation medium
without the use of a moist chamber in order to produce high-viability spores.
To further investigate the effect of cold storage on this microorganism, B. coagulans ATCC 8038
spores stored at 4°C for up to 28 days were used to inoculate tomato juice (pH 4.0) after which
D100°C-values were determined. D100°C values of 2.56 ± 0.00, 2.09 ± 0.34, and 2.87 ± 0.03 were
found after 0, 10 and 28 days storage, respectively. No significant difference (P>0.05) in Dvalues was found. These results demonstrate the relative stability of thermal resistance of those
spores under cold-storage conditions. Therefore, a temperature of 4°Cwas used for storing B.
coagulans ATCC 8038 spores. Similar results were observed in our previous study (7) which
showed that storing Clostridium sporogenes PA 3679 spores at 4°C is satisfactory for
maintaining the viability and heat resistance of those spores during short term storage.
63
3.2. Effect of pH on the thermal resistance of B. coagulans ATCC 8038 spores
According to published data, most authors have revealed that acidification of the heating medium
causes a decrease in microbial heat resistance. Palop et al. (14) investigated the thermal
resistance of B. coagulans spores in homogenized tomato and asparagus at pH 7 and 4 at
temperatures between 105°C and 130°C, and found that the spores were less heat resistant in
both food media at pH 4. Similar results were obtained by Mazas et al. (9) who reported sharp Dvalue reductions for spores of three Bacillus cereus strains by lowering the pH of the heating
medium from 7.0 to 4.0. In the current study, the effect of pH on the thermal resistance of B.
coagulans ATCC 8038 spores in commercial tomato juice was examined by determining the
D100°C-values of the spores in commercial tomato juice adjusted to pH 3.8, 4.0 and 4.3 by adding
sodium citrate or citric acid. Significant differences in D100°C-values were found at different pH
levels (P<0.05). As shown in Fig. 2, D100°C-values increased from 2.85 min at pH 3.8 to 3.85min
at pH 4.3, which indicates that the thermal resistance of B. coagulans ATCC 8038 spores is
influenced by tomato juice pH, decreasing with increased acidification.
3.3. Effect of pre-conditioning on the thermal resistance of B. coagulans ATCC 8038 spores
The influence of pre-conditioning time of commercial tomato juice on the thermal resistance of B.
coagulans ATCC 8038 spores was evaluated by exposing the spores to tomato juice for 1, 2, 3
and 4h at room temperature and 4°C, respectively. As shown in Fig. 3, there were no significant
differences (P>0.05) of D100°C-values for different pre-conditioning times at each treatment
temperature. However, a significant difference (P<0.05) in D100°C-values was found between the
two temperatures. D100°C-values of B. coagulans ATCC 8038 spores exposed to tomato juice at
room temperature were higher than those exposed to 4°C (3.5 min vs 2.86 min, respectively).
64
Therefore, a 1-h exposure at room temperature was considered to be optimum for preconditioning B. coagulans ATCC 8038 spores in tomato juice.
3.4. Thermal resistance of B. coagulans ATCC 8038 spores in tomato juice using a
conventional oil bath
Thermal resistance of B. coagulans ATCC 8038 spores at different temperatures in commercial
tomato juice heated in an oil bath was determined by means of D-value measurements. Fig. 4
shows typical thermal survivor curves of B. coagulans ATCC spores in tomato juice (pH 4.0).
The D-values of B. coagulans ATCC 8038 spores decreased with increasing heating temperature.
D-values of 7.05min at 95°C, 2.56 min at 100°C, 1.18 min at 105°C, and 0.20 min at 110˚C were
obtained. The calculated z-value of B. coagulans ATCC 8038 spores in commercial tomato juice
at pH 4.0 was 10.0°C. This value is higher than that obtained by Milly et al. (10) who obtained a
z-value of 8°C when the spores were treated in tomato juice at pH 4.1. Since only the D100°C
value and z-value were reported in that study, the difference in z-values between the two studies
might be due to the different pH of tomato juice used in both studies. Sandoval et al. (18)
determined D values of a strain of B. coagulans spores in double concentrated tomato paste (pH
4.0) at 75, 80, 85 and 90°C and reported a corresponding z value of 9.5°C. The difference of z
value obtained by Sandoval et al. could be due to the use of different strains, sporulation
temperatures, and water activity.
The D- and z-values of spores in commercial tomato juice at pH 4.3 are shown in Table 2, along
with the thermal inactivation data at pH 4.0. The D-value of B. coagulans ATCC 8038 spores in
tomato juice at pH 4.3 was 4.56 min at 100°C, 1.20 min at 105°C, 0.27 min at 110°C, and 0.07
65
min at 115°C, with a corresponding z-value of 8.3°C. As seen in Table 2, all the D-values
obtained at pH 4.3 under the same heating temperature were higher than those corresponding
values obtained at pH 4.0, which demonstrated that lowering the pH of the heating medium
could reduce the thermal resistance of bacterial spores. This is in agreement with our previous
results shown in Fig. 2. Meanwhile, D values of the spores in tomato juice dropped from 4.56
min (pH 4.3) to 2.56 min (pH 4.0) at 100°C, whereas this value decreased only from 0.27 min to
0.20 min at 110°C. This illustrates the reduced influence of acidification of heating medium to
depress the thermal resistance of bacterial spores at higher temperatures. However, an increase in
z-values of B. coagulans ATCC 8038 spores with acidification was observed from 8.3°C at pH
4.3 to 10.0°C at pH 4.0. Similar trends were observed by Palop et al. (14) who studied the heat
resistance of B. coagulans spores (STCC 4522) in homogenized tomato at pH 7 and 4.
3.5. Thermal resistance of B. coagulans 185A spores at pH 4.3 using oil bath
Thermal resistance of B. coagulans 185A can only be found associated with thermal-assisted
pressure processing (6) among published research. In the present study, the D- and z-values of B.
coagulans 185A spores were determined following the same procedures as for B. coagulans
ATCC 8038 spores and are shown in Table 2. Similar to ATCC strains, the D-values of strain
185A spores decreased with increasing heat treatment temperature, showing D-values of 1.41
min at 100°C, 1.53 min at 105°C and 0.14 min at 110°C. The D100°C value of strain 185A spores
subjected to heat only was higher than that of spores processed by combining heat with high
pressure (D100°C =0.5 min under 600 Mpa) (6). The D115°C-value was not measured due to the
exceptionally brief treatment time required. The z-value of strain 185A spores was calculated to
be10.2°C.
66
As shown in Table 2, when exposed to the same pH and treatment temperature, B. coagulans
ATCC 8038 spores had a much greater thermal resistance than 185A spores, with corresponding
D-values 1-3 times greater. Therefore, B. coagulans ATCC 8038 spores were chosen as the
major target bacterium for further thermal inactivation experiments.
3.6. Thermal resistance of B. coagulans 8038 spores at pH 4.4 (OSU experiments)
The thermal survivor curves (D-values) for B. coagulans ATCC 8038 spores in tomato juice (pH
4.4) from experiments conducted at OSU are shown in Fig.5. A comparison of the D- and zvalues for the OSU and Washington State University (WSU) data are shown in Table 3. Within
each selected temperature, D-values obtained from oil bath and electrical heating methods
showed no significant difference (P>0.05). D-105°C values were 1.20 and 1.32 min obtained from
oil bath and electrical heating methods, respectively; and the corresponding D-110°C values were
0.27 and 0.16 min. Although the D-100°C values obtained from the two heating methods deviated
from one another somewhat, the agreement is quite remarkable, given that the data were obtained
independently using two different heating methods in two different laboratories. The obtained zvalues for the spores were 8.3 and 8.7°C for the WSU and OSU data, respectively. Two key
reasons for differences appear to be the slightly different pH levels (4.4 at OSU vs. 4.3 at WSU)
and the source of juice (commercial juice at WSU vs. freshly blended tomatoes at OSU).
4. Conclusions
The results of this study show that B. coagulans ATCC 8038 strain can produce consistent heatstable spores during refrigerated storage. Compared to strain 185A, B. coagulans ATCC 8038
spores have greater thermal resistance (D-values) and are therefore considered ideal target
67
bacteria for developing and validating thermal processes of tomato products. A storage
temperature of 4°C is appropriate for maintaining viability and thermal resistance of B.
coagulans ATCC 8038 spores during short term storage. Both pH and pre-conditioning
temperature influence D-values of these spores. Thermal resistance data for B. coagulans ATCC
8038 determined independently at two different laboratories were in general agreement, with
differences explainable by slightly different pH levels and juice sources.
Acknowledgements
This work was supported in part by USDA-CSREES-NRICGP Grant No. 2009-55503-05198,
titled: Quality of Foods Processed Using Selected Alternative Processing Technologies. Salaries
and research support provided in part by the Ohio Agricultural Research and Development
Center, The Ohio State University. The senior author acknowledges fellowship supports from
Chinese Scholarship Council.
68
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71
TABLE 1. The effect of 4˚Cstorage on the viability of vegetative cells and spores of B.
coagulans
Strains
(sporulation
environment)
ATCC 8038a
ATCC 8038b
ATCC 7050a
ATCC 7050b
a
Storage time (days)
Vegetative cells (Log CFU/ml)
0
10
23
31
0
8.94±0.00 8.49±0.06 8.71±0.25 8.77±0.16 8.81±0.05
8.57±0.07 8.10±0.10 8.07±0.66 8.27±0.05 7.84±0.18
6.69±0.12 5.63±0.43
NDc
ND
ND
6.54±0.09 5.60±0.46
ND
ND
ND
Spores (Log CFU/ml)
10
23
8.82±0.03 9.35±0.08
7.96±0.04 8.68±0.03
ND
ND
ND
ND
31
9.02±0.14
8.55±0.04
ND
ND
Without moist chamber, plates sealed with Parafilm (Pechiney Plastic Packaging, Menasha, WI,
USA) only.
b
Incubated in moist chamber: plates sealed with Parafilm and Petri dish containing water were
put into a plastic bag to prevent moisture loss of the sporulation medium.
c
Colonies not detectable, detection limit 5 Log CFU/ml. Data are the mean ± SD of replicates.
72
TABLE 2. Comparison of D- and z-values of B. coagulans ATCC 8038 and 185A spores in
commercial tomato juice
D-values (min)
z-value
Strains
pH
95°C
100°C
105°C
110°C
115°C
(°C)
7.05±0.14
2.56±0.15
1.18±0.02
0.20±0.01
-a
10.0
4.0
-
4.56±0.25
1.20±0.01
0.27±0.01
0.07±0.01
8.3
4.3
-
1.41±0.30
0.53±0.10
0.14±0.00
-
10.2
4.3
ATCC 8038
185A
Data are the mean ± SD of replicates.
a
Not tested because of too long or too short heat treatment time.
73
TABLE 3. The effect of heating method on the thermal resistance of spores of B. coagulans
ATCC 8038
D-values (min)
Heating method
pH
95˚C
100˚C
105˚C
110˚C
115˚C
Oil bath
4.3
-a
4.56 ± 0.25
1.20 ± 0.01
0.27± 0.01
0.07 ± 0.01
8.3
Electrical
4.4
10.13 ± 0.31
2.52 ± 0.14
1.32 ± 0.02
0.17 ± 0.05
-
8.7
Data are the mean ± SD of replicates.
a
z-value
Not tested because of too long or too short heat treatment time.
74
Figure legends
FIGURE 1. Schematic of kinetics treatment chamber, with spore suspensions within sample in
capillaries. Gel plugs in the miniature heater are non-conductive, so that the sample heats by heat
transfer from an outer ohmic heater. Samples are withdrawn into a cooling chamber after
processing.
FIGURE 2. D100°C-values of B. coagulans ATCC 8038 spores exposed to different pH levels in
tomato juice. Data are the mean ± SD of three replicates. Data obtained at WSU.
FIGURE 3. Effect of pre-conditioning time on D100°C-value of B. coagulans ATCC 8038 spores
in commercial tomato juice at pH 4.3. D100°C-value of control (spores in tomato juice without
pre-conditioning): <3 min. Data are the mean ± SD of duplicates. Data obtained at WSU.
FIGURE 4. Thermal survivor curves for B. coagulans ATCC 8038 spores at different
temperatures in commercial tomato juice at pH 4.0. Data are the mean ± SD of three replicates.
Data obtained at WSU.
FIGURE 5. Thermal survivor curves for B. coagulans ATCC 8038 spores heated at different
temperatures in tomato juice adjusted to pH 4.4. Data are the mean ± SD of three replicates. Data
obtained at OSU.
75
Figure 1. Schematic of kinetics treatment chamber, with spore suspensions within sample in
capillaries. Gel plugs in the miniature heater are non-conductive, so that the sample heats by heat
transfer from an outer ohmic heater. Samples are withdrawn into a cooling chamber after
processing.
76
Figure 2. D100°C-values of B. coagulans ATCC 8038 spores exposed to different pH levels in
tomato juice. Data are the mean ± SD of three replicates. Data obtained at WSU.
77
Figure 3. Effect of pre-conditioning time on D100°C-value of B. coagulansATCC 8038 spores in
commercial tomato juice at pH 4.3. D100°C-value of control (spores in tomato juice without preconditioning): <3 min. Data are the mean ± SD of duplicates. Data obtained at WSU.
78
Figure 4. Thermal survivor curves for B. coagulans ATCC 8038 spores at different temperatures
in commercial tomato juice at pH 4.0. Data are the mean ± SD of three replicates. Data obtained
at WSU.
79
Figure 5. Thermal survivor curves for B. coagulans ATCC 8038 spores heated at different
temperatures in tomato juice adjusted to pH 4.4. Data are the mean ± SD of three replicates. Data
obtained at OSU.
80
Chapter 4. Kinetics of Carrot Texture Degradation under Pasteurization
Conditions
Abstract: Texture degradation of carrot dices in different solutions (distilled water, 0.1% and
1.4% CaCl2 solutions) under temperatures ranging from 80 to 110ºC was investigated. The
effects of preheating (60ºC for 20 min) before high temperature treatment on carrot texture were
studied and kinetic parameters were estimated. It was found that preheating enhanced the texture
of the final products, and the improvement in texture became more apparent when CaCl 2 was
added. High temperature increased the texture degradation rate. The isotonic solution of carrot
tissue was used to avoid possible ion leakage of carrot tissue during heating, but no significant
differences were found between the texture of carrots immersed in isotonic solution and distilled
water after thermal treatments. The texture degradation of preheated carrot dices under the
investigated pasteurization conditions follows a 2nd order reaction. Kinetic results obtained were
used to recommend processing conditions for carrot products that could control food pathogens
and inactivate enzymes.
Keywords: carrot texture; preheating; isotonic concentration; calcium; microbial curve
81
1.
Introduction
Carrots are one of the most commonly consumed vegetables in the United States, with onefourth of all carrots consumed in processed form, largely canned and frozen (Lucier and Lin,
2007). In processed vegetables, texture is a primary marketable characteristic for the customer.
The texture of processed products is mainly controlled by the chemical composition, physical
structure and amount of cell wall and middle lamella (Bourne, 1989). The various mechanisms of
texture loss during heating of vegetables include turgor loss due to the breakdown of cellular
membranes and cell wall degradation and disassembly resulting from enzymatic and nonenzymatic transformations in pectin structure and composition (Anthon et al., 2005; Greve et al.,
1994ab; Sila et al., 2008). Pectinmethylesterase (PME) and polygalacturonase (PG) are the two
principle enzymes related to the enzymatic degradation of cell wall pectin. PME catalyzes the
de-esterification of pectins, creating binding sites for divalent cations (primarily Ca2+, naturally
present in the tissue or added during processing) on the polygalacturonic acid backbone of the
pectin to form cross-links between pectin chains which improves the texture. Pectin may undergo
nonenzymatic degradation through β-elimination, a chemical reaction that takes place at higher
pH levels (>4.5) and at temperatures higher than 80ºC (Keijbets and Pilnik, 1974; Sila et al.,
2008).
Texture degradation of carrots during thermal processing has previously been investigated in
several studies. Huang et al. (1983) and Bourne (1989) observed a rapid initial softening
followed by a much slower rate of softening during the retort process of diced carrots. The
authors proposed that carrot texture degradation consisted of two simultaneous first order
reactions at different reaction rates during the thermal softening process. Rizvi and Tong (1997)
82
re-determined the kinetic parameters using the fractional conversion technique based on the
published data supporting two substrate mechanisms of tissue softening. They suggested
fractional conversion as an alternate technique which was more accurate and reliable to describe
the overall trends for texture degradation of vegetables. Vu et al. (2004) investigated the kinetic
degradation of sliced carrots in distilled and demineralized water in a temperature range from 80
to 110ºC, and estimated the kinetic parameters using a fractional conversion model. Later, Smout
et al. (2005) studied the thermal texture degradation of carrot cylinders in a 0.5% CaCl2 solution
using different preheating conditions followed by treatments at two heating temperatures (90 and
100°C) and also applied a fractional conversion model. In the current study, the concepts of
“equilibrium texture” and the fraction of texture changes were used to evaluate the kinetic data
of texture degradation of diced carrots. Kinetic models with different reaction order were
evaluated and the best-fit one was selected to estimate the related kinetic parameters.
When heating cut vegetables in aqueous solutions, differences in osmotic pressures within and
outside the cells may result in ion leakage of the higher salt concentration within the cell and loss
of cell integrity, which may influence mechanical properties (e.g. texture) (De EscaladaPla et al.,
2006; Gonzalez et al., 2010). Thus, an isotonic solution may be helpful in reducing the additional
stress that a hypotonic bathing solution places on the already perturbed vegetable membranes.
Gonzalez et al. (2010) found that isotonic solutions help maintain membrane integrity in fresh
onion tissues, and reported that the onion cell membranes ruptured between 50-60ºC. However,
no published literature reported the rupture temperature of carrot cell membranes, nor the impact
of osmotic solutions on carrot texture. In the current study, the isotonic concentration of carrot
83
tissue was determined and the effects of immersing carrot slices in the isotonic solution on the
texture of the tissue at elevated temperatures were studied.
It is known that blanching vegetables at low-temperatures (generally 50-70ºC) prior to hightemperature processing may improve the texture of the final products (Anthon and Barrett, 2006;
Bartolome and Hoff, 1972; Vu et al., 2004; Wu and Chang, 1990). Preheating at these conditions
activates pectin methylesterase (PME), resulting in extensive pectin de-esterification. This
increases the chances for formation of ionically cross-linked pectin complexes and reduces the βelimination reaction. Vu et al. (2004) reported that preheating carrots in distilled and
demineralized water at 50–70°C for 20–40 min prior to high temperature heating could slow
texture degradation, increase the final value of hardness and lower the activation energy of
texture degradation. In the current study, one preheating condition (60°C for 20 min) was
selected. According to published literature, preheating carrots at 60°C for 20 min prior to high
heat treatment should enhance the vegetable texture (Stanley et al., 1995; Vu et al., 2004). The
preheating step also mimics the microwave processing in our further study for pre-packaged
carrot dices, where we always preheat the samples to a certain temperature before microwave
heating (Tang et al., 2008). Since calcium salt is a commonly used firming agent, the effects of
calcium on carrot texture were also investigated in this study. The calcium solution concentration
used was chosen based on the FDA regulation for canned carrot products (0.036% Ca in the final
products), which is far lower than that used in the published literature (Rastogi, et al., 2008;
Smout et al., 2005).
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In addition to investigating the kinetics of texture degradation of carrot dices in solutions with
different calcium levels, the goal of microbial/enzyme inactivation versus texture retention of
carrots during thermal processing predicted by the degradation models was also discussed. This
study provides useful information for determining thermal processing parameters for prepackaged diced carrots, and for predicting quality changes related to texture during processing.
2. Materials and Methods
2.1. Sample preparation
Fresh carrots (Bolthouse Farms, Inc., Bakersfield, CA) purchased from a local grocery store
were diced into 12.7×12.7×6 mm pieces. In order to prepare consistent samples, carrots with
similar length and diameter were selected (the portion between 4-6 cm from the root tip and 4-8
cm from the stem), only those dices that contained a core (xylem) size of 4-7 mm diameter and 6
mm height were used in the study. A specially designed cylindrical aluminum test cell with a net
inner space of 50 mm in diameter and 8 mm in depth was used to hold meaningful sample sizes
for texture analyses while minimizing the come-up time during heating. Eight carrot dices (6.5 ±
0.2 g) were placed in the test cell, then 6 mL solution was added and the test cell was sealed. An
o-ring fitting placed in the groove between the base and lid was used to provide a hermetic seal.
2.2. Determination of isotonic concentration of carrot tissue
The concentration of isotonic solution was determined according to the method of Saltveit
(2002). Briefly, fresh cut carrot dices were rinsed twice in distilled water for about 1 min each
time, blotted dry, and 20 randomly selected pieces were transferred to each tared Petri-dish. The
Petri-dishes were placed into a plastic tub lined with wet paper towels and held overnight (ca.
18h) at room temperature. Twenty-five mL of mannitol solution (0–0.4 M) was added to each
85
dish and shaken at 60 cycles/min for a time period of either 20, 60, 120 or 240 min, and then the
solutions were vacuum aspirated off. The weight gain or loss by the carrot pieces bathed in the
mannitol solutions was recorded. The concentration of mannitol where there was no net weight
gain or loss of the carrot pieces after the initial weight gain was taken to be the isotonic
concentration of the carrot tissue. Experiments were done in triplicates.
2.3. Thermal treatment
Carrot dices immersed in different solutions in each test cell were heated in a thermostated oil
bath (Model HAAKE DC 30, Thermo Electron Corp., Waltham, MA, USA) at 80, 90, 100 and
110°C for different time intervals. The temperatures were selected based on the heat-sensitivity
of carrot texture and pasteurization conditions. Four solutions were investigated in this study:
1) Double distilled water.
2) Isotonic mannitol solution.
3) 0.1% CaCl2 solution (equivalent to containing 0.035% calcium).
4) 1.4% CaCl2 solution (equivalent to containing 0.5% calcium).
For the two calcium levels, the former was chosen according to the FDA regulation which allows
addition of “up to 0.036% calcium to canned carrots” while the latter was within the range of the
most commonly used calcium concentrations (0.5-2.0% CaCl2) added to vegetable products in
published reports (Rastogi, et al., 2008; Smout et al., 2005). Since the diffusion of calcium into
carrot tissues before heating may affect their texture, the time that was sufficient for sample
preparation which resulted in little texture change was pre-determined as the equilibration time
to keep the consistency of the initial carrot texture. According to our preliminary tests, all
samples in the test cells were equilibrated in solutions for 10 min before heating.
86
To evaluate the effects of preheating on carrot texture, test cells containing carrot dices with
different solutions were preheated in a thermostated water bath (Model HAAKE DC 30, Thermo
Electron Corp., Waltham, MA, USA) at 60°C for 20 min, then immediately transferred to an oil
bath (Model HAAKE DL 30, Thermo Electron Corp., Waltham, MA, USA) and followed by a
high heat treatment with preset temperatures ranging from 80 to 110°C. After heating, samples
were cooled in ice water for 2 min, drained, equilibrated to room temperature and texture
analysis was conducted. Unless otherwise stated, the zero-time samples were the samples at the
end of the come-up time for the high heat.
2.4. Texture measurement
The firmness of treated carrot dices was determined using a TA.XT2 Texture analyzer (Stable
Micro Systems Ltd., Godalming, UK) fitted with a 25mm diameter aluminum cylinder probe
following the methods described by Lemmens et al. (2009). The samples were compressed to
70% strain at a cross head speed of 1 mm/s. For each test, one piece of sample was placed under
the probe. The peak force of the first compression cycle of the sample was marked the maximum
force and recorded as the indicator of firmness. At least 6 replicates were measured for each
treatment condition. Statistical analysis (Student’s t-test) was performed using Matlab 7.0 (The
MathWorks, Inc., 2004), and the significance level α was set as 0.05.
2.5. Kinetic analysis
A general form of the reaction equation is expressed as:
(1)
87
where C is a quality index or concentration of a chemical compound, t is the reaction time, k is
the rate constant and n is the order of reaction.
Following the integration of both sides of Eq. (1), it becomes:
For n=1,
(2)
(
For n≠1,
)
(3)
The texture property is presented as the fraction of texture change C, which provides an accurate
way to know the extent of quality change at any time t and can be expressed as
∞
(4)
∞
where F0 is the initial firmness at time 0; Ft is the firmness at time t; F∞ is the firmness at
equilibrium or the nonzero maximum retainable firmness after prolonged heating (Rizvi and
Tong, 1997). In the current study, the F∞ value was obtained by measuring the texture of carrot
after 24 hrs heating at 80 and 90°C, 12 hrs heating at 100 and 110°C, that the firmness was no
longer changed with respect to time (within the standard deviation) (Rizvi and Tong, 1997).
A graph of t against
(n≠1) or lnC (n=1) was plotted and linear regression was performed.
The best-fitted reaction order was determined by comparing the coefficient of determination (r2)
for all the treated temperatures. The rate constant (k) of samples at each temperature was
determined accordingly.
88
The temperature dependence of the reaction rate constant can by represented by the Arrhenius
equation:
(5)
where A is a pre-exponential factor, Ea is activation energy (J/mole), R is the universal gas
constant (8.314 J/K·mole), and T is the temperature (K). Thus, by plotting lnk against 1/T should
result in a straight line, and the activation energy (Ea) can be calculated by the slope of the line
(Ea=8.314×Slope).
3. Results and Discussion
3.1. Determination of the isotonic solution for the carrot tissue and its effect on carrot
texture
Apparent weight gain of carrot dices was observed after the first 20 min in all mannitol solutions,
from 0.0–0.4 M (Figure 1A). After 60 min, carrot dices in the 0.3–0.4 M solutions began to lose
weight; the higher the mannitol concentration, the more the weight loss. For those in 0.0–0.1 M
solutions, the carrot dices kept gaining weight for up to 4h. Only in the 0.2 M solution, carrot
dices did not have any weight change after the initial weight gain. The initial weight gain was
due to the rapid ion diffusion of carrot tissue when it was immersed in the aqueous solution, until
the rate of ion leakage reached a relatively constant point where there was neither weight gain
nor loss. A better understanding of the isotonic concentration of carrot tissue can be seen in
Figure 1B, the weight change with regard to solution concentration. For the 0.2 M solution, the
weight gain of carrot dices maintained 10% when compared to the fresh samples for up to 4h
without any change. Therefore the concentration of the solution was considered to be isotonic
89
with respect to the carrot tissue. Saltveit (2002) also found the concentration of isotonic mannitol
solution for mature green tomato tissue to be 0.2 M.
The texture of carrot dices heated in isotonic solution under temperatures ranging from 80–
110°C up to 2 h was compared with those heated in double distilled water (Figure 2). At each
temperature, no significant difference was found in the texture between samples immersed in
isotonic solution and distilled water at the corresponding heating time. It is likely that carrot cell
membranes were completely ruptured at the test temperatures (80-110ºC) in the thermal
treatments. Gonzales et al. (2010) observed complete loss of membrane integrity of onion cells at
60ºC and above.
3.2. Effects of preheating and calcium treatment on carrot texture
The effect of preheating on carrot texture was investigated by heating at 60°C for 20min
before subjecting the samples to high temperatures ranging from 80 to 110°C in distilled water or
calcium solutions (0.1% and 1.4%). As shown in Figure 3, preheating carrot dices at 60°C for 20
min changed their initial texture very little, with texture expressed by maximum force at 260 ±
22 N in distilled water compared to those fresh ones at 274 ± 23 N. However, the preheating step
retarded the texture degradation of carrots in each solution at the subsequently high temperature;
and the higher calcium concentration, the greater the decrease in degradation rate. The most
possible reason is that PME activity greatly increased at the mild preheating temperature,
resulting in the increase of demethylation of pectins and the number of calcium-binding sites.
This allows increased calcium cross-linking of the pectin chains and improved texture. A more
apparent texture improvement due to preheating was observed at 90 and 100°C. At 110°C,
90
carrots lost most of their texture during the first 5-10 minutes. Nearly 80% loss in firmness took
place within the come-up time (5min) in those immersed in DI water or 0.1% calcium solution,
and around 70% texture loss in firmness occurred within 10 min heating in those immersed in
1.4% calcium solution. Anthon et al. (2005) also observed the texture of diced tomatoes was
reduced to about 1/3 of the original level after 1 min at 100ºC with very little additional change
over the next 4 min.
The reaction order was determined in order to select the texture degradation model with the best
fit. According to the maximum force-time curve of carrot texture which is clearly non-linear, the
reaction order n in equation (1) was set to 1, 1.5 and 2, and the coefficients of determination (r2)
were obtained according to the methods described previously and listed in Table 1. One can
observe that at 80°C, the kinetic model of the three reaction orders all work well for carrots
immersed in each solution, but the 1storder reaction didn’t fit well at high temperatures.
Considering each temperature and immersion solution, the 2nd order reaction had the highest r2
values in all cases, thus it is the best fitting model for the degradation of carrot texture. The plot
of 1/C vs. time of preheated carrots at different temperature (Figure 4) shows a well-fitting linear
regression of the model when n=2 and demonstrates the degradation of carrot texture followed a
2nd order reaction. This indicates complex chemical reaction mechanisms that resulted in the
texture changes of carrots during heating, which could have been affected by many factors such
as the amount of cell wall and middle lamella, physical structures, and enzymes. The 2 nd order
reaction model of this study is different from those obtained by Smout et al. (2005) and Vu et al.
(2004 & 2006) who used a modified 1st order reaction model to analyze carrot texture
degradation. However, in those studies, they didn’t assess the suitability of other reaction orders.
91
In addition, they didn’t experimentally determine
values but rather estimated those values
through regression analyses.
The estimated relative final values of the texture parameter ( /F0) are shown in Figure 5. The
/F0 values decreased with increasing temperature, from 0.037 ± 0.006 to 0.008 ± 0.002 for
samples immersed in distilled water and from 0.129 ± 0.025
immersed in 1.4% CaCl2 solution, due to decreasing
to 0.023 ± 0.004 for those
values with increasing temperature. The
carrots immersed in 1.4% CaCl2 solution had the highest
/F0 while samples in distilled water
exhibited the lowest values at each corresponding temperature. This can be explained by the fact
that the
value increased with increasing calcium concentration due to the firming effects of
calcium. Smout et al. (2005) also reported a general decreasing trend of the final texture values
of carrot discs with higher temperature processing in distilled and demineralized water.
The reaction rate constant (k) of carrot texture degradation behaved in the opposite way, as
illustrated in Figure 6. As the temperature increased, the degradation rate constant increased
from 0.035 min-1 to 1.453 min-1 for samples immersed in distilled water, and from 0.018 min-1 to
0.360 min-1 for those immersed in 1.4% CaCl2 solution. The degradation rate constant started to
increase sharply when the temperature was higher than 90ºC for samples without added calcium,
while for samples with added calcium, this sharp increase occurs when the temperature exceeded
100ºC. It is likely that higher temperature facilitated the breakdown of the pectins, which
resulted in texture softening. However, increasing temperature could increase the calcium
diffusion into the carrot tissue which might have a firming effect on texture. The degradation rate
constant of samples at each temperature ranged in the following order: lowest for those
92
immersed in 1.4% CaCl2 solution, followed by 0.1% CaCl2 solution, and lastly the highest ones
were in distilled water. This increase of the degradation rate constant among samples in the three
solutions was more evident at high temperatures than at low temperatures. At 110ºC, the
degradation rate constant of carrots in 1.4% CaCl2 is 1.453 min-1, almost 4 times smaller than
those in distilled water, and 3 times smaller than those in 0.1% CaCl2 solution. The reason again
is due to the firming effects of calcium, which positively correlates to the calcium concentration
within a certain level depending on the free calcium-binding sites in the carrot pectin chains, and
therefore reduces the degradation of carrot texture.
The Arrhenius equation accurately described the temperature dependence of the reaction rate
constants, and can be used to correlate the reaction rate constant in food systems over typical
temperature ranges associated with preservation processes and storage of food products. The
Arrhenius plot (ln k vs 1/T, the reciprocal absolute temperature) of carrot texture degradation is
given in Figure 7. The activation energy (Ea), which represents the least amount of energy
needed for a chemical reaction to take place, was calculated by the Arrhenius plot based on Eq.
(5). The calculated Ea was 138.9 kJ/mol for carrots immersed in distilled water, 118.3 kJ/mol for
samples in 0.1% CaCl2 solution and 108.0 kJ/mol for samples in 1.4% CaCl2 solution. Vu et al.
(2004) reported activation energies for texture degradation of carrots in distilled and
demineralized water of 117.56 kJ/mol using the conversion fraction model, while Paulus and
Saguy (1980) obtained Ea values of 92–117 kJ/mole for three different carrot varieties during
thermal softening.
93
3.3. Quality versus microbial/enzyme inactivation
Kinetic models for quality degradation are required to predict quality changes for
different processes targeted to achieve an equivalent level of microbial safety, thus helping
optimizing process conditions. For pasteurization of diced carrot products, target bacterium and
processing requirements are associated with product storage temperature and their shelf-life. For
mild pasteurization of low-acid foods to provide a shelf life of 10 days maximum at 5ºC,
traditionally a 6 log or 6 D reduction of Listeria monocytogenes (L. monocytogenes) is
recommended; while for a longer shelf life (up to 6 weeks at 5ºC), a 6 log reduction of nonproteolytic Clostridium botulinum (NP C. botulinum) type E spores is required (ECFF, 2006;
Vervoort et al., 2012).
In addition to pathogen inactivation, pasteurization also aims to inactivate enzymes that cause
quality loss during storage. For carrots, polygalacturonase (PG) is the most heat resistant texturerelated enzyme which is involved in the degradation of pectins and results in texture loss
(Anthon and Barrett, 2002). Anthon and Barrett (2002) reported an Ea-value of PG in carrot juice
as 411 kJ/mol and a reaction rate (k) of 0.0087 s-1 at 80ºC.
Gaze et al. (1989) determined the heat resistance of two strains of L. monocytogenes in carrots
and obtained a z-value of 6.70–7.04ºC; later they studied the thermal resistance of NP C.
botulinum type E spores in carrot from 75–90ºC and reported a z-value of 9.84ºC (Gaze and
Brown, 1990).
The processing times to achieve 4 and 6 log reduction of NP C. botulinum type E spores and L.
monocytogenes in carrots and 90% inactivation of PG under different processing temperatures
were calculated based on the thermal kinetic data obtained from the publications mentioned
94
above, and are presented in Figure 8. For the quality of carrots, texture is selected as the
parameter and the quality retention in this study focuses on carrot dices immersed in 0.1% CaCl 2
solution. The times needed to achieve 20%, 50% and 80% texture loss under each temperature
were calculated from the kinetic texture degradation model obtained previously and are
illustrated in Figure 8. Since the texture of carrots degraded very quickly at 110ºC (nearly 80%
texture loss during the come-up time), the temperature of 110ºC was not considered as a
processing temperature for pasteurization and was not included in Figure 8.
Appropriate processing conditions may be chosen based on the data in Figure 8 using a graphic
approach as suggested by Holdsworth and Simpson (2008). That is, process conditions for
carrots can be selected above the dashed lines in Figure 8 to ensure adequate reduction of target
bacteria (NP CB type E spores or LM) or 90% of inactivation of PG, but below the solid lines to
avoid a chosen level of carrot texture degradation. It is clear from Figure 8A that very short
process times (e.g, 2-10 min at 90oC or 0.2-5 min at 100oC) should be used to achieve 6 log
reduction in NP C. botulinum type E spores while still retaining 50% of the original texture in
diced carrots. For control of LM, up to 20 minutes can be used at 80oC to retain 80% texture
while achieving over 90% of PG inactivation (Figure 8 B and C). More curves for other quality
parameters could be added to this figure to give a comprehensive quality retentionmicrobial/enzyme inactivation chart to facilitate the selection of appropriate process conditions.
4. Conclusions
Thermal degradation of carrot texture with pretreatments (preheating and calcium addition)
under investigated pasteurization conditions follows a 2nd order reaction. Data presented in this
95
paper also show that carrot dices immersed in isotonic solution during preheating treatment
(60°C for 20 min) followed by high temperature heating didn’t help maintain their texture,
compared to those immersed in distilled water. The obtained kinetic model of carrot texture was
used to draw the temperature-time plots for texture retention of carrots during thermal
processing, along with its microbial/enzyme inactivation curves. These provide a useful graphic
approach for selecting appropriate processing conditions for pasteurization processes of carrot
products, and also for predicting texture retention of thermally processed carrots.
Acknowledgements
This research was supported by USDA-NIFA Grant No. 2011-5116-68003-20996, titled: Control
of Food-borne Bacterial & Viral Pathogens using Microwave Technologies. The senior author
would like to thank the Chinese Scholarship Council for fellowship support.
96
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Table 1. Coefficients of determination (r2) from kinetic order (n) models for carrot texture
degradation at four temperatures.
r2 for different kinetic orders (n)
Temp, °C Solution
with preheating treatment
1
1.5
2
DI water
0.921
0.979
0.989
0.1% CaCl2
0.928
0.985
0.990
80
1.4% CaCl2
0.997
0.971
0.939
90
DI water
0.1% CaCl2
1.4% CaCl2
0.846
0.887
0.918
0.955
0.961
0.961
0.975
0.977
0.981
100
DI water
0.1% CaCl2
1.4% CaCl2
0.784
0.787
0.800
0.975
0.938
0.887
0.990
0.980
0.936
110
DI water
0.1% CaCl2
1.4% CaCl2
0.513
0.669
0.809
0.930
0.922
0.970
0.979
0.992
0.992
101
Figure 1. Percent change in weight of excised carrot pericarp discs in 25 ml of aqueous solution
at different mannitol concentrations. The percent change in weight is related to (A) time in
solution, or (B) solution concentration. Data are the means ± S.D. (n≥3).
102
Figure 2. Thermal degradation of texture of carrot dices in isotonic solution or distilled water at
different temperatures. Data are the means ± S.D. (n≥6).
103
Figure 3. Effect of preheating (60°C for 20 min) on the thermal texture degradation of carrots at
different temperatures. For samples without preheating, time-zero equals raw materials before
heating; for preheated samples, time-zero represents the time pre-heated samples were beginning
to be subjected to high temperature heat. Data are the means ± S.D. (n≥6).
104
Figure 4. Plot of 1/C vs. time at different temperatures (n=2). A: 80°C; B: 90°C; C: 100°C; D:
110°C; all the samples were preheated at 60°C for 20min. The line is the regression to the 2nd
order model.
105
Figure 5. The final texture value (F∞/F0) of pretreated carrot dices as a function of temperature in
different solutions. Data are the means ± S.D. (n≥6).
106
Figure 6. Reaction rate k of preheated carrot dices as a function of temperature in different
solutions.
107
Figure 7. Arrhenius plot of texture degradation rates of carrots immersed in different solutions
with pretreatment.
108
Figure 8. TDT curves of target bacteria, enzymes vs. carrot texture. A: Non-proteolytic C.
botulinum type E spores; B: L. monocytogenes; C: Polygalacturonase enzyme. Data for NP CB
type E spores are from Gaze and Brown (1990), LM are from Gaze et al. (1989) and PG are from
Anthon and Barrett (2002). The dash lines represent either 4D (6D) reduction on microbial load,
or 90% inactivation of enzyme activity.
109
Chapter 5. Dielectric Properties of Tomatoes Assisting in the Development of
Microwave Pasteurization and Sterilization Processes
Abstract: Dielectric properties of tomatoes crucially affect their dielectric behaviors in an
electromagnetic field and are essential for developing microwave pasteurization and sterilization
processes for different tomato products. An open-ended coaxial probe technique was used to
determine the dielectric properties of tomatoes over a frequency range of 300–3000 MHz for
temperatures between 22–120°C. Three tomato tissues, the pericarp tissue (including the skin),
the locular tissue (including the seeds) and the placental tissue were studied separately. The
effects of NaCl (0.2g/100g) and CaCl2 (0.055g/100g) on the dielectric properties of tomatoes
were also investigated. The dielectric loss factors were significantly different among the three
tomato tissues, and among the samples with and without salt. However, no significant
differences were found in their corresponding dielectric constants. The loss factors of the three
tomato tissues decreased with increasing frequency and increased with salts added. Increasing
temperature increased the loss factors of the three tomato tissues at 915 MHz, but initially
decreased then increased their corresponding values at 2450 MHz. The differences in the loss
factors of the three tomato tissues were related to their different ionic conductivity. Penetration
depths of the three tomato tissues exhibited similar trends with temperature at a given microwave
frequency.
Keywords: Dielectric properties, tomato, microwave, salt, pasteurization/sterilization
110
1. Introduction
The tomato is one of the most popular vegetables in the United States, second only to potato in
terms of crop yield and consumption. The U.S. is one of the world’s leading producers of
tomatoes, with an annual production of 12 to 15 million metric tons valued around $10 billion
dollars over the last decade (USDA, 2010). Three-fourths of these tomatoes are consumed in
processed form, most of which are thermally processed (Lucier and Glaser, 2009). The U.S.
consumption of processed tomatoes began a steady climb that accelerated in the late 1980s with
the rising popularity of pizza, pasta, and salsa (Lucier and Glaser, 2009). Even with the increase
in consumption of fresh tomatoes in recent years, the demand for processed tomatoes remains
relatively stable and consistent.
As one of the advanced processing technologies, microwave heating provides a relatively short
heating time due to its ability to generate volumetric heating within food materials, and thus has
the potential to be an alternative thermal treatment method for processing tomato products. Two
frequency bands are allocated by the U.S. Federal Communication Commission (FCC) for
microwave heating applications: the 915 MHz band for industrial use and the 2450 MHz band
for both industrial and domestic uses. Dielectric properties of food materials which reflect the
interaction between the foods and electromagnetic energy are essential for successful design of
microwave pasteurization and sterilization processes. The dielectric properties of biological
materials include the dielectric constant (ε') which is related to a material’s ability to store
electric energy when subjected to an electromagnetic field, and dielectric loss factor (ε") which
influences the conversion of electromagnetic energy into thermal energy. They are two elements
of material’s complex relative permittivity (ε*) presented as ε* = ε' ─ jε", where
111
√
. The
dielectric properties of a material can also be used to estimate the thermal energy converted from
electric energy at microwave frequencies. If heat loss is negligible, the increase in temperature
(∆T) of the material can be calculated from (Nelson, 1996):
where
is the specific heat of the material (J/kgºC), ρ is the density of the material (kg/m 3), ∆t
is the time (s), ε0 (8.8542 × 10-12 F/m) is the permittivity of free space or vacuum, E is the
strength of electric filed (V/m), and f is the frequency (Hz).
Several papers have reported the dielectric properties of fruits and vegetables (Seaman and Seals,
1991; Nelson et al., 1994; Ikediala et al., 2000; Feng et al., 2002; Birla et al., 2008; Wang et al.,
2011). There is very limited published information related to dielectric properties of tomatoes in
a wide temperature range for the two microwave frequencies. Experimental data for those
properties are, however, needed for proper design of microwave pasteurization and sterilization
processes. Reyes et al. (2007) obtained the dielectric constant and loss factor of osmotically
dehydrated cherry tomatoes measured at 2450 MHz and 20°C. Ghanem (2010) studied the
dielectric properties and penetration depth of tomato juice from 25 to 45°C at 2450 MHz. Kumar
et al. (2008) measured ε' and ε" of tomato particulates and puree for salsa con queso in a
temperature range of 20–130°C at 915MHz. However, there is no published data on the dielectric
properties of different tomato tissues. In the commercial processing of tomato paste, whole peel
or dice products, the entire tomato containing all three tissue types, is used. For the Roma
tomato, the average wet weight percentage of the percarp tissue (including skin), locular tissue
(including seeds) and placental tissue was 74 ± 6 g/100g, 13 ± 2 g/100g and 13 ± 4 g/100g, based
on our measurements. The differences in the physicochemical properties of different tomato
112
tissues may affect their dielectric properties, which would result in different heating rates and
behaviors in microwave heating. Thus, it is helpful to know their individual dielectric properties
to develop microwave pasteurization and sterilization processes for specific tomato products.
It is known that many factors may influence the dielectric properties of a given food, including
frequency, temperature, moisture content, salts and other food constituents (Tang, 2005). NaCl
and CaCl2 are the two salts most commonly added to canned tomato products; the former is for
improved taste while the latter is a firming agent to retain texture. Several publications have
discussed specific correlations between dielectric properties of foods and salt levels, frequency
and food matrix (Goedeken et al., 1997; Guan et al., 2004; Ahmed et al., 2007; Zhang et al.,
2007; Wang et al., 2011). Little information is available on the influence of salts on the dielectric
properties of tomatoes. Only one analysis conducted by Reyes et al. (2007) studied the dielectric
spectroscopy of cherry tomatoes dehydrated with sucrose, NaCl and calcium lactate solutions at
2450 MHz. However, a very high salt concentration was used in Reyes’s study (1–20 g/100g of
NaCl, and 1–2 g/100g of calcium lactate) in order to create osmotic conditions for dehydration.
In the current study, the effect of NaCl and CaCl2 on the different tomato tissues will be
discussed for typical levels found in commercially canned tomato products.
The objectives of the current study were: (1) measuring the dielectric properties of the three
different tomato tissues (pericarp, locular and placental tissues) in a temperature range of 22–
120°C, over 300–3000MHz; (2) studying the effects of tomato compositions, temperature,
frequency, NaCl and CaCl2 addition on their dielectric properties, particularly at the microwave
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industrial frequencies of 915 and 2450MHz; (3) investigating their loss mechanism; (4)
determining the microwave penetration depths for the three tomato tissues.
2. Materials and methods
2.1. Sample preparation
Fresh Roma tomatoes were purchased from a local grocery store (Safeway, Pullman, WA, USA).
After washing, tomatoes were quartered and the three tissues were separated: pericarp tissue
(including skin), locular tissue (including seeds in the locular cavity) and placental tissue (Figure
1). Each tissue was collected and blended into a homogenate individually. A total of 25 ml
tomato sample made from 2–3 tomatoes was used for each measurement. Separate samples were
prepared with 0.2 g/100g of NaCl and 0.055 g/100g CaCl2 (equivalent to containing 200 mg/kg
Calcium) to evaluate the effects of salt addition on their dielectric properties. The salt
concentrations were chosen based on common practices in the tomato canning industry.
2.2. Moisture content, pH and total soluble solids
Physiochemical properties of the three tomato tissues including moisture content, pH and soluble
solid content were analyzed immediately after samples were prepared as described above.
Determination of moisture content was carried out in a vacuum oven following AOAC method
920.151 (AOAC, 2005). pH was measured using a Fisher Scientific Accumet pH meter. Total
soluble solids were assessed by optical refractometer (Atago Co. LTD, Japan) and expressed as
°Brix. All measurements were conducted in triplicates.
114
2.3. Determination of dielectric properties
The dielectric properties of tomato samples were measured using an open-ended coaxial probe
connected to an HP 8752C network analyzer (Hewlett Packard Corp., Santa Clara, CA, USA)
with system accuracy within 5% of error. An Agilent 85032B type N calibration kit which
included open/short circuits and a 50 ohm load was used to calibrate the network analyzer. Then,
the open-ended coaxial probe was calibrated by an Agilent 85070E dielectric probe kit, with air,
short-circuit, and deionized water (25°C). After the calibration of the analyzer and the probe,
tomato samples were added and tightly sealed in a test cell (Figure 2). The test cell was designed
to hold the sample against the probe while allowing the sample temperature to be raised by a
fluid (circulated from an oil bath) in the jacket wall to the designated temperature (Wang et al.,
2003). To avoid air bubble formation which could influence the probe sensor reading and cause
error of a measurement, tomato samples were vacuum degassed (KOCH Packaging Supplies
Inc., Kansas City, MO) before each filling. Each measurement was repeated three times. The
dielectric properties (ε' and ε") were determined over a frequency range of 300–3000 MHz for
temperatures ranging 22–120°C in 20°C increments. Statistical analysis was performed using
Matlab 7.0 (The MathWorks, Inc., 2004), employing a student’s t-test (α=0.05).
2.4. Measurement of ionic conductivity
The dielectric loss mechanisms of biological materials in electromagnetic energy fields mainly
include polar, electronic, atomic and Maxwell-Wagner responses (Metaxas and Meredith, 1983).
At microwave frequency ranges (915 and 2450 MHz), the dominant loss mechanisms in foods
are dipole dispersion and conductive (ionic) charge migration, and can be expressed as
(Ryynänen, 1995):
115
(1)
where
represents contributions of dipole dispersion to a material’s dielectric loss factor and
represents contributions of ionic conduction to dielectric loss factor. Since
(2)
We can substitute equation (2) into equation (1) and take logarithms of both sides to get
(3)
where ε0 is the permittivity of free space or vacuum (8.854×10-12 F/m), σ is the ionic
conductivity (S/m) of a given material, and f is frequency (Hz).
Ionic conductivity of pureed tomato tissues was measured at 22, 40, 60 and 80°C, using an
electrical conductivity meter (Cole-Parmer Con 500 conductivity meter, Chicago, IL) with a
direct current of 500 mA. Thirty-five ml tomato homogenate was poured into a Corning tube
(Corning Incorporated, NY) and the probe was placed in the center of the sample. The tube was
sealed with Parafilm, and placed in a water bath (Thermo Electron Corporation, Waltham, MA,
USA) to heat to the desired test temperature. A Type-T thermal couple (accuracy ± 0.5°C) was
inserted into the center of the sample to check the temperature. Experiments were done in
triplicate.
2.5. Determination of power penetration depth
The penetration depth of microwaves is a measure of how deep microwave radiation can
penetrate into a material. It is defined as the depth where the dissipated power is reduced to 1/e
of the initial power entering the surface, and can be calculated by the following equation (Von
Hippel, 1954)
116
(4)
√
√
where dp is the penetration depth (m) and c is the speed of light in free space (3×108 m/s).
Microwave power penetration depth is generally used to select appropriate thickness of food
inside packages to ensure a relatively uniform heating along the depth of a food during dielectric
heating processes (Wang et al., 2003; Wang et al., 2008). The penetration depth of microwaves
into tomato samples was calculated at 915 MHz, at temperatures of 22, 40, 60, 80, 100, and
120°C.
3. Results and discussion
3.1. Physicochemical properties of pericarp, locular and placental tissues of raw tomatoes
The moisture content, pH and soluble solids content of the three tissues of raw tomato samples
used in this study are reported in Table 1. The moisture content in the three tissues was very high
and varied from 93–95 g/100g. Both the pH and soluble solids content in the locular tissue were
the highest among the three tissues, with a value of 4.4 and 4.57 °Brix, respectively. Moretti et
al. (1998) studied the chemical composition and physical properties in different tomato tissues
and provided information about other components, including vitamin C, total carotenoids and
chlorophyll. However, due to their large molecular weight and low content in the total sample,
those components have little influence on the dielectric loss factor and thus were not analyzed in
the current study. Since the moisture content in the three tomato tissues was similar and at a high
value, one possible reason for the difference in their dielectric loss factors might be the different
ionic conductivity due to the different amount and mobility of charged ions in the test tissues.
117
This was confirmed by our measurement, in which we obtained an average ionic conductivity of
5.81, 7.42 and 5.01 mS/cm (1 mS/cm=0.1 S/m) for pericarp, locular and placental tissues,
respectively.
3.2. Dielectric properties of pericarp, locular and placental tissues of raw tomatoes
Figure 3 shows typical trends for changes in dielectric constant and loss factor of tomato locular
tissue over temperature and frequency. The dielectric constant decreased linearly with increasing
temperature, about a 3–5 unit reduction for every 20°C temperature increase at the same
frequency. In general, the dielectric constant also decreased with increasing frequency. Unlike
the dielectric constant, the effect of temperature on the loss factor behaved in an opposite
manner, increased with increasing temperature. The loss factor of tomatoes also decreased with
increasing frequency, more sharply at lower frequencies (300–1500 MHz). At low temperatures
(22 and 40°C), a slight increase in loss factor was observed at the higher frequencies after the
value decreased to the minimum at around 2000 MHz. These same trends noted for effects of
temperature and frequency on dielectric properties of the locular tissues were also observed in
tomato pericarp and placental tissues. These trends in tomato tissues are in agreement with those
observed in other fresh fruits and vegetables with high moisture content (Seaman and Seals,
1991; Nelson et al., 1994; Ikediala at el., 2000; Feng et al., 2002).
The dielectric properties of different tomato tissues at 915 and 2450 MHz are shown in Figure 4.
At 915 MHz, the three tomato tissues had the same dielectric constant at each temperature, with
the value decreasing from around 78 at 22°C to 57 at 120°C. However, the dielectric loss factor
of the three tomato tissues differed from each other at each temperature, from 10–17 at 22°C to
118
21–38 at 120°C. The placental tissue had the lowest dielectric loss factor while the locular tissue
showed the highest value at each temperature. About a 3–5 unit increment of the loss factor of
the three tomato tissues was observed at the same temperature, from placental tissue, locular
tissue to pericarp tissue, accordingly. The same trends were observed in dielectric constant of the
three tomato tissues at the frequency of 2450 MHz, with a slightly lower value at each
temperature compared to those at 915 MHz. For the dielectric loss factor of the three tomato
tissues at 2450MHz, their values were still arranged in the same order, with the lowest being the
placental tissue and the highest locular tissue. However, the changes in their dielectric loss
factors at 2450 MHz were different. Raising the temperature at this frequency, the dielectric loss
factor initially decreased then increased when temperature reached or exceeded 80°C, exhibited a
U–shaped trend. At higher temperatures, the differences among the three tomato tissues were
more apparent. The differences in dielectric loss factor of tomatoes with temperature between the
two frequencies (915 and 2450 MHz) might result from the differences in their dominant loss
mechanism. At the lower frequencies, dielectric loss due to ionic conductivity is more important
while at higher frequencies, dipolar rotation of free water is the dominant contributor (Ryynänen,
1995; Calay et al., 1995). The dielectric loss factor of tomatoes continued increasing with
increasing temperatures at 915 MHz because conductive loss played the dominant role and it
increased with temperature increase. But at 2450 MHz, dipolar loss became dominant, which
decreased with temperature increase. Raising temperature initially decreased the overall
dielectric loss factor in tomatoes due to the dominant dipolar loss, and then increased because
conductive losses took over at higher temperatures.
119
3.3. Effect of NaCl
Values for the dielectric constants and loss factors of the three tomato tissues with addition of 0.2
g/100g NaCl at 915 and 2450 MHz are summarized in Figure 5. For the dielectric constant, the
three tissues gave the same results as those without NaCl addition at each frequency, with higher
values at 915 MHz than those at 2450 MHz. At each frequency, although the dielectric constant
of the locular tissue was slightly higher than the other two tissues, no significant difference was
found among these values of the three tomato tissues at each temperature. However, a significant
difference was found in their corresponding dielectric loss factors. At 915 MHz, the dielectric
loss factor was greatly increased compared to the samples without NaCl addition (Figure 4),
from 10–17 to 17–24 at 22°C, and 21–38 to 41–55 at 120°C. The increase in the loss factor of
the three tomato tissues is more pronounced at high temperatures. The same changes in dielectric
loss factor of the three tomato tissues with NaCl addition were observed at 2450 MHz frequency.
These results indicate that 0.2 g/100g NaCl didn’t influence the dielectric constant of tomato
samples, but appparently increased their dielectric loss factor. The increase of loss factor of the
three tomato tissues may be explained by an increase of ionic conductivity due to dissolved ions
coming from NaCl. This positive effect on the loss factor with increasing salt level was
previously found in many foods, such as salmon fillets with 0–0.5 g/100g NaCl (Wang et al.,
2009), potato purees with 0–7 g/100g NaCl ( Guan et al., 2004; Wang et al., 2011), meat with 0–
5 g/100 salt (Lyng et al., 2005; Tanaka et al., 2000; Zhang et al., 2007), surimi with 0–6 g/100g
NaCl (Yaghmaee and Durance, 2001), and butter with 0–0.6 g/100g Na+ (Ahmed et al., 2007).
However, salting a product may also reduce the free water content due to the binding of free
water molecules by the dissolved ions, therefore depressing the dielectric constant (Ahmed et al.,
2007;Calay et al., 1995; Zhang et al., 2007). Since the NaCl concentration used in the current
120
study was only 0.2 g/100g, in such a high moisture-content food these binding effects on
reducing the overall dielectric constant of the tomato tissue maybe negligible. A similar
obsersvation was made by Ikediala et al. (2002), who showed that up to 2 g/100g NaCl addition
to saline water produced little change in its dielectric constant at 915 MHz.
3.4. Effect of CaCl2
Calcium is often added as a firming agent to retain tomato texture. The effect of calcium on the
dielectric properties of the three tomato tissues was investigated. According to the FDA
regulation on calcium addition to canned tomato products (≤ 800 mg/kg calcium by weight in the
finished product) and typical commercial processing practices, a concentration of 200 mg/kg
calcium (equivalent to 0.055 g/100g CaCl2) was added to each tomato sample, along with the 0.2
g/100g NaCl. Dielectric properties of tomato pericarp, locular and placental tissues with these
two added salts are summarized in Table 2. Again, at a specific frequency (915 or 2450 MHz),
no significant difference was found in the value of the measured dielectric constant for the three
tomato tissues, while an apparent difference existed in their dielectric loss factor at each
temperature. The dielectric loss factor of each tissue at the frequency of 915 MHz varied from
20–26 at 22°C, to 50–64 at 120°C. Similar to the situation with NaCl, addition of CaCl2 only
influenced the dielectric loss factor of tomato samples. Ikediala et al. (2002) also reported that
increasing the concentration of CaCl2 solution from 0.1 g/100g to 2.0 g/100g at the frequency of
915 MHz sharply increased its loss factor, but resulted in little change in its dielectric constant.
A better understanding of the effects of NaCl and CaCl2 on the dielectric loss factor of different
tomato tissues can be seen in Figure 6. Although the changing trends of the dielectric loss factors
121
of three tomato tissues with temperatures were different at the two frequencies (915 and 2450
MHz), the effects of either salt addition were the same. Both salts increased the dielectric loss
factor. Figure 6 also shows the increase in loss factor at high temperatures was more evident than
at low temperatures. Besides, adding salts sharply increased loss factor in tomatoes with higher
temperatures at 2450 MHz, lowering the turning point of temperature where their loss factor
started to increase with increasing temperature.
3.5. Effect of ionic conductivity on dielectric loss factor
As shown in Eqs (1)–(3), two major dominant contributors to the value of dielectric loss in food
materials at microwave frequencies are ionic loss which results from migration of ions, and
dipole loss which results from water dipole dispersion. The ionic conductivity is a function of the
concentration and type of ions present, and the temperature. Generally, the ionic conductivity of
a very dilute aqueous solution is proportional to the amount of dissolved ions it contains (Gray,
2004). For high moisture foods, ionic conductivity normally increased with higher temperatures
due to reduced viscosity and increased mobility of the ions (Tang et al., 2002). The values of
ionic conductivity of tomato locular tissue at 22, 40, 60 and 80°C are shown in Figure 7. A sharp
increase in measured ionic conductivity was observed after the addition of salt (NaCl or CaCl2),
and the increase was more pronounced at high temperatures.
The dielectric loss factor value of tomato locular tissue contributed by ionic conduction (εσ")was
calculated according to Eq. (2) and shown in Figure 8, along with the corresponding overall
dielectric loss factor values (ε") measured by the network analyzer. Tomato locular tissue had
similar trends, with or without salt. There is good agreement between measured ε" and calculated
122
εσ" values at all temperatures in the frequency range below 700 MHz. This phenomenon
demonstrated that the dielectric loss factor of tomato locular tissue was governed mainly by ionic
conduction in the lower end of the studied frequency range. The increase in the dielectric loss
factor with increasing temperature was indeed caused by the increase in ionic conductivity
(Figure 7). When the frequency increased above 700MHz, the measured ε" values started to shift
above the calculated εσ" values. The deviation was caused by dipole dispersion which took place
in the tomato samples at the high frequency range. The results are also in agreement with Figure
6, which indicates that the increase in measured ε" of each tissue was not proportional to the
increase of total molar concentration of ions from added salts. The contribution of dipole rotation
to the overall dielectric loss factor became more important when moving towards higher
frequencies. The peak value of ε" due to dipole water at room temperature with respect to
frequency occurs between 16–20 GHz (Mashimo et al., 1987; Tang et al., 2002). Raising
temperature would move this peak towards higher frequency bands. Figure 8 also shows that this
shifting at the higher frequencies was more pronounced at low temperatures than at high
temperatures, which means the dipole loss had a larger influence at low temperatures. At high
temperatures ionic conductivity increases and contributes more to the overall loss factor which
results in a less shifting from the measured ε" to the calculated εσ". These resultsagree with our
previous results of ε" (tomatoes) vs. temperature profiles (Figure 4–6), which were linear at 915
MHz while a U–shaped curve existed at 2450 MHz.
3.6. Penetration depth
The penetration depth of microwaves in the three tomato tissues (pericarp, locular, and placental
tissue) at the two microwave frequencies is summarized in Table 3. Fresh samples had the
123
highest penetration depths, while the samples with the two salts added had the lowest values
under the conditions studied. All of the three tomato tissues showed higher penetration depth at
915 MHz than their corresponding values at 2450 MHz, with average values varied from 7.0–
43.3 mm for the former and 5.8–17.6 mm for the latter. Although the penetration depth of the
three tomato tissues varied from each other, they exhibited very similar trends in their changes
with salt addition, decreasing with increasing salt. For temperature changes, increasing
temperature decreased their penetration depth at 915 MHz; while initially increased then
decreased their penetration depth at 2450 MHz. For every 20°C temperature increment at 915
MHz, the penetration depth decreased around 2–4 mm under the same condition; while the
penetration depth changed around 0.2–2 mm at 2450 MHz. The tendency of change in
penetration depth with increasing temperature is opposite to those in their loss factor, which is
easy to understand because according to Equation (4) ε" is inversely related to dp. The changes in
ε" affected dp more than the influence from ε' in tomato samples used in our studies. A similar
change in the penetration depth with temperature at 915 MHz has been reported for whey protein
gel, macaroni and cheese by Wang et al. (2003), and pink salmon fillets by Wang et al. (2009).
4. Conclusions
Our results showed that dielectric loss factor of three tomato tissues (the pericarp, the locular and
the placental tissues) were significantly different from each other, either with or without salt.
However, no significant differences were found in their corresponding dielectric constant. Salt
addition at the typical commercial canned tomato product level (0.2 g/100g NaCl or 0.055
g/100gCaCl2) sharply increased the loss factor of the three tomato tissues, but didn’t affect their
dielectric constant at the microwave frequencies (915 and 2450 MHz). Similar trends for changes
124
in dielectric loss factor were observed in the three tomato tissues, decreasing with increased
frequency, and increasing with salt addition. For the effects of temperature, increasing
temperature continued increasing their dielectric loss factor at 915 MHz while initially increased
then decreased their corresponding values at 2450 MH, resulting from their different dominant
loss mechanism at the two frequencies. Furthermore, a positive correlation was found between
the loss factor of the tomato tissue and their ionic conductivity. At a specific frequency (either
915 or 2450 MHz), the penetration depth of the three tomato tissues varied from each other, but
again exhibited similar change tendency. Results obtained in this study may be used for
developing microwave pasteurization and sterilization processes for different tomato products,
and also add new information to the database for computer simulation.
Acknowledgements
This research was supported in part by USDA-CSREES-NRICGP Grant No. 2009-55503-05198,
titled: Quality of Foods Processed Using Selected Alternative Processing Technologies and
USDA-NIFA Grant No. 2011-5116-68003-20996, titled: Control of Food-borne Bacterial& Viral
Pathogens using Microwave Technologies. The senior author would like to thank the Chinese
Scholarship Council for fellowship support.
125
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129
Table 1. Moisture content, pH and soluble solids content of pericarp, locular and placental
tissues in raw tomatoes.
Moisture content, (g/100g)
pH
Soluble solids, °Brix
Pericarp tissue
94.6 ± 0.4
4.15 ± 0.01
4.03 ± 0.06
Locular tissue
93.8 ± 0.5
4.41 ± 0.08
4.57 ± 0.06
Placental tissue
94.5 ± 0.4
4.19 ± 0.01
4.30 ± 0.14
130
Table 2. Dielectric properties of tomato pericarp, locular and placental tissues with 0.2 g/100g of
NaCl and 0.055 g/100g of CaCl2 at 915 and 2450 MHz.
Temp (°C)
915MHz
2450MHz
ε'
ε"
ε'
ε"
22
77.9 ± 0.5
22.5 ± 0.2
76.0 ± 0.7
17.1 ± 0.1
40
74.6 ± 0.9
26.3 ± 0.0
73.2 ± 0.9
16.1 ± 0.2
Pericarp
60
70.6 ± 0.8
31.6 ±0.6
69.4 ± 0.8
16.4 ± 0.2
tissue
80
66.3 ± 0.7
38.5 ± 0.4
65.5 ± 0.6
18.0 ± 0.2
100
61.7 ± 0.8
47.3 ± 0.3
60.9 ± 0.6
20.4 ± 0.1
120
57.7 ± 0.7
55.9 ± 0.3
56.9 ± 0.9
23.1 ± 0.2
22
77.7 ± 0.2
25.9 ± 0.4
75.1 ± 0.4
18.5 ± 0.2
40
75.2 ± 0.3
30.4 ± 0.2
73.1 ± 0.1
18.0 ± 0.2
Locular
60
71.2 ± 0.2
36.7 ± 0.3
69.7 ± 0.3
18.9 ± 0.3
tissue
80
67.2 ± 0.5
44.6 ± 0.2
66.0 ± 0.6
20.5 ± 0.4
100
63.0 ± 0.4
54.4 ± 1.0
61.6 ± 0.4
23.5 ± 0.5
120
59.1 ± 0.4
63.3 ± 1.7
58.1 ± 0.2
26.2 ± 0.5
22
76.5 ± 0.1
20.9 ± 0.2
74.3 ± 0.4
16.8 ± 0.1
40
74.0 ± 0.4
24.2 ± 0.2
72.0 ± 0.2
15.7 ± 0.1
Placental
60
69.9 ± 0.3
28.9 ± 0.3
68.5 ± 0.4
15.9 ± 0.3
tissue
80
65.7 ± 0.0
34.8 ± 0.0
64.6 ± 0.1
16.7 ± 0.2
100
61.3 ± 0.3
41.8 ± 0.5
60.3 ± 0.1
18.7 ± 0.1
120
57.3 ± 0.7
49.3 ± 0.4
56.4 ± 0.4
20.7 ± 0.2
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Table 3. Microwave penetration depth into tomato pericarp, locular and placental tissues at
915M Hz.
Penetration Depth (mm)
915MHz
Temp (°C)
Fresh
2450MHz
with 0.2 g/100g
NaCl
with
Fresh
0.2g/100g NaCl
with 0.2g/100g
NaCl
& 0.055 g/100g
with 0.2
g/100g NaCl &
0.055 g/100g
CaCl2
CaCl2
22
33.1 ± 0.3
22.0 ± 0.5
20.7 ± 0.1
12.4 ± 0.3
11.0 ± 0.2
10.0 ± 0.2
40
29.1 ± 0.3
19.2 ± 0.7
17.4 ± 0.1
14.0 ± 0.5
11.5 ± 0.2
10.5 ± 0.2
Pericarp
60
23.9 ± 0.3
15.6 ± 0.4
14.2 ± 0.2
14.5 ± 0.7
11.1 ± 0.1
10.0 ± 0.0
tissue
80
19.3 ± 0.6
12.4 ± 0.3
11.5 ± 0.1
13.9 ± 0.8
10.1 ± 0.0
8.8 ± 0.1
100
15.6 ± 0.7
10.3 ± 0.0
9.2 ± 0.1
12.6 ± 0.8
8.7 ± 0.1
7.6 ± 0.1
120
12.4 ± 0.8
8.6 ± 0.1
7.8 ± 0.1
10.5 ± 0.8
7.4 ± 0.0
6.5 ± 0.0
22
27.1 ± 0.9
19.9 ± 0.5
18.0 ± 0.3
11.4 ± 0.1
10.0 ± 0.0
9.2 ± 0.1
40
24.0 ± 1.0
17.1 ± 0.2
15.2 ± 0.1
12.3 ± 0.1
10.4 ± 0.0
9.3 ± 0.1
Locular
60
19.6 ± 0.7
14.0 ± 0.0
12.4 ± 0.1
12.5 ± 0.2
9.9 ± 0.1
8.7 ± 0.1
tissue
80
16.1 ± 0.6
11.3 ± 0.0
10.1 ± 0.1
11.8 ± 0.1
8.9 ± 0.1
7.8 ± 0.1
100
13.1 ± 0.1
9.4 ± 0.1
8.2 ± 0.1
10.4 ± 0.1
7.7 ± 0.1
6.6 ± 0.2
120
11.0 ± 0.1
7.9 ± 0.0
7.0 ± 0.1
9.2 ± 0.1
6.7 ± 0.0
5.8 ± 0.2
22
43.3 ± 0.4
26.2 ± 0.7
22.0 ± 0.2
13.5 ± 0.1
11.2 ± 0.5
10.1 ± 0.1
40
41.0 ± 0.1
22.7 ± 0.8
18.8 ± 0.1
15.9 ± 0.1
12.1 ± 0.6
10.6 ± 0.1
Placental
60
33.9 ± 0.5
18.6 ± 0.4
15.4 ± 0.1
17.4 ± 0.1
12.2 ± 0.5
10.2 ± 0.2
tissue
80
28.1 ± 0.0
15.0 ± 0.4
12.6 ± 0.0
17.6 ± 0.7
11.2 ± 0.6
9.5 ± 0.1
100
23.4 ± 0.4
12.2 ± 0.2
10.3 ± 0.1
16.7 ± 0.4
9.9 ± 0.4
8.2 ± 0.0
120
18.6 ± 0.5
10.2 ± 0.2
8.6 ± 0.0
14.8 ± 0.2
8.6 ± 0.3
7.2 ± 0.1
132
List of Figure Captions
Figure 1. Illustration of different anatomical structures of tomato fruits.
Figure 2. Schematic diagram of pressure-proof test cells used for dielectric properties
measurement (from Wang et al., 2003).
Figure 3. Dielectric properties of raw tomato locular tissue as a function of temperature and
frequency (♦ 22°C; □ 40°C; ▲ 60°C; × 80°C; ● 100°C; ○ 120°C). Data are the means± S.D.
(n=3).
Figure 4. Dielectric properties of raw pericarp (♦), locular (□) and placental (△ ) tissues at 915
(A) and 2450 (B) MHz. Data are the means± S.D. (n=3).
Figure 5. Dielectric properties of tomato pericarp (♦), locular (□) and placental (△ ) tissues with
0.2 g/100g of NaCl at 915 (A) and 2450 MHz (B). Data are the means± S.D. (n=3).
Figure 6. Dielectric loss factor of tomato pericarp (♦), locular (□) and placental (△ ) tissues at 915
(A) and 2450 (B) MHz. Added NaCl was 0.2 g/100g and CaCl2 was 0.055 g/100g. Data are the
means± S.D. (n=3).
Figure 7. Ionic conductivity of tomato locular tissue as a function of temperature (♦ raw sample;
□ added with 0.2 g/100g of NaCl; △ added with 0.2 g/100g of NaCl and 0.055 g/100g of CaCl2).
Data are the means± S.D. (n=3).
Figure 8. Measured ε" and calculated εσ" of tomato locular tissue (A raw sample; B with NaCl; C
with NaCl & CaCl2) as a function of frequency and temperature (
40ºC, measured;
40ºC,
60ºC, measured;
calculated; 60ºC, calculated;
80ºC, measured;
22ºC, measured;
22ºC, calculated;
80ºC, calculated). Data are the means± S.D.
(n=3).
133
Figure 1. Illustration of different anatomical structures of tomato fruits.
134
Figure 2. Schematic diagram of pressure-proof test cells used for dielectric properties
measurement (from Wang et al., 2003), dimensions are in mm.
135
Figure 3. Dielectric properties of raw tomato locular tissue as a function of temperature and
frequency (♦ 22°C; □ 40°C; ▲ 60°C; × 80°C; ● 100°C; ○ 120°C). Data are the means ± S.D.
(n=3).
136
Figure 4. Dielectric properties of raw pericarp (♦), locular (□) and placental (△ ) tissues at 915
(A) and 2450 (B) MHz. Data are the means ± S.D. (n=3).
137
Figure 5. Dielectric properties of tomato pericarp (♦), locular (□) and placental (△ ) tissues with
0.2 g/100g of NaCl at 915 (A) and 2450 MHz (B). Data are the means ± S.D. (n=3).
138
Figure 6. Dielectric loss factor of tomato pericarp (A1 & B1), locular (A2 & B2) and placental
(A3 & B3) tissues at 915 (A) and 2450 (B) MHz (♦ raw sample; □ added with 0.2 g/100g of
NaCl; △ added with 0.2 g/100g of NaCl and 0.055 g/100g of CaCl2). Data are the means ± S.D
(n=3).
139
Figure 7. Ionic conductivity of tomato locular tissue as a function of temperature(♦ raw sample;
□ added with 0.2 g/100g of NaCl; △ added with 0.2 g/100g of NaCl and 0.055 g/100g of CaCl2).
Data are the means ± S.D. (n=3).
140
Figure 8. Measured ε" and calculated εσ" of tomato locular tissue (A raw sample; B with NaCl; C
with NaCl & CaCl2) as a function of frequency and temperature (
40ºC, measured;
60ºC, measured;
80ºC, measured;
141
22ºC, measured;
22ºC, calculated;
40ºC,
calculated; 60ºC, calculated;
80ºC, calculated). Arrows show the influence of
increasing temperature. Data are the means ± S.D (n=3).
142
Chapter 6. Developing Microwave Sterilization/Pasteurization Processes for
Pre-packaged diced Tomatoes/Carrots
Abstract: Microwave (MW) processing as one of the few emerging food preservation
technologies takes relatively short heating time due to its volumetric heating generated within
food materials, and therefore has the potential to produce high quality shelf-stable or chilled food
products. In the current study, a MW assisted thermal sterilization (MATS) process was
developed for processing diced tomatoes packaged in 8-oz pouches using a semi-continuous,
915MHz single-mode MW system; while a MW assisted thermal pasteurization (MAP) process
was developed for diced carrots using a 14-kW single-mode MW system. A 3-D computer
simulation model that considered temperature dependent dielectric properties of food materials
provided information about heating patterns and the cold spot location in the sample pouches.
The simulation results were validated with a chemical marker based computer-vision method.
Heat penetration tests were conducted to obtain temperature-time data for the cold spot in diced
tomatoes packaged in pouches, from which a MATS process was designed to achieve a 5D
reduction in Bacillus coagulans ATCC 8038 spores (F105°C = 6.0 min). For diced carrots, two
MAP processes were developed: a 6D process for non-proteolytic Clostridium botulinum type E
spores (F90°C=3 min); and a F90°C=10 min process. Incubation tests of the processed tomato
products verified the successful development of MATS processes.
Keywords: Microwave; sterilization; pasteurization; pre-packaged; tomato; carrot
1.
Introduction
Applications of MW energy in food processes have drawn increased attention over the past
decades, due to the rapid heating (usually several minutes) of MW processes by generating
143
volumetric heating within food materials. MW heating overcomes the disadvantages of slow
conductive/convective heat transfer in conventional thermal processes. Therefore, MW
processing has the potential to produce high quality self-stable food products. Tomatoes and
carrots are two of the most commonly consumed vegetables in the US. Three-fourths of the
tomatoes consumed by Americans are in processed forms, most of which are thermally
processed; while one-fourth of carrots are consumed in a processed form, largely canned and
frozen (Lucier and Glaser, 2009; Lucier and Lin, 2007). However, there is no published paper on
developing systematic microwave sterilization or pasteurization processes for the diced tomato
or carrot products.
Non-uniformity in temperature distribution remains the challenge for developing a MW assisted
thermal process for food products to meet the stringent food safety regulation and standards. The
uneven heating results in the hot and cold spots in the MW processed foods. The cold spot is the
area which receives the lowest thermal energy and the hot spot is the area of highest thermal
energy reception. Several factors contribute to the non-uniform heating during the MW
processing, mainly including uneven distribution of the electric field due to electromagnetic
energy dispersion, different dielectric properties of food materials, package geometry, and heat
loss on the boundary. Thus, it is critical to determine the heating pattern, especially the location
of cold spots inside the foods for developing a successful MW process. Kim and Taub (1993)
developed a chemical marker method at the U.S. Army Natick Research Center. This method
correlated heating intensity with brown color caused by the formation of the Maillard browning
product chemical marker M-2 (4-hydroxy-5-methyl-3(2H)furanone). The thermal intensity can
be detected by the yield of M-2 in the homogenous foods through HPLC analysis. Pandit et al.
144
(2007) then improved this chemical marker method for heating pattern determination by
developing a rapid computer vision method to establish a linear correlation between the M-2
marker yield and the cumulative thermal lethality (F0). Considering the processing temperature
and the gelling temperature, whey protein gels containing D-ribose and salt have been used in
our laboratory as a model food to determine heating patterns in foods during high temperature
MATS processes (Lau et al, 2003; Tang, 2008; Wang et al., 2009) while gellan gels, egg whites
and whole eggs were investigated for low temperature MAP processes (Zhang et al., 2013).
Besides the chemical marker methods in experimentation, computer simulation is another
effective way to predict the heating pattern and numerically solve the problems in MW heating.
Chen et al. (2007) successfully developed a commercial electromagnetic software combined with
a customer-built heat transfer model to simulate the coupled electromagnetic-thermal MW
system. They also validated the simulation model with a pilot-scale MW system using direct
temperature measurement data and indirect color patterns in whey protein gels via formation of
the thermally induced chemical marker M-2 (Chen et al., 2008). Resurreccion et al. (2013)
further improved this simulation model for the MATS system. The model considered the coupled
electromagnetic-thermal phenomena in food packages moving in multiple MW cavities. In the
current study, the chemical-marker and computer simulation methods were also used to detect
the heating patterns and cold spots of the foods during MW processing.
When using the model food to simulate the real products and determining the heating patterns
and cold spots, this requires that the dielectric properties of the real foods match that of the
model foods. The dielectric properties of food materials reflect the interaction between the foods
and electromagnetic energy. They include the dielectric constant (ε') and dielectric loss factor
145
(ε"). The former is related to a material’s ability to store electric energy when subjected to an
electromagnetic field, while the latter influences the conversion of electromagnetic energy into
thermal energy. Therefore, the dielectric properties of the diced tomato and carrot products were
also measured in the current study.
In addition to determination of heating pattern and cold spots, temperature profiles of foods
during processing are also essential for a successful design of MW processing to ensure food
safety. Temperature sensors are usually placed at the cold spot inside the foods to trace the
temperature of the cold spot during thermal processing, and the information obtained can be used
to calculate the thermal lethality levels (F value). Using fiber-optic thermometers is the most
reliable and accurate way for direct temperature measurement in electromagnetic fields, and they
are often used in MW and radio frequency heating (Tang et al., 2008). However, this method is
costly, and not convenient in a continuous process. Luan et al. (2013) investigated the feasibility
of using mobile metallic temperature sensors (Ellab sensors) in continuous MATS systems and
verified that the metallic temperature sensor could be used to capture temperature profiles in a
MATS system when placed in a suitable orientation. The same Ellab sensors were used in the
current study to record the temperature profile of the cold spots of diced tomato and carrot
products. Besides this, an incubation test of the MATS processed tomato pouches was used for
microbial validation of the processing procedures developed.
The objectives of this study were to develop a pilot-scale MATS process for diced tomatoes, and
a MAP process for diced carrots prepackaged in 8 oz pouches. To achieve these goals, the
following steps were taken: 1) measuring dielectric properties of the desired products to match
146
those of the model foods; 2) determining heating pattern and cold spots in the food pouches; 3)
conducting heat penetration tests to develop MATS/MAP processes for desired products; and 4)
verifying microbial safety of the MATS processed foods by incubation test. This study should
provide useful information for future commercial MATS/MAP applications in vegetable
processing.
2. Methods and materials
2.1. Preparation of sample pouches
Roma tomatoes were purchased from a local grocery store (Safeway, Pullman, WA) and stored
at 4ºC. Tomatoes with a Hunter color “a” value (redness) of 22-25 were used. Tomato pericarp
(with skin) was quartered and diced into pieces (12.7× 12.7× (6-8) mm), the remainder was
blended into a puree. NaCl and CaCl2 were added into the tomato puree during blending. A total
of 137.5 ± 0.5 g diced tomato and 89.5 ± 0.5 g puree with 0.2% NaCl, 0.055% CaCl 2 (w/w, total
samples in each pouch) were filled into each 8-oz laminate pouch (18.5 × 13.2 ×1.6 cm,
Printpack Inc., Atlanta, GA).
Fresh carrots with their peel (Cultivar Imperator, from Bolthouse Farms, Inc., Bakersfield, CA)
were purchased from the same grocery store as the tomatoes and stored at 4ºC. Carrots were
peeled, cut into dices using a Hallde Flexi RG-7 dicer (Hicksville, NY) the same day just prior to
processing. A total of 136.5 ± 0.5 g diced carrots and 90.5 ± 0.5 g CaCl 2 solution (0.1% or 1.4%,
w/w, total samples in each pouch) were filled into each 8-oz laminate pouches.
147
All filled pouches were sealed with an UltraVac 250 vacuum sealer (KOCH Packaging Supplies
Inc., Kansas City, MO). Prepared sample pouches were then loaded into the MW heating system
to process immediately.
2.2. MW heating system
2.2.1. Microwave assisted thermal sterilization (MATS) system
A single-mode 915 MHz MATS system was used to process the diced tomato pouches. Figure 1a
shows a schematic view of the pilot scale MATS system developed by our laboratory. The
system consisted of preheating, MW heating, holding and cooling sections. Each section was
filled with circulation water from a water conditioning unit. The circulation water at pre-set
temperature provides thermal energy from the outside of food packages, while MW sources
caused volumetric heating inside the food packages. Pre-packaged foods were loaded into
pockets on a mesh belt conveyor that transported samples through each section during thermal
processing. More details about the system can be found in Resurreccion et al. (2013).
2.2.2. Microwave assisted thermal pasteurization (MAP) system
Considering the processing temperature (90°C) and pressure (normal atmosphere), the diced
carrots were processed by a new pilot scale MAP system developed in our laboratory. This pilot
scale MAP system also used single mode 915 MHz cavities. It had four sections, including
preheating, MW heating, holding and cooling (Fig. 1b). The preheating and cooling section also
play the roles of loading and unloading, respectively. Each section has a separated water
circulation system to control water flow at a constant speed and temperature. The MAP had two
148
MW heating cavities, with a total MW power of 14 kW. Food packages were preloaded in
carriers that were transported on rotary wheels through the four sections at a constant speed.
2.2.3. Conventional hot water processing
For the purpose of comparison, conventional hot water (HW) heating was also conducted on the
tomato/carrot pouches using the MATS/MAP systems. During the HW processes, the system
pressure and supply water temperature setting were the same as for the MW processing, only the
MW power was turned off. The holding time of food pouches in the holding section was adjusted
to achieve the same target process.
2.3. Measurement of dielectric properties of tomato/carrot samples
The dielectric properties of tomato/carrot samples were measured using an open-ended
coaxial probe connected to an HP 8752C network analyzer (Hewlett Packard Corp., Santa Clara,
CA, USA, CA, USA). The whole sample pouch of tomatoes was blended into a homogenate
before measurement. For carrot products, drained carrot dices were blended into puree before
test, and the dielectric properties of the two solutions (0.1% and 1.4% CaCl2 in distilled water)
were also determined. An Agilent 85032B type N calibration kit which contains open, short and
load probes was used to calibrate the impedance analyzer. Then, the open-ended coaxial probe
was calibrated by an Agilent 85070E dielectric probe kit, with air, short-circuit, and deionized
water (25°C). After the calibration of the analyzer and the probe, around 25 ml sample was
added into and tight sealed in a test cell. The test cell was designed to hold the sample against the
probe while allowing sample temperature to be raised by a fluid (circulated from an oil bath) in
the jacket wall. A detailed description of the system and calibration procedures is provided in
149
Chapter 5. The dielectric properties (dielectric constant and loss factor) were determined over a
frequency range of 300-3000 MHz for temperatures ranged between 22 and 120°C. Each
measurement was replicated three times.
2.4. Determination of the heating pattern and cold spot in sample pouches
The cold spot location in the sample pouch needed to be determined for developing successful
MW processes. The heating pattern and cold spot location in food pouches were determined by a
chemical–marker based computer–vision method, and predicted by computer simulation to make
sure the results obtained from the two methods agree with each other (Pandit et al., 2007). In our
previous study (Chen et al. 2007), a commercial electromagnetic software combined with a
customer-built heat transfer model was successfully developed to simulate the 915 MHz pilotscale MW system which coupled electromagnetic heating and conventional heat transfer. In the
current study, the same computer simulation models were used for predicting the heating pattern
and cold spot location in sample pouches.
The simulation results of tomato pouches obtained above were compared with heating patterns
experimentally determined using five different whey protein gel (model gel) formulations (Wang
et al., 2009). The five model foods were made of whey protein gels containing salt and D-ribose
at different concentrations. Heating patterns for five model foods in 8-oz pouches determined by
chemical-mark based computer-vision method were in agreement with the results predicted by
computer simulation method, so were the cold spot locations in the five model food pouches
obtained by both methods. Meanwhile, the heating pattern and cold spot location predicted by
computer simulation did not change within a ±20% deviation of dielectric properties of samples.
150
It was our hypothesis that if the dielectric properties of tomato samples fell within the range of ±
20% deviation of the model foods’ dielectric properties, then the heating pattern and cold spot
location inside the model foods could be applied to the tomato pouches (Fig. 2).
For the carrot products, the heating pattern and cold spots in the sample pouches were
determined directly using the chemical marker method. Different from MATS processes using
whey protein gel containing D-ribose and salts as the model food, gellan gel was selected for
MAP processes to determine the heating pattern. Through trial and error, gellan gel model food
(1% gellan gum, 0.2% Ca2+) contained 1% D-ribose and 0.5% L-lysine as chemical marker M-2
precursors has similar dielectric properties as the carrot products, and was used for heating
pattern determination (Data not published). Gellan gel model food was processed under the same
condition as the carrot products, heating pattern and cold spot location in carrot pouches were
determined by the chemical–marker based computer–vision method as we mentioned above.
2.5. Heat penetration test
Heat penetration tests were conducted to determine temperature profiles at the cold spot inside
food packages, and the information was used to design the desired thermal process to achieve
target microbial inactivation.
For diced tomatoes, the temperature of thermal processing was selected based on the heat
resistance of the target bacteria, the spores of B. coagulans ATCC 8038 in tomatoes. According
to the obtained results (Chapter 3), D values of those spores in tomato juice (pH 4.3) at 105°C
was 1.20 min. This temperature was the most appropriate for MW processing among the selected
151
temperatures (90-115°C) and therefore was chosen as the processing temperature for diced
tomato products. The process of diced tomatoes was designed as a 5D process, meaning a 5-log
reduction of the target B. coagulans spores. Since the D value of the spores of B. coagulans
ATCC 8038 strain in tomato juice is 1.20 min at the processing temperature (105°C), a 5D
process for diced tomatoes was designed as a target F value of 6.0 minutes for B. coagulans
spores.
For the processing of diced carrots, 90°C was selected as the pasteurization temperature based on
the kinetic results of carrot texture degradation (Chapter 4). For pasteurization of low-acid foods
which allows a shelf life up to 6 weeks at 5ºC, a common practice to aim for a 6 log reduction of
target pathogen psychrotrophic Clostridium botulinum is suitable (Vervoort et al., 2012; ECEF).
This is called a “6D” process. A general recommendation of F90°C=10 min (or equivalents) is
generally accepted for most foods, which represents at least 6-log reduction of the most heat
resistant non-proteolytic strains of C. botulinum (Vervoort et al., 2012; ECEF). In our current
study, the target microorganism for carrot products is non-proteolytic C. botulinum type E spores
(Vervoort et al., 2012). Gaze and Brown (1990) studied the thermal resistance of NP C.
botulinum type E spores in carrot from 75–90ºC and reported their D value of 0.48 min at 90°C.
Based on this literature, a 6D process of NP C. botulinum type E spores is calculated to be a
process of F90°C=2.88 min. Given the two conditions mentioned above, two thermal treatment
levels were used in processing of carrot product: one was F90°C=3 min which allows a 6D process
of target microorganism, the other one was F90°C=10 min which is generally considered an
adequate process for most of the foods.
The calculation of the F value was based on the following equation:
152
t
F   10
o
T Tref
z
dt
(1)
where T (ºC) is the temperature measured at the cold spot at time t during process, Tref is the
reference temperature, and z is the z-value of the target bacteria in the products. In the current
study, Tref was 105º C for tomato products and 90°C for carrots; z value was 8.31ºC for B.
coagulans ATCC 8038 spores in tomatoes, and 9.84°C for NP C. botulinum type E spores in
carrots (Gaze and Brown, 1990; Peng et al., 2012).
For the heat penetration tests, Ellab sensors (Ellab Inc., Centennial, CO) were used to trace
temperatures inside the sample pouches during thermal processing. The tomatoes were used as
an example to demonstrate this procedure for process development here (Fig. 3). For better
fixation of the Ellab sensor into the tomato dice, a bigger tomato piece (25.4 × 57× 16 mm)
sliced from the middle layer of tomato rather than the real sample size (12.7× 12.7× (6-8) mm)
was used. Computer simulation was performed to see if the change in sample size would cause
any change in the heating pattern or cold spot location in the sample pouch (Fig. 4). A MW
transparent frame was used to fix the position of the Ellab sensor tip at the cold spot of sample
pouch. A total of 100.0 ± 0.5g diced tomato, and 89.5 ± 0.5g puree added with 0.2% NaCl,
0.055% CaCl2 (w/w, total sample in each pouch) were carefully filled into the pouch. The
prepared sample pouches were loaded to the MATS system.
In the heat penetration tests for diced tomatoes, the system pressure for MATS was maintained at
33 psig, and the power for four MW heating cavities were set at 7.0/6.2/2.6/2.5 kW. The supply
water temperatures were set to 56/108/107/15°C for preheating, MW-heating, holding and
153
cooling sections, respectively. The processing time was adjusted by changing the moving speed
of food pouches on the conveyor belt to achieve the target 5D process. For the purpose of
comparison, conventional HW heating was also conducted on the tomato pouches with the MW
system. During the HW processes, the system pressure and circulating water temperature setting
were the same as for the MW processing, the MW power was turned off, and the holding time of
food pouches in the holding section was adjusted to achieve the same 5D process.
The heat penetration tests for carrot samples processed by the MAP system followed the same
procedures as for tomato samples. The differences were that the system pressure was normal
atmosphere for MAP processing; the total power for two MW heating cavities was 14 kW; the
circulating water temperatures were set to 61/93/93/15°C for preheating, MW-heating, holding
and cooling sections, respectively. The moving speed of the package carrier was adjusted to
achieve the target processes of F90°C=3 min or F90°C= 10 min.
2.6. Incubation test for diced tomatoes
Incubation test for diced tomatoes was conducted to validate thermal processing and ascertain
microbiological safety and stability. Processed sample pouches were incubated at 35 ± 1°C for
16 days, 3M Petrifilm plates were used for Aerobic (AC), E.coli/Coliform (EC), and Yeast and
Mold (YM) counts. In addition, microbial assay of the raw material (whole tomato and tomato
products before processing) was also conducted in parallel. Before and after processing, two
replicates were sampled for each process. Pouches were opened aseptically and 100 g of sample
was placed into a Seward 400 circulator stomacher filter bag. The sample was stomached for 2
min at 200 rpm. One ml of sample was pipetted and tenfold-serially diluted in 9 ml of 0.1%
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peptone water. One ml diluents were pipetted onto AC, EC, and YM 3M Petrifilm count plates.
The AC and EC plates were incubated at 35 ± 1ºC for 2 days while the YM plates were
incubated at room temperature for 5 days.
For whole fresh tomatoes, one tomato was sampled per case. A whole Roma tomato was put into
a Seward 400 circulator stomacher bag with 50 ml Difco Buffered Peptone Water (BPW) and the
opening was firmly twisted and closed. The surface of the tomato was gently rubbed and shaken
in the solution by hand for 2 min to dislodge surface micro flora. One ml of BPW solution was
tenfold-serially diluted in 0.1% peptone water. One ml diluents were pipetted onto AC, EC and
YM count plates and the incubation times were same as those described previously.
3. Results and discussions
3.1. Dielectric properties of tomato/carrot samples
The dielectric properties of tomato puree with 0.2% NaCl and 0.055% CaCl2obtained by
blending tomato sample from a whole pouch are shown in Fig. 5A. The dielectric constant of
tomato samples decreased with increasing temperature, about a 3-5 unit reduction for every 20°C
temperature increase at the same frequency. The dielectric constant also decreased with
increasing frequency. Unlike the dielectric constant, the effect of temperature on the loss factor
behaves in the opposite manner; the loss factor increased with increasing temperature. It also
decreased with increasing frequency, more sharply at lower frequencies (300-1500 MHz). The
same trends of dielectric constant and loss factor with temperature and frequency were observed
on carrot puree (Fig. 5B).
155
The dielectric properties of tomato puree with and without added salts (NaCl and CaCl2) as a
function of temperature at 915MHz are shown in Fig. 6A. The agreement of the dielectric
constants at each temperature in both samples with and without salts is remarkable, the two lines
are almost overlapping. This demonstrates that added salts had little effect on the dielectric
constant of tomato puree. However, tomato samples with salts had higher loss factors than those
without salts, and the increasing effect of salts on loss factor was more evident at higher
temperatures.
Fig. 6B shows the dielectric properties of carrot puree and its two solutions (0.2% NaCl with
0.1% or 1.4% CaCl2) at 915 MHz. Overall, the dielectric constant of the carrot puree was very
close to the two solutions at each temperature and ranged from 55-72, although at low
temperatures (22-60°C) it showed slightly lower values than those of the two solutions. The
dielectric loss factor of the carrot puree ranged from 17-40 when the temperature was increased
from 22°C to 120°C, around 3-5 units higher than that of the solution with 0.2% NaCl and 0.1%
CaCl2 at each temperature. When the concentration of CaCl2 increased from 0.1% to 1.4%, the
loss factor of the solution increased sharply, with values ranging from 50-148, around 3 times
increase in its loss factor. This can be explained by the increased ionic conductivity resulting
from the increased dissolved ions, which may lead to the increase in the dielectric loss factor.
More information about the effects of salt on the dielectric properties and their mechanisms can
be found in Chapter 5.
156
3.2. Heating pattern and cold spot in sample pouches
To trace the temperature of tomatoes in pouches, a tomato piece having a larger size than the real
product was used to better fix the position of an Ellab sensor, as mentioned above. Therefore,
computer simulation was conducted to check whether this change would lead to a change in the
heating pattern or heating rate in the sample pouch. The simulation results showed that the same
temperature profile was obtained at the centers of both the large and small tomato pieces (Fig. 7),
and that the sample pouches with a large tomato piece and a small one had the same heating
pattern and cold spot location (Fig. 8a). For comparison, the heating pattern experimentally
obtained in the model foods by the chemical marker method is also shown in Fig. 8.
The previous simulation work showed that the heating pattern and cold spot location did not
change within a ± 20% deviation of dielectric properties of samples. The loss factor of tomato
puree with added salts at 105°C was in the range of those for five model foods, while the
dielectric constant was higher but still in the 20% deviation range (Fig. 9). Therefore, the heating
pattern predicated by computer simulation was the same as that obtained by chemical marker
method (Fig. 8). As seen in Fig 8, the cold spot of the model foods was in the middle layer
concerning depth, located 22.8 mm in the x-direction from the center point and -2.2 mm in the ydirection from the center point in that layer. Therefore, the cold spot location (22.8, -2.2) mm
away from the center point in the middle layer of tomato pouches was applied to their heat
penetration tests.
For the carrot products, the heating pattern obtained by the chemical-mark method using gellan
gel as the model food is shown in Fig. 10. Our results showed that the cold spot in the model
157
food was also located in the middle layer concerning depth. Fig. 10 shows the top view of
heating pattern and cold spot location in the middle layer of model food. As can been seen, the
cold spot in carrot sample pouch was located directly at the center point (0, 0) in the middle
layer, and was used for their heat penetration tests.
3.3. Heat penetration results
Tomato samples were preheated in the preheating section at 55°C for 15 min, then they entered
into the MW heating section, followed by the holding section (105°C), and finally came out from
the cooling section (15°C). To achieve a 5D process (F105°C = 6.0 min), the speed of the conveyor
belt was adjusted to 47 inch/min, with a total MW heating time of 2.7 min, and a total holding
time of 2.8 min. Two Ellab sensors packaged in tomato pouches were used to record the
temperature at the cold spot of the sample pouch in each test. Tests were conducted in triplicate.
The F value was calculated starting from the sample coming out of the last MW cavity and going
into the holding section (Fig. 11). The F values obtained from the 3 tests varied from 7.7 to 16.0
min. Figure 11A shows a typical temperature profile of the tomato sample recorded by the Ellab
sensor under MW processing.
For conventional HW processing of the tomatoes, the holding time at holding section (105°C)
was adjusted to 20.1 min to achieve a target 5D process. Two Ellab sensors packaged in sample
pouches were used to record temperatures at the cold spot in each test, and tests were done in
duplicate. The F values obtained varied from 6.4 to 13.2 min. The typical temperature profile
recorded by the Ellab sensor during hot water heating is shown in Figure 11B.
158
For the MAP processing of diced carrots, samples were first preheated in the preheating section
(60°C) for 20 min, then went through the MW section, the holding section (90°C) and the
cooling section (15°C), consecutively. The speed of the package carrier was adjusted to 42 and
39 inch/min to achieve a process of F90°C = 3 min and F90°C = 10 min respectively. The
corresponding total processing time was 3.22 and 4.96 min for the two MW processes. For HW
processing, the holding time in the holding section was adjusted to 7.80 and 13.94 min to achieve
an equivalent process with an F value at 90°C of 3 min and 10 min, respectively. Fig. 12 shows
typical temperature-time profiles at the cold spot in the diced carrot pouch under MW and HW
processing, for a target process of F90°C= 3 min. The processing parameters to achieve a target
process for each product were determined and summarized in Table 1.
3.4. Incubation results for diced tomatoes
After incubation of the MATS processed tomato pouches at 35 ± 1°C for 16 days, no swollen
pouches were observed. Microbial test results of raw and processed samples are given in Table
2. No colonies were observed in the processed samples under the detection limit (1CFU/g). The
microbiological results validated the success of the MATS processing for the tomato products.
4. Conclusions
Dielectric properties of tomato and carrot products for processing were determined and used for
computer simulation of heating patterns and cold spot locations in the sample pouch. The heating
pattern determined by the computer simulation method was confirmed by that obtained with the
chemical-marker based computer-vision method. The cold spots of the tomato and carrot
pouches were in the middle layer concerning depth. The tomato pouch’s cold spot was located
159
22.8 mm in the x-direction from the center point and -2.2 mm in the y-direction from the center
point, while the carrot pouch’s cold spot was located directly at the center point (0, 0) in the
middle layer of the sample pouch. A MATS process achieving a target F value of no less than 6
min was developed for processing of diced tomatoes packaged in 8-oz pouches, which can
deliver a 5D thermal treatment to B.coagualans ATCC 8038 spores. For diced carrots, MAP
processes with F90°C= 3 min and F90°C=10 min were developed to achieve at least a 6 D reduction
of NP C. botulinum type E spores. Incubation tests and microbial analyses of the processed
tomato pouches verified the safety of the products produced from the developed MATS process.
160
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162
Table 1. Processing conditions for carrot and tomato products for MW and HW processes with
equivalent process severity regarding microbial safety.
Processing Target
Carrot
(pasteurization)
Tomato
(sterilization)
F90°C = 3min
F90°C = 10 min
MW
Water temperature
processing setting (preheating,
heating, holding and
cooling sections)
MW power setting
Preheating time
MW heating time
Holding time
Total processing time
Real F value
61/93/93/15°C
Preheating time
HW
Total processing time
processing Real F value
Processing Target
20 min
20 min
7.80 min
13.96 min
3.9 min
13.4 min
5D process
(F105°C = 6.05 min)
56/108/107/15°C
Water temperature
setting (preheating,
MW
heating, holding and
processing cooling sections)
MW power setting
Preheating time
MW heating time
Holding time
Total processing time
Belt speed
Water temperature
setting (preheating,
HW
heating, holding and
processing cooling sections)
Preheating time
Total processing time
14 kW
20 min
1.36 min
1.86 min
3.22 min
4.9 min
14 kW
20 min
1.46 min
3.50 min
4.96 min
8.9 min
7.0/6.2/2.6/2.5 kW
15 min
2.71 min
2.79 min
5.50 min
47 inch/min
56/108/107/15°C
15 min
20.1 min
The pressure of the sterilization system was 33 psig, and the pasteurization system was at
atmospheric pressure.
163
Table 2. Microbial assay of raw whole tomatoes, and diced tomatoes before and after processing.
Whole tomatoes
Sample before
(CFU/tomato)
process (CFU/g)
(1.21 ± 0.62) ×106
(5.40 ± 0.42) ×104
Coliforms
(1.13 ± 0.40) ×103
(4.35 ± 0.78)×102
Yeasts
(1.11 ± 0.34) ×104
(6.05 ± 0.92)×102
Molds
63 ± 95
±7
3M petrifilm count
Aerobic Plate
Count
5
164
Sample after process
No colonies were
observed
Detection limit: 1
CFU/g
Figure 1a. Front view diagram of four sections in the MATS system at WSU.
Figure 1b. Front view of MAP system at WSU.
165
Figure 2. Flowchart of determination of heating pattern in diced tomato pouches.
166
Figure 3. Preparation of sample pouch with Ellab sensor
Figure 4. Illustration of computer simulation (a): sample pouch with Ellab sensor (b): sample
pouch without Ellab sensor.
167
Figure 5. Dielectric constants and loss factors of tomato puree (A) and carrot puree (B) to which
salts were added, as a function of temperature and frequency.
168
Figure 6. Dielectric properties of processed products with temperatures at 915 MHz (A tomato
puree with and w/o salts; B carrot puree and its solutions).
169
Figure 7. Simulation results of temperature profiles at the cold spot in sample pouches with a
small tomato piece or a large piece for temperature measurement.
170
Figure 8. Heating pattern and cold spot location in tomato sample pouch (top view of the middle
layer). (a): Heating pattern obtained by computer simulation using dielectric properties of tomato
puree. (b): Heating pattern obtained by chemical marker method.
171
Figure 9. Dielectric properties of tomato sample compared with those of five whey protein model
foods.
172
Figure 10. Heating pattern and cold spot in carrot sample pouch (top view of the middle layer,
obtained by chemical mark method).
173
Figure 11. Example of temperature-time profile at the cold spot in the diced tomato pouch under
MW (A) and HW (B) processing. Samples were preheated at 55°C for 15 min.
174
Figure 12. Example of temperature-time profiles at the cold spot in the diced carrot pouch under
MW (A) and HW (B) processing. Preheating was at 60°C for 20 min.
175
Chapter 7. Quality Evaluation of Vegetable Products Thermally Processed
with Microwave and Conventional Methods
Abstract: This chapter presents the results of quality evaluation of diced carrots after microwave
(MW) pasteurization and diced tomatoes after MW sterilization, compared to conventional
thermal processing. For diced carrots, non-proteolytic Clostridium botulinum type E spores were
the target bacteria and the thermal process was designed to be either a 6D process for the target
bacteria (F90°C=3min) or F90°C=10 min. For diced tomatoes, the target bacterium was Bacillus
coagulans spores and the thermal process was designed to achieve a 5D reduction of the target
bacterium. Pre-packaged vegetables (diced carrots or tomatoes) with added salts (NaCl and
CaCl2) were pasteurized or sterilized by the 915 MHz pilot-scale MW systems in a batch
process. Carrot and tomato products were also processed by conventional hot water (HW)
processing to achieve the same level of inactivation for the target microorganisms. Quality
attributes of the products processed by MW and HW heating were compared. Results showed
that compared to raw carrots, carrots heated by MW had a lower total color difference (∆E
values) than those heated by HW processing on equivalent processing conditions, denoting better
color retention. There was no significant difference in texture retention of carrots heated by MW
and HW with the same process severity. No pectin methylesterase activity was detected in any of
the processed carrots. For carotenoids, no significant differences were found in carrot samples
processed by either methods, except in high CaCl2 solutions (1.4%) with intense processing
(F90°C=10 min), where the total carotenoids and β-carotene in samples treated by HW processing
were higher than in those by treated by MW processing. For tomatoes, no significant differences
were found in the color attributes (L*a*b* values), texture, ascorbic acid, or lycopene content of
samples processed by either method using equivalent processing conditions. Adding CaCl2 to
176
tomatoes significantly increased the texture retention and lycopene content in the processed
products. Results showed that the impact of MW processing on the quality of vegetables
depended on the characteristics of the vegetables and the specific quality parameters tested.
Keywords: Microwave processing; carrot; tomato; quality; equivalent
177
1.
Introduction
One of the recent trends in fruit and vegetable processing is using advanced preservation
techniques (e.g., high pressure, microwave, ohmic heating and pulsed electric fields) combined
with new packaging materials and technologies, which help provide consumers with more food
choices (Barrett & Lloyd, 2011; Dauthy, 1995). Microwave (MW) heating can rapidly raise the
temperature inside foods to the desired sterilization or pasteurization temperature (Sun et al.,
2005; Tang et al., 2002). Compared to conventional retorting, MW heating requires a relatively
short heating time (e.g. several minutes) due to its ability to generate volumetric heating within
food materials, and thus has the potential to produce high quality shelf-stable products for food
pasteurization and sterilization processes.
The application of MW heating in food has drawn increased attention over the past few decades.
Many researchers have reported the application of MW heating in vegetable processing and its
effect on a nutrients and quality, such as in carrot juice (Rayman and Baysal, 2011), carrot pieces
(Lemmens et al., 2009), Brussels sprouts (Olivera et al., 2008; Vian et al., 2007), potatoes
(Alvarez and Canet, 2001; Barba at el., 2008), peas and spinach (Hunter and Fletch, 2002),
tomatoes (Begum and Brewer, 2001), Swiss chard and green beans (Villnaueva et al., 2000),
asparagus (Sun et al., 2007), and sweet potato purees (Steed et al., 2008). In most of these
publications, research was carried out in a 2450 MHz domestic MW oven or a modified MW
oven with specially installed temperature sensors. Different heat treatment levels were achieved
by adjusting input power or heating time; normally several minutes are required for blanching or
pasteurization for enzyme inactivation. Limited information is available on the quality of
vegetables processed by MW sterilization and pasteurization. Sun et al. (2007) processed
178
asparagus packaged in 8 oz. pouches by a MW-circulated water combination (MCWC) heating
system and pressurized HW heating (both with F0=3 min), and steam-heating in a retort using the
industrial standard method (121°C for 17 min). They evaluated the quality and antioxidant
activity of the sterilized asparagus and found asparagus processed by MCMC had greater
antioxidant activity and greener color than samples treated by the other two methods. Steed et al.
(2008) sterilized pumpable sweet-potato purees using a continuous flow MW-assisted processing
and aseptic packaging. Compared to unprocessed purees, samples processed by continuous MW
heating using a 60 kW systems howed an increase in total phenolic content and gel strength, a
decrease in anthocyanins, and good retention of the overall color change and antioxidant
activity(Steed et al. 2008). Koskiniemi et al. (2013) evaluated the quality of acidified vegetables
(broccoli, red bell pepper, and sweet-potato) pasteurized by continuous MW processing with a
rotation apparatus and observed good retention of color and after MW pasteurization. The US
Army Natick Soldier Center conducted accelerated shelf-life/sensory studies on three products
processed at Washington State University: chicken breast in 10-oz. trays in 2004; chicken and
dumplings in 8-oz. pouches, and chicken and pasta in 10-oz. trays in 2012. All three MW
processed chicken and chicken products had significantly higher hedonic scores (a sensory
assessment) than those processed by conventional retort processing with an equivalent process
severity (F0=6 min) over a 6-month storage at 100°F (equivalent to three-year storage at 80°F).
These results suggested that MW processed chicken products had a better overall acceptance by
sensory evaluation (data not published). However, there are no systematic studies on the
influence of MW sterilization and pasteurization on the quality of tomatoes and carrots.
179
Tomatoes and carrots are two of the most commonly consumed vegetables in the US. Threefourths of the tomatoes consumed by Americans are in processed form, most of which are
thermally processed; and one-fourth of carrots are consumed in processed form, largely canned
and frozen (Lucier and Glaser, 2009; Lucier and Lin, 2007). In addition to the popularity of the
two vegetables, another reason for selecting carrots and tomatoes was the differences in their pH
and therefore the extent of β-elimination reaction proceeding in the vegetables under high
temperatures. β-elimination is a non-enzymatic reaction that takes place at high pHs (>4.5) and
high temperatures (>80ºC). Studies shows that the early, rapid phase of texture loss in both
tomatoes and carrots under high temperatures is due to the turgor loss resulted from the loss of
membrane integrity, while a 2nd slower, prolonged phase of softening occurred was observed in
carrots, and mainly contributed to the breakdown of pectins through β-elimination reactions
(Grant et al., 1973; Gonzalez et al., 2010; Greve et al., 1994ab; Anthon and Barrett, 2005).
Because the pH of the carrots (5.2-5.8) is much higher than that of the tomatoes (3.9-4.4), βelimination in carrots under thermal processing is much more noticeable than in tomatoes
considering the high pH (pH 4.5) required for this reaction to take place. However, no
information is available on quality changes in tomatoes or carrots processed by microwave
sterilization or pasteurization.
In the current study, diced tomatoes and carrots were pre-packaged in 8-oz pouches with added
salts (NaCl and CaCl2), in accordance with common commercial practices. Pouches were
pasteurized or sterilized by a 915 MHz pilot-scale MW system in a batch process. Tomato or
carrot products heated by conventional HW processing were conducted on an equivalent basis,
achieving the same level of target microbial inactivation. Quality attributes of the products
180
processed with MW and HW heating were compared. This study illustrates the application of
MW pasteurization and sterilization to pre-packaged tomato and carrot products and compares
the impact of MW and HW processing on the products’ quality.
2. Materials and Methods
2.1. Sample preparation
Fresh carrots with their peel (Cultivar Imperator, from Bolthouse Farms, Inc., Bakersfield, CA)
or Roma tomatoes were purchased from a local grocery store (Safeway, Pullman, WA) and
stored at 4ºC. Carrots were cut into dices using a Hallde Flexi RG-7 dicer (Hicksville, NY) and
processed immediately after. A total of 136.5 ± 0.5 g diced carrots and 90.5 ± 0.5 g CaCl 2
solution (0.1% or 1.4%, w/w, total samples in each pouch) were filled into each 8-oz laminate
pouch (18.5 × 13.2 ×1.6 cm, Printpack Inc., Atlanta, GA).
Only tomatoes with dark red color (‘a’ value between 22 and 25) were selected, washed, and cut
into quarters. The pericarps were diced into 12.7× 12.7× (6-8) mm pieces with a food dicer
(Fengxing Trading Co., Ltd, Ningbo, China), and the remainder was blended into puree. Salts
were added to the puree during the blending. Two types of samples were prepared, one with and
one without added CaCl2. For the former, 0.2% NaCl (w/w, total samples in each pouch) and
0.055% CaCl2 (w/w, equivalent to containing 200 ppm calcium) were added. For the latter, only
0.2% NaCl was added to improve the taste. A total of 137.5 ± 0.5 g diced tomatoes and 89.5 ±
0.5 tomato puree with ingredients were filled into each 8-oz laminate pouch.
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All sample pouches were vacuum sealed (Koch Supplies Inc., Kansas city, MO, USA). Prepackaged carrot/tomato pouches were loaded into the MW or HW heating system to process
immediately.
For the processed samples, pH, soluble solids, drain weight, color and texture were measured on
the same day of processing. Sample pouches used to assess the enzymes, carotenoids, ascorbic
acid and lycopene were stored at -30°C until the day of analysis. Frozen samples were thawed at
room temperature (22°C) before analysis. For color and texture assay of the carrot and tomato
dices, samples were drained before analysis, using the procedures described below. For pH,
soluble solids, enzymes, carotenoids, ascorbic acid and lycopene assays, the whole pouch of
carrot or tomato samples was blended until homogenous and used for each measurement.
2.2. Thermal treatments
Based on the kinetic study of carrot texture degradation, 90°C was selected as the pasteurization
temperature for the diced carrot products (Chapter 4). For pasteurization of low-acid foods which
allows a shelf life of up to 6 weeks at 5ºC, a common practice to aim for a 6 log reduction of the
target pathogen psychrotrophic C. Botulinumis suitable (Vervoort et al., 2012; ECEF). This is
called a “6D” process. A general recommendation of F90°C=10 min (or equivalent) is universally
accepted for the pasteurization of most foods, which represents at least 6-log reduction of the
most heat resistant non-proteolytic strains of C. botulinum (Vervoort et al., 2012; ECEF). In the
current study, the target microorganism for carrot products is non-proteolytic C. botulinum type
E spores (Vervoort et al., 2012). Gaze and Brown (1990) studied the thermal resistance of NP C.
botulinum type E spores in carrot from 75–90ºC and reported a D value of 0.48 min at 90°C.
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Based on these data, the required F90°C was 2.88 min to achieve a 6D process for NP C.
botulinum type E spores. Given the two conditions mentioned above, two thermal treatment
levels were used for processing the carrot products: one was F90°C=3 min, which yields a 6-log
reduction of target NP C. botulinum type E spores, and the other process was F90°C=10 min,
which is generally considered an adequate process to pasteurize most foods.
For the diced tomato products, a MW sterilization process was designed to achieve a 5D
reduction in the target bacteria, B. coagulans ATCC 8038 spores. This equated to an F105°C of 6.0
min, which is based on the thermal resistance data of this microorganism obtained in Chapter 3.
A MW assisted thermal pasteurization (MAP) process was developed for diced carrots packaged
in 8-oz pouches using a 14-kW single-mode MW system, while a MW assisted thermal
sterilization (MATS) process was developed for diced tomatoes using a semi-continuous,
915MHz single-mode MW system. Conventional HW processing was also conducted in order to
achieve equivalent process severity with regard to microbial inactivation. Details on the systems
and operation procedures are described in Chapter 6. The processing conditions of the two
products for equivalent MW and HW processes are listed in Table 1.
2.3. Color of carrot and tomato dices
The CIE L*(lightness), a*(redness), and b*(yellowness) color attributes of processed carrot and
tomato dices were determined using a computer vision system following the procedures of Kong
et al. (2007). This system included photography lights, a camera, and a computer with Adobe
PhotoShop to analyze the surface color of food products. A Benchers Copymate copy stand was
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used to fix the camera perpendicular to the food samples. The camera was a Canon EOS 60D
fitted with a Canon EF 100 mm f/2.8 USM macro lens. The Alzo 300 Table Top lighting System
consisted of two parabolic reflectors covered with a diffuser cloth. Each reflector used three Alzo
27 Watt Fluorescent Daylight Balanced light Bulbs with a color temperature of 5500K. A light
meter (Sekonic Corp., Tokyo, Japan) was used to evenly distribute light on the food surface
(12.2 EV). The Canon supplied EOS Utility software was used to remotely control the camera
and to download the color images (.JPG) of the food samples into the computer. The color
images were analyzed using Adobe PhotoShop. In CIE LAB color scale, L* varies from 0-100, a*
and b* between -127 and +127; while these ‘L’, ‘a’ and ‘b’ values under “Lab color model” from
PhotoShop are encoded between 0-255. The color values obtained from Photoshop were
converted to standard CIE L*, a*, and b* values using the following equations (Yam and
Papadakis, 2004):
(1)
(2)
(3)
The hue angle (H) and total color differences (∆E) were calculated by the following equations:
H=tan-1 ( )
∆E=√
(4)
(5)
where the raw samples were used as the control in the calculation of ∆E.
2.4. Texture of carrot and tomato dices
The firmness of the treated, diced carrot pieces was determined using a TA.XT2 Texture
analyzer (Stable Micro Systems Ltd., Godalming, UK) fitted with a 25 mm diameter aluminum
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cylinder probe using the methods described by Lemmens et al. (2009). The samples were
compressed to 70% strain at a cross head speed of 1 mm/s. For each test, one piece of diced
carrot sample was placed under the probe. The peak force of the first compression cycle of the
sample was marked as the maximum force and was the quantitative indicator of sample firmness.
The measurements were made using six replicates for each treatment condition.
The firmness of the diced tomatoes was measured by a TA.XT2 texture analyzer (Stable Micro
Systems Ltd., Godalming, UK) fitted with a mini-Kramer shear cell. A total of 12.30 g drained
samples after processing and 11.30 g fresh diced tomatoes before processing were used. Each
measurement was replicated 6 times according to Anthon et al. (2005) and the maximum
compression force of the first peak was recorded.
2.5. Pectin methylesterase (PME) activity of carrots
PME was extracted from diced carrot samples using a modified method from Vervoort et al.
(2012). Ten g of homogenized sample was mixed with 0.2 M Tris-HCl buffer containing 1M
NaCl (pH 8.0, 1:1.3 w/v) and stirred at room temperature for 30 min.
The PME activity of the sample was quantified as the production of H+ during pectin hydrolysis
as a function of time at pH 7.0 and 30ºC, following the methods by Anthon et al. (2002ab).
Briefly, 30 ml solution containing 0.25 M NaCl and 0.25% citrus pectin was equilibrated to 30ºC
and adjusted to pH 7.0. One ml of homogenized sample was added to the solution and the pH
was readjusted to 7.0 and maintained at this pH for 10 minute by the addition of 0.01 N NaOH.
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The rate was calculated as μmol of NaOH consumed over the 10minute time period. The results
were reported as percentages compared to the raw (unprocessed) control samples.
2.6. Carotene analysis of carrots
The extraction of carotene from the carrot sample followed the methods described by Sadler et
al. (1990) with modifications. Briefly, 5 g of homogenized carrot sample was mixed with 50 ml
extraction solvent hexane with acetone and ethanol (50:25:25) containing 0.1% BHT and mixed
for 20 min. Fifteen ml Milli-Q water was added to the mixture and mixed for an additional 10
min and the mixture was centrifuged at 600 × g for 8 min to separate the organic layer from the
water layer. The organic layer was collected and filtered through a 0.45 μm syringe filter as the
final extract for assay.
The total carotenoid content of the extract was measured by a spectrophotometer (Pharmacia
Biotech Ltd., Cambridge, England) at 450 nm, the maximum absorbance wavelength of βcarotene. Hexane with 0.1% BHT was used as a blank. The total carotenoid concentration of the
extract was calculated by Beer’s law, with the extinction coefficient of β-carotene in hexane
E1%1cm=2560.
The α- and β-carotene contents of the carrot samples were determined by the method described
by Vervoort et al. (2012), using an Agilent RP-HPLC system with a UV-DAD detector. A YMC
Carotenoid column (150×4.6 mm, 5 μm) was used to separate the carotenoids through linear
gradient elution from 100% solution A (81% methanol, 15% methyl-t-butyl ether; 4% milli-Q
water) to 100% solution B (41% methanol, 55% methyl-t-butyl ether; 4% milli-Q water) in 28
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min, held for 5 min, then returned to 100% solution A and equilibrated for 8 min. The flow rate
was 1 ml/min and the detection wavelength was 450 nm. The standards of α- and β-carotenes
were dissolved in hexane and used for standard curves.
The moisture content of the homogenized carrot sample was determined, and the concentrations
of total carotenoids, α- and β-carotenes in carrot samples were calculated on a dry weight basis.
2.7. Determination of pH, soluble solids, drained weight of tomatoes
Analysis of the pH, soluble solids, drained weight and color of the tomato samples were
performed immediately after processing. Fresh tomato samples taken before processing were
used as a control. Total soluble solids were assessed by optical refractometer (Atago Co. LTD,
Japan) expressed as °Brix. Drained weight was determined according to 21 CFR 155.190Canned tomatoes (FDA 2005). The whole sample pouch was emptied onto a tilted U.S. #8 sieve,
and samples were distributed as uniform as possible over the screen. The screen was tilted with a
height of 2 inch. The weight of drained samples after 2 min draining on the screen was drained
weight. The ratio of drained weight to the net weight of samples (weight before draining) was
recorded as the percentage of sample drained weight (%).
2.8. Ascorbic acid content of tomatoes
Ascorbic acid content of the tomato samples was determined following the procedure described
by Nisperos-Carriedo et al. (1992). Briefly, 50 g thawed sample was blended with 50 ml 0.05 N
H3PO4 for 3 min, and centrifuged at 10,000 × g for 10 min. The supernatant was collected and
diluted to 100 ml. Three ml extract was purified by passing a 3cc C18 Sep-Pak cartridge which
was preconditioned by flushing with 2 ml acetonitrile followed by 5 ml Milli Q water. The
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extract was filtered through 0.45 µm syringe filter before injection. Agilent 1100 HPLC system
(Santa Clara, CA) equipped with a XTerra RP18 column (4.6 × 250 mm; 5 µm) from Waters
(Milford, MA) was used for the separation and maintained at 30°C during analysis. Separation
was performed using a 2% KH2PO4 as the mobile phase in the isocratic mode. The flow rate was
0.4 ml/min, and the detection was performed at 260 nm using a diode array detector.
2.9. Lycopene content of tomatoes
Lycopene in the processed tomato samples was extracted and quantified using a modified
method from Halim and Schwartz (2006) and Gupta et. al (2010). Briefly, one gram thawed
sample was mixed with 0.2 g CaCO3 and homogenized with 10ml methanol at 20,000 rpm for 1
min. The sample was centrifuged at 10,000 × g for 10 min, and the supernatant was discarded.
The pellet was re-suspended in 4 ml hexane/acetone (1:1 v/v) solution, 4 ml Milli Q water was
added to induce phase separation. The nonpolar layer was collected and successive extraction
was repeated 3 times. The collected fraction was made up to 10 ml with the extracting solvent
and 1 ml extract was dried under nitrogen. Next, the sample was re-dissolved in 6 ml
methanol/MTBE (1:1 v/v), and filtered through 0.45 µm syringe filter before injection. A YMC
Carotenoid column (4.6 × 250mm; 5 µm) from Waters (Milford, MA) was used for the
separation. The mobile phase was made of A solvent (methanol/MTBE/2% aq. ammonium
acetate, 88:5:7, v/v/v) and B solvent (methanol/MTBE/2% aq. ammonium acetate, 20:78:2,
v/v/v). Separation was achieved by gradient elution of 0-85% B in 20 min, followed by a 10 min
linear gradient to 100% and hold for 5 min, then returning to 0% B and holding for 15 min. The
detection was performed at 471 nm using a UV-Vis detector. All analysis was performed under
dim light to avoid sample degradation.
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2.10. Statistical analysis
Statistical analysis was conducted using SAS 9.2 (SAS Institute Inc., 2008). One-way analysis of
variance (ANOVA) was used for all data. Differences among treatments by means were
determined by least significant difference (LSD) multiple comparison test with significance
defined as P < 0.05.
3. Results and Discussion
3.1. Color
The CIE LAB color values of raw and processed carrot and tomato dices are given in Table 2.
For processed carrots, color values varied depending on the treatment type. Processing
significantly decreased a* values of all carrot samples, and the carrot samples in 1.4% CaCl2
solution after the F90°C=10 min HW processing had the lowest a* value.TheL* and b* values of
carrot samples were relatively less affected by thermal processing. No significant difference was
found in b* values between the samples before and after processing, except for the samples in
0.1% CaCl2 solution after the F90°C=3 min MW processing. Samples in 1.4% CaCl2 solution with
an F value of 3 min by MW processing had a significantly higher L* value compared to the raw
material, while no significant difference was found in samples from other treatments. The
changes in the a* and b* values indicate the a deterioration of the initial intense orange color of
the untreated carrots (Vervoort et al., 2012). The b*/a* value, which is used to represent the
yellowness of carrots, is shown in Fig. 1A. A significant increase in the b*/a* values was found
for processed samples as compared to the untreated samples. No significant differences were
found between samples heated by MW and HW under the same conditions, except for samples in
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1.4% CaCl2 solution with F90°C=10 min. For the hue angle of carrots, raw samples had the lowest
value, which was significantly different from all processed samples, except those in 0.1% CaCl 2
solution with F90°C=3 min by microwave processing. Changes in color were likely related to the
degradation and isomerization of carotenoids during thermal processing, since carotenoids in
carrots are responsible for their orange color. Another possible reason can be due to the
expulsion of air from the tissue matrix during thermal process. This results in the cooking
solution flooding the intercellular spaces where the air was, and makes the cells more translucent
and therefore may affect their L* and a* values.
The color of tomatoes was stable over the course of processing. Neither MW nor HW processing
significantly affected the L*a*b* values, although a decrease in all of these values was observed
compared to the raw samples. The a*/b* values (red-yellow ratio), commonly used to represent
the redness of tomatoes and tomato products, are shown in Figure 1B. There was a slight
decrease in the a*/b* value after processing, but no significant differences existed between
samples before and after processing. The processed samples had an increased hue angle, but
again, no significant differences were found among samples with different treatments. The
stability of the tomato color during thermal processing can be attributed to the heat stability of
lycopene, which is the main pigment responsible for the red color of tomatoes. Shi (2002)
reported that lycopene was relatively stable if heated at temperatures below 100°C, but the
duration of heating must be taken into consideration. These results indicated that heating at
105°C for up to 20 min did not significantly affect the color of diced tomatoes.
Total color differences between the raw and processed carrot and tomato samples are
summarized in Table 3. Theoretically speaking, a ∆E of 1 represents a barely-noticeable color
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difference to the human eyes under ideal viewing conditions; while ∆E values between 2 and 3
are considered equivalent by some viewers in less than ideal lighting (Vervoort, 2012). From
Table 3 it is clear that all the ∆E values were higher than 3, suggesting that the color differences
between the raw and processed carrot or tomato samples are perceptible to the human eye under
normal lighting conditions. Diced carrot samples in 1.4% CaCl2 solution processed by HW with
F90°C=10 min had the largest ∆E value, denoting the least color retention of the carrots. The ∆E
values of processed carrots varied from 5.84 to 10.32, and all carrot samples processed by
microwave heating had lower ∆E values than those processed by hot water under same
conditions, denoting a better color retention. This result could be due to the shorter heating time
of microwave processing compared to hot water processing.
3.2. Texture
Texture changes of diced carrot and tomato samples before and after processing are shown in
Fig. 2. Significant decreases in the texture before and after processing were found in both carrot
and tomato products under all processing conditions. For diced carrots, the texture loss was 2730% for samples in 0.1% CaCl2 with F90°C=3 min, 16-21% for those in 1.4% CaCl2 with F90°C=3
min, 45-50% for those in 0.1% CaCl2 with F90°C=10 min and 21-25% for those in 1.4% CaCl2
with F90°C=10 min. The firming effort of CaCl2on texture was apparent based on these results;
the higher the concentration, the better retention of carrot texture. However, no significant
difference was found between carrot samples heated by MW and HW processing with the same
process severity.
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For diced tomatoes, samples processed by MW and HW heating with added CaCl2 resulted in a
60% and 64% decrease in texture, respectively. For samples processed by MW heating without
added CaCl2, the texture was almost completely lost (90% reduction), which demonstrates the
important role of calcium in texture retention of tomato products. However, no significant
difference was found for diced tomato processed by MW or HW with the same processing
severity. The explanation for this result could be that the degradation of tomato texture occurs at
the very beginning of high temperature processing (105°C). To achieve the 5D process, even a
total processing time of 5.30 min with MW heating could only retain around 2/3 of the initial
texture. Ma and Barrett (2001) found that when holding diced tomatoes at 88°C, a large
reduction in firmness occurred in the 1st min of heating and then remained almost unchanged
over the next 10 min. Greve et al. (1994ab) found that a fast phase of texture loss in vegetables
occurs in the first minute at temperatures near 100°C and this can be attributed to a loss of turgor
resulting from the breakdown of the cellular membranes.
3.3. PME activity of carrots
Pectin methylesterase (PME) is one endogenous enzyme that plays an important role in
stabilizing the cell wall structure of carrots. PME can catalyze the de-esterification of pectins,
creating binding sites for divalent cations, primarily Ca2+ (naturally present in the tissue or added
during processing). The binding sites are on the polygalacturonic acid backbone of the pectin and
the addition of calcium allows the formation of cross-linkages between pectin chains, which
improves the texture. In this study, no PME activity was detected in the processed products.
Results indicated that after a preheating treatment of 60°C for 20 min, heating at 90°C for 3.22
min resulted in a complete loss of PME activity in the processed carrots. This finding is in
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accordance with published papers that indicate PMEs are heat sensitive. Anthon and Barrett
(2002) studied the kinetics of thermal inactivation of PMEs in carrot juice and reported D 65.7°C=
5min for the labile form and D70.5°C=5min for the resistant form of PME. Lemmens et al. (2009)
studied the thermal pretreatments of carrot pieces using different heating techniques and reported
no PME activity detected after blanching at 90°C for 4 min.
3.4. Carotenoids of carrots
Carotenoids are responsible for the orange color of carrots. They are also one of the important
bioactive compounds in carrots, known as vitamin A precursors, and can act as antioxidants to
reduce the risk of developing degenerative diseases. α- and β-carotene are the two major
carotenoids in carrots. The total carotenoids, α-carotene, and β-carotene contents in raw and
processed carrot products are shown in Fig. 4. In raw carrots, the dry weight of the total
carotenoids, α-carotene, and β-carotene contents were 130.13 ± 3.35, 34.15 ± 1.04 and 93.64 ±
3.12 mg/100 g, respectively. Both MW and HW processing significantly decreased the contents
of the total carotenoids, α-carotene, and β-carotene in the carrot samples; the decrease depended
on the intensity of heating. Applied heat treatments with F90°C=3 min caused a loss of 15-23% in
total carotenoids compared to the initial value, while the loss increased to 19-35% for those with
F90°C=10 min. For β-carotene, the loss for processes with F90°C=3 min was 11-20%, and the loss
increased to 17-37% when the processing intensity was increased to F90°C=10 min. The αcarotene seemed to be more heat sensitive than β-carotene at the beginning of a thermal process.
The loss in α-carotene for processes with F90°C=3 min was 22-31% and stayed relatively stable
with increased processing intensity, as the loss value for F90°C=10 min was 30-33%. In most
cases, no significant differences were found in the carotenoids for samples processed by MW
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and HW heating. However, for carrots dices in 1.4% CaCl2 solution by processing with F90°C=10
min, the total carotenoid and β-carotene loss by MW heating was larger than HW processing.
One possible explanation of this result could be that the longer heating time of HW processing
caused more cell disruption of carrot tissue, which resulted in an improved extractability of the
carotenoids. Veervoort et al. (2012) also observed decreased α- and β-carotene content in carrot
products going from mild pasteurization at 70°C to sterilization. However, Knochaert et al.
(2011) reported an increased β-carotene content in sterilized carrots (F0=3min).
3.5. pH, soluble solids, drained weight of diced tomatoes
Results of pH, soluble solids, drained weight for MW and HW processed diced tomatoes are
given in Table 3. Three types of samples were compared, MW processed with added CaCl 2, MW
without added CaCl2, and HW processed with added CaCl2. Samples before processing were
used as controls. The pH of each processed sample was well controlled between 4.30-4.40,
without significant differences. The soluble solids content of the three processed samples were
around 4.6-4.7°Brix and no significant difference was found among samples. For drained weight,
no significant differences were observed among raw and processed samples with CaCl2, yielding
relatively higher values around 76-79%.
However, HW processed samples without CaCl2
showed a much lower drained weight of 70.31 ± 2.05% compared to the other samples. The
higher drained weight of samples processed with CaCl2 could be due to the interaction of calcium
with the pectins in the cell wall, which helped maintain the cell structure during processing so
the samples didn’t lose as much cell contents. This agrees with findings from Bradley (1966),
which showed cell-wall rigidity had an important effect on the final drained weight.
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3.6. Ascorbic acid content of tomatoes
Ascorbic acid is the one of the most important micronutrients in tomato fruits and tomato
products and has numerous physiological benefits with antioxidant values. The ascorbic acid
content in sterilized tomato samples (with and without CaCl2) processed by MW or HW heating
is shown in Fig. 5. The initial concentration of ascorbic acid in raw tomato samples was 7.31 ±
1.22 mg/100g. The value was a little lower than the concentration range reported in other
varieties, which was 10-22 mg/100g fresh matter (Abushita et al., 2000; Marfil et al., 2008; Van
den Broeck et al., 1998). Possible reasons for the relatively low concentration in the raw samples
might be they were less mature tomatoes or grown in cold weather and low light. A decrease of
around 1 mg/100g of ascorbic acid was found in samples processed by either MW or HW with
the addition of calcium, around 14% loss compared to the initial value. Although the amount of
ascorbic acid retained in samples without calcium added was slightly lower than the other two
processing treatments, no significant difference was found between samples processed with and
without calcium. Van den Broeck et al. (1998) studied the thermal degradation kinetics of Lascorbic acid in squeezed tomatoes by heat (120-150°C), and reported D120°C=325-469 min
depending on the variety, and a z value of 30.15°C. Based on these data, D105°C of ascorbic acid
in squeezed tomatoes is calculated to be 1018-1469 min, higher than our results if same
degradation tendency was applied.
3.7. Lycopene content of tomatoes
Lycopene contents of unprocessed and thermally processed tomato samples using MW and HW
are shown in Fig. 6. Total lycopene content in tomatoes decreased in processed samples; the
initial content was 17.45 mg/100g, which decreased to 15.25 (MW processed with CaCl2) and
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15.87 (HW processed with CaCl2). Processed samples without CaCl2 showed a much lower
value (9.08 mg/100g), which was almost a 48% loss. The lycopene lost in processed tomato
samples with added CaCl2 was relatively small and did not differ significantly from the raw
samples, but did have significantly lower values than those without CaCl2 added. One possible
explanation of this result was that the protective effect of calcium on the structural integrity
strengthened the cell walls and better maintained the integrity of carrot cells, therefore reduced
the exposure of lycopene to oxygen in the tissue during the heating process, and resulted in less
lycopene loss. Shi et al. (2003) reported a greater stability of lycopene in tomato puree heated at
100°C or below for within 60 min, with a loss of 3.47% or less.
4. Conclusions
For carrots, the total color differences between all processed and raw products were perceptible
by the human eye under normal viewing conditions. However, samples processed by MW
heating had lower ∆E values than HW heated samples with the same process severity, indicating
the MW heated samples had better color retention. The impact of processing on carrot texture
was significant, but no significant difference in texture retention was found between MW and
HW heated samples treated with the same process severity. Both processing methods completely
inactivated the PMEs in carrots. All processes lowered the total carotenoid, α-carotene, and βcarotene content in carrots. No significant differences were found in the carotenoid content in
samples processed by either method in most cases. However, for diced carrots in high CaCl2
solutions (1.4%) subject to intense processing (F90°C = 10 min), the total carotenoids and βcarotene in samples heated with hot water processing had higher values than those heated with
microwave processing. One possible explanation for this result could be that the longer heating
196
time of hot water processing caused more cell disruption of carrot tissue, which resulted in an
improved extractability of the carotenoids.
For tomatoes, no significant differences were found in the color attributes (L*a*b* values),
ascorbic acid, or lycopene content of samples processed with MW and HW with equivalent
processing severity. Similar to carrots, the impact of processing on the texture loss of tomato
samples was significant. However, the texture loss in samples treated with both processing
methods with the same severity did not show significant differences from each other. Addition of
CaCl2 to tomatoes significantly increased texture retention and increased the lycopene content in
the processed products.
These results indicate that the impact of MW heating on quality attributes of vegetables depends
on the characteristics of the vegetables and the specific quality parameter tested. Some quality
parameters, like texture and enzymes, are quickly reduced, degraded, or inactivated during a very
short time at the beginning of the process (temperature at 105ºC for tomatoes and 90°C for
carrots). In this case, even the MW processing time was not short enough to retain these
characteristics. There are some relatively heat-stable quality parameters, such as lycopene in
tomatoes; results showed that a 20 min holding time at 105ºC was not long enough to lead to a
significant reduction in its total concentration. Other quality parameters have intermediate heatstability, such as color in carrots. Carrots heated with MW processing showed better color
retention than HW processing. These results can be used in optimizing thermal processing for
carrot or tomato products.
197
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Table 1. Processing conditions for carrot and tomato products for MW and HW processes with
equivalent process severity.
Processing Target
Water temperature
setting (preheating,
MW
heating, holding and
processing cooling sections)
MW power setting
Preheating time
MW heating time
Holding time
Carrot
Total processing time
(pasteurization)
Real F value
Preheating time
HW
Total processing time
Processing Real F value
Processing Target
Tomato
(sterilization)
Water temperature
setting (preheating,
MW
heating, holding and
processing cooling sections)
MW power setting
Preheating time
MW heating time
Holding time
Total processing time
Belt speed
Water temperature
setting (preheating,
HW
heating, holding and
processing cooling sections)
Preheating time
Total processing time
F90°C = 3 min
F90°C = 10 min
61/93/93/15°C
14 kW
20 min
1.36 min
1.86 min
3.22 min
4.9 min
14 kW
20 min
1.46 min
3.50 min
4.96 min
8.9 min
20 min
7.80 min
3.9 min
20 min
13.96 min
13.4 min
5D process
(F105°C = 6.05 min)
56/108/107/15°C
7.0/6.2/2.6/2.5 kW
15 min
2.71 min
2.79 min
5.50 min
47 inch/min
56/108/107/15°C
15 min
20.1 min
The pressure of the sterilization system was 33 psig, and the pasteurization system was
atmospheric pressure.
203
Table 2. CIE L*, a*, b* values, total color differences (∆E) and hue angles of carrot and tomato
dices following different treatments. The color attributes of fresh carrots were used as the
control.
Sample
carrot
tomato
L*
a*
b*
Hue angle
Total color difference
(H)
(∆E)
Raw
61.68 ± 2.48a
48.32 ± 3.16a
59.51 ± 2.62abc
0.89 ± 0.01a
Ref
F3min, MW, 0.1% CaCl2
61.15 ± 3.85a
42.60 ± 4.65b
57.05 ± 4.45d
0.92 ± 0.02ae
6.25
a
b
ab
61.14 ± 2.16
0.97 ± 0.04
bcd
6.51
F3min, HW,0.1% CaCl2
61.17 ± 2.35
42.04 ± 4.86
F3min, MW, 1.4% CaCl2
63.07 ± 2.42c
41.84 ± 4.08bc
58.64 ± 3.31c
0.95 ± 0.03bce
6.69
F3min, HW, 1.4% CaCl2
62.24 ± 2.86ab
39.79 ± 4.24bc
60.83 ± 2.35ab
0.99 ± 0.05bd
8.65
ab
b
bc
bcd
5.84
F10min, MW, 0.1% CaCl2
62.92 ± 2.78
42.62 ± 4.22
59.23 ± 3.17
F10min, HW, 0.1% CaCl2
61.19 ± 2.78a
40.16 ± 3.45bc
59.17 ± 2.50bc
0.97 ± 0.03bd
8.18
F10min, MW, 1.4% CaCl2
ab
62.16 ± 2.61
42.26 ± 5.83
bc
cd
ce
6.30
F10min, HW, 1.4% CaCl2
62.57 ± 3.04ab
38.40 ± 5.41c
61.62 ± 2.73a
1.01 ± 0.06d
10.32
Unprocessed w/ CaCl2
48.50 ± 6.44a
48.45 ± 7.06a
53.34 ± 3.21a
0.83 ± 0.08a
Ref
MW w/ CaCl2
44.95 ± 9.00a
40.61 ± 6.51a
50.44 ± 5.37a
0.89 ± 0.09a
9.08
a
a
a
a
7.74
8.83
57.88 ± 3.21
0.99 ± 0.02
0.94 ± 0.05
MW w/o CaCl2
47.51 ± 6.63
41.00 ± 6.78
51.52 ± 3.90
0.90 ± 0.09
HW w/ CaCl2
45.40 ± 8.33a
40.57 ± 6.77a
50.83 ± 4.80a
0.90 ± 0.09a
204
Table 3. pH, drained weight, color, soluble solids of tomato samples before and after processing.
pH (after processing)
°Brix
Drained weight,
%
Unprocessed w/ Ca
4.6 ± 0.0
a
76.16 ± 1.17a
MW w/ Ca
4.32 ± 0.02a
4.7 ± 0.1a
78.04 ± 1.51a
MW w/o Ca
4.36 ± 0.01a
4.7 ± 0.0a
70.31 ± 2.05b
HW w/ Ca
4.30 ± 0.01a
4.7 ± 0.1a
79.13 ± 2.13a
Note: The pH of diced samples before processing was 4.27±0.01, and the corresponding
electrical conductivity was 8.42 ± 0.17 mS/cm. Calcium concentration was 0.02% (w/w) for both
HW and MW processed samples.
205
Figure 1. Color parameter b*/a* of diced carrots (A) and a*/b* of tomatoes (B) following different
treatments. Columns labeled with the same letters are not significant different (p<0.05).
206
Figure 2. Texture of diced carrots (A) and diced tomatoes (B) treated by MW and HW
processing under different conditions.
207
Figure 3. Residual PME activity of carrot dices by MW and HW processing under different
conditions.
208
Figure 4. Total carotenoid content, α-carotene, and β-carotene contents of diced carrots
processed under different treatments.
209
Figure 5. Ascorbic acid content of tomato samples processed under different conditions.
210
Figure 6. Lycopene content of tomato samples under different conditions.
211
Chapter 8. Conclusions and Recommendations
1.
Major conclusions
1.1
B. coagulans ATCC 8038 strain can produce consistent heat-stable spores during
refrigerated storage. The spores of this strain had higher thermal resistance compared to strain
185A, and ATCC 8038 was therefore chosen as the target bacterium for developing and
validating thermal processes of tomato products.
1.2
Thermal degradation of carrot texture with pretreatments (preheating and calcium
addition) under temperatures ranging from 80 to 110ºC followed a 2nd order reaction.
1.3
The dielectric loss factors were significantly different among the three tomato tissues
studied (pericarp tissue with the skin, locular tissue with the seeds, and placental tissue), and
among samples with and without salt. However, no significant differences were found in their
corresponding dielectric constants.
1.4
Salt addition at the typical commercial canned tomato product level (0.2gNaCl/100gor
0.055 gCaCl2/100g) sharply increased the loss factor of the three tomato tissues, but didn’t affect
their dielectric constants at MW frequencies of 915 and 2450 MHz.
1.5
Similar trends for changes in dielectric loss factor were observed for the three tomato
tissues, that is, it decreased with increasing frequency, and increased with salt addition. For the
effects of temperature, increasing temperature resulted in an increase in dielectric loss factor at
915 MHz; while at 2450 MH, temperature increase initially caused an increase, followed by a
decrease of dielectric loss factor, resulting from different dominant loss mechanisms at the two
frequencies.
1.6
The cold spot of tomato pouches was located at (22.8, -2.2) mm from the central point in
the middle layer; while the cold spot was located in the center point (0, 0) mm in carrot pouches.
212
1.7
A MATS process achieving a target F value of no less than 6 min was developed for
processing diced tomatoes packaged in 8-oz pouches, which can deliver a 5D thermal treatment
to B. coagualans ATCC 8038 spores.
1.8
For diced carrots, MAP processes with F90°C= 3 min and F90°C=10 min were developed to
achieve at least a 6 D reduction of NP C. botulinum type E spores.
1.9
Incubation tests and microbial analyses of the processed tomato pouches verified the
safety of products produced by the developed MATS processes.
1.10
Compared to raw carrots, carrots processed by MW heating had lower total color
difference (∆E values) than those produced by HW processing under equivalent processing
conditions, denoting better color retention.
1.11
The impact of processing on carrot texture was significant, but no significant difference
was found in texture retention after MW and HW heating under the same conditions.
1.12
For carotenoids in carrots, all processes lowered levels of total carotenoids, α- and β-
carotene contents in carrots. No significant differences were found in carotenoid content between
samples subjected to MW and HW processing in most cases.
1.13
For tomatoes, no significant differences were found in the color attributes (L*a*b* values),
ascorbic acid, and lycopene content of samples processed by MW and HW on an equivalent
basis.
1.14
The impacts of MW heating on quality attributes of vegetables depend on the
characteristics of the vegetables and the specific quality parameter selected. For some quality
parameters like texture or enzymes, which would be quickly reduced, degraded, or deactivated
during a very short time at the beginning of the process (temperature at 105ºC for tomatoes and
90°C for carrots), even the MW processing time was not short enough to retain these
213
characteristics. For some relatively heat-stable quality parameters like lycopene in tomatoes, our
results show that a 20 min holding time at 105ºC was not long enough to lead to a significant
change in its total concentration. For some parameters, like color in carrots, MW processing
showed a better color retention than HW processing.
2.
Contributions to knowledge
2.1
The thermal resistance of a contaminant (B. coagulans spores) in tomato juice at different
acidic pH levels between 4.0 and 4.5 (the commonly controlled pH range for canned tomato
processes) under high temperatures were characterized. It included cold storage influence on
thermal resistance of the spores, which has not been reported before. This information is useful
for development of thermal processing of tomato products (Chapter 3).
2.2
The obtained kinetic model for carrot texture degradation was used to draw the texture
retention/microbial or enzyme inactivation charts for carrot processing over a relatively large
temperature range (80-110°C), which provide information for recommending processing
conditions for carrot products that could control food pathogens and inactivate enzymes (Chapter
4).
2.3
Obtained results of dielectric properties of different tomato tissues may be used for
developing MW pasteurization and sterilization processes for different tomato products, and also
add new information to the database for computer simulation (Chapter 5).
2.4
This study provides systematic studies on developing pilot-scale MW sterilization
processes for pre-packaged tomato dices and MW pasteurization processes for pre-packaged
carrot dices, and for evaluating the influences of MW sterilization/pasteurization on their quality
attributes (Chapters 6 &7).
214
2.5
Quality evaluation results of the MW/HW processed products suggest that the impacts of
MW processing on the quality of vegetables depend on the characteristics of the vegetables and
their specific quality parameters. This provides information for choosing suitable vegetables and
vegetable products for MW processing in later studies (Chapter 7).
3. Recommendations for future research
3.1
A storage/shelf-life study should be carried out after the pasteurization of carrot products,
and the effects of storage conditions (temperature/time) on the quality of pasteurized products
should be investigated.
3.2
Microbial safety and stability of the pasteurized products should be assessed, along with
the quality/sensory changes with storage to determine the shelf-life of pasteurized products. This
is required for a developed pasteurized product to become a potential commercial product.
215
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