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Polymeric-based multilayer food packaging films for pressure-assisted and microwave-assisted thermal sterilization

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POLYMERIC-BASED MULTILAYER FOOD PACKAGING FILMS FOR PRESSUREASSISTED AND MICROWAVE-ASSISTED THERMAL STERILIZATION
By
SUMEET DHAWAN
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 2013
UMI Number: 3587072
All rights reserved
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a note will indicate the deletion.
UMI 3587072
Published by ProQuest LLC (2013). Copyright in the Dissertation held by the Author.
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To the Faculty of Washington State University:
The members of the Committee appointed to examine the dissertation of SUMEET
DHAWAN find it satisfactory and recommend that it be accepted.
Shyam S Sablani, Ph.D., Chair
Gustavo Barbosa- Cánovas, Ph.D.
Juming Tang, Ph.D.
ii
ACKNOWLEDGEMENTS
I would like to sincerely thank everyone who has helped me complete my Doctoral
degree in Biological and Agricultural Engineering. I would like to express my deepest
appreciation for my committee chair and academic adviser Dr. Shyam S Sablani for providing
me this great opportunity to work with him in his lab and guiding me throughout my program
here at WSU. His constant encouragement and scientific advice have been instrumental in
helping me complete my research work.
I am very grateful to have two Distinguished Food Engineers in my committee: Dr.
Gustavo V Barbosa-Cánovas and Dr. Juming Tang for their valuable suggestions and comments
on my research. They invested a lot of their hours for providing me with technical assistance on
different areas of my research. They also persuaded me to work beyond my comfort zone and I
have learned a lot from the guidance of my committee members.
I am grateful to Dr. Farida Selim from the Department of Physics for helping me conduct
PALS related experiments and for being an excellent mentor. Dr. Selim also provided valuable
comments on the manuscripts related to my PALS work. Special thanks to the pilot plant
manager, Mr. Frank Younce for assisting in high pressure related studies. I would like to
acknowledge technical assistance of Jonathan P Lomber, Galina Mikhaylenko, Xiaoqiao Lu,
Zhouhong Wang, Feng Liu, Zhongwei Tang, and Matthew Smith of Washington State
University. I would like to thank Robert Armstrong and Masakazu Nakaya of EVAL Company
of America for providing the packaging materials for testing. I would also like to thank Scott
McGregor from Shield Bag and Printing Co. for providing packaging materials and technical
assistance with my research.
iii
I would like to thank all my former and current colleagues for their everyday assistance.
Special thanks to Dr. Roopesh M S, Dr. Gopal Tiwari, Dr. Prabhakar Singh, Dr. Ofero Caparino,
Dr. Khanah Mokwena, Dr. Fermin R, Luis Bastarrachea, Pradeep Suriya, Sunil Kumar, and
Kanishka Bhunia for all their valuable advice and help in my experiments and providing moral
support. I would like to thank all the members of Dr. Tang’s laboratory for providing me help
with instruments present in their lab. The members of the Food Engineering Club are not to be
forgotten for their support throughout my research.
My warm appreciation and thanks to all the administrative and technical staff from
Biological Systems Engineering at WSU: John Anderson, Pat Huggins, Joan Hagedorn, Pat
King, Wayne Dewitt, and Vince. I acknowledge the staff of School of Food Science, and
Dr.Valerie from the Francheschi Microscopy and Imaging Center. I am grateful to the
scholarship agencies (WSU Graduate School, Puget Sound IFT, Washington State Potato
Foundation, Department of Biological Systems Engineering at WSU, and IFT) from which I
received the much needed financial support during my graduate school.
Last but not the least, I express my deepest gratitude to my parents, brother and sister-inlaw who have sacrificed a lot towards my education. Without their moral support, it was not
possible to accomplish this research. I thank all the friends I made in Pullman since the time I
have been here.
iv
POLYMERIC-BASED MULTILAYER FOOD PACKAGING FILMS FOR PRESSUREASSISTED AND MICROWAVE-ASSISTED THERMAL STERILIZATION
Abstract
by Sumeet Dhawan, Ph.D.
Washington State University
May 2013
Chair: Shyam S Sablani
Advanced food technologies such as Microwave-Assisted (MATS) and Pressure-Assisted
Thermal Sterilization (PATS) of foods have the advantage of reducing processing times and the
detrimental effects on food quality. However, these processes require food to be processed inside
their packaging and thus, the interaction between food and its packaging during processing must
be studied to ensure package integrity. Gas barrier, thermal, morphological, and free volume
properties are critical packaging characteristics that help determine packaging selection for the
advanced thermal processes. Selecting the appropriate packaging material will help extend the
shelf-life of foods processed with such technologies. The overall objective of this study was to
investigate the performance of multilayered polymeric films after MATS and PATS in terms of
gas barrier, morphological and free volume properties. Influence of microwave processing on
silicon (Si) migration from metal-coated multilayer polymeric films into selected food simulating
v
liquids (FSL, water and 3% acetic acid) using inductively coupled plasma-mass spectroscopy
(ICP-MS), as compared with conventional thermal processing was investigated.
Polyethylene terephthalate (PET) and ethylene vinyl alcohol (EVOH) based multilayered
structures were filled with model foods (mashed potato and water) and subjected to MATS and
PATS, respectively. MATS was performed in a 40kW 915MHz single mode semi-continuous
system. PATS was carried out in a 1.7 L cylindrical high pressure chamber with processing
conditions of 680 MPa for 3 min at 105oC. X-ray diffraction and positron annihilation lifetime
spectroscopy (PALS) were applied to investigate film morphology and free volume
characteristics, respectively.
In conclusion, MATS processing had a lesser influence on gas barrier property of PET
based multilayer structures compared to the conventional retort process. EVOH based structures
could be a suitable for PATS applications in terms of gas barrier requirements. Additionally, Xray diffraction and PALS are powerful techniques that can be used in combination to help
understand the gas barrier changes after food sterilization operations. No significant differences
(P>0.05) between the level of Si migration from films to FSL during microwave processing as
compared to the retort processing. This work provides the basis for understanding the gas-barrier
changes after MATS and PATS application.
vi
TABLE OF CONTENTS
Page
ACKNOWLEDGEMENTS ........................................................................................................... iii
ABSTRACT .................................................................................................................................... v
TABLE OF CONTENTS .............................................................................................................. vii
LIST OF TABLES ....................................................................................................................... xiii
LIST OF FIGURES ...................................................................................................................... xv
CHAPTER ONE ............................................................................................................................. 1
INTRODUCTION .......................................................................................................................... 1
1.
Background .......................................................................................................................... 1
2.
Research Vision ................................................................................................................... 6
3.
Hypothesis and Objectives ................................................................................................... 6
4.
Dissertation Outline ............................................................................................................. 8
References ................................................................................................................................. 10
CHAPTER TWO .......................................................................................................................... 12
OXYGEN BARRIER AND ENTHALPY OF MELTING OF MULTULAYER EVOH FILMS
AFTER PRESSURE-ASSISTED THERMAL PROCESSING AND DURING STORAGE ...... 12
1.
Introduction ........................................................................................................................ 13
2.
Materials and Methods ....................................................................................................... 17
2.1
Multilayer EVOH films .............................................................................................. 17
vii
3.
2.2
Pressure-Assisted Thermal Processing ....................................................................... 17
2.3
Oxygen transmission rate ........................................................................................... 18
2.4
Thermal analysis ......................................................................................................... 19
2.5
X-ray diffraction ......................................................................................................... 20
2.6
Data analysis ............................................................................................................... 20
Results and Discussion ...................................................................................................... 20
3.1
3.1.1
Oxygen transmission rate .................................................................................... 21
3.1.2
Thermal analysis ................................................................................................. 25
3.1.3
X-ray diffraction .................................................................................................. 26
3.2
4.
Film characterization after PATP ............................................................................... 20
Film characterization during long term storage .......................................................... 28
3.2.1
Oxygen transmission rate .................................................................................... 28
3.2.2
Thermal analysis ................................................................................................. 30
Conclusions ........................................................................................................................ 34
References ................................................................................................................................. 35
CHAPTER THREE ...................................................................................................................... 38
PRESSURE-ASSISTED THERMAL STERILIZATION EFFECTS ON GAS BARRIER,
MORPHOLOGICAL, AND FREE VOLUME PROPERTIES OF MULTILAYER EVOH
FILMS ........................................................................................................................................... 38
1.
Introduction ........................................................................................................................ 39
viii
2.
3.
Materials and Methods ....................................................................................................... 41
2.1
Multilayer EVOH films .............................................................................................. 41
2.2
Pressure-assisted thermal sterilization (PATS) .......................................................... 42
2.3
Oxygen transmission rate ........................................................................................... 44
2.4
Water vapor transmission rate .................................................................................... 44
2.5
X-ray diffraction ......................................................................................................... 44
2.6
Positron annihilation lifetime spectroscopy (PALS) .................................................. 45
2.7
Data analysis of OTR and WVTR .............................................................................. 48
Results and Discussion ...................................................................................................... 48
3.1
4.
Film characterization after PATP ............................................................................... 48
3.1.1
Oxygen transmission rate (OTR) ........................................................................ 48
3.1.2
Water vapor transmission rate (WVTR) ............................................................. 49
3.1.3
X-ray diffraction .................................................................................................. 51
3.1.4
Free volume analysis by PALS ........................................................................... 54
Conclusions ........................................................................................................................ 56
References ................................................................................................................................. 59
CHAPTER FOUR ......................................................................................................................... 62
THE IMPACT OF MICROWAVE-ASSISTED THERMAL STERILIZATION ON THE
MORPHOLOGY, FREE VOLUME AND GAS BARRIER PROPERTY OF MULTILAYER
POLYMERIC FILMS ................................................................................................................... 62
ix
1.
Introduction ........................................................................................................................ 63
2.
Materials and Methods ....................................................................................................... 68
3.
4.
2.1
Polymeric Film Composition...................................................................................... 68
2.2
MATS and Retort Treatment ...................................................................................... 69
2.3
Oxygen Transmission Rate ......................................................................................... 71
2.4
Water Vapor Transmission Rate................................................................................. 72
2.5
Thermal analysis ......................................................................................................... 72
2.6
X-ray Diffraction (XRD) ............................................................................................ 73
2.7
Positron Annihilation Lifetime Spectroscopy (PALS) ............................................... 73
2.8
Scanning Electron Microscopy (SEM) ....................................................................... 75
2.9
Data analysis ............................................................................................................... 76
Results and Discussion ...................................................................................................... 76
3.1
Oxygen transmission rate ........................................................................................... 76
3.2
Water vapor transmission rate .................................................................................... 79
3.3
Thermal analysis ......................................................................................................... 79
3.4
X-ray diffraction ......................................................................................................... 81
3.5
Free volume analysis by PALS .................................................................................. 83
3.6
Microscopy analysis ................................................................................................... 86
Conclusions ........................................................................................................................ 89
References ................................................................................................................................. 90
x
CHAPTER FIVE .......................................................................................................................... 93
SILICON MIGRATION FROM HIGH-BARRIER COATED MULTILAYER POLYMERIC
FILMS TO SELECTED FOOD SIMULANTS AFTER MICROWAVE PROCESSING
TREATMENTS ............................................................................................................................ 93
1.
Introduction ........................................................................................................................ 94
2.
MATERIALS AND METHODS ....................................................................................... 98
2.1
Migration test cell ....................................................................................................... 98
2.1.1
2.2
Metal-oxide coated multilayer polymeric films ....................................................... 100
2.3
Characterization of the metal-oxide coated multilayer polymeric film .................... 102
2.3.1
Microwave Digestion of film ............................................................................ 102
2.3.2
Food simulants .................................................................................................. 102
2.4
3.
Design criteria ..................................................................................................... 98
Thermal treatment..................................................................................................... 103
2.4.1
Conventional Heating (CH) .............................................................................. 103
2.4.2
Microwave Heating (MW) ................................................................................ 104
2.5
Inductively coupled plasma-mass spectrometry (ICP-MS) ...................................... 107
2.6
FTIR-ATR spectroscopy .......................................................................................... 109
2.7
Data analysis ............................................................................................................. 110
RESULTS AND DISCUSSION ...................................................................................... 110
3.1
Film Characterization ............................................................................................... 110
xi
3.2
3.2.1
Effect of type of thermal process ...................................................................... 111
3.2.2
Effect of MW process temperature ................................................................... 113
3.2.3
Effect of MW process time ............................................................................... 115
3.3
4.
Migration study......................................................................................................... 111
FTIR-ATR spectroscopy .......................................................................................... 117
CONCLUSIONS.............................................................................................................. 121
REFERENCES ........................................................................................................................ 122
CHAPTER SIX ........................................................................................................................... 126
CONCLUSIONS, CONTRIBUTION TO KNOWLEDGE AND RECOMMENDATIONS .... 126
1.
Major Conclusions ........................................................................................................... 126
2.
Contributions to knowledge ............................................................................................. 127
3.
Research Recommendations ............................................................................................ 128
xii
LIST OF TABLES
Table 1.1
Maximum allowable ingress of oxygen or loss or gain of moisture in shelf-stable
products (Adapted from Armstrong 2002)………………………………………..3
Table 1.2
Preliminary study on the performance of various multilayer polymeric films after
thermal sterilization…………………………………………………………….....5
Table 2.1
Values of oxygen transmission rates (OTRs) obtained for polymeric packaging
films in different studies after high-pressure/high-temperature processing…..…24
Table 2.2
Melting temperature and enthalpy of melting for the EVOH layer in films A and B,
untreated, and after pressure-assisted thermal sterilization (PATS)……………..25
Table 2.3
Oxygen transmission rate (OTR) values (cc/m2 day) for the multilayer EVOH films
after PATS at 680 MPa-5min-100oC……………………………………...………31
Table 2.4
Melting enthalpy (J/g) of individual components and the total melting enthalpy for
multilayer EVOH films after PATS at 680 MPa and 100oC for 5 min during
storage……………………………………………………………………………31
Table 3.1
o-Ps parameters from the Positron annihilation lifetime spectroscopy study for the
films A and B, untreated (control), and after pressure-assisted thermal sterilization
(PATS)…………………………………………………………………………...54
xiii
Table 4.1
Melting temperature and enthalpy of melting for the polymer layers in films A and
B, untreated, and after thermal sterilization……………………………………...81
Table 4.2
o-Ps parameters from the Positron annihilation lifetime spectroscopy study for the
films A and B, untreated, and after thermal sterilization……………...…………84
Table 5.1
Microwave processing conditions used in the current study (Chapter 5)………105
Table 5.2
Concentration (mg kg-1 FSL) of Silicon migrated from Films A (ON// coated
PET//CPP) and B (ON//coated nylon//CPP) to FSL during MW1 and CH1
treatments……………………………………………………………………….113
xiv
LIST OF FIGURES
Figure 2.1 Oxygen transmission rate of films A and B as influenced by the two PATS
conditions………………………………………………………………………...22
Figure 2.2
X-ray diffraction patterns for film A before and after PATS treatments…………27
Figure 2.3
X-ray diffraction patterns for film B before and after PATS treatments………....28
Figure 2.4
The total melting enthalpy of film A after PATS (680 MPa for 5 min at 100oC)
during a storage period of 60 days at room temperature. The DSC scan rate
ranged from 20 to 300oC at a rate of 10oC/min……………………………...…..32
Figure 2.5
The total melting enthalpy of film B after PATS (680 MPa for 5 min at 100oC)
during a storage period of 60 days at room temperature. The DSC scan rate
ranged from 20 to 300oC at a rate of 10oC/min…………………………..….…..33
Figure 3.1 Representative temperature and pressure profile during PATP. The processing
condition is 680 MPa for 5 min at 100oC ……………………….……………....43
Figure 3.2 Oxygen transmission rate of films A and B as influenced by the two PATS
conditions……………………………………………………………….………..50
Figure 3.3
Water vapor transmission rate of films A and B as influenced by the two PATS
conditions………………………………………………………………………...51
Figure 3.4
X-ray diffraction patterns for film A before and after PATS treatments…………52
Figure 3.5
X-ray diffraction patterns for film B before and after PATS treatments…………53
xv
Figure 3.6 An example of the fitting of PALS spectrum of film A after PATS using LT
Program…………………………………………………………………………..55
Figure 3.7 o-Ps lifetime distribution of films A and B before and after the two thermal
sterilization treatments…………………………………………………..……….58
Figure 4.1
Representative temperature and time profile for the cold spot of mashed potato in
polymeric pouches during MATS and retort sterilization (F0 = 6 min)………….71
Figure 4.2 Oxygen transmission rate of films A and B as influenced by the two thermal
sterilization conditions…………………………………………..……………….78
Figure 4.3 Water vapor transmission rate of films A and B as influenced by the two thermal
sterilization conditions…………………………………………………..……….80
Figure 4.4
X-ray diffraction patterns for film A before and after the two thermal sterilization
treatments…………………………………………………………………..…….82
Figure 4.5 o-Ps lifetime distribution of films A and B before and after the two thermal
sterilization treatments………………………………………………………..….85
Figure 4.6 Scanning electron microscopy images of film A (a) control (b) MATS (c) Retort
sterilization treatments………………………………………………...………....87
Figure 4.7 Scanning electron microscopy images of film B (a) control (b) MATS (c) Retort
sterilization treatments…………………………………………………..……….88
Figure 5.1
Picture and schematic diagram of migration test cell …………………….…….101
xvi
Figure 5.2 Picture of the test cell in the CEM microwave system containing the flexible pouch
with FSL. Pouch samples are completely submerged in water in the test cell
during processing……………………………………………………………….106
Figure 5.3 Representative temperature-time profile during conventional (CH1) and microwave
(MW1) heating………………………………………………………………….107
Figure 5.4 Silicon Migration (mg kg-1 FSL) from the two films to aqueous FSL as an influence
of MW process temperature. Mean values with different letters are significantly
different (P<0.05)………………………………………………………………114
Figure 5.5 Silicon Migration (mg kg-1 FSL) from the film A to aqueous FSL as an influence
of MW process time. Mean values of three replicates with different letters are
significantly different (P<0.05)………………………………………………..116
Figure 5.6 Silicon Migration (mg kg-1 FSL) from the film B to aqueous FSL as an influence
of MW process time. Mean values of three replicates with different letters are
significantly different (P<0.05)………………………………………………..117
Figure 5.7
FTIR-ATR spectra of film A (a) Coated metal-oxide layer before (control) and
after MW1 and CH1 treatments. (b) Food contact layer before (control) and after
MW1 and CH1 treatments. Spectrum represents average of three replicates..…119
Figure 5.8
FTIR-ATR spectra of film B (a) Coated metal-oxide layer before (control) and
after MW1 and CH1 treatments. (b) Food contact layer before (control) and after
MW1 and CH1 treatments. Spectrum represents average of three replicates..…120
xvii
DEDICATION
This dissertation is dedicated to my parents for all their support, love, and encouragement
throughout my life.
xviii
CHAPTER ONE
INTRODUCTION
1.
Background
Thermal retorting is the food industry’s most popular processing method to sterilize pre-
packaged, low-acid (pH>4.6) foods. Conventional retort sterilization uses saturated steam,
steam-air mixtures or superheated water to heat pre-packaged food in pressurized vessels at
specific temperature for prescribed lengths of time. However, the slow heat transfer within food
products involved during retorting often lead to long process times which may cause severe
nutritional and quality loss in foods (May, 2000). Consumers’ increasing preference for highquality, shelf-stable foods with improved nutritional and organoleptic properties and the food
processors quest to find more energy-efficient, high throughput, and cost-effective processing
technologies have led to the development of alternative food processing technologies.
Microwave-Assisted Thermal Sterilization (MATS) and Pressure-Assisted Thermal Sterilization
(PATS) are two sterilization technologies that have gained great attention in the food industry
and received approval from the Food and Drug Administration (FDA) as safe technologies for
preserving low-acid foods. These processes have the advantage of decreasing processing times
and the detrimental effects on food quality (Food Production Daily, 2011; Bermúdez-Aguirre
and Barbosa-Cánovas, 2011).
Both MATS and PATS processes require food to be processed inside their packaging. This
exposes the packaging material to temperature, radiation and pressure extremes required in the
production of shelf-stable foods. Food packages are required to protect the shelf-stable food
1
against oxygen and water vapor entry by having low gas transmission rates. Increases in oxygen
permeation into food packaging may severely affect the sensory properties of lipid-containing
foods due to rancidity reactions (Mokwena et al., 2009). Water loss or gain during storage can
cause moisture-sensitive foods to spoil quickly (Bourlieu et al., 2009). Table 1.1 lists the
maximum allowable ingress of oxygen in parts per million (ppm) concentration and loss or gain
of moisture percentage in various shelf-stable foods to avoid food spoilage. Therefore, it is
important to study the interaction between food-processing techniques and packaging material
and storage conditions and this interrelationship forms the basis of the current research. Various
types and forms of packaging materials are available for packaging thermally processed shelfstable foods. Selecting the appropriate packaging material will extend the shelf-life of foods
processed with these advanced food processing technologies (Ozen and Floros, 2001; Guillard et
al., 2010).
Polymeric based packaging materials have attracted attention as choice of packaging because
of their versatility and capability to offer a wide range of properties. Additionally, polymeric
films are easily processed and can be conformed into a range of shapes and sizes. Polymer films
are permeable to oxygen and water vapor at a rate characteristic of the polymer (Mullan and
McDowell, 2003). Thus, polymers that have inherent low gas permeability (i.e. high barrier
polymers) like ethylene vinyl alcohol (EVOH), Nylon, polyethylene terephthalate (PET), etc.,
are considered as suitable candidates as packaging materials. In most cases, the functionality and
properties of gas barrier polymers are further enhanced by combining different polymer layers to
form multilayer structures where each layer contributes to a specific function. For example,
hydrophilic polymers like EVOH and Nylon are protected from contact with moisture during
2
thermal processing by polyolefin layers like polypropylene (PP) and polyethylene (PE)
(Mokwena et al., 2009).
Table 1.1. Maximum allowable ingress of oxygen or loss or gain of moisture in shelf-stable
products (Armstrong, 2002)
Foods
Maximum oxygen ingress
(ppm)
Canned milk, meats, fish,
Maximum moisture
gain (+) or loss (-) %
1-5
- 3%
Beer, wine
1-5
- 3%
Canned fruit
5-15
- 3%
Dried foods
5-15
+1%
Carbonated soft drinks, fruit
10-40
- 3%
50-200
+10%
50-200
3%
poultry, vegetables, soups
juices
Oils, salad dressings, peanut
butter
Jams, jellies, syrups, pickles,
olives vinegar
The last decade has seen the merging of numerous multilayer polymeric based packaging
materials with improved gas barrier and mechanical properties into the market. Multilayer
polymeric films contain two or more polymer layers combined by methods like co-extrusion,
lamination, blending, and coating to achieve the desired gas barrier, as well as optical, thermal,
3
mechanical, morphological properties for a particular food packaging application. Also, the
effort to further enhance gas barrier properties has led to the development of innovative barrier
coating technologies, as well as the application of nanoparticles to improve polymer package
performance (Brody, 2008).
Very limited research has been done to study the performance of high gas barrier
multilayer polymeric films after PATS and MATS processing, and during storage. Table 1.2 lists
the oxygen transmission rates (OTRs) of various multilayer EVOH based polymeric films before
and after retort, PATS, and MATS processes studied by various authors (Koutchma et al., 2009;
Mokwena et al., 2009). These results indicated a significant influence of thermal sterilization on
oxygen barrier properties of multilayer polymeric films with retort sterilization having the most
severe effect. However, as there are wide arrays of likely multilayer structures that result from
combining different polymer layers, polymer processing methods, thicknesses, positioning of the
individual polymer layers, etc., it is very challenging to draw definite conclusions from the above
study. Hence, gaining a basic understanding of the structural and morphological changes of the
polymers during thermal processing and storage could help overcome this difficulty. Materials
Science technique like X-ray diffraction (XRD) and Positron Annihilation Lifetime Spectroscopy
(PALS) could help explain the morphological and free volume characteristics of the polymeric
film which could be related to the mechanism if gas transmission through the polymer matrix
(Yoo et al., 2009; Choudalakis and Gotsis, 2009). Such information could be invaluable for the
polymer industry to help to improve the performance of gas barrier polymers that are utilized for
thermal sterilization processes.
4
Table 1.2. Performance of various multilayer polymeric films after thermal sterilization
Processing Conditions
1
Pressure
(MPa)
T
(oC)
Time
(min)
OTR1 after
processing
(cc/m2 day)
Film
Structure
OTR
before
processing
(cc/m2
day)
Thermal
Process
PET/EVOH/
PP
0.16±0.01
Microwave
-
125
9
0.79±0.01
PET/PP/tie/Ny
lon6/EVOH/
Nylon6/tie/PP
0.096±0.01
Microwave
-
125
9
1.58±0.22
PET/EVOH/
PP
0.16±0.01
Retort
-
121
28
1.75±0.04
PET/PP/tie/Ny
lon6/EVOH/
Nylon6/tie/PP
0.096±0.01
Retort
-
121
28
4.57±0.59
PET/Al/CPP
<0.05
PATS
688
121
3
0.4±0.15
Nylon/Al/PP
<0..05
PATS
688
121
3
The measurements were made in duplicates at 55%RH and 230C
0.44±0.05
Reference
Mokwena et
al., (2009)
Mokwena et
al., (2009)
Mokwena et
al., (2009)
Mokwena et
al., (2009)
Koutchma et
al., (2009)
Koutchma et
al., (2009)
1
In addition to the morphological and gas barrier properties, advanced thermal sterilization
processes may also have an influence on the mass transfer properties of packaging structure
leading to the migration of plastic additives from the package to food. Polymeric packaging
materials contain various classes of chemical additives, such as plasticizers, thermal stabilizers,
antioxidants, antistatic, anti-block, slip agents, etc. to improve their functionality and fabrication
process (Lau and Wong, 2000). These additives have a low molecular weight and may interact
with the food in the package during in-package processing or storage leading to their migration
into the food. Also, monomers and oligomers present in the polymer packaging material could
also migrate into food when exposed to thermal processing conditions. Migrating additives can
5
lead to deterioration in the sensory quality of foods and can cause an increase in toxic level of the
packaged product.
2.
Research Vision
The major vision of this research includes the following:
a. To study the influence of thermal processing on the performance of polymeric films
and contribute to the knowledge of performance of state-of-the-art high gas barrier
polymeric packaging films (some developed and some developing) after PATS and
MATS.
b. To probe into the failure mechanism of packaging using Material Science tools and
develop an understanding of improving the performance of high gas barrier polymers.
c. Study package-food interaction in terms of migration of packaging components into
food during processing and storage.
3.
Hypothesis and Objectives
The central hypothesis of this proposal is that advance thermal processes like MATS and
PATS influence the thermal, mechanical, and mass transfer properties of the multilayer
polymeric food packaging films. These changes can be related to the morphological properties of
the polymeric structure and provide valuable information to design improved packaging for the
advanced thermal processes. The migration kinetics of engineered nanoparticles and plastic
6
additives from the package to food would help understand the food-package interaction during
processing under thermal/MW/high pressure field and during storage.
The objectives of the proposed research are:
I.
a. To determine the influence of PATS on two multilayer ethylene vinyl alcohol (EVOH)
based high barrier films in order to improve the quality and shelf-life of many packaged
foods. This study shall evaluate the impact of processing conditions on oxygen
transmission rates, thermal, and morphological properties of packaging materials. The
changes in oxygen transmission and overall melting enthalpy of the films during an 8month storage period shall also be evaluated.
b. To determine the influence of PATS on two multilayer ethylene-vinyl alcohol (EVOH)
based high barrier films, suitable for high pressure applications, were investigated to
understand the influence of free volume characteristics and film morphology on gasbarrier properties of PATS processed EVOH films.
II.
To investigate the influence of MW treatment on oxygen transmission rate (OTR) of two
multilayer polymeric pouches one of which is coated with a special barrier layer. The
performance of the films is also investigated to understand the influence of free volume
characteristics and film morphology on gas-barrier properties of MATS processed PET
films, as compared to conventional retorting.
7
III.
a. To develop a methodology for examining metal migration from multilayer polymeric
pouches to food simulating liquids (FSLs) after MW and conventional thermal
processing.
b. To determine the influence of MW pasteurization and sterilization treatments on the
migration of Si from metal-oxide coated multilayer polymeric films to FSL compared
with conventional heating.
c. To explore the stability of coating particles and additives in metal-oxide coated,
multilayer food packaging materials as an influence of MW process conditions compared
with conventional heating.
4.
Dissertation Outline
The dissertation was organized into seven chapters. The first chapter gives background
information on the role of packaging for advanced thermal processes and presents the
motivation, vision, and objectives of the research. Chapter 1 is a review on the application of
polymer based multilayer food packaging films for advanced thermal sterilization. Chapters 2
through 5 present and discuss experimental findings from the studies to address the research
objectives outlined above. Specifically, in chapter 2 the oxygen barrier and enthalpy of melting
of multilayer EVOH films after PATS and during storage are evaluated. In chapter 3, PATS
effects on gas barrier, morphological and free volume properties of multilayer EVOH films are
explored. Chapter 4 investigates the impact of microwave-assisted thermal sterilization on the
morphology, free volume and gas barrier property of multilayer polymeric films. In chapter 5,
silicon migration from high-barrier coated multilayer polymeric films to selected food simulants
8
after microwave processing treatments was studied. Chapter 6 is a compilation of the major
conclusions from the dissertation and suggestions for future work. The following manuscripts
have been prepared (or are being prepared) for publication in relevant journals:
(i) Dhawan, S., Barbosa‐Cánovas, G. V., Tang, J., & Sablani, S. S. (2011). Oxygen barrier
and enthalpy of melting of multilayer EVOH films after pressure‐assisted thermal
processing and during storage. Journal of Applied Polymer Science, 122(3), 1538-1545.
(ii) Dhawan, S., Sablani, S. S., Tang, J., Barbosa‐Cánovas, G. V., Ullman, J.L., & Bhunia, K.
(2013). Silicon migration from high-barrier coated multilayer polymeric films to selected
food simulants after microwave processing treatments. Packaging Technology and
Science (Submitted).
(iii) Dhawan, S., Varney, C., Barbosa‐Cánovas, G. V., Tang, J., Selim, F., & Sablani, S. S.
The impact of microwave-assisted thermal sterilization on the morphology, free volume
and gas barrier property of multilayer polymeric films. Prepared for submission to
Polymer Journal.
(iv) Dhawan, S., Varney, C., Barbosa‐Cánovas, G. V., Tang, J., Selim, F., & Sablani, S. S.
Pressure-assisted thermal sterilization Effects on gas barrier, morphological, and free
volume properties of multilayer EVOH films. Prepared for submission to Journal of
Food Engineering.
9
References
Armstrong, R. B. (2002). Effects of polymer structure on gas barrier of ethylene vinyl alcohol
(EVOH) and considerations for package development. In TAPPI 2002 PLACE
Conference.
Bermúdez-Aguirre, D., & Barbosa-Cánovas, G. V. (2011). An update on high hydrostatic
pressure, from the laboratory to industrial applications. Food Engineering Reviews, 3(1),
44-61.
Bourlieu, C., Guillard, V., Vallès-Pamiès, B., Guilbert, S., & Gontard, N. (2009). Edible
moisture barriers: how to assess of their potential and limits in food products shelf-life
extension. Critical reviews in food science and nutrition, 49(5), 474-499.
Brody, A. L. (2008). Packaging-Feeding Astronauts. Food technology, 62(1), 66.
Choudalakis, G., & Gotsis, A. D. (2009). Permeability of polymer/clay nanocomposites: a
review. European Polymer Journal, 45(4), 967-984.
Food Production Daily (2011) Researcher hails ‘major milestone’ for microwave sterilization
technology. Available from: http://www.foodproductiondaily.com/Processing/Researcherhails-major-milestone-for-microwave-sterilization-technology. Accessed Mar 29, 2012.
Guillard, V., Mauricio-Iglesias, M., & Gontard, N. (2010). Effect of novel food processing
methods on packaging: Structure, composition, and migration properties. Critical reviews
in food science and nutrition, 50(10), 969-988.
Koutchma, T., Song, Y., Setikaite, I., Juliano, P., Barbosa‐Cánovas, G. V., Dunne, C. P., &
Patacza, E. (2010). Packaging evaluation for high pressure/high temperature sterilization
of shelf-stable foods. Journal of Food Process Engineering, 33(6), 1097-1114.
10
Lau, O. W., & Wong, S. K. (2000). Contamination in food from packaging material. Journal of
Chromatography A, 882(1), 255-270.
May, N. (2000). Developments in packaging format for retort processing. In Richardson, P.S.
(Ed). Improving the Thermal Processing of Foods. Woodhead Publishing. Cambridge,
England. 138-151.
Mullan, M., & McDowell, D. (2003). Modified atmosphere packaging. In: Coles, R., McDowell,
D., & Kirwan, M.J. (Eds). Food Packaging Technology. CRC Press, Boca Raton, FL.
303-339
Mokwena, K. K., Tang, J., Dunne, C. P., Yang, T., & Chow, E. (2009). Oxygen transmission of
multilayer EVOH films after microwave sterilization. Journal of Food Engineering,
92(3), 291-296.
Ozen, B. F., & Floros, J. D. (2001). Effects of emerging food processing techniques on the
packaging materials. Trends in Food Science & Technology, 12(2), 60-67.
Yoo, S., Lee, J., Holloman, C., & Pascall, M. A. (2009). The effect of high pressure processing
on the morphology of polyethylene films tested by differential scanning calorimetry and
X‐ray diffraction and its influence on the permeability of the polymer. Journal of applied
polymer science, 112(1), 107-113.
11
CHAPTER TWO
OXYGEN BARRIER AND ENTHALPY OF MELTING OF MULTULAYER EVOH
FILMS AFTER PRESSURE-ASSISTED THERMAL PROCESSING AND DURING
STORAGE
Abstract
Pressure-assisted thermal processing (PATP) is an advanced thermal process involving
application of elevated pressures above 600 MPa on a preheated food for a holding time of 3-5
min, causing the volumetric temperature of food to increase above 100oC, to inactivate bacterial
spores and enzymes. This study evaluated the influence of PATP on two state-of-the-art
multilayer EVOH films. Flexible pouches containing water as the food simulant were made from
the two films and processed at 680 MPa for 3 min at 105oC and 680 MPa for 5 min at 100oC.
Each film was investigated for its oxygen transmission rates (OTRs), melting temperature (Tm),
enthalpy of melting (ΔH), and overall crystallinity before (control) and after processing. The
changes in OTRs and total ΔH of the two films were also analyzed during a storage period of 240
days in ambient conditions after processing. Results showed a significant (P<0.05) increase in
the OTRs of the two films after PATP. However, PATP did not cause a significant (P>0.05)
change in the Tm and ΔH of the two films. The overall crystallinity of film A decreased, but
improved slightly for film B after PATP. A recovery in the OTRs of the two films occurred
during storage. The films also showed changes in the total ΔH measured during the storage
period, which was used to explain the changes in the oxygen barrier properties. The OTR of both
films remained below 2cc/m2-day, which is required in packaging applications for shelf-stable
foods with a one-year shelf life. This work demonstrates the advantages of using multilayer films
12
containing EVOH as the barrier layer in PATP applications to produce shelf-stable foods. This
work also highlights the advantage of, DSC analysis for studying the physical ageing of
polymers during storage.
Keywords: Ethylene vinyl alcohol, oxygen transmission, PATP, morphology, food packaging
1.
Introduction
The preservation of foods using thermal energy has been a major milestone in the history of
food preservation. Thermal retorting is now the most popular method utilized in the food
industry to sterilize prepackaged low acid (pH >4.6) foods. However, retorting causes
undesirable changes in the sensory and nutritional aspects of food. An emerging thermal
processing technology, known as Pressure-Assisted Thermal Processing (PATP), has received
great attention due to its ability to process low acid shelf-stable foods with increased sensory and
nutritional benefits (Koutchma et al., 2010; Juliano et al., 2006).
PATP involves application of elevated pressures above 600 MPa on a preheated food for a
holding time of 3-5 min, causing the volumetric temperature of food to increase above 100 oC,
leading to the inactivation of spores and enzymes (Juliano et al., 2010). The advantage of this
technology is the rapid heating and rapid cooling of the food sample during hydrostatic
compression and decompression, respectively (Ratphitagsanti et al., 2009). The synergistic effect
of pressure and temperature leads to a decrease in exposure time of low acid foods to elevated
temperature compared to that of the conventional retort system. In February 2009, PATP
received U.S. Food and Drug Administration approval of a petition to preserve a low-acid food
(Food Processing, 2009).
13
One of the hurdles that must be surmounted before this technology to become
commercially applicable is its compatibility with currently used flexible packaging pouches.
PATP application would involve preheating these flexible packages containing food to an initial
target temperature followed by high-pressure/high-temperature processing. Because packaging
materials undergoing such extreme conditions may be severely damaged, shelf life of the
processed food may decrease. Juliano et al., (2010) described the general requirements for food
packaging pouches for the application of various ranges of pressure and temperature treatments.
Limited studies on the effect of PATP on polymeric based packaging material have been
reported in the literature. A previous work (Koutchma et al., 2010) studied the effect of highpressure/high-temperature sterilization on a few polymeric materials, and found that foillaminated pouches showed minimal changes in terms of gas barrier, and mechanical properties
after the influence of this sterilization technology. However, aluminum foil has the disadvantage
of blistering under the influence of high pressure and also creates a potential problem for solid
waste disposal because of its high mass density (Han et al., 2006). The current polymer industry
has the capability to produce non-foil based multilayer polymeric films with high gas barrier
properties which can withstand thermal sterilization processes. There is a definite need to explore
the influence of PATP on such high barrier multilayer polymeric-based films.
Ethylene vinyl alcohol copolymers (EVOH) are semi-crystalline materials widely used in
food packaging for thermal processes. These materials have the advantage of being an excellent
barrier to oxygen gas and aroma compounds, and have high thermal resistance and fast
crystallization kinetics, as well as good optical characteristics. The hydroxyl group present in
EVOH is responsible for the high cohesive energy offered by the molecule. This leads to a
decrease in the available free volume for exchange of gas and thus the high oxygen barrier
14
property. However, the hydrophilic nature of EVOH causes a significant decrease in its gas
barrier properties when exposed to a high relative humidity (RH) environment. Hence, EVOH is
commonly used in multilayer films protected by hydrophobic polymeric layers of polypropylene
or polyethylene during sterilization operations (López-Rubio et al., 2005; Mokwena et al., 2009;
Tsai et al., 1990).
López-Rubio et al., (2005) studied the influence of high pressure processing on two
commercially available EVOH copolymers. They concluded that this copolymer is scarcely
affected by the application of high pressure processing. They also reported an improvement in
barrier properties due to an increase in crystallinity of EVOH with 26 mol percentage (%) of
ethylene under the influence of high pressure. In another study, the oxygen barrier properties of
Nylon 6/EVOH improved after pasteurization treatment using high pressure processing at 800
MPa for 10 min at 70oC (Halim et al., 2009). The current study builds on the previous studies by
evaluating the impact of PATP on two state-of-the-art multilayer EVOH based packaging
materials subjected to PATP.
Thermal sterilization treatments may impact the gas barrier, thermal properties, and
morphology of packaging materials. These properties have a significant influence on the shelf
life of the packaged products (López-Rubio et al., 2006). Increases in oxygen permeation into
food packaging may severely affect the sensory properties of lipid-containing foods due to
rancidity reactions (Mokwena et al., 2009). The enthalpy of fusion and melting temperature are
important thermal properties of polymers, and can be used to characterize the crystallization of
semi-crystalline materials (Kong and Hay, 2003). The crystallization mechanism influences the
transmission of gases through food packaging films. X-ray diffraction studies assist in analyzing
15
the morphological properties of the packaging materials in terms of percent of crystals. A higher
crystal percentage of polymers would refer to greater orderliness of the polymeric chains, with
lesser void spaces within the polymeric material. An increase in crystallinity of a polymer results
in its superior gas barrier properties along with a greater stiffness and lower transparency (Yoo et
al., 2009).
Thermal processing of the EVOH copolymer leads to a sharp initial increase in its oxygen
permeability and also influences an increase in the steady state permeation of oxygen at a given
relative humidity. This increase in gas permeability is attributed to moisture plasticization of the
EVOH polymer, as well as an increase in the free volume in the polymer during the thermal
process. This free volume increase causes an irreversible change in the film and morphology of
the EVOH polymer (Tsai et al., 1990). It is very important to understand the changes taking
place in the polymer during a storage period after thermal processing as these modifications have
an influence on the shelf life of the processed foods. Hence, studying the gas barriers and thermal
properties of high-pressure/high-temperature processed films during a storage period at ambient
conditions would reveal useful information concerning the free volume of the polymeric films.
To the best of our knowledge, no research has been conducted that correlates thermal property
changes in PATP processed films with oxygen barrier characteristics for storage periods over
200 days. Studies of this nature will help in the selection of polymeric films for sterilization
applications requiring storage of a packaged food beyond one year.
Thus, the objective of this work is to determine the influence of PATP on two multilayer
EVOH based high barrier films in order to improve the quality and shelf-life of many packaged
foods. This study evaluated the impact of processing conditions on oxygen transmission rates,
16
and on the thermal and morphological properties of packaging materials. This research also
determined changes in oxygen transmission and overall melting enthalpy of the films during an
8-month storage period.
2.
Materials and Methods
2.1 Multilayer EVOH films
Two EVOH based multilayer films were developed by EVAL Company of America
(Houston, TX). Film A is laminated, and composed of an outer layer of 12 µm of biaxially
oriented polyethylene terephthalate (PET), a middle layer of 12 µm of EF-XL EVOH resin layer
(32 mol% ethylene); and an inner layer of 75 µm of cast polypropylene (cPP) placed in direct
contact with food surface. Film A is also known as PET//EVOH//PP. Film B was a 7-layer
structure laminated to an outer PET layer, and is denoted as PET//PP/tie/Nylon 6/EVOH/Nylon
6/tie/PP. Film B consists of a 15 µm layer of L171 EVOH resin (27 mol% ethylene) sandwiched
between 10 µm nylon 6 homopolymer and 50 µm polypropylene homopolymer on both sides.
The tie layer in film B was a maleic anhydride acid modified polypropylene. A previous study
gives a detailed description of the structure of the materials.8 Flexible pouches with dimensions
of 6 x 4 inches were prepared from each of the above films.
2.2
Pressure-Assisted Thermal Processing
The pouches were filled with 50 ml distilled water (food simulant) and sealed with a
minimum headspace using an impulse sealer (MP-12; J.J. Elemer Corporation, St. Louis, MO)
with a 4 sec dwell time. Pouches were first preheated in water to 90oC in a tilting steam kettle
17
(DLT-40-1EG, Groen; DI Food Service Companies, Jackson, MS) for 10 min. The pouches were
then placed inside a cylindrical liner made of polypropylene (internal diameter 75 mm, external
diameter 100 mm, height 21.5 mm; McMaster-Carr, Atlanta, GA); the liner was used as an
insulator to prevent heat loss from the packaging material to the pressure walls during holding
time at maximum pressure. The liner was temperature equilibrated prior to loading of pouches, to
ensure that the temperature of the pouch/liner system was maintained at chamber temperature.
The liner was then placed in the 1.7 L cylindrical high pressure chamber measuring 0.1 m
internal diameter and 2.5 m height (Engineered Pressure Systems, Inc., Haverhill, MA), with
pressure vessel walls and compression fluid set at 90oC in order to achieve sterilization process
conditions. The compression fluid was 5% Houghton Hydrolubic 123B soluble oil/water solution
(Houghton & Co., Valley Forge, PA). The high pressure unit was pressurized to operating
pressure in a few seconds using an electrohydraulic pump (Hochdruck-Systeme GmbH, AP 100670-1116, Sigless, Austria). Three thermocouples (K-type; Omega Engineering, Inc., Stamford,
CT) were used to measure the temperature of the liner containing the sample and pressure
medium.
PATP processing conditions for the EVOH pouches were 680 MPa for 3 min at 105oC and
680 MPa for 5 min at 100oC. Figure 2.2 from the previous chapter shows a representative
temperature-pressure profile during processing at 680 MPa for 3 min holding time at 105oC.
2.3
Oxygen transmission rate
Oxygen transmission rates (OTRs) were measured using an Ox-Tran 2/21 instrument
(Modern Control, Minneapolis, MN) at 23oC and 55 ± 1% RH, according to the ASTM standard
18
method D 3985 (ASTM Standard Test Method, 1995). The pouches were first cut into films with
a measurement area of 50 cm2 and then mounted inside the testing chambers. The OTR of the
untreated and PATP processed pouches were measured in replicates. The OTR of the films
processed at 680 MPa for 5 min holding time at 100oC was also measured after 15, 30, 60, and
240 days of storage at ambient conditions. This timeline to measure the OTRs during storage was
selected because EVOH based films undergo dynamic changes immediately after a thermal
process, and the lag time required by EVOH films to reach steady state oxygen permeability is
greater than 200 days (Tsai et al., 1990).
2.4
Thermal analysis
The thermal transition of the EVOH films before and after processing was analyzed using a
differential scanning calorimeter (DSC, Q2000; TA Instruments, New Castle, DE). The pans
containing 2 ± 0.2 mg of the EVOH multilayer samples were heated from 20 to 300oC at a rate of
10oC/min. Melting temperature (Tm, oC) and the enthalpy of melting (ΔH, J/g) of the polymers
present in the multilayer EVOH films were determined using DSC thermograms. The Tm was
determined from the peak temperature of the endotherm and the ΔH was determined by
integrating the respective melting endotherm using the instrument’s software. The sum of the ΔH
of the polymers present in film A was used to calculate its total ΔH [(ΔHtotal)A =
ΔHPET+ΔHEVOH+ΔHPP]. Similarly, the sum of the ΔH of the polymers present in film B was used
to calculate its total ΔH [(ΔHtotal)B = ΔHPET+ΔHEVOH+ ΔHNylon +ΔHPP]. The total ΔH of the films
processed at 680 MPa for 5 min at 100oC was also determined during storage. All measurements
were made in replicate.
19
2.5
X-ray diffraction
X-ray diffraction patterns for all the untreated and processed films were obtained using a
Siemens D-500 diffractometer (Bruker, Karlsruhe, Germany). The diffractometer was operated
at a wavelength of 0.15 nm and the copper target tube was set at 35 KV and 30 mA. The
dimension of the multilayer EVOH sample required for recording the diffraction patterns was 2
inch x 2 inch. The intensity of diffraction was recorded as a function of increasing scattering
angle from 8-35o with a step angle of 0.05o and scan time of 3 sec per step. The XRD patterns
provided an estimate of the crystallinity percentage in the films.
2.6
Data analysis
The OTR and enthalpy of melting data for the two films before and after processing were
studied using a complete randomized design. The data was analyzed using the general linear
model (GLM) and the significant differences (P < α) in properties of the films were determined
through the Fisher’s least significant difference (LSD) test (α = 0.05). Data analysis was
conducted with the statistical software SAS version 9.2 (SAS Inst. Inc., Cary, NC).
3.
Results and Discussion
3.1
Film characterization after PATP
This section will discuss the oxygen barrier and structural changes suffered by the two
multilayer EVOH films immediately after PATP.
20
3.1.1 Oxygen transmission rate
The OTR of films A and B before (control) PATP were 0.24 and 0.11 cc/m 2 day,
respectively. Similar OTR values were observed by Mokwena et al., (2009) for the two films
prior to thermal processing. These values were significantly lower than those of commercially
available polyvinylidene chloride or silicon oxide coated barrier films of similar thickness. These
commercially available films are currently being used to construct pouches for thermal
processing of sterilized foods. Results from Mokwena et al., (2009) also showed that thermal
sterilization influenced the OTRs of both films.
OTR for the two EVOH films was observed immediately after the two PATP processing
treatments. A comparison of OTR of the two films before and immediately after PATP for the
two processing conditions is shown in Figure 2.1. There was a 2.5-fold and 5-fold increase in
the OTR of film A after the 3 min and 5 min PATP processes, respectively. The OTR values of
film A significantly increased (P<0.05) with increased holding time under maximum pressure.
The OTR value for film B increased nearly 4 times after PATP. Results for film B suggest that
its barrier properties were not influenced by the increase of holding time at maximum pressure.
The superior oxygen barrier property of film B with 28 mol% ethylene, compared to film A with
32 mol% ethylene, is in agreement with the study conducted by López-Rubio et al., (2005) who
observed slightly better barrier properties after high pressure processing for the monolayer
EVOH copolymer with lower ethylene content. Also, the influence of individual layers in the
multilayer structure of film B would have played a crucial role in outperforming film A in terms
of the oxygen barrier properties after the thermal process.
21
Figure 2.1. Oxygen transmission rate of films A and B as influenced by the two PATS
conditions. Mean values with different letters are significantly different (P<0.05).
The increase of OTR in EVOH films could be attributed to the poor moisture resistance of
EVOH copolymers. In one study, the authors observed that a decrease in the barrier properties of
films after high-pressure/high-temperature treatment was due to thermal damage of polymers
during preheating (Koutchma et al., 2010). The preheating step in the current study involved
heating for 10 min at 90oC with similar results. It was found that preheating exposed the EVOH
copolymer to a high relative humidity environment causing the plasticization of the hydrophilic
EVOH layer, which led to a decrease in the polymer chain-to-chain interactions, resulting in an
22
increase in the free volume. These modifications may be responsible for the deterioration of
oxygen barrier properties. However, the plasticizing effect is time dependent when the EVOH
layer is protected by hydrophobic polymer layers (López-Rubio et al., 2003). Both films used in
this study were protected by polypropylene, which is a good water barrier. Hence, the increase of
OTR in both films A and B is far below the 2 cc/m2day limit required for packaging application
for sterilized food products (Mokwena et al., 2009).
Table 2.1 compares the OTR of different films after high-pressure/high-temperature
processing observed from various studies. The aluminum foil-laminated films studied by
Koutchma et al., (2010) performed the best in terms of the OTR after processing at 688 MPa and
121oC for 3 min. Nevertheless, the EVOH based films utilized in this study had OTR values
comparable to the foil-based films. These results suggest that multilayer EVOH films have the
potential to replace foil-based films whose drawbacks have been previously discussed.
23
Table 2.1. Values of OTR obtained for polymeric packaging films in different studies after high-pressure/high-temperature processing
Film
PET/EVOH/P
P
PET//PP/tie/
Nylon6/EVO
H/Nylon
6/tie/PP
24
PET/AlOx/
CPP
Biaxial
Nylon/EVOH
Preheating
T (oC)
Time
(min)
High Pressure Conditions
Pressure
T
Time
o
(MPa)
( C)
(min)
Oxygen Transmission Rate
OTR (cc/ day m2 RH
T
o
atm)
(% ( C)
)
1.1±0.09
55
23
References
90
10
680
100
5
90
10
680
100
5
0.43±0.07
55
23
Present Study
90
11
688
121
3
19.6±0.7
*
*
Koutchma et
al., (2010)
12.2
688
121
3
2.0±0.3
*
*
Koutchma et
al., (2010)
90
Present Study
PET/Al/PP
90
8.8
688
121
3
0.4±0.15
*
*
Koutchma et
al., (2010)
Nylon/Al/PP
90
*
688
121
3
0.44±0.05
*
*
Koutchma et
al., (2010)
EVOH
*
*
800
75
5
0.62
0
23
Lopez-Rubio et
al., (2005)
3.1.2 Thermal analysis
The thermal characteristics of the films after PATP were studied using DSC experiments.
Table 2.2 summarizes the melting temperature and enthalpy of melting for the two films before
and after the combined temperature and pressure treatment. The PATP processes had no
influence on the melting temperature (Tm) and melting enthalpy (ΔH) of the EVOH layer in both
films A and B.
Table 2.2. Melting temperature and enthalpy of melting for the EVOH layer in films A and B, untreated,
and after PATS
Film
A (32 mol% ethylene)
B (27 mol% ethylene)
Treatment
Control
Tm (oC)
182.8±0.3a
ΔH (J/g)
6.5±0.2b
680 MPa, 3 min, 105oC
182.2±0.1a
6.7±0.1b
680 MPa, 5 min, 100oC
182.6±0.3a
5.8±0.8b
Control
186.1±0.2c
4±0.6d
680 MPa, 3 min, 105oC
186.1±0.1c
4.1±0.8d
680 MPa, 5 min, 100oC
186.1±0.1c
4.4±0.5d
Tm = melting temperature; ΔH = enthalpy of melting. Values are means ± 1 standard deviation. Means with different
letters within a column are significantly different (P<0.05)
25
The results for film B containing 27 mol% ethylene are in agreement with the study
conducted by López-Rubio et al., (2005) who observed no significant difference in the melting
behavior of high pressure processed monolayer EVOH containing 26 mol% ethylene. On the
other hand, the results of film A are also in agreement with the thermal characteristics of a
multilayer film of nylon/EVOH/polyethylene processed at 690 MPa at 95oC for 10 min in
another study. No significant difference was shown between the untreated film and the high
pressure processed EVOH film containing 32 mol% ethylene (Schauweckeret al., 2002). The
above results indicate that PATP did not influence the thermal characteristics of the EVOH layer
in films A and B.
3.1.3 X-ray diffraction
The XRD patterns for film A for the two PATP processing conditions are presented in
Figure 2.2. The XRD diffractograms show a decrease in peak intensities after both of the PATP
treatments, leading to a decrease in the overall crystallinity of film A. This decrease is reflected
in the loss of the film’s gas barrier property after the PATP treatments, as a decrease in
crystallinity results in a loss of orderliness in the polymeric chains, in turn causing a decrease in
the tortuous path for the gas to travel though the film. This decrease in tortuosity promoted by the
crystalline phase causes more gas to flow through the film, leading to quality deterioration of the
food (López-Rubio et al., 2006; Yoo et al., 2009).
The XRD patterns for film B for the two PATP processing conditions are presented in
Figure 2.3. Unlike film A, the film B showed a small improvement in overall crystallinity. In
another study, the authors observed similar results, in which high pressure processing caused an
26
increase in the percentage of crystallinity of EVOH with 26 mol% ethylene (López-Rubio et al.,
2005). However, the improved crystallinity does not explain the loss of the gas barrier property
for film B after PATP. Hence, more studies involving measurement of the morphology in the
polymeric film are required to gain a clearer understanding of the behavior of the gas barrier
properties for a given polymeric film.
Figure 2.2. X-ray diffraction patterns for film A before and after the two PATS treatments.
27
Figure 2.3. X-ray diffraction patterns for film B before and after the two PATS treatments.
3.2
Film characterization during long term storage
This section reports the influence of storage on the PATP processed multilayer EVOH
films. The alterations suffered by the films are explained by means of oxygen transmission rate,
and total melting enthalpy.
3.2.1 Oxygen transmission rate
The OTRs of the two films were measured before (control) processing, after PATP at 680
MPa for 5 min at 100oC, and during a storage period of 240 days (Table 2.3). Generally, there
was a recovery in the oxygen barrier properties of the films during the storage period after the
28
initial increase of OTR measured immediately after processing. The rate of recovery of the
oxygen barrier properties was slower during the initial storage period, but increased during the
storage period of 60 to 240 days. However, overall, the barrier properties of the films were not
completely recovered during the storage study and the pre-processing values were not attained.
Nevertheless, this study shows an improvement in the gas barrier properties of thermally
processed multilayer films during storage, which will help extent the shelf-life of packaged
foods.
The OTR values for film A observed after a storage period of 15 days did not change
significantly (P>0.05) compared to the initial post-processing value. On the other hand, at the
end of 30 days storage, there was a significant decrease in OTR measured for film A (P<0.05)
(Table 2.3). This observation is in agreement with the study conducted by Mokwena et al.,
(2009) who observed a similar improvement in oxygen barrier properties of film A after
microwave sterilization. As the authors describe, the difference in the vapor pressure between the
EVOH layers and the storage environment force a moisture migration from the film to attain
equilibrium conditions. This migration would have facilitated an improvement in the oxygen
barrier property of the film. The film A reached a quasi-equilibrium condition between day 30
and day 240 of storage and, hence, no significant changes were seen in the oxygen barrier
properties.
On the other hand, the study of storage between 30 and 240 days showed a significant
improvement (P<0.05) in the oxygen barrier property for film B (Table 2.3). The significant
improvement in the barrier properties of film B during this storage period suggests that it would
require more time to attain its quasi-equilibrium state. It is exciting to note that there was no
29
significant difference (P<0.05) between the pre-processing OTR for film B with the postprocessing value obtained after storage for 8 months (Table 2.3). This improvement in the
oxygen barrier property will have a positive impact on the shelf life of PATP processed
packaged foods. To prove this further and to add more insight on the possible reasons for
improvement in the OTR, a thermal analysis was performed during storage to study the
crystallization mechanism of the semi-crystalline polymers. However, the OTR values for the
two films during the storage period was way below 2 cc/m2 day, the value required for packaging
applications for shelf-stable foods (Mokwena et al., 2009).
3.2.2 Thermal analysis
To increase our understanding about the behavior of the oxygen barrier properties and the
morphology of the films during storage, the total ΔH was measured during the 240-day storage
period. Table 2.4 highlights the results obtained for the two films after processing at 680 MPa
and 100oC for 5 min. The (ΔHtotal)A decreased significantly (P<0.05) immediately after
processing. This decrease in the (ΔHtotal)A could have led to a decrease in crystal size and
crystallinity and, hence, led to a significant increase in the OTR immediately after processing.
On the other hand, film B also showed a decrease in the (ΔHtotal)B after processing but the change
was not significant (P>0.05).
30
Table 2.3. OTR values (cc/m2 day) for the multilayer EVOH films after PATS at 680 MPa-5min-100oC
Storage Time (days)
Film
A
B
Control
0.24±0.03a
0.11±0.01a
7
1.1±0.09b
0.43±0.07bc
15
1.06±0.07b
0.4±0.05bc
30
0.84±0.11c
0.43±0.02bc
60
0.86±0.01c
0.47±0.18b
240
0.71±0.04c
0.26±0.01ac
Values are means ± 1 standard deviation. Means with different letters within a row are significantly different (P<0.05)
Table 2.4. Melting enthalpy (J/g) of individual components and the total melting enthalpy for
multilayer EVOH films after PATS at 680 MPa and 100oC for 5 min during storage
31
Storage Time (days)
Film
Components/Total
A
PP
Control
41.81±0.99
7
30.12±4.72
60
37.97±0.64
240
34.84±0.79
EVOH
PET
6.67±0.74
4±1.06
6.17±0.77
3.86±0.09
7.28±0.62
5.31±0.86
7.03±0.65
4.42±0.12
Total melting enthalpy
52.48±0.8a
40.15±5.59b
50.56±2.13a
46.29±1.56ab
PP
EVOH
32.08±1.04
4.37±0.31
28.37±0.56
4.72±0.01
26.51±2.27
4.2±0.02
35.85±4.89
5.11±0.48
Nylon
PET
Total melting enthalpy
5.8±1.35
2.98±0.55
5.52±0.26
3.35±0.15
5.15±0.18
2.58±0.09
6.43±1.05
3.79±0.3
45.23±3.27ab
41.96±0.98ab
38.44±2.52a
51.18±6.73b
B
Values are means ± 1 standard deviation. Means with different letters within a row are significantly different (P<0.05)
During a storage period of 60 days, the film A showed a significant increase (P<0.05) in
the (ΔHtotal)A compared to the previous measurement (Figure 2.4); whereas the (ΔHtotal)B value
remained nearly the same for film B (P>0.05) (Figure 2.5). It can be inferred that the change in
the (ΔHtotal)A could have led to an increase in the crystallinity of the polymers, thereby causing a
recovery in its oxygen barrier property at the end of 60 days.
Figure 2.4. The total melting enthalpy of film A after PATS (680 MPa for 5 min at 100oC)
during a storage period of 60 days at room temperature. The DSC scan rate ranged from 20 to
300oC at a rate of 10oC/min.
32
Nylon
PET
Figure 2.5. The total melting enthalpy of film B after PATS (680 MPa for 5 min at 100oC)
during a storage period of 60 days at room temperature. The DSC scan rate ranged from 20 to
300oC at a rate of 10oC/min
The (ΔHtotal)B at the end of 240 days showed a significant increase (P<0.05) compared to
the previous value obtained at the end of 60 days, which could be related to the significant
decrease (P<0.05) in its OTR value at the end of the storage period. Conversely, for film A, there
was only a slight decrease in the (ΔHtotal)A and, hence, the change in its barrier property during
this period was not significant (P>0.05). These changes in total ΔH could be attributed to the
physical ageing of polymers present in the films A and B. During physical ageing, the polymeric
33
layers exhibit slow thermodynamic changes in order to attain a lower-free energy state; these
modifications lead to significant changes in the mechanical and gas barrier properties of the food
packaging film. The molecular rearrangement during ageing causes a slow decrease in the free
volume within the polymer matrix through which the gas molecules move, and the level of
changes depends on the thermal history and thickness of the polymers present in the film
(Martino et al., 2009). This study shows that the micro structural changes within the polymeric
layers during storage can be utilized to explain the variation in barrier properties of films utilized
for sterilization applications.
4.
Conclusions
PATP had a significant influence on the oxygen barrier properties of the two films. The
state-of-the-art 7-layer film B (PET//PP/tie/Nylon 6/EVOH/Nylon 6/tie/PP) containing 27 mol%
ethylene showed superior oxygen barrier properties compared to film A throughout the study.
The changes in overall crystallinity observed from the XRD diffractograms help explain the
change in oxygen barrier property after PATP. On the other hand, the thermal characterization of
the films with DSC did not show significant changes in the Tm and ΔH after the thermal process.
However, the changes in total ΔH of the EVOH based multilayer films during storage correlated
to the changes in their oxygen barrier properties. Thus DSC analysis is recommended as a useful
technique to reason out the recovery of the oxygen barrier properties in the thermal processed
packaging films during the storage period. Overall, flexible plastic pouches containing EVOH as
the barrier layer is a suitable choice as packaging material for PATP.
34
References
[ASTM] American Society for Testing and Materials. (1995). Standard test method for oxygen
gas transmission rate through plastic film and sheeting using a coulometric sensor.
ASTM Book of Standards, D3985-95. Philadelphia, PA.
Food Processing. Pressure-Assisted Thermal Sterilization Accepted by FDA. (2009).
http://www.foodprocessing.com/articles/2009/032.html. (Accessed July 05, 2010).
Halim, L., Pascall, M. A., Lee, J., & Finnigan, B. (2009). Effect of Pasteurization, High‐Pressure
Processing, and Retorting on the Barrier Properties of Nylon 6, Nylon 6/Ethylene Vinyl
Alcohol, and Nylon 6/Nanocomposites Films. Journal of food science, 74(1), N9-N15.
Han, J., & Yuan, J. (2007). Advances in Packaging for Nonthermal Processes.
Juliano, P., Toldrág, M., Koutchma, T., Balasubramaniam, V. M., Clark, S., Mathews, J. W.,
Dunne, C.P., & Barbosa‐Cánovas, G. V. (2006). Texture and Water Retention
Improvement in High‐pressure Thermally Treated Scrambled Egg Patties. Journal of
food science, 71(2), E52-E61.
Juliano, P., Koutchma, T., Sui, Q., Barbosa-Cánovas, G. V., & Sadler, G. (2010). Polymericbased food packaging for high-pressure processing. Food Engineering Reviews, 2(4),
274-297.
Kong, Y., & Hay, J. N. (2003). The enthalpy of fusion and degree of crystallinity of polymers as
measured by DSC. European polymer journal, 39(8), 1721-1727
35
Koutchma, T., Song, Y., Setikaite, I., Julaino, P., Barbosa-Cánovas, G.V., Dunne, C. P., &
Patazca E. (2010). Packaging evaluation for high pressure/high temperature sterilization
of shelf-stable foods. Journal Food Process Engineering, 33(6), 1097-1114.
López-Rubio, A., Lagaron, J. M., Giménez, E., Cava, D., Hernandez-Muñoz, P., Yamamoto, T.,
& Gavara, R. (2003). Morphological alterations induced by temperature and humidity in
ethylene-vinyl alcohol copolymers. Macromolecules, 36(25), 9467-9476.
López-Rubio, A., Lagarón, J. M., Hernández-Muñoz, P., Almenar, E., Catalá, R., Gavara, R., &
Pascall, M. A. (2005). Effect of high pressure treatments on the properties of EVOHbased food packaging materials. Innovative Food Science & Emerging Technologies,
6(1), 51-58.
Lopez‐Rubio, A., Giménez, E., Gavara, R., & Lagaron, J. M. (2006). Gas barrier changes and
structural alterations induced by retorting in a high barrier aliphatic polyketone
terpolymer. Journal of applied polymer science, 101(5), 3348-3356.
López‐Rubio, A., Gavara, R., & Lagarón, J. M. (2006). Unexpected partial crystallization of an
amorphous polyamide as induced by combined temperature and humidity. Journal of
applied polymer science, 102(2), 1516-1523.
Martino, V. P., Ruseckaite, R. A., & Jiménez, A. (2009). Ageing of poly (lactic acid) films
plasticized with commercial polyadipates. Polymer International, 58(4), 437-444.
Mokwena, K. K., Tang, J., Dunne, C. P., Yang, T., & Chow, E. (2009). Oxygen transmission of
multilayer EVOH films after microwave sterilization. Journal of Food Engineering,
92(3), 291-296.
36
Ratphitagsanti, W., Ahn, J., Balasubramaniam, V. M., & Yousef, A. E. (2009). Influence of
pressurization rate and pressure pulsing on the inactivation of Bacillus amyloliquefaciens
spores during pressure-assisted thermal processing. Journal of Food Protection®, 72(4),
775-782.
Schauwecker, A., Balasubramaniam, V. M., Sadler, G., Pascall, M. A., & Adhikari, C. (2002).
Influence of high‐pressure processing on selected polymeric materials and on the
migration of a pressure‐transmitting fluid. Packaging Technology and Science, 15(5),
255-262.
Tsai, B. C., & Wachtel, J. A. (1990). Barrier properties of ethylene-vinyl alcohol copolymer in
retorted plastic food containers. In Barrier Polymers and Structures. ACS Symposium
Series (Vol. 423, pp. 193-203).
Yoo, S., Lee, J., Holloman, C., & Pascall, M. A. (2009). The effect of high pressure processing
on the morphology of polyethylene films tested by differential scanning calorimetry and
X‐ray diffraction and its influence on the permeability of the polymer. Journal of Applied
Polymer Science, 112(1), 107-113.
37
CHAPTER THREE
PRESSURE-ASSISTED THERMAL STERILIZATION EFFECTS ON GAS BARRIER,
MORPHOLOGICAL, AND FREE VOLUME PROPERTIES OF MULTILAYER EVOH
FILMS
Abstract
Pressure-assisted thermal sterilization (PATS) alters the morphology and free volume
distributions of polymers leading to a decrease in gas-barrier properties of polymer packaging
materials, and hence compromising the quality and shelf life of PATS processed foods. Two
multilayer ethylene-vinyl alcohol (EVOH) films, suitable for high pressure applications, were
investigated to understand the influence of free volume characteristics and film morphology on
gas-barrier properties of PATS processed EVOH films. X-ray diffraction and positron
annihilation lifetime spectroscopy were applied to investigate film morphology and free volume
characteristics,
respectively.
Film
A
was
comprised
of
polyethylene
terephthalate
(PET)/EVOH/polypropylene (PP). Film B consisted of PET laminated to a co-extruded structure
of PP/tie/Nylon6/EVOH/Nylon6/tie/PP. Both oxygen and water vapor transmission rates
increased in the two films after the selected treatment. However, the increase in film A is much
larger which can be understood from the change in free volume distributions measured by
positron lifetime and overall crystallinity observed from X-ray diffraction. This work provides
the basis for understanding the gas-barrier changes after PATS application.
Keywords: High pressure processing, gas transmission, free volume, X-ray diffraction, positron.
38
1.
Introduction
Pressure-Assisted Thermal Sterilization (PATS) is an emerging thermal processing
technology for sterilizing prepackaged low-acid (pH >4.6) foods. Pressures in the range of 600800MPa and initial chamber temperature of 60-90oC is utilized to ensure the inactivation of
spores and enzyme. This synergistic effect of temperature and pressure helps reduce the total
processing time and reduces the exposure of food products to high temperature compared to the
conventional thermal retort system. The lower food processing times help improving the sensory
and nutritional characteristics of low-acid shelf-stable foods. In February 2009, U.S. Food and
Drug Administration approved a petition to preserve a low-acid food using PATS (BermúdezAguirre and Barbosa-Cánovas, 2011).
PATS require food to be packaged during processing and ethylene vinyl alcohol
copolymers (EVOH)-based multilayer polymeric films have been found to be suitable
candidatess of packaging material for withstanding high pressure and temperature (Lopez-Rubio
et al., 2005a). However, a previous study showed that deterioration in oxygen barrier properties
of EVOH-based films has been observed after PATS treatment and during storage which could
have an impact on the quality and shelf life of PATS processed foods. This loss of oxygen barrier
property in EVOH films has been correlated to the changes in thermal and morphological
properties in the polymer encountered during PATS processing. However, the thermal properties
measured by differential scanning calorimeter (DSC) and the morphology measured by X-ray
diffraction (XRD) alone had limitations in providing a clear correlation of gas barrier changes in
the (EVOH)-based multilayer polymeric films (Yoo et al., 2009). Thus, advanced techniques like
positron annihilation lifetime spectroscopy (PALS) is required in addition to XRD to understand
39
the morphological and free volume modifications taking place in the polymeric film after
processing and thus, provide a better understanding to correlate the gas barrier changes.
Inefficient chain packing of the polymer created by folding and molecular architecture of
polymer chain segments leads to the formation of free volumes. The extent of free volumes
present in a polymer matrix help understand the molecular gas transport through the polymer
membrane as the free volumes in a polymer create an easier pathway for the diffusion of solutes
through the solid matrix. PALS is a powerful nondestructive versatile technique utilized for
detecting and characterizing free volume sizes and their distribution in polymers with good
sensitivity at the atomic and nanoscale. The free volume characterization of polymers is possible
because of the capability of positronium (bound state of electron and positron) to preferentially
localize in regions of low electron density such as pores, free volume, interfaces, and holes
(Choudalakis and Gotsis 2009; Awad et al., 2012; Ramya et al., 2012).
Danch et al. (2007) utilized PALS for studying the influence of low temperature and high
pressure on the free volume in polymethylpentene. Temperature rise from 0 to 300 K led to an
increase in free volume, whereas, increase in pressure from 0 to 500 MPa led to a decrease in
free volume in polymethylpentene. However, no studies have been carried out to study the
influence of food processing technologies like PATS on food packaging polymers. Such studies
will help gain a fundamental understanding of influence of both temperature and pressure on the
free volume parameters which in turn could be related to the change in the gas barrier properties
after processing.
Thus, the objective of this work is to determine the influence of PATP on two multilayer
ethylene-vinyl alcohol (EVOH) based high barrier films, suitable for high pressure applications,
40
were investigated to understand the influence of free volume characteristics and film morphology
on gas-barrier properties of PATS processed EVOH films.
2.
Materials and Methods
2.1
Multilayer EVOH films
Two multilayer polymer films containing a thin barrier layer of EVOH were developed by
EVAL Company of America (Houston, TX). Film A is laminated, and composed of an outer
layer of 12 µm of biaxially oriented polyethylene terephthalate (PET), a middle layer of 12 µm
of EF-XL EVOH resin layer (32 mol% ethylene); and an inner layer of 75 µm of cast
polypropylene (cPP) placed in direct contact with food surface. Film A is also known as
PET//EVOH//PP. Film B was a 7-layer structure laminated to an outer PET layer, and is denoted
as PET//PP/tie/Nylon 6/EVOH/Nylon 6/tie/PP. Film B consists of a 15 µm layer of L171 EVOH
resin (27 mol% ethylene) sandwiched between 10 µm nylon 6 homopolymer and 50 µm
polypropylene homopolymer on both sides. The tie layer in film B was a maleic anhydride acid
modified polypropylene. A previous study by Mokwena et al. (2009) gives a detailed description
of the structure of the materials. The above films were used to make flexible pouches with
dimensions of 6 x 4 inches that were utilized for this study. The pouches were filled with 50 ml
distilled water (food simulant) and sealed with a minimum headspace using an impulse sealer
(MP-12; J.J. Elemer Corporation, St. Louis, MO).
41
2.2
Pressure-assisted thermal sterilization (PATS)
Flexible pouches containing water were first preheated in water to 90oC in a tilting steam
kettle (DLT-40-1EG, Groen; DI Food Service Companies, Jackson, MS) for 10 min. The
pouches were then placed inside a cylindrical liner made of polypropylene (internal diameter 75
mm, external diameter 100 mm, height 21.5 mm; McMaster-Carr, Atlanta, GA); the liner was
used as an insulator to prevent heat loss from the packaging material to the pressure walls during
holding time at maximum pressure. The liner was temperature equilibrated prior to loading of
pouches, to ensure that the temperature of the pouch/liner system was maintained at chamber
temperature. The liner was then placed in the 1.7 L cylindrical high pressure chamber measuring
0.1 m internal diameter and 2.5 m height (Engineered Pressure Systems, Inc., Haverhill, MA),
with pressure vessel walls and compression fluid set at 90oC in order to achieve sterilization
process conditions. The compression fluid was 5% Houghton Hydrolubic 123B soluble oil/water
solution (Houghton & Co., Valley Forge, PA). The high pressure unit was pressurized to
operating pressure in a few seconds using an electrohydraulic pump (Hochdruck-Systeme
GmbH, AP 10-0670-1116, Sigless, Austria). Three thermocouples (K-type; Omega Engineering,
Inc., Stamford, CT) were used to measure the temperature of the liner containing the sample and
pressure medium.
PATS processing conditions for the EVOH pouches were 680 MPa for 5 min at 100oC.
Figure 3.1 shows a typical temperature-pressure profile during processing at 680 MPa for 5 min
holding time at 100oC.
42
120
110
Temperature
100
Pressure
80
70
60
50
40
30
20
10
0
0
2
4
Time (Minutes)
6
Pressure (MPa)
Temperature (0C)
90
800
750
700
650
600
550
500
450
400
350
300
250
200
150
100
50
0
8
Figure 3.1. Representative temperature and pressure profile during PATP. The processing
condition is 680 MPa for 5 min at 100oC
43
2.3
Oxygen transmission rate
Oxygen transmission rates (OTRs) were measured according to the ASTM standard
method D 3985 (ASTM, 1995). A Mocon Ox-Tran 2/21 MH permeability instrument (Modern
Control, Minneapolis, MN) was utilized to conduct the measurements at 23oC, 55 ± 1% RH, and
1 atm. The OTR measurement characterizes the ease with which oxygen gas passes through the
films when a gradient in partial pressure of oxygen is present across the films. Film specimens of
surface area 50 cm2 were cut from the polymeric pouches and mounted inside the testing
chambers and readings were measured with the help of a coulometric sensor that was fitted in the
equipment. The OTR of the control (untreated) and PATS processed pouches were measured in
replicates.
2.4
Water vapor transmission rate
Water vapor transmission rates (WVTRs) were measured according to the ASTM standard
method F 372-99. A Mocon Permatran 3/33 tester (Modern Control, Minneapolis, MN) utilizing
an infrared detector was used to characterize the water vapor transmission rate (WVTR) of the
packaging materials at 100%RH and 38oC. Film specimens of surface area 50 cm2 were cut from
the polymeric pouches and mounted inside the testing chambers. The WVTR of the control
(untreated) and PATS processed pouches were measured in replicates.
2.5
X-ray diffraction
A Siemens D-500 diffractometer (Bruker, Karlsruhe, Germany) was utilized to obtain the
X-ray diffraction patterns for the two untreated (control) and processed films A and B. The
44
diffractometer was operated at a wavelength (λ) of 0.15 nm and the copper target tube was set at
35 KV and 30 mA. The dimension of the multilayer EVOH sample required for recording the
diffraction patterns was 2 inch x 2 inch. The intensity of diffraction was recorded as a function of
increasing scattering angle from 8-35oC with a step angle of 0.05oC and scan time of 3 sec per
step. The crystallinity percentage and crystalline thickness were estimated from the XRD
patterns. The ratio of area under the peaks (crystalline region) to the area of the amorphous
region in the diffraction patterns helped estimate the overall crystallinity. The Scherrer equation
was used to determine the crystal thickness (D) as follows (Yoo et al., 2009):
(1)
where
is the full-width at half maximum and
is the angle between the incident rays which
were obtained from the XRD peaks. The initial profiles were refined and processed using the
peak fitting program JADE (Materials Data, Inc., Livemore, CA) for accurate computation.
2.6
Positron annihilation lifetime spectroscopy (PALS)
PALS is a powerful nondestructive versatile technique utilized for detecting and
characterizing free volume sizes and their distribution in polymers and vacancy-type defects in
crystals with good sensitivity. Positrons injected into a solid from a radioactive source annihilate
with electrons, either from a delocalized state in the bulk or from a trapped state in an open
volume such as a lattice vacancy in crystals or an open volume in polymers and porous materials
(Awad et al., 2012). Trapping at defects or open volumes leads to an increase in the average
positron lifetime. In fair approximation, the positron lifetime varies inversely with the electron
45
density at the annihilation site. Consequently, annihilation in vacancies or open volumes, where
electron densities are low, has longer lifetimes. Measured lifetimes are characteristic of the open
volume in which the positrons annihilate, and therefore can be used to discriminate among
different locations where positrons annihilate. A measured lifetime spectrum N(t) consists of a
k 1
sum of components corresponding to each annihilation site:
N (t )  
i 1
Ii
i
t
exp  
 i 
in which k+1 is
the number of lifetime components in the spectrum, corresponding to annihilation in the bulk and
in k defect types, and in which i and Ii are the lifetime and intensity of the ith component in the
spectrum. Fitted lifetimes give information about defect/open volume sizes and characteristics
and the intensities determine defect/open volume concentrations. Therefore, lifetime spectrum
provides information about free volumes in polymers and porous materials just as about defects
in crystalline solids. Positrons also form positronium in polymers which leads to much longer
lifetime (Jean, 1994).
Here, positron lifetimes were measured using a conventional fast-fast time coincidence
spectrometer with two BaF2 gamma-ray detectors mounted on photomultiplier tubes. The
spectrometer has been described in detail by Selim et al. (2013). A positron source was made by
depositing
22
NaCl activity on an 8-microns thick kapton foil that was then folded and
sandwiched between two identical samples. PAL spectra were recorded at room temperature
with a time resolution of 250 ps. Several million counts were accumulated in each lifetime
spectrum for good statistical precision. MELT and LT9 program was employed for analyzing the
lifetime distribution after applying the source correction term (Shukla et al., 1993; Kansy 1996).
The measured spectra were resolved into three components (τ1, τ2, and τ3) with their respective
46
intensities (I1, I2, and I3) for finite-term lifetime analysis. Spectra were fit to the best χ2 with the
most reasonable standard deviation.
The shortest lifetime (τ1) could be attributed to the self-annihilation of para-positronium
(p-Ps) whereas, the intermediate lifetime (τ2) could be related to the free positron annihilation.
The third mean lifetime (τ3) is due to the ortho positronium (o-Ps) pick-off annihilation in freevolume holes of amorphous region. A semi-empirical equation given by the following relation
along with the o-Ps lifetime (τ3) could be used to obtain the mean free-volume hole radius (R).

 2πR  
R
1
-1
(τ3 )-1 = 2 1 +
sin 
  ns
2π  R o  
 Ro
(2)
where τ3 and R are expressed in the units of ns and Å, respectively. R0 equals R+ΔR
where ΔR is the fitted empirical electron layer thickness with a value of 1.66Å. Relative
fractional free volume (%) or the number of free volume content (fv) is expressed as follows
(
)
(3)
where I3(%) is the o-Ps intensity and C is a constant.
The chain folding and molecular architecture of the polymer chains and its segments lead to
formation of free volume holes of varying sizes. Therefore, a distribution of sizes could be
characterized from the measured o-Ps lifetime. MELT program (Shukla et al., 1993) was
employed in this study to measure the free volume distribution of both films A and B before and
after PATP (Cheng et al., 2009; Ramya et al., 2012).
47
2.7
Data analysis of OTR and WVTR
The OTR and WVTR data for the two films before and after PATS were studied using a
complete randomized design. The data was analyzed using the general linear model (GLM) and
the significant differences (P < α) in properties of the films were determined through the Fisher’s
least significant difference (LSD) test (α = 0.05). Data analysis was conducted with the statistical
software SAS version 9.2 (SAS Inst. Inc., Cary, NC).
3.
Results and Discussion
3.1
Film characterization after PATP
The gas barrier, morphological and free volume changes underwent by the two multilayer
EVOH films immediately after PATP will be discussed in this section.
3.1.1 Oxygen transmission rate (OTR)
Figure 3.2 shows the OTR of the two films before (control) thermal treatment and
immediately after treatment by PATS. The OTR of the control films A and B were 0.24 and 0.11
cc/m2 day, respectively. The PATP process led to a nearly 5-fold increase in the OTR of film A
to a final value of 1.1 cc/m2 day. On the other hand, the OTR for film B increased nearly by 4
times to a final value of 0.43 cc/m2 day after PATP. Thus, the PATP process significantly
increased (P<0.05) the OTR values of films A and B. It has been previously observed that
monolayer EVOH copolymer with lower ethylene content showed slightly higher barrier
48
properties after high pressure processing (López-Rubio et al., 2005a). Similar results were
obtained in our study as film B with 28 mol% ethylene showed superior oxygen barrier property
compared to 32 mol% ethylene in film A. Additionally, the EVOH layer in film B is better
protected by the individual polymer layers compared to that of film A.
The deterioration of oxygen barrier properties of EVOH containing films A and B could be
related to the plasticization of the hydrophilic EVOH copolymer in a higher moisture
environment. Such a high moisture environment is exhibited by the preheating step which
exposes the films to 90oC for 10 minutes. Similar results of deterioration in oxygen barrier
properties of biaxial nylon/coextruded EVOH during the preheating step of PATS was found in
another study by Koutchma et al. (2009). It has also been hypothesized that the plasticization of
the EVOH layer could lead to a decrease in the polymer chain-to-chain interactions, resulting in
an increase in the free volume (López-Rubio et al., 2003). Thus, this study further investigated
the influence of processing on the morphological and free volume properties of the films A and
B.
3.1.2 Water vapor transmission rate (WVTR)
Figure 3.3 shows the WVTR of the two films before (control) thermal treatment and
immediately after treatment by PATS. There was a significant increase (P<0.05) in the WVTR of
the two films after processing. However, the deterioration in water vapor barrier property was
higher in film A as compared to film B. The WVTR of film A increased by 74% from 0.73 to
1.27 g/m2 day after PATP processing. On the other hand, there was only a 16% increase in the
WVTR of film B after PATS from 0.61 to 0.71 g/m2 day. It is possible that the better protection
49
of the EVOH layer by hydrophobic polymers in film B could be responsible for the lesser
deterioration of the polymeric chain morphology, compared to that of film A. Similar to oxygen
barrier properties, the WVTR of EVOH based films could have been compromised due to the
structural changes taking place in the films during preheating and the high pressure processing
step. This advocates the need for material science studies to understand the mechanism of gas
transport through the films.
1.4
OTR (cc/m2-day)
1.2
Control
PATS
1
0.8
0.6
0.4
0.2
0
Film A
Film B
Figure 3.2. Oxygen transmission rate of films A and B as influenced by the two PATS
conditions. Mean values with different letters are significantly different (P<0.05).
50
1.4
Control
WVTR (g/m2-day)
1.2
PATS
1
0.8
0.6
0.4
0.2
0
Film A
Film B
Figure 3.3. Water vapor transmission rate of films A and B as influenced by the two PATS
conditions. Mean values with different letters are significantly different (P<0.05).
3.1.3 X-ray diffraction
The diffraction patterns for film A before and after PATS are presented in Figure 3.4.
There was a decrease in peak intensities of the film A after processing leading to a decrease in its
overall crystallinity. Additionally, there was a slight increase in peak width which is an
indication to the disruption of the crystalline structure caused by crystal fractionation (LópezRubio et al., 2005b). The decrease in crystallinity of the film could lead to a reduced orderliness
in the polymeric chains causing an easier path for the gas to travel through the polymer matrix
51
and thus, leading to a reduced oxygen and water vapor barrier property and eventually quality
deterioration of the food (Yoo et al., 2009; López-Rubio et al., 2006) The crystal thickness
estimated from the Scherrer equation showed that PATP led to a decrease in thickness from 133
to 113 Å for the major peak at 20o for film A. Hence, there was clear change in the crystalline
morphology of the polymeric film A after PATS treatment.
Intensity (Counts)
7000
Control
6000
PATS
5000
4000
3000
2000
1000
0
5
15
25
35
Two Thetha (degree)
45
Figure 3.4. X-ray diffraction patterns for film A before and after PATS treatments
On the other hand, Figure 3.5 represents the XRD patterns for film B for the PATS
treatment. The sterilization operation caused a decrease in the peak intensities for all the
characteristic peaks in film B indicating change in the overall crystallinity of the film. However,
52
unlike film A, there was a slight decrease in the peak width indicating lesser distortion to the
crystalline morphology. Also, there was an increase in crystal thickness from 45 to 57 Å for the
characteristic peak at approximately 26o. López-Rubio et al. (2005a) also observed an increase
in the percentage of crystallinity of EVOH with 26 mol% ethylene subjected to high pressure
processing. Hence, an improvement in crystal thickness and reduced distortion of the crystalline
morphology could be responsible for the lesser increase in gas transmission through film B with
27 mol% ethylene as compared to film A with 32 mol% ethylene.
Intensity (Counts)
5000
Control
PATS
4000
3000
2000
1000
0
5
15
25
35
Two Thetha (degree)
Figure 3.5. X-ray diffraction patterns for film B before and after PATS treatments
53
45
3.1.4 Free volume analysis by PALS
Table 3.1 summarizes the o-Ps parameters measured by PALS for the two films before and
after PATS treatment. The PALS raw data were further fitted into lifetime distribution using LT9
program and Figure 3.6 illustrates an example of the fitting of PALS spectrum of film A after
PATP. The sterilization treatment led to a decrease in the o-Ps lifetime for film A from 3.151 to
3.055Å, whereas, there was an increase in o-Ps lifetime for film B from 2.945 to 3.077Å after
PATS. Additionally, free volume fraction (Fv) decreased by 9% for film A and increased by 23%
for film B after PATS treatment. This opposing behavior for the two films could have resulted
from the fact that increase in pressure leads to a decrease in free volume and a temperature
increase causes an increase in thermally induced free volume of polymers (Danch et al., 2007).
Thus, the varying effects of the two factors involved in PATS on the free volume could be
responsible for this opposing behavior.
Table 3.1 – o-Ps parameters from the Positron annihilation lifetime spectroscopy study for the
films A and B, untreated (control), and after pressure-assisted thermal sterilization (PATS)
Film*
Treatment
A
B
o-Ps intensity,
I3 (%)
Control
o-Ps
lifetime, τ3
(ns)
2.34
9.5
Free volume
radius
(Å)
3.15
Free volume
fraction
(FV)
2.24
PATS
2.22
9.4
3.06
2.03
Control
2.10
8.9
2.94
1.71
PATS
2.25
9.6
3.08
2.11
*Film A: PET/EVOH/PP; Film B: PP/tie/Nylon6/EVOH/ Nylon6/tie/PP
54
20
0
5
10
15
20
25
Residue
10
0
-10
55
Counts [a.u]
-20
10
6
10
5
Raw Data
Fit Data
1
2
10
3
4
source
10
3
10
2
0
5
10
15
20
Time (ns)
Figure 3.6. An example of the fitting of PALS spectrum of film A after PATS using LT Program
25
Figure 3.7 illustrates that there was no change in the o-Ps lifetime distribution for film A,
whereas, there was a broadening in the free volume distribution for film B after PATS. The
broader positron lifetime distribution and an increase in positron lifetime of film B after PATS
suggest that there could be overlapping of free volumes leading to gas being trapped in the
overlapped free volumes. The overlapping of free volumes leads to an increase in the tortuous
path of gas flow through the polymer membrane. Hence, the relative increase in OTR and
WVTR of film B is less than that of film A.
In addition, overall crystallinity observed from X-ray diffraction indicated that the level of
amorphous region in both films varied which may influence the level of changes in film
morphology and gas-barrier properties. Thus, the XRD and PALS analysis together help in
providing a clear picture of the polymer morphology and free volume properties in relation to the
gas barrier properties of the polymer food packaging films.
4.
Conclusions
PATS had a significant influence (p<0.05) on the oxygen and water vapor barrier
properties of the two multilayer EVOH-based films. However, the increase in gas barrier
properties of film A was much larger compared to film B which can be understood from the
change in overall crystallinity measured by X-ray diffraction, and free volume distributions
measured by positron lifetime. PALS was successfully applied as a tool to characterize the free
volume properties of multilayer EVOH-based food packaging films before and after PATS. A
broader positron lifetime distribution and an increase in positron lifetime of film B after PATS
56
suggests that there could be overlapping of free volumes leading to gas being trapped in the
overlapped free volumes and hence, the relative increase in gas-barrier properties of B is lesser
than A. This work suggests that X-ray diffraction and PALS are powerful techniques to
investigate film morphology and free volume characteristics which helps understanding the gas
barrier changes after food sterilization operations.
57
Normalized Intensity
0.015
Film A Control
Film A PATS
0.01
0.005
0
0
1
2
3
4
o-Ps lifetime, τ3 (ns)
Normalized Intensity
58
0.025
Film B Control
Film B PATS
0.02
0.015
0.01
0.005
0
0
1
2
3
4
o-Ps lifetime, τ3 (ns)
Figure 3.7.
o-Ps lifetime distribution of films A and B before and after the two thermal sterilization treatment
References
Awad, S., Chen, H. M., Grady, B. P., Paul, A., Ford, W. T., Lee, L. J., & Jean, Y. C. (2012).
Positron Annihilation Spectroscopy of Polystyrene Filled with Carbon Nanomaterials.
Macromolecules, 45(2), 933-940.
Bermúdez-Aguirre, D., & Barbosa-Cánovas, G. V. (2011). An update on high hydrostatic
pressure, from the laboratory to industrial applications. Food Engineering Reviews, 3(1),
44-61.
Cheng, M. L., Sun, Y. M., Chen, H., & Jean, Y. C. (2009). Change of structure and free volume
properties of semi-crystalline poly (3-hydroxybutyrate-co-3-hydroxyvalerate) during
thermal treatments by positron annihilation lifetime. Polymer, 50(8), 1957-1964.
Choudalakis, G., & Gotsis, A. D. (2009). Permeability of polymer/clay nanocomposites: A
review. European Polymer Journal, 45(4), 967-984.
Danch, A., Osoba, W., & Wawryszczuk, J. (2007). Comparison of the influence of low
temperature and high pressure on the free volume in polymethylpentene. Radiation
Physics and Chemistry, 76(2), 150-152.
Jean, Y. J. (1994, November). Positron annihilation in polymers. In Materials Science Forum
(Vol. 175, pp. 59-70).
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Kansy, J. (1996). Microcomputer program for analysis of positron annihilation lifetime spectra.
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Koutchma, T., Song, Y., Setikaite, I., Julaino, P., Barbosa-Cánovas, G. V., Dunne, C, P., &
Patazca, E. (2009). Packaging evaluation for high pressure/high temperature sterilization
of shelf-stable foods. Journal of Food Process Engineering, 33(6), 1097-1114.
Lopez‐Rubio, A., Giménez, E., Gavara, R., & Lagaron, J. M. (2006). Gas barrier changes and
structural alterations induced by retorting in a high barrier aliphatic polyketone
terpolymer. Journal of applied polymer science, 101(5), 3348-3356.
López-Rubio, A., Lagarón, J. M., Hernández-Muñoz, P., Almenar, E., Catalá, R., Gavara, R., &
Pascall, M. A. (2005a). Effect of high pressure treatments on the properties of EVOHbased food packaging materials. Innovative Food Science & Emerging Technologies,
6(1), 51-58.
López‐Rubio, A., Hernández‐Muñoz, P., Gimenez, E., Yamamoto, T., Gavara, R., & Lagarón, J.
M. (2005b). Gas barrier changes and morphological alterations induced by retorting in
ethylene vinyl alcohol–based food packaging structures. Journal of applied polymer
science, 96(6), 2192-2202.
López-Rubio, A., Lagaron, J. M., Giménez, E., Cava, D., Hernandez-Muñoz, P., Yamamoto, T.,
& Gavara, R. (2003). Morphological alterations induced by temperature and humidity in
ethylene-vinyl alcohol copolymers. Macromolecules, 36(25), 9467-9476.
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Mokwena, K. K., Tang, J., Dunne, C. P., Yang, T., & Chow, E. (2009). Oxygen transmission of
multilayer EVOH films after microwave sterilization. Journal of Food Engineering,
92(3), 291-296.
Ramya, P., Ranganathaiah, C., & Williams, J. F. (2012). Experimental determination of interface
widths in binary polymer blends from free volume measurement. Polymer, 53: 842-850.
Selim FA, Varney CR, Rowe MC, Collins GS. 2013. Submitted to Physical Review Letters.
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Yoo, S., Lee, J., Holloman, C., & Pascall, M. A. (2009). The effect of high pressure processing
on the morphology of polyethylene films tested by differential scanning calorimetry and
X‐ray diffraction and its influence on the permeability of the polymer. Journal of Applied
Polymer Science, 112(1), 107-113.
61
CHAPTER FOUR
THE IMPACT OF MICROWAVE-ASSISTED THERMAL STERILIZATION ON THE
MORPHOLOGY, FREE VOLUME AND GAS BARRIER PROPERTY OF
MULTILAYER POLYMERIC FILMS
Abstract
Microwave-assisted thermal sterilization (MATS) is an advanced thermal process that utilizes
microwave energy for in-package food sterilization. It offers the advantage of much shorter
overall process times, when compared to conventional retort sterilization. This research
examined the influence of MATS on the performance of high barrier multilayer polymeric films,
in particular, the impact on the gas barrier, morphological and free volume packaging properties
of these polymeric films, as compared to conventional retort sterilization. These packaging
properties could have a direct influence on the shelf life of shelf-stable foods. Two new high
barrier polyethylene terephthalate (PET) based multilayer pouches with the structure
PET/adhesive/Nylon/adhesive/PP
(film
A)
and
Coated-PET-Coated/adhesive/oriented-
Nylon/adhesive/PP (film B) were selected for this study. The pouches were filled with mashed
potatoes and were processed in a 915 MHz single mode MATS system and retorted in the same
unit without turning on the microwave (MW) generators to achieve the same level of sterilization
(F0=6 minutes). The influence of free volume characteristics and film morphology on gas-barrier
properties of the two MATS processed films was analyzed. X-ray diffraction and positron
annihilation lifetime spectroscopy (PALS) were applied to investigate film morphology and free
62
volume characteristics, respectively. MW processing had a lesser influence on the gas barrier
property of the materials compared to the conventional retort process. The oxygen barrier
property of Film A was better than that of film B after the sterilization treatment. Higher oxygen
transmission rate (OTR) values of film B could be attributed to the increase in fractional free
volume, stability of the barrier coating layer, and lower overall crystallinity compared to film A
after MATS and retort sterilization. However, the oxygen transmission rate (OTR) for both films
remained below 2cc/m2-day after MATS and retort sterilization required for packaging
applications for shelf-stable foods.
Keywords: Microwave processing, gas transmission, polymer packaging, thermal analysis, Xray diffraction, positron.
1.
Introduction
Sterilization is the process of destroying all the viable forms of microbial life, which
includes the bacterial spores, as defined by U.S. Food and Drug Administration (FDA). Thermal
sterilization for preservation of food utilizes heat to disable microorganisms of public health
significance, and food spoilage microorganisms. The time-temperature process schedules for
thermal sterilization are established based on the heat resistance characterization of the target
microorganism. Conventional retort sterilization is the most popular commercial thermal method
utilized in the food industry to sterilize prepackaged low acid (pH >4.6) foods. However, the
high processing temperatures (120-130oC) and relatively long processing times (30-60 min)
involved during retorting cause undesirable changes in the sensory and nutritional aspects of
foods. Consumers’ desire for high-quality, safe foods and the food processors quest to find more
63
energy-efficient, high throughput, and cost-effective processing technologies have led to the
need for the development of advanced food processing technologies.
A forecast provided in 1996 from Food Engineering magazine and 2012 in the Food
Technology magazine identified microwave (MW) heating as one of the most promising
preservation technologies that would dominate the twenty-first century in production of shelfstable foods (Morris, 1996; Brody, 2012). MW heating results from the polarization effect of
electromagnetic radiation on foods at frequencies between 300 MHz and 300 GHz. Industrial
microwave processing uses either one of the two frequencies (915MHz and 2450 MHz) allocated
by Federal Commissions of Communication (FCC) for industrial heating applications to generate
thermal energy inside the food through a process called volumetric heating. Charged ions, water
molecules, and other polar molecules present in the food align in a rapidly-alternating microwave
field and rotate in the electromagnetic environment. This molecular rotation and the agitation of
water molecules and charged ions rapidly raise the product temperature to attain the required
thermal lethality to target microorganism and sharply reduce the process times compared to the
traditional canning processes, which use pressurized high temperature water or steam as the
heating medium
(Barbosa-Cánovas and Bermúdez-Aguirre, 2008; Ramaswamy and Tang,
2008). The reduced processing times provide opportunity for production of high quality and
nutritional shelf-stable products (Guan et al., 2002). Sterilization of in-packaged foods using
MW systems has been commercialized in Europe and Japan (Ramaswamy and Tang , 2008). In
the United States, a 915-MHz, single-mode and semi-continuous microwave-assisted thermal
sterilization (MATS) system for processing low-acid, in-packaged foods was developed by the
advanced thermal processing research team at Washington State University (Tang et al., 2006).
A petition filed by the same research team to preserve a homogenous low-acid food using the
64
MATS system received U.S. Food and Drug Administration acceptance in October 2009. The
MATS system design utilizes the advantages of both the single mode microwave heating at
915MHz and the traditional over-pressure surface water heating, which helps improve the
heating uniformity in the food. In December 2010, a second petition to preserve nonhomogenous foods using the MATS system was also approved (Food Production Daily, 2011). A
US food processing group, AmeriQual, is currently operating the first Microwave Assisted
Thermal Sterilization (MATS) unit on a trial basis at one of their facilities (Food Production
Daily, 2012).
The MATS process requires that food to be processed inside its package. Metal based
flexible meal ready-to-eat (MRE) pouches containing aluminum (Al) foil have been widely used
for retort sterilization. However, the Al layers in the MRE pouches shields electromagnetic fields
from reaching food in packages and, therefore, are not suited for MATS process. Alternatively,
high gas-barrier polymeric based packaging materials have been considered as suitable
candidates for advanced thermal sterilization processes. Similar to conventional canning, the
MATS process exposes the packaging material to high temperatures and radiations that may alter
the gas barrier, mechanical, and morphological properties of the packaging materials. These
physical properties of packaging materials have a significant influence on the shelf life of the
packaged food (Lopez-Rubio et al., 2006). To maintain shelf stability of thermally-sterilized
foods, a high-oxygen barrier property in food package material is required (Mokwena et al.,
2009). Increases in oxygen permeation into food packaging may lead to rancidity reactions,
which would severely negatively affect the sensory properties of lipid-containing foods.
Additionally, there is an urgent need to develop suitable packaging materials that would help
extend the shelf life of MATS processed food and maximize the advantages of this advanced
65
thermal technology (Bermúdez-Aguirre and Barbosa-Cánovas, 2011). Therefore, it is important
to study the interaction between packaging material and MATS food-processing technology.
Most polymeric packaging materials that are selected for packaging foods for advanced
thermal food processing technologies consist of more than one polymeric layer. The last decade
has seen the merging of numerous multilayer polymeric based packaging materials with
improved gas barrier and mechanical properties into the market. Multilayer polymeric films
usually have a core functional barrier layer (polymer layer that is responsible for gas barrier
properties) that provides the necessary shelf life for packaged foods. Ethylene-vinyl alcohol
(EVOH), polyethylene terephthalate (PET), nylon (Ny), and poly (vinylidene chloride) are
functional gas-barrier layers commonly used for packaging shelf-stable foods. Silicon (Si) and
aluminum (Al) metal-oxide coated high-barrier multilayer polymeric films as well as nanoparticles coated gas barrier layer present in the multilayer polymeric films have been developed
in an effort to further improve gas barrier properties, and are commercially available for retort
sterilization treatment.
Few studies have been reported in the literature on the influence of MW sterilization on
polymeric-based packaging material. A previous work by Mokwena et al., (2009) studied the
effect of retort and MW sterilization on two multilayer EVOH based multilayer polymeric lidstocks for low-acid model food packaged in polymeric trays. Both thermal sterilization
technologies resulted in the deterioration of the oxygen barrier of the two films. But, the oxygen
barrier deterioration was higher in retort sterilization compared to MW sterilization, as the longer
processing times of retort sterilization resulted in increasing the plasticization of the hydrophilic
EVOH layer, thus leading to an increased oxygen barrier deterioration (Mokwena et al., 2009).
66
The current study builds on the previous studies by evaluating the impact of MATS on two new
multilayer PET based packaging materials and more importantly, understanding the influence of
free volume characteristics and film morphology on gas-barrier properties of MATS processed
PET films.
Morphology of a polymer refers to the distribution and homogeneity of crystalline and the
amorphous region within the matrix of the polymer material and also describes the polymeric
chain arrangements. A higher overall crystallinity provides improved orderliness of the
polymeric chains and reduces the void spaces within the polymer matrix, thus leading to better
gas barrier properties (Yoo et al., 2009). Degree of crystallinity of a polymer is determined
through fingerprinting X-ray diffraction technique. An inefficient chain packing of the polymer
creates free volume, the size and distribution of which control the rate of gas diffusion and the
permeation properties. Positron annihilation lifetime spectroscopy (PALS), is capable of
detecting the free volume properties of a polymer on the basis that positronium (bound state of
positron and electron, Ps) are formed and localized in low electron density sites, such as free
volumes, interface, and pores (Awad et al., 2012). The shape and size of the free volume in the
polymer have a direct influence on its gas permeation properties (Wang et al., 2005). However,
to date, no known experimental research has been conducted that correlates morphological and
free volume property changes in MW processed polymeric multilayer films with gas barrier
characteristics. Studies of this nature will help in selecting the right packaging material for the
sterilization application and also provide fundamental understanding for the polymer industry to
further improve the barrier properties of polymeric packaging materials.
67
Thus, the objectives of the present work were to investigate the influence of MW treatment
on oxygen transmission rate (OTR) of two multilayer polymeric pouches one of which was
coated with a special barrier layer. The post-processing films were evaluated to understand the
influence of free volume characteristics and film morphology on gas-barrier properties of MATS
processed PET films, as compared to conventional retorting.
2.
Materials and Methods
2.1
Polymeric Film Composition
Two multilayer polymeric films consisting of PET as the functional barrier layer were
subjected to MATS and retort sterilization. Film A was developed by Alcan Packaging (Chicago,
IL) and it consisted of an outer layer of PET, a middle layer of nylon, and an inner layer of
sealant polypropylene (PP) layer (denoted PET/adhesive/Nylon/adhesive/PP). Film B was
developed by Kuraray Co., Ltd (Houston, TX), and had a similar structure with the functional
barrier layer, PET, having a special barrier coating on either side (denoted Coated- PETCoated/adhesive/oriented-Nylon/adhesive/PP). The coating layer was a proprietary barrier
technology and contained both organic and inorganic barrier particles to increase the tortuosity
of gas flow through the polymer matrix. Flexible pouches of dimension 18 cm x 12 cm were
made from the above two multilayer films. They were filled with 225 grams mashed potatoes
(model low acid food), prepared by mixing 15% instant mashed potato flakes (Washington
Potato Company, Warden, WA) with 15% deionized water. Pouches were vacuum sealed with
minimum head space before thermal sterilization.
68
2.2
MATS and Retort Treatment
Thermal treatments were applied in a pilot scale 915-MHz, single mode, semi-continuous
MATS system developed at Washington State University (Tang et al., 2006). This system
consisted of four pressurized sections, namely, preheating, MW heating, holding, and cooling,
arranged in series to simulate the four sequential industrial processing steps. Water at a
controlled temperature was filled in each section from an individual water circulating system.
During processing, the sealed food pouches were loaded on a pocketed mesh conveyor belt
which transports them through the different sections of MATS. The Preheating section, which
included a preheating cavity and a water circulating system, helped to equilibrate the food to a
uniform initial temperature. Pressurized water at 35 psig and 72oC was supplied to the preheating
cavity by a water circulation system, the temperature of which was controlled by resistance
temperature detectors (RTD). Pouches in the conveyor were navigated through the MW heating
section, which included four single-mode MW heating cavities, four MW generators with a
labeled operating frequency at 915 MHz, MW waveguides, and pressurized hot water (35 psig,
124 oC) supplied to each of the four cavities by the water circulation system. The MW generators
supplied a maximum power of 10, 10, 5, and 5 kW to cavities 1, 2, 3, and 4, respectively. The
food in the MW heating section was heated simultaneously by microwave energy infringing
from the four cavities and by circulating hot water (35 psig, 122oC) through
convection/conduction surface heating. The holding section, comprised of the holding cavity was
an extension of the heating system without the microwave energy, where the food achieves the
required thermal lethality. Hot water (35 psig, 123oC) was supplied to the holding cavity by the
water circulation system. Water temperature in the heating and holding section was controlled
69
using a RTD sensor similar to the preheating section. Food pouches were finally cooled down to
room temperature in the cavity of the cooling section using a cold water circulation system. The
forwarded power to and reflected power from each of the four MW-heating cavity was measured
by two directional couplers installed in each of the cavities. Operational parameters were
recorded and monitored by the control and data acquisition system present in the MATS system.
Retort sterilization was also carried out in the same unit without turning on the microwave
generators. In these test runs, the MATS system also functions as a hot water immersion still
retort in the absence of application of microwave power to the system. Temperature
measurements were obtained using Ellab sensors (Ellab Inc.,Centennial, CO) at the cold spot of
the polymeric pouches. Cold spots were identified using a chemical marker-based computer
vision system described in Pandit et al., (2007). The procedure for the thermal treatment was
selected based on its ability to achieve a similar level of sterilization (F0 = 6 min) for both retort
and microwave sterilization. The general method was used to calculate the F0 values at the cold
spot (Downing 1996)
∫
(
)
(1)
where T is the measured temperature at the cold spot of the product (oC);TR is the reference
temperature (121.1 oC); z is the temperature rise required to decrease the thermal death time of
the target microorganism (Clostridium botulinum) by one log cycle (10 oC); and t is the heating
time (minutes). Figure 4.1 shows the representative time-temperature profiles at the cold spot of
the polymeric pouches for both MATS and retort sterilization treatment.
70
140
MATS
Temperature (0C)
120
Retort
100
80
60
40
20
0
0 3 6 9 12151821242730333639424548
Time (min)
Figure 4.1. Representative temperature and time profile for the cold spot of mashed potato in
polymeric pouches during MATS and retort sterilization (F0 = 6 min).
2.3
Oxygen Transmission Rate
A Mocon Ox-Tran 2/21 MH permeability instrument (Modern Control, Minneapolis, MN)
was used to conduct the oxygen transmission rate (OTR) measurements. The conditions of
testing were set at 55 ± 1% relative humidity, 23oC, and 1 atm. The test was conducted according
to the ASTM standard D 3985 method (ASTM, 1995), and readings were measured using a
coulometric sensor that was fitted in the equipment. Film specimens of surface area 50 cm2 were
71
cut from the polymeric pouches and mounted inside the testing chambers. The OTR of the
control (untreated) and MATS processed pouches were measured in replicates.
2.4
Water Vapor Transmission Rate
A Mocon Permatran 3/33 tester (Modern Control, Minneapolis, MN) was used to
characterize the water vapor transmission rate (WVTR) of the packaging materials at 100%RH
and 38oC, according to the ASTM standard method F 372-99. This equipment utilizes an infrared
detector to analyze the transmission rates. Film specimens of surface area 50 cm 2 were cut from
the polymeric pouches and mounted inside the testing chambers. The WVTR of the control
(untreated) and MATS-processed pouches were measured in replicates.
2.5
Thermal analysis
A model Q2000 TA Instruments differential scanning calorimeter (DSC) (New Castle, DE)
was utilized to analyze the effect of MATS and retort sterilization on the thermal transitions of
films A and B. Film samples weighing 2 ± 0.2 mg were placed in pans and heated from 20 to 300
o
C at a rate of 10oC/min in the DSC instrument. The resulting DSC thermograms were analyzed
to determine the melting temperature (Tm, oC) and the enthalpy of melting (ΔH, J/g) of the
polymers present in the multilayer films A and B. The peak temperature of the endotherm
corresponds to the Tm and the ΔH was determined by integrating the respective temperature
versus heat flow melting endotherm using the instrument’s software. All measurements were
conducted in replicates.
72
2.6
X-ray Diffraction (XRD)
X-ray diffractograms for the films before and after thermal sterilization were obtained using
a Siemens D-500 diffractometer (Bruker, Karlsruhe, Germany). The X-ray copper target tube
was set at 35 KV and 30 mA and operated at a wavelength of 0.15 nm. The sample size of the
films size was 2 inch x 2 inch and the diffraction intensity was recorded as a function of
increasing scattering angle from 8-35 degrees with a step angle of 0.05 degrees and the scan time
of 3s per step. The overall percent crystallinity of the films was determined from the XRD
patterns using he instrument’s software.
2.7
Positron Annihilation Lifetime Spectroscopy (PALS)
Positron lifetime spectroscopy is a highly informative technique for microscopic
characterization of vacancy-type defects in crystals and open volumes in polymers. Positrons
injected into a solid from a radioactive source annihilate with electrons, either from a delocalized
state in the bulk or from a trapped state in an open volume such as a lattice vacancy in crystals or
an open volume in polymers and porous materials. Trapping at defects or open volumes leads to
an increase in the average positron lifetime. In fair approximation, the positron lifetime varies
inversely with the electron density at the annihilation site. Consequently, annihilations in
vacancies or open volumes, where electron densities are low, have longer lifetimes. Measured
lifetimes are characteristic of the open volume in which the positrons annihilate, and therefore
can be used to discriminate among different locations where positrons annihilate. A measured
lifetime spectrum N(t) consists of a sum of components corresponding to each annihilation site:
73
k 1
N (t )  
i 1
t
exp  
i
 i 
Ii
(2)
in which k+1 is the number of lifetime components in the spectrum, corresponding to
annihilation in the bulk and in k defect types, and in which i and Ii are the lifetime and intensity
of the ith component in the spectrum. Fitted lifetimes give information about defect/open volume
sizes and characteristics, and the intensities determine defect/open volume concentrations.
Therefore, lifetime spectrum provides information about free volumes in polymers and porous
materials. Positrons also form positronium in polymers which results in a much longer positron
lifetime (Awad et al., 2012).
Here, positron lifetimes were measured using a conventional fast-fast time coincidence
spectrometer with two BaF2 gamma-ray detectors mounted on photomultiplier tubes (Selim et
al., 2013). A positron source was made by depositing
22
NaCl activity on an 8-microns thick
kapton foil that was then folded and sandwiched between two identical samples. PAL spectra
were recorded at room temperature with a time resolution of 250 ps. Several million counts were
accumulated in each lifetime spectrum for good statistical precision. LT9 program was employed
for analyzing the lifetime distribution after applying the source correction term (Kansy 1996).
The measured spectra were resolved into three components (τ1, τ2, and τ3) with their respective
intensities (I1, I2, and I3) for finite-term lifetime analysis. Spectra were fit to the best χ2 with the
most reasonable standard deviation.
The shortest positron lifetime (τ1) could be attributed to the self-annihilation of parapositronium (p-Ps) whereas, the intermediate lifetime (τ2) could be related to the free positron
74
annihilation. The third mean lifetime (τ3) is due to the ortho positronium (o-Ps) pick-off
annihilation in free-volume holes of the amorphous region. A semi-empirical equation given by
the following relation along with the o-Ps lifetime (τ3) could be used to obtain the mean freevolume hole radius (R):

 2πR  
R
1
-1
(τ3 )-1 = 2 1 +
sin 
  ns
2π  R o  
 Ro
(3)
where τ3 and R are expressed in the units of ns and Å, respectively. R0 equals R+ΔR
where ΔR is the fitted empirical electron layer thickness with a value of 1.66Å. Relative
fractional free volume (%), or the number of free volume content (fv), is expressed as follows
(
)
(4)
where I3(%) is the o-Ps intensity and C is a constant.
2.8
Scanning Electron Microscopy (SEM)
Surface topographic images of film surface before and after thermal processing were
analyzed using a Quanta 200F scanning electron microscopy (Field Emmision Instruments,
Hillsboro, OR). Test samples were prepared by cutting 1 cm x 1 cm strips from the packaging
material and mounted on SEM stubs with double sided adhesive tape. The film strips were
sputter coated with gold under vacuum using a Sputter Coater (Technics Hummer V, San Jose,
CA). The stubs were then mounted in the microscope specimen holder and positioned along the
75
electron beam pathway. The accelerating voltage in the specimen chamber was set at 12 KV for
a working distance of 25 mm.
2.9
Data analysis
A completely randomized design was used to evaluate the gas barrier and thermal
properties for the films before and after processing. A general linear model (GLM) was used to
analyze the data and the Fisher’s least significant difference (LSD) test was utilized to determine
the significant differences (P < 0.05) in properties of the films. Data analysis was conducted with
the statistical software SAS version 9.2 (SAS Inst. Inc., Cary, NC).
3.
Results and Discussion
This section discusses the gas barrier, morphological and free volume changes that films A
and B underwent immediately after MATS and retort sterilization.
3.1
Oxygen transmission rate
The OTR of the two films before (control) thermal treatment and immediately after MATS
and retort sterilization is shown in Figure 4.2. Before thermal processing, the OTR of film A and
film B were 0.04 and 0.03 cc/m2 day, respectively. The barrier coated film B had slightly better
oxygen barrier property compared to film A. Nevertheless, the two polymeric packaging
materials used in this study had significantly lower OTRs compared to laminated polyvinylidene
chloride (PVDC) barrier films or silicon oxide coated films, which are currently used as lid films
and flexible pouch materials in the retail market for the production of thermally processed shelf-
76
stable foods. These commercially available films have OTRs in the range of 0.3-2.3 cc/m2 day
(Mokwena et al., 2009).
Both MATS and retort sterilization significantly increased (P<0.05) the OTR of films A
and B immediately after processing. There was a 6-fold and 12-fold increase in the OTR of film
A after the MATS and retort processes, respectively. On the other hand, the OTR for film B
increased by 20 times after MATS process, and increased by about 41 times after the hot water
retort treatment. Consequently, the OTR for films A and B after retort sterilization was twice that
of MATS treatment for the same level of sterilization (F0=6min). Thus, the shorter overall
process time in MATS compared to conventional retort implies lesser deterioration of the
packaging materials as a result of exposure to harsh conditions of heat and moisture during
processing which could have a direct effect on oxygen barrier properties of the packaging films.
Mokwena et al., (2009) also conducted a study on the effect of MW and retort sterilization
on multilayer EVOH films used as lidstock films for rigid polymeric trays. The EVOH films
utilized in the study also exhibited more than twice the level of deterioration in OTR when
processed by retort sterilization as compared to MW treatment. The authors attributed the higher
deterioration level in OTR during retort sterilization to increased plasticization that resulted from
the water absorption by the hydrophilic EVOH layer during processing. Also, the higher
processing time would result in higher water absorption by the films. The hydrophilic nature of
EVOH is one of the major reasons for their reduced success as packaging material in thermal
sterilization application. However, our study involved hydrophobic PET films as the functional
barrier layer and thus, the deterioration in oxygen barrier properties could be related to the
morphological and structural changes in the polymer during and after processing.
77
Even though films A and B had a statistically comparable OTR before thermal treatment, it
is interesting to note that the increase in OTR of film B after MATS and retort sterilization was
significantly greater than that of film A, which had no barrier coating in the PET layer (Figure
4.2). In particular, after the MATS treatment, the OTR of film B was 2.5 times higher than that
of film A. On the other hand, the ratio of OTR of film B to that of film A was 2.7 for the retort
sterilization. The disparity in the performance of the two films may have been caused by the
difference in morphological and free volume properties of the individual polymer layers used in
each multilayer film structure. The stability of the barrier coating layer present in film B during
thermal sterilization may also contribute significantly to how the oxygen barrier property would
OTR (cc/m2- day)
be affected during thermal processing.
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
Control
MATS
Retort
Film A
Film B
Figure 4.2. Oxygen transmission rate of films A and B as influenced by the two thermal
sterilization conditions.
78
3.2
Water vapor transmission rate
Figure 4.3 shows the WVTR of the two films before (control) thermal treatment and
immediately after treatment by MATS and retort sterilization. The WVTR of the two films
before processing (control) were significantly different (p<0.05) from each other, with film B
having nearly 11 times greater transmission than film A. However, after thermal sterilization by
MATS and retort sterilization, the WVTR of film B remained statistically comparable with no
significant changes (P>0.05). On the other hand, film A demonstrated a significant increase in
WVTR after thermal sterilization, with retort sterilization treatment having a greater effect when
compared with MATS. There was a 3.8-fold and 5-fold increase in the WVTR of film A after
the MATS and retort processes, respectively. It is possible that the shorter processing time of
MATS compared to retort sterilization was responsible for the lesser deterioration of the
structural properties of film A and hence, the higher water vapor barrier property.
3.3
Thermal analysis
DSC analysis was utilized to determine the melting temperature (Tm) and enthalpy of
fusion/melting (ΔHm) of the individual film components of film A and film B. The crystalline
morphology of semi-crystalline materials can be characterized using the thermal parameters, Tm
and ΔHm (Kong et al., 2003).The crystallization mechanism influences the rate of gas
transmission through food packaging films. Table 4.1 summarizes the thermal analysis for the
two films before and after the MATS and retort sterilization treatment. The two thermal
processes had no significant influence (P>0.05) on the melting temperature and enthalpy of
melting of the different components of the film A and film B. As the DSC study did not reveal
79
any substantial impact of MATS and retort sterilization on the crystalline morphology of the
individual polymer layers, the issue was further investigated with XRD analysis. This
investigation would help establish the limitation of DSC method in correlating the thermal
WVTR (gm/m2-day)
sterilization conditions with the crystallinity of films A and B.
4.5
4
3.5
3
2.5
2
1.5
1
0.5
0
Control
MATS
Retort
Film A
Film B
Figure 4.3. Water vapor transmission rate of films A and B as influenced by the two thermal
sterilization conditions.
80
Table 4.1. Melting temperature and enthalpy of melting for the polymer layers in films A and B,
untreated, and after thermal sterilization
Film
*
A
B
Tm (oC)
Treatment
Control
PP
Nylon
162.39±0.1 219.75±0.2
MATS
161.65±1.3
219.52±0.1
Retort
162.25±0.1
220.01±0.4
Control
162.95±0.2
219.98±0.1
ΔH (J/g)
PET
255.82±
0.1
255.78±
0.1
256.24±
0.1
PP
20.25±4.6
Nylon
5.96±1.3
PET
3.58±0.4
19.15±3.5
7.32±0.1
5.20±0.6
23.32±3.3
7.76±1.2
5.26±0.5
256.24±
33.52±8.0 11.52±5. 5.01±1.1
0.1
3
MATS
161.54±0.1 218.90±0.1 254.60±
33.71±4.8 8.47±0.9 4.66±0.6
0.2
Retort
161.57±0.3 219.20±0.2 254.55±
27.43±0.3 8.55±0.5 4.44±0.8
0.1
*Film A: PET/adhesive/Nylon/adhesive/PP; Film B: Coated- PET-Coated/adhesive/oriented
Nylon/adhesive/PP
3.4
X-ray diffraction
An illustration of the XRD patterns for film A before (control) and after the two
sterilization treatments are presented in Figure 4.4. The overall crystallinity of the polymeric
films was measured by considering the area under the curve of the peaks for the measured
scattering range. MATS treatment led to an increase in peak area and intensity in the scattering
angle range above 20 degrees, leading to an increase in overall crystallinity of film A. There was
nearly a 5% increase in the crystallinity of film A, implying greater orderliness in the polymeric
chains in film A. On the other hand, the retort sterilization led to a slight decrease in overall
crystalline region for film A. The relatively higher processing time in retort sterilization
compared to MATS would have led to the exposure of the polymeric film to a high-moisture
81
environment, which could result in the plasticization of the Nylon layer which is a hydrophilic
polymer present in the film. This plasticization could cause distortion of some of the crystal
structures of film A and hence, the loss in crystallinity. The superior oxygen and water vapor
barrier property in film A after MATS compared to retort sterilization could be attributed to an
increase in the tortuous path for the gas to travel through the film, resulting from the improved
crystalline morphology of film A after MATS treatment.
6000
Control
5000
Intensity (Counts)
MATS
4000
Retort
3000
2000
1000
0
0
10
20
30
Two Theta (Deg)
40
Figure 4.4. X-ray diffraction patterns for film A before and after the two thermal sterilization
treatments.
82
Film B also showed an increase in overall crystallinity after MATS by nearly 10% whereas
the crystallinity remained statistically comparable after retort sterilization. The improved
crystalline morphology could be responsible for the higher oxygen barrier property of film B
after MATS compared to retort sterilization. Also, film B had lower levels of crystalline region
compared to film A after the two sterilization treatments, which could cause the increased gas
transmission through film B in spite of the barrier coating present on its barrier PET layer.
Additionally, the disparity in the morphology of the individual polymer layers present in the two
films manufactured by different companies could be responsible for the different gas barrier
properties after sterilization treatment.
3.5
Free volume analysis by PALS
Table 4.2 summarizes the effect of thermal sterilization treatment on the o-Ps parameters
measured by PALS. The thermal treatment resulted in an increase in the o-Ps lifetime for the two
films, which validates the increase in free volume size and fraction of the polymer matrix. Free
volume fraction (Fv) for film A increased by 15% and 2% after MATS and retort sterilization,
respectively. On the other hand, film B exhibited a 9% increase in Fv after MATS treatment and a
5% increase after retort sterilization. This thermally-induced increase in Fv could lead to
formation of transient-free volume gaps, which provide a low-resistance avenue for gas
transmission through the polymer matrix. However, Figure 4.5 shows that there was no change
in the o-Ps lifetime distribution for both films A and B after the MATS and retort treatment
which implies that the thermal treatment did not induce a change in the motion of the polymer
chains within the polymer matrix for the two films (Wang et al., 2005).
83
The higher percentage increase in Fv for the two films after MATS compared to retort
sterilization could be attributed to the different heating mechanism involved in MATS compared
to retort sterilization (Table 4.2). The volumetric heating involved in MATS by microwaves
could lead to higher localized temperatures in the polymeric films compared to conventional
retort sterilization. Increase in temperatures leads to formation of thermally-induced free volume
gaps. Nevertheless, it should be noted that that the overall crystallinity of the films after MATS
treatment was higher than the retort sterilization and thus, the level of amorphous and crystalline
region in the two polymeric films varied, which may influence the level of changes in film
morphology and gas-barrier properties. Thus, the free volume studies and crystalline morphology
together helps in understanding the deterioration of oxygen barrier properties for the two films
after thermal sterilization.
Table 4.2. o-Ps parameters from the Positron annihilation lifetime spectroscopy study for the
films A and B, untreated, and after thermal sterilization
Film*
Treatment
A
B
o-Ps intensity,
I3 (%)
Control
o-Ps
lifetime, τ3
(ns)
1.84
15.19
Free volume
radius
(Å)
2.70
Free volume
fraction
(FV)
2.25
MW
1.92
15.91
2.78
2.58
Retort
1.86
15.08
2.72
2.30
Control
1.89
14.00
2.75
2.20
MW
1.97
14.21
2.82
2.40
Retort
1.97
13.68
2.82
2.31
*Film A: PET/adhesive/Nylon/adhesive/PP; Film B: Coated- PET-Coated/adhesive/oriented
Nylon/adhesive/PP
84
0.08
Normalized Intensity
Film A Control
Film A MATS
0.06
Film A Retort
0.04
0.02
0
0
1
2
3
4
o-Ps lifetime, τ3 (ns)
5
Normalized Intensity
85
0.08
Film B Control
Film B MATS
Film B Retort
0.06
0.04
0.02
0
0
1
2
3
4
o-Ps lifetime, τ3 (ns)
5
Figure 4.5 o-Ps lifetime distribution of films A and B before and after the two thermal sterilization treatments.
3.6
Microscopy analysis
Scanning electron microscopy (SEM) images of the barrier side of films A and B before
and after thermal sterilization are shown in Figures 4.6 and4.7, respectively. The coated polymer
surface of film B after MATS and retort sterilization revealed cracks, which could have led to an
increase in the diffusion of the gas molecules. The dimension of cracks was higher in retort
treated films compared to MATS treatment (Figure 4.7). On the other hand, the surface analysis
of film A showed little or no cracks on the PET barrier surface after the two sterilization
treatments (Figure 4.6). These cracks on the surface of film B could be caused as a result of
improper coating operations and hence, could be responsible for the higher gas transmission
value in film B compared to that of film A after the thermal sterilization treatment.
86
a
b
c
87
Figure 4.6. Scanning electron microscopy images of film A (a) control (b) MATS (c) Retort sterilization treatments.
a
b
c
88
Figure 4.7. Scanning electron microscopy images of film B (a) control (b) MATS (c) Retort sterilization treatments
4.
Conclusions
MATS and retort sterilization caused a significant deterioration in oxygen barrier properties
of films A and B. However, the level of deterioration was significantly higher after retort
sterilization compared to MATS treatment. The shorter overall processing time in MATS
compared to conventional retort led to reduced changes in the morphological properties of
polymeric packaging materials which could be responsible for the lesser deterioration in oxygen
barrier property. Thermal characterization studies of the films with DSC did not show significant
changes in the Tm and ΔH after thermal processing. PALS was applied for the first time in MATS
treated polymeric materials. Combining PALS and XRD can reveal the influence of free volume
characteristics and film morphology on gas-barrier properties of MATS and retort processed high
barrier multilayer polymeric films. Microscopic images also highlighted the importance of the
stability of the barrier coating required during and after thermal sterilization. Overall, flexible
plastic pouches containing PET as the barrier layer is a suitable choice as packaging material for
processing shelf-stable foods using the MATS application.
89
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[ASTM] American Society for Testing and Materials. (1995). Standard test method for oxygen
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Bermúdez-Aguirre, D., & Barbosa-Cánovas, G. V. (2011). An update on high hydrostatic
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Brody, A, L. (2012). The coming wave of microwave sterilization and pasteurization. Food
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Cheng, M. L., Sun, Y. M., Chen, H., & Jean, Y. C. (2009). Change of structure and free volume
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Guan, D., Plotka, V. C., Clark, S., & Tang, J. (2002). Sensory evaluation of microwave treated
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Lopez‐Rubio, A., Giménez, E., Gavara, R., & Lagaron, J. M. (2006). Gas barrier changes and
structural alterations induced by retorting in a high barrier aliphatic polyketone
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Mokwena, K. K., & Tang, J. (2012). Ethylene Vinyl Alcohol: A Review of Barrier Properties for
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92
CHAPTER FIVE
SILICON MIGRATION FROM HIGH-BARRIER COATED MULTILAYER
POLYMERIC FILMS TO SELECTED FOOD SIMULANTS AFTER MICROWAVE
PROCESSING TREATMENTS
Abstract
The use of microwave (MW) technology for in-package food sterilization and pasteurization has
the potential for widespread use in the food industry. Since the use of MW technology requires
that food be processed inside its packaging, the interaction between food and its packaging
during processing must be studied to ensure package integrity as well as consumer safety. In this
study, two commercially-available multilayer films developed for retort sterilization were
evaluated for their suitability to microwave processing. Film A was comprised of oriented-nylon
(ON)//coated polyethylene terephthalate (PET)//cast-polypropylene (CPP); Film B consisted of
ON//coated nylon//CPP with overall oxygen transmission rates <0.2 cc/m 2-day. Silicon (Si) was
a major component in the coated PET layer and food-contact CPP layer. This study evaluated the
influence of MW processing on Si migration from films into selected food simulating liquids
(FSL, water and 3% acetic acid) using inductively coupled plasma mass spectroscopy (ICP-MS),
as compared with conventional thermal processing. This study also assessed migration of Si into
FSL in terms of process temperature (70-123oC) and time (18-34 minutes). A Fourier transform
infrared spectrometer was used to evaluate the stability of the silicon-oxygen (Si-O) bonds in the
metal-oxide coated and food-contact layer of the packaging film. Overall, there were no
significant differences (P>0.05) between the level of Si migration from films to FSL and the
93
stability of Si-O-Si bonds during microwave processing as compared to the conventional thermal
processing. However, we found that the final processing temperature and time had a significant
(P<0.05) impact on Si migration into the FSL.
Keywords: Metal-oxide coating; migration; microwave processing; ICP-MS; FTIR
1.
Introduction
The use of microwave (MW) technology for sterilization and pasteurization of in-
packaged, low-acid (pH>4.6) foods is an advanced thermal method with a much shorter
processing time than conventional thermal processes such as retort treatment. MW technology
improves the quality of processed foods and may help meet increasing consumer demand for
high quality, shelf-stable products (Guan et al., 2002). Several MW systems for sterilization of
in-packaged foods have been commercialized in Europe and Japan (Ramaswamy and Tang,
2008). In the United States, the Advanced Thermal Processing Research Team at Washington
State University developed a 915-MHz, single-mode MW sterilization system for processing inpackaged foods (Tang et al., 2006). This research team received U.S. Food and Drug
Administration acceptance for a petition to preserve a homogenous, low-acid food using the MW
sterilization system in October, 2009. The acceptance of this petition was followed in December,
2010 by the acceptance of a second petition to preserve non-homogenous foods using the MW
sterilization system technology (Food Production Daily, 2011). Currently, the MW technology is
being researched for pasteurization of multi-component foods with enhanced physical and
quality attributes (Microwave Pasteurization, 2011).
94
MW processing is a promising preservation technology that is predicted to gain widespread
use in the commercial food industry (Brody, 2012). The first commercial Microwave Assisted
Thermal Sterilization (MATS) unit has been rolled out and is currently being operated on a trial
basis by the U.S. food company, AmeriQual, at one of their facilities (Food Production Daily,
2012). However, during in-package MW pasteurization and sterilization, packaging material is
exposed to temperature and radiation that may alter the mechanical and mass transfer (barrier
and migration) properties of the packaging structure. Therefore, research on the interaction
between packaging material and MW processing during sterilization and pasteurization is
essential to ensure consumer safety. Selecting appropriate packaging materials will not only help
extend the shelf-life of foods, but also ensure minimum chemical and additive migration in the
processed foods (Ozen et al., 2001; Guillard et al., 2010).
The last decade has seen a sharp increase in new multilayer, polymeric-based packaging
materials with a desirable gas barrier and mechanical properties for thermal sterilization
applications. To further improve gas barrier properties, the industry has developed silicon (Si)
and aluminum (Al) metal-oxide coated, high-barrier multilayer polymeric films to withstand
thermal sterilization treatment. Such films are now commercially available. In addition,
polyolefin layers, which function as a food contact and sealant layer in multilayer, polymeric
films, contain various classes of additives, such as antioxidants, antistatic, anti-block, slip agents,
etc. to improve their functionality and fabrication process (Lau et al., 2000). Anti-block agents
help minimize adhesion between the different polymeric layers, and thus improve the
processability of multilayer films. Different types of anti-block additives include synthetic silica,
zeolites, natural silica, talc, which contain the metal Si in various forms (Wells Plastic, 2012).
95
Quantifying silicon and aluminum concentration from packaging materials into food-simulating
liquids (FSL) after thermal processing is one means of determining the migration of coating
particles and additives into real food systems.
Migrating additives and metals can compromise the sensory quality of foods and increase
the toxic substances in packaged products. Therefore, it is imperative to food safety that
researchers examine the influence of thermal food processes on the migration tendencies of
metals and additives (Alin et al., 2010; Mauricio-Iglesias et al., 2010a). The migration tendencies
of metals such as silicon and aluminum can be established by applying a popular analytical
technique for elemental analysis, known as inductively coupled plasma mass spectrometry (ICPMS). This technique has a high sensitivity, in the range of parts per billion (ppb) levels.
Snyder and Breder (1985) developed a new migration cell for evaluation of two-sided
migration of plastic food packaging components such as polymeric additives, as well as chemical
contaminants such as antioxidants, UV absorbers, and monomers etc. into various foodsimulating liquids. In their study, plastic food packaging materials such as polystyrene were
stacked in discs on a copper wire and placed in migration cells filled with a food-simulating
liquid. Small volumes of aliquots were withdrawn from the cell at regular intervals, which
enabled quantitative analysis of the migrant components. However, this type of migration cell
cannot be used to study the migration of coating and additives from flexible polymeric pouches
during thermal processing, due to the inability to simulate thermal process conditions and study
in situ process-package interaction. Therefore, in our study, we developed a test cell that can be
placed in an oil bath for studying chemical or metal migration from polymeric pouches to FSL
under a wide range of thermal processing conditions.
96
Migration from polymeric pouches during microwave processing under controlled
temperature-time process conditions can also be established using a single mode lab scale
microwave digestion system to represent closely the industrial scale MW processing system.
Several studies have been conducted to compare the migration of additives from polymer
packaging to food during domestic microwave heating and conventional heating. Studies by Alin
and Hakkarainen (2010) and Jeon et al. (2006) showed no significant microwave-induced,
nonthermal effects of increasing migration of additives into different FSLs. However, a third
study showed significantly higher overall migration from poly(vinyl chloride) (PVC) during
domestic microwave heating compared to conventional heating (Galotto and Guarda, 1999).
Much of the migration research related to microwave heating has concentrated on domestic
microwave heating as compared to industrial microwave heating. Little research has been
conducted to develop a methodology for studying the migration of additives from packaging
material into food during industrial microwave pasteurization and sterilization. Therefore, this
study aims to ensure the safety of microwave-based industrial processes.
MW processing has significantly less effect on the gas barrier property of multilayer
polymeric packaging films and lidstocks than the conventional retort process for the same level
of sterilization (Mokwena et al., 2009). This implies that the reduced overall MW processing
time compared to conventional retorting plays a role in reducing the deterioration of the gas
barrier layer in the packaging film after thermal treatment. However, to the best of our
knowledge, no experimental research has explored the impact of MW pasteurization and
sterilization processing conditions on the migration of coating particles from high-barrier coated
multilayer polymeric packages to food as compared to conventional retort processes. Studies of
this nature would provide valuable information on the possibility of adopting commercially97
available conventional retort packaging materials to industrial microwave processes and help
ensure food safety.
Thus, the objectives of this work are: (1) to develop a methodology for examining metal
migration from multilayer polymeric pouches to FSL after MW and conventional thermal
processing; (2) to determine the influence of MW pasteurization and sterilization treatments on
the migration of Si from metal-oxide coated multilayer polymeric films to FSL compared with
conventional heating; (3) to explore the stability of coating particles and additives in metal-oxide
coated, multilayer food packaging materials as an influence of MW process conditions compared
with conventional heating.
2.
MATERIALS AND METHODS
2.1
Migration test cell
An aluminum test cell was designed and fabricated for use in migration studies from
flexible multilayer polymeric pouches to food-simulating liquid (FSL) during conventional retort
thermal processing. Retort parameters were simulated by placing the cell in an oil bath, set at the
operating temperature of conventional heating process, to achieve the target sterilization level.
2.1.1 Design criteria
A detailed schematic diagram for the design of the cell is shown in Figure 5.1. The design
was guided by the following criteria:
98

Simulate a come-up time (CUT: the time required for the FSL in the pouch to
reach the target process temperature) similar to that required by flexible pouches
under commercial process conditions for conventional thermal processing (Chung
et al., 2008; Kong et al., 2007).

Incorporate a thermocouple to measure the temperature of the food-simulating
liquid (FSL) accurately at the half depth of the pouch to ensure good contact with
the food medium.

Hold flexible multilayer polymeric pouches of dimensions 3x2.25 inches
containing 13.5 ml of food simulating liquids. Overall, the simulant volume-tosurface-area of the pouch was at least 2 ml/in2, to prevent the possibility of low
solubility of migrant particles in the food-simulating liquid. This prevents the
underestimation of migration, a limitation set by US Food and Drug
Administration for a chemical migration study (US FDA, 2007).

Allow simulation of pasteurization and sterilization temperature-time process
conditions.

Facilitate an accelerated storage study by easily removing the thermally processed
multilayer polymeric pouch from the cell and placing the pouch in a conventional
oven for 30 days at 40oC (US FDA, 2007).
The top part of the test cell is comprised of a retention clip for clamping the multilayer,
polymeric pouch in the cell. The top portion of the cell also includes a screw cap closure,
through which a 0.032 inch Type T thermocouple (Omega Inc., Stamford, CT) is inserted for
monitoring the temperature of the food simulating liquid in the pouch. Once the CUT is
calculated for the FSL, experiments are carried out in similar cells without the thermocouple
99
attachment. The bottom and top portions of the cell are opened and closed along machine lines.
An O-ring in the top portion of the cell helps ensure an hermetic seal of the migration cell
(Figure 5.1).
2.2
Metal-oxide coated multilayer polymeric films
This study evaluated two high gas-barrier, multilayer films fabricated by the EVAL Co. of
America (Houston, TX). Film A was laminated and composed of an outer layer of 15 µm of
oriented nylon (ON), a middle layer of 12 µm of metal-oxide coated polyethylene terephthalate
(PET), and an inner layer of 70 µm of cast polypropylene (CPP) that directly contacts the food
surface. Film A is also known as ON// coated PET//CPP. Film B was also laminated and consists
of a middle layer of 15 µm of metal-oxide coated nylon, sandwiched between an outer layer of
15 µm of oriented nylon and an inner layer of 50 µm of cast polypropylene. Film B is denoted as
ON//coated nylon//CPP. The coating applied to improve the gas barrier properties of the
functional barrier polymer layer (polymer layer that is responsible for gas barrier properties)
usually consists of inorganic, metal-oxide coating particles.
100
101
Figure 5.1. Picture and schematic diagram of migration test cell. Dimensions shown are in cm
2.3
Characterization of the metal-oxide coated multilayer polymeric film
Metal concentration in both of the multilayer polymeric films A and B was analyzed before
and after processing by microwave digestion coupled with ICP-MS.
2.3.1 Microwave Digestion of film
A Discover SP-D CEM microwave system (CEM Corporation, Matthews, NC) was used
for multilayer polymeric film digestion. Film samples (0.05g) of 1 square inch surface area were
weighed into a 35 ml quartz test cell, and 5 ml of nitric acid (HNO3) (69-70% reagent grade,
Mallinckrodt Baker Inc., Phillipsburg, NJ) was added. The following program was used for
digestion: the temperature was raised to 220oC in 5 minutes and held for 6 minutes. After cooling
to ambient temperature in 3 minutes, the digested solution was diluted with Milli-Q water
(Millipore Corporation, Billerica, MA) to 25 ml and analyzed by ICP-MS.
2.3.2 Food simulants
Water was used to simulate aqueous foods, and was prepared by passing distilled water
through Milli-Q water purification system (Millipore Corporation, Billerica, MA). Aqueous
acetic acid (3%) (w/v) (reagent grade, J.T.Baker, Mansfield, MA) was used to simulate low-acid
foods, based on the US Food and Drug Administration recommendation.
21
Thirty grams of
acetic acid was weighed accurately and made up to 1000ml with Milli-Q water in a volumetric
flask to give 3% (w/v) aqueous acetic acid. Similar food simulants are also approved by the
102
European Commission for migration analysis of plastic packaging constituents, which come in
contact with foodstuffs (EEC, 1985).
2.4
Thermal treatment
2.4.1 Conventional Heating (CH)
Flexible pouches with dimensions of 3 inch x 2.25 inch were prepared from each of the
films discussed above. These pouches were then filled with 13.5 ml FSL, for an overall volumeto-specimen surface-area of 2 ml/in2. Pouches were sealed with a minimum headspace using an
impulse sealer (MP-12; J. J. Elemer Corporation, St. Louis, MO) with a 4 sec dwell time. To
study migration during conventional thermal sterilization conditions, the pouches containing FSL
were placed in migration test cells and heated to sterilization temperature (121oC) in an oil bath
(HAAKE W15, Thermo Electron Corporation, Waltham, MA) using fisher bath oil (Fisher
Scientific, Hanover Park, IL) as a heating medium. The two FSL had come-up-time of 5 minutes
to reach the sterilization temperature that was measured using a 0.032 inch type T thermocouple
(Omega Inc., Stamford, CT). The thermocouple was incorporated in the cell to accurately
measure the temperature of the FSL at a position at the half depth of the pouch. The migration
cells were heated for 40 minutes at 121oC (CH1) to simulate industrial sterilization schedules for
conventional thermal retorting for single meal-sized pouches containing low-acid foods. The
process condition represents the level of sterilization at the cold location of single meal pouches
close to F0 = 6 min, which is generally used for commercial retail markets.18 Once heating was
complete, the migration test cell was removed from the oil bath and immediately cooled in a tray
103
containing ice/water mixture. The pouches were removed from the cell and the FSL was
collected for migration studies.
2.4.2 Microwave Heating (MW)
Flexible pouches with dimensions of 2.5 inch x 2 inch were prepared from each of the films
discussed above and filled with 10 ml FSL to have an overall volume-to-specimen surface-area
of 2 ml/in2. A Discover SP-D CEM microwave system (CEM Corporation, Matthews, NC) was
used to simulate the four processing periods of microwave-assisted thermal sterilization, namely
pre-heating, MW heating, holding at target temperature, and cooling (Tang et al., 2008). The
Discover SP-D CEM microwave system has a 35 ml quartz test cell with a maximum working
volume of 25 ml, which was utilized to process the flexible pouches containing FSL in a water
medium (Figure 5.2). Temperatures of the test cell were continuously monitored during
processing using an infrared sensor, which was calibrated using a fiber optic temperature probe
to increase accuracy. The time-temperature combination for CEM-based MW process was
selected to match closely with the commercial sterilization and pasteurization schedules for
single meal-sized pouches containing low-acid foods. This created a fair comparison of the
packaging-food interaction during microwave heating with the conventional heating process
(Tang et al., 2008; Mokwena et al., 2011; Guan et al., 2003). Details of the various treatments are
outlined in Table 5.1. After heating, air at ambient temperature was allowed to pass through the
system to cool the MW cell containing the test pouch to ambient temperature.
104
Figure 5.3 shows representative temperature-time profile during conventional (CH1) and
microwave (MW1) heating for the same level of sterilization at the cold location of single meal
pouches close to F0 = 6 min. FSL collected from the pouch was utilized to quantify the level of
migration.
Table 5.1. Microwave processing conditions used in the current study
Treatment
Stage
Preheating from 25 °C
End
Ramp Hold
Temperature
time
time
°
(min)
(min)
( C)
Total
time
(min)
Processing at target temperature
Temperature
Ramp
Hold
°
time
time
( C)
(min)
(min)
MW1
70
9
1
123
4
4
18
MW2
50
9
1
90
4
4
18
MW3
50
9
1
70
4
4
18
MW4
70
9
1
123
12
12
34
MW5
50
9
1
90
12
12
34
MW6
50
9
1
70
12
12
34
105
35 ml test cell with 25 ml
working volume
2.5 x 2 inches pouch
containing 10 ml FSL
106
Figure 5.2. Picture of the test cell in the CEM microwave system containing the flexible pouch with FSL. Pouch samples are
completely submerged in water in the test cell during processing.
140
Temperature (°C)
120
100
80
60
Conventional (CH1)
40
Microwave (MW1)
20
0
0
10
20
30
40
50
60
Time (min)
Figure 5.3. Representative temperature-time profile during conventional (CH1) and microwave
(MW1) heating.
2.5
Inductively coupled plasma-mass spectrometry (ICP-MS)
The elemental analysis of the FSL in the thermally-processed, flexible polymeric pouches
was performed using an inductively coupled plasma mass spectrometry (ICP-MS) method. ICPMS (Agilent 7500cx system, Agilent Technologies, Santa Clara, CA) equipped with a double
107
bypass quartz spray chamber and a concentric quartz nebulizer was used to analyze the migration
of metals in FSL, with argon used as the carrier gas. ICP-MS operating conditions were: plasma
operated at a power of 1600 W; flow conditions of the argon gas were 15 L/min plasma gas, 1
L/min auxiliary gas, 0.9 L/min nebulization gas, and 0.25 L/min make-up gas. A full quantitative
method was utilized in the ICP-MS system for measuring the 27Al, and 28Si concentration in the
FSL.
Quantitative analysis was conducted, based on a calibration curve developed with the Si,
and Al AccuTrace reference standard (AccuStandard, New Haven, CT). For analyzing the metal
concentration in water as FSL, seven standard solutions with concentrations of 0.1, 0.5, 1.0, 1.5,
2.0, 2.5, 5.0, and 10.0 μg ml-1 were obtained by diluting the stock standard solution (1000 μg ml1
) with 2% HNO3. The responses were linear over the concentration ranges, with a correlation
coefficient greater than 0.995 for both Si and Al. On the other hand, while analyzing metal
concentration in 3% acetic acid as FSL, the standard solutions were obtained by diluting the
stock solution in 3% acetic acid to avoid any potential effect of matrix (extract that can increase
or decrease the analyte signal). Correlation coefficients of 1.000 and 0.997 were obtained for the
Si and Al calibration standard solutions prepared in 3% acetic acid, respectively. All FSL sample
studies for migration were quantified against these calibration curves. Furthermore, collision
reaction cell (CRC) technology in the Agilent ICP-MS was used to reduce the potential
polyatomic interferences that
28
Si suffers. The use of glassware was avoided during preparation
of samples to minimize the influence of dissolution of Si from the glassware. Food simulant
blanks were prepared by keeping the FSL in contact with the unprocessed polymer pouch for a
similar duration of time as that of the thermal process. All measurements were carried out in
triplicate, in which each measurement included analysis of the FSL from an individually
108
processed pouch and subtracted from the blank values. Quality assurance measures were taken
by placing blanks and two samples of known concentration levels after every ten measurements.
This helped establish the repeatability and accuracy of the method.
2.6
FTIR-ATR spectroscopy
Fourier transform infrared (FTIR) spectroscopy was applied to investigate the influence of
thermal processing on the surface characteristics of the multilayer polymeric films with 3%
acetic acid as the food simulating liquid. FTIR spectra of the polymeric films were recorded
using a germanium 45o ATR (Attenuated total reflectance) crystal on a Shimadzu IR Prestige 21
FTIR spectrometer (Shimadzu Corporation, Kyoto, Japan) equipped with KBr beam splitter and
a DLATGS (deuterated L-alanine doped triglycene sulphate) detector. The spectra were collected
over the wave number range of 4000-800 cm-1 by accumulating 64 scans at a resolution of 4cm-1
to study the stability of the silicon-oxygen (Si-O) bonds in the food contact layer (polypropylene)
and the metal-oxide coated layers of the two multilayer films (PET for Film A; Nylon for Film
B). However, since the analysis was performed in reflection mode, only the surface in contact
with the crystal was characterized, because the wave protrudes less than 0.7 microns. To enable
the characterization of the coated PET and nylon layers, separate films were fabricated with the
coated polymeric layers on the surface (Coated PET//ON//CPP; Coated Nylon//CPP). These two
films with coated layers on the surface were processed at selected conditions (see profiles in
Figure 3, CH1 and MW1) and analyzed for the stability of the metal-oxide coating. All spectra
pre-treatments were performed using Omnic v8 (Thermo Fisher Scientific, Madison, WI).
Processing included baseline correction, ATR correction, smoothening, and normalization on the
109
specific band of the polymer matrix. All experiments were performed in triplicate, and results
are displayed as the mean value of measurements.
2.7
Data analysis
The metal migration data for the two films before and after thermal processing were
examined using a completely randomized design. Data was analyzed using the general linear
model (GLM), and significant differences (P < α) in metal concentration at various temperature
and time treatments were determined with Fisher’s least significant difference (LSD) test (α =
0.05). Data analysis was conducted with statistical software SAS version 9.2 (SAS Inst. Inc.,
Cary, NC).
3.
RESULTS AND DISCUSSION
3.1
Film Characterization
The proposed method of microwave digestion coupled with ICP-MS was applied to
analyze the two films for their silicon concentration before and after the microwave (MW1) and
conventional (CH1) thermal processing applications. The concentration of Si in film A decreased
from 64.5±2.1 to 63±3.0 μg /square inch film after the MW treatment, whereas in the retort
treatment, the concentration of Si in film A decreased to 61±2.6 μg /square inch film. The Si
concentration for film B decreased from 61±3.1 to 47±0.6 μg /square inch film after the MW
treatment and 52±6.8 μg /square inch film after the retort one. The decrease in the average Si
concentration after the two thermal treatments indicates a dissociation of Si from the metal-oxide
110
coated multilayer polymeric packaging films. However, there was no significant difference
(P>0.05) in the final Si concentration between the conventional and microwave thermal process
for both films A and B. Thus, microwave heating did not induce any additional Si migration
compared to conventional heating.
3.2
Migration study
The amount of Si and Al migrating into the FSL from the polymeric pouches during
thermal processing was quantified to assess the effect of conventional heating vs. microwave
processing on coating and additive migration. However, the concentrations of Al were found to
be below 10ppb, the detection limit (three times the standard deviation of the signal for blank
measurement) of the ICP-MS employed in the study and hence, only concentration of Si in the
FSL was reported in this section to describe the migration of additives and metal-oxide coating
into food. Additionally, the influence of final MW process temperature, and MW processing
times on the Si migration in water were also evaluated.
3.2.1 Effect of type of thermal process
The effect of MW treatment (MW1) on the migration of Si from Films A and B into the
two FSL was assessed and compared with conventional heating (CH1). The processing
conditions for MW1 and CH1 closely match the temperature-time combinations required for
similar level of sterilization of single meal-sized pouches or trays containing low-acid foods
(Tang et al., 2008; Mokwena et al., 2011). Table 5.2 reports the values for Si migration from
111
both of the films into water and into 3% acetic acid after the two processes. Blank values of 0.04
and 0.02 mg Kg-1 for water and 3% acetic acid as FSL, respectively, was subtracted to attain the
final Si migration concentration. In both FSLs, there was no significant difference (P>0.05) in
the amount of Si migration from the two films when processed with microwaves compared to
conventional heating for the same level of sterilization. Thus, microwave processing had no
significant non-thermal influence on Si migration.
Regarding FSL, the Si migration was higher in water compared to 3% acetic acid for films
A and B processed with both microwave and conventional thermal processes. The higher Si
migration in water could be attributed to the increase in solubility of Si in water at higher
temperatures. Table 5.2 shows no significant difference (p>0.05) in Si migration between films
A and B in both of the FSLs after thermal processing, with and without microwave application.
Therefore, there was substantial alteration between PET and nylon as a functional barrier layer
(the polymer layer responsible for gas barrier properties) in terms of Si migration.
Regarding food regulation, there are no established limits on the migration of metal-oxide
coating particles and metals from additives such as anti-block agents. Previous studies on
migration of clay minerals present in packaging material to food in terms of Al and Si
concentrations suggest that the migration limit of such metals is close to 9 mg kg-1 of food-based
on an opinion by European Food Safety Authority (Mauricio-Iglesias et al., 2010b). However,
the highest value of Si migration obtained in this study was significantly less than the suggested
limit.
112
Table 5.2. Concentration (mg kg-1 FSL) of Silicon migrated from Films A (ON// coated
PET//CPP) and B (ON//coated nylon//CPP) to FSL during MW1 and CH1 treatments
Film
FSL
Process
Water
Conventional Heating
0.92±0.18a
Microwave
1.05±0.17a
3% Acetic Acid
0.55±0.03a
0.60±0.05a
Water
1.25±0.12a
1.07±0.04a
3% Acetic Acid
0.60±0.03a
0.51±0.03a
A
B
Values are means ± 1 standard deviation. Means with different letters within a row are
significantly different (P<0.05).
3.2.2 Effect of MW process temperature
The effect of final MW process temperature on the migration of Si from the two films into
water is illustrated in Figure 5.4. The process temperatures were chosen to represent sterilization
(MW1) and pasteurization (MW2 and MW3) conditions. An increase in MW process
temperature from 70-123oC led to a significant increase (P<0.05) in Si migration into water for
the two films. At 70oC, the amount of Si migration from film A and film B into water was 0.10
and 0.12 mg Kg-1, respectively. There was a 3.5-fold and 10-fold increase in levels of Si
migration from film A, when the temperature was increased from 70oC to 90oC and 123oC,
respectively. On the other hand, Si migration for Film B increased by nearly 4 times and 9 times
to a concentration level of 0.46 and 1.07 mg Kg-1 as the temperature was increased from 70oC to
113
90oC and 123oC, respectively. These observations suggest that the level of Si migration is
strongly dependent on the final treatment temperatures. Temperatures close to sterilization
process conditions could cause metal-oxide coating particles and additives in the coated layer
and food-contact layer, respectively, to undergo physicochemical modifications, possibly leading
to their migration into the FSL.
1.4
70 C
90 C
123 C
Silicon Concentration (mg kg-1)
1.2
c
c
1
0.8
0.6
b
0.4
b
0.2
a
a
0
Film A
Film B
Figure 5.4. Silicon Migration (mg kg-1 FSL) from the two films to aqueous FSL as an influence
of MW process temperature. Mean values with different letters are significantly different
(P<0.05).
114
3.2.3 Effect of MW process time
Results for migration of Si into water during MW processing under two time periods for
film A and film B are shown in Figures 5.5 and 5.6, respectively. MW1, MW2, MW3 underwent
a total processing time of 18 minutes, while MW4, MW5, MW6 underwent a total processing
time of 34 minutes at the three process temperatures (Table 5.1) to elucidate the influence of
increased holding time at the final processing temperature. For film A, there was a significant
increase (P<0.05) in Si migration at all three processing temperatures, when total processing
time increased from 18 to 34 minutes (Figure 5.5). On the other hand, for film B there was a
significant increase (P<0.05) in Si migration when the total processing time was increased from
18 to 34 minutes at 70 and 90oC (Figure 5.6). However, the increase in processing time did not
lead to a significant increase (P>0.05) in migration at 123oC. It is notable that the percentage
increased in migration at 123oC was 10% and 29%, when processing time was increased for film
A and film B, respectively. On the other hand, increasing processing time at pasteurization
temperatures (70 and 90oC) led to 68% more Si migration for both films. Hence, the influence of
total processing time on the migration of Si to water was found to be less at sterilization
temperatures compared to that at pasteurization temperatures.
115
1.6
b
Silicon Concentration (mg kg-1)
1.4
18 min
34 min
1.2
a
1
b
0.8
0.6
a
0.4
0.2
b
a
0
70
90
Temperature (oC)
123
Figure 5.5. Silicon Migration (mg kg-1 FSL) from the film A to aqueous FSL as an influence of
MW process time. Mean values of three replicates with different letters are significantly different
(P<0.05).
116
1.4
18 min
Silicon Concentration (mg kg-1)
1.2
34 min
a
a
1
0.8
b
0.6
a
0.4
b
0.2
a
0
70
90
Temperature (oC)
123
Figure 5.6. Silicon Migration (mg kg-1 FSL) from the film B to aqueous FSL as an influence of
MW process time. Mean values of three replicates with different letters are significantly different
(P<0.05).
3.3
FTIR-ATR spectroscopy
To explore the influence of microwave (MW1) and conventional (CH1) thermal processing
on the chemical structure of the silicon-oxide bonds present in the metal-oxide coated barrier
layer and the food contact layer of films A, corresponding FTIR spectra in the 800-1300 cm-1
range are compared in Figuress. 5.7(a) and 6.7(b), respectively. The characteristic bonds of
117
interest to the study of silica bond stability in the metal-oxide coated layer and the additives in
the food contact layer include the Si-O-Si stretching (1050-1300 cm-1) and Si-O stretching (8001050 cm-1) (Lynch et al., 2008). Figure 5.7 (a) illustrates the small decrease in the absorption of
the peaks at 972, 997, and 1167 cm-1 in the metal-oxide coated layer of film A after both MW
and conventional heat treatment. This decrease in absorption suggests that thermal treatment
altered both the Si-O-Si and Si-O stretching bonds in the metal-oxide coated polymer layer,
which could lead to loss of stability in the coating particles. On the other hand, the spectra from
the food-contact side of film A reveals a slight broadening of the peaks at 1200 and 1265 cm-1
after thermal treatment, implying instability in the Si-O-Si stretching bond present in the
additives in the food contact polymer layer (Figure 5.7b). In a previous study, broadening of
absorption bands corresponding to Si-O stretching was used to explain the distortion of
tetrahedral sheets present in the montmorillonite structures (Mauricio-Iglesias et al., 2011).
Therefore, the minor chemical modifications discussed above in the metal-oxide coated barrier
layer and the food contact layer may explain the small concentrations of Si migration from the
food packaging film into the FSL after thermal processing.
In the case of film B, the FTIR-ATR results for the coated polymeric layer revealed
insignificant changes at 1167 cm-1 (Figure 5.8a) after thermal treatment. Also, the spectra for the
food-contact layer of film B illustrates a slight broadening of the peak at 1200 and 1265 cm -1
after thermal treatment, very similar to the peaks observed for the food contact side of film A
(Figure 5.8b). These observations suggest that both MW and conventional heating at sterilization
temperatures (MW1 and CH1) had little influence on the silicon-oxygen chemical structures in
the coating and additive particles. Hence, there was no marked difference between the influences
that microwave sterilization and the conventional retort sterilization had over Si migration.
118
(a)
(b)
Figure 5.7. FTIR-ATR spectra of film A (a) Coated metal-oxide layer before (control) and after
MW1 and CH1 treatments. (b) Food contact layer before (control) and after MW1 and CH1
treatments. Spectrum represents average of three replicates.
119
(a)
(b)
Figure 5.8. FTIR-ATR spectra of film B (a) Coated metal-oxide layer before (control)
and after MW1 and CH1 treatments. (b) Food contact layer before (control) and after
MW1 and CH1 treatments. Spectrum represents average of three replicates.
120
4.
CONCLUSIONS
The results of this study indicate that Si may migrate from two commercially-available,
high gas barrier, multilayer coated films into FSLs, which represent aqueous and low-acid foods.
No significant difference was found in Si migration after MW heating compared to conventional
retort process conditions. The final MW process temperature had a strong influence in the level
of Si migration for the two films. On the other hand, the total MW processing time had a higher
impact on Si migration at pasteurization temperatures (70 and 90oC) compared with sterilization
temperatures (123oC). FTIR assisted in the study of the chemical stability of the Si-O-Si bonds
present in the metal-coated and food-contact layer. No significant difference was found between
the stability of the bonds when processed with MW vs. conventional retort sterilization.
However, overall migration of Si was found to be <1.5 mg Kg-1 FSL in all cases, suggesting that
selected films can be used for MW processing applications. This study presents the possibility of
utilizing commercially available retortable high barrier coated multilayer polymeric films for inpackage MW sterilization and pasteurization while ensuring consumer safety.
121
REFERENCES
Alin, J., & Hakkarainen, M. (2010). Type of polypropylene material significantly influences the
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125
CHAPTER SIX
CONCLUSIONS, CONTRIBUTION TO KNOWLEDGE AND RECOMMENDATIONS
1.
Major Conclusions
The major findings of this research are summarized below:
a.
PATS had a significant influence on the oxygen barrier properties of EVOH based
multilayer polymeric films investigated in this study. This work also highlights the
advantage of DSC analysis for studying the physical ageing of polymers during
storage.
b.
X-ray diffraction and PALS are powerful techniques to investigate film morphology
and free volume characteristics which helps understanding the gas barrier changes
after food sterilization operations.
c.
The advantages of using multilayer films containing EVOH as the barrier layer in
PATS applications to produce shelf-stable foods which could provide a one-year shelf
life was demonstrated.
d.
The shorter overall processing time in MATS compared to conventional retort led to
reduced changes in the morphological properties of polymeric packaging materials
which could be responsible for the lesser deterioration in oxygen barrier property.
Microscopic images also highlighted the importance of the stability of the barrier
126
coating layers required during and after thermal sterilization. Overall, flexible plastic
pouches containing PET as the barrier layer is a suitable choice as packaging material
for processing shelf-stable foods using the MATS application.
e.
A methodology and test cell was developed for examining metal migration from
multilayer polymeric pouches to FSL after MW and conventional thermal processing.
No significant differences between the level of Si migration from films to FSL and
the stability of Si-O-Si bonds during microwave processing as compared to the
conventional thermal processing. Final processing temperature and time had a
significant (P<0.05) impact on Si migration into the FSL.
2.
Contributions to knowledge
a.
Characterization of high performance polymeric films (multilayer films and barrier
coated films) subjected to MATS and PATS, and during storage.
b.
Utilization of advanced and powerful techniques such as X-ray diffraction and PALS
for studying the morphology and free volume of multilayer polymers treated in high
pressure and electromagnetic fields.
c.
Development of a methodology for studying the migration of additives and coating
particles from packaging to model foods during thermal processing and storage.
127
3.
Research Recommendations
a. Studies showing how the changes in gas barrier properties of polymeric films during
thermal processing translate to modification in shelf-life of selected thermally
processed foods are required in terms of quality parameters (color, texture, weight loss,
pH, Lipid oxidation, protein degradation) immediately after processing and during a
storage period of up to one year.
b. Systematic PALS analysis with single layer barrier polymers subjected to high
pressure and microwave fields to gain insight on the basic mechanisms of how these
food technologies influence the free volume parameters of films should be investigated.
c.
Further studies on the migration kinetics of chemical additives from films to model
foods after MATS and PATS technologies should be performed. Additionally, a
quantitative model should be developed to understand the migration of metals and
additives from packaging materials to food simulating liquids and model food based on
a mechanistic approach.
d. Appropriate selection of the individual layers in the multilayer polymeric packaging
material suitable for MATS and PATS application should be carefully identified. This
would help an increase in scientific understanding of the behavior of the individual
polymer layers when subjected to microwave and high pressure fields through
Materials Science techniques.
128
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