polymeric-based multilayer food packaging films for

POLYMERIC-BASED MULTILAYER FOOD PACKAGING FILMS FOR PRESSURE-

ASSISTED 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

ii

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.

iii

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.

iv

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-in-

law 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.

v

POLYMERIC-BASED MULTILAYER FOOD PACKAGING FILMS FOR PRESSURE-

ASSISTED 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

vi

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, X-

ray 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.

vii

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

viii

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

3. Results and Discussion ...................................................................................................... 20

3.1 Film characterization after PATP ............................................................................... 20

3.1.1 Oxygen transmission rate .................................................................................... 21

3.1.2 Thermal analysis ................................................................................................. 25

3.1.3 X-ray diffraction .................................................................................................. 26

3.2 Film characterization during long term storage .......................................................... 28

3.2.1 Oxygen transmission rate .................................................................................... 28

3.2.2 Thermal analysis ................................................................................................. 30

4. 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

ix


2.1 Multilayer EVOH films .............................................................................................. 41

2.2 Pressure-assisted thermal sterilization (PATS) .......................................................... 42


2.4 Water vapor transmission rate .................................................................................... 44


2.6 Positron annihilation lifetime spectroscopy (PALS) .................................................. 45

2.7 Data analysis of OTR and WVTR .............................................................................. 48


3.1 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

4. 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

x

1. Introduction ........................................................................................................................ 63


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


2.6 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



3.2 Water vapor transmission rate .................................................................................... 79



3.5 Free volume analysis by PALS .................................................................................. 83

3.6 Microscopy analysis ................................................................................................... 86

4. Conclusions ........................................................................................................................ 89

References ................................................................................................................................. 90

xi

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 Design criteria ..................................................................................................... 98

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 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

3. RESULTS AND DISCUSSION ...................................................................................... 110

3.1 Film Characterization ............................................................................................... 110

xii

3.2 Migration study......................................................................................................... 111

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 FTIR-ATR spectroscopy .......................................................................................... 117

4. 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

xiii

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

xiv

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

xv

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 10

oC/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 10

oC/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

xvi

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

xvii

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


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


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

xviii

DEDICATION

This dissertation is dedicated to my parents for all their support, love, and encouragement

throughout my life.

1

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 high-

quality, 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

2

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 shelf-

stable 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

3

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)

Maximum moisture

gain (+) or loss (-) %

Canned milk, meats, fish,

poultry, vegetables, soups

1-5 - 3%

Beer, wine 1-5 - 3%

Canned fruit 5-15 - 3%

Dried foods 5-15 +1%

Carbonated soft drinks, fruit

juices

10-40 - 3%

Oils, salad dressings, peanut

butter

50-200 +10%

Jams, jellies, syrups, pickles,

olives vinegar

50-200 3%

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,

4

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.

5

Table 1.2. Performance of various multilayer polymeric films after thermal sterilization

Processing Conditions

Film

Structure

OTR1

before

processing

(cc/m2

day)

Thermal

Process

Pressure

(MPa)

T

(oC)

Time

(min)

OTR1 after

processing

(cc/m2 day)

Reference

PET/EVOH/

PP

0.16±0.01

Microwave

-

125

9

0.79±0.01

Mokwena et

al., (2009)

PET/PP/tie/Ny

lon6/EVOH/

Nylon6/tie/PP

0.096±0.01

Microwave

-

125

9

1.58±0.22

Mokwena et

al., (2009)

PET/EVOH/

PP

0.16±0.01

Retort

-

121

28

1.75±0.04

Mokwena et

al., (2009)

PET/PP/tie/Ny

lon6/EVOH/

Nylon6/tie/PP

0.096±0.01

Retort

-

121

28

4.57±0.59

Mokwena et

al., (2009)

PET/Al/CPP

<0.05

PATS

688

121

3

0.4±0.15

Koutchma et

al., (2009)

Nylon/Al/PP

<0..05

PATS

688

121

3

0.44±0.05

Koutchma et

al., (2009) 1The measurements were made in duplicates at 55%RH and 23

0C

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

6

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

7

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 8-

month 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 gas-

barrier 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.

8

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


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

9

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.

10

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/Researcher-

hails-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.

http://www.foodproductiondaily.com/Processing/Researcher-hails-major-milestone-for-microwave-sterilization-technology


11

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.

12

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 100

oC.

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

13

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 100oC,

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).

14

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 high-

pressure/high-temperature sterilization on a few polymeric materials, and found that foil-

laminated 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

15

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

16

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,

17

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

18

(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.

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

19

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.

20

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 0.05

o 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.

21

3.1.1 Oxygen transmission rate

The OTR of films A and B before (control) PATP were 0.24 and 0.11 cc/m2 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.

22

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

23

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.

24

Table 2.1. Values of OTR obtained for polymeric packaging films in different studies after high-pressure/high-temperature processing

Film Preheating High Pressure Conditions Oxygen Transmission Rate References

T (oC) Time

(min)

Pressure

(MPa)

T

(oC)

Time

(min)

OTR (cc/ day m2

atm)

RH

(%

)

T

(oC)

PET/EVOH/P

P

90 10 680 100 5 1.1±0.09 55 23 Present Study

PET//PP/tie/

Nylon6/EVO

H/Nylon

6/tie/PP

90

10

680

100

5

0.43±0.07

55

23

Present Study

PET/AlOx/

CPP

90

11

688

121

3

19.6±0.7

*

*

Koutchma et

al., (2010)

Biaxial

Nylon/EVOH

90

12.2

688

121

3

2.0±0.3

*

*

Koutchma et

al., (2010)

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)

25

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 Treatment Tm (oC) ΔH (J/g)

A (32 mol% ethylene) Control 182.8±0.3a 6.5±0.2

b

680 MPa, 3 min, 105oC 182.2±0.1

a 6.7±0.1

b

680 MPa, 5 min, 100oC 182.6±0.3

a 5.8±0.8

b

B (27 mol% ethylene) Control 186.1±0.2c 4±0.6

d

680 MPa, 3 min, 105oC 186.1±0.1

c 4.1±0.8

d

680 MPa, 5 min, 100oC 186.1±0.1

c 4.4±0.5

d

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)

26

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

27

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.

28

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

29

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

30

significant difference (P<0.05) between the pre-processing OTR for film B with the post-

processing 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).

31

Table 2.3. OTR values (cc/m2 day) for the multilayer EVOH films after PATS at 680 MPa-5min-100

oC

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

Film

Components/Total

Storage Time (days)

Control 7 60 240

A PP 41.81±0.99 30.12±4.72 37.97±0.64 34.84±0.79

EVOH 6.67±0.74 6.17±0.77 7.28±0.62 7.03±0.65

PET 4±1.06 3.86±0.09 5.31±0.86 4.42±0.12

Total melting enthalpy 52.48±0.8a 40.15±5.59

b 50.56±2.13

a 46.29±1.56

ab

B PP 32.08±1.04 28.37±0.56 26.51±2.27 35.85±4.89

EVOH 4.37±0.31 4.72±0.01 4.2±0.02 5.11±0.48

Nylon 5.8±1.35 5.52±0.26 5.15±0.18 6.43±1.05

PET 2.98±0.55 3.35±0.15 2.58±0.09 3.79±0.3

Total melting enthalpy 45.23±3.27ab

41.96±0.98ab

38.44±2.52a 51.18±6.73

b

Values are means ± 1 standard deviation. Means with different letters within a row are significantly different (P<0.05)

Storage Time (days)

Film Control 7 15 30 60 240

A 0.24±0.03a 1.1±0.09

b 1.06±0.07

b 0.84±0.11

c 0.86±0.01

c 0.71±0.04

c

B 0.11±0.01a 0.43±0.07

bc 0.4±0.05

bc 0.43±0.02

bc 0.47±0.18

b 0.26±0.01

ac

32

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 10

oC/min.

33

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 10

oC/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

Nylon PET

34

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.

35

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). Polymeric-

based 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

http://www.foodprocessing.com/articles/2009/032.html

36

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 EVOH-

based 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.



92(3), 291-296.

37

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).



X‐ray diffraction and its influence on the permeability of the polymer. Journal of Applied

Polymer Science, 112(1), 107-113.

38

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.

39

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 600-

800MPa 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 (Bermúdez-

Aguirre 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

40

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,

41

were investigated to understand the influence of free volume characteristics and film morphology

on gas-barrier properties of PATS processed EVOH films.


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).

42

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.

43

Figure 3.1. Representative temperature and pressure profile during PATP. The processing

condition is 680 MPa for 5 min at 100oC

0

50

100

150

200

250

300

350

400

450

500

550

600

650

700

750

800

0

10

20

30

40

50

60

70

80

90

100

110

120

0 2 4 6 8

Pressu

re (MP

a) T

emp

era

ture

(0C

)

Time (Minutes)

Temperature

Pressure

44


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 cm

2 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.


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

45

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 0.05

oC 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

46

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

sum of components corresponding to each annihilation site:

i

k

i i

tItN

exp)(

1

1

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

47

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 free-

volume 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).

(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).

-1 -1

3

o o

R 1 2πR(τ ) = 2 1 - + sin ns

R 2π R

48

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).


3.1 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

49

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

50

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.

Figure 3.2. Oxygen transmission rate of films A and B as influenced by the two PATS


0

0.2

0.4

0.6

0.8

1

1.2

1.4

Film A Film B

OT

R (

cc/m

2-d

ay)

Control PATS

51

Figure 3.3. Water vapor transmission rate of films A and B as influenced by the two PATS


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 (López-

Rubio 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

0

0.2

0.4

0.6

0.8

1

1.2

1.4

Film A Film B

WV

TR

(g

/m2-d

ay

)

Control PATS

52

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.

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,

0

1000

2000

3000

4000

5000

6000

7000

5 15 25 35 45

Inte

nsi

ty (

Cou

nts

)

Two Thetha (degree)

Control PATS

53

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.

Figure 3.5. X-ray diffraction patterns for film B before and after PATS treatments

0

1000

2000

3000

4000

5000

5 15 25 35 45

Inte

nsi

ty (

Cou

nts

)

Two Thetha (degree)

Control PATS

54

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 A: PET/EVOH/PP; Film B: PP/tie/Nylon6/EVOH/ Nylon6/tie/PP

Film* Treatment o-Ps

lifetime, τ3

(ns)

o-Ps intensity,

I3 (%)

Free volume

radius

(Å)

Free volume

fraction

(FV)

A Control 2.34 9.5 3.15 2.24

PATS 2.22 9.4 3.06 2.03

B Control 2.10 8.9 2.94 1.71

PATS 2.25 9.6 3.08 2.11

55

Figure 3.6. An example of the fitting of PALS spectrum of film A after PATS using LT Program

0 5 10 15 20 2510

2

103

104

105

106

Raw Data

Fit Data

1

2

3

sourceC

ou

nts

[a

.u]

Time (ns)

-20

-10

0

10

200 5 10 15 20 25

Resid

ue

56

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

57

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.

58

Figure 3.7. o-Ps lifetime distribution of films A and B before and after the two thermal sterilization treatment

0

0.005

0.01

0.015

0 1 2 3 4

No

rma

lize

d I

nte

nsi

ty

o-Ps lifetime, τ3 (ns)

Film A Control

Film A PATS

0

0.005

0.01

0.015

0.02

0.025

0 1 2 3 4

No

rma

lize

d I

nte

nsi

ty


Film B Control

Film B PATS

59

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.



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).

60

Kansy, J. (1996). Microcomputer program for analysis of positron annihilation lifetime spectra.

Nuclear Instruments and Methods in Physics Research Section A: Accelerators,

Spectrometers, Detectors and Associated Equipment, 374(2), 235-244.

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.




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 EVOH-

based 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.

61



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.

Shukla, A., Peter, M., & Hoffmann, L. (1993). Analysis of positron lifetime spectra using

quantified maximum entropy and a general linear filter. Nuclear Instruments and

Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and

Associated Equipment, 335(1), 310-317.





62

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

63

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, X-

ray 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

64

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 shelf-

stable 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

65

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 non-

homogenous 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

66

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 nano-

particles 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 lid-

stocks 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).

67

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.

68

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.


2.1 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- PET-

Coated/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.

69

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

70

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 o

C); z is the temperature rise required to decrease the thermal death time of

the target microorganism (Clostridium botulinum) by one log cycle (10 o

C); 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.

71

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

0

20

40

60

80

100

120

140

0 3 6 9 12151821242730333639424548

Tem

per

atu

re (

0C

)

Time (min)

MATS

Retort

72

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 cm2 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.


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

oC at a rate of 10

oC/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.

73

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:

74

i

k

i i

tItN

exp)(

1

1

i

(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 para-

positronium (p-Ps) whereas, the intermediate lifetime (τ2) could be related to the free positron

75

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 free-

volume hole radius (R):

(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

-1 -1

3

o o

R 1 2πR(τ ) = 2 1 - + sin ns

R 2π R

76

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).


This section discusses the gas barrier, morphological and free volume changes that films A

and B underwent immediately after MATS and retort sterilization.


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-

77

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.

78

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

be affected during thermal processing.

Figure 4.2. Oxygen transmission rate of films A and B as influenced by the two thermal

sterilization conditions.

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

OT

R (

cc/m

2-

da

y)

Film A Film B

Control

MATS

Retort

79

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.


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

80

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

sterilization conditions with the crystallinity of films A and B.

Figure 4.3. Water vapor transmission rate of films A and B as influenced by the two thermal

sterilization conditions.

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

WV

TR

(g

m/m

2-d

ay)

Film A Film B

Control

MATS

Retort

81

Table 4.1. Melting temperature and enthalpy of melting for the polymer layers in films A and B,

untreated, and after thermal sterilization

Film

*

Treatment Tm (oC) ΔH (J/g)

PP Nylon PET PP Nylon PET

A Control 162.39±0.1 219.75±0.2 255.82±

0.1

20.25±4.6 5.96±1.3 3.58±0.4

MATS 161.65±1.3 219.52±0.1 255.78±

0.1

19.15±3.5 7.32±0.1 5.20±0.6

Retort 162.25±0.1 220.01±0.4 256.24±

0.1

23.32±3.3 7.76±1.2 5.26±0.5

B Control 162.95±0.2 219.98±0.1 256.24±

0.1

33.52±8.0 11.52±5.

3

5.01±1.1

MATS 161.54±0.1 218.90±0.1 254.60±

0.2

33.71±4.8 8.47±0.9 4.66±0.6

Retort 161.57±0.3 219.20±0.2 254.55±

0.1

27.43±0.3 8.55±0.5 4.44±0.8

*Film A: PET/adhesive/Nylon/adhesive/PP; Film B: Coated- PET-Coated/adhesive/oriented

Nylon/adhesive/PP


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

82

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.

Figure 4.4. X-ray diffraction patterns for film A before and after the two thermal sterilization

treatments.

0

1000

2000

3000

4000

5000

6000

0 10 20 30 40

Inte

nsi

ty (

Cou

nts

)

Two Theta (Deg)

Control

MATS

Retort

83

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).

84

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 A: PET/adhesive/Nylon/adhesive/PP; Film B: Coated- PET-Coated/adhesive/oriented

Nylon/adhesive/PP

Film* Treatment o-Ps

lifetime, τ3

(ns)

o-Ps intensity,

I3 (%)

Free volume

radius

(Å)

Free volume

fraction

(FV)

A Control 1.84 15.19 2.70 2.25

MW 1.92 15.91 2.78 2.58

Retort 1.86 15.08 2.72 2.30

B 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

85

Figure 4.5 o-Ps lifetime distribution of films A and B before and after the two thermal sterilization treatments.

0

0.02

0.04

0.06

0.08

0 1 2 3 4 5N

orm

ali

zed

In

ten

sity


Film A Control

Film A MATS

Film A Retort

0

0.02

0.04

0.06

0.08

0 1 2 3 4 5

No

rma

lize

d I

nte

nsi

ty


Film B Control

Film B MATS

Film B Retort

86

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.

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

a b c

89

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.

90

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Philadelphia, PA.

[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.

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.

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Barbosa-Canovas, G. V., & Bermúdez-Aguirre, D. (2008). Introduction. Food Science and

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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

properties of semi-crystalline poly (3-hydroxybutyrate-co-3-hydroxyvalerate) during

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Downing, D, L. (1996). A complete course in canning and related processes: Book II. 13th

edition. Baltimore, MD: CTI Publications, Inc.

Food Production Daily. (2011). Researcher hails ‘major milestone’ for microwave sterilization

technology.Available from: http://www.foodproductiondaily.com/Processing/Researcher-


Food Production Daily. (2012). Microwave sterilization system rolled out in US. Available from:

http://www.foodproductiondaily.com/Processing/Microwave-sterilization-system-rolled-

out-in-US. Accessed Oct 31, 2012.

Guan, D., Plotka, V. C., Clark, S., & Tang, J. (2002). Sensory evaluation of microwave treated

macaroni and cheese. Journal of food processing and preservation, 26(5), 307-322.

Kansy, J. (1996). Microcomputer program for analysis of positron annihilation lifetime spectra.

Nuclear Instruments and Methods in Physics Research Section A: Accelerators,

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Kong, Y., & Hay, J. N. (2003). The enthalpy of fusion and degree of crystallinity of polymers as

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Mokwena, K. K., & Tang, J. (2012). Ethylene Vinyl Alcohol: A Review of Barrier Properties for

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Pandit, B, R., Tang, J., Liu, F., & Mikhaylenko, G. (2007). A computer vision method to locate

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Ramaswamy, H., & Tang, J. (2008). Microwave and radio frequency heating. Food Science and


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93

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/m2-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

94

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 in-

packaged 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).

95

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).

96

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 (ICP-

MS). 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 food-

simulating 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.

97

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 commercially-

98

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


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:

99

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-to-

surface-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

100

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.

101

Figure 5.1. Picture and schematic diagram of migration test cell. Dimensions shown are in cm

102

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

103

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 volume-

to-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

104

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.

105

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 Total

time

(min)

Preheating from 25 °C Processing at target temperature

End

Temperature

(°C)

Ramp

time

(min)

Hold

time

(min)

Temperature

(°C)

Ramp

time

(min)

Hold

time

(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

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.

2.5 x 2 inches pouch

containing 10 ml FSL

35 ml test cell with 25 ml

working volume

107

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. ICP-

MS (Agilent 7500cx system, Agilent Technologies, Santa Clara, CA) equipped with a double

0

20

40

60

80

100

120

140

0 10 20 30 40 50 60

Tem

per

atu

re (

°C)

Time (min)

Conventional (CH1)

Microwave (MW1)

108

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 27

Al, and 28

Si 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 ml-

1) 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

109

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

110

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

111

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

112

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.

113

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

Conventional Heating

Microwave

A Water 0.92±0.18a 1.05±0.17

a

3% Acetic Acid 0.55±0.03a 0.60±0.05

a

B Water 1.25±0.12a 1.07±0.04

a

3% Acetic Acid 0.60±0.03a 0.51±0.03

a

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 90

oC and 123

oC,

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

114

90oC and 123

oC, 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.

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).

0

0.2

0.4

0.6

0.8

1

1.2

1.4

Film A Film B

Sil

icon

Con

cen

trati

on

(m

g k

g-1

)

70 C 90 C 123 C

a

b

c

a

b

c

115

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.

116


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).

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

70 90 123

Sil

icon

Con

cen

trati

on

(m

g k

g-1

)

Temperature (oC)

18 min 34 min

a

a

a

b

b

b

117


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

0

0.2

0.4

0.6

0.8

1

1.2

1.4

70 90 123

Sil

icon

Con

cen

trati

on

(m

g k

g-1

)

Temperature (oC)

18 min 34 min

a

a

a

b

b

a

118

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 (800-

1050 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.

119

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.

(b)

(a)

120

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.

(a)

(b)

121

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 in-

package MW sterilization and pasteurization while ensuring consumer safety.

122

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126

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

127

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.

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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.