Investigation of Corrosion in Canned Tomatoes Processed by Retorting

Home , Can seamer, Double seam, Hermetic seal

THESIS

Presented in Partial Fulfillment of the Requirements for the Degree Master of Science in the Graduate School of The Ohio State University

Elliot Dhuey

Graduate Program in Food Science and Technology

The Ohio State University

2019

Master's Examination Committee:

Dr. Melvin Pascall, Advisor

Dr. Gerald Frankel

Dr. John Litchfield

Copyrighted by

Elliot Dhuey

2019

Abstract

This study investigated the presence of volatile and non-volatile compounds in canned processed tomatoes and how these compounds interacted with the Bisphenol A free epoxy-based lining of the cans to cause corrosion of the base metal and the migration of iron and tin compounds to the tomatoes. The tomatoes tested in this study were the

Roma variety. They were sorted, washed, diced, and sealed in two-piece tinplated metal cans. These were processed by retorting at 250°F for 30 minutes then stored at 49°C for up to 50 days. Control samples were packaged and processed in glass jars. The presence and concentrations of the volatile and non-volatile compounds in the processed and unprocessed tomatoes were tested using Selected Ion Flow Tube – Mass Spectrometry

(SIFT-MS) and Ion Chromatography – Mass Spectrometry (IC-MS) respectively. After removing the processed tomatoes from the cans, the linings were removed and analyzed for the volatile and non-volatiles as was mentioned before. Scanning Electron

Microscopy (SEM) paired with Energy Dispersive X-ray Spectroscopy (EDS) was used to confirm the presence of visual corrosion in the processed cans and to analyze its elemental composition. X-ray Diffraction (XRD) and Fourier Transform – Near Infrared

(FT-NIR) was used to characterize changes to the polymeric morphology of the can lining after the retort processing. Also, Inductively Coupled Plasma – Mass

Spectrometry (ICP-MS) was used to determine the rate and level of tin and iron migration from the metal can to the tomato product. The results of the SIFT analyses showed that ii the formation of dimethyl sulfide and other sulfide compounds in the tomatoes resulted from the thermal degradation of methyl methionine. These compounds diffused from the tomatoes to the lining of the cans and the XRD and FT-NIR analyses showed that they interacted with the polymer and led to the reformation of the oxirane ring of the epoxy and binding of water with the polymer lining. The SEM analysis showed that sulfur compounds created breaches in the lining and created avenues for corrosive compounds in the tomatoes to interact with the base metal layer. The presence of nitrates resulted in an increased rate of iron migration when combined with sulfur compounds. From the results obtained, it could be concluded that the combination of volatile and non-volatile compounds found in tomatoes are acting synergistically to initiate corrosion in the tin and steel in the walls of the cans used to package the retorted tomatoes.

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Dedication

Dedicated to my loving parents, Charles and Michele Dhuey, who raised me to believe that with God all things are possible.

Acknowledgments

I would like to sincerely thank my advisor Dr. Melvin Pascall for his incredible support, guidance, and knowledge to help me complete my master’s thesis at The Ohio

State University. The past two years have been incredible to witness the level of effort and detail that Dr. Pascall puts into his students’ research and making sure his students learn and grow is remarkable and inspiring.

I would also like to thank Dr. Gerald Frankel and Dr. Christopher Hadad for their support and knowledge in areas around my research to help make it successful. The support I received from the Dr. Jaesung Lee, Dr. Hardy Castada, and Dr. Jojo Joseph, and

Ryan Hopf have also made this all possible. Kuo-Hsiang Chang’s help with SEM-EDS and processing tomatoes was much needed and appreciated. Additionally, I want to thank Ken Ruffley at PPG for his guidance and being our champion through this project.

The combined financial support from PPG and USDA-NIFA have helped make this project successful.

Lastly, I would like to thank my amazing friends who were able to help me with anything I needed during my masters. Whether it was processing pounds of tomatoes in the food processing plant that was set up by Matt Papic or being there to make my experience at OSU amazing, I’m grateful either way.

Vita

2012...... Sun Prairie High School

2017...... B.S. Food Science & Technology,

University of Wisconsin – Madison

2017 to Present ...... Master’s Student, Food Science &

Technology, The Ohio State University

Fields of Study

Major Field: Food Science and Technology

Table of Contents

Abstract …………………………………………………………………………………...ii

Dedication ………………………………………………………………………………..iv

Acknowledgements ……………………………………………………………………….v

Vita ……………………………………………………………………………………….vi

List of Tables …………………………………………………………………………….xi

List of Figures ………………………………………………………………………..…xii

Chapter 1: Introduction ...... 1

Chapter 2: Literature Review...... 5

2.1 Packaging of food...... 5

2.1.1 Types of packaging materials ...... 6

2.1.2 Cans ...... 7

2.1.2.1 Three-piece cans ...... 8

2.1.2.2 Two-piece cans ...... 11

2.1.2.3 Can coating ...... 12

2.2 Enamel Coating ...... 15

2.2.1 Common types of enamel resins ...... 16

2.2.2 Bisphenol A controversy ...... 17

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2.3 Tomatoes ...... 19

2.3.1 Compounds in tomatoes ...... 19

2.3.1.1 Nitrogenous compounds ...... 22

2.3.1.2 Sulfur compounds ...... 25

2.3.2 Processing of tomatoes ...... 27

2.4 Canned food products...... 29

2.4.1 Canning process ...... 30

2.4.1.1 Filling ...... 30

2.4.1.2 Pressure and headspace ...... 30

2.4.1.3 Can seaming ...... 31

2.4.1.4 Retorting ...... 34

2.4.1.5 Heat penetration ...... 36

2.5 Corrosion in canned food ...... 40

2.5.1 Mechanisms of corrosion...... 40

2.5.2 Factors influencing corrosion in canned food and beverage ...... 43

2.6 Methods of corrosion detection ...... 46

2.6.1 Corrosion inspection technology ...... 46

2.6.1.1 Visual inspection ...... 46

2.6.1.2 Ultrasonic inspection ...... 47

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2.6.1.3 Radiographic inspection...... 48

2.6.2 Scanning Electron Microscopy – X-Ray Microanalysis ...... 49

2.6.3 Inductively Coupled Plasma Mass Spectrometry (ICP-MS) ...... 51

2.7 Select Ion Flow Tube Mass Spectrometry (SIFT-MS) ...... 52

2.8 Ion Chromatography ...... 54

2.9 Attenuated Total Reflectance (ATR) ...... 56

2.10 Dynamic Mechanical Analysis (DMA)...... 57

2.11 Permeability ...... 58

Chapter 3: Materials and Methods ...... 61

3.1 Experimental Design ...... 61

3.1.1 Materials and Ingredients ...... 61

3.1.2 Experimental Design ...... 61

3.1.3 Sample preparation and processing ...... 65

3.2 SIFT-MS Analysis ...... 67

3.3 IC-MS Analysis ...... 72

3.4 ICP-MS Analysis ...... 73

3.5 SEM-EDS Analysis ...... 76

3.6 X-Ray Diffraction Analysis (XRD) ...... 77

3.7 FT-NIR Analysis ...... 78

3.8 Statistical Analysis and Data Interpretation ...... 78

Chapter 4: Results and Discussion ...... 80

4.1 SIFT-MS Analysis ...... 80

4.2 IC-MS Analysis ...... 96

4.3 ICP-MS Analysis ...... 97

4.4 SEM-EDS Analysis ...... 104

4.5 X-Ray Diffraction Analysis (XRD) ...... 111

4.6 FT-NIR Analysis ...... 114

Chapter 5: Conclusion and Future Work ...... 117

List of Tables

Table 2.1. Composition of Dry Matter Content of Tomato (Davies and Hobson 1981) .. 21

Table 3.1. Canned tomato sample experimental design ...... 64

Table 3.2. Diced tomato formulation per 10 oz can...... 67

Table 3.3. Selected volatiles, reagent ions and mass-to-charge ratio of products ...... 71

Table 4.1. Concentrations (ppb) of selected volatile compounds in the tomatoes before and after retort process ...... 81

Table 4.2. Concentrations (ppb) of selected volatile compounds in the can lining before and after retort process ...... 83

Table 4.3. Concentrations (ppb) of selected volatiles in the tomato treatment group during storage at 49OC...... 86

Table 4.4. Concentrations (ppb) of selected volatiles in the various treatment groups during storage at 49OC...... 87

Table 4.5. Concentrations (ppb) of selected volatiles in the nitrate treatment group during storage at 49OC...... 88

Table 4.6. Concentrations (ppb) of selected volatile compounds in the SMM treatment group during storage at 49OC...... 90

Table 4.7. Changes in the concentrations (ppb) of selected volatile compounds of the can lining processed with tomatoes during storage at 49OC...... 91

Table 4.8. Concentrations (ppb) of selected volatile compounds in the can lining processed with SMM treatment group during storage at 49OC...... 92 xi

Table 4.9. Post hoc LSD multiple comparison of DMS concentration between the tomato and SMM treatment groups at day 0 and 50 ...... 93

Table 4.10. Post hoc LSD multiple comparison of DMS concentration in the can lining between the tomato and SMM treatment groups at day 0, 10 and 50 of storage at 49oC. 94

Table 4.11. Concentration of iron and tin of the various treatment groups over time ...... 99

Table 4.12. Post hoc LSD multiple comparison of tin concentration between treatment groups at day 0 and 40 ...... 100

Table 4.13. Linear regression summary of the various treatment groups on the iron concentration over time (ppb/day) ...... 101

Table 4.14. Post hoc LSD multiple comparison of iron concentration between the treatment groups at day 0 and 40 ...... 103

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List of Figures

Figure 1.1. Synthesis of BPA epoxy resin (Chow et al. 2016) ...... 3

Figure 2.1. Parts of the metal can ...... 9

Figure 2.2. Stages in the formation of a double seam (Page et al. 2011) ...... 10

Figure 2.3. Metal can with ring-pull tab ...... 11

Figure 2.4. Tinplated steel layers (Piergiovanni and Limbo 2016)...... 14

Figure 2.5. Cross-Sectional Structure of tomato fruit ...... 20

Figure 2.6. Reaction scheme of proposed mechanism of the formation of dimethyl sulfide

(1) from a thermal degradation of S-methylmethionine (2) (Scherb et al. 2009) ...... 26

Figure 2.7. Parts and mechanisms of the can seaming operation (Marino et al. 2002) .... 32

Figure 2.8. Double seam terminology of a metal can with first operation (left) and second operation (right) (Dixie Canner Company 2018)...... 34

Figure 2.9. Vertical still retort (Albaali and Farid 2006) ...... 35

Figure 2.10. Heat transfer in container by conduction (left) and convection (right)

(Heldman and Hartel 1997) ...... 37

Figure 2.11. Lethal-rate curve for typical retort process (Heldman and Hartel 1997) ..... 39

Figure 2.12. Schematic of SEM-EDS (Hamilton and Quail 2011) ...... 50

Figure 2.13. Schematic of ICP-MS (Gilstrap and Allen 2009) ...... 52

Figure 2.14. Schematic of SIFT-MS Apparatus (Smith and Španěl 2005) ...... 53 xiii

Figure 3.1. Experimental design for studying corrosion mechanism in canned diced tomatoes...... 63

Figure 3.2. Filled 211 x 400 can of diced tomatoes ...... 66

Figure 3.3. Pyrex storage media bottles (500 mL) with 5 grams of tomato sample ...... 69

Figure 3.4. 211 x 400 metal can cut cross-sectionally into four quadrants ...... 70

Figure 3.5. Standard curve of iron (56Fe) intensity prepared at increasing concentration

(ppb) and measured using ICP-MS ...... 74

Figure 3.6. Standard curve of tin (120Sn) intensities prepared at increasing concentration

(ppb) and measured using ICP-MS ...... 75

Figure 4.1. The correlation of dimethyl sulfide with increasing levels of SMM expressed as the average of 4 replicates ...... 89

Figure 4.2. Nitrate concentration of tomato processed in a glass jar or metal can during the storage at 49oC...... 97

Figure 4.3. Concentration of iron in each treatment group during the storage period .... 102

Figure 4.4. SEM Image of internal wall of the unprocessed can lining ...... 105

Figure 4.5. SEM Image of the internal wall of the day 50 tomato treatment group showing an area of breaches ...... 106

Figure 4.6. SEM Image of the internal wall of the day 40 Nitrate treatment group showing an area of rupture ...... 107

Figure 4.7. EDS Spectrum of the internal wall of the unprocessed can ...... 108

Figure 4.8. EDS spectrum of the internal wall of the day 50 tomato treatment group ... 109

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Figure 4.9. Proposed corrosion scheme of tomatoes in BPA-free epoxy-lined metal cans

...... 110

Figure 4.10. XRD patterns of unprocessed and processed can and lining in tomato ..... 113

Figure 4.11. XRD patterns of unprocessed and processed can lining in tomato ...... 114

Figure 4.12. FTIR spectra of unprocessed and processed in tomato epoxy-based lining in the near ranges ...... 116

Chapter 1: Introduction

The packaging of food in metal cans has emerged as an excellent method to extend its shelf life because cans have high barrier to gases, vapors, light, filth, and microorganisms. At the same time, the complexity of food products greatly varies from one another and many natural and added compounds are known to contribute to the initiation of corrosion in metal packaging. Some factors that are known to accelerate this corrosion include: oxygen, color pigments, nitrates, sulfur compounds, sodium chloride and trimethylamines (Robertson 2012).

The tomato fruit is known to be a very nutritious vegetable. It is high in vitamin

C and provides antioxidant protection against cancer and it helps to optimize the functions of certain organs in the human body (Yamaguchi 2012). In addition, the cultivation of tomato has economic importance since it is one of the major crops in the world. In 2017 the United States produced 14.7 million tons of tomatoes with a value of

1.67 billion dollars (AgMRC 2018). On the other hand, processed tomato is considered to be one of the most aggressive products to initiate corrosion when packaged in metal cans (Razdan and Mattoo 2006; AgMRC 2018).

The tomato fruit has nearly 400 identified volatile compounds derived from various enzymatic or kinetic reactions of larger molecules (Petro‐Turza 1986). Dimethyl sulfide is one of the most prevalent volatile compounds in tomatoes and can be easily detected in heat processed tomato-based products (Maarse 2017). Tomatoes also contain many nonvolatile compounds including nitrates. A portion of the nitrates found in 1 tomatoes come from the fertilizers used in the soil to grow the fruit. During harvest time on large commercial farms, tomato plants are mechanically removed from the ground and sometimes this causes soil to be smeared on the surface of the fruits. This could cause nitrogen fertilizers in the soil to be readily absorbed by the tomatoes in the form of ammonia, nitrate and nitrite compounds (Gould 1992; Rao and Puttanna 2000). The natural low acidic character of tomatoes is capable of oxidizing these nitrates and nitrites into compounds with corrosion potential. Nitrates are also known as depolarizers which contribute to the detinning of the metal in food cans (Albu-Yaron and Feigin 1992;

Palmieri et al. 2004).

Since many compounds have potential to cause corrosion if they interact with the metal surface of food cans, passivated metal and polymeric lining technology have been developed and applied to cans in order to minimize these interactions (Robertson 2012).

Examples of passivated metals include tin and chromium. Examples of polymers used as protective linings include: oleoresins, phenolic compounds, vinyl, and polyesters. In selecting an appropriate liner, consideration must be given to the nature of the food plus the method used to process the packaged product. This is necessary in order to minimize the incidence of corrosion in the cans. Epoxy resins are selected as a protective coating because of its strong mechanical properties, adhesion, and resistance to corrosion (Chow et al. 2016). The epoxide group (the ring shown in the epichlorohydrin in Figure 1.1) reacts with bisphenol A (BPA) to produce a BPA epoxy resin. The hydroxyl groups at both ends of the BPA monomer breaks the epoxide group on the epichlorohydrin to form strong intermolecular bonds between adjacent chains of the polymeric backbone. The

2 newly formed BPA epoxy resin is a thermoset polymer which has a high degree of crosslinking that can withstand high temperature processing conditions. The addition of

BPA to the epoxy also increases its molecular weight and this increases the melting point of the polymer (Jin et al. 2015).

Figure 1.1. Synthesis of BPA epoxy resin (Chow et al. 2016)

While the use of BPA epoxy resins serve as a useful protective coating for canned food products, various concerns about the adverse effects of BPA on the human body have been published (Vandenberg et al. 2009; Rubin 2011). These include its effects on the endocrine system, time of puberty, estrous cycles, prostrate, and mammary gland development. BPA has also been linked to human diseases such as diabetes, cardiovascular symptoms, recurrent miscarriages, increased inflammation and oxidative stress, and decreased in semen quality (Vandenberg et al. 2009; Rubin 2011). As a result, 3 many regulatory agencies in the United States and in other countries have enacted legislations limiting the use of BPA in food cans and in plastic containers (Rubin 2011).

This has forced many food processors to turn to BPA-free coatings in order to meet these demands. However, most alternative coatings are less effective in preventing corrosion when compared with BPA-based coatings (Piergiovanni and Limbo 2016).

The long-term goal of this study is to develop BPA-free coatings for food cans with corrosion protecting capabilities as good as BPA-based linings. In order to accomplish this, it is necessary to understand the types of compounds that are associated with foods that are heated under pressure within sealed metal cans. This process must first begin by investigating the types of food associated with corrosion in metal cans.

This must be followed by locating the parts of the cans where the corrosion is occurring.

This is an essential piece of knowledge because corrosion occurring in the headspace of the can is an indication of the action of volatile compounds, while those occurring in the lower body of the can indicate the action of higher molecular weight compounds.

The objectives of this study are: (1) to identify and quantify the volatile and nonvolatile compounds associated with processed tomatoes under retort conditions; (2) to understand which of these compounds are responsible for causing corrosion in retorted cans; (3) to measure the quantities of the compounds of concern in the tomato product and the quantities that migrate to the coating of the tested cans; (4) to understand how these migrated compounds affected the properties of the coating of the cans; and (5) to quantify metallic compounds from the corroded areas of the cans that migrate to the packaged tomatoes.

Chapter 2: Literature Review

2.1 Packaging of food

Packaging is essential for the processing, manufacturing, storage, marketing, transportation and handling of food. The genesis of modern food packaging began during the Industrial Revolution when new manufacturing processes and materials were developed. At that time, large numbers of rural agricultural workers began migrating towards the cities where they sought employment. At the same time, house-hold items began to be mass-produced, new shops opened, and the initiation of modernized techniques and developments led to an increased demand for packaging. This translated into demands for barrels, boxes, kegs, baskets and bags to transport many of the mass- produced goods to the cities, and thus the modern role of packaging began (Soroka 2002).

Initially, metal cans were made for tobacco packaging because of its high barrier to moisture and gases. Later, Nicholas Appert developed a method to preserve military food using glass bottles closed with cork. Many of the modern packaging types in use today were developed during World War II (Risch 2009). Globally, two thirds of all packaging material are used to package food. This translated into $797 billion in sales in

2013 and it was expected to grow at an annual rate of 4% by 2018 (Piergiovanni and

Limbo 2016).

Packaging serves three primary functions: protection, utility and communication

(Risch 2009). The most obvious function of a package is to contain the contents that it holds in order to protect the food quality and prevent contamination from moisture, gases, 5 microorganisms, and dust, as well as provide stability from shock, vibrations, and other modes of physical stresses. The package acts as a barrier to many of these potential contaminants thus prolonging the shelf-life of the food (Robertson 2012).

2.1.1 Types of packaging materials

Packaging materials vary by their barrier properties to light, gases and vapors, and their abilities to preserve the quality of the contained food. Common food packaging materials include metal, glass, paper, plastic, and combinations of these called composites.

Glass is a form of inorganic amorphous ceramic packaging material that is formed by the fast cooling of molten raw materials. This fast cooling prevents the crystallization of glass, thereby creating its many functional properties designed to maintain the quality of the packaged food. Glass used for food packaging is commonly composed of silica sand, cullet (recycled glass), sodium carbonate (soda), calcium chloride (lime) and other ingredients that are melted at approximately 1500°C to form this amorphous structure

(Piergiovanni and Limbo 2016). Some of the advantages of glass packaging include: (1) inert interaction with chemicals and food; (2) impermeability to external conditions, and thus providing for long-term storage of gases and vapors; and (3) it is stable at high and low temperatures (Singh et al. 2017).

Plastics (synthetic derived or bioplastics) are polymers that are composed of subunits, plasticizers and other additives. The classification of plastic by its properties is determined by its chemical and physical nature, which includes its molecular weight, degree of crystallinity, and chemical composition (Paine and Paine 1992a). These

6 properties are known to affect the gas barrier, mechanical and thermal properties of the material. Plastics are permeable and the degree of this depends on the segmental mobility of its polymeric chains and its storage condition. This influences how it allows the migration or sorption of external or internal chemical compounds. This permeation of chemicals can lead to adulterated or contaminated food products that are contained within the plastic packaging. Thus, the barrier, mechanical and thermal properties of the plastic are essential for the maintenance of the integrity of the package and the quality of the packaged food (Cantor and Watts 2011).

Metal packaging for food products is commonly made from aluminum and metal alloys such as tinplated and tin-free steel. The benefits of metal packaging include its compact molecular structure which provides impermeability to the diffusion of light, vapors, and gases (Jasse and Mathlouthi 1994). The malleability of metals increases its functional and machinable properties to meet the needs of industrial applications such as food cans, closures (caps and lids), drums, kegs, or aerosol containers. Additionally, metals have high thermal conductivity and this makes it an excellent medium for pasteurized or other commercially heat sterilized foods (Singh et al. 2017).

2.1.2 Cans

The metal can is an intricately designed packaging vessel that can be composed of a base metal and/or multiple layers of other materials, including a protective coating.

Furthermore, the shape of the can is taken into consideration when selecting the materials. The base metal of the package must be selected with a full understanding of the nature of the packaged contents in order to limit corrosion and increase functionality.

Aluminum (a base metal material) has a high barrier to light, gases and vapors and is created from aluminum oxide that undergoes an electrolytic process. Because aluminum is malleable, it can be cast, rolled, or extruded into thin sheets. It can also be vaporized and allowed to form very thin films on paper or plastic, a process called sputtering. Additionally, its high thermal conductivity makes it a great material for thermally processed foods because it reduces that time to heat the product (Page et al.

2011).

Compared to aluminum, steel also has high barrier properties. However, it lacks the malleability of aluminum because it is made from iron alloys with carbon which creates a rigid structure. Carbon helps to secure the iron in a lattice that enhances the tensile strength and brittleness of the material (Paine and Paine 1992a). The most common form of steel in food packaging is carbon-steel that is comprised of manganese, silicon, copper, phosphorus, sulphur, and carbon (Piergiovanni and Limbo 2016). During the formation of steel, the uncoated steel sheet is referred to as black plate because it is covered in black iron oxide. This makes it susceptible to corrosion because there is no galvanic protection. Thus, it is necessary to plate the steel with tin or chromium in order to protect it against severe corrosion (Robertson 2012).

2.1.2.1 Three-piece cans

A three-piece can consists of two ends and a body. Three-piece cans in food packaging are made exclusively of steel as the base metal because the body must be welded to provide a hermetic seal. The process begins with a flat rectangle steel sheet which is coated or left uncoated and then rolled to form a cylinder. The overlap of the

8 cylinder forms the side seam of the body which can be soldered, welded, or joined by mechanical clinching. Additionally, the can body has sidewall beading to help increase the axial load resistance which improves resistance to collapse due to the external pressure during retorting (Malin 1980). Furthermore, the beading allows the can to withstand internal vacuum that can cause deformation. The can body is then sent through a flanging machine where the top and bottom are flanged outward so that the manufacturer’s end and packer’s end can be fitted to the body as shown in Figure 2.1

(Page et al. 2011).

Figure 2.1. Parts of the metal can

The can ends are designed to tolerate a small amount of deformation which occurs during the heat processing of the food. This causes the can to expand from the internal and external pressures and heat that build up during the retorting process. During thermal processing, the can ends will expand and upon cooling, the created vacuum will cause the ends to return to their original shape. As the can ends are attached to the body of the can,

9 they require a rubber sealing compound that helps to provide a hermetic seal. Using a double seam mechanical operation, the can ends are joined to the cylindrical body. The interlocking of the can body and the ends occurs in two operations. The first operation gradually rolls the end curl inwards radially into the flange on the body of the can. The second operation will then tighten this coming together of these parts to form a double seam as shown in Figure 2.2 (Malin 1980).

Figure 2.2. Stages in the formation of a double seam (Page et al. 2011)

Because of demands for convenience, can ends are now fitted with an easy-open access. An example of such a convenience is the incorporation of a ring-pull tab that allows for full opening of the end (Figure 2.3). The can lid is made of aluminum, tin-free steel, tinplated steel, chrome-plated steel or other forms of alloy plated steels (Matsukawa et al. 2011).

Figure 2.3. Metal can with ring-pull tab

2.1.2.2 Two-piece cans

In the 1970s the development of the two-piece can created containers without as many seams as three-piece cans. Two-piece cans are manufactured using the drawn and re-drawn (DRD) or the drawn and ironed (D&I) process. Both processes could be used to make tinplated steel or aluminum cans. The choice of either process depends on the flow characteristics of the material and its ability to rearrange its crystal structure under compound stresses (Robertson 2012).

The D&I manufacturing process starts with a circular disc of material. The disc is stamped from a metal sheet and formed into a shallow cup. By punch drawing with a circular die, the material is progressively stretched into the correct depth. The wall thickness remains uniform throughout the process. The cup is then transferred to an ironing press where it passes through iron dies which cause the wall thickness to decrease

11 and the body height to increase. Because overdrawing frequently occurs, any unevenness in the height at the rim is trimmed. The cans are then cleaned and flanged (Page et al.

2011). The thin walls of the D&I process make it unsuitable for intense processing or thermal treatment, therefore, these type of cans are often used for carbonated beverages

(Soroka 2002).

The DRD manufacturing process produces cans with a higher height-to-diameter ratio than the D&I process. While, the D&I process stretches the walls of the can to decrease the thickness and increase the height, the DRD process causes the metal to flow from the base to the walls. This gives a uniform base and wall thickness throughout the can. DRD give a greater wall thickness than the D&I process which means that it is better for the packaging of heat processed foods (Malin 1980).

2.1.2.3 Can coating

Metal packaging that uses steel as the base metal is usually coated because of its lack of inertness to certain foods. Reaction between the food and the metal could lead to the migration of metal compounds into the food due to the degradation of the steel. The primary function of coatings is to prevent interaction between the content of the can and the metal itself. By preventing these interactions, the shelf-life of the canned product will be extended, and its initial quality maintained for a long time. Therefore, the coating must be inert, resist physical deformation during fabrication, and be resistant to chemicals in the food (Grassino et al. 2007). One example of a coating is an epoxy polymer which provides protection from corrosion due to its strong inter-molecular bonds (Jin et al.

2015).

The most common coating for steel is tinplate (Figure 2.4). During this process, the steel is coated by a thin layer of metallic tin in a sulfuric acid bath with tin sulfate.

The tin adheres to the steel through an electrolytic process that is followed by a thermal step that quenches the material and this produces a tinplated steel alloy. The alloy then undergoes chemical passivation with sodium dichromate. During this step, tin and chromium oxides and tin oxides are produced on the surface. The oxide layer on the surface and passivation help to make the can more stable and resistant to any corrosive atmosphere. The final step is oil lubrication to help protect against scratches or corrosion

(Sharretts Plating 2015; Piergiovanni and Limbo 2016). The combination of tin and steel creates a material that has good strength, ductility, drawability, solderability, weldability, and other beneficial attributes. Additionally, the tinplated steel is often coated with an enamel to provide greater inertness to the food (Ball 2019).

Figure 2.4. Tinplated steel layers (Piergiovanni and Limbo 2016).

Metal packaging can also be made without tin as a Tin-free steel (TFS), or it could be made with other coatings including electrolytically chromium-coated steel (ECCS) that are less protective than tinplate. TFS is protected with a layer of metal oxides, whereas ECCS uses chromium and chromium oxide as a protection layer on the steel.

Because chromium is one of the few metals that can undergo spontaneous passivation, this makes it a good alternative to tinplate. However, ECCS does not have as great of a resistance to extreme pH conditions as tinplate. Thus, it is most commonly used for

14 closures. To be effective as a protective coating, ECCS must have a enamel to protect against corrosion (Chen 2015; Melvin et al. 2018).

For enamel coatings, the enamel is applied to the metal before it is formed into the cylinder. For cans made by the D&I process the enamel is applied after formation of the cylinder because it could be damaged by the metal deformation. This enamel is usually applied as thin layer. If it is applied too thin, it will not be functional because it might not cover the entire internal surface. If the lining is applied too thick, it will be too brittle, and it will not be effective as a protective agent. Therefore, understanding the properties and types of each can coating is important. (Oldring and Nehring 2007).

2.2 Enamel Coating

Enamels used to coat food cans are classified into three groups. These are general- purpose, sulfur resistant, and special enamels. General-purpose enamels are typically epoxy phenolic resins and used for more acidic products. Sulfur resistant enamels are used to prevent staining that sulfur produces on tinplate surfaces. Foods with sulfur containing amino acids will breakdown during heat processing and will release sulfides that could cause a black tin sulfide stain to accumulate in the headspace region of the can.

Thus, these enamels are pigmented with zinc oxide or aluminum powder to react with the sulfur. The sulfur staining that occurs is often noted as being harmless but it is unsightly.

Special enamels are compounds with additives like waxes to help consumers to easily empty the contents of the can (Robertson 2012).

Enamels are either applied to metal cans as coatings or by spraying. The resin is mixed with an organic or aqueous solvent that acts as a carrier and allows it to be applied

15 wet. Once applied wet, the coating is dried to remove the solvent. The baking or curing process is done in a convection oven at 210°C for approximately 15 minutes. The coating can be applied as a powder using electrostatic fields and then cured using high frequency induction heating or radiation (Oldring and Nehring 2007).

2.2.1 Common types of enamel resins

Common enamel types include oleoresins, phenolic compounds, vinyl resins, epoxies, and polyesters. Oleoresins coatings have a natural plant-based origin and are blended with other components like zinc oxide to protect from discoloration of sulphur- containing foods. Unlike oleoresins, phenolic compounds are resistant to sulfur compounds, and additionally, oils and acids. They are commonly used in combination with epoxy resins (Piergiovanni and Limbo 2016). Phenolic resins are made from formaldehyde with a phenol. They are limited by their low flexibility and heat resistance, therefore they are better suited for three-piece cans where flexibility is not an issue

(Robertson 2012).

Vinyl enamels are created by the dissolution of vinyl chloride and vinyl acetate copolymer in solvents. They have long carbon-carbon chains that make them a thermoplastic that can be blended with other resins (Robertson 2012). They have low heat stability; however, vinyl enamels are beneficial because they are odorless, tasteless, and resistant to harsh conditions (high and low pH). This make them a good choice for canned dry foods, beer, and beverages but not for retorted products (Piergiovanni and

Limbo 2016). Epoxy resins are formed by condensation polymerization of epichlorohydrin and bisphenol A. Because of their high barrier properties to gases and

16 vapors, they are one of the most commonly used coatings in metal cans (Jin et al. 2015).

The high chemical resistance of epoxies makes them a great application for metal packaging for a large range of food products (Melvin et al. 2018).

2.2.2 Bisphenol A controversy

In the early 1960s, the FDA approved bisphenol A (BPA) as a chemical to be used in certain food contact materials (Nutrition, 2019). Globally, 6 billion pounds of BPA are processed each year and it releases over 100 tons of the chemical into the atmosphere

(Vandenberg et al. 2009). Evidence has shown that BPA can migrate from the packaging and into the food in many products including metal cans with BPA epoxy-liners.

Because BPA has widespread use in various food and cosmetic products, it is found in nearly every person in developed nations. The continuous exposure to BPA raises the concerns about its impact on the human cell functions and on certain diseases (Lim et al.

2009). Concerns about exposure to BPA have focus on its impact on the endocrine system, time of puberty, estrous cycles, prostrate, and mammary gland development.

BPA has also been linked to human diseases such as diabetes, cardiovascular symptoms, recurrent miscarriages, increased inflammation and oxidative stress, and decreased in semen quality (Vandenberg et al. 2009; Rubin 2011). As a result, many regulatory agencies in the United States and in other countries have enacted legislations limiting the use of BPA in food cans and in plastic containers (Rubin 2011).

A 2018, U.S. Department of Health and Human Services report on BPA exposed to rats defined BPA as a “high-production-volume industrial chemical that is used as a monomer in the production of polycarbonate plastics and epoxy resins. These have broad

17 applications in consumers products, including storage containers for foods and beverages and in medical devices” (National Toxicology Program (NTP) 2018). Because of its wide use in consumer products, the safety of BPA usage has been questioned. Human exposure to BPA is estimated to be less than 1 μg/kg body weight (bw/day) which may come from multiple sources including but not limiting to the environment and food contaminants (Vandenberg et al. 2007). Currently, regulatory agencies declare that BPA at its current levels (0.5 μg/kg bw/day) doesn’t have any risk associated with human health. However, this is contradicted by some regulatory agencies that deemed BPA as a risk and have banned it in consumer products. California has listed BPA as a reproductive toxicant under Proposition 65 (National Toxicology Program (NTP) 2018).

Because of many conflicting regulations around the usage of BPA in consumer products, it has become a public concern.

Previously, the National Toxicology Program conducted a two-year study on BPA in rats, however, the validity of the study was questioned after it concluded “that there was no convincing evidence of carcinogenesis”. The questioning of the study’s validity was due to the dosage level of BPA given to the rats; it was not representative of the human exposure of BPA. Therefore, a new study was conducted using a broad range of

BPA doses in rats. In this study, the rats were dosed with 2.5 through 250,000 μg BPA/ kg bw/day with 6 log-spaced doses between. The study concluded that treatment groups were not dose responsive with no clear pattern of consistent responses (National

Toxicology Program (NTP) 2018).

2.3 Tomatoes

Tomato (Solanum lycopersicum) has been cultivated for approximately 400 years since its original discovery in Peru. Today, tomatoes have become one of the major crops worldwide (Razdan and Mattoo 2006). In 2017, approximately 1.42 million tons of fresh tomatoes and 14.7 million tons of processed tomatoes were produced with a value of 1.67 billion dollars. Because tomato is a warm season crop and sensitive to frost, it is practical to process it for extended shelf life and year round availability (AgMRC 2018).

Most of the tomatoes produced in the US is processed in the form of canned tomatoes, ketchup, chili sauce, juice, paste, and others. Because of its popularity, it is one of the major sources of selected vitamins and minerals in the human diet (Razdan and Mattoo

2006).

2.3.1 Compounds in tomatoes

Tomato fruits are fleshy berries, that when ripened, the color can be red, pink, orange, yellow, or colorless. Most western cultures are familiar with the red pigmentation of the tomato fruit. This is caused by lycopene. Yellow is seen because of the carotenoid pigments present. When the fruit is matured, the locules are filled with a gelatinous material surrounding the seeds (Figure 2.5) (Saltveit 2005a).

Figure 2.5. Cross-Sectional Structure of tomato fruit

The composition of the fruit is 90% water, sugars and acids. During ripening, glucose and fructose increase while the malic acid content decreases. The sucrose and citric acid contents generally remain constant throughout the development of the fruit. The pH is typically around 4.3 to 4.7, however, when processed the pH is lowered to 4.5 to prevent the growth of Clostridium botulinum. The tomato fruit is high in pro-vitamin A and vitamin C with small amounts of tomatine and other alkaloids (which decrease upon ripening) (Yamaguchi 2012).

Tomatoes are generally grown in various soil types from sandy to fine clays. They grow in temperatures that are frost-free or warm (above 16°C). To successfully grow the

- tomato crop, it is treated with nitrogenous fertilizers. During this process, the NO3 is

+ mixed with NH4 at higher proportion to obtain an optimal plant growth. The nitrogen fertilizer is added before planting and then phosphorous and potassium are added after

20 planting the tomato seeds. Many steps are done to ensure the complete growth and optimal quality of the tomato fruit (Peet 2005) .

The average dry matter content of the tomato fruit is between 5 to 7.5%, which has various components that play a role in flavor (Table 2.1). Sugars make up approximately

50% of the dry matter with glucose and sucrose being the most abundant. During ripening, the sugar content increases then decreases when the fruit is processed due to caramelization and browning reactions between the sugars and amino acids. The organic acids comprise approximately 10% of the dry content in the fruit, with citric and malic acids being the two main organic acids. When the tomato ripens, citric becomes the predominant acid. Among the other non-volatiles are the free amino acids which make up 2 to 2.5% of dry matter (Petro‐Turza 1986).

Table 2.1. Composition of Dry Matter Content of Tomato (Davies and Hobson 1981)

Constituent Percent of total content (%) Fructose 25 Glucose 22 Saccharose 1 Citric acid 9 Malic acid 4 Protein 8 Dicarboxylic amino acids 2 Pectic substances 7 Cellulose 6 Hemicellulose 4 Minerals 8 Lipids 2 Ascorbic acid 0.5 Pigments 0.4 Other amino acids, vitamins, and polyphenols 1 Volatiles 0.1

The majority of the amino acids (around 80%) are comprised of glutamic acid, glutamine, gamma-amino-butyric acid, and aspartic acid. The amino acid development depends on the soil – whether it has a high or low nitrogen and phosphate content. The amino acids present in the tomato are known to influence the final chemical composition of the fruit and these have a correlation with the development of flavor and the promotion of browning during processing (Petro‐Turza 1986).

The volatile compounds in tomatoes make up approximately 0.1% of the total dry matter content and they contribute significantly to the development of flavors. Free amino acids and fatty acids play a major role and serve as potential precursors to a majority of the volatiles found in the tomato fruits. With around 400 volatile compounds identified in tomato fruits, they are formed through enzymatic and/or kinetic reactions involving larger molecules found in the fruit (Petro‐Turza 1986; Whitfield and Last

2017). Examples of volatile found in tomato fruits include but are not limited to methanol, ethanol, acetone, geranyl butanoate, and dimethyl sulfide. Dimethyl sulfide is particularly important in heat treated tomato based products (Whitfield and Last 2017).

2.3.1.1 Nitrogenous compounds

Nitrogen can be absorbed by tomatoes in the form of either ammonium or as nitrates. This absorption is largely dependent on the species, cultivar, age and soil conditions of plants. Once absorbed, nitrate reductase will reduce nitrate to ammonium and it later becomes assimilated by glutamate. The origin of nitrogen in tomato fruits may come from nitrogen fertilizers directly added to the soil or pollution of fertilizers in ground water (Rao and Puttanna 2000). Nitrogen can also be added to the soil during

22 thunderstorms especially by the action of lightening on atmosphere nitrogen. This nitrogen reacts with minerals in the soil to form nitrates. Irrespective of the source of the nitrate it is later reduced and incorporated into an organic nitrogenous compound with glutamate and glutamine and this contributes to the formation of tomato amino acids

(Sanderson and Cocking 1964).

The presence of nitrates in canned foods and serve to initiate the process of corrosion in the materials, especially if it is not properly protected. Nitrates are capable of acting as depolarizers and they will contribute to the detinning of tin-plated steel in metal food cans. During this process, the nitrates are reduced to nitrite in a slow step and then they will be quickly reduced to nitrogen oxides, molecular nitrogen and ammonia as illustrated in the following equations:

4푆푛 → 4푆푛++ + 8푒−

− − + − Slow: 푁푂3 + 2푒 + 2퐻 → 푁푂2 + 퐻2푂

− − + + Fast: 푁푂2 + 6푒 + 8퐻 → 푁퐻4 + 2퐻2푂

The slow step is activated by a small amount of tin (II) that is oxidized to tin (IV) (Albu-

Yaron and Feigin 1992; Palmieri et al. 2004). This nitrate reduction produces nitrous oxide (N2O), nitric oxide (NO), hydroxylamine (H3NO), and ammonia/ ammonium ions

(Mannheim and Passy 1982). Ammonia is found to be the principle nitrate reduction product at pH levels below 5.0. In the equations above, the nitrate reacts with tin to produce tin (IV) and 1 mole of nitrate is reduced to ammonia for every 4 moles of tin that dissolves (Farrow et al. 1970). Once reduced to ammonia, it then acts as a corrosion accelerator. This oxidizing potential of nitrates and nitrites are enhanced by the low

23 acidity of tomato products, and this works to promote the corrosion process (Albu-Yaron and Feigin 1992; Palmieri et al. 2004). A study by Farrow et al. (1970) shows that little detinning occurs at higher pH levels, whereas more detinning occurs at pH values between 3.0 to 5.0 in canned food. During the corrosion process, the ammonia concentration initially increases as it reacts with tin and then stops increasing after all available tin reacts with the nitrogenous compounds. Furthermore, any residual oxygen present in the container may trigger the reduction reaction of the nitrates and nitrites

(Board and Holland 1969). Other nitrate reduction products such as hydroxylamine and nitric oxide were also detected in canned tomato products, but they were low in concentrations and were not considered to be significantly involved in the detinning process. In Farrow et al., 1970 study , they reported that nitrous oxide was implicated and contributed to the detinning process, but it did so at lower concentrations than ammonia (Farrow et al. 1970). Overall, these results show that low concentrations of nitrates can result in extensive detinning. It was seen that concentrations as low as 50 ppm resulted in 50% of the tin removed in experimental cans (Farrow et al. 1970). The study also showed that nitrates level in food products will decrease over time during the shelf life of canned commodities as a result of nitrate reduction.

Another study by Albu-Yaron and Feigin, (1992) showed that canned tomatoes spiked with a nitrate increased the level of corrosion in the material and at the same time, a decrease in the nitrate concentration in tomatoes was also observed. When the canned tomato was spiked with chloride alone, the processing and storage time showed no change in the chloride content. However, when spiked with chloride and nitrate, the

24 study found a greater decrease in the nitrate and chloride concentrations (Albu-Yaron and

Feigin 1992).

The studies discussed in this section clearly demonstrated that nitrates in tomato products play a significant role in contributing to corrosion in canned products. This was demonstrated by the reduction in the nitrate content during storage of the experimental cans. Because ammonia is produced in smaller quantities after the corrosion process, it may be difficult to detect using current analytical quantification tools. Therefore, in a study to understand the process of corrosion in canned tomato products, it is better to directly measure, monitor and correlate the nitrate concentration with any corrosion occurring in the package during its shelf-life.

2.3.1.2 Sulfur compounds

Cysteine and methionine are two major amino acids found in plants and they account for approximately 90% sulfur compounds formed (Giovanelli 1987). In particular, S-methylmethionine (SMM) is a metabolite that is synthesized from L- methionine (Met) via s-methyltransferase. A proposed biosynthetic pathway with S- adenosyl-L-methionine (AdoMet) and S-adensylhomocysteine occurs as follows:

퐴푑표푀푒푡 + 푀푒푡 → 푆푀푀 + 푆 − 푎푑푒푛표푠푦푙ℎ표푚표푐푦푠푡푒𝑖푛푒

During the reaction a methyl group is transferred from AdoMet and is catalyzed by the enzyme methyltransferase to form the SMM. Once formed, the SMM is further broken down into dimethyl sulfide and homoserine by the enzyme methionine sulfonium lyase

(SMM hydrolase). This methylation process occurs when the plant is growing and can be accelerated under certain conditions, such as the applications of heat (Mudd and Datko

1990). Research on tomatoes have shown that when the concentration of methylmethionine sulfonium salt (SMM) decreases in a heated solution, there is an equimolar increase in homoserine (Wong and Carson 1966). Research as shown that the reaction rate of methylmethionine sulfonium doubled with a 5-6°C increase in temperature during the formation of dimethyl sulfide. The degradation of methylmethionine sulfonium bromide (MMSBr) displays first order kinetic reaction at any pH and temperature:

[푀푀푆퐵푟 ] ln 0 = 푘푡 [푀푀푆퐵푟] where MMSBr0 is the initial concentration and MMSBr is the concentration after reaction time t (min). The rate of degradation is directly proportional to pH and temperature

(Williams and Nelson 1974).

Figure 2.6. Reaction scheme of proposed mechanism of the formation of dimethyl sulfide (1) from a thermal degradation of S-methylmethionine (2) (Scherb et al. 2009)

2.3.2 Processing of tomatoes

The tomato plant is easily cultivated since it grows well in diverse ranges of latitude, soil types, temperatures, and crop propagation methods. However, the plant will not survive under poor lighting conditions, extreme temperatures, lack of drainage, and excessive fertilization. In order to stimulate the plant to produce fruit, various methods can be used. For example, pollen release can be stimulated by naphthalene acetic acid added at low temperatures. Once the plant produces flowers that go on to yield fruits they require 40 to 60 days to fully ripen. During ripening, the chlorophyll degrades, and carotenoids are synthesized. To help speed the ripening process, ethephon can be used to generate ethylene (Rick 1978). This ethephon will rapidly degrade into phosphate, ethylene, and chloride (Extoxnet 1995). Specifically, it is the presence of ethylene that speeds the ripening process.

During harvest time, large commercial farms employ modern techniques to help keep the fruit from microbiological contamination via contact with soil and a subsequent reduction in the fruit quality. The process of harvesting initially involves the complete removal of the plant from the ground. If the harvester smears or clods soil onto the surface of unbroken tomato fruits, it can be washed off without transferring contaminants from the soil to the crop. However, if the skin of the fruit is broken it becomes more difficult to remove any soil contamination and this can result in rapid spoilage to those fruits. If such contamination fruits are consumed without proper sanitization or sterilization, the concern is that they could be inoculated with Clostridium botulinum, a

27 deadly pathogen that is ubiquitous in the soil. In addition to causing fruit damage, mechanical harvesting has the potential to harvest fruits that are over or under ripened

(Gould 1992).

After harvest time, the tomatoes are shipped to a processing facility where they are cleaned. The tomatoes are first sorted while dry and contaminants and defective ones are removed. The tomatoes then soaked, washed with water to remove soil, and then spray rinsed. The tomatoes then go through a final sorting before being peeled steam or a solution of sodium hydroxide. If used during the peeling process, a stream of commercial grade steam is applied long enough to loosen the skin on the tomato fruit. However, the use of sodium hydroxide in this process helps to avoid softening of the fruit since it attacks the cuticular tissue and causes its dissolution. The solution then softens the skin while leaving the rest of the tomato intact so that the skin can be easily removed by spraying water. The concentration of the sodium hydroxide depends on the wetting agent

(sodium 2-ethylhexyl sulfate and sodium mono—and dimethylnaphthalene sulfonates) and the temperature of the solution (Rock et al. 2012).

Once the tomato skin has been peeled, the fruit can be trimmed or cored, diced, juiced, pureed or chopped depending on the end use application. During processing, the fruit tends to soften because of damage to the structure of the cells and this causes a leaching of pectin and certain enzymes. This causes pectin to convert to pectic acid. If there is a need to address this issue, certain calcium salts such as calcium chloride, calcium sulfate, calcium citrate, and monocalcium phosphate can be added to maintain the firmness. These salts increase the firmness by interacting with the pectic acid to form

28 a gel. Additionally, citric acid is often added to adjust the pH of canned tomatoes between 4.1 and 4.3 in order to prevent the growth of spoilage microorganisms. (Saltveit

2005b).

The heat treatment of the pulverized tomato can be done by one of two different methods. The first is called the hot break (>170°F) method. The second is the cold break

(150°F) method. The hot break process leads to a more viscous product whereas the cold break method allows for a more natural color and fresher tomato flavor. Most processors use the hot break process because the homogenization of the product and destruction of pectic enzymes occur at its temperature range (Gould 1992). However, when pectic enzymes are deactivated, lipoxygenase is also inactivated during the hot break process.

Lipoxygenase is known to initiate fresh aroma compounds by breaking down unsaturated fatty acids. Thus, when comparing the hot break and cold break methods, less major fresh aroma compounds are formed during the hot break method because lipoxygenase is inactivated (Goodman et al. 2002).

2.4 Canned food products

In the beginning of the 1800s, after Napoleon Bonaparte offered a prize to the first person to develop a novel method to preserve food, heat processed tinplated canned food was created and sold in Bermondsey, East London. This technology was quickly integrated into America. By the 1900s, the inefficient method of soldering can ends to the body was replaced by a mechanical method. With greater speed to the canning operation, metal packaging grew even further. Today, approximately 400 billion units of

29 metal containers with food, drinks, aerosols, and other products are packaged each year

(Emblem 2012).

2.4.1 Canning process

2.4.1.1 Filling

The U.S. Department of Agriculture grade standards for canned fruits and vegetables are given in Title 7 of the Code of Federal Regulations. Standards for canned tomatoes are regulated by the Food and Drug Administration in 21CFR 155.190. The processing of acidified canned tomatoes is specified by the Food and Drug

Administration in 21 CFR 114.3 and in 114.80. U.S. Grade A canned tomatoes must have a drained weight ≥ 66% by weight of the capacity of the container. Whereas Grade

B must be ≥ 58% and Grade C ≥ 50% by weight. These standards are designed to ensure that consumer get what they pay for when purchasing canned tomato products (Gould

1992).

2.4.1.2 Pressure and headspace

The headspace region of the can is the unfilled area located at the top of the can where it is affected by three variables: gas, volume, and pressure. The headspace gas consists of air and water vapor. The oxygen present in the headspace accelerates the corrosion mechanism. Therefore, it is often flushed with steam or nitrogen gas to minimize the oxygen concentration prior to sealing of the can. A larger headspace volume equates to an increase concentration of oxygen, and this has the potential to influence the heat transfer properties of the retorting process because of the specific heat capacity of water versus oxygen. The headspace pressure is the intensity of the vacuum

30 created within the sealed can. The vacuum serves three purposes: 1) it helps to create and keep the ends of the can in a concave position, 2) it reduces the oxygen present in the container, and 3) it prevents distortion of the can ends and reduces strain during the retort process. Hot filling, mechanical air evacuation, and steam displacement are used during the canning process to control the headspace volume. When hot filled, the contraction of the contents of the can occurs after seaming and cooling and it produces a vacuum. For the mechanical evacuation operation, air in the headspace is sucked out prior to steaming.

During steam displacement, culinary steam is injected into the headspace of the can immediately prior to its sealing. This displaces the air in the headspace and increases the temperature of the can. Upon cooling, the contraction of the air in the headspace creates the vacuum within the sealed can (Robertson 2012).

2.4.1.3 Can seaming

The proper sealing of a food can is important for maintaining the quality and safety of the content. Among the variable sealing types, the use of a double seam helps to create a hermetic seal in a food can. The hermetic seal is created when the package provides an airtight seal where the outside and inside environment around the sealed can are not in contact with each other (Paine and Paine 1992b). The double seam is the method used to seal the lid to the body of a metal food can. This process involves the folding together of 5 layers of metal sheets and pressed them firmly together using a two- phase operation (Rong C. Lin et al. 2001). This is illustrated in Figure 2.7 and it shows how the lid is folded into the body of the can.

Figure 2.7. Parts and mechanisms of the can seaming operation (Marino et al. 2002)

This two-phase operation finishes when the can seaming equipment clenches the end curl of the lid and the flange of the body tightens them together. During the first phase (first operation), the bottom of the unsealed can is placed on a baseplate and is clamped firmly in place be a chuck that fits into the countersink area of the upper lid of the can. The can seamer is fitted with two rollers called the first operation roller and the second operation roller. The actual seaming process begins when the first operation roller rotates around the circumference of the can and begins the process of folding the end-curl into the flange before it disengages from the can. During the second phase, the second operation roller engages the can and tightens the seam to give the correct final dimensions for the hermetic seal before it disengages (Emblem 2012). When the double seam is created, the sealing compound in the end curl presses onto the uppermost rim of the body of the can and creates an airtight seal, and fills any voids left between the folds 32 of the overlapping metal (Rong C. Lin et al. 2001). Control of can closures is described by the Food and Drug Administration in 21CFR 113.60.

Various methods of analyses have been used to inspect and ensure the quality and integrity of the sealed can. During a manual statistical sampling of a product lot, a random representative sample of the cans is selected. These are then visually inspected for external evidence of leakage, pinholes, rusting, dents, buckling, and other defects. A fluorescein dye can be used to detect the presence of any leaks (Rong C. Lin et al. 2001).

The double seam of metal cans can also be measured to determine its dimensions using two methods: the seam scope and can teardown. The seam scope method first uses a can saw to cut the metal can cross-section and then uses a projector to magnify the image of the cross-section area to visually inspect. Measurements are taken on the overlap, body hook, and cover hook as shown in Figure 2.8 (IFSH 2019). The can teardown method requires a nipper and micrometer. The nipper is used to remove the can end from the body which will result in disengaging the body hook and cover hook form each other.

The micrometer can then be used to measure the seam width, thickness, and countersink as shown in Figure 2.8 (Double Seam 2019). An improper double seam seal can result in various defects including but not limited to a false seam where the body flange fails to engage with the cover hook and bends back against the can body instead. This information can aid in determining the cause of the defect to correct the operation (CFIA

1993).

Figure 2.8. Double seam terminology of a metal can with first operation (left) and second operation (right) (Dixie Canner Company 2018).

2.4.1.4 Retorting

Conventionally, the canning process consists of preparing the food, filling it in a container, sealing the container, inspecting it for quality and integrity, heating it with sufficient energy to achieve commercial sterility, cooling and a final inspection for defects. Retorting is a process that occurs inside the package at a high temperature

(~125°C) for a sufficient period of time that will inactivate microorganisms and spores.

This will result in the food having an extended shelf-life at room temperature in a hermetically sealed container. Several types of retorts are used commercially in the processing of food cans. These include still and agitating retorts. An agitated retort works by rotating the containers in the retort vessel. This movement of the sealed metal cans shifts the headspace within the package, causing a stirring of the contents and results in greater heat transfer process from the retort itself to the canned product (Kumar and

Sandeep 2014). A still retort functions without agitation to the canned product. This means that a slower rate of heat transfer takes place within the package. Figure 2.9

34 shows an illustration of a still retort. Food and Drug Administration regulations for vertical and horizontal still retorts are presented in 21CFR 113.40.

Figure 2.9. Vertical still retort (Albaali and Farid 2006)

This still retort is made of an iron pressure vessel where the cans are vertically aligned during the heat treatment. During the heat processing of a sealed can, the retort is loaded and closed, followed by steam introduction until a calculated temperature for a predetermined length of time is reached before the steam is shut off. Cold water is then used to cool the containers to 40°C before the retort is opened. The cooled cans are then removed and air-dried (Al-Baali and Farid 2006). A still retort that is above 15 psi at

250°F is referred to as overpressure. Other forms for retorts include the hydrostatic and the continuous rotary retorts. The hydrostatic retort has a continuous process that uses constant temperature, cascading water with overpressure from steam as the heating

35 medium, and agitation of the containers to create a commercially sterile product. The continuous rotary retorts use cylindrical vessels to move the container through a series of steps to continuous retort incoming containers with intermittent agitation (Podolak et al.

2015).

2.4.1.5 Heat penetration

Heat can be transferred between two materials through either conduction, convection or radiation. This process is called heat transfer (Ghoshdastidar 2012).

Conduction occurs when heat is transferred by direct contact with a solid. Convection heat transfer occurs when the medium itself flows from areas of different temperatures therefore it is often a liquid or gas medium. Radiation heat transfer can occur in either a solid, liquid or gas medium through the use of electromagnetic waves to radiate energy

(Karwa 2017). Retorting processing involves both conduction and convection to transfer heat into the product. The heat medium can be steam, hot water, cascaded water, or and mixture of steam and air. The aim of heating a retort is to direct heat from the source to the slowest heating region of the can that is intended to reach the predetermined temperature for the determine length of time. This part is called the “cold spot”

(Podolak et al. 2015). Heat must transfer to the cold spot of the can to effectively create a commercial sterile product. A commercially sterile product is one that becomes shelf- stable due to processing conditions that destroy all microorganisms that can grow under nonrefrigerated conditions (>50°F) (Podolak et al. 2015). This concept will be discussed in more details later in this section.

Heat transfer during the retort process of metal containers occurs through conduction and/or convection as shown in Figure 2.10. Conduction heat transfer occurs when the heat from the outside walls of the package transfers through the package material and in toward the geometric center of the container. This occurs in solid food whereas convection heat transfer occurs in liquid food. Convection heat transfer initially starts with conduction, however, as the heat of the liquid food product increases near the inside walls it results in a change in density. This results in redistribution of the liquid contents due to variances in density within the container. The liquid movement creates convection heating and changes the cold spot of the can from the geometric center to the lower center (Heldman and Hartel 1997).

Figure 2.10. Heat transfer in container by conduction (left) and convection (right) (Heldman and Hartel 1997)

The location of the cold spot is determined by heat penetration studies. This requires multiple thermocouples to be placed throughout the can during retorting. Once the cold spot is located then the target microorganism must be identified to determine the optimal time and temperature for that cold spot to reach to achieve commercial sterility

(Kumar and Sandeep 2014).

High temperatures are used to destroy or inactivate target microorganisms to a certain degree by denaturing the cellular components in microorganisms. The degree of sterilization can be determined using D-values. Microorganisms are affected by heat on a logarithmic scale. A thermal death rate curve can be created by plotting the number of survivors against the corresponding times at a constant temperature. The time required to destroy 90% of the spores of an organism is called the decimal reduction time or D-value.

With 90% destruction being equal to 1D or one log cycle to reduce the microbial spore concentration 1/10 of its initial concentration. Therefore, increasing the temperature will decrease the D-value. The D-value is affected by pH, type of microbe, water activity, and composition of food (Al-Baali and Farid 2006). The lethality temperature on the microorganism over time at the cold spot of the can is shown in Figure 2.11.

Figure 2.11. Lethal-rate curve for typical retort process (Heldman and Hartel 1997)

The thermal conditions needed to obtain commercial sterility depend on factors such as: nature of the food (pH and Aw), storage conditions, heat resistance of microbes, heat transfer of the food, container shape and material, heating medium, and the initial load of microorganisms. Commercial sterility is often defined as a 12D process, where

12 the 12 complete decimal reductions reduces the spores present from 1X10 cfu (colony forming unit) of spores to 1 cfu. To standardize the lethality, the F value has been used.

The unit chosen for the F value is one minute at 121.1°C. Therefore, an F value of 4 implies that the sum of all lethal effect of all the time-temperature combinations in the

39 process is equivalent to the lethal effect of 4 minutes of heating at 121.1°C. The concept of the F value is applied to many foods; for example, low acid foods must receive a minimum of 2.8 F (Robertson 2012).

2.5 Corrosion in canned food

2.5.1 Mechanisms of corrosion

Corrosion is defined as a chemical reaction between a metal and its environment to form derivative compounds of the metal. In aqueous solutions, the reaction the of metal is known as wet corrosion and it is electrochemical in nature. This involves the transfer of an electrical charge across the boundary between the metal surface and the environment. An electrolyte is a liquid conductor that allows the movement of ions from one source to another and as a result, it conducts electricity (Robertson 2012). An anodic reaction occurs when the metal atoms are lost from the surface in the form of cations thus leaving behind electrons at the surface. The anode (metal surface or M) releases electrons called oxidation of the metal atoms:

푀 → 푀푛+ + 푛 푒푙푒푐푡푟표푛푠

The metal surface with the residual electrons is now called the cathode. At the same time as the anodic reaction is occurring, a cathodic reaction is occurring when the reagents in the electrolyte solution react with the metal surface (the cathode) to remove electrons from the metal (Scully 1990). This reaction occurs at the surface of the cathode and results in the reduction of the metal atoms. This process involves energy as dictated by the metals within an electrolyte solution. The corrosion reaction will proceed in an electrolyte solution if a metal of high reaction energy is connected to one of low reaction

40 energy. The electrons will then flow from high to low energy. This is shown in the formula where ΔG is Joule/mole, n is the number of electrons, E is the potential energy, and F is the Faraday constant:

∆퐺 = −푛퐸퐹

Thus, if ΔG is more negative, then the reaction is more likely to proceed. Though a greater negative ΔG helps the reaction proceed, it does not correlate with the rate of corrosion reaction rate. The reaction rate depends on the difference in reaction energy between connected metals (the free metal ions in solution and base metal of the package).

When the metal is more electropositive, it will be more willing to give up electrons and therefore have greater potential to corrode. The greater the negativity of E, the greater will be its tendency to corrode. In aqueous environments, the corrosion reactions becomes balanced by the reduction of hydrogen-ion to form hydrogen gas (Revie and

Uhlig 2008):

+ − 2퐻 + 2푒 → 퐻2

There are three stages in the process of corrosion in deaerated cans filled with acidic food. In the first stage, the oil and tin oxides layers are removed from the can surface with a high rate of tin dissolution. During the second stage, the corrosion rate slows down, and the steel becomes exposed because of the tin dissolution. Cathodic reaction occurs at the sites of the exposed steel. Hydrogen evolves and is taken up by the depolarizers in the food. This stage lasts the longest. Stage three has a high rate of tin and iron dissolution. Hydrogen evolves at a faster rate because larger steel areas are

41 exposed, causing the can to swell. If sufficient hydrogen gas is produced, the ends of the can will bulge and no longer maintains a hermetic seal (Mannheim and Passy 1982).

There are four possible tin-iron couple situations that can occur during corrosion in a sealed can filled with food. Normal detinning is caused by the continued dissolution of the tin and this causes an enlargement of existing pores and scratches, leading to greater exposure of the steel. Since complexed tin is anodic, it creates cathodic protection to the steel. The tin forms complexes with components in the food and the hydrogen attaches to the depolarizers. Normal detinning is typically observed in low pH products in tinplate cans. Rapid detinning occurs when the couple current is high, while tin sufficiently provides anodic protection to the steel. When the tin coating is not thick enough, or the canned food product is highly corrosive (contains corrosion accelerators), rapid detinning will occur. Partial detinning and pitting will be observed when the tin becomes anodic to iron in the internal wall of the can. This results in hydrogen swelling and could lead to perforation of the can if the steel corrodes sufficiently. Lastly, pitting corrosion is the opposite of the normal situation where iron is more anodic than the tin.

Because of this reversal, the tin does not corrode and the steel will have pitting corrosion in areas of imperfection in the tin layer (Mannheim and Passy 1982).

The mechanism of corrosion in enameled can is different from plain tinplate cans.

The pattern of corrosion depends on the steel layer, the tin-iron alloy layer, the enamel coating, and the passivation layers. If there is an area of exposure in the enamel and it exposes the tinplated area, corrosion accelerators in the food product will be available to react with the exposed metal. This has the potential to cause a rapid rate of corrosion and

42 breaching of the steel under the exposed area, resulting in hydrogen swelling or perforation of the can. Thus, enamel coating can be more harmful if applied improperly.

The purpose of coating metal cans is to provide a protective barrier to gases, liquids, and ions. The transfer of ions through the enamel coating depend on the electrical charge and the concentration of the electrolyte. Because the enamel coating is designed to protect the interior surface of the can, a failure of the coating will create the potential for dissolution of the tin to occur at sites of scratches or pores, and this will be called anodic undermining. When this occurs, a small exposed area of tin will increase in size and ultimately cause detachment of the enamel. The enamel coating may then act as a cathode because of the diffusion of protons through it (Robertson 2012).

In another scenario, the alloy and iron layers are more anodic than the tin and are rapidly dissolving. Pitting corrosion occurs and there is no undermining of the enamel as was seen in the first scenario. When this occurs, the corrosion starts at the location of a scratch or pore in the enamel coating. At first glance, such cans will not appear to be corroded but with a lens, it will be seen that the steel became deeply penetrated. When the enamel coating is not properly adhered to the tinplate surface it contributes to the anodic reactions. The loss of adhesion can occur prior to corrosion or during corrosion as a result of a breach in the material under the coating (Robertson 2012).

2.5.2 Factors influencing corrosion in canned food and beverage

The complex nature of food products and the processing conditions create many factors that contribute to the corrosion of metal containers. Combined with the storage conditions of the cans, these work synergistically to influence the type and to accelerate

43 the rate of corrosion. Oxygen, anthocyanins, nitrates, sulfur compounds, and trimethylamines are some of the most important corrosion accelerators in foods

(Montanari 2015).

While it is often thought that acidity and pH cause corrosion of canned food containers, these two factors have no direct proportionality with the degree of corrosion in tinplated containers. Organic acids alone were seen to be less corrosive compared to organic acids in fruit juices. This seems to indicate that the fruit juices likely enhanced the corrosive action of the organic acids due to depolarizers in the juice. The pH of a solution can be a factor that could affect the relative cathodic protection given to steel surfaces. For example, tinplating offers cathodic protection to steel, however if the pH of the solution is <4, accelerated corrosion can occur (valdez salas et al. 2012).

Sulfur and sulfur compounds can be introduced to canned food products by agricultural chemicals, preservatives, and/or proteins that are denatured during heat processing. These compounds are capable of releasing free sulfides, hydrosulfide ions, and the evolution of hydrogen sulfide gas in the headspace of the can. With sulfur compounds, pitting corrosion can occur and this could lead to a loss of cathodic protection from tin. Thereby reducing the rate of tin dissolution and providing no electrochemical protection to the steel from the tin coating. Sulfur dioxide, as a depolarizer, may accelerate corrosion by shifting the potential of the double layer from a negative to positive direction. The sulfur reactions with the tinplated steel can cause two types of staining: iron sulfide and tin sulfide. Iron sulfide staining causes black colorations in the headspace region of the can. This forms during or immediately after

44 heat processing. Iron sulfide reactions tend to occur at pH levels above 6, and it usually occurs in the headspace region of the can because of volatilization of reacting chemicals.

Tin sulfide staining tends to be located throughout the can rather than just the in the headspace region. It is blue-black or brown in color and it occurs in two stages. During the first stage, the tin is oxidized, followed by an insoluble deposit of tin sulfide on the surface of the can. Tin sulfide staining occurs during or after heat processing of the cans

(Mannheim 1987).

Nitrates also act as potential corrosion accelerators and can be found in many fruits and vegetables due to fertilizers in the soil and from water polluted with fertilizer runoffs.

Nitrates are very efficient cathode depolarizers and can be reduce to ammonia during the heat treatment of the food cans. The nitrate detinning reaction rate initially depends on the nitrate concentration and the pH of the solution in the can. The process causes an attack on the tin and iron surfaces and precipitates the corrosion. The formation of ammonium ion is a major conversion product that forms at pH 5 or less. Above pH 5, products like nitrous oxide are formed instead (Mannheim 1987).

Along with the intrinsic factors of the foodstuff to cause corrosion acceleration, the processing conditions and storage also play a major role. It is essential to remove oxygen from the canned product because of its depolarizing nature and potential to react with hydrogen during cathodic reactions. Therefore, it is important to have a smaller headspace to reduce residual oxygen and hydrogen swelling. During thermal processing, degradation products that are formed can become involved in the corrosion process also.

To reduce this from occurring, it is important to cool the cans quickly after the retorting

45 process is completed. After cooling, it is important to store the cans in a cool and dry environment. An increase in the storage temperature can increase the rate of chemical reactions within the sealed cans. In general, this rate doubles for every 10°C increase in temperature (Robertson 2012).

2.6 Methods of corrosion detection

The global cost of corrosion is estimated to be $2.5 trillion annually (NACE

International 2016). Because electrons flow from one material to another, all materials will degrade in its environment over time. Due to the major impact that corrosion has on multiple industries, it is essential that it be detected and monitored and prevented as early as possible.

2.6.1 Corrosion inspection technology

2.6.1.1 Visual inspection

Corrosion can be manually inspected by production personnel or through machine vision to inspect for signs of leaks, distortions, or other physical change to the cans. In both inspection methods, a metal can is compared with a standard. Mechanical visual methods of inspection can be accomplished by the use of equipment such as cameras, magnifying lenses, borescopes or mirrors. Any can that is outside of the limits of differences with the standard are rejected from the production lot. Manual visual inspection requires personnel training, but the disadvantages of manual inspection include human errors due to fatigue and/or improper training. Therefore, a machine vision approach is more accepted in the industry in order to limit errors (IAEA 2012).

An example of such an approach is by the use of charged coupled devices (or CCDs)

46 which uses optical scanning to record images that are processed through a computer system to identify flaws or imperfections in the material that may be undergoing corrosion (Agarwala et al. 2000). Visual inspection through machine vision is beneficial because it is a nondestructive approach that can be continuously performed in real-time.

2.6.1.2 Ultrasonic inspection

Ultrasonic inspection uses high frequency sound waves (1 to 20 MHz) emitted through a transducer that is aimed at the metal can. The system works by generating ultrasonic waves that transfer from a transducer in the equipment through the substrate of the sample that is being inspected. The waves that are transmitted through a homogeneous sample are reflected back to the detector of the equipment at a frequency that is characteristic of the sample. However, imperfections within the sample are transmitted at a different frequency. Depending on the type of ultrasonic system, various methods are used to detect the echoes that are transmitted back to the detector. Variances in the ultrasonic waves that are echoed back to the detector can be via the original transducer, if they are made to reflect back from the sample. However, the ultrasonic waves could be made to pass through the material and be picked up by a second transducer on the opposite side of the material. The measured waves received by the detector can then be used to determine discontinuities, a lack of homogeneity or other defects in the tested material based on attenuation of the waves and reflective patterns

(Pascall et al. 2002). An ultrasonic wave is characterized by its amplitude and frequency.

As the waves travel through the material, the amplitude decreases due to the attenuation of the waves. The attenuation is typically caused by adsorption or scattering of the

47 waves. During adsorption, the ultrasonic waves are converted into energy by the material. During scattering, the dispersion or homogeneity of the material causes scattered direction of the incident wave. The software of the detection system in the equipment is used to calculate the variances in ultrasonic waves as they are returned via the transducer. When a package is tested by this method, the software is programmed to either reject the package or accept it based on how it deviates from the programmed standard (McClements and Gunasekaran 1997).

Ultrasonic inspection can be used to examine packages by an airborne or immersion technique. In the immersion technique, the ultrasonic transducer transmits the ultrasonic waves to the substrate through a water or a viscous gel as a coupling. In the airborne technique, the coupling between the transducer and the substrate is air. In airborne testing systems, the absence of liquid coupling requires the use of piezoelectric, bimorphic, electrostatic or ceramic air coupled transducers. The advantage of using the immersion technique is that it produces high quality images due to high frequencies compared to the airborne technique (IAEA 2012).

2.6.1.3 Radiographic inspection

Gamma or X-rays can be used to monitor corrosion using imaging techniques.

Because X-ray is generally a safer form of radiation than gamma rays, X-ray vision is more widely used to detect the presence of defects or contaminants in packaged food products (Agarwala et al. 2000). This can be operated in-line with the production to continuously monitor the quality of the packaging. In an energized tunnel, a package will emit X-ray photons. The X-rays will vary depending on the homogeneity of the material.

Thus, any corroded areas will absorb or scatter the X-rays and cause attenuation of the emitted photons. These photons can then be converted to electrical signals that can be used to create an image of the tested packages (Nave 2016). Corrosion can be detected by comparing the electrical signals coming out of the corroded areas compared with those from the uncorroded areas using the computer software to accept or reject the test package (Connolly 2007).

2.6.2 Scanning Electron Microscopy – X-Ray Microanalysis

Scanning electron microscopy (SEM) is a technique that focuses a beam of electrons on the surface of a material under controlled pressure to develop an image of the targeted area of the material (as illustrated in Figure 2.12). The energy of the electron beam usually ranges from 0.1 to 30 keV with a current from 1 pA to 1 μA. The focused beam interacts with the atoms on the surface or within the material creating scattering events. The energy from the electron beam is transferred to the material and alters the direction of the beam which creates backscattered electrons, secondary electrons, and x- rays, all of which give information about the targeted material. Information on the topography, composition, crystal structures, and local electrical and magnetic fields can be obtained from the use of SEM (Goldstein et al. 2003).

Images of the material can be obtained through two energy scattering events: inelastic and elastic. During inelastic scattering, the energy of the beam is reduced as a result of energy transfer from the beam to the atoms of the targeted material by interaction of inner-shell atoms and valence electrons. Energy that is lost during the transfer will result in valence electrons being ejected and formed into secondary electrons

and ejection of inner-shell electrons that will contribute to X-rays. During elastic

scattering events, energy is not lost, but rather, the energy is being redirected. The

electron beam is deflected by the electrical field of the atom and takes a new trajectory

(Newbury 2013).

Pairing SEM with energy-dispersive spectrometer (EDS) allows for an elemental

analysis of the targeted material. EDS rapidly analyzes the X-rays from the major

elements to give the composition of the sample. Furthermore, the scan also allows for the

mapping the elements on the targeted area of the sample. Prior to the analysis, the

Figure 2.12. Schematic of SEM-EDS (Hamilton and Quail 2011)

50 samples are coated with gold and other coatings (such as nickel or molybdenum) to reduce the noise or image defects of the scan (Goldstein et al. 2003).

2.6.3 Inductively Coupled Plasma Mass Spectrometry (ICP-MS)

Inductively coupled plasma and mass spectrometry are combined to create a sensitive instrument to analyze trace elements. An illustration of the ICP-MS technique is shown in Figure 2.13. At a high temperature of 6000°C, ICP can completely decompose a sample into its constituent atoms to efficiently ionize elements at a single charge without complexes with other compounds. During the test of a sample, a liquid sample is introduced into the system and then converted into an aerosol by a nebulizer.

The nebulized droplets enter a spray chamber and into the quartz torch which is the central channel of the ICP (Ebdon et al. 1998). Typically, the plasma is made from argon gas which is flowing through the quartz tube. The argon gas is formed into the plasma by a high voltage Tesla discharge which creates energized electrons (Montaser 1998).

During the test, the droplets of the liquid sample become desolvated and decompose by the heat of the plasma and then become ionized. During thermal ionization, the electrons of the sample absorb energy from ion-atom and atom-atom collisions. In ICP-MS, these ions are transferred from the hot plasma to the mass spectrometer. The mass-to-charge ratio is analyzed by separating the ions through the use of a quadrupole. The quadrupole has four rods that emit an electric field between the rods where the ions are tunneling through. Depending on the charge of the ion, it will

51 pass through the quadrupole are various speeds. This allows the ions to be separated due to their mass to charge ratio (Ebdon et al. 1998).

Figure 2.13. Schematic of ICP-MS (Gilstrap and Allen 2009)

2.7 Select Ion Flow Tube Mass Spectrometry (SIFT-MS)

Selected Ion Flow Tube Mass Spectrometry (SIFT-MS) is an analytical method that allows for the real-time measurement of trace gases through ionization by selected precursor cations in a flow tube streamed with helium. An illustration of the SIFT-MS apparatus is shown in Figure 2.14. Using a quadrupole mass filter, positive ions are formed in a current of a carrier gas (helium) through a flow tube. The ions are then detected by a channeltron multiplier/ pulse counting system. To elaborate, a sample is introduced to the system by an injection of volatiles accumulated in the headspace of the

52 test package. The volatile organic compounds are carried by the mobile helium through the flow tube under adjusted pressure and temperature. Three ionic species are added

+ + + into the system to aid in the determination of the volatiles: H3O , NO , and O2 . These three precursor ions were found to be the only suitable species for SIFT-MS application because they do not undergo biomolecular reactions with N2, O2, H2O, CO2, or Ar which would impact the results (Smith and Španěl 2005). By factoring in the loss of precursor ions to the walls of the flow tube and the loss from reactions with the sample ions, the calculated flow rate can be used to determine the compounds of interest (Smith and

Španěl 2005).

Figure 2.14. Schematic of SIFT-MS Apparatus (Smith and Španěl 2005)

During the test of a sample, a solution of the sample is placed in a vessel at atmospheric pressure and sealed with a gas-tight septum. This allows a needle connected 53 to the entry port of the equipment to puncture the vessel, retrieve the headspace gas, and allows the septum to reseal the vessel once the needle is withdrawn. After withdrawal of the sample, the headspace pressure in the vessel will reduce because the instrument is creating a vacuum as it removes the volatiles compounds into the SIFT-MS flow tube.

The unknown volatile compounds are mixed with the reactant gases and the helium carrier gas at the same time. This results in the formation of different product ions which help in identifying and quantifying an unknown ion. SIFT-MS measures the precursor and product ion count rates via the mass spectrometer. This system of detection allows for the real time quantification and analysis of trace gases in complex matrices (Smith and Španěl 2005).

SIFT-MS has several functional modes, one of which is the Full-Scan mode which obtains a complete mass spectrum. The detection quadrupole ion is swept over a mass-charge ratio range while the sample is introduced into the system at a steady flow rate. In the multiple ion monitoring mode specific ions can be monitored which keeps track the count rates of the precursor ions of selected product ions, this scan is limited, however, to around 14 ions (Smith and Španěl 2005).

2.8 Ion Chromatography

Chromatography is a separation technique that functions with both a stationary and mobile phase to separate individual compounds in a complex chemical mixture

(Weiss 2016). The mobile phase consists of gas, liquid, or supercritical fluid that contains electrolytes or other species necessary for the separation. The stationary phase is made of solids, gels, liquids immobilized in solids, and coatings on the capillary walls

54 of a cylindrical column. As a sample is introduced into the equipment, it is transported through the column by the mobile phase. Depending on the chemical nature of the sample and the packing of the column, the chemicals in the mixture are separated from one another based on their affinities for the column. As the separated compounds arrive at the detector of the equipment, they are quantified after comparison with appropriate standards (Small 2013).

Ion chromatography (IC) belongs to the class of liquid chromatography which contains a liquid mobile phase and a stationary phase in a column or capillary. Ion chromatography functions by separating the components of a chemical mixture based on the ion charge and their electrical conductance. In ion-exchange chromatography, the ions in the sample solution are carried through the column by an ionic mobile phase. The carried ions are separated out by the packed column via ion exchange. For example, if measuring cations in a sample then the stationary phase column with be packed with fixed negative charges to attract the cations. Thus, ionic compounds with greater affinity the stationary phase will have longer retention times. The compound that are eluted are then measured by conductivity using a detector. In ion-exclusion chromatography has a stationary phase that has the same charge as the sample ion. Thus, cations in the mobile phase will be repulsed by cations in the stationary phase. The advantage of using ion chromatography is its speed, sensitivity, selectivity, simultaneous detection, and stability of the columns (Weiss 2016).

2.9 Attenuated Total Reflectance (ATR)

Electromagnetic radiation in the infrared region is absorbed by nearly all organic compounds which contain covalent bonds. Infrared radiation wavelengths (2.5 μm to 25

μm) are in a region that is between visible light (400 to 800 nm) and microwaves (longer than 1 mm). Molecules that absorb the infrared radiation are excited to a higher energy state resulting in stretching and bending vibrational frequencies of the molecular covalent bonds. There are two types of infrared spectrometers: dispersive and Fourier transform

(FT). Dispersive infrared spectrometers create a beam of infrared radiation that is divided by mirrors into two parallel beams. A sample a placed in one of the two beams and the other beam is used as a reference. As the beams pass through the monochromator, they are dispersed into a spectrum of infrared light. Through diffraction grating, the radiation frequency varies as it reaches the detector. Alternatively, Fourier transform uses an interferogram which consists of wave-like patterns that contain all frequencies rather than varying frequencies. By plotting intensity verses time, a mathematical operation called Fourier transform can be computed to separate the interferogram into individual absorption frequencies. This creates a spectrum that is virtually identical to one produced by dispersive technique but can be obtained in less than a second (Pavia et al. 2008). Because traditional methods of characterization of an analyte in a polymer are not appropriate, attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) is employed. ATR-FTIR is capable of analyzing the aqueous solutions in polymer films which can be an issue in traditional IR where water is diffused into the polymer films or analyte is adsorbed with the film (Jones et al. 2005).

2.10 Dynamic Mechanical Analysis (DMA)

Dynamic mechanical analysis (DMA) is a technique that measures the viscoelastic response of a material as a function of mechanical solicitation through temperature, time, stress, atmosphere and frequency. DMA (also known as Dynamic

Mechanical Thermal Analysis, DMTA) is used to understand the molecular thermal transitions of a material. It consists of a displacement sensor, furnace, linear drive motor, drive shaft support and control system, and sample holding clamps (Badia et al. 2015).

An oscillatory force which is at a set frequency is applied to a mounted sample. With high sensitivity, the DMA can obtain information on the modulus of a material. This oscillatory force will apply a sinusoidal deformation to the material under controlled stress or strain. The linear drive motor generates the sinusoidal wave which is then transmitted to the sample by the drive shaft. This motion will measure the stiffness and damping of the material (or modulus and tan delta). The storage modulus will provide information on the elastic behavior of the material whereas the loss modulus provides information on the viscous component to stress or the material’s nature to dissipate stress through heat. Tan delta is the ratio of loss to the storage modulus (also known as damping) which measures the dissipation of energy of a material (PerkinElmer, Inc

2013).

The dynamic mechanical analyzer can is capable of measuring the strain and stress of a sample. A polymer is known to exhibit a symptom called creep, which is its ability to absorb energy by the segmental rearrangement of its bonds. The DMA can also

57 determine the glass-rubber relaxation temperature or the glass transition temperature of a sample (Badia et al. 2015). The glass transition temperature can be located as a transition in the storage modulus when plotted logarithmically as a function of temperature. A multi-frequency scan and the calculated activation energy of the glass transition is used to verify that drop is in fact the glass transition temperature (PerkinElmer, Inc 2013).

2.11 Permeability

Fick’s first law states that the flux in x-direction (Fx) is proportional to the concentration gradient (∂c/∂x) of a given compound. The flux is defined as the amount of compound (gas or vapor) that diffuses across a given area of a material, where D is the diffusion coefficient of the material. Fick’s first law is shown in the following equation:

휕퐶 [1] 퐹푥 = −퐷( ) 휕푥

The first law is only applied during steady state diffusion (when the concentration doesn’t change over a period of time). Therefore, Fick’s second law of diffusion considers non- steady state diffusion. The following equation models Fick’s second law with limited x- direction:

휕 휕2푐 퐶 [2] = 퐷( 2) 휕푡 휕푥

The substance will move from high concentration to low concentration which drives the permeation through a material. The substance can also become a sorbed concentration, c, and the penetrant concentration, C, are both in contact with polymer or material surface.

The Nernst equation helps to explain the distribution of the penetrant on the polymer surface where K is function of temperature:

푐 = 퐾퐶 [3]

With Henry’s law the pressure, p, on both sides of the material with a particular thickness, l, and solubility S(p) can be considered:

[4] 퐹 = 퐷푆(푝1 − 푝2)/푙

The penetrants have the ability to plasticize the polymer and this could change the segmental mobility and increase the rate of diffusion of the substance through the polymer. In such cases, the mass transfer of the penetrant is non-fickian and its concentration will exponentially increase in the polymer, it will further plasticize the molecular chains, increase the segmental mobility and decrease the glass transition temperature (Comyn 2012).

Because changes in a polymeric material can occur due to its interaction with the environment, it is essential to monitor the storage conditions of plastic packaging in order to determine the extent of any penetrant sorption and its effect on plasticization of the material. The rate of permeation can be monitored with various instrumentation including a water vapor and gas transmission analyzer. In this test, the film is mounted between two half cells of a permeation test chamber with gas or vapor flowing from one side of the film and collected by a carrier gas on the opposite side of the film to measure the rate of permeation. The permeant is then carried to a detector where it monitors and

59 quantifies the permeant over a period of time to determine the transmission rate. Various materials will have different transmission rates to gases and vapors depending the molecular composition of the polymer.

Chapter 3: Materials and Methods

3.1 Experimental Design

3.1.1 Materials and Ingredients

Two types of packaging materials were tested in this study: BPA-free epoxy-lined metal cans and 8oz Ball Regular Mouth Mason glass jars. The metal cans were 211 x

400 D&I in size, and supplied by PPG Industries Inc. (Mason, OH). The glass jars were purchased from a local grocery store in the Columbus, OH area. Roma tomatoes and salt

(sodium chloride) were purchased from a local food supermarket in the Columbus, OH area. Red mature tomatoes of uniform size were chosen and stored at room temperature for 3 days after being purchased. Citric acid, calcium chloride, sodium nitrate, and methionine were purchased from Sigma-Aldrich, Co. (St. Louis, MO).

3.1.2 Experimental Design

The experimental design is illustrated in Figure 3.1. The tomatoes were peeled, diced, canned, sealed, and retorted using the facilities in the Food Processing Pilot Plant in the Department of Food Science and Technology at The Ohio State University,

Columbus, OH. The tomato processing technique and equipment are explained in further details in Section 3.1.3. The canned diced tomato samples were stored at 49°C prior to testing. After the containers were opened, the diced tomato mixtures were tested for volatile and nonvolatile compounds using Selected-Ion Flow Tube Mass Spectrometry

(SIFT-MS), Ion Chromatography Mass Spectrometry (IC-MS), and Fourier-Transform

Infrared Spectroscopy (FTIR) methods. The concentrations of iron and tin in the diced 61 tomato mixture were analyzed using Inductively Coupled Plasma Mass Spectrometry

(ICP-MS). The internal can coating was removed and analyzed for the presence and concentrations of organic volatiles and nonvolatile using SIFT-MS and IC-MS. The coating was also tested by X-Ray Diffraction and FTIR to analyze for changes in the crystallinity and functional groups of the polymer coating before and after processing and during the shelf-life storage period. The recipe of the canned diced tomatoes was modified to selectively eliminate each ingredient, thereby creating various treatments of the canned product (Table 3.1). Either the tomato, citric acid, calcium chloride, and sodium chloride was selectively left out from each treatment of the cans. The control was one set of all the ingredients, and it was heat treated in glass jars and metal cans.

Furthermore, one set of all ingredients was packaged in glass jars but was not heat treated. This was also used as a control.

Robertson (2012) reported that sulfur and nitrogen compounds found in canned foods are known to accelerate corrosion in the metal container. Therefore, methionine and nitrate were targeted as possible contributors to the corrosion occurring in this study.

S-methyl methionine is also reported to contribute to the formation of dimethyl sulfide in tomatoes when they are heat treated (Petro‐Turza 1986; Scherb et al. 2009). Because of the presence of sulfur in the molecular structure of the tomatoes, methionine was suspected of playing an important role in creating breaches in the can coating. Nitrates were also found to accelerate corrosion, especially when combined with the acidic compounds such as those found in canned tomato products (Ninčević Grassino et al.

2009). Because of the possible contribution of these two corrosion accelerators (nitrates

62 and sulfur), sodium nitrate and methyl methionine were added as treatment groups in this study. As shown in Table 3.1, the methionine group was formulated with citric acid, sodium chloride, and calcium chloride in a solution of distilled water. The nitrate group was formulated using the same chemicals as the methionine group. All sample treatment groups were stored at -80°C prior to data collection to delay chemical reactions until the necessary instruments were available for use.

Figure 3.1. Experimental design for studying corrosion mechanism in canned diced tomatoes.

Table 3.1. Canned tomato sample experimental design

Treatments Ingredients Unprocessed Control Citric Calcium Salt Methionine Nitrate SMM Nit+SMM Tomato (Processed) Acid Chloride Diced Tomato X X Tomato Juice X X Salt X X X X X Citric Acid, X X X X X X X X X Anhydrous Calcium X X X Chloride Sodium Nitrate X X Methionine X X Distilled Water X X X X X X X S-Methyl X X Methionine

3.1.3 Sample preparation and processing

The canned diced tomatoes formulation is summarized in Table 3.2. The whole tomatoes were washed by submersion in a tank of potable water that was agitated by a circulation pump. The washed tomatoes were sorted to remove damaged and oddly shaped fruits. The tomatoes were then soaked in 18% sodium hydroxide and 0.1%

Faspeel (Collinsville, IL) solutions for 30 seconds at 190°F in order to soften the outer skin. They were then placed on a conveyor belt fitted with rotary rubber disks that acted to provide a scrubbing action to the tomatoes as they passed under a low-pressure water spray that completely removed the skins. The peeled tomatoes were then diced into 1.27- cm3 cubes. One-third of the diced tomatoes were sent to be juiced. The tomatoes were reduced in size to a pumpable slurry using a W.J. Fitzpatrick Model D Comminuting

Machine (Chicago, IL) equipped with a ¾ inch screen. The slurry was then pumped to a tubular heat exchanger. This was heated using steam to reach a hot break temperature of

±190°F. The juice was extracted using a Chisholm-Ryder Model CLE-360-D28 screw type extractor (Kalamazoo, MI) with a 0.20-inch mesh screen. A 278 grams aliquot of diced tomatoes was transferred to each 211 x 400 two-piece can. To this was added 36 grams of tomato juice, to create a headspace of 7.14mm (Figure 3.2) in each can. The cans were then closed using a Model#6 American Can Seamer (Greenwich, CT). After similarly filling the 8oz glass jars with the diced tomatoes and juice, they were tightened by hand to a torque level that prevented leaking. These were used as controls. The hermetically sealed cans were sterilized in a Dixie Canner Still Retort at a hold

65 temperature of 250°F for 30 min. The glass jars were retorted separately from the metal cans because of the difference in heat penetration properties of glass compared with tinplated steel. For the glass jars, they were heated at 250°F for 40 minutes to achieve commercial sterility. The heating times and temperatures were selected based on recommendations from the USDA, to ensure that the cold spot of the containers filled with the diced and juiced tomatoes reached a temperature of 121°C (USDA 2015).

Figure 3.2. Filled 211 x 400 can of diced tomatoes

All samples were adjusted to a pH of 3.55 using citric acid prior to filling and sealing the cans and glass jars. To prepare the methionine (15 ppm) samples 0.90 g of L- 66 methionine was added to 6 L of buffer solution that consisted of 11.4 g citric acid, 25.2 g sodium chloride and 3 g calcium chloride in 6 L of distilled water. To prepare the nitrate

(25 ppm) samples, 0.15 g of sodium nitrate was added to 6 L of buffer solution. The buffer solution consisted of 0.90 g L-methionine, 11.4 g citric acid, 25.2 sodium chloride, and 3 g calcium chloride in 6 L of distilled water.

To obtain a hermetically sealed can, the double seam was required to have a body hook between 0.075 – 0.085 inches, a cover hook between 0.070 – 0.080 inches and an overlap > 0.045. The average seam width measurement of the tested cans used in this study was 0.113 inches. The average body hook was 0.076 inches, cover hook was

0.075 inches, and overlap was 0.048 inches. After sealing, all samples were shaken to ensure proper mixing of the ingredients.

Table 3.2. Diced tomato formulation per 10 oz can.

Ingredients Weight per can (g) % by weight Diced Tomatoes 278.33 87.94% Tomato Juice 36.08 11.40% Sodium Chloride 1.32 0.42% Citric Acid, Anhydrous 0.60 0.19% Calcium Chloride 0.16 0.05% Total 316.50 100.00%

3.2 SIFT-MS Analysis

A selected ion flow-tube – mass spectrometry (SIFT-MS) instrument SYFT

Voice100, Syft Ltd. (Christchurch, New Zealand) was used for the headspace analysis of volatile organic compounds at the parts-per-billion level by volume in the test containers.

Prior to the SIFT-MS analysis, all samples were stored at 49°C after retorting for 50 days.

Each treatment group illustrated in Table 1.1 was opened and tested on days 0, 3, 6, 10,

20, 30, 40, and 50. If samples were not tested on the day they were opened, the samples were stored in 8oz Ball Glass Mason jars with aluminum foil covering the seal between the lid and the glass jar, and then stored in a -80°C Thermo ScientificTM FormaTM 89000

Series ultra-low freezer (Waltham, MA). The storage times were selected after preliminary results showed the average times when headspace and body corrosions were likely to be visible in the different ingredient groups.

For quantification of the volatile compounds in the canned diced tomato treatment groups, the mixtures were homogeneously blended using a Waring, Dynamic Corp. commercial blender (Torrington, Conn., USA) for 30 s at the highest speed setting. An aliquot of 5 g of all aqueous solutions were transferred to 500 mL Pyrex media storage bottles (Figure 3.3). Each bottle was capped with an open top screw cap fitted with an airtight silicone septum and incubated at 70°C in a water bath for 5 minutes to allow for headspace equilibration. For quantification volatile compounds in the can coating, the coating was removed from the headspace region of each test can after the can was cut into four quadrants (Figure 3.4). A mass of 6 mg of coating was transferred to 500 mL

Pyrex media storage bottles. Each bottle was sealed and incubated at 90°C in a water- bath for 5 minutes. Before the analyses all media glass storage bottles and septa were incubated at 100°C for 12 hours to remove all traces of residual volatile compounds.

Figure 3.3. Pyrex storage media bottles (500 mL) with 5 grams of tomato sample

The headspace of the solution in each test bottle was analyzing using the selected- ion monitoring (SIM) mode of the SIFT-MS instrument. The headspace sampling was performed using a 14-gauge 3.8 cm long passivated sampling needle connected to the inlet port of the SIFT-MS instrument. It was inserted into each test sample bottle through the septum of the bottle cap. Prior to introducing the samples into the equipment, a 5 mL aliquot of a 50°C HPLC grade water was injected into the system and scanned in order to

69 clean the loading tube of the instrument. A blank analysis was done before and after each test and they were used to zero the equipment. Room air was scanned between each sample to also zero the instrument before running the subsequent sample. The scan duration was 1 minute.

Figure 3.4. 211 x 400 metal can cut cross-sectionally into four quadrants

The analyses targeted sulfur and acid compounds as well as other compounds of interest reported to be found in tomatoes. Table 3.3 summarizes the information used to identify the 22 volatile organic compounds of interest in the headspace of the bottles. 70

The data collected were expressed as concentrations in parts-per-billion by volume of the compounds. Each treatment group showed the average of 6 data points (2 batches and 3 observations per batch).

Table 3.3. Selected volatiles, reagent ions and mass-to-charge ratio of products

Compounds Formula m/z Reagent Reference Sulfur compounds + dimethyl disulfide C2H6S2 95 H3O (Castada et al. 2015) 94 NO+ + 94 O2 + dimethyl sulfide C2H6S 62 NO (Castada et al. 2015) + 62 O2 + dimethyl trisulfide C2H6S3 126 NO (Castada et al. 2015) + 126 O2 + 2-isobutylthiazole C7H11NS 142 H3O (Xu and Barringer + 99 O2 2010) + methyl mercaptan CH4S 49 H3O (Španěl and Smith + 48 O2 1999) + 1-propanethiol C3H8S 76+106 NO (Španěl and Smith 1999) Acids + acetic acid CH3COOH 90+108 NO (Castada et al. 2015) + butanoic acid C4H8O2 89 H3O (Castada et al. 2015) 118 NO+ + hexanoic acid C6H12O2 146 NO (Olivares et al. 2010) + hexyl acetate C8H16O2 174 NO (Španěl and Smith 1999) Other compounds + Ammonia NH3 18+36 H3O (Castada et al. 2015) + (E)-2-hexenal C6H10O 97 NO (Xu and Barringer 2010) + (E)-2-octenal C8H14O 125+156 NO (Xu and Barringer 2010) + (E)-2-pentenal C5H8O 83 NO (Xu and Barringer 2010) + furfural C5H4O2 96 NO (Španěl and Smith + 96 O2 1999)

+ hexanal C6H12O 101+119+137 H3O (Xu and Barringer 99 NO+ 2010) + phenylacetaldehyde C8H8O 121+139+157 H3O (Xu and Barringer 120 NO+ 2010); (Castada et al. 2015) + acetaldehyde C2H4O 45+81 H3O (Xu and Barringer 43+61+79 NO+ 2010); (Castada et al. 2015) + ethanol C2H6O 45+63+81 NO (Castada et al. 2015) + furaneol C6H8O3 129+147 H3O (Castada et al. 2015) 128 NO+ + 128 O2 + methanol CH3OH 33+51+69 H3O (Xu and Barringer 2010) + acetone C3H6O 59 H3O (Xu and Barringer 88 NO+ 2010)

Additionally, to determine if sulfonium methyl methionine (SMM) thermally degraded to dimethyl sulfide, increasing levels of SMM were added to 2 mL distilled water in a 50 mL Pyrex jar fitted with a septum in its cap. The samples (in replicates of

4) were heated at the same retort processing conditions as the tomatoes (121°C for 30 minutes) in the closed system and then analyzed for dimethyl sulfide by the SIFT-MS.

3.3 IC-MS Analysis

This test was done using a ThermoScientific Dionex ICS-1500 Ion chromatography instrument to determine the nitrate concentrations in the tomato samples.

Prior to the analysis, the samples were stored at 49°C. The canned tomatoes were tested on days 0, 3, 6, 10, 30, and 50 of storage, while those in the glass jar were tested on days

0, 20, and 50. The unprocessed tomato samples were stored in a glass jar prior to analysis. All samples were frozen at -80°C prior to analysis to delay chemical reactions before the instrument was available to be used. The samples were tested from 2 batches

72 with 3 replicates per batch. All samples were homogenized using the commercial blender for 30 s at the highest speed setting. The blended samples were centrifuged at 5000 r/min for 30 minutes using in a Sorvall Legend X1 Centrifuge manufactured by Thermo

Scientific (Waltham, MA). The aqueous supernatant was pipetted out and recentrifuged a second time at the same rpm and time. The supernatant was removed and then diluted by a factor of 8 with distilled water.

The test samples were then analyzed for the nitrate concentrations using the Ion chromatography instrument. The equipment was fitted with a Dionex IonPac AS22 anion exchange column (250 mm x 4 mm) and an AG22 Guard column. The mobile phase was

1.4 mM sodium bicarbonate ⁄ 4.5 mM sodium carbonate in water, and it was filtered with a 0.45 µm nylon membrane at a flow rate of 1 mL⁄ min. The total analysis duration time was 10 min or 20 min depending on the test sample. All sample vials were staked in the autosampler of the instrument and the injected volume was 5 ml.

3.4 ICP-MS Analysis

To measure the migration of the tin and iron compounds from the metal can to the packaged tomato, a ThermoFisher Element 2 ICP-MS (Bremen, Germany) instrument was used. All chemicals used in the analysis were of analytical-reagent grade and were not further purified. Deionized water processed by an 18 MΩcm-1, Millipore Milli-Q-

Plus water purifier (Bedford, MA) was used in all testing on the ICP-MS. The standard solutions for the iron and tin analyses were obtained from dilutions of stock solutions which included the single element iron (CPI International, Santa Rosa, CA) and the single element tin (CPI Internal, Santa Rosa, CA). From these, standard curves were

73 plotted for iron and tin in concentrations of 10, 25, 50, 100, 150, and 200 ppb. Colbalt-59

(CPI International, Santa Rose, CA) was used as an internal standard in all sample solutions at 10 ppb. The stock solutions were diluted with a solution containing 2%

HNO3 and 0.05% HF. Iron-56 and Tin-120 were selected for monitoring the concentration of iron and tin migration from the package to the tomato contents. The concentrations of iron and tin were determined from the standard curves by plotting the intensities versus the concentrations of iron (Figure 3.5) and the concentrations of tin

(Figure 3.6) from the external standards and interpolating them with the intensities for the test samples.

Figure 3.5. Standard curve of iron (56Fe) intensity prepared at increasing concentration (ppb) and measured using ICP-MS

Figure 3.6. Standard curve of tin (120Sn) intensities prepared at increasing concentration (ppb) and measured using ICP-MS

The tomato products from the test cans and the glass jars (controls) were tested on days 0, 10, 30, and 40 and days 0, 20, and 40, respectively after being held at a storage temperature of 49°C. The treatment groups containing S-methyl methionine (SMM) and sodium nitrate with SMM were both tested on days 0, 20, and 40 of the storage period.

The samples were tested from 2 batches with 3 replicates per batch. The samples were homogenized using the commercial blender for 30 s at the highest speed setting. An aliquot of 2 grams from each sample was weighed using either plastic spoons or micro pipetted into 50 mL polypropylene plastic tubes. Aliquots of 5.0 mL of a 69% HNO3 and

2.5 mL a 30% H2O2 were added to each tube including a blank tube. The tubes were 75 loosely capped for 10 minutes to allow for partial digestion and then tighten before transferring to a digestor. The tubes were then incubated at 105°C for 2.5 hours in a

DigiPREP MS digestion system manufactured by SCP Science, Inc. (Champlain, NY).

After digestion, the tubes were cooled to room temperature (23°C ±2). The samples were then diluted with a solution of 0.05% HF and 10 ppb of cobalt (as an internal standard) in

50 mL of deionized water.

The samples were analyzed by ICP-MS where the intensities of 56Fe and 120Sn were used to determine the concentrations of iron and tin in the tested samples. The samples were introduced into the instrument at 100 microliters/min (μL/min) through a concentric PFA micronebulizer into a PFA double pass spray chamber. The ICP RF power was 1250 watts. A rinse of 2% HNO3 was scanned before each analysis and between the samples to clean the loading tube. A standard check and calibration blank was scanned after every 10th test sample reading. The standard solutions were scanned from low to high concentration to create a calibration curve. The results were collected using the Thermo ELEMENT software. Each sample was automatically measured five times and the average concentration calculated. The concentration of each test sample time point was averaged between 2 batches and 3 replicates per batch.

3.5 SEM-EDS Analysis

The elemental composition of the surfaces of the unprocessed, processed, and aged cans were analyzed using SEM-EDS. All cans were processed with tomato and stored at elevated temperatures to accelerate the corrosion process. After retorting at day 0 and after storage at day 50 of the shelf-life study, the cans were opened, the contents removed

76 and the empty cans rinsed in distilled water and into four quadrants as shown in Figure

3.4. The unprocessed cans that were not treated or filled with tomato and retorted were used as controls. Evidence of corrosion in the can samples from the day 0 storage were analyzed and the results compared with the corrosion evident in the cans from day 50.

To perform the SEM-EDS test, all samples were cut in 1 x 1 cm sections and coated in carbon. Each sample was then loaded into the chamber of the FEI Quanta 200 SECM-

EDS equipment manufactured by FEI Company (Hillsboro, OR). The EDS analysis was carried out using the TEAMTM EDS Analysis System for SEM the was manufactured by

The EDAX Company (Mahwah, NJ). X-ray microanalysis was performed at an acceleration voltage of 20 kV and the emission current was 0.80 nA. The scans focused specifically on the corroded and non-corroded areas of the can with a magnification of x86.

3.6 X-Ray Diffraction Analysis (XRD)

Diffraction patterns of the can coating before and after retort processing were obtained using a Rigaku MiniFlex 600 Diffractometer (Tokyo, Japan) to determine the crystallinity of the samples. Samples of unprocessed can and day 0 and 20 of processed cans were cut and the lining was removed in order properly fit and mount on to the sample platform used to collect data on the XRD instrument. The diffraction patterns were recorded using copper Kα radiation and with a secondary monochromator at 40kV and 15 mA. The relative intensity was recorded at a scattering angle (2θ) range of 3-90° by a scintillation counter at a scanning speed of 10°/min and a 0.02° step.

3.7 FT-NIR Analysis

Fourier Transform – Infrared in the near region spectra were recorded with a Varian

Excalibur Series FTS 3100 Spectrophotometer (Randolph, MA) to determine changes in the functional groups of the polymeric lining before and after retort processing. To accomplish this, the can linings were removed from the metal base for analysis. The data were collected for samples in processed cans stored on days 0, 10, 30, 40, and 50 and from unprocessed cans. For each test, the background spectrum was eliminated in order to zero the equipment. For each sample tested. The bands were recorded in the near infrared region of 4000 to 10,000cm-1 with 64 scans and a resolution of 4 cm-2 using a quartz near-IR beam splitter.

3.8 Statistical Analysis and Data Interpretation

Each sample was test in triplicate and the experiments done in duplicate. The statistical analyses of the data were carried out using an IBM® SPSS® Statistics Program

Version 24 Software. Linear regression was used to compare the significant effects of each treatment group on the rate of the tin and iron migration over the storage period. An analysis of variance (univariate and multivariate) was used with a post hoc tests of

Fisher’s Least Significant Difference (LSD) at an alpha level of 0.05 to investigate the following: (1) the significant effect of heat processing to the concentrations of the volatile and non-volatile compounds before and after processing the tomatoes and during the storage period; (2) the volatile compounds that had significant interaction with the epoxy coating during the storage period; and (3) the significant effect that each treatment group

78 had on the migration of tin and iron compounds from the walls of the cans to the tomato product during the storage period.

Chapter 4: Results and Discussion

4.1 SIFT-MS Analysis

The concentrations of selected volatile compounds were investigated using the

SIFT-MS to determine which compounds were found at the highest concentrations in the tomatoes and in the can lining. Table 4.1 shows the mean concentrations of the selected compounds in the tomatoes before and after the retorting process. After retorting, there was a significant increase in sulfur compounds such as dimethyl sulfide, dimethyl disulfide, and dimethyl trisulfide in the tomatoes (p-value < 0.05). The concentration of dimethyl sulfide (DMS) increased by approximately twenty-fold after the retorting process. Along with the sulfur compounds, methanol, ethanol, acetaldehyde, and acetone also had the high volatile concentrations in the headspace of the package after retort processing. Among the selected acid compounds, acetic acid increased significantly after retort processing (p-value<0.05).

Table 4.1. Concentrations (ppb) of selected volatile compounds in the tomatoes before and after retort process

Concentration of analytes in 3 grams of sample (ppb) Volatile Compounds Unprocessed Processed p-value

Sulfur dimethyl disulfide 17 28 0.004 dimethyl sulfide 259 5,208 <0.001 dimethyl trisulfide 30 62 <0.001 methyl mercaptan 80 70 0.272 1-propanethiol 19 21 0.309 2-isobutylthiazole 18 20 0.439 Acid hexanoic acid 5 2 0.016 hexyl acetate 2 1 0.541 butanoic acid 94 83 0.252 acetic acid 65 118 <0.001 Other methanol 176,373 184,909 0.590 ethanol 17,996 17,042 0.308 furaneol 9 11 0.286 furfural 7 140 <0.001 hexanal 273 83 <0.001 phenylacetaldehyde 4 79 <0.001 (E)-2-hexenal 29 58 <0.001 (E)-2-octenal 40 0 <0.001 (E)-2-pentenal 135 87 <0.001 acetaldehyde 2,332 4,156 <0.001 acetone 630 2,145 <0.001 ammonia 173 194 0.500 *significant at α=0.05 **Values expressed as the mean of 2 batches by 3 replicates per batch

To determine which compounds were sorbed from the tomato to the lining of the cans after retorting, changes in the concentrations of the selected volatile compounds in

81 the lining were examined before and after the retorting process. The results in Table 4.2 show that the DMS was the only compound (p-value<0.05) that remained significantly high in the lining after the retort processing. All other compounds found at high concentrations in the tomato content were not found to be significant in the lining (Table

4.2). The sorbed DMS could have the potential to complex with and alter the polymer that comprised the lining of the cans. When a molecule is sorbed by a polymeric matrix, it can plasticize the chains of the polymer and this can facilitate an exponential uptake of additional molecules by the polymer (Comyn 2012). A study by Kontominas et al. showed that sulfur and other compounds in canned fish products were sorbed by the polymeric lining causing pores and cracks which resulted in corrosion and the migration of tin and iron into the package contents (Kontominas et al. 2006).

Table 4.2. Concentrations (ppb) of selected volatile compounds in the can lining before and after retort process

Concentration of analytes in 6 mg of can lining (ppb) Volatile Compounds Unprocessed Processed in tomato p-value Sulfur

dimethyl disulfide 0 0 0.168 dimethyl sulfide 0 62 <0.001 dimethyl trisulfide 0 3 0.002 methyl mercaptan 0 0 0.017 1-propanethiol 0 1 0.540 2-isobutylthiazole 0 0 <0.001 Acid hexanoic acid -1 0 0.143 hexyl acetate -2 0 0.002 butanoic acid -1 -1 0.954 acetic acid -3 -3 0.932 Other methanol 8 44 0.081 ethanol 29 78 0.206 furaneol 0 0 0.092 furfural 0 1 0.002 hexanal 1 2 0.005 phenylacetaldehyde 1 1 0.689 (E)-2-hexenal 0 1 <0.001 (E)-2-octenal 0 0 0.345 (E)-2-pentenal 0 1 0.024 acetaldehyde 12 11 0.804 acetone 1 6 0.251 ammonia -4 16 0.125 *significant at α=0.05 **Values expressed as the mean of 2 batches by 2 replicates per batch

Table 4.3 shows changes in the concentrations of the selected volatile compounds in the tomato treatment groups during the storage period at 49oC. The concentrations of the sulfur containing compounds in the tomatoes showed a continuous decrease during the 50-day storage period. Additionally, the concentrations of the aldehyde containing

83 compounds and butanoic acid also decreased during the storage time. As corrosion occurred during the storage period, it could have resulted in the formation of ferrous sulfide and stannous sulfide. These complex interactions could increase the molecular weight and decrease the volatility of the sulfur compounds (Nagu et al. 2018). To confirm that these compounds were originally from the tomato fruit, their concentrations in the citric acid only, calcium chloride only, and salt only treatment groups were measured (Table 4.4). None of the identified compounds were detected at the same levels as found in the tomato treatment group. Table 4.4 shows the results of the methionine only treatment group which was tested to determine whether it contributed to the formation of the DMS. It shows that there was no detection of the DMS at day 0, however, a noticeable increase in the dimethyl disulfide and methyl mercaptan concentrations were detected during the storage time of the methionine treatment group.

These results may indicate that the methionine may have reacted to give rise to the dimethyl disulfide during the storage of the retorted product. Table 4.5 shows similar results for the nitrate only treatment group, in which the same level of methionine was added to the nitrate solution. The literature reports that nitrates will be reduced into nitrous oxide (N2O), nitric oxide (NO), hydroxylamine (H3NO), and ammonia/ammonium ions when exposed to low pH conditions (Mannheim and Passy

1982). Therefore, the nitrate only treatment group was expected to have increased levels of ammonia during the storage time in the presence of corrosion. Farrow et al. (1970) reports that the nitrate will react with tin (II) to oxidize it to the more stable tin (IV) where 1 mole of nitrate will be reduced to ammonia for every 4 moles of tin that will

84 oxidize. This will be so because ammonia will be the principle nitrate reduction product at pH levels below 5.0 (Farrow et al. 1970). However, the concentration of ammonia was not high in this study as can be seen in the preceding tables. The low detection of ammonia might be due to the loss of it during the opening the cans to remove the contents after retorting. The concentration would also expect to be low in cans that did not show or lacked substantial corrosion.

Table 4.3. Concentrations (ppb) of selected volatiles in the tomato treatment group during storage at 49OC.

Concentration of analytes in 3 grams of sample (ppb) Volatile Compounds Day 0 Day 3 Day 6 Day 10 Day 20 Day 30 Day 40 Day 50 Sulfur dimethyl disulfide 19 13 13 9 11 12 13 12 dimethyl sulfide 3,836 3,258 3,403 2,508 3,158 3,159 3,546 3,076 dimethyl trisulfide 46 27 25 19 24 24 29 20 methyl mercaptan 49 31 28 23 26 25 29 24 1-propanethiol 27 20 20 17 22 21 25 21 2-isobutylthiazole 15 9 8 6 8 7 9 6 Acid

hexanoic acid 0 2 2 1 2 2 3 2 hexyl acetate -1 0 1 0 1 0 0 1 butanoic acid 68 58 53 45 43 37 42 32 acetic acid 76 61 56 52 69 74 97 147 Other

methanol 136,369 96,608 97,194 79,675 90,670 92,142 104,443 91,277 ethanol 14,563 12,948 12,505 11,070 11,841 11,077 13,051 11,232 furaneol 6 3 3 3 4 4 6 5 furfural 57 47 60 66 106 129 183 223 hexanal 76 61 57 45 52 50 54 43 phenylacetaldehyde 32 30 31 24 22 20 22 17 (E)-2-hexenal 32 20 19 15 19 20 28 28 (E)-2-octenal -3 2 2 1 2 2 2 2 (E)-2-pentenal 64 43 42 32 37 36 42 34 acetaldehyde 2,640 1,853 1,707 1,240 1,228 1,070 1,120 908 acetone 1,143 1,072 1,502 1,086 1,218 1,424 1,321 1,294 ammonia 115 65 60 42 48 43 55 39 *Values expressed as the mean of 2 batches by 3 replicates per batch

Table 4.4. Concentrations (ppb) of selected volatiles in the various treatment groups during storage at 49OC. Concentration of analytes in 3 grams of sample in various treatment groups Citric Acid Salt Calcium Chloride Methionine Volatile Compounds Day 0 Day 20 Day 40 Day 0 Day 20 Day 50 Day 0 Day 20 Day 40 Day 0 Day 20 Day 50 Sulfur dimethyl disulfide 0 0 0 0 0 0 0 0 0 2 47 223 dimethyl sulfide 1 1 1 1 1 1 1 1 2 2 2 2 dimethyl trisulfide -1 0 0 -1 0 -1 -1 0 -2 -2 1 5 methyl mercaptan 0 0 0 0 0 0 0 0 0 1 7 23 1-propanethiol 0 0 0 0 0 0 0 0 3 7 27 19 2-isobutylthiazole 0 0 0 0 0 0 0 0 0 0 0 0 Acid hexanoic acid -1 -1 -1 -1 -1 -1 -1 -1 -2 -3 -2 -2 hexyl acetate -1 -1 -1 -1 -1 -2 -2 -1 0 -3 -1 0 butanoic acid 0 -1 -1 0 -1 0 0 0 0 -1 0 2 acetic acid -3 0 -1 -4 1 2 -3 1 8 -11 5 18 Other methanol 66 60 58 67 50 44 60 58 86 96 108 105 ethanol 57 66 64 29 55 62 63 76 219 71 305 321 furaneol 0 0 0 0 0 0 0 0 1 -1 0 0 furfural 0 0 0 0 0 0 0 0 0 0 5 20 hexanal 1 1 1 1 1 0 2 1 0 1 1 1 phenylacetaldehyde -1 0 0 -1 0 0 0 0 -1 -11 -3 -3 (E)-2-hexenal 0 0 0 0 0 0 0 0 0 0 0 0 (E)-2-octenal 0 0 0 0 0 0 0 0 0 -6 0 0 (E)-2-pentenal 0 0 0 0 0 0 0 0 0 0 0 0 acetaldehyde 16 32 36 17 35 40 18 51 108 18 72 141 acetone 92 210 253 86 1,000 1,196 72 703 1887 83 490 1220 ammonia 0 3 2 3 1 3 2 6 14 -19 -9 -10 *Values expressed as the mean of 2 batches by 3 replicates per batch

Table 4.5. Concentrations (ppb) of selected volatiles in the nitrate treatment group during storage at 49OC.

Concentration of analytes in 3 grams of Nitrate treatment groups (ppb) Volatile Day 0 Day 3 Day 6 Day 10 Day 20 Day 30 Day 40 Day 50 Compounds Sulfur dimethyl disulfide 4 0 4 4 37 144 189 132 dimethyl sulfide 2 1 2 1 2 2 2 1 dimethyl trisulfide -2 -1 0 0 2 7 9 5 methyl mercaptan 2 0 3 3 8 19 28 28 1-propanethiol 8 4 13 14 24 12 11 13 2-isobutylthiazole 0 0 0 0 0 0 0 0 Acid hexanoic acid -3 0 -1 -1 -1 -2 -1 0 hexyl acetate -4 -2 -1 -1 -1 -2 -2 0 butanoic acid -1 0 -1 -1 -1 -1 -1 -1 acetic acid -11 -4 -2 -3 3 9 14 11 Other methanol 102 67 75 70 81 81 80 72 ethanol 79 27 72 77 9 41 30 95 furaneol -1 0 0 0 0 0 0 0 furfural 0 0 0 0 4 14 17 12 hexanal 1 1 1 1 2 1 1 1 phenylacetaldehyde -14 -1 0 0 0 1 1 0 (E)-2-hexenal 0 0 0 0 0 0 1 1 (E)-2-octenal -6 0 -1 -1 0 0 0 0 (E)-2-pentenal 0 0 0 1 0 0 0 0 acetaldehyde 14 18 25 25 54 108 117 85 acetone 84 48 148 132 348 817 1,099 726 ammonia -15 3 1 0 -4 -5 -5 -8 *Values expressed as the mean of 2 batches by 3 replicates per batch

Williams and Nelson (1974) reported that the formation of DMS in tomatoes occurred due to the thermal degradation of methyl methionine. Figure 4.1 shows that the increasing trend of dimethyl sulfide concentration was directly proportional with increasing levels of the SMM after being heated for 30 minutes at 250°F in an oven. The

88 standard curve was then used to quantify the level of the SMM found in the tomatoes obtained for this study.

Figure 4.1. The correlation of dimethyl sulfide with increasing levels of SMM expressed as the average of 4 replicates

Table 4.6 shows that the concentrations of the DMS gradually decreased during the storage of the SMM treatment group. The initial high concentration of DMS seems to indicate that the degradation of SMM was responsible for its formation. Table 4.7 and

Table 4.8 show that the DMS was also detected in the linings of both tomato and SMM treated cans respectively. The concentration of DMS in the SMM lining gradually

89 decreased during the storage time, whereas the DMS showed no change in the tomato treated lining.

Table 4.6. Concentrations (ppb) of selected volatile compounds in the SMM treatment group during storage at 49OC.

Concentration of analytes in 3 grams of SMM sample (ppb) Volatile Compounds Day 0 Day 3 Day 6 Day 10 Day 20 Day 30 Day 40 Day 50 Sulfur dimethyl disulfide 8 8 21 9 49 91 65 46 dimethyl sulfide 3390 3129 2261 2495 1028 470 14 19 dimethyl trisulfide 1 1 4 1 2 5 2 1 methyl mercaptan 1 1 3 1 4 5 2 2 1-propanethiol 1 2 5 2 5 4 0 0 2-isobutylthiazole 0 0 0 0 0 0 0 0 Acid hexanoic acid 0 0 0 0 0 0 0 0 hexyl acetate 0 0 0 -1 -1 -1 -1 -1 butanoic acid 0 0 0 1 1 1 0 1 acetic acid -1 -2 0 -3 0 5 5 4 Other methanol 79 77 80 65 81 79 86 88 ethanol 294 283 222 230 146 83 54 67 furaneol 0 0 0 0 0 0 0 0 furfural 0 0 2 0 4 6 5 3 hexanal 1 1 0 0 0 1 1 1 phenylacetaldehyde 0 0 0 0 0 0 0 0 (E)-2-hexenal 0 0 0 0 0 0 0 0 (E)-2-octenal 0 0 0 0 0 0 0 0 (E)-2-pentenal 0 1 0 12 0 41 0 0 acetaldehyde 25 28 34 26 42 52 74 71 acetone 161 228 408 222 583 914 1105 1052 ammonia -3 -6 -5 -8 -4 -3 -4 -6 *Values expressed as the mean of 2 batches by 3 replicates per batch

Table 4.7. Changes in the concentrations (ppb) of selected volatile compounds of the can lining processed with tomatoes during storage at 49OC.

Concentration of analytes in 6 mg of can lining (ppb) processed in tomato Volatile Compounds Day 0 Day 3 Day 6 Day 10 Day 20 Day 30 Day 40 Day 50 Sulfur

dimethyl disulfide 0 1 1 0 0 0 0 0 dimethyl sulfide 62 165 157 166 193 145 164 103 dimethyl trisulfide 3 5 5 4 5 4 5 3 methyl mercaptan 0 1 1 1 1 1 1 0 1-propanethiol 1 2 1 1 1 1 1 0 2-isobutylthiazole 0 1 1 1 1 0 1 0 Acid hexanoic acid 0 0 0 -1 1 -1 -1 -1 hexyl acetate 0 1 0 0 0 -3 -3 -3 butanoic acid -1 1 1 1 1 1 1 0 acetic acid -3 0 -3 -3 -3 -1 0 -3 Other methanol 44 171 370 337 377 115 146 83 ethanol 78 159 251 260 279 82 94 61 furaneol 0 0 1 0 1 0 0 0 furfural 1 2 2 3 4 5 6 6 hexanal 2 4 4 4 4 5 4 3 phenylacetaldehyde 1 1 0 0 1 1 1 0 (E)-2-hexenal 1 2 1 1 1 1 1 0 (E)-2-octenal 0 0 0 0 0 0 0 0 (E)-2-pentenal 1 1 1 1 2 1 1 1 acetaldehyde 11 27 20 16 22 13 17 18 acetone 6 22 23 13 20 7 12 7 ammonia 16 42 27 18 31 -2 -2 -18 *Values expressed as the mean of 2 batches by 2 replicates per batch

Table 4.8. Concentrations (ppb) of selected volatile compounds in the can lining processed with SMM treatment group during storage at 49OC.

Concentration of analytes in 6 mg of can lining processed with SMM (ppb) Volatile Compounds Day 0 Day 3 Day 6 Day 10 Day 20 Day 30 Day 40 Day 50 Sulfur dimethyl disulfide 1 0 4 1 14 19 22 12 dimethyl sulfide 156 95 92 89 47 16 1 1 dimethyl trisulfide 0 -2 -1 -2 -1 -1 -1 -1 methyl mercaptan 1 0 2 1 2 2 2 2 1-propanethiol -4 -7 -7 -9 -9 -9 -9 -9 2-isobutylthiazole 0 -1 0 0 0 -1 -1 0 Acid hexanoic acid 3 -2 -2 -3 -2 -2 -2 -2 hexyl acetate -2 -3 -2 -4 -3 -3 -3 -4 butanoic acid -3 -3 -3 -4 -3 -3 -3 -3 acetic acid -7 -10 -9 -12 -10 -10 -9 -10 Other methanol 10 0 34 27 37 43 34 43 ethanol 302 300 462 365 613 672 749 829 furaneol -1 -2 -1 -1 -1 -1 -1 -1 furfural 0 0 0 0 2 2 3 1 hexanal -1 -2 -1 -2 -2 -2 -2 -2 phenylacetaldehyde -2 -4 -4 -5 -5 -4 -4 -4 (E)-2-hexenal 0 -1 0 -1 -1 -1 -1 -1 (E)-2-octenal 0 0 0 -1 -1 0 0 -1 (E)-2-pentenal 0 69 0 -1 0 0 0 0 acetaldehyde 17 6 12 3 10 8 14 13 acetone -4 -3 -4 -9 -3 0 5 1 ammonia -44 -51 -47 -59 -51 -52 -47 -48 *Values expressed as the mean of 2 batches by 2 replicates per batch

Table 4.9 shows a post hoc LSD test that was performed on the processed and unprocessed tomato and SMM groups on day 0 and day 50 of the accelerated shelf life study. At day 0, the concentrations of the DMS in the tomato and the SMM treatment groups were not significantly different (p-value>0.05). However, both concentrations

92 were significantly higher than that of the unprocessed tomato group at the same time period (p-value<0.05). After 50 days of storage at 49oC, the DMS concentration in the

SMM treatment group significantly declined from a mean of 3390 to 19 ppb, while the level in the tomato treatment group declined slightly (3836 to 3076 ppb).

Table 4.9. Post hoc LSD multiple comparison of DMS concentration between the tomato and SMM treatment groups at day 0 and 50

Sample Mean [DMS] (ppb) Std. Error 95% Confidence Interval Lower Bound Upper Bound Day 0 Unprocessed Tomato 258.53A 220.45 -211.35 728.42 Processed Tomato 3836.14B 220.45 3366.26 4306.03 Processed SMM 3390.20B 220.45 2920.32 3860.09 Day 50 Processed Tomato 3076.12A 126.519 2794.216 3358.021 Processed SMM 19.28B 126.519 -262.624 301.181 *Letter connected are significant at α=0.05

Table 4.10 shows a post hoc LSD test that was performed on the can lining of the processed and unprocessed tomato and SMM groups on day 0, 10, and 50 of the accelerated shelf life study. At day 0, the concentration of the DMS in the can lining was significantly different among the three groups (p-value<0.05). The lining of unprocessed can had no detectable levels of DMS, while 62 and 156 ppb of DMS were detected in the tomato treatment group and the SMM treatment group, respectively. Table 4.10 also shows that the DMS concentrations in the linings of the tomato and the SMM treatment groups were significantly different (p-value<0.05) during the storage periods. At day 10, the concentration of the DMS in the lining increased to 166 ppb for the tomato treatment 93 group, while it decreased to 89 ppb for the SMM treatment group. At day 50, the DMS concentration in the lining decreased for both the tomato treatment group (103 ppb) and the SMM treatment group (0.60 ppb). These results suggested that the rate at which the

DMS interacted with the lining was slower for the tomato treatment group when compared with the SMM treatment group.

Table 4.10. Post hoc LSD multiple comparison of DMS concentration in the can lining between the tomato and SMM treatment groups at day 0, 10 and 50 of storage at 49oC.

Sample Mean [DMS] (ppb) Std. Error 95% Confidence Interval Lower Bound Upper Bound Day 0 Unprocessed 0.00A 9.60 -21.73 21.73 Processed in Tomato 62.02B 9.60 40.30 83.75 Processed in SMM 155.84C 9.60 134.11 177.56 Day 10 Processed in Tomato 165.94A 21.58 113.13 218.75 Processed in SMM 88.98B 21.58 36.18 141.79 Day 50 Processed in Tomato 103.29A 5.24 90.47 116.12 Processed in SMM 0.60B 5.24 -12.22 13.43 *Letter connected are significant at α=0.05

In summary, retort processing of tomatoes produces many volatile compounds that have the potential to interact with the polymeric lining of the metal can. Among the targeted volatile compounds in this study, the DMS was shown to have the highest concentration for significant interaction with the can lining. The formation of the DMS was due to the thermal degradation of methyl methionine which is naturally found in the

94 fresh tomato plant (Mudd and Datko 1990). The retort process provides sufficient heat to drive the reaction that converts methyl methionine into the DMS and the subsequent interaction of DMS with the polymeric matrix of the can lining. This interaction has the potential to form breaches in the lining which can allow sulfur and other acidic and potential corrosive compounds (penetrants) to diffuse through the polymer to the base metal. It is known that certain penetrants have the ability to plasticize the polymer, increase its segmental mobility and increase the rate of diffusion of the substance through the polymer. Comyn (2012) confirmed this theory when he reported that increase sorption of a penetrant will decrease the segmental mobility of the polymer but increase the polymeric interactions (Comyn 2012). This interaction was observed by Kontoninas et al (2006) where canned fish resulted in sulfur induced black spots on the enamel surface. These black spots were generated from ferrous sulfide and brown spots can be formed by stannous sulfide (Kontominas et al. 2006). Other studies have found that sulfur compounds in fruits and vegetables can contribute to metal corrosion where the sulfur compounds are involved in the initial stages of corrosion (Helwig and Biber 1990;

Charbonneau 1997). The SMM treatment group in this present study showed an initial increase in the DMS concentration (in both the tomato content and the lining of metal can), then the concentration decreased over the 50-day storage period. This result might be closely related with previous observations on corrosions. These observations showed that as the can corroded, the sulfur compounds interacted with the tin and iron and caused a decrease in the observed volatility due to the increased molecular weight. Nagu et al.

(2018) reported that the sulfur compounds at high temperatures in a wet environment will

95 oxidize metal such as iron and reduce the sulfur to form ferrous sulfide in the following reaction scheme (Nagu et al. 2018):

퐹푒2+ + 푆2− → 퐹푒푆

4.2 IC-MS Analysis

Figure 4.2 shows the concentration of nitrate in tomato processed in metal cans and glass jars (control) that were analyzed using IC-MS during a 50-day storage period.

There were no significant differences between any of the treatment groups or time points

(p-value>0.05). The nitrate concentration was expected to decrease over the shelf life study due to its reduction in the corrosion process. Nitrates have been shown to cause corrosion and become reduced to nitrite and ammonia (Albu-Yaron and Feigin 1992;

Palmieri et al. 2004).

4푆푛 → 4푆푛++ + 8푒−

− − + − Slow: 푁푂3 + 2푒 + 2퐻 → 푁푂2 + 퐻2푂

− − + + Fast: 푁푂2 + 6푒 + 8퐻 → 푁퐻4 + 2퐻2푂

These researchers reported that corrosion will cause a decrease in nitrate levels in the canned food products due to nitrate reduction. However, this was not observed in this present study, as shown by the IC-MS data, in which no decline was seen in the nitrate concentration during the storage period. This may have occurred if the analytical method used to quantify the nitrate was not sensitive enough to detect small changes in the nitrate

96 concentration. Also, the storage period may not have been long enough to cause a greater decrease in the nitrate concentration.

Figure 4.2. Nitrate concentration of tomato processed in a glass jar or metal can during the storage at 49oC.

4.3 ICP-MS Analysis

Table 4.11 shows the concentrations of iron and tin in the tomato, SMM, and

SMM+Nitrate treatment groups that were packaged in glass and metal containers. The iron and tin concentrations remained constant during the storage time in the control tomato glass jar group. These results show the natural levels of iron and tin found in the tomato itself because there was no expectation of migration of iron and tin from the glass packaging. Compared to the control group, the iron and tin concentrations in the 97 packaged tomatoes in the metal containers were higher. The iron and tin concentrations both increased over time from day 0 to day 40 of the shelf life study, indicating that these elements originated from the metal packaging material.

To understand the corrosion process that took place in this study, an observation of Table 4.11 shows the concentrations of iron and tin in the SMM treatment group compared with the SMM+Nitrate treatment group. The comparison of the two treatment groups with each other and against the control, contributed to an understanding of the impact of the SMM, in the absence of nitrate, and on the can lining followed by the addition of the nitrate. Table 4.11 shows that the iron content increased from day 0 to day 20 in the SMM group. After day 20, the iron content had a slight change, which indicated that the rate of iron migration may have slowed. When the nitrate was added to

SMM mixture, the iron concentration continuously increased after day 20. These results seem to indicate that the nitrate increased the rate of corrosion and this in turn increased the rate of iron and tin migration from the cans to the packaged tomato product.

No concentration change in the tin was seen during the storage period in either the

SMM or the SMM+Nitrate treatment groups. These results seem to indicate that SMM and nitrate did not affect the tin-plate layer of the metal can but instead affected the steel layer. However, the tomato treatment group showed an increase in the concentration of tin during the storage period. Because the tomato group showed tin migration and the

SMM and SMM+Nitrate groups did not, there might be another contributing factor that led to the tin corrosion in the canned tomatoes.

Table 4.11. Concentration of iron and tin of the various treatment groups over time

Average metal concentration (ppb) Sample [56Fe] ±SD [120Sn] ±SD Control (Glass Jar), Day 0 74.89±8.88 10.05±1.61 Control (Glass Jar), Day 20 72.58±4.59 9.70±1.69 Control (Glass Jar), Day 40 74.55±5.54 11.58±1.95 Tomato, Day 0 141.88±18.81 22.19±1.34 Tomato, Day 10 170.41±24.43 63.03±10.77 Tomato, Day 30 225.23±14.38 93.10±14.77 Tomato, Day 40 318.88±60.69 163.98±27.02 Tomato, Day 50 279.80±91.37 135.69±48.87 Distilled Water, Day 0 1.37±0.25 1.19±0.02 SMM, Day 0 2.31±1.14 1.50±0.15 SMM, Day 20 204.14±46.43 1.63±0.41 SMM, Day 40 215.38±23.52 1.35±0.02 SMM + Nitrate, Day 0 2.06±0.69 1.48±0.03 SMM + Nitrate, Day 20 201.86±27.27 1.39±0.02 SMM + Nitrate, Day 40 451.58±97.91 1.40±0.03

Table 4.12 shows the multiple comparisons between tin concentrations of the treatment groups on day 0 and day 40. The tin concentrations on day 0 and day 40 during the storage of the cans were significantly different (p-value<0.05) between the tomato treatment group and both the SMM and SMM+Nitrate groups. Whereas, no differences in tin concentrations were found between the SMM and SMM+Nitrate treatment groups on day 0 and day 40 (p-value>0.05). This phenomenon can be best explained by the mechanisms of pitting corrosion which occurs in areas of imperfection in the tin-plated steel layer. This could also occur if there is a breach in the polymeric lining and the exposed tin-plated layer experiences localized corrosion that exposes the steel layer.

During pitting corrosion, the iron is more anodic than tin. Therefore, the tin does not corrode first, and the steel will have pitting corrosion in exposed areas that are tin-free.

This phenomenon often occurs with sulfur compounds which will result in a loss of cathodic protection from tin due to tin sulfide interactions. This reduces the rate of tin dissolution and provides no electrochemical protection to the steel from the tin-plate coating (Robertson 2012). Pits were observed in the cans of this present study and have been reported in other studies with canned tomato products (Albu-Yaron and Feigin

1992; Dey and Agrawal 2018).

Table 4.12. Post hoc LSD multiple comparison of tin concentration between treatment groups at day 0 and 40

Sample Mean [120Sn] (ppb) Std. Error 95% Confidence Interval Lower Bound Upper Bound Day 0 Tomato 9.107A 1.082 6.749 11.464 SMM 0.316B 1.082 -2.042 2.673 SMM + Nitrate 0.295B 1.082 -2.063 2.652 Day 40 Tomato 152.399A 3.691 144.357 160.441 SMM 0.161B 3.691 -7.881 8.202 SMM + Nitrate 0.209B 3.691 -7.833 8.251 *Letter connected are significant at α=0.05

To determine the effect of each treatment on the corrosion in the metal packaging,

Table 4.13 compares the rate of iron migration for each treatment group during the 40- day period using a linear regression model. The linear fit was significant for each treatment group (p-value <0.05), indicating a linear upward trend in the iron migration over the 50-day storage period (shown in Figure 4.3). When compared to the

SMM+Nitrate treatment group (11.238), the tomato and the SMM treatment groups

100

(4.495 and 5.327, respectively), had a lower rate of iron migration. Because the same concentrations of SMM and nitrate were also found in the tomato itself, one would expect that the rate of iron and migration would have been the same in the SMM and

SMM+Nitrate treatment groups. However, Table 4.13 shows that the concentrations of iron at day 40 in the SMM+Nitrate treatment group (451.58 ppb) was higher than that of the tomato treatment group (318.88 ppb). Therefore, the availability of the nitrates and the SMM to interact with the surface of the metal, which lead to an increase in the rate of corrosion and chemical migration, was greater than that of the tomato group.

Table 4.13. Linear regression summary of the various treatment groups on the iron concentration over time (ppb/day)

K R 95% CI for K 95% CI for K p- Treatment (Rate) Square (Lower) (Upper) value SMM 5.327 0.732 3.619 7.035 0.000 SMM+Nitrate 11.238 0.918 9.458 13.018 0.000 Tomato 4.495 0.780 3.441 5.549 0.000 *Level of significance of fit to linear regression model determined by α= 0.05

101

Figure 4.3. Concentration of iron in each treatment group during the storage period

Table 4.14 shows the multiple comparisons between the iron concentrations of the treatment groups on day 0 and day 40. The iron concentrations on day 0 was significantly higher (p-value<0.05) in the tomato treatment group than that of the SMM and the SMM+Nitrate treatment groups. However, no significant differences were found on day 0 between SMM and SMM+Nitrate treatment groups (p-value>0.05). These results indicate that the iron layer of the metal package was immediately affected by the ingredients in the tomato. After 40 days of storage, significance was found among those three groups (p-value <0.05). The samples from the SMM+Nitrate treatment group had

102 the highest average concentration of iron (450.218 ppb), while those of the SMM group had the lowest average concentration (214.015 ppb).

Table 4.14. Post hoc LSD multiple comparison of iron concentration between the treatment groups at day 0 and 40

Sample Mean [56Fe] (ppb) Std. Error 95% Confidence Interval Lower Bound Upper Bound Day 0 Tomato 46.994A 7.778 30.046 63.941 SMM 0.941B 7.778 -16.007 17.888 SMM + Nitrate 0.694B 7.778 -16.253 17.642 Day 40 Tomato 244.326A 8.196 226.468 262.184 SMM 214.015B 8.196 196.157 231.873 SMM + Nitrate 450.218C 8.196 432.360 468.076 *Letter connected are significant at α=0.05

Overall, the results from the ICP-MS study showed that the effect of the SMM on the corrosion in the BPA-free coated tin-plate metal cans was significant. This was concluded because the ICP-MS results showed that the SMM was associated with an increase in the level of iron migration from the walls of the cans to the packaged tomato products. The level of iron migration was even higher in the presence of the nitrates. At the same time, neither the SMM alone or the combination of SMM with nitrate had a significant impact on the migration of tin. The corrosion occurring in the canned tomatoes affected both the tin-plated and the iron layers of the metal package. Thus, both the SMM and nitrate may have synergistically contributed to the corrosion mechanism of iron but not tin. Therefore, other compounds in the tomatoes may have contributed to the

103 migration of tin from the package. Corrosion resulting in the migration of iron and tin into the contents of the cans was observed in other shelf life studies with tomato products

(Perring and Basic-Dvorzak 2002; Ninčević Grassino et al. 2009). Furthermore, the literature supports findings of nitrate at low concentrations in tomatoes and that they resulted in metal migration due to corrosion. In one study, nitrate as low as 50 ppm resulted in the removal of 50% of the tin in canned acidified solutions with nitrate

(Farrow et al. 1970). Another study showed that canned tomatoes spiked with nitrate experienced an increase in corrosion and a resultant decrease in the nitrate content in the packaged solution. This study concluded that nitrate in tomatoes works synergistically with oxygen, organic acid, and chloride to cause delamination and detinning in lacquered cans (Albu-Yaron and Feigin 1992). While these studies showed that corrosion occurs in cans containing nitrates, acids, and other ingredients, these studies didn’t report the effect of the individual ingredients on corrosion at concentrations naturally in the tomatoes.

4.4 SEM-EDS Analysis

The corroded and non-corroded cans were analyzed using SEM-EDS to determine the nature of the compounds in the corroded area. For this test, a metal can that has not been processed with tomato and without signs of corrosion analyzed. A different can that had been processed with tomato and was incubated for 50 days, at which time visible signs of corrosion were observed, was also tested. The corrosion (on day 50) and lack of corrosion (unprocessed sample) observations were confirmed by the SEM images shown in Figure 4.4, Figure 4.5, and Figure 4.6. Figure 4.4 shows an image of the internal wall

104 of a can that was not processed with tomato. The image shows no breaches in the polymeric lining, whereas Figure 4.5 shows breaches in the lining after the 50-day storage period. Figure 4.6 shows an area of pitting corrosion where the metal layer is exposed, and it also shows a corroded area that went all the way through the wall of the sample to the outside of the can.

Figure 4.4. SEM Image of internal wall of the unprocessed can lining

105

Figure 4.5. SEM Image of the internal wall of the day 50 tomato treatment group showing an area of breaches

106

Pitting corrosion that Breached polymeric perforated the wall of Exposed metal layer lining the can

Figure 4.6. SEM Image of the internal wall of the day 40 Nitrate treatment group showing an area of rupture

107

These results are further confirmed by the EDS spectra. Figure 4.7 shows the

spectrum of the unprocessed can and it shows a lower concentration of the iron

compounds in the lining when compared with Figure 4.8, the 50-day aged can with

visible signs of corrosion. It can also be seen that a higher concentration of carbon and a

higher ratio of carbon to oxygen were detected in the unprocessed can in Figure 4.7 when

compared to the corroded can in Figure 4.8. These results are understandable because the

can lining was made of an epoxy-based thermoset polymer which contained both carbon

and oxygen. There was a larger concentration of carbon in the unprocessed non-corroded

can than the corroded day 50 can. This seems to indicate that there were interactions

between the polymeric hydrocarbon lining and the chemicals of the tomato product.

Figure 4.7. EDS Spectrum of the internal wall of the unprocessed can

108

Figure 4.8. EDS spectrum of the internal wall of the day 50 tomato treatment group

The EDS spectrum in Figure 4.8 also shows higher relative intensities of iron and tin compared to the intensities seen in the unprocessed sample. The relative intensities of oxygen and sulfur increased in comparison with the rest of the elements present. The sulfur present likely came from the dimethyl sulfide and other sulfur containing compounds found in the tomato. This was discussed before when the SIFT-MS results were presented. After the sulfur interacted with the lining it formed breaches which allowed avenues for potential corrosive compounds to interact with the base metal. This proposed mechanism is illustrated in Figure 4.9. Additionally, because the cans were not sealed under vacuum, residual oxygen was present in the cans during the storage period.

In summary, the relative intensity of the carbon compared to the iron and tin was shown to be larger in the non-corroded can when compared with the corroded can. The sulfur shown in the non-corroded can likely was produced from the tomato during retort processing. This sulfur may have formed breaches that allowed for corrosive compounds

109 to interact with the base metal and to initiate the corrosion. A study by Dey and Agrawal

(2018) found that corrosion in canned tomato juice resulted in initial discontinuities in the enamel which allows electrolytes to corrode the metal layer first and then the metal subsequently punctured new areas of the enamel. This then created additional areas for increased levels of corrosion (Dey and Agrawal 2018). Dey and Agrawal (2018) didn’t look at the compounds in tomatoes that could have interacted with the enamel which contributed to the discontinuities.

Figure 4.9. Proposed corrosion scheme of tomatoes in BPA-free epoxy-lined metal cans

110

4.5 X-Ray Diffraction Analysis (XRD)

In contrast with Figure 4.10, Figure 4.9 shows the X-ray diffraction patterns of an unprocessed and processed samples with the lining bonded to the can. The retort processed sample was filled with tomato and stored at 49°C for 50 days using XRD. The diffractograms show crystalline patterns for the metal and polymeric structures. These

X-ray diffraction patterns are consistent with findings from a corrosion study of a tinplated material exposed to a salt solution (Ma et al. 2018). Ma et al. (2018) concluded that defects in the tin-plate layer resulted in exposure of the steel layer to the electrolyte solution, resulting in corrosion of the metal surface. The peaks shown on the diffractograms in Figure 4.9 increased in intensity after the can was retort processed when compared to unprocessed can. The resultant increase in intensity of the peaks after processing and aging of the cans, indicate that there was low temperature recrystallization of the tin on the sample. This low temperature recrystallization of tin was reported in the literature by Paine et al. (1999).

Figure 4.10 shows the X-ray diffraction patterns of the polymeric linings before and after processing. These diffractograms are of the linings after they were removed from the metal surfaces. The unprocessed lining shows 4 strong peaks at 2θ = 18.2, 44.4,

64.7, and 77.8. Whereas the processed lining shows a singular peak at 2θ = 18.2. Paul and Sindhu (2014) found that peaks at 2θ = 45.3, 65.7 and 78.8 are characteristic to aluminum and 17.8 is characteristic to an epoxy resin. Similar diffractograms were reported by Bello et al. (2018) where aluminum particles were incorporated into an epoxy composite. Various nanoparticles, like aluminum, are added to epoxy resins in order to

111 fill small voids and increase the tortuosity of the lining and inhibit corrosive compounds which can diffuse through resin to the metal surface of the can (Shi et al. 2009; Alam et al. 2017; Yang et al. 2018). Figure 4.10 shows that after processing, the crystalline aluminum is no longer present in the polymeric matrix. These changes could have been due to the loss and/ or change of plasticizers in the epoxy polymer lining. Plasticizers interact with polymers to increase their flexibility by increasing the spaces between adjacent polymeric chains (Mouritz 2012). Therefore, the interaction of dimethyl sulfide or other sulfides produced during the retorting process, and which were sorbed into the can lining may have changed the polymer chemistry by impacting the role of the plasticizers. Additionally, the heat of the retorted process was capable of evaporating some of the plasticizers from the polymer (Marcilla and Beltrán 1998). This could have the effect of decreasing the spaces between the polymeric chains. Additionally, it should be noted that the literature reports that epoxy resins are notorious for high water uptake, especially at high temperatures such as in a retort (Zhou and Lucas 1999). This also has potential to change the thermomechanical properties of the polymer, including that of the glass transition temperature and crystallinity due to chain scission and secondary crosslinking (González et al. 2012). In conclusion, the combination or independent action of tomato compounds like dimethyl sulfide and water present in the can may have impacted the epoxy lining by changing the void spaces within the polymeric matrix which increased available pathways for corrosive compounds to contact the metal surface.

112

Figure 4.10. XRD patterns of unprocessed and processed can and lining in tomato

113

Figure 4.11. XRD patterns of unprocessed and processed can lining in tomato

4.6 FT-NIR Analysis

The sample cans tested by FT-NIR were filled with tomatoes, sealed, retorted, and stored for 0, 10, 30, 40, and 50 days of storage at 49°C. The control can was unprocessed. The results are shown in Figure 4.11. The fingerprint region of the epoxy resin is shown from 4000-4500 cm-1 with the C-H stretching of the epoxy ring at 4530 cm-1. After processing the lining, the spectra indicate that the intensity of the epoxy functional group increased. Musto et al. (2000) observed at the 4524 cm-1 region (which is attributed to the oxirane ring of the epoxy resin) decreased as the resin was cured due to breaking of the ring which led to the formation of the polymeric network (Musto et al. 114

2000). Thus, the results indicate that the oxirane ring was being reformed. Additionally, the scans show that the region associated with O-H asymmetric stretching and bending of the hydroxyl groups (5239 cm-1) increased in intensity after the unprocessed can was retort processed with the tomatoes. The resulting increase may be due to water uptake.

Water uptake in epoxy resins is known to lead to deterioration of the lining, resulting in changes to the thermomechanical properties, adhesion, and swelling of the polymeric material (González et al. 2012). These water molecules can react with the material through Type I and Type II bonding. During Type I bonding, the water diffuses into the polymer and binds with the chains via Van der Waals forces and hydrogen bonding.

Type II bonding occurs when the polymer is in contact with water for prolonged periods of time and at higher temperatures. While Type I is known to break bonds between adjacent polymer chains, Type II promotes secondary crosslinking via water-resin interactions (Zhou and Lucas 1999). Hydration of the epoxy-based polymeric chain has potential to cause scission or breakage of the chains due to the hydrogen bonding of the water with the functional groups of the chain. This interchain hydrogen bonding can result in plasticizing the polymer as was previously discussed (Zhou and Lucas 1999;

González et al. 2012). Thus, the increase in intensity of the O-H at the 5239 cm-1 was due to an increased uptake of bound water by the lining. This has potential to cause swelling of the lining, delamination and a loss of corrosion protection abilities (González et al.

2012). Chain scission could result in the reformation of the epoxy ring which attribute to the increased C-H stretching associated with the oxirane ring at the 4530 cm-1 region

(Xiao and Shanahan 1998).

115

Figure 4.12. FTIR spectra of unprocessed and processed in tomato epoxy-based lining in the near ranges

116

Chapter 5: Conclusion and Future Work

This study showed that dimethyl sulfide was one of the highest produced volatile compounds that formed during the retort process of the canned tomatoes. Its formation was due to the thermal degradation of methyl methionine. SIFT-MS analysis demonstrated that dimethyl sulfide had significant retention in the lining after the retorting process. This was also shown by the SEM-EDS analysis where the sulfur compounds were sorbed by the lining. At the same time, the FT-NIR revealed that water interacted with the epoxy lining. The synergistic effects of water bonding and sulfur interaction with the polymer led to the loss of corrosion protection of the lining. XRD results showed the loss of aluminum nanoparticles in the lining. The tomato compounds interactions with the polymer resulted in the formation of breaches which created avenues for corrosive compounds to diffuse through and react with the base metal to initiate corrosion. The corrosion reaction caused tin and iron compounds to migrate from the metal surface of the package into the tomato product. Sorption of sulfur compounds and water binding in the polymeric lining occurred substantially during the retort process.

Therefore, to optimize the effectiveness of the lining to prevent corrosion, it must become inert to these interactions at the retort processing conditions of tomato products.

This thesis was successful in determining an effective way at investigating the contributors of corrosion in canned food products using a combination of instruments throughout this study. In particular, this study demonstrates an investigative approach to determine how compounds within the food product can interact and affect the can lining. 117

However, future studies need to consider how the buffering capacity of fruits and vegetables may change during storage which will result in available hydrogen-ions to interact with the metal surface.

As a generalized summary of this study, the actions of water uptake and the sorption of DMS into the polymeric matrix led to its plasticization which contributed to the reformation of the oxirane ring and secondary water-resin crosslinking. These actions then led to the delamination of the lining from the metal surface and an increase in voids in the lining which functioned as potential pathways for corrosive compounds to diffuse from the tomato contents to the metal surface. These mechanisms led to the loss of corrosion protection of the polymeric lining and therefore the migration of iron and tin from the metal surface to the packaged contents.

Additionally, future studies should focus on changes to the polymer characteristics before and after retorting by monitoring: (1) the permeability of the polymer to water vapors and gases; (2) the mechanical properties of the polymer; (3) the thermal stability of the polymer; and (4) the glass transition temperatures of the polymer before and after processing.

118

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