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Applying and curing coatings
Internal coatings for metal packaging are typically applied by either roller coating or spraying before baking. In three-piece cans, coatings are usually applied before deformation of the metal to form the container or cap, in this case the coating has to withstand this severe mechanical deformations. In contrary, two-piece cans are coated after forming the object, but nearly always further deformation is required before the final object is obtained, e.g. necking the cans. Coatings can be applied more than once, after the first coating is dried. The curing conditions are determined by the industry depending on the materials used. Generally, the curing time ranges from few seconds to 12 min, while the temperature is in the range 195-270°C. The typical weight of coating per area and per can is, respectively, in the range of 5–15 g/m2 and 110–180 mg/330 mL can [10].
Commercially available coatings
Internal lacquer must be inert, must provide a good barrier, and must also have good mechanical resistance, as well as thermal resistance especially that almost all canned products undergo thermal processing. These coatings must be approved for food contact, i.e. only substances that had successful migration or extraction tests and that do not impart any flavor to the contents, can be used. Therefore, there are limited of different chemical functionalities available for food contact coating, resulting in a limited number of different types of resins that can be used for coatings for metal packaging[12]. Table 1.1 shows the properties of most used internal can coatings.
Coatings for different foodstuffs
Corrosion due to extremely acidic soft drinks, being based upon phosphoric acid, represents a serious problem; therefore, it is vital that internal coatings provide complete protection. Since soft drinks are more aggressive than beer, higher film weights must be used. On the other hand, beers are susceptible to flavor contamination by metallic traces present particularly by the slightest trace of contamination with iron or tin which requires higher film weights in case of steel beverage cans compared to aluminum cans [10]. DWI cans are sprayed with coating after fabrication, usually with two coats of lacquers. These cans are mostly coated with a water-based epoxy acrylic system, normally with an amino resin for cross-linking [10].
Coatings for food cans
The choice of lacquer mainly depends on the nature of the food and the can to be used. The most demanding foods are meat, fish, high-sulfur vegetables (peas and sweet corn), highly acidic foods, and highly colored foods (fruits). Accordingly, the lacquers must resist the contents of the can during processing and any produced by-products in food, such as hydrogen sulfide. The bulk of the food can coatings are based upon epoxy resins, primarily epoxy phenolics (gold lacquer) and to a lesser extent epoxy anhydride (white lacquer), with PVC-based organosols being the next most-used category [10]. High-sulfur foods usually cause sulfur staining. Solid packs including ham, luncheon meat or solid fish such as tuna, which are not in a covering liquid, requires a lacquer that presents a barrier to the sulfur products in the pack and will therefore prevent sulfur blackening of the tinplate. In this case aluminum pigmentation on the lacquer is used to obscure the brown-violet tin-sulfide formed [10]. Titanium dioxide also provides a clean white appearance of the coating and masking sulfide stains because of its good hiding power [2].
Migration of organic contaminants
Many migrants from all different can coatings belong to the group of non-intentionally added substances (NIAS), which may be structurally and toxicologically characterized or even completely unknown [21]. In case of epoxy-based lacquers, which are epoxy phenolic resins, the lacquer is synthesized from bisphenol A (BPA) and epichlorohydrin, forming bisphenol A-diglycidyl ether (BADGE) epoxy resins [22]. Numerous studies from all over the world demonstrated that the occurrence of BPA in epoxy can coatings and its migration into food and beverages are common (e.g. [19], [23]–[26]). Migration of BPA mainly occurred during can processing and sterilization [25].
Moreover, BADGE, which is the main monomer of epoxy coatings, has also commonly been added to organosol coatings as scavenger for hydrochloric acid which is formed as unwanted by-product after exposure to heat [27]–[29]. Depending on the intended function of BADGE and the production and storage conditions of the can, different reaction products are formed [30]. The epoxy groups of BADGE can hydrolyze in the presence of water to BADGE·H2O and BADGE·2H2O. When BADGE is used as scavenger for hydrochloric acid or in the presence of salty food, BADGE·HCl, BADGE·HCl·H2O and BADGE·2HCl are formed. Furthermore, a cyclic product (cyclo-diBPA) is a common by-product from BPA and BADGE during the production of epoxy resins [31], [32]. In general, the total migration of BADGE and its derivatives was higher from organosols than from epoxy coatings because of its different functions in the two materials [2]. In 2010, more complex reaction products of BADGE with food ingredients such as sugars and peptides were identified [33]. Similarly, NOGE has commonly been used as scavenger for hydrochloric acid in organosols; in the EU, it has replaced BADGE for certain years until regulatory action banned the use of NOGE in can coatings [28]. NOGE is a complex mixture of epoxidized molecules based on the three isomers of bisphenol F (p,p-BPF, o,p-BPF, o,o-BPF) and its 3- to 8-ring derivatives [28], [34], it typically contains 30-40% BFDGE [2]. Other bisphenol analogues, including bisphenol S (BPS), bisphenol AF (BPAF), and bisphenol B (BPB), have been gradually developed as substitutes for BPA in the production of epoxy resins [35]. The presence of this group of compounds has received attention lately owing to its suspected toxicity. BPA has been identified as an endocrine disruptor due to its potential to elicit developmental and reproductive toxicity [36], [37]. Acute toxicity, genotoxicity and estrogenic activity of BPS, BPB and BPF have been reported [38]–[40]. One study showed that exposure to BPAF considerably reduced testosterone levels in adult male rats [41]. A few other studies have shown that BPF and BPS are more resistant to degradation in the environment than is BPA [42], [43]. In 2004, the toxicity of BADGE was reviewed and it was concluded that it neither affects reproduction and developmental endpoints nor acts as endocrine toxicant [44]. In the same year, EFSA concluded in a scientific opinion that BADGE and its derivatives do not raise concern for genotoxicity and carcinogenicity in vivo [45]. However, more recent studies showed effects of BADGE e.g. on the testes of rats [46], on adipocytes in vitro [47], and on the development of amphibians [48]. The lack of toxicity data for NOGE and BFDGE led to the prohibition of their use and presence in food contact materials (FCMs) in Europe [2]. Finally, the variety of monomers used in coatings makes the prediction, analysis and quantification of oligomers very challenging and analytical standards are generally not available yet [49].
Migration of metal trace elements
Metals are also common migrants from non-coated cans; yet these elements can still migrate in the presence of coatings. In fact, the release of metals occurs due to corrosion of the metal material. As this mechanism differs from diffusion the correct term concerning metal materials is release instead of diffusion [50]. In the case of coated tinplate cans, corrosion can occur in two possible occasions:
(1) Corrosion under the varnish film: the tin undergoes corrosion. A black area of corrosion may develop under the varnish. Although, in this case, the life of the cans is not threatened, there is still a high risk of metallic contamination of the product.
(2) Perforation of the can: the iron undergoes corrosion. The preserved food can be enriched in iron and the perforation of the cans is the ultimate evolution. Iron has an adverse effect on the taste and color of certain food products [51].
As a result, tin (Sn) and iron (Fe) are released in the food products. Other metals can also be intentionally or non-intentionally present in the alloy; including: nickel (Ni) that enhances the corrosion resistance, zinc (Zn) that is used in galvanized iron to protect iron from rusting due to its stronger reducing ability, copper (Cu) which hardens the cans, chromium (Cr) (usually present in the Cr(III) form) is used in TFS lids that has better corrosion resistance than Sn, and finally cadmium (Cd) and lead (Pb) that are expected to have higher potential toxicity than other elements and are present as contaminants rather than basic constituents [50]. Although metals generally play important roles in our life functioning in wide spectrum, these metals can become toxic when consumed excessively (e.g. very high doses of iron indicates acute damage of gastrointestinal, hepatic, pancreatic and cardiovascular structures[50]). Other metals, such as Cd and Pb, are toxic even at low doses and usually imitate the action of an essential element in the body, interfering with the metabolic processes to cause illnesses [50]. At this point it is important to mention that consumers‘ demand for safer products has enhanced the study of food/packaging interaction, which can lead to the migration of either intentionally or accidentally contaminants from packaging material.
Regulation relative to cans
According to the European Framework Regulation EC 1935/2004 [52] on food contact materials (FCMs), the can coatings generally have to comply with Article 3 in compliance with good manufacturing practice (GMP), so that they do not transfer chemical substances to food in quantities which could harm the consumer or affect the composition of the food or deteriorate the organoleptic characteristics of the food (i.e. affect the smell and the taste). The regulation (EU) No 10/2011 [53] consists of a consolidation of existing Directive 2002/72/EC [54]. These Directives set the migration tests conditions and appropriate food simulants used for specific measures of migrants from plastic materials and articles. As the use of food simulants greatly simplifies the compliance testing, six food simulants are stated in the regulation (EU) No 10/2011 for testing migration from FCMs, which cover the whole range of foods as shown in Table 1.2 [53]. One amendment over the Directive 2002/72/EC [54] is the introduction of new food simulants, for example water was replaced by 10% ethanol to simulate aqueous food. In fact, water is considered now as a food and not as a food simulant. However testing can be performed into water only for plastic materials intended to come into contact with water.
Table of contents :
LIST OF PUBLICATIONS AND COMMUNICATIONS
INTRODUCTION:
CHAPTER 1: BIBLIOGRAPHI REVIE
1. CANNED FOOD MARKET
1.1. Worldwide scale
1.2. Lebanese market
2. MIGRANTS FROM FOOD CANS
2.1. Can constituents and fabrication
2.1.1. Principal materials of cans
2.1.2. Tinplate cans
2.1.3. Electrolytic chromium coated steel (ECCS)
2.1.4. Aluminum cans
2.1.5. Types of cans
2.1.6. Coatings
2.1.7. Main migrants
2.1.8. Regulation relative to cans
2.2. Trace metals
2.2.1. Regulation relative to trace metals in foods
2.2.2. Trace metals in canned food
2.2.3. Summary
2.3. Bisphenol compounds
2.3.1. Regulation relative to bisphenol compounds
2.3.2. Migration of bisphenol compounds in food simulants
2.3.3. Bisphenol compounds in canned foods
2.3.4. Summary
2.4. Global summary
3. ANALYTICAL METHODS FOR DETERMINING MIGRANTS FROM PACKAGING
3.1. Trace metals
3.1.1. Sample treatments
3.1.2. Analytical techniques
3.1.3. Overall method performances
3.2. Bisphenol compounds
3.2.1. Sample treatment
3.2.2. Analysis of bisphenolic compounds
3.2.3. Overall method performance
3.3. Global summary
4. MIGRATION PREDICTIVE MODELING
4.1. Deterministic models based on Ficks’s law
4.2. Other models
4.2.1. Limm and Hollified
4.2.2. Helmroth
4.2.3. Fauconier
4.3. Discussion
5. THESIS OVERVIEW
6. REFERENCES
CHAPTER 2: EFFECT OF STERILIZATION AND STORAGE CONDITIONS ON THE MIGRATION OF BISPHENOL A FROM TINPLATE CANS OF THE LEBANESE MARKET
INTRODUCTION
ARTICLE TITLE PAGE
ABSTRACT
KEYWORDS
1. INTRODUCTION
2. MATERIALS AND METHOD
2.1. Reagents
2.2. Sample collection
2.3. Analytical instruments and conditions
2.3.1. UPLC/Fluorescence
2.3.2. UPLC/MS
2.4. Sample preparation
2.5. Storage conditions
2.6. Procedure used in the experimental design
3. RESULTS AND DISCUSSION
3.1. Quality assurance
3.1.1. Analytical performance
3.1.2. Detection and quantification limits
3.1.3. BPA pre-concentration recovery
3.1.4. Sterilization repeatability
3.2. Peak confirmation by UPLC/MS analysis
3.3. Effect of heat processing
3.4. Effect of storage time and temperature
3.5. Types of cans
3.6. Estimation of long term migration of BPA
4. CONCLUSION
5. REFERENCES
CONCLUSION
CHAPTER 3: BUILDING EMPIRICAL MODELS TO PREDICT THE EFFECT OF STERILIZATION AND STORAGE ON BISPHENOLS MIGRATION FROM METALLIC CAN COATING INTO FOOD SIMULANTS
INTRODUCTION
ARTICLE TITLE PAGE
ABSTRACT
KEYWORDS
1. INTRODUCTION
2. MATERIALS AND METHOD
2.1. Choice of input variables
2.2. Building experimental designs
2.3. Building the models
2.4. Models validation
2.5. Tinplate cans
2.6. Measuring bisphenols migration in food simulants
2.6.1. Standards and reagents
2.6.2. Instrumentation
2.6.3. Sample treatment
3. RESULTS AND DISCUSSION
3.1. Migration of bisphenols in the simulants after can contact
3.2. Migration modeling
3.2.1. Effect of food simulant and storage with non-sterilized cans (Design I)
3.2.2. Effect of process and storage conditions with sterilized cans (Design II)
3.3. Testing models adequacy and validation
3.4. Valuable models application for the industry
4. CONCLUSION
5. REFERENCES
CONCLUSION
SUPPLEMENTARY MATERIAL CHPT.3
CHAPTER 4: PARAMETERS INFLUENCING THE MIGRATION OF TRACE METALS IN UNCOATED FRUIT CANS
INTRODUCTION
ARTICLE TITLE PAGE
GRAPHICAL ABSTRACT
ABSTRACT
KEYWORDS
1. INTRODUCTION
2. MATERIALS AND METHOD
2.1. Reagents
2.2. Sample collection and storage conditions
2.3. Sample treatment
2.4. Instrumental conditions
2.5. Method accuracy
2.6. Statistical analysis
3. RESULTS AND DISCUSSION
3.1. Method performance
3.2. Metal content in canned fruits
3.3. Effect of storage time at room temperature
3.4. Effect of storage temperature
3.5. Effect of can denting
3.6. Effect of leaving opened cans in the fridge
4. CONCLUSION
5. REFERENCES
COMPLEMENTARY ANALYSIS: PRINCIPLE COMPONENT ANALYSIS (PCA)
CONCLUSION
SUPPLEMENTARY MATERIAL CHPT.4
CHAPTER 5: SIMULTANEOUS MIGRATION OF BISPHENOL COMPOUNDS AND TRACE METALS IN CANNED VEGETABLE FOOD .
INTRODUCTION
ARTICLE TITLE PAGE
ABSTRACT
KEYWORDS
1. INTRODUCTION
2. MATERIALS AND METHODS
2.1. Reagents and standards
2.1.1. Analysis of bisphenol compounds
2.1.2. Analysis of metal elements
2.2. Processing and storage conditions of food samples
2.3. Sample preparation
2.3.1. Bisphenol compounds extraction from food
2.3.2. Bisphenol compounds extraction from empty tinplate cans
2.3.3. Dissolution of trace metals
2.4. Analysis of studied contaminants
2.4.1. Analytical instruments and conditions for bisphenol compounds
2.4.2. Analytical instruments and conditions for trace metals
2.5. Method validation
2.6. Statistics
3. RESULTS AND DISCUSSION
3.1. Occurrence of bisphenol compounds and metals in raw and canned food
3.2. Influence of heat treatment
3.3. Migration as a function of storage time and temperature
3.3.1. Migration of BPA and BADGE.2H2O
3.3.2. Migration of iron
3.3.3. Migration of chromium
3.3.4. Migration of lead and cadmium
3.3.5. Migration of nickel, copper and zinc
3.4. Correlation of contaminants levels with food product, can brand and storage conditions
3.5. Effect of can denting
3.6. Effect of heating food directly in the can
4. CONCLUSION
5. REFERENCES
CONCLUSION
SUPPLEMENTARY MATERIAL CHPT.5
CHAPTER 6: GENERAL DISCUSSION
1. INTRODUCTION
2. EFFECT OF HEAT TREATMENT
2.1. Influence of heat treatment on the migration of bisphenol compounds
2.1.1. Contents of bisphenol compounds in food simulants vs. real food
2.1.2. Extent of bisphenols migration after sterilization
2.2. Influence of heat treatment on the migration of trace metals
3. EFFECT OF STORAGE TIME
3.1. Influence of storage time on the migration of bisphenol compounds
3.2. Influence of storage time on the migration of trace metals
4. EFFECT OF FOOD/FOOD SIMULANT
4.1. Contents of bisphenol compounds in canned aqueous, acidic and semi fatty foods/food simulants
4.2. Contents of metal elements in canned aqueous, acidic and semi fatty foods
5. DIFFERENCE BETWEEN PACKAGING AND BRAND
5.1. Contents of targeted contaminants between food packaging
5.1.1. Bisphenol compounds between types of coated cans and glass jars
5.1.2. Release of metal traces from coated and uncoated food cans
5.2. Contents of targeted contaminants between brands of canned food
5.2.1. Difference in the levels of bisphenol compounds between brands
5.2.2. Reduction of bisphenol compounds in can coating
5.2.3. Difference in the levels of trace metals between brands
6. EFFECT OF STORAGE TEMPERATURE
6.1. Influence of storage temperature on the migration of bisphenol compounds
6.2. Influence of storage temperature on the migration of trace metals
7. EFFECT OF CAN DENTING
8. OTHER PARAMETERS
9. WORRYING CONCENTRATIONS
10. VALUABLE ADVICES FOR INDUSTRIES AND CONSUMERS
11. CONCLUSION
12. REFERENCES
