Metal food contact materials (FCMs) cover a very diverse range of products from metal tanks, pipes and components in food manufacturing machinery, through cutlery, bowls and work surfaces (commercial or domestic) to metal packaging for foodstuffs (either food or drink). Only metal foodstuff packaging will be dealt with here. For guidance on the wider application of metal FCMs the Council of Europe (CoE) Guidelines on Metals and Alloys Used as Food Contact Materials (CoE 2001) provides a useful reference. Metal foodstuff packaging itself covers a wide range of packaging types including food and beverage cans and ends, closures, tubes, trays, drums and pails. The focus of this chapter will be on metal cans, ends and closures.

Although metal is the defining component of these packages providing strength and integrity, the additional materials required to make a functional package are more often the primary food contact materials in the package and much of this chapter covers these materials. As for all FCMs, consumer safety is of prime importance and is assured through a combination of regulatory compliance and industry risk assessment. In the USA coated metal packaging is specifically regulated, but there is currently no EU harmonised legislation covering this sector beyond the Framework Regulation (1935/2004) and certain substance specific legislation. Some EU national member state legislation exists and is used in demonstrating compliance with the Framework Regulation, but this provides only limited help.

For this reason harmonised EU legislation that includes metal foodstuff packaging is needed. Extension of the existing harmonised legislation on plastics materials and articles is not appropriate without significant modifications due to the particular nature of metal packaging and the filling and subsequent processing operations specific to this sector. These differences are particularly significant for those food cans that do not have an internal protective coating, and these packages will also be covered in this chapter, as will other areas of special consideration.

11.1.1 Scope of metal packaging for food and beverage

A common factor of metal foodstuff packaging is that it provides long term ambient stable storage with excellent abuse resistance and protection from environmental contamination ensuring food safety and quality retention with extended shelf life. Flexible packaging which is covered elsewhere in this book may also use thin layers of metal either as discrete foil layers or as metallised plastic or paper layers for improved barrier properties, but the main structural components are non-metallic and so are not covered here.

The metals used to manufacture cans, ends and closures are either steel (tin plated or chromium passivated) or aluminium. In most cases they are coated on the food contact surface with a resinous or polymeric protective coating to avoid interaction between the foodstuff and the metal. However, there is a well defined sector of the tinplate food packaging market where no protective organic coating is needed or used.

Cans may either be 'three piece' consisting of a body made up of a welded cylinder with one end seamed in place which is then closed with a second end, or 'two piece' consisting of a body formed from a single piece of metal closed with an end (see Fig. 11.1). Aluminium cans are always 'two piece' whereas steel cans may be either 'three piece' or 'two piece'. The end or ends are seamed onto the can body which involves the forming of a 'double seam' of folded metal (see Fig. 11.2). A hermetic seal (a key element of metal food and beverage packaging) is ensured by the incorporation of a

Three-piece can Two-piece can

Fig. 11.1 Major can technologies.

Three-piece can Two-piece can

Fig. 11.1 Major can technologies.

Fig. 11.2 Can double seam.

very thin layer of rubber based compound buried within the seam. Metal closures always incorporate some form of sealing gasket to ensure that an effective seal is maintained. These various elements of the construction of metal packaging mean that a number of different material types have to be considered in the assessment of the safety and regulatory compliance of the finished package:

• metals, from uncoated packaging and from failure of the internal coating

• internal protective coatings

• can end sealants (buried within the double seam)

• gaskets for metal closures

• residual lubricants, etc., from the package manufacturing process

• adventitious contamination from the manufacturing process.

These last two categories, which are common to most packaging manufacturing processes, could be considered as 'non-intentionally added substances' if they contaminate the foodstuff.

11.1.2 Particular features of metal packaging influencing migration

Metal packaging for food and beverage has particular features that differentiate it from other food packaging materials and which also influence the management of overall food safety and regulatory compliance. These features differ in some aspects between food and beverage packaging.

Food packaging

Foods packaged using metal packaging are almost always ambient stable and have long shelf lives of between one and five years. Other than with dried foods and some intrinsically microbiologically stable foods this is achieved by a heat sterilisation or pasteurisation process of the foodstuff after sealing into the packaging. Long-term microbiological stability of the foodstuff is then assured by eliminating post-process contamination. Maintenance of the integrity of the structure and seal of the package is therefore critical to the safety of the foodstuff. It also plays a vital role in retaining the food quality, nutrition and wholesomeness by excluding oxygen ingress that would lead to product deterioration. This latter feature can also be important for those intrinsically stable foods which do not rely on the seal integrity for microbiological safety. The integrity of the metal packaging must be retained throughout the life of the product. This includes withstanding damage and abuse during distribution, retail and consumer handling. This puts significant demands on the performance of the can end sealants, closure sealing gaskets and internal protective coatings and constrains the potential choice of these materials. The thermal sterilisation process, which may be at temperatures in excess of 130 °C, puts significant demands on the packaging material with regard both to migration and material performance and sets particular challenges to compliance testing which will be covered later in this chapter.

Beverage packaging

Depending on the nature of the beverage, the cans or bottles may either be heat processed post-filling (beer and cider, fruit juices, tea, coffee and milk based drinks) or ambient filled where the product formulation and pH ensure microbiological stability. Although post-filling contamination of the non-heat-processed products is less critical, it remains important in retaining product quality. Exclusion of oxygen and retention of carbonation are also important in ensuring long-term product quality and consumer acceptability.

11.1.3 Internal protective coatings, sealants and gaskets

Internal protective coatings

These coatings, typically 2-20 micrometres (mm) thick, protect the metal package surface from the corrosive properties of the foodstuffs throughout filling, long-term storage, and in some cases, consumer heating. This role requires very high performance, as the coating must survive without any loss of integrity during metal forming operations and high temperature food processing, as well as abuse during distribution and retailing. The coatings need to have very good substrate adhesion combined with flexibility, temperature resistance, food product resistance and inertness. To achieve this level of performance for the wide range of foodstuffs, package styles and manufacturing routes, many different coating formulations are needed, used either as a single coating or for some critical applications in combination in multi-coat systems. Although most coatings belong to a small number of basic chemistries, a typical metal packaging manufacturer may use a range of over 100 different coating formulations which adds to the complexity of managing food safety and regulatory compliance. Most coatings are thermoset systems and are formulated from resins, cross-linkers and additives. These are generally dispersed in solvent mixtures which are driven off in the early stages of the thermal stoving sequence leading to the highly cross-linked, impervious, inert finished coating film. Powder coatings which are usually thermoplastics with some thermoset modification, may be used as a side stripe to protect the welded side seam of cans. In addition, increasing use is being made of thermoplastic polymer coated metal, either extrusion coated onto the substrate or laminated as a film. Difficulties in welding polymer coated metal limit its use currently to 'two piece' can technology and end/ closure manufacture and the market share of this material is still low. A summary of coating chemistries with their typical applications and properties is given in Table 11.1.

Can end sealants

As already described, the ends on food and beverage cans are mechanically seamed onto the can bodies. Although the 'double seam' gives a very tight, strong seal, a thin layer of rubber based sealant is incorporated, buried within the seam, to ensure a hermetic seal. This seal must accommodate the differential expansion and contraction of the can components during the heating and cooling of the sterilisation process as well as withstand the abuse that distribution and storage entails. Although the potential for contact between the foodstuff and the sealant is limited by the positioning of the sealant, it still needs to be considered in the management of the food safety and regulatory compliance of canned foodstuffs. These sealants are generally dispersions of rubber or latex either in an organic solvent or in aqueous dispersion which are applied to the curl of the end and allowed to dry at ambient or elevated temperatures before seaming onto the can. Control of sealant application and end seaming ensures the correct placement in the finished seam.

Closure gaskets

There are a large variety of metal closures for glass, plastic or metal containers which may be used for thermally processed foods, ambient stable foods, dry foods, still and carbonated beverages, etc. A common requirement for all these closures is an effective seal, in much the same way as with can ends. For thermally sterilised or pasteurised foods this seal is responsible for preventing post-process microbiological contamination of the foodstuff and its performance is critical to the safety of the finished product. In a similar way to can ends, the performance of the seal also ensures retention of the quality of the foodstuff though control of oxygen ingress and, for carbonated products, retention of carbonation. The range of materials that have the required properties is very limited with plasticised polyvinyl chloride (PVC)

Table 11.1 Typical protective coatings used for metal foodstuff packaging

Coating chemistry


Pack/process resistance



High molecular weight epoxy resins cross-linked with phenolic resole resins


Very good

Most widely used system

Universal gold lacquer for three piece cans Shallow drawn

Epoxy-amino and epoxy-acrylate



Thermoset polyester

Thermoplastic polymer coated

High molecular weight epoxy resins cross-linked with amino or acrylate resins Water reducible for reduced environmental impact

PVC dispersed in an appropriate varnish and conventionally stabilised with a low molecular weight epoxy , resin or epoxidised bean/seed oils

High molecular weight epoxy resins cross-linked with anhydride hardeners

Polyester resins cross-linked with amino or phenolic resins

May contain lower molecular weight epoxy resin

Extrusion coated or laminated film of thermoplastic polyester, polypropylene, nylon or combinations - high molecular weight



Very good Very good


Very good

Very good

Pack dependent

Very good Good

Universal lacquer for beer and beverage cans (water reducible) Side seam stripes Some food systems

Drawn cans Easy-open ends* Closures* *Often used over epoxy-phenolic basecoat

Internal white for three piece cans

May not be suitable for very acidic and aggressive foods

Shallow drawn cans

Easy-open and standard ends Closures cans plastisols being used for virtually all food applications and either plasticised PVC (plastisols) or thermoplastic polymers used for beverage applications.

11.1.4 Holistic management of food safety

In most cases, foodstuffs packed in metal packaging make use of its unique properties to provide long-term storage of foods and beverages with retention of food safety, quality and nutrition. This is achieved by maintaining the package integrity, which prevents any post process contamination of the foodstuff. The integrity of metal packaging relies heavily on the properties of the different materials used in its construction, particularly the internal protective coatings, the can end sealants and closure gaskets. As well as providing the long-term integrity of the package, these materials must not themselves contaminate the foodstuff at levels that may be harmful to health and in addition they must not taint the foodstuff in any way. Because of the complex interactions between real foods and metal packaging, only limited use can be made of predictive testing of package performance and long-term pack testing with real foods is invariably required to ensure the safety of the package over the one- to five-year shelf life. The packaging manufacturer and the food packer have responsibility for all aspects of the safety of the packaged food which includes package integrity as well as the potential for migration. It is essential that any restriction of materials available to the packaging manufacturer on the grounds of precaution do not compromise the existing high degree of microbiological as well as chemical safety that metal packaging provides.

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