Review on metal packaging: materials, forms, food applications, safety and recyclability

Abstract

Metal based packaging materials provide excellent barrier properties and hence, being used widely in food packaging applications. They are used in different package forms and also as closures such as for glass bottles and composite cans. Major health and product safety concerns of metal packaging comprise migration of bisphenol A, lead, cadmium, mercury, aluminium, iron, nickel, bulging of cans, tin dissolution, blackening and corrosion. Metals are not inert to food products, hence coated with protective lacquers to prevent metal–food interaction and migration of metal components. Metal packaging materials have lower global warming potential and higher recyclability due to their magnetic properties which helps in easier segregation. An attempt has been made in this article to review the metal packaging materials used in food industry and Indian Standard specifications, their safety and recyclability aspects.

Keywords:

Metal packaging, Lacquers, Corrosion, Migration, Safety, Recyclability

Introduction

The global demand of packaging was estimated to reach US $ 974 billion by 2018 with Asia, North America and Western Europe accounting to a little over 40%, 21% and 19%, respectively of this demand (Essuman 2018). Among them metal packaging materials represents 15% of the total packaging materials being used across the globe (FICCI 2016). According to a market intelligence report, the metal cans market was valued at US $ 47.88 billion in 2018 and is expected to reach US $ 58.25 billion by 2024 with a cumulative annual growth rate (CAGR) of 3.39% (Mordor Intelligence 2019).

Various metals like aluminium, tin plate, tin free steel and stainless steel, and metal based packaging material in both rigid and semi-rigid forms such as cans, foil wraps and retort pouches are most commonly used for food packaging applications. The historical developments in metal packaging materials is given in Table  . The global production of metal cans in 2014 was 364.4 billion cans and is estimated to reach 430 billion cans by 2020, at a CGAR of 2.9% with North America (32%) and Europe (30%) leading the market share. Among the food products, beverages industry leads with a market share of 75% in 2014, of which, alcoholic beverages constitute 45% of the market share and remaining 30% is from non-alcoholic beverages (Business Wire 2015). Metal cans are manufactured mostly by aluminum, tinplate, tin-free steel (TFS) and stainless steel (SS), of which aluminum cans are widely used. Cans are available in various shapes and sizes but conventional round cans represent 90% of the total market (Joshi et al. 2003). Due to rising demand for healthy beverages, carbonated soft drinks, fruit and vegetable juices, among all the geographical regions, North America and Asia-Pacific are projected to positively impact the metal cans market. Metal packaging industry has both positive and negative impacts on health and environment. Metal based packaging provides excellent barrier to light, gas and moisture, recyclability, easy conversion into various shapes, ability to withstand high heating temperatures, rigid structure, transportation to long distances and unique decorating possibilities. However, all these benefits come relatively at a higher price. The negative aspects comprise of global warming due to carbon dioxide emission from metal manufacturing units, leaching of harmful toxic chemicals from containers to food and depletion of resources. A balance between both the aspects of the metal food packaging need to be upheld with better research and policies.

Table 1

YearMilestones in metal packaging1699Manufacture of tinplate in England1720Manufacture of tinplate in France1760Dutch navy preserved roasted beef in tinned iron canisters1804Nicholas Appert discovered process for conserving food in containers1810Peter Durand patented canning process titled as “substitution of glass jars and bottles with tin cases”1811Successful trial of canning with Royal Navy on Durand’s request1812Patent of Peter Durand acquired by Bryan Donkin in 1000 euros1822Introduction of tin cans in the United States by Thomas Kennsett and Ezra Daggett1825Chemical isolation of aluminium using potassium amalgam by Hans C. Oersted1892Establishment of first pineapple cannery in Hawaii1896Max Ams of New York patented the double seaming of can ends1913Commercial production of aluminium foil in the USA for wrapping candy bars1920sMarketing of household foil1921Lamination of aluminium foil on paperboard for folding cartons1930sTinplating of steel by electro-deposition1935Introduction of 3-piece soldered tin plate beer can by Krueger Brewery Company, USA1938Development of heat sealable aluminium foil1950sIntroduction of two piece aluminium can; development of retort pouches1958Launching of first aluminium drawn and wall ironed (DWI) cans by a USA Brewery1960Initial work on development of electrolytically chromium coated steel (ECCS) in Japan1960Use of aluminium for beer can ends by Schlitz1962Easy open ends for canned beverages patented by Ermal Cleon Fraze of the USA1992Maximum tin level in food products restricted to 200 mg/kg in the United Kingdom1970sIntroduction of tinplate can1975Stay on tab can ends patented by Daniel F. Cudzik from Virginia1975Reports of corrosion related problems with canned apple sauce1995Prohibition of tin/lead solder for food container by United States FDA2000Launching of aluminium bottlecan by Japanese breweries2001Use of aluminium bottlecan for energy drinks in the USAOpen in a separate window

Food packaging is not only half (by weight) of the total packaging material sales but also accounts for 67% (by volume) of the total packaging waste out of which 10% is metal packaging waste. In 2016, in the European Union alone, 170 kg of packaging waste was generated per inhabitant which varied between 55 kg (Croatia) and 221 kg (Germany) per inhabitant among the member countries. Paper and cardboard (41%), plastic (19%), glass (19%), wood (16%) and metal (5%) are the most common types of packaging waste (EUROSTAT 2019). According to the United States Environmental Protection Agency, 1.8 million tons of aluminum was generated as packaging waste in 2015, which represented about 0.8% of the municipal solid waste and out of which 670 thousand tons of aluminum beverage cans was recycled (USEPA 2019a). On an average 2% of the world’s energy is utilized for producing aluminium (The World Counts 2019) and packaging industry of the USA alone utilizes approximately 27% of the total aluminium consumed. Recently, aluminium cans with thinner gauge material had resulted in 26% weight reduction which directly permits production of 34 cans as compared to 27 cans from a pound of aluminium. Furthermore, steel can weight had reduced by 40% since 1970. Recycling of aluminium can is swinging between 50 and 52% because of better collection, segregation and recycling (Marsh and Bugusu 2007). An attempt has been made in this article to review the metal packaging materials used in food industry and Indian Standard specifications (Table  ), their safety and recyclability aspects.

Table 2

Indian Standard regulationsSubjectsIS 2471 (1963)Methods of test for metal containersIS 6941 (1973)Method for sampling metal closuresIS 7182 (1973)Methods of test for aluminium collapsible tubesIS 8932 (1978)Preformed metal screw caps for glass containersIS 4638 (1981)Seamless rectangular fish tinsIS 2134 (1981)Round tins for general purposesIS 9991 (1981)Condensed milk cansIS 11079 (1984)Metal cans for tobacco based productsIS 1394 (1984)Glossary of terms related to metal containersIS 1994 (1987)Crown closuresIS 9396-1 (1987)Round open top sanitary cans for foods and drinks, part 1: tinplateIS 5818 (1988)Specifications for lacquers and decorative finishes for food cansIS 3603 (1988)Seamless aluminium bottlesIS 1783-2 (1988)Drums, large, fixed ends, part 1: grade B drumsIS 2087 (1988)Square tins for general purposeIS 2552 (1989)Specifications of steel drums (galvanized and ungalvanized)IS 8996 (1988)20-L steel jerricansIS 8970 (1991)Specifications of aluminium foil laminate for packagingIS 1783-1 (1993)Drums, large, fixed ends, part 1: grade A drumsIS 1406 (1995)Rectangular tins for liquidsIS 3101 (1995)Aluminium collapsible tubesIS 14407 (1996)Aluminium cans for beveragesIS 10325 (2000)Square tins-15 kg/L for ghee, vanaspati, edible oils and bakery shorteningsIS 10339 (2000)Ghee, vanaspati, Edible oil tins up to 10 kg/L capacityIS 1993 (2006)Cold-reduced electrolytic tinplateIS 2034 (2012)Round open top sanitary cans for butter and cheeseIS 9396-2 (2012)Round open top sanitary cans for foods and drinks, part 2: sizes and general requirementsIS 11078 (2012)Round open top sanitary cans for milk powderOpen in a separate window

Materials for metal packaging

Coated steel

Several alloys of iron are called as steels, with all of them having carbon content ranging between 0.2 and 2% which binds the iron atoms in rigid lattice and contributes to mechanical properties of steels, especially exceptional tensile strength. The alloys of iron used for food packaging applications can be categorized as ‘carbon steel’ with carbon content not exceeding 1% (Lee et al. 2008). Tin plate, tin free steel and polymer coated steels are the three majorly used coated steel for food packaging applications.

Tin plate

Tin plate is the most important coated steel used in food packaging applications. Tin plate usually refers to steel (base steel) coated with tin on each side. Historically, dipping was used for coating and it was known as hot-dipped tin plate, but now electroplating is most commonly used (known as electrolytic tin plate) because of its ability to have coatings of different thickness on both sides. The process of tin plate production (Fig.  ) involves: tinning which involves covering of steel base plate with thin layer of tin; flow melting comprising thermal treatment above tin’s melting point (260–270 °C) and rapid quenching in water leading to formation of tin iron compound (FeSn2); chemical passivation in a sodium dichromate electrolyte generating tin and chromium oxides on the surface thus providing more stability and resistance to atmosphere; coatings with oily lubricant such as dioctyl sebacate and acetyl tributyl citrate for resistance against scratch, environmental corrosion and finally passage of tin plate sheets through container forming machines (Barnes et al. 2006). Tin plate is cheaper and heavier than aluminum, recyclable, has magnetic property which helps in its easy segregation, easy to decorate, impermeable to moisture and gases, and also withstands high temperature of product processing which makes it suitable for sterile products including beverages for longer storage. However, it requires surface coatings as it may react with food and its containers generally requires an opener to access product (Catala et al. 2005). According to the Bureau of Indian Standards (IS: 9396 1987), the tin plate for food and drink cans should have tin plate of 0.15–0.49 mm thickness and coated on both sides either by dipping or electro-deposition. As per IS:5818 (1988), permitted thickness and properties of tin coatings on metal plate had been reported in Table  . For thermally and non-thermally processed food products, MR type (medium residual) baseplate may be used and type L (low metalloid) baseplate is recommended for highly aggressive products and special applications as copper toxicity (i.e. copperiedus) occurs in contact with acidic food products.

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Table 3

Protective coatingsDry coating weight as per the Indian Standard specification (g/m2)PropertiesOleoresinous4–10Inexpensive, naturally occurring oils with synthetic modifications, golden colouredVinyl lacquers2–7Vinyl chloride or vinyl acetate co-polymers, free from flavour taints, sensitive to soldering, used as top coating for beer and beverage cansPhenolic lacquers1.5–5Self-cross-linking phenolic resins, used in container where flexibility is not critical, excellent for aggressive foodsEpoxy lacquers3–5Epoxy resins with phenolic resins or zinc oxide, not suitable for acidic productsOpen in a separate window

Tin free steel (TFS)

Tin free steel or electrolytically chromium/chromium oxide coated steel (ECCS) is similar to tin plate except non-involvement of flow melting and chemical passivation during its production (Fig.  ). The production process involves dual electroplating of chromium and chromium sesquioxide and finally coating with an oil such as butyl stearate oil. ECCS is slightly less expensive as compared to tin plate and more susceptible to corrosion in acidic environment because of absence of sacrificial tin layer and therefore usually coated. Conversely, it is more acceptable for protective enamel coatings than tin plate because of low melting point (232 °C). The use of TFS is less as compared to tin plate and mainly utilized for food can ends, crown caps, and vacuum closures for glass containers (Li et al. 2011). Removal of coatings as a prerequisite for welding of TFS hinders its extensive usage for single use containers and recyclability. Moreover, its low cost over tinplate makes it the best choice for drums used in bulk storing and transportation of finished products.

Polymer coated steel

Various efforts for the passivation of steel against corrosion has led to the utilization of conducting polymers like polyaniline, polythiopen and polypyrrole for coating steel cans. A bilayered polypyrrole coating prevented corrosion of steel in 3.5% sodium chloride solution for 200 h (Ohtsuka 2012). Similarly, thermally sprayed coatings of synthetic fluoropolymer like polyvinylidene fluoride (PVDF), ethylene chlorotrifluoroethylene (ECTFE), perfluoroalkoxy alkane (PFA) and fluorinated perfluoroethylenepropylene (FEP) had been successfully tested for avoidance of corrosion using optical microscope, liquid immersion and salt spray tests (Leivo et al. 2004). Studies on utilization of polyethylene terephthalate (PET) and polypropylene (PP) as coatings on deep drawn cans had indicated good effectiveness as well as no solvent emission during process (Boelen et al. 2004). Polymer coated steels are highly abrasion and corrosion resistant with outstanding appearance and moisture barrier properties.

Stainless steel

Stainless steel is the iron alloy which possess extensive corrosion resistance and chemical inertness property due to chromium content (normally above 11%). Although, chromium is highly active metal but in contact with atmospheric oxygen, it forms an inert layer of chromium oxide (Cr2O3) on steel surface leading to its auto-passivation against corrosion. Stainless steel, owing to its corrosion resistance and inertness, is used in food industry as a packaging material and for development of food processing and storage equipment. Austenitic, ferritic and martensitic are the three major types of stainless steel based on their crystalline structure. Austenitic types are considered as food grade and most commonly used for packaging applications. Stainless steel is costly as compared to aluminium and tin therefore it is mainly used for returnable containers in food packaging (kegs for beer, wine and soft drinks). However, for large storage or transport containers, stainless steel is the leading material. Food and dairy processing industries mainly utilizes austenitic 304 grade (18% chromium and 8% nickel) for mild treatment applications and austenitic 316 grades (16% chromium and 10% nickel) for excessive corrosion resistant surfaces due to presence of 2% molybdenum in the latter (Cvetkovski 2012).

Aluminium

Aluminium, with 8.8% of earth’s crust, is the most abundant metallic constituent. Aluminium production process involves the conversion of alumina to aluminium hydroxide (Al(OH)3) in a solution of sodium hydroxide at 175 °C. The insolubles are filtered off and soluble Al(OH)3 precipitated as white fluffy solid. The cost of aluminium is higher as compared to almost all coated steels and mostly preferred for seamless containers because of its incompetency to get welded. Aluminium is mainly used as light weight packaging material in its pure form for sea foods, soft-drink cans, pet foods etc. while addition of manganese enhances its strength (Morris 2011). It is also used for making foil, cans, laminated and metallized packaging material in combination with paper and plastics. Aluminium is used for food packaging application in different forms like collapsible tubes, bottles, caps, closures, retort pouches, laminated and metallized films, which are discussed in subsequent sections. Aluminium is considered to be the best material for recyclability because of its easy conversion to new products but foils from recycled aluminium usually contain pinholes and its non-magnetic property creates segregation glitches.

Metal packaging forms

Three piece cans

As the name indicates, three-piece structure consists of two end lids and a body (flat metal sheet rolled with longitudinal side joined to form cylindrical structure with open top and bottom). Three piece cans are easier to form in any combination of height and diameter, providing mixed specification cans with ease. Firstly, the cut sheets of metal are coated, printed and rolled as per the requirement and longitudinal sides of the flat metal sheet is joined either by soldering or welding (Fig.  ). However, soldering is usually avoided for food cans because of concerns related to migration of lead from tin/lead solder into foods. Welding is preferred for side seaming in food industries, which not only overcomes safety concerns of soldering but also reduces the metal usage as overlapping is a prerequisite for welding which requires less metal as compared to interlocking for soldering. After the formation of round hollow structure (cylindrical body) by side seaming, necking and flanging for beverage cans and beading and flanging for food cans are performed. Necking usually refers to reducing the diameter of flat ends in perfectly cylindrical seamed body to concave ends thus offering metal usage reduction. The beading treatment of food cans gives spherical curves (corrugations) to the round structure of can for extra strength required during retorting of food products. Flanging refers to creation of outward flanges (hooks) for better fitting of can lids. Can ends (lids) are attached mechanically using double seaming process, involving interlocking of flange and lid’s edge (Hosford and Duncan 1994).

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Two piece cans

Two piece cans were a major innovation in can making, consisting of one end (lid) and seamless body (flat metal sheet stretched to form cylindrical cup type structure with closed bottom) without any joint (Fig.  ). Two piece cans are economic, hygienic and have high printing area as compared to three piece cans because of absence of side seam. Two piece cans are approximately 35% lighter and provide better integrity since absence of side seam doesn’t require coatings or enamel usage. The production process for two-piece cans involves two major process namely, drawn and wall ironed (DWI) and drawn and redrawn (DRD).

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Drawn and wall ironed (DWI)

The first DWI can was introduced in 1958 by a United States Brewery. The production process involves stamping of circular discs from lubricated metal sheets; forming process punch draws circular disc to shallow cup leading to uniform wall thickness throughout the structure; ironing process involving passage of shallow cup through a series of tungsten carbide dies resulting into wall thickness reduction and increase of body height. Drawn structures are later trimmed to have same height and cleaned for removal of lubricants. Finally, bottom end is domed or profiled and similar to three piece cans they are necked or beaded and flanged (Silbereis 2009). The base thickness for DWI can is more than wall thickness as during ironing process only side wall is stretched and thinned.

Drawn and redrawn (DRD)

The concept of DRD container is based on the production of cartridge cases in Switzerland during Second World War. The initial stages of stamping and forming for DRD can development is similar to DWI cans. The ironing stage of DWI cans is replaced by multistage drawing in DRD, which is sequential stretching of can’s initial structure (shallow cups). The multistage drawing leads to metal flow from base to the wall of container and providing similar wall and base thickness for DRD cans. The final treatments including trimming, cleaning, flanging and lidding for DRD cans are similar to DWI cans (Silbereis 2009).

Aluminium foil

The term aluminium foil indicates aluminium with 99% purity which is available in the form of thin rolled flat sheets varying in thickness from 4 to 150 microns. Commercially, for the first time, it was introduced in United States in 1913 for wrapping candy and chewing gums (Life Savers™). The first step of aluminium foil production is casting of metal into rectangular blocks followed by scalping to remove oxides. Hot or cold rolling of thick heated aluminium sheets through a series of rolling mills until desired thickness is achieved. Finally, annealing or softening is done to remove hardening which occurred during rolling. Aluminium is mainly preferred because it is the lightest weighing metal with exceptionally good malleability. Different alloys of aluminium foils are available, which includes 1100, 1124, 1235 and 3003. Alloy 1235 is most commonly used in flexible food packaging applications while alloy 3003 for thicker and stiffer aluminium foil (Lee et al. 2008). At a minimum thickness of 15 microns, it is impermeable to moisture and gas and takes the shapes of the food or product over which it is wrapped, which is referred to as dead fold property without spring back (Bayus et al. 2016).

Aluminium collapsible tubes

Collapsible tubes are flexible, light-weight, tamper-proof and hygienic food packages with multi-usability option, delivering the product in required amount when desired. It is formed by extrusion of aluminium slug using a plunger. Annealing at 600 °C relieves hardness and final treatments like enameling and printing enhances its appearance. End sealing is performed by folding and heat sealing after applying some latex or lacquer. In food industry, aluminium collapsible tube was mainly used for cheese spread, mustard cream, butter, honey, condensed milk, mayonnaises, tomato ketchup, jams and jellies. Currently, availability of cheaper alternatives in the form of plastic laminates had considerably reduced their use in food (Robertson 2013). According to the BIS (IS: 3101 1995), collapsible aluminium tube’s outside diameter, wall thickness, shoulder angle and shoulder thickness shall be 12.7–57 mm, 0.10–0.18 mm and 30°, respectively.

Aluminium bottles

Aluminium beverage bottle, which is also referred as bottle can, was launched in 2000 by Japanese breweries. Until 2008, bottle cans were made by impact extrusion but now coil-to-can process quickly and efficiently produces bottle cans of varying shapes and sizes at a lower cost. Impact extrusion technique involves extrusion of aluminium slug contained in a die cylinder by a spindle which produces a seamless can of very small sizes. Aluminium bottles are mainly used for aerosol and beer. Recently, a bottle can with easy opening cap having reduced wall thickness and weight while maintaining the necessary features was patented by Nakagawa et al. (2016).

Laminated and metallized films

When two substrates are bonded together by heating or adhesive it is referred as laminated (Bayus et al. 2016). Aluminium foil is mainly laminated with a variety of papers and plastics using water or solvent based adhesives and waxes to improve the functionality especially in terms of barrier properties. The aluminium foil laminate used for packaging shall be plain or printed, soft or hard annealed, smooth, free from specks, holes, objectionable flavour and 0.006–0.15 mm thick. The arsenic, copper, iron and lead content should not be more than 2, 30, 70 and 20 ppm (IS:8970 1991). Adhesive is applied to foil and pressed against other substrate with heat application for linking of diverse materials and solvent removal. Recently, Fujii et al. (2019) patented a heat bonded laminate of metal foil and biaxially oriented polyester film with enhanced corrosion, heat, acid resistance and high tensile strength possessing suitability for DRD and DWI can manufacturing process. A cheaper and more flexible alternative to laminated films is physical deposition of aluminium fumes/vapours over any plastic compatible packaging material by condensation which is called as metallization. During the process of metallization, aluminium is heated to 1500–1800 °C to form vapours which are allowed to condense on the plastic film. In contrast to thickness of metallic layers in laminate structures, which is about 5–12 µm, thickness of aluminum coating due to metallization is in the range of 10–40 nm (Piergiovanni and Limbo 2004a, b). Oriented polypropylene (OPP) and PET films are most commonly used as metallized films which results in decorative, reflective and more durable films (Lange and Wyser 2003). Aluminium laminated films are used for premium category of food products like dried herbs and soups while metallized films are used for bakery products, snacks and certain food powders.

Retort pouches

Retort pouch is a multilayered, flexible and hermetically sealed pouch which mainly consists of aluminium foil as a barrier layer. A typical retort pouch consists of an outer PET layer for strength, middle aluminium foil layer for barrier to air, moisture and light, central nylon layer for abrasion resistance and inner food contact polypropylene (PP) layer for heat seal. It is mainly used for ready-to-eat pouches. Major advantages of retort pouches over cans are comparative barrier properties at lower thickness which also allows rapid heating and ease of disposal as it can be flattened. However, segregation of different layers for recycling and its susceptibility to rupture during rapid cooling after high heating are concerns of retort pouches. Recent advances consist of transparent and resealable retort pouches achieved by replacing central aluminium foil layer with oriented polyamide, EVOH, PVDC and zippers, respectively (Chonhenchob et al. 2017). Poly (trimethylene terephthalate), a bio-based material had shown promising results for the replacement of nylon and PET from retort pouches thus making them more environment friendly (Kim et al. 2018).

Metal drums

Drum is an industrial packaging material which is mainly used for bulk storage and transportation of food materials in processed or semi-processed form. Drums are large containers with 100–250 L volume while smaller counterpart ‘pails’ are of 5–25 L capacity. Drum consists of body, lids (top and bottom) and fittings such as locker ring for open top. The production process of drum involves shearing of metal sheets, grinding aimed at sharp edge reduction, body forming process which converts flat sheet into cylindrical shape, spot and seam welding intended for joining overlapped structure, corrugating process that creates symmetrical corrugations and finally flanging for co-joining lid with drum. The whole body of the steel drum should be made from a single sheet. Drums up to 25 L capacity should be provided with a handle for convenience. When drums are used for liquid products they should not show any signs of leakage when exposed to a pressure of 40 kPa. In food industry, metal drums are mainly used for handling concentrates, powders or highly corrosive products.

Metal caps and closures

Metals are also used for sealing of beverage glass bottles in the form of pilfer proof closures (Fig.  ). The crown cap is usually made either from tinplate, tin free steel or black plate with a nominal thickness of 0.25 mm. The internal and external surface of the crown cap should be finished in order to provide protection and decoration and shall be free from any pit holes and off-flavors. The inside surface of the crown cap should be lined with cork or plastisol liner using hot melt type adhesive. Twist-off, press-twist, pry off and deep-press are the commonly used metal closures in the canning industry. All these closures are made up of tin plate and applied to the container by screwing or applying pressure under vacuum and opens up by unscrewing or pulling off the lever provided with the containers. Lids on glass jars and bottle tops are normally made up of 20–70 micron aluminium foils with polyacrylic or polyvinyl chloride coatings. Vacuum closures like lug caps, twist-off type, etc. are used for baby foods, jams, pickles and dry powders (Peter and Ulrich 2007). Aluminium foil is used for sealing of milk bottles and lids of yoghurt, fruit drinks, jams, cheese spreads etc. as tagger or tamper or pilfer proof diaphragm for containers.

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Metal corrosion

Corrosion of metal is a type of destructive food–metal electrochemical interaction which is confined to tin, iron and lead in case of food cans (Montanari and Zurlini 2018). Owing to the presence of moisture in food can, galvanic cells are formed on coupling of two metals at different dissolution potential. Anode i.e. the active metal dissolves, leading to ion formation, which precipitates in the form of insoluble salts at the surface. Spontaneous corrosion occurs at cathode due to hydrogen release, oxygen reduction or electron consumption (Mannheim et al. 1983). Stress corrosion cracking is an example of localized corrosion at stressed areas which is caused due to sulphur dioxide, hydrogen sulfide and chloride present in food products. Sulfide black corrosion or sulphur staining is blackening of metal container or product due to reaction between metal and product constituents such as oxygen, sulphur and phosphorus. Pitting corrosion includes rapid dissolution of iron. Chlorides present in water or sweetener cause perforations in the aluminium. Filiform corrosion is external corrosion which generally occurs when storage temperature is 20–35 °C and relative humidity is 60–95%. These food–metal interactions not only affect the shelf-life of food products but can also affect human health in case of food consumption from corroded containers (Jellesen et al. 2006). External corrosion of metal cans may occur due to acidic or alkaline label adhesive, poor venting of retorts, low external tin coating, moisture from humid environment, leaking product from neighboring cans and rust particles from rusty retorts (Page et al. 2003).

Protective and decorative coatings on metal cans

Tin plate is not inert totally to environmental corrosion and contact food materials (especially highly acidic products like fruits juices), therefore, it is coated with some substances called as coatings or lacquers or enamels (Barilli et al. 2003). Lacquers on interior of food cans are intended to prevent interaction between can and its contents that may result in deterioration of the product or reduces performance. They basically consist of a solution of resin or resin-drying oil complex in volatile solvents, which after application to tinplate or any other metal sheet suitable for can manufacture, dries and leaves resin or resin-drying oil complex as a hard film on the metal surface. External finishes, either lacquers or decorative finishes, should be impermeable to moisture, resistant to abrasion and capable of withstanding normal can manufacturing (Peter and Ulrich 2007). Food can internal lacquers are generally applied by roller coating and baked in an oven but materials suitable for application by brushing or spraying had also been used. According to Bureau of Indian Standards (BIS) (IS:5818 1988), can lacquers and decorative finishes are categorized into three types, category A as internal food can lacquers, category B as external food can lacquers and category C as decorative finishes. The category A consists of lacquers for processed foods (class-I) and for non-processed foods (class-II). Depending up on the end use, class-I materials are further classified into three types. Type 1 is an acid resistant lacquer suitable for a range of food stuffs including coloured fruits and type 2 is a sulphur resistant lacquer suitable for certain vegetables, meat and fish based foods. While type 3 is a sulphur impermeable lacquer suitable for meat products which require long sterilization at high temperature. Oleoresins, vinyl, phenolic and epoxy lacquers are the permitted lacquer materials as per Indian standard requirements (Table  ). It is also desirable to heat or dry (stove) lacquered sheets at a temperature not more than 204 °C as tin melts at 231.9 °C.

Applications of metal in food packaging

Dairy products

The main role of packaging is to prevent deteriorating factors such as light, moisture, oxygen and microorganisms from affecting the shelf-life of milk products. Pasteurized milk for retail sale is generally packed in polyethylene pouches. The premium milk variants like ultra-heat treated flavoured milk, condensed milk and evaporated milk are packed in metal based containers in the form of cans or multilayered packages such as retort pouches and aseptic containers. Ultra-high temperature treated milk is packed in multilayer packages which consists aluminium foil as an internal barrier layer. Evaporated and condensed milk packed in metal cans with lacquer coatings remains shelf-stable for 6 months to 1 year. Fat rich dairy products like butter, butter oil and cream require protection against fat oxidation due to light and oxygen. Butter wrapped in parchment paper develops an objectionable oxidised flavour while butter cubes wrapped in aluminium foil are acceptable even after 48 days of storage. Light transmittance of metallized paper is less than 10% and negligible for foil which makes it an appropriate packaging material for butter to prevent oxidative degradation. Fermented products like dahi, yoghurt, kefir, kumiss, etc. are mainly packed in high impact polystyrene (HIPS) and polypropylene containers for retail purpose and short-shelf life (Raju and Singh 2016). However, aluminium foil and plastic or paperboard and foil laminate are highly preferred as lids for yoghurt and dahi. Metallized films of PET are used for fresh cheese varieties like cottage, mozzarella, cream, feta, etc. while cheese slices or spreads are widely wrapped in printed or plain aluminium foil across the globe.

Ethnic Indian milk based confections such as khoa, rasogolla, gulabjamun, rasomalai, paneer, chhana and ghee are packaged in tin containers for extending their shelf-life and promoting exports. Lacquered tin containers with capacities varying between 1 and 15 L are widely used for packaging of ghee (Sabikhi et al. 2018). Sachets and stand-alone cartons having multilayered laminated structures of polyvinyliedene chloride/aluminium foil/polypropylene are also used for packaging of ghee in small sizes intended for better aroma protection and longer storage life. Burfi stored in tin cans at 30 °C possessed a shelf-life of 150 days. Gulabjamun is hot filled in tin cans for extending its shelf-life to 6 months at room temperature (Vasava et al. 2018). Chhana and paneer stored in tin cans offers maximum shelf-life with lowest chemical deterioration. The shelf-life of rasogolla stored in tin cans with permissible preservatives like sodium benzoate possess a shelf-life of 6 months. Sandesh, another traditional sweet of eastern India, when stored in tin cans at 30 °C remains acceptable for 45 days. Chennapoda prepared with different amount of semolina, sugar and cottage cheese remained acceptable in three layered retort pouches [12 μm polyethylene terephthalate/9 μm aluminium foil/62.5 μm copolymer of nylon and cast polypropylene (CPP)] for 30 days at refrigerated storage (Pal et al. 2019).

Ice-cream is predominantly packaged in reusable plastics with very little use of steel cans. Reusable ice-cream cans are lead soldered cylindrical tin cans having ‘slip-on-lids’ with rounded corners. In order to have better presentation at scooping shop, ice-cream (especially ‘Gelato’) is often packed in reusable stainless steel containers (Goff and Hartel 2013). Powdered products of milk including whole milk powder (WMP), skim milk powder, butter milk powder, ice-cream powder, casein, caseinate, infant milk powder, whey powder, lactose powder, cream powder and cheese powder are packed in metal or tin cans having easy opening closures. Fat containing powder like WMP, butter milk powder, cream powder are packed in tin cans with nitrogen gas flushing to achieve very low oxygen concentration (approximately less than 4%) in the headspace in order to prevent deteriorative reactions related to fat oxidation.

Beverages

Soft drinks usually contains sugar dissolved in treated water with additional ingredients while carbonated drinks also contain carbon dioxide. Therefore, carbonated soft drinks require containers which can hold internal pressure of CO2 and being corrosion resistant at the same time, which is provided by enameled two piece cans. Two piece cans overcome the problem of flavour deterioration related to iron pick from side seams of three piece cans as the former lacks side seams (Bernardo et al. 2005). The first three piece beer can was introduced in 1935 by Krueger Brewing firm in the United States (Barak 2018). Till 1950s, tin plate was majorly used for beer cans, but iron pick up from the seamed end led to objectionable metal based turbidity in the product. Aluminium cans with easy opening ends called as “stay on tab” provided the best barrier properties to deteriorating factors of beer i.e. oxygen and light, which had created monopoly of aluminium beer cans in the present market. However, aluminium cans had never been accepted as packaging of wine because of sulphur based foul smell. Oxygen scavenging by immobilized yeast, self-cooling beer cans and temperature indicator based on thermochromic ink for beer cans are some of the recent advances in beer packaging (Ramos et al. 2015).

Fruits and vegetables

The selection of metal based packaging material and protective coatings for canning of fruits and vegetables depends on its type, processing (majorly thermal treatment) and storage conditions. The inherent mildness or aggressiveness activity is dependent on the natural components and treatments given to fruits and vegetables. Very high concentration of nitrates in vegetables like spinach, lettuce, radish and green beans, anthocyanins in raspberries and red fruits, and sulphur dioxide added as preservative in fruit juices reacts with metallic containers and corrodes it. Sulphur rich vegetables like garlic, onion and asparagus reacts with metal and form black spots of metallic sulphides accompanied with the release of hydrogen sulphide gas. Zinc oxide based or type II coatings are used to avoid the black staining of cans. Interestingly, pineapple canned in plain tin plate cans without any coating endorsed reaction between tinplate and ingredients of pineapple, creating an attractive yellow colour of product (Robertson 2013). However, this could be unsafe to consume but no such incidents and reports are available.

Fish, meat, poultry and sea foods

Several other ready to eat products like crab, shrimp curry and prawn products are widely canned for longer storage and distribution. Various fish products like mackerel in brine and oil, mussel in oil and brine, fish curry, tuna in oil and prawns in brine were canned in polymer coated tin free steel (TFS) which resulted in a shelf-life of more than 24 months at a temperature of 28 ± 2 °C (Mallick et al. 2006a). Tuna (Euthynnus affinis) fish canned in open top sanitary (OTS) cans with sulphur resistant lacquer and ECCS containers with clear PET coating possessed a shelf-life of more than 5 months at room temperature (Maheswara et al. 2011). Similarly, rohu (Labeo rohita) fish curry canned in polyester coated TFS in 60:40 ratio of fish and curry remained acceptable in terms of chemical, texture and sensory attributes up to 6 months at 37 °C (Mallick et al. 2006b). Seer fish (Scomberomorus guttatus) curry and mackerel fish curry packaged in three layered flexible retort pouch (12µ polyester/15µ aluminium foil/75µ polypropylene) remained acceptable for 24 and 12 months, respectively (Gopal et al. 2001). Four layered retort pouches (12μ polyester/12μ aluminium foil/75μ cast polypropylene/biaxially oriented nylon 15μ) were found suitable for heat treatment and 12 months ambient temperature storage of Rogan josh, a traditional meat curry of Kashmir (Shah et al. 2017). Ready-to-eat black clam (Villorita cyprinoides) remained acceptable in three layered retort pouches (12.5μ polyester/12.5μ aluminium foil/80μ cast polypropylene) for 1 year at ambient temperature (Bindu et al. 2007). Heating or storage of food in aluminium foil had been reported to increase the aluminium concentration of stored food commodity irrespective of the side i.e. shiny/dull surface (Ertl and Goessler 2018).

Bakery and confectionary products

Historically, unsealed aluminium foil of 0.009 mm thickness was used to wrap chocolate blocks but presently, heat sealable laminates (aluminium foil and LDPE) are most commonly used. Premium quality chocolates and other bakery products like biscuits, cookies and crackers are packed in metal boxes as gift packs (Fig.  ). Nitrogen gas flushing in metal cans is also used for packaging superior quality fried snack products while use of metallized films is widespread in confectionary and bakery products (Mexis et al. 2011). Aluminium foil with varying thickness is also used for biscuit overwraps.

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Coffee and tea

Tinplate cans were the first commercialized container for packaging of roasted and milled coffee which provides barrier to loss of volatiles and holds the pressure created by CO2 emission during their storage. Flexible laminated packages having aluminium as the middle layer (approximately 12 µm thickness) are most widely used for packaging of roasted and powdered coffee. Initially, instant coffee was packed in metal cans for retail sale but flexible packaging material having aluminum foil layer between PET and LDPE (PET-Al foil-LDPE) had replaced it because of 1 year shelf-life at lower cost by latter one. Paperboard cartons with aluminium foil liner are used for packaging loose tea while premium tea products are packed in metal containers with snap-on lids. Aluminium foil is used for bulk packaging of tea in the form of tea chest liner. Metallized multilayer plastic films provides the best aroma and moisture barrier properties for longer storage of tea. Ready to drink tea is also packed in retort pouches (Kim et al. 2011).

Food safety issues of metal packaging

The adverse health effects of metal containers for food packaging are mainly related to two major process: migration and interaction. Migration refers to the transfer of packaging components to food product or vice versa during their storage or processing. The common migrants from metal packaging includes tin, bisphenol A (BPA), lead, aluminium, chromium, coatings and contaminants from metal. Overall migration for metal packaging is usually carried out using plastic’s protocol with distilled water, 3% acetic acid, 10% ethanol, 50% ethanol and n-heptane as food simulants. However, no use of acetic acid had been suggested for coatings on metal surface by the Commission of the European Communities (CEC) (Peter and Ulrich 2007). Interaction is the physical, chemical or microbiological reaction at food–package interface or compatibility of food products with metal containers depending on their chemical composition, pH, processing treatments, container’s material, coatings on the package, storage temperature and humidity. Interaction of metal and food results in corrosion, pitting, perforation, loss of coating and product deterioration and discolouration. Some of the common catalyst for enhancing the reaction between food and metal include nitrates, phosphates, plant pigments, synthetic colours, copper and sulphur compounds.

Bisphenol A (BPA), chemically known as 2,2-(4,4′-dihydroxydiphenyl)propane is used as a monomer in the production of polycarbonate and epoxy-resins (lacquer), as antioxidants in some plasticizers and as an inhibitor of polymerization of vinyl chloride in the production of plasticized PVC. It represents major migrant from tin cans into food products which is an endocrine-disrupting chemical and causes failure of reproductive system and also possess carcinogenic activity. A study revealed higher level of BPA migration from heat processed tuna fish cans (121 °C for 90 min) as compared to non-heated cans coated with organasol and epoxy phenolic coatings. Commercially heated and organosol resin coated cans filled with food simulants was reported to contain 646.5 µg/kg of BPA, which is much above the safe limit (Munguia-Lopez et al. 2005). A positive relationship was reported in migration level of BPA from coffee cans with caffeine content. Different levels of caffeine (0.05, 0.1, 0.5 and 1.0 mg/mL) in canned coffee had 21.5, 23.8, 58.9 and 79.7 ng/mL of BPA, respectively (Kang and Kondo 2002). The partitioning tendency of BPA in solid portion of canned foods as compared to liquid portion was investigated using liquid chromatography mass spectrometry method (Noonan et al. 2011). Canned food items (fruits, vegetables, soups, fish and meat) in Belgian market contained 40.3 ng/g of BPA on an average, which was also found to be dependent on the type of can and sterilization treatment given (Geens et al. 2010). The chlorohydrins of bisphenol A diglycidyl ether (BADGE) and of bisphenol F diglycidyl ether (BFDGE), used as starting materials for epoxy-resins, were detected in canned vegetables and coffee samples in Japan market. However, the migration were within the safe limit (< 600 µg/kg of food) defined by European Union, but concerns related to formation of more toxic products of BADGE and BFDGE still remains unanswered (Uematsu et al. 2001). An exposure assessment study of BPA in New Zealand indicated 7% contribution of BPA to oestrogenicity whose impact remains unclear. Contrarily, water stored in uncoated stainless steel and aluminium lined with epoxy resin showed no detectable BPA contamination representing their aptness for use as ‘BPA free’ container (Cooper et al. 2011). However, considering the adverse effect of bisphenol based coating materials, use and presence of its derivative BFDGE and novolac glycidyl ethers (NOGE) had been prohibited in the manufacture of food contact surface during 2005 itself by European Union (Peter and Ulrich 2007).

Studies had revealed presence of aluminium in food samples (pastries and ready to eat meals) contained in aluminium trays. Animal experiments in past had showed connection between aluminium and Alzheimer’s disease, owing to which aluminium weekly uptake safe limit was reduced from 7 to 1 mg/kg body weight by FAO/WHO Joint Committee on Food Additives (Stahl et al. 2011). Three different tomato sauce samples packed in aluminium foil containers showed aluminium leaching which increased with decrease in pH and increase in temperature which is alarming for consumers and regulatory bodies as Al foil is extensively used in such products (Joshi et al. 2003). Tin at higher concentrations can cause gastrointestinal perturbations like nausea, vomiting, diarrhoea, abdominal cramps, bloating, fever and headache owing to which the maximum permissible levels of tin in solid foods is 250 mg/kg and 150 mg/kg in beverages as per Food and Agriculture Organization (FAO). Tin content in canned fruits (pineapple, lichies, pear, mushrooms, apricot, guava, fruit cocktail and mango) was evaluated and for canned papaya and apricots it was 269.8 and 153.4 mg/L. Acidic juices may corrode the tin plate which results into substantial level of tin in food. Higher concentrations are generally observed in canned vegetables and fruits (Morte et al. 2009). Various other contaminants from metal packaging may include chromium, used for tin plate treatment, which causes carcinogenic and mutagenic toxicity; fatty acid and esters used as lubricant leads to stale, rancid or woody flavour in canned beverages, and lead used for soldering is highly toxic to bones and brains of infants (Arvanitoyannis and Kotsanopoulos 2014). More studies are needed for detection of tin and aluminium in food items contained in metal based containers along with their threshold and toxicological concerns.

Environmental aspects of metal packaging

The consumption of packed and processed food had increased the proportion of packaging waste in total mass of municipal solid waste. Packaging waste in Italy is 35% in weight and 50% in volume of total municipal waste (Norgate et al. 2007). The raw materials for metal packaging material iron (iron ore and scrap iron), aluminium (aluminium ores) and tin (tin ores) are limited, moderately limited and severely limited (Pongracz 2007). Owing to the magnetic properties of the steel, its segregation and recycling is quite easy and it has a higher recycling rate (about 60% in European Union countries) as compared to other metal packaging materials. Different management approaches/pathways like recycling, landfilling and combustion for energy recovery are utilized for reducing the metal packaging material waste and its adverse effects. After disposal and subsequent incineration, a tin can decomposes in 6–12 months but takes 6–10 years for complete reduction by oxidation without incineration (Griese 1971). The workers of the aluminium production plants are exposed to carcinogenic polyaromatic hydrocarbons (PAHs) and additives involved in metal packaging like adhesives, coatings and decorative inks are a significant source of hydrocarbon pollution (Pongracz 2007).

Global warming potential (GWP) indicates the amount of greenhouse gases (carbon dioxide, methane, nitrogen oxide) and halogenated hydrocarbons emitted while cumulative energy demand (CED) refers to renewable and non-renewable energy collected throughout the life cycle of a product. A study on environmental feasibility of 330 mL of beer aluminium can (67.9 g aluminium per liter of container) with 1 L aseptic juice carton (35.2 g of 6 layered material per liter of container with 1.4 g HDPE closure) and 1.5 L plastic water bottle (19.3 g PET per liter of container and 0.9 g HDPE closure) highlighted 826 g CO2 equivalents for beer can in comparison to 113 and 78 g CO2 equivalents for juice and water container. CED of beer can, juice carton and plastic bottle were 19.06, 2.68 and 1.53 mega Joules (MJ), respectively. Incineration of 1  kg aluminium produces 0 MJ of electric energy (electricity generated was regarded as avoided environmental load) and 1.6 kg of slag. Aluminium cans are 100% recyclable with losses including only coatings and inks, and with disposal prioritization order as recycling followed by incineration and landfill (Pasqualino et al. 2011). Similarly, recycling of aluminium and polyethylene (Al foil-PE) laminated film was the best option for environmental benefit and energy saving as compared to incineration and landfilling (Xie et al. 2016). GWP of plastic, paper, glass and metal packaging material during recycling was 2.23, − 0.28, − 0.08 and − 0.12, where ‘−’ symbol represents savings in energy or emission reduction (Beigl and Salhofer 2004).

Aluminium beverage cans are usually thrown in waste streams causing blockage and pollution of water bodies. Aluminum of virgin grade consumes huge amount of energy and also results in generation of huge amount of solid waste and sludge as only 20% of raw material goes into final product. Nearly 8 kg of bauxite, 4 kg chemical and 14 kWh of electricity is saved by recycling of 1 kg aluminum and on an average within 6–8 weeks an aluminium can is re-melted and packed with fresh product (Pongracz 2007). Recycling of aluminium is a better deal as it consumes 11.7 giga Joules (GJ)/ton energy as compared to 140 GJ/ton for smelting of aluminium ore and recycling also reduces the extraction of virgin ore bauxite. Further, during processing of virgin aluminium, potent greenhouse gases like perfluorocarbons (PFCs) are generated while recycling involves no such PFCs. Similarly, processing of steel from raw materials require 25.2 GJ/ton while recycling requires 9.43 GJ/ton. During melting of aluminium scraps about 10% of the material is lost irreversibly due to its mixing with slag in induction furnace and reaches to 38% loss in gas and oil furnace (Diaz and Warith 2006). Environmental impact studies of three packaging materials namely aluminium foil, metallized oriented polypropylene (MOPP) and metallized polyethylene terephthalate (MPET) revealed 71% and 52% less impact on metal depletion by MOPP and MPET as compared to aluminium foil. Global warming potential of MOPP and MPET was almost half that of aluminium foil (Bayus et al. 2016). In an evaluation study using life cycle assessment approach, stainless steel production by smelting, aluminium by vertical electrode cells and titanium by FFC Cambridge process were more effective in reducing the adverse environmental impacts of metal production (Norgate et al. 2007).

Conclusion

Strict regulations on level of emerging migrants from metal packaging material into food products need to be framed similar to plastics. Detailed studies in context of metal food packaging and relationship between food components, migrants, heat treatment and their adverse health impacts need to be focused. Specular reflectance/transmittance methods like FT-IR (Fourier Transform Infrared Spectroscopy), gas chromatography (GC), liquid chromatography mass spectrometry (GC–MS) etc. provides a quick and easy means to screen parameters of can coatings. In similarity, modern and quick methods for determination of migrants from metals, coatings and lacquers, their threshold and toxicological concerns are to be urgently researched and regulated. WasteWise, a voluntary conglomerate between United States Environmental Protection Agency (EPA), institutions, non-profit and government organizations promotes waste reduction, reuse of materials, sustainable management of materials and recycling. The members of WasteWise prevented 8.5 million tons of waste in 2016 which would have burdened or harmed the environment (USEPA 2019b). Analogous to WasteWise, other countries should also start similar initiatives for control and reduction of packaging waste materials. Environmental enforcement agencies need to create and streamline the metal packaging materials’ disposal methods based on their initial and end use since same treatment could not be given to all types of packaging materials. Moreover, similar to plastic identification codes some codes for metal packaging may be needed to be introduced for their improved segregation, recycling and reusability.

Footnotes

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