Barrier packaging materials
Posted: 23 April 2015 | Kata Galić, Faculty of Food Technology and Biotechnology, University of Zagreb | 2 comments
An important requirement in selecting food packaging systems is the barrier properties of the packaging material. Barrier properties include permeability of gases (such as O2, CO2, and N2), water vapour, aroma compounds and light. These are vital factors for maintaining the quality of packaged foods.
Traditional packages (glass containers, metal cans) as well as plastic bottles, and laminates (such as paper laminated with aluminium foil) provide a proper barrier to oxygen. However, some differences exist between the various packaging systems. In the case of metal cans and glass containers, these can be regarded as impermeable to the passage of gases, odours and water vapour. Plastics-based packaging materials provide varying degrees of protection, depending largely on the nature of the polymers used in their manufacture1.
Barrier packaging materials: Metals
Metals such as tinplate (tin coated steel) and aluminium are used in can and tray manufacture.
Metal packaging offers the longest shelf life and superior safety for foods and beverages by preventing microbes, light and oxygen from damaging the product inside the container2. Metal can be manufactured into the traditional three-piece can, which includes a base, cylinder and lid; or a two-piece can, consisting of a base and cylinder in one piece and a lid without a seam.
Both plain and lacquered tinplate cans are used for food and beverage packaging. Plain internal tinplate cans are used for specific food types (white fruits (e.g. pineapple, pear) and some vegetables (e.g. mushrooms, asparagus)). The presence of a bare tin surface inside the can leads to protection of the natural flavour and appearance of the food, through oxidation of the tin surface in preference to oxidative degradation of the food. This process retains the quality attributes that consumers expect from these products throughout the entire shelf life. One of the disadvantages of metal cans is that they are prone to corrosion.
The general pattern of corrosion in lacquered cans is different from that in plain cans, and is generally more complex. It depends not only on the quality of the base steel plate, the tin–iron alloy layer and the tin coating, but also on the passivation layers and the nature of the lacquered coating. The effectiveness of a lacquer coating is related directly to its ability to act as an impermeable barrier to gases, liquids and ions, thereby preventing corrosive action on the protected surface1.
High purity grades of aluminium (99.5%) and its alloys are still preferred for some foods due to the reasonable corrosion resistance of the metal. This resistance is attributable to the easy and rapid formation of a thin, continuous, adherent oxide film on exposed surfaces3. In packaging, aluminium can also be unprotected or protected by a lacquer or a plastic film. Aluminium is easily formed into cans with hermetic seals. Aluminium foil laminates are also used to package oxygen or moisture sensitive foods, in the form of pouches, sachets and tubes, and as tamper evident closures. Aluminium foil containers and trays are used to bake pies and to pack takeaway meals and snacks.
This was the preferred material for food packaging due to its inertness and non-toxic nature. Moreover, it has very high barrier properties and hence acts as an excellent material for preserving the aroma of its contents and also protecting the food from external influences4. A clear glass bottle is transparent to visible light. This is why light sensitive food should be packaged in coloured glass. Technological advances in glass packaging have led to improvements in strength and weight, as well as colour and shape. The critical aspect of glass packaging is the closure which can consist of a cap, lid, cork or plug to seal the jar or bottle. Although glass provides an excellent barrier to water vapour, gases and odours, an incorrectly designed or applied closure may compromise the benefits that packaging offers in protecting food products from deterioration and lead to a reduction in shelf life1.
Due to their flexibility, variability in size and shape, thermal stability and barrier properties, plastic packaging products is the fastest growing sector, replacing the traditional materials of glass, metal, paper and board. In general, the permeability of plastic packaging depends on its characteristics (such as crystallinity, molecular orientation, chain stiffness, free volume, cohesive energy density, etc.), permeate properties (such as molecule size and nature) and external conditions (such as temperature, moisture etc.).
The temperature and humidity conditions to which a product is likely to be exposed in the supply chain are vital in calculating the required barrier. Thus, it is essential to specify these and check that the data being quoted are applicable to the conditions expected. Haze and gloss are extremely important properties in plastic packages, since many users demand a highly transparent material with a glossy and brilliant appearance. On the other hand, food can be adversely affected by prolonged exposure to light in transparent polymeric films.
Improvements in the barrier properties of plastic can be obtained by biaxial orientation processes. Biaxial orientation results in increased toughness, increased stiffness, enhanced clarity, improved oil and grease resistance, and enhanced barrier properties to water vapour and oxygen5,6. This process is performed for many polymers, such as: biaxially oriented polypropylene (PP-BO), biaxially oriented polyethylene terephthalate (PET-BO), and biaxially oriented polyamide (PA-BO).
As it is difficult to obtain all the desirable barrier properties from a single polymeric film, multi-layered, laminated, coextruded, coated, and metallised films are manufactured to meet the varied barrier requirements of food packaging.
Laminates are multi-layers of foil, paper and/or plastics that may be utilised selectively according to the specific food packaging need. In combination, the various laminates provide more strength and barrier protection than the individual material (monofilm). Laminates of paper/foil/polyethylene composition rely on plastic layers for heat sealing (forming leak-tight containers). Aluminium foil provides a barrier to moisture, gases and light, whereas paper provides stiffness, strength and shape. This pack format is used in the fruit juice and milk sectors.
Apart from aluminium foil, the barrier layer can consist of other barrier resins such as ethylene/vinyl-alcohol (EVAL) or a barrier coating such as polyvinyl alcohol (PVAL), polyvinylidene chloride (PVDC), metallized aluminum, or one of the ‘glass coatings’ (silicon oxides (SiOx) or aluminum oxide (AlOx)). The SiOx coating offers a greatly improved barrier towards oxygen, carbon dioxide and moisture, which is not affected by temperature and humidity7,8. Transparent SiOx is usually coated on the surface of polymers such as PET, PP, PA, polyethylene naphthalate (PEN), and PVAL films9,10.
Aluminium metallised films are mostly used for packaging nuts and salty snacks (e.g. crisps). The oxides of SiOx and AlOx are commonly used to replace aluminium foil in applications such as fibre-based packages for dry food mixes, drinks, sauces and seasonings, polymer composite cans, and packages for snacks, coffee and pet foods11,12.
Additives and enhanced packaging
The incorporation of nano-sized and sub-micron-sized additives is under intense investigation for enhancement of barrier properties in both rigid and flexible packaging. Nanoscale is considered where the dimensions of the particle, platelet or fibre modification are in the range of 1–100nm13. Commonly used fillers to prepare nanocomposites for food packaging applications are montmorillonite clays, kaolinite, carbon nanotube and graphene nanosheet. However, the problem of the detection and analysis of nanomaterials is complex and there is strong need for appropriate analytical methods to identify nanomaterials properly14,15.
Barrier technology continues to improve and so-called ‘active and intelligent’ packaging is becoming a reality16. Active packaging can been seen as a means of maintaining the optimum (desired barrier) conditions to which a food is exposed, while intelligent packaging detects the status of foods (e.g. quality, maturity) inside the package.
The advancement in this technology will require researchers to continue to use non-traditional packaging approaches to meet new challenges. Thus, packaging science, food science, biotechnology, sensor science, information technology, nanotechnology and other disciplines are coming together to develop a breakthrough packaging technology. Issues such as those relating to legislation, economics and consumer privacy also need to be addressed17.
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About the author
Kata Galić is a Professor at the University of Zagreb, Faculty of Food Technology and Biotechnology. Her research interests include permeability characterisation of polymeric materials and factors affecting barrier changes of polymers aimed for food packaging. She has published 79 scientific and 24 professional papers and has been a mentor of 33 diplomas, four masters and four doctoral theses. She is co-author of two university textbooks and three book chapters, and has been a Vice-Dean (2001-2003) and Congress Secreatary (2001, 2003, and 2008) for the International Scientific Cooperation. She has also received state awards for science in the area of biotechnical sciences (2008 and 2011).