The quality and safety of flexible packaging materials

Food contact materials (FCMs) comprise a broad and complex area, using many different types of materials and articles, as well as many different chemical substances such as additives in the materials and articles. The intent of packaging is to maintain its function of protecting the integrity, quality, freshness and safety of foods from the manufacturing plant through transport, shelf life and storage.

With the tremendous variety of plastic packaging materials available today, there are many possible combinations. The simplest structures may consist of just one or two layers. The more complicated structures can easily exceed eight layers, including all of the components such as primers, inks and tie layers.

The impact of plastics has been so widespread in the food industry. The use of polymers has been growing rapidly and occupying areas traditionally held by metals and glass. Besides, many polymeric-based coatings are being used on glass and metal containers. Plastics, in addition to the basic polymers, also contain some chemical components or additives, which are added in a small amount during manufacture and processing to impart desired properties to the polymer or to aid in their processing. These may be anti-oxidants, anti-blocking agents, antistatic agents, stabilisers, plasticisers, pigments, fillers, antislip agents, etc. The plastic packaging materials may also contain small amounts of monomers, oligomers, catalysts, polymerisation residues etc. These substances are generally low molecular weight components.

Package integrity testing is a measure of the package’s barrier material and seal i.e., a ‘leak test’ of the whole package. In addition to seal bonding failures or disrupted seals, leakage can be the result of large holes, pinholes or cracks in package materials. Either source of leakage represents the potential for product contamination – elements of the ambient atmosphere outside of the package entering the package – or for the materials inside the package to escape.

Incorrect handling of pouches during processing and post process could cause physical damage to the pouch and seal, which could weaken the seal or compromise the pouch hermeticity.

Multilayer packaging, consisting of different layers joined by using an adhesive or an extrusion process, is widely used to promote different products, such as food, cosmetics, etc. The bonding agents (adhesives, tie layers) are literally the glue that holds everything together. A poor choice in this area means the package seal will delaminate either immediately or, worse, upon aging. Special challenges include retort and autoclave applications, bonding of greatly dissimilar materials. Heat sealing is a widely used joining technique in the packaging industry1. Two films are pressed together by two heated bars or platens to cause a (partially) melted surface on both foils. If a small pressure is applied, fusion between the two foils is achieved. The bonding is achieved by the inter-diffusion of polymer chains across the interface. This is a time and temperature controlled mechanism and thus the sealing parameters such as interface temperature and heat seal time are most important to the seal properties like seal strength, toughness and failure mode, of which the sealing strength is the most important1,2. At a certain temperature, the seal starts to form. This stage is denoted as the seal initiation temperature. With increasing temperature, the seal strength increases to a maximum value. If the temperature rises, the melt becomes less viscous and finally, because of the applied pressure, thinning occurs, resulting in a reduction of the heat seal strength. It must be noted that the failure mechanism is highly dependent on the strain rate as thermoplastics are known to be very sensitive to strain rate and low strain rates will generally lead to peeling and/or delaminating failure modes whilst high strain rates lead more to tearing failure. Peel strength indicates how difficult it is to peel one substrate from another, but adhesive fracture toughness is indicative of how well the two substrates are stuck together. Different surface analysis techniques are used today to study multilayer packaging delamination mechanisms3,4.

Another key consideration is the level of barrier required. A product often needs to be protected from oxygen, moisture vapour or light. The influence of pores and leaks in a package on the total permeation depends primarily on whether or not the package is vacuum packed, or at atmospheric pressure5.

With the increase of application and demand of polymer materials in the packaging market and with the fast development of new functional materials, the development and testing of barrier films is not only limited to parameters such as permeability coefficient and transmission rate, it is more frequently to test the diffusion and solubility coefficient that affect permeation parameters directly.

In general, the permeability of plastics depends on crystallinity, molecular orientation, chain stiffness, free volume, cohesive energy density, temperature and moisture sensitivity. Higher crystallinity, molecular orientation, chain stiffness and cohesive energy density lead to lower permeability. Gas transport through polymers mainly takes place through the amorphous regions, but it may be strongly influenced by the presence of other phases. These effects can have different backgrounds. First by circumventing the impermeable regions, like crystallites, the effective path length of diffusing molecules will be increased, which is reflected in a decrease in the effective diffusion coefficient. Secondly, the other phases can act as physical cross-links which may reduce the chain mobility in the amorphous phase, which results in a lower diffusion coefficient as well6.

The selection of a barrier polymer for a particular packaging application depends not only on its barrier properties but also on other physical properties and a comparison of physical, mechanical and optical properties as well. Oxygen is a critical mass transfer component in a number of deteriorative reactions that can have an effect on the shelf life of many packaged foods. The permeability of gases such as oxygen, nitrogen and carbon dioxide through polymeric materials increases as temperature increases but the extent of these changes varies for different polymers. Knowledge of the quality kinetics associated with specific food products has permitted development of mathematical models to predict shelf life from data collected at elevated storage temperatures5,7.

Quality assurance of the packaged food and therefore the guarantee of consumer safety will always have priority and must remain the most important criterion for optimisation (Figure 1). The fulfilment of these requirements assumes complete knowledge of possible interactions between packaging and food during their contact time (Figure 2). Here, possible interactions between the two parts play an important role in the quality assurance of the food5. The term interaction encompasses the sum of all mass transports from the package into the food as well as mass transport in the opposite direction. The mass transfers, often coupled with chemical reactions, lead to quality changes in the food and packaging material.

Figure 1: Quality and safety assurance of flexible food packaging

The polymers themselves, being of very high molecular weight, are inert and of limited solubility in aqueous and fatty systems and are unlikely to be transferred into food to any significant extent5. The low molecular weight substances and additives possess high mobility and therefore there is a likelihood of their transfer (migration) from the packaging material into the package contents, thereby contaminating the food with a possible toxic hazard to the health of the consumer8. Furthermore, most flexible packaging, used in contact with food, has a laminate structure. This is due to demands to reduce pack weight, to improve physical properties such as strength and tear, to control the atmosphere surrounding the food etc. If the materials in the laminate differ in terms of polymer type, then in most cases adhesive tie-layers are required to bond the different layers together (adhesive laminating). The adhesive formulations used represent a chemistry that is chosen to withstand the processing and distribution environment of the filled product. Although adhesive layers in laminates are behind a barrier layer of the food contacting plastic, there exists the possibility that components of the adhesive could permeate from the adhesive, through the plastic, into food.

The safety of packaging materials used in direct contact with food is of critical importance, with extensive regulations covering chemical migration and food at all levels. Monitoring migration from packaging materials ensures the safety of the product. Within European Union (EU) legislation, two types of migration limits have been established. Firstly, an overall migration (OM) limit fixes the total quantity of migrated substances to a maximum of 10 mg/dm2 of the food contact material or 60 mg/kg of the food. Secondly, a specific migration limit (SML) applies to individual chemicals, based on the toxicological evaluation of the substance. The extent to which a substance migrates depends on the nature of the migrant (the chemical), the nature of the material(s) from which it can be released and the nature of the food with which it comes into contact. Taking plastic FCMs as an example, the EU Plastics Directive contains a positive list of monomers and additives permitted for use in the manufacture of plastic for food contact. Specific migration limits have been assigned to some of these substances following their toxicological assessment. Testing for compliance with these specific migration limits can be achieved in several ways. The food itself can be tested. The food contact material or article can be tested before it is used to ensure that it does not contain residues that can migrate at levels that could cause problems. Finally, uniquely for food contact materials, the food contact material or article can be tested for its suitability before use by employing food simulants that are intended to mimic the migration properties of different categories of foods. These simulants mimic aqueous foods, acidic foods, alcoholic foods and fatty foods under worst case conditions.

Figure 2 Intrinsic and extrinsic factors affecting food quality and safety

Storage temperature, storage duration and contact surface are also important factors. A high storage temperature accelerates transfer while longer storage duration raises the amount of transfer5. In addition, non-volatile migrants and flavours can be transferred when the food and packaging are in contact.

Food packaging has gained widespread importance in food safety due to the possibility of migration of chemicals from FCMs into the food contained therein. Identification of unknown compounds in complex matrices (e.g. foods, food simulants and food contact materials) utilises a range of advanced chromatographic and mass spectrometric techniques including headspace gas chromatography-mass spectrometry (GC-MS) and liquid chromatography (LC)-time of flight (TOF)-MS. To allow ‘design for compliance’, the required performance of the packaging material must be identified clearly during the development phase.

References

1. Mueller C., Capaccio G., Hiltner A., Baer E. (1998) Heat Sealing of LLDPE: Relationships to Melting and Interdiffusion, Journal of Applied Polymer Science 70, 2021-2030

2. Morris B.A. (2002) Predicting the Heat Seal Performance of Ionomer Films, Journal of Plastic Film & Sheeting 18, 157-167

3. Garrido-López Á., Tena M.T. (2010) Study of multilayer packaging delamination mechanisms using different surface analysis techniques, Applied Surface Science 256, 3799–3805

4. Moore D. R., Williams J. G. (2010) A protocol for determination of the adhesive fracture toughness of flexible laminates by peel testing: fixed arm and T-peel methods, An ESIS protocol, Revised June 2007, Nov 2010

5. Piringer O.G., Baner A.L. (2008) Plastic packaging, Interactions with food and pharmaceuticals, 2nd edition. Wiley-VCH Verlag GmbH & Co KGaA

6. Michaels A.S. and Bixler H.J., (1961) Solubility of gases in polyethylene. J. Polym. Sci., 50, 413-439

7. Blackburn, C. de W. (2000). Modelling shelf-life. In: The Stability and Shelf-life of Food. D. Kilcast, P. Subramaniam, Eds., CRC Press, Woodhead Publishing Limited, Cambridge

8. Grob K., Stocker J., Colwell R. (2009) Assurance of compliance within the production chain of food contact materials by good manufacturing practice and documentation – Part 1: Legal background in Europe and compliance challenges, Food Control, 20, 476–482

About the Author

Dr. Kata Galić is Professor at the University of Zagreb, Faculty of Food Technology and Biotechnology. She graduated in 1982, received her MSc in 1985 and PhD degree in 1989, from the University of Zagreb, Faculty of Food Technology and Biotechnology. She received The British Council Fellowship, from 1990 to 1991, and J.W.T. Jones Fellowship in 1994. Her research interests include permeability characteristics of polymeric materials and factors affecting barrier changes of polymers used for food packaging. Her scientific (46) and professional (21) papers are published in international and national journals and presented at national (25) and international (35) meetings. Her teaching experience includes undergraduate and graduate lectures as well as supervision of diploma (32), master (3) and doctoral (1) thesis.

She is a member of Editorial Board of the Trade Journal for Packaging, as well as member of Scientific Council at the Institute for packaging and material handling in Croatia. She was a vicedean for the international scientific cooperation (2001-2003). She is the member of the executive committee of the European Federation of Food Science and Technology (EFFoST), Croatian Society of food technologists, biotechnologists and nutritionists (CROFoST), and marketing task force member of the International Union of Food Science and Technology (IUFoST). She is also the member of the Royal Society of Chemistry (UK). She received State award for scientific promotion in the area of biotechnical sciences in 2009.