Main types of food packaging

4.2 Main types of food packaging

  Packaging can be grouped into a number of basic types. These types are dis- cussed below:

  • plastics and plastic laminates • glass • paper board • paper plastic laminates and metal composites • metal • regenerated cellulose films.

4.2.1 Plastics and plastic laminates

  Plastic laminates here include all polymer laminates as well as paper-to- metal laminates. Laminates are used because, in spite of recent advances in polymer science, there is still no one polymer type that meets all the cost

  Packaging materials as a source of taints 67

  and performance requirements needed for food packaging. The layers in a laminate are bonded together using adhesives, which can contain solvents. In addition, the laminates may be printed with solvent-based inks. Addi- tives are added to the plastics to protect them from thermal degradation during processing into sheets, bottles etc. All of these chemicals, if present in the final packaging material, can sometimes migrate into the packaged food, and then may occasionally give rise to a taint.

  Common polymers include polyethylene (PE), linear lowdensity (LLDPE), lowdensity (LDPE), highdensity (HDPE), polypropylene (PP), polystyrene (PS), polyvinyl chloride (PVC) and polyethylene terephthalate (PET). The ease of migration of many tainting compounds through these polymers follows the order of polymers given, with the greatest migration through PE and the least through PET. In general, the more crystalline the polymer, the better the barrier properties. Typically, polyethylene is the polymer that is in contact with the food, with higher melting point poly- mers, barrier layers and coatings on the outer surface. Often the print has an outer layer applied to it. This protects the print and gives a gloss image. The result is that any tainting component is likely to migrate backwards into the food rather than out through the higher barrier polymers on the outer surface. Polyolefins (polyethylene or polypropylene) can also affect the taste of packaged food and drink by removing flavour components. This is called aroma scalping. Marin et al. (1992) analysed packaged orange juice. They showed the absorption of key components of orange juice into polyolefins.

4.2.2 Glass

  Glass is one of the packaging types least likely to give taint problems. Taints from glass packaging usually occur as a result of contamination of the food from a component or contaminant in the closures, or a contaminant deposited on the inside of the glass container. Contamination from the closure may occur from the metal lacquers used or the polymeric seal. Glass is often coated with thin coatings of polyethylene or aliphatic esters. This protects the glass from scuffing and also confers a lubricating or slip prop- erty to the surface, for example on screw threads. These coatings may, on rare occasions, degrade and give rancid odours.

4.2.3 Paperboard

  Paper is used as secondary packaging, for example for breakfast cereal, where a plastic inner bag is used to maintain the shelf-life of the product and the paperboard box provides the aesthetic and brand appearance as well as pack handling convenience. Migration of components from the paperboard into food in such packaging is usually low. Where taints do occur, they usually result from the contamination of the paper carton from

  68 Taints and off-flavours in food printing inks and solvents, or a chemical in the coatings or adhesives. Sizing

  agents (agents that control the absorbtivity of the paper) and wet strength agents are added to paper. These very rarely cause taint. The majority of paper used for packaging has some form of coating applied. The coating serves to provide a good surface for printing and thereby improves the appearance of the packaging. The most common coatings on paper consist of filler, a pigment and a binder. Common fillers are china clays, i.e. silicates. Pigments are often calcium carbonate or titanium dioxide. Binders are often styrene butadiene rubber (SBR) latex. Styrene odours can sometimes arise from these. Other binders used are styrenemaleic anhydride, acrylate copolymers, or alkyd polyesters. Impurities and monomers such as ethyl benzene and butyl acrylate can give rise to odours.

  Bleaching the pulp with chlorine dioxide can result in chlorinated phe- nolics. These are highly tainting and can be chemically converted, for example by bacteria, into even more powerful tainting compounds. A review of taints from paperboard packaging was carried out by Tice and Offen (1994). Odour can be caused by microbial action. Under anaerobic conditions volatile fatty acids (VFAs) can be produced. These VFAs include acetic acid (vinegar) and butyric acid (rancid butter). Wood, from which paperboard pulp is produced, contains glycerides, resin acids, alcohols, waxes and fatty acids. Hydrolysis of the glycerides can occur via lipase enzy- matic activity during storage. Autoxidation of unsaturated compounds occurs to form hydroperoxides, which in turn decompose to aldehydes and ketones with hexanal (a boardy mown grass odour) being a common product. Contamination of the paper pulp during paper production can occur. The use of recycled paper carries with it the risk of contamination resulting in taint. The most common causes of taint from paper are chloro- cresols, phenols and anisoles. Bromine derivatives of these compounds have also been reported (Whitfield et al., 1989; 1997)

  Kraft paper can contain a series of phenol derivatives that are poten- tially tainting. These include 2-methoxyphenol (guaiacol), 4-methylphenol (p-cresol), 4-vinyl-2-methoxyphenol, 2, 6-dimethylphenol and 4-hydroxy-3- methoxybenzaldehyde (vanillin). All of these compounds have been observed in one sample of paper at Pira International Analytical Labora- tory. There is good evidence in the literature that these compounds are all derived from the oxidation of lignin during paper manufacture (Kurschner, 1926). The p-cresol and guaiacol are highly tainting. The odour threshold for guaiacol and p-cresol is 21 and 200 ppb (parts per billion) above water, respectively (Saxby, 1992). Guaiacol can also be produced by biological action on vanillin (Kilcast, 1996).

4.2.4 Paper plastic laminates and metal composites

  Paper and plastic laminates are widely used in food packaging. Paper and aluminium foil are used on a large scale in drink cartons and paper tubes

  Packaging materials as a source of taints 69

  for chocolate and crisp packaging. In almost all cases, the foil has an extruded layer of polyethylene on both surfaces to protect the aluminium and provide a heat seal layer. Cardboard tubes are manufactured from spiral wound paper on metal mandrills. The mandrills have to be lubricated and the choice of lubricant is important if odour is to be avoided. If vegetable-based oils are used, aldehydes such as hexanal can be generated from free radical degradation of the vegetable oil. Minute traces of degraded oil containing the aldehydes are sufficient to cause taint in the sealed tubes. Drink cartons are manufactured by extruding a polyethylene layer onto paperboard. If the polyethylene is overheated, thermal oxidation of the polyethylene layer occurs. This results in a taint in the drink. Numerous workers have studied the causes of taints from this type of packaging. Hoff and Jacobsson (1981) discuss the mechanisms involved in the thermal oxi- dation of polyethylene. They found that out of 44 compounds identified and quantified, fatty acids and aldehydes predominated. Bravo et al. (1992) iden- tified odorous compounds resulting from thermal oxidation of polyethyl- ene. These are predominantly C6 to C9 saturated or unsaturated aldehydes and ketones, the important ones being hexanal, 1-hepten-3-one, 1-octene- 3-one, octanal, 1-nonen-3-one, nonanal, trans-2-nonenal and diacetyl (butane 2,3-dione). The effect of processing temperature and time on poly- ethylene has been investigated (Bravo and Hotchkiss, 1993). Hexanal, a well-known tainting compound is often the most abundant aldehyde pro- duced. a-Unsaturated aldehydes and ketones were found to be responsible for much of the odour associated with thermally oxidised polyethylene. Villberg et al. (1997) identified aldehydes and ketones that were responsi- ble for taints in a range of polyethylenes.

  Paper and metal composites are used in packaging of cakes and fish such as salmon where a decorative ‘upmarket’ presentation is required.

  A polyethylene or a coating may be applied to the metal as metal can catalyse deterioration reactions with foods. Paper is also used with a mineral coating containing finely divided metal particles applied to the food contact surface. This coating is called a susceptor layer. It becomes very hot and browns the food. The elevated temperatures can greatly accelerate taint forming reactions and migration rates of tainting compounds into the food.

4.2.5 Metal

  In most cases the metal is not in direct contact with the food. Direct metal contact can affect the taste and appearance of the food. Coatings are widely used on metal packaging; these are often organosol PVC, epoxyphenolic, polyester or PET. Taints are not that common compared to other types of packaging. This may be due to the stoving processes carried out on the coated metal during can production and retorting of the food in the cans before sealing.

  70 Taints and off-flavours in food

4.2.6 Regenerated cellulose films

  Twist wraps for boiled sweets are typical examples of regenerated cellulose film packaging. Taint transfer is rare and when it occurs it is often as a result of poor quality inks. A softener is often used which has the effect of plasticising the film. Softeners used include glycerol and sorbitol. These compounds do not have particularly strong odours or tastes.