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Food Focus August 2008

Don’t let plastics take the wrap

Plastic packaging plays a significant role in the shelf life and ease of storage and cooking for many foods but there have been some concerns about chemicals migrating from the plastic packaging or cling film into food

There are over 30 types of plastics used as packaging materials, although polyethylene and polypropylene are the most common. In New Zealand’s food industry, it is up to food manufacturers and sellers to ensure their products are safe and comply with the relevant legislation. Packaging suppliers should also ensure their products are suitable for the intended use.

Types of plastic packaging

Polyethylene plastic comes in high or low density. High-density polyethylene is stiff and strong and used for milk bottles, water and juice bottles, cereal box liners, margarine tubs, grocery, rubbish and retail bags, but is not heat stable (ie, it melts at a relatively low temperature). Low-density polyethylene is relatively transparent and used to make films of various sorts (including domestic/household cling film), bread bags, freezer bags, flexible lids and squeezable food bottles.

Polyethylene terephthalate (PET or PETE) is a polyester. It is commonly used in soft drink bottles, jars and tubs, thermoformed trays, bags and snack wrappers because it is strong and resistant to heat, gases and acidic foods.

Polypropylene is more heat resistant, harder, denser and more transparent than polyethylene so is used for heat-resistant microwavable packaging and sauce or salad dressing bottles.

Polycarbonate is clear, heat resistant and durable, and often used instead of glass for refillable water bottles and sterilisable baby bottles.

Polyvinyl chloride (PVC) is heavy, stiff and transparent and often used with added plasticisers such as phthalates or adipates. Common uses of PVC with plasticisers include commercial-grade cling films for over-wrap of trays in supermarkets and filled rolls at delicatessens.

How stable are plastics with food?

Plastics in common household use here are very stable if used appropriately. Most plastics in contact with food have a basic composition of high molecular weight and therefore are unlikely to migrate into food.

To make plastics more useful, low molecular weight additives are used to increase flexibility, make them more ‘sticky’ (for cling film), heat stable or, for example, have anti-microbial compounds in them. Small amounts of low molecular weight compounds may potentially leach into food during cooking or storage.

When polycarbonate bottles are washed with harsh detergents or bleach (eg, sodium hypochlorite) tiny amounts of bisphenol A are formed. At high levels of exposure, bisphenol A is potentially hazardous because it mimics the female hormone estrogen.

Diethylhexyl adipate (DEHA) is a food-compatible phthalate plasticiser found in commercial cling films made from PVC. Tiny amounts of DEHA may migrate into fatty food (such as meat or cheese), especially with heating. DEHP (diethylhexyl phthalate) is another plasticiser that can migrate. It has been used for jar or bottle seals and lid inserts of bottles, spreads and juices, and may be in printing ink for labels.

Taking precautions

Proper use of plastic packaging reduces any chemical migration. Follow manufacturers’ instructions when using household plastics, such as cling films and oven bags, and when cleaning containers, bottles and lids. Use the correct type of plastic for the role, eg, only use microwave-safe plastics in the microwave.

Meat can be safely frozen in its plastic tray and wrap, but keep it at a cool temperature if left in the wrapping to thaw. DEHA can migrate into fatty food if heated.

If using household cling film when microwave cooking don’t let it touch the food as it melts at a low temperature. Leaving a corner of the dish uncovered to let steam escape reduces the risk of the film blowing off and settling on to the food.

If you re-use plastic containers, only re-use those that are food compatible, and use them in the way the original food was presented. For example, you can freeze food in ice-cream containers but don’t heat them in the microwave – they were designed for use on cold food. Some types of plastics are not microwave proof and will melt at moderate temperatures, such as margarine containers.

More information on migration of chemicals from plastic into food is available here.