Metals safe and efficient for microwave, says report

The latest findings conducted by Fraunhofer Institute for Process Engineering and Packaging, suggest that during 1,000 trials of aluminium and steel packaging in a microwave oven, there was not a single spark or similar danger recorded.

With consumers increasingly demanding both convenience and sustainability from their food purchases, processors are looking for innovative ways of attracting a consumer, both through the product itself and the packaging.

The use of electro conductive materials, such as metal has long been deemed unsuitable for microwavable foods due to fears that it could cause damage to the oven.

However, the latest findings suggest that using metallic materials like steel and aluminium, which are fully recyclable, could help processors meet customer demands for convenience, while also allowing them to be safely heated in a microwave.

Along with its suitability for use, the study also looked at quality benefits of steel and aluminium packaging over popular plastic alternatives like CPET, commonly used in convenience packaging.

When placed in a microwave, metal packaging was found to distribute heat more evenly through food than in the same tests for plastic at a serving temperature of 75°C.

The testing was performed on a number of food types including egg batter, chili con carne and infant meals.

Using the example of chilli con carne, the testing found that in metal containers, the difference between the hottest and coldest parts of the food was between 20°C to 40°C, depending on container size and oven type.

Under the same conditions, the temperature difference at different parts of the food in plastic containers was between 40°C to 60°C.

The study suggested this was in part due to the additional time needed to heat food in metal packaging, which is less heat efficient, the study added.

In some testing, metal containers took about three times as long to heat foods in a microwave, then that needed for plastics.

The report added that packaging size heavily affected heating efficiency in food with larger metal containers, posting a reduced time difference for cooking compared to plastic products of similar size.

However, to ensure the safe use of metal packaging in the microwave, the institute said that a number of guidelines had to be met.

These include ensuring only one metal container is heated at a time, and that it is kept at the centre of a glass turntable 2.5 centimetres away from the floor and sides of the oven.

The use of an additional plastic dome to cover the food also helped to reduce the danger of the metal coming into contact with the walls, the report added.

In a case of their not being a turntable, the report claims a ceramic dish can be used as a substitute.

Additionally any product with a metal lid, must also be uncovered before heating.

The testing, compiled by the Fraunhofer Institute's Thomas Pfeiffer, used five different forms of metal packaging chosen due to their large open surface and shallow profile, which are preferable characteristics for microwave heating.

These products included two round steel bowls, 99mm by 35 mm and 127 mm by 30 mm, filled respectively with 200g and 250g of food.

A square steel container 125mm by 125mm by 25mm, which contained 300g was also used.

Other products included a rectangular aluminium container 160mm by 99mm by 35 mm used with 400g of food and a round steel container 153mm by 36mm containing 425g.

The packaging was tested in four common household microwave ovens, each with their own individual nominal power ratings and cavity sizes.

They were all used at 100 per cent efficiency with only the heating time being adjusted.

The latest study, which was commissioned by a number of metal organisations including the Association of European Producers of Steel (APEAL), also made use of earlier testing performed by the Fraunhofer Institute in 2006, entitled the "Microwaveability of Aluminium Foil Packages".

This report was commissioned by the European Aluminium Foil Association (EAFA).