can metal containers be heated in the microwave?

In Germany they have carried out an experimental study with different rigid containers in both steel and aluminium for food use in order to analyse the safety and performance of microwave heating.

This research was funded by the organisations, Alcan Rhenalu, Biesheim, France APEAL, Association of European Packaging Steel Producers, Brussels, Belgium; Crown Food Europe, Paris, France; FGM, Fördergesellschaft Metallverpackungen, Düsseldorf, Germany; Impress, Deventer, The Netherlands; Novelis Deutschland, Göttingen, Germany.

Microwave oven, gas oven, steam oven and water oven heating tests were carried out with food packaged in five metal containers of different sizes. Specifically, a round steel bowl (99 mm diameter x 35 mm height) used with 200 g of filling, a round steel bowl (127 mm diameter x 30 mm height) used with 250 g of filling, a square steel bowl (125 mm x 125 mm x 25 mm) used with 300 g of filling, a rectangular aluminium bowl (160 mm x 99 mm x 35 mm) used with 400 g of filling, a round steel bowl (153 mm diameter, 36 mm height) used with 425 g of filling were used in the experiments.

These vessels were chosen for their large open surface area and shallow profile. These were compared in the various subsequent tests with plastic containers of similar shape and size, which were filled with the same amount of test foods. The plastic containers were made of C-PET and were specified for microwave heating. In all experiments, the furnace power was 100% and only the heating time was adjusted to achieve the heating target.

Some of the parameters studied were effects of misuse of the metal containers in the microwave ovens and the stability of the microwave power level of the oven during the experimental series was tested.

Heating safety

The conclusions have been clear, one of the main ones revealing that heating metal containers is safe if they are cooked in microwaves. When the tested metal containers were used in microwave ovens in standard food heating experiments, they were safe to use as not a single spark was produced and no hazardous situation was observed.

However, it is necessary to comply with some recommendations such as removing the lid completely from the container before heating the container in the microwave. As with all other containers, it should only be inserted one at a time into the centre of the turntable, leaving an air space of at least 2.5 centimetres between the walls of the appliance when the container is full, so it is important not to insert several containers at the same time. If a glass turntable is not available, the pan should be placed on a ceramic plate.

Longer than plastic ones

Another peculiarity that many people don’t know is that heating food in the microwave takes less time in plastic containers than in metal ones, in some cases even twice as long. In other words, to obtain the same heating effect in metal containers, a longer heating time is required, since the plastic containers can be accessed from all sides, whereas the metal container is accessed with the lid open.

For example, the time to heat a portion of food in an equivalent plastic container was 3.5 minutes. With larger metal vessels, the heating efficiency increases slightly and a smaller time difference is observed.