At first glance it seems practical: The herbs from the garden you want to freeze fit the washed-out margarine carton perfectly. And the empty ice cube trays can be filled up with leftover soup.

However, this can be harmful to health, consumer health experts say. This is because most plastic food packaging is single-use, meaning they're intended for specific food and a specific purpose.

If you use the packaging for something else, you run the risk of substances in the plastic contaminating your food, warns the Bavarian Consumer Advice Centre in Germany.

Studies in recent years have shown that traces of plastic can be found in human faeces after we eat some products packaged in plastic, while the internal organs of humans are also being contaminated with microplastics.

An empty ice cube tray can certainly be refrozen with new contents because that is what it was made for.

But the rest of the hot soup does not belong in there, and it should not be heated in the microwave either, as this can cause unwanted components to be released into the food.

For the same reasons, single-use plastic is not meant to be put in the dishwasher and exposed to high temperatures.

Similarly, packaged food not designed to be frozen shouldn't be. Instead, it needs to be removed from its packaging and placed in a freezer bag.

Glass or porcelain containers that can also be frozen or heated in the microwave are better for storing food. For polypropylene cans, look for the appropriate pictogram.

Caution is also advised with aluminium foil. Aluminium is not a problem in bought packaging because it is coated. But using the aluminium foil from the roll in the kitchen drawer can be dangerous: Acidic and salty foods or contact with other metals can cause components to dissolve from the foil and pass into the food.

For example, plates with sausage and gherkins should not be covered with aluminium foil – cling film is much safer. In fact, it is best to avoid wrapping food in aluminium foil at all, according to the consumer advice center.