Why Margarine Pots Should Not Go in the Microwave

Margarine and soft cheese pots are reused in many households, but they should not be used to heat food in the microwave. They are designed for chilled storage, not for the direct heating of food.

Why should a margarine pot not go in the microwave?

A margarine pot is generally made to hold its product at cool or refrigerated temperatures, within a restricted temperature range. Putting it in the microwave can cause it to warp, soften or become unstable, raising the risk of food contamination.

Polypropylene is a plastic polymer found in many types of packaging and kitchenware, but not every plastic container is suitable for heating. Its safety depends on its formulation, thickness, intended use and the manufacturer's guidance.

Before heating food, check the following:

• Margarine: the pot is made as packaging, not for cooking.
• Heat: unsuitable plastic may distort and release residues.
• Symbol: only use containers marked as microwave-safe.
• Glass: suitable ovenproof dishes are safer for heating meals.
• Base of the pot: inspect the markings before heating anything.

What happens to plastic when it is heated?

In a microwave, the food heats up and passes heat to the container. If the plastic was not designed for that temperature, it may bend, soften, crack or transfer substances from the material into the food, particularly in dishes containing fat and sauce.

This is a greater concern with reused pots, as they may already have been washed, rubbed, cut, had their lids forced on and come into contact with fat. Scratches, stains and warping make a container even less suitable for heating packed meals or leftovers.

How can you tell whether a pot is microwave-safe?

Start by turning the container over and looking on its base for a microwave-use marking. This is often shown as small waves, a microwave oven symbol or the words “microwave-safe”. If there is no such information, it is best not to heat food in it.

Do not heat it without a symbol.

The plastic number alone is not enough.

Even if a pot displays a recycling mark, it must also state that it can be used in the microwave.

If in doubt, move the food into suitable glass or crockery.

The number inside the recycling triangle identifies the type of plastic, but it does not guarantee that it is safe for heating. A pot marked with number 5, for instance, may be polypropylene, but it must still clearly state that it can withstand microwave use in the home.

Check the following before use:

• A microwave symbol on the base or packaging.
• The maximum temperature the container can withstand.
• No cracks, stains, cuts or distortion.
• A microwave-safe lid, if one will be used.
• The manufacturer's instructions on heating with or without the lid.

Why is glass safer for heating food in the microwave?

Glass intended for cooking does not release plastic compounds into food and is better able to withstand the heating of meals. Microwave-safe dishes, containers and ovenproof glassware are safer choices for rice, beans, meat, vegetables and ready-made packed meals.

Even so, glass must also be used properly. It is best to avoid thermal shock, not to take a cold container straight into intense heat, and to check that the item is labelled for microwave use. Plastic lids should be removed, or used only when they are compatible with heat.

For safer heating:

• Transfer food into suitable glass, ceramic or crockery.
• Do not heat food in margarine, soft cheese or ice-cream pots.
• Avoid plastic containers that are scratched, stained or distorted.
• Leave an opening for steam to escape when using a suitable lid.
• Stir the food halfway through heating to spread the heat evenly.

What else can margarine pots be used for?

As with the discussion of glass pots versus plastic in food safety, the issue is not reuse itself, but using each material for the right purpose. Margarine pots can be used for cold storage, but not to heat meals.

Use these containers to store small dry items, separate cold food for a short time, organise drawers or freeze portions only when the manufacturer permits it. When heating food, the safest rule is straightforward: remove it from the reused pot and heat it in glass, ceramic or microwave-safe crockery.